Photochemistry of a proton relay – system with triple fluorescence

[Display omitted] •Photophysical processes of three 3-carboxy substituted bis-salicylidenes in polar solvents are discussed.•The excited state proton transfer competes with proton donation to the solvent.•Strongly interacting solvents break the internal hydrogen bond and lead to a solvent – assisted excited state proton transfer.•The charged species and the twisted enols outnumber the neutral keto structures in alcohols.•A comprehensive photochemical scheme of the S1 state deactivation is proposed. The excited state of proton transfer and the dynamics of the excited states of three 3-carboxy substituted bis-salicylidenes (H4L1-3) have been studied by combining steady state and time-resolved absorption and emission methods, and with quantum chemical calculations. The 3-carboxy substituted bis-salicylidenes contain two coupled intramolecular hydrogen bonds of the OH…OH…N type. This system has two proton transfer sites. The compounds have excitation – dependent emission and a high sensitivity to the solvent polarity. ESIPT and deprotonation result in the co-existence of enol, keto, anionic and zwitterionic species with or without quinoid structure, whose emission bands are located from the blue to the yellowish-green region. The photophysical processes in the nano-to-microseconds timescale have been linked to the intermolecular interactions with the solvent, where hydrogen bonding leads to the formation of a cyclic 1:2 solute − solvent complex. Transient absorption spectroscopy revealed that the generation of charged structures of H4L1-3 occurs through a solvent-assisted proton transfer along the chain of two solvent molecules, which act as a proton-relay system. The computational study of the energy profiles and of the enol-to-keto tautomerization with explicit solvent molecules has been performed for the first time for this kind of ESIPT system.