Photophysics of proton transfer in hydrazides: a combined theoretical and experimental analysis towards OLED device application

Abstract

Hydrazides generate phototautomers and thus, a mechanistic interpretation to uncover the excited state dynamics of such systems is highly necessary to theorize principles based on experimental speculations. Accordingly, focus on the proton transfer barrier, which is a questionable step-wise or hypothetical simultaneous double proton transfer on structurally favored species, is quintessential; however, to the best of our knowledge, theoretical insights into such findings remain rare. Thus, TX, PX and FX (where X = 2 and 3) were designed and synthesized by incorporating hydrazides, which exhibit the phenomenon of excited state intramolecular proton transfer (ESIPT). Some of the molecules exhibited electroluminescence when employed as an active emitter material in fabricated OLED devices. Theoretical predictions support the presence of extended conjugation in TX, PX and FX (where X = 2 and 3) to support ESIPT efficiently in comparison with TX, PX and FX (where X = 1). The solvatochromic study revealed that TX, PX and FX (where X = 2 and 3) exhibit a distinct double peak in THF solvent, characteristic of ESIPT. Interestingly, for some of the molecules, emission in thin film form showed a double peak, which indicates ESIPT in the solid state. However, it was found that aggregation induced emission (AIE) was inactive in these molecules. The geometrical attributes of the molecules and the nature of electronic orbital distribution well underline the principle supporting excited state proton translocation. The theoretically estimated energy transitions exhibited good correlation with the experimental results. Also, the potential energy scans revealed the molecules possess a lower forward barrier at their excited state in comparison with that of their ground state, promoting ESIPT. The potential energy surface scans performed on structurally favored species confirmed the impossible double proton transfer and highly difficult step-wise double proton transfer.