Solid state Raman and IR spectra of [(η²-dppe)(η⁵-C₅Me₅)Fe(CC–C₆H₄–X)ⁿ⁺][PF₆ ⁻]_n (n = 0, 1; dppe = 1,2-(diphenylphosphino)ethane; X = NO₂, CN, CF₃, F, Br, H, Me, ^tBu, OMe, NH₂) complexes (1a–j/[1a–j⁺][PF₆ ⁻]) were recorded. The Raman spectra are discussed in connection with the corresponding solid state IR spectra. In the present set of compounds, Raman allows a firm identification of ν_CC stretching modes and confirms the occurrence of Fermi coupling with some substituents in a given oxidation state. Linear correlations between the IR frequencies or between the square roots of the intensities of the alkynyl stretch and the electronic substituent parameters (ESPs) allow establishment of the electronic origin of the substituent influence on the strength and polarization of the triple bond.

Two 1,1,4,4-tetracyanobutadiene (TCBD) derivatives were prepared by [2+2]cycloaddition–retroelectrocyclization from ynamides bearing either a pyrene (1) or a perylene unit (2). In addition to panchromatic absorptions in 2, in the solid state, both compounds unexpectedly display NIR photoluminescence that could be detected up to about 1350 nm.