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. 2023 Dec 28;63(2):1317–1327. doi: 10.1021/acs.inorgchem.3c03810

Trinuclear Cyclometalated Iridium(III) Complex Exhibiting Intense Phosphorescence of an Unprecedented Rate

Marsel Z Shafikov †,*, Andrey V Zaytsev ‡, Valery N Kozhevnikov ‡,*
PMCID: PMC10792602  PMID: 38154085

Abstract

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Herein, we present two novel cyclometalated Ir(III) complexes of dinuclear and trinuclear design, Ir2(dppm)3(acac)2 and Ir3(dppm)4(acac)3, respectively, where dppm is 4,6-di(4-tert-butylphenyl)pyrimidine ligand and acac is acetylacetonate ligand. In both cases, rac-diastereomers were isolated during the synthesis. The materials show intense phosphorescence of outstanding rates (kr = ΦPL/τ) with corresponding radiative decay times of only τr = 1/kr = 0.36 μs for dinuclear Ir2(dppm)3(acac)2 and still shorter τr = 0.30 μs for trinuclear Ir3(dppm)4(acac)3, as measured for doped polystyrene film samples under ambient temperature. Measured under cryogenic conditions, radiative decay times of the three T1 substates (I, III, and III) and substate energy separations are τI = 11.8 μs, τII = 7.1 μs, τIII = 0.06 μs, ΔE(II–I) = 7 cm–1, and ΔE(III–I) = 175 cm–1 for dinuclear Ir2(dppm)3(acac)2 and τI = 3.1 μs, τII = 3.5 μs, τIII = 0.03 μs, ΔE(II–I) ≈ 1 cm–1, and ΔE(III–I) = 180 cm–1 for trinuclear Ir3(dppm)4(acac)3. The determined T1 state ZFS values (ΔE(III–I)) are smaller compared to that of mononuclear analogue Ir(dppm)2(acac) (ZFS = 210–1 cm). Theoretical analysis suggests that the high phosphorescence rates in multinuclear materials can be associated with the increased number of singlet states lending oscillator strength to the T1 → S0 transition.

Short abstract

A trinuclear Ir(III) complex is reported with an unprecedented rate of phosphorescence corresponding to radiative decay time of only 0.3 μs. The properties of the emitting triplet state are investigated in detail with cryogenic steady-state spectroscopy and theoretical methods.

Introduction

Transition metal complexes exhibiting phosphorescence—radiative relaxation of the lowest excited triplet state to the singlet ground state (T1 → S0)—have found application in various fields.1−11 This strongly stimulates the research aimed to understand the molecular design principles allowing us to fine-tune their photophysical properties for a particular function. The high rate of phosphorescence, for instance, is a desirable property when fast and efficient utilization of the excited state’s energy is advantageous.12,13 For instance, organic light emitting diodes (OLEDs) benefit from the ability of phosphorescent metal complexes to utilize both types of excitons, singlet and triplet, formed in the emitting layer, but the associated triplet–triplet annihilation (TTA) processes, competing with phosphorescence, appear to be a problem limiting the lifespan of the devices. The efficiency of such a deleterious process can be diminished through the enhanced rate of T1 → S0 phosphorescence. Therefore, materials with enhanced phosphorescence rates and design approaches affording such materials are highly sought after. Complexes of Ir(III) emerged as the most prominent class of materials in terms of the high phosphorescence rate. This is due to the strong metal-induced spin–orbit coupling (SOC) of the lowest excited triplet state T1 with singlet states that open the otherwise spin-forbidden T1 → S0 transition path.14−16 A particularly strong SOC of state T1 with singlets in Ir(III) complexes is conditioned by a large SOC constant of iridium (ζ = 3909 cm–1)17 and, importantly, also by the energetical ease of fulfilling the total momentum conservation requirement based on El-Sayed’s rule.15,16,18 The latter is due to the (quasi)-degeneracy of the occupied t2g symmetry orbitals of d6 Ir(III) center (dxy, dxz, dyz) resulting from the splitting of the 5d-orbitals in the (quasi)-octahedral ligand field.19 As the SOC of state T1 with singlet states in Ir(III) complexes is predominantly brought by the metal center, the phosphorescence rate depends on the contribution of the metal to those states. Indeed, a prominent correlation of phosphorescence rate with the extent of metal to ligand charge transfer (MLCT) character of state T1 has previously been discussed.16 Furthermore, analyzing a number of investigated mononuclear Ir(III) complexes, Yersin et.al showed a major trend of radiative decay times (τr = 1/kr) of phosphorescence in these materials asymptotically approaching the 1 μs value.16 Therefore, the development of design approaches affording phosphorescent materials with radiative decay time significantly shorter than 1 μs is an actual challenge. Presently, only a few exceptional examples of mononuclear Ir(III) complexes with τr value below 1 μs are known.20,21 One of those exceptional complexes has a heteroleptic design (Ir(dppm)2(acac) in Chart 1) and utilizes alignment of the two cyclometalating ligands to enhance the oscillator strength of an excited singlet state which, through SOC to the T1 state, translates to the enhanced phosphorescence rate.21 In recent years, dinuclear molecular design has been shown as advantageous to reach higher phosphorescence rates.22−28 In particular, radiative decay times (τr) of below 1 μs were demonstrated as easily attainable for dinuclear Ir(III) complexes with the T1 state of strong MLCT character27−30 (compare to the cases with T1 state of relatively weak MLCT character in refs (31−34)). This renders the multinuclear design of Ir(III) complexes as particularly promising to afford materials with previously unreachable phosphorescence rates.

Chart 1. Chemical Structures of Ir(dppm)2(acac), Reported Previously,21 and of the Dinuclear Ir2(dppm)3(acac)2 and Trinuclear Ir3(dppm)4(acac)3, Reported in This Work.

Chart 1

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Molecular Design and Synthesis

Recently we reported a detailed investigation of a mononuclear Ir(III) complex Ir(dppm)2(acac) (Chart 1), comprising derivatives of 4,6-diphenylpyrimidine (dppm) and acetylacetonate (acac) as ligands, to show its previously unnoticed extraordinarily fast phosphorescence as for a mononuclear complex.21 Since dppm ligand has two C^N cyclometalating sites, we became interested in multinuclear analogues of Ir(dppm)2(acac) with potentially further enhanced phosphorescence rates. However, the octahedral metal center created by bidentate ligands exhibits chirality. Consequently, linking two or more metal centers within a single molecule could generate stereomers, necessitating difficult separation. Previously, we had to overcome this challenge by replacing the bidentate ligands with symmetrical tridentate ones.24,29,35 Nevertheless, there are examples of stereoselective synthesis of dinuclear Ir(III) complexes using bidentate ligands.36 Usually, the meso-product is evidently less favored due to steric interactions arising from the proximity of N^C ligands on adjacent metal centers. Indeed, previously, we noticed that during the synthesis of Ir(dppm)2(acac),37 there is the formation of trace amounts of deeply red byproducts, which we suspected to be polynuclear species. We, therefore, decided to carry out the synthesis on a larger scale to isolate and characterize these products. To aid the formation of polynuclear complexes, we used three molar equivalents of the ditopic ligand dppm and two molar equivalents of IrCl3. After heating under reflux in pure ethoxyethanol, sodium acetylacetonate was added to cleave the dichloro-bridged intermediates. Conveniently, the main product of the reaction, mononuclear Ir(dppm)2(acac), is only moderately soluble in ethanol, and most of it was easily removed by simple filtration. The polynuclear complexes are more soluble and accumulate in the ethanolic filtrate. The desired multinuclear complexes were then chromatographically isolated. Thus, with significant effort, we were able to prepare dinuclear Ir2(dppm)3(acac)2 and trinuclear Ir3(dppm)4(acac)3 (Chart 1), although in very low yields. The low yields and the necessity for chromatographic separation represent primary obstacles that must be addressed in the future optimization of the synthesis. The 1H NMR spectrum of the dinuclear Ir2(dppm)3(acac)2 shows one set of signals for the two singly cyclometalated (terminal) dppm ligands and one set of signals for the doubly cyclometalated bridging dppm ligand. We interpret that, at each coordination center, both cyclometalating (C^N) ligands are quasi-equal with respect to the ancillary acac ligand, where each coordinated nitrogen is trans to the coordinated nitrogen of the other dppm ligand, and each coordinated carbon is trans to an oxygen of the acac ligand. This configuration was found for the mononuclear Ir(dppm)2(acac) where the two dppm ligands appeared to be equal on the 1H NMR spectrum. Indeed, this configuration avoids strong trans influence of the two coordinated carbons exerted on each other and is thermodynamically favorable. Due to the possible Λ/Δ-isomerism of the coordination centers, we modeled different stereoisomers of Ir2(dppm)3(acac)2 and Ir3(dppm)4(acac)3. We found that alternation of the Λ and Δconfigurations between adjacent coordination centers results in steric hindrances between the tert-butyl groups of two dppm ligands as well as between the two acac ligands, which strain the coordination center geometries. Therefore, having only one set of 1H NMR signals, we believe that the isolated samples are ΛΛ/ΔΔ-isomers of dinuclear Ir2(dppm)3(acac)2 and ΛΛΛ/ΔΔΔ-isomers of trinuclear Ir3(dppm)4(acac)3. The compositions of Ir2(dppm)3(acac)2 and Ir3(dppm)4(acac)3 are further supported by elemental (C, H, N) and mass-spectrometry analyses. Unfortunately, despite several attempts, we were not able to obtain single crystals suitable for X-ray diffraction analysis for either of the new complexes.

The synthetic details and characterization data are given in the Supporting Information.

Optical Spectroscopy

The optical spectroscopy measurements were carried out with dilute solutions of Ir2(dppm)3(acac)2 and Ir3(dppm)4(acac)3 in toluene (c ≈ 10–5 M). The corresponding pertinent data are collected in Table 1. The absorption spectrum of both Ir2(dppm)3(acac)2 and Ir3(dppm)4(acac)3 feature relatively low-intensity bands at the lower energy end (longer wavelengths), presumably due to the significant contribution of charge transfer (CT) excitations, such as metal to ligand charge transfer (d → π*, MLCT), to the character of the associated excited states. Toward the higher energies, the absorption bands gradually gain intensity manifesting the increase of ligand-centered (π → π*, LC) character of the corresponding excited states. These results and assignments are characteristic of intensely emissive Ir(III) complexes and agree well with our DFT calculations (vide infra).

Table 1. Summary of Key Photophysical Properties of Complexes Ir(dppm)2(acac),21Ir2(dppm)3(acac)2 and Ir3(dppm)4(acac)3 Measured as Dilute Toluene Solutions (c ≈ 10–5 M) and Doped Polystyrene (PS) Films (c ≪ 1 wt %)a.

  Ir(dppm)2(acac) Ir2(dppm)3(acac)2 Ir3(dppm)4(acac)3
absorption λmax/nm (ε/M–1cm–1) 530 (6600), 500 (5900), 426 (11800), 379 (16900), 310 (47100) 576 (10800), 536 (16100), 474 (15500), 427 (22400), 373 (34500), 321 (71600) 580 (21000), 538 (23700), 475 (24260), 425 (31300), 372 (50400), 321 (89500)
photoluminescence in toluene at 300 K
λmax/nm 570 598, 640 600, 645
ΦPL/% 80 65 60
τ/μs 0.73 0.27 0.20
τr = τ/ΦPL /μs 0.91 0.42 0.33
kr/106 s–1 1.10 2.41 3.00
knr/106 s–1 0.27 1.30 2.00
photoluminescence in toluene at 77 K
λmax/nm 563 590, 638 593, 644
ΦPL/% 100 100 90
τ/μs 10.90 2.40 1.40
τr = τ/ΦPL/μs 10.90 2.40 1.56
kr/106 s–1 0.09 0.42 0.64
knr/106 s–1 <0.003b <0.010b 0.064
photoluminescence in PS film at 300 K
λmax/nm 568 600, 642 602, 642
ΦPL/% 90 80 70
τ/μs 0.66 0.29 0.21
τr = τ/ΦPL/μs 0.73 0.36 0.30
kr/106 s–1 1.4 2.8 3.3
knr/106 s–1 0.15 0.69 1.4
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a

The absorption spectrum was measured in toluene under ambient conditions. The ambient temperature (300 K) emission decay time and quantum yield values were measured for degassed toluene solution and PS films under nitrogen. The radiative decay rates are calculated as kr = ΦPL/τ; the non-radiative decay rates are calculated as knr = (1−ΦPL)/τ; the radiative decay times are calculated as τr = τ/ΦPL = 1/kr.

b

These values are estimated assuming 3% error in the emission quantum yield value, e.g., assuming ΦPL = 0.97 (97%).

Both complexes, Ir2(dppm)3(acac)2 and Ir3(dppm)4(acac)3, exhibit intense red photoluminescence, with the maxima at ca. 598 nm and ca. 600 nm, respectively, as measured for dilute toluene solution and doped polystyrene (PS) film samples under ambient conditions (Figure 1 and Table 1). In both cases, the emission spectrum strongly overlaps with the lowest energy absorption band (Figure 1). Tentatively assigning the observed emission to T1 → S0 phosphorescence, similarly to that in mononuclear Ir(dppm)2(acac),21 such an spectral overlap is evident of a relatively small energy separation of states S1 and T1 in Ir2(dppm)3(acac)2 and Ir3(dppm)4(acac)3. With the help of TD-DFT calculations (see below), we substantiate that this is a consequence of the combined effect of the lowest excited states having strong charge transfer (d → π*) character and of their relatively extended delocalization within more metal centers and cyclometalated ligands, compared to mononuclear complexes such as Ir(dppm)2(acac).21 Indeed, both charge transfer character and extended delocalization ought to decrease the exchange energy (K) of the electronic configuration and decrease the gap between the corresponding singlet and triplet states (2K). Hence, a relatively strong spectral overlap of T1 → S0 phosphorescence with S1 → S0 absorption band in Ir2(dppm)3(acac)2 and Ir3(dppm)4(acac)3 seems rational. It is noted that in frozen toluene glass at T = 77 K, the emission spectra of the two complexes are slightly hypsochromically shifted, which we rationalize to be due to the limited reorganization of the media affecting the stabilization of the emitting state. Also, at T = 77 K, vibrational shoulders of emission spectra become resolved at 638 nm for Ir2(dppm)3(acac)2 and at 644 nm for Ir3(dppm)4(acac)3 (Figure 1).

Figure 1.

Figure 1

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Absorption spectrum (blue trace) and emission spectrum at T = 300 K (solid red line) and at T = 77 K (dashed red line) measured for dilute toluene solutions (c = 10–5 M) of (a) Ir2(dppm)3(acac)2 and (b) Ir3(dppm)4(acac)3.

The emission decay time constants and quantum yields, measured for degassed toluene solutions (c ≈ 10–5 M) under ambient temperature, are τ = 0.27 μs and ΦPL = 0.65 (65%) for Ir2(dppm)3(acac)2; and τ = 0.20 μs and ΦPL = 0.60 for Ir3(dppm)4(acac)3. The traces of time-correlated single photon counting (TCSPC) measurements are shown in Figure 2. The corresponding radiative rate (kr = ΦPL/τ) and radiative decay time constant (τr = τ/ΦPL = 1/kr) values are kr = 2.41 × 106 s–1 and τr = 0.42 μs for dinuclear Ir2(dppm)3(acac)2; and kr = 3.0 × 106 s–1 and τr = 0.33 μs for trinuclear Ir3(dppm)4(acac)3.

Figure 2.

Figure 2

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Emission decay traces measured for degassed toluene solution (c ≈ 10–5 M) of (a) Ir2(dppm)3(acac)2 and (b)Ir3(dppm)4(acac)3 at 300 K and at 77 K as indicated in the insets. The white line on the black dots of experimental data represents the best fit of the exponential decay function.

The obtained radiative decay time value of the dinuclear Ir2(dppm)3(acac)2 is in the range of the fastest emitting dinuclear Ir(III) complexes reported so far and about two times shorter than that of the mononuclear Ir(dppm)2(acac) at the same conditions.21 Meanwhile, the radiative decay time of trinuclear Ir3(dppm)4(acac)3 of τr = 0.33 μs is unprecedentedly short for T1 → S0 phosphorescence, making the complex the fastest phosphorescing material known so far. At T = 77 K in frozen toluene glass, phosphorescence efficiency increases to unity for Ir2(dppm)3(acac)2 and reaches 0.9 (90%) for Ir3(dppm)4(acac)3 (Table 1), which is explained by the significantly reduced rate of nonradiative relaxation processes in rigid media at low temperatures. Interestingly, the radiative rates of the two complexes are also lower at T = 77 K compared to those under ambient conditions, though the decrease is not as significant as for nonradiative decay rates. This, in Ir(III) complexes, occurs due to a zero field splitting (ZFS) of the emitting T1 state, causing a relatively inefficient thermal population of the fastest emitting (highest energy) substate of T1 at lower temperatures. Another noteworthy finding is that in doped polystyrene films (c < 1 wt %), both complexes, Ir2(dppm)3(acac)2 and Ir3(dppm)4(acac)3, demonstrate the increase in the phosphorescence efficiency, as compared to the toluene solutions under ambient conditions (Table 1). This is associated with both increased radiative rates and reduced nonradiative relaxation rates in polystyrene films (Table 1). Indeed, a more rigid media of the film may suppress extensive geometry reorganizations of the molecule in the excited state and hence the nonradiative relaxation processes too. Also, the relatively high rigidity of the film may suppress the rotation of the noncoordinated terminal phenyl groups of dppm ligands in both complexes, keeping them more in plane with the rest of the ligand and improving the molecule’s chromophore properties. This results in a higher average oscillator strengths of the excited singlet states (f(Sn ↔ S0)) spin–orbit coupled with the phosphorescing T1 state, and hence in a faster T1 → S0 phosphorescence.14 A similar increase in the phosphorescence rate in PS film was also observed for mononuclear Ir(dppm)2(acac).21 It is noted that in PS film under ambient temperature, the radiative decay time of phosphorescence of trinuclear Ir3(dppm)4(acac)3 is as short as only τr = 0.3 μs.

Low Temperature Optical Spectroscopy

The rate of phosphorescence stemming from a triplet substate of a mixed ππ* (LC) and dπ* (MLCT) character in Ir(III) complexes is largely defined by the strength of spin–orbit coupling (SOC) of the triplet substate with the higher laying singlets and by the oscillator strengths of those singlets with respect to the ground state as approximated by the following equation:15,38−40

graphic file with name ic3c03810_m001.jpg 1

herein T1,i is the i-th substate of T1, kr(T1,i → S0) is the radiative rate of the T1,i → S0 transition, τr(T1,i → S0) is the corresponding radiative decay time, h and c are Planck’s constant and the speed of light, ⟨T1|ĤSO|Sn⟩ is the SOC matrix element (SOCME), and ⟨S0|μ̂|Sn⟩ is the transition dipole moment of Sn → S0 related to the oscillator strength of Sn as f(Sn ↔ S0) ∝ |⟨S0|μ̂|Sn⟩|2, respectively. The experimental phosphorescence rate of the T1 state represents (assuming efficient equilibration between the three triplet substates (T1,i) a Boltzmann average of its three substates:41,42

graphic file with name ic3c03810_m002.jpg 2

Herein I, II, and III are the three substates (i-th) of the T1 state, according to ascending energy, ΔEII–I and ΔEIII–I are energy separations between the T1 substates, kB is the Boltzmann constant, and T is the absolute temperature, respectively. ΔEIII–I representing the energy difference between the lowest (I) and highest triplet substates is also denoted as zero-field splitting (ZFS). Thus, according to eq 2, the overall rate of T1 → S0 phosphorescence will vary with temperature that governs the population efficiency of the higher laying T1 substates. Assuming emission quantum yields of the complexes in this temperature range remain near unity, similar to that measured at T = 77 K, so that measured decay time constants are close to radiative decay time constants, eq 2 can be simplified to:

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which is now a convenient relation to use for analysis of measured decay time constants as the function of properties of T1’s substates and energy gaps between them. This opens the possibility for temperature-dependent investigations to reveal individual emission rates of T1 substates and ZFS value.43,44 Thus, the dilute toluene solution samples of Ir2(dppm)3(acac)2 and Ir3(dppm)4(acac)3 were investigated at cryogenic temperatures in the range 1.6 K ≤ T ≤ 120 K where toluene remains frozen under vacuum.

Figure 3a shows the measured emission decay times of Ir2(dppm)3(acac)2 plotted as a function of temperature. The decay time at 1.6 K is τ(1.6 K) = 11.8 μs, several times shorter than that of mononuclear Ir(dppm)2(acac) (τ(1.6 K) = 66 μs)21 measured under the same conditions. It is interpreted that emission at this temperature mainly stems from the lowest T1 substate (T1,i). With the increase in temperature, the emission decay time drops to reach a quasi-plateau at the 10–25 K temperature range where substrates I and II are thermally equilibrated with an average decay time value of about 9.7 μs. With a further increase in temperature, the emission decay time drops due to the thermal population of the fastest-emitting T1 substate III, and at T = 120 K reaches the value of 0.75 μs. Analysis of the experimental data using eq 3. with fixed τ(I) = τ(1.6 K) = 11.8 μs suggests the following parameter values for the best fit: τ(II) = 7.1 μs, τ(III) = 0.06 μs (60 ns), ΔE(II–I) = 7 cm–1 and ΔE(III–I) = 175 cm–1. According to these data, dinuclear Ir2(dppm)3(acac)2 has notably faster emitting T1 substates I and III, compared to those of mononuclear Ir(dppm)2(acac) (τ(I) = τ(1.6 K) = 66 μs, τ(II) = 7.3 μs, τ(III) = 0.19 μs, ΔE(II–I) = 14 cm–1 and ΔE(III–I) = 210 cm–1).21 A similar study of trinuclear Ir3(dppm)4(acac)3 is shown in Figure 3b. The two lowest substates of T1, I and II, are thermally equilibrated early at low temperatures due to a particularly small energy gap between them of about 1 cm–1. The plateau of the average emission decay time of the two lowest T1 substates continues up to T = 35 K. The average value of ca. 3.2 μs is about three times shorter than that in the dinuclear Ir2(dppm)3(acac)2, indicating a stronger average SOC mixing of the T1 substates with singlet states. An increase in temperature above 35 K is associated with the population of the highest T1 substate III and a significant drop in the emission decay time reaching 0.4 μs at T = 120 K. The fit of eq 3 suggests ZFS = 180 cm–1 and τ(III) = 0.03 μs (30 ns) for Ir3(dppm)4(acac)3 (Figure 3b).

Figure 3.

Figure 3

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Emission decay times of (a) Ir2(dppm)3(acac)2 and (b) Ir3(dppm)4(acac)3 as a function of temperature (black dots), and the best fit of eq 3 to the experimental values (red line).

Extrapolation of the thus obtained low-temperature data for the T1 state properties to the room temperature conditions, using T = 300 K in eq 3, suggests radiative phosphorescence decay time of τav(300 K) = 0.32 μs for dinuclear Ir2(dppm)3(acac)2 and τav(300 K) = 0.19 μs for trinuclear Ir3(dppm)4(acac)3. It is noted that the value of the trinuclear complex is corrected to account for the emission quantum yield in glassy toluene of 90% as measured at T = 77 K, e.g., the given value is after division by 0.9. These extrapolated values predict a bit faster T1 → S0 radiative rate than what is found experimentally at room temperature (Table 1). Such a deviation may arise from modification of the T1 state’s properties in liquid media at room temperature, as well as from the relative crudeness of the data obtained at low temperatures, for example, due to the possible nonuniformity of emission quantum yield in the temperature range of investigation. It is noted that in the case of mononuclear Ir(dppm)2(acac), the rate value extrapolated from cryogenic temperature data agreed very well with the experimental room temperature value.21 Importantly, from the data analysis above, it can be concluded that the photoluminescence of both materials, dinuclear and trinuclear, is completely governed by the T1 → S0 transition up to room temperature. Indeed, at the determined high T1,i → S0 transition rates in these materials, there is little chance for emission through thermal activation of a state above T1,III, even at its close energetic proximity. Another noteworthy finding is that with more than a two-fold increase in the T1 → S0 phosphorescence rate from mononuclear Ir(dppm)2(acac) to dinuclear Ir2(dppm)3(acac)2, the analogous increase from mononuclear Ir(dppm)2(acac) to trinuclear Ir3(dppm)4(acac)3 is less than three-fold (Table 1). We attribute this to the different electronic properties of the central metal atom from the two peripheral metal atoms in the trinuclear complex, which may compromise the SOC efficiency of T1 with singlet states (vide infra).

Interestingly, the T1 ZFS value of 175 cm–1 in dinuclear Ir2(dppm)3(acac)2 and of 180 cm–1 in trinuclear Ir3(dppm)4(acac)3 are both slightly smaller than T1 ZFS of 210 cm–1 found for the mononuclear Ir(dppm)2(acac).21 Such a decrease in the ZFS value in multinuclear complexes is unexpected and contrasts with the findings for other pairs of mononuclear and dinuclear complexes.24,27 In the transition metal complexes, the ZFS size is determined by different SOC strengths for different orientations of electron spins caused by the filed anisotropy at the SOC center(s) (metal ion(s)).45 Therefore, the decreased ZFS value in the dinuclear and trinuclear complexes is indicative of lowered overall field anisotropy at the metal centers. In the mononuclear Ir(dppm)2(acac), two dppm ligands are aligned (Chart 1), giving the molecule and orbitals involved in SOC of the T1 state a shape elongated in one direction (along an arbitrary z-axis), which works for a relatively strong field anisotropy at the SOC center (gives a relatively large ZFS parameter D).21 In the dinuclear Ir2(dppm)3(acac)2 and trinuclear Ir3(dppm)4(acac)3, however, the alignment of the two dppm ligands at different metal centers cover different spatial directions (close to mutually perpendicular) so that the molecules and the orbitals appear less elongated along one axis. This, we conclude, decreases the field anisotropy (a smaller ZFS parameter D) “seen” by the spin–orbit coupled electron. Indeed, the above-referenced dinuclear complexes, with T1 ZFS values larger than that in their mononuclear analogues,24,27 do not feature such mutually compensating alignment of the ligands at the two metal centers. It is also noteworthy that the decrease of the ZFS parameter D of the T1 state with the expansion of electron delocalization to a new dimension was reported for π-stacked dimers of porphyrin complexes in comparison to their monomers.46,47 This structural feature of Ir2(dppm)3(acac)2 and Ir3(dppm)4(acac)3, affording relatively small T1 ZFS, should improve the thermal population of the higher laying and faster emitting T1 substates and thus contribute to the enhancement of phosphorescence rate under ambient temperature.

DFT Calculations and Theory

Electronic structures of Ir2(dppm)3(acac)2 and Ir3(dppm)4(acac)3 were computed utilizing the density functional theory (DFT) approach at the M11L48,49/def2-SVP50 level with effective core potentials (ECP) for iridium atoms and using Gaussian 0951 code. Vertical excitations were computed with the time-dependent DFT (TD-DFT) approach. The C-PCM polarizable continuum model52 with parameters of toluene was applied to all the calculations to account for the solvation effect. The ground state optimized ΛΛ-isomer of Ir2(dppm)3(acac)2 was calculated 11.16 kcal/mol more stable than the ΛΔ-isomer, which is due to the steric hindrances in the latter, as mentioned above. For calculation of excited states and electronic structure analysis, we therefore used ΛΛ-isomer of Ir2(dppm)3(acac)2 and ΛΛΛ-isomer of Ir3(dppm)4(acac)3. The structures were optimized for both ground state (S0) and the lowest excited triplet state (T1) electronic configurations. The corresponding atomic Cartesian coordinates are given in the Supporting Information. Unless stated otherwise, the data discussed below refer to the T1 state optimized geometries as more relevant to the photoluminescent properties of these complexes.

According to TD-DFT calculations, the state T1 of dinuclear Ir2(dppm)3(acac)2 has HOMO → LUMO (98%) orbital parentage (Table S5). The HOMO is localized over the two metals evenly with 44% of net contribution; on the doubly cyclometalated bridging dppm ligand (dppm2) with 24% contribution; over two terminal dppm ligands (dppm1 and dppm3) evenly with 22% of net contribution; and over two ancillary acac ligands evenly with 8% of net contribution (Table S3). The LUMO is almost entirely localized on the bridging dppm (dppm2 in Chart 1) ligand, predominantly on the pyrimidine ring, with 90% contribution (Table S3). Contour plots of the MOs are shown in Figure 4. Hence, the T1 state of Ir2(dppm)3(acac)2 has strong metal-to-ligand charge transfer (3MLCT, 3dπ*) character contribution mixed with ligand-centered (3LC) character and, to a lesser extent, ligand-to-ligand charge transfer (LL′CT) character contributions. An unpaired electron in such a state, partially localized on the d-orbital of iridium, has a large chance of occurring at the Ir center with a strong SOC effect. Then, through its 3dkπ* character, the T1 state can directly spin–orbit couple with a singlet state (Sn) bearing 1dk′π*′ character. The corresponding SOC matrix element in eq 1 can be approximated as follows:

graphic file with name ic3c03810_m004.jpg 4

Here, ck and ck′ are the normalized contributions of the dk and dk′ atomic orbitals to the molecular orbitals involved to direct excitations forming T1 and Sn states, respectively; aT1 and aSn are normalized configuration interaction (CI) coefficients of orbital transitions contributing to S0 → T1 and S0 → Sn excitations, respectively; ζ is the SOC constant of the center(s) associated with the d-orbitals, l is the angular momentum operator, and s is the spin momentum operator. To demonstrate, the HOMO → LUMO orbital transition contribution to the T1 state (S0 → T1 excitation) of Ir2(dppm)3(acac)2 has the corresponding configuration interaction coefficient of aT1 = 0.98 (98%), and metal contribution to the HOMO has the corresponding coefficient of ck = 0.44 (44%) (Tables S3 and S5). Coefficients aT1 and aSn with coefficients ck and ck′ are to account for the probability of the electron occurring at the same heavy atom in both states. This is because the SOC effect, proportional to the fourth power of the nucleus’s charge (Z4) and to the inverse of the third power of distance to the nucleus (1/r3), is strong only close to a heavy atom nucleus.18,53 Therefore, a spin–orbit coupled electron can “mix” only those state wave functions, which are contributed by the same SOC-providing center(s). This, basically, is a requirement that dk and dk′ belong the same atom(s) and that π* = π*′ as an electron on a π* orbital is not spin–orbit coupled to an appreciable extent and must remain the same for the two states. Also, to conserve the total momentum (spin+orbital) of the spin–orbit coupled electron upon spin-flip, it is required that dk ≠ dk′ (also known as El-Sayed’s rule54). Then, the flip of spin vector of the spin–orbit coupled electron between the two states can be compensated by the change of angular momentum vector to keep the total momentum vector conserved. Hence, SOC matrix elements in eq 4 will be appreciable only for pairs of states fulfilling both dk ≠ dk′, with dk and dk′ belonging to the same SOC center(s), and π* = π*′, and will vanish for other pairs of states as mathematically ensured by the spin and angular momentum operators and matrix multiplication rules. It is noted that although El-Sayed’s rule is only a “thumb rule” and may miss some details, it still is important for the general trend of SOC efficiency between the states depending on their electronic properties and can be a reliable guide for qualitative and semi-quantitative analyses such as presented in this contribution.

Figure 4.

Figure 4

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Iso-surface (iso-value = 0.05) contour plots of selected molecular orbitals of Ir2(dppm)3(acac)2.

Equation 4 shows that the higher the MLCT (dπ*) character extent of a given state, the larger the SOC matrix elements with other states it can have. Then, from the requirements dk ≠ dk′ and π* = π*′, it follows that the T1 state (1.95 eV, 98% HOMO → LUMO) can have a large SOC matrix element with singlet states of HOMO–n → LUMO orbital parentage, where HOMO–n has a significant contribution by a metal atomic orbital (dk′) with angular momentum orientation different from that of dk atomic orbital contributing to the HOMO. In Ir(III) complexes, dk and dk′ orbitals are represented by the three quasi-degenerate t2g symmetry 5d-orbitals (dxy, dxz, dyz). In dinuclear complexes of C2 symmetry, electronic coupling of the two coordination sites, symmetric and antisymmetric to C2 rotation, doubles the number of the t2g 5d-orbital contributed MOs, where each MO is evenly contributed by both metals. In other words, instead of three MOs contributed by the t2g atomic orbitals of one metal atom and another three MOs contributed by the t2g atomic orbitals of the other metal atom, like in two independent mononuclear molecules, the same net metal contribution goes to six MOs each evenly contributed by the t2g orbitals of both metal atoms (Figure 4). Accordingly, in dinuclear Ir2(dppm)3(acac)2, the T1 state bearing 3dkπ* character from both metals can spin–orbit couple with four 1dk′π*′ character singlet states also contributed by both metal centers, with individual SOC matrix elements comparable to those in the analogous mononuclear complex. The analogous mononuclear Ir(III) complex, however, will have only three occupied MOs with significant t2g atomic orbital contribution and hence a triplet state contributed by one of those MOs will have only two singlet states that can be directly spin–orbit coupled with.16,21 Hence, the number of singlet states that the T1 state can borrow oscillator strength from via SOC in a symmetric dinuclear complex is also doubled, compared to its mononuclear analogue, whereas the SOC matrix elements of each contributing path remains similar to that in the mononuclear complex.24,27 From the TD-DFT data we obtained, such singlet states in the dinuclear Ir2(dppm)3(acac)2 are S7 (2.30 eV, 98% HOMO–3 → LUMO), S8 (2.35 eV, 98% HOMO–2 → LUMO), S16 (2.80 eV, 54% HOMO–4 → LUMO), and S17 (2.81 eV, 79% HOMO–5 → LUMO) (Table S5). Note that state S2 of HOMO–1 → LUMO orbital parentage does not fulfill the dk ≠ dk′ requirement for SOC with the T1 state as HOMO and HOMO–1 are contributed by the same d-orbitals of the iridium atoms and differ only by symmetry to C2 symmetry operations. The doubled number of singlet states spin–orbit coupled to T1, compared to the mononuclear analogue, gives a larger sum of the SOC matrix elements in eq 1 and, consequently, a strong enhancement of the phosphorescence rate in the dinuclear complex. This agrees well with the experimental data for Ir(dppm)2(acac)(21) and Ir2(dppm)3(acac)2 (Table 1).

The case of trinuclear Ir3(dppm)4(acac)3 is rather interesting. On the one hand, one could expect that trinuclear design would triple the SOC paths of the T1 state, compared to the mononuclear case, and enhance the phosphorescence rate yet further. On the other hand, the molecule has C2 symmetry, with the C2 axis going through the central Ir atom (Ir2 on Chart 1) and bisecting the acac ligand coordinated to it, so that the two terminal iridium atoms are electronically identical to each other and different from the central one. Indeed, as the calculations show, the nine highest occupied MOs of Ir3(dppm)4(acac)3, from HOMO to HOMO−8, comprise even t2g contribution from the two terminal iridium atoms, electronically coupled symmetric and antisymmetric to C2 rotation, and a different amount of t2g contribution from the central iridium atom (Figure 5 and Table S4). According to the TD-DFT data, the T1 state (1.98 eV) of Ir3(dppm)4(acac)3 is of HOMO → LUMO (95%) orbital parentage (Table S6). The HOMO is localized on the two terminal iridium atoms evenly with a net contribution of 20%; on the central metal atom with a contribution of 24%; on the two doubly cyclometalated dppm ligands (dppm2 and dppm3 on Chart 1) evenly with a net contribution of 40%; on the two singly cyclometalated dppm ligands evenly with a net contribution of 8%; and on the three acac ligands with a net contribution of 8% (Figure 5 and Table S4). The LUMO is localized on the two doubly cyclometalated dppm ligands (dppm2 and dppm3 on Chart 1) evenly with a net contribution of 92%. Hence, the T1 state of Ir3(dppm)4(acac)3 is, as expected, of mixed 3MLCT/LC character. The total metal contribution to the HOMO of 44%, and hence, the MLCT extent of the T1 state is similar to that in mononuclear Ir(dppm)2(acac)(21) and dinuclear Ir2(dppm)3(acac)2. Importantly, according to the composition of the HOMO shown above, the contribution of the central iridium atom to the MLCT character of T1 state outweighs the net contribution of the two terminal Ir atoms. Hence, the central iridium atom will be the main SOC center for the T1 state. As such, for the correct description of spin–orbit coupling matrix elements, eq 4 should be modified to split the part of the terminal iridium atoms from the part of the central iridium atom.

graphic file with name ic3c03810_m005.jpg 5

On the right side of eq 5 , the first term in the bracket accounts for the terminal iridium atoms. Here dk and dk′ are atomic orbitals of the terminal Iridium atoms contributing to the MLCT character of the T1 and Sn states with weighs ck and ck′, respectively. The second member accounts for the part of the central Iridium atom where dm and dm′ are orbitals of the central iridium atom with cm and cm′ contribution weighs to the MLCT character of the T1 state and Sn state, respectively. Coefficients aT1aSn represent the same as in eq 4.

Figure 5.

Figure 5

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Iso-surface (iso-value = 0.05) contour plots of selected molecular orbitals of Ir3(dppm)4(acac)3.

Similar to the dinuclear case, in trinuclear Ir3(dppm)4(acac)3, a singlet state directly spin–orbit coupled with the T1 state should have HOMO–n → LUMO character where HOMO–n is contributed by the same Ir center(s) as HOMO but with different t2g 5d-orbital(s) (dk ≠ dk′ and/or dm ≠ dm′). Accordingly, HOMO–n can be HOMO–3 through HOMO–8 (Figure 5). HOMO–1 has only a minor contribution of the central iridium atom fulfilling dm ≠ dm′. The corresponding singlet states are S11 (2.32 eV, 82% HOMO–3 → LUMO), S14 (2.34 eV, 85% HOMO–4 → LUMO), S15 (2.36 eV, 74% HOMO–5 → LUMO), S30 (2.76 eV, 82% HOMO–6 → LUMO), S32 (2.78 eV, 23% HOMO–7 → LUMO), S33 (2.80 eV, 39% HOMO–7 → LUMO), S38 (2.87 eV, 39% HOMO–8 → LUMO), and S40 (2.88 eV, 32% HOMO–8 → LUMO). All these states are within a 1 eV gap to the T1 state (1.98 eV, 95% HOMO → LUMO). However, due to the differences in MLCT character distribution between the central metal atom and the two terminal metal atoms in the two states, neither of these SOC paths exploits the MLCT character of the T1 state and of the corresponding singlet state to the full scale. For instance, HOMO–3 has an orbital node at the central Iridium atom (Figure 5) and is not contributed by it. Consequently, SOC of state S11 with the T1 state is limited to the terminal iridium atoms, meaning that less than half of the original MLCT character of the T1 state will contribute to this SOC path. In other words, the term accounting for the SOC part at the central iridium atom in eq 5 vanishes due to cm′ = 0. Thus, although, trinuclear Ir3(dppm)4(acac)3 has more SOC paths of T1 state with singlets, the SOC matrix element of each path is compromised, compared to those in mononuclear Ir(dppm)2(acac)(21) and dinuclear Ir2(dppm)3(acac)2. We rationalize that this is manifested by the relatively moderate enhancement of T1 → S0 phosphorescence rate from dinuclear Ir2(dppm)3(acac)2 to trinuclear Ir3(dppm)4(acac)3.

Concluding Remarks

Complexes Ir2(dppm)3(acac)2 and Ir3(dppm)4(acac)3 are found to exhibit intense phosphorescence with extraordinarily high rates (kr = ΦPL/τ). The corresponding radiative decay times (τr = 1/kr = τ/ΦPL) are only τr = 0.36 μs for dinuclear Ir2(dppm)3(acac)2 and τr = 0.30 μs for trinuclear Ir3(dppm)4(acac)3, as measured in doped neat polystyrene film at room temperature. These values are unprecedented and set a new milestone for the phosphorescence rates of transition metal complexes. In agreement with these rate values, the cryogenic temperature investigations revealed a drastic increase in the emission rates of the individual T1 substates in the dinuclear Ir2(dppm)3(acac)2 and, especially, in trinuclear Ir3(dppm)4(acac)3, as compared to the mononuclear Ir(dppm)2(acac). Evidently, such a high rate of phosphorescence in Ir2(dppm)3(acac)2 and Ir3(dppm)4(acac)3 is brought by the multinuclear design of these materials. Based on the electronic structure analysis, we interpret that multinuclear structure can afford a larger number of excited singlet states spin–orbit coupled with T1 state and lending oscillator strength to T1 → S0 transition. In dinuclear Ir2(dppm)3(acac)2, for instance, the sum of SOC matrix elements between T1 state and singlet states is roughly doubled (multiplied by the number of SOC centers (“nuclei”)), as compared to the mononuclear analogue. The effect of multinuclearity on the phosphorescence rate can be less prominent if the SOC centers of the molecule differ. This is because the matrix elements of each SOC path in such case is reduced, as found for Ir3(dppm)4(acac)3 where the central iridium atom is slightly different from the two terminal iridium atoms. In the case of Ir3(dppm)4(acac)3, however, the larger overall number of SOC paths seems to take over the relatively reduced efficiency of each path and its phosphorescence rate still appears to be higher than that of dinuclear Ir2(dppm)3(acac)2. Nevertheless, to exploit the multinuclear design for enhancement of phosphorescence rate to the full extent, the molecular symmetry must ensure that all the SOC centers (“nuclei”) are electronically equal and coupled, similarly to the case of dinuclear Ir2(dppm)3(acac)2. Also, one can anticipate that the SOC centers in the molecule better be rigidly bridged to prevent symmetry-breaking distortions in the T1 state, which may lift the electronic equality of the SOC centers.

The smaller T1 state ZFS of both Ir2(dppm)3(acac)2 (175 cm–1) and Ir3(dppm)4(acac)3, (180 cm–1) compared mononuclear Ir(dppm)2(acac) (210 cm–1)21 is another finding that knows no precedence. We rationalize that MOs involved in SOC of state T1 in Ir2(dppm)3(acac)2 and Ir3(dppm)4(acac)3 appear less elongated in one direction due to the alignment of the coordinated ligands in different directions at different coordination centers. Hence, the overall field anisotropy, seen by the spin–orbit coupled electron, is reduced (smaller ZFS parameter D), resulting in a smaller ZFS value. This structural effect may also be of use in designing fast phosphors as a smaller ZFS means more efficient thermal activation of the higher laying and faster emitting T1 substates.

Acknowledgments

The financial support from German Research Foundation (DFG) (Project No 389797483), and EPSRC (Project No EP/S01280X/1) is gratefully acknowledged.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.inorgchem.3c03810.

  • Experimental information; DFT geometries; DFT and TD-DFT output data; and NMR characterization, mass spectrometry, and elemental analysis data (PDF)

The authors declare no competing financial interest.

Supplementary Material

ic3c03810_si_001.pdf (634.5KB, pdf)

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