Quinoidal π-extension of dipyranylidene derivatives: towards efficient dopants for n-type organic semiconductors

Abstract

Quinoidal π-extended dipyranylidene derivatives, 2,2',6,6'-tetraphenyl-2,2'-p-quinobis(4,4'-dipyranylidene) (DP8) and its tetrakis(dimethylamino) derivative (DP9), were newly synthesized and characterized. DP8, with the HOMO energy level (E_HOMO) of −4.2 eV, was isolated as a stable compound, whereas DP9, with a very high-lying E_HOMO of −3.9 eV, was unstable under ambient air conditions. Nevertheless, they were well characterized by means of single-crystal X-ray analysis. Based on their bond lengths, the closed-shell quinoidal molecular structures in the crystalline state were most likely. DP9, with the highest E_HOMO among the DP series of n-type dopants, acted as an efficient dopant to polybenzimidazobenzophenanthroline (BBL) with a low-lying LUMO energy level (E_LUMO) of −4.3 eV and to N2200 with an E_LUMO of −3.8 eV. Although the ambient stability of DP9 was still an issue, the quinoidal π-extension is an efficient strategy to develop n-type dopants with “Weitz type” molecules.