Multiple fluorescence within a single ESIPT system: Orchestrating the emission of Lophine-Benzothiazole dyads via excitation wavelength and solvent

Lophine-benzothiazole dyads were synthesized via the amine-isocyanate reaction involving N-alkylamino lophine and 2-(5 '-isothiocyanate-2-hydroxyphenyl)benzothiazole. The resultant coupling generated a thiourea linkage, connecting the two primary fluorophores with varying chain lengths (2 and 8 methylene groups). These newly formed dyads exhibit distinct combinations of fluorescence processes, modulated by both the excitation wavelength and the solvent. Varying the excitation wavelength allowed for the observation of emissions from both lophine and benzothiazole (enol species) fluorophores. Notably, a tautomeric equilibrium in the excited state was also identified, resulting in emissions from both enol and keto forms, transitioning to the anionic species with ICT emission. Finally, multiple color emissions from orange to blue can be obtained using a binary DCM:DMSO mixture via simultaneous solvatochromic effect and ratiometric emission from excited-state intramolecular proton transfer (ESIPT) lophine-benzothiazole dyads. Theoretical calculations corroborate the proposed emission mechanism. The results indicate that the ESIPT dyads are promising for developing sensitive fluorescent probes, allowing their fluorescence modulation in solution.