Photoreduction of alkylmethylviologens in dioctadecyldimethylammonium chloride vesicles: combined effects of the alkylmethylviologen chain length and the addition of cholesterol to control the net photoreduction yield
摘要:
Electron spin resonance spectroscopy has been used to detect the photoreduction yields of four alkylmethylviologens (AV2+) in rapidly frozen dioctadecyldimethylammonium chloride (DODAC) vesicles containing concentrations of cholesterol from 0 to 33 mol %. The abundant radical is photoreduced alkylmethylviologen cation radical (AV+) together with a lesser amount of an alkyl radical (DAC) from DODAC formed by photoconversion from AV+. The DAC intensity decreases with increasing cholesterol concentration even though electron spin echo modulation data shows that AV+ is in a less hydrated environment with increasing cholesterol concentration which should promote DAC formation. This is explained by high mole percentages of cholesterol increasing the distance between AV+ and the DODAC alkyl chains which decreases the DAC yield. The AV+ yield increases with alkyl chain length but decreases with increasing cholesterol. Analysis shows that the alkyl chain length effect is stronger than the cholesterol effect. This also holds for the total photoreduction yield. Although the alkyl chain length predominantly controls the photoreduction yield, it is shown that cholesterol addition can tune the magnitude of this effect over a narrow range.