Chromophore-radical excited state antiferromagnetic exchange controls the sign of photoinduced ground state spin polarization

Abstract

A change in the sign of the ground-state electron spin polarization (ESP) is reported in complexes where an organic radical (nitronylnitroxide, NN) is covalently attached to a donor–acceptor chromophore via two different meta-phenylene bridges in (bpy)Pt(CAT-m-Ph-NN) (mPh-Pt) and (bpy)Pt(CAT-6-Me-m-Ph-NN) (6-Me-mPh-Pt) (bpy = 5,5′-di-tert-butyl-2,2′-bipyridine, CAT = 3-tert-butylcatecholate, m-Ph = meta-phenylene). These molecules represent a new class of chromophores that can be photoexcited with visible light to produce an initial exchange-coupled, 3-spin (bpy˙⁻, CAT⁺˙ = semiquinone (SQ), and NN), charge-separated doublet ²S₁ (S = chromophore excited spin singlet configuration) excited state. Following excitation, the ²S₁ state rapidly decays to the ground state by magnetic exchange-mediated enhanced internal conversion via the ²T₁ (T = chromophore excited spin triplet configuration) state. This process generates emissive ground state ESP in 6-Me-mPh-Pt while for mPh-Pt the ESP is absorptive. It is proposed that the emissive polarization in 6-Me-mPh-Pt results from zero-field splitting induced transitions between the chromophoric ²T₁ and ⁴T₁ states, whereas predominant spin–orbit induced transitions between ²T₁ and low-energy NN-based states give rise to the absorptive polarization observed for mPh-Pt. The difference in the sign of the ESP for these molecules is consistent with a smaller excited state ²T₁ – ⁴T₁ gap for 6-Me-mPh-Pt that derives from steric interactions with the 6-methyl group. These steric interactions reduce the excited state pairwise SQ-NN exchange coupling compared to that in mPh-Pt.

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Introduction

Molecular quantum information science (QIS) requires the synthesis and study of new molecules that can enable the generation, manipulation, and readout of specifically prepared quantum states¹ in order to advance new nanoscale technologies that include, computing,^2–5 sensing,^6,7 and communications.⁸ QIS exploits quantum bits, or qubits, that are unlike two-level classical bits (e.g., binary 0 and 1) since they can be entangled and exist as superposition states described by linear combinations of these 0 and 1 binary representations.^1,9 An unpaired electron spin (S = 1/2, m_s ±1/2) represents such a two-level quantum system that can be created with relative ease in a molecular environment. However, expanding such a system to include multiple unpaired spins and generating the desired coherences within the system represents a significant challenge.

Building on our earlier work,^10–16 we have shown that photoexcitation into the low-energy ligand-to-ligand charge transfer (LL′CT) bands of bipyridine-metal-catecholate ((bpy)M(CAT)) complexes with M = Pt or Pd provide a convenient way to generate charge transfer excited states with considerable biradical spin character.^{11–13,16,17} When these molecules are elaborated with a persistent radical attached to the CAT donor, the LL′CT excited states can be described as having three localized spin qubits that possess different pairwise exchange interactions.^16,17 A particular benefit of this tripartite entanglement^9,18–20 is that when the spins in the open-shell excited singlet- (S) and triplet (T) states of the chromophore exchange couple with the radical, two doublet states (²S₁ and ²T₁) and a quartet state (⁴T₁) result, and magnetic exchange interactions admix the ²S₁ and ²T₁ states.¹⁷ This excited state exchange mixing effectively mediates an enhanced intersystem crossing^21–24 that permits access to the localized high-spin triplet configuration of the chromophore. The triplet state of the (bpy)M(CAT) parent chromophore is otherwise inaccessible due to non-competitive intersystem crossing (ISC) rates compared to efficient non-radiative relaxation back to the singlet ground state.^13,15

Chromophores with appended radicals have been used to provide high-resolution information regarding the nature of their low-energy excited states,^11,12,16,17 illustrate how multiple, pairwise, excited state magnetic exchange interactions influence photophysical processes,¹¹ and highlight how the excited state chromophore triplet – radical magnetic exchange interaction and associated excited state dynamics can affect electron spin polarization (ESP).^{11,16,17,22,23,25–34} In most systems of this type, ESP is a result of the spin selectivity of the transitions between nearly degenerate ²T₁ and ⁴T₁ states and, in the absence of intermolecular collisions, ESP is observed when these states are relatively long lived. These radical-elaborated chromophores possess spin-polarized quartet ⁴T₁ excited states with lifetimes that compete with or exceed the spin-lattice relaxation time (T₁) of the appended radical, and this effectively inhibits the transfer of ESP to the electronic ground state. Although the observation of ground state ESP is rare in glassy matrices, a few examples are known and they occur via the reversed quartet mechanism (RQM, Fig. 1).^35,36

Fig. 1 Left (A): reversed quartet mechanism (RQM), antiferromagnetic J_CR. Spin-selective transitions (k_QD/DQ) between doublet and quartet result in non-Boltzmann population (red dots) of the doublet m_s levels (non-Boltzmann population of quartet not shown) resulting in enhanced emissive ESP. Right (B): RQM, ferromagnetic J_CR. Spin-selective transitions (k_QD/DQ) between doublet and quartet states result in non-Boltzmann population (blue dots) of the doublet m_s levels (non-Boltzmann population of quartet not shown) resulting in enhanced absorptive ESP.

Electron spin polarization,^23,37,38 including that generated by RQM,^35,36 derives from non-Boltzmann m_s sublevel populations (Fig. 1) and can be detected using time resolved electron paramagnetic resonance (TREPR) spectroscopy as either an enhanced emissive (Fig. 1A) or absorptive (Fig. 1B) signal. An emissive TREPR signal via the RQM requires the ⁴T₁ state to be higher in energy than ²T₁ (and ΔE_D1Q ∼ k_BT where k_B is Boltzmann's constant and T is temperature). In this case, spin selective transitions (k_QD/DQ, Fig. 1) between the ²T₁ and ⁴T₁ states result in an excess population of the higher energy m_s = +1/2 sublevel of the doublet and enhanced emissive ESP. Conversely, an absorptive TREPR signal via the RQM (Fig. 1B) requires the ⁴T₁ quartet state to be lower in energy than the ²T₁ doublet (and ΔE ∼ k_BT). Here, spin selective transitions (k_QD/DQ, Fig. 1) between ²T₁ and ⁴T₁ result in an excess population of the lower-energy m_s = −1/2 level of the doublet and enhanced absorptive ESP. This photogenerated ²T₁ ESP can then be transferred to the ground state if ²T₁ → ²S₀ relaxation is faster than the T₁ spin relaxation of the appended radical in the ²S₀ state. A key point of the RQM is that the sign of the exchange parameter determines the relative energy ordering of the ²T₁ and ⁴T₁ states, and whether the observed ESP will be absorptive or emissive.^35,36 This mechanism predicts that antiferromagnetic chromophore-radical exchange (J_CR <0), with E(²T₁) < E(⁴T₁), results in emissive ESP (Fig. 1A), while ferromagnetic chromophore-radical exchange (J_CR >0), with E(⁴T₁) < E(²T₁), yields absorptive ESP (Fig. 1B).^35,36

Recently, we demonstrated that despite the short ²T₁ excited state lifetime and absence of intermolecular collisions, strong ground state ESP can be generated in (bpy)Pt(CAT-m-Ph-NN) (mPh-Pt, NN = nitronylnitroxide radical).¹⁶ The intensity of the photoinduced ESP in mPh-Pt was also shown to be dramatically altered by changing the metal ion from Pt to Pd in what are otherwise identical complexes.¹⁶ In the present work, we show that the nature of the ground state ESP is also related to the magnitude of the excited state chromophore-radical antiferromagnetic exchange (J_SQ-NN (= 2J_CR) Fig. 2A), which can be controlled by tailored bond torsions that reduce π-system mediated superexchange coupling. Thus the sign of the photogenerated ESP is not determined by changing the sign of J_SQ-NN, as predicted by the RQM.^35,36 Here, changing the magnitude of the excited state exchange affects how the coupled chromophore-radical ⁴T₁–²T₁ states interact with localized states of the radical (e.g., D_NN, Q_NN, vide infra), suggesting a new mechanism for modulating the sign of ground state ESP.

Fig. 2 (A) (Left) ground doublet- (²S₀/D₀) and LL′CT excited state manifold. Vertical doublet ²S₁(D₂) undergoes rapid internal conversion (k_IC) to ²T₁(D₁), which has (bpy)Pt(CAT) triplet character. Nonradiative decay of the ²T₁ state (k_NR) provides the ²S₀ ground state with non-Boltzmann m_s populations. (Right) depictions of ground- (²S₀/D₀), and LL′CT excited states; magnetic exchange coupling parameters, J_SQ-bpy and J_SQ-NN defined; J_SQ-NN is related to J_CR in Fig. 1. (B) Electronic absorption spectra (300 K; CH₂Cl₂), for mPh-Pt (blue line) and 6-Me-mPh-Pt (red line), indicating TREPR photoexcitation energy relative to the ∼17 000 cm^{−1 2}S₀(D₀) → ²S₁(D₂) LL′CT transition

(Bipyridine)Pt^II(catecholate-bridge-radical) complexes

The (bpy)M(CAT-Ph-NN) chromophoric system (Fig. 2A and B)^11,16,17 offers a synthetically flexible platform to explore magnetic exchange contributions to both the sign and magnitude of the photoinduced ground state ESP. Fig. 2B shows the virtually identical solution electronic absorption spectra of mPh-Pt and (bpy)Pt(CAT-6-Me-m-Ph-NN) (6-Me-mPh-Pt), where the characteristic low-energy CAT(HOMO) → bpy(LUMO) ligand-to-ligand charge transfer (LL′CT) transitions are observed at ∼17 000 cm⁻¹.^11,15–17 Interestingly, localized aryl-NN π→π* transitions are also observed in this spectral region,^39,40 but they are obscured in mPh-Pt and 6-Me-mPh-Pt by the more intense LL′CT band.¹⁶ Despite the remarkably similar electronic absorption spectra observed for (bpy)Pt(CAT-bridge-NN) complexes,¹⁷ the nature of the magnetic exchange coupling between spins in the excited states of (bpy)Pt(CAT-bridge-NN) systems can dramatically affect excited state processes when compared to the parent (bpy)Pt(CAT) chromophore that lacks an appended radical.¹¹ This was previously exemplified by the observation of both magnetic circular dichroism (MCD) activity,¹⁷ and a dramatic modulation of excited state lifetimes in (bpy)Pt(CAT-bridge-NN) molecules.¹¹

Optical excitation into the mPh-Pt and 6-Me-mPh-Pt spin-allowed ²S₀ → ²S₁ LL′CT transition leads to a high degree of CAT → bpy charge separation, and a bpy¹CAT¹NN¹ (i.e., a bpy˙SQ˙NN˙ tri-radical; SQ = semiquinone; hereafter “˙” is omitted) electronic configuration that has ²S₁, ²T₁, and ⁴T₁ excited states (Fig. 2A and B).¹⁷ The cross-conjugated meta-phenylene bridges in mPh-Pt and 6-Me-mPh-Pt promote an antiferromagnetic chromophore – radical exchange interaction J_SQ-NN, Fig. 2A,⁴¹ with the primary consequence being that the ²T₁ state resides at lower energy than the high-spin ⁴T₁ quartet excited state (Fig. 1A and 2A). Moreover, the spatial separation of the unpaired spins in the LL′CT state leads to large differences in the pairwise magnetic exchange couplings to the NN radical, which results in mixing of the ²S₁ and ²T₁ states, and ultrafast enhanced intersystem crossing (EISC)^21,22,24 from the ²S₁ to the ²T₁ state. Thus, photoexcitation followed by EISC (k_IC, Fig. 2A) results in ²S₀ → ²S₁ → ²T₁ (Fig. 2A) for both mPh-Pt and 6-Me-mPh-Pt. Fast non-radiative decay (k_NR, Fig. 2A) of the ²T₁ state yields the ²S₀ ground state.^11,16,17

The effect of bond torsions on excited state exchange

The key structural difference between mPh-Pt and 6-Me-mPh-Pt is a single methyl substituent on the meta-phenylene bridge fragment of 6-Me-mPh-Pt that results in increased CAT-bridge bond torsion. As seen in Fig. 2B, the CAT-bridge bond torsion results in modest changes in the LL′CT band by attenuating CAT-bridge delocalization.¹⁰ The increased CAT-bridge torsion reduces the LL′CT excited state SQ-mPh-NN π-overlap relative to that of mPh-Pt.^10,14 This reduction in π-overlap decreases the magnitude of J_SQ-NN in the LL′CT excited state of 6-Me-mPh-Pt so that ΔE_D1Q(6-Me-mPh-Pt) < ΔE_D1Q(mPh-Pt) (Fig. 2A). An experimentally-derived estimate of J_SQ-NN for the LL′CT excited state of 6-Me-mPh-Pt is obtained from J_SQ-NN in the ground state biradical ligand of LZnSQ-6-Me-mPh-NN (Fig. 3). The SQ-6-Me-mPh-NN linkage in LZnSQ-6-Me-mPh-NN is, by analogy, the two-spin donor half of the charge separated LL′CT excited state for 6-Me-mPhPt.

Fig. 3 Variable-temperature paramagnetic susceptibility data for LZn(SQ-6-Me-mPh-NN) (L = hydro-tris(cumenyl, methyl-pyrazolyl)borate), see ESI† for details. The best fit of eqn (2) to the data yields J_SQ-NN = −4 cm⁻¹.

The magnitude of the antiferromagnetic J_SQ-NN mediated by an unsubstituted m-Ph bridge has previously been determined from solid state magnetic susceptibility measurements on LZnSQ-mPh-NN (J_SQ-NN = −32 cm⁻¹).⁴¹ The same Heisenberg-Dirac-Van Vleck spin Hamiltonian (eqn (1)) is used here to describe the exchange split singlet and triplet components of the LZnSQ-6-Me-mPh-NN ground state. The corresponding expression for the magnetic susceptibility is given by eqn (2), which is used to determine the singlet-triplet energy gap (Δ_S–T = −2J_SQ-NN) for LZn(SQ-6-Me-mPh-NN). In eqn (2), x is percentage of the biradical that contributes to the susceptibility, (1 − x) is the mole fraction of the monoradical impurity, and TIP is the temperature-independent paramagnetism.^42,43

The Δ_S–T value that we obtain by fitting eqn (2) to the magnetic susceptibility data (Fig. 3) for LZn(SQ-6-Me-mPh-NN) is Δ_S-T = 8.0 cm⁻¹ (J_SQ-NN = −4.0 cm⁻¹), which corresponds to an 87% decrease in the magnetic exchange coupling relative to LZn(SQ-mPh-NN).^10,44 For conjugated systems, the attenuation of π-type superexchange coupling between SQ and NN is approximated by the product of the cosine squared of bond torsion angles for the SQ-bridge and bridge-NN linkages.^10,44 Based solely on the differences in SQ-bridge-NN torsion angles, this is expected to reduce the magnitude of J_SQ-NN for LZn(SQ-6-Me-mPh-NN) by >99% relative to LZn(SQ-mPh-NN). The larger experimental J_SQ-NN value observed for LZn(SQ-6-Me-mPh-NN), compared to that predicted solely by the bond torsion angles, derives from a combination of weak σ- and configuration interaction contributions to the exchange.^10,41 Critically, the bridge methyl group attenuates antiferromagnetic J_SQ-NN exchange in LZn(SQ-6-Me-mPh-NN) and this translates to a similar attenuation of J_SQ-NN in the LL′CT excited state of 6-Me-mPh-Pt that results in ΔE_D1Q(6-Me-mPh-Pt) < ΔE_D1Q(mPh-Pt) (Fig. 2A).

Excited state exchange determines ground state electron spin polarization

Low-temperature EPR and time-resolved EPR (TREPR) spectra of mPh-Pt and 6-Me-mPh-Pt are presented in Fig. 4A and B, respectively, and these data highlight the dramatic difference in their spin polarized EPR signals at virtual parity of their molecular (but not conformational) structure, ground-state EPR spectra, and LL′CT energy. Note that the direct-detection method used to measure the TREPR spectra is insensitive to static EPR signals and the difference between EPR absorbance before, and after, the laser flash is taken. Thus, a signal is only observed when time-dependent spin polarization is generated. We note that no ESP is generated in (DMSO)₂Pt(CAT-m-Ph-NN), which lacks an LL′CT excited state, indicating that the LL′CT excited state is necessary to observe ground state ESP in these systems.¹⁶

Fig. 4 X-band cw-EPR (A); 20 K; 2-MTHF, and TREPR (B); 20 K; 2-MTHF spectra for mPh-Pt (blue lines) and 6-Me-mPh-Pt (red lines). (A) Field modulation detected cw-EPR spectra of the ground states without light excitation. (B) Light-induced, spin-polarized TREPR spectra, extracted from time/field datasets 4 μs after the laser flash. Light excitation at 532 nm, concentrations ∼0.5 mM. Abs = absorption, Em = emission.

Remarkably, and contrary to the predictions of the RQM, we observe emissive ESP in the recovered ground state of 6-Me-mPh-Pt and absorptive ESP in the recovered ground state of mPh-Pt. Thus, the non-Boltzmann population distribution within the ²S₀m_s = ±1/2 sublevels generated by the laser-induced excitation/relaxation photocycle (Fig. 2A and 5) differs markedly for these two complexes. The differences in the nature of the ESP for mPh-Pt and 6-Me-mPh-Pt correlate with the bridge-dependent energy gaps between ²T₁ and the other states (⁴T₁, ²NN, ⁴NN) in this energy region, which are affected by CAT/SQ-bridge bond torsions. Since the degree of charge separation in the excited state manifolds of mPh-Pt and 6-Me-mPh-Pt is near unity,¹³ the J_SQ-NN value for LZn(SQ-6-Me-mPh-NN) can be used as a proxy for J_SQ-NN in the excited state of 6-Me-mPh-Pt. The J_SQ-NN exchange interaction that we obtain for LZn(SQ-6-Me-mPh-NN) is related to the ²T₁–⁴T₁ energy gap (ΔE_D1Q) in the three-spin excited states of mPh-Pt and 6-Me-mPh-Pt according to ΔE_D1Q = 1.5 J_SQ-NN when J_bpy˙_-SQ ≫ J_SQ-NN.¹⁷ Thus, the ²T₁–⁴T₁ energy gaps (ΔE_D1Q) are 48 cm⁻¹ for mPh-Pt and 6 cm⁻¹ for 6-Me-mPh-Pt, respectively.

Fig. 5 (A) The reversed quartet mechanism (RQM). Mixing of chromophoric ²T₁ and ⁴T₁ states, mediated by conformational J_SQ-NN-modulation, gives rise to emissive ESP for 6-Me-mPh-Pt. (B) Spin–orbit induced mixing of chromophoric ²T₁ and the NN-based quartet state, ⁴NN, gives rise to absorptive ESP for mPh-Pt. The ²NN state can be observed spectroscopically (see ESI†).

These ΔE_D1Q values indicate that <6% of the mPh-Pt⁴T₁ state would be populated at thermal equilibrium at 20 K (the temperature at which the EPR spectra were recorded), but this increases dramatically to ∼56% ⁴T₁ population for 6-Me-mPh-Pt at the same temperature. These populations are significant because the RQM (Fig. 1A and 4) requires thermal population of the ⁴T₁(Q) state and spin–orbit coupling (SOC) induced zero-field mixing between the ²T₁–⁴T₁ states to observe ground- and excited state ESP in mPh-Pt and 6-Me-mPh-Pt. SOC induced interconversion rates between Zeeman split ²T₁ and ⁴T₁ states are given by the rate constants k_DQ and k_QD (Fig. 5), with k_DQ and k_QD being related by the equilibrium Boltzmann populations between the two levels (i.e.).^35,36 Given that J_SQ-NN is antiferromagnetic in 6-Me-mPh-Pt, the RQM correctly predicts emissive ground state ESP due to E(⁴T₁) > E(²T₁), ²T₁–⁴T₁ SOC state mixing, and k_QD > k_DQ.^35,36 Since mPh-Pt displays absorptive ground state ESP and a significantly larger ²T₁(D₁)–⁴T₁(Q) energy gap, a different ESP mechanism must be operational for this complex.

Recently, we described how localized NN radical ²NN(D_NN) and ⁴NN(Q_NN) states (Fig. 4B) may be nearly isoenergetic with the ²T₁ state of mPh-Pt.¹⁶ We propose that the dynamic population of the ⁴NN state effectively competes with the higher energy ⁴T₁ state to determine the sign of the ground state ESP. This competition is readily apparent in mPh-Pt and results from the larger value of J_SQ-NN for this complex, which leads to a larger ΔE_D1Q energy gap, a more stabilized ²T₁ state, and ΔE_D1Q > ΔE_D1-QNN between chromophoric ²T₁ and Q_NN states (Fig. 5). The inclusion of additional states in the context of the RQM is key, as it correctly predicts the experimentally observed absorptive ESP via²T₁ ↔ Q_NN mixing for mPh-Pt. Thus, the magnitude of the antiferromagnetic excited state exchange interaction, J_SQ-NN, controls how the ²T₁ state mixes with neighboring excited states to switch the sign of the ground state ESP. This observation highlights the effectiveness of TREPR spectroscopy for revealing details of fast non-radiative processes that are difficult to detect using other spectroscopic methods. In the present case, the sign of the ground state ESP reveals how the small energy gap between charge transfer and localized excited states, coupled with the relative magnitudes of interstate mixing, critically alters the spin selectivity of the electronic relaxation.

Conclusions

In summary, we observe dramatic differences in ground state ESP for mPh-Pt and 6-Me-mPh-Pt that are attributed to the energies of their ²T₁ states relative to chromophoric ⁴T₁(Q) states and a localized radical-based Q_NN state (Fig. 6). For 6-Me-mPh-Pt, an LL′CT excited state SQ-bridge bond torsion effectively attenuates the magnitude of the excited state J_SQ-NN antiferromagnetic exchange interaction, which leads to ΔE_D1Q < ΔE_D1QNN and emissive ESP as described by the RQM (Fig. 1A) and observed experimentally (Fig. 4B). Conversely, the comparatively planar CAT-mPh-NN π-system of mPh-Pt results in ΔE_D1QNN < ΔE_D1Q, and absorptive ESP is both predicted and observed (Fig. 1B and 4B, respectively). Thus, small conformational changes along low-frequency torsional modes of donor–acceptor molecules that have been elaborated with pendant persistent radicals can lead to conformational J-modulation, and this affects the sign and magnitude of the ground state ESP. This indicates that the sign of the excited state chromophore-radical exchange predicted by the RQM is not universal, and our work points to a previously unobserved change in ground state ESP as a function of the magnitude of the excited state exchange interaction, J_SQ-NN. The present results highlight the extreme importance of understanding how excited state electronic structure affects excited state properties and dynamics, and demonstrates additional mechanisms to control ESP relevant to QIS. Ongoing research efforts focus on further exploration of synthetic design principles directed toward determining the electronic and geometric structure factors that control optically-generated ESP in these and related radical-elaborated donor–acceptor chromophores, and how these can be exploited for QIS applications.

Fig. 6 Origins of emissive and absorptive ESP for NN-elaborated (bpy)Pt(CAT) complexes. Left: RQM with emissive ESP is observed. Right: RQM with absorptive ESP is observed.

Experimental

General considerations

Reagents and solvents were used as received and purchased from commercial vendors. See ESI† for compound syntheses and characterization.

Magnetic susceptibility measurements

Magnetic susceptibility measurements were collected on a Quantum Design MPMS-XL7 SQUID magnetometer. Variable temperature magnetometry experiments were performed with a constant field of 0.7 T on a microcrystalline sample (∼15 mg) which was loaded into a gelcap/straw sample holder and mounted to the sample rod with Kapton tape. Collected raw data was initially corrected with a straight line for diamagnetic response of sample and container as a first approximation, where the slope of the line represents the residual diamagnetic correction.

EPR and TREPR spectroscopies

Samples for steady state and transient EPR measurements were prepared by dissolving solid (bpy)Pt(CAT-mPh-NN), (mPh-Pt) or (bpy)Pt(CAT-6-Me-mPh-NN), (6-Me-mPh-Pt) in 2-methylTHF to a concentration of ∼0.5 mM. The samples were placed in 4 mm O.D. quartz EPR tubes and degassed by repeated freeze-pump-thaw cycles. The frozen, degassed samples were then transferred without thawing from liquid nitrogen to the spectrometer cryostat, which was at 20 K. The steady state spectra were collected using field modulation detection with a modulation amplitude of 0.1 mT and a microwave power of 6.3 mW. TREPR time/field dataset were collected with direct detection at the microwave power as for the steady state experiments. The samples were irradiated at 532 nm using 10 ns laser flashes from a frequency doubled NdYAG laser. The modified Bruker EPR 200D-SRC X-band spectrometer used for the EPR experiments has been described in detail elsewhere.⁴⁵

Data availability

The datasets supporting this article have been uploaded as part of the ESI.†

Author contributions

M. L. K. supervised the research, vetted the data, and wrote the manuscript. D. A. S. supervised the research, vetted the data, and wrote the manuscript. P. H. synthesized and characterized the compounds. D. E. S. synthesized and characterized the compounds. J .C. performed electronic structure computations. A. v. d. E. supervised the research, vetted the data, and wrote the manuscript.

Conflicts of interest

No competing financial interests have been declared.

Acknowledgements

M. L. K. and D. A. S. acknowledge financial support from DOE (DE-SC0020199). A. v. d. E. acknowledges support from NSERC (Discovery grant 2015-04021).

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Footnotes

† Electronic supplementary information (ESI) available: Synthesis and characterization of 6-Me-mPh-Pt and related complexes, X-ray crystallographic analysis for LZn(SQ-6-Me-mPh-NN). CCDC 2087657. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/d1sc02965g

‡ These authors contributed equally.

§ Present address: Cree, RTP, Durham, NC.

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