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  Journal of Spectroscopy and Dynamics 2014, 4: 2
  Research Article
Photophysics of donor-acceptor kind of styryl dyes: Involvement of twisted intramolecular charge transfer (TICT) state and the effect of solvent polarity
  Mhejabeen Shaikh, Haridas Pal  
Radiation & Photochemistry Division, Bhabha Atomic Research Center, Trombay, Mumbai-400085, India
  Photophysical properties of two styryl dyes containing both electron donating and accepting substituents in a single molecule have been investigated in protic solvents of varying polarities. Understanding the involvement of twisted intramolecular charge transfer (TICT) state in the excited state deexcitation process and its modulation with the changing solvent polarity have been the aim of the present study. Strong solvatochromism of the absorption and fluorescence spectral characteristics of present dyes indicate that the fluorescent states of these dyes are of intramolecular charge transfer (ICT) character. For both the dyes, fluorescence quantum yields (Φf) and fluorescence lifetimes (τf) show very sharp decrease with an increase in the solvent polarity, suggesting the involvement of highly polar TICT state in the excited state deexcitation mechanism. While the radiative decay rate (kf = Φf/τf) shows only a moderate solvent polarity dependence, the nonradiative decay rate (knr = 1/τf - Φf/τf) shows very strong and exponential decrease with solvent polarity, in accordance with the involvement of a non-fluorescent TICT state in the nonradiative deexcitation pathways. A suitable mechanistic scheme has been proposed considering an activation controlled ICT to TICT conversion in the excited state and accordingly a theoretical basis has been made to correlate the observed solvent polarity dependent changes in the nonradiative rates following an intriguing amalgamation of the Lippert-Mataga formulation of dielectric solvent effect on electronic energy states and the Arrhenius concept of the activation controlled reaction rates. Present study provides an unique and very explicit analysis for the TICT mediated nonradiative rates that has so far not been reported by any other groups.
  Styryl dyes; Photophysics; Solvent polarity effect; Nonradiative decay; TICT state  

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