CHARGE-TRANSFER COMPLEXES OF TETRACYANOETHYLENE WITH ALKYL AND ARYL DERIVATIVES OF SULFUR, SELENIUM, AND TELLURIUM

Spectral characteristics (lambda(CT), Delta nu(1/2), epsilon) and asso ciation constants (in dichloromethane) of charge-transfer (CT) complex es of tetracyanoethylene (T) with donors containing CXH, CXC, and CXXC linkages (X = S, Se, Te) are presented. The donor orbitals from which CT transitions originate are identified by correlation of lambda(CT) and Delta nu(1/2) values of CT bands of the complexes with the ionizat ion bands of the photoelectron spectra of the donor molecules. CT ener gies of complexes are influenced by intramolecular conjugative and ind uctive effects between functional groups as well as geometric factors that control the angular orientation of these groups. Alkyl thiols nor mally react very rapidly with T; however, highly hindered thiols react slowly so that complexes can be partially characterized. Alkyl thiols and sulfides donate electrons from the n(p(y)) orbital of sulfur to t he pi(b(3g)) orbital of T to form weak CT complexes which exhibit a s ingle absorbance band. lambda(max) depends upon the identity of alkyl substituent, increasing in the order of the inductive effect for acycl ic alkyls (Me < Et < i-Pr < t-Bu) and in the order of ring size (3 < 4 < 5 < 6) for cyclic alkyls. Aryl thiols, chalcogenides, and chalcogen ophenes donate electrons from the two highest occupied pi orbitals to form CT complexes exhibiting two optical bands. In these donors the ch alcogen atom bound to the aryl substituent conjugates with phenyl pi o rbitals through the nonbonded electron pair, although the tendency for conjugation diminishes in the order S > Se > Te. The HOMO in an aryl telluride is essentially a localized n(p(y)) orbital that interacts wi th T to give rise to a band lambda(1) > 650 nm. The conjugative effect and tendency to complex with T are optimal when the CXC or CXX moiety is coplanar with the aryl ring.