The one-dimensional nature of polyynes

Carbyne is a hypothetical carbon allotrope that consists of sp-hybridized carbon atoms in an infinitely-long, one-dimensional (1-D) linear chain. Polyynes, the oligomeric cousins of carbyne, with a dense delocalized-electron framework, could offer groundbreaking electronic properties. We have studied the linear and third-order nonlinear optical properties of both triisopropylsilyl end-capped (TIPS-PY) and phenyl end-capped (p-PY) polyynes containing pure sp-hybridized carbon backbones. Analysis of the TIPS-polyyne UV-vis absorption spectra shows that the absorption gap, Eg, in these materials scales very precisely as a power-law with increasing oligomer length, n, with Eg~n-0.379±0.002. The phenylated polyynes show a similar trend of Eg~n-0.36±0.01. Ultrafast molecular second-hyperpolarizabilities, γ, were obtained in solution using 800nm, 100fs pulses in a differential optical Kerr effect (DOKE) setup. Polyyne second-hyperpolarizabilities also scale with a power-law, and, surprisingly, with exponents higher than that of any other reported oligomer system, yielding a behavior of γ~n4.3±0.1 and γ~n3.8±0.1 for the TIPS-polyynes and phenylated-polyynes, respectively. These findings contrast direct theoretical predictions that increases in gamma with increasing conjugation length for polyynes should be considerably lower than those of polyenes and polyenynes. Furthermore, the combined linear and nonlinear optical results agree with recent theoretical studies on ideal 1-D conjugated systems, suggesting that polyynes display true 1-D behavior.

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