Synthesis, Photophysical and Electrochemistry of Near-IR Absorbing Bacteriochlorins Related to Bacteriochlorophyll a

A series of new bacteriochlorins was synthesized using 132-oxo-bacteriopyropheophorbide a (derived from bacteriochlorophyll a) as a starting material, which on reacting with o-phenylenediamine and 1,10-diaminonaphthalene afforded highly conjugated annulated bacteriochlorins with fused quinoxaline, benzimidazole, and perimidine rings, respectively. The absorption spectra of these novel bacteriochlorins demonstrated remarkably red-shifted intense Qy absorption bands observed in the range of 816–850 nm with high molar extinction coefficients (89,900–136,800). Treatment of 132-oxo-bacteriopyropheophorbide a methyl ester with diazomethane resulted in the formation of bacterioverdins containing a fused six-membered methoxy-substituted cyclohexenone (verdin) as an isomeric mixture. The pure isomers which exhibit long-wavelength absorptions in the near-IR region (865–890 nm) are highly stable at room temperature with high reactivity with O2 at the triplet photoexcited state and favorable redox potential and could be potential candidates for use as photosensitizers in photodynamic therapy (PDT).