Regioisomeric Effects on the Electronic Features of Indenothiophene-Bridged D-π-A′-A DSSC Sensitizers

Two D-pi-A'-A regioisomers (A-IDT-D and D-IDT-A) featuring 4,4'-di-p-tolyl-4H-indeno[1,2-b]-thiophene as a pi linker (pi) between the diarylamino donor (D) and the pyrimidine-cyanoacrylic acid acceptor (A'-A) have been successfully synthesized and characterized as efficient sensitizers for the dye-sensitized solar cells (DSSCs). The different arrangements of the D and A'-A blocks on the unsymmetrical indenothiophene (IDT) core render the dipole of IDT being along (A-IDT-D) or opposite (D-IDT-A) to the direction of intramolecular (donor-to-acceptor) charge transfer, and thus induce variations in the physical properties. The experimental observations correlated well with the theoretical analyses, clearly revealing the trade-off between the molar extinction coefficient (epsilon) and the S-0 -> S-1 transition energy. As a result, a superior e value was observed for D-IDT-A, whereas a bathochromic shift in the absorption occurred in A-IDT-D. The larger e value of D-IDT-A together with its more favorable energy level relative to TiO2 led to a higher power conversion efficiency of 7.41% for the D-IDT-A-based DSSC, retaining approximately 95% of the N719-based DSSC efficiency. This work manifests the clear structure-property relationship for the case of donor and acceptor components being connected by an unsymmetrical pi linker and provides insights for molecular engineering of organic sensitizers.