Valentin D. Vachev

Stilbene isomerization dynamics on multidimensional potential energy surface. Molecular dynamics simulation

Abstract The dynamics in the photoisomerization of isolated cis-stilbene, especially the dynamics leading to the intermediate twisted configuration is explored by molecular dynamics simulation. Comparison of the dynamics of the trans and cis isomers shows that for cis-stilbene the time required to reach the twisted configuration is an order of magnitude or two less than for trans-stilbene, due only to the specific steric interactions, and not to any difference in the potential functions or parameters. A barrier of between 510 and 640 cm−1 along the torsional coordinate from the cis side is estimated for isolated molecule, accounting for a transition to the intermediate in approximately 300 fs.

The (n, π∗) absorption spectrum of benzophenone. A new model for the excited state dynamics

Abstract We present a new model for the excited state dynamics and S 1 ← S 0 ( n , π ∗ ) absorption spectrum of benzophenone. As part of this model, we present compelling evidence for the activity of a symmetric bending mode (ν62) with frequency of approximately 140 cm−1 in the excited state. This result contrasts with previous work based on models in which the second low-frequency mode is the asymmetric twist whose activity is due to coupling with the symmetric twist. We analyze the effects of deuterium and fluorine substituents, and show how the results could be used to test the identity of the optically active modes.

Excited State Dynamics and Chemical Control of Large Molecules

A detailed knowledge of the vibrational dynamics that occurs on excited states is important in understanding photochemical processes and in controlling their outcome. We examine ways in which spectroscopic observations can be used to construct models for electronic excited states and infer the relevant nuclear dynamics on them. Our methods are applied to the well-studied photoisomerization reactions of stilbene and used to explore two photon (IR + UV) schemes for manipulating the photoproduct branching ratio.

Optical switching through photoisomerization: basics of the mechanism in rhodopsin and stilbene.

Vertebrate vision is based on the unique conversion of light into nerve impulses. Detailed understanding of the primary photochemical events is important with respect to potential applications in molecular optoelectronic devices. In our studies we apply modern theory, computer simulations, and experimental knowledge to develop new understanding of the photochemical processes and ultimately devise means to control their outcome. We use a combination of quantum theory and quasiclassical calculations to model excitation with laser pulses, excited state dynamics, internal conversion, and relaxation to photoproducts. In rhodopsin, the primary photochemical event involves an 11-cis to 11- trans photoisomerization. Recent time-resolved measurements provide the time-scales for disappearance of the reactant and the appearance of products. We find microscopical details of the internal conversion which are in accordance with the experimental results. This new perspective to the microscopic mechanism reveals the optimization of the electronic structure achieved in evolutionary development in nature. Cis-Stilbene is studied as a model system for which a double resonance laser excitation can be used for influencing the reaction. UV excitation leads to two photoproducts, trans-stilbene and dihydrophenanthrene (DHP). We show that IR excitation can be used to prepare ground sate cis-stilbene in an appropriate vibrational state before electronic excitation. We find that excitation of some modes leads to up to 4.5 times increase in the quantum yield of DHP or up to 4 times increase in trans-stilbene. Because of the shifted absorption spectra of the products, if such system is realized experimentally it can be used as switching bistable device.

Potential energy surface hopping algorithms for polyatomic molecules : Theoretical study

The applicability of a computer simulation technique for investigating photoinduced dynamics of polyatomic molecules, based on a combination of classical dynamics and a quantum surface hopping algorithm is discussed. The Wigner representation of a molecules time-dependent density matrix is calculated and the corresponding approximation of the molecules dynamics as a series of quantum jumps between the electronic states with classical movement on single electronic levels between jumps is presented. The effectiveness of this approach in computer simulations of a molecules photodissociation dynamics in the presence of a strong laser field is shown and computer simulation results on IR photostimulated dissociation in HCl+ molecule are reported.

Computer simulation of the structure, torsional potentials, excited state dynamics, and spectra of conjugated π-electron systems

We present a theoretical-computational method for studying the excited structure and dynamics of conjugated (pi) -electron systems using spectra calculations, a normal mode analysis and a theoretical derivation of the correct Hamiltonian for constrained systems. We illustrate the method for the example of benzophenone. It is possible to reproduce very well the S1 implied by S0 absorption spectrum using a model with only four active modes, which we identify as (nu) 64, (nu) 62, (nu) 27, and (nu) 25. We present compelling evidence for the activity of a symmetric bending mode ((nu) 62) with frequency of approximately 140 cm-1 in the excited state. This result contrasts previous work based on models with activity in the asymmetric twist with frequency of approximately 70 or 100 cm-1. We analyze the effects of deuteration and fluorine substituents, and show how the results could be used for an experimental test of the identity of the optically active modes.

Ultrafast dynamics of surface-adsorbed conjugated molecules

A detailed computer examination of the excited state dynamics of stilbene, both isolated in the gas phase and physisorbed on an inert surface is presented. It is discussed how the surface affects stilbenes dynamics in comparison with the isolated molecule case.

Simple models for excited-state dynamics of conjugated molecules

The photochemistry of large molecules with conjugated (pi) -systems plays an important role in many systems of biological interest, including vision and photosynthesis. The size of these systems, however, often precludes a detailed knowledge of the electronic states and associated potential energy surfaces on which the photochemical dynamics takes place. We present some recent progress in the construction of empirical excited state potential surfaces of conjugated molecules based on an analysis of spectroscopic, ab initio, and ultrafast lifetime measurement data. These surfaces are then used to investigate the excited state dynamics of both reduced- and full-dimensionality molecular models, including nonadiabatic processes. Applications to the photoisomerizaton reactions of the model system stilbene are featured.