SYNTHESIS AND CHARACTERIZATION OF 11,11,12,12-TETRACYANO-1,4-ANTHRAQUINODIMETHANES (1,4-TCAQS) - NOVEL ELECTRON ACCEPTERS WITH PHOTOINDUCED CHARGE-TRANSFER PROPERTIES

The synthesis of novel substituted 11,11,12,12-tetracyano-1,4-anthraquinodimethanes (1,4-TCAQs) 3 from the corresponding 1,4-anthraquinones 2 by reaction with the Lehnert's reagent is described, thus confirming the validity of this procedure to obtain different substitution patterns on the benzene fused quinone ring. W-vis spectra of the novel compounds show the presence of an intramolecular charge-transfer band in the blue part of the visible region. Depending upon the substitution pattern, the cyclic voltammetry (CV) measurements on 1,4-TCAQs reveal one reduction wave involving two electrons to the dianion or two One-electron reduction waves to the corresponding radical-anion and dianion. Acceptor properties similar to those of TCNQ are expected for 3 when the TCNQ moiety is substituted with chlorine atoms. The molecular and electronic structures of compounds 2 and 3 are investigated using the semiempirical PM3 method and the nonempirical VEH approach, respectively. PM3 calculations predict that 1,4-TCAQs are nonplanar and adopt a butterfly-type structure where the planarity of the lateral naphthalene unit is preserved. VEH calculations show that the naphthalene moiety behaves as an electron donor since the HOMO --> LUMO transition corresponds to an electronic charge transfer from this moiety to the acceptor TCNQ ring, thus supporting the experimental UV-vis results. The evolution of the geometric structure of compounds 3 upon reduction evidences a gain of aromaticity of the TCNQ ring, even if the anions are predicted to remain nonplanar. This absence of planarity justifies the low stability found experimentally for the anions and explains the observation of only one reduction wave involving two-electrons for the most hindered derivatives.