Charge transfer dynamics in β- and meso-substituted dithienylethylene porphyrins

Transient optical studies, at the nanosecond and femtosecond time scales, have been carried out on three structurally different porphyrin dyes to probe their charge transfer dynamics relevant to spectral sensitization and dye-sensitized solar cell (DSSC) operation. Two of them, showing a β substitution pattern, were previously reported: ZnB bearing an ethynylphenyl cyanoacrylic acceptor group and ZnBT bearing an additional π-conjugated dithienylethylene (DTE) chromophore in the acceptor pendant. The new porphyrin ZnMT, showing a donor-acceptor system in 5,15-meso position and the DTE unit as well, is characterized by a smaller HOMO-LUMO energy gap and by a more significant push-pull character which guarantees more intense and red-shifted absorption bands compared to β-substituted dyes. Besides, the introduction of the DTE unit promotes a remarkable gain in spectral sensitization both in β and meso positions. We have demonstrated that the population of the absorption manifold resulting from the presence of the additional DTE chromophore leads to charge separation on a time scale of ca. 200 fs, irrespective of either the meso or β substitution position, resulting in the generation of a long-lived charge-separated state surviving for several microseconds after injection. Thus, dyes equipped with the DTE bridge display the ability to convert photons in the 450-550 nm range, where more common push-pull designs do not show optimal absorption. Further, we have confirmed that β substitution may represent an effective strategy to combine extended spectral sensitization and optimal charge transfer rates with a synthetic and up-scalable feasibility.