Syntheses and spectroscopic studies of novel chlorins with fused quinoxaline or benzimidazole ring systems and the related dimers with extended conjugation

Abstract

Condensation of 13²-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.