A Theoretical Study of the Radiationless Decay Mechanism of Cyclic Alkenes in the Lowest Triplet State

Abstract

The radiationless decay mechanisms of cyclic alkenes C n H 2n-2 (n ) 4, 5, 6), norbornene, their phenyl derivatives, and styrene in their lowest triplet state have been investigated by unrestricted density functional, ab initio CASSCF, and MRD-CI calculations. The potential energy surfaces of the ground (S 0 ) and lowest triplet state (T 1 ) have been explored along double bond twisting and anti pyramidalization reaction pathways to explain the experimentally observed inverse proportionality between ring size and triplet-state lifetime. The calculations for the transition probabilities between T 1 and S 0 states are based on Fermi's golden rule including spin-orbit coupling (SOC) constants. According to the older "free-rotor model", the hindered twist around the double bond in small ring alkenes has been assumed so far to be the main factor determining the T 1 -state lifetimes. All computational results show, however, that only a combined reaction coordinate of anti pyramidalization and twisting at the double bond provides a low-energy pathway which reproduces the experimentally observed transition probabilities. For the relative transition rates, the different Franck-Condon (FC) factors in the series of compounds are found to be much more important than the SOC constants (FCcontrolled mechanism). On the basis of the theoretical model, the effect of substitution of vinylic hydrogen atoms by phenyl groups is discussed.

Investigation of photoinduced electron transfer in model system of vitamin E-duroquinone by time-dependent density functional theory

“…32 The minimum geometries of all the species are confirmed by a frequency analysis. The geometry optimization of DQ is performed under D 2h symmetry.…”

Exploring Two‐State Reaction Pathways in the Photodimerization of Cyclohexadiene

“…The adiabatic UB3LYP/6‐31G* CA22 (S 0 )– CA22 (T 1 ) energy gap of 48.0 kcal mol −1 is lower than that associated with the singlet–triplet vertical transition for CA22 (S 0 ). This energy difference is due mainly to geometrical relaxation of the triplet state, which allows twisting of the planar C7C8 double bond in CA22 (S 0 ) to the perpendicular arrangement in CA22 (T 1 ), in which the two biradical centers C7 and C8 are pyramidalized (see below) 27…”

“…In CA22 (S 0 ) the H‐C1‐C2‐H and H‐C7‐C8‐H dihedral angles are about 0°, whereas in CA22 (T 1 ) the H‐C1‐C2‐H dihedral angle is 0° and the H‐C7‐C8‐H dihedral angle is −76.9°. This latter dihedral angle indicates anti pyramidalization of the vinylic atoms C7 and C8, which reduces Coulombic repulsion between the electrons27 and favors relaxation of the torsional energy associated with the C7C8 double bond of the cyclohexene system.…”

Ultrafast Intramolecular Relaxation and Wave‐Packet Motion in a Ruthenium‐Based Supramolecular Photocatalyst

“…18 This affects, not only IC and IVR, but also ISC as spin–orbit coupling (SOC) is not the only parameter governing ISC rates 9c. 18b, c, e, h The availability of optically driven vibrational modes to explore the crossing region of PES can greatly influence the crossing rates 18a. f–h Hence, due to the differing localization of the initial excitation in the ligand sphere (tbbpy and/or tpphz 1 MLCT state), triplet population rates might differ with changing excitation conditions.…”