Deciphering the Photophysical Role of Conjugated Diyne in Butadiynyl Fluorophores: Synthesis, Photophysical and Theoretical Study

The present work focuses on the current interest in diyne bridged chromophores necessitating a clearer understanding of the photophysics of such molecules. The significance of the diyne moiety in the photophysics has been investigated by synthesizing simple substituted diphenyl butadiynyl derivatives following a quick and efficient microwave assisted Eglinton coupling of terminal alkynes. Emission of the fluorophores is observed from the usual locally excited (LE) state and intramolecular charge transfer (ICT) state. Separation of pure ICT emission from pure LE emission has been carried out by Gaussian/Lorentzian curve fitting. The vibronic coupling in the local transitions appears to be confined to the normal mode involving the C–C triple bond stretching of the diyne moiety. This implies that the LE transition involves the diyne moiety, a conclusion supported by quantum chemical calculations. The resolved ICT emission follows double linear dependence on ET(30) solvent polarity scale. The important role of the diyne moiety in the photophysics of this class of molecules is clearly discernible in this study.