Electronic absorption spectra of charge-transfer complexes and effects of substituents in radical cations

The effects of X substituents on the energies of charge-transfer bands hv(C T) in electronic absorption spectra of charge-transfer complexes of pi-, n- , or sigma-donors (DX) with pi- or sigma-acceptors (A) as well as on the io nization potentials ID of individual DX molecules are described by the equa tion hv(CT)(I-D) = a + b sigma(I) + c sigma(R) + d sigma(alpha). When DX an d A are fixed, the inductive (b sigma(I)), resonance (c sigma(R)(+)), and p olarization (d sigma(alpha)) contributions to hv(CT) and I-D are virtually identical. The electronic structure of the D.+X donor component of the comp act [A(.-), D X.+] radical-ionic pair in a solution is similar to that of t he radical cation generated upon photoionization of the individual DX molec ule in the gaseous phase.