Chapter Open for the Excited-State Intramolecular Thiol Proton Transfer in the Room-Temperature Solution.

We report here, for the first time, the experimental observation on the excited-state intramolecular proton transfer (ESIPT) reaction of the thiol proton in room-temperature solution. This phenomenon is demonstrated by a derivative of 3-thiolflavone (), namely, 2-(4-(diethylamino)phenyl)-3-mercapto-4-chromen-4-one (), which possesses an -S-H···O═ intramolecular H-bond (denoted by the dashed line) and has an S absorption at 383 nm. Upon photoexcitation, exhibits a distinctly red emission maximized at 710 nm in cyclohexane with an anomalously large Stokes shift of 12 230 cm. Upon methylation on the thiol group, , lacking the thiol proton, exhibits a normal Stokes-shifted emission at 472 nm. These, in combination with the computational approaches, lead to the conclusion of thiol-type ESIPT unambiguously. Further time-resolved study renders an unresolvable (<180 fs) ESIPT rate for , followed by a tautomer emission lifetime of 120 ps. In sharp contrast to , both and 3-mercapto-2-(4-(trifluoromethyl)phenyl)-4-chromen-4-one () are non-emissive. Detailed computational approaches indicate that all studied thiols undergo thermally favorable ESIPT. However, once forming the proton-transferred tautomer, the lone-pair electrons on the sulfur atom brings non-negligible π* contribution to the S' state (prime indicates the proton-transferred tautomer), for which the relaxation is dominated by the non-radiative deactivation. For , the extension of π-electron delocalization by the diethylamino electron-donating group endows the S' state primarily in the ππ* configuration, exhibiting the prominent tautomer emission. The results open a new chapter in the field of ESIPT, covering the non-canonical sulfur intramolecular H-bond and its associated ESIPT at ambient temperature.