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. 2024 Aug 9;63(33):15323–15330. doi: 10.1021/acs.inorgchem.4c02058

Dual Emission with Efficient Phosphorescence Promoted by Intermolecular Halogen Interactions in Luminescent Tetranuclear Zinc(II) Clusters

Fumiya Kobayashi †,*, Yuta Takatsu †, Daisuke Saito ‡, Masaki Yoshida ‡, Masako Kato ‡, Makoto Tadokoro †,*
PMCID: PMC11337158  PMID: 39119626

Abstract

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The development of Zn-based phosphorescent materials, associated with a ligand-centered (LC) transition, is extremely limited. Herein, we demonstrated dual emissions including fluorescence and phosphorescence in luminescent tetranuclear Zn(II) clusters [Zn4LI4(μ3-OMe)2X2] (HLI = methyl-5-iode-3-methoxysalicylate; X = I, Br, Cl), incorporating iodine-substituted ligands. Single-crystal X-ray structural analyses and variable-temperature emission spectra studies revealed the presence of iodine substitutions, and intermolecular halogen interactions produced the internal/external heavy-atom effects and yielded strong green phosphorescence with a long emission lifetime (λmax = 510–522 nm, Φem = 0.28–0.47, τav = 0.78–0.95 ms, at 77 K). This work provided a new example that the introduction of halogen interactions is an advantageous approach for inducing phosphorescence in fluorescent metal complexes.

Short abstract

An example of dual emissions and temperature-dependent emission color modulation promoted by intermolecular halogen interactions, including the expression of efficient phosphorescence, has been demonstrated in tetranuclear zinc(II) clusters. In this system, the substituted iodine atom is the key for the corresponding luminescence properties.

Introduction

Luminescent Zn(II) complexes have attracted attention as a functional material in organic light-emitting devices and molecular probes; owing to their low cost and low toxicity, these complexes are an alternative to rare metal ions with strong phosphorescence, such as IrIII, PtII, RuII, and AuI.1−9 However, the intersystem crossing (ISC) promoted by spin–orbit coupling (SOC) is intrinsically weak in Zn(II) complexes, as their emission origin is mainly associated with a ligand-centered (LC) transition, resulting in fluorescent emission.6−9 Thus, achieving a high phosphorescence quantum yield (QY), which is industrially important for Zn(II) complexes, remains challenging. In the area of pure organic molecules, one direct and efficient way to facilitate ISC is the introduction of heavy atoms (e.g., halogens).10−12 The external heavy-atom effect (EHE) through orbital interactions between the heavy atoms and the luminophore increases both the ISC rate and the phosphorescence decay time of the excited-state luminophore.13−16 Kim et al. have reported purely organic phosphorescence molecules promoted by crystal engineering, through halogen bonding and the resulting EHE.17 In the recent decade, there has been a notable increase in reports concerning phosphorescence materials that use halogen bonding, indicating their significance and desirability.18−23 Among these, the development of a highly luminescent material that exhibits dual emission from the singlet excited state (fluorescence) and triplet excited state (phosphorescence)24−29 is significant owing to its contribution to time-resolved imaging technology, which has application in various fields such as in anticounterfeiting systems.30−33

Although halogen bonding is commonly used to enhance the phosphorescence of organic molecules, there have been very few reports on the application of EHE through halogen bonding in metal complex systems. The further modification of fluorescent metal complexes using halogen bonding is an attractive avenue to develop pioneering multifunctional phosphorescent metal complexes. In our previous study, we demonstrated that a heptanuclear Zn(II) cluster [Zn7L6(μ3-OMe)2(μ3-OH)4]I2 (HL = methyl-3-methoxysalicylate), incorporating iodide counteranions, exhibited strong phosphorescence with an exceptionally long emission lifetime, which can be attributed to the EHE.34 This study offered a unique case of EHE-induced phosphorescence in metal complex systems, where halogen interactions acted as the trigger for the corresponding phosphorescence. This mechanism provides an efficient means of introducing SOC into luminescent metal complexes, particularly those associated with LC transitions. Consequently, extending the application of this system to similar types of multinuclear Zn(II) clusters is promising for the further development of multifunctional luminescent materials. Herein, we report novel tetranuclear Zn(II) clusters [Zn4L4I(μ3-OMe)2X2] (HLI = methyl-5-iode-3-methoxysalicylate; X = I, Br, Cl), where the ligand HLI is iodine-substituted (Scheme 1). These Zn(II) clusters exhibit highly efficient phosphorescence and temperature-dependent emission color modulation, which are characteristics not observed in substitution-free Zn(II) clusters [Zn4L4(μ3-OMe)2X2] (X = NCS, Cl, Br).34

Scheme 1. Molecular Structure of HL (3-Methoxysalicylic Acid Methyl Ester) and HLI (5-Iode-3-methoxysalicylic Acid Methyl Ester).

Scheme 1

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Experimental Section

Synthesis

All reagents and solvents were obtained from Tokyo Kasei Co. and Wako Pure Chemical Industries and were of reagent grade; they were used without further purification. All reactions were carried out under an ambient atmosphere.

Preparation of Methyl-5-iode-3-methoxysalicylate (HLI)

HLI was prepared according to the method reported previously.35 The single crystal suitable for single X-ray structural analysis was obtained by recrystallization from ethyl acetate/hexane (4:1) as the colorless crystals. 1H NMR (400 MHz, DMSO-d6): 3.80 (s, 3H), 3.85 (s, 3H), 7.40 (d, 1H), 7.58 (d, 1H), 10.41 (s, 1H) (Figure S1). IR (KBr/cm–1): 3134, 3086, 2947, 2723, 1676, 1572, 1473, 1439, 1392, 1346, 1290, 1254, 1233, 1198, 11,167, 1059, 885, 862, 785, 725, 690, 629, 571.

Preparation of Tetranuclear Zinc(II) Complexes

Tetranuclear Zn(II) clusters were synthesized according to the method we described previously (with minor modifications).36

[Zn4L4I(μ3-OMe)2I2] (1)

Triethylamine (0.202 g, 2.00 mmol) in methanol (10 mL) was added with stirring to methanol (20 mL) containing HLI (0.31 g, 1.00 mmol) and ZnI2 (0.31 g, 1.00 mmol). The reaction mixture was stirred for 30 min at room temperature under air. The white microcrystals were participated and collected by suction filtration, washed with a small amount of methanol, and dried in air. Yield 78%. The single crystal suitable for the single X-ray structural analysis was obtained by allowing the mixed solution to stand for a few days to yield 1 as colorless block crystals. Anal. Calc. for 1 (C38H38I6O18Zn4): C, 25.28; H, 2.12; I, 42.17%. Found: C, 24.95; H, 2.34; I, 41.32%. IR (KBr/cm–1): 3087, 2956, 2819, 1664, 1647, 1583, 1552, 1464, 1441, 1348, 1323, 1228, 1186, 1105, 1061, 976, 895, 849, 808, 793, 704, 602, 567, 553.

[Zn4L4I(μ3-OMe)2Br2] (2)

Complex 2 was prepared by a procedure similar to that employed for 1, except that ZnBr2 was used instead of ZnI2. 2 was obtained as colorless crystals which were collected by suction filtration, washed with a small amount of methanol, and dried in air. Yield 47%. Anal. Calc. for 2·H2O (C38H38I4Br2O18Zn4): C, 26.12; H, 2.42; Br, 9.14%. Found: C, 25.83; H, 2.35; Br, 9.39%. IR (KBr/cm–1): 3087, 2952, 2827, 1662, 1645, 1583, 1552, 1464, 1442, 1348, 1323, 1228, 1188, 1107, 1063, 974, 983, 850, 808, 793, 704, 667, 592, 566, 553.

[Zn4L4I(μ3-OMe)2Cl2] (3)

Complex 3 was prepared by a procedure similar to that employed for 1, except that ZnCl2 was used instead of ZnI2. 3 was obtained as colorless crystals, which were collected by suction filtration, washed with a small amount of methanol, and dried in air. Yield 54%. Anal. Calc. for 3 (C38H38I4Cl2O18Zn4): C, 28.13; H, 2.36; I, 31.28; Cl, 4.37%. Found: C, 28.03; H, 2.14; I, 31.28; Cl, 4.84%. IR (KBr/cm–1): 3086, 2945, 2843, 1674, 1606, 1572, 1473, 1439, 1394, 1346, 1315, 1288, 1254, 1232, 1198, 1167, 1061, 978, 885, 862, 845, 785, 725, 692, 629, 571.

Physical Measurements

1H NMR spectrum for HLI was measured with a JEOL JNM-ECZS instrument. Elemental analyses (C, H, N, Cl, Br, and I) were performed on a J-Science Lab JM10 CHN analyzer. Infrared (IR) spectra measurements were performed on a HORIBA FT-730 instrument equipped with the KBr pellet method. Fast atom bombardment (FAB) mass spectra for 1 were measured with a JEOL JMS-AX505HA instrument with 3-nitrobenzyl alcohol (NBA) matrix.

Single-Crystal and Powder X-ray Diffraction

The single-crystal X-ray diffraction data for 1–3 and HLI were recorded on a Bruker D8 QUEST diffractometer employing graphite monochromated Mo Kα radiation generated from a sealed tube (λ = 0.7107 Å). Data integration and reduction were undertaken with APEX3. Using Olex2 software, the structure was solved with the SHELXT structure solution program using Intrinsic-Phasing Methods and refined with the SHELXL refinement package using least squares minimization. Hydrogen atoms were included in idealized positions and refined using a riding model. Powder X-ray diffraction data (PXRD) for 1–3 were collected on a Rigaku MiniFlex II (40 kV/15 mA) X-ray diffractometer using Cu Kα radiation (λ = 1.5406 Å) in the 2θ range of 5–30° with a step width of 3.0°.

Luminescence Property Measurements

Emission spectra at 298 and 77 K (Figures 2a,b and S5–S7) were measured using a JASCO FP-6600 spectrofluorometer. Emission quantum yields were recorded using a Hamamatsu Photonics C9920-02 absolute photoluminescence quantum yield measurement system equipped with an integrating sphere apparatus and 150 W CW xenon light source. The accuracy of the instrument was confirmed by the measurement of quantum yield of anthracene in ethanol solution (Φ = 0.27).37,38 Emission lifetimes were recorded by using a Hamamatsu Photonics C4780 system equipped with a streak camera (Hamamatsu Photonics C4334) as a photodetector and a nitrogen laser (Usho KEN-X) for the 337 nm excitation. The emission decays of complexes were analyzed using two exponentials, i.e., I = A1 exp(−t/τ1) + A2 exp(−t/τ2), where τ1 and τ2 denote the lifetimes, and A1 and A2 are the pre-exponential factors. Therefore, for the determination of radiative and nonradiative rate constants, the averaged emission lifetimes (τav) were estimated using the following equation:39

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Variable-temperature emission spectra in the solid state and MeOH solution at 300–77 K (Figures 2c,d, and 5) were measured using a SHIMADZU RF-6000 spectrofluorometer equipped with a Unisoku Cryostat (CoolSpek USP-203) thermostated cryostat cell holder.

Figure 2.

Figure 2

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Emission spectra for (a) HLI and (b) 1 in the solid state at 298 K (black) and 77 K (red) (inset: emission images under UV light (365 nm)). (c, d) Temperature dependence of emission spectra for 1 at 300–100 K. (e) Emission-integrated intensity ratio (Iphosphorescence/Ifluorescence) for 1 at 200–77 K in the solid state (red) and MeOH solution (blue) (inset: enlarged figure). (f) CIE 1931 chromaticity diagrams for emission spectra of HLI and 1–3 in the solid state at 298 and 77 K.

Figure 5.

Figure 5

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(a) Temperature dependence of emission spectra for 1 at 300–77 K in MeOH (1.0 × 10–5 M). (b) Enlarged area in (a).

Results and Discussion

Crystal Structures

HLI was prepared according to the method reported previously (Figure S1).35 The three tetranuclear Zn(II) clusters [Zn4L4I(μ3-OMe)2X2] (X = I (1), Br (2), Cl (3)) were synthesized using our previously reported methods with minor modifications.34,36 Colorless crystals of 1–3 were obtained by allowing a mixed solution of the required Zn(II) salt, HLI, and triethylamine in methanol to stand at room temperature for a few days (Scheme 2). The obtained compounds were characterized by elemental analysis and single-crystal and powder X-ray diffraction (XRD) measurements.

Scheme 2. Synthetic Scheme of 1–3.

Scheme 2

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Single-crystal X-ray structural analyses for 1–3 were carried out at 173 K. Individual structures of the tetranuclear Zn(II) clusters 1–3 are shown in Figures 1 and S2a, with the crystallographic data presented in Table S1. Each compound features a “defective” double-cubane core [Zn4O6], where four Zn(II) ions are bridged by μ2-O atoms and μ3-methoxo groups from the deprotonated LI– ligands (Figure S3b). One of the ligands LI– is disordered. In 1, two distinct octahedral Zn(II) coordination spheres are present (Figure S3a). One sphere is formed by six oxygen atoms: two from bridging methoxo groups and four from bridging phenoxo group and carbonyl group on the LI– ligands. The other sphere consists of an anion and four oxygen atoms: one from a bridging methoxo group and three from bridging phenoxo and methoxy groups on the LI– ligands. 2 and 3 have structures identical to 1 (Figures S2 and S4). The tetranuclear structures of 1–3 are similar to those reported for Zn(II) clusters, which incorporate HL ligand derivatives.34,40

Figure 1.

Figure 1

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Crystal structure of tetranuclear Zn(II) cluster [Zn4L4I(μ3-OMe)2I2] (1) at 173 K. Color code: gray, Zn; red, O; light gray, C; purple, I; white, H.

Luminescence Properties

The solid-state excitation and emission spectra of the HLI ligand at 298 and 77 K are shown in Figures 2a and S5 (Table S2). At 298 K, the crystalline solid sample of HLI exhibited light blue emission. The emission spectrum exhibited a fluorescence band with a maxima (λmax) at 473 nm attributed to the 1ππ* transition. The solid-state excitation and emission spectra of compounds 1–3 at 298 and 77 K are presented in Figures 2b and S6, S7 (Table S2). At 298 K, crystalline solid samples of 1–3 displayed weak blue luminescence. The emission spectra for 1–3 featured broad, unstructured emission bands with maxima (λmax) at 425, 438, and 425 nm, respectively. While slight variations in the emission maxima for 1–3 were noted, the overall emission profiles were similar. The observations described above for 1–3 suggest that the emission maxima of 1–3 are largely independent of the coordinating anions. Importantly, in the spectra at 77 K, emission maxima of 510–522 nm were observed, which can be attributed to phosphorescence, alongside the emission bands at 425–438 nm. In addition, we evaluated the temperature dependence of the emission behavior at 300–100 K for 1 (Figure 2c,d). Although the emission intensities of the maxima increased slightly, the emission spectra remained almost unchanged below 250 K. Additional emission bands at 510 nm appeared from 200 K, corresponding to the emission-integrated intensity ratio of Iphos./Ifluo. (=Iphosphorescence/Ifluorescence) increased to 1.3, which was almost 1:1, at 150 K (Figure 2e). Although the phosphorescence intensity increased significantly with decreasing temperature, the fluorescence intensity was almost saturated. The emission intensity ratios of Iphos./Ifluo. at 100 and 77 K were 12.5 and 66.2, respectively, indicating that the main contributor to the presented emission in the low-temperature region was phosphorescence. In our previous work for the heptanuclear Zn(II) cluster [Zn7L6(μ3-OMe)2(μ3-OH)4]I2,34 the emission intensity ratio of Iphos./Ifluo. was 0.66 at 77 K, and no significant emission color change was observed even at 77 K (Figure S8). Therefore, efficient promotion of the formation of the triplet state by internal and/or external heavy-atom effects via intermolecular halogen interactions is expected for 1–3. The emission images and CIE 1931 coordinates for the emission spectra of HLI and 1–3 clearly illustrate the color change of the emission from deep blue to green for 1–3, attributed to the expression of phosphorescence at low temperature (Figure 2b, inset, and Figure 2f), whereas the emission colors of HLI were almost unchanged even at 77 K (Figure 2a, inset).

Emission QYs (Φem) and emission lifetimes (τ) were measured for HLI and 1–3 (Tables 1 and S3, Figure S9). The emission QYs of HLI and 1–3 were identical (0.02) at 298 K. Due to the low emission QYs, the emission lifetimes at 298 K could not be detected. At 77 K, the emission QYs of 1, 2, and 3 clearly increased to 0.47, 0.31, and 0.28, respectively, whereas that of HLI was almost unchanged (0.03). The emission lifetimes (τav) of 1, 2, and 3 were in the millisecond range of 0.78, 0.91, and 0.95 ms, respectively. These values are consistent with the emission band attributed to phosphorescence involving the lowest-excitation triplet state (T1 state). HLI exhibited a QY value at 0.03 that was significantly smaller than that found for the Zn(II) clusters (0.28–0.47). This difference in values is likely due to the ISC generated by the EHE of the intermolecular halogen interactions present in this case, as described below. The radiative decay rate constants (kr) for 1–3 are 5.99 × 102, 3.39 × 102, and 2.95 × 102 s–1, respectively. The order of magnitudes of these values may be reflected in the order of the heavy-atom effect for the coordinated halogen anions (I– > Br– > Cl–). Although phosphorescence was not clearly observed at room temperature owing to thermal deactivation, the overall results highlight the significant contribution of the triplet excited states of 1–3, which are generated by the presence of iodine substitutions and EHE. The emission origin of the Zn-based phosphorescent molecular materials reported so far is predominantly associated with metal-to-ligand charge transfer (MLCT),41−44 halogen-to-ligand charge transfer (XLCT),45−47 and intraligand charge transfer (ILCT),48 with extremely limited instances of phosphorescence reported to originate from LC transitions.34,49 Additionally, due to a scarcity of reports evaluating photophysical parameters, such as rate constants, the investigation of the estimated kr values in this study is highly important. Notably, the fact that these kr values for 1–3 are comparable to those reported for a Zn(II) complex exhibiting phosphorescence arising from Zn–Zn interactions (4.2 × 102 s–1, at 77 K)50 is noteworthy.

Table 1. Photophysical Properties for HLI and 1–3 in the Solid State at 77 K.

  HLI 1 2 3
λema/nm 475 510 522 519
Φemb 0.03 0.47 0.31 0.28
τemc/ms   0.784 0.914 0.948
krd/s–1   5.99 × 102 3.39 × 102 2.95 × 102
knre/s–1   6.76 × 102 7.55 × 102 7.59 × 102
kr/knr   0.89 0.45 0.39
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a

Emission maximum, λex = 300 nm.

b

Photoluminescence quantum yields, λex = 337 nm.

c

Emission lifetime.

d

Radiative decay rate constants (kr) were estimated using the equation: Φem/τem.

e

Nonradiative decay rate constants (knr) were estimated using the equation: kr(1 – Φem)/Φem.

The observed differences in the photophysical properties of 1–3 and HLI can be attributed to the differences in their crystal structures and the resulting intermolecular interactions. The crystal packing diagrams of compounds 1–3 are shown in Figures 3 and S10–S13. For 1, intermolecular interactions via halogen atoms, such as I−π interaction (I(1)···C(6) = 3.58(1) Å, I(1)···C(7) = 3.657(9) Å), CH–I interactions (C(9)–H(9A)···I(1) = 3.142 Å), and I–I interactions (I(1)···I(3) = 3.827(1) Å), were observed, indicating that all halogen atoms strongly interact with neighboring molecules (Figure 3 and Table S4). Similar intermolecular interactions via halogen atoms were presented in 2 and 3 (Figures S10–S13, Tables S5 and S6). Meanwhile, for the crystal packing structure for HLI at 173 K, HLI dimerized by π–π interactions (C(7)···C(7)* = 3.260(4) Å, O(2)···O(2)* = 3.026(3) Å) (Figure 4a and Table S7). Each dimer interacted with neighboring dimers by CH–O interactions (C(1)–H(1A)···O(2) = 2.518 Å, C(3)–H(3)···O(1) = 2.695 Å), forming three-dimensional supramolecular interactions (Figure 4b). Based on the crystal structure analysis, halogen interactions occurred between the LI– ligands of each molecule in 1–3, whereas no halogen-related interactions occurred for HLI. This is a key factor in the observed phosphorescence differences between 1–3 and HLI in the solid state.

Figure 3.

Figure 3

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Packing structure of 1 at 173 K. All disordered atoms have been omitted for the sake of clarity. Blue dashed lines represent halogen-related interactions (CH–I, I–I, and I−π interactions).

Figure 4.

Figure 4

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(a) Dimeric structure of HLI at 173 K. (b) Crystal packing structure of HLI at 173 K. Blue dashed lines represent π–π interactions, and orange-dashed lines represent CH–O interactions between each dimer.

To evaluate the contribution of the internal heavy-atom effect (IHE) and EHE, we investigated the variable-temperature emission spectrum for 1 in solution (MeOH, 1.0 × 10–5 M) (Figure 5). At 300–200 K, 1 exhibited a weak blue emission with an emission maximum at 425 nm. This is because nonradiative transitions reflect the violent molecular vibrations of 1 in solution. At 150 K, the emission intensity at 425 nm increased drastically (approximately 9 times). This can be attributed to the inhibition of molecular vibrations induced by the frozen MeOH solution. However, phosphorescence at 510 nm was not clearly observed at 150 K. Upon further decreasing the temperature, the emission bands remarkably appeared at 510 nm, which can be attributed to the phosphorescence. The emission-integrated intensity ratio of Iphos./Ifluo. at 150 and 77 K was 0.2 and 2.6, respectively (Figure 2e). Compared with that in the solid state, phosphorescence in the solution state decreased, which indicates the key role played by EHE, rather than IHE, induced by intermolecular halogen interactions. To reveal the luminescent species, FAB mass spectrum (MS) analysis was performed for 1 in a MeOH solution. The observed spectrum showed dominant peaks at m/z = 1267.53, attributable to the heptanuclear species of {[Zn7L6I(μ3-OMe)(μ3-OH)5]I}2+, rather than a tetranuclear species. This indicates the possibility that dissolution in MeOH or ionization by MS leads to structural conversion to the heptanuclear species. However, it should be noted that these results do not lead to a precise identification of the correct luminescent species. On the other hand, the results of variable-temperature emission spectra for HLI in MeOH solution showed clearly different behaviors from those of 1 (Figure S14). At 77 K, HLI exhibited a structured fluorescence band at 405 nm and a broad phosphorescence band at 510 nm, both with similar intensity. Therefore, the difference in luminescence behavior between the solid state and solution of 1 can still be attributed to EHE due to intermolecular halogen interactions. The CIE 1931 coordinates for the temperature dependence of the emission spectra of 1 clearly demonstrate the emission color differences and modulations from deep blue to green, attributed to the expression of phosphorescence in both the solid state and the solution (Figure 6). The above unique photophysical properties observed in 1–3 were not observed in the substitution-free Zn(II) clusters.34,40

Figure 6.

Figure 6

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CIE 1931 chromaticity diagrams for the temperature dependence of emission spectra of 1 at 300–77 K in (a) the solid state and (b) MeOH solution.

Conclusions

In conclusion, we synthesized tetranuclear luminescent Zn(II) clusters [Zn4L4I(μ3-OMe)2X2] (X = I, Br, Cl) and demonstrated that the intermolecular halogen interactions produced the EHE for the expression of strong phosphorescence with long emission lifetime. An improvement over previous studies is noteworthy, as the observed temperature for clear phosphorescence has increased to around 200 K. Further improvement for the development of novel Zn(II) complexes exhibiting room-temperature phosphorescence is currently in progress. Importantly, the formation of halogen interactions by introducing halogen substitutions is an advantageous approach, which can result in highly efficient phosphorescence and functions related to the dual emissions51,52 in both organic molecules and fluorescent metal complexes. This result also suggests the possibility of achieving luminescence switching induced by structural rearrangements triggered by other external stimuli, such as mechanical force, and the development of novel optofunctional molecular systems.

Acknowledgments

This work was supported by the JSPS KAKENHI Grant-in-Aid for Early-Career Scientists JP23K13767 and the Tokuyama Science Foundation.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.inorgchem.4c02058.

  • NMR spectrum, crystallographic data, crystal structures, PXRD, excitation and emission spectra, photophysical data, and packing diagrams (PDF)

Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

The authors declare no competing financial interest.

Supplementary Material

ic4c02058_si_001.pdf (1.3MB, pdf)

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