Anomalous Thermal Expansion and Luminescence Thermochromism in Silver(I) Dicyanamide

Regulating luminescence thermal enhancement in negative thermal expansion metal-organic frameworks

Overcoming thermal quenching is generally essential for the practical application of luminescent materials. It has been recently found that frameworks with negative thermal expansion (NTE) could be a promising candidate to engineer unconventional luminescence thermal enhancement. However, the mechanism through which luminescence thermal enhancement can be well tuned remains an open issue. In this work, enabled by altering ligands in a series of UiO-66 derived Eu-based metal-organic frameworks, it was revealed that the changes in the thermal expansion are closely related to luminescence thermal enhancement. The NTE of the aromatic ring part favors luminescence thermal enhancement, while contraction of the carboxylic acid part plays the opposite role. Modulation of functional groups in ligands can change the thermal vibration of aromatic rings and then achieve luminescence thermal enhancement in a wide temperature window. Our findings pave the way to manipulate the NTE and luminescence thermal enhancement based on ligand engineering.

Thermal Enhancement of Luminescence for Negative Thermal Expansion in Molecular Materials

Overcoming thermal quenching is an essential issue in the practical application of luminescent materials. Herein, we found that negative thermal expansion (NTE) can achieve the thermal enhancement of luminescence in molecular materials based on three metal-organic frameworks CuX-bpy (X = Cl, Br, I; bpy = 4,4'-bipyridine). All complexes exhibit NTE on the c-axis, and the strongest NTE leads to a contraction of the Cu...Cu distance in CuCl-bpy, which further intensifies the luminescence emission. This phenomenon indicates the existence of thermally enhanced charge transfer. Moreover, the origin of the distinction in charge transfer attributed to the different valence states of the copper is investigated through the combined studies of X-ray photoelectron spectroscopy, X-ray absorption near-edge structure, and density functional theory calculations. This research provides a new approach to modulating the luminescence thermal enhancement by NTE.

Syntheses and Characterization of Two Dicyanamide Compounds Containing Monovalent Cations: Hg2[N(CN)2]2 and Tl[N(CN)2]

Crystals of Hg2[N(CN)2]2 were grown by a slow diffusion-reaction between aqueous Hg2(NO3)2·2H2O and Na[N(CN)2]. Hg2[N(CN)2]2 adopts the triclinic space group P 1 ¯ (no. 2) with a = 3.7089(5), b = 6.4098(6), c = 8.150(6) Å, α = 81.575(6)°, β = 80.379(7)°, γ = 80.195(7)°, and Z = 1. Crystals of Tl[N(CN)2] were obtained from the reaction of TlBr with Ag[N(CN)2] in water. Single-crystal structure analyses evidence that Tl[N(CN)2] is isotypic to α-K[N(CN)2] and adopts the orthorhombic space group Pbcm (no. 57) with a = 8.5770(17), b = 6.4756(13), c = 7.2306(14) Å, and Z = 4. Regarding volume chemistry, the dicyanamide anion occupies ca. 44 cm3·mol−1, and so it corresponds to a large pseudohalide. The IR spectra of both compounds exhibit vibrational modes that are characteristic of the dicyanamide anion.

Negative thermal expansion in molecular materials

Some mechanisms resulting in negative thermal expansion in molecular materials are summarized.

About the air- and water-stable copper(I) dicyanamide: synthesis, crystal structure, vibrational spectra and DSC/TG analysis of Cu[N(CN)2]

Light-yellow microcrystalline samples of Cu[dca] ([dca]−≡[N(CN)2]−≡dicyanamide anion) were obtained by blending an in-situ generated aqueous Cu+ brine with stoichiometric amounts of Na[dca] dissolved in water. The crystal structure of Cu[dca] was solved and refined from powder X-ray diffraction (PXRD) data. Cu[dca] crystallizes in the orthorhombic space group Cmcm (no. 63) with the lattice parameters of a=356.28(3), b=611.10(9) and c=1525.87(10) pm. The crystal structure contains undulated chains of alternating Cu+ and boomerang-shaped [N≡C–N–C≡N]− ions with C 2v symmetry running along [100]. It is closely related to that of Ag[dca] crystallizing in space group Pnma (no. 62). The vibrational spectra for Cu[dca] and Cu[dca]2 were recorded exhibiting modes typical for the dicyanamide anion. Comparative DSC/TG measurements were performed for both copper dicyanamides and the cyanide Cu[CN].