Cyclic Polyynes: Generation, Characterization, and Reactions - Polyynes: Synthesis, Properties, and Applications - page 97

bonded with two neighboring atoms. That they have two-dimensional ring-
type structures, thus having no terminable end, makes them constitute
another class of molecular polyynes. The ring-shaped all-carbon molecules
C n are also referred to as cyclic carbon clusters or cyclo[n]carbons in relation
to cluster science or organic chemistry [1-9]. Concerning the electronic
structure of the polyyne ring, two orthogonal p -electron systems are not
identical due to the strained bent arrangement of bonds. Incorporating
trivalent sp 2 or tetravalent sp 3 carbons as atomic junctions, the polyyne
chains can extend large branching structures forming two- or three-
dimensional polygons. With such characteristic features in mind, researchers
have thought that cyclic polyynes may serve as new building blocks of
hybrid sp-sp 2 -sp 3 carbon networks or carbynoid structures, and in some
cases such compounds have been successfully constructed and characterized
in organic chemistry [1b-d]. More generally, one may realize that the
strained bent carbon chains are found commonly within the polymeric
systems that form during the aggregation of linear polyynes.
The formation of carbon molecules, including cyclic polyynes, has been

performed so far by laser vaporization of graphite that was initiated with

relevance to astrophysics [10] and finally leading to the discovery of fullere-
nes [11,12] and nanotubes [13]. For the cyclic polyynes, generation from
organic precursors of well-defined structures having a dehydroannulene
framework has been a topic in organic chemistry in recent years [1-9]. Also
noted for cyclic polyynes is reactivity of their highly unsaturated bonds. The
carbon-rich compounds are so reactive as to condense easily forming
polymers. Similarly to the linear polyynes, the unsaturated carbon atoms
form cross-linking bonds towards the other molecules upon collisions (see

Section 6.3.2 ). To avoid the undesired reactions, characterization of such

transient species has been performed in vacuum or in matrix-isolated states.
For instance, under the collision-free conditions in vacuum, the organic pre-
cursors having carbon-ring structures of C n (n

¼

12-30) that are stabilized
by some protecting groups successfully liberate bare carbon molecules C n
through expulsion of the ligand molecules upon UV laser irradiation
(Section 6.2.2) [2-5,9], and characterized by anion photoelectron spectro-
scopy (Sections 6.2.3 and 6.2.4) [6]. In solution, on the other hand, the
product materials observed upon UV irradiation are uncharacterized poly-
mers or ligand-substituted compounds [5c]. In order to trap the molecules
containing highly strained polyyne units, the precursor molecules are co-
condensed in an inert matrix at low temperatures, then irradiated with UV
light to observe the IR absorption spectra in which the high frequency
stretching vibrations characteristic of these polyyne chains are detected [2b].
The approach to form carbon ring molecules from organic precursors
has been extended to produce three-dimensional macrocyclic polyynes of
larger sizes (Section 6.4). Despite the fragility and explosive reactivity, the
precursory molecules containing polyyne structures are successfully isolated
and well characterized. The liberated pure carbon molecules have been