Polyynes as a Model for Carbyne: Synthesis, Physical Properties, and Nonlinear Optical Response

With the Fritsch−Buttenberg−Wiechell rearrangement as a primary synthetic route, a series of
conjugated, triisopropylsilyl end-capped polyynes containing 2−10 acetylene units has been assembled.
In a few steps, significant quantities of the polyynes are made available, which allow for a thorough analysis
of their structural, physical, and optical properties. Molecules in the series have been characterized in
detail using 13C NMR spectroscopy, differential scanning calorimetry, mass spectrometry, and, for four
derivatives including octayne 6, X-ray crystallography. UV−vis spectroscopy of the polyynes 1−7 shows
a consistent lowering of the HOMO−LUMO gap (Eg) as a function of the number of acetylene units (n),
fitting a power-law relationship of Eg ∼ n-0.379±0.002. The third-order nonlinear optical (NLO) properties of
the polyyne series have been examined, and the nonresonant molecular second hyperpolarizabilities (γ)
increase as a function of length according to the power-law γ ∼ n4.28±0.13. This result exhibits an exponent
that is larger than theoretically predicted for polyynes and higher than is observed for polyenes and
polyenynes. The combined linear and nonlinear optical results confirm recent theoretical studies that suggest
polyynes as model 1-D conjugated systems. On the basis of UV−vis spectroscopic analysis, the effective
conjugation length for this series of polyynes is estimated to be ca. n = 32, providing insight into
characteristics of carbyne.