Solvatochromic multi-colour fluorescence driven by thiol-type ESIPT in a 1-mercapto-naphthaldiimide compound†

A novel thiol (SH)-type excited-state intramolecular proton transfer (ESIPT) compound, 1SNT-IC, was designed and synthesized. 1SNT-IC exhibited bright red fluorescence (FL) with large Stokes shifts (∼7200 cm⁻¹) under UV irradiation, mediated by ESIPT. This is the first example of an SH-type ESIPT imide compound. When dispersed in amorphous polymers, 1SNT-IC exhibited dual FL emission from the excited normal (N*) and tautomeric (T*) forms, with the latter's wavelength varying according to the polarity of the matrices. In various solvents, FL from the excited anion (A*) form was also observed, in addition to those from N* and T*. The FL wavelengths of T* and A* also varied with the polarity of the solvent. These environmental sensitivities have rarely been observed for the NH-type ESIPT compounds. This could be attributed to the weak intramolecular hydrogen bond (S–H⋯O) strength and the electron-withdrawing naphthalimide moiety.