Rational Design of an Air-Stable, High-Spin Diradical with Diazapyrene

Photoinduced Radical Luminescence of Diazapyrene Derivatives upon Complexation with Cucurbit[n]uril Hosts

Molecular design with an electron donor-acceptor (D-A) structure and host-guest complexation are effective strategies for stabilizing radicals. Herein, we report the design and synthesis of cationic diazapyrene derivatives featuring a D-A structure, as well as the influence of host-guest complexation on their photoinduced organic radical photophysical properties. Compared to the methylated diazapyrene 1, the other three derivatives 2-4, containing triphenylamine, coumarin, and naphthalene units as D groups, respectively, are more favorable for the generation of radicals. Binding studies reveal that cucurbit[8]uril (CB[8]) forms 1 : 1 inclusion complexes with derivatives 2-4, while CB[10] forms a 2 : 2 inclusion complex with 2 and supramolecular polymers with 3 and 4. The radical luminescence of derivatives 1-4 in aqueous solution is significantly promoted upon complexation with CB[8] or CB[10]. Additionally, the emission color of 3 shifts from cyan to yellow-green upon light irradiation in the presence of CB[8].

A Kinetically Stabilized Dianthraceno[2,3-a:3',2'-h]-s-Indacene: Stable Kekulé Diradical Polycyclic Hydrocarbon with Triplet Ground State

High-spin polycyclic hydrocarbons (PHs) hold significant potential in organic spintronics and organic magnets. However, their synthesis is very challenging due to their extremely high reactivity. Herein, we report the successful synthesis and isolation of a kinetically blocked derivative (1) of dianthraceno[2,3-a : 3',2'-h]-s-indacene, which represents a rare persistent triplet diradical of a Kekulé PH. Its triplet ground state was unambiguously confirmed by electron paramagnetic resonance and superconducting quantum interference device measurements. Its structure was also unequivocally confirmed through X-ray crystallographic analysis, and its electronic properties were systematically investigated by both experiments and theoretical calculations. The key design principle is to extend the π-conjugation for achieving the decrease of the bonding interaction and the increase of the exchange interaction between unpaired electrons, which are essential for accessing the stable triplet ground state. Due to kinetic blocking, 1 shows a reasonable stability with a half-life time of 64 h under ambient conditions. It has a narrow HOMO-LUMO energy gap and displays amphoteric redox behavior. Notably, its dication and dianion exhibit a closed-shell ground state and near-infrared absorption, and the structures were identified by X-ray crystallographic analysis. This study will shed new light on the design and synthesis of novel stable PHs with high-spin multiplicity.

Triplet-ground-state nonalternant nanographene with high stability and long spin lifetimes

High-spin carbon-based polyradicals exhibit significant potential for applications in quantum information storage and sensing; however, their practical application is hampered by limited structural diversity and chemical instability. Here, we report a straightforward synthetic and isolation method for synthesizing a nonalternant nanographene (1) with a triplet ground state. Moving beyond the classic m-xylylene scaffold for high-spin organic molecules, seven-five-seven (7-5-7)-membered rings are introduced to create stable high-spin diradicals with half-lives (t1/2) as long as 101 days. Moreover, considering the spin relaxation of compound 1, with a spin-lattice relaxation time (T1) of 53.55 ms and a coherence time (Tm) of 3.41 μs at 10 K, the compound 1 shows great promise for applications in spin-based information retention and quantum computing.

Stable Mono-Radical and Triplet Diradicals Based on Allylic Radical-Embedded All-Benzenoid Polycyclic Hydrocarbons

High-spin organic radicals are notable for their unique optical, electronic, and magnetic properties, but synthesizing stable high-spin systems is challenging due to their inherent reactivity. This study presents a novel strategy for designing stable high-spin polycyclic hydrocarbons (PHs) by incorporating allylic radical into fused aromatic benzenoid rings. To enhance stability, large steric hindrance groups with a synergistic captodative effect were added to the allylic radical centers. This approach was applied to synthesize derivatives of two open-shell all-benzenoid PHs: benzo[fg]tetracene (1) and dibenzo[fg,lm]heptacene (2). Both compounds exhibited excellent stability, with diradical 2 showing a half-life of up to 17.2 days under ambient conditions. Bond length analysis and theoretical calculations suggest that 1 and 2 predominantly feature Clar's aromatic sextet structures with embedded allylic radicals. Compound 2 was confirmed to have a triplet ground state through DFT calculations and experimental methods, including pulse EPR spectroscopy and SQUID measurements. This work introduces a new design strategy for stable high-spin hydrocarbons, paving the way for future developments in high-spin organic materials.

A stable open-shell peri-hexacene with remarkable diradical character

[n]Peri-acenes ([n]PA) have attracted great interest as promising candidates for nanoelectronics and spintronics. However, the synthesis of large [n]PA (n > 4) is extremely challenging due to their intrinsic open-shell radical character and high reactivity. Herein, we report the successful synthesis and isolation of a derivative (1) of peri-hexacene in crystalline form. The structure of 1 is unequivocally confirmed by X-ray crystallographic analysis. Its ground state, aromaticity and photophysical properties are systematically studied by both experimental methods and theoretical calculations. Although the parent peri-hexacene is calculated to have a very large diradical character (y0 = 94.5%), 1 shows reasonable stability (t1/2 = 24 h under ambient conditions) due to the kinetic blocking. 1 exhibits an open-shell singlet ground state with a small singlet-triplet energy gap (-1.33 kcal/mol from SQUID measurements). 1 has also a narrow HOMO-LUMO energy gap (1.05 eV) and displays amphoteric redox behavior. This work opens new avenues for the design and synthesis of stable zigzag-edged graphene-like molecules with significant diradical character.

Rational design of organic diradicals with robust high-spin ground state based on antiaromatic linkers

Fully-organic molecules with high-spin ground states are promising building blocks for new lightweight flexible magnetic materials for emerging technological applications (e.g. spintronics). In this study, we explore the potential of diradicals made of two diphenylmethyl-based open-shell cores covalently linked via different types of pentalene and diazapentalene-based antiaromatic couplers (including dibenzopentalenes and acene-inserted derivatives). Accurate electronic structure calculations have been employed to target non-bonding and non-disjoint frontier molecular orbitals that favor high-spin configurations, leading to the identification of diradicals displaying robust triplet ground states. These candidates exhibit singlet-triplet energy gaps that are up to ten times the thermal energy at room temperature. These substantial gaps emerge from strong interactions between the π-systems of the open-shell centers and the antiaromatic coupler. These interactions not only result in high spin states but are also found to lead to an enhanced stability of the diradicals by drastically dampening their inherent antiaromatic character as compared to the bare couplers, and promoting a high degree of spin density delocalization. These findings highlight the potential of pentalene-based diradicals as building blocks for developing new advanced fully organic magnetic materials.

1,1'-Biolympicenyl: A Stable Non-Kekulé Diradical with a Small Singlet and Triplet Energy Gap

Dimerization of delocalized polycyclic hydrocarbon radicals is a simple and versatile method to create diradicals with tailored electronic structures and accessible high-spin states. However, the synthesis is challenging, and the stability issue of the diradicals remains a concern. In this study, we present the synthesis of a stable non-Kekulé 1,1'-biolympicenyl diradical 1 using a protection-oxidation-protection strategy. Diradical 1 demonstrated exceptional stability, with a solution half-life time exceeding 3.5 years and a solid state thermal decomposition temperature above 300 °C. X-ray crystallographic analysis revealed its intersected molecular structure and tightly bound dimer configuration. A singlet ground state with a small singlet-triplet energy gap is consistently identified using electron paramagnetic resonance (EPR) and a superconducting quantum interference device (SQUID) in a rigid matrix, and the triplet state is thermally accessible at room temperature. The solution phase properties were systematically examined through EPR, absorption spectroscopy, and cyclic voltammetry, revealing a rotational motion in the slow-motion regime and multistage redox characteristics. This study presents an efficient synthetic and stabilization strategy for organic diradicals, enabling the development of various high-spin functional materials.

Nonalternating π-System Mediated Spin Coupling in Azulene Nitronyl Nitroxide Diradicals

To explore the distinctions in spin coupling between the molecular bridges of alternating and nonalternating π-systems, we synthesized a pair of isoelectronic compounds, namely, 2,6-Na-NN and 2,6-Az-NN, by utilizing naphthalene and azulene (naphthalene = Na and azulene = Az) as the bridges, respectively. Moreover, we conducted assessments to predict the coupling paths for nonalternating azulene. Variable-temperature EPR (VT-EPR) and SQUID results consistently reveal that both 2,6-Na-NN and 2,6-Az-NN exhibit antiferromagnetic coupling interactions, with coupling constants of J(2,6-Na-NN) = -22.3 cm-1 and J(2,6-Az-NN) = -30.1 cm-1, respectively. Density functional theory computations support these discoveries by revealing negative coupling constants (J < 0) and the spin densities population of the diradicals are observed to delocalize into the molecular bridges. This work suggests the most suitable coupling path for 2,6-Az-NN. In addition, we have investigated the potential spatial resistance of the diradicals in conjunction with single-crystal data. Theoretical calculations underestimating the torsion angle of the diradicals and overestimating the value of the magnetic coupling provide an explanation for this phenomenon. The final experimental results and theoretical calculations show that the 2,6-Az-NN coupling path prefers short paths.

An Air-Stable Carbon-Centered Triradical with a Well-Addressable Quartet Ground State

Organic polyradicals with a high-spin ground state and quantum magnetic properties suitable for spin manipulation are valuable materials for diverse innovative technologies, including quantum devices. However, the typically high reactivity and low stability of conventional polyradicals present a major obstacle to such applications. In this study, a highly stable carbon-centered triradical TR with a quartet ground state and excellent stability (τ1/2 of ∼90 days in air-saturated toluene at room temperature) is achieved, which shows apposite magnetic anisotropy and Zeeman splitting partition with favorable addressability. By virtue of the optimal stability, thorough structural and magnetic characterizations are realized. With X-ray crystallography unambiguously proving the molecular structure, the quartet ground state (ΔED-Q = 0.78 kcal/mol) is confirmed by the SQUID measurements, while the cw- and pulsed EPR techniques offer additional supportive evidence for the high-spin nature. Remarkably, owing to the easily attained magnetic anisotropy, selective excitations between different Zeeman splitting levels are successfully demonstrated with TR in its frozen toluene solution without the requirement for special alignment, which is unprecedented for organic polyradicals. Along with the millisecond spin-lattice relaxation and microsecond coherence time manifested by TR, this triradical is promising for potential coherent spin manipulation applications as a multienergy-level quantum information carrier.

A Stable Chichibabin Diradicaloid with Near-Infrared Emission

Conjugated molecules with multiple radical centers such as the iconic Chichibabin diradicaloid hold promise as building blocks in materials for quantum sensing and quantum information processing. However, it is a considerable challenge to design simple analogues of the Chichibabin hydrocarbon that are chemically inert, exhibit high diradical character and emit light at a distinct wavelength that may offer an optical readout of the spin state in functional ensembles. Here we describe the serendipitous discovery of the stable TTM-TTM diradicaloid, which exhibits high diradical character, a striking sky-blue color and near-infrared (NIR) emission (in solution). This combination of properties is unique among related diradicaloids and is due to the presence of hydrogen and chlorine atoms in "just the right positions", allowing a perfectly planar, yet predominantly benzenoid bridge to connect the two sterically stabilized radical centers. In-depth studies of the optical and magnetic properties suggest that this structural motif could become a mainstay building block of organic spin materials.

Triple para-Functionalized Cations and Neutral Radicals of Enantiopure Diaza[4]helicenes

Modulation of absorbance and emission is key for the design of chiral chromophores. Accessing a series of compounds absorbing and emitting (circularly polarized) light over a wide spectral window and often toward near-infrared is of practical value in (chir)optical applications. Herein, by late-stage functionalization on derivatives bridging triaryl methyl and helicene domains, we have achieved the regioselective triple introduction of para electron-donating or electron-withdrawing substituents. Extended tuning of electronic (e.g., E1/2red -1.50 V → -0.68 V) and optical (e.g., emission covering from 550 to 850 nm) properties is achieved for the cations and neutral radicals; the latter compounds being easily prepared by mono electron reductions under electrochemical or chemical conditions. While luminescence quantum yields can be increased up to 70% in the cationic series, strong Cotton effects are obtained for certain radicals at low energies (λabs ∼ 700-900 nm) with gabs values above 10-3. The open-shell electronic nature of the radicals was further characterized by electron paramagnetic resonance revealing an important spin density delocalization that contributes to their persistence.

bay/ortho-Octa-substituted Perylene: A Versatile Building Block toward Novel Polycyclic (Hetero)Aromatic Hydrocarbons

ConspectusPolycyclic (hetero)aromatic hydrocarbons (PAHs) have emerged as a focal point in current interdisciplinary research, spanning the realms of chemistry, physics, and materials science. Possessing distinctive optical, electronic, and magnetic properties, these π-functional materials exhibit significant potential across diverse applications, including molecular electronic devices, organic spintronics, and biomedical functions, among others. Despite the extensive documentation of various PAHs over the decades, the efficient and precise synthesis of π-extended PAHs remains a formidable challenge, hindering their broader application. This challenge is primarily attributed to the intricate and often elusive nature of their synthesis, compounded by issues related to low solubility and unfavored stability.The development of π-building blocks that can be facilely and modularly transformed into diverse π-frameworks constitutes a potent strategy for the creation of novel PAH materials. For instance, based on the classic perylene diimide (PDI) unit, researchers such as Würthner, Wang, and Nuckolls have successfully synthesized a plethora of structurally diverse PAHs, as well as numerous other π-functional materials. However, until now the availability of such versatile building blocks is still severely limited, especially for those simultaneously having a facile preparation process, adequate solubilizing groups, favored material stability, and critically, rich possibilities for structural extension spaces.In this Account, we present an overview of our invention of a highly versatile bay-/ortho-octa-substituted perylene building block, designated as Per-4Br, for the construction of a series of novel PAH scaffolds with tailor-made structures and rich optoelectronic and magnetic properties. First, starting with a brief discussion of current challenges associated with the bottom-up synthesis of π-extended PAHs, we rationalize the key features of Per-4Br that enable facile access to new PAH molecules including its ease of large-scale preparation, favored material stability and solubility, and multiple flexible reaction sites, with a comparison to the PDI motif. Then, we showcase our rational design and sophisticated synthesis of a body of neutral or charged, closed- or open-shell, curved, or planar PAHs via controlled annulative π-extensions in different directions such as peripheral, diagonal, or multiple dimensions of the Per-4Br skeleton. In this part, the fundamental structure-property relationships between molecular conformations, electronic structures, and self-assembly behaviors of these PAHs and their unique physiochemical properties such as unusual open-shell ground states, global aromaticity, state-associated/stimuli-responsive magnetic activity, and charge transport characteristics will be emphatically elaborated. Finally, we offer our perspective on the continued advancement of π-functional materials based on Per-4Br, which, we posit, may stimulate heightened research interest in the versatile structural motifs typified by Per-4Br, consequently catalyzing further progress in the realm of organic π-functional materials.