| Abstract
| - Two series of push−pull chromophores built around thiophene-based π-conjugating spacers rigidifiedeither by covalent bonds or by noncovalent intramolecular interactions have been synthesized andcharacterized by UV−vis spectroscopy, electric field induced second harmonic generation (EFISH)and differential scanning calorimetry. Comparison of the linear and second-order nonlinear opticalproperties of chromophores based on a covalently bridged dithienylethylene (DTE) spacer with thoseof their analogues based on open chain DTE shows that the rigidification of the spacer produces aconsiderable bathochromic shift of the absorption maximum together with a dramatic enhancementof the molecular quadratic hyperpolarizability (μβ) which reaches values among the highest reportedso far. A second series of NLO-phores has been derived from a 2,2‘-bi(3,4-ethylenedioxythiophene)(BEDOT) π-conjugating spacer. As indicated by X-ray and UV−vis data, rigidification of the spaceroriginates in that case, from noncovalent intramolecular interactions between sulfur and oxygenatoms. Again, comparison with the parent compounds based on an unsubstituted bithiophene spacerreveals a marked red shift of the absorption maximum and a large enhancement of μβ. In an attemptto 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 unsubstitutedthiophene shows that, despite a red shift of λmax, introduction of alkoxy groups leads to a decreaseof μβ. Theoretical calculations indicate that this effect results from a decrease of the dipole moment(μ) caused by the auxiliary electron-donor alkoxy groups on the thiophene ring. In contrast,replacement of BT by BEDOT produces an increase of μ, which associated with the noncovalentrigidification of the BT system accounts for the observed enhancement of μβ.
|
