Thermochromic Uranyl Isothiocyanates: Influencing Charge Transfer Bands with Supramolecular Structure

The synthesis and structural characterization of seven new [UO₂(NCS)₅]^3-- and [UO₂(NCS)₄Cl]^3--containing materials charge balanced by 4-phenylpyridinium or 4,4'-bipyridinium cations are reported. Assembly of these materials occurs via a diverse set of noncovalent interactions, with the most prevalent involving the terminal sulfur atoms, which can both accept hydrogen bonds and/or form S···S and S···O_yl interactions. The electrostatic potential of the [UO₂(NCS)₅]^3- and [UO₂(NCS)₄Cl]^3- anions was calculated and mapped on the 0.001 au isodensity surface to rationalize the observed assembly modes and to provide an electrostatic basis to elucidate the role of the S atoms as both donors and acceptors of noncovalent interactions. Compounds 1-7 display a range of colors (red to yellow) as well as pronounced thermochromism. A computational treatment (time-dependent density functional theory, TDDFT) of the absorbance properties supports the temperature dependence on the ratio of inter- to intramolecular ligand to metal charge transfer (LMCT) bands as obtained from UV-vis diffuse reflectance analysis. Finally, the luminescence profiles of these materials feature additional peaks atypical for most uranyl-containing materials, and a combined spectroscopic (Raman, IR, and fluorescence) and computational (harmonic frequency calculations) effort assigns these as originating from vibronic coupling between the ν₁ U═O symmetric stretch and bending modes of the isothiocyanate ligands.