Diagnosing the Role of Hydrogen Bonding in the Organization, Aggregation, and Optical Properties of Phthalhydrazide-Functionalized Molecules in Solution and Solid State

Abstract

To realize optoelectronics based on self-assembling organic semiconductors, it is imperative to have both excellent molecular design and conditions that facilitate well-behaved molecular assembly. We previously reported how hydrogen-bond (HB)-directed self-assembly of π-conjugated donor materials with a phthalhydrazide (PH) functional group can improve active layer morphology and charge transport in organic photovoltaics (OPVs). In this work, we improved the molecular design by synthesizing new hydrogen-bonding (H-bonding) capable molecules featuring narrower optical gaps and enhanced solubility. We evaluated their self-assembling properties in a variety of environments (solvents, temperatures, concentrations, etc.) and in the solid state to establish procedures to prepare films with predictable structural and optical properties. The new H-bonding molecules (iQPH and iQPH-BO) and their comparator compounds (QPMe and QPMe-BO) were prepared by fusing a phthalhydrazide unit and quinoxaline unit to achieve donor-acceptor functionality. By characterizing iQPH in solution using ultraviolet-visible (UV-vis) spectroscopy, nuclear magnetic resonance (NMR) methods, density functional theory (DFT), and single crystal X-ray diffraction, we discovered its surprisingly complex structural, self-assembling, and optical properties. Thin film structural characterization by atomic force microscopy (AFM) and grazing-incidence wide-angle X-ray scattering (GIWAXS) revealed “locked-in” morphology and superior thermal stability in iQPH-BO films compared to QPMe-BO films due to H-bonded supramolecular assemblies. Negligible changes in UV-vis absorption were observed for iQPH-BO films even when annealed at temperatures over 150 °C. This work supports the idea that H-bonding compounds can effectively control the morphology of photoactive layers and enhance the robustness of optoelectronic devices. This work also underscores the complexity of using H-bonding to mediate the self-assembly of π-conjugated small molecule compounds. In doing so, we contribute to the growing body of molecular design principles aimed at achieving programmable supramolecular assemblies in organic optoelectronics.