25 research outputs found
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<sup>IV</sup>-product complexes with xanthine
oxidase (XO) and xanthine dehydrogenase. These Mo<sup>IV</sup>-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 metalâorganic 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 metalâorganic
frameworks. Here, we use KohnâSham 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>-phenyleneÂ(R) bridge
(where R = âNH<sup>â˘</sup>/âNH<sub>2</sub>, âO<sup>â˘</sup>/OH, âCH<sub>2</sub><sup>â˘</sup>/CH<sub>2</sub>). These model compounds are based on a fully characterized
SQâ<i>meta</i>-phenyleneâNNO diradical with
moderately strong antiferromagnetic coupling. Replacing closed-shell
substituents CH<sub>3</sub> and NH<sub>2</sub> with their radical
counterparts CH<sub>2</sub><sup>â˘</sup> and NH<sup>â˘</sup> leads to an increase in stabilization of the ground state with respect
to other low-lying spin states by a factor of 3â6, depending
on the exchangeâcorrelation functional. For OH compared with
O<sup>â˘</sup> 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 DonorâAcceptor Complex
Interligand
charge transfer is examined in the novel metallo-dithiolene complex
MoOÂ(SPh)<sub>2</sub>(<sup><i>i</i></sup>Pr<sub>2</sub>Dt<sup>0</sup>) (where <sup><i>i</i></sup>Pr<sub>2</sub>Dt<sup>0</sup> = <i>N</i>,<i>N</i>â˛-isopropyl-piperazine-2,3-dithione).
The title complex displays a remarkable 70° âenvelopeâ-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 âź18âŻ000 cm<sup>â1</sup> is assigned as a thiolate â dithione ligand-to-ligand
charge transfer (LLâ˛CT) transition that also possesses MoÂ(IV)
â dithione charge transfer character. Strong orbital mixing
between occupied and virtual orbitals with MoÂ(<i>x</i><sup>2</sup>â<i>y</i><sup>2</sup>) orbital character
is derived from a strong pseudo JahnâTeller 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 DonorâAcceptor Biradical Analogues of Charge-Separated Excited States
A new donorâacceptor 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 JSQâVD = +77 cmâ1 (H = â2JSQâVDSĚSQ¡SĚVD) 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
donorâacceptor biradical complex TpCum,MeZn(SQ-NN)
(NN = nitronyl nitroxide, S = 1/2), where JSQâNN â
+550 cmâ1. This represents a dramatic
reduction in the biradical exchange by a factor of âź7, 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 â 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 singletâtriplet 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 CofactorâProtein 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
Mocoâprotein 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
enzymeâproduct 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 âNH<sub>2</sub> terminus of the PDT, and between Q197 and the terminal oxo of the
MoîźO group. The Q102A, Q102G, Q197A, and Q197E variants do
not appreciably affect the kinetic parameters <i>k</i><sub>red</sub> and <i>k</i><sub>red</sub>/<i>K</i><sub>D</sub>, 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 electronâhole
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 âreadâ 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<sup>Cum,Me</sup>ZnÂ(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 SQÂ(Ď)<sub>SOMO</sub> â NN-Ph
(Ď*)<sub>LUMO</sub> D â 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
PhÂ(8a) + SQÂ(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 â A interaction given by the magnitude
of the electronic coupling matrix element, <i>H</i><sub><i>ab</i></sub>
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 polyÂ[2-methoxy-5-(3â˛,7â˛-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<sup>II</sup>(O<sub>2</sub>CCH<sub>3</sub>)<sub>2</sub>[12-MC<sub>Mn<sup>III</sup>(N)shi</sub>-4](DMF)<sub>6</sub>, <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<sup>II</sup> ion surrounded by four peripheral Mn<sup>III</sup> 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<sup>III</sup> ions (<i>J</i> = â6.3 cm<sup>â1</sup>) and between the central Mn<sup>II</sup> ion and the Mn<sup>III</sup> ones (<i>J</i>Ⲡ= â4.2 cm<sup>â1</sup>). In order to obtain a reasonable fit, a relatively large single ion magnetic anisotropy (<i>D</i>) value of 1 cm<sup>â1</sup> was necessary for the central Mn<sup>II</sup> 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 (Ď<sub>o</sub>) of 14.7 cm<sup>â1</sup> and 1.4 Ă 10<sup>â7</sup> s, respectively, demonstrating the single molecule magnetic behavior in solution
Electronic and Exchange Coupling in a Cross-Conjugated DâBâA 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<sup>Cum,Me</sup>ZnÂ(SQ-<i>m-</i>Ph-NN) (<b>1-meta</b>) a donorâbridgeâacceptor
(DâBâA) 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 singletâtriplet splitting in this cross-conjugated
DâBâA 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 DâBâA biradical possessing a <i>para</i>-phenylene bridge, admixture of a single low-lying singly excited
D â A type configuration into the cross-conjugated DâBâA
biradical ground state makes a negligible contribution to the ground-state
magnetic exchange interaction. Instead, an excited state formed by
a Ph-NN (HOMO) â Ph-NN (LUMO) one-electron promotion configurationally
mixes into the ground state of the <i>m-</i>Ph bridged DâA
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 DâBâA 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> âź
0 cm<sup>â1</sup>. We hypothesize that similar superexchange
mechanisms are common to all cross-conjugated DâBâA
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