Chain bending of polyynes investigated with Raman and IR spectroscopies

Polyynes are molecular systems characterized by π‐conjugated chains of carbon atoms in sp hybridization state. Unlike sp2 π‐conjugated systems, polyynes are formed by carbon chains with alternating single and triple bonds with no hydrogen bonded to the conjugated carbons [1]. Under this regard, polyynes constitute a truly one-dimensional system, being essentially atomic wires made of carbon. Among sp2 π-conjugated carbon systems, graphene is usually considered a flat planar system, but studies have proven that its peculiar out‐of‐plane dynamics is responsible for the negative in‐plane thermal expansion coefficient of graphite [2]. Similarly, while polyynes could be naively considered to be essentially linear systems, a recent spectroscopic study has shown the presence of an out‐of‐line bending effect as the polyyne chain length is increased [3]. In this contribution, IR and Raman spectroscopy are used to examine the vibrational characteristics of a series of three macrocyclic tetraynes [4] in comparison to an acyclic analog. By changing the length of the alkyl tether of the macrocycles, varying degrees of bending of the tetrayne moiety can be achieved. The joint use of IR and Raman spectroscopy provides a quantitative probe of the bending of the sp‐chain. The spectroscopic data show a general trend toward increasing activation of Raman bands in the IR spectra, and vice versa, as bending of the polyyne chain is increased. It will be also shown how Density Functional Theory calculations can offer a detailed rationalization of the experimental observations.