Unusual light spectroscopic properties of a 2-pyridone-based multi-ring-fused fluorescent scaffold.

UV-VIS absorption and fluorescence spectroscopic properties of six related polyaromatic 2-pyridones have been studied. Excitation of the lowest and rather weak and structure-less transition [ε (max) (430 nm) ≈ 3,000 mol(-1)dm(3)cm(-1)] gives rise to a broad fluorescence band in the visible region, for these compounds. These S (0) ↔ S (1) transitions are compatible with symmetrically forbidden transitions, promoted by intensity borrowing, as is revealed by fluorescence depolarisation data. With one exception, all compounds exhibit strong fluorescence, with quantum yields in glycerol varying between 40% and 70%. The corresponding fluorescence lifetimes range from 11 ns to 17 ns, while the radiative lifetimes are very similar (≈26 ns), for all compounds. Interestingly and rarely observed, the calculated radiative lifetimes for the weak absorption band are significantly longer, i.e. between 37 and 40 ns.