Synthesis and Light-Harvesting Potential of Cyanovinyl β-Substituted Porphyrins and Dyads

Knoevenagel condensation has been utilized as an alternative way to synthesize a series of β-vinyl-substituted porphyrins and porphyrin dyads with good to excellent yields. The condensation of β-formyl porphyrins and phenylacetonitriles allows control of the substitution pattern and metal centres in the porphyrin dyads, allowing the use of metallated synthons. While the optical and electronic properties of the resulting porphyrin dyes are perturbed by the presence of the cyano substituent, this does not significantly affect their use. For example, Raman spectroscopy, in agreement with density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations, show porphyrin electronic transitions with delocalization of frontier molecular orbital electron density onto the β substituent. A comparison of the photovoltaic performance of a carboxylated cyanostyryl condensation product and the unsubstituted analogue in dye-sensitized solar cells (DSSCs) was made. Although the devices showed similar efficiency, the device containing the cyano-substituted dye showed an extended incident photon-to-current conversion efficiency (IPCE) due to a slight red-shift in absorption and an increase in photovoltage as a result of a longer electron lifetime. This minimal change in light-harvesting performance highlights the potential of this Knoevenagel synthetic methodology for producing light-harvesting porphyrin dyes.