A Theoretical Study on Rate Constants of Excited State Proton Transfer Reaction in Anthracene-Urea Derivatives

We elucidate the origin of the two types of observed fluo-rescence in a complex consisting of nPUA (1-anthracen-n-yl-3-phenylurea; n = 1, 2, 9) and an acetate ion. By calculating the molecular properties related to the proton transfer reaction in the excited state, we clarified a correlation between the acid dissociation constant in the excited state (pKa*) in the urea moiety and the rate constant of the excited state proton transfer reaction. The computed pKa* suggests that the proton on the anthracenyl group side is transferred in the case of 1PUA and 2PUA, whereas the proton on the phenyl group side is trans-ferred in 9PUA. Low pKa* and activation barriers were calcu-lated for 9PUA, which causes the stability of 9PUA due to the absence of the planarity after the proton transfer reaction.