Influence of Sulfur Oxidation on the Absorption and Electronic Energy Levels of Poly(thienothiophene) Derivatives

Abstract

Two poly(thienothiophene) derivatives containing thieno[3,2-b]thiophene-4,4-dioxide unit were synthesized by Pd-catalyzed Stille coupling method. They were poly(3,6-dihexyl-thieno[3,2-b]thiophene-4,4-dioxide vinylene) (P2) and poly(2,5-diyl-3,6-dihexyl-thieno[3,2-b]thiophene-4,4-dioxide)-co-(2,5-diyl-thiophene) (P4). Poly(3,6-dihexyl-thieno[3,2-b] thiophene vinylene) (P1) and poly(2,5-diyl-3,6-dihexyl-thieno[3,2-b] thiophene)-co-(2,5-diyl-thiophene) (P3) were synthesized for comparison with P2 and P4. After sulfur oxidation on the thienothiophene units, the absorption peaks of the polymer solutions were red-shifted from 540 nm of P1 to 625 nm of P2 and from 445 nm of P3 to 520 nm of P4. The absorption peaks of the polymer films were red-shifted more significantly from 542 nm of P1 to 630 nm of P2 and from 480 nm of P3 to 564 nm of P4. The lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) energy levels also decreased a lot after the sulfur oxidation. In comparison with P1, the LUMO and HOMO energy levels of P2 decreased by 0.59 and 0.35 eV, respectively. The levels were 0.87 and 0.39 eV lower in the LUMO and HOMO energy levels of P4 than in that of P3.