Development and synthesis of a “push – pull” tetrathia[7]helicenes for NLO applications

The development of telecommunications is pushing the research toward the development of efficient systems that manipulate and modulate an incoming optical signal. Materials which show these properties must interact strongly with the electric field of an incident laser radiation and generate “nonlinear optical” (NLO) responses (i.e. second and third armonic generation). The so-called “first-generation” organic chromophores for second order NLO have the general structure in which a pi-conjugated bridge links together both a strong electron-accepting (A) unit and a strong electron-donating (D) one, producing A-π-D system. In this communication we report the development, synthesis and NLO characterization of the first A-π-D system where the π-conjugated bridge is constituted by the chiral tetrathia[7]helicene. Thiahelicenes are polycondensed heteroaromatic molecules characterized by a chiral helix-like structure including multiple ortho-condensed thiophene-benzene units.Theoretical calculations led to the design of compound the compound where D = N,N-dimethylhydrazone group and A = alkylpyridinium salt. This compound is one of the most promising, helicene-based, NLOphore due to its high calculated NLO response. The synthesis and characterization of will be described. Solvatochromic studies performed on the A-π-D helicene show an ipsochromic effect related to the longest wavelength transition (485nm in hexane) as the solvent polarity increases, consistent with an excited state less polar that the ground state; the NLO response (EFISH) of is mu*beta = 321*10-48 esu (CHCl3, 10-3 M), and mu*beta = 1140*10-48 esu (10-4 M), respectively one or two order of magnitude higher with respect to previously reported helicenes.