Effects of Strong Electronic Coupling in Chlorin and Bacteriochlorin Dyads

Achieving tunable, intense near-infrared absorption in molecular architectures with properties suitable for solar light harvesting and biomedical studies is of fundamental interest. Herein, we report the photophysical,, redox, and molecular-orbital characteristics of nine hydroporphyrin dyads and associated benchmark monomers that have been designed and synthesized to attain enhanced light harvesting. Each dyad contains two identical hydroporphyrins (chlorin or bacteriochlorin) connected by a linker (ethynyl or butadiynyl) at the macrocycle beta-pyrrole (3- or 13-) or meso (15-) positions. The strong electronic communication between constituent chromophores is indicated by the doubling of prominent absorption features, split redox waves, and paired linear combinations of frontier molecular orbitals. Relative to the benchmarks, the, chlorin dyads in toluene show substantial bathochromic shifts of the long-wavelength absorption band (17-31 nm), modestly reduced singlet excited-state lifetimes (tau(s) = 3.6-6.2 ns vs 8.8-12.3 its), and increased fluorescence quantum yields (Phi(f) = 0.37-0.57 vs 0.34-0.39). The bacteriochlorin, dyads in toluene show significant bathochromic shifts (25-57 nm) and Modestly reduced tau(S) (1.6-3.4 ns vs 3.5-5.3 ns) and dot (0.09-0.19 vs 0.17-0.21) values. The tau(S) and Of values for the bacteriochlorin dyads are reduced substantially (up to similar to 20-fold) benzonitrile. The quenching is due primarily to the increased Si, So internal, conversion that is likely induced by increased contribution of charge-resonance configurations, to the S-1, excited, state in the polar medium. The fundamental insights gained into the physicochemical properties of the strongly coupled hydroporphyrin dyads may aid their utilization in solar-energy conversion and photomedicine.