Room temperature phosphorescence of 2-thioxo-coumarins in aqueous solution promoted by octa acid inclusion

Abstract

Unlike coumarins, the sulfur analogues, 2-thioxo-coumarins (TCs) have not attracted attention. Given the large energy gap between S2 and S1 and high spin–orbit coupling between S1 and T1 in these systems lack of attention is surprising. In this manuscript, we are concerned with the phosphorescence of seven TCs at room temperature. The TCs investigated here belong to two groups, one with the amino substitution at the 7-position and the other with an assorted collection of the parent, alkoxy and acetoxy substituted ones. All seven TCs show phosphorescence at 77 K while the ones with amino substitution exhibit both fluorescence and phosphorescence. The inability of phosphorescence to compete fully in this set is attributed to intramolecular electron/charge flow from the lone pair of the amino functionality. Occurrence of such a process opens opportunities to examine the solvation dynamics and TICT process in these systems. The most important result relates to the observation of room temperature phosphorescence (RTP) of these molecules in water with the help of octa acid host. TCs that are not soluble can be solubilized in water with OA capsule and this strategy suppresses self-quenching and oxygen quenching in favor of radiative process in the triplet state. When the chemical reaction is facile as in one system investigated here OA fails to bring about RTP. In our opinion, octa acid capsule can serve as an excellent medium to bring about RTP from organic molecules in water.