Synthesis of luminescent coumarin-substituted phosphinoamide-bridged polynuclear gold(I) metallacycles and reactivity studies

Abstract

Herein, we present the first report of coumarin-based phosphinoamine ligands [CoumPNH] and their gold(I) complexes. The deprotonation and chloride abstraction from phosphinoamine gold(I) complexes [(CoumPNH)Au^ICl] led to the unexpected formation of polynuclear gold(I) metallacycles. The size of the cycles depends on the steric bulk of the phosphine groups. The gold(I) metallacycles feature only weak intramolecular aurophilic interactions and do not exhibit intermolecular interactions. Additionally, these metallacycles show significant luminescent properties. Quantum chemical calculations support the photophysical properties of the complexes. Notably, these compounds are the first polynuclear cyclic rings bridged by phosphinoamide ligands. Moreover, the reactivity of the cyclic compounds was explored in this study.

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Introduction

Cyclic gold(I) complexes are a unique class of metallacycles featuring intra- and intermolecular metallophilic interactions.^1–5 These molecular architectures have garnered significant interest in various research fields, including supramolecular assemblies, optoelectronics and acid–base chemistry.^6–11 Cyclic motifs containing Au^I cations can be constructed of different nuclearity, varying from trinuclear to hexanuclear complexes.^12–16 Such complexes were synthesized using several angular ditopic anionic bridging ligands, which include, e.g., pyrazolate, carbeniate, imidazolate, pyridinate, and 1,2,4-triazolate ligands (Fig. 1).^{6–8,17–19} Additionally, cyclic gold(I) rings can also be constructed from phosphanyl ligands and phosphorus ylides (Fig. 1).^20–22

Fig. 1 Previously reported cyclic gold(I) metallacycles.

Phosphinoamides ([PN]) are another such class of bridging ligands containing PN⁻ units,²³ which are analogous to pyrazolates and 1,2,4-triazolates containing NN⁻ units.²⁴ Phosphinoamide metal complexes are synthesized by deprotonating the respective phosphinoamine ligand, followed by its reaction with metal precursors or by transmetallation of the alkali metal phosphinoamides.^24–27 These ligands have been used as both chelating and bridging ligands to access several monometallic and heterobimetallic complexes with applications in the field of catalysis.^27–31 However, only a handful of [PN] based coinage metal complexes have been reported (Fig. 2).^32–35 Even though this ligand motif is isoelectronic to pyrazolates, until now, only bimetallic complexes have been reported with phosphinoamides.^36,37 Specifically, Braunstein and co-workers reported a phosphanyl iminolate-bridged Au–Pd monomer (Fig. 2a) and an unprecedented Ag–Pd coordination polymer (Fig. 2b).³⁸ Later, they also reported a N-(diphenylphosphino)-thiazolin-2-amine-bridged Au–Pt bimetallic complex (Fig. 2c).³⁹ Breher and co-workers reported an unusual dimeric Ag^I complex bridged by a PN motif (Fig. 2d).⁴⁰ Nagashima and co-workers used a phosphinoamide ligand to synthesize heterobimetallic complexes, yielding dimer complexes (Fig. 2e).⁴¹ However, these complexes were not explored for their photophysical properties.

Fig. 2 Previously reported [PN]-bridged coinage metal-containing complexes.

Recently, we have shown that incorporating 7-amino-4-methylcoumarin (dye) in the ligand backbone can significantly affect the photophysical properties of bisphosphinoamine and their copper(I) and silver(I) complexes.^9,42 The complexes exhibited visually visible ultralong phosphorescence at 77 K. The properties of these class of compounds were strongly phase-dependent.⁴²

Inspired by the promising luminescent nature of the coumarin-based metal complexes^43–46 and the lack of [PN]-bridged gold(I) complexes, herein, we designed phosphinoamine ligands with 7-amino-4-methylcoumarin in the ligand backbone to access luminescent gold(I) complexes. The attempts to synthesize [PN]-bridged gold(I) complexes led to the isolation of polynuclear metallacycles with significant photoluminescence properties. Further, the effect of substituents on the phosphine group and their reactivity was investigated. To the best of our knowledge, the complexes reported in this study are the first examples of luminescent polynuclear cyclic gold(I) complexes supported by phosphinoamide ligands.

Results and discussion

For the synthesis of dye functionalized luminescent polynuclear gold(I) complexes, we first designed and isolated a phosphinoamine ligand 1 featuring 7-amino-4-methylcoumarin in the backbone. 7-Amino-4-methylcoumarin was reacted with bis(4-(tert-butyl)phenyl)chlorophosphine in an equimolar ratio in the presence of triethylamine as a base. The reaction yielded the desired ligand 1 as colorless solid, which was crystallized by layering a tetrahydrofuran (THF) solution with n-pentane. Compound 1 crystallizes in the triclinic space group P1̄, and its molecular structure in the solid state is depicted in Fig. 3a. The P–N bond distance is 1.7099(10) Å, indicating the single bond character.⁴⁷ The deprotonation of the N–H proton was confirmed by the integration of the N–H signal in the ¹H NMR spectrum, and it appears as a doublet at δ 5.91 ppm (compound 1). The phosphine resonance of the ligand 1 appears as a singlet at δ 24.4 ppm in the ³¹P{¹H} NMR spectrum.

Fig. 3 Molecular structure of (a) ligand 1 and (b) complex 2 in the solid state. Hydrogen atoms (except N–H for ligand 1) and non-coordinating solvents are removed for clarity. Selected bond distances (Å) and angles (°): 1: N–P 1.7099(10); 2: Au–Cl 2.2892(10), Au–P 2.2181(11), P–N 1.685(4); P–Au–Cl 178.85(4).⁴³

The synthesized ligand 1 was reacted with [Au(tht)Cl] to obtain its corresponding gold(I) complex 2 (Scheme 1). The molecular structure of the complex is displayed in Fig. 3b. As expected, the gold(I) cation is coordinated by a phosphorous atom and a chloride ion, and therefore, it adopts a linear geometry with the P–Au–Cl angle of 178.85(4)°. The P–N and Au–P bond distances are 1.685(4) Å and 2.2181(11) Å, respectively, comparable to similar reported molecules.⁴⁸ The complex features a singlet at δ 57.2 ppm in the ³¹P{¹H} NMR spectrum.

Scheme 1 Synthesis of ligand 1 and complex 2.

Subsequently, the deprotonation of the gold(I) complex 2 with potassium tert-butoxide resulted in the isolation of an metallacycle complex 3 (Scheme 2), featuring four gold(I) ions in the core, supported by four bridging phosphinoamide ligand moieties (Fig. 4a).

Scheme 2 Synthesis of the polynuclear complex 3.⁴³

Fig. 4 Molecular structure of (a) polynuclear gold(I) complex 3 in the solid state and (b) simplified view of the core structure without the coumarin and para-(tert-butyl)phenyl rings featuring in a ladder-like configuration (right). Hydrogen atoms and non-coordinating solvents are removed for clarity. Selected bond distances (Å) and angles (°): Au1–Au2 3.2718(4), Au1–P2′ 2.2253(15), P1–Au2 2.2195(15), Au1–N1 2.055(5), Au2–N2 2.072(4), N1–P1 1.632(5), N2–P2 1.655(6); P2–N2–Au2 119.7(3), P1–Au2–N2 177.4(2), P1–N1–Au1 113.8(3), N1–Au1–P2′ 175.04(14).⁴³

Yellow colored crystals of complex 3 were grown from a concentrated toluene solution and X-ray crystallographic studies show that the asymmetric unit is comprised of only half of the molecule with two gold(I) cations. Unlike the pyrazolate coordinated saddle-shaped tetramer [AuAg(μ-pz^R)(BF₄)]₄ (pz^R = 3,5-bis(organothiomethyl)-pyrazolate),¹² in complex 3, the ring adopts a ladder-like arrangement (Fig. 4b). The four gold(I) ions in unique twelve membered ring structure are bridged by a N and P atom of the phosphinoamide motif in a typical linear geometry (∠P–Au–N: >175°). In the ladder-like tetramer structure, two gold(I) ions and two phosphine groups are situated in a spatially different environment, leading to two distinct intramolecular Au–Au distances. The shortest Au1⋯Au2 distance of 3.2718(4) Å is indicative of weak intramolecular aurophilic interactions.^49–52 The Au⋯Au distance is also in close agreement with the values obtained from optimized geometry (Fig. 10). The longest distance between the gold(I) ions is greater than 4.5 Å, suggesting no metallophilic interactions between Au1′ and Au2. The Au–N and Au–P bond distances are ∼2.1 Å and 2.2 Å, respectively, comparable to reported compounds with similar bonding situations.⁴⁸ The P–N bond distances are ∼1.6 Å, slightly shorter than those in the protonated ligand (1.71 Å) and the precursor complex (1.68 Å).

The characteristic resonances of two non-equivalent phosphorous atoms in complex 3 appear at δ 54.9 and δ 60.9 ppm in the ³¹P{¹H} NMR spectrum. An additional singlet is observed at δ 57.9 ppm in the ³¹P{¹H} NMR spectrum. Even after ten days, there were no changes in the relative intensities among the peaks, which indicates that the additional singlet doesn't appear due to decomposition. Hence, we speculated that solution dynamics might exist between the tetramer and trimer molecules. The singlet at δ 57.9 ppm in the ³¹P{¹H} NMR spectrum might belong to the latter compound (based on the resonance observed for the trimer complex 6, discussed below), which is present in approximately 13% ratio upon the dissolution of crystals of complex 3.

Therefore, we performed variable temperature ³¹P{¹H} NMR studies to gain insights into the solution dynamics. From the variable temperature NMR experiments, we noticed that the intensity of the resonance corresponding to the trimer decreases on lowering the temperature, and it disappears below 243 K. These results confirm our assumptions concerning the dynamics between the trimer and tetramer structure in the solution. Additionally, NMR shielding tensors were calculated using the Gauge-Independent Atomic Orbital (GIAO) method at the CAM-B3LYP level with the def2TZVP basis set, showing consistency with experimental observations.

In order to establish the generality of the reaction shown in Scheme 2, another ligand 4, with two tert-butyl groups at the meta-position of the phenyl rings, was designed. Compound 4 was synthesized by following a similar procedure as that of ligand 1 except that bis(3,5-(di-tert-butyl)phenyl)chlorophosphine was used instead of bis(4-(tert-butyl)phenyl)chlorophosphine (Scheme 3). The molecular structure of the ligand with expected planar N and pyramidal phosphorous geometry is displayed in Fig. 5a. From the single crystal X-ray diffraction studies, the bond distances and angles of the ligand are similar to that of compound 1. The N–H resonance appears as a doublet at δ 4.89 ppm in the ¹H NMR spectrum. The phosphine resonance of the ligand appears as a singlet at δ 32.7 ppm in the ³¹P{¹H} NMR spectrum, which is downfield-shifted compared to ligand 1. In the next step, its corresponding gold(I) chloride complex was synthesized by reacting it with [Au(tht)Cl] in a stoichiometric ratio to give the gold(I) complex 5. Similar to complex 2, the gold(I) ion is coordinated to a phosphine P atom and chloride ion with a Au–P bond distance of 2.287(13) Å and P–Au–Cl angle of 179.26(6)° (Fig. 5b), in accordance with similar reported structures.⁵³ The complex exhibits a singlet at δ 60.1 ppm in the ³¹P{¹H} NMR spectrum, which is downfield-shifted compared to the free ligand 4 (δ 32.7 ppm) upon coordination. As expected, the introduction of an additional t-butyl group on the backbone of phenyl rings of ligand 4 resulted in enhanced solubility of the gold(I) complexes.

Scheme 3 Synthesis of the ligand 4 and the gold(I) complex 5.

Fig. 5 Molecular structure of (a) ligand 4 and (b) complex 5 in the solid state. Hydrogen atoms (except N–H of the ligand 4) and non-coordinating solvents are removed for clarity. Selected bond distances (Å) and angles (°): 4: N–P 1.705(3); 5: Au–Cl 2.2742(14), Au–P 2.287(13), N–P 1.682(5); P–Au–Cl 179.26(6).

The deprotonation and chloride abstraction of the complex 5 using KO^tBu resulted in the macrocyclic complex 6 (Scheme 4). X-ray crystallographic studies revealed that complex 6 exists as a trinuclear phosphinoamide-bridged motif [Au(μ-PN)]₃ (Fig. 6) rather than the tetramer seen in compound 3. The formation of a trimeric complex rather than a tetramer could result from the increased steric bulk of the substituents in the ligand 4.²⁰ The central [Au(μ-PN)]₃ is almost planar with Au⋯Au distances of ∼3.5 Å, indicating the absence of notably intramolecular aurophilic interactions. These distances are significantly longer than that of complex 3 and other similar pyrazolate-bridged trimers. In the nine-membered ring containing three gold(I) ions, each metal center is bridged by a N and P atom of the phosphinoamide ligand moiety 4 nearly in a linear coordination geometry with ∠P–Au–N: ∼173°. The P–N bond distances are ∼1.65 Å, comparatively shorter than the precursor complex 5 (1.68 Å).

Scheme 4 Synthesis of the trimer gold(I) complex 6 and its reactivity with HCl.

Fig. 6 Molecular structure of trimeric gold(I) complex 6 in the solid state. Hydrogen atoms and non-coordinating solvents are removed for clarity. Selected bond distances (Å) and angles (°): Au1–N1 2.057(4), Au1–P3 2.2325(12), Au2–N2 2.082(4), Au2–P1 2.2414(13), Au3–N3 2.067(4), Au3–P2 2.2417(12), N1–P1 1.653(4), N2–P2 1.660(4), N3–P3 1.651(4); N1–Au1–P3 172.47(12), Au1–P3–N3 112.80(14), P3–N3–Au3 115.6(2), N3–Au3–P2 173.26(11), Au3–P2–N2 109.80(15), P2–N2–Au2 115.7, N2–Au2–P1 173.39(10), Au2–P1–N1 111.41(2), P1–N1–Au1 117.4(2).

The trimer compound exhibits a single set of resonances in the ¹H NMR spectrum and a singlet at δ 59.4 ppm in the ³¹P{¹H} NMR spectrum which corroborates the structural integrity of the solid state structure in solution. The phosphine resonance is upfield shifted compared to the complex 5 on formation of the metallacycle 6. The absence of additional signals confirms the existence of only one trimeric species in the solution. It is noteworthy that the metallacycles reported in this work are not photosensitive.

To investigate the π-acidity of the trimer compound, we reacted complex 6 with ethylene.⁵⁴ The trimer didn't show any changes in the NMR and only crystals of the trimer complex were isolated upon reaction with ethylene. Further, to probe the reactivity of the trimer 6, we reacted the complex with phenylacetylene (Scheme 5). The reaction yielded a monomer complex 7, wherein the acidic proton of the phenylacetylene migrated to the N atom of the coumarin ring, and the phenylacetylene moiety was coordinated to the Au^I ion as an ancillary anionic ligand. Low quality single crystals of 7 only provided the connectivity owing to the low precision of the data quality. Hence, only a pictorial representation of the structure is presented in Fig. S38.† However, the complex was fully characterized by NMR and IR spectroscopy as well as combustion analysis.

Scheme 5 Reactivity of the trimer 6 with phenylacetylene.

The ¹H NMR resonances of complex 7 unambiguously corroborate the chemical structure shown in Scheme 5, with a characteristic N–H resonance as a doublet at δ 4.98 ppm. The complex exhibits a singlet in the ³¹P{¹H} NMR spectrum at δ 74.7 ppm, which is downfield shifted compared to the precursor complex 6 (δ 59.4 ppm). The presence of N–H group was further confirmed by IR spectroscopy showing characteristic absorption at 3265 cm⁻¹.

Based on the observations made in this reaction (migration of acidic proton and the formation of monomer), we planned to react the trimer complex with HCl to yield its precursor complex 5. As expected, upon addition of HCl to the yellow-colored solution of the trimer (Scheme 4), the reaction mixture immediately turned colorless and the NMR studies confirm complete conversion of the trimer 6 to complex 5 within 15 min.

Photophysical properties

The absorption spectra of the precursor gold(I) complexes (2 and 5) and polynuclear gold(I) metallacycles (3 and 6) are depicted in Fig. 7. UV-Vis spectra of all the complexes in dichloromethane show strong absorption band centered at ∼348 nm (molar absorptivity of order 10⁴ M⁻¹ cm⁻¹). The coumarin motif in the ligand backbone mainly contributes to the absorption band between 340–350 nm.^42,55,56 The high energy absorption band below 300 nm is caused by ligand centered π–π* transition. The absorption spectra of the polynuclear complexes 3 and 6 differ from the precursor complexes 2 and 5 due to the substantial contribution from the metal centers as corroborated by the theoretical studies.

Fig. 7 Normalized absorption spectra of complexes 2, 3, 5 and 6 in dichloromethane at ambient temperature.

For the tetranuclear complex 3, the low-energy absorption band is broad, and the absorption onset is shifted to the visible region. The trinuclear complex 6 features an additional shoulder centered at 375 nm, with absorption onset shifted to the visible region. These observations indicate that the formation of the polynuclear complex significantly affects the optical properties.

Further, the synthesized metallacycles were investigated for their photophysical properties. The photoluminescence spectra of gold(I) precursor complex 2 at 77 K and 295 K in the solid state is shown in Fig. 8. At low temperatures, the complex exhibits a PL band at 400 nm (contributed by the coumarin motif) and minor bands at 495 nm and 530 nm. The PL band decay rapidly, and the luminescence lifetimes are below our detector's detection limit. At room temperature, only the high-energy emission band is observed in the PL spectra.

Fig. 8 Normalized photoluminescence excitation (PLE) and emission (PL) spectra of polycrystalline sample of 2 in the solid state at room temperature and 77 K. PLE and PL spectra were recorded at the depicted wavelengths (λ_em and λ_ex).

In contrast to the precursor complex 2, the tetramer 3 exhibits a broad and structureless band centered at ∼590 nm in the solid state at 77 K (Fig. 9a). The formation of the tetramer led to a shift of the emission band to lower energy by ∼1.03 eV, and the excited states now decay biexponentially with lifetimes in the scale of microseconds, indicating phosphorescence (τ₁ = 5.5 μs and τ₂ = 32.8 μs). This could be in good agreement with a contribution of the metal d orbitals to the electronic transitions involved in the excited states. Complex 3 features a broad band at room temperature with λ_max = 585 nm. The excited states now decay rapidly with lifetime less than 6 μs.

Fig. 9 Normalized photoluminescence excitation (PLE) and emission (PL) spectra of (a) polycrystalline samples (b) Me-THF solutions of complexes 3 and 6 at room temperature and 77 K. PLE and PL spectra were recorded at the depicted wavelengths (λ_em and λ_ex).

In the solid state at 77 K, the trimer complex 6 features also a broad band centered at 550 nm upon excitation at 400 nm (Fig. 9a). The luminescence decay kinetics of complex 6 is also biexponential and in the order of milliseconds (τ₁ = 7.5 μs and τ₂ = 1.06 ms). Such long lifetimes are rarely reported for analogous pyrazolate-bridged cyclic trinuclear complexes lacking intermolecular interactions.^57,58 On increasing the temperature, the luminescence intensity rapidly decreases and features an emission band at λ_max = 590 nm (with a lifetime of 7.8 μs). Even though, the metallaycles exhibit long lifetimes, their PL efficiency (Φ_PL) is very poor (1–2%) in the solid state. In contrast, their precursor Au^I complexes have comparatively high Φ_PL values of 21% and 9% for complexes 2 and 5, respectively.

The polynuclear compounds were also investigated for their luminescence properties in solution (Fig. 9b). The measurements were carried out in a frozen 2-methyl-THF glass matrix at 77 K. The complexes 3 and 6 feature a complicated PL spectra with several decay processes. A fluorescent band of low intensity was observed at 400 nm with lifetime of 4 ns. Additionally, the complexes feature a strong phosphorescent band at ∼480 nm. The decay kinetics in the milliseconds range are clearly non-monoexponential for both the tetramer 3 (τ₁ = 6.0 μs and τ₂ = 3.45 ms) and trimer 6 (τ₁ = 4.4 μs and τ₂ = 4.01 ms). It is noteworthy that such long phosphorescence was not observed for the gold(I) chloride precursors.

The excitation spectra of the compounds align with their respective absorption spectrum, which strongly supports our notion that the polynuclear compounds are indeed the emissive species. At 295 K in 2-methyl-THF, both the complexes (3 and 6) feature only one emission band centered at 400 nm, which is attributed to the coumarin moiety. The excited states decay rapidly with a lifetime of ∼3 ns, indicative of emission from a singlet excited state.

This work can further be extended to synthesize analogous copper(I) and silver(I) complexes. Copper(I) complexes can potentially feature visible light absorption and low-energy emission band, which can be promising compounds for photocatalysis.⁵⁹ Further, the silver(I) complexes may show reactivity with ethylene,⁵⁴ which was not observed with the gold(I) complexes in this study.

Quantum chemical calculations

For a deeper understanding of the electronic structure of complexes, we conducted density functional theory (DFT) calculations and time-dependent density functional theory (TDDFT) calculations (for detailed methodology, refer to the ESI†). We optimized the geometries of complexes 3 and 6 in the gas phase without constraints (Fig. 10), and the calculation reveals that they maintain their original structural motifs. Frontier molecular orbitals of the gold complexes are depicted in Fig. S43–S46.†

Fig. 10 CAM-B3LYP optimized geometries of (a) complex 3 and (b) complex 6 with bond lengths in Å as calculated using def2TZVP basis set for gold atom and 6-31G** for all other atoms. Parentheses contain the experimental value obtained from the respective X-ray structures.

Across all complexes, the highest occupied molecular orbitals (HOMOs) primarily originate from coumarin-centered orbitals with minimal contribution from gold atoms and none from coordinating phosphorus atoms, while the lowest unoccupied molecular orbitals (LUMOs) are predominantly ligand-localized. In tri- and tetra-metallic complexes, LUMOs exhibit significant contributions from the [Au₃(PN)₃] and [Au₄(PN)₄] cores, respectively. Absorption and emission properties of precursor gold(I) complexes (2 and 5) primarily arise from intraligand π to π* transitions. However, polynuclear gold(I) metallacycles (3 and 6) display distinct luminescent properties compared to their precursor complexes. Selected excited state energy levels along with main orbital configurations are summarized in Tables S2 and S3;† however, it's worth noting that these values tend to underestimate experimental results, particularly for states with significant charge transfer character. TDDFT calculation explains the broad and red-shifted absorption band in complexes 3 and 6. Specifically, the red-shifted shoulder at 375 nm in complex 6 originates from transitions from coumarin-centered HOMOs to the [Au₃(PN)₃] core, including transitions from HOMO to LUMO+1, HOMO−2 to LUMO, and HOMO−1 to LUMO (Table S3†). Similarly, the broad absorption features for complex 3 are attributed to transitions from HOMO−2 to LUMO, HOMO−3 to LUMO, and HOMO−1 to LUMO+1.

Complexes 3 and 6 feature favourable triplet states energetically proximal to S₁, with energy differences typically ≤0.4 eV, and exhibit similar orbital configurations between singlet (S₁) and triplet (T_n) states.⁴² These properties position them as promising candidates for intersystem crossing (ISC), contributing to their observed red-shifted phosphorescence attributed to the [Au(PN)] core. Additionally, NMR shielding tensors were calculated using the Gauge-Independent Atomic Orbital (GIAO) method at the B3LYP level with the def2TZVP basis set, showing consistency with experimental observations.

Conclusions

In this work, we have reported the synthesis of new coumarin based phosphinoamine ligands 1 and 3, and their respective gold(I) chloride complexes 2 and 5. The reaction of the gold(I) chloride complexes led to the formation of unexpected cyclic structures containing four or three gold(I) ions. The number of gold(I) ions is dependent on the steric bulk of the organic substituents on the phosphine moiety. The cyclic structures show yellow-orange emission in the solid state and coumarin centered emission in solution both at room temperature and 77 K. At low temperatures in frozen Me-THF matrix, the excited states of the cyclic compounds decay on a timescale of milliseconds. The quantum chemical calculations reveal that in the cyclic complexes, the HOMOs primarily originate from the coumarin-centered orbitals, while the LUMOs show significant contributions from [Au_x(PN)_x] cores. The absorption and emission properties are altered compared to the precursor complexes. This work showcases the possibility to alter the optical properties of 7-amino-4-methyl coumarin based phosphinoamine ligands. Furthermore, the trimer compound was explored for its reactivity towards phenylacetylene and HCl.

Author contributions

V. R. N. and J. K. performed the synthesis, characterization, photophysical studies and analysed the data with the help of S. and M. I. A. K. S. performed the quantum chemical calculations. P. W. R. conceived the idea and supervised the work. All authors provided suggestions and comments on the manuscript.

Data availability

The data supporting this article have been included as part of the ESI.† Crystallographic data for 1–6 has been deposited at the Cambridge Crystallographic Data Centre as a supplementary publication no. 2362899–2362904.†

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

V. R. N., S., and P. W. R. acknowledges GRK 2039 Molecular Architectures for Fluorescent Cell Imaging for the financial support. P. W. R. acknowledges Deutsche Forschungsgemeinschaft for support within the project 540378534, RO 2008/22-1.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. CCDC 2362899–2362904 (1–6). For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d4qi01747a

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