High-Level Quantum Chemical Methods for the Study of Photochemical Processes (original) (raw)
Related papers
2004
Multireference configuration interaction with singles and doubles ͑MR-CISD͒ calculations have been performed for the optimization of conical intersections and stationary points on the ethylene excited-state energy surfaces using recently developed methods for the computation of analytic gradients and nonadiabatic coupling terms. Basis set dependence and the effect of various choices of reference spaces for the MR-CISD calculations have been investigated. The crossing seam between the S 0 and S 1 states has been explored in detail. This seam connects all conical intersections presently known for ethylene. Major emphasis has been laid on the hydrogen-migration path. Starting in the V state of twisted-orthogonal ethylene, a barrierless path to ethylidene was found. The feasibility of ethylidene formation will be important for the explanation of the relative yield of cis and trans H 2 elimination.
Photochemistry from first principles — advances and future prospects
Journal of Photochemistry and Photobiology A: Chemistry, 2001
Detailed simulation of photochemistry poses considerable challenges because quantum mechanical effects are important in determining both the electronic potential energy surfaces and the subsequent nuclear dynamics. We provide a brief overview of the ab initio multiple spawning (AIMS) method which addresses the problem by solving both the electronic and nuclear Schrödinger equations simultaneously. We discuss our recent AIMS simulations of cis-trans photoisoimerization in ethylene as an example application. The prospects of the method for modeling of photochemistry in large organic molecules and condensed phases are assessed.
A comparison of methods for theoretical photochemistry: Applications, successes and challenges
Annual Reports in Computational Chemistry, 2019
Herein we highlight recent studies and active areas of interest in the ongoing challenge to model photochemical processes in a wide variety of molecules. We also discuss recent significant methodological improvements and developments that may aid future investigations. Studies using the wide range of techniques available in modern electronic structure software packages have been included, with their successes and shortcomings forming part of the discussion. This study should therefore aid in the design of future computational studies.
Nonadiabatic Excited-State Molecular Dynamics: Modeling Photophysics in Organic Conjugated Materials
Accounts of Chemical Research, 2014
CONSPECTUS: To design functional photoactive materials for a variety of technological applications, researchers need to understand their electronic properties in detail and have ways to control their photoinduced pathways. When excited by photons of light, organic conjugated materials (OCMs) show dynamics that are often characterized by large nonadiabatic (NA) couplings between multiple excited states through a breakdown of the Born−Oppenheimer (BO) approximation. Following photoexcitation, various nonradiative intraband relaxation pathways can lead to a number of complex processes. Therefore, computational simulation of nonadiabatic molecular dynamics is an indispensable tool for understanding complex photoinduced processes such as internal conversion, energy transfer, charge separation, and spatial localization of excitons. Over the years, we have developed a nonadiabatic excited-state molecular dynamics (NA-ESMD) framework that efficiently and accurately describes photoinduced phenomena in extended conjugated molecular systems. We use the fewest-switches surface hopping (FSSH) algorithm to treat quantum transitions among multiple adiabatic excited state potential energy surfaces (PESs). Extended molecular systems often contain hundreds of atoms and involve large densities of excited states that participate in the photoinduced dynamics. We can achieve an accurate description of the multiple excited states using the configuration interaction single (CIS) formalism with a semiempirical model Hamiltonian. Analytical techniques allow the trajectory to be propagated "on the fly" using the complete set of NA coupling terms and remove computational bottlenecks in the evaluation of excited-state gradients and NA couplings. Furthermore, the use of state-specific gradients for propagation of nuclei on the native excited-state PES eliminates the need for simplifications such as the classical path approximation (CPA), which only uses ground-state gradients. Thus, the NA-ESMD methodology offers a computationally tractable route for simulating hundreds of atoms on ∼10 ps time scales where multiple coupled excited states are involved. In this Account, we review recent developments in the NA-ESMD modeling of photoinduced dynamics in extended conjugated molecules involving multiple coupled electronic states. We have successfully applied the outlined NA-ESMD framework to study ultrafast conformational planarization in polyfluorenes where the rate of torsional relaxation can be controlled based on the initial excitation. With the addition of the state reassignment algorithm to identify instances of unavoided crossings between noninteracting PESs, NA-ESMD can now be used to study systems in which these so-called trivial unavoided crossings are expected to predominate. We employ this technique to analyze the energy transfer between poly(phenylene vinylene) (PPV) segments where conformational fluctuations give rise to numerous instances of unavoided crossings leading to multiple pathways and complex energy transfer dynamics that cannot be described using a simple Forster model. In addition, we have investigated the mechanism of ultrafast unidirectional energy transfer in dendrimers composed of poly(phenylene ethynylene) (PPE) chromophores and have demonstrated that differential nuclear motion favors downhill energy transfer in dendrimers. The use of native excited-state gradients allows us to observe this feature.
Theoretical photochemistry of the photochromic molecules based on density functional theory methods
Lecture Notes in Computer Science (including subseries Lecture Notes in Artificial Intelligence and Lecture Notes in Bioinformatics), 2009
Mechanism of photoswitching in diarylethenes involves the lightinitiated symmetry-allowed disrotatory electrocyclic reaction. Here we propose a computationally inexpensive Density Functional Theory (DFT) based method that is able to produce accurate potential surfaces for the excited states. The method includes constrained optimization of the geometry for the ground and two excited singlet states along the ring-closing reaction coordinate using the Slater Transition State method, followed by single-point energy evaluation. The ground state energy is calculated with the broken-symmetry unrestricted Kohn-Sham formalism (UDFT). The first excited state energy is obtained by adding the UDFT ground state energy to the excitation energy of the pure singlet obtained in the linear response Time-Dependent (TD) DFT restricted Kohn-Sham formalism. The excitation energy of the double excited state is calculated using a recently proposed (Mikhailov, I. A.; Tafur, S.; Masunov, A. E. Phys. Rev. A 77, 012510, 2008) a posteriori Tamm-Dancoff approximation to the second order response TD-DFT.
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.
The Journal of Chemical Physics, 2009
Using previously developed potential energy surfaces and their couplings, non-Born–Oppenheimer trajectory methods are used to study the state-selected photodissociation of ammonia, prepared with up to six quanta of vibrational excitation in the symmetric (ν1) or antisymmetric (ν3) stretching modes of NH3(Ã). The predicted dynamics is mainly electronically nonadiabatic (that is, it produces ground electronic state amino radicals). The small probability of forming the excited-state amino radical is found, for low excitations, to increase with total energy and to be independent of whether the symmetric or antisymmetric stretch is excited; however some selectivity with respect to exciting the antisymmetric stretch is found when more than one quantum of excitation is added to the stretches, and more than 50% of the amino radical are found to be electronically excited when six quanta are placed in the antisymmetric stretch. These results are in contrast to the mechanism inferred in recen...