The role of Raman spectroscopy in the research on sp-hybridized carbon chains: carbynoid structures polyynes and metal polyynides

This paper is an account of the author's works on the synthesis of carbynoid structures through the Glaser and the Cadiot-Chodkiewicz reactions. Related topics are covered, such as the synthesis of the polyynes in solution with the submerged electric arc technique or with an improved use of the Glaser synthesis. The isolation of polyyne chains in the solid state as stable solids, i.e. as copper or silver complexes called polyynides, is covered as well. In all these research works, the Raman spectroscopy emerges as a powerful and indispensable tool for the characterization of such molecules and materials. The use of Raman spectroscopy in combination with solid-state C-13-NMR showed that the carbonaceous solids produced in the early attempts of carbyne synthesis were not made of pure carbyne as desired and as claimed in the past, but consisted of three types of hybridized carbon atoms simultaneously present in the solid: the sp, sp(2) and sp(3). In such carbonaceous solids, the sp structures are called carbynoid structures. The reason for the formation of these mixed forms of carbon was ascribed to the fact that the carbon chains are not stable and in the solid state they tend to cross-link forming in fact sp(2)- and sp(3)-carbon atoms from the original sp-hybridized carbon atoms. Polyynes are easily accessible through the submerged carbon-arc synthesis or alternatively, with the laser ablation of graphite particles submerged in an opportune solvent. Surface enhanced Raman spectroscopy (SERS) is a powerful tool to record the spectra of these molecules in dilute solutions. The best SERS spectra were recorded by using colloidal silver nanoparticles. However, polyynes can be complexed with copper or silver to form polymeric derivatives called polyynides. In this way, the polyyne chains are stabilized in air at room temperature and beautiful Raman spectra have been recorded showing the triple bonds stretching bands. Copyright (C) 2007 John Wiley & Sons, Ltd.