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(t riacetylene) (PTA) oligomers ranging from monomer to hexamer were synthesiz ed by oxidative Hay oligomerization under end-capping conditions. The corre sponding D-D and A-A end-substituted polymers with an average degree of pol ymerization (DP) of n approximate to 18 and n approximate to 12, respective ly, were also prepared and served as reference points for the corresponding infinitely long polymers. These terminally functionalized PTA oligomers an d polymers are yellow- to orange-colored compounds, displaying excellent so lubility in aprotic solvents with melting points above 200 degreesC for the hexamers. For the 4-(dimethylamino)phenyl substituted compounds, a consist ent first oxidation potential around + 0.42 V versus Fc/Fc(+) (ferrocene/fe rricinium) was observed, whereas the 4-nitrophenyl functionalized systems u nderwent 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 ele ctronic absorption spectra. Saturation of the linear optical properties in the D-D series occurs at significantly shorter chain-length [effective conj ugation length (ECL) of n approximate to 4 monomer units] than in the A-A s ubstituted or the previously reported Me3Si- and Et3Si-end-capped PTA oligo mer series (ECL: n approximate to 10 monomer units). Similar observations w ith respect to the ECL were made by measurement of the Raman-active <(<nu>) over tilde>((C=C)) stretches. Third-harmonic generation (THG) and degenerat e four-wave mixing (DFWM) experiments showed that shorter oligomers of term inally D-D or A-A functionalized PTAs display higher second hyperpolarizabi lities gamma than the corresponding R3Si-end-capped series (R=alkyl), Moreo ver, they disclose a distinct peak of the nonlinearity per monomer unit at intermediate backbone lengths. In THG experiments, the second hyperpolariza bilities for long D/A-functionalized PTA oligomers attained the same satura tion values as observed for the corresponding R3Si-end-capped rods. The non linearities 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 cl oseness of the two-photon resonance.