Synthesis and Opto-Electrical Properties of Novel Conjugated Small Molecule Bearing a B←N Moiety

Effects of the B←N unit on the energy level of conjugated small molecule were investigated by synthesizing a new small molecule BPDPP (3,8-bis(5-(2,5-dioctyl-3,6-dioxo-4-(thiophen-2-yl)-2,3,5,6-tetrahydropyrrolo[3,4-c]pyrrol-1-yl)-thiophen-2-yl)-6,6-dioctyl-6H-benzo[3,4] [1,2]azaborolo[1,5-a]pyridin-5-ium-6-uide) via Suzuki coupling. For comparison, CPDPP (6,6′-(5,5′-(9,9-dioctyl-9H-fluorene-2,7-diyl)bis(thiophene-5,2-diyl))bis(2,5-dioctyl-3-(thiophen-2-yl)pyrrolo[3,4-c] pyrrole-1,4(2H,5H)-dione)) having the same chemical structure with BPDPP except the B←N unit is synthesized through the same coupling reaction. Absorption spectra and cyclovoltammetry analysis for these small molecules revealed that the B←N unit reduces the band gap of BPDPP by maintaining large oxidation potential. As a result, the blend film of BPDPP and PC 71 BM maintained 87.4% of the initial absorption intensity after 1sun light soaking test for 315 h. The photo stability of the BPDPP is significantly higher than the blend film of P3HT: PC 71 BM, which maintained only 19.8% of the initial absorption intensity only after light soaking for 23 h.