Electronic characteristics of arylated tetraethynylethenes: A cooperative computational and electrochemical investigation

A systematic analysis of a series of donor- and/or acceptor-substituted tetraethynylethenes (TEEs, TEE = 3,4-diethynylhex-3-ene-1,5-diyne) was conducted by means of electrochemical analysis and ab initio calculations to determine the ability of the conjugated carbon core to promote electronic communication between pendant functionality. The electronic behavior as a function of the degree and pattern of Substitution was examined and compared to the theoretical results. Experimentally, the study indicates that the electrochemically generated charges localize, since the presence of one redox center on the TEE core has a minimal effect bn the redox characteristics of the other centers. Upon reduction of p-nitrophenyl-substituted TEEs, however, a decrease in bond length alternation in the TEE core and a corresponding increase in the phenyl rings were predicted by ab initio calculations. This cumulenic/ quinoid structure was validated experimentally by the electrochemical isomerization of cis-1,6-bis(4-nitrophenyl)-3,4-bis{[(tert-butyldimethylsilyl)oxy]methyl}hex-3-ene-1,5-diyne to its trans-isomer during cyclic voltammetry. Thus, the findings show that although the multiple p-nitrophenyl redox centers present on the TEE core apparently behave independently from one another in electrochemical reduction steps, delocalization of the incurred charges is effectively conveyed by the alkynes into the carbon framework. This delocalization imparts a sufficiently high single bond character to the central TEE double bond in the dianion to allow rotation and cis-trans-isomerization.