Oliver Schalk

Excited state non-adiabatic dynamics of pyrrole

The dynamics of pyrrole excited at wavelengths in the range 242-217 nm are studied using a combination of time-resolved photoelectron spectroscopy and wavepacket propagations performed using the multi-configurational time-dependent Hartree method. Excitation close to the origin of pyrroles electronic spectrum, at 242 and 236 nm, is found to result in an ultrafast decay of the system from the ionization window on a single timescale of less than 20 fs. This behaviour is explained fully by assuming the system to be excited to the A2(πσ(∗)) state, in accord with previous experimental and theoretical studies. Excitation at shorter wavelengths has previously been assumed to result predominantly in population of the bright A1(ππ(∗)) and B2(ππ(∗)) states. We here present time-resolved photoelectron spectra at a pump wavelength of 217 nm alongside detailed quantum dynamics calculations that, together with a recent reinterpretation of pyrroles electronic spectrum [S. P. Neville and G. A. Worth, J. Chem. Phys. 140, 034317 (2014)], suggest that population of the B1(πσ(∗)) state (hitherto assumed to be optically dark) may occur directly when pyrrole is excited at energies in the near UV part of its electronic spectrum. The B1(πσ(∗)) state is found to decay on a timescale of less than 20 fs by both N-H dissociation and internal conversion to the A2(πσ(∗)) state.

Following the excited state relaxation dynamics of indole and 5-hydroxyindole using time-resolved photoelectron spectroscopy

Time-resolved photoelectron spectroscopy was used to obtain new information about the dynamics of electronic relaxation in gas-phase indole and 5-hydroxyindole following UV excitation with femtosecond laser pulses centred at 249 nm and 273 nm. Our analysis of the data was supported by ab initio calculations at the coupled cluster and complete-active-space self-consistent-field levels. The optically bright (1)L(a) and (1)L(b) electronic states of (1)ππ∗ character and spectroscopically dark and dissociative (1)πσ∗ states were all found to play a role in the overall relaxation process. In both molecules we conclude that the initially excited (1)L(a) state decays non-adiabatically on a sub 100 fs timescale via two competing pathways, populating either the subsequently long-lived (1)L(b) state or the (1)πσ∗ state localised along the N-H coordinate, which exhibits a lifetime on the order of 1 ps. In the case of 5-hydroxyindole, we conclude that the (1)πσ∗ state localised along the O-H coordinate plays little or no role in the relaxation dynamics at the two excitation wavelengths studied.

Through-Bond Interactions and the Localization of Excited-State Dynamics

The influence of through-bond interactions on nonadiabatic excited-state dynamics is investigated by time-resolved photoelectron spectroscopy (TRPES) and ab initio computation. We compare the dynamics of cyclohexa-1,4-diene, which exhibits a through-bond interaction known as homoconjugation (the electronic correlation between nonconjugated double bonds), with the nonconjugated cyclohexene. Each molecule was initially excited to a 3s Rydberg state using a 200 nm femtosecond pump pulse. The TRPES spectra of these molecules display similar structure and time constants on a subpicosecond time scale. Our ab initio calculations show that similar sets of conical intersections (a [1,2]- and [1,3]-hydrogen shift, as well as carbon-carbon bond cleavage) are energetically accessible to both molecules and that the geometry and orbital composition at the minimum energy crossing points to the ground state are directly analogous. These experimental and computational results suggest that the excited-state dynamics of cyclohexa-1,4-diene become localized at a single double bond and that the effects of through-bond interaction, dominant in the absorption spectrum, are absent in the excited-state dynamics. The notion of excited-state dynamics being localized at specific sites within the nuclear framework is analogous to the localization of light absorption by a subsystem within the molecule, designated a chromophore. We propose the utility of the analogous concept, denoted here as a dynamophore.

Initial processes of proton transfer in salicylideneaniline studied by time-resolved photoelectron spectroscopy.

Proton transfer is a fundamental and important process in biological systems. To understand this process in complicated biological activities, one fruitful approach is the investigation of simpler molecular systems to gain insight into the proton transfer dynamics. Salicylideneaniline (SA), shown in Fig. 1, is one of the most attractive molecules and was investigated extensively so far [1]. The basic understanding of excited state proton transfer in SA is as follows: In the ground state, the enol form is more stable than the keto form, while in excited states, the proton-transferred keto form is preferred. Thus, the proton of the hydroxy group migrates on a fs-timeacsle upon photoexcitation. From there, cis-trans isomerization takes place by passing a conical intersection with the ground state and produces a photochromic product : the trans-keto form.

Ultrafast non-adiabatic dynamics of methyl substituted ethylenes: the π3s Rydberg state.

Excited state unimolecular reactions of some polyenes exhibit localization of their dynamics at a single ethylenic double bond. Here we present studies of the fundamental photophysical processes in the ethylene unit itself. Combined femtosecond time-resolved photoelectron spectroscopy (TRPES) and ab initio quantum chemical calculations was applied to the study of excited state dynamics in cis-butene, trans-butene, trimethylethylene, and tetramethylethylene, following initial excitation to their respective π3s Rydberg states. The wavelength dependence of the π3s Rydberg state dynamics of tetramethylethylene was investigated in more detail. The π3s Rydberg to ππ(∗) valence state decay rate varies greatly with substituent: the 1,2-di- and tri-methyl substituted ethylenes (cis-butene, trans-butene, and trimethylethylene) show an ultrafast decay (∼20 fs), whereas the fully methylated tetramethylethylene shows a decay rate of 2 to 4 orders of magnitude slower. These observations are rationalized in terms of topographical trends in the relevant potential energy surfaces, as found from ab initio calculations: (1) the barrier between the π3s state and the ππ∗ state increases with increasing methylation, and (2) the π3s∕ππ∗ minimum energy conical intersection displaces monotonically away from the π3s Franck-Condon region with increasing methylation. The use of systematic methylation in combination with TRPES and ab initio computation is emerging as an important tool in discerning the excited state dynamics of unsaturated hydrocarbons.

Femtosecond UV Excitation in Imidazolium-Based Ionic Liquids

Femtosecond pump-probe absorption spectroscopy was employed to investigate ultrafast dynamics in various room temperature ionic liquids (RTILs) based on imidazolium cations, i.e., 1,3-dimethylimidazolium iodide ([DMIM]I), 1-butyl-3-methylimidazolium iodide ([BMIM]I), 1-hexyl-3-methylimidazolium iodide ([HMIM]I), 1-hexyl-3-methylimidazolium chloride ([HMIM]Cl), and 1-methyl-3-octylimidazolium chloride ([MOIM]Cl). Immediately after photoexcitation, an induced absorption was observed at various probe wavelengths (555-1556 nm). Afterward, the decay of the induced absorption was found to be independent of the alkyl chain length and viscosity of the ionic liquids. Two alternative mechanisms were proposed to explain the dynamics. In a first scenario excess electrons are generated through one-photon photodetachment of halides analogous to aqueous halide photodetachment. The dynamics in this case were analyzed with the help of a competing kinetic model proposed for geminate recombination in aqueous chloride photodetachment. Alternatively, imidazolium cations may be subject to photoionization. The transient NIR absorption can then be assigned to imidazolium dimer radical cations and/or excess electrons which may be formed upon association of imidazolium radicals with their parent cations. Both scenarios suggest that a thorough explanation of the ultrafast dynamics probably requires the implication of cooperative effects in the ionic liquids upon photoexcitation.

Near-Infrared Excitation of the Q Band in Free Base and Zinc Tetratolyl-porphyrins

The photophysics of 5,10,15,20-tetra-p-tolyl-21H,23H-porphyrin (TTP-H2) and 5,10,15,20-tetra-p-tolyl-porphyrinato zinc II (ZnTTP) have been investigated by means of pump-probe and transient anisotropy experiments. After excitation to the Q band, the molecules were probed by NIR pulses in the range between 950 and 1350 nm in order to study states of gerade symmetry in the vicinity of the Soret band. Examination of transient spectra and anisotropy delivered the first direct observation and the excitation energies of the two lowest so-called dark states. The experimental results were compared with predictions from theoretical calculations.

On the condensed phase ring-closure of vinylheptafulvalene and ring-opening of gaseous dihydroazulene.

Dihydroazulenes are interesting because of their photoswitching behavior. While the ring-opening to vinylheptafulvalene (VHF) is light induced, the back reaction is known to proceed thermally. In the present paper, we show the first gas phase study of the ring-opening reaction of 2-phenyl-1,8a-dihydroazulene-1,1-dicarbonitrile (Ph-DHA) by means of time-resolved photoelectron spectroscopy which permits us to follow the ring-opening process. Moreover, we investigated s-trans-Ph-VHF in a series of transient absorption experiments, supported by ab initio computations, to understand the origin of the absence of light-induced ring-closure. The transient absorption results show a biexponential decay governed by a hitherto unknown state. This state is accessed within 1-2 ps and return to the ground state is probably driven through a cis-trans isomerization about the exocyclic C1═C2 double bond. The rapid decrease in potential energy disfavors internal rotation to s-cis-Ph-VHF, the structure that would precede the ring-closure reaction.

Excited state non-adiabatic dynamics of pyrrole: A time-resolved photoelectron spectroscopy and quantum dynamics study

The dynamics of N-methylpyrrole following excitation at wavelengths in the range 241.5-217.0 nm were studied using a combination of time-resolved photoelectron spectroscopy (TRPES), ab initio quantum dynamics calculations using the multi-layer multi-configurational time-dependent Hartree method, as well as high-level photoionization cross section calculations. Excitation at 241.5 and 236.2 nm results in population of the A2(πσ(∗)) state, in agreement with previous studies. Excitation at 217.0 nm prepares the previously neglected B1(π3py) Rydberg state, followed by prompt internal conversion to the A2(πσ(∗)) state. In contrast with the photoinduced dynamics of pyrrole, the lifetime of the wavepacket in the A2(πσ(∗)) state was found to vary with excitation wavelength, decreasing by one order of magnitude upon tuning from 241.5 nm to 236.2 nm and by more than three orders of magnitude when excited at 217.0 nm. The order of magnitude difference in lifetimes measured at the longer excitation wavelengths is attributed to vibrational excitation in the A2(πσ(∗)) state, facilitating wavepacket motion around the potential barrier in the N-CH3 dissociation coordinate.

Pseudo-Bimolecular [2+2] Cycloaddition Studied by Time-Resolved Photoelectron Spectroscopy

The first study of pseudo-bimolecular cycloaddition reaction dynamics in the gas phase is presented. We used femtosecond time-resolved photoelectron spectroscopy (TRPES) to study the [2+2] photocycloaddition in the model system pseudo-gem-divinyl[2.2]paracyclophane. From X-ray crystal diffraction measurements we found that the ground-state molecule can exist in two conformers; a reactive one in which the vinyl groups are immediately situated for [2+2] cycloaddition and a nonreactive conformer in which they point in opposite directions. From the measured S(1) lifetimes we assigned a clear relation between the conformation and the excited-state reactivity; the reactive conformer has a lifetime of 13 ps, populating the ground state through a conical intersection leading to [2+2] cycloaddition, whereas the nonreactive conformer has a lifetime of 400 ps. Ab initio calculations were performed to locate the relevant conical intersection (CI) and calculate an excited-state [2+2] cycloaddition reaction path. The interpretation of the results is supported by experimental results on the similar but nonreactive pseudo-para-divinyl[2.2]paracyclophane, which has a lifetime of more than 500 ps in the S(1) state.