THE ROLE OF THE TRIPLET-STATE IN THE PHOTOSENSITIZATION OF CATIONIC POLYMERIZATIONS BY ANTHRACENE

The photosensitization mechanism for cationic polymerizations initiate d by diaryliodonium salts photosensitized by anthracene was investigat ed using fluorescence and phosphorescence spectroscopy. In situ photos ensitizer fluorescence measurements confirmed that the photosensitizat ion reaction proceeds by an electron transfer process. Transient phosp horescence studies demonstrated that electron transfer occurred from t he triplet excited state of anthracene to the initiator, with an intri nsic kinetic rate constant of 2 X 10(8) L/mol s. Further evidence for the role of the triplet state was provided by an observed seven-fold d ecrease in the polymerization rate upon addition of a tripler state qu encher. Finally, numerical solution of the photophysical kinetic equat ions indicated that the triplet state concentration was approximately three orders of magnitude higher than that of the singlet state, and t hat 94-96% of the active cationic centers are produced by reaction of the initiator with the tripler state. These results indicate that the electron transfer occurs primarily from the triplet state of anthracen e, with the singlet state providing only a minor contribution to the p hotosensitization reaction. (C) 1995 John Wiley and Sons, Inc.