Photoionization of (alkoxyphenyl)triphenylporphyrins (CnOPTPP) in dihexadecyl phosphate (DHP), dipalmitoylphosphatidylcholine (DPPC), and dioctadecyldimethylammonium chloride (DODAC) frozen vesicle solutions at 77 K occurs via electron transfer from CnOPTPP to bulk water at the interface of the vesicles. This process results in CnOPTPP+ formation which is characterized and quantitated by electron spin resonance. The cation yield decreases with increasing C-n alkyl chain length of CnOPTPP due to an increasing interaction distance between the porphyrin headgroup and interface water. This change in interaction distance is directly measured with electron spin echo modulation by measuring the deuterium modulation depth associated with CnOPTPP+ interactions with deuterated water at the interface. The cation yield also decreases from cationic DODAC to neutral DPPC to anionic DHP vesicles because of a charge effect on electron transfer through the interface.