Synthesis and Thermochromic Luminescence of Ag(I) Complexes Based on 4,6-Bis(diphenylphosphino)-Pyrimidine

Abstract

Two Ag(I)-based metal-organic compounds have been synthesized exploiting 4,6-bis(diphenylphosphino)pyrimidine (L). The reaction of this ligand with AgNO₃ and AgBF₄ in acetonitrile produces dinuclear complex, [Ag₂L₂(MeCN)₂(NO₃)₂] (1) and 1D coordination polymer, [Ag₂L(MeCN)₃]_n(BF₄)_2n (2), respectively. In complex 1, µ₂-P,P′-bridging coordination pattern of the ligand L is observed, whereas its µ₄-P,N,N′,P′-coordination mode appears in 2. Both compounds exhibit pronounced thermochromic luminescence expressed by reversible changing of the emission chromaticity from a yellow at 300 K to an orange at 77 K. At room temperature, the emission lifetimes of 1 and 2 are 15.5 and 9.4 µs, the quantum efficiency being 18 and 56%, respectively. On account of temperature-dependent experimental data, the phenomenon was tentatively ascribed to alteration of the emission nature from thermally activated delayed fluorescence at 300 K to phosphoresce at 77 K.

1. Introduction

Recently, Ag(I)-based metal-organic compounds have attracted increased attention as promising antibacterial agents [1,2], luminescent sensorics [3,4], and potential emitters for lighting application [5,6,7]. The rich coordination abilities of Ag(I) ion coupled with easy accessibility of the organic ligands provide ample opportunities for the design of most diverse coordination architectures covering both simple molecules and sophisticated metal-organic frameworks [8,9,10,11,12,13]. The charge balancing anions (NO₃⁻, OAc⁻, OTf⁻, ClO₄⁻, BF₄⁻, PF₆⁻, halides, etc.) can also determine the structure of products, which are self-assembled via the reactions of Ag(I) precursors with organic ligands [14,15,16]. Remarkably, such giant structural diversity of the Ag(I) compounds provides various tools for tuning their functional properties. For instance, they can be regulated by adjusting the electronic properties of organic ligands. Not the least, of the factors are supramolecular interactions of Ag∙∙∙Ag, Ag∙∙∙π, and Ag∙∙∙X kind, which also can influence the properties of the Ag(I) compounds, e.g., luminescence [17,18,19,20,21].

Concerning luminescence of silver(I) metal-organic compounds, it was almost neglected for a long time, probably because of the preconception on photosensitivity of this class in principle. It was not until fairly recently the Ag(I) complexes were recognized as promising emitters, which can exhibit enhanced quantum efficiency coupled with short decay time [22,23,24,25,26,27]. Note that the luminescent properties of Ag(I) complexes primarily depend on the structure of the ligand environment. Compared to Cu(I) complexes showing a metal-to-ligand charge transfer (MLCT) luminescence [28,29], the Ag(I) analogues commonly emit metal-perturbed ligand-centered fluorescence [30,31,32], ligand-centered phosphorescence [33], or dual emission [34]. This stems from the fact that the MLCT excitation is hampered for Ag(I) complexes because of the higher ionization potential of Ag⁺ ion in comparison with the Cu⁺ one [35]. This restriction, however, can be overcome using highly electron-donating ligands (e.g., phosphines) coupled with π-acceptors (chelating diimines, azines, etc.). On account of highly electron-donating ligands, the silver d¹⁰ orbitals begin to contribute to HOMO and near-HOMO, while the π-acceptors facilitate charge transfer from the metal. As a result, (M + L’)LCT excited states can be generated, thereby inducing thermally activated delayed fluorescence (TADF) in Ag(I) compounds [22,23,24,25,26,27,36,37,38,39,40,41,42]. In the context of OLED application, it is relevant to note that the (M + L’)LCT emission of Ag(I) benefits over that of Cu(I) analogues since the former (i) is shorter in the lifetimes [36,41,42], and (ii) generally appears in the higher energy domain [36,37,39,43,44,45]. On the whole, Ag(I) compounds that emit apparent luminescence at ambient temperature, especially, that of TADF nature, are still rare.

Herein, we report on the synthesis and investigation of two Ag(I) compounds derived from 4,6-bis(diphenylphosphino)pyrimidine and AgNO₃ or AgBF₄. Both compounds manifest pronounced thermochromic luminescence, which appears as a reversible changing of the emission color from yellow at 300 K to orange/red at 77 K. It should be noted in this regard that the Ag(I) complexes endowed with such property are very limited in number [13,46,47,48,49].

2. Results and Discussion

Synthesis and Characterization

The starting ligand, 4,6-bis(diphenylphosphino)pyrimidine (L), has been synthesized by the treatment of 2,6-dichloropyrimidine with 2 equiv. of lithium diphenylphosphide [50]. The ligand has been tested in the reaction with AgNO₃ and AgBF₄ in acetonitrile using different reactant’s molar ratios. It has been revealed that the interaction of AgNO₃ with ligand L in a 1:1 molar ratio affords dinuclear complex [Ag₂L₂(MeCN)₂(NO₃)₂] (1) isolated as solvate 1·MeCN (Scheme 1). Meanwhile, using AgBF₄ under similar conditions, the reaction provides cationic 1D coordination polymer (CP), [Ag₂L(MeCN)₃]_n(BF₄)_2n (2) that also crystallizes as solvate 2·MeCN (Scheme 1). The preparative yields of products 1 and 2 are 69 and 91%, respectively.

The products obtained are off-white powders, which are well soluble in acetonitrile. Upon storage on air, both 1·MeCN and 2·MeCN easily lose acetonitrile molecules. Note that the desolvation is reversible: the recrystallization of the powders formed from acetonitrile leads again to the crystals of the above solvates. Both compounds have been characterized by single crystal X-ray diffractometry (sc-XRD), FT-IR, and UV-Vis abortion spectroscopy.

Complex 1·MeCN crystallizes in the monoclinic P2₁/n space group with one half molecule per asymmetric unit. As seen from Figure 1, the scaffold of 1 is formed by two Ag atoms bridged by two ligands L through phosphorus atoms so that the pyrimidine rings become coplanar. The latter are sandwiched in a “head-to-tail” manner with the distance between the average planes being 3.861 Å. The O atom of the NO₃ group and acetonitrile N atom complete the coordination sphere of Ag1 and Ag2 atoms to the distorted {Ag@P₂ON} tetrahedron. The bond lengths around metal atoms are comparable with those of most related Ag(I) complexes [51,52,53]. In the crystal, molecules of 1 are associated together and with MeCN molecules via weak C–H∙∙∙O and C–H∙∙∙C contacts forming 3D supramolecular structure.

CP 2·MeCN crystallizes in the orthorhombic P2₁2₁2₁ space group, and its crystals contain zig-zag chains propagating along a axis. The chains are built up from alternating ligand molecules and [Ag₂(MeCN)₃] units, which are linked via Ag–N and Ag–P bonds (Figure 2). The metal atoms of the [Ag₂(MeCN)₃] units are bridged by two P,N-faces of the adjacent ligands L in a “head-to-tail” fashion. The formed eight-membered cycles feature short Ag···Ag contact of 3.3352(4) Å that is consistent with twice van der Waals radius of Ag atom (3.44 Å [54]). Both Ag1 and Ag2 atoms are ligated by one MeCN molecule (d_Ag–N ≈ 2.33 Å); besides, a second MeCN molecule is weakly associated (d_Ag–N ≈ 2.65 Å) with Ag1 atom. As a result, Ag1 atom adopts a T-shaped {Ag@N₂P} geometry, while Ag2 center has a distorted see-saw {Ag@N₃P} environment. The Ag–N_L and Ag–P distances are nearly the same as those in 1·MeCN. The non-coordinated tetrafluoroborate anions and MeCN solvate molecules are associated with the [Ag₂L(MeCN)₃]_n chains by means of van der Waals contacts such as C–H∙∙∙F, C–H∙∙∙N, Ag∙∙∙F, and C∙∙∙F.

FT-IR spectra of solid 1 and 2 are in agreement with sc-XRD data, showing characteristic vibrations of the ligand L along with stretching vibrations of the counter-ions (Figure S1). The N–O and B–F stretchings of the NO₃⁻ and BF₄⁻ groups appear as strong bands at 1380–1417 and 950–1200 cm^–1, correspondingly. The solid state UV-Vis spectra of 1 and 2 (plotted as Kubelka-Munk function, Figure S2) display broad bands expanding from the far-UV edge and falling close at about 400 and 440 nm, respectively (Figure S2). Each absorption band has two pronounced shoulders. The high-energy (HE) shoulders, with λ_max ≈ 280 nm, are nearly the same for both compounds, while the low-energy (LE) ones maximize at ≈350 nm for 1 and at ≈390 nm for 2. Considering the literature data [38], the HE absorption band can be attributed to intraligand π–π * and n–π * transitions. The LE band is likely associated with promotions of MLCT kind, which is typical for emissive Ag(I) complexes [22,23,24,25,26,27,36,37,38,39,40].

When UV-irradiated, solid compounds 1 and 2 exhibit yellow luminescence at ambient temperature. Upon cooling down to liquid nitrogen temperature, the luminescence strongly enhances, and its color changes to red-orange (for 1) or red (for 2) (Figure 3d and Figure 4d). The thermochromic luminescence found appears to be reversible: warming the samples to 300 K recovers the initial emission chromaticity. Inspirited by these noticeable findings, we have studied the emission properties of the titled compounds at 77–300 K range. Temperature-dependent emission and excitation spectra of 1 and 2 are plotted in Figure 3 and Figure 4, and the corresponding photophysical data are summarized in

Table 1. Photophysical data for solid 1 and 2.

	1	2
λmax (300 K) (nm)	550	580
ΦPL (300 K) (%)	18 a	56 b
τ (300 K) (µs)	15.5	9.4
λmax (77 K) (nm)	580	602
τ (77 K) (µs)	3970	300

^a λ_ex = 365 nm; ^b λ_ex = 410 nm.

. As seen from the graphs, the emission spectra of 1 and 2 contain a broad band maximized at about 550 and 580 nm, accordingly. The corresponding emission colors on the CIE chromaticity diagram are consistent with those observed by the naked eye. The associated emission lifetimes (τ_obs) of 1 and 2 measured at 300 K are 15.5 and 9.4 µs, and the photoluminescence quantum yields (Φ_PL) are 18 and 56%, respectively. The excitation profiles of 1 and 2 are presented by smooth bands that fall close at about 420 and 440 nm (Figure 3b and Figure 4b). The excitation curves, therefore, resemble the absorption patterns (Figure S2). Note that the compounds studied do not possess excitation-dependent properties, which are quite common for Ag(I) complexes [55,56,57]. When the temperature is gradually lowered to 77 K, the emission bands of 1 and 2 are red-shifted by 30 and 22 nm (Figure 3a and Figure 4a), thereby changing the emission color to red-orange and red, respectively (Figure 3c,d). Simultaneously, the lifetimes rise to 3970 µs (1) and 300 µs (2).

Taken together, these observations suggest that TADF is likely responsible for the room temperature emission of 1 and 2. The temperature dependence of the lifetimes, τ_obs(T), measured in 77–300 K window, supports this suggestion, following the equation intended for the TADF model [58]:

$${\tau }_{obs}\left(T\right)=(3+\exp \left(-\frac{\mathsf{\Delta }{E}_{\mathrm{ST}}}{k_{B}T}\right))/(\frac{3}{{\tau }_T}+\frac{1}{{\tau }_S}\exp (-\frac{\mathsf{\Delta }{E}_{\mathrm{ST}}}{k_{B}T}))$$

(1)

wherein ${\tau }_S$ and ${\tau }_T$ are the lifetimes of prompt fluorescence and phosphorescence, respectively, $\mathsf{\Delta }{E}_{ST}$ is the energy gap between the respective excited states (S₁ and T₁), and $k_B$ is the Boltzmann constant. Applying this equation for fitting the datasets of Figure 5, the following values have been roughly estimated for 1: ΔE(S₁−T₁) = 750 cm⁻¹, fluorescence lifetime τ(S₁) = 400 ns, and phosphorescence lifetime τ(T₁) = 4000 µs. Analogously, the following values have been estimated for 2: ΔE(S₁−T₁) = 1000 cm⁻¹, τ(S₁) = 35 ns, and τ(T₁) = 300 µs. It should be emphasized that the given ΔE(S₁−T₁) values are purely evaluative because the Eq. 1 can be applied correctly when the emission quantum yields do not change much than the investigated temperature range [58]. Nevertheless, the estimated ΔE(S₁−T₁) magnitudes agree well with the common values for TADF-emitting Ag(I) and Cu(I) complexes, thus allowing to assume manifestation of TADF by 1 and 2.

As seen from Figure 5, the luminescence of both compounds at ambient temperature represents TADF because the τ_obs(T) curves attain the high-temperature plateau. The pure phosphorescence begins to appear when the τ_obs(T) curve reaches the low-temperature plateau. In the case of 2, it occurs below 120 K, while complex 1, possessing a narrower ΔE(S₁−T₁) gap, emits pure phosphorescence at 77 K and below. On the drop-down range of the τ_obs(T) curves, the contribution of the TADF gradually increases up to ~100% at 300 K, owing to the thermal population of the higher-lying S₁ state from the T₁ state. As a result, the emission energy shifts in the blue region upon warming from 77 to 300 K, resulting in thermochromic luminescence of 1 and 2. Considering the literature data on the related Ag(I) complexes exhibiting TADF [22,23,24,25,26,27,36,37,38,39,40,41,42], we believe that the S₁ and T₁ excited states of 1 and 2 are of MLCT or (M + L’)LCT nature. The DFT computations performed on complex 1 support this suggestion revealing that HOMO and near-HOMOs are contributed by silver d-orbitals and p-orbitals of nitrate oxygen atoms, while LUMO and near-LUMOs are pure pyrimidine π-orbitals (Figure S10). Note the previously reported Ag(I) complexes feature a similar HOMO/LUMO distribution pattern [22,23,24,25,26,27,36,37,38,39,40,41,42]. Thus, it can be assumed that the emissive excited state of 1 has a ^1,3(M + L’)LCT character (L’ = NO₃, and L = pyrimidyldiphosphine).

3. Materials and Methods

3.1. General

Synthesis of the ligand L was performed under argon atmosphere, while the compounds 1 and 2 were prepared under ambient conditions. AgNO₃ (≥99.9%, Aldrich, St. Louis, Missouri, MO, USA), AgBF₄ (≥99.9%, Aldrich), n-BuLi (2.5 M in hexanes, Aldrich), 4,6-dichloropyrimidine (97%, Aldrich), and diphenylphosphine (98%, Aldrich) were used as purchased. Prior to use, commercial tetrahydrofuran (THF, anhydrous, ≥99.9%, Aldrich) was purified by distillation over sodium/benzophenone under argon flow. Acetonitrile and dichloromethane were distilled over phosphorus pentoxide.

FT-IR spectra were measured on a Bruker Vertex 80 spectrometer (Bruker, Billerica, Massachusetts, MA, USA) at ambient temperature. The microanalyses were performed on a MICRO cube analyzer.

¹H, ¹³C, and ³¹P{¹H} NMR spectra were recorded using a Bruker AV-500 spectrometer at 500.13, 125.8 MHz and 202.46 MHz, respectively, with solvent peaks as reference. The ³¹P{¹H} NMR shifts are expressed with respect to 85% H₃PO₄/D₂O as an external standard.

The microanalyses were performed on a MICRO cube analyzer Photoluminescence spectra were recorded on a Fluorolog 3 spectrometer (Horiba Jobin Yvon, Kyoto, Japan) with a cooled PC177CE-010 photon detection module equipped with an R2658 photomultiplier. The luminescence decays (Figures S5 and S6) were measured on the same instrument. The absolute values of PLQYs were recorded using a Fluorolog 3 Quanta-phi device (Horiba Jobin Yvon). The luminescence quantum yield at 77 K was obtained relative to the quantum yield of the same sample at 300 K. Independently, these relative quantum yields were calibrated by using the absolute PLQY values measured at 77 K. Temperature dependences of luminescence were carried out using Optistat DN optical cryostats (Oxford Instruments, Abingdon, UK).

The solid-state reflectance spectra were recorded on a Shimadzu UV-3101 spectrophotometer (Shimadzu Corporation, Kyoto, Japan). Samples were prepared by a thorough grinding of a mixture of a complex (ca. 2 mol %) with BaSO₄. The reflectance data were converted into a spectrum applying a Kubelka-Munk function using BaSO₄ as a standard.

DFT computations of 1 were performed using the hybrid B3LYP functional [59] combined with the def2TZVP basis sets [60]. The calculations were performed for single point geometry taken from X-ray coordinates for non-hydrogen atoms. The computations were proceeded using Gaussian 09 suite [61].

3.2. Synthesis of 4,6-Bis(diphenylphosphino)pyrimidine (L)

n-BuLi in hexanes (2.5 M, 15 mL) was added dropwise to a solution of diphenylphosphine (6.139 g, 0.033 mol) in absolute THF (40 mL) at −20 °C. The mixture was kept at −20 °C and stirred for 1 h. Then, at the same temperature, suspension of 4,6-dichloropyrimidine (2.384 g, 0.016 mol) in THF (10 mL) was added dropwise. The resulting mixture was warmed to 40 °C and stirred for 4 h. After that H₂O (50 mL) was added and the quenched mixture was extracted with CH₂Cl₂ (3 × 30 mL). The organic extracts were washed with H₂O (3 × 10 mL), dried with Na₂SO₄, and evaporated in vacuum. The crude product obtained was recrystallized from MeOH/CH₂Cl₂ (10:1, v/v) to give colorless crystals of L. Yield: 4.087 g (57%). ¹H NMR (500.13 MHz, CDCl₃) δ 9.20 (s, 1H, C²–H in Pym), 7.35–7.31 (m, 20H in Ph), 6.69 (s, 1H, C⁵–H in Pym). ¹³C{¹H} NMR (126 MHz, CDCl₃) δ 174.4 (d, J = 7.0 Hz, C⁴ and C⁶ in Pym), 156.8 (t, J = 9.3 Hz, C² in Pym), 134.4 (s, o-Ph), 134.3 (s, o-Ph), 133.6 (d, J = 8.8 Hz, i-Ph), 132.1 (d, J = 9.6 Hz, C⁵ in Pym), 129.7 (s, p-Ph), 128.9 (d, J = 7.8 Hz, m-Ph). ³¹P{¹H} NMR δ (202.47 MHz, CDCl₃) −2.62. FT-IR (KBr, cm⁻¹): 424 (w), 434 (w), 444 (m), 463 (w), 484 (m), 500 (vs), 544 (w), 608 (w), 619 (w), 696 (vs), 743 (vs), 766 (m), 783 (w), 889 (w), 978 (w), 999 (m), 1026 (m), 1070 (w), 1097 (m), 1157 (w), 1184 (w), 1260 (s), 1310 (w), 1331 (w), 1435 (s), 1479 (s), 1535 (s), 1584 (w), 2984 (vw), 3049 (w), 3071 (vw).

3.3. [Ag₂L₂(NO₃)₂(MeCN)₂]·MeCN (1·MeCN)

A solution of L (50 mg, 0.11 mmol) and AgNO₃ (20 mg, 0.11 mmol) in CH₃CN (1 mL) was stirred at room temperature for 30 min. The precipitated white powder of the 1·CH₃CN was centrifuged and dried in air. White powder. Yield: 50 mg (69%). Single crystals of 1·CH₃CN were grown by vapor diffusion of diethyl ether into the CH₃CN solution for overnight. FT-IR (thin film, cm⁻¹): 474 (m), 505 (s), 692 (vs), 746 (s), 997 (w), 1028 (w), 1099 (m), 1265 (m), 1288 (s), 1385 (s), 1420 (s), 1437 (vs), 1481 (m), 1495 (m), 1539 (vs), 2251 (w), 2294 (vw), 2921 (w), 2936 (vw), 2994 (vw), 3056 (w). Anal. Calcd: C₅₆H₄₄Ag₂N₆P₄O₆ (1236.62) C, 54.4; H, 3.6; N, 6.8. Found: C, 54.4; H, 3.4; N, 6.7. Since the solvate 1·CH₃CN quickly loses the molecules of acetonitrile upon storage, the microanalysis was calculated on [Ag₂(L)₂(NO₃)₂].

3.4. [Ag₂L(MeCN)₃]_n(BF₄)_2n·MeCN (2·MeCN)

A solution of L (50 mg, 0.11 mmol) and AgBF₄ (43 mg, 0.22 mmol) in CH₃CN (1 mL) was stirred at room temperature for 30 min. To the resulting solution, diethyl ether (5 mL) was then added and the precipitate formed was centrifuged and dried in air. White powder. Yield: 100 mg (91%). Single crystals of 2·CH₃CN were grown by vapor diffusion of diethyl ether into the CH₃CN solution for overnight. FT-IR (cm⁻¹): 478 (w), 507 (m), 519 (m), 692 (S), 746 (m), 997 (m), 1063 (vs), 1084 (vs), 1097 (vs), 1163 (w), 1184 (w), 1287 (w), 1308 (w), 1437 (s), 1454 (w), 1481 (w), 1497 (w), 1560 (m), 1634 (w), 2253 (w), 2298 (w), 2388 (w), 2971 (w), 3009 (vw), 3063 (w). Anal. Calcd: C₂₈H₂₂Ag₂B₂F₈N₂P₂ (837.78) C, 40.1; H, 2.6; N, 3.3. Found: C, 40.0; H, 2.6; N, 3.5. Since the solvate 2·CH₃CN quickly loses the molecules of acetonitrile upon storage, the microanalysis was calculated on [Ag₂L](BF₄)₂.

3.5. X-ray Crystallography

Single crystals of 1∙MeCN and 2·MeCN were grown by diffusion of diethyl ether vapors into a MeCN solutions at ambient temperature for overnight. The X-ray data and the details of the refinement are summarized in Table S1. Diffraction data were collected on an automated Agilent Xcalibur diffractometer equipped with an area AtlasS2 detector (graphite monochromator, λ (Mo Kα) = 0.71073 Å, ω-scans, Agilent, Santa Clara, California, CA, USA). Integration, absorption correction, and determination of unit cell parameters were performed using the CrysAlisPro program package [62]. The structures were solved by dual space algorithm (SHELXT [63]) and refined by the full-matrix least squares technique (SHELXL [64]) in the anisotropic approximation (except hydrogen atoms). Positions of hydrogen atoms of organic ligands were calculated geometrically and refined in the riding model.

The crystallographic data and details of the structure refinements are summarized in Table S1. CCDC 2020455 and 2020456 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Center at http://www.ccdc.cam.ac.uk/data_request/cif.

4. Conclusions

Thus, two Ag(I) metal-organic compounds have been synthesized by the treatment of 4,6-bis(diphenylphosphino)pyrimidine (L) with AgNO₃ and AgBF₄ in acetonitrile. It has been revealed that the interaction with AgNO₃ results in neutral dinuclear complex, [Ag₂L₂(MeCN)₂(NO₃)₂], while the reaction with AgBF₄ produces cationic 1D zig-zag polymer, [Ag₂L(MeCN)₃]_n(BF₄)_2n. The structure of the complex is built up from two Ag(I) ions bridged by the two ligands in a µ₂-P,P′-manner. The 1D chains of the polymer are assembled by alternating ligand (µ₄-N,P,N′,P′) and [Ag₂(MeCN)₃] units, interconnected through Ag–N and Ag–P bonds. Both title compounds feature pronounced thermochromic luminescence, which appears as reversible yellow-to-orange changing of the emission color during the cooling-warming cycling (300–77 K). The detailed temperature-dependent photophysical study has shown that the ambient temperature photoluminescence of the above compounds may be tentatively ascribed to TADF. At 77 K, they certainly emit pure phosphorescence. The distinct thermochromic behavior of the complexes designed makes them promising materials for luminescent thermometry. From the fundamental viewpoint, the findings reported contribute to coordination chemistry and photophysics of Ag(I)-based metal-organic compounds.