Hyperconjugation in light-switched intramolecular charge transfer of anthrapyridinium derivatives: New insight into molecular motion and fluorescent materials

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Light-induced intramolecular charge transfer (ICT) is important in chemical and physical processes involving organic reactions and materials. However, because of the fast light-absorption rates and short excited state lifetime, charge-transfer separation and the resulting molecular motion in the ICT states are generally expected to be unidirectional and uncontrollable. Herein, an unexpected hyperconjugation derived light-switched ICT and tunable fluorescence emissions are described, and these can be rapidly and reversibly controlled in anthrapyridinium dyes by irradiation with light of different wavelengths such as 365 nm and 254 nm. Most importantly, the subtle differences between the dyes in terms of the σ–π (C–H and C–C) hyperconjugation between the alkyl chains and the pyridinium, which were confirmed from the 1H NMR spectra for the first time, and which was further proved to be the key factor affecting the light-induced ICT of the dyes.