title: Thermochromism of Cu(I) Tetrakisguanidine Complexes: Reversible Activation of Metal-to-Ligand Charge-Transfer Bands.
abstract: Tetranuclear, intensely blue-coloured Cu(I) complexes were synthesised in which two Cu2 X3 (-) units (X=Br or I) are bridged by a dicationic GFA (guanidino-functionalised aromatic) ligand. The UV/Vis spectra show a large metal-to-ligand charge-transfer (MLCT) band around 638 nm. The tetranuclear "low-temperature" complexes are in a temperature-dependent equilibrium with dinuclear Cu(I) "high-temperature" complexes, which result from the reversible elimination of two CuX groups. A massive thermochromism effect results from the extinction of the strong MLCT band upon CuX elimination with increasing temperature. For all complexes, quantum chemical calculations predict a small and method-dependent energy difference between the possible electronic structures, namely Cu(I) and dicationic GFA ligand (closed-shell singlet) versus Cu(II) and neutral GFA ligand (triplet or broken-symmetry state). The closed-shell singlet state is disfavoured by hybrid-DFT functionals, which mix in exact Hartree-Fock exchange, and is favoured by larger basis sets and consideration of a polar medium.
authors.affiliation: Anorganisch-Chemisches Institut, Ruprecht-Karls Universität Heidelberg, Im Neuenheimer Feld 270, 69120 Heidelberg (Germany), Fax: (+49) 6221-545707.
authors.forename: Sven
authors.initials: S
authors.lastname: Wiesner
authors.affiliation: Anorganisch-Chemisches Institut, Ruprecht-Karls Universität Heidelberg, Im Neuenheimer Feld 270, 69120 Heidelberg (Germany), Fax: (+49) 6221-545707.
authors.forename: Arne
authors.initials: A
authors.lastname: Wagner
authors.affiliation: Anorganisch-Chemisches Institut, Ruprecht-Karls Universität Heidelberg, Im Neuenheimer Feld 270, 69120 Heidelberg (Germany), Fax: (+49) 6221-545707.
authors.forename: Olaf
authors.initials: O
authors.lastname: Hübner
authors.affiliation: Anorganisch-Chemisches Institut, Ruprecht-Karls Universität Heidelberg, Im Neuenheimer Feld 270, 69120 Heidelberg (Germany), Fax: (+49) 6221-545707.
authors.forename: Elisabeth
authors.initials: E
authors.lastname: Kaifer
authors.affiliation: Anorganisch-Chemisches Institut, Ruprecht-Karls Universität Heidelberg, Im Neuenheimer Feld 270, 69120 Heidelberg (Germany), Fax: (+49) 6221-545707. hans-jorg.himmel@aci.uni-heidelberg.de.
authors.forename: Hans-Jörg
authors.initials: HJ
authors.lastname: Himmel
keywords: charge-transfer; coordination compounds; copper; electronic structure; guanidine; redox-active ligands