Enhanced luminescence properties through heavy ancillary ligands in [Pt(C^N^C)(L)] complexes, L = AsPh₃ and SbPh₃

Abstract

In the frame of our research aiming to develop efficient triplet-emitting materials, we are exploring the concept of introducing additional heavy atoms into cyclometalated transition metal complexes to enhance intersystem-crossing (ISC) and thus triplet emission through increased spin–orbit coupling (SOC). In an in-depth proof-of-principle study we investigated the double cyclometalated Pt(II) complexes [Pt(C^N^C)(PnPh₃)] (HC^N^CH = 2,6-diphenyl-pyridine (H₂dpp) or dibenzoacridine (H₂dba); Pn = pnictogen atoms P, As, Sb, or Bi) through a combined experimental and theoretical approach. The derivatives containing Pn = P, As, and Sb were synthesised and characterised comprehensively using single crystal X-ray diffraction (scXRD), UV-vis absorption and emission spectroscopy, transient absorption (TA) spectroscopy and cyclic voltammetry (CV). Across the series P < As < Sb, a red-shift is observed concerning absorption and emission maxima as well as optical and electrochemical HOMO–LUMO gaps. Increased photoluminescence quantum yields Φ_L and radiative rates k_r from mixed metal-to-ligand charge transfer (MLCT)/ligand centred (LC) triplet states are observed for the heavier homologues. Transient absorption spectroscopy showed processes in the ps range that were assigned to the population of the T₁ state by ISC. The heavy PnPh₃ ancillary ligands are found to enhance the emission efficiency due to both higher Pt–Pn bond strength and stronger SOC related to increased MLCT character of the excited states. The experimental findings are mirrored in hybrid (TD-)DFT calculations. This allowed for extrapolation to the rather elusive Bi derivatives, which were synthetically not accessible. This shortcoming is attributed to the transmetalation of phenyl groups from BiPh₃ to Pt, as supported by experimental NMR/MS as well as DFT studies.

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Introduction

Luminescent organometallic transition metal complexes with efficient phosphorescence at ambient temperature are the subject of academic and technological interest with promising applications in illumination, display technology, sensors, therapeutic agents, and biological probes.^1–15 High quantum efficiency of such materials is due to strong spin–orbit coupling (SOC), which allows for triplet (electro)luminescence.^5,7 However, many transition metal complexes are scarcely emissive at room temperature, often because their excited metal-centred (d–d*) states are subject to efficient non-radiative deactivation.^5,8,9,11 Cyclometalated heteroaromatic ligands appear to be a good strategy towards effective emitters in many regards. The very strong ligand field of the cyclometalated carbanions raises the energy of the dissociative d–d* states impeding the population of these non-emissive, so-called dark states.^8,11 The rigidity introduced by these ligands prohibits large distortions of the excited state compared with the electronic ground state. High rigidity of both the organic ligand and the coordination polyhedron around the metal are considered to be important to generate long-lived and efficient triplet emitting complexes.^5,9,16–42 Such a strategy has led to many cyclometalated Pt(II) and a number of related Pd(II) complexes, which are emissive either from triplet states with primarily ligand-centred ³LC (³π–π*) character, from metal-to-ligand charge transfer ³MLCT states, or mixtures thereof.^5,8,16–29

On the way to introducing Ni(II) to the field, replacing Pt or Pd, the intrinsically much smaller spin–orbit coupling (SOC) and ligand field splitting of Ni(II) pose an obstacle to the realisation of efficient phosphorescence.^{29,34,42–46} Considering the question of how to facilitate spin-forbidden transitions and to also reach significant phosphorescence rates in Ni(II) complexes has led us to the idea of introducing additional heavy main group elements into the compounds, specifically in the form of PnPh₃ (Pn = P, As, Sb, Bi) ancillary ligands for tridentate ligand systems. The ancillary ligand or coligand represents an important point of access for tuning the properties of square planar complexes with tridentate ligands.^{17,26,47–49} In order to evaluate whether our concept of improving phosphorescence using heavy ancillary ligands for tridentate ligand systems is viable, we initially resorted to Pt(II) complexes as a synthetically well-accessible model system. After a first study on a series of Pt(II) complexes [Pt(N^N^N)(PnPh₃)] (Pn = P, As, Sb) containing the tridentate N^N^N = 2,6-bis(3-(trifluoromethyl)-1H-1,2,4-triazol-5-yl)pyridine and 2,6-bis(3-(tert-butyl)-1H-1,2,4-triazol-5-yl)pyridine ligands found little systematic variation between the PnPh₃ homologues due to a saturation effect from Pt on the emission efficiency in this N^N^N system,³⁰ we moved on to cyclometalated C^N^C ligand systems.³¹

Herein, we report two series of double cyclometalated Pt(II) complexes [Pt(C^N^C)(PnPh₃)] (Pn = P, As, Sb) based on the two established C^N^C ligand systems dpp (H₂dpp = 2,6-diphenyl-pyridine)^31,50 and dba (H₂dba = dibenzo[c,h]acridine)^31,51 which showed an enhancement of the phosphorescence quantum yields for their heavier congeners. The role of radiative and radiationless deactivation rates was studied by combining extensive photoluminescence (PL) and transient absorption spectroscopy (TAS) with density functional theory (DFT) calculations.

Results and discussion

Synthesis and structural characterisation

The new complexes [Pt(C^N^C)(AsPh₃)] and [Pt(C^N^C)(SbPh₃)] were synthesised in the same way as the previously reported lighter congeners [Pt(C^N^C)(PPh₃)]^50,51via a facile ligand exchange reaction from the corresponding [Pt(C^N^C)(dmso)] precursors as shown in Scheme 1 in good yields ranging from 64% to 84% (more information in the ESI†). The new complexes were characterised by nuclear magnetic resonance (NMR) spectroscopy (see Fig. S1–S20 in the ESI†) as well as high-resolution electrospray ionisation mass spectrometry (HR-ESI-MS) and single-crystal X-ray diffraction (scXRD).

Scheme 1 General synthetic procedure for the complexes.

Similar to our previous report on a series of Pt(II) complexes [Pt(N^N^N)(PnPh₃)] (N^N^N = 2,6-bis(3-(trifluoromethyl)-1H-1,2,4-triazol-5-yl)pyridine and 2,6-bis(3-(tert-butyl)-1H-1,2,4-triazol-5-yl)pyridine; Pn = P, As, Sb)³⁰ the BiPh₃ derivatives of the dpp and dba complexes were not isolated, but MS and NMR evidence suggests their transient formation. Based on our observation of progressively de-phenylated fragments of BiPh₃ and progressively phenylated derivatives of the dpp ligand in ESI-MS reaction control samples as well as the relative depletion of BiPh₃ next to [Pt(dpp)(dmso)] during in situ¹H NMR studies (Fig. S21, ESI†), we suspect that the reason for our failure to isolate [Pt(dpp)(BiPh₃)] lies in the catalytic decomposition of BiPh₃ in the presence of [Pt(dpp)(dmso)]. A free DFT geometry optimisation of the postulated structure of [Pt(dpp)(BiPh₃)] yielded a pentacoordinate geometry [Pt(C^N^C)(Ph)(BiPh₂)] where a phenyl group from BiPh₃ moved to the Pt centre in the C^N^C coordination plane while the remaining BiPh₂ fragment is displaced into a distal position with a Pt⋯Bi distance of 2.72 Å (Fig. S26 and Table S5, ESI†). This further supports the assumption that the decomposition of BiPh₃ under the reaction conditions is triggered by the transmetalation of phenyl groups to Pt as initial reaction, which is then followed by a cascade of decomposition reactions. Detailed information on our experimental studies leading to this conclusion as well as computational details can be found in the ESI.† For further DFT calculations on the elusive [Pt(dpp)(BiPh₃)] and [Pt(dba)(BiPh₃)] complexes, the predicted transmetalation of a phenyl group was suppressed during the geometry optimisation by excluding the N-Pt-Bi angle from the optimisation process (Fig. S26 and Table S5, ESI†).

Crystal structures and DFT-calculated molecular structures

The structures of the four new complexes [Pt(C^N^C)(AsPh₃)] and [Pt(C^N^C)(SbPh₃)] (C^N^C = dpp, dba) were determined by scXRD (Fig. 1 and Figs. 22–24, ESI†). Details on data collection, structure solution and refinement as well as selected structural parameters are listed in Tables S1–S3 (ESI†) alongside with the data for the previously reported [Pt(dpp)(PPh₃)] and [Pt(dba)(PPh₃)].^50,51 The crystal structures feature long Pt⋯Pt distances ranging from about 7 to 10 Å (far beyond a significant interaction between the Pt atoms, typically below 3.5 Å); thus, no significant Pt⋯Pt interactions nor π-stacking interactions were found.^52,53 In contrast, the dba containing structures show pronounced π-stacking of the acridine moieties with centroid distances between 3.5 and 3.6 Å. However, due to the antiparallel alignment of the [Pt(dba)] fragments as dictated by the bulky PnPh₃ ancillary ligands, significant Pt⋯Pt interactions can still be ruled out from large Pt⋯Pt distances ranging from 6.9 to 7.7 Å (Fig. 1).

Fig. 1 Molecular structures of the complexes [Pt(dpp)(AsPh₃)] (left) and [Pt(dba)(AsPh₃)] (right) with numbering, as obtained from scXRD. Displacement ellipsoids are shown at 50% probability, H atoms omitted for clarity.

As expected from the increasing radii of the progressively heavier pnictogens, the Pt–Pn bond lengths increase by about 0.11 Å from the P to the As derivatives and by another 0.15 to 0.18 Å from the As to the Sb derivatives for both C^N^C ligands. This aligns with our findings for the previously reported [Pt(N^N^N)(PnPh₃)] series. However, the Pt–Pn distances are overall slightly shorter in the herein reported C^N^C complexes by about 0.07 to 0.02 Å.³⁰ This indicates a slightly higher bond strength, possibly due to an increased covalent character of the Pt–Pn bond in the herein studied system. The differences between Pt–Pn bond lengths (if comparing the C^N^C versus N^N^N complexes) are largest for the PPh₃ complexes and smallest for the SbPh₃ derivatives. This may be related to the Pt–Sb bond being generally more covalent than the Pt–P or Pt–As bond due to better orbital overlap between atoms of more similar atomic or ionic radii and polarisability. The averaged C–Pn–C angles decrease slightly along P > As > Sb for both series of complexes, from P (∼104°) to As (∼103°) and more markedly to Sb (99° for dpp, 101° for dba) (data in Tables S2 and S3, ESI†), exceeding the C–Pn–C angles of the free ligands by around 3 to 4°. We have previously described this widening of the angles upon binding to Pt as compensation of the hybridisation defect at the Pn atom.³⁰

Notably for the complexes with the more flexible dpp ligand, a significant deviation from a coplanar orientation between the pyridine moiety and one of the phenyl moieties is observed in the solid state, with dihedral angles reaching up to 12° in the PPh₃ derivative.⁵⁰ This pronounced unilateral twist is not predicted by DFT geometry optimisations using BP86/def2-TZVP/CPCM(CH₂Cl₂)^54–58 (Tables S2 and S3, ESI†) and the underlying reason are likely packing effects in the solid state. Apart from this deviation, the experimentally observed structural trends in the solid state are well reproduced by DFT. The optimised structures were used as the basis for all further calculations using the hybrid functional TPSSh, which was found to provide qualitatively good to excellent results for organometallic transition metal complexes in previous studies.^34,59–62

Electrochemistry and DFT-calculated frontier orbitals

Cyclic voltammograms showed irreversible oxidations at around +0.6 V (Fig. 2, Fig. S27–S32 and Table S6, ESI†) for all complexes with slightly higher values for the dba derivatives and almost no variation within the dpp and dba series. They are assigned to essentially metal centred Pt(II)/Pt(III) couples. The DFT-calculated frontier molecular orbital (FMO) landscapes (Fig. S33 and S34, ESI†), which show energetically virtually invariant highest occupied molecular orbitals (HOMOs) with strong metal centred character for the complexes within each series, support this assignment.

Fig. 2 Electrochemical data from cyclic voltammetry in 0.1 M n-Bu₄NPF₆ THF solutions at a scan rate of 50 mV s⁻¹ (dots); DFT-calculated frontier orbitals, and energies for [Pt(dpp)(PnPh₃)] (left) and [Pt(dba)(PnPh₃)] (right; Pn = P, As, Sb). Values for the redox potentials are half half-wave potentials E_1/2 for the reversible first reduction and potentials E_p for the irreversible oxidations and second and third reductions (Table S6, ESI†).

Two reduction processes were observed for all complexes (Fig. 2). Interestingly, their potentials are virtually identical for [Pt(dpp)(PPh₃)] and [Pt(dpp)(AsPh₃)] and appear at −2.36 and −2.96 V (vs. ferrocene/ferrocenium). For the dba complexes three reduction waves were observed, the first ones at markedly less negative potentials. In both series the SbPh₃ complexes exhibit the least negative values. The DFT-calculated lowest unoccupied molecular orbitals (LUMO) represent exclusively the π* levels of the C^N^C ligands. This fits well to the general separation of the reduction potential of the two systems into dpp and dba complexes. The observed PnPh₃ ancillary ligand-dependent differences (with the Sb derivatives having the least negative potentials) are fully in line with the DFT-calculated LUMO+1 showing contributions from the PnPh₃ ligands. These assignments are further substantiated through spectroelectrochemical (UV-vis-SEC) studies (Fig. S39–S42, ESI†). Upon reduction, the complexes show several bands indicative of reduced phenyl-pyridine or acridine moieties, and pointing to ligand-centred LUMOs mirroring what has been reported for related complexes.^{25,34,51,63,64} Oxidation of the dpp complexes leads to a general bleaching of the long-wavelength bands in line with a depopulation of the metal-dominated HOMO. For the dba derivatives, the long-wavelength bands also bleach upon oxidation, but also a broad absorption band ranging from 500 to 350 nm appears. This slightly resembles the acridinyl radical (also called “C radical”), which absorbs in this range^65,66 and agrees with the more pronounced contribution of the dba ligand to the HOMO of the complex, particularly when compared with dpp.

The electrochemical HOMO–LUMO gaps range from about 3.0 to 2.6 eV narrowing along the series P > As > Sb for both the dpp and the dba complexes. The smaller gaps for the dba complexes (2.72 to 2.62 eV) compared with the dpp derivatives (3.01 to 2.87 eV) by about 0.2 to 0.3 eV can be rationalised by the more extended π-system in the dba complexes and are reproduced by the DFT-calculations (Fig. 2, Fig. S33 and S34, ESI†). Parallel trends are predicted by DFT, which shows a decrease in the HOMO–LUMO gaps between the PPh₃ and SbPh₃ derivatives by 0.07 eV for the dpp complexes and 0.06 eV for the dba complexes. The DFT calculated values are generally slightly smaller than the experimental values. It should be noted that the predictive power of DFT reproduces trends much better than absolute numbers, as specifically the HOMO–LUMO gap depends strongly on how much Hartree–Fock exchange is included in the functional.^67,68 Energetically above the strongly ligand-centred LUMOs (LUMO and LUMO+1 for dpp; LUMO to LUMO+2 for dba), a metal centred MO of strong d_x²–y² character is found in the DFT calculated FMO landscapes. This orbital is significantly destabilised across both series from P to Sb, indicating that the ligand-field splitting increases down the group 15 elements due to their increasing polarisability and atomic radii, as discussed earlier. The described trends for the PPh₃, AsPh₃, and SbPh₃ derivatives of both the dpp and dba complexes are continued in the calculated FMO landscapes for the hypothetical BiPh₃ derivatives.

UV-vis absorption spectroscopy and TD-DFT calculations

The UV-vis absorption spectra of the dba complexes [Pt(dba)(PnPh₃)] (Pn = P, As, Sb) feature absorption bands down to almost 600 nm, which are structured into four double bands around 290, 340, 400 and 520 nm (Fig. 3, Table 1, Fig. S35 and S37, ESI†). The dpp complexes [Pt(dpp)(PnPh₃)] (Pn = P, As, Sb) show only two intense, structured absorption bands, one at about 250 to 300 nm and a second at around 350 nm. Additionally, very weak absorption bands trail down to about 500 nm. UV-bands in both systems have been previously assigned to transitions into LC (π–π*) states, whereas mixed LC/metal-to-ligand charge transfer (MLCT) states were discussed for increasingly long-wavelength bands.^31,51,69,70

Fig. 3 Left: experimental UV-vis absorption spectra (molar absorption coefficient ε as a function of wavelength) of [Pt(C^N^C)(SbPh₃)] in CH₂Cl₂ solution at 298 K. Inset: magnification of the 350–630 nm region. Right: experimental UV-vis absorption spectrum in CH₂Cl₂ solution at 298 K and TD-DFT-calculated spectrum with selected transition difference densities for [Pt(dba)(SbPh₃)].

Table 1 Experimental and TD-DFT-calculated UV-vis absorption maxima for the complexes [Pt(dba)(PnPh₃)] (Pn = P, As, Sb, Bi)^a,b

		λ 1	λ 2	λ 3	λ 4	λ 5	λ 6	λ 7	λ 8
a In CH2Cl2; absorption maxima λ in nm. b TPSSh/def2-TZVP/CPCM(CH2Cl2), maxima from convoluted spectrum.
[Pt(dba)(PPh3)]	Exp.^a	286	296	333	348	387	409	499	533
	Calc.^b	306		363		430		552
[Pt(dba)(AsPh3)]	Exp.^a	285	296	333	354	390	412	505	539
	Calc.^b	307		364		432		560
[Pt(dba)(SbPh3)]	Exp.^a	286	296	335	351	392	416	511	546
	Calc.^b	308		365		434		567

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TD-DFT calculated absorption spectra excellently match the experimental results and support these assignments (Fig. 3, Table 1, Fig. S35–S38, Tables S6 and S7, ESI†). Fig. 3 (right) shows selected transition difference densities for the TD-DFT calculated transitions for [Pt(dba)(SbPh₃)] as a representative example, while illustrating their mixed yet predominantly ¹LC and ¹MLCT character, in agreement with previously reported data for related compounds.^{25,28–31,34,35,51,52} Across the series P > As > Sb, a slight red-shift of the UV-vis absorption energies is both observed experimentally and predicted theoretically, matching the electrochemical data and the electronic structure modelled by DFT.

The absorption cut-off extrapolated from the low-energy slope of the lowest energy absorption band, representing the optical HOMO–LUMO gap, is about 565 nm (2.19 eV) for [Pt(dba)(PPh₃)] to 580 nm (2.14 eV) for [Pt(dba)(SbPh₃)]. While the optical HOMO–LUMO gap is expected to exceed the electrochemical HOMO–LUMO gap (which probes the molecule in its geometric ground state), the opposite is the case here, as the electrochemical HOMO–LUMO gaps determined from CVs are larger by about 0.5 eV than the optical gaps obtained from UV-vis absorption spectroscopy. Since negative reorganisation energy (i.e., reorganisation into a higher energy conformation upon oxidation/reduction) is physically not reasonable, we have to consider the contribution of direct absorption from the S₀ ground state into lowest ³MLCT states producing the low-energy absorption bands and thus the low optical gaps. This is discussed for Pt(II) systems in the literature,^71–74 with a lower energy and weakly intense band related to a ³MLCT state mirroring the absorption into the ¹MLCT state. Remarkably, our TD-DFT calculations also predict these very low energy transitions, even if they do not include such triplet admixtures. DFT calculated T₁–S₀ energy gaps of 1.97 eV for [Pt(dba)(PPh₃)] and 1.95 eV for the As and Sb derivatives (Table 2) are roughly consistent with this assumption and reinforcing the assumed correlation between optical and actual HOMO–LUMO gaps.

Table 2 Experimental and calculated photophysical data for the complexes [Pt(C^N^C)(PnPh₃)] (Pn = P, As, Sb)^a

	T	λ max (nm)	Φ L ± 0.02	τ (μs)^e	k r (10^−4 s^−1)	k nr (10^−4 s^−1)	〈Sn|HSO|T1〉^2 (cm^−2)^f	ΔE(T1–S0) (eV / nm)^f
a Excitation wavelength λexc = 350 nm. b Measured in a frozen glassy matrix of CH2Cl2/MeOH (1 : 1) at 77 K. c Measured in fluid CH2Cl2 solution upon purging with Ar. d Shoulder. e For bi-exponential photoluminescence decays, amplitude-weighted average lifetimes are shown. Raw time-resolved photoluminescence decays along with the fitting parameters are shown in the ESI.† f (TD-)DFT calculated, computational details in the ESI.†
[Pt(dpp)(PPh3)]	77 K^b	511, 550, 590^d	0.48	12.70 ± 0.01	3.8 ± 0.2	4.1 ± 0.2	9.18 × 10^5 (n = 0)	2.06 / 602
							1.23 × 10^4 (n = 2)
[Pt(dpp)(AsPh3)]	77 K^b	517, 557, 588^d	0.70	11.80 ± 0.01	5.9 ± 0.2	2.5 ± 0.2	9.06 × 10^5 (n = 0)	2.04 / 608
							1.41 × 10^4 (n = 2)
[Pt(dpp)(SbPh3)]	77 K^b	521, 562, 592^d	0.68	11.60 ± 0.01	5.6 ± 0.2	3.0 ± 0.2	8.14 × 10^5 (n = 0)	2.05 / 605
							1.11 × 10^4 (n = 2)
[Pt(dba)(PPh3)]	77 K^b	576, 625, 684	0.90	27.90 ± 0.06	3.2 ± 0.1	0.5 ± 0.1	2.99 × 10^5 (n = 0)	1.97 / 629
	298 K^c	590, 635^d, 695^d	0.03	0.81 ± 0.01	0.4 ± 0.2	12.0 ± 0.3	1.20 × 10^5 (n = 2)
[Pt(dba)(AsPh3)]	77 K^b	580, 632, 692	0.90	23.70 ± 0.07	3.8 ± 0.1	0.4 ± 0.1	2.94 × 10^5 (n = 0)	1.95 / 636
	298 K^c	594, 636^d, 700^d	0.04	0.270 ± 0.001	1.5 ± 0.7	35.4 ± 0.8	1.38 × 10^5 (n = 2)
[Pt(dba)(SbPh3)]	77 K^b	584, 635, 694	0.94	21.90 ± 0.02	4.3 ± 0.1	0.3 ± 0.1	2.22 × 10^5 (n = 0)	1.95 / 636
	298 K^c	598, 640^d, 702^d	0.07	0.470 ± 0.001	1.5 ± 0.4	19.8 ± 0.4	1.41 × 10^5 (n = 2)

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Time-resolved and steady-state photoluminescence spectroscopy

Photoluminescence spectroscopy showed the dpp complexes [Pt(dpp)(PnPh₃)] (Pn = P, As, Sb) to be emissive at 77 K in frozen CH₂Cl₂/MeOH (1 : 1) glassy matrices, but not in solution at 298 K. The spectra (Fig. S43–S45, ESI†) show two sharp emission bands around 515 and 560 nm and a shoulder around 590 nm. This corresponds to an approximate energy of around 2.4 eV for the 0–0 transitions, which is markedly larger than the DFT calculated ΔE(T₁–S₀) values (around 2.05 eV, see Table 2). The dba derivatives [Pt(dba)(PnPh₃)] (Pn = P, As, Sb) are emissive both at 77 K in frozen glassy matrices and, albeit weakly, at 298 K in solution (spectra in Fig. 4 and Fig. S46–S48, ESI†). At 77 K, three sharp bands with emission maxima around 580, 630 and 690 nm are found. At 298 K, these maxima are shifted to around 595, 640 and 700 nm, where the latter two are not sharp bands but shoulders. Also in this case, the energy of the 0–0 transitions according to spectroscopy (around 2.1 eV) exceeds the DFT calculated ΔE(T₁–S₀) values discussed above, but not as markedly as for the dpp series. Like for the absorption spectra, a red-shift is observed for the emission spectra across the series P < As < Sb for both systems, whereas the calculated ΔE(T₁–S₀) values show a marginal (and in the case of dpp imperfect) downwards trend on the second decimal place. All data is compiled in Table 2.

Fig. 4 Photoluminescence spectra of the complexes [Pt(dba)(PnPh₃)] (Pn = P, As, Sb) at 77 K in a frozen glassy matrix of MeOH/CH₂Cl₂ (1 : 1) (solid lines) and in CH₂Cl₂ solution at 298 K (dashed lines).

For the dpp series, photoluminescence quantum yields (Φ_L) of 0.70 and 0.68 were found for the AsPh₃ and SbPh₃ derivatives, significantly exceeding the value of 0.48 for [Pt(dpp)(PPh₃)]. The Φ_L are in concordance with literature reported values for similar Pt(II) complexes with flexible double cyclometalated C^N^C ligands.^{49,51,64,69,75,76} For the dba series at 77 K, values of Φ_L near unity (0.90 for PPh₃ and AsPh₃, 0.94 for SbPh₃) were determined. For similar complexes bearing several different ancillary ligands on the [Pt(dba)] fragment, we recently reported Φ_L at 77 K between 0.80 and 0.90.⁵¹ At 298 K, the dba complexes exhibit much lower Φ_L with values (below 0.1, see Table 2). At room temperature (298 K), all three dba complexes are practically non-emissive, with their Φ_L values approaching the experimental uncertainty (except for the SbPh₃ complex, which exhibits a modest yet measurable Φ_L). The excited state lifetimes (τ) determined for both series at 77 K are within the range typically reported for similar C^N^C Pt(II) complexes at 77 K (ref. 47–49, 51, 64, 69, 75 and 76) and decrease across the series P > As > Sb.

Based on the obtained Φ_L and excited-state lifetimes, we estimated the average radiative (k_r) and non-radiative (k_nr) rate constants, as detailed in Table 2 and Fig. 5.

Fig. 5 Visualisation of k_r (pink) and k_nr (green) for the complexes [Pt(dpp)(PnPh₃)] (A) and [Pt(dba)(PnPh₃)] at 77 K (B) and 298 K (C) (Pn = P, As, Sb).

At 77 K, we see a trend of increasing k_r values and decreasing k_nr values from P to Sb in both the dpp and the dba series (Fig. 5A and B). We assume that the observed deviations from the linear trend (especially for k_nr) are due to 3d-contraction anomaly related to the first filling of a d-shell before the element As.^77,78 At 298 K, k_r for the dba complexes also increases from P to Sb, but no clear trend can be deduced from the k_nr values, which we attribute to more complex roto-vibronic relaxation pathways between T₁ and S₀ or possibly via the population of dissociative ³MC states at ambient temperature (Fig. 5C).

To further our understanding of the interplay between SOC and roto-vibronic aspects in the photophysics of the two series of complexes, we expanded our computational work to include TD-DFT spin–orbit coupling calculations at the DFT optimised T₁ geometries (Table S4, ESI†).^79–81 The results indicate significant mixing of the T₁ states with the S₀ and S₂ states. Surprisingly, the S₁ states only marginally couple with the T₁ states. The corresponding SOC matrix elements 〈S_n|H_SO|T₁〉² are listed in Table 2. In general, the values of 〈S₂|H_SO|T₁〉² are larger for the dba series than in the dpp series by a factor of 10. This may be related to the rigidity of the dba framework, thus allowing for a stronger coupling with higher excited states as more geometric overlap is preserved.

For the dba complexes, an increase in 〈S₂|H_SO|T₁〉² across the P < As < Sb series is predicted alongside a decrease in 〈S₀|H_SO|T₁〉². For the dpp derivatives, the 〈S₀|H_SO|T₁〉² values again decrease when moving down the pnictogen group. Interestingly, an unexpected trend in 〈S₂|H_SO|T₁〉² is observed for the dpp series, namely Sb < P < As. Thus, in the particular case of [Pt(dpp)(SbPh₃)], our calculations predict a counterintuitively diminished SOC.

A possible explanation for these differences between the dpp and dba complexes lies in the character of the emissive states. The MLCT character of the emissive state is expected to decrease in frozen matrix at 77 K. Furthermore, the extended π-system of the dba ligand leads to smaller MLCT and larger LC contributions compared with the dpp system. We assume that for the dpp complexes at 77 K and for the dba derivatives at 298 K we have a larger MLCT contribution compared with dba at 77 K. With a higher MLCT character in the excited state, the heavier SbPh₃ ligand is not able to develop its potential for higher SOC as the effect is already strong due to the participation of the Pt centre.³⁰ Only in case of the LC-dominated dba systems at 77 K, SbPh₃ reveals its superior SOC capacity over the lighter homologues.

To confirm this, we assessed the character of the emissive T₁ states for both series of complexes as well as for the hypothetical BiPh₃ theoretically via exciton analysis using the open-source software TheoDORE (Fig. 6, see ESI† for details).⁸² On first view, they confirm that for the dpp series that MLCT character of the T₁ excited state is higher than for the dba complexes (∼22% vs. ∼17%) and the LC character dominates the latter (∼65% vs. ∼75%).

Fig. 6 Decomposition of the emissive T₁ states into MLCT, L′MCT, LL′CT, LC and MC contributions for [Pt(dpp)(PnPh₃)] (left) and [Pt(dba)(PnPh₃)] (right; Pn = P, As, Sb, Bi) based on TD-DFT calculations (TPSSh/def2-TZVP/CPCM(CH₂Cl₂)).⁸²

In both series, the LC character decreases slightly down the series from P to Sb, while the MLCT contribution increases. This is in line with the above discussed higher polarisability of the PnPh₃ ligands leading to the observed red-shifts in absorption and emission as well as to the reduced electrochemical gaps.

Femtosecond transient absorption spectroscopy

The excited state dynamics of the two series of complexes [Pt(dpp)(PnPh₃)] and [Pt(dba)(PnPh₃)] (Pn = P, As, Sb) were studied further using femtosecond transient absorption (fsTA) spectroscopy under ambient conditions. A particular focus was on observing the ISC from the singlet to the triplet manifold and obtaining time constants for this process in the different PnPh₃ derivatives. The ISC from singlet excited states into the triplet manifold is an ultrafast process for Pt(II) complexes, with reported ISC time constants for organometallic Pt(II) complexes being typically <5 ps and often even <1 ps.^83–86

When exciting the dpp complexes at 340 nm we observed intense excited state absorption (ESA) with broad features around 430 and 550 nm and no ground state bleach (spectra in Fig. 7a and Fig. S49, ESI†). Global analysis of the TA data using an exponential kinetic scheme reveals two characteristic time constants, one very fast (<10 ps) and one slower process beyond 10 ns after excitation. The first process is fastest in the PPh₃ derivative and significantly slower in the SbPh₃ complex. We assign this to an ISC in accordance with literature values for similar complexes.^83–86 The counterintuitive trend in the <10 ps time constants (P ≈ As < Sb) roughly parallel the trend in decreasing SOC calculated by TD-DFT for the dpp series. Another concurring factor is probably the increasing Pt–Pn bond strength slowing down all radiationless processes (including ISC). The second process with time constants around 1.7 to 2.9 ns can be assigned to the relaxation from the T₁ state to the ground state. This is consistent with absence of phosphorescence in the μs time range at 298 K. For [Pt(dpp)(dmso)], a relaxation pathway via T₂ is described in the literature based on detailed theoretical analysis.⁷² It is therefore in principle also possible that the observed ISC occurs between S₁ and T₂ followed by a rapid decay from T₂ to T₁, even though we did not make any experimental observations specifically pointing in that direction.

Fig. 7 Transient absorption spectra (TAS) at selected delay times (a–c) and decay-associated spectra (d–f) for [Pt(dpp)(SbPh₃)] following excitation at 340 nm (a and d), [Pt(dba)(SbPh₃)] upon excitation at 340 nm (b and e) and 500 nm (c and f) in THF. The thicker lines in TAS shows smoothed data.

For the dba complexes, we performed transient absorption experiments upon excitation at 340 and 500 nm. In all cases, we also observed intense ESA with maxima at 430 nm and shoulders at 500 and 580 nm. The evolution of signals could be fitted to a model taking into account two distinct processes (see Fig. 7 and Fig. S50 and S51, ESI†), with a slower process that is assigned to the decay of the emissive T₁ state exceeding the 10 ns window.

Upon 340 nm excitation, we observed a fast process characterised by time constants around 30 ps for all complexes Pn = P, As, and Sb. We again assign this sub-100 ps kinetics to ISC, based on the characteristic time constants and the similar shapes regarding the decay-associated spectra for the fast process in the dpp derivatives (i.e. decrease of ESA at 580 nm and increase of ESA at 430 and 500 nm). For the dba derivatives, this process is overall slower than in the dpp complexes due to the larger LC contributions to the excited states.

Upon excitation at 500 nm, we see less pronounced spectral changes in the process represented by the first component, if compared to that observed at 340 nm excitation, leading us to assume the population of a different state (with significant triplet admixtures already upon initial excitation). It is possible that intermediate steps (like partial ISC) are not resolved from the relatively slow effective τ₁. The slower ISC rate could be attributed to increased LC character. The second component is longer-lived and its spectral profile resembles that observed upon 340 nm excitation.

Generally, the lifetimes of the S₁ states for the more rigid dba complexes are significantly longer than for the dpp derivatives, pointing to a slower ISC. For both series, the ISC under ambient conditions is not significantly faster for the heavy SbPh₃ derivatives compared to their lighter PPh₃ and AsPh₃ congeners as originally expected. As discussed above, the effect of the PnPh₃ variation is multifaceted. The initially predicted enhancement of SOC for the heavier derivatives is observed only in the dba series, due to the predominant LC character of their emissive triplet states. With increasing MLCT character of the emissive triplet state, the heavy atom effect is progressively masked by the dominating effect of the 5d metal, as seen in the dpp complexes. Furthermore, the variation of the Pn atom induces not only changes in SOC, but also in the Pt–Pn bond strength, influencing the photophysical properties by affecting the vibronic properties.

Conclusions

The effects of progressively heavy pnictogen-based ancillary ligands PnPh₃ on the electronic and photophysical properties two series of Pt(II) C^N^C cyclometalated complexes [Pt(dpp)PnPh₃] and [Pt(dba)PnPh₃] (Pn = P, As, Sb; H₂dpp = 2,6-diphenyl-pyridine, H₂dba = dibenzoacridine) were explored. The electrochemical HOMO–LUMO gaps and the long-wavelength UV-vis absorption and emission maxima are red-shifted along the series P < As < Sb for both the dpp and the dba framework, in line with increasing electron density at the Pt centre, which we interpret as stronger σ-donation of the heavier PnPh₃ ligands to Pt. The latter correlates with an increasing ligand-field splitting along P < As < Sb. The photoluminescence quantum yields Φ_L are increased for the heavy congeners reaching almost unity for the dba complex. The radiative rate constants k_r are increased by introduction of the heavier homologues while the non-radiative rate constants k_nr are reduced. As an important factor for this behaviour we identify increased Pt–Pn bond strength in the heavier derivatives, especially with SbPh₃, due to better orbital overlap resulting from more similar atomic/ionic radii and polarisability. The initially sought-after heavy atom effect with increased spin orbit coupling (SOC) for the heavier PnPh₃ ancillary ligands is modulated by the character of the excited states. In our two series of complexes the increased SOC is most evident for the dba complexes at 77 K, where the LC character of the excited state is dominant over the MLCT character. Thus, the enhancement of phosphorescence via introduction of heavy ligands is less prominent in systems with emissive states of strong MLCT character.

Based on this analysis, we are confident that the present design principle of combining a cyclometalating, rigid tridentate ligand with a heavy element containing ancillary ligand can be applied to further systems, especially with LC-dominated excited states to enhance triplet emission. For the present series of complexes, we were unable to obtain the Bi derivative, which therefore remains an open synthetic challenge. In the future, the isolation of this or related Bi-containing complexes will show whether its heavy atom effect can further enhance k_r in complexes with weak SOC. Further promising candidates for heavy ancillary ligands may also include the anionic tetrelate derivatives GeR₃⁻, SnR₃⁻, and PbR₃⁻.

Author contributions

R.J. carried out syntheses, cyclic voltammetry, UV-vis absorption spectroscopy and spectroelectrochemistry measurements as well as the computational work. I.M. carried out the emission spectroscopy. S.S.N and R.J. carried out the transient absorption spectroscopy measurements. A.K., B.D.-I., and C.A.S. supervised the project. R.J. and A.K. wrote the original draft, all authors revised the manuscript. All authors agreed with the final version.

Data availability

All relevant data are available from the corresponding authors upon request.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We thank the Deutsche Forschungsgemeinschaft [DFG Priority Programme 2102 “Light-controlled Reactivity of Metal Complexes” STR 1186/6-1 and 6-2 (C. A. S.), KL1194/16-1 and 16-2 (A. K.), and DI1517/19-1 (B. D.-I.)] and the Studienstiftung des Deutschen Volkes e.V. (R. J.) for funding of this project. We would also like to thank the Regional Computing Center of the University of Cologne (RRZK) for providing computing time on the DFG-funded High Performance Computing (HPC) system CHEOPS as well as for technical support.

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Footnote

† Electronic supplementary information (ESI) available: All other data including experimental procedures and characterisation data for all new compounds. CCDC 2149899, 2257280, 2194357 and 2208288 for [Pt(dpp)(AsPh₃)], [Pt(dpp)(SbPh₃)], [Pt(dba)(AsPh₃)] and [Pt(dba)(SbPh₃)]. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d3dt03225f

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