The photophysics of the hydrogen-bonded (HB) state and the protonated (PR) state of a photosensitizer, 2,6-bis(4'-diethylaminobenzylidene)cyclopentanone (DBC) in a polymer matrix containing carboxyl groups was investigated by fluorescence spectroscopy and decay curve analysis. The fluorescence spectrum of the excited DBC HB with the carboxyl unit was composed of emission from two species peaking at 567 and 627 nm, which were assigned to the emission from the weakly HB state (HB1) and the strongly HB state (HB2) of DBC, respectively. The excitation energy is able to migrate through the HB1 states and then transfers to the HB2 site and the PR DBC site. At low DBC concentrations, not only PR but also HB2 acts as a deep trap of the excitation energy. The concentration of PR was estimated to be ca. 40% of DBC by the fluorescence decay analysis based on the Forster kinetics. These results indicate that the photoinitiation reaction of this photopolymer system proceeds in competition with the energy transfer to these large amounts of trap sites formed by interaction between the DBC dye and the carboxyl unit of the matrix polymer. (C) 2003 Elsevier Ltd. All rights reserved.