A theoretical study of luminescent vapochromic compounds including an AuCu2(NHC)2 core

Abstract

Vapochromic complexes [Au(im(CH₂py)₂)₂(Cu(MeCN)₂)₂]³⁺1, [Au(im(CH₂py)₂)₂(Cu(MeOH))₂]³⁺2 and [Au(im(CH₂py)₂)₂(Cu(H₂O))₂]³⁺3 were theoretically investigated. The Au–Cu distances of 1 and 2 (4.631 Å and 2.767 Å, respectively) optimized by the SCS-MP2 method in this work agree with the literature experimental values (4.591 Å and 2.792 Å). Their structural features are explained by computational results: (i) in 1, two MeCN molecules coordinate with the Cu center, because of the strong coordination ability of MeCN, to afford a four-coordinate tetrahedral-like Cu center. This geometry needs a long Au–Cu distance. (ii) In 2 and 3, only one MeOH or H₂O molecule coordinates with the Cu center because of their weak coordination abilities, to afford a three-coordinate planar Cu center. Because the three-coordinate Cu center is flexible, the Au–Cu distance becomes short due to the Au–Cu metallophilic interaction, the strength of which is 5.3 kcal mol⁻¹ at the SCS-MP2 level. The emission energies of 1, 2 and 3 (2.62, 2.40 and 2.38 eV, respectively) calculated here by the B3PW91 agree with their literature experimental values (2.68, 2.47, and 2.39 eV). The lowest energy triplet excited state (T₁) is assigned as the excitation from the Cu d to the pyridine π* orbital in 1 and that from the Au–Cu 5d–3d anti-bonding MO to the Au–Cu 6p–4sp bonding MO in 2 and 3. As a result, the emission energy from the T₁ to the ground state is different between these compounds. The difference in Au–Cu distance is one of the important factors for the differences in emission energy and assignment between 1 and others (2 and 3). The vapochromism of these compounds arises from the difference in Au–Cu distance which is determined by the balance between the strengths of the coordination of a gas molecule and the Au–Cu metallophilic interaction; in other words, the Au–Cu heterometallophilic interaction is important for the vapochromic activity of the complex.