Time-dependent density functional theory (TDDFT) was used to investigate the excited-state proton transfer (ESPT) dynamics of 6-hydroxyquinolinium (6HQc) in aqueous solvent, resulting in the excited zwitterionic form (6HQz). The optimized excited-state energy profiles of 6HQc:(H2O)(n) complexes have been calculated along the phenolic O-H bond to simulate the minimum energy pathway (MEP) in the excited state. The results suggested that the threshold of the size of the water cluster is 3 for the excited-state proton transfer of 6HQc in aqueous solvent, since the conformation of the stable hydrated proton requires proton transferring to the second or deeper shell of water solvent. Moreover, the stability of the hydrated proton can be improved significantly by adding one more H2O molecule to form an Eigen cation in the excited-state 6HQz:H9O4+. The effect of the size of water cluster on the proton transfer is investigated theoretically in the excited state for the first time. (C) 2011 Elsevier B.V. All rights reserved.