Dual Phosphorescence of Rhodamine B Lactone

Abstract

Rhodamines are well-known as highly coloured dyes and potent, bright fluorophores with negligible phosphorescence, and thus often neglected triplet states. Rhodamines with a carboxyphenyl group exist in spirocyclic lactone and as open-ring colour zwitterion or cation forms. The colourless lactone is virtually the only form of rhodamines in aprotic environment and, upon ultraviolet photoexcitation at room temperature, it undergoes ultrafast intramolecular electron transfer and emits — depending on solvent polarity — single or dual fluorescence from a charge-transfer state and  zwitterion  populated  upon  adiabatic  photodissociation of the C–O bond. The photophysics and mechanisms of complex excited state processes in rhodamine lactones are still poorly understood, and especially the nature of nonradiative deactivation and the role of triplet states remain unknown. Here, we present a photophysical study of rhodamine B lactone in rigid organic solvent glasses at 77 K and report highly efficient phosphorescence generated by intramolecular charge transfer, demonstrating the potential of rhodamine lactones for triplet generation by charge recombination. A mechanism behind the formation of triplet states in rhodamine lactones is proposed.