Anomalous Water Fluorescence Induced by Solutes

The origin of recently reported anomalous fluorescence emissions from aqueous solutions of nonaromatic solutes remains elusive. To determine whether the solute nature influences the fluorescence characteristics and to identify a potential common mechanism, we measured the fluorescence spectra of 21 different solutions. We observed similar emission characteristics across all samples, suggesting that the solute nature plays a minimal role in the emission mechanism. Using time-dependent density functional theory on large water, NaCl/water, and glycerol/water clusters, we attributed the anomalous emission to the decay of charge-transfer-to-solvent excitations (CTTS) which populate a diradical zwitterionic excited state localized at hydrogen-bond network defects. The Arrhenius-like plots for NaCl and glycerol solutions revealed that the S₁ nonradiative decay pathway involves the diradical recombination via librational motion. We propose that the presence of solute molecules slows this process, thus increasing the lifetime of the CTTS excited states and facilitating emission.