Investigating the Structural Features and Spectroscopic Properties of Bis(tetrazolato)-Based Coordination Polymers

The luminescent compound xylene-bis(2H-tetrazol-5-yl) (H2BTZPX) was prepared and employed in the construction of the novel coordination polymers (CPs) Zn(BTZPX), Ag2(BTZPX), and Hg(BTZPX), containing d10 transition metal ions. Powder X-ray diffraction (PXRD) structure determination enabled us to disclose the crystal and molecular structure of the three CPs: while Zn(BTZPX) features a 3-D polymeric network, Ag2(BTZPX) and Hg(BTZPX) show 2-D corrugated layers. Thermogravimetric analysis and variable-temperature PXRD revealed the appreciable thermal robustness of the three CPs, peaking up to 370 °C, in air, in the case of Ag2(BTZPX). Given the sizable fluorescence emission of the ligand in the solid state, both H2BTZPX and the three CPs were characterized as to their electronic-state transition spectroscopic properties. UV–visible absorption unveiled a bathochromic shift for both the ligand and the CPs with respect to the absorption maximum of the main chromophore of the spacer, i.e., the benzene ring. In order to understand this peculiarity, quantum mechanical calculations were performed using the time-dependent density-functional theory approach. Furthermore, absorption spectra of H2BTZPX in different organic solvents were recorded. To investigate how the luminescence properties of H2BTZPX are influenced by complexation, the fluorescence emission spectra of H2BTZPX and the three CPs were recorded, and the fluorescence quantum yields determined by comparison to anthracene. Zn(BTZPX) exhibited enhanced fluorescence with respect to the ligand, while fluorescence was notably reduced for Ag2(BTZPX) and barely detectable for Hg(BTZPX). This occurrence suggests different decay pathways, which were further investigated by reconstructing the time-resolved fluorescence decays by means of time-correlated single-photon counting.