Formation of C H Ablation of Graphite, Coal or C Particles Suspended in Selected Solvents - Polyynes: Synthesis, Properties, and Applications - page 147

The relative abundance of polyynes increased with increasing the laser
irradiation time and leveled off at longer irradiation time above 60min. This
may be due to the fact that the relative formation rates of precursor C 2n
clusters from graphite-like carbon are smaller than those from C 60 . Black
graphite-like carbon absorbs more laser light than nearly colorless C 60
particle does. Thus, the decrease in the relative abundance of polyyne at
high C 60 particle concentration can be attributed to the scattering of
laser light by graphite-like carbon. We found a similar peak in the depend-
ence of the relative abundance of polyyne on the graphite concentration.
The decrease in the relative abundance of polyyne at high particle con-
centration of graphite is also attributable to the scattering of laser light by
graphite particles. On the other hand, a major reason for the decrease in
the formation rates of polyynes from graphite at long irradiation time
may be due to a change in graphite structure to other amorphous
carbon structures for which the formation rates of precursor C 2n clusters
are small.

7.5 CONCLUSIONS

C 2n H 2 (n 4-8) polyynes are produced by laser ablation of graphite, coal,

and C 60 particles suspended in solutions. relative abundance of polyynes
rapidly increased with decreasing laser wavelength due to an increase in the
photochemical effects. All substrates favor C 8 H 2 formation, which was
consistent with reported gas phase reactions of carbon clusters with added
gases. Thus, it was concluded that solvents play the same role as that of
foreign added gases for the polymerization and hydrogenation of carbon
clusters. The relative abundance of C 8 H 2 polyyne from graphite, coals, and
C 60 in hexane at 355 nm laser irradiation was 0.55:0.11 or 0.24:1.0,
respectively. The small relative abundance of polyynes from coals was
attributed to the existence of OH, CO, and COOH chemical groups in coals.
The n-distribution of polyynes depended on the solvent. The larger
distributions of long-chain polyynes from graphite in aromatic hydro-
carbons than those in aliphatic hydrocarbons and methanol were explained
by the fact that H concentrations in the latter solvents are larger than
those in aromatic hydrocarbons, and/or that H-abstraction reactions of C 2n
radicals with the latter solvents are faster than those with the former
solvents.
We reported here that C 2n H 2 (n

¼

4-8) polyynes could be synthesized by
laser ablation of various carbon materials in one pot, though their relative
abundance depended strongly on the experimental parameters. The present
technique will contribute to the development of an efficient method for one-
pot synthesis of polyynes, one of the materials for potential use in molecular
electronics.

¼