HOMO-LUMO Energy-Gap Tuning of π-Conjugated Zwitterions Composed of Electron-Donating Anion and Electron-Accepting Cation.

π-Conjugated molecules with small highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) energy gaps (ΔEH-L) have been extensively studied because of their unique optoelectronic properties. Although the expansion of π-conjugation is one of the well-known approaches for designing molecules with small ΔEH-L values, such an approach inevitably gives large π-conjugated molecules sometimes suffering from synthetic difficulty and low solubility toward organic solvents. To develop relatively small donor-acceptor molecules with small ΔEH-L values, we have designed and synthesized π-conjugated zwitterions composed of electron-donating anions, such as phenoxide and anthroxide, and electron-accepting cations, such as pyridinium and acridinium. The energy difference between the HOMO of the anion and the LUMO of the cation (ΔED-A) and the interplanar angle between them (φDA) have a crucial effect on ΔEH-L, and hence, on the electronic structures and optoelectronic properties of these zwitterions. The zwitterions with small ΔED-A and large φDA have a small ΔEH-L of ca. 1 eV and show amphoteric redox properties and near-infrared (NIR) electronic absorption exceeding λ = 1000 nm. The NIR absorption responds to solvent polarity, temperature, and acid addition. This molecular design will generate small π-conjugated donor-acceptor molecules with small ΔEH-L values.