Emission enhancement in a luminescent polychlorinated diphenylpyridylmethyl radical through coordination to silver(I)

Abstract

A luminescent silver(I) complex containing a luminescent radical ligand was prepared for the first time. Coordination to Ag^I enhanced and red-shifted the radical-centered emission. This study demonstrates similar effects in the luminescence of the radical by complexation with group 11 d¹⁰-metal ions.

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Introduction

Luminescent radicals exhibit fluorescence based on the transition from the lowest doublet excited state (D₁) to the doublet ground state (D₀).¹ This enables a range of unusual photophysical characteristics including the absence of heavy-atom effects^2,3 and efficient carrier-to-photon conversion in electroluminescence.⁴ Furthermore, the luminescence of assemblies of luminescent radicals responds to a magnetic field through a mechanism unique to open-shell molecular systems.⁵ Overall, luminescent radicals appear promising for use in the next generation of photonic and spintronic devices.⁶

Coordination with metal ions is an attractive way to modulate the emission properties of organic radicals. For example, complexes comprising Au^I ligated with (3,5-dichloro-4-pyridyl)bis(2,4,6-trichlorophenyl)methyl radical (PyBTM) and its analogues have shown longer emission wavelengths and higher photoluminescence quantum yields than the radical ligands alone,^7–9 and a PyBTM-Au^I complex has recently been utilized as an X-ray scintillator.¹⁰ In addition, several metal–organic frameworks¹¹ and supramolecular cages¹² with distinctive emission characteristics have been developed using luminescent radical ligands. The emission characteristics of these radical complexes depend on the choice of metal ion, e.g., Au^I, Zn^II, and La^III enhance the emission,^2,7 whereas Cu^II, Fe^III, and Ir^III weaken or quench the emission.^2,13 Despite the increasing number of examples of luminescent radical–metal complexes, the range of metal ions employed remains limited, and more importantly, differences and similarities in the effects of different metal ions on the emission of radicals have rarely been discussed. Further investigation is thus necessary to comprehensively understand the effect of different metal ions on the luminescence of radicals.

This study investigated the complexation of PyBTM with Ag^I ions and the luminescence of the resulting complex in solution, to elucidate the effect of the metal ion on the electronic structure and emission characteristics of this class of radical complexes (Chart 1). Single-crystal X-ray diffraction (SCXRD) and titration studies revealed that two PyBTM molecules coordinate to one Ag^I ion in the crystalline state, whereas PyBTM formed a 1 : 1 complex with Ag^I in solution. The 1 : 1 complex exhibited enhanced and redshifted photoluminescence compared with PyBTM alone in solution. Comparison of the synthesized 1 : 1 Ag^I complex with some well-studied Au^I complexes (e.g., [Au^I(PyBTM)(PPh₃)]BF₄, labelled 2BF₄ ⁷ in Chart 1) revealed commonalities between the effects of Ag^I and Au^I (i.e., group 11 d¹⁰) metal ions on the radical's luminescence.

Chart 1 Chemical structures of PyBTM and some of its metal complexes.

Results and discussion

Details of the synthesis and characterization of the silver complex [Ag^I(PyBTM)₂]SbF₆ (1aSbF₆) is described in the Experimental section. Slow vapor diffusion of diisopropyl ether into a mixed solution of PyBTM and AgSbF₆ in dichloromethane at room temperature afforded crystals of 1aSbF₆. A SCXRD analysis revealed 1a⁺ to have a linear two-coordinate geometry around the Ag^I ion, with the two PyBTM molecules coordinating to the Ag^I ion through their pyridyl nitrogen atoms with a N1–Ag1–N2 bond angle of 176.0(1)° (Fig. 1a). Trigonal planar geometries around the central carbon atoms (C6 and C24) of the PyBTM units indicate their sp² hybridization and radical character (Fig. 1b). The shortest intermolecular Ag⋯Ag distance of 8.5341(5) Å indicates no significant interaction between the Ag^I ions. The shortest intermolecular distance between the radical centers (C6 and C24) is 7.212(5) Å, much shorter than the intramolecular C6⋯C24 distance of 12.880(7) Å. Several intermolecular Cl⋯Cl contacts were also detected (Fig. S1†).

Fig. 1 (a) Molecular structure of [Ag^I(PyBTM)₂](SbF₆) determined by SCXRD analysis with thermal ellipsoids set at 50% probability. The SbF₆ anion shows positional disorder; one is shown for clarity. C, gray; N, light blue; F, yellow green; Cl, light green; Sb, purple; H, white. (b) Side view of one of the PyBTM moieties in [Ag^I(PyBTM)₂]⁺. (c) Temperature-dependent χ_MT of [Ag^I(PyBTM)₂](SbF₆) at 1 T. The solid black line is a fitting curve following the Curie–Weiss law.

The magnetic properties of 1aSbF₆ were investigated using a SQUID magnetometer (Fig. 1c and Fig. S3†). The temperature dependence of χ_MT (χ_M is molar magnetic susceptibility) was analyzed by the Curie–Weiss law to afford a Curie constant (C) of 0.732 cm³ K mol⁻¹ and a Weiss temperature (θ) of −1.5 K (Fig. 1c). The C value is close to that expected for two non-interacting S = 1/2 spins (0.75 cm³ K mol⁻¹), confirming the radical character of the two PyBTM ligands in 1aSbF₆. The negative θ value indicates weak antiferromagnetic interactions dominant between the spin centers, which would be mediated via intermolecular atomic contacts.

1aSbF₆ in dichloromethane demonstrated concentration-dependent UV-vis absorption characteristics (Fig. S4†). The absorption spectrum of a dilute solution (4.93 μM) was almost identical to that of PyBTM in dichloromethane, whereas a concentrated solution (149 μM) showed absorption similar to that of the gold(I) complex with ligated PyBTM, 2BF₄.⁷ The result suggests the presence of an equilibrium between the dissociation and association of PyBTM from and to the Ag^I ion in solution. The equilibrium behaviour was further investigated by titration studies (Fig. 2a). The incremental addition of AgSbF₆ to PyBTM in dichloromethane caused a new absorption band to appear around 420 nm and the lowest-energy absorption band to shift to a longer wavelength. Two isosbestic points were observed at 373 and 320 nm, suggesting that only one UV-vis detectable species was in equilibrium with PyBTM under the tested conditions. Two possible chemical species, both Ag^I-PyBTM complexes, formed in this titration: one with a ligand–metal stoichiometry of 2 : 1 (i.e., 1a⁺) and another with a 1 : 1 stoichiometry (i.e., [Ag(PyBTM)L]⁺, where L represents a ligand other than PyBTM). Simulations suggest that the selective formation of the 2 : 1 Ag^I complex requires an unfeasibly strong positive cooperativity between the first and second binding of the ligand to an Ag^I ion (Fig. S6†).¹⁴ Furthermore, no additional change in the absorption spectral shape was observed upon the addition of a large excess of Ag^I ions. We thus tentatively conclude that the 1 : 1 Ag^I complex formed as the major product in this titration experiment. The titration plots show a good fit with a 1 : 1 binding model, giving an association constant (K) of (8.7 ± 0.4) × 10³ M⁻¹ (Fig. 2a and Fig. S5†).

Fig. 2 (a) Absorption spectral changes of PyBTM (30 μM, black line) in dichloromethane upon addition of 0–4.8 equiv. silver(I) hexafluoroantimonate (0.5 equiv., green line; 4.8 equiv., blue line) at room temperature. (b) Oxygen K-edge X-ray absorption spectrum of soln_99 containing 10 mM of 1b⁺ at room temperature.

The 1 : 1 Ag^I–PyBTM complex [Ag(PyBTM)L]⁺ is expected to possess an auxiliary ligand L, which is likely to be H₂O according to soft X-ray absorption spectroscopy. Fig. 2b shows an oxygen K-edge X-ray absorption spectrum of a dichloromethane solution containing [Ag(PyBTM)L]⁺ and PyBTM in a molar ratio of 99 : 1 (soln_99). The molar ratio was determined based on the association constant K (further details are given in ESI†). A distinct pre-edge peak assignable to H₂O was detected around 534 eV. It was not observed from either the dichloromethane solvent or AgSbF₆ in dichloromethane (Fig. S7†). The results suggest that the peak is attributable to the H₂O molecule coordinating to the Ag^I ion in [Ag(PyBTM)(H₂O)]⁺ (1b⁺) rather than the very small amount of free H₂O molecules that were under the detection limit of X-ray absorption spectroscopy.

ESR spectroscopy for soln_99 and PyBTM in CH₂Cl₂ was performed to characterize the spin density distribution of 1b⁺. The measured ESR spectrum of PyBTM in CH₂Cl₂ at 175 K matched the simulated spectrum involving hyperfine splitting due to six ¹H, one ¹⁴N, and one ¹³C atoms, indicating the delocalization of the unpaired electron over the triarylmethyl skeleton (Fig. S8†). This result is consistent with the previous report.¹⁵ On the other hand, the spectrum of soln_99 at 175 K showed a different hyperfine structure, which was reproduced by simulation considering hyperfine coupling with six ¹H, one ¹⁴N, one α-¹³C, and one ¹⁰⁷Ag/¹⁰⁹Ag atoms (Fig. S9†). This indicates the coordination of PyBTM to Ag^I and partial distribution of the spin density on the Ag^I ion in 1b⁺.

Absorption and emission spectroscopic investigations of soln_99 enabled us to characterize 1bSbF₆ as the first luminescent open-shell Ag^I complex (Fig. 3a). soln_99 displayed absorption peaks around 420 and 620 nm in the visible region. The similarity of the absorption spectra of soln_99 and 2BF₄ suggests the coordination of PyBTM to Ag^I in 1bSbF₆.⁷soln_99 exhibited distinct photoluminescence with a broad and structureless emission band at λ_em = 620 nm. The excitation spectrum of soln_99 was similar to its absorption spectrum, indicating the same origin of the light absorption and emission (Fig. S10†). Importantly, the decay profile of this emission fitted well with a mono-exponential function with a lifetime (τ) of 19.0 ns (Fig. 3b), which differed from that of PyBTM (τ = 6.4 ns).¹⁵ These results confirm that the emission originated mostly from a single substance, namely 1bSbF₆.

Fig. 3 (a) Absorption (solid lines) and emission (dashed lines, λ_ex = 375 nm) spectra of soln_99 containing 30 μM of 1b⁺ (red) and PyBTM in dichloromethane (30 μM, black). Inset shows a photograph of soln_99 under 365 nm UV light. (b) Emission decay of soln_99 (red curve, λ_ex = 375 nm, λ_em = 620 nm). The black line represents the mono-exponential fit.

Table 1 summarizes the photophysical characteristics of 1bSbF₆ and the related compounds, PyBTM¹⁵ and 2BF₄, in dichloromethane. The emission maximum wavelength of 1bSbF₆ (λ_em = 620 nm) was bathochromically shifted with respect to that of PyBTM (585 nm). In addition, its absolute photoluminescence quantum yield (ϕ = 0.12) was about five times that of PyBTM (ϕ = 0.025). The radiative and non-radiative rate constants (k_r and k_nr, respectively) estimated from ϕ and τ suggest that the increased ϕ of 1SbF₆ was due to both increasing k_r and decreasing k_nr. These coordination-induced changes in the emission characteristics are qualitatively similar to those previously observed in 2BF₄.

Table 1 Photophysical parameters of PyBTM, 2BF₄, and 1bSbF₆ in dichloromethane

Compound	λ em/nm	ϕ em	τ/ns	k r/10^6 s^−1	k nr/10^8 s^−1
a Photophysical parameters were determined using soln_99.
PyBTM ^15,16	585	0.025	6.4	3.9	1.5
2BF4 ^7	653	0.08	13.1	6.0	0.70
1bSbF6 ^a	620	0.12	19.0	6.1	0.46

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Density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations also suggested that Ag^I and Au^I ions similarly affect the electronic structure and excited state characteristics of PyBTM. Fig. 4 shows the frontier orbital energies of PyBTM, 1b⁺, and 2⁺, along with the respective frontier orbitals mainly involved in their lowest-energy excitation. The lowest-energy excitations of all three compounds were attributed to the transitions of β-spin electrons between orbitals mainly distributed on the PyBTM moiety. This suggests that photoexcitation and the resulting emission were centred on the PyBTM moiety. The frontier β-orbital energies of 1b⁺ and 2⁺ are lower than those of PyBTM, with the energy lowering of β-singly occupied molecular orbital (β-SOMO) being especially remarkable (Fig. 4a). In addition, the distributions of the frontier orbitals in 1b⁺ and 2⁺ are similar to each other but different from those in PyBTM, whose molecular orbitals are more evenly distributed on the three aryl rings (Fig. 4b).

Fig. 4 (a) Frontier orbital energy diagrams and (b) molecular orbitals mainly involved in the lowest-energy excitation of PyBTM, 1b⁺, and 2⁺, calculated by DFT (UM06/SDD(Ag, Au), 6-31G(d)(H, C, N, O, Cl, P)). The solvent (CH₂Cl₂) effect was treated using the polarizable continuum model.

This series of experimental and calculated results indicates that coordination of PyBTM to an Ag^I ion has a qualitatively similar effect on its luminescence to its coordination to an Au^I ion. In accordance with previous studies of several Au^I complexes of PyBTM derivatives including 2⁺,^7,8 inductive and electrostatic effects of the Ag^I ion stabilized the β-SOMO and broke the symmetry of the frontier orbital distribution of the PyBTM moiety. The former contributed to the red-shifting of the emission, and the latter contributed to the increase of oscillator strength of the D₁ ← D₀ transition (and thus the increase of the k_r for emission), because this transition is forbidden for ideal three-fold symmetric triarylmethyl radicals. In addition, the reduced overlap of the empty and occupied orbitals might suppress vibronic coupling between the D₁ and D₀ states, thereby decreasing k_nr.¹⁷

The photoluminescence characteristics of 1b⁺ and 2⁺ are quantitatively slightly different, with 1bSbF₆ having a shorter maximum wavelength of emission and higher ϕ than 2⁺ (Table 1).⁷ The higher ϕ is due to the smaller k_nr, which can be explained by the energy gap law.¹⁸ One reason for the blueshifting of the emission of 1bSbF₆ relative to that of 2BF₄ might be Ag^I complexes having longer metal–nitrogen bonds than the corresponding Au^I complexes.¹⁹ The average Ag^I–N bond length in crystalline [Ag^I(PyBTM)₂](SbF₆) was 2.158 Å, larger compared with the Au^I–N bond length of 2.101 Å in crystalline 2ClO₄.⁷ DFT calculations also indicated longer metal–nitrogen bonds in 1b⁺ (2.178 Å) than in 2⁺ (2.150 Å). The more distal Ag^I ion and its consequently weaker electrostatic effects may stabilize the β-SOMO less than in Au^I complexes, resulting in 1bSbF₆ having a higher emission energy than 2BF₄.

Conclusions

In conclusion, the complexation of PyBTM with Ag^I ions was investigated. While two PyBTM molecules coordinated to one Ag^I ion in the crystalline state, PyBTM formed a 1 : 1 complex with Ag^I in solution. The labile nature of the Ag^I–radical coordination bond can aid the construction of more elaborate molecular and supramolecular architectures including metal–organic frameworks and self-assembled cage-like supramolecules. In addition, facile exchange of the auxiliary ligands on an Ag^I ion could serve new photofunctions such as sensing. The 1 : 1 complex exhibited enhanced redshifted photoluminescence compared with free PyBTM, as did the corresponding Au^I complex. This study demonstrated the similarity in the effect of Ag^I and Au^I ions on the electronic structure and excited states of the radical complexes, which would contribute to the rational design of luminescent radical complexes with controllable and predictable emission characteristics.

Experimental

Materials and methods

Unless otherwise noted, solvents and reagents were purchased from TCI Co., Ltd, Wako Pure Chemical Industries, Ltd, or Kanto Chemical Co., Inc. and used without further purification. Dichloromethane was purified with AS ONE Ultimate Solvent System 4S.

UV-Vis absorption spectra were recorded on a JASCO V-770 spectrophotometer. Steady-state emission spectra and fluorescence decay curves were measured using a measurement system with a picosecond diode laser with the emission wavelength of 375 nm (Advanced Laser Diode Systems PIL037X) as light source, a single grating spectrometer (Andor Kymera193i-B1), a multichannel CCD detector (Andor iDus DV420A-OE), and a photon counting detector (MPD SPD-050-CTE) operated with a time-correlated single photon counting (TCSPC) technique. Solvents used for the spectroscopic measurements were air-saturated. Absolute photoluminescence quantum yields were determined by a Hamamatsu Photonics Quantaurus-QY absolute PL quantum yield spectrometer (C11347-01). MALDI-TOF mass data were recorded by a Bruker microflex LRF system.

Soft X-ray absorption spectra were obtained using a transmission-type liquid cell at the soft X-ray beamline BL3U of the UVSOR-III Synchrotron.²⁰ The liquid layer was sandwiched between two 100 nm thick Si₃N₄ membranes, and the thickness of the liquid layer was precisely controlled from 20 nm to 40 μm for obtaining the appropriate absorbance of soft X-rays. The liquid samples were introduced to the liquid cell using a syringe pump keeping the inert condition.

The data for single crystal X-ray diffraction (SCXRD) analysis was collected at 123 K on a ROD, Synergy Custom system (Rigaku Oxford Diffraction) equipped with mirror monochromated Mo-Kα radiation. A suitable single crystal was mounted on a looped film (micromount) with Paraton-N. Data were processed using CrysAlisPro 1.171.39.43c (Rigaku Oxford Diffraction). The structures were solved using ShelXT²¹ and the whole structure was refined against F² with SHELXL-2018/3.²² Hydrogen atoms were located in idealized positions and were refined using a riding model with fixed thermal parameters. Crystal structure data (CIF, CCDC 2331661† for [Ag(PyBTM)₂]SbF₆) can be obtained free of charge from The Cambridge Crystallographic Data Centre. Powder X-ray diffraction measurements were performed at room temperature using a Rigaku MiniFlex600 diffractometer (Cu-Kα radiation, λ = 1.5406 Å) operating at 40 kV/15 mA with a Kβ foil filter.

The temperature dependence of the magnetic susceptibility was measured with a quantum design MPMS-7 SQUID magnetometer. Aluminium foil was used as a sample container, whose magnetic contribution was subtracted as background by measuring its own magnetic susceptibilities in every measurement. The diamagnetic correction χ_dia for [Ag(PyBTM)₂](SbF₆) was carried out with Pascal's constants (χ_dia = 5.35 × 10⁻⁴ cm³ mol⁻¹).

DFT and TD-DFT calculations were carried out using the Gaussian 16 Revision C.01 program package.²³ The TD-DFT calculations were performed after the geometry optimization in the electronic ground state at the UM06/SDD(Ag, Au);6-31G(d)(H, C, N, Cl, O, P) level under the SCRF (solvent = CH₂Cl₂) effect.

Synthetic procedures

Synthesis of [Ag(PyBTM)₂]SbF₆ (1aSbF₆). PyBTM was prepared according to the procedure in the literature.¹⁵ A solution of PyBTM (52.1 mg, 0.100 mmol) in dichloromethane (3 mL) was added to a solution of silver(I) hexafluoroantimonate (17.4 mg, 0.0506 mmol) in dichloromethane (3 mL). After being stirred for 10 min at room temperature in the dark, the mixed solution was slowly concentrated under a diisopropyl ether atmosphere at room temperature to give 1aSbF₆ as red crystals, which is suitable for the SCXRD measurement (58.4 mg, 0.0422 mmol, 83% yield). Anal. calcd for C₃₆H₁₂N₂Cl₁₆AgSbF₆ (1aSbF₆): C, 31.26; H, 0.87; N, 2.03. Found: C, 31.02; H, 1.08; N, 2.03; MALDI-TOF MS m/z [M]⁺ calcd for C₃₆H₁₂N₂Cl₁₆Ag ([Ag(PyBTM)₂]⁺) 1146.5 (base peak), found 1145.9.

Preparation of a solution containing 1b⁺ and PyBTM with their molar ratio of 99 : 1 (soln_99).  soln_99 was prepared based on the equilibrium eqn (1) with K value of 8.7 × 10³ M⁻¹, which was determined by titration experiments (Fig. S5†). A dichloromethane solution containing 2.31 × 10⁻² M silver(I) hexafluoroantimonate and 3.0 × 10⁻⁵ M PyBTM was mixed with a dichloromethane solution of 3.0 × 10⁻⁵ M PyBTM with a 1 : 1 v/v ratio to obtain soln_99 suitable for absorption, emission, and ESR spectroscopy. Similarly, a dichloromethane solution containing 21.3 mM silver(I) hexafluoroantimonate and 10 mM PyBTM was prepared to obtain soln_99 suitable for Soft X-ray absorption spectroscopy.

Author contributions

T.M.: conceptualization, project administration, investigation, formal analysis, visualization, and writing – original draft. R.M.: investigation, formal analysis, visualization, and writing – original draft. M.N.: methodology, resources, and investigation (X-ray absorption spectroscopy). T.K.: conceptualization, funding acquisition, project administration, and supervision. All the authors contributed equally to writing – review & editing.

Data availability

Crystallographic data for 1aSbF₆ has been deposited at the CCDC under 2331661.† The other data supporting this article have been included as part of the ESI.†

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The present study was supported by the JSPS KAKENHI (Grant Numbers JP23H01980, JP23H04863, and JP23K04699) and the JST FOREST (Grant Number JPMJFR221Q). A part of this work was conducted in Institute for Molecular Science, supported by Advanced Research Infrastructure for Materials and Nanotechnology in Japan (JPMXP1222MS5001 and JPMXP1223MS5002) of the Ministry of Education, Culture, Sport, Science and Technology (MEXT), Japan. The soft X-ray absorption spectroscopy was measured at the BL3U of UVSOR Synchrotron Facility, Institute for Molecular Science (IMS Program 23IMS6620). The computation was performed using Research Centre for Computational Science, Okazaki, Japan (Project: 23-IMS-C244, 22-IMS-C191). This work was partly supported by the Spintronics Research Network of Japan (Spin-RNJ).

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Footnote

† Electronic supplementary information (ESI) available: Materials and methods, X-ray crystallography, magnetic properties, photophysical properties, X-ray absorption spectroscopy, Titration analyses, and DFT calculations. CCDC 2331661. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d4dt03129f

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