Tailoring the Optical Properties of Organic D-pi-A Photosensitizers: Effect of Sulfur Introduction in the Acceptor Group

In this work, we present for the first time the synthesis and characterization of potential DSSC organic sensitizers whose cyanoacrylic acceptor/anchoring group was modified by replacement of oxygen with one or more sulfur atoms. Using known carboxylic acid dye DF15 as a reference, their TD-DFT computational analysis indicated that oxygen-sulfur substitution should induce a significant red-shift of the corresponding UV/Vis absorption spectra. Whereas synthesis of monothio-carboxylic derivatives of DF15 was successfully carried out, isolation of the analogous dithiocarboxylic acid proved impossible due to its very low stability: despite that, their relative spectroscopic properties could be compared by analyzing the corresponding benzylic esters. Combined computational and transient absorption spectroscopy studies suggested that photoexcited thiocarboxylic acid 1-SO and thioamide 1-SN were able to inject electrons into nanocrystalline TiO2 and indicated a similar charge injection efficiency for 1-SO relative to DF15. Preliminary DSSC studies showed that, when tert-butylpyridine was present in the electrolyte solution, 1-SO was rapidly degraded; however, in the absence of basic additives 1-SO provided a sufficiently stable device, giving a slightly lower efficiency compared to carboxylic sensitizer DF15.