Theory guided systematic molecular design of benzothiadiazole–phenazine based self-assembling electron-acceptors

Abstract

We report two new self-assembling n-type materials in which the design process was systematically analyzed using theoretical calculations prior to experimental synthesis. Benzothiadiazole (A′) and alkoxyphenazine (A) serve as our basic electron-deficient building blocks to construct two candidate molecules with an acceptor (A) – acceptor (A′) – acceptor (A) configuration. We conducted a computational characterization (B3LYP/6-31G*) of the electronic properties for structural subunits linked together and analyzed in a step-by-step fashion, thereby culminating in the target molecules of interest. We found that E_LUMO was controlled primarily by benzothiadiazole as was evident by orbital localization on this subunit. Meanwhile, E_HOMO was influenced by the dihedral angle between A and A′. The molecule with A and A′ coupled with a C–C triple bond (BTD-P-T) was found to be planar with a more stabilized E_LUMO and a reduced E_gap when compared to its C–C single bond counterpart (BTD-P-S). The two molecules were synthesized and characterized to verify the theoretical findings. Optical, electrochemical, and thermal properties were characterized with UV-vis absorption and fluorescence spectroscopy, cyclic voltammetry, and differential scanning calorimetry, respectively. In addition, the molecular packing was examined by X-ray powder diffraction. As predicted by theory, BTD-P-T exhibited a lower E_gap based on the system's lower E_LUMO in comparison to BTD-P-S. BTD-P-T exhibited a higher T_m and crystallinity due to its planarity. Both BTD-P-S and BTD-P-T exhibited excellent fibrillation ability upon solvent casting. Bulk heterojunction (BHJ) organic solar cell (OSC) devices were fabricated using BTD-P-S or BTD-P-T as an acceptor and P3HT as a donor at different weight ratios. For both P3HT:BTD-P-S and P3HT:BTD-P-T BHJs, the weight ratio of 6 : 1 produced the highest power conversion efficiency (PCE) of 0.17% and 0.44%, respectively. These results are consistent with fluorescence quenching experiments in which 90% of P3HT fluorescence was quenched at that ratio. Nearly two times higher PCE was observed for the BTD-P-T based device compared to that of the BTD-P-S based system, mainly due to the higher J_sc. Presumably, the flat geometry of BTD-P-T allows for more efficient intermolecular π-orbital overlap, enhancing charge transport.