Photoinduced electron transfer at the core/shell interface of polystyrene-anthracene-poly(methacrylic acid) diblock copolymer micelles in aqueous media

We have characterized the photophysics and excited-state electron transfer reactions of polymer micelles formed from diblock polystyrene-(anthracene)-block-poly(meth acid). In these studies (anthracene) comprises either a single 1-(2-anthryl)-1-phenylethylene (An) or an average of two vinyl-9,10-diphenylanthracene (vDPA). This architecture is expected to place the chromophores in the interfacial region between the polystyrene core and poly(methacrylic acid) corona. Quenching of the anthryl fluorescence by Tl+ and two viologens (SPV, 4,4'-bipyridyl-1,1'-bis(propanesulfonate), and MV(2+), methyl viologen) demonstrated that access to these chromophores was limited at all pHs, and the rate of diffusion of the viologens was relatively slow. Similar conclusions were drawn from quenching of the (3)An* state and the anion radical SPV.- by O-2. We believe this illustrates that the interfacial region of the micelle is not fully deprotonated even at pH 9. It was observed that the fraction of 3An* that was quenched by SPV was smaller than (1)An*. This demonstrates the existence of a heterogeneous distribution of anthryl sites such that some (3)An* cannot be quenched, which is not the case for (1)An*. We conclude that the spatial requirements for quenching these two excited states are not equivalent. Electron transfer quenching by SPV produces SPV.- that is very long-lived, but some unknown reaction removes the anthryl cation radical. As a consequence, we can build up a concentration of SPV.- under steady-state photolysis. The quantum yield of charge separation per quenching event is similar to previous cases we have studied, ca. 0.5, but unlike linear polyacids, there is very little pH dependence. This is consistent with the idea that there is minimal deprotonation near the core-corona interface of the micelle.