Several dyes containing benzylideneimidazopyridine moiety (BIPDs) were synthesized and evaluated as photoinitiators for free-radical polymerization induced with the visible emission of an argon-ion laser. One method of dye structure change was applied in our study. The modification was based on the character of the substituent introduced into both the imidazopyridine skeleton and phenyl ring. Several different groups were tested including heavy atoms (-CI, -Br) as well as electron-accepting (-NO2), and electron-donating groups [-N(CH3)(2), -OCH3]. Analysis of the dye properties demonstrated that there is a significant heavy atom effect on the photoinitiation ability of the novel dyes in both cases, for example, when a heavy atom is introduced into the phenyl ring as well as into the imidazopyridine part of the molecule. The introduction of an electron-acceptor or electron-donor group into the phenyl part of the dye caused a dramatic decrease in its photosensitivity. The type of applied counterion had no effect on the overall sensitivity of a dye. BIPDs are not particularly good photoinitiators. Further modification of the dye structure involved the elimination of the motion of a C-C bond by the coplanarization of the styrylium residue with other parts of the dye. This approach decreased the degree of branching of the dye, which stabilized the molecule in its excited state. The formed dye, quinoline[2,3-b]-2,3-dihydro-1H-imidazo[1,2-a]pyridinium bromide (QDIPB), exhibited dramatically enhanced sensitivity. QDIPB possessed broad structured spectra with a long-wavelength part shifted to the blue as compared to other BIPD dyes. The change of the absorption spectra and its high photoinitiation ability makes QDIPB a good candidate for the photoinitiating system applied in dental restorative materials. (C) 2003 Wiley Periodicals, Inc.