Core-Modified Porphyrin Diradicals with a CC Unit: Redox-Driven Magnetic Switching

We explore the intramolecular spin interactions of the core-modified porphyrin diradicals with a CC unit (R-(CC) and R-(CC)²⁺) featuring (CC)­porphyrin and (CC)­porphyrin²⁺ as the couplers and verdazyl, nitronyl nitroxide, and imino nitroxide as spin sources (R) at the B3LYP/6-31G­(d) level and the CC effect through comparison with the porphine-coupled diradicals (R-(Null)). Structurally, modifications of porphine through introducing radical groups to the edge sites and a CC unit to its core lead to a nonplanar diradical structure featuring a curved (CC)­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 CC unit noticeably enhances the spin couplings in R-(CC) and R-(CC)²⁺ compared with R-(Null) with a planar porphine coupler, and R-(CC) possess mild ferromagnetic couplings but R-(CC)²⁺ 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-(CC), and R-(CC)²⁺. 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-(CC)²⁺. 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(s²)···Be­(s²) 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 −NN– 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^–1 for the trans form and from −388.1 to −1227.9 cm^–1 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

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<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 CO₂ 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

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<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 CO₂ 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

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<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 CO₂ 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

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<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 CO₂ 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