Spin-forbidden transitions in flavone

The ground and low-lying excited electronic states of flavone were investigated by means of quantum chemical methods including Spin-orbit coupling. Minimum structures were determined employing (time-dependent) Kohn-Sham density functional theory. Spectral properties were computed utilizing a combined density functional and multi-reference configuration interaction (DFT/MRCI) method. Inter-system crossing (ISC) rate constants for the S-1 (sic) T-1 transition were Computed using a discretized Fermi golden rule approach. For the evaluation of phosphorescence lifetimes a multi-reference spin-orbit configuration interaction procedure (DFT/MRSOCI) was invoked. According to the calculations the phenyl ring is twisted Out of the benzopyrone plane by 281, in the electronic ground state whereas the nuclear frame is nearly planar in the lowest excited (1)(n pi*) (S-1) state and is slightly V-shaped in the (3)(pi pi*) (T-1) and (1)(pi pi*) (S-2) states. The calculations clearly show that the T-1 state has mainly pi pi* character. The large spin-orbit coupling of the S, and T, states and their small energy gap explain the high S-1 (sic) T-1 ISC rate for which a value of k(ISC) approximate to 3 x 10(11) s is computed, in good agreement with experimental build-up times of the T-n <- T-1 absorption. In the absence of collisions and other nonradiative processes, the T-1 state of flavone is prediced to be long-lived with a pure phosphorescence lifetime of tau(p) approximate to 4s, in qualitative agreement with low-temperature measurements. The much faster decay of triplet flavone observed in fluid solutions is ascribed to nonradiative processes. (C) 2009 Elsevier B.V. All rights reserved.