Luminescent Complexes of the Trinuclear Silver(I) and Copper(I

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Cite This: Inorg. Chem. 2019, 58, 8645−8656

Luminescent Complexes of the Trinuclear Silver(I) and Copper(I) Pyrazolates Supported with Bis(diphenylphosphino)methane A. A. Titov,† O. A. Filippov,† A. F. Smol’yakov,†,∥,⊥ I. A. Godovikov,† J. R. Shakirova,‡ S. P. Tunik,*,‡ I. S. Podkorytov,§ and E. S. Shubina*,†

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A. N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences, Vavilov Str., 28, 119991 Moscow, Russia ‡ Institute of Chemistry, St. Petersburg State University Universitetskii pr., 26 198504, St. Petersburg, Russia § St. Petersburg State University, Laboratory of Biomolecular NMR, St. Petersburg, Universitetskaya nab. 7/9, 199034, Russia ∥ Inorganic Chemistry Department, Peoples’ Friendship University of Russia, Miklukho-Maklaya str. 6, 117198, Moscow, Russia ⊥ Plekhanov Russian University of Economics, Stremyanny per. 36, Moscow, 117997, Russian Federation S Supporting Information *

ABSTRACT: The first example of trinuclear copper(I) and silver(I) pyrazolates adducts with a tertiary diphosphine (Ph2PCH2PPh2) retaining trimeric [MPz]3 core is reported. Despite rather strong M−P bonding, the complexes are able to undergo the dissociation of one M−P bond leading to the “merry-go-round” movement of P atoms over the M3 triangle. The copper complex displays emission from 1MLCT and 3 MLCT states. The triplet and singlet states are separated by a relatively small energy gap (1080 cm−1) that triggers the thermally activated delayed fluorescence (TADF) behavior and leads to the worthy quantum yield of 41% at 298 K. The silver complex in the solid state and frozen solution shows dual emission originating from the 1IL and 3 MLCT states that is dictated by the much higher energy difference between the emissive singlet and triplet as well as by the essentially different nature of these states.

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INTRODUCTION Growing interest in OLED technology in modern science correlates with an increasing requirement for efficient and low energy consumption. It is well established that phosphorescent emitters based on transition metal complexes theoretically may harvest up to 100% of applied energy that allows for fabrication of highly efficient devices, but a considerable part of these compounds contain expensive metals, such as Ir(III), Ru(II), and Pt(II).1 Therefore, it is not surprising that the use of inexpensive and easily available materials is one of the challenging problems in further development of this technology. All these reasons stimulate increased attention to the Cu(I) and Ag(I) luminescent complexes and light emitting devices based on them. In addition, it is worth noting that these metal ions are not toxic compared to many other widely used heavy metals.2 Moreover, coinage metal complexes may display thermally activated delayed fluorescence (TADF) due to their potentials for harvesting both triplet and singlet excitation that can be used for producing highly effective electroluminescent devices.3,4 Complexes of group 11 metals with N-donor ligands are widely represented in modern coordination chemistry, and the structural diversity of these classes of compounds is enormous.5 Pivotal works are focused on the complexes of simple inorganic metals salts with such diimine type ligands as © 2019 American Chemical Society

bipyridine or phenanthroline (more than 1000 structures in Cambridge Structural Database) and in particular on the complexes showing highly efficient fluorescence and phosphorescence. Neutral cyclic pyrazolate complexes of group 11 metals [M(Pz)]n, where M = Cu or Ag and n depends on the type of substituents in the pyrazolate ligand and the nature of the metal atom, constitute a very special class of coordination macrocyclic compounds, which display unique luminescent behavior6−9 and interesting structural features of the supramolecular aggregates due to acid−base10−13 or metallophilic interactions.7,14 The structural chemistry of cyclic copper(I) and silver(I) pyrazolates and their ability to form stable complexes with the bases of different nature are widely presented in the literature.13,15−19 However, there are very few studies devoted to the reactions of these complexes with phosphorus donors,20−22 but the photoluminescence properties of the products obtained have been discussed only in our works.23 It is well documented that phosphine ligands in the copper diimine complexes, in particular those containing bulky substituents, increase the lifetime and quantum yield of emission by blocking nonradiative relaxation through prevention of structural distortions in the excited state and Received: April 5, 2019 Published: June 19, 2019 8645

DOI: 10.1021/acs.inorgchem.9b00991 Inorg. Chem. 2019, 58, 8645−8656

Article

Inorganic Chemistry

Figure 1. Molecular structures of 2a (left) and 2b (right). Metal, nitrogen, phosphorus, and carbon atoms in Pz ligand and methylene group are shown as ellipsoids (30% probability level); other C, F, and H atoms are shown in “stick model”.

increasing the energy of unoccupied d-orbitals, which are commonly responsible for emission quenching in transition metal complexes.24−26 Herein we present synthesis, structural characterization, and photophysical and theoretical study of the {[3,5-(CF3)2Pz]M}3, (M = Ag, Cu) complexes with coordinated bis(diphenylphosphino)methane (dppm). The complexes display dynamic behavior in solution and intriguing emission properties; the latter are determined by the nature of the metal ion to give multiple fluorescence in the silver complex and TADF type behavior in the case of the copper derivative.

Table 1. Selected Bond Lengths (Å) and Angles of Complexes 2a and 2b 2a Ag1−P1 Ag1−N1 Ag1−N6 Ag2−P2 Ag2−N3 Ag2−N2 Ag3−N5 Ag3−N4 P1−C23 P1−C16 P1−C17 P2−C29 P2−C16 P2−C35

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RESULTS AND DISCUSSION Trinuclear Ag(I) (2a) and Cu(I) (2b) complexes were prepared by treating the corresponding metal macrocyclic pyrazolate [{3,5-(CF3)2Pz}M]3 (M = Ag (1a), M = Cu (1b)) with bis(diphenylphosphino)methane (dppm) in a 1:1 molar ratio in aromatic hydrocarbons (benzene, toluene). Complexes 2a and 2b are readily soluble in common organic solvents (dichloromethane (DCM), CHCl3, benzene, acetone, and boiling hexane). Crystal Structure Determination. Colorless crystals of complexes 2a and 2b were obtained by crystallization from DCM/hexane solution (v/v = 1:1). The complexes contain trinuclear macrocyclic pyrazolate cores ([{3,5-(CF3)2Pz}M]3) and bis(diphenylphosphine)methane in a 1:1 ratio. The structures of 2a and 2b were established by single-crystal XRD study (Figure 1, Table S1). Selected bond lengths and angles are listed in Table 1. Each phosphorus atom of the diphosphine is coordinated to one metal atom in the macrocycle. The third metal atom does not take part in coordination with the diphosphine that leads to significant distortion of the macrocycle planarity. Similar bending of the macrocycle plane was observed for the copper macrocycle 1b complex with pentaphosphaferrocene,18 but it should be noted that distortion in the case of the diphosphine ligand is slightly smaller. The angle between the Ag1N1N2Ag2 and Ag1Ag2N3N4Ag3N5N6 planes is 75.84° in 2a and 85.61° between Cu1N1N2Cu2 and Cu1Cu2N3N4Cu3N5N6 in 2b. In the product of the reaction between 1b and Cp*Fe(η5-P5), this deviation amounts up to ca. 89 degrees. The M−P bonds are 2.3363(8) Å, 2.3735(7) Å for 2a and 2.175(1) Å, 2.200(1)

∠N1−Ag1−N6 ∠N2−Ag2−N3 ∠N4−Ag3−N5 ∠P1−Ag1−N1 ∠P1−Ag1−N6 ∠P2−Ag2−N2 ∠P2−Ag2−N3 ∠P1−C16−P2

2b bond lengths (Å) 2.3363(8) Cu1−P1 2.211(3) Cu1−N1 2.278(2) Cu1−N6 2.3735(7) Cu2−P2 2.271(3) Cu2−N3 2.289(2) Cu2−N2 2.085(3) Cu3−N5 2.083(3) Cu3−N4 1.818(2) P1−C23 1.831(2) P1−C16 1.817(3) P1−C17 1.816(3) P2−C29 1.841(3) P2−C16 1.820(2) P2−C35 Angles, deg 92.63 ∠N1−Cu1−N6 92.21 ∠N2−Cu2−N3 175.62 ∠N4−Cu3−N5 138.24 ∠P1−Cu1−N1 128.40 ∠P1−Cu1−N6 137.78 ∠P2−Cu2−N2 129.38 ∠P2−Cu2−N3 115.92 ∠P1−C16−P2

2.175(1) 1.970(2) 2.010(1) 2.200(1) 2.028(2) 1.994(1) 1.872(2) 1.873(2) 1.826(2) 1.835(2) 1.820(1) 1.826(1) 1.841(2) 1.828(2) 104.67 104.81 177.14 132.89 122.18 132.97 121.81 114.52

Å for 2b, respectively. The Cu−P bonds in 2b are significantly shorter than those in the complex of 1b with pentaphosphaferrocene (2.307−2.324 Å). There are no contacts of metal atoms in macrocycles with π-electron systems of the diphosphine phenyl rings. Coordination of dppm to macrocycles leads to significant elongation of the M−N bonds from 2.081(3)−2.095(3) Å in the starting 1a to 2.211(3)−2.289(2) Å for 2a and from 1.855(2)−1.863(2) Å in 1b to 1.970(2)− 2.028(2) Å 2b. At the same time the fragment containing “phosphorus-free” metal atom, the N4−M3−N5 bond angle, remains almost intact (Figure 1). For example, the angles N4− M3−N5 of 175.62° and 177.14° are practically equal to those in the starting macrocycles, in contrast to the angles N6−M1− N1 and N2−M2−N3, which are 92.63° and 92.21°, 104.67° 8646


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Inorganic Chemistry

Figure 2. IR spectra of macrocycle 1b (black) and complex 2b (red) in the region of δ(CH) in KBr pellets (left) and in the DCM solution (right).

Figure 3. Gas phase optimized geometries of 2a and 2b. Hydrogen and fluorine atoms are omitted for clarity.

macrocycle and approximately equivalent M−P bonds. Similar to the XRD geometry Cu−P distances are shorter than Ag−P; the angle between the planes M1N1N2M2 and M1M2N3N4M3N5N6 is 75.6° in 2a and is smaller than that of 83.9° in 2b. The energy decomposition analysis (EDA) shows that electrostatic attraction energy between dppm and [ML]3 fragments in 2a and 2b is mostly counterbalanced by the Pauli repulsion, making their sum ΔEsteric = ΔEelstat + ΔEPauli somewhat positive. Therefore, the most important attractive energetic term would be an orbital interaction ΔEorb. This ΔEorb energy term was further decomposed to the individual electron density transfer channels in the framework of the ETS-NOCV method (Table 2). As could be expected, the two highest energy channels are due to the donation of the electron density from the P-lone pairs (which are two highest energy HOMO’s of the dppm fragment) to the [ML]3 empty orbitals located on the metal atoms (Figure S10). The channels of electron density transfer ##3−8 are the metal-to-ligand π-back-donation (Figures S10− S12). Those 6 channels feature transfer of electron density from the metal (Cu or Ag) filled d-orbitals (dz2, dxz and dyz) to the antibonding orbitals of dppm located on the P atoms as outer lobes of the P−C antibonding orbitals (σ*P−C), as dppm has six σ*P−C orbitals. Notably the overall energy of the σL→M donation is at the same threshold as the πM→L back-donation, with the latter being even 7 kcal/mol larger for the copper complex. Similarly for the [CuL]3/Cp*FeP5 complex, the complexes 2a and 2b have a large negative impact on the macrocycle deformation energy ΔEprep, which can be as high as

and 104.81° for 2a and 2b, respectively. Configurations of the ligand environment of the metal atoms bound to phosphorus can be described as planar trigonal with angles N−M−N and N−M−P for 2a being equal to 92°, 129°, and 138° for 2b 105°, 122°, and 133°. Coordination of dppm to macrocycles leads to nonsignificant shortening of all P−C(CH2) and P− C(Ph) bonds for ca. 0.01−0.02 Å. In the solid state the molecules of both complexes form pseudodimers due to short H(Ph)−π(N−NPz) contacts: 2.495 and 2.624 Å in 2a; 2.631 and 2.719 Å in 2b. Additional H−F and F(Pz)−π(Ph) contacts give infinite supramolecular chains. There are not any close M···M intermolecular contacts. The similarity of the structures of these complexes both in solution and in the solid state was confirmed by using IR spectroscopy as exemplified by the study of the complex 2b. The local C3v symmetry of unsubstituted macrocycles usually results in the appearance of only one intense band in the region of δ(CH) of pyrazolate ligand in the IR spectra (1038 cm−1). Bending of the macrocycle plane leads to the symmetry distortion and appearance of new bands in the δ(CH) region of the IR spectra both in solid state and DCM solution. The position and number of the new bands corresponds to the presence of three inequivalent CH groups (Figure 2) and indicates the similarity of the structures in both solid and solution. These data correlate with our previous results obtained in the investigation of the interaction of macrocycle 1b with pentaphosphaferrocene.18 Nature of M−P Bonding. Gas phase optimization (BP86D3/TZ2P) nicely reproduces the XRD geometry of 2a and 2b(Figure 3). DFT study indicates the bending of the 8647


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Inorganic Chemistry Table 2. Results of EDA and ETS-NOCV Analysis of the [ML]3/Phosphine Complexes (in kcal/mol) ΔEPauli ΔEelstat ΔEsteric ΔEorb ΔEdisp ΔEint σL→Ma πM→Lb ΔEprep(dppm) ΔEprep([ML]3) ΔEtotal

2a

2b

[CuL]3/P5c

274.3 −246.3 28.1 −108.7 −34.7 −115.4 −46.2 −53.4 2.5 52.5 −60.4

246.6 −228.0 18.6 −86.6 −29.4 −97.4 −40.9 −36.2 3.2 53.5 −40.7

304.9 −229.8 75.1 −158.3 −24.7 −107.9 40.5 79.2 3.1 50.9 −53.2

a

Sum of NOCVs 1 and 2. bSum of NOCVs from 3 to 8. cData from ref 18 provided for comparison.

+52.5 and +53.5 kcal/mol for copper and silver macrocycles correspondingly. It should be noted that the availability of the [ML]3 macrocycles filled d orbitals for the π-back-donation strictly depends on the bending of the macrocycle (see18). The same could be said about the HOMO energies; EHOMO and EHOMO−1 are −6.41 and −6.42 eV for free [CuL]3 macrocycle being −5.85 and −6.08 eV in the bent one. The [AgL]3 macrocycle possesses higher HOMO’s energies (−6.95 and −6.96 eV for the flat [AgL]3) and smaller energy lowering upon bending (−6.85; −6.79 eV for bent [AgL]3) which is in line with the weaker binding in the silver complex 2a. Analysis of the complexes geometry and energy decomposition reveals several common trends. First, we should mention the ability of the phosphorus bases to accept the electrons to form the π-back-donation channel. This feature was initially revealed for the cyclo-P5 ligand,18 where it was not very surprising, but in the current work, we confirm the conclusion for a simple chelating dppm ligand. In the case of the phosphines with aliphatic substituents, the σ-holes of the P−C bonds play a key role in the electron withdrawing ability of the phosphine ligands. The second feature is an ability of the metal atoms in the bent macrocycle to display electron donation. Obviously, the same d-lone pairs exist in the flat macrocycle but bending makes them available for the interaction and also increases their energies strengthening back-donation. Last but not least, the energetic demands for bending are not so high (ca. 50 kcal/mol) and they are almost counterbalanced by the appearing π-back-donation (40−50 kcal/mol). Therefore, we could suggest that the interaction of the macrocyclic Cu(I) and Ag(I) pyrazolates with the phosphorus bases features some specific bonding characteristics. NMR Investigation. Complexes 2a and 2b demonstrate dynamic behavior. According to the VT 1H NMR spectra of 2b shown in Figure 4 at low temperature (blue and red traces), the exchange is frozen. In accordance with the solid state structure, the three CHPz groups give two singlets with the relative intensities of 2 and 1 in the aromatic region (6.96 and 6.92 ppm). The two protons of the CH2 group of the dppm ligand are nonequivalent and produce a characteristic AB quartet (3.41 and 3.14 ppm, 2J(1H1H) = 13.8 Hz) in the 1 H{31P} NMR spectrum (Figure 4, blue trace). In the case of the conventional 1H NMR spectrum (Figure 4, red trace), the AB quartet turns into a doublet of multiplets due to the

Figure 4. 300 MHz 1H NMR spectra of complex 2b in CD2Cl2 at different temperatures. Blue (195 K): 1H NMR spectrum with 31P decoupled, 1H {31P}. Red (195 K), green (253 K), and black (292 K): conventional 1H NMR spectra.

presence of 2J(1H31P) coupling. When the temperature rises, the exchange rate becomes higher, and at 253 K (Figure 4, green trace) the three CHPz protons as well as the two protons of dppm of CH2 coalesce and give singlets (very broad in the last case). Finally, at 292 K (Figure 4, black trace) the CHPz proton remains a singlet and the CH2 signal of dppm looks as a triplet due to coupling to phosphorus 2J(1H31P) = 10.0 Hz. These observations are indicative of dynamic behavior, which results at low temperature in the static structural pattern identical to that found in the solid state and C 3v symmetrization of the molecule at room temperature. In the same temperature range the 31P{1H} NMR spectra of complex 2b display only one broaden signal, corresponding to the phosphorus atoms coordinated to copper atoms. On the contrary, the 31P{1H} NMR spectra of 2a demonstrated a complicated temperature dependent behavior. In particular, the temperature dependent (198−298 K) 31P{1H} NMR spectra (Figure 5) display the transformation, which could be assigned to intramolecular dynamic process related to the movement of the diphosphine ligand over the [Ag3Pz3] core. Suppose that at 198 K the exchange is frozen, the molecular structure is then identical to that found in the solid state and belongs to the Cs symmetry group. In this case the experimental 31P{1H} NMR spectrum can be considered as a superposition of three static AA′XX′, AA′XY, and AA′YY′ spectra (A = 31P, X = 107Ag, Y = 109Ag) corresponding to the isotopomers with all possible combinations of silver isotopes. The parameters extracted from the spectrum and used for subsequent simulation of dynamic spectra are 2J(PP) = 151.1 Hz, 1J(31P−107Ag) = −596.86 Hz, 3J(31P−107Ag) = 3.63 Hz; note that the last two coupling constants have different signs. The couplings involving the 109Ag isotope were set according to magnetogyric ratios: nJ(31P−109Ag) = (109γ/107γ)·nJ(31P−107Ag), n = 1 or 3. All other long-range couplings were neglected. It is worth noting that these values 8648


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Figure 5. 162 MHz 31P{1H} NMR variable temperature spectra (T = 198, 218, 238, 258, 278, and 298 K) of 2a in toluene-d8. Black traces show the experimental results; red traces are corresponding simulations.

Figure 6. Schematic enthalpy profile of 2b (red line) and 2a (black line) isomerization.

displays the signal of the AB system of CH2 protons together with two CHPz signals in the 2:1 ratio that indicate that the conformational dynamics, in contrast to the “merry-go-round” movement, is not frozen even at this temperature. In the simulation of the temperature dependent NMR spectra of 2a, the rate of the dynamic process is an essential model parameter, and treatment of the rate data for the “merry-goround” dynamics with the Eyring equation gave ΔH# = 12 ± 1 kcal/mol (Table S2, Figure S13). Simultaneously the calculations revealed high energy minima located on the potential energy surface (PES), with the dangling diphosphine arm, one M−P bond, and flattened macrocycle. We suppose that this is metastable high energy intermediate near the transition state (TS) of the “merry-go-round” exchange, which occurs through breaking of the one M−P bond and rotation of the dppm ligand around the other M−P bond. The calculated isomerization barriers ca. 10−15 kcal/mol (see reaction energy profile shown in Figure 6 ) are in good agreement with the activation barrier obtained from analysis of the NMR data, vide supra. Note that the optimized structure of the trimetallic core in the high energy intermediate is not flat, which is in agreement with retaining of one phosphorus atom bond with a metal center. Photophysical Properties. Both silver and copper complexes are luminescent in solution and in the solid state but their emission behavior is substantially different due to the delicate balance between the energy of singlet and triplet excited states, relative rates of their radiative and nonradiative transitions, and rates of intersystem crossing processes. The absorption spectra of the compounds 2a and 2b contain strong high energy (HE) bands in the short wavelength area (λabs < 250 nm) and low energy (LE) shoulders at about 250 and 265 nm, respectively, with the tail stretching below 350 nm for the latter; see Table 3 and Figure 7. The HE absorption may be assigned to LC (Ligand Centered) transitions whereas LE shoulders are evidently generated by MLCT (metal-to-ligand charge transfer) that is in agreement with their long wavelength shift in the case of copper complex compared to the silver analogue due to destabilization of the metal orbitals. The room temperature (RT) emission spectrum of 2a in 1,2-dichloroethane (DCE) solution (Figure 8) displays a weakly structured band centered at 362 nm with the lifetime in the nanosecond domain (τ = 3.8 ns) that indicates the LC origin of the emission observed, i.e. fluorescence. Cooling the solution to 77 K results in emission splitting into two

of spin−spin coupling constants are not exceptional for the phosphine complexes containing coordination Ag−P bonds.27−29 The simulated spectrum (red trace at the bottom Figure 5) is in excellent agreement with the experimental one that clearly indicates the right choice of the structural model. The temperature increase results in gradual transformation of a doublet of multiplets at 198 K (Figure 5, bottom) into a quartet of quartets at 298 K (Figure 5, top) as a high temperature limiting spectrum for the dynamic process under study. We assumed that the exchange process consists in a stepwise “merry-go-round” movement of dppm ligand over the Ag3 triangle resting after each “step” at a certain triangle edge. The stages of this process most probably include decoordination of one phosphorus atom from its initial position and recoordination to another free Ag vertex, thus changing the dppm coordinated edge of the {Ag3Pz3} triangle. The second phosphorus does not change its parent position at the Ag atom during this stage. This movement of the diphosphine ligand may be described from the viewpoint of molecular symmetry as averaging of the phosphorus atoms position over the triangle to eventually give the C3v symmetry group of the molecule as a whole. This model allows for simulation of the complete set of 31 P NMR variable temperature spectra using the values of coupling constants derived from the low temperature spectrum. The simulations are shown as red traces in Figure 5 and are in a good agreement with the experimental data; the deviations in line positions do not exceed 6 Hz, which is an acceptable discrepancy in view of a relatively simple model used and possible temperature dependence of J couplings. DFT Investigation of the Dynamic Process. DFT analysis on the PBE/def2-TZVP/SMD level reveals two possible conformers of both 2a and 2b, which differ in dppm methylene group position relative to the macrocycle skeleton. The conformers 2 and 2′ (Figure 6) have nearly the same energies and M−P bond lengths (Δr < 0.01 Å), and therefore 31 P NMR chemical shifts, as well as P−X coupling constants, should also be similar. Moreover, fast interconversion of the conformers can easily occur through the changes in conformation of the five-membered cycle {M−M−P−C−P} analogously to cyclohexane “chair-boat” dynamics. The 2 ↔ 2′ isomerization evidently leads to the averaging of CH2 group signals as well as pyrazole CH signals in the 1H NMR. At the low temperature limit the 1H NMR spectrum of 2b (Figure 4) 8649


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Inorganic Chemistry Table 3. Photophysical Parameters of 2a and 2b in Solution and Solid State Temp.

λabs./nm (ε × 10−3/ cm−1 M−1)

λexc./ nma

λem./nm

τd/μs

b

2a

298 K 77 K

2a

298 K 77 K

2a

298 K

77 K

2b

298 K 77 K

DCE solution 224 (48), 252 (20) 312 310

294, 325 224 (52), 265 (21), 293, 340 (tail) 325 314 Solid statec 328, 366 328, 365sh 328, 366 328, 365sh 259, 312 259, 312

348sh, 362 341, 357, 370 505

0.0038 0.0073

532

1.6

572

195

430

0.0042

ca. 530sh

22.1

ca. 430sh

0.03

530 514

215 (38) 2700 (62)e 32.7

554

148.6

3.7

Figure 8. Excitation (short dash line−monitored at 360 nm, dash-dot line−monitored at 515 nm) and emission spectra (solid line) of complex 2a in DCE solution at 77 K (red) and 298 K (blue), λex = 280 nm.

temperature because of its effective quenching in fluid solution under ambient conditions. In the solid state RT spectrum of 2a one can observe a broad band centered at 430 and a shoulder at ca. 530 nm (Figure 9),

a Excitation spectra are measured on emission maxima. bEmission spectra and emission lifetimes measured with excitation at 320 nm. c Emission spectra and emission lifetimes measured with excitation at 340 nm. dEmission lifetimes were measured at the corresponding emission maxima. eThe values in parentheses give relative (%) contributions of the corresponding exponent in the double exponential decay.

Figure 9. Normalized emission (solid lane, λexc = 320 nm) and excitation (dash line) spectra of complex 2a in solid state at 298 K (blue) and 77 K (red); excitation spectra were monitored at 410 and 550 nm, respectively.

which display the lifetimes 4.2 ns and 22.1 μs, respectively. These data are indicative of the presence of two independent emissive excited states of essentially different nature, which can be assigned to 1LC (fluorescence) and 3MLCT (phosphorescence) similarly to the description of the dual emission in a solution of the same complex at 77 K, vide supra. The appearance of the phosphorescent band at ambient conditions in the solid state sample is due to the rigidity of the emitter environment that suppresses nonradiative vibrational relaxation of the excited 3MLCT state. The effect is essentially similar to the influence of the frozen matrix on the behavior of 2a in solution. In the solid state spectrum recorded at 77 K the triplet contribution expectedly becomes dominant due to the further suppression of nonemissive vibrational relaxation of this excited state. It is also worth noting that the triplet state decay at low temperature can only be fit with biexponential analysis with a nearly equal contribution of microsecond and millisecond components. This observation can be rationalized by the presence of the different environment of the triplet chromophoric center in the crystal cell that is not a unique

Figure 7. Absorption spectra of complexes 2a (blue) and 2b (red) in 1,2-dichloroethane solution.

components, one of which is nearly identical to the band observed in the RT spectrum (λem = 341 nm, τ = 7.3 ns), but demonstrates clearly visible vibronic progression (ca. 1200 cm−1) that additionally confirms its 1LC origin. The second broad structureless component of emission appears in the lower energy range (505 nm) with the lifetime in the microsecond domain (τ = 3.7 μs). This band displays a large Stokes shift of about 14 212 cm−1 and the excitation spectrum, which is very different from that (2 932 cm−1) detected for the HE fluorescent band. These observations indicate 3MLCT origin of the green emission, which is operative only at low 8650


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Figure 10. Normalized emission (solid line, λexc = 320 nm) and excitation (dash line) spectra of complex 2b in DCE solution (A) and solid state (B) at 298 K (green) and 77 K (red).

Figure 11. Normalized emission spectra of complex 2b in solid state in the 77−383 K temperature range, λex = 340 nm (A); graphical dependence of lifetime on temperature, solid line represents the fit made using eq 1) (B).

terms of the TADF behavior, vide infra, which has also been successfully applied for description of temperature dependent luminescent for closely related planar triangular and tetrahedral copper33 and silver33,35,36 complexes. The major characteristic feature of TADF emitters is a relatively small energy gap (less than 1300 cm−1) between the lowest lying S1 and T1 excited states that makes possible reverse population of the S1 state from T1 through their thermal equilibration at room temperature. This also gives a red shift of emission band upon temperature lowering and extremely high values of excited state lifetime for emission from the S1 state, up to a few hundred microseconds. In contrast to 2a, the room temperature spectrum of the copper complex 2b in DCE displays a structureless band at 532 nm, which shows a red shift (40 nm) upon solution cooling to 77 K (Figure 10A) This is in contrast to the “normal” blue shift of emission at low temperature, which is dictated by stabilization of the ground state. The lifetime at liquid nitrogen temperature is unusually long, 195 μs. These observations indicate that TADF is operative in the emission observed for this complex. Under the framework of this model the band observed at low temperature has to be ascribed to the triplet 3 MLCT with a possible admixture of 3LLCT state, similar to the assignment made earlier33,37 for structurally analogous complexes. The band observed at room temperature evidently originates from the singlet 1MLCT state, which has an extremely long lifetime because of its population from the closely disposed triplet due to the relatively small energy gap between these states and very fast intersystem crossing dynamics. Essentially similar behavior was observed for solid

situation in solid state photophysics of coordination compounds.30,31 It has to be noted that the photophysical behavior of the nonplanar {[ML]3(dppm)} complexes in the solid state is very different from that typical for the parent planar [ML]3 compounds. Stacking of planar motifs with the formation of metallophilic bonding dominates in the solid state structures of the planar relatives that also promotes M−M exciplex generation, which determine the specific features of their emissive behavior and distinctions to the photophysics observed for isolated molecules in solution. The variations in photophysical characteristics observed of the [ML]3 complexes in a solid state depend strongly on the stacking mode, the number of metal−metal bonds formed, and the strength of bonding.32,33 On the contrary, the nonplanar structure of the central {M3(L)3} fragment in 2a and 2b and the out of plane disposition of the diphosphine ligand prevent the stacking of the pseudoaromatic cores in solid state that dictates the difference in the photophysical behavior observed for these types of cyclic trinuclear pyrazolates. Therefore, the photophysics of 2a and 2b in the solid state and in solution is rather determined by their intramolecular characteristics than intermolecular interactions, either π-stacking or metallophilic bonding. The coordination environment of the metal ions in the complexes under study (distorted planar triangular motif) is closer to that of triply coordinated copper(I) complexes,32,34 which display thermally activated delayed fluorescence (TADF).4 In fact, the photophysical characteristics revealed for 2b both in solution and in solid state may be interpreted in 8651


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Inorganic Chemistry state emission (Figure 10B) with the only difference, which consists in a small blue shift of the bands observed at room temperature and at 77 K relative to the band maxima found in solution. Variations in emission spectra of the solid state sample and graphical representation of lifetime dependence on temperature in the 77−383 K range are shown in Figure 11A. The Stype curve obtained for the variations of lifetime with temperature is a typical feature of TADF emission and was observed for the other copper complexes of this class.33,38,39 This {τobs−T} dependence is described by the following equation:38 1+ τ(obs) =

1 τ(T1)

+

1 3

(

exp −

1 3τ(S1)

ΔEST kBT

(

exp

)

ΔE − k TST B

)

(1)

which allows for fitting the experimental data (Figure 11B) and obtaining essential parameters of the singlet and triplet states: τ(T1) = 144 ± 2 μs, τ(S1) = 0.06 ± 0.02 μs and ΔEST = 1080 ± 60 cm−1, operative in the emission of 2b in a wide temperature range, with the quantum yield up to 41%. The parameters of the TADF process obtained for 2b fit well the range of values obtained by the present time for TADF copper complexes, which have been summarized in the excellent review by Yersin et al.4 TD-DFT. For the isomers of copper and silver complexes 2 and 2′ the electronic structure was analyzed by using TD-DFT calculations. We have found that singlet and triplet exited states can be optimized only for the lowest energy isomers 2a′ and 2b; therefore, the calculation data given below refer to this isomeric species. The calculated absorption spectra (Figure S14, S15) fits well the experimental data. The lowest energy absorption bands include S0 → S1, S0 → S2 transitions for 2a′ and S0 → S2, S0 → S3 for 2b. In the former case the ligand (dppm) centered transitions have the major contribution into the low energy band with the admixture from MLCT (metal → dppm), whereas MLCT character dominates in the latter case (Figure 12) with some contribution from interligand charge transfer (ILCT, Pz → dppm). The dominating LC transition in the low energy absorption of 2a′ and low spin−orbit coupling is evidently the main reason for singlet emission of this complex in solution at room temperature. On the contrary, the copper complex 2b displays triplet emission in solution at room temperature. The triplet states Tn were optimized using TD DFT. The results of calculations showed that degenerated T1−T4 states are not emissive due to their energy surface intersection with the ground state (Figure S16) and consequently effective nonradiative relaxation. The emission observed can be ascribed to the system relaxation from the T5 state, the energy of which is in good agreement with the experimental emission wavelength. It is also worth noting that the value of Δ = (ES1 − ET5) is 0.17 eV (1364 cm−1) that evidently allows for thermal equilibrium between emissive excited triplet and singlet states. This is in good agreement with the experimental value obtained from the lifetime dependence on temperature and can be considered as theoretical support of the TADF model applied to describe the emission behavior of the copper complex.

Figure 12. Electron density transfer maps in the S0 → S1, S0 → S2 excitation of 2a′ (left) and S0 → S2, S0 → S3 in 2b (right) as isosurface at 0.002 au. Upon the electronic transition the electron density increases in the blue areas and decreases in the red areas. Hydrogen and fluorine atoms are omitted for clarity.

containing ligands. Bis(diphenylphosphino)methane coordination leads to the macrocycle bending similar to that found previously for pentaphosphaferrocene.18 Despite the lack of the apparent back-donation to the dppm compared to the complexes containing the cyclo-P5 moiety, the silver (2a) and copper (2b) still display an impact of back-donation to the σholes of the P−C bonds comparable to those of the ligand-tometal donation. The complexes under study have essentially similar structure in the solid state and in solution, with the bent macrocycle fragment, as shown by the IR spectroscopic data. Despite rather strong M−P bonding, complexes 2a and 2b in solution display intramolecular dynamics related to the “merry-goround” movement of the diphosphine about the trinuclear metal core, clearly visible in the NMR spectra in the temperature range 200−300 K. The results of the VT NMR spectra simulation of this dynamic process are in perfect agreement with the DFT calculations of the isomerization process. The exploration of the photophysics of trinuclear silver and copper pyrazolate diphosphine complexes 2a and 2b revealed essentially different behavior of these compounds. The copper complex 2b displays emission from the singlet (1MLCT) and triplet (3MLCT) states. The triplet and singlet states are separated by a relatively small energy gap (1080 cm−1) that makes possible the reversible population of singlet excited state from lower lying triplet state at room temperature, thus paving the way to TADF behavior in both solution and solid state, with the quantum yield of 41%. The TADF property has not yet been reported for the family of cyclic trinuclear Cu(I) pyrazolates and their adducts. The TD-DFT calculations support the possibility of the TADF behavior; however, they show the fifth triplet state T5 is responsible for this phenomenon.

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CONCLUSIONS The trinuclear copper(I) and silver(I) pyrazolates feature a unique geometry in their complexes with the phosphorus 8652


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and multiple-event time digitizer P7887 (FAST ComTec GmbH) were used for lifetime measurements. The absolute emission quantum yields of the crystalline samples were determined on a FluoroLog 3 Horiba spectrofluorometer equipped with a Quanta-phi integration sphere. The simulations of 31P NMR spectra were performed by using the SpinWorks program41 and Spinach simulation package.41,42 X-ray Diffraction Study. Single-crystal X-ray diffraction experiments were carried out with a Bruker SMART APEX II diffractometer (graphite-monochromated Mo Kα radiation, λ = 0.71073 Å, ω-scan technique). The APEX II software43 was used for collecting frames of data, indexing reflections, determination of lattice constants, integration of intensities of reflections, scaling, and absorption correction while SHELXTL44 and OLEX245 were applied for space group and structure determination, refinements, graphics, and structure reporting. The structures were solved by direct methods and refined by the full-matrix least-squares technique against F2 with anisotropic thermal parameters for all non-hydrogen atoms. The CF3 groups in complex 2a are disordered. The hydrogen atoms were placed geometrically and included in the structure factor calculations in the riding motion approximation. CCDC 1883126−1883127 contain the supplementary crystallographic data for 2a, 2b. Crystallographic data for complexes 2a, 2b are presented in Table S1. Computation Details. Density functional theory (DFT) optimizations of the complexes geometries were performed by the ADF201446 software suite utilizing the BP86 functional with the Grimme D3 correction47 and the TZ2P basis set. All optimizations showed good agreement with the XRD data. The same geometries of complexes were obtained with the PBE/Def2-TZVP48 Gaussian0949 optimizations in the DCM solution (SMD50 model). The dynamic behavior of the complexes was studied with this level of theory. With the EDA51−53 approach, complexes 2a and 2b were considered constructed from dppm and [ML]3 fragments, and their interaction energy, ΔEint, was decomposed into the terms ΔEint = ΔEelstat + ΔEPauli + ΔEorb + ΔEdisp. ΔEorb was further decomposed with the ETSNOCV approach to the electron density transfer channels, Δρi(r).54 The energy of complex formation was determined as ΔEtotal = ΔEint + ΔEprep, where ΔEprep is the energy of deformation of fragments (dppm and [ML]3) from the free “relaxed” state to their state in the complex. TD-DFT computations were performed with the Gaussian0949 software suite using the CAM-B3LYP55 functional applying the SVP fitting56,57 basis set. Full optimization of the ground state, first singlet, and fifth triplet excited states for 2a and 2b complexes were performed at this level.

On the contrary, the silver complex in solid state and at low temperature in solution shows dual singlet/triplet emission originating from the 1IL and 3MLCT states, respectively, that is dictated by the much higher energy difference between emissive singlet and triplet as well as by the essentially different nature of these states. The solution room temperature spectrum of the silver complex only displays the 1IL emission due to effective quenching of phosphorescence through nonradiative vibrational relaxation. This could be rationalized in terms of the significant energy difference between singlet and triplet obtained by the TD-DFT calculations. The TADF photoluminescent behavior of substituted copper pyrazolates opens the new horizons for application of this type of compounds as active components of OLED devices.

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EXPERIMENTAL SECTION

General Considerations. All reactions were performed under argon atmosphere using anhydrous solvents or solvents treated with an appropriate drying reagent. Commercially available bis(diphenylphosphino)methane was used without additional purification. Macrocycles 1a and 1b were prepared by the previously described method.40 No unexpected or significant safety hazards occurred in the present work. Synthetic Procedures. Synthesis of 2a. A mixture of 0.04 g (0.043 mmol) of 1a and 0.025 g of dppm (0.066 mmol) was stirred overnight at room temperature in 7 mL of toluene. The solvent was evaporated to dryness under reduced pressure, and the residue was dissolved in DCM (1 mL). Hexane (3 mL) was added to the solution, and white crystalline product was obtained by crystallization at 5 °C and isolated by filtration. Yield: 0.045 g (79.4%). 1H NMR (CD2Cl2, 298 K): δ 7.55−7.20 (m, 20H, CHPh), 6.95 (s, 3H, CHPz), 3.43 (bm, 2H, CH2). 19F NMR (CD2Cl2, 298 K): δ −60.84 (s, 18F, CF3Pz). 31 1 P{ H} NMR (CD2Cl2, 298 K): δ = 7.37 (m, 2P); (toluene-d6, 198 K): δ 7.1 (m, 2P, AA′XX′ system, 2J(PP) = 151.1 Hz, 1J(31P-107Ag) = −596.86 Hz, 3J(31P-107Ag) = 3.63 Hz). IR (KBr, cm−1): ν 3138 (νCHPz), 3080, 3062, 3030 (νCHAr), 1626, 1547 (δCNPz), 1529, 1438, 1356, 1258. Calc. for C40H25F18N6P2Ag3(%): C, 36.47; H, 1.91; N, 6.38; F, 25.96; P, 4.70. Found (%): C, 36.62; H, 2.01; N, 6.42; F, 25.88; P, 4.75. Synthesis of 2b. Complex 2b was similarly prepared according to the procedure for 2a. Yield: 0.046 g (88.3%). 1H NMR (CD2Cl2)δ 7.48−7.12 (m, 20H, CHPh), 6.92 (s, 3H, CHPz), 3.27 (t, 2H, CH2, J(HP) = 10.0 Hz). 19F NMR (CDCl3): δ= −60.72 (s, 18F, CF3Pz). 31 1 P{ H} NMR (CDCl3) δ = −6.72. IR (KBr, cm−1): ν 3194 (νCHPz), 3100, 3082, 3036 (νCHAr), 1587 (δCNPz), 1444, 1439, 1353, 1201. Calc. for C40H25F18N6P2Cu3(%): C, 40.57; H, 2.13; N, 6.38; F, 28.88; P, 5.23. Found (%): C, 40.50; H, 2.21; N, 6.33; F, 28.88; P, 5.23.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.9b00991. NMR spectra, detailed results of DFT and TDDFT calculations, crystal data for complexes 2a and 2b (PDF)

MATERIALS AND METHODS

Accession Codes

Crystals of the complexes {[3,5-(CF3)2Pz]M}3(dppm) (M = Ag, Cu) were prepared from DCM/hexane solution (1:1) by slow solvent evaporation at 5 °C. 1H, 19F, and 31P NMR measurements were carried out on a Bruker Avance 400, Bruker Avance-III-500 spectrometer with CryoProbe Prodigy and Bruker Avance 600 instruments. Infrared (IR) spectra were collected on a Shimadzu IRPrestige-21 FT-IR spectrometer using KBr pellets and in KBr cuvettes. The steady-state emission and excitation spectra of complexes in the solid state and solution at 298 and 77 K were recorded on a FluoroMax 4 and a FluoroLog 3 Horiba spectrofluorometers. The xenon lamps (300 and 450 W) were used as excitation sources. A helium−nitrogen optical cryostat optCryo 105-40 with a temperature control system was used for the samples cooling in the range of temperatures 383−77 K. A pulse laser DTL399QT “Laser-export Co. Ltd” (351 nm, 50 mW, pulse width 6 ns, repetition rate 1 kHz), a monochromator MUM (LOMO, slit bandwidth 1 nm), a photon counting head H10682 (Hamamatsu),

CCDC 1883126−1883127 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Authors

*S.P.T. e-mail: [email protected]. *E.S.S. e-mail: [email protected]. ORCID

A. A. Titov: 0000-0003-3685-5329 O. A. Filippov: 0000-0002-7963-2806 8653


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S. P. Tunik: 0000-0002-9431-0944 E. S. Shubina: 0000-0001-8057-3703 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS E.S.S. is grateful for the financial support from the Russian Science Foundation (Project No. 17-73-30036). J.R.S. thanks the Russian Foundation for Basic Research (Project 16-3360109) for the support of photophysical studies. The elemental analysis was performed with financial support from the Ministry of Science and Higher Education of the Russian Federation using the equipment of the Center for molecular composition studies of INEOS RAS. The XRD studies performed by A.F.S. were supported by the RUDN University Program “5-100”.

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Luminescent Complexes of the Trinuclear Silver(I) and Copper(I

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