Influence of (de)protonation on the photophysical properties of phenol-substituted diazine chromophores: experimental and theoretical studies

In this contribution, a series of seven new push-pull systems has been designed by combining a protonable diazine heterocycle (pyrimidine/pyrazine) with a deprotonable phenol unit through various pi-conjugated linkers (phenylene, thienylene, thienylenevinylene, and phenylenevinylene). The (de)protonation in solution resulted in a systematic bathochromic shift both in the absorption and emission maxima compared to the neutral forms. Extensive Density Functional Theory (DFT) and its Time Dependent counterpart (TD-DFT) calculations were performed to rationalize this behavior and understand the impact of (de)protonation on the different optical transitions. These computations showed that (de)protonation affects both the energy and the nature of the vertical transitions, with a significant increase in the Intramolecular Charge Transfer (ICT) character of the (de)excitations. Some of the compounds remained moderately luminescent after (de)protonation, giving a mixture of complementary emitting species that were used to obtain white light emission.