Synthesis and Physical Investigation of Donor–Donor and Acceptor–Acceptor End‐Functionalized Monodisperse Poly(triacetylene) Oligomers

Two series of monodisperse, terminally donor–donor [D–D, D=4‐(dimethylamino)phenyl] and acceptor–acceptor [A–A, A=4‐nitrophenyl] functionalized poly(triacetylene) (PTA) oligomers ranging from monomer to hexamer were synthesized by oxidative Hay oligomerization under end‐capping conditions. The corresponding D–D and A–A end‐substituted polymers with an average degree of polymerization (DP) of n ≈18 and n ≈12, respectively, were also prepared and served as reference points for the corresponding infinitely long polymers. These terminally functionalized PTA oligomers and polymers are yellow‐ to orange‐colored compounds, displaying excellent solubility in aprotic solvents with melting points above 200 °C for the hexamers. For the 4‐(dimethylamino)phenyl substituted compounds, a consistent first oxidation potential around +0.42 V versus Fc/Fc + (ferrocene/ferricinium) was observed, whereas the 4‐nitrophenyl functionalized systems underwent a reversible reductive two‐electron transfer around −1.40 V versus Fc/Fc + . The nature of the end‐groups has a dramatic influence on the electronic absorption spectra. Saturation of the linear optical properties in the D–D series occurs at significantly shorter chain‐length [effective conjugation length (ECL) of n ≈4 monomer units] than in the A–A substituted or the previously reported Me 3 Si‐ and Et 3 Si‐end‐capped PTA oligomer series (ECL: n ≈10 monomer units). Similar observations with respect to the ECL were made by measurement of the Raman‐active $\tilde {\nu}$ (C≡C) stretches. Third‐harmonic generation (THG) and degenerate four‐wave mixing (DFWM) experiments showed that shorter oligomers of terminally D–D or A–A functionalized PTAs display higher second hyperpolarizabilities γ than the corresponding R 3 Si‐end‐capped series (R=alkyl). Moreover, they disclose a distinct peak of the nonlinearity per monomer unit at intermediate backbone lengths. In THG experiments, the second hyperpolarizabilities for long D/A‐functionalized PTA oligomers attained the same saturation values as observed for the corresponding R 3 Si‐end‐capped rods. The nonlinearities measured by DFWM of the D–D and A–A substituted PTAs were found to be larger than for the silylated ones, which can be explained by the closeness of the two‐photon resonance.