Photoinduced Reactions of Nitrate in Aqueous Microdroplets by Triplet Energy Transfer.

In-situ Raman spectroscopy of single levitated charged aqueous microdroplets irradiated by dual-beam (266 and 532 nm) lasers demonstrates that the nitrate anion (NO 3 - ) can be depleted in the droplet through an energy transfer mechanism following excitation of sulfanilic acid (SA), a UV-absorbing aromatic organic compound. Upon 266 nm irradiation, a fast decrease of the NO 3 - concentration was observed when SA is present in the droplet. This photoinduced reaction occurs without the direct photolysis of NO 3 - . Instead, the rate of NO 3 - depletion was found to depend on the initial concentration of SA and the pH of the droplet. Based on absorption-emission spectral analysis and excited-state energy calculations, triplet-triplet energy transfer between SA and NO 3 - is proposed as the underlying mechanism for the depletion of NO 3 - in aqueous microdroplets. These results suggest that energy transfer mechanisms initiated by light-absorbing organic molecules may play a significant role in NO 3 - photochemistry.