Effect of cation alkyl chain length on photo-luminescence dynamics of ionic liquids containing dicyanoaurate(I) anion

Abstract

Viscosity and photo-luminescence dynamics of N-alkyl-N-methylpyrrolidinium dicyanoaurate (I) ([Py1n][Au(CN)2]) (n = 4, 6, 8, and 12) and 1-alkyl-3-methylimidazolium dicyanoaurate (I) ([Cnmim][Au(CN)2]) (n = 4, 8, and 12) were studied. The viscosity increased with increasing alkyl chain length for both cations, and the temperature dependence of the viscosity showed a good fit with the Vogel–Fulcher–Tammann equation. Steady-state luminescence of both [Py1n][Au(CN)2] and [Cnmim][Au(CN)2] showed dual luminescent bands at around 370 nm and 460 nm. The band with the shorter wavelength most likely arose owing to fluorescence of the trimer of the anion, and the longer-wavelength band could be assigned to phosphorescence of the linear oligomers of the anion. The relative ratio of the intensity of the fluorescence to that of the phosphorescence of [Py1n][Au(CN)2] was significantly smaller than that of [Cnmim][Au(CN)2]. The luminescent dynamics measured by a streak camera revealed a short lifetime of fluorescence, and the phosphorescence peak shift with time. The phosphorescence peak shift was attributed to two contributions from the amplitudes and the time constants. Based on the dependence on the solvent viscosity and the anion concentration, the slower time constant was assigned to the dynamics of tetramer formation owing to a reaction between the trimer and the monomer. The faster dynamics were assigned to the solvation dynamics, which were accompanied by the structural change of the trimer after intersystem crossing.