D–A–π–A Fused Pentacyclic Organic Dyes as Sensitizers for Dye‐Sensitized Solar Cells Operating under Low‐Light Conditions

In this work, the highly conjugated pentacyclic [1]benzothieno[3,2-b]thieno[2,3-f][1]benzothiophene (TBTBT) moiety has been introduced, for the first time, as a mild donor group in the structure of three donor-acceptor organic dyes, named PCD1-3. After dye design assisted by DFT and TD-DFT computational studies, the fused TBTBT system with the appropriate substitution pattern was synthesized by means of a three-step sequence, featuring a Pd-catalyzed direct arylation of 1-bromo-2-(methylsulfinyl)benzene and a subsequent interrupted Pummerer reaction as the key steps. Further elaboration of the dyes’ structures was then achieved by application of additional C(sp2)-C(sp2) direct arylation protocols, thereby making the overall pathway particularly concise. Spectroscopic and electrochemical characterization revealed that dyes PCD1-3 presented the appropriate properties for employment as photosensitizers in dye-sensitized photovoltaic cells (DSSCs) for indoor applications. Preliminary tests conducted on DSSCs fabricated with the new dyes highlighted differences in their compatibility with electrolyte solutions of different compositions, as well as in their response to different indoor and outdoor (simulated) light sources, with the best results obtained under illumination with a cold white LED (900 lux).