Ultrafast excited state relaxation dynamics of pyran-based D--A systems: solvent polarity controls the triplet state

The excited state relaxation dynamics of V-shaped D-pi-A systems having 4H-pyranylidene appended barbituric acid as an acceptor and diphenylamine (TPAPBA) and diethyl amine (EAPBA) as donors were investigated using steady-state and time-resolved spectroscopy along with theoretical optimization. The steady-state photophysical characterization exhibited the bathochromic shift of the emission maximum (similar to 6400 cm-1) and large change in the dipole moment (similar to 24D) with an increase of solvent polarity, reflecting the occurrence of the intramolecular charge transfer state (ICT) in the excited state. The nanosecond and femtosecond transient absorption spectra of these derivatives in a non-polar solvent, toluene, reveal that the excited state relaxation pathway involving a local excited state (LE) decayed to ICT followed by the formation of a twisted ICT state by conformational relaxation, finally leading to the triplet state. The lack of observation of a triplet state in the polar solvent, acetonitrile, signifies that the relaxation dynamics of V-shaped triads in the excited state are influenced by the polarity of the solvent. The excited state relaxation dynamics of V-shaped D-pi-A systems, EAPBA and TPAPBA were investigated using steady-state and time-resolved spectroscopy along with theoretical optimization.