Excited-state dynamics of small organic molecules studied by time-resolved photoelectron spectroscopy

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(1)Excited-state dynamics of small organic molecules studied by time-resolved photoelectron spectroscopy Ting Geng.

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(3) Excited-state dynamics of small organic molecules studied by timeresolved photoelectron spectroscopy Ting Geng.

(4) ©Ting Geng, Stockholm University 2017 ISBN print 978-91-7649-758-6 ISBN PDF 978-91-7649-759-3 Printed by Universitetsservice US-AB, Stockholm 2017 Distributor: Department of Physics, Stockholm university.

(5) Abstract Ultra-violet and visible light induced processes in small organic molecules play very important roles in many fields, e.g., enviromental sciences, biology, material development, chemistry, astrophysics, and many others. Thus it is of great importance to better understand the mechanisms behind these processes. To achieve this, a bottom-up approach is most effective, where the photo-induced dynamics occurring in the simplest organic molecule (ethylene) are used as a starting point. Simple substituents and functional groups are added in a controlled manner to ethylene, and changes in the dynamics are investigated as a function of these modifications. In this manner, the dynamics occurring in more complex systems can be explored from a known base. In this thesis, the excited state dynamics of small organic molecules are studied by a combination of time-resolved photoelectron spectroscopy and various computational methods in order to determine the basic rules necessary to help understand and predict the dynamics of photo-induced processes. The dynamics occurring in ethylene involve a double bond torsion on the ππ* excited state, followed by the decay to the ground state coupled with pyramidalization and hydrogen migration. Several different routes of chemical modification are used as the basis to probe these dynamics as the molecular complexity is increased (i) When ethylene is modified by the addition of an alkoxyl group (-OCnH2n+1), a new bond cleavage reaction is observed on the πσ* state. When modified by a cyano (-CN) group, a significant change in the carbon atom involved in pyramidalization is observed. (ii) When ethylene used to build up small cyclic polyenes, it is observed that the motifs of the ethylene dynamics persist, expressed as ring puckering and ring opening. (iii) In small heteroaromatic systems, i.e., an aromatic ring containing an ethylene-like sub-structure and one or two noncarbon atoms, the type of heteroatom (N: pyrrole, pyrazole O: furan) gives rise to different bond cleavage and ring puckering channels. Furthermore, adding an aldehyde group (-C=O) onto furan, as a way to lengthen the delocalised ring electron system, opens up additional reaction channels via a nπ* state. The results presented here are used to build up a more complete picture of the dynamics that occur in small molecular systems after they are excited by a visible or UV photon, and are used as a basis to motivate further investigations. Key words: time-resolved photoelectron spectroscopy, excited-state dynamics, organic molecules Stockholm 2017.

(6) This thesis is dedicated to my mom.

(7) List of Papers. The following papers, referred to in the text by their Roman numerals, are included in this thesis PAPER I: Cyclohexadiene revisited: a time-resolved photoelectron spectroscopy and ab initio study. O. Schalk, T. Geng, T. Thompson, N. Baluyot, R. D. Thomas, E. Tapavicza, and T. Hansson, Journal of Physical Chemistry A, 120, 2320 (2016). PAPER II: Excited state dynamics of acrylonitrile: substituent effects at conical intersections interrogated via time-resolved photoelectron spectroscopy and ab initio simulation. R. J. MacDonell, O. Schalk, T. Geng, R. D. Thomas, R. Feifel, T. Hansson, and M. S. Schuurman, Journal of Chemical Physics, 145, 114306 (2016). PAPER III: Influence of alkoxy groups on the photoinduced dynamics of organic molecules exemplified on alkyl vinyl ethers. O. Schalk, M. Stenrup, T. Geng, R. Lindh, R. D. Thomas, R. Feifel, and T. Hansson, Journal of Phyiscal Chemistry A, 119, 11105 (2015). PAPER IV: Substituent effects on the relaxation dynamics of furan, furfural and β-furfural: A combined theoretical and experimental approach. S. Oesterling, O. Schalk, T. Geng, R. D. Thomas, T. Hansson, and R. de Vivie-Riedle, Phys. Chem. Chem. Phys, 19, 2025 (2017). PAPER V: Dynamics in higher lying excited states: Valence to Rydberg transitions in the relaxation paths of pyrrole and methylated derivatives. T. Geng, O. Schalk, S. P. Neville, T. Hansson, and R. D. Thomas, submitted to Journal of Chemical Physics, January 2017. PAPER VI: Time-resolved photoelectron spectroscopy studies on pyrazole and its derivates. T. Geng, I. F. Galván, O. Schalk, R. Lindh, T. Hansson, and R. D. Thomas, in manuscript..

(8) Reprints were made with permission from the publishers..

(9) Author’s contribution. My contributions to the papers are as follows: Paper I: Cyclohexadiene revisited: a time-resolved photoelectron spectroscopy and ab initio study. • Participated in setting up the optics. • Participated in conducting the experiments. • Participated in the discussion of the results and the analysis presented in the paper. PAPER II: Excited state dynamics of acrylonitrile: substituent effects at conical intersections interrogated via time-resolved photoelectron spectroscopy and ab initio simulation. • Participated in setting up the optics. • Participated in conducting the experiments. • Participated in the discussion of the results and the analysis presented in the paper. PAPER III: Influence of alkoxy groups on the photoinduced dynamics of organic molecules exemplified on alkyl vinyl ethers. • Participated in setting up the optics. • Participated in conducting the experiments. • Wrote the first draft of the article and prepared the figures for the paper. PAPER IV: Substituent effects on the relaxation dynamics of furan, furfural and β-furfural: A combined theoretical and experimental approach. • Participated in setting up the optics. • Participated in conducting the experiments. • Analysed the spectra and prepared the figures for the paper. PAPER V: Dynamics in higher lying excited states: Valence to Rydberg transitions in the relaxation paths of pyrrole and methylated derivatives. • Participated in setting up the optics • Participated in conducting the experiments. • Analysed the data and prepared the figures for the paper..

(10) •. Wrote the first draft of the article.. PAPER VI: Time-resolved photoelectron spectroscopy studies on pyrazole and several methylated derivatives • Participated in setting up the optics • Participated in conducting the experiments. • Analysed the data and prepared the figures for the paper. • Wrote the first draft of the article..

(11) Contents. Abstract. V. List of Papers. VII. Author’s contribution. IX. Abbreviations. XIII. 1. Introduction ............................................................................................... 1 2. Photoinduced reactions and concepts ..................................................... 6 2.1 Electronic transition .............................................................................. 7 2.2 Franck-Condon region ........................................................................ 10 2.3 Propagation ......................................................................................... 11 2.4 Conical intersection, internal conversion and intersystem crossing ... 12 3. Excited state dynamics of molecules...................................................... 15 3.1 Excited state dynamics of ethylene ..................................................... 15 3.2 Excited state dynamics of polyenes .................................................... 17 3.2.1 Linear polyenes ............................................................................ 17 3.2.2 Cyclo-polyenes ............................................................................ 21 3.3 Excited state dynamics of molecules with heteroatoms ...................... 27 3.3.1 Acrolein........................................................................................ 27 3.3.2 Heterocycles................................................................................ 29 4. Experimental apparatus ......................................................................... 34 4.1 Laser system ....................................................................................... 35 4.2 Optical setup- two colour experiment ................................................. 36 4.3 Interaction region and magnetic bottle spectrometer .......................... 38 5. Time-resolved photoelectron spectroscopy and data analysis ............ 41 5.1 Time-resolved photoelectron spectroscopy ........................................ 41 5.2 Time-resolved photoelectron spectrum (TRPES) and data analysis ... 42 6. Discussion of the attached papers .......................................................... 49 6.1 Dynamics of polyenes – cyclohexadiene ............................................ 50.

(12) 6.2.1 Ethylene molecules with a cyano group - acrylonitrile................ 53 6.2.2 Ethylene molecules with alkoxy group- alkyl vinyl ethers .......... 56 6.3 Dynamics of cyclopolyenes with heteroatoms.................................... 59 6.3.1 Dynamics of cyclopolyenes with oxygen - furan......................... 59 6.3.2 Dynamics of cyclopolyenes with nitrogen - pyrrole .................... 62 6.3.2 Dynamics of cyclopolyenes with two nitrogen - pyrazole ........... 66 7. Conclusion and outlook .......................................................................... 69 Sammanfattning. LXXII. Acknowledgements. LXXV. References. LXXVI. Attached papers. LXXXI.

(13) Abbreviations. BAC BBO BOA BP CoIn CPA DAS FC FHG FWHM HOMO IC ISC LUMO MCP SHG TDC THG TOF TP TRPES Z-state. Bond alternation coordinate Beta-barium borate Born-Oppenheimer approximation Bicylco [2,1,0] pentene Conical intersection Chirped pulse amplification Decay associated spectrum Franck-Condon Fourth harmonic generation Full width at half-maximum Highest occupied molecular orbital Internal conversion Intersystem crossing Lowest unoccupied molecular orbital Microchannel plate Second order harmonic generation Time-to-digital conversion card Third order harmonic generation Time-of-flight Tricyclo [2,1,0,0]pentane Time-resolved photoelectron spectroscopy Zwitterionic state.

(14) Declaration Portions of this work have been taken from my licentiate thesis. With a few modifications, chapters/sections 1, 2.4, 3.1, 5.2 are taken from my licentiate thesis. With minor modifications, chapter 4 is taken from section 4 in the licentiate thesis..

(15) 1. Introduction. Ultra-violet and visible light induced processing of a molecule can be described as the absorption of a photon which subsequently induces a chemical reaction in the molecule, possibly leading to a change in chemical conformation of the molecule. Such processing plays significant roles in various fields, including chemistry, physics, astrophysics, biology, and material sciences, and is exemplified by light induced photochemical smogs [1], the light induced isomerisation of retinal [2], and the operation of photoswitches [3]. In early studies, nanosecond and picosecond lasers were used to investigate these photochemical reactions, e.g. probing the lifetime of excited states and radiationless decay pathways [4, 5]. Then, with the invention of the femtosecond laser at the end of 20th century, more detailed investigations of these ultrafast chemical processes became possible, as well as giving control over the mechanisms of the chemical reaction [6]. Ultrashort laser pulses have the characteristics of a large spectral bandwidth and a high intensity: the short pulse duration allows monitoring of the molecular dynamics on the femtosecond time-scale; the large spectral bandwidth allows molecules to be excited to a number of vibrational states; the high intensity allows multiphoton absorption processes to be investigated [7]. To date, much work has been done, and a series of techniques has been developed, in order to investigate and understand the different kinds of processes and the various reaction pathways which occur in molecules upon photoexcitation [8]. Despite these advances, there are still many unresolved questions related to the various reaction mechanisms that occur after excitation. This is because even with cutting-edge tools and techniques, disentangling the important dynamics remains a non-trivial and time-consuming complex issue. This is due to i) the photon-molecule interaction itself, where molecules absorbing different photon energies can populate different excited states, e.g. pyrrole can be excited to a ππ*-state or a πσ*-state [9], and ii) the electronic environment and functionality of the molecules, where the addition of a functional group introduces states of different electronic character and opens up new and different reaction pathways, e.g. the addition of an aldehyde group to ethylene introduces the nπ* state, where the molecules can now decay to a triplet state[10], or the existence of a πσ* state in alkyl vinyl ethers, where C-O bond cleavage occurs [11]. 1.

(16) In order to both obtain a broader picture and unravel the finer details, new models are needed. In this thesis I present a time-resolved photoelectron spectroscopic experimental study which is combined with various computational methods. Here, a bottom-up approach is used with the aim of working towards obtaining the information necessary to develop a consistent picture that could explain and predict the outcome of these photochemical processes. In a bottom up approach, the “recipe” to understand the excited-state dynamics is to start with a small system, and then undertake chemical modification in a controlled manner while following the effects of these modifications on the dynamics. In this context, the “starter” molecule is ethylene (H2C=CH2): the simplest unsaturated hydrocarbon. With ethylene as the “bottom” system, the dynamics may therefore be influenced/steered by the addition of extra CHx groups, e.g., making polyenes (molecules which contain one or more sequences of alternating double and single carbon– carbon bonds) or changing the topology, as in ring-structures, e.g., cycloalkenes, benzene, or by adding functional groups/heteroatoms (atoms that are not carbon or hydrogen). It has been shown that the dynamics of ethylene can be used to describe the dynamics of more complex polyenes, where two of these processes are hydrogen migration and twisting of the C=C double bond (a detailed description is given in Chapter 3) [12]. For examples, hydrogen migration happens in cyclohexene [13] and cycloheptatriene [14, 15]. However, the twist of a C=C double bond either leads to ring opening, as observed in cyclohexa-1,4-diene [13], or leads to ring puckering, as exhibited in cyclopentadiene [16, 17]. The dynamics occurring on the excited state surfaces may also be influenced by the addition of heteroatoms. Rather than introducing purely structural constraints to the molecule, heteroatoms will change the polarity and electronic energy levels, the character of states, and lead to the opening of new reaction channels, e.g. extra bond cleavage channels, charge transfer states, intersystem crossing channels, due to the addition of spin-orbit couplings, etc. With larger structures, especially rings, the location of two or more heteroatoms may also have different influences: e.g. the potential for N-N bond cleavage in pyrazole. Starting with simple molecules, coupled with inserting different functional groups at specific positions in these molecule, enables us to better understand the dynamics in increasingly complex systems, as well as to tailor functional materials used in light sensitive devices, such as molecular photoswitches [3]. Then the important questions arise. Which part of the molecule is absorbing the photon? Which parts of the molecule participate in the dynamics? Light induced processes happen due to photon-absorption by the 2.

(17) chromophore of a molecule, while the dynamics occur in a region of the molecule called the dynamophore [13]. Briefly, the chromophore is “the part of molecular entity for which the electronic transition responsible for a given spectral band is approximately localised” [18] and the dynamophore is the “part of the molecule where the dynamics are approximately localised”. In order to localise dynamics from a delocalised wavefunction, as generated by the absorption process, we need a mixture of different states, which in turn is achieved by perturbations. Take a simple example of H2+, illustrated in Figure 1. In H2+, the electronic wavefunction is distributed equally among the two hydrogen nuclei. When the molecules are excited, the wavefunction is transferred from the bonding ground state to the antibonding excited state, where the molecule eventually dissociates forming H and H+. As the reaction progress, the wavefunction is still distributed equally at both nuclei. In order to achieve localisation in which the electronic wavefunction is localised on only one hydrogen atom, mixing of the bonding (Ψa) and antibonding orbital wavefunctions (Ψb) is required. This mixing is achieved through interaction with the environment (a heat bath, e.g. black body radiation). See reference 19 for a more detailed discussion [19].. Figure 1. Potential energy curve of H2+ and the wavefunctions of the bonding and antibonding molecular orbitals. A more complex example is cyclohexa-1,4-diene, shown in Figure 2 [20]. The two homoconjugated double bonds create a delocalised π-system which is the chromophore of the system. After excitation, the dynamics first evolve along delocalised vibrations according to the vibrations in the ground state. Localisation occurs through mixing of different electronic states (see Figure 2b) which allows the dynamics to occur at one of the double bonds, while 3.

(18) the other acts as a “spectator”. Here, again, this localisation is obtained by interaction with the “environment” which, in this case, is provided by localised molecular vibrations.. Figure 2. (a) The chromophore in cylcohexa-1,4-diene is delocalised through the two double bonds, while the dynamophore is localised only at one of the two double bonds. (b) A linear combination forms the localisation on one of the double bonds (modified from ref [20]) Cylcohexa-1,4-diene contains a π-system, which is delocalised over the whole molecule, and, with a conjugated or homoconjugated π-system, the excitation is delocalised. However, if a molecule contains a sub-unit, for example -NH or -C=O, then an n-π* transition is often a localised excitation. It also has been observed that even though the same chromophore is responsible for absorption of the photon, the same dynamophore is not involved in the dynamics. For example, although cyclohexa-1,3-diene also contains a delocalised conjugated system, which acts as the chromophore, the dynamophore includes the whole carbon skeleton, and so is significantly larger than the chromophore [21, 22, 23, 24]. Conversely, 1,3-butaidene, which contains a delocalised conjugated system (due to the two double carbon bonds) that acts as the chromophore, the dynamics involves only one. 4.

(19) C=C bond, here a bond torsion mode, and in this case the dynamophore is smaller than the chromophore [25, 26]. This thesis is focused on the studies of the excited state dynamics of small organic molecules arising from the effects of the substituents and comparison among molecules from the point of view of dynamophore. The experimental method used is time-resolved photoelectron spectroscopy. In the following chapters, the background to photon-induced dynamics is introduced in Chapter 2. Extending these ideas from simple molecules, e.g. ethylene, to more complex ones, is illustrated in Chapter 3 to get a better picture on how to study the dynamics of complex molecules. Chapter 4 presents the experimental apparatus used in the measurements, while the data acquisition and the subsequent data analysis are described in Chapter 5. Chapter 6 summarizes the results and conclusions of the attached articles. Finally, Chapter 7 presents future perspectives.. 5.

(20) 2. Photoinduced reactions and concepts. This chapter will give a general background on photodynamical processes, and review some of the concepts relevant during these processes, to give a better idea on how to describe the electronic structure and the nuclear motions on the relevant electronically excited states. Figure 3 shows a schematic of a general photochemical process.. Figure 3. A schematic of a photochemical process. A detailed description is given in the main text. For a reaction to occur on the ground state (denoted as S0 in Figure 3), the dynamics start from the reactants passing over a barrier to form the final products. All of these processes happen on the ground state surface. In photochemistry, the reaction pathway is different. A typical photoinduced chemical process is also summarized in Figure 3 where, in a simple approximation, such a process can be roughly described in four separate steps, 1-4 (these steps are discussed in the following subsections according to the numbers in the figure). Briefly, molecules absorbing the photon 6.

(21) energy are excited (1) to the Franck-Condon (FC) region of one excited state. Then molecules leave the FC region (2), flow along the excited state (3), and return to the ground state (4), e.g., through a conical intersection (CoIn).. 2.1 Electronic transition In a photochemical process, the molecules, which are typically in the singlet electronic ground state (S0), are promoted to a singly electronically excited state (denoted as Sn in Figure 3) by absorbing a photon (denoted as process 1 in Figure 3).. Figure 4. (a) Formation of molecular σ and σ* bonds by overlap of atomic s or p orbitals. (b) Formation of molecular π and π* bonds by atomic p orbital overlap. (Adapted from ref [27]) The initial electronic transition in this process can be described in the molecular orbital picture. A molecular orbital [28, 29] is used to describe the wave-like behaviour of an electron in a molecule using a mathematical function. Molecular orbitals are represented as a linear combination of atomic orbitals. The efficiency of the atomic orbital interaction depends on the overlap between these orbitals. Figure 4 illustrates several examples. A constructive interaction creates a bonding molecular orbital, such as an σ 7.

(22) bond which is formed from the overlap of 1s orbitals or head-on overlapping of the (pz) atomic orbital (directed along the bond axis), or a π-bond which is formed by the overlap of the two lobes of the involved atomic (px) orbitals perpendicular to the bond-axis. A destructive interaction of atomic orbitals creates an anti-bonding molecular orbital, represented by σ* or π*. When there is no interaction between the atomic orbitals, it is termed nonbonding, and is represented by n with free electron pairs, and such molecular orbitals generally occur in molecules containing oxygen or nitrogen atoms, for example in furan, pyrrole, etc.. Figure 5. The π orbitals of the ethylene molecule (Adapted from ref [27]) Take ethylene as an example: the π and π* molecular bonds are shown in Figure 5. In the molecular ground state, two electrons are in the π orbital, which is the highest occupied molecular orbital (HOMO). The lowest unoccupied valence molecular orbital (LUMO) is the π* orbital In this example, the excited state created after a one electron transition is called a singly excited state. There are also electronic transitions from a π orbital to an anti-bonding σ* orbital, or from a non-bonding orbital to an anti-bonding π* orbital, forming the π-σ* state or the n-π* state, respectively, as observed in different molecules [9, 10, 27]. For example, in pyrrole, the existence of the π-σ* state is due to the orbital overlap integral of the σ and π orbitals allowing a π-σ* transition. The n-π* (one electron forbidden) transition in furfural (2-aldehyde furan), is an electron transition from a nonbonding oxygen p-orbital to a perpendicular carbon π orbital. These orbitals are described in Figure 6.. 8.

(23) Figure 6. (a) σ* orbital in pyrrole and (b) n orbital in furfural. If a process excites two electrons from an occupied orbital to an unoccupied orbital, this excited state is called a doubly excited state. Generally, doubly excited states are expected to require twice as large an excitation energy as the singlet excited state, due to the requirement of exciting two electrons from the HOMO to the LUMO. However, in polyenes the lowest-lying excited state often is a state with doubly excited character. This could be characterized as a HOMO2 → LUMO2, and is explained by two reasons: one is due to a large singlet-triplet splitting, which leads to an increase in the energy of the lowest-lying singlet excited state; the other is an admixture of the singlet excited configuration with the doubly excited state [30]. According to the spin direction of the electrons, the excited state can be called a singlet state or a triplet state. If the electronic excited state has spinpaired electrons, i.e. the electrons have opposite spins, then this is called a singlet state, e.g., 1(ππ*). If an electron flips its spin direction, it is called a triplet state, e.g., 3(ππ*). However, the electronic transitions do not happen arbitrarily, they need to obey the selection rules, e.g. singlet to singlet transition is more favourable, and a singlet to triplet state transition is forbidden. Moreover, the selection rules also constrain the allowed excited state according to the number of photons absorbed. If a molecule with inversion symmetry absorbs only one photon, the excited states which the molecules can populate are called optically “bright” states, while if two photons are absorbed, the molecules will be promoted to an excited state called an optically “dark” state. The efficiency of the absorption is generally described by the oscillator strength. In addition to the above electronic transition states, there exist molecular Rydberg states. A molecular Rydberg state is a state in which an electron is excited into a molecular orbital that resembles a hydrogen-like atomic 9.

(24) orbital, and where the energy of these electronically excited states follows the Rydberg formula (shown in equation 1) as they converge to a state with an ionic core [31]. The energy of these states is described by . Ε = − . (1). where R is the Rydberg constant, and n is the principle quantum number describing the orbital. Comparing the above formula to the standard Rydberg formula for the allowed Bohr-energies of the hydrogen atom [31], δ is called the quantum defect, which works as “screening” parameter, because the ionic core containing electrons is larger than the bare nucleus, and the quantum defect decreases both with increasing angular momentum quantum number and with increasing principle quantum number. Typical values are 0.9 for an s-, 0.5 for a p-, and 0.1 for a d-Rydberg state series.. 2.2 Franck-Condon region Under normal conditions, the process of absorbing a photon is faster than the nuclear motion in the molecule, which can be considered to be static in a very good approximation. This is called the Franck-Condon approximation and, consequently, the region on the excited state to where the molecule is excited is called the Franck-Condon (FC) region (number 2 in Figure 3). On a bound excited state, the excitation induces nuclear motion through a displacement of the potential energy surfaces. These induced vibrations are called accepting modes or “Franck-Condon” active modes. These modes give rise to very fast dynamics, and are totally symmetric vibrations. Conversely, there exist the so-called promoting modes which are non-totally symmetric but vibronically active modes. These kinds of modes give an explanation to the possible dynamics which happen on the excited state. When a molecule in the vibronic ground state is excited to different vibrational modes in the excited state, it forms a so-called wavepacket which is not an eigenstate of the excited state [32]. Therefore, the wavepacket will evolve in time regardless of whether excitation occurs to a bound or nonbound state. Generally, some limited information on the electronic transition and possible dynamics can be obtained through the absorption spectrum of the molecule. The absorption spectrum consists of a series of electronic transitions which can have a superimposed vibrational structure. The spectral bandwidth of the peaks depends on various influences. It is important for this thesis that the lifetime of the excited state can be estimated from the absorption spectrum. Moreover, the existence of vibrational structure indicates that the molecule is returning to the Franck-Condon point while a structureless peak indicates a non-bound excited state. 10.

(25) When the wavepacket of a small organic molecule is lifted up to the Sn state, as illustrated in Figure 3, it will start in the Franck-Condon region as described earlier. However, it will often experience non-bound conditions, and will propagate away from the Franck-Condon region along the different gradients of the potential energy surface. Since different Franck-Condon active modes exist, the wavepacket can travel in different directions. The wavepacket can propagate with the non-Franck-Condon active modes on the excited state and move towards crossing points with lower lying excited states. These processes are indicated by the number 3 in Figure 2, and are described next.. 2.3 Propagation The pathway the excited molecule takes from the initial excited state to the lower-lying state may influence the outcome of a photochemical reaction. In some cases, there may be several intermediate states but only one channel and one observed product. However, at every crossing between the different surfaces, and sometimes even on an excited surface, there can be a bifurcation, and the coupling of the wavepacket to the other surface depends on the momentum of the wavepacket and the topology of the intersection (see Chapter 2.4), e.g. allowing access to different final excited states. On the other hand, the intermediate states can also change the direction of the wavepacket. For example, in unsaturated hydrocarbon molecules, the states which play the main roles in the dynamics are the ππ* states. However, on the reaction pathway, interference from different intermediate states induces different dynamics where, e.g., low-lying Rydberg states have been studied as an intermediate state that is involved in the dynamics of ethylene [33, 34]. For unsaturated hydrocarbon molecules with heteroatoms, e.g., nitrogen, oxygen, excitation occurs mainly to the ππ* states, but the existence of closelying πσ* states is necessary to explain the dynamics. Such a state induces bond cleavage of, e.g., N-H in pyrrole [35], or C-O in alkyl vinyl ether [11]. Furthermore, for molecules with a carbonyl group, nπ* states also become involved in the dynamics, in which it is observed that the population further decays to the nπ* or ππ* triplet states [10]. On the potential energy surface, the linking of the two minima along the reaction coordinate which has higher energy is called a barrier. Then, upon photoexcitation of molecules, the wavepacket splits on the excited state, in which some part of wavepacket tunnels through the barrier and decays to the lower-lying state. For example, in pyrrole upon excitation with low photon energies, N-H dissociation occurs on the πσ* state, and takes a long time due. 11.

(26) to the existence of a barrier through which the wavepacket needs to tunnel [36].. 2.4 Conical intersection, internal conversion and intersystem crossing The description of potential energy surfaces is based on the so called Born-Oppenheimer approximation (BOA). This approximation states that the electronic and nuclear motions can be separated such that the electrons adiabatically adopt any change in nuclear configuration. In this approximation, it can be shown that electronic states with the same symmetry cannot cross [37, 38]. However, when the two potential energy surfaces get closer in energy, the BOA breaks down and there is a mixing of the electronic and nuclear degrees of freedom. This is called a non-adiabatic coupling. The two states may be coupled by a “photochemical funnel” which is also called a conical intersection (CoIn) (indicated by the number 4 in Figure 3). CoIns are not isolated molecular geometries; they are collections of geometries that form a high dimensional “seam” [39]. For nonlinear molecules, it has 3N-8 dimensions (here, N is the number of atoms, 6 degrees of freedom are for translation and vibrations and two dimensions lift the degeneracy). The potential energy surfaces intersect with each other forming a double cone, which is plotted against two special internal geometric coordinates, called “branching space coordinates”, (g) and (h), where (g) indicates the gradient difference and (h) represents the gradient of interstate coupling [40]. Motion along these coordinates lifts the degeneracy of the two excited states. This is depicted in Figure 7. As a result, in the photochemical reaction, passing through the CoIn can lead to two or more products, due to the different valleys on the ground state potential energy surface, unlike the single product obtained on the ground state in the thermal chemistry reactions. The potential energy surface may have several minima which enable access to various CoIns that lead to the different photoproducts. As such, the nuclear motions at the conical intersections are often different. A CoIn is classified into two categories according to its topology [39, 41, 42]. One type is the “peaked” CoIn, where the nuclei have trajectories directly towards the intersection. The other type is called a “sloped” CoIn, where the potential energy surfaces have downhill slopes, and there is an increased possibility of nuclear trajectories passing from the higher excited state to the lower state and returning. At the simplest level, the “peaked” CoIn results in a high probability of transition, i.e., an ultrafast electronic transition. The “sloped” CoIn induces a low transition rate. However, Malhado et al [43] 12.

(27) assert that with the same crossing velocity (momentum), the non-adiabatic transition probability is the same for both the “peaked” and “sloped” CoIns. The higher efficiency at a “peaked” CoIn rather than at a “sloped” CoIn is due to dynamical effects, and does not depend on the topology. This is in agreement with the Landau-Zener model [44].. Figure 7. A conical intersection. The double cone is plotted against the two branching space coordinates. As the population transitions from one state to the other, if the two states are both singlet states the process is called internal conversion (IC), while if one state is a singlet state and the other is a triplet state, the process is called intersystem crossing (ISC). ISC is driven by spin-orbit coupling, which involves the coupling of an electron’s spin with orbital angular momenta. Generally, for molecules which do not contain heavy atoms, this coupling is weak, such that the ISC rate is low. However, as the El-Sayed rules assert [45, 46, 47], if the radiationless transition involves a change of molecular orbital type, the ISC rate increases significantly, e.g. the transition from a 1 (ππ*) to a 3(nπ*) state or from a 1(nπ*) to 3(ππ*) in the carbonyl compounds. Furthermore, ISC is enhanced if the energy gap between the singlet and triplet states is small. As a consequence, competition between ISC and IC has also been found in benzene, which is a hydrocarbon molecule that has no obvious large spin-orbit coupling, but where this is motivated by the near degeneracy of the S1 and T2 states [48]. 13.

(28) Non-radiative decay pathways have emerged as a dominant mechanism for photochemical reactions in many molecular systems. They allow for the energy transfer from the electronic excitations to the nuclear degrees of freedom of the molecules. In summary, the dynamics of small organic molecules initiated by a UV or visible photon can be described by an electronic transition from the ground state to the FC region of the excited state, which follows the standard selection rules for one-photon absorption. The molecule leaves the FC region and propagates to different final configurations through the available conical intersections via different vibrational modes. In addition, there are other factors which also need to be considered to describe the excited state dynamics of molecules. Two of these are the time taken by the molecules populated on the excited state to access to the CoIn (called the time delay), and the time taken for molecules to pass through the conical intersection to the lower-lying excited state or the ground state (called the time constant). These aspects will be discussed in more detail in Chapter 5.. 14.

(29) 3. Excited state dynamics of molecules. Excited state dynamics of molecules have been at the centre of interest for a long time. Here, I would like to review some important aspects from the viewpoint of the dynamophore. The chapter starts with the excited state dynamics of ethylene as the “bottom” system, using the 4-step description of the photodynamics discussed in Chapter 2. I then give an introduction to the effect of substituent groups, e.g. methylation, on the dynamics before proceeding to more complex systems, i.e., by adding more double bonds, to form linear and cyclic polyenes. Finally, I finish by explaining the effects of functional groups containing heteroatoms on the dynamics. In this way, I provide a clear picture on the change in the dynamics with controlled chemical modification of the molecules.. 3.1 Excited state dynamics of ethylene Ethylene is the smallest organic molecule containing a π orbital. As the basic unit of unsaturated molecules, ethylene attracts much interest. Extensive theoretical [33, 49-55] and experimental [34, 56-60] studies have been undertaken to better understand its excited state photodynamics. Furthermore, being the “bottom”-molecule of this thesis, the ethylene dynamics serves as a model for understanding the photochemical and photobiological processes in larger systems. Ethylene is a planar molecule in the ground state. Photoexcitation to the lowest-lying ππ* state induces a rapid C=C twist motion as the molecule moves from the Franck-Condon region towards the conical intersection region, where an ultrafast internal conversion to the electronic ground state occurs [34, 60]. A sketch of the dynamics is given in Figure 8, where Φ indicates the C=C torsion angle. Upon population of the ππ* state, the molecules leave the FC region and start twisting. When the excited state potential reaches a minimum, at a 900 twist angle [52, 61, 62], the potential energy can be further lowered by a break of molecular symmetry [63, 64]. Here two different pathways are open. The first accesses a conical intersection which leads to a hydrogen-atom migration from one carbon atom to the other, forming an ethylidene-like configuration. The second. 15.

(30) accesses a CoIn leading to pyramidalization of one CH2 group. The ethylidene-like and pyramidalization structures are shown in Figure 8.. Figure 8. Sketch of the excited state dynamics of ethylene. The molecules are excited to the ππ* states by the pump pulse (blue arrow) from the ground state (S0). The population on the ππ* state proceeds along the C=C bond torsion coordinate. After reaching the minimum, two reaction pathways are pyramidalization or hydrogen migration to form CH3CH. Furthermore, also on the hot ground state, hydrogen-atom elimination will occur. Φ, on the xaxis, indicates the C=C bond torsion angle. It has been identified that there is a two-electron excited zwitterionic state (π*)2 (Z state) crossing the ππ* state, and which lies slightly below it at a C=C twist angle of 900 [65-67]. Hence, the excited state experiences “sudden polarization” [67, 68], yielding a structure with a planar partially positive CH2 group and a pyramidalized partially negative CH2 group. The pyramidalization of the CH2 group means it is formally identical with a sp3hybridization of the carbon atom which is able to stabilize the negative charge. Ethylidene (CH3CH), which can only be formed by hydrogen migration, has been detected in experiments [59]. Several investigations indicate that hydrogen elimination also occurs on the hot ground state after internal conversion [58, 69, 70]. In the context of the current discussion, the C=C group can be considered as the dynamophore, as it is the dynamics (the motion) of this bond which drives the processes along. Furthermore, along the twisting motion on the 16.

(31) excited state, it is also possible to transiently populate the π3s-Rydberg state [33, 34]. As the wavepacket progresses on the valence state, the C=C bond torsion leads to a coupling of the ππ* state and the π3s Rydberg state. As a result, there is an ultrafast population transfer from the initially excited state to the lower-lying Rydberg state. Even though the time scale is very short, it indicates the involvement of the Rydberg state in the relaxation of photoexcited ethylene. The Rydberg state dynamics following direct population of the π3s Rydberg state upon photoexcitation has also been investigated [59]. The π3s Rydberg state is lowered in energy with CC twisting, reaching a minimum at around 25-300 [71], while the ππ* state is lowered in energy by twisting to 900. The two states intersect each other, and electron population flows from the π3s Rydberg state to the ππ* state, accompanied by C=C twisting and C=C stretching. The controlled addition of substituents on a molecule may help elucidate the dynamics, where both the number of substituents and their location in the molecule could cause a change in the photodynamics. Methylation, exchanging a H-atom for a CH3-group, is one simple way to do this. The Stolow group investigated the effect of methylation on the dynamics in ethylene upon photoexcitation to the π3s Rydberg state [72]. They report that with increasing methylation the decay time of the Rydberg state increases, and that population transfer occurs on the picosecond time scale for the tetramethylethylene molecule. This is explained by an increased energy barrier to accessing the π3s/ππ* conical intersection. As a result, upon methylation, the energy barrier between the Franck-Condon region (the maximum of the torsional potential) of the π3s Rydberg state and the π3s/ππ* conical intersection increases, and causes a slowing of the C=C bond torsion mode. Methylation also affects the dynamics on the ππ* state which, in turn, affects the H-migration and pyramidalization of the CH2 group. It can be concluded that addition of the CH3-group not only affects the inertia but has other implications, as it shifts excited states in energy which can become the determining factor if they are close together. As a result, for ethylene and the methylated ethylene molecules, the chromophore and the dynamophore are the C=C double-bond.. 3.2 Excited state dynamics of polyenes 3.2.1 Linear polyenes For the ethylene and methylated ethylene molecules, the chromophore and dynamophore is the C=C bond. I now increase the complexity slightly 17.

(32) by moving to linear polyenes and cyclic systems, in which we add additional C=C double bonds, i.e. the ethylene moiety, and see how these influence the dynamics. Polyenes attract much interest due to their importance in biological systems [73, 74]. Polyenes are generally considered as a family of molecules with alternating single and double carbon bonds, and constitute fundamental chromophores. Molecules containing four or more double bonds are called “longer polyenes”, while butadiene and hexatriene are “shorter polyenes”. The structure of several selected polyenes is shown in Figure 9.. Figure 9. The structure of 1,3-butadiene, 1,3,5-hextriene, and 1,3,5,7octatetraene Comparing the absorption spectrum of polyenes to that of ethylene shows that the lowest-lying singlet excited state is a ππ* state [75, 76]. Studies of polyenes indicate that there are two low-lying excited states involved. One is a strong singly excited state 1B (ππ*) transition, corresponding to a HOMO > LUMO excitation, and the other is a linear combination of a doubly excited configuration with higher lying singlet excitations, the 2A state. Schuleten and Karplus [77] found the HOMO2 → LUMO2 configuration mixes with different higher single excitations, e.g. HOMO-1 → LUMO, and HOMO → LUMO+1, which brings down the energy of the 2A state and makes it lower-lying than the 1B state. The 2A state is optically “dark”, which means that it is not directly accessible via a single photon excitation. In the photodynamics of ethylene, the 2A state, often referred to as the Z state, lies higher in energy in the Franck-Condon region than the 1B (ππ*) state compared with the longer polyenes. This is because of a decreased interaction with the singly excited states since HOMO-1 and LUMO+1 do not have π-character. Thus, when the ethylene molecules are excited to the 1B state, the 2A state does not play role until the molecule is significantly distorted. Conversely, in the shorter polyenes, like butadiene, the 2A state plays a role before the torsion distortion makes the symmetry label irrelevant. However, for the longer polyenes, the 2A state is only involved when the 18.

(33) torsion is so severe that the symmetry label becomes meaningless [25]. We can look separately at what happens to the dynamics on going from the shorter to the longer polyenes.. Figure 10. A sketch of dynamics of 1,3-butadiene. The blue arrow indicates the pump pulse. The singly excited 11Bu state and the doubly excited 21Ag states are shown. The red arrows indicates the channel from the excited state returning back to the ground state Among polyenes, trans-1,3-butadiene is the simplest conjugated diene, i.e., with two double bonds, that has been investigated experimentally [7881] and theoretically [25, 82-84] to understand its relaxation process after photoexcitation. Many researchers have studied whether the 2A state is involved in the photodynamics of butadiene from a transition from the 1B state. Martínez et al. [25] report a theoretical study of the dynamics of excited butadiene, and the corresponding dynamics are depicted in Figure 10. Upon photoexcitation to the 1B state, during the initial few femtoseconds, part of the initially excited population remains on the bright 1B state whilst the remaining population has transferred to the optically dark 2A state. The dynamics, which involve either torsion of the one C-C single bond and the terminal C=C double bond, with the other bonds remaining planar, or a twist plus pyramidalization at one of the two ethylenes, occurs either on the 1B or the 2A state. The twist motion causes the conical intersection to the ground state. Here it is found that charge transfer plays a role in the way that the wavepackets decay to the ground state. The population on the S1 state faces 19.

(34) one or two funnels to form two oppositely polarized charge transfer states, which means one twisted methylene unit is positively charged and the rest forms a negative charge. The proposed reaction channels are drawn in figure 11.. Figure 11. The proposed reaction channels happen on the excited state: pyramidalization and the charge transfer state. Excitation of butadiene to directly populate the optically dark 2A state has been investigated via two photon absorption [78, 80]. In the FranckCondon region, the 2A state lies slightly higher in energy than the 1B state [85-87]. The C=C bond becomes weaker in the 2A excitation, being almost like a C-C single bond, and then it is broken by the twist of the terminal C=C bond. Access to the π3s- and π3d-Rydberg states via two photon absorption has also been studied by Schalk et al. [80]. The lowest-lying 3s-Rydberg state is strongly coupled with the valence manifold, and so the decay rate of the π3sRydberg state is larger. Furthermore, for the π3d-Rydberg state dynamics, it is found that the different excitation wavelength does not affect the position of the Rydberg state peak, i.e., no energetic shift is found. This is due to, when the Rydberg state is ionized, the diagonality of Franck-Condon factor indicates the preservation of internal vibrational energy upon photoionization. In comparison with the dynamics of ethylene on the ππ* state, the dynamics of butadiene have a localised dynamophore, which keeps the ethylene-like torsion of the C=C bond. However, this only happens on the terminal C in one of the C=C double bonds, and the butadiene dynamics also displays delocalised charge transfer dynamics on the way from the 2A state to the ground state. Hexatriene contains one/two more double bonds than butadiene/ethylene, respectively. The dynamics of hexatriene is useful for understanding the photodynamics of cyclohexadiene, as it is a photoproduct from ring-opening in 1,3-cyclohexadiene. It is reported that the 2A state lies lower in energy than the 1B state, which reinforces the general trend for polyenes that the lowest singlet excited state has a doubly excited configuration [88]. The molecules are predominantly excited to the 1B state, which has a very short lifetime (20~40 fs) due to a rather steep slope along the double-bond 20.

(35) lengthening and single-bond shortening co-ordinate [89]. The nuclei are strongly accelerated, and leave the Franck-Condon region and undergo internal conversion to the 2A state. Quantum calculations indicate the crossing point of the 1B and 2A states is located near the Franck-Condon region, then the internal conversion proceeds along the symmetric deformation mode [90]. Several groups measured the lifetime of the 2A state and found values between 250 and 600 fs [88, 91, 92]. On the 2A state, the molecules twist around the C=C double bond and the adjacent C-C bond, and the population decays to the ground state where isomerization of the C-C single bond occurs, forming trans and cis conformers [93]. For the longer polyenes, octatetraene is taken as an example. All-trans1,3,5,7-octatetraene contains four double bonds. Similar dynamics has been found for all trans-1,3,5,7-octatetraene as for the shorter polyenes. Upon photoexcitation, molecules predominantly populate the 1B state, and the population decays to the 2A state [93]. Calculations indicate that for all trans-1,3,5,7-octatetraene internal conversion from the 2A state to the ground state occurs due to a non-adiabatic trans-cis isomerization around the central C3 and C4 C=C double bond [94]. In comparison with 1,3,5-hexatriene, the lifetime of the 1B state is much higher in 1,3,5,7-octatetraene, which could be explained by the difference in the vibronic coupling strengths between the 1B and 2A states. Furthermore, the lifetime of the 2A state is also longer than for hexatriene, being a few nanoseconds long [93]. From butadiene to octatetraene, the photodynamics involves both the 2A and 1B states. From the point view of the dynamophore, butadiene has one localised dynamophore, due to one terminal double bond torsion, whilst the other remains planar on the 1B or 2A state and also displays a delocalised dynamophore due to the charge transfer state. For hexatriene and octatetraene, with the increased length of the double-bond chain, the cistrans isomerization conformers obtained also indicates localised dynamics, although the dynamics are so slow that a dynamics picture of the reaction almost makes no sense.. 3.2.2 Cyclo-polyenes Having reviewed the photoexcitation dynamics of linear polyenes, I now focus on ring-systems with double bonds whose dynamics include hydrogen shift, ring-puckering, and bond cleavage. Recall the dynamics of the ethylene molecule, where it undergoes a C=C double bond torsion when decaying to the ground state followed by the pyramidalization of one carbon bond. Similar dynamic processes are also observed in trans-1,3-butadiene, which displays a double bond torsion and pyramidalization of the double bond to the central single C-C bond. How will the double bond affect the 21.

(36) dynamics in a ring system? Here, cyclohexene and cyclohexa-1,4-diene will be taken as two examples to illustrate the dynamics in six-membered ring systems.. 3.2.2.1 Cyclohexene and cyclohexa-1, 4-diene Cylcohexene is a six-membered ring system with only one double bond. According to early quantum calculations [95], the photochemical reactions of cyclohexene include hydrogen shift, carbene formation, bond cleavage, and ring puckering. These are depicted in Figure 12. [13, 96].. Figure 12. The photoreaction dynamics of cyclohexene. After a first arrangement on the excited state, the next reactions occur on the hot ground state (adapted from reference [13]). In the photodynamics of cyclohexene (as shown in figure 13), two excited states are involved, the ππ* state and the π3s Rydberg state [97]. The ππ* state is optically dark to one-photon absorption while the Rydberg state is weakly absorbing [98]. Upon photoexcitation to the π3s Rydberg state the molecules rapidly depart from the Franck-Condon region of the Rydberg state to the ππ* state, accompanied by a C=C bond stretch and twist. For molecules travelling along the ππ* surface, several low-lying conical intersections to the ground state can be identified: a [1,2] H-bridge, a [1,3] 22.

(37) H-shift, a [1,2] H-shift, and a bond cleavage in the α position and in the β position [99, 100]. Furthermore, recent unpublished calculations for the dynamics of cyclohexene on the ππ*-state indicate the C=C bond twisted pyramidalization is the dominant channel (95%), while the bond cleavage and hydrogen shift channels are insignificant [101].. Figure 13. Sketch of the photodynamics of cylcohexene and cyclohexa-1,4diene upon excitation at 200 nm. Cyclohexa-1, 4-diene can be considered as adding one more double bond to cyclohexene, where the two double bonds are separated by one –CH2– group. The photochemical pathways are summarized in Figure 14, which are similar to cyclohexene, and which include the hydrogen shift, ring puckering and bond cleavage channels. For a comparison of the photodynamics of cylcohexene and cyclo-1,4hexadiene, their photoelectron spectra [13] show similar features indicating similar dynamics. In cyclohexa-1,4-diene, the molecules are mainly excited to the π3s Rydberg state followed by transfer to the lower-lying ππ* state. According to recent unpublished quantum calculations on the dynamics of cyclohexa-1,4-diene on the ππ* state, the majority (~75%) of the dynamics are localised at a single double bond [101]. The molecules return from the excited state to the ground state, following the hydrogen shift or the twisted pyramidalization pathways. This indicates that the presence of the second double bond had nearly no influence on the dynamics, and the molecular orbitals also show no involvement of this second π-bond. Interestingly, there is still around 25% of the population which undergoes a twist at both double 23.

(38) bonds, meaning a twist of the entire ring to form a “chair-like” structure [101].. Figure 14. Photoreaction pathways of cyclo1,4-hexadiene. After the first arrangement upon photoexcitation, the next reactions occur on the hot ground state (modified from reference [13]). From the idea of the chromophore and the dynamophore for these two molecules, the chromophore and dynamophore of cyclohexane are both localised on one double bond, which mainly involves the twisted pyramidalization. In cyclohexa-1,4-diene, the chromophore is delocalised into the homoconjugated double bonds. A large fraction of the molecules upon photoexcitation have the same dynamophore as cyclohexene, where the large amplitude nuclear motion is localised only at single double bond, meaning that bond interaction between the two double bonds on the excited state is a minimum. However, there is still a fraction of the cyclohexa-1,4diene molecules upon photoexcitation which have the same chromophore and dynamophore, and that the dynamics are delocalised into the whole ring system. Comparing these to the ethylene dynamics, the [1,2] hydrogen shift in a cyclo-system corresponds to the hydrogen migration in ethylene to form the ethylidene-like molecule. Moreover, in the ring system, the ethylene-like double bond twist and pyramidalization causes a similar twisted pyramidalization and, even, ring-opening in cyclohexene and cyclohexa-1,4diene. 24.

(39) 3.2.2.2 Cyclopentadiene The focus now moves to 5-membered ring systems. Cyclopentadiene is taken as an example, which is a five-membered ring system with two double bonds. This molecule is a very important precursor in organic synthesis [102]. The photochemical pathways [16, 17, 103, 104] of cyclopentadiene are described in Figure 15, and are similar to those observed in cyclohexene and cyclohexa-1,4-diene: involving a ring puckering channel, and products like bicylo[2,1,0]pentene (Bp) and tricyclo[2,1,0,0]pentane (Tp), which are highly strained structures formed via electrocyclic ring closure [103].. Figure 15. Photochemical reaction of cyclopentadiene. tricyclo[2,1,10,0] pentane and Bp is bicylco[2,1,0]pentene.. Tp. is. From previous studies, the optically dark 2A state has been thought to be involved in the photodynamics [103, 104]. However, recent experimental and theoretical results show that there is only one excited state involved in the photodynamics [16, 17]. These studies indicate that after cyclopentadiene is excited to the ππ* state, the initial dynamics happen on the in-plane modes and then follows the out-of-plane modes, where the wavepacket acquires doubly excited character along these modes when accessing the conical intersection between the ππ* state and the ground state. This two-step mechanism was misunderstood as occurring separately on two different excited states. A sketch of the photodynamics of cyclopentadiene is depicted in Figure 16. The out-of plane motion causes a significant torsion in one of the double bonds, leading to the out-of-plane bend of the CH2 group and the neighbouring hydrogen, and accesses the conical intersection to the ground 25.

(40) state. Furthermore, the out-of plane motion favours the reaction pathway leading to the formation of Bp. Several methylated derivatives of cyclopentadiene are investigated by Schalk et. al to unravel the substituent effect on the dynamics [104]. The substituents clearly slow down the motion, and it could be shown that substitutions out of the ring-plane only slowed down the out of plane motion, while in-plane substitutions affected both time constants. This interpretation agrees with theoretical findings [16]. The methyl substituent has only a minor effect on the topography of the conical intersection, but adds inertia to the vibrational motion.. Figure 16. Sketch of the photodynamics of cyclopentadiene. We can make a comparison with linear polyenes and cylcopentadiene. For linear polyenes, the 2A state is the lowest-lying doubly excited state. For these molecules, upon excitation to the 1B bright state, the populations transfer to the 2A state following the C=C bond stretching and torsion. However, in cyclopentadiene, the bond torsion gives access to the conical intersection between the 1B state and the ground state, although the relative proximity of the 2A state leads to a partial doubly excited character of the potential energy surface. From the idea of the dynamophore, cyclopentadiene is similar to butadiene, cyclohexene, and a high fraction of cyclohexa-1,4-diene, upon photoexcitation, in which the dynamics is localised on one double bond and there is torsion of the H2C=CH-CH3 moiety. As mentioned earlier, there is a charge transfer state which plays a role in the dynamics of butadiene when the population decays from 2A state to the ground state. However, in 26.

(41) cyclopentadiene, the constraint of the ring structure does not allow a complete out-of-plane twist as was found in butadiene, and a smaller charge transfer was observed [16]. When considering a system with more double bonds, e.g. the cycloheptatrienes [15, 105], the bright and the dark valence states come closer together. After excitation to the ππ* state, cyclopeheptatriene reaches the dark state within a few tens of femtoseconds, and reaches the ground state via a conical intersection with a fast [1,7] hydrogen shift. This motion can be considered as a localised motion since the ring planarization coordinate seems to be decoupled from the nonadiabatic dynamics [105, 106].. 3.3 Excited state dynamics of molecules with heteroatoms The photodynamics of ethylene and polyenes has been discussed which indicates that some polyenes retain the ethylene-like dynamics. Now the focus is changed to molecules with heteroatoms. These kinds of molecules are considered from two aspects; one from adding heteroatoms to ethylene, e.g. acrolein, acrylonitrile, alkyl vinyl ethers, etc. and the other is from adding the heteroatom into the ring structure, i.e. heterocycles, e.g. pyrrole and furan.. 3.3.1 Acrolein The first molecules to be discussed are created by adding a heteroatom to the ethylene moiety. Acrolein is taken as one of the examples. The structure of acrolein is shown in Figure 17 (a).. Figure 17. The structure of: (a) acrolein, (b) furan and (c) pyrrole Unlike linear polyenes, the acrolein molecule contains a –C=O functional group which introduces an additional excited state and different photochemical pathways due to the existence of nonbonding electrons from the oxygen atom. In addition to the lowest-lying ππ* state, there is another low-lying excited state, the 1nπ* state, which is due to an excitation from the 27.

(42) nonbonding lone pair orbital of the oxygen atom to the delocalised antibonding π* orbital. There are two reported major product channels from measuring acrolein in the liquid phase: one is the [1,3] Hydrogen migration channel, and the other is the α-hydrogen loss process [107].. Figure 18. The photodynamics of acrolein upon photoexcitation to the ππ* state. Upon photoexcitation to the ππ* state, the population leaves the FranckCondon region and undergoes internal conversion to the 1nπ* state within 100 fs [108]. On the 1nπ* state, there are two possible relaxation mechanisms [109]; internal conversion to the singlet ground state, and intersystem crossing to the 3nπ* state or 3ππ* states, of which the latter is allowed by the El-sayed’s rule. Regarding the quantum calculations [109-111], acrolein has a planar structure at room temperature, and the dynamics happens on the ππ* state, involving the terminal –CH2 group rotating out of the plane by around 900. The photodynamics of acrolein is depicted in Figure 18. Experimental studies on acrolein and its derivatives have been undertaken in both liquid phase and gas phase [10, 107, 108, 112]. Lee and co-workers [108] investigated the methylation effect on the photodynamics of acrolein. The decay time on the 1nπ* state is significantly influenced by the methylation position on the acrolein molecule. This is explained by the terminal methylene torsion on the 1nπ* state to the conical intersection between the 1nπ* state and the ground state. Furthermore, the study reported by Schalk et. al. [10] also indicates the methylation effect on the dynamics of acrolein, especially to the decay rate of the IC and ISC processes. As mentioned in Chapter 2, IC through a conical intersection has a more rapid decay rate compared to that of ISC. However, in those molecules with an aldehyde group, the rate ISC can become competitive with IC. This is both 28.

(43) because of a large spin-orbit coupling effect over an extended region of near degeneracy between the singlet and the triplet state, and that the rate of IC slows down due to a sloped conical intersection. Conversely to studies on hydrocarbons, increasing methylation does not necessarily slow down the dynamics in acroleins. For example, crotonaldehyde (γ-methyl acrolein) decays twice as fast as acrolein due to a better access to the CoIn region [108]. Acrolein displays different reaction dynamics as compared to ethylene and the linear polyenes, which now includes a 1nπ* state in the photodynamics. The factors influencing the IC rate for decay from 1nπ* state to the ground state are the twist motion of the terminal CH2 group and the velocity of the wavepacket through the conical intersection involving the two branching coordinates (the h and g vectors), i.e. the twist motion and pyramidalization at the C2 atom, respectively, where the pyramidalization dominates. Furthermore, the aldehyde group effect on the ring system, e.g. in cyclopentenone, shows dynamics similar to acrolein [10]. Here, the dynamics slows down due to an increased barrier between the 1nπ* state to the ground state. This opens the door for ISC, but the photoproduct formed after return to the ground state of cyclopentenone involves the α-cleavage reaction at the carbonyl group, and, if a γ-hydrogen is present, leading to the creation of dienole or cyclobutane [113]. Another recent study indicates that in comparison with acrolein and cyclopentenone the latter has a slower ISC rate owing to the ring-constraint, resulting in a smaller spin-orbit coupling [114]. From a study of methylated cyclopentenone, the decay rate of IC and ISC decreases with increasing methylation [10]. This is an example of the aldehyde group effect on the ethylene molecule, which changes the photodynamics but keeps the ethylene-like channel of terminal CH2 torsion. However, other molecules, like acrylonitrile, which introduces the effect of a –CN group onto ethylene, and alkyl vinyl ethers, e.g. methyl vinyl ether which adds a –OCH3 onto ethylene, have also been studied in our work, and this will be described in detail for their photodynamics and comparison with acrolein and ethylene in Chapter 6.. 3.3.2 Heterocycles Heterocycle compounds can be regarded as replacing one or more of the ring-carbon atoms with different atoms in the cyclopolyene structure, e.g. pyrrole, furan, pyrazole, oxazole, etc. These molecules play an important role in organic chemistry, pharmaceutical chemistry, and in biological systems. The dynamics of cyclopolyenes has been discussed earlier, where the general trend for molecules is the out-of-plane mode motion on the ππ* state which causes one double bond torsion and ring puckering. For 29.

(44) heterocycles, the exchanged atoms will introduce new excited states and different dynamics. For example, a new excited πσ* state participating in the pyrrole, furan, and pyrazole dynamics [115, 116], the nπ* state in the oxazole dynamics [117], and, in addition to the similar dynamics of ringpuckering, bond cleavage channels have also been observed. The detailed dynamics and comparison of heterocycles will be described below, with the following two molecules taken as an example; furan (the structure is shown in figure 17(b)), to show the effect from the oxygen atom, and pyrrole (the structure is shown in figure 17 (c)), to investigate the effect of introducing a nitrogen-hydrogen bond. Furan is a five-membered ring system, which is obtained by replacing a carbon atom in cyclopentadiene with an oxygen atom. In contrast to acrolein and cyclopentenone, the oxygen atom in the ring-system does not connect to the adjacent carbon atoms with a double bond, but forms a cyclic 6-electron π-system. Therefore, these systems are also called heteroaromatics. The interest in furan is due to it is role as a pollutant [118, 119] in atmospheric chemistry, and in combustion it has been considered as a potential biofuel [120,121]. The two lowest-lying excited states of furan are the 1B2(ππ*) state and the 1 A1(ππ*) state, respectively. The lowest-lying Rydberg state is the π3s Rydberg state, whose symmetry label is 1A2, which weakly couples with the 1 B2(ππ*) state. The other lower-lying Rydberg states are the 3p Rydberg states, which could split into three types: 1B1(3py), 1B2(3px) and 1A2(3pz). Photoexcitation to both the 1B2(ππ*) state and the 1A2(π3s) state has been studied [4, 122-124]. On the 1B2(ππ*) state, the wavepacket motion involves the C-O stretching motion together with an out-of-plane deformation mode, and the population transfers to the ground state through a conical intersection. Quantum calculations also show [125] the two competing deactivation pathways upon photoexcitation to the ππ* state. It is found that the reduced energy of the πσ* state along the Franck-Condon active in-plane ring-breathing mode allows access the ππ* state via a conical intersection. From here, the population flows to the πσ* state, and, on the way to accessing the conical intersection with the ground state, undergoes a ringopening, which requires a large torsion of the atomic framework. Conversely, the ring-puckered pathway is less complicated, in which relaxation along the ππ* state accesses the conical intersection with the ground state via the ring deformation. The CoIn of the ring opening channel is energetically more stable than the CoIn of the ring puckering channel; however, the barrier to access the πσ* state might cause a low quantum yield. The photodynamics of furan is described in Figure 19. According to the photodynamics of furan, the photoproducts can be determined after excitation to the 1B2(ππ*) state, and these are also shown in Figure 19. The C-O bond-cleavage channel could form 3-butadienal and cyclopropen-3-carbaldehyde. 30.

(45) The photodynamics upon excitation to the 1A1(ππ*) state has also been investigated [124]. On the 1A1(ππ*) state, there is a rapid structural deformation. Here, the population could flow to the lower-lying 3s and 3p Rydberg states through a conical intersection. Liu et al [126] investigated the ultrafast internal conversion of furan, and reported two rapid relaxation pathways: one from the S2 state to the hot S1 state, and one involving formation of α- and β-carbene by H migration.. Figure 19. Photodynamics of furan From the point of view of photodynamics, furan is similar to cyclopentadiene upon excitation to the 1B2(ππ*) state, but the existence of the oxygen atom introduces an accessible πσ* state along the O-C bonding coordinate. This state also exists in cyclopentadiene but the barrier to access it is too high. As such, the population has another possible decay pathway from the ππ* state to the πσ* state. For cyclopentadiene on the excited state, the in-plane plus out-of plane modes lead to the ring-puckering channel, while, for furan, the in-plane plus out-of-plane modes cause ring puckering on the ππ* state and C-O bond stretch causes ring-opening on the πσ* state. From the basic ideas obtained from the photodynamics of furan and the aldehyde group effect on the acrolein and cyclopentenone, we investigated the effect of an aldehyde group on furan. These molecules, and the details are given in Chapter 6. 31.

(46) Having introduced the effect of oxygen on a heterocycle, I now move to introducing a nitrogen atom, specifically an N-H bond. This is the pyrrole molecule. Pyrrole is a prototypical heteroatom molecule, and the most important interest in it arises from the fact that it is a significant subunit for large biological system, for examples the DNA bases, aromatic amino acids and so forth. As such, pyrrole has been measured extensively in theory and experiments [9, 35, 36, 127-136]. The absorption bands with excitation energies between 5.5 eV and 6.5 eV have been assigned by quantum calculations [137], and from low to high energy involve the A2(πσ*), B1(πσ*), π3p Rydberg, A1(ππ*), and B2(ππ*) states. The π3p Rydberg state separates into the 3pz and 3py states, and, especially for the π3py Rydberg state, the absorption structure shows a prominent, sharp feature [137, 138]. The (πσ*) states have a strong s-Rydberg admixture, and are typically respect to the bond dissociation in the dynamics. According to Barbatti [35] et al., who investigated the pyrrole dynamics theoretically, there are two main relaxation pathways: one involving the N-H bond dissociation, and the other involving ring-puckering on the excited state which accesses the conical intersection with the ground state. If the pyrrole molecules are excited initially to the B2(ππ*) state, then the population crosses the conical intersection between the B2(ππ*) state and the A2(πσ*) state, leading to N-H bond cleavage and ring puckering. This is described in Figure 20. However, if the molecules are initially excited to the A2(πσ*) state, this directly induces the N-H bond cleavage and the out-ofplane motions.. Figure 20. One kind of photodynamics in pyrrole. 32.

(47) Different excitation wavelengths between 200 nm and 250 nm have been used to investigate the photodynamics of pyrrole [9]. These indicate that the longer wavelengths, e.g., 250 nm, lead to longer lifetimes on the A2(πσ*) state. Furthermore, Wu et. al. studied the dynamics on the A2(πσ*) state with different wavelengths [36]. They report that there are two pathways leading to N-H dissociation. One is a fast N-H cleavage due to the wavepacket lying above the barrier on the πσ*-state with a higher excitation energy to the dissociation channel, while the other is slow due to a lower excitation energy which puts the wavepacket below the barrier, requiring it to tunnel through the barrier to dissociate. As a result, the excitation wavelength determines the photodynamics of pyrrole and the lifetime of the dynamics. Pyrrole also can be directly excited to the B1(πσ*) state, where there is a decay pathway for internal conversion to the A2(πσ*) state. Quantum calculations indicates that around 34% undergo N-H cleavage directly on the B1(πσ*) state, while the remainder follow the decay path to the A2(πσ*) state [36]. Methylated pyrrole, like N-methyl pyrrole, is studied with excitation to the π3p Rydberg state [139]. Here, the population transfers to the A2(πσ*) state where the ring-puckering and N-CH3 dissociation channels occur. The common point for cyclo-system molecules, like cyclopolyenes, furan, and pyrrole, is that they all undergo ring-puckering dynamics. Adding heteroatoms opens up new and different channels for different molecules, e.g. for furan, the C-O bond stretch on the excited state causes a ring-opening, and, for pyrrole, the N-H stretch leads to N-H dissociation. Conversely, comparable with ethylene and polyenes, the pyrrole and furan molecules have an additional excitation state, since there is a πσ* state where bond cleavage occurs. As the π3p Rydberg state lies lower in energy than the B2(ππ*) state, there is a possibility of transiently populating the Rydberg state upon photoexcitation to the B2(ππ*) state which can have a guiding influence on which of the NH dissociation channels is accessed. We have investigated this interaction and the details are given in Chapter 6.. 33.

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