Amplified Spontaneous Emission from Zwitterionic Excited-State Intramolecular Proton Transfer

作者：Atul J Shukla, Van T. N. Mai, Velayudhan V. Divya, Cherumuttathu Hariharan Suresh, Megha Paul, Venugopal Karunakaran, Sarah K. M. McGregor, Ilene Allison, K. N. Narayanan Unni, Ayyappanpillai Ajayaghosh, Ebinazar B. Namdas, Shih‐Chun Lo · 发表于：Journal of the American Chemical Society · 年份：2022 · DOI：10.1021/jacs.2c02163 · 被引用次数：62 · 研究领域：Photochemistry and Electron Transfer Studies、Porphyrin and Phthalocyanine Chemistry、Luminescence and Fluorescent Materials

The unique four-level photocycle characteristics of excited-state intramolecular proton transfer (ESIPT) materials enable population inversion and large spectral separation between absorption and emission through their respective enol and keto forms. This leads to minimal or no self-absorption losses, a favorable feature in acting as an optical gain medium. While conventional ESIPT materials with an enol–keto tautomerism process are widely known, zwitterionic ESIPT materials, particularly those with high photoluminescence, are scarce. Facilitated by the synthesis and characterization of a new family of 2-hydroxyphenyl benzothiazole (HBT) with fluorene substituents, HBT-Fl1 and HBT-Fl2, we herein report the first efficient zwitterionic ESIPT lasing material ( HBT-Fl2 ). The zwitterionic ESIPT HBT-Fl2 not only shows a remarkably low solid-state amplified spontaneous emission (ASE) threshold of 5.3 μJ/cm 2 with an ASE peak at 609 nm but also exhibits high ASE photostability. Coupled with its substantially large Stokes shift (≈236 nm ≈10,390 cm –1 ) and an extremely small overlap of excited-state absorption with ASE emission, comprehensive density functional theory (DFT) and time-dependent DFT studies reveal the zwitterionic characteristics of HBT-Fl2 . In opposition to conventional ESIPT with π-delocalized tautomerism as observed in analogue HBT-Fl1 and parent HBT, HBT-Fl2 instead shows charge redistribution in the proton transfer through the fluorene conjugation. This structura...