Synthesis of novel organic sensitizers for artificial photosynthesis through Pd-catalyzed cross-coupling reactions

In the field of artificial photosynthesis, dye-sensitized photoelectrochemical cells (DS-PECs) for water-splitting have been extremely investigated in the last years in the effort to provide a reliable system to produce solar fuels from water and sunlight. Given the crucial role of the dye in the working mechanism of the device, the design and synthesis of novel sensitizers with a broad optical response, appropriate energy levels, and good stability represents a key-point. Among the different classes of sensitizers employed for such a process, organic metal-free dyes, and especially D (donor)-pi-A (acceptor) sensitizers, have been intensely investigated thanks to their low cost, tunable spectroscopic properties, good stability and relatively easy synthesis. In this study, we report the design and synthesis of a family of five new metal-free organic dyes, based on quinoxaline (compound 1a-c) and pyrido-pirazine (compounds 2,3) central cores. Moreover, we explored their potential application as sensitizers in DS-PECs in combination with the chosen water oxidation catalyst: Ru(bda)(pyP)2 (pyP = pyridin-4-methyl phosphonic acid). All the compounds have been successfully synthesized, with good overall yields, through well-known Pd-catalysed C-C cross coupling reactions, such as Suzuki-Miyaura and Stille reactions, or, when possible, through more recent approaches like direct C-H activation reactions. Preliminary studies, such as absorption and emission spectra and cyclic voltammetry, have been also carried out in order to evaluate their photoelectrochemical properties and applicability in the device.