Insight into the drastically different triplet lifetimes of BODIPY obtained by optical/magnetic spectroscopy and theoretical computations

The triplet state lifetimes of organic chromophores are crucial for fundamental photochemistry studies as well as applications as photosensitizers in photocatalysis, photovoltaics, photodynamic therapy and photon upconversion. It is noteworthy that the triplet state lifetime of a chromophore can vary significantly for its analogues, while the exact reason was rarely studied. Herein with a few exemplars of typical BODIPY derivatives, which show triplet lifetimes varying up to 110-fold (1.4–160 μs), we found that for these derivatives with short triplet state lifetimes (ca. 1–3 μs), the electron spin polarization (ESP) pattern of the time-resolved electron paramagnetic resonance (TREPR) spectra of the triplet state is inverted at a longer delay time after laser pulse excitation, as a consequence of a strong anisotropy in the decay rates of the zero-field state sublevel of the triplet state. For the derivatives showing longer triplet state lifetimes (>50 μs), no such ESP inversion was observed. The observed fast decay of one sublevel is responsible for the short triplet state lifetime; theoretical computations indicate that it is due to a strong coupling between the T_z sublevel and the ground state mediated by the spin–orbit interaction. Another finding is that the heavy atom effect on the shortening of the triplet state lifetime is more significant for the T₁ states with lower energy. To the best of our knowledge, this is the first systematic study to rationalize the short triplet state lifetime of visible-light-harvesting organic chromophores. Our results are useful for fundamental photochemistry and the design of photosensitizers showing long-lived triplet states.

The electron spin polarization inversion and anisotropic decay of triplet substates explain the short triplet state lifetime of BODIPY derivatives.     

Broad-band spectroscopy of a vanadyl porphyrin: a model electronuclear spin qudit

We explore how to encode more than a qubit in vanadyl porphyrin molecules hosting a S = 1/2 electronic spin coupled to a I = 7/2 nuclear spin. The spin Hamiltonian and its parameters, as well as the spin dynamics, have been determined via a combination of electron paramagnetic resonance, heat capacity, magnetization and on-chip magnetic spectroscopy experiments performed on single crystals. We find low temperature spin coherence times of micro-seconds and spin relaxation times longer than a second. For sufficiently strong magnetic fields (B > 0.1 T, corresponding to resonance frequencies of 9–10 GHz) these properties make vanadyl porphyrin molecules suitable qubit realizations. The presence of multiple equispaced nuclear spin levels then merely provides 8 alternatives to define the ‘1’ and ‘0’ basis states. For lower magnetic fields (B < 0.1 T), and lower frequencies (<2 GHz), we find spectroscopic signatures of a sizeable electronuclear entanglement. This effect generates a larger set of allowed transitions between different electronuclear spin states and removes their degeneracies. Under these conditions, we show that each molecule fulfills the conditions to act as a universal 4-qubit processor or, equivalently, as a d = 16 qudit. These findings widen the catalogue of chemically designed systems able to implement non-trivial quantum functionalities, such as quantum simulations and, especially, quantum error correction at the molecular level.

We show that a sizeable electronuclear entanglement of the S = 1/2 and I = 7/2 spins of a vanadyl porphyrin provides the conditions to act as a universal 4-qubit processor, and thus implement quantum error correction at the molecular level.     

A highly emissive AIE-active luminophore exhibiting deep-red to near-infrared piezochromism and high-quality lasing

Further development of high-efficiency and low-cost organic fluorescent materials is intrinsically hampered by the energy gap law and spin statistics, especially in the near-infrared (NIR) region. Here we design a novel building block with aggregation-induced emission (AIE) activity for realizing highly efficient luminophores covering the deep-red and NIR region, which originates from an increase in the orbital overlap and electron-withdrawing ability. An organic donor–acceptor molecule (BPMT) with the building block is prepared and can readily form J-type molecular columns with multiple C–H⋯N/O interactions. Notably, such synthesized materials can emit fluorescence centered at 701 nm with extremely high photoluminescence quantum yields (PLQYs) of 48.7%. Experimental and theoretical investigations reveal that the formation of the hybridized local and charge-transfer (HLCT) state and substantial C–H⋯N/O interactions contribute to a fast radiative decay rate and a slow nonradiative decay rate, respectively, resulting in high PLQYs in the solid state covering the NIR range. Remarkably, such BPMT crystals, as a first example, reveal strong-penetrability piezochromism along with a distinct PL change from the deep-red (λ_max = 704 nm) to NIR (λ_max = 821 nm) region. Moreover, such typical AIE-active luminophores are demonstrated to be a good candidate as a lasing medium. Together with epoxy resin by a self-assembly method, a microlaser is successfully illustrated with a lasing wavelength of 735.2 nm at a threshold of 22.3 kW cm⁻². These results provide a promising approach to extend the contents of deep-red/NIR luminophores and open a new avenue to enable applications ranging from chemical sensing to lasing.

A HLCT-type luminophore is prepared with bright deep-red fluorescence, showing high-performance piezochromism and lasing.     

Light- and temperature-assisted spin state annealing: accessing the hidden multistability

Among responsive multistable materials, spin crossover (SCO) systems are of particular interest for stabilizing multiple spin states with various stimulus inputs and physical outputs. Here, in a 2D Hofmann-type coordination polymer, [Fe(isoq)₂{Au(CN)₂}₂] (isoq = isoquinoline), a medium-temperature annealing process is introduced after light/temperature stimulation, which accesses the hidden multistability of the spin state. With the combined effort of magnetic, crystallographic and Mössbauer spectral investigation, these distinct spin states are identified and the light- and temperature-assisted transition pathways are clarified. Such excitation-relaxation and trapping-relaxation joint mechanisms, as ingenious interplays between the kinetic and thermodynamic effects, uncover hidden possibilities for the discovery of multistable materials and the development of multistate intelligent devices.

Two new two-stage manipulation protocols, namely light- and temperature-assisted spin state annealing (LASSA/TASSA), are applied to a spin crossover coordination polymer, [Fe(isoq)₂{Au(CN)₂}₂], revealing the hidden multistability of spin states.     

A clustering-triggered emission strategy for tunable multicolor persistent phosphorescence

A clustering-triggered emission (CTE) strategy, namely the formation of heterogeneous clustered chromophores and conformation rigidification, for achieving tunable multicolor phosphorescence in single-component compounds is proposed. Non-conventional luminophores comprising just oxygen functionalities and free of π-bonding, i.e., d-(+)-xylose (d-Xyl), pentaerythritol (PER), d-fructose (d-Fru) and d-galactose (d-Gal), were adopted as a simple model system with an explicit structure and molecular packing to address the hypothesis. Their concentrated solutions and crystals at 77 K or under ambient conditions demonstrate remarkable multicolor phosphorescence afterglows in response to varying excitation wavelengths, because of the formation of diverse oxygen clusters with sufficiently rigid conformations. The intra- and inter-molecular O⋯O interactions were definitely illustrated by both single crystal structure analysis and theoretical calculations. These findings shed new light on the origin and simple achievement of tunable multicolor phosphorescence in single-component pure organics, and in turn, have strong implications for the emission mechanism of non-conventional luminophores.

A clustering-triggered emission strategy is proposed to readily realize tunable multicolor afterglows in single-component pure organic compounds.     

Chemical tuning of spin clock transitions in molecular monomers based on nuclear spin-free Ni(ii)

We report the existence of a sizeable quantum tunnelling splitting between the two lowest electronic spin levels of mononuclear Ni complexes. The level anti-crossing, or magnetic “clock transition”, associated with this gap has been directly monitored by heat capacity experiments. The comparison of these results with those obtained for a Co derivative, for which tunnelling is forbidden by symmetry, shows that the clock transition leads to an effective suppression of intermolecular spin–spin interactions. In addition, we show that the quantum tunnelling splitting admits a chemical tuning via the modification of the ligand shell that determines the crystal field and the magnetic anisotropy. These properties are crucial to realize model spin qubits that combine the necessary resilience against decoherence, a proper interfacing with other qubits and with the control circuitry and the ability to initialize them by cooling.

We have directly monitored spin level anti-crossings, or “clock transitions”, in Ni(ii) molecular monomers and shown that the quantum tunnelling gap admits a chemical tuning.     

Controlling the structure and photophysics of fluorophore dimers using multiple cucurbit[8]uril clampings

A modular strategy has been employed to develop a new class of fluorescent molecules, which generates discrete, dimeric stacked fluorophores upon complexation with multiple cucurbit[8]uril macrocycles. The multiple constraints result in a “static” complex (remaining as a single entity for more than 30 ms) and facilitate fluorophore coupling in the ground state, showing a significant bathochromic shift in absorption and emission. This modular design is surprisingly applicable and flexible and has been validated through an investigation of nine different fluorophore cores ranging in size, shape, and geometric variation of their clamping modules. All fluorescent dimers evaluated can be photo-excited to atypical excimer-like states with elongated excited lifetimes (up to 37 ns) and substantially high quantum yields (up to 1). This strategy offers a straightforward preparation of discrete fluorophore dimers, providing promising model systems with explicitly stable dimeric structures and tunable photophysical features, which can be utilized to study various intermolecular processes.

Dimerisation of a wide range of fluorophores through multiple CB[8] clampings leads to constrained intracomplex motion and distinct photophysical properties.     

Single-ion magnetism in the extended solid-state: insights from X-ray absorption and emission spectroscopy

Large single-ion magnetic anisotropy is observed in lithium nitride doped with iron. The iron sites are two-coordinate, putting iron doped lithium nitride amongst a growing number of two coordinate transition metal single-ion magnets (SIMs). Uniquely, the relaxation times to magnetisation reversal are over two orders of magnitude longer in iron doped lithium nitride than other 3d-metal SIMs, and comparable with high-performance lanthanide-based SIMs. To understand the origin of these enhanced magnetic properties a detailed characterisation of electronic structure is presented. Access to dopant electronic structure calls for atomic specific techniques, hence a combination of detailed single-crystal X-ray absorption and emission spectroscopies are applied. Together K-edge, L_2,3-edge and Kβ X-ray spectroscopies probe local geometry and electronic structure, identifying iron doped lithium nitride to be a prototype, solid-state SIM, clean of stoichiometric vacancies where Fe lattice sites are geometrically equivalent. Extended X-ray absorption fine structure and angular dependent single-crystal X-ray absorption near edge spectroscopy measurements determine Fe^I dopant ions to be linearly coordinated, occupying a D_6h symmetry pocket. The dopant engages in strong 3dπ-bonding, resulting in an exceptionally short Fe–N bond length (1.873(7) Å) and rigorous linearity. It is proposed that this structure protects dopant sites from Renner–Teller vibronic coupling and pseudo Jahn–Teller distortions, enhancing magnetic properties with respect to molecular-based linear complexes. The Fe ligand field is quantified by L_2,3-edge XAS from which the energy reduction of 3d_z² due to strong 4s mixing is deduced. Quantification of magnetic anisotropy barriers in low concentration dopant sites is inhibited by many established methods, including far-infrared and neutron scattering. We deduce variable temperature L₃-edge XAS can be applied to quantify the J = 7/2 magnetic anisotropy barrier, 34.80 meV (∼280 cm⁻¹), that corresponds with Orbach relaxation via the first excited, M_J = ±5/2 doublet. The results demonstrate that dopant sites within solid-state host lattices could offer a viable alternative to rare-earth bulk magnets and high-performance SIMs, where the host matrix can be tailored to impose high symmetry and control lattice induced relaxation effects.

Taking advantage of synchrotron light source methods, we present the geometric and electronic structure of iron doped in lithium nitride.     

Tuning phenoxyl-substituted diketopyrrolopyrroles from quinoidal to biradical ground states through (hetero-)aromatic linkers

Strongly fluorescent halochromic 2,6-di-tert-butyl-phenol-functionalised phenyl-, thienyl- and furyl-substituted diketopyrrolopyrrole (DPP) dyes were deprotonated and oxidised to give either phenylene-linked DPP1˙˙ biradical (y₀ = 0.75) with a singlet open shell ground state and a thermally populated triplet state (ΔE_ST = 19 meV; 1.8 kJ mol⁻¹; 0.43 kcal mol⁻¹) or thienylene/furylene-linked DPP2q and DPP3q compounds with closed shell quinoidal ground states. Accordingly, we identified the aromaticity of the conjugated (hetero-)aromatic bridge to be key for modulating the electronic character of these biradicaloid compounds and achieved a spin crossover from closed shell quinones DPP2q and DPP3q to open shell biradical DPP1˙˙ as confirmed by optical and magnetic spectroscopic studies (UV/vis/NIR, NMR, EPR) as well as computational investigations (spin-flip TD-DFT calculations in combination with CASSCF(4,4) and harmonic oscillator model of aromaticity (HOMA) analysis). Spectroelectrochemical studies and comproportionation experiments further prove the reversible formation of mixed-valent radical anions for the DPP2q and DPP3q quinoidal compounds with absorption bands edging into the NIR spectral region.

By variation of spacer aromaticity, a spin crossover from thienylene/furylene-linked quinones DPP2q/DPP3q to phenylene-bridged biradical DPP1˙˙ (y₀ = 0.75) with a singlet open shell ground state (ΔE_ST = 19 meV) was achieved.     

Origin of the temperature dependence of 13C pNMR shifts for copper paddlewheel MOFs

An efficient protocol for the calculation of ¹³C pNMR shifts in metal–organic frameworks based on Cu(ii) paddlewheel dimers is proposed, which involves simplified structural models, optimised using GFN2-xTB for the high-spin state, and CAM-B3LYP-computed NMR and EPR parameters. Models for hydrated and activated HKUST-1 and hydrated STAM MOFs with one, two and three Cu dimers have been used. The electronic ground states are low-spin and diamagnetic, with pNMR shifts arising from thermal population of intermediate- and high-spin excited states. Treating individual spin configurations in a broken symmetry (BS) approach, and selecting two or more of these to describe individual excited states, the magnetic shieldings of these paramagnetic states are evaluated using the approach by Hrobárik and Kaupp. The total shielding is then evaluated from a Boltzmann distribution between the energy levels of the chosen configurations. The computed pNMR shifts are very sensitive to temperature and, therefore, to the relative energies of the BS spin states. In order to reproduce the temperature dependence of the pNMR shifts seen in experiment, some scaling of the calculated energy gaps is required. A single scaling factor was applied to all levels in any one system, by fitting to experimental results at several temperatures simultaneously. The resulting scaling factor decreases with an increasing number of dimer units in the model (e.g., from ∼1.7 for mono-dimer models to 1.2 for tri-dimer models). The approach of this scaling factor towards unity indicates that models with three dimers are approaching a size where they can be considered as reasonable models for the ¹³C shifts of infinite MOFs. The observed unusual temperature dependencies in the latter are indicated to arise both from the “normal” temperature dependence of the pNMR shifts of the paramagnetic states and the populations of these states in the thermal equilibrium.

Thermal averaging over DFT-computed paramagnetic spin states can reproduce observed temperature-dependent shifts in ¹³C pNMR spectra of MOFs with Cu(ii) paddlewheel dimers.     

Electron spin polarization generated by transport of singlet and quintet multiexcitons to spin-correlated triplet pairs during singlet fissions

Singlet fission (SF) is expected to exceed the Shockley–Queisser theoretical limit of efficiency of organic solar cells. Transport of spin-entanglement in the triplet–triplet pair state via one singlet exciton is a promising phenomenon for several energy conversion applications including quantum information science. However, direct observation of electron spin polarization by transport of entangled spin-states has not been presented. In this study, time-resolved electron paramagnetic resonance has been utilized to observe the transportation of singlet and quintet characters generating correlated triplet–triplet (T + T) exciton-pair states by probing the electron spin polarization (ESP) generated in thin films of 6,13-bis(triisopropylsilylethynyl)pentacene. We have clearly demonstrated that the ESP detected at the resonance field positions of individual triplet excitons is dependent on the morphology and on the detection delay time after laser flash to cause SF. ESP was clearly explained by quantum superposition of singlet–triplet–quintet wavefunctions via picosecond triplet-exciton dissociation as the electron spin polarization transfer from strongly exchange-coupled singlet and quintet TT states to weakly-coupled spin-correlated triplet pair states. Although the coherent superposition of spin eigenstates was not directly detected, the present interpretation of the spin correlation of the separated T + T exciton pair may pave new avenues not only for elucidating the vibronic role in the de-coupling between two excitons but also for scalable quantum information processing using quick T + T dissociation via one-photon excitation.

Singlet fission (SF) is expected to exceed the Shockley–Queisser theoretical limit of efficiency of organic solar cells.     

Persistent organic room temperature phosphorescence: what is the role of molecular dimers?

Molecular dimers have been frequently found to play an important role in room temperature phosphorescence (RTP), but its inherent working mechanism has remained unclear. Herein a series of unique characteristics, including singlet excimer emission and thermally activated delayed fluorescence, were successfully integrated into a new RTP luminogen of CS-2COOCH₃ to clearly reveal the excited-state process of RTP and the special role of molecular dimers in persistent RTP emission.

The first purely organic room temperature phosphorescence (RTP) luminogen, with singlet excimer emission and thermally activated delayed fluorescence (TADF) effect, was successfully developed.      

Regioselective addition/annulation of ferrocenyl thioamides with 1,3-diynes via a sulfur-transfer rearrangement to construct extended π-conjugated ferrocenes with luminescent properties

Herein a regioselective addition/annulation strategy of ferrocenyl (Fc) thioamides with alkynes to construct thienylferrocene (ThienylFc) structures, involving a rhodium-catalyzed C–H activation, an unusual C2-selective addition of 1,3-diyne, and an unexpected intramolecular sulfur-transfer rearrangement process is described. In this protocol, thioamide not only serves as a directing group to activate the ortho-C–H bond of the ferrocene, but also as a sulfur source to form the thiophene ring. The resulting carboxylic ester group after sulfur transfer can act as a linkage to construct extended π-conjugated ferrocenes (OCTFc) with luminescent properties. ThienylFc displays effective fluorescence quenching due to the photoinduced electron transfer (PET) from the Fc unit to the excited luminophore, which turns out to be a promising type of redox molecular switch. OCTFc exhibit relatively strong emission owing to their intramolecular charge transfer (ICT) characteristics. The ring-fused strategy is herein employed for the first time to construct luminescent materials based on ferrocenes, which provides inspiration for the development of novel organic optoelectronic materials, such as electroluminescent materials based on ferrocenes.

Regioselective addition/annulation of ferrocenyl thioamides with 1,3-diynes has been developed to construct extended π-conjugated ferrocenes with luminescent properties.     

Exploiting clock transitions for the chemical design of resilient molecular spin qubits

Molecular spin qubits are chemical nanoobjects with promising applications that are so far hampered by the rapid loss of quantum information, a process known as decoherence. A strategy to improve this situation involves employing so-called Clock Transitions (CTs), which arise at anticrossings between spin energy levels. At CTs, the spin states are protected from magnetic noise and present an enhanced quantum coherence. Unfortunately, these optimal points are intrinsically hard to control since their transition energy cannot be tuned by an external magnetic field; moreover, their resilience towards geometric distortions has not yet been analyzed. Here we employ a python-based computational tool for the systematic theoretical analysis and chemical optimization of CTs. We compare three relevant case studies with increasingly complex ground states. First, we start with vanadium(iv)-based spin qubits, where the avoided crossings are controlled by hyperfine interaction and find that these S = 1/2 systems are very promising, in particular in the case of vanadyl complexes in an L-band pulsed EPR setup. Second, we proceed with a study of the effect of symmetry distortions in a holmium polyoxotungstate of formula [Ho(W₅O₁₈)₂]⁹⁻ where CTs had already been experimentally demonstrated. Here we determine the relative importance of the different structural distortions that causes the anticrossings. Third, we study the most complicated case, a polyoxopalladate cube [HoPd₁₂(AsPh)₈O₃₂]⁵⁻ which presents an unusually rich ground spin multiplet. This system allows us to find uniquely favorable CTs that could nevertheless be accessible with standard pulsed EPR equipment (X-band or Q-band) after a suitable chemical distortion to break the perfect cubic symmetry. Since anticrossings and CTs constitute a rich source of physical phenomena in very different kinds of quantum systems, the generalization of this study is expected to have impact not only in molecular spin science but also in other related fields such as molecular photophysics and photochemistry.

We employ a python computational tool to compare 3 relevant case studies with increasingly complex ground states: vanadyl complexes, Ho(iii) square antiprisms and Ho(iii) cubic structures.     

Amination of the Gd@C82 endohedral fullerene: tunable substitution effect on quantum coherence behaviors

The core–shell structure of endohedral fullerene-based anisotropic magnetic molecules of high spin with long coherence time could help scale up quantum systems. In this research, by amination of Gd@C₈₂ with morpholine, three derivatives are functionalized with 5, 7 and 9 morpholine groups providing an interesting model to investigate the relationship between the quantum coherence and the spin environment. The original radical located on the carbon cage is successfully quenched, affording a quantum phase memory times (T_M) over 5 μs at 5 K. By increasing the number of substitution groups, spin–lattice relaxation times (T₁) also show significant enhancement due to the interaction variation between the molecules and the environments. We found that the T_M of the three molecules show no obvious difference below 10 K, while they are limited by T₁ at higher temperatures. In this work, the variable functional groups are able to tune both T₁ and T_M, offering the possibility for application of high-spin magnetic molecules in the field of quantum information processing.

Amination of the endohedral fullerene Gd@C82 exhibits enhanced quantum phase memory time and tunable substitution effect for quantum information processing.     

Photo-induced ultralong phosphorescence of carbon dots for thermally sensitive dynamic patterning

Stimuli-responsive films with a dynamic long afterglow feature have received considerable attention in the field of optical materials. Herein, we report the unique dynamic ultralong room temperature phosphorescence (URTP) in flexible solid films made of luminescent carbon dots (CDs) and polyvinylpyrrolidone (PVP). Impressively, fully reversible photo-activation and thermal deactivation of the dynamic long afterglow was achieved in this material, with a lifetime on–off ratio exceeding 3900. Subsequently, ultra-fine URTP patterns (resolution > 1280 dpi) with thermally sensitive retention time were readily photo-printed onto the films and utilized as time–temperature indicating logistics labels with multi-editing capacity. These findings not only enrich the library of dynamic URTP materials, but also extend the scope of the potential applications of luminescent CDs.

A flexible CD–polymer composite with a reversibly editable photo-induced URTP long afterglow was rationally designed and successfully applied in dynamic optical patterning with built-in time–temperature indicating functionality.     

A quantum algorithm for spin chemistry: a Bayesian exchange coupling parameter calculator with broken-symmetry wave functions

The Heisenberg exchange coupling parameter J (H = −2JS_i · S_j) characterises the isotropic magnetic interaction between unpaired electrons, and it is one of the most important spin Hamiltonian parameters of multi-spin open shell systems. The J value is related to the energy difference between high-spin and low-spin states, and thus computing the energies of individual spin states are necessary to obtain the J values from quantum chemical calculations. Here, we propose a quantum algorithm, B̲ayesian ex̲change coupling parameter calculator with b̲roken-symmetry wave functions (BxB), which is capable of computing the J value directly, without calculating the energies of individual spin states. The BxB algorithm is composed of the quantum simulations of the time evolution of a broken-symmetry wave function under the Hamiltonian with an additional term jS², the wave function overlap estimation with the SWAP test, and Bayesian optimisation of the parameter j. Numerical quantum circuit simulations for H₂ under a covalent bond dissociation, C, O, Si, NH, OH⁺, CH₂, NF, O₂, and triple bond dissociated N₂ molecule revealed that the BxB can compute the J value within 1 kcal mol⁻¹ of errors with less computational costs than conventional quantum phase estimation-based approaches.

A quantum algorithm “Bayesian exchange coupling parameter calculator with broken-symmetry wave function (BxB)” enables us to calculate Heisenberg exchange coupling parameter J without inspecting total energies of individual spin states, within 1 kcal mol⁻¹ of energy tolerance.     

Substitutional doping of symmetrical small fullerene dimers

Magnetic carbon nano-structures have potential applications in the field of spintronics as they exhibit valuable magnetic properties. Symmetrically sized small fullerene dimers are substitutional doped with nitrogen (electron rich) and boron (electron deficient) atoms to visualize the effect on their magnetic properties. Interaction energies suggests that the resultant dimer structures are energetically favorable and hence can be formed experimentally. There is significant change in the total magnetic moment of dimers of the order of 0.5 μ_B after the substitution of C atoms with N and B, which can also be seen in the change of density of states. The HOMO-LUMO gaps of spin up and spin down electronic states have finite energy difference which confirm their magnetic behaviour, whereas for non-magnetic doped dimers, the HOMO-LUMO gaps for spin up and down states are degenerate. The optical properties show that the dimers behave as optical semiconductors and are useful in optoelectronic devices. The induced magnetism in these dimers makes them fascinating nanocarbon magnetic materials.     

Tuning the stability of organic radicals: from covalent approaches to non-covalent approaches

Organic radicals are important species with single electrons. Because of their open-shell structure, they are widely used in functional materials, such as spin probes, magnetic materials and optoelectronic materials. Owing to the high reactivity of single electrons, they often serve as a key intermediate in organic synthesis. Therefore, tuning the stability of radicals is crucial for their functions. Herein, we summarize covalent and non-covalent approaches to tune the stability of organic radicals through steric effects and tuning the delocalization of spin density. Covalent approaches can tune the stability of radicals effectively and non-covalent approaches benefit from dynamicity and reversibility. It is anticipated that the further development of covalent and non-covalent approaches, as well as the interplay between them, may push the fields forward by enriching new radical materials and radical mediated reactions.

Covalent and non-covalent approaches to tune the stability of organic radicals through steric effects and the delocalization of spin density.     

J-dimer emission in interwoven metal–organic frameworks

J-dimer emission is an emergent property that occurs when pairs of ground state fluorophores associate, typically in a dilute solution medium. The resulting fluorescence is shifted with respect to the monomer. J-dimer emission, however, has never been observed in concentrated dispersions or in the solid state. We posited that multivariate (MTV) MOFs with double interwoven structures would help to isolate these dimers within their crystalline matrix. Using this strategy, J-aggregate density was controlled during crystallization by following a substitutional solid solution approach. Here, we identified the presence of J-dimers over the entire composition range for interwoven PIZOF-2/NNU-28 structures with variable amounts of a diethynyl-anthracene aggregate-forming link. We produced bulk crystals that systematically shifted their fluorescence from green to red with lifetimes (up to 13 ns) and quantum yields (up to 76%) characteristic of π–π stacked aggregates. Photophysical studies also revealed an equilibrium constant of dimerization, K_D = 1.5 ± 0.3 M⁻¹, enabling the first thermodynamic quantification of link–link interactions that occur during MOF assembly. Our findings elucidate the role that supramolecular effects play during crystallization of MTV MOFs, opening pathways for the preparation of solid-state materials with solution-like properties by design.

J-dimer emission is an emergent property that occurs when pairs of ground-state fluorophores associate within multivariate MOFs producing tunable red shifted emission.