Solvent effects on electronic absorption spectra of donor-substituted 11,11,12,12-tetracyano-9,10-anthraquinodimethanes (TCAQs)

Solvent effects on the electronic absorption spectra of donor-substituted 11,11,12,12-tetracyano-9, 10-anthraquinodimethanes (TCAQs) 1-3 have been investigated in 32 well-selected solvents. These compounds were chosen as model structures for charge-transfer chromophores featuring second- and third-order nonlinear optical properties. The resulting data were evaluated by means of theoretical models and (semi)empirical correlations determining the optical properties related to electron distribution and polarizability. We found that solvent effects on a polar D-pi-A system do not depend on the donor/acceptor orientation (HOMO/LUMO localization) but especially on the length of the pi-system in between. The observed solvent effects are described with high accuracy by the applied theoretical models and linear combinations of physical quantities. Solvent polarization, permanent dipole moment, and molar volume substantially affect the longest-wavelength absorption maxima. Solvent-induced bathochromic shift resulting from the solvent polarity is described with high accuracy by the Born function. On the other hand, hypsochromic effects of the solvent permanent dipole moment are caused due to the slower reorganization of molecular dipoles compared with the rate of excitation. Solvent polarizability shifts the longest-wavelength absorption maxima bathochromically with increasing length of the pi-conjugated system. Whereas this effect could be suitably described by the Onsager-induced polarizability, the orientation polarizability was not found to be important. The solvent molar volume as a hypsochromic shift-inducing factor is only relevant if the size of the solute and solvent molecules are comparable. If the size of the solute is considerably larger than that of the solvent molecules, the solvent behaves as a 'shape continuum.' Copyright (c) 2008 John Wiley & Sons, Ltd.