Thermochromic Uranyl Isothiocyanates: Influencing Charge Transfer Bands with Supramolecular Structure

The synthesis and structural characterization of seven new [UO<sub>2</sub>(NCS)<sub>5</sub>]<sup>3–</sup>- and [UO<sub>2</sub>(NCS)<sub>4</sub>Cl]<sup>3–</sup>-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<sub>yl</sub> interactions. The electrostatic potential of the [UO<sub>2</sub>(NCS)<sub>5</sub>]<sup>3–</sup> and [UO<sub>2</sub>(NCS)<sub>4</sub>Cl]<sup>3–</sup> 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 <b>1</b>–<b>7</b> 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 <i>inter-</i> to <i>intra</i>molecular 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 ν<sub>1</sub> UO symmetric stretch and bending modes of the isothiocyanate ligands.