Photoinduced electron transfer from alkylpyrenes embedded into DHP, DPPC and DODAC vesicles studied with electron paramagnetic resonance and electron spin echo modulation spectroscopies

Abstract

Photoinduced electron transfer from alkylpyrene derivatives solubilized in cationic dioctadecyldimethylammonium chloride (DODAC), neutral dipalmitoylphosphatidylcholine (DPPC) and anionic dihexadecylphosphate (DHP) vesicles to interface water (D₂O) was monitored by changing the pendant alkyl chain length of the alkylpyrenes and the interface charge of the vesicles. The amount of photoproduced pyrene cation radical defined as the photoyield decreased with increasing alkyl chain length of the alkylpyrene and with the interface charge from cationic DODAC to neutral DPPC to anionic DHP vesicles. The interpretation is that the increased alkyl chain length of alkylpyrene increases the electron transfer distance between the pyrene moiety as an electron donor and the interface water (D₂O) as an electron acceptor. This occurs by deeper penetration of the alkylpyrene into the hydrophobic region of the vesicles for longer alkyl chains due to greater hydrophobic interaction of the alkylpyrene with the surfactant alkyl chains. This interpretation is supported by a decreasing deuterium modulation depth trend. The decreasing photoyields of alkylpyrene from DODAC to DPPC to DHP vesicles are due to an increasing energy barrier for electron transfer through the vesicular interface.