Luminescent Properties of Phosphonate Ester-Supported Neodymium(III) Nitrate and Chloride Complexes

This study examines the synthesis of two geminal bisphosphonate ester-supported Ln3+ complexes [Ln(L3)2(NO3)3] (Ln = Nd3+ (5), La3+ (6)) and optical properties of the neodymium(III) complex. These results are compared to known mono-phosphonate ester-based Nd3+ complexes [Nd(L1/L2)3X3]n (X = NO3−, n = 1; Cl−, n = 2) (1–4). The optical properties of Nd3+ compounds are determined by micro-photoluminescence (µ-PL) spectroscopy which reveals three characteristic metal-centered emission bands in the NIR region related to transitions from 4F3/2 excited state. Additionally, two emission bands from 4F5/2, 2H9/2 → 4IJ (J = 11/2, 13/2) transitions were observed. PL spectroscopy of equimolar complex solutions in dry dichloromethane (DCM) revealed remarkably higher emission intensity of the mono-phosphonate ester-based complexes in comparison to their bisphosphonate ester congener. The temperature-dependent PL measurements enable assignment of the emission lines of the 4F3/2 → 4I9/2 transition. Furthermore, low-temperature polarization-dependent measurements of the transitions from R1 and R2 Stark sublevel of 4F3/2 state to the 4I9/2 state for crystals of [Nd(L3)2(NO3)3] (5) are discussed.


Introduction
Luminescent lanthanide(III) (Ln 3+ ) ions have attracted considerable research interest due to their versatile photophysical properties [1][2][3][4][5], which are related to their 4f electrons: Due to their small radial distribution, 4f electrons exhibit minimal interaction and little involvement in chemical bonding with surrounding ligands, since they are effectively shielded by electrons of the 5s and 5p shell. Thus, Ln 3+ luminescence exhibits characteristic narrow emission bands (FWHM < 10 nm) [6,7] along with relatively long emission lifetimes in a micro-or millisecond range [8][9][10]. This UV/Vis-to-NIR photoluminescence results from intra-configurational 4f-4f transitions [11] which are Laporte forbidden, but partially permitted by mixing of 4f and 5d orbitals or with charge transfer states of neighboring ligands [6,12].

Room Temperature Emission Properties of Nd 3+ Complexes 1-5 from Amorphous Solids and Solutions
Neodymium(III) complex [Nd(L3) 2 (NO 3 ) 3 ] (5) exhibits a broad absorption band in the UV range due to ligand absorption as well as sharp absorption bands between 500 to 850 nm, characteristic of Nd 3+ ions (see Figure S7 in SI file). Excitation of the synthesized Nd 3+ complexes at 750 nm, which is resonant with the 4 I 9/2 → 4 F 7/2 , 4 S 3/2 transition, results in the detection of three emission bands in the NIR region (centered around 890 nm, 1060 nm, and 1350 nm). These are associated with the electronic transitions 4 F 3/2 → 4 I 9/2 , 4 F 3/2 → 4 I 11/2 , and 4 F 3/2 → 4 I 13/2 , respectively. Pumping into the 4 F 7/2 , 4 S 3/2 levels also enable the detection of two emission lines from the 4 F 5/2 , 2 H 9/2 excited states ( 4 F 5/2 , 2 H 9/2 Molecules 2023, 28, 48 4 of 15 → 4 I J ) centered around 960 nm (J = 11/2) and 1180 nm (J = 13/2). However, following non-radiative decay and due to the small energy gap between 4 F 5/2 , 2 H 9/2 levels, and 4 F 3/2 , emission from the lower excited state represents the dominant process. Figure 2 depicts RT emission spectra of [Nd(L1) 3 Cl 3 ] 2 (2) highlighting the relatively weak transition bands from the 4 F 5/2 , 2 H 9/2 states as insets, which were detected also for the other Nd 3+ complexes. These emission bands from 4 F 5/2 , 2 H 9/2 states cannot be observed from conventional laser materials such as Nd:YAG, but were observed from Nd-doped lead halides [49][50][51]. Lead halides represent solid-state host materials with low maximum phonon energy and therefore less quenching of luminescence from 4 F 5/2 , 2 H 9/2 excited states. and 1350 nm). These are associated with the electronic transitions 4 F3/2 → 4 I9/2, 4 F3/2 → 4 I11/2, and 4 F3/2 → 4 I13/2, respectively. Pumping into the 4 F7/2, 4 S3/2 levels also enable the detection of two emission lines from the 4 F5/2, 2 H9/2 excited states ( 4 F5/2, 2 H9/2 → 4 IJ) centered around 960 nm (J = 11/2) and 1180 nm (J = 13/2). However, following non-radiative decay and due to the small energy gap between 4 F5/2, 2 H9/2 levels, and 4 F3/2, emission from the lower excited state represents the dominant process. Figure 2 depicts RT emission spectra of [Nd(L1)3Cl3]2 (2) highlighting the relatively weak transition bands from the 4 F5/2, 2 H9/2 states as insets, which were detected also for the other Nd 3+ complexes. These emission bands from 4 F5/2, 2 H9/2 states cannot be observed from conventional laser materials such as Nd:YAG, but were observed from Nd-doped lead halides [49][50][51]. Lead halides represent solid-state host materials with low maximum phonon energy and therefore less quenching of luminescence from 4 F5/2, 2 H9/2 excited states.  Since the mono-phosphonate ester-supported neodymium chloride complexes are less soluble in DCM, only nitrate-based complexes with mono-phosphonate and geminal bisphosphonate esters are considered. The complexes are dissolved in dry DCM (c = 4 × 10 −3 mol/L) and their optical characteristics were investigated at RT using µ-PL spectroscopy (same laser power and acquisition time for all three complexes). Since there is less rotation and vibrational modes along the Nd-O bonds, geminal bisphosphonate ester-based complexes are more rigid than their mono-phosphonate ester congeners. As a result, enhanced luminescence intensity can be anticipated for complexes based on a geminal bisphosphonate ester such as L3. In contrast, the emission spectra shown in   Since the mono-phosphonate ester-supported neodymium chloride complexes are less soluble in DCM, only nitrate-based complexes with mono-phosphonate and geminal bisphosphonate esters are considered. The complexes are dissolved in dry DCM (c = 4 × 10 −3 mol/L) and their optical characteristics were investigated at RT using µ-PL spectroscopy (same laser power and acquisition time for all three complexes). Since there is less rotation and vibrational modes along the Nd-O bonds, geminal bisphosphonate esterbased complexes are more rigid than their mono-phosphonate ester congeners. As a result, enhanced luminescence intensity can be anticipated for complexes based on a geminal bisphosphonate ester such as L3. In contrast, the emission spectra shown in Figure 3d-  4 F 3/2 → 4 I 11/2 (e), and 4 F 3/2 → 4 I 13/2 (f) transition. For data acquisition, laser power and excitation duration were kept constant. The spiky signal of the 4 F 3/2 → 4 I 13/2 transition (c,f) between 1350 nm and 1380 nm is related to setup noise.
As the morphology has a significant impact on the emission spectrum, crystals of the geminal bisphosphonate ester-supported compound [Nd(L3) 2 (NO 3 ) 3 ] (5) are investigated in this respect. The normalized PL spectra of the amorphous bulk and crystalline complexes are depicted in Figure 6a. As expected, the amorphous bulk material has a wider linewidth than the crystalline sample due to structural disorder.
As the morphology has a significant impact on the emission spectrum, crystals of the geminal bisphosphonate ester-supported compound [Nd(L3)2(NO3)3] (5) are investigated in this respect. The normalized PL spectra of the amorphous bulk and crystalline complexes are depicted in Figure 6a. As expected, the amorphous bulk material has a wider linewidth than the crystalline sample due to structural disorder.  When a perfect crystal considered under ideal conditions is excited with a laser, the emission bands have a narrow Lorentzian shape since all emitting molecules have the same orientation in the crystal. Due to differences in the local environment of the Nd 3+ centers in amorphous bulk samples and grown crystals, broadened Gaussian band shapes are observed [54]. As described by Lenz et al. [54] the PL intensity of transition lines is strongly affected by the crystal orientation. This orientation-dependence was also observed for crystalline [Nd(L3) 2 (NO 3 ) 3 ] (5) when applying a polarizer in front of the detector. As the polarization angle increases from 0 • to 90 • , the intensity of the transition lines associated with the R 1 sublevel decrease, whereas transition lines associated with the R 2 sublevel increase, as illustrated in Figure 6b.

Discussion
According to Figure 4, ten, twelve and fourteen emission lines are expected for the 4 F 3/2 → 4 I 9/2 , 4 F 3/2 → 4 I 11/2 , and 4 F 3/2 → 4 I 13/2 transitions, respectively. Due to spectral overlap, not all transition lines can be resolved in the emission spectra as demonstrated by the 4 F 3/2 → 4 I 9/2 transition of [Nd(L3) 2 (NO 3 ) 3 ] (5), where only eight of ten emission lines can be observed (Figure 6a). However, temperature-dependent PL measurements can be used to assign the emission lines. At RT, the R 2 Stark sublevel is easily populated due to the small energy difference between R 1 and R 2 [55,56]. With decreasing temperature, R 2 is less populated leading to decreasing PL intensity of the corresponding emission lines. Figure 5a illustrates the increase in emission intensities of transition lines centered at 869.3 nm, 875.3 nm, 879.8 nm, 892.6 nm, and 901.2 nm with rising temperature for the 4 F 3/2 → 4 I 9/2 transition of [Nd(L2) 3 (NO 3 ) 3 ] (3). A radiative depopulation of the R 2 Stark sublevel of the 4 F 3/2 state to the 4 I 9/2 manifold results in the emission of these lines. Increased intensity of emission lines related to transitions from the R 1 sublevel centered at 878.1 nm, 895.7 nm, and 904.6 nm may be caused by spectral overlap with R 2 Stark sublevel emission lines. The emission bands of the 4 F 3/2 → 4 I 11/2 , and 4 F 3/2 → 4 I 13/2 transitions were not assigned, as the emission lines overlap strongly even at low temperatures.
Due to spectral overlap and low intensity of some of the emission lines, its peak positions (λ max ) cannot be determined precisely. A powerful tool for resolving overlapping spectral bands is the so-called derivative spectroscopy [57,58], which gives detailed infor-mation about emission lines and λ max values. Figure 7 depicts the zero-order (dashed line) PL spectrum of [Nd(L2) 3 (NO 3 ) 3 ] (3) (recorded at 5 K) and its second-order derivative (D2) spectrum (solid line). When compared to the original (zero-order) PL spectrum, the D2 spectrum's peaks are reversed, revealing minima at λ max of the zero-order spectrum. In addition, a positive satellite band is also present on either side of each dip. In general, sharp peaks of zero-order spectra become even narrower in D2 spectra, while broad peaks will be flattened, leading to a reduction in broad background but also to unwanted enhancement of sharp noise-signals. Thus, PL spectra were smoothed to increase the signal-to-noise ratio. The λ max values of the Nd 3+ complexes are derived from the second-order derivative spectra and are summarized in Table 1   As previously stated, the 4f shell of lanthanides is well shielded by electrons of the 5s and 5p orbitals resulting in only minor influence from neighboring ligands. However, for Stark level splitting, ligand parameters such as interatomic distances and electric charge are critical [54]. As a result, nitrate and chloride anions have a significant influence on the PL spectra.    As previously stated, the 4f shell of lanthanides is well shielded by electrons of the 5s and 5p orbitals resulting in only minor influence from neighboring ligands. However, for Stark level splitting, ligand parameters such as interatomic distances and electric charge are critical [54]. As a result, nitrate and chloride anions have a significant influence on the PL spectra. Figure 8a-c compares the 5 K emission bands associated with the three NIR transitions of Nd 3+ complexes 1-4. The emission bands of the monomeric NO 3 − and dimeric Cl − based Nd 3+ complexes under investigation are similar, nevertheless, there are two noteworthy differences: First, NO 3 − based complexes exhibit emission lines for the 4 F 3/2 → 4 I 9/2 transition that start at shorter wavelengths and are spread across a wider spectral range. Second, the transition lines of the neodymium(III) chloride complexes, apart from the two outer lines of each transition band, are much less prominent than those of the nitrate-based congeners. It appears that the different organic ligands do not significantly influence the µ-PL spectrum, as similar spectra were observed for [Nd(L1) 3

(NO 3 ) 3 ] (1) and [Nd(L2) 3 (NO 3 ) 3 ] (3), and [Nd(L1) 3 Cl 3 ] 2 (2) and [Nd(L2) 3 Cl 3 ] 2 (4), respectively.
Molecules 2023, 28, x FOR PEER REVIEW 10 of 16 In line with the already mentioned orientation-dependence of the PL intensity of transition lines for crystalline samples, we explored this aspect for the geminal bisphosphonate ester complex [Nd(L3)2(NO3)3] (5) by introducing a polarizer in front of the detector. Since the first two transition lines of 4 F3/2 → 4 I9/2 transition are spectrally most isolated, the following investigations focus on the R2 → Z1 and R1 → Z1 transition lines. The relative emission intensities as a function of the polarization angle, as extracted from careful fits of many spectra, are shown in Figure 9a. As can be seen, an increase in peak intensity corresponds to transitions from the upper Stark sublevel R2 while a decrease in peak intensity corresponds to the transition from the lower Stark R1 sublevel and vice versa. Figure 9b shows the transition energies of the two transition lines as a function of the linear polarization angle. The oscillatory behavior of both lines stems from two   (Figure 8d). In both NO 3 based complex types, the transition line shape is similar.
In line with the already mentioned orientation-dependence of the PL intensity of transition lines for crystalline samples, we explored this aspect for the geminal bisphosphonate ester complex [Nd(L3) 2 (NO 3 ) 3 ] (5) by introducing a polarizer in front of the detector. Since the first two transition lines of 4 F 3/2 → 4 I 9/2 transition are spectrally most isolated, the following investigations focus on the R 2 → Z 1 and R 1 → Z 1 transition lines. The relative emission intensities as a function of the polarization angle, as extracted from careful fits of many spectra, are shown in Figure 9a. As can be seen, an increase in peak intensity corresponds to transitions from the upper Stark sublevel R 2 while a decrease in peak intensity corresponds to the transition from the lower Stark R 1 sublevel and vice versa. Figure 9b shows the transition energies of the two transition lines as a function of the linear polarization angle. The oscillatory behavior of both lines stems from two perpendicularly linearly polarized components. The two transitions show anticorrelated shifts when changing the polarization angle, confirming the above assigned transitions. While polarized emission has been observed, the exact correlation between polarization and crystal orientation has not yet been examined. It is necessary to conduct further investigations, which is beyond the scope of this study. The FWHM was also found to be polarization-dependent as shown in Figure 9c. The two transitions show as well anticorrelated broadening when changing the polarization angle. Here only the R 1 → Z 1 transition line is plotted. Since the peak intensity of the R 2 → Z 1 transition is weak, the fitting of the emission line was not unambiguous, therefore it is not shown. perpendicularly linearly polarized components. The two transitions show anticorrelated shifts when changing the polarization angle, confirming the above assigned transitions. While polarized emission has been observed, the exact correlation between polarization and crystal orientation has not yet been examined. It is necessary to conduct further investigations, which is beyond the scope of this study. The FWHM was also found to be polarization-dependent as shown in Figure 9c. The two transitions show as well anticorrelated broadening when changing the polarization angle. Here only the R1 → Z1 transition line is plotted. Since the peak intensity of the R2 → Z1 transition is weak, the fitting of the emission line was not unambiguous, therefore it is not shown.

Materials and Methods
Starting materials for synthesis were purchased commercially and were used as received, unless stated otherwise. The ligands L1-L3 as well as complexes 1-4 have been prepared according to literature protocols [42,43]. NMR experiments were performed with a Varian 500 MHz spectrometer, and spectra were processed with MestReNova (v11.0.4-18998, Mestrelab Research S.L.). 1 H-and 13 C NMR spectra are referenced relative to TMS using the residual solvent signals as internal standards [59]. IR spectra were recorded with a diamond probe ATR IR spectrometer by Bruker. Elemental analyses were performed using a HEKAtech Euro EA-CHNS elemental analyzer. For analyses, samples

Materials and Methods
Starting materials for synthesis were purchased commercially and were used as received, unless stated otherwise. The ligands L1-L3 as well as complexes 1-4 have been prepared according to literature protocols [42,43]. NMR experiments were performed with a Varian 500 MHz spectrometer, and spectra were processed with MestReNova (v11.0.4-18998, Mestrelab Research S.L.). 1 H-and 13 C NMR spectra are referenced relative to TMS using the residual solvent signals as internal standards [59]. IR spectra were recorded with a diamond probe ATR IR spectrometer by Bruker. Elemental analyses were performed using a HEKAtech Euro EA-CHNS elemental analyzer. For analyses, samples were prepared in tin cups with V 2 O 5 as an additive to ensure complete combustion.

Crystallographic Details
X-ray diffraction experiments were performed with either a STOE IPDS 2 with an image plate (Ø 34 cm) using a Mo-GENIX source (λ = 0.71073 nm) or a STOE StadiVari instrument with DECTRIS PILATUS 200 K using a Cu-GENIX source (λ = 1.54186 nm). All structures were solved using direct methods (SHELXT) [60] and refined against F 2 using the full-matrix least-squares methods of SHELXL [61] within the SHELXLE GUI [62] or with OLEX2 [63]. CCDC 2201668 (5) and 2201669 (6) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/ data_request/cif.

Micro-Photoluminescence (µ-PL) Measurments
Luminescence characteristics of phosphonate ester-supported Nd 3+ complexes are investigated by µ-PL spectroscopy. For the characterization of Nd 3+ complexes in solid form, the samples were mounted in a liquid helium flow cryostat. The compounds are attached to silicon wafer pieces by partially melting or, if non-meltable, by sticking with vacuum grease, to fix the solid in position and to ensure good thermal conductivity when cooling the sample down to liquid helium temperature. For RT PL measurements in solution, the complexes were dissolved in dry DCM, filled into a cuvette, and attached to the holder of an open cryostat. The Nd 3+ complexes are excited at 750 nm, using a CW Ti:Sapphire laser. A microscope objective (NA = 0.7) focuses the laser onto the sample and collects the photoluminescence light of the complexes. The emitted light is guided to a monochromator equipped with a liquid nitrogen-cooled InGaAs detector. For polarization-dependent measurements, a polarizer is inserted in front of the detector.
Low-temperature measurements were conducted on amorphous solids except for [Nd(L3) 2 (NO 3 ) 3 ] (5) where a crystalline sample was used. From this compound, crystals were obtained from vapor diffusion of pentanes into a saturated THF solution of the complex. The amorphous bulk material as well as a crystalline sample are depicted in Figure 10a,b.

Conclusions
The preparation of two geminal bisphosphonate ester-supported Ln 3+ complexes [Ln(L3)2(NO3)3] (Ln = Nd 3+ , La 3+ ) has been presented. Emission intensities of equimolar solutions of the germinal bisphosphonate ester-supported Nd 3+ nitrate complex 5 and related NO3 − based Nd 3+ complexes featuring mono-phosphonate esters were compared obtaining unexpected higher emission intensities for the latter compounds. Emission bands from 4 F5/2, 2 H9/2 → 4 IJ (J = 11/2, 13/2) transitions were detected, which are rarely presented for Nd 3+ containing materials. The three emission bands characteristic for transitions from 4 F3/2 excited state, of mono-and dimeric phosphonate ester-supported Nd 3+ nitrate and chloride complexes as well as of the geminal bisphosphonate-based complex at liquid helium temperature (5 K) were examined. PL spectra of all three complex types depict similar features with slight shifts of peak positions. Temperature-dependent PL spectroscopy enabled assignment of the transition lines corresponding to the 4 F3/2 → 4 I9/2 transition. At 5 K polarization-dependence of a crystalline sample was observed showing opposite change in peak intensity of transitions related to the depopulation of the R1 and R2 Stark sublevel, respectively.
This study shows that the investigated neodymium(III) complexes exhibit interesting luminescence properties. With improved synthesis processes, their optical properties could be further enhanced. In the next step, molecules will be integrated onto microcavities to examine molecule-cavity coupling.

Supplementary Materials:
The following are available online at www.mdpi.com/xxx/s1, Figure S1: 1 H NMR spectrum of 6 in DMSO-d6, Figure S2: 13 C{ 1 H} NMR spectrum of 6 in DMSO-d6, Table S1: Crystallographic data for complex 5 and 6, Figure S3: Asymmetric unit of 5, Figure S4 Funding: The federal state of Hesse, Germany is kindly acknowledged for financial support of the SMolBits project within the LOEWE program. This work was also financially supported by the DFG Heisenberg grant-BE 5778/4-1.

Conclusions
The preparation of two geminal bisphosphonate ester-supported Ln 3+ complexes [Ln(L3) 2 (NO 3 ) 3 ] (Ln = Nd 3+ , La 3+ ) has been presented. Emission intensities of equimolar solutions of the germinal bisphosphonate ester-supported Nd 3+ nitrate complex 5 and related NO 3 − based Nd 3+ complexes featuring mono-phosphonate esters were compared obtaining unexpected higher emission intensities for the latter compounds. Emission bands from 4 F 5/2 , 2 H 9/2 → 4 I J (J = 11/2, 13/2) transitions were detected, which are rarely presented for Nd 3+ containing materials. The three emission bands characteristic for transitions from 4 F 3/2 excited state, of mono-and dimeric phosphonate ester-supported Nd 3+ nitrate and chloride complexes as well as of the geminal bisphosphonate-based complex at liquid helium temperature (5 K) were examined. PL spectra of all three complex types depict similar features with slight shifts of peak positions. Temperature-dependent PL spectroscopy enabled assignment of the transition lines corresponding to the 4 F 3/2 → 4 I 9/2 transition. At 5 K polarization-dependence of a crystalline sample was observed showing opposite change in peak intensity of transitions related to the depopulation of the R 1 and R 2 Stark sublevel, respectively.
This study shows that the investigated neodymium(III) complexes exhibit interesting luminescence properties. With improved synthesis processes, their optical properties could be further enhanced. In the next step, molecules will be integrated onto microcavities to examine molecule-cavity coupling.
Institutional Review Board Statement: Not applicable.

Informed Consent Statement: Not applicable.
Data Availability Statement: NMR, IR, and crystallographic data presented in this study are available in the Supplementary Materials.