Structure and dynamics of copper complexes with 2,2′:6′,2"-terpyridines in glassy matrices

Continuous-wave and pulse electron paramagnetic resonance (EPR) as well as electron-nuclear double resonance (ENDOR) techniques are applied for determination of the electronic and geometric structure of copper(II) complexes with terpyridine and related ligands that are relevant in the context of supramolecular chemistry. The results are analysed in conjunction with density functional theory Computations and are compared to the crystal structure of the bis(terpyridyl) copper(II) complex in the limit of static Jahn-Teller distortion (R. Allmann, W. Henke and D. Reinen, Inorg. Chem. 1978, 17, 378, ref. 6). The static structure in disordered environments is subject to some strain in both the g-values and copper-ligand distances, but otherwise is rather similar to the structure in crystals. The formation of coordination oligomers is indicated by broadening of the lineshape and a decrease in the transverse relaxation time at low fields which are both related to exchange coupling between copper centres. At high fields of approximately 3 T and a temperature of 15 K, the transverse relaxation rate is governed by modulation of the g-values induced by small-amplitude libration along the Jahn-Teller active mode. A study of the dynamics in a temperature range from below the glass transition temperature to above the melting point of ethanol by CW EPR reveals that the complex is a sensitive probe for matrix dynamics, which detects dynamic heterogeneities and the transition from the structural glass to the crystalline phase. Jahn-Teller dynamics is completely unfrozen only on melting of the matrix.