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 S 1 ↝ 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 28 ° in the electronic ground state whereas the nuclear frame is nearly planar in the lowest excited ( n π ∗ ) 1 ( S 1 ) state and is slightly V-shaped in the ( π π ∗ ) 3 ( T 1 ) and ( π π ∗ ) 1 ( S 2 ) states. The calculations clearly show that the T 1 state has mainly π π ∗ character. The large spin–orbit coupling of the S 1 and T 1 states and their small energy gap explain the high S 1 ↝ T 1 ISC rate for which a value of k ISC ≈ 3 × 1 0 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 τ P ≈ 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.