Long-range spin dependent delocalization promoted by the pseudo Jahn-Teller effect

Origin of Ferromagnetic Exchange Coupling in Donor-Acceptor Biradical Analogues of Charge-Separated Excited States

A new donor-acceptor biradical complex, Tp^Cum,MeZn(SQ-VD) (Tp^Cum,MeZn⁺ = zinc(II) hydro-tris(3-cumenyl-5-methylpyrazolyl)borate complex cation; SQ = orthosemiquinone; VD = oxoverdazyl), which is a ground-state analogue of a charge-separated excited state, has been synthesized and structurally characterized. The magnetic exchange interaction between the S = 1/2 SQ and the S = 1/2 VD within the SQ-VD biradical ligand is observed to be ferromagnetic, with J_SQ-VD = +77 cm^-1 (H = -2J_SQ-VDŜ_SQ·Ŝ_VD) determined from an analysis of the variable-temperature magnetic susceptibility data. The pairwise biradical exchange interaction in Tp^Cum,MeZn(SQ-VD) can be compared with that of the related donor-acceptor biradical complex Tp^Cum,MeZn(SQ-NN) (NN = nitronyl nitroxide, S = 1/2), where J_SQ-NN ≅ +550 cm^-1. This represents a dramatic reduction in the biradical exchange by a factor of ∼7, despite the isolobal nature of the VD and NN acceptor radical SOMOs. Computations assessing the magnitude of the exchange were performed using a broken-symmetry density functional theory (DFT) approach. These computations are in good agreement with those computed at the CASSCF NEVPT2 level, which also reveals an S = 1 triplet ground state as observed in the magnetic susceptibility measurements. A combination of electronic absorption spectroscopy and CASSCF computations has been used to elucidate the electronic origin of the large difference in the magnitude of the biradical exchange coupling between Tp^Cum,MeZn(SQ-VD) and Tp^Cum,MeZn(SQ-NN). A Valence Bond Configuration Interaction (VBCI) model was previously employed to highlight the importance of mixing an SQ_SOMO → NN_LUMO charge transfer configuration into the electronic ground state to facilitate the stabilization of the high-spin triplet (S = 1) ground state in Tp^Cum,MeZn(SQ-NN). Here, CASSCF computations confirm the importance of mixing the pendant radical (e.g., VD, NN) LUMO (VD_LUMO and NN_LUMO) with the SOMO of the SQ radical (SQ_SOMO) for stabilizing the triplet, in addition to spin polarization and charge transfer contributions to the exchange. An important electronic structure difference between Tp^Cum,MeZn(SQ-VD) and Tp^Cum,MeZn(SQ-NN), which leads to their different exchange couplings, is the reduced admixture of excited states that promote ferromagnetic exchange into the Tp^Cum,MeZn(SQ-VD) ground state, and the intrinsically weaker mixing between the VD_LUMO and the SQ_SOMO compared to that observed for Tp^Cum,MeZn(SQ-NN), where this orbital mixing is significant. The results of this comparative study contribute to a greater understanding of biradical exchange interactions, which are important to our understanding of excited-state singlet-triplet energy gaps, electron delocalization, and the generation of electron spin polarization in both the ground and excited states of (bpy)Pt(CAT-radical) complexes.

Excited State Magneto-Structural Correlations Related to Photoinduced Electron Spin Polarization

Photoinduced electron spin polarization (ESP) is reported in the ground state of a series of complexes consisting of an organic radical (nitronylnitroxide, NN) covalently attached to a donor-acceptor chromophore either directly or via para-phenylene bridges substituted with 0-4 methyl groups. These molecules represent a class of chromophores that undergo visible light excitation to produce an initial exchange-coupled, three-spin [bpy^•-, CAT^•+ (= semiquinone, SQ) and NN^•], charge-separated doublet ²S₁ (S = chromophore spin singlet configuration) excited state that rapidly decays by magnetic exchange-enhanced internal conversion to a ²T₁ (T = chromophore excited spin triplet configuration) state. The ²T₁ state equilibrates with chromophoric and NN radical-derived excited states, resulting in absorptive ESP of the recovered ground state, which persists for greater than a millisecond and can be measured by low-temperature time-resolved electron paramagnetic resonance spectroscopy. The magnitude of the ground state ESP is found to correlate with the excited state magnetic exchange interaction between the CAT^+• and NN^• radicals, which in turn is controlled by the structure of the bridge fragment.

Excited State Exchange Control of Photoinduced Electron Spin Polarization in Electronic Ground States

Ground-state electron spin polarization (ESP) is generated in radical elaborated (bpy)Pt(CAT-NN) and (bpy)Pt(CAT-p-Me₂PhMe₂-NN) (bpy = 5,5'-di-tert-butyl-2,2'-bipyridine, CAT = 3-tert-butylcatecholate, p-Ph = para-phenylene, NN = nitronylnitroxide). Photoexcitation produces an exchange-coupled, three-spin, charge-separated doublet ²S₁ (S = chromophore excited spin singlet configuration) excited state that rapidly decays to a ²T₁ (T = chromophore excited spin triplet configuration) excited state. The SQ-bridge-NN bond torsions affect the magnitude of the excited state exchange interaction (J_SQ-NN), which determines the ²T₁-⁴T₁ energy gap. Ground state ESP is dependent on the magnitude of J_SQ-NN, and we postulate that this results from differences in ²T₁ and ⁴T₁ state mixing. Mechanisms that lead to the rapid transfer of the excited state ESP to the ground state are discussed. Although subnanosecond ²T₁ state lifetimes are measured optically in solution, the ground state ESP decays very slowly at 20 K and is observable for more than a millisecond.

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

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.

Metal Ion Control of Photoinduced Electron Spin Polarization in Electronic Ground States

Both the sign and intensity of photoinduced electron spin polarization (ESP) in the electronic ground state doublet (²S₀/D₀) of chromophore-radical complexes can be controlled by changing the nature of the metal ion. The complexes consist of an organic radical (nitronyl nitroxide, NN) covalently attached to a donor-acceptor chromophore via a m-phenylene bridge, (bpy)M(CAT-m-Ph-NN) (1) (bpy = 4,4'-di-tert-butyl-2,2'-bipyridine, M = Pd^II (1-Pd) or Pt^II (1-Pt), CAT = 3-tert-butylcatecholate, m-Ph = meta-phenylene). In both complexes, photoexcitation with visible light produces an initial exchange-coupled, three-spin (bpy^•-, CAT^•+ = semiquinone (SQ), and NN^•), charge-separated doublet ²S₁ (S = chromophore excited spin singlet configuration) excited state that rapidly decays to the ground state via a ²T₁ (T = chromophore excited spin triplet configuration) state. This process is not expected to be spin selective, and only very weak emissive ESP is found for 1-Pd. In contrast, strong absorptive ESP is generated in 1-Pt. It is postulated that zero-field-splitting-induced transitions between the chromophoric ²T₁ and ⁴T₁ states (1-Pd and 1-Pt) and spin-orbit-induced transitions between ²T₁ and NN-based quartet states (1-Pt) account for the differences in polarization.

Delocalization tunable by ligand substitution in [L2Al]n− complexes highlights a mechanism for strong electronic coupling

Organo-aluminum mixed-valent complexes combine properties of both organic and transition element mixed-valent compounds. This supports delocalized electronic structures that are structurally and electronically tunable.