Benjamin Lasorne

Photophysics of the π,π* and n,π* States of Thymine: MS-CASPT2 Minimum-Energy Paths and CASSCF on-the-Fly Dynamics

The photodynamics along the main decay paths of thymine after excitation to the lowest pi,pi* state have been studied with MS-CASPT2 calculations and semiclassical CASSCF dynamics calculations including a surface hopping algorithm. The static calculations show that there are two decay paths from the Franck-Condon structure that lead to a conical intersection with the ground state. The first path goes directly to the intersection, while the second one is indirect and involves a minimum of the pi,pi* state, a small barrier, and a crossing between the pi,pi* and n,pi* states. From the static calculations, both paths have similar slopes. The dynamics calculations along the indirect path show that, after the barrier, part of the trajectories are funneled to the intersection with the ground state, where they are efficiently quenched to the ground state. The remaining trajectories populate the n,pi* state. They are also quenched to the ground state in less than 1 ps, but the static calculations show that the decay rate of the n,pi* state is largely overestimated at the CASSCF level used for the dynamics. Overall, these results suggest that both direct and indirect paths contribute to the subpicosecond decay components found experimentally. The indirect path also provides a way for fast population of the n,pi* state, which will account for the experimental picosecond decay component.

Excited-state dynamics

Excited‐state dynamics is the field of theoretical and physical chemistry devoted to simulating molecular processes induced upon UV‐visible light absorption. This involves nuclear dynamics methods to determine the time evolution of the molecular geometry used in concert with electronic structure methods capable of computing electronic excited‐state potential energy surfaces. Applications concern photochemistry (see Chapter CMS‐030: Computational photochemistry) and electronic spectroscopy. Most of the work in this field looks at unsaturated organic molecules as these provide widely used chromophores with a straightforward photochemistry that can be described by a small number (usually two) of electronic states. The electronic ground state of closed‐shell organic molecules is a singlet (electronic spin zero) termed S0. Molecules are promoted to their electronic excited states through absorption of UV‐visible light (200–700 nm), usually to the first or second singlet, S1 or S2. Typical examples are well represented as a one‐electron transition from the π or n highest occupied molecular orbital to a π* or σ* low‐lying unoccupied molecular orbital. The photo‐excited system will deactivate and return to the electronic ground state over a timescale that can be as short as about 100 fs for ultrafast mechanisms. For example, the initial event of vision is a photo‐isomerization of the retinal chromophore in the rhodopsine protein that occurs in ca. 200 fs. 1 , 2 The goal of a computational approach to the simulation of photo‐induced processes is the complete description of what happens at the molecular level from the promotion to the excited electronic state to the formation of products or regeneration of reactants back in the electronic ground state.

Controlling S1/S0 Decay and the Balance between Photochemistry and Photostability in Benzene: A Direct Quantum Dynamics Study†

In this work, we investigate general mechanistic principles that control reaction selectivity following S(1)/S(0) internal conversion in benzene. A systematic relationship is drawn between the varying topology of an extended seam of conical intersection and the balance between two competitive radiationless decay channels: photophysical (benzene reactant regeneration) and photochemical (prefulvene product formation). This is supported by a model quantum dynamics study, using a direct dynamics approach based on variational multiconfiguration Gaussian wavepackets, where initial excitation of specific vibrational modes is designed to generate dynamical pathways that reach selected targets regions of the seam. High-energy regions of the seam are found to be sloped and in favor of the photophysical channel, while lower-energy regions are peaked and give access to the photochemical channel. This changeover could in principle be exploited to define targets for optimal control, by exciting different combinations of specific vibronic levels in S(1), accessing different regions of the seam, and giving different products.

A Straightforward Method of Analysis for Direct Quantum Dynamics: Application to the Photochemistry of a Model Cyanine†

We present a new way of analyzing direct quantum dynamics simulations based on a Mulliken-type population analysis. This provides a straightforward interpretation of the wavepacket in much the same way as semiclassical trajectories are usually analyzed. The result can be seen as a coupled set of quantum trajectories. We apply this to the study of the photochemistry of a 12-atom model cyanine to explore possibilities for intelligent optimal control. The work presented here builds on previous semiclassical dynamics simulations [ Hunt , P. A. ; Robb , M. A. J. Am. Chem. Soc. 2005 , 127 , 5720 ]. Those calculations suggested that, by controlling the distribution of momentum components in the initial wavepacket, it should be possible to drive the system to a specific region of the conical intersection seam and ultimately control the product distribution. This was confirmed experimentally by optimal control methods [ Dietzek , B. ; Bruggemann , B. ; Pascher , T. ; Yartsev , A. J. Am. Chem. Soc. 2007 , 129 , 13014 ]. This paper aims to demonstrate this in a quantum dynamics context and give further insight into the conditions required for control. Our results show that directly addressing the trans-cis torsional modes is not efficient. Instead, one needs to decrease the momentum in the skeletal deformation coordinates to prompt radiationless decay near the minimum conical intersection at large twist angles.

Towards converging non-adiabatic direct dynamics calculations using frozen-width variational gaussian product basis functions.

In this article, we investigate the convergence of quantum dynamics calculations with coupled variationally optimized gaussian product basis functions, describing wavepacket motion on regions of molecular potential energy surfaces calculated on the fly. As a benchmark system, we model the radiationless decay of fulvene from its first electronic excited state through an extended S(1)∕S(0) conical intersection seam and monitor two associated properties: the spatial extent to which the conical intersection seam is sampled and the timescale and stepwise nature of the population transfer. We suggest that the fully variational description reviewed here (direct dynamics-variational multi-configuration gaussian) provides a way to balance accuracy against computational cost for molecules of comparable sizes by choosing the number of coupled gaussian product basis functions and a middle way forward between grid based and trajectory surface hopping approaches to non-adiabatic molecular quantum dynamics calculations.

Controlling the mechanism of fulvene S1/S0 decay: switching off the stepwise population transfer

Direct quantum dynamics simulations were performed to model the radiationless decay of the first excited state S(1) of fulvene. The full space of thirty normal mode nuclear coordinates was explicitly considered. By default, ultrafast internal conversion takes place centred on the higher-energy planar region of the S(1)/S(0) conical intersection seam, giving the stepwise population transfer characteristic of a sloped surface crossing, and leading back to the ground state reactant. Two possible schemes for controlling whether stepwise population transfer occurs or not-either altering the initial geometry distribution or the initial momentum composition of the photo-excited wavepacket-were explored. In both cases, decay was successfully induced to occur in the lower-energy twisted/peaked region of the crossing seam, switching off the stepwise population transfer. This absence of re-crossing is a direct consequence of the change in the position on the intersection at which decay occurs (our target for control), and its consequences should provide an experimentally observable fingerprint of this system.

Non-adiabatic effects in the photodissociation of bromoacetyl chloride

Abstract The competitive photodissociation of bromoacetyl chloride has been investigated by means of ab initio methods. Quantum dynamics in full dimensionality is prohibitive for such a system and therefore a reduced dimensionality method based on constrained Hamiltonians has been used. A one-dimensional (1-D) non-adiabatic wave packet treatment in the C S optimized geometry (trans Cl and Br) on the first excited states leads to encouraging results when compared to experimental data. The slow relaxation of the torsion angle is assessed by a 2-D dynamics in the subspace including the CO bond length.

Wave packet dynamics along bifurcating reaction paths

The problem of bifurcating reaction paths is revisited by wave packet (WP) dynamics. The pitchfork model connecting five stationary points—a reactive, two transition structures and two enantiomeric products—is characterized by a Valley Ridge inflection point (VRI) where WP could leave the standard intrinsic reaction path. We question the role of such a VRI point to determine whether the mechanism is sequential or concerted. WP simulations on two-dimensional minimum energy surfaces are carried out in the benchmark case of the methoxy radical isomerization H3CO→H2COH. The ab initio potential energy surface (PES) is fitted to an analytical model which is bent to analyze the incidence of geometrical parameters on the WP behavior. For each of these generated PES, the WP width in the entrance valley is the main factor which conditions the behavior on the unstable ridge. The WP evolution is also analyzed in terms of nonadiabatic transitions among adiabatic channels along the reaction coordinate. Finally, the loc...

Controlling product selection in the photodissociation of formaldehyde: direct quantum dynamics from the S1 barrier.

Controlling the selectivity between H(2)+CO and H+HCO in the S(1)/S(0) nonadiabatic photodissociation of formaldehyde has been investigated using direct quantum dynamics. Simulations started from the S(1) transition state have suggested that a key feature for controlling the branching ratio of ground-state products is the size of the momentum given to the wavepacket along the transition vector. Our results show that letting the wavepacket fall down from the barrier to the conical intersection with no initial momentum leads to H(2)+CO, while extra momentum toward products favors the formation of H+HCO through the same conical intersection. Quantum dynamics results are interpreted in semiclassical terms with the aid of a Mulliken-like analysis of the final population distribution among both products and the reactant on each electronic state.

Intramolecular Charge Transfer in 4-Aminobenzonitrile Does Not Need the Twist and May Not Need the Bend

A study combining accurate quantum chemistry and full-dimensional quantum dynamics is presented to confirm the existence of an ultrafast radiationless decay channel from the charge-transfer state to the locally excited state in 4-aminobenzonitrile. This intramolecular charge-transfer pathway proceeds through a newly found planar conical intersection, and it is shown to be more efficient in the presence of acetonitrile than in the gas phase. Our results are consistent with recent experimental observations.