Cheng-Yen Pan

Organic Solar Cells (OSCs) are promising platforms to generate environmentally sustainable electricity. Unfortunately, the morphology of the device active layer and its consequences on efficiency remain unpredictably sensitive to molecular structure. Our previous studies demonstrated how the H-bond(HB)-directed self-assembly of phthalhydrazide(PH)-terminated π-conjugated donor materials can improve active layer morphology and charge transport properties in OSCs. In this work, we seek to improve this design by preparing narrow gap, donor-acceptor (D-A) molecules featuring interior rather than terminal PH groups. Positioning the PH heterocycle at the center of the π-system facilitates synthesis (through increased target symmetry) and is expected to improve π-stacking and therefore charge transport properties in the solid state. By selecting from various π-conjugated moieties, the self-assembling electron-deficient (ED) and electron-rich (rich) building blocks have been designed, prepared, and subjected to optoelectronic evaluation. Unexpectedly, the ED design changes the tautomeric preferences of the PH unit, and the NH/OH tautomer required for hydrogen-bonded rosette formation becomes thermodynamically favored. Gas-phase DFT computations confirm that ED-centered molecules demonstrate narrow HOMO-LUMO gaps of 2.40 eV and 1.56 eV for QPH and TQPH, respectively, promising for efficient light harvesting. A systematic tautomer and conformational evaluation has been performed to determine the most stable structural arrangement of the PH monomers. Also discussed will be the solvatochromic and self-assembly behavior of the molecules in solution determined from UV-vis spectroscopy and NMR methods.