Synthesis, Structure and Near Infrared Fluorescence Property of a New Nd-MOF Based on a Triangular Benzylamine Ligand

A new 3D metal–organic framework (Nd-MOF) {[Nd2L2]·2NH2(CH3)2·3H2O} was successfully established via a solvothermal method with Nd3+ ion and 5-(bis(4-carboxybenzyl) amino)–isophthalicacid (H4L), and has also been characterized by X-ray diffraction, powder X-ray diffraction (PXRD), IR and photoluminescence(PL)spectrum. The neodymium ions are free of coordinated solvents, and the Nd-MOF exhibits strong near-infrared (NIR) fluorescence. Besides, Its NIR fluorescence property shows low temperature resistance, which is favorable for being used in the low temperature environment. Besides, the fluorescence lifetime of Nd-MOF is 6.03 μs, and the quantum yield is 1.2%. The small quantum yield may owe to large energy gap between the T1 of the ligand H4L and the resonance energy level 4F3/2 of the Nd3+ ion, or due to large crystal size of the Nd-MOF.


Introduction
Fluorescence lanthanide metal organic frameworks (Ln-MOFs), a member of metal-organic frameworks (MOFs), combine the advantages of both MOFs and the intrinsic luminescence properties of lanthanides, such as characteristic sharply emission peaks, strong luminescence intensity, high fluorescent quantum efficiency, wide spectrum scope from ultraviolet (UV) to near-infrared (NIR) [1][2][3]. Furthermore, NIR fluorescence materials have attracted more and more interest as the functional materials, for its higher tissue penetration depth, signal-to-noise ratio, sensitivity and less damage to biological samples in biological detection [4][5][6] and biological imaging [7][8][9]. Besides, the invisible to the naked eye of the infrared makes it a special material for barcoding [10,11].
So near-infrared fluorescence Ln-MOFs may be a much better candidate for the application in biological detection [12,13], biological imaging [14] and barcoding [15].
However, the near-infrared emission signals of Ln-MOFs are easily quenched due to nonradiative relaxation when there are N-H, O-H, and C-H oscil lators on ligands and coordinated solvents which are located close to lanthanide ions [16,17]. So, it's difficult to obtain a NIR fluorescence Ln-MOFs with much strong luminescence intensity.

Materials and Instruments
Common reagents were purchased from general commercial channels and used without further purification. The compound was synthesized under solvothermal conditions. Elemental analyses (C, H, N) were tested on a Vario El-Cube elemental analyser. The powder X-ray diffraction (PXRD) data were collected on a Rigaku MiniFlex II diffractometer by CuKα radiation. Simulated PXRD pattern was derived from the Mercury Version 3.10.2 software using the X-ray single crystal diffraction data. The infrared spectrum was measured on FTIR-650 infrared spectrometer (KBr pellet). All fluorescence emission spectra, lifetime and absolute quantum yield were recorded by the Edinburgh Instruments FLS980P fluorescence spectrometer, and absolute quantum yield with an integrating sphere (BaSO 4 coating), low temperature fluorescence emission spectra with optistatDN2.

Single-crystal X-ray Diffraction
The crystallographic data of the Nd-MOF was collected on a Bruker SMARTAPEX CCD diffractometer with MoKa radiation (λ = 0.71073 Å). The crystal structure was solved by direct methods, refined through anisotropic thermal parameters using full-matrix least-squares techniques based on F 2 values with Shelxtl-2014 [18] and combined with the program OLEX2 [19]. The crystallographic data and structure refinement parameters of Nd-MOF were summarized in Table 1, and part of the bond distances and angles were listed in the Table S1. The CCDC number of Nd-MOF is 2121479.

Description of Crystal Structure
Nd-MOF crystallizes in the monoclinic space group P2/c. All the Nd 3+ have the same coordination environment, and are nine coordinated in a Muffin configuration by nine oxygen atoms from six different ligands (Fig. 1). And two Nd 3+ display a symmetric binuclear structure connecting with nine carboxyl, among which four carboxyl are chelated to one Nd 3+ , two carboxyl are bridged with binuclear metals, and another two carboxyl are both chelated with one Nd 3+ and bridged with the other one respectively (Figs. S2 and S3). Besides, every ligand is further connected with other four binuclear structure to generate a 3D anionic network with Me 2 NH 2 + filling in the channels (Fig. 2).

Powder X-ray Diffraction
The powder X-ray diffraction (PXRD) pattern of the Nd-MOF is well consistent with the simulated one calculated from the single crystal diffraction data, which indicates the phase purity of the sample is much high (Fig. 3).

Fluorescent Properties
The solid state fluorescence emission spectrum of the Nd-MOF was investigated at room temperature. As shown in Fig. 4, upon excited at 370 nm, Nd-MOF exhibits strong NIR fluorescence, and the three major characteristic emission bands (882 nm, 1060 nm, 1330 nm) are ascribed to the 4 F 3/2 -4 I J (J = 9/2, 11/2, 13/2,). The obvious fluorescent signal of Nd 3+ suggests that the Nd 3+ ions are sensitized successfully by ligands, and intramolecular energy transfer occur from the triplet excited state (T 1 ) of H 4 L to the resonance levels of the Nd 3+ [20]. Besides, there is no coordinated solvent in the structure, which can reduce some energy loss [21]. We also invested the temperature stability of fluorescence, and measured the fluorescence emission spectra from 110 K to 290 K (Fig. 5), and found that the fluorescence intensities are much stable and show less change (Fig. 6) [22,23], which may be favorable for being used in the low temperature environment, such as biological detection [12,13], biological imaging [14], barcoding [15] and so on.

Fluorescence Lifetime of Nd-MOF
The fluorescence lifetime for Nd-MOF was measured with excitation wavelength 370 nm and emission wavelength 1060 nm. The result was shown in Fig. 7, and the lifetime is 6.03 μs, which is comparable with other Nd-MOFs (Table S2).

Fluorescence Quantum Yield of Nd-MOF
The absolute luminescence quantum yield of Nd-MOF was measured using integrating sphere with excitation   Table S3]. And the reason may be the inefficient energy transfer for Nd-MOF [24] and large crystal size [25]. In our previous work, the T 1 of the H 4 L is 28406 cm −1 , and the resonance energy level 4 F 3/2 of the Nd 3+ ion is 11,690 cm −1 , energy gap ΔE 11,567 cm −1 , which is too large to waste the energy from ligands in the nonradiative intersystem crossing [26,27]. Besides, the crystallite size of the Nd-MOF is 500 ~ 1000 nm [ Fig. S4], which may be related to the quantum yield [25].

Conclusion
In this study, a new 3D porous Nd-MOF has been fabricated successfully, in which every Nd 3+ atom are nine coordinated in a Muffin configuration with nine carboxyl oxygen atoms and are free of coordinated solvents. The Nd-MOF shows strong near-infrared (NIR) fluorescence because of the antenna effect. And the fluorescence intensities show less change versus temperature from 110 K to 290 K, which shows it is low temperature resistance and may be a much better candidate for the application in biological detection [12,13], biological imaging [14], barcoding [15] in low temperature. Besides, the fluorescence lifetime of Nd-MOF is 6.03 μs, and the quantum yield is 1.2%. The small quantum yield may owe to large energy gap between the T1 of the ligand H 4 L and the resonance energy level 4 F 3/2 of the Nd 3+ ion, or due to large crystal size of the Nd-MOF.