Synthesis of C3-substituted chlorophyll-a derivatives fused by a π-conjugated E-ring and their physical properties with unique near-infrared absorption

Abstract

Chlorophylls (Chls) are one of the most important pigments for the light-harvesting systems in photosynthesis. Methyl pheophorbide-a derivative fused by a π-conjugated E-ring with the C131=C132 double bond was found as a synthetic example of the unique Chl-a derivatives, which showed split Soret absorption bands in the purple and ultraviolet-A regions and a broad absorption band at a near-infrared (NIR) region as its monomeric state, but not its self-aggregated species. The 3-vinyl group of the Chl-a derivative was chemically transformed into ethyl, formyl, hydroxymethyl, and dialkoxymethyl groups. All the resulting chlorins exhibited unique wide bands at a NIR region. The substitution effect on the peaks of their wide NIR bands was similar to that on the sharp redmost Qy maxima at a visible region of the corresponding pheophorbides with the C131–C132 single bond. Protons at the C5-, C10-, and C20-positions as well as of the peripheral C2-, C3-, C7-, and C8-substituents of methyl 3-substituted 131,132-didehydro-131-deoxo-pheophorbides-a were upfield-shifted in 1H nuclear magnetic resonance, compared with those of intact pheophorbides. The shifts were ascribed to the less ring current of the cyclic tetrapyrroles by π-conjugation of the E-ring. The non-fluorescent C3-substituted Chl-a derivatives could be useful as new materials absorbing NIR wavelength light.