Time-dependent density functional theory studies of the electronic absorption spectra of N,N′-disubstituted 2,3-dialkynyl-1,4-diazabuta-1,3-dienes

Abstract

The electronic structures and transition energies of the title compounds are studied computationally using time-dependent density functional theory. A computationally tractable model chemistry is established, involving the local density approximation in conjunction with double-zeta plus polarisation basis sets. Solvent effects are included via the conductor-like screening model. Good agreement is found between the computationally determined geometric structures of several of the title systems and those found experimentally for related molecules. The electronic transition energies of all of the systems studied are found to be strongly influenced by the dihedral angle between the aryl rings and the backbone of the diazabutadiene unit. Satisfactory interpretation (at the molecular orbital level) of experimentally determined electronic absorption spectra is obtained using computational model compounds in which the acetylenic ⁱPr₃Si substituents are replaced by hydrogen atoms. Inclusion of the ⁱPr₃Si units in the calculations leads to a simulated spectrum which is more complicated than that of the H-substituted system. Comparison with experiment does not conclusively establish if the ⁱPr₃Si units are required in the computational model. The implications for future simulations of the electronic absorption spectra of diazabutadiene complexes of transition metals are discussed.