Synthesis and photophysical and photovoltaic properties of porphyrin-furan and -thiophene alternating copolymers

Conjugated polymers with alternating main chain structures of zinc porphyrin-furan (PZnPF) and zinc porphyrin-thiophene (PZnPT) have been synthesized by palladium(0)-catalyzed Stille coupling reaction. The optical, electrochemical, photophysical, and photovoltaic properties of PZnPF and PZnPT were investigated to elucidate the effects of the heterole bridges (i.e., furan vs. thiophene) in the porphyrin polymers. The optical bandgap of PZnPF (1.75 eV) is smaller than that of PZnPT (1.90 eV), implying the high derealization of the π-electrons along the polymer main chain of PZnPF relative to PZnPT. The more extended π-conjugation in PZnPF results from the smaller steric repulsion of the meso-furan moiety with the porphyrin rings than that of the meso-thiophene. The time-resolved fluorescence spectrum of PZnPF showed a gradual Stokes shift to the longer wavelength in the subnanosecond time domain due to the relaxation from a twisted conformation with the large dihedral angles between the porphyrins and the furan rings to a coplanar conformation with the small dihedral angles, whereas the fluorescence spectrum of PZnPT did not exhibit the dynamic Stokes shift. Both PZnPF and PZnPT are electrochemically active in the oxidation and reduction regions and have suitable HOMO/LUMO levels that enable photoinduced electron transfer from the polymer to [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM) in the blend films. Indeed, the blend films displayed strong fluorescence quenching from the porphyrin moieties together with appearance of charge-transfer emission arising from the interaction between the porphyrin and the C₆₀ moieties. This is the first observation on charge-transfer emission between conjugated porphyrin polymers and fullerenes. Bulk heterojunction solar cells were fabricated by using the blend films of PZnPF:PCBM and PZnPT:PCBM as a photoactive layer. The PZnPF:PCBM and PZnPT:PCBM devices revealed power conversion efficiencies of 0.048% and 0.027% under standard AM 1.5 sunlight (100 mW cm^-2). These results obtained here will provide fundamental information on the design of large chromophore-embedded conjugated polymers for solar energy conversion.