PHOTOPHYSICAL INVESTIGATION OF SIMILARITIES BETWEEN STARBURST DENDRIMERS AND ANIONIC MICELLES

The dynamics of the electron-transfer quenching of photoexcited Ru(phen)3(2+) by methyl viologen in solutions containing various anionic micelles and anionic starburst dendrimers were investigated by the single-photon-counting technique by monitoring the luminescence decay of the excited complex. Analysis of the kinetics of luminescence quenching revealed that the quenching process in higher generation (G > 3.5 and higher) starburst dendrimer solutions obeys a general kinetic model previously employed for micellar solutions. In these cases the intramicellar or intrastarburst quenching rate constants (k(q)) were found to be unimolecular and decreased with increasing the size of the host. In the case of lower generation starburst dendrimers (G = 2.5 and lower) as with smaller micelles (C-7 and C-8 alkyl sodium sulfates), the quenching reaction was found to be bimolecular in nature. This bimolecular quenching is attributed to the rapid exist of the probe from these macromolecules into the aqueous phase during its life time. Quenching studies using an anionic quencher such as K4[Fe(CN)6] substantiates this conclusion. Thus, in spite of the obvious structural differences between micellar aggregates and dendrimer molecules, some striking similarities exist in the ability of both types of restricted spaces to mediate and control electron-transfer processes conducted on their surfaces.