POLY(TRIACETYLENE) OLIGOMERS - SYNTHESIS, CHARACTERIZATION, AND ESTIMATION OF THE EFFECTIVE CONJUGATION LENGTH BY ELECTROCHEMICAL, UV VIS, AND NONLINEAR-OPTICAL METHODS/

Poly(triacetylene)s [PTAs, -(C=C-CR=CR-C=C)(n)-] are a new class of li nearly conjugated polymers with a nonaromatic all-carbon backbone. To explore structure-property relationships in PTAs, we prepared a series of monodisperse oligomers ranging from monomer to hexamer by oxidativ e Glaser-Hay coupling of a bifunctional (''chain-forming'') (E)-hex-3- ene-1,5-diyne in the presence of an endcapping unit. All six oligomers are amazingly stable towards exposure to light, air, and temperatures beyond their melting points. They have been fully characterized and a re readily soluble in a wide range of solvents. The conjugated rods ar e reversibly reduced in one-electron transfer steps and cannot be oxid ized below +1.23 V vs. Fc/Fc(+). The effective conjugation length in P TAs was estimated from the electronic absorption (UV/Vis) data, and va rious evaluation methods yielded convergence of the optical properties in the range of 7 to 10 monomer units. The nonresonant second-order m olecular hyperpolarizability gamma was measured in CHCl3 by means of t he third harmonic generation (THG) at lambda = 1.907 mu m. A plot of g amma/n vs. n revealed a power law gamma approximate to n(a) for gamma with a fitted exponent n = 2.5 +/- 0.1. From the THG measurements, an effective conjugation length of about 10 monomer units was found, in s urprisingly good agreement with the value obtained from UV/Vis spectro scopy data.