Design of rigid donor-acceptor systems with a low-lying charge-transfer state. An INDO model study of barrelene-based compounds

As a case study for the design of donor-acceptor compounds with tailor-made properties we present an INDO/S molecular orbital investigation of organic molecules where aromatic donors are rigidly linked to a maleic ester group via a barrelene-type bridge. We discuss the ground state and various excited states both in vacuo and in solution and investigate the influence of the donor size, the donor orientation, and the variation of the acceptor group Solvent effects are incorporated by use of an electrostatic cavity model which allows for a cavity shape that is adapted to the solute molecule. We identify the pertinent structural features that favor a low-lying charge-transfer (CT) state in the region of the first aromatic transitions, even in the gas phase. Recent spectroscopic measurements support the findings of the present study.