Design, synthesis, and photophysical characterization of novel pentacyclicred shifted azine dyes

As part of our continuing efforts to develop second generation photodynamic therapeutic agents [1], we synthesized three pentacyclic azine dyes that w ere designed with the aid of MNDO calculations to absorb visible light havi ng wavelengths longer than 600 nm. Photophysical measurements for the azine dyes 1,4,8,11-tetraethyl-1,2,3,4,8,9,10,11,13-nonahydrodipyrazino[2,3-b:2' ,3'-i]phenazinium acetate, 13, 1,4,8,11-tetraethyl-2,3,4,8,9,10,11,12,13-me thyldipyrazino[2,3-b:2',3'-i]phenazinium iodide, 14, and 1,4,8,11,13-pentae thyl-2,3,4,8,9,10,11,12,13-octahydrodipyrazino[2,3-b:2',3'-i]phenazinium io dide, 15, are highlighted by a 35 nm red shift in their absorption spectra and a 5-7 fold increase in their singlet oxygen quantum yield relative to t ricyclic model compounds 3,7-bis(diethylamino)phenazinium chloride, 20, and 3,7-bis(diethylamino)-5-ethyl-phenazinium iodide, 21, which were also synt hesized for this study. Incorporation of rigid peripheral tetrahydropyrazin o ring systems in the pentacyclic azines 13, 14, and 15 are responsible for the improved fluorescence and singlet oxygen quantum yields relative to th e tricyclic azines 20 and 21.