Restriction of Photoinduced Twisted Intramolecular Charge Transfer

• Published in: Chemphyschem 2011-02, Vol.12 (2), p.397-404
• Main Authors: Qian, Yan, Cai, Min-Min, Xie, Ling-Hai, Yang, Guo-Qiang, Wu, Shi-Kang, Huang, Wei
• Format: Article
• Language: English
• Subjects: aggregation; Atomic and molecular physics; benzothiazoles; charge transfer; Exact sciences and technology; fluorescence; Fluorescence and phosphorescence spectra; Fluorescence and phosphorescence; radiationless transitions, quenching (intersystem crossing, internal conversion); Molecular properties and interactions with photons; photophysics; Physics
• ISSN: 1439-4235; 1439-7641
• DOI: 10.1002/cphc.201000457

An intensive investigation of structure–property relationships in the aggregation‐induced enhanced emission (AIEE) of luminescent compounds is essential for the rational design of highly emissive solid‐state materials. In the AIEE‐active compounds N,N′‐bis[3‐hydroxy‐4‐(2′‐benzothiazolyl)phenyl]isophthalamide and N,N′‐bis[3‐hydroxy‐4‐(2′‐benzothiazolyl)phenyl]‐5‐tert‐butylisophthalamide, fast photoinduced twisted intramolecular charge transfer (TICT) of the enol excited state is found to be mainly responsible for the weak emission of their dilute solutions. The photoinduced TICT enol excited state is formed with a greatly distorted configuration, due to the large rotation about the CN single bond. This facilitates nonradiative TICT decay from the normal enol excited state to the highly twisted enol excited state, rather than proton‐transfer decay to the keto excited state. In aggregates, photoinduced nonradiative deactivation of TICT is strongly prohibited, so that excited‐state intramolecular proton transfer (ESIPT) becomes the dominant decay, and hence contributes greatly to the subsequent emission enhancement of the keto form. Molecular design and investigation of analogous single‐armed compounds further verifies this kind of AIEE mechanism. The restriction of geometrical distortion induced by aggregation effectively prohibits the formation of the nonemissive photoinduced twisted intramolecular charge transfer (TICT) excited state of 2‐hydroxybenzothiazole‐based compounds and hence greatly contributes to the enhanced emission of their aggregates (see schematic).