Effects of Conformation on Doping Efficiency in π-Extended Bipyranylidene Molecules: Relationship between Molecular Structure and Electron-Doping Ability for Developing n-Type Organic Thermoelectrics

Abstract

Abstract Electron doping is an essential process for developing n-type organic thermoelectric materials, and thus the search for efficient n-type dopants is critically important. By replacing the central 1-methylpyrrole ring in 2,5-bis((2,6-diphenyl-4H-pyran-4-ylidene)methyl)-1-methylpyrrole (1) with electron-rich 3,4-ethylenedioxythiophene and 2,2′-bis(3,4-ethylenedioxythiophene) moieties, we synthesized new candidate molecules (2 and 3, respectively) as n-type dopants. The single-crystal X-ray analyses of 1 and 3 elucidated that 3 has a totally planar π-conjugated structure over the whole molecule, whereas 1 has a non-planar structure. Although the energy levels of the highest occupied molecular orbitals of 1–3 evaluated by the electrochemical measurement in solution were not significantly different, the work function of 3 thin film evaluated by the Kelvin probe method was slightly higher than those of 1 and 2. Furthermore, 3 was capable of electron-doping to an n-type semiconducting polymer, poly(benzimidazobenzophenanthroline) (BBL), and the resulting doped BBL showed decent thermoelectric characteristics with the power factor of 1.25 × 10−3 µW m−1 K−2, which was higher by one order of magnitude than those of 1- and 2-doped BBL thin films. These results imply that the high planarity of 3 can contribute to electron-doping ability, which could be useful information for further development of n-type dopants for organic thermoelectric applications.