Minimalistic Cationic Scaffolds Exhibiting Excited State Intramolecular Proton Transfer (ESIPT), White Light Emission and Photo-click reactions

The excited state intramolecular proton transfer (ESIPT) process significantly influences the photophysics of (bio)molecules. ESIPT is facilitated by the spatial proximity of the proton donating and accepting residues as well as by the acidity of the proton being transferred. While neutral and zwitterionic molecules undergoing ESIPT are well reported in the literature, cationic scaffolds capable of ESIPT are conspicuously absent. Here, we present two cationic organic scaffolds viz. DADAB.TFA and DATzTz.TFA that exhibit rapid Anilinium-to-Azonium (Ano-Azo) ESIPT resulting in unique photophysical and photochemical outcomes. Strong 5-membered and 6-membered intramolecular hydrogen bonding (IHB) in DADAB.TFA and DATzTz.TFA, respectively, ensured the structural rigidity and planarity necessary for ESIPT. It also rendered DADAB.TFA emissive, with a moderate Stokes shift of 6597 cm-1 in H2O along with Anti-Kasha behaviour of excitation-dependent (Ex-De) emission. On the other hand, the DATzTz.TFA showed a large Stokes shift of 10209 cm-1 (in acetone). The emission of DADABH2+2 from its tautomeric azonium state gained prominence with the acidity of the protonating species. This was employed to achieve single-component white light emission (WLE) using DATzTz.TFA. Transient absorption spectroscopy (TAS) provided the lifetime of excited state absorption (ESA) as 0.85 ps for DADAB.TFA, whereas the lifetime of 1.2 ps was observed for stimulated emission (SE). Upon photoexcitation, DADAB.TFA also underwent photo-click reaction with aldehydes to yield more emissive benzimidazole frameworks. This work presents the design of new minimalistic cationic scaffolds undergoing ESIPT and expands the repertoire of photophysically-active and photochemically-reactive charged organic scaffolds.