Exciplex formation in solid state blends of charge-transfer type AFX dyes and bisimide compounds

We have been studying exciplex formation in nonlinear optical materials containing a high concentration of 2PA chromophores (AFX dyes) as a means to enhance the nonlinear optical properties. A number of dipolar AFX dyes having various electron-accepting moieties (pi-excess and pi-deficient examples) and three bisimide compounds having substituents with varying electron-withdrawing power were synthesized to study exciplex formation in their solid state blends. In substrate supported thin films of various equimolar blends containing an AFX dye and bisimide, the dye monomer emission was severely quenched, and a new emission, red-shifted by up to 110 nm, appeared. The emission energies were consistent with the charge recombination energy calculated from the energy levels of the donor and acceptor present in the blend, which confirmed the emission was from an exciplex. Time resolved emission measurements also indicated the presence of much longer lived transients in the blends, consistent with exciplex formation. Spectroelectrochemistry confirmed that the radical cations of these dyes had strong absorption in the NIR region, so exciplex formation is a means to enhance nonlinear optical absorption of the dyes in this spectral region.