Bacteriochlorins with a Twist: Discovery of a Unique Mechanism to Red-Shift the Optical Spectra of Bacteriochlorins

Owing to their intense near infrared absorption and emission properties, to the ability to photogenerate singlet oxygen, or to act as photoacoustic imaging agents within the optical window of tissue, bacteriochlorins (2,3,12,13-tetrahydroporphyrins) promise to be of utility in many biomedical and technical applications. The ability to fine-tune the electronic properties of synthetic bacteriochlorins is important for these purposes. In this vein, we report the synthesis, structure determination, optical properties, and theoretical analysis of the electronic structure of a family of expanded bacteriochlorin analogues. The stepwise expansion of both pyrroline moieties in near-planar meso-tetraarylbacteriochlorins to morpholine moieties yields ruffled mono- and bismorpholinobacteriochlorins with broadened and up to 90 nm bathochromically shifted bacteriochlorin-like optical spectra. Intramolecular ring-closure reactions of the morpholine moiety with the flanking meso-aryl groups leads to a sharpened, blue-shi...