Charge-transfer complexes of non-coplanar derivatives of 4,4′-dimethoxystilbene and of cis-trans isomer pairs of substituted stilbenes

Charge-transfer absorption spectra of tetracyanoethylene complexes with a number of stilbene derivatives as donor molecules (1-15, Table 1) have been measured in dichloromethane as solvent. Job tests showed that the complexes present in the solutions must be 1 : 1 complexes. All curves show two absorption maxima. The longest wavelength absorption band has been ascribed to a complex in which the complete stilbene molecule acts as the electron donor. Its position varies with increasing lack of coplanarity of the donor molecule. The other peak may belong to a configuration in which one of the aromatic rings in the stilbene molecule is the donating part. Values of the equilibrium constants (K) and extinction coefficients (ϵ) of the charge-transfer complexes have been derived from Benesi-Hildebrand graphs. A statistical analysis of the results has been given in the appendix. In the series of alkylated 4,4′-dimethoxystilbenes (1–9) K decreases with increasing hindrance of coplanarity reaching a nearly constant value when the angle of twist of the phenyl nuclei in the stilbene component becomes large. Parallel to this decrease the character of the charge-transfer absorption seems to change from complex to contact charge-transfer. Finally the existence of linear relationships between ionisation potentials of donors or electron affinities of acceptors and vmax of their charge-transfer absorptions has been tested.