Twisted intra-molecular electron transfer phenomenon of dansyl immobilized on chitosan film and its sensing property to the composition of ethanol-water mixtures

A new fluorescent chitosan film bearing dansyl as a fluorophore has been prepared. The film shows dual fluorescence phenomenon due to twisted intra-molecular charge transfer (TICT) in the excited state of the fluorophore. The position of the maximum emission of the film depends on the polarity of the medium, and it shifts from 460 nm in ethanol to 505 nm in water. The two emissions have been attributed to the emission from the “locally excited” state or non-charge transfer excited state of dansyl and that from the TICT excited state of the fluorophore, respectively. Existence of TICT phenomenon of the immobilized dansyl has been confirmed and studied by various fluorescence techniques, such as fluorescence lifetime measurement, steady-state and time-resolved fluorescence emission spectroscopy measurements, etc. The ratio, I505/I460, of the intensities of the two emission bands depends linearly on the concentration of water in ethanol-water mixture provided the concentration is less than 40%. Furthermore, the sensing property of the film to the mixture is reversible.