Tuning the Properties of Donor-Acceptor and Acceptor-Donor-Acceptor Boron Difluoride Hydrazones via Extended π-Conjugation.

Abstract

Molecular materials with π-conjugated donor–acceptor (D–A) and acceptor–donor–acceptor (A–D–A) electronic structures have received significant attention due to their usage in organic photovoltaic materials, in organic light-emitting diodes, and as biological imaging agents. Boron-containing molecular materials have been explored as electron-accepting units in compounds with D–A and A–D–A properties as they often exhibit unique and tunable optoelectronic and redox properties. Here, we utilize Stille cross-coupling chemistry to prepare a series of compounds with boron difluoride hydrazones (BODIHYs) as acceptors and benzene, thiophene, or 9,9-dihexylfluorene as donors. BODIHYs with D–A and A–D–A properties exhibited multiple reversible redox waves, solid-state emission with photoluminescence quantum yields up to 10%, and aggregation-induced emission (AIE). Optical band gaps (or highest occupied molecular orbital (HOMO)–lowest unoccupied molecular orbital (LUMO) gaps) determined for these compounds (2.02–2.25 eV) agree well with those determined from cyclic voltammetry experiments (2.05–2.42 eV). The optoelectronic properties described herein are rationalized with density functional theory calculations that support the interpretation of the experimental findings. This work provides a foundation of understanding that will allow for the consideration of D–A and A–D–A BODIHYs to be incorporated into applications (e.g., organic electronics) where fine-tuning of band gaps is required.