The high pressure reactivity of substituted acetylenes: a vibrational study on diphenylacetylene

This article is part of an extended study concerning the reactivity of acetylenic systems at high pressure and aims to the investigation of the high pressure synthesis and stabilization of polyacetylenic chains. The data for diphenylacetylene (DPA) are interpreted by establishing a comparison with the results obtained for acetylene, phenylacetylene, DPA and benzene. The room temperature high-pressure reaction of DPA was studied by means of Fourier Transform Infrared Spectroscopy (FTIR) and Raman spectroscopy using a membrane diamond anvil cell (MDAC). The appearance of the sp(3) C-H stretching absorption in the Infrared (IR) spectrum at about 9 GPa, indicates the formation of chemical bonds among the aromatic rings belonging to different DPA molecules. For higher pressures, the Raman data indicate the involvement of the internal C equivalent to C moiety in the chemical transformation. Therefore, due to the steric hindrance of the phenyl rings, the pressure threshold for the reactivity of the two molecular fragments appears in this case to be reversed with respect to other molecular systems. The reaction smoothly occurs until the highest investigated pressure (26.6 GPa), and further proceeds on decompression. The recovered sample, deeply red coloured, was characterized by means of FTIR spectroscopy as an amorphous hydrogenated carbon containing locally ordered polymeric or oligomeric structures, where unreacted DPA molecules may be embedded.