Polyynes: Synthesis with the Submerged Electric Arc - Polyynes: Synthesis, Properties, and Applications - page 151

8.1 INTRODUCTION

Polyynes are a fascinating class of molecules because they are involved in
many processes and in many different fields of science. For instance, it has
been demonstrated that they are the precursors in fullerene synthesis [1-7].
Since nanotubes and nanohorns are obtained under electric arc conditions
similar to those adopted for polyyne production [8-11], it may be supposed
that they may also play a role in nanotube formation. Furthermore, poly-
ynes are also considered the key intermediates in the mechanism of soot
formation in flames or in thermal decomposition processes of hydrocarbons
[12]. Polyynes also represent a very wide class of naturally occurring organic
molecules synthesized by plants, microorganisms, and fungi; about 1000
of naturally occurring polyynes are known [13]. They display biological
activity, including antibiotic and anticancer activity [13] and may find appli-
cation in the treatment of certain diseases. Carbon chains are also produced
from carbon-rich late-type stars and are released in the interstellar medium.
It is quite possible that in this medium the carbon chains are the pre-
cursors of several exotic molecules and also of interstellar carbon dust.
Polyyne derivatives known as cyanopolyynes have been identified by radio-
astronomy in the molecular clouds of the interstellar medium [14]. It is quite
astonishing that the largest molecule identified till now in molecular clouds
of the interstellar space is just a cyanopolyyne: C 11 N. Polyynes are thought
to also play a major role in the interstellar medium and have been advocated
as a source of the diffuse interstellar bands (DIBs), although this has not
been definitively proven [15-17], and they may play a role in the formation
mechanism of interstellar carbon dust. Polyynes and cyanopolyynes are
present in the atmosphere of Titan, the giant moon of Saturn, and are
thought to be involved in the haze formation in this satellite [18-20].
Furthermore, the formation of polyynes and derivatives is expected also in
the atmospheres of the giant gaseous planets such as Jupiter and Saturn and
have been found in cometary dust [21]. Polyyne chains are also considered
as models of molecular wires and in the synthesis of push-pull chemical
structures [22].
The polyynes are described by the general formula R-(C

C) n -R 0 where
R and R 0 are the end groups which can be any chemical group in general or
metal atom and n an integer 1,2,3, . In the present chapter we will limit our

discussion to the hydrogen-terminated polyynes where R

...

R 0 ¼ H. Polyynes

with other end groups are treated in other chapters of this topic.
Polyynes are made by a sequence of sp-hybridized carbon atoms. For an
extremely long carbon chain (n

¼

) the resulting high polymer is called
carbyne. Thus, by using the language of polymer chemistry, it is possible to
affirm that the polyynes are oligomers of acetylene while carbyne is the
resulting high polymer.
The first systematic synthesis of polyynes was achieved in 1972 [23] with
an elegant multi-step synthetic approach. Polyyne synthesis by coupling

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