Intramolecular electron transfer in diastereomeric naphthalene–amine dyads: a fluorescence and laser flash photolysis study

Abstract

Two dyads containing a naphthalene-like chromophore linked to a pyrrolidine-derived moiety, namely (S,S)- and (R,S)-NPX–PYR, have been synthesised by esterification of (S)- or (R)-naproxen (NPX) with (S)-N-methyl-2-pyrrolidinemethanol (PYR) and submitted to photophysical studies (steady-state and time-resolved fluorescence, as well as laser flash photolysis). The emission spectra of the dyads in acetonitrile were characterised by a typical band centred at 350 nm, identical to that of the reference compound (S)-NPX. However the intensities were clearly different, revealing a significant intramolecular quenching in the dyads, as well as a remarkable stereodifferentiation (factor of 1.6). Accordingly, the fluorescence lifetimes of the two dyads were different from each other and markedly shorter than that of (S)-NPX. The quenching mechanism is intramolecular electron transfer, that is thermodynamically favoured. Exciplex formation, that is nearly thermoneutral, does not compete efficiently. The electron transfer rate constants for (S,S)- and (R,S)-(NPX–PYR) were 1.8 × 10⁸ and 2.8 × 10⁸ s⁻¹, respectively. By contrast, no significant intramolecular quenching was observed for the excited triplet states (λ_max = 440 nm), generated by laser flash photolysis; this is in agreement with the fact that intramolecular electron transfer is thermodynamically disfavoured, due to the lower energy of excited triplets.