19 research outputs found
Core-Modified Porphyrin Diradicals with a CC Unit: Redox-Driven Magnetic Switching
We
explore the intramolecular spin interactions of the core-modified
porphyrin diradicals with a CC unit (R-(CC) and R-(CC)<sup>2+</sup>) featuring (CC)porphyrin and (CC)porphyrin<sup>2+</sup> as the couplers and verdazyl, nitronyl nitroxide, and imino
nitroxide as spin sources (R) at the B3LYP/6-31G(d) level and the
CC effect through comparison with the porphine-coupled diradicals
(R-(Null)). Structurally, modifications of porphine through introducing
radical groups to the edge sites and a CC unit to its core
lead to a nonplanar diradical structure featuring a curved (CC)porphyrin
coupler and twist linkages of radical groups. Although such nonplanar
structures seem unfavorable to the spin coupling between spin sources,
our results suggest that the core modification with a CC unit
noticeably enhances the spin couplings in R-(CC) and R-(CC)<sup>2+</sup> compared with R-(Null) with a planar porphine coupler, and
R-(CC) possess mild ferromagnetic couplings but R-(CC)<sup>2+</sup> present strong antiferromagnetic ones, indicating that two-electron
redox can switch the magnetisms. The differences in the magnetic properties
and coupling magnitudes should be attributed to distinctly different
spin-interacting pathways among R-(Null), R-(CC), and R-(CC)<sup>2+</sup>. Besides, the energies of the lowest unoccupied molecular
orbitals of the couplers regulate the magnetic couplings, and the
linking modes of the radical groups to the couplers also affect the
magnetic coupling strengths especially for R-(CC)<sup>2+</sup>. The observed magnetic coupling regularities are reasonably analyzed
by the modified spin alternation rule. This work provides a promising
strategy for rational designs of the porphyrin-based diradicaloids
and new insights into the spin interaction mechanisms in such diradicaloids
which are useful bases for further applications in the future
Remarkable Magnetic Coupling Interactions in Multi-Beryllium-Expanded Small Graphene-like Molecules with Well-Defined Polyradical Characters
Multi-beryllium-expanded
small graphene-like molecules including oligoacenes (<i>m</i>Be-<i>n</i>A) and graphene patches (<i>m</i>Be-GP)
are computationally designed through introducing two or three Be atoms
into the specific benzenoid rings of the graphene-like molecules,
leading to replacement of some C–C bonds by the C–Be–C
linkages with elongated C···C distances of about 3.3
Å in them. As a result, the elongation of the C···C
bonds and insertion of more Be atoms make the two radical moieties
in each molecule relatively separated and their interaction relatively
weak. Both density functional theory and CASSCF calculations indicate
that all these multi-Be-expanded graphene-like molecules exhibit well-defined
polyradical characters: an open-shell singlet diradical for all <i>m</i>Be-<i>n</i>A and an open-shell singlet diradical
or quintet tetraradical for <i>m</i>Be-GP depending on the
Be-insertion patterns of the patches. The main findings in this work
are that (i) a switching from the parent graphene-like closed-shell
molecules (e.g., linear oligoacenes and graphene patches) to the open-shell
singlet (diradical) or quintet (tetraradical) ground states can be
realized by introducing Be as linkers into the graphene-like molecules;
(ii) more importantly, the spin-coupling interactions of such <i>m</i>Be-<i>n</i>A and <i>m</i>Be-GP are
remarkably large; and (iii) in these Be-modified molecules the Be–C
bonds exhibit considerable covalent character and the Be···Be
distances are 2.67–2.84 Å, implying weak Be(s<sup>2</sup>)···Be(s<sup>2</sup>) metallophilic interaction. This
work would open a new perspective for the rational design of perfect
and stable singlet diradicals or polyradicals with large spin-coupling
constants on the basis of small closed-shell graphene-like molecules
by multimetal incorporation and also encourage experimentalists to
pursue and realize these interesting structures with enhanced magnetic
properties in the future
Protonation-Enhanced Antiferromagnetic Couplings in Azobenzene-Bridged Diradicals
Proton-induced magnetic
enhancement in an organic diradical is
an appealing phenomenon. Here, taking two nitroxide groups as spin
sources, we predict the magnetic properties of the trans and cis forms
of azobenzene (AB)-bridged diradicals in which the central −NN–
unit can undergo single protonation to convert to its protonated counterpart
or vice versa. The calculated results for these two pairs of diradicals
(protonated versus unprotonated trans and cis forms) indicate that
the signs of their magnetic coupling constants <i>J</i> do
not change, but the magnitudes remarkably increase after protonation
from −716.4 to −1787.1 cm<sup>–1</sup> for the
trans form and from −388.1 to −1227.9 cm<sup>–1</sup> for the cis form, respectively. Such noticeable magnetic enhancements
induced by protonation are mainly attributed to the strong mediating
role of the coupler AB between two radical groups through its lowest
unoccupied molecular orbital (LUMO) with a lower energy level after
protonation. The planar structure for the protonated trans diradical
and two reduced CCNN torsional angles due to protonation for the cis
one are responsible for the significant magnetic enhancements. Protonation
not only supports the development of π conjugation among the
spin groups and coupler but also creates a very favorable condition
for spin transmission through the coupler AB LUMO by lowering the
LUMO energy level and improving spin polarization and charge delocalization
and thus enhances the spin coupling effectively. In addition, different
spin sources and linking modes of the radical groups are also considered
to confirm our conclusions, and the possibilities of protonation of
such diradical systems are further discussed. The studied diradicals
could be the promising candidates for the rational design of magnetic
molecular switches
Redox-Modulated Magnetic Transformations between Ferro- and Antiferromagnetism in Organic Systems: Rational Design of Magnetic Organic Molecular Switches
Organic
molecules with switchable magnetic properties have extensively
technological applications due to the fact that magnetic conversion
can be realized through diverse methods. In particular, the redox-induced
magnetic reversal is easy to accomplish and exhibits promising application
in the field of magnetic materials, and thus it is an imperative task
to find magnetism-switchable systems. Herein, we computationally design
two couples of nitroxy–pyrazinyl–nitroxy diradicals
in which two nitroxy radical groups are connected to a redox-active
pyrazinyl coupler in the para or meta modes. We find that the magnetic
conversion can occur from ferromagnetic to antiferromagnetic exchange
coupling or vice versa by means of the redox method in these designed
magnetic organic molecules, and their magnetic exchange coupling constants
are considerably large no matter for ferromagnetic or antiferromagnetic
couplings, as evidenced at both the B3LYP and M06-2X levels of theory.
Analyses indicate that redox-induced structural change of the coupler
leads to conversion of its aromaticity and considerable spin delocalization
from the π-conjugated structure and spin polarization from non-Kekule
structure, which thus determine the spin coupling between two spin
centers in the magnetic molecules. In addition, the spin alternation
rule, singly occupied molecular orbital (SOMO) effect, and SOMO–SOMO
energy splitting of triplet state are utilized to analyze the diradical
characters of the molecules, suggesting effective tools for predicting
molecular ground states (ferromagnetic, antiferromagnetic, or nonmagnetic).
This work provides helpful information for the rational design of
promising organic magnetic switches
Molecular Vibrations Induced Potential Diradical Character in Hexazapentacene
While the photoelectrochemical behavior
of azapentacene has been
investigated successfully, insight into the dynamic electronic properties
of azapentacene triggered by different energy pulses is very scarce.
The present work reports a fascinating phenomenon about potential
diradical character governed by structural vibrations in hexazapentacene.
In complete contrast to the static equilibrium configuration of hexazapentacene
without diradical character, due to the vibration-based structural
perturbation, DFT calculations show that some of the transient configurations
possess diradical character and thus magnetism, which exhibit different
periodic pulse behavior in time evolution. Since each vibrational
mode refers to two distortion ways (positive/negative distortions
from equilibrium configuration), 7 different possibilities are observed
for the vibration-induced diradical character for all vibrational
modes (e.g., a combination of nonradical, singlet diradical, or triplet
diradical for positive distortion and those of for negative distortion
for each vibrational mode). This intriguing diradical character is
rationalized by structural distortions with considerable changes of
some energy quantities. The structural distortions cause the HOMO
energy raising and LUMO energy lowering and thus an efficient reduction
of the HOMO–LUMO energy gap and singlet–triplet gap
of the system, which are favorable to the formation of the broken-symmetry
open-shell singlet or triplet states. The periodic pulsing behavior
is attributed to persistent molecular vibrations and is thus vibrational
mode controlled. Compared with pentacene, the remarked effects of
nitrogen substitution on the diradical properties and their pulsing
behaviors are mainly due to the decreases of both the HOMO and the
LUMO energies and considerable narrowing of their gaps in the vibrations-distorted
configurations. This intriguing potential diradical character and
its different dynamic behavior suggest hexazapentacene potential applications
as promising building blocks in the rational design of novel electromagnetic
materials because of its controllable magnetism through energy pulses.
This work provides comprehensive understanding of the nature of dynamic
variations of the electronic structures and properties of the nitrogen-rich
acene derivatives and other materials molecules
Multi-Zinc-Expanded Oligoacenes: An Intriguing Class of Well-Defined Open-Shell Singlet Diradicals
Two classes of multi-Zn-expanded oligoacenes from benzene
to pentacene
are computationally designed through introducing a Zn array into acene
rings in two ways: acene-chain axial versus single-ring quasi-transversal
direction. Combined density functional theory, CASSCF, and CCSD calculations
predict that all these multi-Zn-expanded oligoacenes have the open-shell
singlet diradical ground states, in contrast with the common fact
that their parent oligoacenes are closed-shell systems or may have
a triplet ground state and only acenes larger than octacene have open-shell
singlet diradical ground states. These results offer the first theoretical
prediction that the multi-Zn introduction into the acene ring(s),
forming the Zn-expanded oligoacenes, can lead them to diradical structures.
The diradical character of the ground states of these molecules arises
from the Zn-participation-induced disjoint nature of the nonbonding
molecular orbitals that are singly occupied in the diradicals. This
work provides a strategy to design perfect and stable singlet diradicals
from oligoacenes or their derivatives
Image_8.JPEG
<p>In this study, we evaluated suitable selected markers and optimized transformation protocols to develop a new genetic transformation methodology for DHA-producing Crypthecodinium cohnii. Additionally, ribulose 1,5-bisphosphate carboxylase/oxygenase (RuBisCO), potentially involved in CO<sub>2</sub> fixation under autotrophic conditions, was selected as the target for construction of a gene knockdown mutant. Our results show that the constructs were successfully inserted into the C. cohnii chromosome by homologous recombination. Comparative analysis showed that deletion of the RuBisCO gene promoted cell growth and increased the lipid content of C. cohnii under heterotrophic conditions compared with those of the wild-type. The liquid chromatography-mass spectrometry (LC-MS) based metabolomic analysis showed that the metabolites involved in energy metabolism were upregulated, suggesting that the deletion of the RuBisCO gene may contribute to the re-direction of more carbon or energy toward growth and lipid accumulation under heterotrophic conditions.</p
Table_2.PDF
<p>In this study, we evaluated suitable selected markers and optimized transformation protocols to develop a new genetic transformation methodology for DHA-producing Crypthecodinium cohnii. Additionally, ribulose 1,5-bisphosphate carboxylase/oxygenase (RuBisCO), potentially involved in CO<sub>2</sub> fixation under autotrophic conditions, was selected as the target for construction of a gene knockdown mutant. Our results show that the constructs were successfully inserted into the C. cohnii chromosome by homologous recombination. Comparative analysis showed that deletion of the RuBisCO gene promoted cell growth and increased the lipid content of C. cohnii under heterotrophic conditions compared with those of the wild-type. The liquid chromatography-mass spectrometry (LC-MS) based metabolomic analysis showed that the metabolites involved in energy metabolism were upregulated, suggesting that the deletion of the RuBisCO gene may contribute to the re-direction of more carbon or energy toward growth and lipid accumulation under heterotrophic conditions.</p
Image_6.JPEG
<p>In this study, we evaluated suitable selected markers and optimized transformation protocols to develop a new genetic transformation methodology for DHA-producing Crypthecodinium cohnii. Additionally, ribulose 1,5-bisphosphate carboxylase/oxygenase (RuBisCO), potentially involved in CO<sub>2</sub> fixation under autotrophic conditions, was selected as the target for construction of a gene knockdown mutant. Our results show that the constructs were successfully inserted into the C. cohnii chromosome by homologous recombination. Comparative analysis showed that deletion of the RuBisCO gene promoted cell growth and increased the lipid content of C. cohnii under heterotrophic conditions compared with those of the wild-type. The liquid chromatography-mass spectrometry (LC-MS) based metabolomic analysis showed that the metabolites involved in energy metabolism were upregulated, suggesting that the deletion of the RuBisCO gene may contribute to the re-direction of more carbon or energy toward growth and lipid accumulation under heterotrophic conditions.</p
Image_4.JPEG
<p>In this study, we evaluated suitable selected markers and optimized transformation protocols to develop a new genetic transformation methodology for DHA-producing Crypthecodinium cohnii. Additionally, ribulose 1,5-bisphosphate carboxylase/oxygenase (RuBisCO), potentially involved in CO<sub>2</sub> fixation under autotrophic conditions, was selected as the target for construction of a gene knockdown mutant. Our results show that the constructs were successfully inserted into the C. cohnii chromosome by homologous recombination. Comparative analysis showed that deletion of the RuBisCO gene promoted cell growth and increased the lipid content of C. cohnii under heterotrophic conditions compared with those of the wild-type. The liquid chromatography-mass spectrometry (LC-MS) based metabolomic analysis showed that the metabolites involved in energy metabolism were upregulated, suggesting that the deletion of the RuBisCO gene may contribute to the re-direction of more carbon or energy toward growth and lipid accumulation under heterotrophic conditions.</p