A spectroscopic investigation of the temperature and solvent sensitivities of resorufin

Abstract

The electronic absorption spectra, fluorescence spectra and fluorescence lifetimes of resorufin anion in protic (ethanol–methanol 4 : 1 v/v) and aprotic (propiononitrile–butyronitrile 4 : 5 v/v) solvents in the temperature range 87–293 K are reported. Interaction of the resorufin probe with protic solvent results in the appearance of a spectroscopically identified species, not detected at room temperature. The resemblance of such a species to the neutral form of resorufin, as detected in water at pH < 5, indicates the involvement of a strong hydrogen bond between the solvent and the alkoxide ion of resorufin. Formation of such a complex is consistent with ‘super-stick’ behaviour of resorufin in alcohols. At temperatures < 150 K, where both protic and aprotic solvents are rigid, quenching of the emitting state of the resorufin anion is seen. Analysis of the lifetime behaviour vs. temperature provides the activation energy, ΔE_s, for the solvent rearrangement affecting the emission in the 110–160 K temperature interval (ΔE_s= 1980 and 3600 cm^–1 for alcohol and nitrile solvents, respectively). These results are discussed in terms of ion–solvent interaction and of shielding of the anionic charge by primary solvation in alcohols.