PHOTOCHEMISTRY AND PHOTOCURING ACTIVITIES OF NOVEL SUBSTITUTED 4'-(4-METHYLPHENYLTHIO)BENZOPHENONES AS PHOTOINITIATORS

The photoinduced polymerization activities of 19 novel substituted 4'- (4-methylphenylthio)benzophenones (or 4'-(4-tolylthio)- benzophenones) were determined and compared with that of the unsubstituted derivativ e itself in different monomers and prepolymers using real-time infrare d (RTFTIR) and pencil hardness methods. Absorption, fluorescence and p hosphorescence analyses, as well as photoreduction/ photolysis studies , were undertaken on the compounds, and the data were related to the p hotopolymerization activities. Solvent shift studies on the absorption maxima of all the compounds indicate the presence of a long-wavelengt h singlet pi pi() state with some n pi(*) character. The absorption m axima of the bistolylthio, amino, chloro and nitro derivatives are mor e red shifted than those of the simpler alkyl, alkoxy, phenyl and phen oxy derivatives. Fluorescence and phosphorescence analyses indicate a high rate of intersystem crossing to the tripler state. The fluorescen ce emission maxima are markedly red shifted with increasing solvent po larity, indicating that the lowest excited singlet state exhibits a hi gh degree of charge transfer character. The lowest excited triplet sta te of all the 4-substituted 4-tolylthiobenzophenones is essentially a pi pi() configuration, with some n pi(*) character, as indicated by t he phosphorescence lifetime data. The lowest excited triplet state of the unsubstituted derivative, on the other hand, is essentially pi pi( ) in character with a longer emission lifetime and a lower quantum yi eld. The emission maxima are independent of the nature of the substitu tion, indicating the presence of a rigid molecular structure. The latt er is confirmed by the observation of very high phosphorescence quantu m yields. The bistolylthio derivatives exhibit shorter phosphorescence emission lifetimes, indicating that their lowest excited triplet stat e is n pi() in nature. The triplet pi pi(*) character of the amino de rivatives is indicated by their increased emission lifetimes. The elec tron-withdrawing effect of the chloro rind nitro groups markedly reduc es the phosphorescence quantum yield. These effects are accounted for by the relative spacings of close-lying lowest excited singlet pi pi( ) and second excited tripler n pi() states controlling the competitiv e processes of internal conversion and intersystem crossing. Compared with the unsubstituted 4-tolylthio derivative, all the alkyl-, alkoxy- , phenyl- and phenoxy-substituted derivatives exhibit higher activitie s as photoinitiators in the absence of an amine cosynergist. Lengtheni ng or broadening of the alkoxy substituent also gives rise to an incre ased photoinitiation activity, whereas alkyl groups are less effective . The presence of an amine cosynergist enhances photocuring, giving ri se to variable effects depending on the nature of the triplet exciplex interaction. The bistolylthio derivatives exhibit the highest photoin itiation activity, whereas the 4-nitro group has a marked deactivating effect. The presence of the 4-amino group also enhances the photoinit iation activity, but only by RTFTIR measurements. The photoreduction/p hotolysis rates of all the 4-substituted derivatives are greater than that of the unsubstituted derivative and show some relation to the nat ure of the lowest excited triplet state. The photophysical and photoch emical data are discussed in relation to the relative photoactivities of the compounds as photoinitiators for curing acrylated multifunction al monomers. (C) 1997 Elsevier Science S.A.