Enhanced intersystem crossing in carbonylpyrenes.

Ultrafast intersystem crossing of carbonylpyrenes in chloroform was investigated by femtosecond pump-probe spectroscopy. When compared to the dominant fluorescence decay pathway in pyrene, carbonyl functionalized pyrenes display near-unity triplet formation upon photoexcitation. The excited singlet state (Sp) undergoes rapid intersystem crossing (kISC) concomitantly with internal conversion (kIC) to lower excited singlet states (Sn) within a timescale of 5-11 ps (1/τ2 = kIC + kISC). Furthermore, intersystem crossing from lower excited singlet states (Sn) proceeds through coupling with receiver triplet states, eventually leading to high triplet quantum yields (ΦT = 97%; tetraacetylpyrene). Followed by internal conversion in the triplet manifolds, phosphorescence decay on a microsecond timescale is observed from the emitter triplet state.