Large Singlet-triplet Energy Gap Research Articles

Highly efficient organic light‐emitting diodes and light‐emitting electrochemical cells employing multiresonant thermally activated delayed fluorescent emitters with bulky donor or acceptor peripheral groups

AbstractMultiresonant thermally activated delayed fluorescence (MR‐TADF) emitters have been the focus of extensive design efforts as they are recognized to show bright, narrowband emission, which makes them very appealing for display applications. However, the planar geometry and relatively large singlet–triplet energy gap lead to, respectively, severe aggregation‐caused quenching (ACQ) and slow reverse intersystem crossing (RISC). Here, a design strategy is proposed to address both issues. Two MR‐TADF emitters triphenylphosphine oxide (TPPO)‐tBu‐DiKTa and triphenylamine (TPA)‐tBu‐DiKTa have been synthesized. Twisted ortho‐substituted groups help increase the intermolecular distance and largely suppress the ACQ. In addition, the contributions from intermolecular charge transfer states in the case of TPA‐tBu‐DiKTa help to accelerate RISC. The organic light‐emitting diodes (OLEDs) with TPPO‐tBu‐DiKTa and TPA‐tBu‐DiKTa exhibit high maximum external quantum efficiencies (EQEmax) of 24.4% and 31.0%, respectively. Notably, the device with 25 wt% TPA‐tBu‐DiKTa showed both high EQEmax of 28.0% and reduced efficiency roll‐off (19.9% EQE at 1000 cd m−2) compared to the device with 5 wt% emitter (31.0% EQEmax and 11.0% EQE at 1000 cd m−2). The new emitters were also introduced into single‐layer light‐emitting electrochemical cells (LECs), equipped with air‐stable electrodes. The LEC containing TPA‐tBu‐DiKTa dispersed at 0.5 wt% in a matrix comprising a mobility‐balanced blend‐host and an ionic liquid electrolyte delivered blue luminance with an EQEmax of 2.6% at 425 cd m−2. The high efficiencies of the OLEDs and LECs with TPA‐tBu‐DiKTa illustrate the potential for improving device performance when the DiKTa core is decorated with twisted bulky donors.

Achieving highly efficient long-wavelength phosphorescence emission of large singlet-triplet energy gap materials by host-guest doping

Achieving highly efficient long-wavelength phosphorescence emission of large singlet-triplet energy gap materials by host-guest doping

A Kinetically Stabilized Nitrogen-Doped Triangulene Cation: Stable and NIR Fluorescent Diradical Cation with Triplet Ground State.

A kinetically-stabilized nitrogen-doped triangulene cation derivative has been synthesized and isolated as the stable diradical with a triplet ground state that exhibits near-infrared emission. As was the case for a triangulene derivative we previously synthesized, the triplet ground state with a large singlet-triplet energy gap was experimentally confirmed by magnetic measurements. In contrast to the triangulene derivative, the nitrogen-doped triangulene cation derivative is highly stable even in solution under air and exhibits near-infrared absorption and emission because the alternancy symmetry of triangulene is broken by the nitrogen cation. Breaking the alternancy symmetry of triplet alternant hydrocarbon diradicals by a nitrogen cation would therefore be an effective strategy to create stable diradicals possessing magnetic properties similar to the parent hydrocarbons but with different electrochemical and photophysical properties.

Confining donor conformation distributions for efficient thermally activated delayed fluorescence with fast spin-flipping

Fast spin-flipping is the key to exploit the triplet excitons in thermally activated delayed fluorescence based organic light-emitting diodes toward high efficiency, low efficiency roll-off and long operating lifetime. In common donor-acceptor type thermally activated delayed fluorescence molecules, the distribution of dihedral angles in the film state would have significant influence on the photo-physical properties, which are usually neglected by researches. Herein, we find that the excited state lifetimes of thermally activated delayed fluorescence emitters are subjected to conformation distributions in the host-guest system. Acridine-type flexible donors have a broad conformation distribution or bimodal distribution, in which some conformers feature large singlet-triplet energy gap, leading to long excited state lifetime. Utilization of rigid donors with steric hindrance can restrict the conformation distributions in the film to achieve degenerate singlet and triplet states, which is beneficial to efficient reverse intersystem crossing. Based on this principle, three prototype thermally activated delayed fluorescence emitters with confined conformation distributions are developed, achieving high reverse intersystem crossing rate constants greater than 106 s−1, which enable highly efficient solution-processed organic light-emitting diodes with suppressed efficiency roll-off.

Cationic Zinc(II) Complexes with Carbazole-Type Counter-Anions: Intracomplex Donor/Acceptor Pairs Affording Exciplexes with Thermally Activated Delayed Fluorescence.

Two cationic zinc(II) complexes with carbazole-type counter-anions, namely, [Zn(tpy)2]2+[CAZ-p-BF3-]2 (Zn-p) and [Zn(tpy)2]2+[CAZ-o-BF3-]2 (Zn-o), have been designed and synthesized, where tpy is 2,2':6',2″-terpyridine, CAZ-p-BF3- is 4-((9H-carbazol-9-yl)phenyl)trifluoroborate, and CAZ-o-BF3- is (2-(9H-carbazol-9-yl)phenyl)trifluoroborate. The complex cation [Zn(tpy)2]2+ (as the acceptor) and the carbazole-type counter-anion CAZ-p-BF3- or CAZ-o-BF3- (as the donor) form an intracomplex donor/acceptor pair. Single-crystal structures reveal that compared to Zn-p, Zn-o exhibits a stronger π-π stacking interaction between the carbazole group (as the donor unit) of the counter-anion and the tpy ligand (as the acceptor unit) of [Zn(tpy)2]2+ because of the different anchoring position of the BF3- anion in the counter-anion. In a doped film, Zn-p and Zn-o afford an isolated exciplex formed between the carbazole group and the tpy ligand within the single complex, which gives green-yellow emission with a thermally activated delayed fluorescence (TADF) feature. In crystalline states, Zn-p and Zn-o afford exciplexes with blue emission centered at 468 nm and green-blue emission centered at 508 nm, respectively. The Zn-p crystalline sample shows a relatively large singlet-triplet energy gap (ΔEST) (0.33 eV) and no TADF, whereas the Zn-o crystalline sample exhibits a small ΔEST (0.06 eV) and distinct TADF, with a reverse intersystem crossing rate at 3.3 × 105 s-1. Zn-p and Zn-o both exhibit intriguing mechanochromic luminescence, with largely red-shifted (by over 70 nm) emission and modulated TADF properties upon mechanically grinding the crystalline samples. The work demonstrates that donor/acceptor pairs affording exciplexes can be formed within cationic metal complexes using counter-anions with donor nature, which opens a new avenue toward photo-active metal complexes with rich photophysical properties.

Prediction of donor-acceptor-type novel noble gas complexes in the triplet electronic state.

Closed-shell noble gas (Ng) compounds in the singlet electronic state have been extensively studied in the past two decades after the revolutionary discovery of 1HArF molecule. Motivated by the experimental identification of very strong donor-acceptor-type singlet-state Ng complex 1ArOH+, in the present article, for the first time, we report new donor-acceptor-type noble gas complexes in the triplet electronic state (3NgBeN+ (Ng = He-Rn)), where most of the Ng-Be bond lengths are smaller than the corresponding covalent limits. The newly proposed complexes are predicted to be stable by various computational tools, including coupled-cluster and multireference-based methods, with strong Ng-Be bonding (40.4-196.2 kJ mol-1). We have also investigated 3NgBeP+ (Ng = He-Rn) complexes for the purpose of comparison. Various computational results, including the structural parameters, bonding energies, vibrational frequencies, and atoms-in-molecule properties suggest that it may be possible to prepare and characterize these triplet state complexes through suitable experimental technique(s).

Unveiling the intersystem crossing dynamics in N-annulated perylene bisimides.

The efficient population of the triplet excited states in heavy metal-free organic chromophores has been one of the long-standing research problems to molecular photochemists. The negligible spin-orbit coupling matrix elements in the purely organic chromophores and the large singlet-triplet energy gap (ΔES-T) pose a hurdle for ultrafast intersystem crossing (ISC). Herein we report the unprecedented population of triplet manifold in a series of nitrogen-annulated perylene bisimide chromophores (NPBI and Br-NPBI). NPBI is found to have a moderate fluorescence quantum yield (Φf = 68 ± 5%), whereas Br-NPBI showcased a low fluorescence quantum yield (Φf = 2.0 ± 0.6%) in toluene. The femtosecond transient absorption measurements of Br-NPBI revealed ultrafast ISC (kISC = 1.97 × 1010 s-1) from the initially populated singlet excited state to the long-lived triplet excited states. The triplet quantum yields (ΦT = 95.2 ± 4.6% for Br-NPBI, ΦT = 18.7 ± 2.3% for NPBI) calculated from nanosecond transient absorption spectroscopy measurements showed the enhancement in triplet population upon bromine substitution. The quantum chemical calculations revealed the explicit role of nitrogen annulation in tuning the excited state energy levels to favor the ISC. The near degeneracy between the singlet and triplet excited states observed in NPBI and Br-NPBI (ΔES-T = -0.01 eV for NPBI, ΔES-T = 0.03 eV for Br-NPBI) facilitates the spin flipping in the molecules. Nitrogen annulation emerges as a design strategy to open up the ISC pathway and the rate of which can be further enhanced by the substitution of a heavier element.

Intrinsically-ionic donor-acceptor molecules featuring thermally-activated delayed fluorescence for high-performance host-guest blue light-emitting electrochemical cells

High-performance blue light-emitting electrochemical cells (LECs) have remained a formidable challenge. Ionic donor–acceptor molecules, especially those featuring thermally-activated delayed fluorescence (TADF), are promising hosts for developing high-performance blue LECs, but such hosts have remained undeveloped. Here, we report two intrinsically-ionic donor–acceptor molecules, 1-(3-(4-(4-(3,6-di-tert-butyl-9H-carbazol-9-yl)phenyl)-6-phenylpyrimidin-2-yl)phenyl)-3-ethyl-1H-imidazol-3-ium hexafluorophosphate (H1) and 1-(3-(4-(2-(3,6-di-tert-butyl-9H-carbazol-9-yl)phenyl)-6-phenylpyrimidin-2-yl)phenyl)-3-ethyl-1H-imidazol-3-ium hexafluorophosphate (H2), and their use as hosts for high-performance blue LECs. 3,6-di-tert-butylcabazole and imidazolium-substituted 2,4-diphenylpyrimidine are used as the donor and acceptor, respectively, which are located para (for H1) or ortho (for H2) to each other on the central phenyl linker. While H1 shows a low triplet energy (<2.6 eV), a large singlet–triplet energy gap (ΔEST) (>0.4 eV) and no TADF, H2 exhibits a high triplet energy (>2.8 eV), a small ΔEST (∼0.2 eV) and distinct TADF, due to its twisted donor–acceptor conformation. Both H1 and H2 show excellent electrochemical stability. Host-guest blue LECs using H2 as the host and a phosphorescent cationic Ir(III) complex as the guest afford sky-blue electroluminescence with high blue-color stability and peak brightness/peak external quantum efficiency/half-lifetime at 1078 cd m−2/8.6%/66 min under a constant-current driving. Upon adding LiPF6 into the active layer, a half-lifetime of ∼ 2 h is achieved at 918 cd m−2. These performances are the highest for host–guest blue LECs reported so far. The efficiencies and operational stability achieved at the high brightness are also the highest for blue LECs with inert-metal cathodes reported so far.

Capture of CO2 by novel diiodo‐N,N‐imidazoliumvinylidene: A theoretical quest

AbstractCapture of CO2 by diiodo‐N,N‐imidazoliumvinylidene, with singlet and triplet minima (1s and 1t, respectively), is probed at M06‐2X/6‐311++G** level of theory. Ground state appears to be triplet 1t, which exhibits a rather large singlet–triplet energy gap (ΔEs–t = +10.30 kcal mol−1). Adsorption of CO2 over 1t releases −3.70 kcal mol−1 energy and gives the initial CO2 adduct, with no transition state (TS). Sequentially, the latter goes through a reactant‐like TS, which leads to an internal exothermic attack of the carbene carbon to the adsorbed CO2 and forms 3t with conversion energy (Econ) of −6.77 kcal mol−1. Additional calculations involve endothermic capture of CO2 by the rather unstable 1s. The immense potential of the natural bond orbital (NBO) and atom in molecule (AIM) data in elucidating a reaction mechanism is unveiled by employing the electron‐pushing technique. Results appear consistent with the proposed mechanism.

First-principles study of luminescence of fullerene-like clusters

Thermally activated delayed fluorescence (TADF), a unique molecular fluorescence mechanism, plays a key role in designing emitters of high efficiency. Carbon fullerenes such as C&lt;sub&gt;60&lt;/sub&gt; and C&lt;sub&gt;70&lt;/sub&gt; exhibit strong TADF with intensity even higher than that of the prompt fluorescence, owing to their long lifetimes of triplet state and modest singlet-triplet energy gaps. Thus, there arises the intriguing question whether other fullerene-like clusters can also have fluorescence and host the TADF effect. In this work, by time-dependent density functional theory (TD-DFT) calculations, we explore the excited-states of the experimentally reported boron nitride cage clusters B&lt;sub&gt;12&lt;/sub&gt;N&lt;sub&gt;12&lt;/sub&gt;, B&lt;sub&gt;24&lt;/sub&gt;N&lt;sub&gt;24&lt;/sub&gt; and B&lt;sub&gt;36&lt;/sub&gt;N&lt;sub&gt;36&lt;/sub&gt;, as well as compound clusters B&lt;sub&gt;12&lt;/sub&gt;P&lt;sub&gt;12&lt;/sub&gt;, Al&lt;sub&gt;12&lt;/sub&gt;N&lt;sub&gt;12&lt;/sub&gt; and Ga&lt;sub&gt;12&lt;/sub&gt;N&lt;sub&gt;12&lt;/sub&gt; with the same geometry as B&lt;sub&gt;12&lt;/sub&gt;N&lt;sub&gt;12&lt;/sub&gt;. Using the HSE06 hybrid functional, the predicted energy gaps of these fullerene-like clusters are obtained to range from 2.83 eV to 6.54 eV. They mainly absorb ultraviolet light, and their fluorescence spectra are all in the visible range from 405.36 nm to 706.93 nm, including red, orange, blue, and violet emission colors. For the boron nitride cages, the energy gap of excited states increases with the cluster size increasing, accompanied by a blue shift of emission wavelength. For the clusters with B&lt;sub&gt;12&lt;/sub&gt;N&lt;sub&gt;12&lt;/sub&gt; geometry and different elemental compositions, the excited energy gap decreases as the atomic radius increases, resulting in a red shift of emission wavelength. In addition, the highest occupied molecular orbitals (HOMOs) and lowest unoccupied molecular orbitals (LUMOs) of these compound cage clusters are distributed separately on different elements, resulting in small overlap between HOMO and LUMO wavefunctions. Consequently, these fullerene-like clusters exhibit small singlet-triplet energy differences below 0.29 eV, which is beneficial for the intersystem crossing between the excited singlet state and triplet state, and hence promoting the TADF process. Our theoretical results unveil the fluorescence characteristics of cage clusters other than carbon fullerenes, and provide important guidance for precisely modulating their emission colors by controlling the cluster sizes and elemental compositions. These experimentally feasible fullerene-like compound clusters possess many merits as fluorophors such as outstanding stabilities, non-toxicity, large energy gap, visible-light fluorescence, and small singlet-triplet energy gap. Therefore, they are promising luminescent materials for applications in display, sensors, biological detection and labelling, therapy, and medicine.

Tricyclo[2.1.0.02,5]pent-3-ylidene: Stereoelectronic Control of Bridge-Flapping within a Nonclassical Nucleophilic Carbene.

Tricyclo[2.1.0.02,5]pent-3-ylidene is a carbene foreseen to rearrange to pyramidane (c-C4H4)C, a highly strained molecule featuring an inverted C atom. Modeling of the carbene, using the (U)MPWB1K/cc-pVTZ//(U)MPWB1K/6-311G(d) theoretical model, indicated a large singlet–triplet energy gap (ΔES–T = −45 kcal/mol), a high gas-phase proton affinity (PA = 258 kcal/mol), and a preference for electron-poor alkenes. These properties are consistent with those of nucleophilic carbenes. Structural differences between the Cs-symmetric singlet (ωflap = ±44 deg) and C2v-symmetric triplet (ωflap = 0 deg) stem from nonclassical electron delocalization in the former and the lack thereof in the latter. Degenerate bridge-flapping of the singlet’s main bridge, which comprises the reactive divalent C3 atom, is computed to be slow due to a high activation barrier of the C2v-symmetric transition state (TS) (Ea = 17 kcal/mol). The position of the conformeric equilibrium is subject to stereoelectronic control. 1-Substituted derivatives of the carbene (R ≠ H) are sensitive to σ inductive effects. A proximal conformation is preferred when R is electron-donating and a distal one is favored when R is electron-withdrawing. Finally, carbene rearrangements via 1,2-C atom shift or enyne fragmentation were computed. The C2v-symmetric bridge-flapping TS has the proper geometry to initiate enyne fragmentation.

Ferromagnetically Coupled S=1 Chains in Crystals of Verdazyl-Nitronyl Nitroxide Diradicals.

Thermally stable organic diradicals with a triplet ground state along with large singlet-triplet energy gap have significant potential for advanced technological applications. A series of phenylene-bridged diradicals with oxoverdazyl and nitronyl nitroxide units were synthesized via a palladium-catalyzed cross-coupling reaction of iodoverdazyls with a nitronyl nitroxide-2-ide gold(I) complex with high yields. The diradicals exhibit high stability and do not decompose in an inert atmosphere up to 180 °C. For the diradicals, both substantial AF (ΔEST ≈-64 cm-1 ) and FM (ΔEST ≥25 and 100 cm-1 ) intramolecular exchange interactions were observed. The sign of the exchange interaction is determined both by the bridging moiety (para- or meta-phenylene) and by the type of oxoverdazyl block (C-linked or N-linked). Upon crystallization, diradicals with the triplet ground state form unique one-dimensional exchange-coupled chains with strong intra- and weak inter-diradical ferromagnetic coupling.

Structure, optical and magnetic properties of radical anion, dianion salts and coordination complexes of organic dye 3,4:9,10-perylenetetracarboxylic dianhydride (PTCDA)

Reduction of organic dye 3,4:9,10-perylenetetracarboxylic dianhydride (PTCDA) yields a series of radical anion and dianion salts of PTCDA (1–5). Their formation is accompanied by essential red shift and splitting of absorption bands of pristine PTCDA (493 and 560 nm) which appear nearly in the whole visible range, namely, at 660, 700 and 760–770 nm for the radical anions, and 514–526, 544–569, 633–644 and 700–713 nm for the dianions. Weaker bands are also observed in the NIR range at 954 and 938–940 nm, respectively. Reduction elongates double CO bonds, and the shape of the anions becomes non-planar due to twisting. A dihedral angle between two planar halves of PTCDA increases together with anion charge. The PTCDA•− radical anions form {PTCDA}22− dimers in 1–3 with short interplanar distances of 3.220–3.299 Å. These dimers show intense charge transfer bands at 1460–1480 nm and they are diamagnetic up to 360 K indicating large singlet-triplet energy gap for them. The dianions are diamagnetic in the salts. The PTCDA2− dianion can also form coordination complexes coordinating two potassium cations in {cryptand(K+)}2(PTCDA2−)⋅C6H4Cl2 (6) or vanadocene cation in {cryptand(K+)}(Cp2V+)(PTCDA2−)⋅C6H4Cl2 (7). Mixed {cryptand(K+)}2(PTCDA2−){GaIIICl(Pc•3−)}•−⋅2C6H4Cl2 (Pc: phthalocyanine, 8) salt containing two strongly absorbing anionic chromophores has also been obtained. Both anions are chequered in the layers forming short CO(PTCDA2−)⋅⋅⋅H[GaIIICl(Pc•3−)}•−] hydrogen bonds. Most probably, namely these bonds provide rather effective antiferromagnetic coupling of the [GaIIICl(Pc•3−)}•−] spins (S = 1/2) within the layers with large negative Weiss temperature of −25 K.

The Photophysical Properties of Triisopropylsilyl-ethynylpentacene—A Molecule with an Unusually Large Singlet-Triplet Energy Gap—In Solution and Solid Phases

The process of singlet-exciton fission (SEF) has attracted much attention of late. One of the most popular SEF compounds is TIPS-pentacene (TIPS-P, where TIPS = triisopropylsilylethynyl) but, despite its extensive use as both a reference and building block, its photophysical properties are not so well established. In particular, the triplet state excitation energy remains uncertain. Here, we report quantitative data and spectral characterization for excited-singlet and -triplet states in dilute solution. The triplet energy is determined to be 7940 ± 1200 cm−1 on the basis of sensitization studies using time-resolved photoacoustic calorimetry. The triplet quantum yield at the limit of low concentration and low laser intensity is only ca. 1%. Self-quenching occurs at high solute concentration where the fluorescence yield and lifetime decrease markedly relative to dilute solution but we were unable to detect excimer emission by steady-state spectroscopy. Short-lived fluorescence, free from excimer emission or phosphorescence, occurs for crystals of TIPS-P, most likely from amorphous domains.

Benzidine/Quinoidal-Benzidine-Linked, Superbenzene-Based π-Conjugated Chiral Macrocycles and Cyclophanes.

Synthesis of fully conjugated cyclophanes containing large-size polycyclic aromatics is challenging. Now, three benzidine-linked, hexa-peri-hexabenzocoronene (superbenzene)-based ortho-, para-, and meta-cyclophanes are synthesized through intermolecular Yamamoto coupling reaction of structurally pre-organized precursors. Subsequent oxidative dehydrogenation gave the corresponding quinoidal benzidine-linked cyclophanes. Their geometries were confirmed by X-ray crystallographic analysis and their electronic properties were investigated by electronic absorption, cyclic voltammetry, and DFT calculations. The quinoidal benzidine-linked cyclophanes show thermally populated paramagnetic activity with a relatively large singlet-triplet energy gap. Two enantiomers for the ortho-cyclophanes (1-NH and 1-N) were isolated and their chiral figure-of-eight macrocyclic structures were identified. The cage-like cyclophanes 2-NH and 3-NH with concave surface can selectively encapsulate fullerene C70 .

Stabilization of Open-Shell Single Bonds within Endohedral Metallofullerene.

The open-shell single covalent bond composed of two electrons is unstable under normal conditions, because the closed-shell electronic configuration is generally beneficial to minimize the energy of the system. This classical rule always governs the chemical bonding of s- and p-block homonuclear diatomic molecules, such as the stable σ2 electron-pair bonds in hydrogen. In this work, surprisingly, we found that the diversified open-shell single bonds between two f-block atoms (e.g., thorium) can be stabilized within a tight "carbon-confined-space" using relativistic quantum chemical calculations. We first identified a stable dithorium endohedral metallofullerene (EMF), Th26+@Ih-C806-, with a Th-Th distance of 3.803 Å inside the Ih-C80 cage, which displays a unique spin-polarized σ1π1 2-fold single-electron Th3+-Th3+ bond that is collaboratively dominated by 5f6d7s7p orbitals. The Th3+-Th3+ bond can further evolve into a 5f6d dominated spin-polarized π2 configuration by compressing the Th-Th distance further down to 2.843 Å, within a smaller Ih-C60 cage. On the other hand, elongating the Th-Th distance to 4.063 Å by encapsulating Th2 into a long diametric D3h-C78 fullerene returns the Th3+-Th3+ bond to the normal closed-shell (6d7s7p)σ2 form. Hence, a new rule is unambiguously revealed through the carbon-confinement induced spin-polarization of a single bond. The key point of this rule is the size of the carbon cage, because the squeezed effect is conducive to the effective overlap of the Th(5f) orbitals, reducing and further reversing the original large singlet-triplet energy gap of the Th26+ unit. This discovery provides pioneering guidance for exploring new chemical bonds and thorium-based endofullerenes.

Intramolecular Cyclopropanation of Alkali-Metal-Substituted Silylene with the Aryl Substituent of an N-Heterocyclic Framework.

Reaction of the bulky N-heterocyclic silylene Si[NB(NArCH)2CH2]2 (3; Ar = 2,6-iPr2C6H3) with BCl3 resulted in ring expansion with formation of the six-membered C2N2SiB heterocycle 4. Reduction of 4 with lithium afforded the first silanorcaradienyllithium 5, which presumably formed by intramolecular cyclopropanation of a lithium-substituted borylsilylene intermediate with one of the aryl C═C bonds. Treatment of 5 with MeOTf and Et3NHCl led to electrophilic substitution accompanied by the SiC2 ring expansion to give silacycloheptatrienes (silepins) 6 and 7, respectively. The addition of 1,3,4,5-tetramethylimidazol-2-ylidene (NHCMe) to silepin 7 yielded the NHC-stabilized hydrosilylene 8 with the silicon atom free from the silepin ring. The formation of 5 could be attributed to the unique electronic structure of the lithium-substituted silylene, which has been predicted to be in a triplet ground state with a relatively large singlet-triplet energy gap by theoretical calculations.

The Puzzling Monopentamethylcyclopentadienyltitanium(III) Dichloride Reagent: Structure and Properties.

Following the track of the useful titanocene [Ti(η5-C5H5)2Cl] reagent in organic synthesis, the related half-sandwich titanium(III) derivatives [Ti(η5-C5R5)Cl2] are receiving increasing attention in radical chemistry of many catalyzed transformations. However, the structure of the active titanium(III) species remains unknown in the literature. Herein, we describe the synthesis, crystal structure, and electronic structure of titanium(III) aggregates of composition [{Ti(η5-C5Me5)Cl2} n]. The thermolysis of [Ti(η5-C5Me5)Cl2Me] (1) in benzene or hexane at 180 °C results in the clean formation of [{Ti(η5-C5Me5)Cl(μ-Cl)}2] (2), methane, and ethene. The treatment of 1 with excess pinacolborane in hexane at 65 °C leads to a mixture of 2 and the paramagnetic trimer [{Ti(η5-C5Me5)(μ-Cl)2}3] (3). The X-ray crystal structures of compounds 2 and 3 show Ti-Ti distances of 3.267(1) and 3.219(12) Å, respectively. Computational studies (CASPT2//CASSCF and BS DFT methods) for dimer 2 reveal a singlet ground state and a relatively large singlet-triplet energy gap. Nuclear magnetic resonance spectroscopy of 2 in aromatic hydrocarbon solutions and DFT calculations for several [{Ti(η5-C5Me5)Cl2} n] aggregates are consistent with the existence of an equilibrium between the diamagnetic dimer [{Ti(η5-C5Me5)Cl(μ-Cl)}2] and a paramagnetic tetramer [{Ti(η5-C5Me5)(μ-Cl)2}4] in solution. In contrast, complex 2 readily dissolves in tetrahydrofuran to give a green-blue solution from which blue crystals of the mononuclear adduct [Ti(η5-C5Me5)Cl2(thf)] (4) were grown.

Thermally and Magnetically Robust Triplet Ground State Diradical.

High spin ( S = 1) organic diradicals may offer enhanced properties with respect to several emerging technologies, but typically exhibit low singlet triplet energy gaps and possess limited thermal stability. We report triplet ground state diradical 2 with a large singlet-triplet energy gap, Δ EST ≥ 1.7 kcal mol-1, leading to nearly exclusive population of triplet ground state at room temperature, and good thermal stability with onset of decomposition at ∼160 °C under inert atmosphere. Magnetic properties of 2 and the previously prepared diradical 1 are characterized by SQUID magnetometry of polycrystalline powders, in polystyrene glass, and in other matrices. Polycrystalline diradical 2 forms a novel one-dimensional (1D) spin-1 ( S = 1) chain of organic radicals with intrachain antiferromagnetic coupling of J'/ k = -14 K, which is associated with the N···N and N···O intermolecular contacts. The intrachain antiferromagnetic coupling in 2 is by far strongest among all studied 1D S = 1 chains of organic radicals, which also makes 1D S = 1 chains of 2 most isotropic, and therefore an excellent system for studies of low-dimensional magnetism. In polystyrene glass and in frozen benzene or dibutyl phthalate solution, both 1 and 2 are monomeric. Diradical 2 is thermally robust and is evaporated under ultrahigh vacuum to form thin films of intact diradicals on silicon substrate, as demonstrated by X-ray photoelectron spectroscopy. Based on C-K NEXAFS spectra and AFM images of the ∼1.5 nm thick films, the diradical molecules form islands on the substrate with molecules stacked approximately along the crystallographic a-axis. The films are stable under ultrahigh vacuum for at least 60 h but show signs of decomposition when exposed to ambient conditions for 7 h.

Thiophene and its sulfur inhibit indenoindenodibenzothiophene diradicals from low-energy lying thermal triplets.

Many qualitative structure-property correlations between diradical character and emerging molecular properties are known. For example, the increase of diradical character further decreases the singlet-triplet energy gap. Here we show that inclusion of thiophenes within a quinoidal polycyclic hydrocarbon imparts appreciable diradical character yet retains the large singlet-triplet energy gap, a phenomenon that has no precedent in the literature. The low aromatic character of thiophene and its electron-rich nature are the key properties leading to these unique findings. A new indenoindenodibenzothiophene scaffold has been prepared and fully characterized by several spectroscopies, magnetic measurements, solid-state X-ray and state-of-the-art quantum chemical calculations, all corroborating this unique dichotomy between the diradical input and the emerging magnetic properties. New structure-property relationships such as these are not only extremely important in the field of diradical chemistry and organic electronics, but also provide new insights into the versatility of π-electron chemical bonding.