Symmetrical and Non-symmetrical 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 ( 1 O 2 ) photosensitization, and electronic properties have been examined. Marked bathochromic shift of the long-wavelength Q y absorption band and increase in fluorescence quantum yields in dyads, compared to the corresponding monomers, are observed. Non-symmetrical dyads (except bacteriochlorin-bacteriochlorin) show two distinctive Q y bands, corresponding to the absorption of each dyad component. A nearly quantitative S 1 -S 1 energy transfer between hydroporphyrins in dyads, leading to an almost exclusive emission of hydroporphyrin with a lower S 1 energy, have been determined. Several symmetrical and all non-symmetrical dyads exhibit a significant reduction of fluorescence quantum yields in solvents of high dielectric constants; this is attributed to the photoinduced electron transfer. The complexation of one macrocycle by Cu(II) or Pd(II) enhances intersystem crossing in the adjacent, free base dyad component, which is manifested by a significant reduction in fluorescence and increase in quantum yield of 1 O 2 photosensitization.