Amphiphilic terpolymers [consisting of ca. 40 mol % sodium sulfonate group, ca. 60 mol % lauryl(La) or cyclododecyl(Cd) group, and 0.1-0.2 mol % zinc(II) tetraphenylporphyrin (ZnTPP) moiety] form unimer micelles in aqueous solution, and the ZnTPP moieties are compartmentalized in the hydrophobic microdomains in the unimer micelles, leading to an extraordinarily long-lived triplet excited state of the ZnTPP((ZnTPP)-Zn-3*). Electron transfer from the long-lived (ZnTPP)-Zn-3* to phenylmethylphenacylsulfonium p-toluenesulfonate (PMPS), a self-destructive electron acceptor, which is electrostatically concentrated on the surface of the unimer micelle, generates porphyrin cation radicals (ZnTPP.+) within the hydrophobic microdomain. Since the ZnTPP chromophores are protected from the bulk aqueous phase, subsequent reaction between the resulting ZnTPP.+ and phenacyl radical from PIMPS is prevented. Thus, the porphyrin cation radicals are efficiently accumulated under steady-state irradiation of visible light. The unimer micelle consisting of Cd groups provides a much better protection for ZnTPP.+ than that consisting of La groups; a significant amount of the porphyrin cation radical persisted for more than 1 day in the unimer micelle consisting of the Cd groups.