Pyranopterin Dithiolene Distortions Relevant to Electron Transfer in Xanthine Oxidase/Dehydrogenase

The reducing substrates 4-thiolumazine and 2,4-dithiolumazine have been used to form Mo^IV-product complexes with xanthine oxidase (XO) and xanthine dehydrogenase. These Mo^IV-product complexes display an intense metal-to-ligand charge-transfer (MLCT) band in the near-infrared region of the spectrum. Optical pumping into this MLCT band yields resonance Raman spectra of the Mo site that are devoid of contributions from the highly absorbing FAD and 2Fe2S clusters in the protein. The resonance Raman spectra reveal in-plane bending modes of the bound product and low-frequency molybdenum dithiolene and pyranopterin dithiolene vibrational modes. This work provides keen insight into the role of the pyranopterin dithiolene in electron-transfer reactivity

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

Large Ligand Folding Distortion in an Oxomolybdenum Donor–Acceptor Complex

Interligand charge transfer is examined in the novel metallo-dithiolene complex MoO­(SPh)₂(^<i>i</i>Pr₂Dt⁰) (where ^<i>i</i>Pr₂Dt⁰ = <i>N</i>,<i>N</i>′-isopropyl-piperazine-2,3-dithione). The title complex displays a remarkable 70° “envelope”-type fold of the five-membered dithiolene ring, which is bent upward toward the terminal oxo ligand. A combination of electronic absorption and resonance Raman spectroscopies have been used to probe the basic electronic structure responsible for the large fold-angle distortion. The intense charge transfer transition observed at ∼18 000 cm^–1 is assigned as a thiolate → dithione ligand-to-ligand charge transfer (LL′CT) transition that also possesses Mo­(IV) → dithione charge transfer character. Strong orbital mixing between occupied and virtual orbitals with Mo­(<i>x</i>²–<i>y</i>²) orbital character is derived from a strong pseudo Jahn–Teller effect, which drives the large fold-angle distortion to yield a double-well potential in the electronic ground state

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

Vibrational Probes of Molybdenum Cofactor–Protein Interactions in Xanthine Dehydrogenase

The pyranopterin dithiolene (PDT) ligand is an integral component of the molybdenum cofactor (Moco) found in all molybdoenzymes with the sole exception of nitrogenase. However, the roles of the PDT in catalysis are still unknown. The PDT is believed to be bound to the proteins by an extensive hydrogen-bonding network, and it has been suggested that these interactions may function to fine-tune Moco for electron- and atom-transfer reactivity in catalysis. Here, we use resonance Raman (rR) spectroscopy to probe Moco–protein interactions using heavy-atom congeners of lumazine, molecules that bind tightly to both wild-type xanthine dehydrogenase (wt-XDH) and its Q102G and Q197A variants following enzymatic hydroxylation to the corresponding violapterin product molecules. The resulting enzyme–product complexes possess intense near-IR absorption, allowing high-quality rR spectra to be collected on wt-XDH and the Q102G and Q197A variants. Small negative frequency shifts relative to wt-XDH are observed for the low-frequency Moco vibrations. These results are interpreted in the context of weak hydrogen-bonding and/or electrostatic interactions between Q102 and the −NH₂ terminus of the PDT, and between Q197 and the terminal oxo of the MoO group. The Q102A, Q102G, Q197A, and Q197E variants do not appreciably affect the kinetic parameters <i>k</i>_red and <i>k</i>_red/<i>K</i>_D, indicating that a primary role for these glutamine residues is to stabilize and coordinate Moco in the active site of XO family enzymes but to not directly affect the catalytic throughput. Raman frequency shifts between wt-XDH and its Q102G variant suggest that the changes in the electron density at the Mo ion that accompany Mo oxidation during electron-transfer regeneration of the catalytically competent active site are manifest in distortions at the distant PDT amino terminus. This implies a primary role for the PDT as a conduit for facilitating enzymatic electron-transfer reactivity in xanthine oxidase family enzymes

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

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>

Charge Transfer Doping Induced Conformational Ordering of a Non-Crystalline Conjugated Polymer

Charge transfer doping of a nominally disordered conjugated polymer induces long-range conformational ordering (stiffening) of backbone segments. Addition of [2,3-dichloro-5,6-dicyano-<i>p</i>-benzoquinone (DDQ) to dilute solutions of poly­[2-methoxy-5-(3′,7′-dimethyloctyloxy)-1,4-phenylenevinylene] (MDMO-PPV) results in quantitative charge transfer in the ground electronic state of the polymer. Following charge (hole) injection, greater MDMO-PPV monomer coplanarity is evident in electronic, Raman, and electron paramagnetic resonance (EPR) spectra over a broad range of dopant loadings. New transitions emerge at lower energies with spectral patterns distinct from pristine materials but closely resemble minority low energy conformers selectively that can be prepared by careful control of processing conditions. We further demonstrate that characteristic Raman patterns of PPV systems actually contain signatures of a minority ordered form that interacts preferentially with the dopant. Subsequent additions of dopant also show that most chains convert into the low energy form. This notion is consistent with greater backbone planarity and, hence, lower torsional reorganization energies required to access the cation form. Preresonant excitation of the emergent red-shifted optical transition reveals long overtone-combination progressions due to enhanced electronic delocalization along planarized backbone segments and diminished coupling the surroundings. We propose that planarity enhancements from doping also lead to the eventual formation of spinless bipolarons, evident from EPR spectra. Facile charge transfer doping induced conversion into the ordered MDMO-PPV conformer suggests that better control of polymer conformations and carrier levels could be harnessed to improve charge and energy transport efficiency in optoelectronic devices

Single Molecule Magnet Behavior of a Pentanuclear Mn-Based Metallacrown Complex: Solid State and Solution Magnetic Studies

The magnetic behavior of the pentanuclear complex of formula Mn^II(O₂CCH₃)₂[12-MC_Mn^III(N)shi-4](DMF)₆, <b>1</b>, was investigated using magnetization and magnetic susceptibility measurements both in the solid state and in solution. Complex <b>1</b> has a nearly planar structure, made of a central Mn^II ion surrounded by four peripheral Mn^III ions. Solid state variable-field dc magnetic susceptibility experiments demonstrate that <b>1</b> possesses a low value for the total spin in the ground state; fitting appropriate expressions to the data results in antiferromangetic coupling both between the peripheral Mn^III ions (<i>J</i> = −6.3 cm^–1) and between the central Mn^II ion and the Mn^III ones (<i>J</i>′ = −4.2 cm^–1). In order to obtain a reasonable fit, a relatively large single ion magnetic anisotropy (<i>D</i>) value of 1 cm^–1 was necessary for the central Mn^II ion. The single crystal magnetization measurements using a microsquid array display a very slight opening of the hysteresis loop but only at a very low temperature (0.04 K), which is in line with the ac susceptibility data where a slow relaxation of the magnetization occurs just around 2 K. In frozen solution, complex <b>1</b> displays a frequency dependent ac magnetic susceptibility signal with an energy barrier to magnetization reorientation (<i>E</i>) and relaxation time at an infinite temperature (τ_o) of 14.7 cm^–1 and 1.4 × 10^–7 s, respectively, demonstrating the single molecule magnetic behavior in solution

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