Symmetrical and Nonsymmetrical Meso–Meso Directly Linked Hydroporphyrin Dyads: Synthesis and Photochemical Properties

A series of a rigid meso–meso directly linked chlorin–chlorin, chlorin–bacteriochlorin, and bacteriochlorin–bacteriochlorin dyads, including free bases as well as Zn(II), Pd(II), and Cu(II) complexes, has been synthesized, and their absorption, emission, singlet oxygen (1O2) photosensitization, and electronic properties have been examined. Marked bathochromic shifts of the long-wavelength Qy absorption band and increase in fluorescence quantum yields in dyads, in comparison to the corresponding monomers, are observed. Nonsymmetrical dyads (except bacteriochlorin–bacteriochlorin) show two distinctive Qy bands, corresponding to the absorption of each dyad component. A nearly quantitative S1–S1 energy transfer between hydroporphyrins in dyads, leading to an almost exclusive emission of hydroporphyrin with a lower S1 energy, has been determined. Several symmetrical and all nonsymmetrical dyads exhibit a significant reduction in fluorescence quantum yields in solvents of high dielectric constants; this is attr...