Construction of 3,7-Dithienyl Phenothiazine-Based Organic Dyes via Multistep Direct C–H Arylation Reactions

Herein, an effective and feasible approach to prepare 3,7-dithienyl phenothiazine-based organic dyes has been developed. In this synthetic procedure, the Pd-catalyzed multistep direct C-H arylation of thiophene derivatives with phenothiazine bromides was employed for the first time to construct the 3,7-dithienyl phenothiazine core scaffold, which greatly streamlines access to this class of organic dyes. 3-Thienyl phenothiazine-based dyes were also synthesized via the direct C-H arylation of thiophenes as references. Most of the 3,7-dithienyl phenothiazine-based dyes exhibit better photovoltaic performances than the 3-thienyl phenothiazine-based dyes. Among these organic dyes, the solar cell device based on 6d exhibits the highest conversion efficiency of 8.9%. Compared with 6d, organic dyes with longer π-conjugation, also including bithiophene as the π-spacer, show dramatically reduced conversion efficiencies of cell devices. The introduction of the more electron-rich 3,4-ethylenedioxythiophene to the C3- and/or C7-position of phenothiazine instead of thiophene does not significantly improve the photoelectric conversion performance. The highly efficient synthetic strategy herein developed and these primary results may be helpful to design and synthesize a variety of new 3,7-dithienyl phenothiazine-based organic dyes.