FULL PAPER z Poly(triacety1ene) Oligomers: Synthesis, Characterization, and Estimation of the Effective Conjugation Length by Electrochemical, UV/Vis, and Nonlinear Optical Methods Rainer E. Martin, Ulrich Gubler, Corinne Boudon, Volker Gramlich, Christian Bosshard, Jean-Paul Gisselbrecht, Peter Giinter, Maurice Gross, and Franqois Diederich* zyx Dedirutcd to Profe,tou Dieter Seehaih on the zyxwvutsrq occmion of 1717 60th birthduy Abstract: zyxwvutsrqpon Poly( triacety1ene)s [PTAs, -(C-C-CR=CR-C-C),-] 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 zyxwvutsrqp by oxidative Glaser-Hay coupling of a bifunctional ("chain-forming") (El-hex-3-ene-I ,5- diyne in the presence of an eridcapping unit. All six oligomers are amazingly stable towards exposure to light, air, and temperatures beyond their melting points. Introduction They have been fully characterized and are readily soluble in a wide range of sol- vents. 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 (UVjVis) data, and various Keywords conjugation length . electrochemistry * nonlinear optics * oligomers poly- (triacety1ene)s evaluation methods yielded convergence of the optical properties in the range of 7 to 10 monomer units. The nonresonant second-order molecular hypcrpolarizabil- ity y was measured in CHCI, by means of the third harmonic generation (THG) at i = 1.907 pm. A plot of ;I//? vs. zyx /I revealed a power law y zn" for y with a fitted expo- nent zyxw CI = 2.SkO.1. From the THG iiica- surements, an effective conjugation length of about 10 monomer units was found, in surprisingly good agreement with the val- ue obtained rrom UVWs spectroscopy data. An increasing number of conjugated organic materials and polymers are widely explored as advanced materials for elec- tronic and photonic applications," since they possess inherent synthetic flexibility, potential ease of processing, and the possi- bility of tailoring material characteristics to suit a desired prop- erty. Conjugated organic molecules of multinanometer length are of particular interest for use as molecular wires with poten- tial application in the emerging field of molecular electronics.['' Oligomers of defined length also play an important role for establishing structure-property relationships in the corre- sponding long-chain conjugated polymers.[3] [*] Prof. F. Diederich, Rainci- E. Martin Laboratorium fur Organische Chemie, ETH-Zentruni Univeryilitstrasse 16, CH-8092 Zurich (Switzerland) Fax: lnt. code +(1)632-1109 Prof. P. Gunter, Dr. C. Bosshard. U. Cubler lnslitut fur Quanlenelcktroiiik ETH-Henggerberg, CH-8093 Zurich (Switrerland) Prof. Dr. V. Gramlich Institut fur Kristallographie und Pelrographic. ETli-Zcntriim Sonneggstrasse 5, CH-8092 Zurich (Switzerland) Prof. M. Gross, Dr C. Boudon, Dr. J.-P. Gisselbrecht Lnhoratoirc d'Elcctrochiniie ct de Chimir Physique du Corps Solide. Faculte de Chimic. UniversitC Louis Pastcur 1 et 4, rue Blaise Pascal. B: P. 296, F-6700X Strasbourg Cedex (France) The number of repeat units in a conjugated polymcr required to furnish size-independent redox, optical, or other properties is of great general interest. Although the effective n-electron dclo- calization length cannot be measured directly, the idea of "con- jugation length' or "effective conjugation length" has become one of the central concepts in thc thcoretical and experiniental understanding of many properties of conjugated polymers.'""] The usefulness of monodisperse oligomers for the experimental determination of the effective conjugation length has been wide- ly We recently reported poly(triacety1enc)s [PTAs. -(CrC-CR=CR-C=C),-][5,61 as the third linearly conjugat- ed polymers with a nonaromatic all-carbon backbone in the progression which starts with polyacetylene [-(CR=CR),-; PA] and poly(diacety1ene) [-(C=C-CR=CR),-; PDA], and ulti- mately leads to carbyne [-(C=C),-]. To explore structure- property relationships in PTAs, we had also prepared a series of stable monodisperse oligomers ranging from phenylacetylene endcapped monomer 1 a to pcntainer 1 e."] Although trends in electrochemical[81 and linear optical properties in this series have been investigated, difficulties in large-scale preparation and poor solubility prevented a more extensive investigation of the physical properties of these molecular rods as a function of their length. zyxwv Chrm Lur J IYY7.3. No 9 ( WILEY-VCII Veildg CmbH D-69451 Weinhelm. 19Y7 0947-6539 97 0309-1505 '$17 50+ 50 0 1 SO5