Design, synthesis, and photophysical characterization of novel pentacyclic red shifted azine dyes

As part of our continuing efforts to develop second generation photodynamic therapeutic agents [1], we synthesized three pentacyclic azine dyes that were designed with the aid of MNDO calculations to absorb visible light having 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-methyldipyrazino[2,3-b:2',3'-i]phenazinium iodide, 14, and 1,4,8,11,13-pentaethyl-2,3,4,8,9,10,11,12,13-octahydrodipyrazino[2,3-b:2',3'-i]phenazinium iodide, 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 tricyclic model compounds 3,7-bis(diethylamino)phenazinium chloride, 20, and 3,7-bis(diethylamino)-5-ethyl-phenazinium iodide, 21, which were also synthesized for this study. Incorporation of rigid peripheral tetrahydropyrazino ring systems in the pentacyclic azines 13, 14, and 15 are responsible for the improved fluorescence and singlet oxygen quantum yields relative to the tricyclic azines 20 and 21.