Photoreduction of Methyleneblue by the two-equivalent electron donor N-methyl-9-phenylacridane and the use of the system for the spectrally sensitized dediazoniation of p-N,N-dimethylamino benzenediazonium tetrafluoroborate

The photoreduction of methyleneblue by N-methyl-9-phenylacridane (ACH) is studied in acetonitrile by means of flash photolysis and quantum yields. In the first step, due to fast proton shift within the original electron transfer product protonated semi-methyleneblue MBH+. and the deprotonated donor radical (N-methyl-9-phenylacridanyl radical AC.) are formed with a rate constant of 2 x 10(8) M-1 s-1. In this radical pair a second electron is transferred very fast from AC. to MBH+. with a rate constant k(e2) almost-equal-to 10(10) s-1 to form leuco-methyleneblue and N-methyl-9-phenyl-acridinium salt (AC+). About 80% of the two-equivalent reduction product, leuco-methyleneblue, are formed within the first solvent cage during the flash. The maximum quantum yields of photo-reduction approach phi-isc of MB+ as expected for a two-equivalent reduction reaction. The out-of-cage reaction consists of the known disproportionation of the protonated semi-methyleneblue MBH+. and its reduction by the N-methyl-9-phenylacridanyl radical AC. From the decay kinetics k(red) = 3 x 10(9) M-1 s-1 and k(dis) = 8 x 10(8) M-1 s-1 is derived. The system sensitizes the dediazoniation of p-N,N-dimethylamino benzenediazonium tetrafluoroborate efficiently even at very low diazonium salt concentrations (PHI = 0.6).