Facile, Regioselective Synthesis of Highly Solvatochromic Thiophene-Spaced N-Alkylpyridinium Dicyanomethanides for Second-Harmonic Generation

TLDR: In this paper, the facile and clean synthesis of a novel class of highly solvatochromic chromophores has been reported, which are push-pull systems containing a negatively charged dicyanomethanide as a donor group and a positively charged N-alkylpyridinium as an acceptor group.

Abstract: The facile and clean synthesis of a novel class of highly solvatochromic chromophores 1 is reported. Compounds 1 are push−pull systems containing a negatively charged dicyanomethanide as a donor group and a positively charged N-alkylpyridinium as an acceptor group. The terminal polar functions are spaced by a thiophene-based moiety containing one or two heterocyclic rings and none, one, or two ethylene bridges. Chromophores 1 have been obtained through a general synthetic scheme involving, as the last step, the 100% regioselective alkylation of the precursor bidentate anions 2, where two competing nucleophilic sites, one neutral at the pyridic nitrogen and one anionic at the carbanionic carbon of the dicyanomethanide group, are present. The unprecedented highly regioselective attack of the alkylating agent onto the neutral pyridic nitrogen rather than the highly charged carbanionic carbon has been also confirmed in the case of the intermolecular competition. Multinuclear (13C and 15N) NMR spectroscopy has...