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-4,4′-(cyclohexa-1,4-diene-3,6-diylidene)dipyran (DP8) and its tetrakis(dimethylamino) derivative (DP9), were newly synthesized and characterized. DP8, with the HOMO energy level (EHOMO) of −4.2 eV, was isolated as a stable compound, whereas DP9, with a very high-lying EHOMO 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 the bond lengths and theoretical calculations, it was concluded that they mainly have a quinoidal character in the crystalline state. DP9, with the highest EHOMO among the dipyranylidene-based n-type dopants, acted as an efficient dopant to polybenzimidazobenzophenanthroline (BBL) with a low-lying LUMO energy level (ELUMO) of −4.3 eV and to N2200 with an ELUMO 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.