Spin-forbidden transitions in flavone

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 (nπ∗)1 (S1) state and is slightly V-shaped in the (ππ∗)3 (T1) and (ππ∗)1 (S2) states. The calculations clearly show that the T1 state has mainly ππ∗ 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≈3×1011 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 prediced to be long-lived with a pure phosphorescence lifetime of τP≈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.