Integrating multiple emission centers for photoluminescence regulation in copper–antimony/bismuth halides

Abstract

Zero-dimensional (0-D) heterometallic halides containing multiple metal–halogen units are emerging optoelectronic materials with diverse photophysical properties. In this work, eight 0-D ionic compounds, including heterometallic halides [(Tp)₃CuX]MX₆ (Tp = protonated thiomorpholine; X = Cl, Br; M = Sb, Bi), monometallic halides (Tp)₃MX₆ (M = Bi when X = Cl; M = Bi, Sb when X = Br), and (Tp)Br have been synthesized. Their structures and photoluminescence have been comparatively studied. Inserting a CuX unit into the three (Tp)⁺ cation moieties of (Tp)₃MX₆ results in the formation of [(Tp)₃CuX]MX₆ with a complex cation of [(Tp)₃CuX]³⁺. The assembly of two distinct metal–halogen units in the heterometallic halide enables charge transfer between the complex cation and anion unit, as verified by DFT calculations, and meanwhile achieves the wide modulation of the luminescent color (green to red region) resulting from the integration of both complex cationic and anionic emission centers in one single lattice of [(Tp)₃CuBr]MBr₆. The luminescence mechanisms of monometallic and heterometallic halides are elucidated by detailed structural and spectral comparisons. Moreover, the compound [(Tp)₃CuBr]SbBr₆ shows significant potential for low-temperature optical temperature sensing applications based on the fluorescence intensity ratio of its dual emissions, with absolute sensitivity (S_a) and relative sensitivity (S_r) values of 0.078 K⁻¹ and 5.38% K⁻¹, respectively. This study not only provides a new strategy for developing heterometallic halide photoluminescence materials, but also offers new ideas for a better understanding of the photophysical mechanism of 0-D heterometallic halides.

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Introductions

In recent years, zero-dimensional (0-D) inorganic–organic metal halides (IOMHs) have attracted much attention due to their excellent photophysical properties as well as the designable dual functionality of inorganic and organic moieties.^1–7 IOMHs have potential applications in various optoelectronic fields such as light-emitting diodes,^8,9 information storage and anti-counterfeiting encryption,^10,11 and X-ray scintillation.^12–14 IOMHs containing metal ions of the ns² type (e.g., Sb³⁺ and Bi³⁺) are well known for their unique photophysical properties.^4,15 The lone-pair stereo-activity of trivalent Sb³⁺ and Bi³⁺ leads to the formation of various metal halide anionic units with different geometrical configurations.^16,17 However, their corresponding photoluminescence (PL) features are relatively humdrum. For example, Sb³⁺ usually emits orange-yellow light^18,19 and Bi³⁺ usually emits blue light.²⁰ Thus, to enrich the PL of Sb³⁺ or Bi³⁺ based halides, introducing multiple emission centers into them is an expectable strategy.

Indeed, the integration of different metal–halogen units in metal halides can enrich their structural diversity and designability of photophysical properties.^21–36 For example, Zhi-Guo Xia et al. presented a series of compounds (C₉NH₂₀)₉[Pb₃X₁₁](MX₄)₂ (C₉NH₂₀ = 1-buty-1-methylpyrrolidinium, X = Br and Cl, M = Mn, Fe, Co, Ni, Cu, and Zn) and emphasized the synergistic effects of the different metal halide units on photophysical properties.³⁷ Bi-Wu Ma et al. reported a novel 0-D (bmpy)₉[SbCl₅]₂[Pb₃Cl₁₁] (bmpy = 1-butyl-1-methylpyrrolidinium), in which the warm white light emissions could be obtained by combining green emission from [Pb₃Cl₁₁]⁵⁻ and orange emission from [SbCl₅]²⁻ unit.²¹ Ze-Wei Quan et al. reported a novel 0-D hybrid metal halide [Pb(C₁₂H₂₄O₆)Cl]₂[Mn₂Cl₆] with broad yellow emission originating from the d–d transition of the [Mn₂Cl₆]²⁻ dimer anion.³⁰ However, these reports on 0-D heterometallic halides mainly focus on lead-based heterometallic halides,^{25,26,28–30,37,38} which are not environmentally friendly due to the biotoxicity of Pb. Cu(I)-based IOMHs (Cu_nX_m(L)_z; X = Cl, Br, I; L = N, P, S-based ligands) are inexpensive and environmentally friendly, with a diversity of structures and PL properties.^8,39–43 Therefore, the combination of ns² metal–halogen and copper(I)–halogen units would endow IOMHs with interesting photophysical properties and even enable broadband luminescence modulation, which are still blank in the 0-D IOMHs family.

In this work, we synthesized eight 0-D ionic compounds, including [(Tp)₃CuX]MX₆ (Tp = protonated thiomorpholine; X = Cl, Br; M = Sb, Bi), (Tp)₃MX₆ (M = Bi when X = Cl; M = Bi, Sb when X = Br) and (Tp)Br to clarify the structure–property relationship in 0-D heterometallic halides. Their crystal structures, stability, and photophysical properties have been studied and compared. The heterometallic halides [(Tp)₃CuX]MX₆ are formed by introducing a CuX unit into the corresponding monometallic halides. The assembly of two distinct metal–halogen units in the heterometallic halide achieves charge transfer between the complex cation and anion unit, as verified by Density Functional Theory (DFT) calculations. Furthermore, such an assembly allows for the wide modulation of the emission color (green to red region), which results from the integration of both complex cationic and anionic emission centers in one single lattice. Through the detailed structural and spectral comparisons, their photophysical mechanisms are expounded. Our study not only introduces a new strategy for novel metal halides with rich luminescent behiviors, but also provides new insights into the photophysical mechanism of 0-D heterometallic halides.

Results and discussion

Crystal structure and stability

Considering the good coordination ability of S and Cu atoms, we therefore chose a S-containing organic, namely thiomorpholine, as a reactant. Single crystals of (Tp)Br were synthesized in a mixture of thiomorpholine and hydrohalic acid, which was heated to complete dissolution and then cooled naturally. Three monometallic halides (Tp)₃MX₆ were synthesized by mixing SbBr₃ or BiX₃ (X = Cl, Br) and thiomorpholine in a hydrohalic acid solution, which was heated to a completely clear solution and then cooled naturally. By using a similar synthetic method, the addition of CuX (X = Cl, Br) as a reactant would result in two copper–antimony heterometallic halides [(Tp)₃CuX]SbX₆ (X = Cl, Br) and two copper–bismuth heterometallic halides [(Tp)₃CuX]BiX₆ (X = Cl, Br) (Scheme 1). Detailed synthetic methods for all compounds are given in the Experimental section of the ESI.† The crystal structures of all compounds are determined using single crystal X-ray diffraction at both room temperature and 100 K. Tables S1–S20† provide crystallographic data, structural refinement details, selected bond lengths and bond angles for all title compounds, as well as hydrogen bonding data.

Scheme 1 Synthesis scheme of title compounds.

All title compounds are ionic 0-D structures. The asymmetric unit of (Tp)Br consists of a protonated thiomorpholine cation and a bromide anion (Fig. S1a and b†). The asymmetric unit of monometallic halides consists of three protonated thiomorpholine cations and an octahedral [MX₆]³⁻ unit (X = Cl or Br, M = Sb or Bi) (Fig. 1a and b). The asymmetric unit of heterometallic halides has a complex cation [(Tp)₃CuX]³⁺ and an octahedral anionic unit [MX₆]³⁻ (Fig. 1d and e). Detailed structural descriptions are presented in ESI.†

Fig. 1 Single-crystal molecular structural diagrams of (Tp)₃SbBr₆ (a) and [(Tp)₃CuBr]SbBr₆ (d). Packing diagrams of (Tp)₃SbBr₆ (b) and [(Tp)₃CuBr]SbBr₆ (e) viewed along the b axis. Topological nets of pcu type for (Tp)₃SbBr₆ (c) and [(Tp)₃CuBr]SbBr₆ (f), in which the reddish-purple ball represents [SbBr₆]³⁻ and the dark blue balls represents three (Tp)⁺ moieties and the light blue ball represents the [(Tp)₃CuX]³⁺.

Comparing the structures of monometallic and heterometallic halides using (Tp)₃SbBr₆ and [(Tp)₃CuBr]SbBr₆ as examples, it is found that their anionic unit [SbBr₆]³⁻ is arranged in a similar manner in the packing structure (Fig. 1a–f). However, the coordination of CuBr reduces the shortest distance of organic components from 10.03 Å in (Tp)₃SbBr₆ to 7.21 Å in [(Tp)₃CuBr]SbBr₆. By contrast, the shortest distance between the anionic [SbBr₆]³⁻ units in [(Tp)₃CuBr]SbBr₆ increases by 0.21–0.78 Å compared to that in (Tp)₃SbBr₆ (Fig. S2a–f†). More molecular structural diagrams and structure packing diagrams for each compound are given in Fig. S3–S5.† The bond lengths of Bi–X and Sb–X in all compounds are comparable to those previously reported for antimony(III) and bismuth(III) halides.^44–48 In addition, the degree of distortion of the anionic octahedra of all compounds at 100 and 297 K is evaluated using eqn (S1) and (S2),† and their σ² and Δd values are listed in Table S5.† All of the experimental powder X-ray diffraction (PXRD) patterns for all compounds match well with the corresponding simulated ones (Fig. S6a–d and S7a–d†), indicating that the synthesized compounds are high-purity phases.

The PXRD patterns (Fig. S8a–d†) for the powder samples of [(Tp)₃CuCl]SbCl₆ and [(Tp)₃CuCl]BiCl₆ after storing at ambient condition for 6 months and [(Tp)₃CuBr]SbBr₆ and [(Tp)₃CuBr]BiBr₆ after storing at ambient condition 12 months are consistent with that of corresponding as-synthesized samples, indicating their good stability. The high stability of four heterometallic halides can be attributed to their abundant hydrogen bonding in the structures.^12,49 The thermal stability of all compounds was also investigated through thermogravimetric (TG) analysis. Compounds [(Tp)₃CuCl]SbCl₆, [(Tp)₃CuCl]BiCl₆, and (Tp)₃BiCl₆ have no weight loss until around 225 °C (Fig. S9a and S9c†). Compounds [(Tp)₃CuBr]SbBr₆ and (Tp)₃SbBr₆ have no weight loss until around 250 °C (Fig. S9b and S9d†). Similarly, compounds [(Tp)₃CuBr]BiBr₆ and (Tp)₃BiBr₆ have no weight loss until around 280 °C (Fig. S9b and S9d†). This is consistent with that of other reported 0D metal halides with good thermal stability.^30,50,51 Based on the above characterizations, the title compounds have excellent stability.

Optical absorption and DFT calculation

The [(Tp)₃CuCl]SbCl₆ and [(Tp)₃CuCl]BiCl₆ are colorless and transparent under ambient light, while [(Tp)₃CuBr]SbBr₆ and [(Tp)₃CuBr]BiBr₆ show a similar yellowish transparency. (Tp)₃SbBr₆ and (Tp)Br are yellowish transparent and compounds (Tp)₃BiCl₆ and (Tp)₃BiBr₆ are light-pink transparent, which is consistent with their optical absorption spectra shown in Fig. S10a–d and S11a–d.† The optical absorption edges for [(Tp)₃CuCl]SbCl₆, [(Tp)₃CuCl]BiCl₆, [(Tp)₃CuBr]SbBr₆, and [(Tp)₃CuBr]BiBr₆ are 385, 380, 450, and 440 nm, respectively (Fig. S10a–d†). The optical absorption edges for (Tp)₃BiCl₆, (Tp)₃BiBr₆, (Tp)₃SbBr₆, and (Tp)Br are 373, 422, 425, and 426 nm, respectively (Fig. S11a–d†).

To further understand the effect of organic components on the bandgap transition and their electronic structure, DFT calculations were performed for [(Tp)₃CuCl]SbCl₆, [(Tp)₃CuBr]SbBr₆, and (Tp)₃SbBr₆. The calculated band gaps for [(Tp)₃CuCl]SbCl₆, [(Tp)₃CuBr]SbBr₆, and (Tp)₃SbBr₆ are 3.01 eV, 2.71 eV, and 2.94 eV, respectively, all with an indirect band gap (Fig. S12a–c†). Their DFT calculated band gap results show that the introduction of CuBr narrows the band gap of [(Tp)₃CuBr]SbBr₆, which is comparable to the experimental absorption edge. The density of states (DOSs) for heterometallic halides [(Tp)₃CuCl]SbCl₆ and [(Tp)₃CuBr]SbBr₆ were analysed (Fig. 2a and d). The valence-band maximum (VBM) of [(Tp)₃CuCl]SbCl₆ and [(Tp)₃CuBr]SbBr₆ is mainly contributed by Cu d electrons, halogen p electrons and protonated thiomorpholine (Fig. 2b and e), while the conduction-band minimum (CBM) is dominated by Sb p electrons, halogen p electrons and protonated thiomorpholine (Fig. 2c and f). The DOSs of the monometallic bromide (Tp)₃SbBr₆ (Fig. 2g) reveal that the VBM is mainly contributed by Sb s electrons, Br p electrons, and protonated thiomorpholine, whereas the CBM is mainly contributed by Sb p electrons and Br p electrons (Fig. 2h and i).

Fig. 2 Density of states (DOSs) for [(Tp)₃CuCl]SbCl₆ (a), [(Tp)₃CuBr]SbBr₆ (d), and (Tp)₃SbBr₆ (g). The C₄H₁₀NS refers to the protonated thiomorpholine. The valence-band maximum (VBM) diagram of [(Tp)₃CuCl]SbCl₆ (b), [(Tp)₃CuBr]SbBr₆ (e), and (Tp)₃SbBr₆ (h). The conduction-band minimum (CBM) diagram of [(Tp)₃CuCl]SbCl₆ (c), [(Tp)₃CuBr]SbBr₆ (f), and (Tp)₃SbBr₆ (i). The round-like electronic cloud represents the s-electrons, the spindle-like electronic cloud represents the p-electrons, and the four-dumbbell-shaped electronic cloud represent the d-electrons.

Photoluminescence

The PL spectra, the photoluminescence excitation (PLE) spectra, and time-resolved PL spectra have been performed for all compounds. As shown in Fig. 3a, the PLE bands of [(Tp)₃CuCl]SbCl₆ around 365 nm can be attributed to the triplet state transition of Sb³⁺.^52–56 [(Tp)₃CuCl]SbCl₆ shows orange-yellow emission at ambient temperature and orange-red emission at 80 K (Fig. 3a). At 300 K, [(Tp)₃CuCl]SbCl₆ has a maximum emission peak at 590 nm, and a large Stokes shift of 225 nm with full width at half maximum (FWHM) of 133 nm (Fig. S13a†). At 80 K, the maximum peak of [(Tp)₃CuCl]SbCl₆ emission is 615 nm with FWHM of 95 nm and a large Stokes shift of 250 nm (Fig. 3a). Notably, upon cooling to 80 K, the emission peaks show a variable redshift of 25 nm compared to that at room temperature. The PL lifetimes of the emission peaks of [(Tp)₃CuCl]SbCl₆ at 590 nm was 15.57 ns at 300 K, and that of [(Tp)₃CuCl]SbCl₆ at 615 nm was 4.21 μs at 80 K (Fig. S13b and c†), which suggested that the orange-yellow emission may originate from a partially permissive ³P₁ → ¹S₀ transition of the [SbCl₆]³⁻ units.^44,46,56 The thermal quenching effect should be the reason of the decreased PL lifetime along with the increased temperature.⁴⁴ It is noteworthy that the isomeric heterometallic bromide [(Tp)₃CuBr]SbBr₆ exhibits distinct luminescence at different temperatures. [(Tp)₃CuBr]SbBr₆ emits an orange light at room temperature, which changes to a red light as the temperature decreases to about 160 K, and to a yellow-green light when the temperature reaches 80 K (Fig. 3b and g). At 300 K, the maximum emission peak of [(Tp)₃CuBr]SbBr₆ is at 625 nm under 380 nm and 430 nm excitation (Fig. S14a and b†). At 80 K, under 380 nm or 355 nm excitation, the maximum emission peak of [(Tp)₃CuBr]SbBr₆ is at 650 nm with a shoulder peak at 545 nm (Fig. 3b). Interestingly, the excitation spectra of the two emission peaks (650 nm and 545 nm) are different, suggesting that the origin of the two emission peaks is quite different. Different excitation wavelengths and temperatures result in different intensity ratios of the main and shoulder peaks.

Fig. 3 The PLE and PL spectra for [(Tp)₃CuCl]SbCl₆ (a), [(Tp)₃CuBr]SbBr₆ (b), (Tp)₃SbBr₆ (c), (Tp)₃BiCl₆ (d), (Tp)₃BiBr₆ (e), and [(Tp)₃CuBr]BiBr₆ (f). (g) Luminescence of six title compounds under 365 nm UV lamp at room temperature and low temperature.

To further determine the origin of the red and green emissions in [(Tp)₃CuBr]SbBr₆, the PL spectra of the monometallic halide (Tp)₃SbBr₆ and the organic bromide salt (Tp)Br are tested. At 300 K, the highest emission peak of (Tp)₃SbBr₆ is at 610 nm (Fig. S15a†), whereas at 80 K, the highest emission peak of (Tp)₃SbBr₆ is slightly red-shifted to 620 nm (Fig. 3c). At 300 K, (Tp)Br exhibits broadband emission with a maximum peak of 575 nm at 355 nm excitation and 545 nm at 450 nm excitation (Fig. S16a†). Comparison of the luminescence behavior of compounds [(Tp)₃CuBr]SbBr₆, [(Tp)₃CuCl]SbCl₆, and (Tp)₃SbBr₆ shows that the source of their emission around ∼620 nm can be attributed to the ³P₁ → ¹S₀ transition of the anionic octahedral [SbX₆]³⁻ unit.^55–58 and the difference in the positions of their emission peaks may be due to the different halogens in the anionic octahedral [SbX₆]³⁻ unit.⁵⁹ Based on the structural features and luminescent properties of the complex cation, it is suggested that the 545 nm shoulder PL peak of [(Tp)₃CuBr]SbBr₆ should come from the metal–halide-to-ligand charge transfer [(M + X)LCT] of the complex cation,^39,60–62 which is in good agreement with the results of DFT calculation (Fig. 2d–f).

The PL properties of Bi-based halides have been studied as well. There are three bands around 310 nm, 415 nm, and 518 nm in the PLE of (Tp)₃BiCl₆ at 80 K (Fig. 3d). Compared with the PLE of (Tp)Br (Fig. S16a†), we can infer that the major contribution of (Tp)₃BiCl₆ PLE should be from the organic component. Under the excitation of these three PLE bands, there are four emission bands in the PL spectra, that is, a narrow peak at 355 nm, a shoulder band around 490 nm, and two broad peaks around 575 and 615 nm (Fig. 3d). The PL band around 575 nm is mirror-symmetric with PLE band around 518 nm, which is similar with the PLE of (Tp)Br (Fig. S16a†). Thus, the two broad emission bands (i.e., 575 nm and 615 nm) should originate from the organic cation. The 490 nm shoulder peak should be from the Bi³⁺ ion, which is consistent with the previous reports.^20,44,51 The sharp peak at 355 nm should have originated from the free exciton emission,⁴⁴ whose absorption can be observed in the PLE spectrum of [(Tp)₃CuBr]BiBr₆ (Fig. 3e). The PL and PLE of (Tp)₃BiBr₆ at 80 K are similar to those of (Tp)₃BiCl₆ as shown in Fig. 3e. There are three emission bands around 420, 495, and 585 nm under the excitation wavelengths of 355, 420, and 525 nm. The sharp peak around 420 nm should be from the free exciton emission.⁵¹ The shoulder peak around 495 nm is from the Bi³⁺ ion. The emission band around 585 nm is from the organic component.

The Bi-based heterometallic halides [(Tp)₃CuCl]BiCl₆ and [(Tp)₃CuBr]BiBr₆ are non-luminous at room temperature. Even more, [(Tp)₃CuCl]BiCl₆ is non-luminescent at 80 K. Fig. 3f shows that compound [(Tp)₃CuBr]BiBr₆ emits yellow-green double emission at 80 K. Under excitation at 359 nm, the highest emission peak of [(Tp)₃CuBr]BiBr₆ is at 655 nm with a shoulder peak at 545 nm. Under the excitation at 415 nm, [(Tp)₃CuBr]BiBr₆ has the maximum emission peak at 655 nm and the shoulder peak at 470 nm (Fig. 3f and S17a†). The shoulder peak 470 nm might be from the Bi³⁺ ion. Under the excitation at 430 nm, the broadband emission peak of [(Tp)₃CuBr]BiBr₆ is around 525 nm, which is very similar to that of (Tp)Br at 545 nm (Fig. 3f and S16a†). Thus, the peaks around 545 nm should be from the organic component. However, It is hard to explain the peak of 655 nm in [(Tp)₃CuBr]BiBr₆. In our opinion, the 655 nm peak should be from the organic component, as the same as the 615 nm peak of (Tp)₃BiCl₆. Typically, 0-D Bi-based bromides exhibit blue emission around 460 nm.^20,51 However, (C₅N₂H₁₄)₂[BiBr₆]Br·H₂O (C₅N₂H₁₄ = double-protonated homopiperazine) has a red emission (650 nm) at 77 K, which was ascribed to the triplet emission of the [BiBr₆]³⁻ anion.⁴⁵

In order to gain insight into the luminescence mechanism, we have performed the temperature-dependent emission spectroscopy of [(Tp)₃CuCl]SbCl₆, [(Tp)₃CuBr]SbBr₆, and [(Tp)₃CuBr]BiBr₆. As shown in Fig. 4a and b, the intensity of the emission (around 620 nm) of [(Tp)₃CuCl]SbCl₆ in the temperature range of 80 to 140 K increases gradually with the increase of temperature, and then decreases with the increase of temperature from 140 K to 320 K (Fig. 4a and b). The temperature-dependent PL behavior of [(Tp)₃CuCl]SbCl₆ can be attributed to thermally activated delayed fluorescence (TADF).^61–63 The emission color changes from orange-yellow to orange-red during the whole process (Fig. S18a†). The redshift of the emission peak from 590 nm to 615 nm and the shrinkage of the FWHM from 133 nm to 95 nm with decreasing temperature can be the result of the suppressed thermal expansion and electron–phonon interaction, respectively.^56,64 Since the Stokes shift is different for [(Tp)₃CuCl]SbCl₆ at low temperature (250 nm) and room temperature (225 nm), the distortion of the anionic octahedral [SbCl₆]³⁻ at 100 and 297 K is evaluated using eqn (S1) and (S2)† with σ² of 14.70 and 11.75, Δd of 25.79 and 21.63, respectively (Table S5†). At low temperatures, the greater distortion of the [SbCl₆]³⁻ anion in [(Tp)₃CuCl]SbCl₆ leads to a larger recombination distortion of [SbCl₆]³⁻ in the excited state, which results in a smaller radiative transition energy and ultimately in a large Stokes shift.^46,65,66

Fig. 4 (a) Temperature-dependent PL spectra of [(Tp)₃CuCl]SbCl₆ in the temperature range of 80–320 K. (b) Temperature-dependent PL spectra contour map of [(Tp)₃CuCl]SbCl₆ from 80–320 K. (c) Temperature-dependent PL spectra of [(Tp)₃CuBr]SbBr₆ from 10–120 K. (d) Temperature-dependent PL spectra of [(Tp)₃CuBr]SbBr₆ from 100–250 K. (e) Temperature-dependent PL spectra contour map of [(Tp)₃CuBr]SbBr₆ from 100–250 K. (f) Temperature-dependent PL spectra contour map of [(Tp)₃CuBr]SbBr₆ from 10–120 K. (g) Temperature-dependent PL spectra of [(Tp)₃CuBr]BiBr₆ from 100–250 K. (h) Temperature-dependent PL spectra contour map of [(Tp)₃CuBr]BiBr₆ from 100–250 K. (i) Excitation-dependent PL spectra of [(Tp)₃CuBr]SbBr₆ at 10 K.

The electron–phonon coupling interaction can be evaluated by the Huang–Rhys factor (S) and electron–optical phonon coupling energy (Γ_op) from eqn (S3) and (S4),† respectively. The temperature-dependent spectral fitting of compound [(Tp)₃CuCl]SbCl₆ shows S and Γ_op values of 45.87 and 129 meV, respectively (Fig. S18b and c†). The S value is higher than that of previously reported metal halides, such as [Na(DMSO)₂]₃SbBr₃Cl₃ (10.7) and (PPZ)₂SbCl₇·5H₂O (28.0), which indicates that single crystals of [(Tp)₃CuCl]SbCl₆ exhibit strong electron–phonon coupling.^55,64,67 The Arrhenius formula (eqn (S5)†) is used to estimate the activation energy (E_a) for nonradiative recombination. The fitting results indicate that E_a is 253.87 meV (Fig. S18d†). The temperature-dependent PL analysis further suggests the soft lattice and strong electron–phonon coupling in [(Tp)₃CuCl]SbCl₆.

To investigate the emission phenomena of compound [(Tp)₃CuBr]SbBr₆, we conducted temperature-dependent spectra of [(Tp)₃CuBr]SbBr₆ from 100–250 K under the excitation of 380 nm. The emission peak of [(Tp)₃CuBr]SbBr₆ red shifts from 625 nm to 650 nm with increased intensity and the FWHM narrows from 144 nm to 102 nm, when the temperature decreases from 250 to 100 K (Fig. 4d, e, and S19a†). Similarly, the temperature-dependent PL spectra of [(Tp)₃CuBr]BiBr₆ from 250 to 100 K shows that the highest emission peak of [(Tp)₃CuBr]BiBr₆ is slightly red-shifted from 644 nm to 655 nm with improved intensity, and the FWHM narrows from 139 nm to 109 nm (Fig. 4g, h, and S19b†). The temperature-dependent PL behaviors can be attributed to the suppressed thermal expansion and electron–phonon interaction at low temperatures.^56,64 Their Stokes shifts are enlarged along with the decreased temperature, which should be related to the enhanced distortion (Table S5†).

The temperature-dependent dual emission of [(Tp)₃CuBr]SbBr₆ is further carried out in the range of 10–120 K under 355 nm excitation. Under 355 nm excitation, the green emission peak at 545 nm gradually decreases, while the red emission peak at 650 nm gradually increases as the temperature increases from 10 K to 120 K (Fig. 4c, f, and S20a†). Different dual peak shapes should be due to the varied energy transfer between cation and anion. At 10 K, the double peaks from the cation and [SbBr₆]³⁻ exist simultaneously due to their inhibited energy transfer at low temperatures. At 100 K, the emission peaks of the cations begin to disappear and only red emission of [SbBr₆]³⁻ is shown (Fig. 4m). This is due to the activation of energy transfer from the complexing cation to [SbBr₆]³⁻. The possibility of energy transfer is supported by the DOS result of [(Tp)₃CuBr]SbBr₆ (Fig. 2d). The excitation-dependent spectra of [(Tp)₃CuBr]SbBr₆ at 10 K show that the green light at 545 nm and the red emission at 650 nm coexist at the excitation wavelength from 320 to 400 nm, while it only displays red emission at the excitation wavelength above 400 nm (Fig. 4i and S19c†). Thus, the lower temperature and shorter excitation wavelength could enhance the emission of [(Tp)₃CuBr]³⁺, thus showing fluorescence emission with dual emission centers (Fig. 4f and i).

The pronounced change in the emission color of [(Tp)₃CuBr]SbBr₆ with temperature is further used for optical thermometry in the temperature range of 10–120 K (Fig. S20a–c and S19a†). The fluorescence intensity ratio (FIR) of the emission peaks at 545 nm and 650 nm of [(Tp)₃CuBr]SbBr₆ in the range of 10–120 K was used as an assessment of the sensitivity. The FIR (I₆₂₀/I₅₄₅) as a function of temperature was fitted by eqn (1) ^68,69 to obtain ΔE_T:

FIR = B + Ce^{−ΔE_T/k_BT} (1)

As important indicators, absolute sensitivity (S_a) and relative sensitivity (S_r) were calculated using the following eqn (2) and (3) to evaluate the properties of the luminescent thermometer.^23,68–70

S_a = Ce^{−ΔE_T/k_BT} × ΔE_T/k_BT² (2)

where B and C are constants, ΔE_T is the energy difference between two coupled levels, k_B is the Boltzmann constant, and T is the temperature. The energy gap of ΔE_T is calculated to be 29.66 meV (Fig. S21a†). At 80 K, the values of S_a and S_r calculated from eqn (2) and (3) are 0.078 K⁻¹ and 5.38% K⁻¹, respectively, which are comparable to those of other reported multicolor luminescence-based materials.^23,68,71 It is noteworthy that the emission color of [(Tp)₃CuBr]SbBr₆ changes from yellow-green to red as the temperature rises between 10–120 K (Fig. S20a†), and then gradually changes from red to orange-red as the temperature increases to 300 K, with the intensity of the light gradually becoming weaker (Fig. S21c†). This distinct color change can be a useful visual indicator of the temperature change of a substance, and the temperature dependence of the emission color of [(Tp)₃CuBr]SbBr₆ allows optical thermometry and thermochromism at low-temperature in the range 10–120 K (Fig. S20a–c†).

Conclusions

In summary, we have synthesized three 0-D monometallic halides (Tp)₃MX₆ using protonated thiomorpholine as the cationic ligand and have successfully synthesized four novel 0-D copper–antimony/bismuth heterometallic halides [(Tp)₃CuX]MX₆, taking into account that Cu has a better coordination capacity with S atoms. Their crystal structures, thermal stability, and photophysical properties have been investigated. DFT calculations reveal that in monometallic halides the electronic states are highly localized in the anionic octahedral unit, whereas in heterometallic halides charge transfer occurs between the complexing cations and the anionic octahedral unit. The PL data of [(Tp)₃CuCl]SbCl₆, [(Tp)₃CuBr]SbBr₆, and (Tp)₃SbBr₆ show that the PL mechanism of their broadband orange-red emission around 620 nm can be attributed to the ³P₁ → ¹S₀ transition of the anionic octahedral unit. Comparison of heterometallic halides [(Tp)₃CuBr]SbBr₆ and [(Tp)₃CuBr]BiBr₆ with the monometallic halides (Tp)₃SbBr₆, (Tp)₃BiCl₆, and (Tp)₃BiBr₆, and (Tp)Br verifies that their green light at 545 nm is attributable to the complex cation [(Tp)₃CuBr]³⁺. Based on the PL characteristics of [(Tp)₃CuBr]SbBr₆, which changes from yellow-green to red and then to orange-red light with temperature, it can be considered that it has the potential to be used as a low-temperature sensor. To the best of our knowledge, this is the first report on the in-depth study of the relationship of structure and photoluminescence properties for 0-D copper–antimony/bismuth heterometallic halides. This work provides a new strategy for designing 0-D heterometallic halides with low-cost and multiple luminescence centers.

Author contributions

A. A. and X. Y. H. conceived the project. A. A. designed and performed most of the experiments. A. A. performed DFT calculations and analyzed the data. A. A. performed the PL, PLE, PL decay, TG, UV, and XRD experiments. S. M. Z. offered help in the synthesis of compounds. G. Y. C. and J. H. L. offered help with crystal photographs and UV testing. A. A. prepared the manuscript. A. A., H. W. L., Z. P. W., K. Z. D. and X. Y. H. revised the manuscript. All the authors discussed the results and commented on the manuscript. X. Y. H. supervised the project.

Data availability

CCDC no. 2365627–2365634 and 2391052–2391054 contains the supplementary crystallographic data for this paper.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (No. 92261115, 22205236, and 22373014) and the Natural Science Foundation of Fujian Province (No. 2022J05089).

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Footnote

† Electronic supplementary information (ESI) available: Experimental section, crystal structure and bond length analyses, figures for PXRD, TG, and optical properties, and crystallographic data tables. CCDC 2365627–2365634 and 2391052–2391054 for all title compounds. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d4qi02614d

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