Synthesis and Physical Investigation of Donor–Donor and Acceptor–Acceptor End-Functionalized Monodisperse Poly(triacetylene) Oligomers
作者:Rainer E. Martin、Ulrich Gubler、Corinne Boudon、Christian Bosshard、Jean-Paul Gisselbrecht、Peter Günter、Maurice Gross、François Diederich
DOI:10.1002/1521-3765(20001201)6:23<4400::aid-chem4400>3.0.co;2-a
日期:2000.12.1
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 approximate to 18 and n approximate to 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 degreesC 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 approximate to 4 monomer units] than in the A-A substituted or the previously reported Me3Si- and Et3Si-end-capped PTA oligomer series (ECL: n approximate to 10 monomer units). Similar observations with respect to the ECL were made by measurement of the Raman-active <()over tilde>((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 gamma than the corresponding R3Si-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 R3Si-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.