Electron Accepting Low Bandgap Conjugated Polymers Based on Aza-borondipyrromethene Dyes

摘要

Stable n-type conjugated compounds with high electron affinity (EA > 4) are relatively rare, and very few combine solution processability and low bandgaps < 1.5 eV. Compounds with this unique combination of properties would be especially desirable for organic photovoltaics (OPV), where the dominant electron-acceptors are fullerene-derivatives that do not absorb light at these longer wavelengths. Our approach to create n-type low bandgap conjugated materials is to synthesize alternating copolymers incorporating tetraphenylazadipyrromethenes (Aza-DIPYs) in the main chain.~1 Aza-DIPYs are strong red light absorbers, are easy to reduce, and the "pocket" nitrogens in aza-DIPY allow their electronic properties to be readily tuned by chelation.~2 In particular, chelation with boron difluoride further red shifts the absorption of aza-DIPY by reducing rotational freedom and increases electron affinity. Here, we synthesized linear alternating copolymers of aza-DIPY and phenylene acetylene.~3 The alkyl and alkoxy substituents were varied to tune solubility. The aza-DIPY moieties were chelated with BF_2, which increased solubility, lowered the bandgap and stabilized the HOMO and LUMO energy levels. Introducing difluoroboron in key to obtain the lowest bandgap and greatest electron accepting properties.