Polyynes: a new class of carbon allotropes. About the formation of dicyanopolyynes from an electric arc between graphite electrodes in liquid nitrogen

A mixture of dicyanopolyynes having the general formula Nequivalent toC-(Cequivalent toC)(n)-Cequivalent toN with n = 2, 3 and 4 is formed and collected in a simple apparatus when graphite electrodes are arced in liquid nitrogen. As secondary products regular polyynes, i.e. hydrogen-terminated polyynes with the general formula H-(Cequivalent toC)(n)-H with n = 4, 5 and 6 are formed together with the cyanopolyynes. It is shown that the hydrogen for carbon chain end-capping is supplied by the plasmalysis of traces of water present in the liquid nitrogen while nitrogen end groups of cyanopolyynes derive from the plasmalysis of molecular nitrogen. This study has confirmed the key role played by the graphite electrodes as the source of elemental carbon under the form of sp-hybridized carbon chains which are then terminated by atoms derived from the medium where the electric arc is struck. The analysis of the reaction products was made by electronic absorption spectroscopy and the separation and identification of the molecular species produced was made with a liquid chromatographic analysis (HPLC) coupled with a diode-array (DAD) detector. The importance of the polyynes and their role as a class of carbon allotropes has been discussed. (C) 2004 Elsevier Ltd. All rights reserved.