Design of an open-shell nitrogen-centered diradicaloid with tunable stimuli-responsive electronic properties

Organic diradicaloids usually display an open-shell singlet ground state with significant singlet diradical character (y₀) which endow them with intriguing physiochemical properties and wide applications. In this study, we present the design of an open-shell nitrogen-centered diradicaloid which can reversibly respond to multiple stimuli and display the tunable diradical character and chemo-physical properties. 1a was successfully synthesized through a simple and high-yielding two-step synthetic strategy. Both experimental and calculated results indicated that 1a displayed an open-shell singlet ground state with small singlet-triplet energy gap (ΔE_S-T = -2.311 kcal mol^-1) and a modest diradical character (y₀ = 0.60). Interestingly, 1a was demonstrated to undergo reversible Lewis acid-base reaction to form acid-base adducts, which was proven to effectively tune the ground-state electronic structures of 1a as well as its diradical character and spin density distributions. Based on this, we succeeded in devising a photoresponsive system based on 1a and a commercially available photoacid merocyanine (MEH). We believe that our studies including the molecular design methodology and the stimuli-responsive organic diradicaloid system will open up a new way to develop organic diradicaloids with tunable properties and even intelligent-responsive diradicaloid-based materials.

Quinoidal Azaacenes: 99 % Diradical Character

Quinoidal azaacenes with almost pure diradical character (y=0.95 to y=0.99) were synthesized. All compounds exhibit paramagnetic behavior investigated by EPR and NMR spectroscopy, and SQUID measurements, revealing thermally populated triplet states with an extremely low-energy gap ΔEST' of 0.58 to 1.0 kcal mol-1 . The species are persistent in solution (half-life≈14-21 h) and in the solid state they are stable for weeks.

A BODIPY-Bridged Bisphenoxyl Diradicaloid: Solvent-Dependent Diradical Character and Physical Properties

We report a new boron dipyrromethene (BODIPY)-bridged bisphenoxyl diradicaloid (2), which showed closed-shell diamagnetic character in less polar solvents such as dichloromethane but open-shell diradical character with paramagnetic activity in the very polar solvent N,N-dimethylformamide. X-ray crystallographic analysis of 2 revealed an anti-parallel stacked dimer structure via intermolecular dipole-dipole interaction, and the observed solvent-dependent diradical character can be explained by the different dihedral angles between the phenoxyl units and the BODIPY bridge, and structural flexibility of the molecule in different solvents. Compound 2 also exhibited solvent-dependent optical and electrochemical properties.

A near-infrared benzoquinone-coupled BODIPY: Synthesis, spectroscopic and electrochemical properties

A near-infrared absorbing boron-dipyrromethene (BODIPY) chromophore coupled with two benzoquinone moieties at its 3,5-positions, 3, was prepared via Knoevenagel condensation of 1,3,5,7-tetramethyl-8-(4[Formula: see text]-benzonitrile) BODIPY 1 with 3,5-di-tert-butyl-4-hydroxybenzaldehyde to afford 1,7-dimethyl-3,5-di-(4[Formula: see text]-hydroxy-3[Formula: see text],5[Formula: see text]-di-tert-butyl styryl)-8-(4[Formula: see text]-benzonitrile) BODIPY 2, followed by oxidization with Ag2O in good yield (91%). The UV-vis-NIR absorption spectrum of 3 exhibits two major bands at 795 and 895 nm in the near-IR region, while 2shows maximum absorbance at 661 nm and strong fluorescence at 692 nm ([Formula: see text] 0.59). The cyclic voltammetry of 3 consists of two pairs of reversible one-electron reductions at -0.61 V and -0.88 V and two pairs of one-electron oxidation waves at 0.26 V and 0.54 V. Compared with the redox potentials of 2([Formula: see text] V and [Formula: see text] 0.25 V), the first reduction of 3 is anodically shifted for 710 mV, whereas the first oxidation potential is close. Theoretical calculation reveals that conjugation with the benzoquinone moieties on the BODIPY chromophore significantly lowers the LUMO energy level and the HOMO–LUMO energy gap, resulting in a dramatic bathochromic shift of the S0–S1 transition of 3 compared with that of 2. X-ray crystallographic analysis of 3 reveals that the whole molecule adopts a V-type twisted conformation along the delocalized [Formula: see text]-conjugated pathway.

Nitrogen analogues of Chichibabin's and Müller's hydrocarbons with small singlet-triplet energy gaps

Nitrogen analogues of Chichibabin's and Müller's hydrocarbons, DPh-D and TPh-D, based on 1,2,4-benzotriazinyl (Blatter) were studied. The two diradicaloids with good chemical and thermal stability exhibit smaller singlet-triplet energy gaps (ΔES-T from -1.05 to -1.27 kcal mol-1) than the hydrocarbon diradicaloids with the same bridges.

MoS2 Quantum Dot Growth Induced by S Vacancies in a ZnIn2S4 Monolayer: Atomic-Level Heterostructure for Photocatalytic Hydrogen Production

It is highly demanded to steer the charge flow in photocatalysts for efficient photocatalytic hydrogen reactions (PHRs). In this study, we developed a smart strategy to position MoS₂ quantum dots (QDs) at the S vacancies on a Zn facet in monolayered ZnIn₂S₄ (Vs-M-ZnIn₂S₄) to craft a two-dimensional (2D) atomic-level heterostructure (MoS₂QDs@Vs-M-ZnIn₂S₄). The electronic structure calculations indicated that the positive charge density of the Zn atom around the sulfur vacancy (Vs) was more intensive than other Zn atoms. The Vs confined in monolayered ZnIn₂S₄ established an important link between the electronic manipulation and activities of ZnIn₂S₄. The Vs acted as electron traps, prevented vertical transmission of electrons, and enriched electrons onto the Zn facet. The Vs-induced atomic-level heterostructure sewed up vacancy structures of Vs-M-ZnIn₂S₄, resulting in a highly efficient interface with low edge contact resistance. Photogenerated electrons could quickly migrate to MoS₂QDs through the intimate Zn-S bond interfaces. As a result, MoS₂QDs@Vs-M-ZnIn₂S₄ showed a high PHR activity of 6.884 mmol g^-1 h^-1, which was 11 times higher than 0.623 mmol g^-1 h^-1 for bulk ZnIn₂S₄, and the apparent quantum efficiency reached as high as 63.87% (420 nm). This work provides a prototype material for looking into the role of vacancies between electronic structures and activities in 2D photocatalytic materials and gives insights into PHR systems at the atomic level.

The Suzuki–Miyaura reaction as a tool for modification of phenoxyl-nitroxyl radicals of the 4H-imidazole N-oxide series

2-(3,5-Di-tert-butyl-4-hydroxyphenyl)-5-(4-iodophenyl)-4,4-dimethyl-4H-imidazole 3-oxide reacts with phenylboronic acid and its substituted derivatives in a cross-coupling reaction of the Suzuki–Miyaura type to form 5-biphenyl derivatives of 4H-imidazole-N-oxide. Interaction of the same compound with B₂(pin)₂ in the presence of PdCl₂(PPh₃)₂ proceeds through the formation of intermediate 1,3,2-dioxoborolane and leads to the product of homocoupling: biphenyl-bis(imidazole). Oxidation of the resultant imidazoles with lead dioxide quantitatively yields stable conjugated phenoxyl-nitroxyl mono- and diradicals, which are of interest as electroactive paramagnetic materials. The crystal structure of the monoradical, 2,6-di-tert-butyl-4-[1-oxido-4-(biphenyl-4-yl)-5,5-dimethyl-1H-imidazole-2(5H)-ylidene]cyclohex-2,5-dienone, its magnetic susceptibility, EPR spectra of the obtained hybrid radicals in solution, and cyclic voltammetry characteristics of 4H-imidazoles were studied.

Pd-catalyzed cross-coupling reaction as a tool for modification of hybrid phenoxyl-nitroxides.