Syntheses and Spectroscopic Studies of Novel Chlorins with Fused Quinoxaline or Benzimidazole Ring Systems and the Related Dimers with Extended Conjugation

Abstract Condensation of 13 2 -oxopyropheophorbide a 1 with various aromatic diamines under acid-catalyzed conditions afforded a series of novel chlorins with fused quinoxaline or benzimidazole polyaromatic ring systems. This methodology was extended for the preparation of laterally bridged pyropheophorbide a dimers. Reaction of 1 with 1,2,4,5-benzenetetramine produced a bis-quinoxaline-bridged symmetrical chlorin dimer 16 and an unsymmetrical benzimidazole/pyrazine bridged analog 17 . Spectroscopic data of novel conjugated dimers show a significant perturbation of the extended bis-chlorin π-system in a coplanar arrangement. The excited state properties of some novel chlorin derivatives 4 – 6 , 14 , and dimers 16 and 17 , which contain two laterally bridged pheophorbides have been examined. Compounds 4 – 6 , 16 and 17 are weakly fluorescent ( φ f =0.08–0.16) with singlet state lifetimes ranging from 2–5 ns. These compounds show high intersystem crossing efficiencies ( φ T =0.39–0.58). A close match was observed between the quantum yields for singlet oxygen generated and their triplet quantum yields. The excited state of 14 undergoes deactivation mainly through a non-radiative route with fluorescence and triplet state quantum yields being negligible.The monomeric and dimeric structures of annulated chlorins show a characteristic vinylogous-type enolization of the 12-methyl group adjacent to the fused ring systems. The crystallographic and the modeling studies of these novel chlorins indicated a rigid construction of condensed aromatic heterocycles in a planar arrangement.