Session G26: Focus Session: Non-Adiabatic Dynamics: New Insights from Experiment and Theory III

The Non-Adiabatic dynamics of Singlet Fission in Polyacenes

Singlet fission involves the splitting of a single excitation into two coupled triplet excitations and is manifested in an increasing range of aromatic crystals and amorphous thin films. If the energy of the lowest triplet state is one half (or less) of the first singlet excited state, as it is for tetracene or pentacene and their derivatives, singlet fission may occur between two adjacent chromophores. Since there is no change in the overall spin state of the system, singlet fission can be exceptionally fast, occuring on the fs -- ps range. If the triplets can diffuse away from the fission site they are available for harvesting as a dissociated carriers with up to two charge carrier pairs per absorbed photon. The possibility of recovering excess energy above the material band gap (in this case determined by the triplet energy) when a higher energy photon is absorbed has led to great recent interest in exploiting this process for increased efficiency solar energy harvesting. The nature of the electronic couplings between the chromophores, intermediate electronic configurations, and the role of entropy in the spin-allowed primary fission event have all come under great scrutiny. Results from a series of femtosecond spectroscopy experiments on a variety of amorphous thin films, nanoparticles and isolated acene dimer compounds will be presented that shed light on the electronic intermediate states key to the efficiency and speed of this process.   

Extracting Molecular Dynamics from Ion Imaging Experiments of Carbonyl Sulfide

Photodissociation of carbonyl sulfide at 215nm are studied in details with sliced ion imaging experiments. Energy partitioning as well as vector correlations between carbonyl sulfide transition dipole moments, CO recoil velocity vector and angular momentum will be revealed. They can provide valuable information about symmetry of excited states which are involved in photodissociation process. These results can reveal information about non-adiabatic dynamics in carbonyl sulfide excited states. The results will be compared with both computational chemistry study conducted by G. McBane and coworkers. The result will also be compared with previous study on dynamics from carbonyl sulfide photodissociation at longer wavelength by Bersohn and coworkers.   

Vibronic interactions in multi-chromophores

Understanding and control of excitation energy transfer and electron-phonon interactions is quintessential for advances in solar energy utilization. Recently, we developed a vibronic model that is capable of predicting vibronic spectra in complex multi-chromophore systems. Parameters to the model are obtained from electronic structure calculations on monomer units of a multi-chromophore. This model can account for multiple vibrational modes, asymmetric wave functions, and inter-chromophore vibrations. Using this model, we explored vibronic spectra in a series of flexible bichromophores with available high-resolution experimental spectra. One of the goals of this work was to understand the effects of asymmetry in monomer units on vibronic interactions in bichromophores. Detailed investigation of diphenylmethane, partially deuterated diphenylmethane, and diphenylethane resulted in intriguing observation that asymmetry leads to a partial localization of one of the exciton states but leaves the other one delocalized. Extension of the developed methodology to modeling spectroscopy and dynamics in synthetic and biological multi-chromophore systems such as photosynthetic proteins will be also discussed.   

Non-Adiabatic Dynamics in the UV Photodissociation of Alkyl Radicals

This presentation focuses on the ultraviolet (UV) photodissociation dynamics of a series of prototypical alkyl radicals (ethyl, propyl, and butyl) using the high-$n$ Rydberg-atom time-of-flight (HRTOF) technique. Upon excitation to the 3$s$ state at 245-nm, ethyl dissociates into H atom and ethylene. Bimodal profile in the product translational energy distribution and energy-dependent product angular distribution indicate two different dissociation pathways that are influenced by conical intersection. A slow and isotropic component corresponds to unimolecular dissociation of the hot radical after internal conversion from the 3$s$ state to the ground state. A fast and anisotropic component corresponds to a direct, rapid H-atom scission via a nonclassical H-bridged transition state from the 3$s$ state to yield H + C$_2$H$_4$. Upon excitation to the 3$p$ state at 237 nm, $n$- and $i$-propyl radical dissociate into the H atom and propene products. The product translational energy release of both $n$- and $i$-propyl radicals also have bimodal distributions. The H-atom product angular distribution in $n$-propyl is anisotropic, while that in $i$-propyl is isotropic. The bimodal translational energy distributions indicate two dissociation pathways: (i) a unimolecular dissociation pathway from the ground-state propyl after internal conversion from the 3$p$ state, and (ii) a repulsive pathway directly connected with the excited state of the propyl radical. The UV photodissociation dynamics of the $n$-, $s$-, and $i$-butyl radicals are also investigated. The photodissociation mechanisms and the possible role of conical intersections will be discussed.   

Ultraviolet photodissociation dynamics of the cyclohexyl radical

Cycloalkanes are important components in conventional fuels and oil shale derived fuels and the combustion of cyclohexane fuels leads to the production of benzene, a pollutant precursor. One of the pathways from cyclohexane to benzene is through sequential hydrogen loss, including the cyclohexyl radical as an intermediate. The ultraviolet (UV) photodissociation dynamics of the cyclohexyl ($c$-C$_{\mathrm{6}}$H$_{\mathrm{11}})$ radical was studied for the first time using the high-$n$ Rydberg atom time-of-flight (HRTOF) technique in the range of 232-262 nm. The translational energy distributions of the H-atom loss product channel, $P(E_{\mathrm{T}})$'s, show a large translational energy release and a large fraction of average translational energy in the total excess energy, $\langle f_{\mathrm{T}}\rangle $, from 232-262 nm. The H-atom product angular distribution is anisotropic with a positive $\beta $ parameter. The most likely H-atom loss pathway is an axial H ejection from the $\beta $-carbon in cyclohexyl to form cyclohexene $+$ H, which along with the positive $\beta $ parameter, indicates that the transition dipole moment, $\mu $, is perpendicular to the ring. The $P(E_{\mathrm{T}})$ and anisotropy of the H-atom loss product channel are significantly larger than those expected for a statistical unimolecular dissociation of a hot radical, indicating a non-statistical dissociation mechanism. The dissociation mechanism is consistent with direct dissociation on a repulsive excited state surface or on the repulsive part of the ground state surface to produce cyclohexene $+$ H, possibly mediated by a conical intersection. Cyclohexyl is the largest radical so far showing a direct dissociation mechanism.   

Photoisomerization dynamics of a rhodopsin-based molecule (potential molecular switch) with high quantum yields

It is worthwhile to explore the detailed reaction dynamics of various candidates for molecular switches, in order to understand, e.g., the differences in quantum yields and switching times. Here we report density-functional-based simulations for the rhodopsin-based molecule 4-[4-Methylbenzylidene]-5-p-tolyl-3,4-dihydro-2H-pyrrole (MDP), synthesized by Sampedro et al. We find that the photoisomerization quantum yields are remarkably high: 82\% for \textit{cis}-to-\textit{trans}, and 68\% for \textit{trans}-to-\textit{cis}. The lifetimes of the S$_{1}$ excited state in \textit{cis}-MDP in our calculations are in the range of 900-1800 fs, with a mean value of 1270 fs, while the range of times required for full \textit{cis}-to-\textit{trans} isomerization are 1100-2000 fs, with a mean value of 1530 fs. In \textit{trans}-MDP, the calculated S$_{1}$ excited state lifetimes are 860-2140 fs, with a mean value of 1330 fs, and with the full \textit{trans}-to-\textit{cis} isomerization completed about 200 fs later. In both cases, the dominant reaction mechanism is rotation around the central C$=$C bond (connected to the pyrroline ring), and de-excitation occurs at an avoided crossing between the ground state and the lowest singlet state, near the midpoint of the rotational pathway.   

Photoexcited energy transfer in a weakly coupled dimer

Nonadiabatic excited-state molecular dynamics (NA-ESMD) simulations have been performed to study the time dependent exciton localization during energy transfer between two chromophore units of the weakly coupled dimer dithia-anthracenophane (DTA). The initial photoexcitation creates an exciton which is primarily localized on a single monomer unit. Subsequently, the exciton experiences an ultrafast energy transfer becoming localized on either one unit or the other whereas delocalization between both monomers never occurs. In half of the trajectories, the electronic transition density becomes completely localized on the same monomer as the initial excitation, while in the other half, it becomes completely localized on the opposite monomer. Here we present an analysis of the energy transfer dynamics and the effect of thermally induced geometry distortions on the exciton localization. Finally, simulated fluorescence anisotropy decay curves for both DTA and the monomer dimethyl anthracene (DMA) reveal that changes in the transition density localization caused by energy transfer in DTA is not the only source of depolarization and relaxation within a single monomer unit can also cause reorientation of the transition dipole.