Liquid-phase photolysis. Part XIII. Charge-transfer transitions in complexes of benzene and alkylbenzenes with tetracyanoethylene, p-chloranil, and maleic anhydride: use of the aromatic donors as solvents

Abstract

U.v. and visible charge-transfer absorption spectra have been examined for dilute solutions of tetracyanoethylene, p-chloranil, and maleic anhydride in benzene and various alkylbenzenes, using very short liquid-path lengths where necessary. Tetracyanoethylene and p-chloranil were found to be essentially completely complexed in such solutions, and it appears that the extinction coefficients of the complexes can be measured directly without recourse to graphical procedures of the Benesi-Hildebrand type. These extinction coefficients show no marked dependence on donor strength or steric effects, and most lie in the range 2000–3000 cm.–1 mol–1 l.In contrast, the weaker acceptor maleic anhydried is incompletely complexed in liquid aromatic-hydrocarbon solvents. The degree of complexing appears to be determined both by the donor strength and steric effects. Since alkyl substituents increase the donor character of the benzene ring while decreasing its accessibility, the extent to which an alkylaromatic hydrocarbon complexes with maleic anhydride reflects a compromise.The relevance of these results to the photochemistry of the corresponding systems is discussed.