Effects of proximity on the relaxation dynamics of flindersine and 6(5H)-phenanthridinone

The role played by the carbonyl group in the antenna system of a naturally occurring photochromic chromene, flindersine (FL), has been experimentally investigated and compared with that of a carbonyl group present in a structurally related unreactive heterocyclic compound, 6(5H)-phenanthridinone (PH). Through stationary and time-resolved absorption and emission techniques, the excited-state relaxation dynamics after UV irradiation were determined for FL and PH. The presence of a carbonyl group in both compounds entails the existence of two close-lying, strongly coupled electronic excited states, having n,pi* and pi,pi* character, respectively. Their coupling can be modulated by a careful choice of the solvent proticity and temperature. Moreover, in the case of strong coupling between the n,pi* and pi,pi* states, we have proved that the relaxation dynamics can involve transitions in which the upper of the coupled states acts as an intermediate for radiationless decay, bypassing the lowest emissive state, whereby the fluorescence quantum yield becomes a function of the excitation wavelength.