Twisted non-charge-transfer electronically excited triplet states of aromatic aldehydes and ketones revealed by resonance Raman scattering and transient dielectric loss/interconversion between two populated triplet states

Abstract

Optical excitation of solutions of 2-naphthaldehyde (NA) or 2-acetonaphthone in cyclohexane at 300 K leads in both cases to the formation of two triplet states. Time-resolved optical absorption spectroscopy reveals that these states decay relatively slowly with a decay rate amounting either to 3 × 10⁶ or to 20 × 10³ s^–1(respectively in non-deoxygenated and deoxygenated solutions in cyclohexane), while they are interconverted at a higher rate. One of these two triplet states exhibits resonance Raman scattering and is characterized as a non-planar ππ* triplet state. The other triplet state may be characterized as an nπ* triplet state on the basis of the photochemical behaviour of 2-naphthaldehyde towards cycloheptatriene. A non-planar ππ* triplet state is also formed upon optical excitation of 9-anthraldehyde or 9-acetylanthracene, but in these cases no evidence for the existence of a dynamic equilibrium between two triplet states could be found. The measurement of transient dielectric losses at microwave frequencies indicates that the non-planar ππ* triplet states do not have the character of a twisted intramolecular charge-transfer state.