Size effect of water cluster on the excited-state proton transfer in aqueous solvent

► We study the excited-state proton transfer of photoacid in aqueous solvent. ► An effective model has been built up for the excited-state proton transfer. ► The excited-state potential energy curves have been sufficiently optimized. ► The threshold of the size of water cluster is 3 for the excited-state proton transfer. ► The hydrated proton favors Eigen form in the excited-state contact ion.