Trinuclear Cyclometalated Iridium(III) Complex Exhibiting Intense Phosphorescence of an Unprecedented Rate.

Herein, we present two novel cyclometalated Ir(III) complexes of dinuclear and trinuclear design, Ir2(dppm)3(acac)2 and Ir3(dppm)4(acac)3, respectively, where dppm is 4,6-di(4-tert-butylphenyl)pyrimidine ligand and acac is acetylacetonate ligand. In both cases, rac-diastereomers were isolated during the synthesis. The materials show intense phosphorescence of outstanding rates (kr = ΦPL/τ) with corresponding radiative decay times of only τr = 1/kr = 0.36 µs for dinuclear Ir2(dppm)3(acac)2 and still shorter τr = 0.30 µs for trinuclear Ir3(dppm)4(acac)3, as measured for doped polystyrene film samples under ambient temperature. Measured under cryogenic conditions, radiative decay times of the three T1 substates (I, III, and III) and substate energy separations are τI = 11.8 µs, τII = 7.1 µs, τIII = 0.06 µs, ΔE(II-I) = 7 cm-1, and ΔE(III-I) = 175 cm-1 for dinuclear Ir2(dppm)3(acac)2 and τI = 3.1 µs, τII = 3.5 µs, τIII = 0.03 µs, ΔE(II-I) ≈ 1 cm-1, and ΔE(III-I) = 180 cm-1 for trinuclear Ir3(dppm)4(acac)3. The determined T1 state ZFS values (ΔE(III-I)) are smaller compared to that of mononuclear analogue Ir(dppm)2(acac) (ZFS = 210-1 cm). Theoretical analysis suggests that the high phosphorescence rates in multinuclear materials can be associated with the increased number of singlet states lending oscillator strength to the T1 → S0 transition.

Aligning π-Extended π-Deficient Ligands to Afford Submicrosecond Phosphorescence Radiative Decay Time of Mononuclear Ir(III) Complexes.

Herein, we report a profound investigation of the photophysical properties of three mononuclear Ir(III) complexes fac-Ir(dppm)3 (Hdppm-4,6-bis(4-(tert-butyl)phenyl)pyrimidine), Ir(dppm)2(acac) (acac-acetylacetonate), and Ir(ppy)2(acac) (Hppy-phenylpyridine). The heteroleptic Ir(dppm)2(acac) is found to emit with efficiency above 80% and feature a remarkably high rate of emission. As measured under ambient temperature, Ir(dppm)2(acac) emits with the unusually short (sub-µs) radiative decay time of τr = τem/ΦPL = 1/kr = 0.91 µs in degassed toluene and τr = 0.73 µs in a doped polystyrene film under nitrogen. Investigations at cryogenic temperatures in glassy toluene showed that the emission stems from the T1 state and thus represents T1 → S0 phosphorescence with individual decay times of the T1 substates of T1,I = 66 µs, T1,II = 7.3 µs, T1,III = 0.19 µs, and energy gaps between the substates of ΔE(T1,II-T1,I) = 14 cm-1 and ΔE(T1,III-T1,I) = 210 cm-1. Analysis of the electronic structure of Ir(dppm)2(acac) showed that such a high rate of phosphorescence may stem from the two dppm ligands, with extended π-conjugation system and π-deficient character due to the pyrimidine ring, being serially aligned along one axis. Such alignment, along with the quasi-symmetric character of Jahn-Teller distortions in the T1 state, affords a large chromophore, comprising four (het)aryl rings of the two dppm ligands. This affords an exceptionally large oscillator strength of the MLCT-character singlet state spin-orbit coupled with the T1 state and thus brings about enhancement of the phosphorescence rate. These findings reveal molecular design principles paving the way to new phosphors of enhanced emission rates.

Construction and performance of OLED devices prepared from liquid-crystalline TADF materials.

The device performance is reported for three compounds which show both thermally activated delayed fluorescence and liquid crystallinity, and use the donor 3,6-bis(3,4-didodecyloxyphenyl)carbazole. Two of the compounds, whose photophysics were reported previously, are based on a terephthalonitrile acceptor. A third and new compound is based on an isophthalonitrile acceptor and shows a more temperature-accessible mesophase and enhanced solution emission quantum yield. Two of the compounds show device external quantum efficiencies of between 2-3% and exhibit very small efficiency roll off. The responses are evaluated in terms of the specific nature of the materials.

Non-Stereogenic Dinuclear Ir(III) Complex with a Molecular Rack Design to Afford Efficient Thermally Enhanced Red Emission.

Cyclometalated complexes containing two or more metal centers were recently shown to offer photophysical properties that are advantageous compared to their mononuclear analogues. Here we report the design, synthesis, and luminescent properties of a dinuclear Ir(III) complex formed by a ditopic N^C^N-N^C^N bridging ligand (L1) with pyrimidine as a linking heterocycle. Two dianionic C^N^C terminal ligands were employed to achieve a charge-neutral and nonstereogenic dinuclear complex 5. This complex shows a highly efficient red emission with a maximum at λem = 642 nm as measured for a toluene solution. The decay time and emission quantum yield of the complex measured for the degassed sample are τ = 1.31 µs and ΦPL = 80%, respectively, corresponding to the radiative rate of kr = 6.11·105 s-1. This rate value is approximately fourfold faster than for the green-emitting mononuclear analogue 3. Cryogenic temperature measurements show that the three substrates of the lowest triplet state T1 of 5 emit with decay times of τ(I) = 120 µs, τ(II) = 7 µs, and τ(III) = 1 µs that are much shorter compared to those of the mononuclear complex 3, which has values of τ(I) = 192 µs, τ(II) = 65.6 µs, and τ(III) = 3.6 µs. These data indicate that the spin-orbit coupling of state T1 with the singlet states is much stronger in the case of complex 5, which results in a much higher T1 → S0 emission rate. Indeed, a computational analysis suggests that in the dinuclear complex 5 the T1 state is spin-orbit coupled with twice the number of singlet states compared to that of mononuclear 3, which is a result of the electronic coupling of two coordination sites. The investigation of the temperature dependence of the emission rates of 3 and 5 shows that the room-temperature emission of both complexes is mainly contributed by a thermally populated excited state lying above the T1 state. To the best of our knowledge, complexes 3 and 5 are the first examples of Ir(III) complexes that show photophysical behavior reminiscent of thermally activated delayed fluorescence (TADF).

Halide-Enhanced Spin-Orbit Coupling and the Phosphorescence Rate in Ir(III) Complexes.

The spin-forbidden nature of phosphorescence in Ir(III) complexes is relaxed by the metal-induced effect of spin-orbit coupling (SOC). A further increase of the phosphorescence rate could potentially be achieved by introducing additional centers capable of further enhancing the SOC effect, such as metal-coordinated halides. Herein, we present a dinuclear Ir(III) complex Ir2I2 that contains two Ir(III)-iodide moieties. The complex shows intense phosphorescence with a quantum yield of ΦPL(300 K) = 90% and a submicrosecond decay time of only τ(300 K) = 0.34 µs, as measured under ambient temperature for the degassed toluene solution. These values correspond to a top value T1 → S0 phosphorescence rate of kr = 2.65 × 106 s-1. Investigations at cryogenic temperatures allowed us to determine the zero-field splitting (ZFS) of the emitting state T1 ZFS(III-I) = 170 cm-1 and unusually short individual decay times of T1 substates: τ(I) = 6.4 µs, τ(II) = 7.6 µs, and τ(III) = 0.05 µs. This indicates a strong SOC of state T1 with singlet states. Theoretical investigations suggest that the SOC of state T1 with singlets is also contributed by halides. Strongly contributing to the higher occupied molecular orbitals of the complex (e.g., HOMO, HOMO - 1, and so forth), iodides work as important SOC centers that operate in tandem with metals. The examples of Ir2I2 and of earlier reported analogous complex Ir2Cl2 reveal that the metal-coordinated halides can enhance the SOC of state T1 with singlets and, consequently, the phosphorescence rate. A comparative study of Ir2I2 and Ir2Cl2 shows that the share of halides in total contribution (halides plus metals) to the SOC of state T1 with singlets increases strongly upon exchange of chlorides for iodides. The exchange also led to the decrease in values of ZFS of the T1 state from ZFS(III-I) = 205 cm-1 for Ir2Cl2 to T1 ZFS(III-I) = 170 cm-1 for Ir2I2. This results in a more efficient thermal population of the fastest emitting T1 substate III, thus further enhancing the room-temperature phosphorescence rate.

Near Infrared Phosphorescent Dinuclear Ir(III) Complex Exhibiting Unusually Slow Intersystem Crossing and Dual Emissive Behavior.

A dinuclear iridium(III) complex IrIr shows dual emission consisting of near infrared (NIR) phosphorescence (λmax = 714 nm, CH2Cl2, T = 300 K) and green fluorescence (λmax = 537 nm). The NIR emission stems from a triplet state (T1) localized on the ditopic bridging ligand (3LC). Because of the dinuclear molecular structure, the phosphorescence efficiency (ΦPL = 3.5%) is high compared to those of other known red/NIR-emitting iridium complexes. The weak fluorescence stems from the lowest excited singlet state (S1) of 1LC character. The occurrence of fluorescence is ascribed to relatively slow intersystem crossing (ISC) from state S1 (1LC) to the triplet manifold. The measured ISC rate corresponds to a time constant τISC of 2.1 ps, which is an order of magnitude longer than those usually found for iridium complexes. This slow ISC rate can be explained in terms of the LC character and large energy separation (0.57 eV) of the respective singlet and triplet excited states. IrIr is internalized by live HeLa cells as evidenced by confocal luminescence microscopy.

Unusually Fast Phosphorescence from Ir(III) Complexes via Dinuclear Molecular Design.

The design and detailed photophysical study of two novel Ir(III) complexes featuring mono- and dinuclear design are presented. Emission quantum yield and decay times in solution are ΦPL = 90% and τ(300 K) = 1.16 µs for the mononuclear complex 5, and ΦPL = 95% and τ(300 K) = 0.44 µs for the dinuclear complex 6. These data indicate an almost 3-fold increase in the phosphorescence rate for dinuclear complex 6 compared to 5. Zero-field splitting (ZFS) of the T1 state also increases from ZFS = 65 cm-1 for the mononuclear complex to ZFS = 205 cm-1 for the dinuclear complex and is accompanied by a drastic shortening of the individual decay times of T1 substates. With the help of TD-DFT calculations, we rationalize that the drastic changes in the T1 state properties in the dinuclear complex originate from an increased number of excited states available for direct spin-orbit coupling (SOC) routes as a result of electronic coupling of Ir-Cl antibonding molecular orbitals of the two coordination sites.

Can Coumarins Break Kasha's Rule?

Coumarin C-2 was reported ( Signore et al., J. Am. Chem. Soc. , 2010 , 132 , 1276 and Brancato et al., J. Phys. Chem. B , 2015 , 119 , 6144 ) to break Kasha's rule. However, the two lowest excited singlet states of C-2 are separated by less than 0.5 eV. To slow down the S2 → S1 internal conversion and thus to enable the Kasha's rule-breaking S2 fluorescence, a much larger energy separation seems to be necessary. Thus, the photophysical behavior reported for C-2 raised very basic questions concerning mechanisms of nonradiative transitions in organic molecules. Herein we reinvestigated luminescence of C-2 and found that thoroughly purified C-2 does not show any dual fluorescence in steady-state experiments, contrary to the previous findings. The higher-energy emission, previously erroneously assigned as S2 → S0 fluorescence of C-2, stems from persistent impurity of the synthetic precursor (C-1).

Ag(i) complex design affording intense phosphorescence with a landmark lifetime of over 100 milliseconds.

A highly emissive Ag(i) complex comprising 2,9-dimethyl-1,10-phenanthroline (dmp) and bis[(2-diphenylphosphino)phenyl] ether (dpep) ligands was synthesized, characterized and investigated for its photophysical properties both experimentally and theoretically. The material exhibits intense phosphorescence from the triplet state of ligand centered (3LC) character featuring an unprecedented long lifetime of τ = 110 ms and a quantum yield of ΦPL = 50%, as measured for a doped PMMA matrix under ambient conditions. This is an efficient yet exceptionally slow emission decay, breaking the previous record by several orders of magnitude. Such properties are attributed to two factors: (i) the Ag(i) ion introduces weak spin-orbit coupling and efficient population of the emitting triplet state; (ii) the rigid molecular design combined with a matrix-based rigidity largely suppresses non-radiative relaxations.

Dinuclear Design of a Pt(II) Complex Affording Highly Efficient Red Emission: Photophysical Properties and Application in Solution-Processible OLEDs.

The light-emitting efficiency of luminescent materials is invariably compromised on moving to the red and near-infrared regions of the spectrum due to the transfer of electronic excited-state energy into vibrations. We describe how this undesirable "energy gap law" can be sidestepped for phosphorescent organometallic emitters through the design of a molecular emitter that incorporates two platinum(II) centers. The dinuclear cyclometallated complex of a substituted 4,6-bis(2-thienyl)pyrimidine emits very brightly in the red region of the spectrum (λmax = 610 nm, Φ = 0.85 in deoxygenated CH2Cl2 at 300 K). The lowest-energy absorption band is extraordinarily intense for a cyclometallated metal complex: at λ = 500 nm, ε = 53 800 M-1 cm-1. The very high efficiency of emission achieved can be traced to an unusually high rate constant for the T1 → S0 phosphorescence process, allowing it to compete effectively with nonradiative vibrational decay. The high radiative rate constant correlates with an unusually large zero-field splitting of the triplet state, which is estimated to be 40 cm-1 by means of variable-temperature time-resolved spectroscopy over the range 1.7 < T < 120 K. The compound has been successfully tested as a red phosphor in an organic light-emitting diode prepared by solution processing. The results highlight a potentially attractive way to develop highly efficient red and NIR-emitting devices through the use of multinuclear complexes.

Unexpected, photochemically induced activation of the tetrabutylammonium cation by hexachloroplatinate(iv).

A dinuclear, butadiene-bridged complex, trans-µ2:η2,η2-1,3-butadiene-bis(trichloroplatinate(ii)) (1) was unexpectedly obtained on photolysis of acetone solutions of (NBu4)2[PtCl6].

Dinuclear Ag(I) Complex Designed for Highly Efficient Thermally Activated Delayed Fluorescence.

The dinuclear Ag(I) complex has been designed to show thermally activated delayed fluorescence (TADF) of high efficiency. Strongly electron-donating terminal ligands are introduced to destabilize the d orbitals of the Ag+ ions. Consequently, the orbitals distinctly contribute to the HOMO, whereas the LUMO is localized on the bridging ligand. This ensures charge transfer character of the lowest excited singlet S1 and triplet T1 states. Accordingly, a small energy gap ΔE(S1-T1) is obtained, being essential for TADF behavior. Photophysical investigations show that at ambient temperature the complex exhibits TADF reaching a quantum yield of ΦPL = 70% with the decay time of only τ = 1.9 µs, manifesting one of the fastest TADF decays observed so far. Such an outstanding TADF efficiency is based on a small value of ΔE(S1-T1) = 480 cm-1 combined with a large transition rate of k(S1 → S0) = 2.2 × 107 s-1.

Thermally Activated Delayed Fluorescence from Ag(I) Complexes: A Route to 100% Quantum Yield at Unprecedentedly Short Decay Time.

The four new Ag(I) complexes Ag(phen)(P2-nCB) (1), Ag(idmp)(P2-nCB) (2), Ag(dmp)(P2-nCB) (3), and Ag(dbp)(P2-nCB) (4) with P2-nCB = bis(diphenylphosphine)-nido-carborane, phen = 1,10-phenanthroline, idmp = 4,7-dimethyl-1,10-phenanthroline, dmp = 2,9-dimethyl-1,10-phenanthroline, and dbp = 2,9-di-n-butyl-1,10-phenanthroline were designed to demonstrate how to develop Ag(I) complexes that exhibit highly efficient thermally activated delayed fluorescence (TADF). The substituents on the 1,10-phenanthroline ligand affect the photophysical properties strongly (i) electronically via influencing the radiative rate of the S1 → S0 transition and (ii) structurally by rigidifying the molecular geometry with respect to geometry changes occurring in the lowest excited S1 and T1 states. The oscillator strength of the S1 ↔ S0 transition f(S1 ↔ S0)-an important parameter for the TADF efficiency being proportional to the radiative rate-can be increased from f(S1 ↔ S0) = 0.0258 for Ag(phen)(P2-nCB) (1) to f(S1 ↔ S0) = 0.0536 for Ag(dbp)(P2-nCB) (4), as calculated for the T1 state optimized geometries. This parameter governs the radiative TADF decay time (τr) at ambient temperature, found to be τr = 5.6 µs for Ag(phen)(P2-nCB) (1) but only τr = 1.4 µs for Ag(dbp)(P2-nCB) (4)-a record TADF value. In parallel, the photoluminescence quantum yield (ΦPL) measured for powder samples at ambient temperature is boosted up from ΦPL = 36% for Ag(phen)(P2-nCB) (1) to ΦPL = 100% for Ag(dbp)(P2-nCB) (4). This is a consequence of a cooperative effect of both decreasing the nonradiative decay rate and increasing the radiative decay rate in the series from Ag(phen)(P2-nCB) (1), Ag(idmp)(P2-nCB) (2), and Ag(dmp)(P2-nCB) (3) to Ag(dbp)(P2-nCB) (4). Another parameter important for the TADF behavior is the activation energy of the S1 state from the state T1, ΔE(S1-T1). Experimentally it is determined for the complexes Ag(dmp)(P2-nCB) (3) and Ag(dbp)(P2-nCB) (4) to be of moderate size of ΔE(S1-T1) = 650 cm-1.

TADF Material Design: Photophysical Background and Case Studies Focusing on CuI and AgI Complexes.

The development of organic light emitting diodes (OLEDs) and the use of emitting molecules have strongly stimulated scientific research of emitting compounds. In particular, for OLEDs it is required to harvest all singlet and triplet excitons that are generated in the emission layer. This can be achieved using the so-called triplet harvesting mechanism. However, the materials to be applied are based on high-cost rare metals and therefore, it has been proposed already more than one decade ago by our group to use the effect of thermally activated delayed fluorescence (TADF) to harvest all generated excitons in the lowest excited singlet state S1 . In this situation, the resulting emission is an S1 →S0 fluorescence, though a delayed one. Hence, this mechanism represents the singlet harvesting mechanism. Using this effect, high-cost and strong SOC-carrying rare metals are not required. This mechanism can very effectively be realized by use of CuI or AgI complexes and even by purely organic molecules. In this investigation, we focus on photoluminescence properties and on crucial requirements for designing CuI and AgI materials that exhibit short TADF decay times at high emission quantum yields. The decay times should be as short as possible to minimize non-radiative quenching and, in particular, chemical reactions that frequently occur in the excited state. Thus, a short TADF decay time can strongly increase the material's long-term stability. Here, we study crucial parameters and analyze their impact on the TADF decay time. For example, the energy separation ΔE(S1 -T1 ) between the lowest excited singlet state S1 and the triplet state T1 should be small. Accordingly, we present detailed photophysical properties of two case-study materials designed to exhibit a large ΔE(S1 -T1 ) value of 1000 cm-1 (120 meV) and, for comparison, a small one of 370 cm-1 (46 meV). From these studies-extended by investigations of many other CuI TADF compounds-we can conclude that just small ΔE(S1 -T1 ) is not a sufficient requirement for short TADF decay times. High allowedness of the transition from the emitting S1 state to the electronic ground state S0 , expressed by the radiative rate kr (S1 →S0 ) or the oscillator strength f(S1 →S0 ), is also very important. However, mostly small ΔE(S1 -T1 ) is related to small kr (S1 →S0 ). This relation results from an experimental investigation of a large number of CuI complexes and basic quantum mechanical considerations. As a consequence, a reduction of τ(TADF) to below a few µs might be problematic. However, new materials can be designed for which this disadvantage is not prevailing. A new TADF compound, Ag(dbp)(P2 -nCB) (with dbp=2,9-di-n-butyl-1,10-phenanthroline and P2 -nCB=bis-(diphenylphosphine)-nido-carborane) seems to represent such an example. Accordingly, this material shows TADF record properties, such as short TADF decay time at high emission quantum yield. These properties are based (i) on geometry optimizations of the AgI complex for a fast radiative S1 →S0 rate and (ii) on restricting the extent of geometry reorganizations after excitation for reducing non-radiative relaxation and emission quenching. Indeed, we could design a TADF material with breakthrough properties showing τ(TADF)=1.4 µs at 100 % emission quantum yield.

Mitochondria Targeting with Luminescent Rhenium(I) Complexes.

Two new neutral fac-[Re(CO)3(phen)L] compounds (1,2), with phen = 1,10-phenanthroline and L = O2C(CH2)5CH3 or O2C(CH2)4C≡CH, were synthetized in one-pot procedures from fac-[Re(CO)3(phen)Cl] and the corresponding carboxylic acids, and were fully characterized by IR and UV-Vis absorption spectroscopy, ¹H- and 13C-NMR, mass spectrometry and X-ray crystallography. The compounds, which display orange luminescence, were used as probes for living cancer HeLa cell staining. Confocal microscopy revealed accumulation of both dyes in mitochondria. To investigate the mechanism of mitochondrial staining, a new non-emissive compound, fac-[Re(CO)3(phen)L], with L = O2C(CH2)3((C5H5)Fe(C5H4), i.e., containing a ferrocenyl moiety, was synthetized and characterized (3). 3 shows the same mitochondrial accumulation pattern as 1 and 2. Emission of 3 can only be possible when ferrocene-containing ligand dissociates from the metal center to produce a species containing the luminescent fac-[Re(CO)3(phen)]⁺ core. The release of ligands from the Re center was verified in vitro through the conjugation with model proteins. These findings suggest that the mitochondria accumulation of compounds 1-3 is due to the formation of luminescent fac-[Re(CO)3(phen)]⁺ products, which react with cellular matrix molecules giving secondary products and are uptaken into the negatively charged mitochondrial membranes. Thus, reported compounds feature a rare dissociation-driven mechanism of action with great potential for biological applications.

Tuning the Excimer Emission of Amphiphilic Platinum(II) Complexes Mediated by Phospholipid Vesicles.

Two new amphiphilic platinum(II) complexes, [Pt(2-(4-fluorophenyl)-5-(4-dodecyloxyphenyl)pyridine) (acac)] (Pt-1) and [Pt(2-(4-dodecyloxyphenyl)-5-(thien-2-yl)-c-cyclopentenepyridine) (acac)] (Pt-2), where acac is acetylacetonate, were synthesized and characterized. Apart from conventional phosphorescence of single molecules (ME-monomer emission), complexes Pt-1 and Pt-2 also exhibit excimer emission (EE) when embedded into phospholipid vesicles, that is assigned to emissive Pt-Pt excimers. The EE intensity in vesicular media appeared to depend on the viscosity of the vesicles and the concentration of the embedded complex. Differences in the EE properties of complexes Pt-1 and Pt-2 are correlated with the energies of the π-character frontier orbitals defined by the design of the cyclometalating phenylpyridine ligand. Higher energies of the frontier π-orbitals (HOMO and LUMO) naturally promote stronger π-π interactions, thus obstructing the PtII-PtII interaction.

Modulation of Intersystem Crossing Rate by Minor Ligand Modifications in Cyclometalated Platinum(II) Complexes.

Photophysical properties of four new platinum(II) complexes comprising extended ppy (Hppy = 2-phenylpyridine) and thpy (Hthpy = 2-(2'-thienyl)pyridine) cyclometalated ligands and acetylacetonate (acac) are reported. Substitution of the benzene ring of Pt-ppy complexes 1 and 2 with a more electron-rich thiophene of Pt-thpy complexes 3 and 4 leads to narrowing of the HOMO-LUMO gap and thus to a red shift of the lowest energy absorption band and phosphorescence band, as expected for low-energy excited states of the intraligand/metal-to-ligand charge transfer character. However, in addition to these conventional spectral shifts, another, at first unexpected, substitution effect occurs. Pt-thpy complexes 3 and 4 are dual emissive showing fluorescence about 6000 cm(-1) (∼0.75 eV) higher in energy relative to the phosphorescence band, while for Pt-ppy complexes 1 and 2 only phosphorescence is observed. For dual-emissive complexes 3 and 4, ISC rates kISC are estimated to be in order of 10(9)-10(10) s(-1), while kISC of Pt-ppy complexes 1 and 2 is much faster amounting to 10(12) s(-1) or more. The relative intensities of the fluorescence and phosphorescence signals of Pt-thpy complexes 3 and 4 depend on the excitation wavelength, showing that hyper-intersystem crossing (HISC) in these complexes is observably significant.

Mesomorphism and Photophysics of Some Metallomesogens Based on Hexasubstituted 2,2':6', 2''-Terpyridines.

The luminescent and mesomorphic properties of a series of metal complexes based on hexacatenar 2,2':6',2''-terpyridines are investigated using experimental methods and density functional theory (DFT). Two types of ligand are examined, namely 5,5''-di(3,4,5-trialkoxyphenyl)terpyridine with or without a fused cyclopentene ring on each pyridine and their complexes were prepared with the following transition metals: Zn(II) , Co(III) , Rh(III) , Ir(III) , Eu(III) and Dy(III) . The exact geometry of some of these complexes was determined by single X-ray diffraction. All complexes with long alkyl chains were found to be liquid crystalline, which property was induced on complexation. The liquid-crystalline behaviour of the complexes was studied by polarising optical microscopy and small-angle X-ray diffraction. Some of the transition metal complexes (for example, those with Zn(II) and Ir(III) ) are luminescent in solution, the solid state and the mesophase; their photophysical properties were studied both experimentally and using DFT methods (M06-2X and B3LYP).

Detection of nitroaromatic explosives by new D-π-A sensing fluorophores on the basis of the pyrimidine scaffold.

A series of D-π-A- type dyes based on pyrimidines, bearing various thiophene linkers, have been studied as sensing fluorophores. Fluorescence studies have demonstrated that the emission of all derivatives is sensitive to the presence of nitroaromatic explosives, such as 2,4,6-trinitrophenol (PA), 2,4,6-trinitrotoluene (TNT), and 2,4-dinitrotoluene (DNT), in their acetonitrile solutions. The detection limits of fluorophores to PA, TNT, and DNT proved to be in the range from 5.83 × 10(-6) to 2.38 × 10(-7) mol/L, 1.70 × 10(-4) to 8.40 × 10(-6) mol/L, and 8.39 × 10(-5) to 6.87 × 10(-6) mol/L, respectively. The theoretical investigation into the quenching mechanism in the presence of fluorophore has been performed. All compounds have shown a good efficiency as sensor materials when tested as elements of the original device «Nitroscan¼ for detecting nitro-containing explosives in vapor phase (Plant "Promautomatika", Ekaterinburg, Russia). Graphical Abstract ᅟ.

Combined experimental and theoretical studies of regio- and stereoselectivity in reactions of ß-isoxazolyl- and ß-imidazolyl enamines with nitrile oxides.

Reactions of ß-azolyl enamines and nitrile oxides were studied by both experimental and theoretical methods. (E)-ß-(4-Nitroimidazol-5-yl), (5-nitroimidazol-4-yl) and isoxazol-5-yl enamines smoothly react regioselectively at room temperature in dioxane solution with aryl, pyridyl, and cyclohexylhydroxamoyl chlorides without a catalyst or a base to form 4-azolylisoxazoles as the only products in good yields. The intermediate 4,5-dihydroisoxazolines were isolated as trans isomers during the reaction of (E)-ß-imidazol-4-yl enamines with aryl and cyclohexylhydroxamoyl chlorides. Stepwise and concerted pathways for the reaction of ß-azolyl enamines with hydroxamoyl chlorides were considered and studied at the B3LYP/Def2-TZVP level of theory combined with D3BJ dispersion correction. The reactions of benzonitrile oxide with both E- and Z-imidazolyl enamines have been shown to proceed stereoselectively to form trans- and cis-isoxazolines, respectively. The preference of E-isomers over Z-isomers, driven by the higher stability of the former, apparently controls the stereoselectivity of the investigated cycloaddition reaction with benzonitrilе oxide. Based on the reactivity of azolyl enamines towards hydroxamoyl chlorides, a novel, effective catalyst-free method was elaborated to prepare 4-azolyl-5-substituted isoxazoles that are otherwise difficult to obtain.