[Figure: see text]. A quantum chemical study has been performed to assess changes in aromaticity along the T1 state Z/E-isomerization pathways of annulenyl-substituted olefins. It is argued that the point on the T1 energy surface with highest substituent aromaticity corresponds to the minimum. According to Baird (J. Am. Chem. Soc. 1972, 94, 4941), aromaticity and antiaromaticity are interchanged when going from S0 to T1. Thus, olefins with S0 aromatic substituents (set A olefins) will be partially antiaromatic in T1 and vice versa for olefins with S0 antiaromatic substituents (set B olefins). Twist of the C=C bond to a structure with a perpendicular orientation of the 2p(C) orbitals (3p*) in T1 should lead to regaining substituent aromaticity in set A and loss of aromaticity in set B olefins. This hypothesis is verified through quantum chemical calculations of T1 energies, geometries (bond lengths and harmonic oscillator measure of aromaticity), spin densities, and nucleus independent chemical shifts whose differences along the T1 PES display zigzag dependencies on the number of -electrons in the annulenyl substituent of the olefin. Aromaticity changes are reflected in the profiles of the T1 potential energy surfaces (T1 PESs) for Z/E-isomerizations because olefins in set A have minima at 3p* whereas those in set B have maxima at such structures. The proper combination (fusion) of the substituents of set A and B olefins could allow for design of novel optical switch compounds that isomerize adiabatically with high isomerization quantum yields.

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.

Nonlocal gradient-corrected and hybrid density functional theory (DFT) have been used to calculate T1 potential energy surfaces (PES), spin densities, and geometries of ethylene and aromatic olefins of various sizes:  ethylene (1), styrene (2), stilbene (3), 1,1-diphenylethylene (4), 1,4-bis-(1-propenyl)benzene (5), 1,3-divinylbenzene (6), and 2-(1-propenyl)anthracene (7). Calculated properties were used to determine differences in electronic structure of olefins that follow adiabatic vs diabatic Z/E-isomerization mechanisms. In the planar T1 structure, the CC bond in 1 is elongated to a single bond, but in 7 it remains a double bond, archetypal of excitations in the olefinic bond and in the substituent, respectively. Changes in geometries and spin-density distributions of 2 − 7 reveal that substituent aromaticities vary along the T1 PES. For systems that isomerize diabatically (e.g., 2), substituent aromaticity is regained in the 90° twisted structure of the CC bond (3p*). This leads to stabilization and a minimum on the PES at 3p*. If the substituent of the planar T1 olefin fully can accommodate the triplet biradical and still remain aromatic as in 7, aromaticity is instead reduced upon twist to 3p*, so that the T1 PES has a barrier that is suitable for adiabatic isomerizations. The planar structures of olefins with substituents that are partially antiaromatic in T1 (e.g., phenyl) can be stabilized by radical accepting groups in the proper positions (e.g., 5). In summary, our calculations indicate that for an aryl-substituted olefin the structure with the highest substituent aromaticity in T1 corresponds to the minimum on the T1 PES of Z/E-isomerizations.