Triplet State Z/E-Photoisomerizations of Polyenes:  A Comparison of ab Initio and Density Functional Methods

Abstract

In the search for less CPU-costly methods for study of triplet state Z/E-photoisomerization of olefins, Kohn−Sham density functional theory (DFT) has been tested on 1,3-butadiene (BD) and 1,3,5-hexatriene (HT). Computed T1 energies were compared to those from CASSCF, CASPT2, and spin-projected UMP4(SDTQ) calculations as well as experiments. For both molecules it is necessary that nonlocal gradient corrections are made to the exchange functional since usage of the local spindensity approximation for exchange in most cases leads to vertical and relaxed T1 energies that are too high. Gradient-corrected DFT as well as hybrid functional methods lead to T1 energies that are bracketed by the corresponding UMP4(SDTQ) and CASPT2 energies and lie at most 4 kcal/mol below measured values. The relaxed T1 energies for planar geometries are in slightly better agreement with experiment when calculated by pure nonlocal gradient-corrected DFT than by hybrid functional methods. However, T1-state potential energy surfaces obtained by either type of method explain the experimental observations on triplet-state Z/E-photoisomerizations of BD and HT, and geometries of T1 isomers of BD and HT compare well with those from UMP4(SDQ), UMP2, and CASSCF calculations. Finally, it should be noted that for both molecules UHF deviates from the higher computational levels in T1 energies by 20−30 kcal/mol and should be avoided in all computations of T1 states of olefins.

Temperature Dependence of the 1,6-Diphenyl-1,3,5-hexatriene Triplet Lifetime in Solution and Theoretical Evaluation of Triplet Conformer Interconversion

“…26 The calculations employed density functional theory (DFT), a less-CPU-costly approach, that was recommended for such systems, after a critical evaluation of different theoretical methods for the calculation of triplet 1,3-diene and 1,3,5-hexatriene potential energy surfaces. 27 In all of the cases cited above, experiment and theory agree that photoisomerization in the lowest triplet state occurs as a consequence of decay from triplet conformers that are twisted (close to perpendicular) about the isomerizing bond. The twisted conformers ( 3 p for stilbene; 3 pt and 3 pc for the dienes) represent minima on the triplet potential energy surface.…”

“…Pertinent to the research described here are the satisfactory theoretical predictions of the experimental relative energies and structures of conformers, isomers, and transition states for their interconversion in the ground and lowest triplet states for the stilbenes , and the 1,4-dimethyl- and 1,4-diaryl-1,3-butadienes . The calculations employed density functional theory (DFT), a less-CPU-costly approach, that was recommended for such systems, after a critical evaluation of different theoretical methods for the calculation of triplet 1,3-diene and 1,3,5-hexatriene potential energy surfaces . In all of the cases cited above, experiment and theory agree that photoisomerization in the lowest triplet state occurs as a consequence of decay from triplet conformers that are twisted (close to perpendicular) about the isomerizing bond.…”

Excited-State Geometry Optimization with the Density Matrix Renormalization Group, as Applied to Polyenes

“…For example, adiabatic relaxation gives rise to a central polaronic feature in the bright 1 1 B u state 65,69,75,76 , as well as two-soliton or four-soliton structures for the dark 2 1 A g state 54,56,65 . 2) For short polyenes ranging from ethylene to octatriene, studies of low-lying excited state relaxation pathways suggest that non-planar molecular conformations are important at energy crossings [79][80][81][82][83][84][85][86][87][88][89][90][91][92][93][94][95] . This opens up the question of the nature of energy crossings and their associated geometries in the excited states of longer polyenes.…”

Detailed Mechanism for Trans−Cis Photoisomerization of Butadiene Following a Femtosecond-Scale Laser Pulse

“…Recently, a nonresonant multphoton ionization experiment has demonstrated that trans → cis isomerization of butadiene proceeds within about 420 fs . Theoretical approaches attempt to provide a detailed knowledge of the potential energy surfaces of the ground and electronically excited states as a tool for understanding the isomerization reaction paths. − Semiclassical molecular dynamics calculations have been frequently used to study the photoisomerization dynamics of polyenes. − Some early calculations were based on a one-dimensional approximation (torsion mode only). , These calculations provide a fairly comprehensive picture of the role of the C−C single bond but are unable to fully reveal the detailed mechanisms and irreversible nature of the reaction.…”