Polyyne-Type Materials - Polyynes: Synthesis, Properties, and Applications - page 192

graphite and diamond. The discovery that polyyne compounds with long
p -conjugation and amorphous carbynoid materials are quite unstable has
caused the development of newly devised synthetic methods for polyynes.
The instability is mainly due to the high reactivity of linear carbon-chain
species with oxygen and radical reactions between the carbon chains,
including Diels-Alder-type addition. In order to overcome the difficult
handling of the polyyne materials caused by the instability, stabilization
of the polyyne chains by end capping with steric and electronic effects is
frequently attempted, affording some stable end-capped polyynes. Such
methods are quite effective for obtaining stable polyyne-type materials. The
synthesis of Si-capped, long, conjugated oligoynes [9], the detection of end-
capped linear carbons consisting of more than a hundred sp-carbons [10],
and the isolation of long polyynes with end-capped metal complexes [11] are
impressive and successful cases.
In this chapter, stable polyyne-type compounds and materials are
surveyed with regard to their synthesis, properties, and possible applica-
tions. They are classified into end-capped polyynes with or without metals
and alternating co-polymers having a polyyne unit with or without metals.
In order to determine the possible application of polyynes, the detailed
chemical and physical properties in solution, and in the liquid, and solid
states, must be known.

10.2 END-CAPPED POLYYNES

Various end-capped polyynes have been synthesized in a stepwise manner
by classical methods using Glaser, Hay, Eglinton, or Cadiot-Chodkiewicz

couplings with appropriate modifications. The most convenient method of

characterizing polyynes is absorption spectroscopy. Oligoyne usually shows
a well separated comb-like absorption spectrum due to vibronic effect.
HOMO-LUMO energy gaps (E g ) of these compounds can be estimated
from the longest absorption maximum ( max ) in solution. As the polyyne

conjugation becomes longer, the energy gap becomes the narrower. A pro-
portional relation between max 2 values and the numbers of triple bonds (n)

has been observed in several oligoynes (R-(C

C) n -R) [9,12]. However, the

max 2 vs n) frequently did not fit well into the large n
region. Here I show better fitting of the optical results [9,12] with power

Lewis-Calvin plot (

function (ln ¼ a þ b ln n) as shown in Figure 10.1 , but (n ¼ 8) should

approach to a limit value.
Crystallographic characterizations of the polyynes provide significant
information with respect to crystalline carbyne structure. Crystallographic
results of a number of end-capped tetraynes and octaynes are reviewed by
Szafert and Gladysz [13], where bond lengths and angles, the space group,
the volume of the unit cell, the number of molecules in the unit cell (Z), and