Larger Diradical Character Research Articles

Enhancing stability of diradical polycyclic hydrocarbons via P=O-attaching

Diradical polycyclic hydrocarbons (PHs) have unique open-shell structures and interesting physical properties. However, owing to high reactivity of unpaired electrons, such open-shell organic diradicaloids are usually less stable than closed-shell systems, limiting their practical applications. In this study, we report P=O-attaching of diradical PHs as a new strategy to enhance their stability while maintaining diradical properties. Three P=O-attached PHs containing the indeno[1,2-b]fluorene, fluoreno[3,2-b]fluorene and indeno[2,1-b]fluorene π-skeletons, respectively, were designed and synthesized. As theoretically and experimentally proved, two of them have the relatively large diradical characters and open-shell singlet diradical nature. In comparison to their all-carbon analogues, the attached electron-withdrawing P=O groups endow them with much lower LUMO/HOMO energy levels but preserved magnetic activities and physical properties, such as thermally accessible triplet species and multi-redox ability. Moreover, the P=O groups effectively decrease their oxidation activities and thereby lead to their remarkably excellent ambient stabilities. Thus, this P=O-attaching strategy will be applicable to other diradical PH systems and may promote the generation of stable organic diradicaloids for radical chemistry and materials.

77 % Photothermal Conversion in Blatter-Type Diradicals: Photophysics and Photodynamic Applications.

Diradicals based on the Blatter units and connected by acetylene and alkene spacers have been prepared. All the molecules show sizably large diradical character and low energy singlet-triplet gaps. Their photo-physical properties concerning their lowest energy excited state have been studied in detail by steady-state and time-resolved absorption spectroscopy. We have fully identified the main optical absorption band and full absence of emission from the lowest energy excited state. A computational study has been also carried out that has helped to identify the presence of a conical intersection between the lowest energy excited state and the ground state which produces a highly efficient light-to-heat conversion of the absorbed radiation. Furthermore, an outstanding photo-thermal conversion 77.23 % has been confirmed, close to the highest in the diradicaloid field. For the first time, stable diradicals are applied to photo-thermal therapy of tumor cells with good stability and satisfactory performance at near-infrared region.

77 % Photothermal Conversion in Blatter‐Type Diradicals: Photophysics and Photodynamic Applications

AbstractDiradicals based on the Blatter units and connected by acetylene and alkene spacers have been prepared. All the molecules show sizably large diradical character and low energy singlet‐triplet gaps. Their photo‐physical properties concerning their lowest energy excited state have been studied in detail by steady‐state and time‐resolved absorption spectroscopy. We have fully identified the main optical absorption band and full absence of emission from the lowest energy excited state. A computational study has been also carried out that has helped to identify the presence of a conical intersection between the lowest energy excited state and the ground state which produces a highly efficient light‐to‐heat conversion of the absorbed radiation. Furthermore, an outstanding photo‐thermal conversion 77.23 % has been confirmed, close to the highest in the diradicaloid field. For the first time, stable diradicals are applied to photo‐thermal therapy of tumor cells with good stability and satisfactory performance at near‐infrared region.

From closed-shell edge-extended kekulenes to open-shell carbonylated cycloarene diradicaloid.

The precise synthesis of cycloarenes remains a challenging topic in both organic chemistry and materials science due to their unique fully fused macrocyclic π-conjugated structure. Herein, a series of alkoxyl- and aryl-cosubstituted cycloarenes (kekulene and edge-extended kekulene derivatives, K1-K3) were conveniently synthesized and an unexpected transformation of the anthryl-containing cycloarene K3 into a carbonylated cycloarene derivative K3-R was disclosed by controlling the temperature and gas atmosphere of the Bi(OTf)3-catalyzed cyclization reaction. All their molecular structures were confirmed by single-crystal X-ray analysis. The crystallographic data, NMR measurements, and theoretical calculations reveal their rigid quasi-planar skeletons, dominant local aromaticities, and decreasing intermolecular π-π stacking distance with extension of the two opposite edges. The much lower oxidation potential for K3 by cyclic voltammetry explains its unique reactivity. Moreover, carbonylated cycloarene derivative K3-R shows a remarkable stability, large diradical character, a small singlet-triplet energy gap (ΔES-T = -1.81 kcal mol-1), and weak intramolecular spin-spin coupling. Most importantly, it represents the first example of carbonylated cycloarene diradicaloids as well as the first example of radical-acceptor cycloarenes and will shed some light on synthesis of extended kekulenes and conjugated macrocyclic diradicaloids and polyradicaloids.

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.

A Stable [4,3]Peri‐acene Diradicaloid: Synthesis, Structure, and Electronic Properties

AbstractThe synthesis of peri‐fused acenes (peri‐acenes) with two or more rows is challenging due to their intrinsic open‐shell diradical character. Herein, we report the isolation of a derivative (4) of [4,3]peri‐acene in crystalline form. The parent [4,3]peri‐acene, containing three rows of tetracene, has a large diradical character (y0=94.8 %) originating from aromatic stabilization. Due to kinetic blocking, 4 showed a reasonable stability with a half‐life time of ≈157 h under ambient conditions. Its structure was determined by X‐ray crystallographic analysis, and bond‐length analysis revealed eight localized Clar's sextets. 4 exhibited an open‐shell singlet ground state with a narrow electrochemical energy gap (1.13 eV) and a small singlet–triplet energy gap (−0.57 kcal mol−1 from SQUID measurements). Its electronic properties are compared with previously reported peri‐tetracene and teranthene derivatives.

A Stable [4,3]Peri-acene Diradicaloid: Synthesis, Structure, and Electronic Properties.

The synthesis of peri-fused acenes (peri-acenes) with two or more rows is challenging due to their intrinsic open-shell diradical character. Herein, we report the isolation of a derivative (4) of [4,3]peri-acene in crystalline form. The parent [4,3]peri-acene, containing three rows of tetracene, has a large diradical character (y0 =94.8 %) originating from aromatic stabilization. Due to kinetic blocking, 4 showed a reasonable stability with a half-life time of ≈157 h under ambient conditions. Its structure was determined by X-ray crystallographic analysis, and bond-length analysis revealed eight localized Clar's sextets. 4 exhibited an open-shell singlet ground state with a narrow electrochemical energy gap (1.13 eV) and a small singlet-triplet energy gap (-0.57 kcal mol-1 from SQUID measurements). Its electronic properties are compared with previously reported peri-tetracene and teranthene derivatives.

A Stable Nitrogen‐centered Bis(imino)perylene Dimer‐based Diradicaloid

AbstractCarbon‐centered radicals are usually reactive and there are only few stable heteroatom‐centered organic radicals. Herein, we report a nitrogen‐centered bis(imino)perylene dimer based diradicaloid 2Per‐2 N, which has a large diradical character (y0=72%) but still shows reasonable stability due to kinetic blocking of the aminyl radical sites by the bulky and electron‐deficient 2,4,6‐trichlorophenyl groups and effective spin delocalization. It displays thermally populated paramagnetic activity with a moderate singlet‐triplet energy gap (ΔES–T=−3.55 kcal/mol). Compared with a fully fused bis(imino)quaterrylene analogue (QR‐2 N) which is a typical closed‐shell compound, 2Per‐2 N possesses a much larger diradical character mainly due to the steric repulsion between the neighboring perylene units in the rigid quinoidal form. 2Per‐2 N exhibits a much smaller energy gap and longer absorption wavelength compared with QR‐2 N. 2Per‐2 N represents a rare example of stable nitrogen‐centered diradicaloid with a large diradical character.

Understanding the nature of quinoidal and zwitterionic states in carbazole-based diradicals.

We report two carbazole-based diradicals, out of which the m-isomer shows a large diradical character y0 (0.89) and a small singlet-triplet energy gap ΔES-T (-0.98 kcal mol-1), whereas the p-isomer exhibits smaller y0 (0.79) but a much larger ΔES-T (-6.16 kcal mol-1). DFT calculations reveal that this tendency is also suitable for nitrogen and carbon-centered diradicals.

Frontispiece: Stable Diindeno‐Fused Corannulene Regioisomers with Open‐Shell Singlet Ground States and Large Diradical Characters

Frontispiece: Stable Diindeno‐Fused Corannulene Regioisomers with Open‐Shell Singlet Ground States and Large Diradical Characters

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.

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

AbstractThe 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.

Nitrogen-coupled blatter diradicals: the fused versus unfused bridges

We report the synthesis and characterization of nitrogen-coupled diradicals NDR (unfused) and CDR (fused).

Fluoreno[2,1-a]fluorene: an ortho-naphthoquinodimethane-based system with partial diradical character.

Fluoreno[2,1-a]fluorene, a molecule comprising fused ortho-quinodimethane units in a 1,5-napthoquinodimethane core, has been prepared and investigated with spectroscopy (UV-Vis-NIR, 1H-NMR and Raman), SQUID magnetometry, spectroelectrochemistry and quantum chemistry. While para-quinodimethanes with a 2,6-substitution pattern exist as closed-shell species and meta-quinodimethanes with 2,7-substitution favour a ground electronic state with very large diradical character, our 1,5-substituted ortho-naphthoquinodimethane-based system exhibits an intermediate degree of diradical character.

Superoctazethrene: An Open-Shell Graphene-like Molecule Possessing Large Diradical Character but Still with Reasonable Stability.

Laterally extended zethrenes can be regarded as Z-shaped nanographenes with four zigzag edges, but their synthesis is very challenging. Herein, we report the successful synthesis of by far the largest zethrene molecule, a superoctazethrene (SOZ) derivative SOZ-Cl, in crystalline form. Although the parent SOZ is calculated to have a very large diradical character ( y0 = 81.0%), SOZ-Cl shows reasonable stability ( t1/2 = 64 h under ambient conditions) and can be purified by silica gel column chromatography. It exhibits a small electrochemical energy gap ( Eg = 1.01 eV) and characteristics for open-shell singlet diradicaloids. Compared with a previously reported octazethrene derivative (OZ-TIPS), SOZ-Cl shows much larger diradical character ( y0 = 76.3% vs 43.4%) and smaller singlet-triplet gap (Δ ES-T = -2.30 kcal/mol vs -3.87 kcal/mol). Calculations also demonstrate global aromatic character of SOZ, but the smaller size octazethrene shows local aromaticity.

Toward helical-shaped diradicaloids: cyclobutenyl o-quinodimethane-bridged indeno[1,2-b]fluorenes.

Open-shell diradicaloids have been intensively investigated recently due to their attractive and unique optical, electronic and magnetic properties. However, open-shell diradicaloids with non-planar conjugated frameworks are less studied, especially helical-shaped π systems. In this work, we synthesized tetraindeno-based singlet biradicals in a helical geometry, bridged by a curved cyclobutenyl o-quinodimethane molecule. Large diradical characters and extremely small singlet-triplet energy gaps, as well as unusual half-field transitions (forbidden ΔMs = 2) were revealed for these 3D diradicals.

The double exciton state of conjugated chromophores with strong diradical character: insights from TDDFT calculations.

A peculiar characteristic of open-shell singlet diradical molecules is the presence of a double exciton state (H,H → L,L) among low lying excited states. Recent high-level quantum-chemical investigations including a static and dynamic electron correlation have demonstrated that this state can become the lowest singlet excited state, a diagnostic fingerprint of the diradical system. Here we investigate the performance of less computationally demanding TDDFT calculations by employing two approaches: the spin-flip TDDFT scheme and TD calculations based on unrestricted broken symmetry antiparallel-spin reference configuration (TDUDFT). The calculations are tested on a number of recently synthesized, large conjugated systems displaying from moderate to large diradical character and showing experimental trace of the double exciton state. We show that spin-flip (SF) TDB3LYP calculations in the collinear approximation generally underestimate the excitation energy of the double exciton state. When the molecule displays a strong diradical character, the unrestricted antiparallel-spin reference configuration of TDUDFT calculations is characterized by strongly localized frontier molecular orbitals. We show that under these conditions the double exciton state is captured by TDUB3LYP calculations since it is described by singly excited configurations and its excitation energy can be accurately predicted. Owing to the improved description of the ground state, also the excitation energy of the single exciton H → L state generally improves at the TDUB3LYP level. With regard to the double exciton state, SF TDB3LYP performs slightly better for small to medium diradical character while a large diradical character (and strong orbital localization) is a prerequisite for the success of TDUB3LYP calculations whose quality otherwise deteriorates.

A comparative computational study of C[sbnd]N and C[sbnd]C bonding visible to NIR absorbing croconines

The lowest electronic excitations and charge transfer properties in two series of croconine dyes; 1) molecules with CN bonding, having an absorption in the visible region (400–600 nm) and 2) molecules with CC bonding, showing absorption in visible to near infrared (NIR) region (600–1100 nm) are analyzed by quantum-chemical calculations. The absorption maxima in CC bonding croconines (CCR) are always having 200–300 nm red shifted than its corresponding CN bonding croconines (NCR). The reason for this drastic red shift in CCR series than its corresponding NCR has been systematically studied by DFT, TDDFT and SAC-CI methods. It is found that, CCR series are with less charge transfer in nature and are having larger diradical character, whereas NCR series molecules showing larger charge transfer with lower diradical character. The change in bonding mode of central five membered croconate ring, from CN to CC, destabilization and/stabilization of HOMO LUMO levels were observed. This study may helpful in the design and synthesis of new visible to NIR absorbing croconine dyes which are useful in materials applications.

Stable Oxindolyl-Based Analogues of Chichibabin's and Müller's Hydrocarbons.

Chichibabin's and Müller's hydrocarbons are classical open-shell singlet diradicaloids but they are highly reactive. Herein we report the successful synthesis of their respective stable analogues, OxR-2 and OxR-3, based on the newly developed oxindolyl radical. X-ray crystallographic analysis on OxR-2 reveals a planar quinoidal backbone similar to Chichibabin's hydrocarbon, in accordance with its small diradical character (y0 =11.1 %) and large singlet-triplet gap (ΔES-T =-10.8 kcal mol-1 ). Variable-temperature NMR studies on OxR-2 disclose a slow cis/trans isomerization process in solution through a diradical transition state, with a moderate energy barrier (ΔG≠298K =15-16 kcal mol-1 ). OxR-3 exhibits a much larger diradical character (y0 =80.6 %) and a smaller singlet-triplet gap (ΔES-T =-3.5 kcal mol-1 ), and thus can be easily populated to paramagnetic triplet diradical. Our studies provide a new type of stable carbon-centered monoradical and diradicaloid.

Stable Oxindolyl‐Based Analogues of Chichibabin's and Müller's Hydrocarbons

AbstractChichibabin's and Müller's hydrocarbons are classical open‐shell singlet diradicaloids but they are highly reactive. Herein we report the successful synthesis of their respective stable analogues, OxR‐2 and OxR‐3, based on the newly developed oxindolyl radical. X‐ray crystallographic analysis on OxR‐2 reveals a planar quinoidal backbone similar to Chichibabin's hydrocarbon, in accordance with its small diradical character (y0=11.1 %) and large singlet–triplet gap (ΔES‐T=−10.8 kcal mol−1). Variable‐temperature NMR studies on OxR‐2 disclose a slow cis/trans isomerization process in solution through a diradical transition state, with a moderate energy barrier (ΔG≠298K=15–16 kcal mol−1). OxR‐3 exhibits a much larger diradical character (y0=80.6 %) and a smaller singlet–triplet gap (ΔES‐T=−3.5 kcal mol−1), and thus can be easily populated to paramagnetic triplet diradical. Our studies provide a new type of stable carbon‐centered monoradical and diradicaloid.