Thermochromic Uranyl Isothiocyanates: Influencing Charge Transfer Bands with Supramolecular Structure

• Published in: Inorganic chemistry 2018-03, Vol.57 (5), p.2455-2471
• Main Authors: Ducati, Lucas C, Autschbach, Jochen, Deifel, Nicholas P, Cahill, Christopher L
• Format: Article
• Language: English
• Subjects: Chemistry
• ISSN: 0020-1669; 1520-510X
• DOI: 10.1021/acs.inorgchem.7b02702

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···Oyl 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.