Influence of Radical Bridges on Electron Spin Coupling

Increasing interactions between spin centers in molecules and molecular materials is a desirable goal for applications such as single-molecule magnets for information storage or magnetic metal–organic frameworks for adsorptive separation and targeted drug delivery and release. To maximize these interactions, introducing unpaired spins on bridging ligands is a concept used in several areas where such interactions are otherwise quite weak, in particular, lanthanide-based molecular magnets and magnetic metal–organic frameworks. Here, we use Kohn–Sham density functional theory to study how much the ground spin state is stabilized relative to other low-lying spin states by creating an additional spin center on the bridge for a series of simple model compounds. The di- and triradical structures consist of nitronyl nitroxide (NNO) and semiquinone (SQ) radicals attached to a <i>meta</i>-phenylene­(R) bridge (where R = −NH^•/–NH₂, −O^•/OH, −CH₂^•/CH₂). These model compounds are based on a fully characterized SQ–<i>meta</i>-phenylene–NNO diradical with moderately strong antiferromagnetic coupling. Replacing closed-shell substituents CH₃ and NH₂ with their radical counterparts CH₂^• and NH^• leads to an increase in stabilization of the ground state with respect to other low-lying spin states by a factor of 3–6, depending on the exchange–correlation functional. For OH compared with O^• substituents, no conclusions can be drawn as the spin state energetics depend strongly on the functional. This could provide a basis for constructing sensitive test systems for benchmarking theoretical methods for spin state energy splittings. Reassuringly, the stabilization found for a potentially synthesizable complex (up to a factor of 3.5) is in line with the simple model systems (where a stabilization of up to a factor of 6.2 was found). Absolute spin state energy splittings are considerably smaller for the potentially stable system than those for the model complexes, which points to a dependence on the spin delocalization from the radical substituent on the bridge

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

A new donor–acceptor biradical complex, TpCum,MeZn(SQ-VD) (TpCum,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 JSQ‑VD = +77 cm–1 (H = −2JSQ‑VDŜSQ·ŜVD) determined from an analysis of the variable-temperature magnetic susceptibility data. The pairwise biradical exchange interaction in TpCum,MeZn(SQ-VD) can be compared with that of the related donor–acceptor biradical complex TpCum,MeZn(SQ-NN) (NN = nitronyl nitroxide, S = 1/2), where JSQ‑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 TpCum,MeZn(SQ-VD) and TpCum,MeZn(SQ-NN). A Valence Bond Configuration Interaction (VBCI) model was previously employed to highlight the importance of mixing an SQSOMO → NNLUMO charge transfer configuration into the electronic ground state to facilitate the stabilization of the high-spin triplet (S = 1) ground state in TpCum,MeZn(SQ-NN). Here, CASSCF computations confirm the importance of mixing the pendant radical (e.g., VD, NN) LUMO (VDLUMO and NNLUMO) with the SOMO of the SQ radical (SQSOMO) for stabilizing the triplet, in addition to spin polarization and charge transfer contributions to the exchange. An important electronic structure difference between TpCum,MeZn(SQ-VD) and TpCum,MeZn(SQ-NN), which leads to their different exchange couplings, is the reduced admixture of excited states that promote ferromagnetic exchange into the TpCum,MeZn(SQ-VD) ground state, and the intrinsically weaker mixing between the VDLUMO and the SQSOMO compared to that observed for TpCum,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 Magnetic Exchange Interactions Enable Large Spin Polarization Effects

Excited state processes involving multiple electron spin centers are crucial elements for both spintronics and quantum information processing. Herein, we describe an addressable excited state mechanism for precise control of electron spin polarization. This mechanism derives from excited state magnetic exchange couplings that occur between the electron spins of a photogenerated electron–hole pair and that of an organic radical. The process is initiated by absorption of a photon followed by ultrafast relaxation within the excited state spin manifold. This leads to dramatic changes in spin polarization between excited states of the same multiplicity. Moreover, this photoinitiated spin polarization process can be “read” spectroscopically using a magnetooptical technique that is sensitive to the excited state electron spin polarizations and allows for the evaluation of wave functions that give rise to these polarizations. This system is unique in that it requires neither intersystem crossing nor magnetic resonance techniques to create dynamic spin-polarization effects in molecules

Synthesis, Characterization, and Photophysical Studies of an Iron(III) Catecholate–Nitronylnitroxide Spin-Crossover Complex

The synthesis and characterization of an Fe^III catecholate–nitronylnitroxide (CAT-NN) complex (<b>1-NN</b>) that undergoes Fe^III spin-crossover is described. Our aim is to determine whether the intraligand exchange coupling of the semiquinone–nitronylnitroxide Fe^II(SQ-NN) excited state resulting from irradiation of the CAT → Fe^III LMCT band would affect either the intrinsic photophysics or the iron spin-crossover event when compared to the complex lacking the nitronylnitroxide radical (<b>1</b>). X-ray crystallographic analysis provides bond lengths consistent with a ferric catecholate charge distribution. Mössbauer spectroscopy clearly demonstrates Fe^III spin-crossover, hyperfine couplings, and a weak ferromagnetic Fe^III–CAT-NN exchange, and spin-crossover is corroborated by variable-temperature magnetic susceptibility and electronic absorption studies. To explore the effect of the NN radical on photophysical processes, we conducted room-temperature transient absorption experiments. Upon excitation of the ligand-to-metal charge transfer band, an Fe^IISQ state is populated and most likely undergoes fast intersystem crossing to the ligand field manifold, where it rapidly decays into a metastable low-spin Fe^IIICAT state, followed by repopulation of the high-spin Fe^IIICAT ground state. The decay components of <b>1-NN</b> are slightly faster than those obtained for <b>1</b>, perhaps due to the higher number of microstates present within the LMCT and LF manifolds for <b>1-NN</b>. Although the effects of the NN radical are manifest in neither the spin-crossover nor the photophysics, our results lay the groundwork for future studies

Spectroscopic Studies of Bridge Contributions to Electronic Coupling in a Donor-Bridge-Acceptor Biradical System

Variable-temperature electronic absorption and resonance Raman spectroscopies are used to probe the excited state electronic structure of Tp^Cum,MeZn­(SQ-<i>Ph</i>-NN) (<b>1</b>), a donor-bridge-acceptor (D-B-A) biradical complex and a ground state analogue of the charge-separated excited state formed in photoinduced electron transfer reactions. Strong electronic coupling mediated by the <i>p</i>-phenylene bridge stabilizes the triplet ground state of this molecule. Detailed spectroscopic and bonding calculations elucidate key bridge distortions that are involved in the SQ­(π)_SOMO → NN-Ph (π*)_LUMO D → A charge transfer (CT) transition. We show that the primary excited state distortion that accompanies this CT is along a vibrational coordinate best described as a symmetric Ph­(8a) + SQ­(in-plane) linear combination and underscores the dominant role of the phenylene bridge fragment acting as an electron acceptor in the D-B-A charge transfer state. Our results show the importance of the phenylene bridge in promoting (1) electron transfer in D-Ph-A systems and (2) electron transport in biased electrode devices that employ a 1,4-phenylene linkage. We have also developed a relationship between the spin density on the acceptor, as measured via the isotropic NN nitrogen hyperfine interaction, and the strength of the D → A interaction given by the magnitude of the electronic coupling matrix element, <i>H</i>_<i>ab</i>

Electronic and Exchange Coupling in a Cross-Conjugated D–B–A Biradical: Mechanistic Implications for Quantum Interference Effects

A combination of variable-temperature EPR spectroscopy, electronic absorption spectroscopy, and magnetic susceptibility measurements have been performed on Tp^Cum,MeZn­(SQ-<i>m-</i>Ph-NN) (<b>1-meta</b>) a donor–bridge–acceptor (D–B–A) biradical that possesses a cross-conjugated <i>meta</i>-phenylene (<i>m-</i>Ph) bridge and a spin singlet ground state. The experimental results have been interpreted in the context of detailed bonding and excited-state computations in order to understand the excited-state electronic structure of <b>1-meta</b>. The results reveal important excited-state contributions to the ground-state singlet–triplet splitting in this cross-conjugated D–B–A biradical that contribute to our understanding of electronic coupling in cross-conjugated molecules and specifically to quantum interference effects. In contrast to the conjugated isomer, which is a D–B–A biradical possessing a <i>para</i>-phenylene bridge, admixture of a single low-lying singly excited D → A type configuration into the cross-conjugated D–B–A biradical ground state makes a negligible contribution to the ground-state magnetic exchange interaction. Instead, an excited state formed by a Ph-NN (HOMO) → Ph-NN (LUMO) one-electron promotion configurationally mixes into the ground state of the <i>m-</i>Ph bridged D–A biradical. This results in a double (dynamic) spin polarization mechanism as the dominant contributor to ground-state antiferromagnetic exchange coupling between the SQ and NN spins. Thus, the dominant exchange mechanism is one that activates the bridge moiety via the spin polarization of a doubly occupied orbital with phenylene bridge character. This mechanism is important, as it enhances the electronic and magnetic communication in cross-conjugated D–B–A molecules where, in the case of <b>1-meta</b>, the magnetic exchange in the active electron approximation is expected to be <i>J</i> ∼ 0 cm^–1. We hypothesize that similar superexchange mechanisms are common to all cross-conjugated D–B–A triads. Our results are compared to quantum interference effects on electron transfer/transport when cross-conjugated molecules are employed as the bridge or molecular wire component and suggest a mechanism by which electronic coupling (and therefore electron transfer/transport) can be modulated

Modification of Molecular Spin Crossover in Ultrathin Films

Scanning tunneling microscopy and local conductance mapping show spin-state coexistence in bilayer films of Fe­[(H₂Bpz₂)₂bpy] on Au(111) that is independent of temperature between 131 and 300 K. This modification of bulk behavior is attributed in part to the unique packing constraints of the bilayer film that promote deviations from bulk behavior. The local density of states measured for different spin states shows that high-spin molecules have a smaller transport gap than low-spin molecules and are in agreement with density functional theory calculations

Determining the Conformational Landscape of σ and π Coupling Using <i>para</i>-Phenylene and “Aviram–Ratner” Bridges

The torsional dependence of donor–bridge–acceptor (D–B–A) electronic coupling matrix elements (<i><b>H</b></i>_{<i><b>DA</b></i>}, determined from the magnetic exchange coupling, <i><b>J</b></i>) involving a spin S_D = 1/2 metal semiquinone (Zn-<b>SQ</b>) donor and a spin S_A = 1/2 nitronylnitroxide (<b>NN</b>) acceptor mediated by the σ/π-systems of <i>para</i>-phenylene and methyl-substituted <i>para</i>-phenylene bridges and by the σ-system of a bicyclo[2.2.2]­octane (<b>BCO</b>) bridge are presented and discussed. The positions of methyl group(s) on the phenylene bridge allow for an experimentally determined evaluation of conformationally dependent (π) and conformationally independent (σ) contributions to the electronic and magnetic exchange couplings in these D–B–A biradicals at parity of D and A. The trend in the experimental magnetic exchange couplings are well described by CASSCF calculations. The torsional dependence of the pairwise exchange interactions are further illuminated in three-dimensional, “Ramachandran-type” plots that relate D–B and B–A torsions to both electronic and exchange couplings. Analysis of the magnetic data shows large variations in magnetic exchange (<i><b>J</b></i> ≈ 1–175 cm^–1) and electronic coupling (<i><b>H</b></i>_{<i><b>DA</b></i>} ≈ 450–6000 cm^–1) as a function of bridge conformation relative to the donor and acceptor. This has allowed for an experimental determination of both the σ- and π-orbital contributions to the exchange and electronic couplings

Superexchange Contributions to Distance Dependence of Electron Transfer/Transport: Exchange and Electronic Coupling in Oligo(<i>para</i>-Phenylene)- and Oligo(2,5-Thiophene)-Bridged Donor–Bridge–Acceptor Biradical Complexes

The preparation and characterization of three new donor–bridge–acceptor biradical complexes are described. Using variable-temperature magnetic susceptibility, EPR hyperfine coupling constants, and the results of X-ray crystal structures, we evaluate both exchange and electronic couplings as a function of bridge length for two quintessential molecular bridges: oligo­(<i>para</i>-phenylene), β = 0.39 Å^–1 and oligo­(2,5-thiophene), β = 0.22 Å^–1. This report represents the first direct comparison of exchange/electronic couplings and distance attenuation parameters (β) for these bridges. The work provides a direct measurement of superexchange contributions to β, with no contribution from incoherent hopping. The different β values determined for oligo­(<i>para</i>-phenylene) and oligo­(2,5-thiophene) are due primarily to the D–B energy gap, <b>Δ</b>, rather than bridge–bridge electronic couplings, <b>H</b>_<b>BB</b>. This is supported by the fact that the <b>H</b>_<b>BB</b> values extracted from the experimental data for oligo­(<i>para</i>-phenylene) (<b>H</b>_<b>BB</b> = 11 400 cm^–1) and oligo­(2,5-thiophene) (12 300 cm^–1) differ by <10%. The results presented here offer unique insight into the intrinsic molecular factors that govern <b>H</b>_<b>DA</b> and β, which are important for understanding the electronic origin of electron transfer and electron transport mediated by molecular bridges

Superexchange Contributions to Distance Dependence of Electron Transfer/Transport: Exchange and Electronic Coupling in Oligo(<i>para</i>-Phenylene)- and Oligo(2,5-Thiophene)-Bridged Donor–Bridge–Acceptor Biradical Complexes

The preparation and characterization of three new donor–bridge–acceptor biradical complexes are described. Using variable-temperature magnetic susceptibility, EPR hyperfine coupling constants, and the results of X-ray crystal structures, we evaluate both exchange and electronic couplings as a function of bridge length for two quintessential molecular bridges: oligo­(<i>para</i>-phenylene), β = 0.39 Å^–1 and oligo­(2,5-thiophene), β = 0.22 Å^–1. This report represents the first direct comparison of exchange/electronic couplings and distance attenuation parameters (β) for these bridges. The work provides a direct measurement of superexchange contributions to β, with no contribution from incoherent hopping. The different β values determined for oligo­(<i>para</i>-phenylene) and oligo­(2,5-thiophene) are due primarily to the D–B energy gap, <b>Δ</b>, rather than bridge–bridge electronic couplings, <b>H</b>_<b>BB</b>. This is supported by the fact that the <b>H</b>_<b>BB</b> values extracted from the experimental data for oligo­(<i>para</i>-phenylene) (<b>H</b>_<b>BB</b> = 11 400 cm^–1) and oligo­(2,5-thiophene) (12 300 cm^–1) differ by <10%. The results presented here offer unique insight into the intrinsic molecular factors that govern <b>H</b>_<b>DA</b> and β, which are important for understanding the electronic origin of electron transfer and electron transport mediated by molecular bridges