Heptamethine cyanine dyes: a promising class of light harvesting antennas for biohybrid photosynthetic assembly

The bioconjugation of photosynthetic proteins with efficient organic light harvesting antennas is a very intriguing approach to build novel hybrid organic-biological machineries that, mimicking nature, employ solar energy to generate photocurrents or to drive thermodynamically unfavoured reactions, reaching efficiencies higher than those obtainable by their natural conterparts. Such hybrid systems are potentially useful as active materials in new generation devices for photovoltaics and biosensing. In the frame of our studies on organic-biological hybrids for solar energy conversion,[1] here we present the design, synthesis and preliminary characterization of a series of heptamethine cyanine dyes (such as Cian-1 in Figure 1a) particularly suitable as light harvesting antennas for the photosynthetic Reaction Center (RC) of the purple bacterium Rhodobacter sphaeroides strain R26. These molecules have been properly tailored to have efficient light absorption in the visible spectral range, where the RC absorbance is very low, and efficient emission in the near infrared region, in correspondence of the highest RC absorption peaks. Moreover, the charged sites within their molecular structure make these molecules highly soluble in detergent aqueous environment where the RC is stable, this allowing them to approach the bioconjugation sites of the protein. Finally, the synthesized cyanines are endowed with a carboxylic moiety useful for their covalent binding to the amino groups of the RC lysine residues. Our preliminary results show that the bioconjugation of these organic antennas to the RC is expected to be a very profitable strategy to afford highly efficient organic-biological hybrids for solar energy conversion.