Studies on the Chemistry of Heterocyclics. XV. The Synthesis of Thiophene Polyenes

Design and synthesis of push-pull chromophores for second-order nonlinear optics derived from rigidified thiophene-based pi-conjugating spacers

Two series of push-pull chromophores built around thiophene-based pi-conjugating spacers rigidified either by covalent bonds or by noncovalent intramolecular interactions have been synthesized and characterized by UV-vis spectroscopy, electric field induced second harmonic generation (EFISH) and differential scanning calorimetry. Comparison of the linear and second-order nonlinear optical properties of chromophores based on a covalently bridged dithienylethylene (DTE) spacer with those of their analogues based on open chain DTE shows that the rigidification of the spacer produces a considerable bathochromic shift of the absorption maximum together with a dramatic enhancement of the molecular quadratic hyperpolarizability (mu beta) which reaches values among the highest reported so far. A second series of NLO-phores has been derived from a 2,2'-bi(3,4-ethylenedioxythiophene) (BEDOT) pi-conjugating spacer. As indicated by X-ray and UV-vis data, rigidification of the spacer originates in that case, from noncovalent intramolecular interactions between sulfur and oxygen atoms. Again, comparison with the parent compounds based on an unsubstituted bithiophene spacer reveals a marked red shift of the absorption maximum and a large enhancement of mu beta. In an attempt to distinguish the contribution of the electronic and geometrical effects of the ethylenedioxy group, a third series of NLO-phores based on 3,4-ethylenedioxythiophene (EDOT) and 3,4-dihexyloxythiophene spacers has been synthesized. Comparison with compounds based on unsubstituted thiophene shows that, despite a red shift of lambda(max), introduction of alkoxy groups leads to a decrease of mu beta. Theoretical calculations indicate that this effect results from a decrease of the dipole moment (mu) caused by the auxiliary electron-donor alkoxy groups on the thiophene ring. In contrast, replacement of BT by BEDOT produces an increase of mu, which associated with the noncovalent rigidification of the BT system accounts for the observed enhancement of mu beta.

Bridged Dithienylethylenes as Precursors of Small Bandgap Electrogenerated Conjugated Polymers

Bridged dithienylethylenes (DTEs) bearing solubilizing alkyl chains at various positions (2-5) have been synthesized by McMurry dimerization of cyclopenta[b]thiophen-6-ones. In order to introduce alkyl substituents at different positions of the DTE molecule, the precursor ketones have been prepared by different strategies based on a combination of Mannich or Wittig-Horner reaction and Friedel-Craft intramolecular cyclization. The position and the length of the alkyl substituents exert a strong effect on the ability of the precursor to undergo electrochemical polymerization. Thus, whereas substitution at the alpha-position of the ethylene linkage (3) results in a rapid inhibition of electropolymerization, introduction of alkyl chains at the beta-position (4, 5) greatly improves the efficiency of the polymerization process. The analysis of the electrochemical and optical properties of the polymers shows that rigidification of the DTE molecule leads to a significant decrease of the oxidation potential and bandgap. A comparative analysis of DTE and its bridged analogues by means of X-ray diffraction reveals, in agreement with experimental and theoretical results, that the observed reduction of both the HOMO-LUMO gap of the precursor and the bandgap of the corresponding polymers are related to a relaxation of bond length alternation in the DTE moiety.