Unlocking Arene Phosphorescence in Bismuth-Organic Materials.

Three novel bismuth-organic compounds, with the general formula [Bi2(HPDC)2(PDC)2]·(arene)·2H2O (H2PDC = 2,6-pyridinedicarboxylic acid; arene = pyrene, naphthalene, and azulene), that consist of neutral dinuclear Bi-pyridinedicarboxylate complexes and outer coordination sphere arene molecules were synthesized and structurally characterized. The structures of all three phases exhibit strong π-π stacking interactions between the Bi-bound PDC/HPDC and outer sphere organic molecules; these interactions effectively sandwich the arene molecules between bismuth complexes and thereby prevent molecular vibrations. Upon UV irradiation, the compounds containing pyrene and naphthalene displayed red and green emission, respectively, with quantum yields of 1.3(2) and 30.8(4)%. The emission was found to originate from the T1 → S0 transition of the corresponding arene and result in phosphorescence characteristic of the arene employed. By comparison, the azulene-containing compound displayed very weak blue-purple phosphorescence of unknown origin and is a rare example of T2 → S0 emission from azulene. The pyrene- and naphthalene-containing compounds both display radioluminescence, with intensities of 11 and 38% relative to bismuth germanate, respectively. Collectively, these results provide further insights into the structure-property relationships that underpin luminescence from Bi-based materials and highlight the utility of Bi-organic molecules in the realization of organic emission.

Efficient Europium Sensitization via Low-Level Doping in a 2-D Bismuth-Organic Coordination Polymer.

A new bismuth-organic compound containing 1,10-phenanthroline (phen) and 2,5-pyridinedicarboxylic acid (PDC) was synthesized and structurally characterized by single-crystal X-ray diffraction. The structure consists of 2-D {Bi(phen)(HPDC)(PDC)}n sheets wherein the PDC ligands bridge metal centers via three unique bonding modes. The 2-D sheets are further connected through strong hydrogen-bonding interactions to form a 3-D supramolecular network. The parent compound displayed yellow photoluminescence in the solid state at room temperature. Doping studies were undertaken to incorporate Eu3+ into the structure, statistically replacing Bi3+ in small quantities (1, 5, and 10 mol % Eu3+ relative to Bi3+). All three compounds displayed characteristic Eu3+ emission, with total quantum yields as high as 16.0% and sensitization efficiencies between 0.21 and 0.37 depending on the Eu3+ doping percentage.

Lanthanide Luminescence and Thermochromic Emission from Soft-Atom Donor Dichalcogenoimidodiphosphinate Ligands.

The luminescence properties of two divalent europium complexes of the type Eu[N(SPPh2)2]2(THF)2 (1) and Eu[N(SePPh2)2]2(THF)2 (2) were investigated. The first complex, Eu[N(SPPh2)2]2(THF)2 (1), was found to be isomorphous with the reported structure of complex 2 and exhibited room temperature luminescence with thermochromic emission upon cooling. We found the complex Eu[N(SePPh2)2]2(THF)2 (2) was also thermochromic but the emission intensity was sensitive to temperature. Both room temperature and low temperature (100 K) single crystal X-ray structural investigation of 1 and 2 indicate geometric distortions of the metal coordination, which may be important for understanding the thermochromic behavior of these complexes. The trivalent europium complex Eu[N(SPPh2)2]3 (3) with the same ligand as 1 was also structurally characterized as a function of temperature and exhibited temperature-dependent luminescence intensity, with no observable emission at room temperature but intense luminescence at 77 K. Variable temperature Raman spectroscopy was used to determine the onset temperature of luminescence of Eu[N(SPPh2)2]3 (3), where the 615 nm (5D0 → 7F2 transition) peak was quenched above 130 K. The UV-visible diffuse reflectance of 3 provides evidence of an LMCT band, supporting a mechanism of thermally activated LMCT quenching of Eu(III) emitting states. A series of ten isomorphous, trivalent lanthanide complexes of type Ln[N(SPPh2)2]3 (Ln = Eu (3) Pr (4), Nd (5), Sm (6), Gd (7), Tb (8)) and Ln[N(SePPh2)2]3 (Ln = Pr (9), Nd (10, structure was previously reported), Sm (11), and Gd (12) for Q = Se) were also synthesized and structurally characterized. These complexes for Ln = Pr, Nd, Sm, and Tb exhibited room temperature luminescence. This study provides examples of temperature-dependent luminescence of both Eu2+ and Eu3+, and the use of soft-atom donor ligands to sensitize lanthanide luminescence in a range of trivalent lanthanides, spanning near IR and visible emitters.

A spectroscopic, structural, and computational study of Ag-oxo interactions in Ag+/UO22+ complexes.

Twelve novel Ag+/UO22+ heterometallic complexes have been prepared and characterized via structural, spectroscopic, and computational methods to probe the effects of Ag-oxo interactions on bonding and photophysical properties of the uranyl cation. Structural characterization reveals Ag-oxo interaction distances ranging from 2.475(3) Å to 4.287(4) Å. These interactions were probed using luminescence and Raman spectroscopy which displayed little effect on the luminescence intensity and the energy of the Raman active UO symmetric stretch peak as compared to previously reported Pb-oxo interactions. Computational efforts via density functional theory-based natural bond orbital analysis revealed that the highest stabilization energy associated with the Ag-oxo interaction had a value of only 11.03 kcal mol-1 and that all other energy values fell at 7.05 kcal mol-1 or below indicating weaker interactions relative to those previously reported for Pb2+/UO22+ heterometallic compounds. In contrast, quantum theory of atoms in molecules analysis of bond critical point electron density values indicated higher electron density in Ag-oxo interactions as compared to Pb-oxo interactions which suggests more covalent character with the Ag+. Overall, this data indicates that Ag+ has a less significant effect on UO22+ bonding and photophysical properties as compared to other Pb2+, likely due to the high polarizability of the cation.

Harnessing Bismuth Coordination Chemistry to Achieve Bright, Long-Lived Organic Phosphorescence.

A new bismuth(III)-organic compound, Hphen[Bi2(HPDC)2(PDC)2(NO3)]·4H2O (Bi-1; PDC = 2,6-pyridinedicarboxylate and phen = 1,10-phenanthroline), was synthesized, and the structure was determined by single-crystal X-ray diffraction. The compound was found to display bright-blue-green phosphorescence in the solid state under UV irradiation, with a luminescent lifetime of 1.776 ms at room temperature. The room temperature and low-temperature (77 K) emission spectra exhibited the vibronic structure characteristic of Hphen phosphorescence. Time-dependent density functional theory studies showed that the excitation pathway arises from an energy transfer from the dimeric structural unit to Hphen, with participation from a nine-coordinate Bi center. The triplet state of Hphen is believed to be stabilized via supramolecular interactions, which, when coupled with the heavy-atom effect induced by Bi, leads to the observed long-lived luminescence. The compound displayed a solid-state quantum yield of over 27%. To the best of our knowledge, this is the first such compound to exhibit phenanthrolinium phosphorescence with such long-lived, room temperature lifetimes in the solid state. To further elucidate the energy-transfer mechanism, Ln3+ (Ln = Eu, Tb, Sm) ions were successfully doped into the parent compound, and the resulting materials exhibited dual emission from Hphen and Ln, promoting tunability of the emission color.

Impact of Noncovalent Interactions on the Structural Chemistry of Thorium(IV)-Aquo-Chloro Complexes.

Five novel tetravalent thorium (Th) compounds that consist of Th(H2O)xCly structural units were isolated from acidic aqueous solutions using a series of nitrogen-containing heterocyclic hydrogen (H) bond donors. Taken together with three previously reported phases, the compounds provide a series of monomeric ThIV complexes wherein the effects of noncovalent interactions (and H-bond donor identity) on Th structural chemistry can be examined. Seven distinct structural units of the general formulas [Th(H2O)xCl8-x]x-4 (x = 2, 4) and [Th(H2O)xCl9-x]x-5 (x = 5-7) are described. The complexes range from chloride-deficient [Th(H2O)7Cl2]2+ to chloride-rich [Th(H2O)2Cl6]2- species, and theory was used to understand the relative energies that separate complexes within this series via the stepwise chloride addition to an aquated Th cation. Electronic structure theory predicted the reaction energies of chloride addition and release of water through a series of transformations, generally highlighting an energetic driving force for chloride complexation. To probe the role of the counterion in the stabilization of these complexes, electrostatic potential (ESP) surfaces were calculated. The ESP surfaces indicated a dependence of the chloride distribution about the Th metal center on the pKa of the countercation, highlighting the directing effects of noncovalent interactions (e.g., Hbonding) on Th speciation.

Synthesis and photoluminescence of three bismuth(III)-organic compounds bearing heterocyclic N-donor ligands.

Three bismuth(iii)-organic compounds, [Bi4Cl8(PDC)2(phen)4]·2MeCN (1), [BiCl3(phen)2] (2), and [Bi2Cl6(terpy)2] (3), were prepared from solvothermal reactions of bismuth chloride, 2,6-pyridinedicarboxylic acid (H2PDC), and 1,10-phenanthroline (phen) or 2,2';6',2''-terpyridine (terpy). The structures were determined through single crystal X-ray diffraction and the compounds were further characterized via powder X-ray diffraction, Raman and infrared spectroscopy, and thermogravimetric analysis. The photoluminescence properties of the solid-state materials were assessed using steady state and time-dependent techniques to obtain excitation and emission profiles as well as lifetimes. The compounds exhibit visible emission ranging from the yellow-green to orange region upon UV excitation. Theoretical quantum mechanical calculations aimed at elucidating the observed emissive behavior show that the transitions can be assigned as predominantly ligand-to-ligand and ligand-to-metal charge transfer transitions. The solid-state structural chemistry, spectroscopic properties, and luminescence behavior of the bismuth compounds are presented herein.