Mechanochromism induced through the interplay between excimer reaction and excited state intramolecular proton transfer

Excited-state intramolecular proton transfer (ESIPT) and excimer formation are unimolecular and bimolecular reactions, respectively. The coupling between these two reactions has been rarely observed. Here we show the interplay between ESIPT and excimer formation in CF3-HTTH (2,2′-(thiazolo[5,4-d]thiazole-2,5-diyl)bis(4-(trifluoromethyl)phenol) solid crystal, resulting in the extensively broad multiple emissions spanning from 420 to 750 nm. Comprehensive structural, time-resolved spectroscopic, and two-photon microscopic studies of CF3-HTTH in crystal lattice unveil the pre-equilibrium ESIPT between the normal (N*) and tautomer (T*) form, for which the slow population decay and well-ordered lattice packing facilitate excimeric (E*) formation, rendering N* (460 nm), T* (520 nm) and E* (600 nm) triple emissions. In contrast, the lower degree of packing on the solid surface prohibits excimer formation, showing only the ESIPT process. The correlation between luminescence properties and packing structure sheds light on the corresponding mechanochromic effect based on molecular solid architecture and provide new insight into the aggregation-induced properties.