Synthetic, Optical and Theoretical Study of Alternating Ethylenedioxythiophene–Pyridine Oligomers: Evolution from Planar Conjugated to Helicoidal Structure towards a Chiral Configuration

A series of alternating 3,4-ethylenedioxythiophene–alkynylpyridine oligomers (DA)_n with increased solubility are synthesized and their photophysical properties and nonlinear optical properties are investigated. Their quadratic polarizabilities are determined from hyper-Rayleigh scattering experiments to obtain information on their conformations in solution. These chromophores, based on the alternation of electron-rich (D) and electron-deficient (A) moieties, exhibit optical properties that arise from the combination of dipolar and helicoidal features in the (DA)_n homologue series where n=1–4. The transition from dipolar conjugated planar structures (n=1, 2) to helicoidal structures (n=3, 4) is clearly evidenced by results from symmetry-sensitive second-order nonlinear optical experiments. This suggests an approach towards highly efficient chiral chromophores for second-order nonlinear optics. Interestingly, this structural evolution also has significant impact on the photophysical properties: both absorption and fluorescence emission show bathochromic and hyperchromic shifts with increasing number of repeating units in the dipolar planar derivatives (n=1–2) but show saturation effects in the helicoidal structures (n=2–4). In addition, the helicoidal structures show sizeable two-photon absorption at 700–750 nm (40–100 GM) for compounds lacking either electron-donating or electron-withdrawing substituents.