The host/guest type of excited-state proton transfer; A general review

Contemporary progress regarding guest/host types of excited-state double proton transfer has been reviewed, among which are the biprotonic transfer within doubly H-bonded host/guest complexes, the transfer through a solvent bridge relay, the intramolecular double proton transfer and solvation dynamics coupled proton transfer. Of particular emphases are the photophysical and photochemical properties of excited-state double proton transfer (ESDPT) in 7-azaindole and its corresponding analogues. From the chemical aspect, two types of ESDPT reaction, namely the catalytic and non-catalytic types of ESDPT, have been classified and reviewed separately. For the case of static host/guest hydrogen-bonded complexes both hydrogen-bonding strength and configuration (i.e. geometry) play key roles in accounting for the reaction dynamics. In addition to the dynamical concern, excited-state thermodynamics are of importance to fine-tune the proton transfer reaction in the non-catalytic host/guest type of ESDPT. The mechanisms of protic solvent assisted ESDPT, depending on host molecules and proton-transfer models, have been reviewed where the plausible resolution is deduced. Particular attention has been given to the excited-state proton transfer dynamics in pure water, aiming at its future perspective in biological applications. Finally, the differentiation in mechanism between solvent diffusive reorganization and solvent relaxation to affect the host/guest ESPT dynamics is made and discussed in detail.