Fluorescence Properties and Density Functional Theory Calculation of a Structurally Characterized Heterotetranuclear [Zn II2 –Sm III2 ] 4,4 (cid:48) -Bipy-Salamo-Constructed Complex

: A new heterotetranuclear complex, validated via elemental analysis, powder X-ray di ﬀ raction (PXRD) analysis, infrared spectroscopy, and ultraviolet–visible (UV–Vis) absorption spectroscopy. The X-ray single crystal di ﬀ raction analysis of the [Zn II2 –Sm III2 ] complex was carried out via X-ray single-crystal crystallography. The crystal structure and supramolecular features were discussed. In addition, while studying the ﬂuorescence properties of the [Zn II2 –Sm III2 ] complex, the density functional theory (DFT) calculation of its structure was also performed.


General Details
2-Hydroxy-3-Methoxybenzaldehyde (99%) was purchased from Meryer Chemical Technology Co., Ltd., 3-Ethoxysalicylaldehyde (>97%) was purchased from Tokyo Chemical Industry Co., Ltd., and other reagents and solvents were purchased from the Tianjin Chemical Reagent Factory and used as received without further purification. X-ray single-crystal diffraction data of the [Zn II 2 -Sm III 2 ] complex were measured and recorded using a Bruker APEX-II CCD surface inspection diffractometer (Bruker, Germany). The PXRD experiment with the [Zn II 2 -Sm III 2 ] complex was recorded in the range of 2θ = 5-50 • using a D/max-2400 powder X-ray diffractometer (Rigaku, Japan  [33,35,47,54]. The test methods of other instruments used in this paper are the same as those in the previous literature [27,66].

Synthesis of H 2 L
As depicted in Scheme 1, 1,2-bis(aminooxy)ethane was obtained according to a previously reported method [66]

X-Ray Crystallography Analysis
The description of X-ray crystallography is presented in the Supplementary Information (SI). Crystallographic data of the [Zn II 2-Sm III 2] complex and the refinement parameters are presented in Table 1. CCDC: 1959376.

Synthesis of the [Zn II 2 -Sm III 2 ] Complex
A solution of H 2 L (7.5 mg, 0.02 mmol) in dichloromethane (2 mL) was added to Zn(OAc) 2 ·2H 2 O (4.50 mg, 0.02 mmol) and Sm(NO 3 ) 3 ·6H 2 O (9.0 mg, 0.02 mmol) in methanol (4 mL). After the mixture was stirred for about 10 minutes, a solution of 4,4 -bipyridine (3.50 mg, 0.02 mmol) in methanol (2 mL) was added and continued to be stirred for an additional 10 minutes. The mixture was filtered off and the filtrate was sealed with a foil paper. It was placed in an undisturbed environment for about two weeks, and transparent white block-like single crystals suitable for X-ray crystallographic analysis were obtained. Anal.

X-ray Crystallography Analysis
The description of X-ray crystallography is presented in the Supplementary Information (SI). Crystallographic data of the [Zn II 2 -Sm III 2 ] complex and the refinement parameters are presented in Table 1. CCDC: 1959376.

PXRD Analysis
The PXRD experiment with the [Zn II 2 -Sm III 2 ] complex was performed with an X-ray diffractometer using Cu-Kα radiation (λ = 0.154 nm) in the range of 2θ = 5-50 • . A comparison of the simulated and experimental PXRD patterns of the [Zn II 2 -Sm III 2 ] complex is shown in Figure 1. The experimental pattern was in good agreement with the simulated pattern, which confirmed the phase purity and isomorphism of the [Zn II 2 -Sm III 2 ] complex. It further illustrated that the [Zn II 2 -Sm III 2 ] complex had sufficient purity to be used to study its spectral characteristics and fluorescence properties.

PXRD Analysis
The PXRD experiment with the [Zn II 2-Sm III 2] complex was performed with an X-Ray diffractometer using Cu-Kα radiation (λ = 0.154 nm) in the range of 2θ = 5-50°. A comparison of the simulated and experimental PXRD patterns of the [Zn II 2-Sm III 2] complex is shown in Figure 1. The experimental pattern was in good agreement with the simulated pattern, which confirmed the phase purity and isomorphism of the [Zn II 2-Sm III 2] complex. It further illustrated that the [Zn II 2-Sm III 2] complex had sufficient purity to be used to study its spectral characteristics and fluorescence properties.

IR Spectra
The infrared spectra of H 2 L and the [Zn II 2 -Sm III 2 ] complex are shown in Figure S1 (Supplementary Materials) and

UV-Vis Spectra
The UV-Vis absorption spectra of H 2 L and the [Zn II 2 -Sm III 2 ] complex were measured at room temperature in concentrations of 1.0 × 10 −5 M, respectively. As depicted in Figure 2, the absorption spectrum of H 2 L mainly exhibited two relatively strong absorption peaks, which appeared at approximately 270 and 319 nm. The former absorption peak at 270 nm was part of the π-π* transition of the benzene ring conjugate system, while the latter absorption peak at 319 nm was part of the π-π* transition of the chromophore C=N groups. In the [Zn II 2 -Sm III 2 ] complex, the absorption peak formed by the π-π* transition of the benzene ring conjugate system still existed, but only moved to a high wave number at 3 nm. The absorption peak at 319 nm disappeared, and a new absorption peak appearing at approximately 343 nm was found in the [Zn II 2 -Sm III 2 ] complex. This peak may be part of the L→M charge-transfer transition [68], which is characteristic absorption peak of an N 2 O 2 -donor metal complex.

Crystal Structure and Supramolecular Interactions
As depicted in Figure 3a, two chemically and crystallographically identical dinuclear [Zn(L)Sm] units and one 4,4′-bipyridine linker were self-assembled to obtain a heterotetranuclear [Zn II 2-Sm III 2] salamo-like complex [{Zn(L)Sm(NO3)3}2(4,4′-bipy)]·2CH3OH. The coordination polyhedrons are depicted in Figure 3b. The coordination environments of the two Zn II and Sm III ions were the same in each heterobinuclear [Zn(L)Sm(NO3)3] unit. Compared with the previously reported Salamo-type Zn-La complex [65], the coordination environments of Zn II ions are similar, while the coordination environments of Sm III ions are different. The penta-coordinated Zn II ion (Zn1) was located at the N2O2 coordination cavity (N1, N2, O2, and O5) of the (L) 2− unit, and the axial position was occupied by the nitrogen atom (N6) of 4,4′-bipyridine, bearing a slightly twisted square pyramidal coordination geometry with τ = 0.1845 [27,69]. The four atoms (N1, N2, O2, and O5) in the N2O2 cavity of the ligand formed an equatorial plane of the square pyramid, and the nitrogen atom (N6) on the pyridine ring occupied the apex of the square pyramid. The deca-coordinated Sm III ion (Sm1) was coordinated to the O4 coordination environment (O2, O5, O1, and O6) of the (L) 2− unit, and the six oxygen atoms (O7, O8, O10, O11, O13, and O14) from the three bidentate nitrates. Therefore, the Sm III ion (Sm1) bore a twisted double-capped tetragonal anti-prism geometry. The 4,4′-bipyridine

Fluorescent Properties
The solid fluorescence spectrum of the [Zn II 2 -Sm III 2 ] complex is shown in Figure 5. This emission spectrum showed that the [L-Zn 2+ ] units and rare-earth ions (Sm III ) exhibited co-luminescence in the solid state.

Fluorescent Properties
The solid fluorescence spectrum of the [Zn II 2-Sm III 2] complex is shown in Figure 5. This emission spectrum showed that the [L-Zn 2+ ] units and rare-earth ions (Sm III ) exhibited co-luminescence in the solid state. The ethanol solutions of H2L and the [Zn II 2-Sm III 2] complex were prepared in concentrations of 1.0 × 10 −5 M, respectively, and the fluorescence spectra were measured at an excitation wavelength of 320 nm at room temperature (Figure 6a). When the excitation wavelength was 320 nm, the emission spectrum of the ligand H2L exhibited a broad emission band at 397 nm, which can be assigned to the π-π* transitions in the ligand H2L. For the [Zn II 2-Sm III 2] complex, in addition to a large broad peak at 401 nm, several visible emission bands of Sm III ions as the lanthanide ions were observed at 564, 598, and 644 nm. These peaks correspond to the energy level transitions of 4 G5/2-6 HJ (J = 5/2, 7/2, 9/2), respectively. The appearance of characteristic emission peaks indicated that the ligand H2L can act as an antenna group, sensitizing the emission of Sm III ions through the [L-Zn 2+ ] units, thereby making the luminescence of Sm III ions sensitive [27]. The ethanol solutions of H 2 L and the [Zn II 2 -Sm III 2 ] complex were prepared in concentrations of 1.0 × 10 −5 M, respectively, and the fluorescence spectra were measured at an excitation wavelength of 320 nm at room temperature (Figure 6a). When the excitation wavelength was 320 nm, the emission spectrum of the ligand H 2 L exhibited a broad emission band at 397 nm, which can be assigned to the π-π* transitions in the ligand H 2 L. For the [Zn II 2 -Sm III 2 ] complex, in addition to a large broad peak at 401 nm, several visible emission bands of Sm III ions as the lanthanide ions were observed at 564, 598, and 644 nm. These peaks correspond to the energy level transitions of 4 G 5 / 2 -6 H J (J = 5/2, 7/2, 9/2), respectively. The appearance of characteristic emission peaks indicated that the ligand H 2 L can act as an antenna group, sensitizing the emission of Sm III ions through the [L-Zn 2+ ] units, thereby making the luminescence of Sm III ions sensitive [27].
The fluorescence titration spectra of H 2 L and the [Zn II 2 -Sm III 2 ] complex in ethanol solution are depicted in Figure 6b,c. In the fluorescence titration experiment with the [Zn II 2 -Sm III 2 ] complex, the fluorescence emission peak intensity decreased gradually during the addition of Zn II ions (1.0 × 10 −3 M) to the free ligand H 2 L (1.0 × 10 −5 M). When Zn II ions started from 5 µL and gradually increased to 55 µL in increments of 5 µL, the fluorescence intensity began to stabilize and the titration reached the end point, indicating that the coordination of Zn II ions with H 2 L was completed and the stoichiometric ratio was 1:1. Immediately thereafter, when the amount of Sm III ions added reached the equivalent of 1.0 equivalent, the fluorescence intensity was no longer lowered, indicating the coordination of the Sm III ions with the [L-Zn 2+ ] units. This titration curve also clearly indicated that the stoichiometric ratio of H 2 L:Zn II :Sm III was 1:1:1. This result also corresponds to the coordination of the actually obtained crystal structure.

Fluorescence Print Imaging
From the fluorescence spectra, Sm III ions were effectively sensitized, and exhibited characteristic excitation and emission spectra, so the [Zn II 2 -Sm III 2 ] complex had the pink light that is characteristic emission of Sm III ions. Therefore, we attempted to refer this special property to fluorescent print imaging. As shown in Figure 7, the fluorescence lifetime was measured to further investigate the luminescent properties of H 2 L and the [Zn II 2 -Sm III 2 ] complex. It was determined from the correlation data of the spectrua that the fluorescence lifetime of H 2 L was 1.5222 ns, and for the [Zn II 2 -Sm III 2 ] complex, the decay of the excited state became shorter (1.5048 ns). Two bottles with the same concentrations (2.5 × 10 −5 M) and volumes of the ligand H 2 L and the [Zn II 2 -Sm III 2 ] complex were selected as research objects. Under natural light, both the ligand H 2 L and the [Zn II 2 -Sm III 2 ] complex exhibited a colorless and transparent state (Figure 8a). Under 365 nm ultraviolet (UV) light, the ligand H 2 L showed a pale light, at which time the [Zn II 2 -Sm III 2 ] complex showed a pink light distinctly from the ligand, as depicted in Figure 8b. These excellent optical properties could easily allow for the [Zn II 2 -Sm III 2 ] complex to be used in fluorescent print imaging under specific conditions. Compared to many other nanoparticle inks, the [Zn II 2 -Sm III 2 ] complex had a lower concentration (0.44 mg/mL) [71][72][73], which also reflects the environmental friendliness and economy of selecting such a complex.
fluorescence emission peak intensity decreased gradually during the addition of Zn II ions (1.0 × 10 −3 M) to the free ligand H2L (1.0 × 10 −5 M). When Zn II ions started from 5 μL and gradually increased to 55 μL in increments of 5 μL, the fluorescence intensity began to stabilize and the titration reached the end point, indicating that the coordination of Zn II ions with H2L was completed and the stoichiometric ratio was 1:1. Immediately thereafter, when the amount of Sm III ions added reached the equivalent of 1.0 equivalent, the fluorescence intensity was no longer lowered, indicating the coordination of the Sm III ions with the [L-Zn 2+ ] units. This titration curve also clearly indicated that the stoichiometric ratio of H2L:Zn II :Sm III was 1:1:1. This result also corresponds to the coordination of the actually obtained crystal structure.

Fluorescence Print Imaging
From the fluorescence spectra, Sm III ions were effectively sensitized, and exhibited characteristic excitation and emission spectra, so the [Zn II 2-Sm III 2] complex had the pink light that is characteristic emission of Sm III ions. Therefore, we attempted to refer this special property to fluorescent print imaging. As shown in Figure 7, the fluorescence lifetime was measured to further investigate the luminescent properties of H2L and the [Zn II 2-Sm III 2] complex. It was determined from the correlation data of the spectrua that the fluorescence lifetime of H2L was 1.5222 ns, and for the [Zn II 2-Sm III 2] complex, the decay of the excited state became shorter (1.5048 ns). Two bottles with the same concentrations (2.5 × 10 −5 M) and volumes of the ligand H2L and the [Zn II 2-Sm III 2] complex were selected as research objects. Under natural light, both the ligand H2L and the [Zn II 2-Sm III 2] complex exhibited a colorless and transparent state (Figure 8a). Under 365 nm ultraviolet (UV) light, the ligand H2L showed a pale light, at which time the [Zn II 2-Sm III 2] complex showed a pink light distinctly from the ligand, as depicted in Figure 8b. These excellent optical properties could easily allow for the [Zn II 2-Sm III 2] complex to be used in fluorescent print imaging under specific conditions. Compared to many other nanoparticle inks, the [Zn II 2-Sm III 2] complex had a lower concentration (0.44 mg/mL) [71][72][73], which also reflects the environmental friendliness and economy of selecting such a complex.  In order to reduce the interference from background UV fluorescence, a non-fluorescent filter paper was used as the printing paper. The fluorescent imaging of H2L and the [Zn II 2-Sm III 2] complex solutions as the ink are shown in Figure 8c,d. The word "Crystals" was marked with each of the H2L and the [Zn II 2-Sm III 2] complex on the same filter paper and dried in an oven. When observed under 365 nm UV light, there was almost no pattern at the mark with the ligand solution, and a pink image of "Crystals" was displayed at the mark with the [Zn II 2-Sm III 2] complex. This further confirmed that the [Zn II 2-Sm III 2] complex can be used to create a fluorescent print imaging.

DFT Calculation
Molecular orbital calculations were performed by density functional theory. The DFT method used was the gradient-corrected functional proposed by B3LYP, and basis sets with SDD were used to expand the Kohn-Sham orbitals. Surface plots of some selected In order to reduce the interference from background UV fluorescence, a non-fluorescent filter paper was used as the printing paper. The fluorescent imaging of H 2 L and the [Zn II 2 -Sm III 2 ] complex solutions as the ink are shown in Figure 8c,d. The word "Crystals" was marked with each of the H 2 L and the [Zn II 2 -Sm III 2 ] complex on the same filter paper and dried in an oven. When observed under 365 nm UV light, there was almost no pattern at the mark with the ligand solution, and a pink image of "Crystals" was displayed at the mark with the [Zn II 2 -Sm III 2 ] complex. This further confirmed that the [Zn II 2 -Sm III 2 ] complex can be used to create a fluorescent print imaging.

DFT Calculation
Molecular orbital calculations were performed by density functional theory. The DFT method used was the gradient-corrected functional proposed by B3LYP, and basis sets with SDD were used to expand the Kohn-Sham orbitals. Surface plots of some selected molecular orbitals of H 2 L and the [Zn II 2 -Sm III 2 ] complex are shown in Figure 9. It is worth noting that the HOMO

Conclusions
A heterotetranuclear 3d-4f complex was prepared and characterized structurally. In the [Zn II 2-Sm III 2] complex, the auxiliary ligand 4,4′-bipyridine acted as a bidentate connecting rod, introducing its pyridine nitrogen atoms, which tended to coordinate with the axial position of the Zn II ions, further linking the two [Zn(L)Sm] units. The Zn II ion (Zn1) was located in the coordination

Conclusions
A heterotetranuclear 3d-4f complex was prepared and characterized structurally. In the [Zn II 2 -Sm III 2 ] complex, the auxiliary ligand 4,4 -bipyridine acted as a bidentate connecting rod, introducing its pyridine nitrogen atoms, which tended to coordinate with the axial position of the Zn II ions, further linking the two [Zn(L)Sm] units. The Zn II ion (Zn1) was located in the coordination environment of the N 2 O 2 donor and considered to have a twisted square pyramidal geometry (τ = 0.1845). In addition, sensitizing the emission of Sm III ions through the [L-Zn 2+ ] units, the [Zn II 2 -Sm III 2 ] complex exhibited a pink characteristic fluorescence that can be applied to fluorescent print imaging under specific conditions. The molecular orbital energy levels of the ligand H 2 L and the [Zn II 2 -Sm III 2 ] complex were discussed by DFT calculation. The calculated results further indicated that the lower the energy gap, the more active and the easier it is to develop in a stable direction.