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 linearly 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 oxidative 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 are readily soluble in a wide range of solvents. The conjugated rods are reversibly reduced in one-electron transfer steps and cannot be oxidized below +1.23 V vs. Fc/Fc(+). The effective conjugation length in PTAs was estimated from the electronic absorption (UV/Vis) data, and various evaluation methods yielded convergence of the optical properties in the range of 7 to 10 monomer units. The nonresonant second-order molecular hyperpolarizability gamma was measured in CHCl3 by means of the third harmonic generation (THG) at lambda = 1.907 mu m. A plot of gamma/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 surprisingly good agreement with the value obtained from UV/Vis spectroscopy data.