AN ELECTRON MAGNETIC-RESONANCE STUDY ON THE PHOTOIONIZATION OF N-ALKYLPHENOTHIAZINES IN DIOCTADECYLDIMETHYLAMMONIUM CHLORIDE FROZEN VESICLES - THE EFFECT OF UREA, 1,3-DIMETHYLUREA, 1,3-DIETHYLUREA, AND 1,1',3,3'-TETRAMETHYLUREA

Photoionization of N-alkylphenothiazines was carried out in frozen dioctadecyldimethylammonium chloride (DODAC) vesicle suspensions to which various concentrations of urea, 1,3-dimethylurea (DMU), 1,3-dimethylurea (DEU), 1,1',3,3'-tetramethylurea (TMU) had been added. The photoproduced cation radical yields were investigated by electron spin resonance (ESR). Electron spin echo modulation (ESEM) and electron nuclear double resonance (ENDOR) spectroscopies were used to probe the microenvironment of the photoproduced cation. The concentration of each of the urea derivatives was systematically varied. Urea, DMU, and DEU showed a similar trend in the photoyield versus concentration, with the highest yield obtained at 1 M additive. For TMU the photoyield of phenothiazine increased as a function of additive concentration. The different photoyield trends are explained in terms of the effect of urea on the DODAC vesicule interface. This interpretation is supported by the ESEM and ENDOR studies. Urea and its derivatives appear to interact with water through the "direct mechanism". The ESR data for these species suggest that impregnation of the vesicle interface with urea decreases the strength of the photoproduced cation-water interactions which leads to a decrease in the photoyield. The ESE and ENDOR results for TMU suggest that this bulky additive opens up the vesicle interface to water penetration. The steady increase in the photoyield with TMU concentration together with the ESEM and ENDOR results support this interpretation of the ESR data.