Highly Electron‐Donating Bipyranylidene Derivatives: Potential n‐Type Dopants for Organic Thermoelectrics

The design, synthesis, and characterization of new electron‐donor molecules with the closed‐shell electronic structure are described. The molecular design features the use of a bipyranylidene 7π‐electron system together with strong electron‐donating substituents, namely, methoxy and dimethylamino groups, which realizes the highest occupied molecular orbitals (HOMOs) as high as or higher than 4.0 eV below the vacuum level. Such high‐lying energy levels of the HOMOs make them potential n‐type dopants for organic thermoelectrics, and it turned out that they are capable of electron‐doping to representative n‐type semiconducting polymers, poly([ N,N ′‐bis(2‐octyldodecyl)‐naphthalene‐1,4,5,8‐bis(dicarboximide)‐2,6‐diyl]‐ alt ‐5,5′‐(2,2′‐bithiophene)) (N2200) and poly(benzimidazobenzophenanthroline) (BBL). The resulting doped polymers show decent thermoelectric characteristics with the power factors of 5.7 × 10 −3 (N2200) and 1.62 μW m −1  K −2 (BBL), respectively. These values are almost comparable with those of the polymers doped with 4‐(2,3‐dihydro‐1,3‐dimethyl‐1 H ‐benzimidazol‐2‐yl)‐ N , N ‐dimethylbenzenamine ( N ‐DMBI), a representative n‐type dopant frequently used in the development of n‐type polymer thermoelectrics. These results indicate that even with the molecules possessing the closed‐shell electronic structure, efficient n‐type dopants can be realized, implying that versatile molecular fragments with electron‐donating natures could be used to develop novel superior n‐type dopants for organic thermoelectric applications.