Anilinium-to-Azonium Excited State Intramolecular Proton Transfer in a Cationic Scaffold Exhibiting Multitunable Emission

Excited state intramolecular proton transfer (ESIPT) confers multiple useful photophysical characteristics to organic fluorophores, such as a large Stokes shift and tunable emission. Herein, we have designed an ESIPT-active scaffold viz. diaminoazobenzene.TFA (DAB.TFA) that shows an anilinium-to-azonium (Ano-Azo)-type ESIPT. Planarity in DAB.TFA is enforced through strong five-membered intramolecular hydrogen bonds (IHBs). It exhibited a Stokes shift of 6474 cm-1 (in acetonitrile), although the existence of deactivating pathways involving twisted quinoidal structures makes it weakly emissive. DAB.TFA exhibited excitation-dependent (Ex-De) ratiometric dual emission wherein the high-energy excitation led to emission from the anilinium excited state (N*), whereas low-energy excitation produced emission from the azonium excited state (Z*). Further control of the emission profile of DAB.TFA could be exerted by isotopic substitution as well as pKa of the protonating species─the use of deuterated acid (TFA-D) led to dual emission from both the N* and Z* states, while increasing the acidity of the protonating acid produced stronger Z* emission. The lifetimes of the excited state absorption (ESA) and stimulated emission (SE) of DAB.TFA were determined through transient absorption spectroscopy (TAS) to be 1.5 and 0.6 ps, respectively. Computational investigations further confirmed the ESIPT process in the DAB.TFA.