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

pi-Conjugated molecules with small highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) energy gaps (Delta EH-L) have been extensively studied because of their unique optoelectronic properties. Although the expansion of pi-conjugation is one of the well-known approaches for designing molecules with small Delta EH-L values, such an approach inevitably gives large pi-conjugated molecules sometimes suffering from synthetic difficulty and low solubility toward organic solvents. To develop relatively small donor-acceptor molecules with small Delta E-H-(L) values, we have designed and synthesized p-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 (Delta ED-A) and the interplanar angle between them (phi(DA)) have a crucial effect on Delta EH-L, and hence, on the electronic structures and optoelectronic properties of these zwitterions. The zwitterions with small Delta ED-A and large phi(DA) have a small Delta EH-L of ca. 1 eV and show amphoteric redox properties and near-infrared (NIR) electronic absorption exceeding lambda = 1000 nm. The NIR absorption responds to solvent polarity, temperature, and acid addition. This molecular design will generate small pi-conjugated donor-acceptor molecules with small Delta EH-L values.