Intriguing photophysical aspects of nitro substituted 2-(2′-hydroxyphenyl) benzothiazole (HBT) derivative: AIE in non-polar media?

Excited photophysics of a synthesized molecule namely 2-(benzo[d]thiazol-2-yl)-5-nitrophenol (BTNP) have been studied using steady state and time resolved techniques. BTNP exhibits ESIPT exclusively in non-polar media with exceptional Stokes' shift. In polar solvents anion formation of BTNP hampers the ESIPT process. Solid state emission spectral pattern matches reasonably well with that in non-polar medium. Dynamic light scattering showed the increment of particle size in heptane in comparison to water, a trend reversed to conventional trait of Aggregation Induced Emission (AIE) active molecules. Nitro substituent in BTNP reverses the conventional definition of a 'good solvent' and 'bad solvent', thereby promoting ESIPT in non-polar medium via aggregation. As all reports on AIE active compounds claim water to be the solvent capable of producing the effect, we have reversed the effect by simple change in substitution. Reversal of AIE phenomenon would certainly open up new avenues in future research on AIE active fluorophores.