Introduction

Luminescent rare earth ions have an unusual position in the fields of optical materials and devices for their remarking properties such as high color purity caused by their line-like emission, large Stokes shifts and wide lifetime range from microsecond to millisecond lifetimes [1–8]. But the 4f-4f transition of rare earth ion itself is spin-forbidden and can directly affect the efficiency of the luminescence output. Rare earth coordination compounds are ideal system to sensitize the luminescence of rare earth ions with so-called “antenna effect” [9–13]. Generally, the photophysical sensitization process involves the energy transfer from the triplet excited state of an organic ligand containing an antenna chromophore to the f-block ions [14, 15].

Rare earth coordination polymers with repeating coordination entities extending in 1, 2, or 3 dimensions, have attracted much attention for their potentials and advantages as inorganic–organic hybrid materials with infinite polymeric structure [16]. The variety of rare earth ions, organic linkers, and structural motifs affords an essentially infinite number of possible combinations [17–19]. These materials have shown their potential applications in luminescent thin film, biological imaging, and chemical sensors, etc. [20–25]. The luminescent properties of rare earth coordination compounds are very sensitive to their structural characteristics, coordination environment, and their interactions with guest species, which endows coordination compounds with inherent advantage in luminescent sensing [26–29]. Recently, these rare earth coordination compounds have unlimited potentials as chemical sensors, detecting cations, anions, small molecules, pH value and temperature [30–39].

Furthermore, rare earth coordination polymers are easily to realize the different rare earth ions substitution from each other for their similar physical properties [40–45]. This can develop a lot of rare earth hybrids based with coordination polymers, just like rare earth phosphors. On the other hand, there are a lot of metallic (including rare earth) coordination polymers with cationic dimethyl amino group (H2NMe2 +) in the DMF solution reaction systems [46–49]. So it can be expected to further introduce other rare earth ions through ionic exchange reaction with H2NMe2 +. For example, bio-MOF-1 (Zn8(ad)4(BPDC)6O⋅2Me2NH2, BPDC = biphenyl-4,4’-dicarboxylate, Ad = adeninate) has been proved to be functionalized with rare earth ions to show the characteristic luminescence of them [50, 51]. We have also studied rare earth ions exchanged bio-MOF-1 and other coordination polymers, whose luminescence can be tuned and further applied to fluorescent sensing [52, 53]. Therefore, for these kinds of rare earth coordination polymers with H2NMe2 +, both ion substitution and ion exchange can be used to functionalize them to construct the functional hybrid systems.

Among rare earth ions, inert ions such as Y3+ are often used as matrices for traditional luminescent material, while other active ions such as Eu3+, Tb3+, Sm3+ and Dy3+ ions, act as vital activators. The synthesis of [H2NMe2]3[Eu(Tb)(DPA)3] and rare earth ions doped [H2NMe2]3[RE(DPA)3] systems through the ion substitution of framework rare earth ions have been reported [54–56]. Herein, different from the work, [H2NMe2]3[Y(DPA)3] coordination polymer is synthesized and further functionalized to RE3+ ⊂ [Y(DPA)3] by ion exchange with rare earth ions considering the existence of H2NMe2 +. The multi-color luminescence of these coordination polymers are obtained and even the white emission is tuned. Moreover, Tb3+ ⊂ [Y(DPA)3] are selected to detect sensing properties.

Experimental Section

Materials and Instruments

All the solvents and chemicals were available as A.R grade commercially. RE(NO3)3·xH2O (RE = Y, Eu, Tb, Sm, Dy) were prepared from their oxide by dissolving in nitric acid. Dimethylformamide (DMF) and 2,6-dipicolinic acid (2,6-H2DPA) were used as received. The contents of RE3+ ions in the hybrids were determined with ICP-AES. The elemental analyses of C, H and N elements of the hybrids were measured with a CARIO-ERBA 1106 elemental analyzer. X-Ray powder diffraction patterns (XRD) were obtained Bruker Foucs D8 at 40 kV, 40 mA of Cu-Kα with a speed size of 0.02 and a scan speed of 0.10 s per step. We collected the data within 2θ range from 5 to 50°. A Nexus 912 AO446 infrared spectrum radiometer was used to measure Fourier transforms infrared spectra (FTIR) from 4000 to 400 cm−1. The luminescence spectra were carried out by an Edinburgh FLS 920 phosphorimeter using a 450 W xenon lamp as excitation source. The luminescent lifetimes and quantum yields were also tested by the phosphorimeter. The spectra were corrected for variations in the output of the excitation source and for variations in the detector response. The quantum yield can be defined as the integrated intensity of the luminescence signal divided by the integrated intensity of the absorption signal. The absorption intensity was calculated by subtracting the integrated intensity of the light source with the sample in the integrating sphere from the integrated intensity of the light source with a blank sample in the integrating sphere.

Synthesis of [H2NMe2]3[Y(DPA)3] (H2DPA = 2,6-Dipicolinic Acid)

The complex was obtained according to solvothermal method similar to ref. [54, 56]. 0.50 mmol Y(NO3)3·xH2O and 2 mmol H2DPA were dissolved in a 50 mL of Teflon-lined stainless steel vessel, which was placed mixed solvent of 14 mL DMF and 2 mLH2O previously. The mixture was hated to 120 °C and kept this temperature for 3 days. Then, cooling down to room temperature naturally, the products were collected after washing with DMF (5 × 3 mL) and drying under vacuum for 12 h. The contents of C, H, N and Y3+ were determined by elemental analysis, whose data are shown in Table S1.

Cation Exchanging Experiment

[H2NMe2]3[Y(DPA)3] (30 mg) was immersed in the 10 mL of DMF solutions of RE(NO3)3·xH2O and M(NO3)x (RE3+ = Eu3+, Tb3+, Sm3+, Dy3+; Mx+ = Na+, Mg2+, Al3+, Cd2+, Cr3+, Co2+, Fe2+, Fe3+, Ag+, Cu2+) with the concentration of 1 mmol/L for 3 days. We collected the PXRD after washing with DMF (5 × 3 mL) and drying under vacuum for 12 h. Among the cation exchange functionalized hybrid systems were named as RE3+ ⊂ [Y(DPA)3] (RE = Eu, Tb, Sm, Dy). The contents of C, H, N and RE3+ were determined by elemental analysis, whose data are shown in Table S1.

Luminescence Sensing Experiment

Tb3+ ⊂ [Y(DPA)3] (10 mg) was simply immersed in the DMF solutions of M(NO3)x with the concentration of 10−4 mol⋅L−1 and 10−5 mol⋅L−1 respectively at room temperature (Mx+ = Na+, Mg2+, Al3+, Cd2+, Cr3+, Co2+, Fe2+, Fe3+, Ag+, Cu2+). The luminescent was then determined after vibrating under ultrasonic for 5 min.

Results and Discussion

Yttrium coordination polymer, [H2NMe2]3[Y(DPA)3] ([H2NMe2]+ = dimethyl amino cation; H2DPA = 2,6-dipicolinic acid) is hydrothermally synthesized, whose scheme for the structure is shown in Fig. S1. Their crystal structures belong to a coordination sphere with N3O6 chromophore, whose coordination geometry can be described as a distorted tricapped trigonal prism consisting six carboxylato oxygen atoms and three pyridine nitrogen atoms. Among amine is more basic than the carboxylate group and so the protons are located on the dimethylamine molecules. The X-ray diffraction patterns of [H2NMe2]3[Y(DPA)3] and rare earth ions exchanged hybrid systems RE3+ ⊂ [Y(DPA)3] are also checked and shown in Fig. 1, whose crystal diffractions are similar to unexchanged [H2NMe2]3[RE(DPA)3] system. So the exchanged functionalization of [H2NMe2]3[Y(DPA)3] cannot have influence on the crystal framework structure of [Y(DPA)3]3+.

Fig. 1
figure 1

PXRD patterns of [H2NMe2]3[Y(DPA)3] (a) and RE3+ ⊂ [Y(DPA)3] (RE = Eu (b), Tb (c), Sm (d), Dy (e))

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The coordination interaction between rare earth ions and DPA can be shown from the selected FT-IR spectra analyses of [H2NMe2]3[Y(DPA)3] (Fig. S2) and Eu3+ ⊂ [Y(DPA)3] (Fig. S3). The absorption band located at ~1619 cm−1 is assigned to the asymmetric stretching vibrations of C = O. Comparing with the free carboxyl groups whose absorption band of C = O is at ~1700 cm−1, the lower wavenumber indicates the coordination between carboxyl groups and Y3+.The peak located at ~ 1434 cm−1 can be assigned to the stretching vibration of C-C. The peak at ~1375 cm−1 may result from the amino C-N stretch. The absorption band at ~732 cm−1 can be ascribed to the C-H bending vibrations of the aromatic ring. All of these suggest similar coordination interactions between rare earth ions and DPA to construct the whole framework [Y(DPA)3]3−. In addition, three characteristic peaks of benzene ring stretching vibrations (1600 ~ 1500 cm−1) remain after reaction. The asymmetric vibrations (1685 cm−1) of carboxylate ions and its symmetric vibrations (1427 cm−1) have changed simultaneously, proving the carboxyl groups have coordinated with rare earth ions [54, 55].

The emission and excitation spectra of these pure yttrium coordination polymer [H2NMe2]3[Y(DPA)3] is measured and shown in Fig. 2. In the visible region, a number of very weak and narrow peaks (characteristic of the Laporte-forbidden f-f transitions of the Ln3+ ions) are observed. [H2NMe2]3[Y(DPA)3] possess the emission of free DPA ligand exhibits excitation at 313 nm and emission at 414 nm which is presumably due to π → π* electron transitions of pyridine cycle, while Y3+ ion only have little disturbance of the emission of DPA ligand for its coordination interaction.

Fig. 2
figure 2

The excitation and emission spectra of [H2NMe2]3[Y(DPA)3] and the corresponding luminescent pictures under xenon lamp (inset)

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After the ion exchange between rare earth ions with H2NMe2 + ion to form RE3+ ⊂ [Y(DPA)3], different from [H2NMe2]3[Y(DPA)3], the characteristic luminescence of rare earth ions (Eu3+, Tb3+, Sm3+, Dy3+) can be observed. This may be due to the energy transfer between [Y(DPA)3]3− and RE3+ through the ionic interaction. Figure 3 shows the luminescent spectra of Tb3+ ⊂ [Y(DPA)3] and Dy3+ ⊂ [Y(DPA)3]. For Tb3+ ⊂ [Y(DPA)3] hybrid systems in Fig. 3a, similar to [H2NMe2]3[Tb(DPA)3], an apparent wide spectrum ranges in 250–400 nm, which has two excitation peaks. The excitation peaks to f-f transition of Tb3+are too weak to be checked. The emission of Tb3+ ⊂ [Y(DPA)3] also shows the characteristic transitions (5D4 → 7F J , J = 6–3) of Tb3+ at 491, 545, 585, and 622 nm, respectively [57]. For Dy3+ ⊂ [Y(DPA)3] hybrids, it shows the excitation spectrum identical to [Y(DPA)3] and the weak f-f transition excitation of Dy3+ to the transition from 6H15/2 to 6P3/2 (325 nm), 6P7/2 (351 nm), 6P5/2 (365 nm), and 4 K17/2 (381 nm) [58]. The emission mainly displays two bands to Dy3+ characteristic transitions (4F9/2 → 6H J , J = 15/2, 13/2) at 482, 573 nm (Fig. 3b). The inset pictures in the spectra show the color of these systems under xenon lamp. Under characteristic excitation of each material, various colours can be obtained such as blue-green for Dy3+ ⊂ [Y(DPA)3], and green for Tb3+ ⊂ [Y(DPA)3], respectively. Here Tb3+ ⊂ [Y(DPA)3] also exhibit the bright green luminescence due to the strong wide excitation band, while Dy3+ ⊂ [Y(DPA)3] possess the comparable intensity of both blue and yellow color to show the close white color emission.

Fig. 3
figure 3

The excitation and emission spectra of RE3+ ⊂ [Y(DPA)3] (RE = Tb (a), Dy (b)) and the corresponding luminescent pictures under xenon lamp

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Figure 4 displays the luminescent spectra of Eu3+ ⊂ [Y(DPA)3] and Sm3+ ⊂ [Y(DPA)3]. Both of their excitation spectrum shows the similar feature to the excitation spectrum of [H2NMe2]3[Y(DPA)3] with a wide band to the DPA ligands to form the charge transfer state Eu-O or Sm-O while no f-f transitions excitation of Sm3+ or Eu3+ can be checked. The emission spectra of Eu3+ ⊂ [Y(DPA)3] in Fig. 4a shows five narrow emission peaks at 580, 591, 614, 652 and 700 nm are observed and assigned to the characteristic 5D0 → 7F0, 5D0 → 7F1, 5D0 → 7F2, 5D0 → 7F1 and 5D0 → 7F2 transitions of Eu3+, respectively [59]. And red luminescence is obtained (inset picture in Fig. 4a). The emission spectrum are originated from the characteristic transitions (4G5/2 → 6H J , J = 5/2, 7/2, 9/2, 11/2) of Sm3+ at 561, 596, 644 and 703 nm, respectively (Fig. 4b) [58]. It is interesting that another wide emission band can be observed in the emission spectrum of the weak emission of Sm3+, some disturbing peaks can be observed in the emission spectra of Sm3+ ⊂ [Y(DPA)3], corresponding to the [Y(DPA)3]3−.So it is predicted that the two emission bands may be integrate white-color luminescent output (see inset picture in Fig. 4b).

Fig. 4
figure 4

The excitation and emission spectra of RE3+ ⊂ [Y(DPA)3](RE = Eu (a), Sm (b)) and the corresponding luminescent pictures under xenon lamp

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Moreover, considering the spectra with different luminescent region, some systems may be expected to realize to tune the close white luminescence. For samarium ion exchanged Sm3+ ⊂ [Y(DPA)3] hybrid system, both luminescence of Sm3+ ⊂ [Y(DPA)3] hybrid system, both luminescence of Sm3+ and [Y(DPA)3]3− framework can be observed, resulting in the white color luminescence with CIE coordinates (0.3810, 0.2875). (See Fig. 5).

Fig. 5
figure 5

CIE Chromaticity diagram of Sm3+ ⊂ [Y(DPA)3] when excited at 281 nm

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Furtherly, we determine the photoluminescent data of lifetimes and quantum yields of these hybrid systems, whose data are summarized in Table 1. Here it is worthy pointing out that they are all the data from the monitoring of rare earth ions’ luminescence except for [H2NMe2]3[Y(DPA)3] which is mainly due to the luminescence of DPA ligand coordinated to Y ions. It is interesting that the rare earth ions exchanged hybrid systems RE3+ ⊂ [YDPA)3] display the comparable value of lifetimes and quantum yields to pure coordination polymers [H2NMe2]3[RE(DPA)3], which reveals that the ion exchanged functionalization of such rare earth coordination polymers is an effective approach to construct hybrid systems. So for rare earth coordination polymers with [H2NMe2]+ ion, two kinds of strategies can be utilized to functionalize, ion substitution and ion exchange, both of which are benefit to realize the effective energy transfer and luminescence of photoactive rare earth ions [55, 56].

Table 1 Luminescent data of [H2NMe2]3[RE(DPA)3] and RE3+ ⊂ [YDPA)3]
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To examine and compare the sensing potential of Tb3+ ⊂ [Y(DPA)3] which is obtained through cation exchange, were suspended in DMF solutions containing different metal ions (Na+, Ag+, Mg2+, Al3+, Co2+, Cr3+, Cd2+, Fe2+, Fe3+, Cu2+) at the concentration of 10−4 mol⋅L−1 at first. The luminescent properties were recorded in Fig. S4 and Fig. S5, different with [H2NMe2]3[Tb(DPA)3], Tb3+ ⊂ [Y(DPA)3] shows highly pronounced to Co2+, Cr3+, Fe3+ and Cu2+, the selective sensing of Fe3+ is not as obvious as [H2NMe2]3[Tb(DPA)3] [56]. The KSV of these ions is displayed in Table S2. Subsequently, the concentration of metal solutions has been decreased to 10−5 mol⋅L−1 to explore the sensing properties of Tb3+ ⊂ [Y(DPA)3] furtherly. To our surprise, it shows high selectively towards Cr3+, as is illustrated in Fig. 6, Further work is carried out to examine how the concentration of Cr3+ influences the luminescence of Tb3+ ⊂ [Y(DPA)3]. As is exhibited in Fig. 7, the luminescent intensity of Tb3+ ⊂ [Y(DPA)] suspension decreased gradually as the concentration of Cr3+ varying from 0 to 100 μM. The Ksv value is calculated as 3.64 × 104, which reveals a strong quenching effect on the luminescence of Tb3+ ⊂ [Y(DPA)3]. The quenching effects on luminescence of MOFs with the addition of metal ions may be attributed to the following factors: (i) interaction between the metal ions and organic ligands; (ii) collapse of the crystal structure; (iii) cation exchange between the central cations of coordination polymer and the targeted cations. PXRD was employed to study the structural data of original [H2NMe2]3[Tb(DPA)3], Mz+ ⊂ [Tb(DPA)3], Tb3+ ⊂ [Y(DPA)3], Mz+,Tb3+ ⊂ [Y(DPA)3]. For Mz+ ⊂ [Tb(DPA)3], the quenching effect should originate from the less-effective transfer process of ligand to the central, which is due to the interaction between metal cations and organic ligands. For Mz+, Tb3+ ⊂ [Y(DPA)3], besides the less-effective transfer process of ligand to the central, the exchange between the Tb3+ and Mz+ may also cause the quenching effect in a degree, which can lead to the decreasing selection of Tb3+ ⊂ [Y(DPA)3] to Mz+.

Fig. 6
figure 6

a PL spectra of Tb3+ ⊂ [Y(DPA)3] interacting with different metal cations DMF solutions (10−5 mol/L) when excited at 278 nm; b The luminescent intensity of the 5D4 → 7F5 transition of Tb3+ ⊂ [Y(DPA)3] interacting with different metal cations DMF solutions

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Fig. 7
figure 7

a Emission spectra of Tb3+ ⊂ [Y(DPA)3] in various concentrations of Cr3+ when excited at 278 nm; b KSV curve between I0/I with the concentration of Cr3+

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Conclusions

In summary, [H2NMe2]3[Y(DPA)3] is prepared and further a novel strategy is used to modify [H2NMe2]3[Y(DPA)3] through the ion exchange reaction between RE3+ and [H2NMe2]+, resulting in hybrid systems RE3+ ⊂ [Y(DPA)3]. It is interesting that the ion exchange functionalized hybrid systems possess the comparable luminescent lifetimes and quantum yields to the pure coordination polymers, revealing that it is an effective path to functionalize rare earth coordination polymers with [H2NMe2]+ by means of ion exchange interaction. Furthermore, Tb3+ ⊂ [Y(DPA)3] is selected to explore their potential for sensing metal ions, which performs apparently selective and sensitive luminescence sensor for Cr3+ ion.