INTRAMOLECULAR SENSITIZED ELECTRON RELAYS 1,1'',1'''-TRIALKYL[4,2',4',4'',6',4''']QUATERPYRIDINIUM TRICHLORIDES

Four new electron relay substances (1,1",1"'-trimethyl[4,2';4',4";6',4"']quaterpyridinium trichloride (1), 1,1",1"'-triethyl[4,2';4',4";6',4"']quaterpyridinium trichloride (2), 1,1",1"'-tripropyl[4,2';4',4";6',4"']quaterpyridinium trichloride (3) and 1,1",1"'-tribenzyl[4,2';4',4";6',4"']quaterpyridinium trichloride (4)) have been found to combine the features of a sensitizer and electron relay for photolytic water splitting systems. Compounds 1-4 have absorption maxima at 320 nm with epsilon-max = 9700, 9330, 9800 and 21 200 1 mol-1 cm-1 respectively. Excitation at L1,ex = 313 nm gives fluorescence at L1fl,max = 372 nm (1), 376 nm (2), 374 nm (3) and 378 nm (4), whereas excitation at L2,ex = 436 nm gives fluorescence at L2fl,max = 570 nm (1-3) and 575 nm (4). The fluorescence of compounds 1-4 decreases with increasing concentration when excited at L1,ex = 313 nm. Irradiation at L2,ex gives the usual dependence on concentration. The quantum yields of the hydrogen-producing systems (compounds 1-4, ethylenediaminetetraacetic acid, platinum catalyst and acetate buffer; high pressure mercury lamp, greater than 280 nm) are 4% (1), 5% (2), 12% (3) and 4% (4), calculated for light absorbed from 280 to 360 nm. The syntheses, spectroscopy and photochemistry of compounds 1-4 are described.