FACILE, REGIOSELECTIVE SYNTHESIS OF HIGHLY SOLVATOCHROMIC THIOPHENE-SPACED N-ALKYLPYRIDINIUM DICYANOMETHANIDES FOR 2ND-HARMONIC GENERATION

The facile and clean synthesis of a navel class of highly solvatochrom ic chromophores 1 is reported. Compounds 1 are push-pull systems conta ining a negatively charged dicyanomethanide as a donor group and a pos itively charged N-alkylpyridinium as an acceptor group. The terminal p olar functions are spaced by a thiophene-based moiety containing one o r two heterocyclic rings and none, one, or two ethylene bridges. Chrom ophores 1 have been obtained through a general synthetic scheme involv ing, as the last step, the 100% regioselective alkylation of the precu rsor bidentate anions 2, where two competing nucleophilic sites, one n eutral at the pyridic nitrogen and one anionic at the carbanionic carb on of the dicyanomethanide group, are present. The unprecedented highl y regioselective attack of the alkylating agent onto the neutral pyrid ic nitrogen rather than the highly charged carbanionic carbon has been also confirmed in the case of the intermolecular competition. Multinu clear (C-13 and N-15) NMR spectroscopy has been used to investigate th e structure and the extent of intramolecular charge transfer in 1, whi ch are shown to exist in the ground state as highly charge-separated z witterionic systems. Experimental results are discussed and compared w ith semiempirical (PM3) computations. The solvatochromic response of c ompounds 1, among the highest ever reported in the literature for simi lar systems, candidates this class of compounds as very attractive act ive components of nonlinear optical materials.