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

Abstract

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 approximately 18 and n approximately 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 degrees 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 -1.40V 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 approximately 4 monomer units] than in the A-A substituted or the previously reported Me3Si- and Et3Si-end-capped PTA oligomer series (ECL: n approximately 10 monomer units). Similar observations with respect to the ECL were made by measurement of the Raman-active v(C triple bound 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.

2-Pyridones as a New Photochemically Stable Structural Design for the Off-Resonant Optical Kerr Effect

“…According to the different approaches of this issue, some studies have reported molecular chromophores or conjugated polymers exhibiting a large third-order nonlinear susceptibility in their transparency range, or a large nonlinear index satisfying the requirements imposed by the material's figures-of-merit [2] defined for photonic switching applications. Typically, this has been addressed in several push±pull molecules (stilbenes, [4a,b] azobenzenes, [4b±d] tetraethynylethenes, [4e] diphenylpolyenes, [4f] 3,3¢-bipyridines [4g] ) and conjugated polymers (para-toluene sulfonate, [5] polyphenylvinylenes [6] ) based on the classical dipolar electronic structure, but also, with further prospects, in non-dipolar chromophores or polymers exhibiting an apolar (tetraethynylethenes, [4e] 3,3¢-bipyridines, [4g] polytriacetylenes [7] ), quadrupolar (tetraethynylethenes [4e,8] ), or octupolar (thiophene-based triazines [9] ) molecular geometry.…”

Highly Functionalized Dimeric Tetraethynylethenes and Expanded Radialenes: Strong Evidence for Macrocyclic Cross-Conjugation

“…TEE dimer 13 is oxidized in two twoelectron steps at 0.37 and 0.54 V, centered on its four dimethylanilino groups. [37] The reduction at À 1.61 V is a reversible one-electron transfer occurring on the central carbon core; the same reduction potential was measured for TEE dimer 6 bearing two dimethylanilino groups. A similar redox behavior is expectedly found for 14 with four (H 25 C 12 ) 2 NC 6 H 4 groups.…”

“…The electron-donating dialkylanilino groups present in 13 and 14 make the first reduction more difficult by about 100 mV relative to the dimers 9 and 12. The push ± pull dimer 16 is reduced in two one-electron steps, centered on the two nitrophenyl groups, [37] which potentials (À 1.35 and À 1.28 V) are separated by 70 mV. Similarly, the oxidation of its two dialkylanilino groups occurs in two distinct one-electron oxidation steps at 0.45 and 0.56 V. In the extended chromophore 17, the four dialkylanilino groups are oxidized in two two-electron steps, similar to 13.…”

“…[37] The expanded [4]radialene 27 b underwent three well-defined reduction steps, each one being a reversible one-electron transfer, and one oxidation step close to the electrolyte discharge. The expanded [6]radialene 27 d gave four reduction steps as well as one oxidation step also close to the electrolyte discharge.…”

Functional Conjugated Materials for Optonics and Electronics by Tetraethynylethene Molecular Scaffolding

“…The PTA mono-, di-, tri-, tetra-, penta-, and hexamers 41a-f and 42a-f ( Figure 18) provide useful information regarding the influence of donor-donor (D-D) and acceptor-acceptor (A-A) substitution in the end groups of PTAs based on the trans-1,2-diethynylethene repeat unit [45]. As reference points for the infinitely long polymers, compounds 41g and 42g (@18 and 12 DEE units) were studied.…”