Singlet Biradical Character Research Articles

Activation of carbon-hydrogen bonds by complexes involving multiply bonded Group 13 elements

In the present research, Group 13 dimetallene complexes have been modeled for the activation of carbon-hydrogen bonds in small molecules using dispersion-corrected DFT calculations. It has been hypothesized that Group 13 multiple bonds, namely those between aluminum, can be extremely reactive due to the Lewis acidity of the metals and the high singlet biradical character of triel–triel bonds. Interestingly, from MCSCF (multiconfiguration SCF) calculations, it was found these molecules lack high levels of biradical character and contain 92% double bond character. Despite this, it was calculated that the activation of toluene with these dimetallene catalysts have reasonable free energy barriers as low as 30 kcal/mol with gallium being the most favorable among the Group 13 elements studied. Various substitutions made on the peripheral ligands of the catalyst models made little impact, but substitutions on the toluene being activated showed a decrease in energy barriers. The results give insight into the design of molecules containing Group 13 multiple bonds to activate carbon-hydrogen bonds.

A cyclic thioketone as biradical heterocyclopentane-1,3-diyl: synthesis, structure and activation chemistry

Reaction of [(μ-NR)P˙]2 with CSCl2 and Mg gives a deep blue, highly labile cyclic thioketone with singlet biradical character. It can be converted to a housane-type isomer by light and to bicyclic cage compounds by addition of CS2 and benzaldehyde.

Synthesis, crystal structure and charge transport characteristics of stable peri-tetracene analogues.

peri-Acenes have shown great potential for use as functional materials because of their open-shell singlet biradical character. However, only a limited number of peri-acene derivatives larger than peri-tetracene have been synthesized to date, presumably owing to the low stability of the target compounds in addition to the complicated synthesis scheme. Here, a very simple synthesis route for the tetrabenzo[a,f,j,o]perylene (TBP) structure enables the development of highly stable peri-tetracene analogues. Despite a high degree of singlet biradical character, the compounds with four substituents at the zigzag edge show a remarkable stability in solution under ambient conditions, which is better than that of acene derivatives with a closed-shell electronic configuration. The crystal structures of the TBP derivatives were obtained for the first time; these are valuable to understand the relationship between the structure and biradical character of peri-acenes. The application of peri-acenes in electronic devices should also be investigated. Therefore, the semiconducting properties of the TBP derivative were investigated by fabricating the field-effect transistors.

Room-temperature chemical synthesis of C2

Diatomic carbon (C2) is historically an elusive chemical species. It has long been believed that the generation of C2 requires extremely high physical energy, such as an electric carbon arc or multiple photon excitation, and so it has been the general consensus that the inherent nature of C2 in the ground state is experimentally inaccessible. Here, we present the chemical synthesis of C2 from a hypervalent alkynyl-λ3-iodane in a flask at room temperature or below, providing experimental evidence to support theoretical predictions that C2 has a singlet biradical character with a quadruple bond, thus settling a long-standing controversy between experimental and theoretical chemists, and that C2 serves as a molecular element in the bottom-up chemical synthesis of nanocarbons such as graphite, carbon nanotubes, and C60.

Duality of Reactivity of a Biradicaloid Compound with an o-Quinodimethane Scaffold.

Sigmarene, which is a Kekulé hydrocarbon with appreciable singlet biradical character originating from an o-quinodimethane scaffold, is isolated as a doubly σ-bonded dimer. The dimer dissociates into a monomeric sigmarene upon heating or photoirradiation. The monomeric species undergoes a rapid [4 + 4] cycloaddition reaction under dark conditions even at room temperature to produce the dimer. Contrarily, the monomeric sigmarene undergoes a [4 + 2] cycloaddition reaction in the presence of dienophile as an orbital symmetry allowed process. Therefore, the sigmarene shows high reactivity for both symmetry-forbidden and allowed processes in the framework of the orbital symmetry rule. This duality of reactivity of the sigmarene is consistent with the intermediate singlet biradical character (44%) estimated by a density functional theory (DFT) calculation.

Azoniadibenzo[a,j]phenalenide: A Polycyclic Zwitterion with Singlet Biradical Character

AbstractA polycyclic zwitterion, azoniadibenzo[a,j]phenalenide, having a singlet biradical character, was synthesized and isolated for the first time. The negative charge was found to be delocalized over the periphery of the main core of the zwitterion and the positive charge was localized near the nitrogen atom. Its singlet biradical character was confirmed by a small HOMO–LUMO energy gap with their spatial overlap, and observation of thermally excited triplet species together with DFT calculations.

Azoniadibenzo[a,j]phenalenide: A Polycyclic Zwitterion with Singlet Biradical Character.

A polycyclic zwitterion, azoniadibenzo[a,j]phenalenide, having a singlet biradical character, was synthesized and isolated for the first time. The negative charge was found to be delocalized over the periphery of the main core of the zwitterion and the positive charge was localized near the nitrogen atom. Its singlet biradical character was confirmed by a small HOMO-LUMO energy gap with their spatial overlap, and observation of thermally excited triplet species together with DFT calculations.

Toward Full Zigzag-Edged Nanographenes: peri-Tetracene and Its Corresponding Circumanthracene.

Zigzag-edged nanographene with two rows of fused linear acenes, called as n- peri-acene (n-PA), is considered as a potential building unit in the arena of organic electronics. n-PAs with four ( peri-tetracene, 4-PA), five ( peri-pentacene, 5-PA) or more benzene rings in a row have been predicted to show open-shell character, which would be attractive for the development of unprecedented molecular spintronics. However, solution-based synthesis of open-shell n-PA has thus far not been successful because of the poor chemical stability. Herein we demonstrated the synthesis and characterization of the hitherto unknown 4-PA by a rational strategy in which steric protection of the zigzag edges playing a pivotal role. The obtained 4-PA possesses a singlet biradical character ( y0 = 72%) and exhibits remarkable persistent stability with a half-life time ( t1/2) of ∼3 h under ambient conditions. UV-vis-NIR and electrochemical measurements reveal a narrow optical/electrochemical energy gap (1.11 eV) for 4-PA. Moreover, the bay regions of 4-PA enable the efficient 2-fold Diels-Alder reaction, yielding a novel full zigzag-edged circumanthracene.

(Invited) Strongly Antiaromatic Porphyrins with Singlet Biradical Character

Recently, antiaromatic compounds with 4n π-electrons have received much attention due to their intriguing properties, which are different from those of conventional aromatic compounds. They are expected to be applicable for ambipolar semiconductors and secondary batteries. Antiaromatic π-systems generally have a closed-shell configuration because of the lowering of the symmetry. Consequently, antiaromatic compounds generally show substantial bond-length alternation. It is quite challenging to investigate the properties of 4n π-conjugated systems upon enhancing their symmetry. In this study, we have succeeded in preparing Ni(II) tetrabenzonorcorrole TBNc. Interestingly, the introduction of fused benzene rings resulted in bond-length equalization of the π-system of norcorrole NC. Moreover, we found that the enhancement of the symmetry of the norcorrole conjugation significantly decreases the HOMO–LUMO gap, which results in the emergence of a singlet diradical character and significant increase of the antiaromatic character. Figure 1

Synthesis and Characterization of Dibenzo[a,f]pentalene: Harmonization of the Antiaromatic and Singlet Biradical Character.

Mesityl derivatives of the unknown dibenzopentalene isomer dibenzo[a,f]pentalene were synthesized. The molecular geometry and physical properties of dibenzo[a,f]pentalene were investigated. Dibenzo[a,f]pentalene combines a large antiaromatic and appreciable singlet open-shell character, properties not shared by well-known isomer dibenzo[a,e]pentalene.

Biphenalenylidene: Isolation and Characterization of the Reactive Intermediate on the Decomposition Pathway of Phenalenyl Radical.

First isolation and characterization of biphenalenylidenes, which have long been unidentified reactive intermediates on the decomposition pathway of phenalenyl radical, were accomplished. Photoinduced electrocyclic ring-opening reaction of anti-dihydroperopyrene resulted in a successful conversion to E-biphenalenylidene, which enabled a detailed investigation of the electronic structure of E-biphenalenylidene by means of spectroscopic techniques. A stereoisomer, Z-biphenalenylidene, was also observed by suppressing a facile E-Z isomerization to E-biphenalenylidene in a rigid matrix. Furthermore, Z-biphenalenylidene demonstrated a thermal ring-closure in conrotatory process, which is not conforming to the Woodward-Hoffmann rule. These unusual reactivities of biphenalenylidene are ascribed to the ground states destabilized by its singlet biradical character, which was fully supported by theoretical calculations. The presence of E-biphenalenylidene on the decomposition pathway of phenalenyl was confirmed experimentally, leading to the full understanding of the decomposition mechanism of phenalenyl.

Perylene Bisimide Radicals and Biradicals: Synthesis and Molecular Properties

AbstractUnprecedented neutral perylene‐3,4:9,10‐tetracarboxylic acid bisimide (PBI) radicals and biradicals were synthesized by facile chemical oxidation of 4‐hydroxyaryl‐substituted PBIs. Subsequent characterization by optical and magnetic spectroscopic techniques, as well as quantum chemical calculations, revealed an open‐shell singlet biradical ground state for the PBI biradical OS‐2.. (〈s2〉=1.2191) with a relatively small singlet–triplet energy gap of 0.041 eV and a large singlet biradical character of y=0.72.

Perylene Bisimide Radicals and Biradicals: Synthesis and Molecular Properties.

Unprecedented neutral perylene-3,4:9,10-tetracarboxylic acid bisimide (PBI) radicals and biradicals were synthesized by facile chemical oxidation of 4-hydroxyaryl-substituted PBIs. Subsequent characterization by optical and magnetic spectroscopic techniques, as well as quantum chemical calculations, revealed an open-shell singlet biradical ground state for the PBI biradical OS-2(..) (〈s(2)〉=1.2191) with a relatively small singlet-triplet energy gap of 0.041 eV and a large singlet biradical character of y=0.72.

ChemInform Abstract: Phenalenyl‐Based Open‐Shell Polycyclic Aromatic Hydrocarbons

AbstractReview: [67 refs.

Phenalenyl-based open-shell polycyclic aromatic hydrocarbons.

The phenalenyl radical is a polycyclic aromatic hydrocarbon (PAH) radical. Owing to its widely distributed spin structure, phenalenyl is relatively stable compared to other hydrocarbon radicals and has been studied from the viewpoint of its application to electroconductive and magnetic materials. In addition, a strong intermolecular spin-spin coupling nature is another feature of phenalenyl. This account summarizes my studies so far into PAH radicals containing the phenalenyl scaffold in terms of their amphoteric redox properties and singlet biradical character, which strongly rely on the characteristic electronic structure, that is, non-bonding character and sixfold symmetry of a singly occupied molecular orbital of the phenalenyl radical.

Cerium(IV)-driven oxidation of water catalyzed by mononuclear ruthenium complexes

This paper reviews results from study of mononuclear ruthenium complexes capable of catalyzing the oxidation of water to molecular oxygen. These catalysts may be classified into three groups, with different rate laws associated with O2 evolution. In one class, O2 evolution proceeds via radical coupling of the oxygen atom of an RuV=O species with a hydroxocerium(IV) ion. O2 evolution catalyzed by the second class occurs via acid–base reaction of the oxygen atom of an RuV=O species with a water molecule. In the third group, the dominant mechanism is oxo–oxo radical coupling between two RuV=O species. Several significant properties of the oxidant Ce(IV) are also discussed, including the singlet biradical character of the hydroxocerium(IV) ion.

Indolo[2,3-b]carbazoles with tunable ground states: how Clar's aromatic sextet determines the singlet biradical character

Polycyclic hydrocarbons (PHs) with a singlet biradical ground state have recently attracted extensive interest in physical organic chemistry and materials science. Replacing the carbon radical center in the open-shell PHs with a more electronegative nitrogen atom is expected to result in the more stable aminyl radical. In this work, two kinetically blocked stable/persistent derivatives (1 and 2) of indolo[2,3-b]carbazole, an isoelectronic structure of the known indeno[2,1-b]fluorene, were synthesized and showed different ground states. Based on variable-temperature NMR/ESR measurements and density functional theory calculations, it was found that the indolo[2,3-b]carbazole derivative 1 is a persistent singlet biradical in the ground state with a moderate biradical character (y0 = 0.269) and a small singlet–triplet energy gap (ΔES–T ≅ −1.78 kcal mol−1), while the more extended dibenzo-indolo[2,3-b]carbazole 2 exhibits a quinoidal closed-shell ground state. The difference can be explained by considering the number of aromatic sextet rings gained from the closed-shell to the open-shell biradical resonance form, that is to say, two for compound 1 and one for compound 2, which determines their different biradical characters. The optical and electronic properties of 2 and the corresponding aromatic precursors were investigated by one-photon absorption, transient absorption and two-photon absorption (TPA) spectroscopies and electrochemistry. Amphoteric redox behaviour, a short excited lifetime and a moderate TPA cross section were observed for 2, which can be correlated to its antiaromaticity and small biradical character. Compound 2 showed high reactivity to protic solvents due to its extremely low-lying LUMO energy level. Unusual oxidative dimerization was also observed for the unblocked dihydro-indolo[2,3-b]carbazole precursors 6 and 11. Our studies shed light on the rational design of persistent aminyl biradicals with tunable properties in the future.

Zethrenes, extended p-quinodimethanes, and periacenes with a singlet biradical ground state.

Researchers have studied polycyclic aromatic hydrocarbons (PAHs) for more than 100 years, and most PAHs in the neutral state reported so far have a closed-shell electronic configuration in the ground state. However, recent studies have revealed that specific types of polycyclic hydrocarbons (PHs) could have a singlet biradical ground state and exhibit unique electronic, optical, and magnetic activities. With the appropriate stabilization, these new compounds could prove useful as molecular materials for organic electronics, nonlinear optics, organic spintronics, organic photovoltaics, and energy storage devices. However, before researchers can use these materials to design new devices, they need better methods to synthesize these molecules and a better understanding of the fundamental relationship between the structure and biradical character of these compounds and their physical properties. Their biradical character makes these compounds difficult to synthesize. These compounds are also challenging to physically characterize and require the use of various experimental techniques and theoretic methods to comprehensively describe their unique properties. In this Account, we will discuss the chemistry and physics of three types of PHs with a significant singlet biradical character, primarily developed in our group. These structures are zethrenes, Z-shaped quinoidal hydrocarbons; hydrocarbons that include a proaromatic extended p-quinodimethane unit; and periacenes, acenes fused in a peri-arrangement. We used a variety of synthetic methods to prepare these compounds and stabilized them using both thermodynamic and kinetic approaches. We probed their ground-state structures by electronic absorption, NMR, ESR, SQUID, Raman spectroscopy, and X-ray crystallography and also performed density functional theory calculations. We investigated the physical properties of these PHs using various experimental methods such as one-photon absorption, two-photon absorption, transient absorption spectroscopy, electrochemistry, and spectroelectrochemistry. These systematic studies revealed that aromaticity played a very important role in determining their singlet biradical character, which is critically related to both their physical properties and their chemical reactivity. In particular, we found that Clar's aromatic sextet rule, which is useful for the closed-shell PAHs, can also predict the relative biradical character of benzenoid PH-based singlet biradicaloids. Other factors, such as structural flexibility of the biradical and quinoid resonance forms and the participation of the substitution in the π-conjugation, also influence the biradical character. These molecular materials demonstrate a number of unique properties such as near-infrared absorption, redox amphotericity, large two-photon absorption cross section, short excited state lifetime, stimuli-responsive magnetic activity, and singlet fission, which suggests promise for future applications.

Organic nonlinear optical materials: the mechanism of intermolecular covalent bonding interactions of Kekulé hydrocarbons with significant singlet biradical character.

The ground- and excited-state properties of benzene-linked bisphenalenyl (B-LBP), naphthaline-linked bisphenalenyl (N-LBP), and anthracene-linked bisphenalenyl (A-LBP) Kekulé molecules and their respective one-dimensional (1D) stacks are investigated using time-dependent density functional theory (TD-DFT) and a range of extensive multidimensional visualization techniques. The results reveal a covalent π-π bonding interaction between overlapping phenalenyl radicals whose bond length is shorter than the van der Waals distance between carbon atoms. Increasing the linker length and/or number of molecules involved in the 1D stack decreases the HOMO-LUMO energy gap and increases the wavelength of the systems. The charge-transfer mechanism and electron coherence both differ with changes in the linker length and/or number of molecules involved in the 1D stack.

Remarkable Solvatochromic Color Change via Proton Tautomerism of a Phenol-Linked Imidazole Derivative

A unique solvatochromic 2-phenyl-4,5-diarylimidazole derivative linked with a phenol moiety and a p-quinonemethide moiety at the 4- and 5-positions of a imidazole ring, which shows remarkable color change via proton tautomerism, was synthesized and the mechanism of the solvatochromic color change was investigated. The yellow-colored OH tautomeric form (1_OH) exists as a dominant species in nonpolar solvents, whereas the blue-colored NH tautomeric form (1_NH) is stabilized in polar solvents. The molecular structures of these tautomers were determined by X-ray crystallographic analysis. The p-quinonemethide moiety and the imidazole ring of 1_OH are coplanar to one another and possess a planar quinoidal structure. On the other hand, 1_NH has a nonplanar twisted quinoidal structure causing large bathochromic shift in the visible absorption spectrum. Moreover, the X-ray crystallographic analysis and the DFT calculations support the closed-shell singlet character of 1_OH. In contrast 1_NH possesses partial single bond character that leads to the open-shell singlet biradical character and the decrease in the singlet-triplet energy gap. The twisting of the π-conjugated electron system induced by the proton tautomerization was found to be the origin of the open-shell biradical character of 1_NH and the enhanced solvatochromic color change.