Strategic molecular design of closo-ortho-carboranyl luminophores to manifest thermally activated delayed fluorescence

In this paper, we propose a strategic molecular design of closo-o-carborane-based donor–acceptor dyad system that exhibits thermally activated delayed fluorescence (TADF) in the solution state at ambient temperature. Planar 9,9-dimethyl-9H-fluorene-based compounds with closo- and nido-o-carborane cages appended at the C2-, C3-, and C4-positions of each fluorene moiety (closo-type: 2FC, 3FC, 4FC, and 4FCH, and nido-type: nido-4FC = [nido-form of 4FC]·[NBu₄]) were prepared and characterized. The solid-state molecular structure of 4FC exhibited a significantly distorted fluorene plane, which suggests the existence of severe intramolecular steric hindrance. In photoluminescence measurements, 4FC exhibits a noticeable intramolecular charge transition (ICT)-based emission in all states (solution at 298 K and 77 K, and solid states); however, emissions by other closo-compounds were observed in only the rigid state (solution at 77 K and film). Furthermore, nido-4FC did not exhibit emissive traces in any state. These observations verify that all radiative decay processes correspond to ICT transitions triggered by closo-o-carborane, which acts as an electron acceptor. Relative energy barriers calculated by TD-DFT as dihedral angles around o-carborane cages change in closo-compounds, which indicates that the structural formation of 4FC is nearly fixed around its S₀-optimized structure. This differs from that for other closo-compounds, wherein the free rotation of their o-carborane cages occurs easily at ambient temperature. Such rigidity in the structural geometry of 4FC results in ICT-based emission in solution at 298 K and enhancement of quantum efficiency and radiative decay constants compared to those for other closo-compounds. Furthermore, 4FC displays short-lived (∼0.5 ns) and long-lived (∼30 ns) PL decay components in solution at 298 K and in the film state, respectively, which can be attributed to prompt fluorescence and TADF, respectively. The calculated energy difference (ΔE_ST) between the first excited singlet and triplet states of the closo-compounds demonstrate that the TADF characteristic of 4FC originates from a significantly small ΔE_ST maintained by the rigid structural fixation around its S₀-optimized structure. Furthermore, the strategic molecular design of the o-carborane-appended π-conjugated (D–A) system, which forms a rigid geometry due to severe intramolecular steric hindrance, can enhance the radiative efficiency for ICT-based emission and trigger the TADF nature.

The first example of a closo-o-carboranyl compound demonstrating thermally activated delayed fluorescence (TADF) nature in solution is shown, and a strategic molecular design of a closo-o-carboranyl luminophore to exhibit TADF is proposed.     

Construction of Covalent Organic Cages with Aggregation-Induced Emission Characteristics from Metallacages for Mimicking Light-Harvesting Antenna

A series of covalent organic cages built from fluorophores capable of aggregation-induced emission (AIE) were elegantly prepared through the reduction of preorganized M₂(L_A)₃(L_B)₂-type metallacages, simultaneously taking advantage of the synthetic accessibility and well-defined shapes and sizes of metallacages, the good chemical stability of the covalent cages as well as the bright emission of AIE fluorophores. Moreover, the covalent cages could be further post-synthetically modified into an amide-functionalized cage with a higher quantum yield. Furthermore, these presented covalent cages proved to be good energy donors and were used to construct light-harvesting systems employing Nile Red as an energy acceptor. These light-harvesting systems displayed efficient energy transfer and relatively high antenna effect, which enabled their use as efficient photocatalysts for a dehalogenation reaction. This research provides a new avenue for the development of luminescent covalent cages for light-harvesting and photocatalysis.     

Chiral phenylazomethine cage

Enantiopure phenylazomethine cages ((S)- and (R)-PAC) were selectively obtained by one-pot dehydration of chiral (S)- and (R)-syn-tris(norborneno)benzene with p-phenylenediamine in moderate yields (56% and 50%), respectively. The structure and chirality of PACs in solution and solid state were confirmed by NMR, CD, X-ray diffraction and DFT calculations.     

Near-IR emission of Nd(III) encaged in nanosized zeolites

The near-IR emission of Nd(III) with the highest quantum yield (9.5%) in organic media was successfully observed for the first time by using bis-(perfluoromethylsulfonyl)amide (PMS) as a low vibrational ligand of the ion and TMA⁺-containing FAU zeolite nanocrystallites (TMA-nanoFAU) as a host matrix. Treatments such as deuteration and thermal treatments at high temperatures were ineffective for the strong emission of Nd(III) within TMA-nano-FAU. Judd-Ofelt analysis revealed that the ligation of PMS with the Nd(III) ion occurred easily, because the ions remained in the super cages without migrating into inner cages due to the hindrance of TMA⁺ ions occupying in the sodalite cages. The emission intensity of TMA-nano-FAU treated with PMS increased with the Nd(III)-loading level. The emission decays did not follow simple first-order kinetics and the average lifetime became longer with increasing Nd(III)-loading level. The short lifetimes at low loading levels and the long lifetimes at high loading level were attributed to Nd(PMS)₃ complexes formed with coordinating water molecules and [Nd(PMS)]-zeolite complexes without coordinating water molecules, respectively.     

Strong dual-state emission of unsymmetrical and symmetrical thiazolothiazole-bridged imidazolium salts

A novel thiazolothiazole-bridged imidazole derivative (1) was found to exhibit blue fluorescence in gaseous state or in methanol and yellow fluorescence in solid state. The N-alkylation of imidazole subunit(s) in 1 using n-propyl iodide generated unsymmetrically or symmetrically alkylated thiazolothiazole-bridged imidazolium salts with good water solubility and remarkably strong emission in solution. Furthermore, the replacement of iodide counter-anion by triflate or bis(trifluoromethane sulfonyl)imide achieved remarkably strong emission in solid state and in solution as well as good water solubility. The strong fluorescence of dicationic salts with triflate and NTf₂^– counter-anions in solid state can be ascribed to their twisted and rigid structures induced by interionic C?H···F hydrogen bonding.     

Studies of chromium cages and wheels

Studies of two distinct classes of chromium(III) cage complexes are discussed. The first are compact oxo- and carboxylate cages, made by heating precursors to high temperature under a flow of nitrogen. One of these cages, [Cr₁₂O₉(OH)₃(O₂CCMe₃)₁₅], has an S = 6 spin ground state which proves a very interesting subject for study by EPR and MCD spectroscopy. Use of other carboxylates leads to other octa- and dodeca-nuclear complexes. The second class of compounds are homo- and hetero-metallic wheels and chains bridged by fluoride and carboxylates. These include the first heterometallic anti-ferromagnetically coupled ring systems and are being widely studied in areas as diverse as magnetic cooling and quantum information processing. The mechanism by which these unusual cyclic and acyclic structures form is discussed.     

An atropisomeric M2L4 cage mixture displaying guest-induced convergence and strong guest emission in water

Introduction of atropisomeric axes into a bent bispyridine ligand leads to the quantitative formation of a complex mixture of atropisomeric M₂L₄ cages upon treatment with metal ions. Whereas the isomer ratio of the obtained cage mixture, consisting of up to 42 isomers, is insensitive to temperature and solvent, the quantitative convergence from the mixture to a single isomer is accomplished upon encapsulation of a large spherical guest, namely fullerene C₆₀. The observed isomerization with other guests depends largely on their size and shape (e.g., <10 and 82% convergence with planar triphenylene and bowl-shaped corannulene guests, respectively). Besides the unusual guest-induced convergence, the present cage mixture displays the strongest guest emission (Φ_F = 68%) among previously reported M_nL_m cages and capsules, upon encapsulation of a BODIPY dye in water.

A complex mixture of atropisomeric M₂L₄ cages is shown to undergo perfect convergence to a single isomer upon encapsulation of spherical C₆₀ in water. Moreover, the cage mixture displays very strong guest emission upon encapsulation of a BODIPY dye.     

Azobenzene‐Functionalized Cage Silsesquioxanes as Inorganic–Organic Hybrid,Photoresponsive, Nanoscale,Building Blocks

Mono‐ and octa‐azobenzene‐functionalized cage silsesquioxanes were easily synthesized by the reaction of 4‐bromoazobenzene with monovinyl‐substituted octasilsesquioxane and cubic octavinylsilsesquioxane through the Heck coupling reaction. Excited‐state energies obtained from time‐dependent density functional theory (TDDFT) and the CAM‐B3LYP functional correlate very well with experimental trans–cis photoisomerization results from UV/Vis spectroscopy. These azobenzene‐functionalized cages exhibit good thermal stability and are fluorescent with maximum emission at approximately 400 nm, making them potential materials for blue‐light emission.     

Self-assembled metallasupramolecular cages towards light harvesting systems for oxidative cyclization

Designing artificial light harvesting systems with the ability to utilize the output energy for fruitful application in aqueous medium is an intriguing topic for the development of clean and sustainable energy. We report here facile synthesis of three prismatic molecular cages as imminent supramolecular optoelectronic materials via two-component coordination-driven self-assembly of a new tetra-imidazole donor (L) in combination with 180°/120° di-platinum(ii) acceptors. Self-assembly of 180° trans-Pt(ii) acceptors A1 and A2 with L leads to the formation of cages Pt₄L₂(1a) and Pt₈L₂(2a) respectively, while 120°-Pt(ii) acceptor A3 with L gives the Pt₈L₂(3a) metallacage. PF₆⁻ analogues (1b, 2b and 3b) of the metallacages possess a high molar extinction coefficient and large Stokes shift. 1b–3b are weakly emissive in dilute solution but showed aggregation induced emission (AIE) in a water/MeCN mixture as well as in the solid state. AIE active 2b and 3b in aqueous (90% water/MeCN mixture) medium act as donors for fabricating artificial light harvesting systems via Förster resonance energy transfer (FRET) with organic dye rhodamine-B (RhB) with high energy efficiency and good antenna effect. The metallacages 2b and 3b represent an interesting platform to fabricate new generation supramolecular aqueous light harvesting systems with high antenna effect. Finally, the harvested energy of the LHSs (2b + RhB) and (3b + RhB) was utilized successfully for efficient visible light induced photo-oxidative cross coupling cyclization of N,N-dimethylaniline (4) with a series of N-alkyl/aryl maleimides (5) in aqueous acetonitrile with dramatic enhancement in yields compared to the reactions with RhB or cages alone.

Synthesis of Pt(ii) based metallacages as aggregation induced emissive supramolecular architectures for fabricating artificial light harvesting systems for cross coupling cyclization under visible light is achieved.     

Ultraviolet emission of molecular chlorine

The UV emission of Cl₂ from a new valence-shell state having 0⁺_u symmetry (T_c ≈ 59774 cm^?1, r_c ≈ 3.0 Å) was observed by focusing ≈ 500 nm laser radiation to gaseous chlorine. Excitation was achieved by virtual two-photon absorption from the B ³Π_0⁺u state formed by single-photon absorption stepwisely. The emission spectra showed transitions to the ground state as well as to the repulsive grade estate dissociating to Cl²P) + Cl(²P) products.     

Construction and structural analysis of mono- and heterobimetallic bis(titanate) molecular cages

To explore the utility of bis(dihydroxynaphthalene) ligands for the construction of supramolecular structures, we demonstrated the preparation of cage-shaped complexes by combining these ligands with hexacoordinate titanium(IV). The reaction of biphenylenebis(dihydroxynaphthalene) with TiO(acac)₂ proceeded in the presence of N-methylmorpholine in DMF and an M₂L₃-type cage was obtained by self-organization. As an extension of this work, the preparation of heterobimetallic molecular cages was examined by using combinations of titanium(IV), palladium(II) or platinum(II), and pyridyldihydroxynaphthalenes. Ti(IV)/Pd(II) cages were prepared in one pot by treatment of the pyridyldihydroxynaphthalene ligands with TiO(acac)₂ followed by PdCl₂(MeCN)₂. In the preparation of Ti(IV)/Pt(II) cages, platinum(II)-bridged bis(dihydroxynaphthalene) ligands were isolated in advance from the reaction of pyridyldihydroxynaphthalene ligand precursors with K₂PtCl₄, which were then deprotected and reacted with TiO(acac)₂ in the same conditions as those for biphenylenebis(dihydroxynaphthalene). The precise structures of the Ti(IV)/Pd(II) and Ti(IV)/Pt(II) heterobimetallic cages were fully elucidated by X-ray crystallographic analysis.     

An efficient route to the synthesis of symmetric and asymmetric diastereomerically pure fullerene triads

A new synthetic route based on the stepwise functionalisation of fullerene cages allows the facile formation of linear, diastereomerically pure triads incorporating two different fullerene cages linked by an organic spacer group. The critical coupling step of two fullerene cages via activation by N,N′-dicyclohexylcarbodiimide was systematically investigated to reveal that the yield of the coupling is maximised in o-dichlorobenzene at high concentrations of the reactant fullerene nucleophile, while in more polar solvents or at lower concentrations of reactants the formation of unwanted side-products (such as guanidine-, N-acylurea- and anhydride-functionalised fullerenes) is favoured. The resultant triads possess an atypically good solubility for this class of compound, which enabled full detailed characterisation by ¹H and ¹³C NMR, IR and UV–vis spectroscopies and by MALDI-TOF mass spectrometry.     

Highly efficient blue emission from boron complexes of 1-(o-hydroxyphenyl)imidazo[1,5-a]pyridine

Two boron complexes of 1-(o-hydroxyphenyl)imidazo[1,5-a]pyridine, which were named as BOHPIP, have been synthesized. These complexes exhibited blue emission in solution with high quantum yields (ΦFL?=?up to 0.71). In addition, 1-(o-hydroxyphenyl)imidazo[1,5-a]pyridine-BPh2 complex showed the strong blue emission even in the solid state (ΦFL?=?0.58).     

Topological prediction of palladium coordination cages

The preparation of functionalized, heteroleptic Pd_xL_2x coordination cages is desirable for catalytic and optoelectronic applications. Current rational design of these cages uses the angle between metal-binding (∠B) sites of the di(pyridyl)arene linker to predict the topology of homoleptic cages obtained via non-covalent chemistry. However, this model neglects the contributions of steric bulk between the pyridyl residues—a prerequisite for endohedrally functionalized cages, and fails to rationalize heteroleptic cages. We describe a classical mechanics (CM) approach to predict the topological outcomes of Pd_xL_2x coordination cage formation with arbitrary linker combinations, accounting for the electronic effects of coordination and steric effects of linker structure. Initial validation of our CM method with reported homoleptic Pd₁₂LFu₂₄ (LFu = 2,5-bis(pyridyl)furan) assembly suggested the formation of a minor topology Pd₁₅LFu₃₀, identified experimentally by mass spectrometry. Application to heteroleptic cage systems employing mixtures of LFu (∠B = 127°) and its thiophene congener LTh (∠B = 149° ∠B_exp = 152.4°) enabled prediction of Pd₁₂L₂₄ and Pd₂₄L₄₈ coordination cages formation, reliably emulating experimental data. Finally, the topological outcome for exohedrally (LEx) and endohedrally (LEn) functionalized heteroleptic Pd_xL_2x coordination cages were predicted to assess the effect of steric bulk on both topological outcomes and coordination cage yields, with comparisons drawn to experimental data.

A molecular mechanics approach enables the accurate prediction of polyhedral topology for homoleptic and heteroleptic palladium M_xL_2x coordination cages, allowing for new insight and design when considering endo- and exo-hedral functionalization.     

Luminescence and electroluminescence of bis (2-(benzimidazol-2-yl) quinolinato) zinc. Exciplex formation and energy transfer in mixed film of bis (2-(benzimidazol-2-yl) quinolinato) zinc and N,N′-bis-(1-naphthyl)-N,N′-diphenyl-1,1′-biphenyl-4,4′-diamine

The absorption and emission properties of benzimidazol-2-yl-quinoline (BIQ) and bis (2-(benzimidazol-2-yl) quinolinato) zinc (ZnBIQ) a new emitter used for organic light emitting device (OLED) were reported. Exciplexes are observed for ZnBIQ with N,N′-bis-(1-naphthyl)-N,N′-diphenyl-1,1′-biphenyl-4,4′-diamine (NPB) system, in both electro- and photoluminescent processes. The identification of exciplex emission in co-evaporated and multi-layer ZnBIQ thin film was reported for the first time. The optical formation of the exciplex involves the excitation of a single molecule, followed by the relaxation of that exciton into a lower energy exciplex state. Both BIQ and ZnBIQ possess very high thermal stabilities and can be purified by subliming under the high vacuum condition. Devices consisting of ZnBIQ as the emitting layer have been fabricated, and the emission spectra of ZnBIQ-base devices gave a voltage-dependent spectrum, with the red emission observed (3-7 V), switching over to strong white emission as the bias was raised.     

Tunable construction of transition metal-coordinated helicene cages

We report a facile and tailored method to prepare globally twisted chiral molecular cages through tunable coordination of bis-bipyridine-terminated helicene ligands to a series of transition metals including Fe(II), Co(II), Ni(II) and Zn(II). This system shows an efficient remote transfer of stereogenecity from the helicene core to the bipyridine-metal coordination sites and subsequently the entire cages. While the Fe(II), Co(II) and Ni(II)-derived M₂L₃ (M for metal and L for ligand) cages exhibit quasi-reversible redox features, the Zn(II) analogues reveal prominent yellow circularly polarized luminescence. Interestingly, with the addition of Na₂SO₄, the Zn₂L₃ cages reassemble into sextuple-stranded Zn₆L₆(SO₄)₄ cages in which three Zn₂L₂ units are bound together by four sulfates and further coalesced by offset inter-ligand π-π interactions.     

Spectroscopic properties of lanthanoid benzene carboxylates in the solid state: Part 2. Polar substituted benzoates

Europium and terbium complexes of ortho, meta and para substituted benzoate ligands including nitrobenzoate (NBA), aminobenzoate (ABA), hydroxybenzoate (OHBA) and methoxybenzoate (MeOBA) have been synthesised by metathesis reactions, carried out in aqueous media. The complexes were characterised by elemental, compositional and structural investigations, including microanalysis, EDTA titrations, differential thermal analysis, infra red spectroscopy, X-ray powder diffraction and single crystal structural analyses. Besides this, strong emphasis was on the determination of the optoelectronic properties of the compounds in the solid state. In this regard, reflectance, excitation and emission spectra were recorded. From these, the emission and excitation efficiencies were determined. The relative intensities as well as the splitting patterns of the ⁵D₀ → ⁷F_J transitions in the europium emission spectra are discussed.     

A computational investigation of the electronic properties of Octahedral Al n N n and Al n P n cages (n = 12, 16, 28, 36, and 48)

Density functionla theory (DFT) calculations are performed to characterize geometric and electronic features of the octahedral Al _n N _n and Al _n P _n cages (n = 12, 16, 28, 32, and 48). Toward this aim, ¹⁵N, ²⁷Al, and ³¹P chemical shielding (CS) tensors as well as natural charge analyses are calculated for the optimized structures. CS parameters detect three distinct electronic environments for atoms within the Al _n N _n and Al _n P _n cages. The chemical shifts of N2 sites belonging to a hexagon and surrounded by three hexagons and a square obtained are different from those of N3 sites belonging to a hexagon that is surrounded only by hexagons—due to different curvatures exerted at the sites with different local structures. In addition, there is an increasing tendency in the Δσ values of the three local structures, Δσ (N1) > Δσ (N2) > Δσ (N3), N1 sites belonging to four-membered rings. The chemical shieldings of those Al and P sites belonging to a hexagon that is surrounded only by hexagons in the cages (360.7–366.7 and 496.5–514.7 ppm) are close to those previously reported for AlP nanotubes. Three distinct electrostatic environments around the N, Al, and P nuclei are also confirmed by the calculated natural charges. It should be noted that the positively charged Al atoms on the cages turn out to be the available sites for adsorption of H2 molecules.     

Modulation of charge transfer by N-alkylation to control photoluminescence energy and quantum yield

Charge transfer in organic fluorophores is a fundamental photophysical process that can be either beneficial, e.g., facilitating thermally activated delayed fluorescence, or detrimental, e.g., mediating emission quenching. N-Alkylation is shown to provide straightforward synthetic control of the charge transfer, emission energy and quantum yield of amine chromophores. We demonstrate this concept using quinine as a model. N-Alkylation causes changes in its emission that mirror those caused by changes in pH (i.e., protonation). Unlike protonation, however, alkylation of quinine''s two N sites is performed in a stepwise manner to give kinetically stable species. This kinetic stability allows us to isolate and characterize an N-alkylated analogue of an ‘unnatural’ protonation state that is quaternized selectively at the less basic site, which is inaccessible using acid. These materials expose (i) the through-space charge-transfer excited state of quinine and (ii) the associated loss pathway, while (iii) developing a simple salt that outperforms quinine sulfate as a quantum yield standard. This N-alkylation approach can be applied broadly in the discovery of emissive materials by tuning charge-transfer states.

A versatile N-alkylation strategy controls the presence of charge-transfer excited states and the emission colour of N-heterocyclic chromophores.     

Photodissociation of acryloyl chloride in the gas phase

The 193 nm photodissociation dynamics of CH2 CHCOCl in the gas phase has been examined with the technique of time-resolved Fourier transform infrared emission (TR-FTIR) spectroscopy.Vibrationally excited photofragments of CO (≤ 5),HCl (≤ 6),and C2H2 were observed and two photodissociation channels,the C-Cl fission channel and the HCl elimination channel have been identified.The vibrational and rotational state distributions of the photofragments CO and HCl have been acquired by analyzing their fully rotationally resolved v→v-1 rovibrational progressions in the emission spectra,from which it has been firmly established that the mechanism involves production of HCl via the four-center molecular elimination of CH2 CHCOCl after its internal conversion from the S1 state to the S0 state.In addition to the dominant C-Cl bond fission along the excited S1 state,the S1→S0 internal conversion has also been found to play an important role in the gas phase photolysis of CH2 CHCOCl as manifested by the considerable yield of HCl.