Photoinduced intramolecular charge transfer in a cross-conjugated push–pull enediyne: implications toward photoreaction
Push–pull organic fluorophores are important owing to their interesting optoelectronical properties. Here we report the photophysics of a new cross-conjugated push–pull enediynyl dye which belongs to an unexplored class of π-conjugated donor–acceptor systems. Two N,N-dimethylaniline moieties serve as donors and one pyrene ring functions as an acceptor via a common Y-shaped ‘enediyne’ bridge which facilitates the cross-electronic communication. The dye exhibits dual emission from locally excited (LE) and intramolecular charge transfer (ICT) states. While the LE emission is dominant in non-polar solvents, the ICT emission predominates in polar solvents. Time-resolved fluorescence decay experiments reveal a relatively shorter lifetime component (∼0.5–0.9 ns) belonging to an ICT state and a relatively longer lifetime species (∼1.6–2.8 ns) corresponding to the LE state. The strong ICT behavior of the dye is manifested through the huge red-shift (4166 cm−1) of the emission spectra from non-polar cyclohexane to polar N,N-dimethylformamide. In contrast to many small push–pull organic dyes, the LE and ICT states of the push–pull enediynyl dye follow the same excitation pathway. The dominant red-shifted ICT emission (∼550 nm) intensity of the dye in polar solvent decreases with a concomitant appearance of the blue-shifted LE emission (∼385 nm) upon prolonged exposure to photons. This opens up a new photophysical strategy of achieving high contrast two fluorescence color conversion from yellow to blue.