Effect of solvent polarity on the spectroscopic properties of an alkynyl gold(I) gelator. The particular case of water

Abstract

The spectroscopic properties of aggregates obtained from the hydrogelator [Au(4-pyridylethynyl)(PTA)] were studied in solvents of different polarities. Inspection of the absorption and emission spectra of diluted solutions showed that the singlet ground state of the monomeric species is sensitive to polarity and is stabilized in more polar solvents whereas the triplet excited state is rather insensitive to changes in polarity. The study of relatively concentrated solutions revealed the presence of new emission and excitation bands at 77 K that was attributed to the presence of different kinds of aggregates. Particularly interesting behaviour was revealed in water where aggregation is observed to be more efficient. For this, absorption, emission quantum yields and luminescence lifetimes of aqueous solutions at different concentrations were investigated in more detail. These data permitted one to correlate the increase of nonradiative and radiative rate constants of the low lying triplet emissive state with concentration, and therefore with the low limit concentration for aggregation, due to the shortening of the Au. Au average distances in the aggregates and consequent enhancement of the spin-orbit coupling in the system.