We have investigated the mechanism of proton transfer in the lowest photoexcited state of 7-hydroxyquinoline·(NH3)3 using TDDFT based molecular dynamics. We observe a concerted mechanism according to which all protons are transferred simultaneously in a fast process (~100 fs) that amounts to the net transport of one proton from the oxygen to the nitrogen of 7-hydroxyquinoline. In addition, the observed proton transfer pathway involves all three ammonia molecules and not only two as previously proposed. These differences arise from dynamical effects that occur at finite temperature. Our simulations provide a detailed time-resolved description of the proton transfer reaction mechanism for a prototypical molecular cluster and thus help to shed further light on the nature of this important and ubiquitous process.