A newly designed isoindigo/thiophene medium-sized molecule containing a π (D–A–D) bridge with unexpected organic photovoltaic performance†
4,7-Bis(2,3-dihydrothieno[3,4-b][1,4]dioxin-5-yl)benzo[c][1,2,5]thiadiazole (ED-BT-ED) is used as a π (donor–acceptor–donor) bridge between two arms of (E)-1-(2-octyldodecyl)-3-(1-(2-octyldodecyl)-2-oxo-6-(5-(5-(thiophen-2-yl)thiophen-2-yl)thiophen-2-yl)indolin-3-ylidene)indolin-2-one (ID-T3) to form a novel medium-sized molecule BT(EDIDT3)2, using a Pd(AcO)2 catalyzed direct C–H arylation method. The presence of a π(D–A–D) bridge induced π conjugation extension, across the molecule backbone structure, the octyldodecyl groups improved the solubility and the terminal thiophene units enhanced the absorption in the UV-vis region. The optical and electrochemical properties of BT(EDIDT3)2 were studied. The BT(EDIDT3)2 : PC70BP (1 : 1) blend was prescreened as an active layer for organic solar cells and showed a power conversion efficiency (PCE)% = 2.25%, as an unexpected result compared with its counterparts with shorter conjugation lengths. The unexpected photovoltaic performance of BT(EDIDT3)2 highlights the importance of the careful selection of the inserted π-aromatic groups because their proper choice is essential to enhance the light-harvesting efficiency. Calculations by density functional theory (DFT) and time-dependent density functional theory (TD-DFT) were performed to confirm this deduction. We deduced that the central benzothiadiazole core in BT(EDIDT3)2 made big distortions that could inhibit the benefit behind the extension of conjugated molecular length.