A quantum dynamics study of the benzopyran ring opening guided by laser pulses (original) (raw)
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The Journal of Chemical Physics
We present a model for the lowest two potential energy surfaces (PESs) that describe the photoinduced ring-opening reaction of benzopyran taken as a model compound to study the photochromic ring-opening reaction of indolinobenzospiropyran and its evolution toward its open-chain analog. The PESs are expressed in terms of three effective rectilinear coordinates. One corresponds to the direction between the equilibrium geometry in the electronic ground state, referred to as the Franck-Condon geometry, and the minimum of conical intersection (CI), while the other two span the two-dimensional branching space at the CI. The model correctly reproduces the topography of the PESs. The ab initio calculations are performed with the extended multiconfiguration quasidegenerate perturbation theory at second order method. We demonstrate that accounting for electron dynamic correlation drastically changes the global energy landscape since some zwitterionic states become strongly stabilized. Quantum dynamics calculations using this PES model produce an absorption spectrum that matches the experimental one to a good accuracy.
A generalised vibronic-coupling Hamiltonian model for benzopyran
The Journal of Chemical Physics, 2014
A new general model for describing intersecting multidimensional potential energy surfaces when motions of large amplitude are involved is presented. This model can be seen as an extension of the vibronic coupling models of Köppel et al. ["Multimode molecular dynamics beyond the Born-Oppenheimer approximation," Adv. Chem. Phys. 57, 59 (1984)]. In contrast to the original vibronic coupling models, here the number of diabatic states is larger than the number of adiabatic states and curvilinear coordinates are used in a systematic way. Following general considerations, the approach is applied to the fitting of the potential energy surfaces for the very complex nonadiabatic photodynamics of benzopyran. Preliminary results are presented at the complete active space self-consistent field level of theory and with up to 12 active degrees of freedom. Special emphasis is placed on the physical interpretation of the diabatic states and on the influence of the various degrees of freedom on the fit. © 2014 AIP Publishing LLC. [http://dx.
Quantum dynamics of the photostability of pyrazine
Physical chemistry chemical physics : PCCP, 2015
We investigate the radiationless decay of photoexcited pyrazine to its ground electronic state using multireference electronic structure and quantum dynamics calculations. We construct a quadratic vibronic coupling Hamiltonian, including the four lowest electronic states and ten vibrational modes, by fitting to more than 5000 ab initio points. We then use this model to simulate the non-adiabatic excited state dynamics of the molecule using the multi-configuration time-dependent Hartree method. On the basis of these calculations, we propose a new mechanism for this decay process involving a conical intersection between the Au(nπ*) state and the ground state. After excitation to the B2u(ππ*) state, the molecule decays to both the B3u(nπ*) and Au(nπ*) states on an ultrashort timescale of approximately 20 fs. The radiationless decay to the ground state then occurs from the Au(nπ*) state on a much longer timescale.
International Journal of Quantum Chemistry, 2019
A vector-based representation of the chemical bond is introduced, which we refer to as the bond-path framework set B = {p, q, r}, where p, q, and r represent 3 eigenvector-following paths with corresponding lengths H * , H, and the familiar quantum theory of atoms in molecules (QTAIM) bond-path length (BPL). The intended application of B is for molecules subjected to various types of reactions and distortions, including photoisomerization reactions, applied torsions θ, or normal modes of vibration. The lengths H * and H of the eigenvector-following paths are constructed using the e 1 and e 2 Hessian eigenvectors, respectively, along the bond path, these corresponding to the least and most preferred directions of charge density accumulation. In particular, the paths p and q provide a vector representation of the scalar QTAIM ellipticity ε.
Laser control of reactions of photoswitching functional molecules
The Journal of Chemical Physics, 2006
Laser control schemes of reactions of photoswitching functional molecules are proposed based on the quantum mechanical wave-packet dynamics and the design of laser parameters. The appropriately designed quadratically chirped laser pulses can achieve nearly complete transitions of wave packet among electronic states. The laser parameters can be optimized by using the Zhu-Nakamura theory of nonadiabatic transition. This method is effective not only for the initial photoexcitation process but also for the pump and dump scheme in the middle of the overall photoswitching process. The effects of momentum of the wave packet crossing a conical intersection on the branching ratio of products have also been clarified. These control schemes mentioned above are successfully applied to the cyclohexadiene/hexatriene photoisomerization ͑ring-opening͒ process which is the reaction center of practical photoswitching molecules such as diarylethenes. The overall efficiency of the ring opening can be appreciably increased by using the appropriately designed laser pulses compared to that of the natural photoisomerization without any control schemes.
MD Investigations of Photoinduced Transformations in Organic Molecules
1992
A computer simulation procedure is developed for modeling intramolecular dynamics in polyatomic molecules. Electronic-vibrational excitation by ultrashort laser pulses (20 fs-1 ps) is treated explicitly using quantum theory in harmonic approximation. MD simulation is used for studying the excited state dynamics. Stilbene photoinduced isomerization is modeled. Model potential energy surfaces (PES) for the ground and first excited singlet states are obtained using experimental absorption spectra in supersonic jet. Using a symmetrical along the torsional coordinate PES, it is shown that cis-stilbene undergoes the first stage of the isomerization reaction, i. e. transition to the twisted configuration, much faster than Lrans-stilbene, only due to the specific conformational properties. 2. INTRODUCTION Many important processes in polyatomic molecules take place in excited electronic states or use them as transition states. Between these are photoinduced isomerization of retinal systems and photosynthetic bacteria, electron transfer in biological, interfacial, or electrochemical systems, vibrational relaxation in liquids, and photodissociation. These processes occur typically on pico-and subpicosecond timescales, and recent advances in generation of ultrashort and broadly tunable laser pulses are highly promising for experimental studies of the basic mechanisms realized in Nature.'4 Such investigations are important also in the search of new materials for nonlinear optics, electro-optics and molecular electronics. The complexity of the objects under consideration, however, makes difficult a direct interpretation of the data obtained from different laser spectroscopy methods in both time and frequency domains. Thus, appropriate approximations and computer simulation methods are necessary. Molecular Dynamics (MD) is a computer-based technique for modeling gases, liquids and solids on microscopic scales of distance and time, and is therefore an ideal technique for studying molecular behavior in many physical processes.57 The method is based on the assumption that atomic motions are governed by classical mechanics provided some appropriate multidimensional force-field is used. Limitations of the method are well known. A fundamental one results from the basic assumptions of the method, namely, quantum-mechanical behavior is neglected and a single potential energy surface is assumed to govern the motion. The quantum nature of vibrational and electronic motion, however, is important and must generally be accounted for. Other problems are connected with practical difficulties in constructing accurate force-field, including large number of atoms, integrating over long times, or achieving accurate statistical sampling. All of these depend on the efficiency of the computational procedure and models used. A general formulation of the problem and the approach used can be understood from Figure 1. A molecule being initially in a ground electronic state 1) after irradiation is excited to an upper electronic level 2). The initial state of the molecule after excitation depends on the two potential energy surfaces (PES) and laser pulse characteristics such as laser frequency, time duration and coherence length. The laser pulse duration r should be compared with the vibrational period r of the molecule. For instance, ultrashort laser pulse, r, <<TV, would bring the molecule to the excited electronic state without sufficient changes in the nuclei configuration (Franck-Condon transition). In the quasi-stationary case a long laser pulse with sufficient coherence length would excite (under appropriate conditions) a single vibrational level (SVL). Actually a 15 ps laser pulse may be long enough for a SVL excitation.8 In the intermediate case of comparable T and r the excited state is determined by a complex interference between vibrational movements in the two electronic states.
Physical Chemistry Chemical Physics, 2010
Under the usual assumption of noninteracting v-representability, density-functional theory (DFT) together with time-dependent DFT (TDDFT) provide a formally exact single-reference method suitable for the theoretical description of the electronic excited-states of large molecules, and hence for the description of excited-state potential energy surfaces important for photochemistry. The quality of this single-reference description is limited in practice by the need to use approximate exchange-correlation functionals. In particular it is far from clear how well approximations used in contemporary practical TDDFT calculations can describe funnel regions such as avoided crossings and conical intersections. These regions typically involve biradical-like structures associated with bond breaking and conventional wisdom would seem to suggest the need to introduce explicit double excitation character to describe these structures. Although this is lacking in ordinary spin-preserving (SP) TDDFT, it is present to some extent in spin-flip (SF) TDDFT. We report our tests of Wang-Ziegler noncollinear SF-TDDFT within the Tamm-Dancoff approximation for describing the avoided crossing in the C 2v CC ring-opening reaction of oxirane and for describing the conical intersection relevant for the more physical asymmetric CO ring-opening reaction of oxirane. Comparisons are made with complete active space self-consistent field and quantum Monte Carlo benchmark results from two previous papers on the subject [J. Chem. Phys., 2007, 127, 164111; ibid 129, 2008, 124108]. While the avoided crossing in the C 2v pathway is found to be reasonably well described, the method was found to be only partially successful for the conical intersection (CX) associated with the physically more important asymmetric pathway. The origin of the difficulties preventing the noncollinear SF-TDDFT method from giving a completely satisfactory description of the CX was traced back to the inability of SF-TDDFT based upon a single triplet reference state to correlate all potentially relevant configurations involving not just two but three nearly degenerate orbitals (n, s CO , and s à CO ). This article is also the first report of our implementation of SF-TDDFT within the DEMON2K program.
Journal of physics. Condensed matter : an Institute of Physics journal, 2015
The computational simulation of photo-induced processes in large molecular systems is a very challenging problem. Firstly, to properly simulate photo-induced reactions the potential energy surfaces corresponding to excited states must be appropriately accessed; secondly, understanding the mechanisms of these processes requires the exploration of complex configurational spaces and the localization of conical intersections; finally, photo-induced reactions are probability events, that require the simulation of hundreds of trajectories to obtain the statistical information for the analysis of the reaction profiles. Here, we present a detailed description of our implementation of a molecular dynamics with electronic transitions algorithm within the local-orbital density functional theory code FIREBALL, suitable for the computational study of these problems. As an example of the application of this approach, we also report results on the [2 + 2] cycloaddition of ethylene with maleic anhy...
The photoinduced ring opening reaction of benzo(2H)chromenes: a kinetic and thermodynamic approach
Chemical Physics, 2005
The pyran-ring opening process in the benzo(2H)chromenes depends on the degree and nature of vibronic excitation in the first two or three excited electronic states when fundamental, harmonic and combination bands of two optically active modes are selectively excited by monochromatic radiation. Based on a previously proposed model, these findings are interpreted as being the result of competition between vibrational relaxation and photochemistry at each vibronic level. In this work, the model is further expanded and a new equation is developed to describe the relaxation from combination bands, which allows the quantum yields of the individual processes which contribute to deactivation of vibronically excited levels to be determined. To gain more insights into the relaxation dynamics of the excited states, the Heisenberg uncertainty principle was applied to evaluate the upper limit values of the rate constants for the relaxation processes of the excited states from the widths of the vibronic peaks. The energy barriers between reactants and photoproducts were estimated by using an Arrhenius-type relationship. The low barriers that were found ($2 kJ mol À1) satisfactorily explain the competition between the reaction path and the energy degradation processes.