Spin-forbidden transitions in flavone

Abstract

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. Intersystem crossing (ISC) rate constants for the S1-->T1 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 28 degrees in the electronic ground state whereas the nuclear frame is nearly planar in the lowest excited (npi*)1 (S1) state and is slightly V-shaped in the (pipi*)3 (T1) and (pipi*)1 (S2) states. The calculations clearly show that the T1 state has mainly pipi* character. The large spin-orbit coupling of the S1 and T1 states and their small energy gap explain the high S1-->T1 ISC rate for which a value of kISC approximately 3x10(11) s is computed, in good agreement with experimental build-up times of the Tn<--T1 absorption. In the absence of collisions and other nonradiative processes, the T1 state of flavone is predicted to be long-lived with a pure phosphorescence lifetime of tauP approximately 4 s, in qualitative agreement with low-temperature measurements. The much faster decay of triplet flavone observed in fluid solutions is ascribed to nonradiative processes.