Band gap engineering of donor-acceptor co-crystals by complementary two-point hydrogen bonding

We report a detailed investigation of a series of new charge-transfer (CT) complexes assembled via a two-point complementary hydrogen bonding (H-bonding) of diindolopyrrole (DIP) electron donors with o-quinone and diazafluorenone acceptors. Unidirectional polarization through the DD⋯AA type H-bonding leads to a dramatic perturbation of electronic levels of the donor and the acceptor. π-Stacking of the H-bonded pairs results in strong charge-transfer (HOMO-LUMO) interactions in their ground state, manifested in low energy optical absorption. Density functional theory (DFT) calculations predict a H-bonding induced rise of the HOMOD (by up to 0.5 eV) and lowering of the LUMOA (by up to 0.7 eV). As a result, the complexes of relatively weak electron donors and acceptor ability exhibit remarkably low optical energy gaps (down to <0.8 eV), that can be tuned by varying the ionization potential and electron affinity of the individual components. Single crystal X-ray analysis for 6 complexes displayed H-bond lengths between 1.9 and 2.3 Å and short π-stacking distances (≥3.2 Å), in line with strong donor-acceptor interactions. Thin-film transistors of such a H-bonded complex, fabricated by vacuum co-sublimation of PhDIP and pyrenetetraone, showed ambipolar charge transport with unusual 'double dip' characteristics.