Tuning the HOMO energy levels in quinoline and biquinoline based donor-acceptor polymers

Four donor-acceptor type polymers based on quinoline and biquinoline have been synthesized by Pd catalyzed direct C-H (hetero)arylation reaction. Polymers P1 and P2 are alternate copolymers of thiophene-benzothiadiazole-thiophene (TBTT) unit with quinoline and biquinoline unit, respectively. P3 is a random copolymer containing cyclopentadithiophene (CPDT), benzothiadiazole and quinoline moieties in the backbone whereas P4 contains CPDT unit with randomly distributed benzothiadiazole and biquinoline units. All the polymers show good thermal stability and solubility in common organic solvents. CPDT based polymers P3 and P4 exhibit higher absorbance maxima, higher lying Highest Occupied Molecular Orbital (HOMO) energy levels and smaller band gap as compared to thiophene based polymers P1 and P2 as a result of better electron-donating ability of the former leading to stronger intramolecular charge transfer. Also, quinoline based polymers P1 and P3 show a red-shift in the absorbance maxima compared to biquinoline based polymers P2 and P4, respectively due to non-planar transoid conformation of the two quinoline rings in the biquinoline unit. It is found that the use of N-heterocycle based comonomers allows the tuning of the HOMO level over a remarkably wide range (~0.8 eV). Additionally, the use of quinoline or biquinoline along the conjugated chain leads to deeper lying HOMO levels suggesting good oxidative stability for this class of materials.