Cyanine borates revisited. Study of the kinetics of photoinitiated free radical polymerization via intermolecular electron transfer process

Abstract

Three series of cyanine butyl triphenyl borate salts were prepared and tested as initiators of free radical polymerization photoinitiated via the photoinduced electron transfer process. To illustrate the influence of the primary process on the rate of photoinitiated polymerization the fluorescence lifetime of the tested dye containing non-electron donating counterion as well as electron donating borate ion were measured. The fluorescence lifetime measurements were performed using a pico/femtosecond spectrometer allowing a successful analysis of fluorescence decay signals in the range of single picoseconds. As obtained results show, for cyanine dyes bi-exponential decay of fluorescence was observed, while for cyanine with a substituent at the meso position, for the majority of dyes, mono-exponential decays were obtained. Based on fluorescence measurements the rate of the electron transfer process between excited dyes and borate anions was calculated and a short discussion on photophysics of cyanine dyes is given. Several important conclusions arrive from the theoretical and experimental data: (i) the experimental data reveal that the relationship between the rate of the electron transfer and the free energy for this reaction display typical “normal Marcus region” kinetic behavior; (ii) the experimental results show that the relationship between the logarithm of the rate of polymerization and the logarithm of the rate constant for the electron transfer is linear in character. This finding permits the conclusion that the rate of photopolymerization is a function of the free energy change of the electron transfer (ΔGel). This type of relationship is observed only for cyanines with no substituent in the meso position. These observations allow the conclusion that in the case of meso-cyanines the fluorescing state of the dye is not the state that is reduced by the borate. The main conclusion derived from the data presented in the paper is that the Marcus theory of electron transfer can be extended for the description of certain types of photoinitiated polymerization.