14 research outputs found
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
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
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
Synthesis, Characterization, and Photophysical Studies of an Iron(III) CatecholateâNitronylnitroxide Spin-Crossover Complex
The
synthesis and characterization of an Fe<sup>III</sup> catecholateânitronylnitroxide
(CAT-NN) complex (<b>1-NN</b>) that undergoes Fe<sup>III</sup> spin-crossover is described. Our aim is to determine whether the
intraligand exchange coupling of the semiquinoneânitronylnitroxide
Fe<sup>II</sup>(SQ-NN) excited state resulting from irradiation of
the CAT â Fe<sup>III</sup> 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. MoĚssbauer spectroscopy
clearly demonstrates Fe<sup>III</sup> spin-crossover, hyperfine couplings,
and a weak ferromagnetic Fe<sup>III</sup>âCAT-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<sup>II</sup>SQ 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<sup>III</sup>CAT state, followed by repopulation of the high-spin Fe<sup>III</sup>CAT 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<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>
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
Modification of Molecular Spin Crossover in Ultrathin Films
Scanning tunneling microscopy and
local conductance mapping show
spin-state coexistence in bilayer films of FeÂ[(H<sub>2</sub>Bpz<sub>2</sub>)<sub>2</sub>bpy] 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 Ď and Ď Coupling Using <i>para</i>-Phenylene and âAviramâRatnerâ Bridges
The
torsional dependence of donorâbridgeâacceptor
(DâBâA) electronic coupling matrix elements (<i><b>H</b></i><sub><i><b>DA</b></i></sub>, determined from the magnetic exchange coupling, <i><b>J</b></i>) involving a spin S<sub>D</sub> = 1/2 metal semiquinone
(Zn-<b>SQ</b>) donor and a spin S<sub>A</sub> = 1/2 nitronylnitroxide
(<b>NN</b>) acceptor mediated by the Ď/Ď-systems
of <i>para</i>-phenylene and methyl-substituted <i>para</i>-phenylene bridges and by the Ď-system of a bicyclo[2.2.2]Âoctane
(<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 (Ď) and
conformationally independent (Ď) contributions to the electronic
and magnetic exchange couplings in these DâBâA 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, âRamachandran-typeâ plots that
relate DâB and BâA torsions to both electronic and exchange
couplings. Analysis of the magnetic data shows large variations in
magnetic exchange (<i><b>J</b></i> â 1â175
cm<sup>â1</sup>) and electronic coupling (<i><b>H</b></i><sub><i><b>DA</b></i></sub> â 450â6000
cm<sup>â1</sup>) as a function of bridge conformation relative
to the donor and acceptor. This has allowed for an experimental determination
of both the Ď- and Ď-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 DonorâBridgeâAcceptor Biradical Complexes
The
preparation and characterization of three new donorâbridgeâacceptor
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: oligoÂ(<i>para</i>-phenylene), β = 0.39 Ă
<sup>â1</sup> and oligoÂ(2,5-thiophene), β = 0.22 Ă
<sup>â1</sup>. 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 oligoÂ(<i>para</i>-phenylene) and oligoÂ(2,5-thiophene) are due primarily
to the DâB energy gap, <b>Î</b>, rather than bridgeâbridge
electronic couplings, <b>H</b><sub><b>BB</b></sub>. This
is supported by the fact that the <b>H</b><sub><b>BB</b></sub> values extracted from the experimental data for oligoÂ(<i>para</i>-phenylene) (<b>H</b><sub><b>BB</b></sub> = 11â400 cm<sup>â1</sup>) and oligoÂ(2,5-thiophene)
(12â300 cm<sup>â1</sup>) differ by <10%. The results
presented here offer unique insight into the intrinsic molecular factors
that govern <b>H</b><sub><b>DA</b></sub> 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 DonorâBridgeâAcceptor Biradical Complexes
The
preparation and characterization of three new donorâbridgeâacceptor
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: oligoÂ(<i>para</i>-phenylene), β = 0.39 Ă
<sup>â1</sup> and oligoÂ(2,5-thiophene), β = 0.22 Ă
<sup>â1</sup>. 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 oligoÂ(<i>para</i>-phenylene) and oligoÂ(2,5-thiophene) are due primarily
to the DâB energy gap, <b>Î</b>, rather than bridgeâbridge
electronic couplings, <b>H</b><sub><b>BB</b></sub>. This
is supported by the fact that the <b>H</b><sub><b>BB</b></sub> values extracted from the experimental data for oligoÂ(<i>para</i>-phenylene) (<b>H</b><sub><b>BB</b></sub> = 11â400 cm<sup>â1</sup>) and oligoÂ(2,5-thiophene)
(12â300 cm<sup>â1</sup>) differ by <10%. The results
presented here offer unique insight into the intrinsic molecular factors
that govern <b>H</b><sub><b>DA</b></sub> and β,
which are important for understanding the electronic origin of electron
transfer and electron transport mediated by molecular bridges