Open-shell Singlet Ground State Research Articles

Antiaromatic Dicyclopenta[b,g]/[a,f]naphthalene Isomers Showing an Open-Shell Singlet Ground State with Tunable Diradical Character.

Since the first isolation of 1,3,5,7-tetra-tert-butyl-s-indacene in 1986, core-expanded s- and as-indacenes have attracted intensive interest. However, there is no reported synthesis of such type of molecules due to their high reactivity for over 30 years. Herein, we report the successful synthesis of two relatively stable, core-expanded indacene isomers, dicyclopenta[b,g]-naphthalene (5) and dicyclopenta[a,f]naphthalene (6). X-ray crystallographic analyses reveal that the backbone of 5 adopts a bond-delocalized structure, while that of 6 exhibits a bond-localized character. Variable-temperature 1H NMR/ESR measurements, electronic absorption spectra, and theoretical calculations confirm that both molecules are globally antiaromatic and have an open-shell singlet ground state. However, 6 shows stronger antiaromaticity, a larger diradical character (y0 = 48%), and a smaller singlet-triplet energy gap (ΔES-T = -0.99 kcal mol-1) compared to 5 (y0 = 30%, ΔES-T = -6.88 kcal mol-1), which can be explained by their different quinoidal structures.

Diindeno[2,1-b:2',1'-h]biphenylenes: Syntheses, Structural Analyses, and Properties.

A series of diindeno[2,1-b:2',1'-h]biphenylenes with open-shell singlet ground states and interesting properties were prepared. The studied compounds consist of p-quinodimethane moieties, which suffer from geometric perturbation with bond angles of around 90°. The substituent effects on structural parameters, local aromaticity, and properties were systematically explored.

On-Surface Synthesis and Characterization of Super-nonazethrene.

Beginning with the early work of Clar et al. in 1955, zethrenes and their laterally extended homologues, super-zethrenes, have been intensively studied in the solution phase and widely investigated as optical and charge transport materials. Superzethrenes are also considered to exhibit an open-shell ground state and may thus serve as model compounds to investigate nanoscale π-magnetism. However, their synthesis is extremely challenging due to their high reactivity. We report here the on-surface synthesis of the hitherto largest zethrene homologue-super-nonazethrene-on Au(111). Using single-molecule scanning tunneling microscopy and spectroscopy, we show that super-nonazethrene exhibits an open-shell singlet ground state featuring a large spin polarization-driven electronic gap of 1 eV. Consistent with the emergence of an open-shell ground state, high-resolution tunneling spectroscopy reveals singlet-triplet spin excitations in super-nonazethrene, characterized by a strong intramolecular magnetic exchange coupling of 51 meV. Given the paucity of zethrene chemistry on surfaces, our results therefore provide unprecedented access to large, open-shell zethrene compounds amenable to scanning probe measurements, with potential application in molecular spintronics.

DFT Calculations of InP Quantum Dots: Model Chemistries, Surface Passivation, and Open-Shell Singlet Ground States

DFT Calculations of InP Quantum Dots: Model Chemistries, Surface Passivation, and Open-Shell Singlet Ground States

Kinetic stability and NBO analysis of the C20-nAln nanocages (n = 1–5) using DFT investigation

Following our quest for novel heterofullerenes, we have compared and contrasted substitution effects on C20-nAln structures with n = 1–5, from kinetically view point at density functional theory (DFT). Structures with the odd numbered heteroatom (C19Al1, C17Al3 and C15Al5) appear open shell singlet ground state while the even ones (C18Al2-a, C18Al2-b, C16Al4-a and C16Al4-b) show closed shell singlet ground state. The high NBO charge distribution on the surface of C15Al5 nanocage provokes the more investigation for hydrogen storage. The overall trend for kinetic stability based on their band gap (ΔEHOMO-LUMO) in eV is C15Al5 (3.36) > C20 (2.31) > C18Al2-a (2.25) > C17Al3 (1.96) ≥ C19Al1 (1.95) > C16Al4-b (1.86) > C16Al4-a (1.56) > C18Al2-b (1.03). Evidently, the C15Al5 heterofullerene is as the most kinetic stable species against electronic excitation from the occupied HOMO orbital to the corresponding unoccupied LUMO orbital and contains five aluminum atom alternatively in equatorial. Furthermore, C15Al5 displays the highest energy of HOMO (EHOMO = -0.23321 a.u.), the lowest energy of LUMO (ELUMO = -0.10981 a.u.), the lowest nucleophilicity (N = 3.11 eV), the lowest electrophilicity (ω = 3.24 eV), the highest hardness (η = 3.36 eV) and the lowest softness (S = 0.15 eV) among the scrutinized species. Isolation of the two Al substitutions from each other by means of strong CC double bonds, increases N (4.48 eV), ω (9.69 eV) making C18Al2-b as the most nucleophilic and electrophilic species.

Single-layer polymeric tetraoxa[8]circulene modified by s-block metals: toward stable spin qubits and novel superconductors.

Tunable electronic properties of low-dimensional materials have been the object of extensive research, as such properties are highly desirable in order to provide flexibility in the design and optimization of functional devices. In this study, we account for the fact that such properties can be tuned by embedding diverse metal atoms and theoretically study a series of new organometallic porous sheets based on two-dimensional tetraoxa[8]circulene (TOC) polymers doped with alkali or alkaline-earth metals. The results reveal that the metal-decorated sheets change their electronic structure from semiconducting to metallic behaviour due to n-doping. Complete active space self-consistent field (CASSCF) calculations reveal a unique open-shell singlet ground state in the TOC-Ca complex, which is formed by two closed-shell species. Moreover, Ca becomes a doublet state, which is promising for magnetic quantum bit applications due to the long spin coherence time. Ca-doped TOC also demonstrates a high density of states in the vicinity of the Fermi level and induced superconductivity. Using the ab initio Eliashberg formalism, we find that the TOC-Ca polymers are phonon-mediated superconductors with a critical temperature TC = 14.5 K, which is within the range of typical carbon based superconducting materials. Therefore, combining the proved superconductivity and the long spin lifetime in doublet Ca, such materials could be an ideal platform for the realization of quantum bits.

3D global aromaticity in a fully conjugated diradicaloid cage at different oxidation states.

Aromaticity is a vital concept that governs the electronic properties of π-conjugated organic molecules and has long been restricted to 2D systems. The aromaticity in 3D π-conjugated molecules has been rarely studied. Here we report a fully conjugated diradicaloid molecular cage and its global aromaticity at different oxidation states. The neutral compound has an open-shell singlet ground state with a dominant 38π monocyclic conjugation pathway and follows the [4n + 2] Hückel aromaticity rule; the dication has a triplet ground state with a dominant 36π monocyclic conjugation pathway and satisfies [4n] Baird aromaticity; the tetracation is an open-shell singlet with 52 π-electrons that are delocalized along the 3D rigid framework, showing 3D global antiaromaticity; and the hexacation possesses D3 symmetry with 50 globally delocalized π-electrons, showing [6n + 2] 3D global aromaticity. Different types of aromaticity were therefore accessed in one molecular cage platform, depending on the symmetry, number of π-electrons and spin state.

2,6-/1,5-Naphthoquinodimethane bridged porphyrin dimer diradicaloids

2,6-Naphthoquinodimethane (2,6-NQDM)- and 1,5-naphthoquinodimethane (1,5-NQDM)-bridged porphyrin dimers, 2,6-P2 and 1,5-P2, were synthesized as relatively stable compounds. Both exhibit open-shell singlet ground state according to variable-temperature (VT) NMR and magnetic measurements, as well as restricted active space spin-flip (RAS-SF) calculations. The 1,5-P2 isomer has a larger diradical character ([Formula: see text], based on the RAS-SF calculations) and smaller singlet-triplet energy gap ([Formula: see text] kcal/mol, based on SQUID measurements) compared to the 2,6-P2 isomer ([Formula: see text], [Formula: see text] kcal/mol). In addition, 2,6-P2 shows intense one-photon absorption (OPA) ([Formula: see text] nm, [Formula: see text] M[Formula: see text] cm[Formula: see text] and a large two-photon absorption (TPA) cross-section ([Formula: see text] GM at 1400 nm) in the near-infrared region, while 1,5-P2 with larger diradical character displays red-shifted but weaker OPA ([Formula: see text] nm, [Formula: see text] M[Formula: see text] cm[Formula: see text] and a smaller TPA cross-section ([Formula: see text] GM at 1600 nm). Both compounds show four reversible redox waves and 1,5-P2 has a smaller electrochemical energy gap (1.06 eV vs.1.16 eV for 2,6-P2). Therefore, the bridge structure has a significant impact on the diradical character, electronic properties, and magnetic behaviors of the obtained porphyrin-based diradicaloids.

Sulfur-Doped (Dibenzo)heptazethrene and (Dibenzo)octazethrene Diradicaloids.

We report the synthesis and characterization of sulfur-doped dibenzoheptazethrene (DBHZ-2S) and dibenzooctazethrene (DBOZ-2S) dications. Their neutral precursors 9a/9b were synthesized by acid-mediated intramolecular cyclization reaction of sulfoxides and the dications were prepared by treating 9a/9b with NOSbF6 or AgSbF6, respectively. Both dications displayed open-shell singlet ground state with a moderate diradical character (y0 = 0.44 for DBHZ-2S and 0.66 for DBOZ-2S) and a small singlet-triplet energy gap (ΔES-T = -3.9 kcal/mol for DBHZ-2S and -3.2 kcal/mol for DBOZ-2S). The open-shell singlet diradical nature of DBHZ-2S and DBOZ-2S were supported by electronic absorption spectroscopy, electron spin resonance spectroscopy, variable-temperature NMR studies, together with spin-unrestricted density functional theory calculations. However, synthesis and isolation of the sulfur-doped heptazethrene (HZ-2S) and octazethrene (OZ-2S) dications upon chemical oxidation of their corresponding neutral precursors 5a/5b were not successful owing to their high reactivity.

Tunable Reduction of 2,4,6-Tri(4-pyridyl)-1,3,5-Triazine: From Radical Anion to Diradical Dianion to Radical Metal-Organic Framework.

The reduction of 2,4,6-tri(4-pyridyl)-1,3,5-triazine (TPT) with alkali metals resulted in four radical anion salts (1, 2, 4 and 5) and one diradical dianion salt (3). Single-crystal X-ray diffraction and electron paramagnetic resonance (EPR) spectroscopy reveal that 1 contains the monoradical anion TPT.- stacked in one-dimensional (1D) with K+ (18c6) and 2 can be viewed as a 1D magnetic chain of TPT.- , while 4 and 5 form radical metal-organic frameworks (RMOFs). 1D pore passages, with a diameter of 6.0 Å, containing solvent molecules were observed in 5. Variable-temperature EPR measurements show that 3 has an open-shell singlet ground state that can be excited to a triplet state, consistent with theoretical calculation. The work suggests that the direct reduction approach could lead to the formation of RMOFs.

Multi-copper incorporation into ring-expanded base pairs: An ab initio study

In this work, we design four multi-copper-mediated ring-expanded base pairs, which possess improved electromagnetic properties due to the introduction of cyclopentadienyl radicals and copper ions. These designed base pairs have smaller HOMO-LUMO gaps and lower ionization potentials than those of the corresponding natural base pairs. Trans-rG3CurC and rA2CurT possess open-shell singlet ground states. They exhibit variable degrees of magnetic exchange interaction. In addition, their transverse electronic communication by the charge-transfer transitions in the UV absorption spectra is enhanced significantly.

An Ab initio study of double-electron oxidized base pairs with diradical character through ring-expansion modification

In this work, our calculations show the Watson-Crick base pairs remain stable through ring-expansion modification. DFT calculations reveal the optimized structures and novel electromagnetic properties of the designed base pairs. It is found the optimized structures possess open-shell singlet ground states except for [yGC]2+ and [yyGC]2+. They exhibit variable degrees of magnetic exchange interaction. As the size of adenine increases, [yAT]2+ and [yyAT]2+ exhibit stronger antiferromagnetic properties. There is no significant increase in the magnetism of [yG-tC]2+ and [yyG-tC]2+, which is due to the lack of hydrogen bond in WC H-bonding zones. For the other base pairs, the magnetic interaction between the bases becomes weaker due to the increase of the size of the cytosine and thymine. The [yyTA]2+ base pair even exhibits a weak ferromagnetic property.

Cyclobis(7,8-(para-quinodimethane)-4,4'-triphenylamine) and Its Cationic Species Showing Annulene-Like Global (Anti)Aromaticity.

π-Conjugated macrocycles containing all-benzenoid rings usually show local aromaticity, but reported herein is the macrocycle CBQT, containing alternating para-quinodimethane and triphenylamine units displaying annulene-like anti-aromaticity at low temperatures as a result of structural rigidity and participation of the bridging nitrogen atoms in π-conjugation. It was easily synthesized by intermolecular Friedel-Crafts alkylation followed by oxidative dehydrogenation. X-ray crystallographic structures of CBQT, as well as those of its dication, trication, and tetracation were obtained. The dication and tetracation exhibited global aromaticity and antiaromaticity, respectively, as confirmed by NMR measurements and theoretical calculations. Both the dication and tetracation possess open-shell singlet ground states, with a small singlet-triplet gap.

Open-Shell Nonbenzenoid Nanographenes Containing Two Pairs of Pentagonal and Heptagonal Rings

Nonbenzenoid carbocyclic rings are postulated to serve as important structural elements toward tuning the chemical and electronic properties of extended polycyclic aromatic hydrocarbons (PAHs, or namely nanographenes), necessitating a rational and atomically precise synthetic approach toward their fabrication. Here, using a combined bottom-up in-solution and on-surface synthetic approach, we report the synthesis of nonbenzenoid open-shell nanographenes containing two pairs of embedded pentagonal and heptagonal rings. Extensive characterization of the resultant nanographene in solution shows a low optical gap, and an open-shell singlet ground state with a low singlet-triplet gap. Employing ultra-high-resolution scanning tunneling microscopy and spectroscopy, we conduct atomic-scale structural and electronic studies on a cyclopenta-fused derivative on a Au(111) surface. The resultant five to seven rings embedded nanographene displays an extremely narrow energy gap of 0.27 eV and exhibits a pronounced open-shell biradical character close to 1 (y0 = 0.92). Our experimental results are supported by mean-field and multiconfigurational quantum chemical calculations. Access to large nanographenes with a combination of nonbenzenoid topologies and open-shell character should have wide implications in harnessing new functionalities toward the realization of future organic electronic and spintronic devices.

Diindeno-Fused Dibenzo[a,h]anthracene and Dibenzo[c,l]chrysene: Syntheses, Structural Analyses, and Properties.

Diindeno-fused dibenzo[a,h]anthracene 6 and diindeno-fused dibenzo[c,l]chrysene 9 contain the key moieties 1,4-quinodipropene (1,4-QDP) and 2,6-naphthoquinodipropene (2,6-NQDP), respectively, and they both have an open-shell singlet ground state. The latter compound exhibits a strong biradical character and interesting properties, including a low ΔET-S (2.44 kcal mol-1 ), a small HOMO-LUMO gap (1.06 eV), a wide photoabsorption range (250-1172 nm), and a large two-photon absorption cross-section (σ=1342±56 GM). This work verifies that 6 has a slightly larger HOMO-LUMO gap and ΔET-S than its helical isomer diindeno[2,1-f:1',2'-j]picene (DIP), but is a much stronger two-photon absorber, verifying the important effect of geometry on the photophysical properties.

NMR chemical shifts of urea loaded copper benzoate. A joint solid-state NMR and DFT study

We report solid-state 13C NMR spectra of urea-loaded copper benzoate, Cu2(C6H5CO2)4·2(urea), a simplified model for copper paddlewheel-based metal-organic frameworks (MOFs), along with first-principles density functional theory (DFT) computation of the paramagnetic NMR (pNMR) chemical shifts. Assuming a Boltzmann distribution between a diamagnetic open-shell singlet ground state (in a broken-symmetry Kohn-Sham DFT description) and an excited triplet state, the observed δ(13C) values are reproduced reasonably well at the PBE0-⅓/IGLO-II//PBE0-D3/AE1 level. Using the proposed assignments of the signals, the mean absolute deviation between computed and observed 13C chemical shifts is below 30 ppm over a range of more than 1100 ppm.

Stable Diindeno-Fused Corannulene Regioisomers with Open-Shell Singlet Ground States and Large Diradical Characters.

The synthesis of open-shell polycyclic hydrocarbons with large diradical characters is challenging because of their high reactivities. Herein, two diindeno-fused corannulene regioisomers DIC-1 and DIC-2, curved fragments of fullerene C104 , were synthesized that exhibit open-shell singlet ground states. The incorporation of the curved and non-alternant corannulene moiety within diradical systems leads to significant diradical characters as high as 0.98 for DIC-1 and 0.89 for DIC-2. Such high diradical characters can presumably be ascribed to the re-aromatization of the corannulene π system. Although the DIC compounds have large diradical characters, they display excellent stability under ambient conditions. The half-lives are 37 days for DIC-1 and 6.6 days for DIC-2 in solution. This work offers a new design strategy towards diradicaloids with large diradical characters yet maintain high stability.

Orthogonal Oriented Bisanthrancene-Bridged Bis(Triarylamine) Diradical Dications: Isolation, Characterizations and Crystal Structures.

Two bis(triarylamine) diradical dication salts 12+ 2[Al(ORF )4 ]- and 22+ 2[Al(ORF )4 ]- bridged with an orthogonal oriented bisanthrancene motif were synthesized. Their solid state and electronic structures were investigated by various experimental approaches in conjunction with theoretical calculations. Superconducting quantum interference device measurements and density functional theory calculations reveal that both of 12+ and 22+ feature open-shell singlet ground states with rather small singlet-triplet energy gaps; thus, they can be regarded as nearly pure diradicals (biradicals). Their diradical characters are enhanced by the orthogonal configuration of the bisanthrancene moiety which demonstrates the importance of the geometry of bridging units in turning the electronic structures of bis(triarylamine) dications.

Exploring the mechanism of alkene hydrogenation catalyzed by defined iron complex from DFT computation.

UB3LYP computation including dispersion and toluene solvation has been carried to elucidate the mechanisms of alkene hydrogenation catalyzed by bis(imino)pyridine iron dinitrogen complex (iPrPDI)Fe(N2)2, which has low stability towards N2 dissociation. The coordinatively unsaturated complexes, (iPrPDI)Fe(N2) and (iPrPDI)Fe(1-C4H8), favor open-shell singlet ground states. On the basis of our computations, we propose a new mechanism of 1-butene coordination and hydrogenation after N2 dissociation. The hydrogenation of 1-butene undergoes a concerted open-shell singlet transition state involving H2 dissociation, C-H bond formation and C=C bond elongation, as well as the subsequent C-H reductive elimination. In the whole alkene hydrogenation, the H-H bond cleavage is the rate-determining step. Graphical abstract The alkene hydrogenation catalyzed by redox-active pyridine(diimine)-chelate iron complex follows the open-shell singlet state path.

The Diradical-Dication Strategy for BODIPY- and Porphyrin-Based Dyes with Near-Infrared Absorption Maxima from 1070 to 2040 nm.

Four stable boron dipyrromethene (BODIPY)- and porphyrin-based bis-arylamine diradical dications were synthesized by two-electron oxidation of their neutral molecules. The two BODIPY-based dications have open-shell singlet ground states. UV/Vis absorption spectra of all four dications showed large redshifts in the NIR region compared to their neutral precursors with absorption maxima at 1274 and 1068 nm for the two BODIPY-based dications and 1746 and 2037 nm for the two porphyrin-based dications. Thus, two new types of NIR dyes with longer wavelengths are provided by the diradical-dication strategy, which can be applied for the generation of other NIR dyes with a range of different chromophores and auxochromes.