T1 Excited State Research Articles

Encapsulation-induced hypsochromic shift of emission properties from a cationic Ir(III) complex in a hydrogen-bonded organic cage: A theoretical study.

Encapsulation of coordination complexes within the confined spaces of self-assembled hosts is an effective method for creating supramolecular assemblies with distinct chemical and physical properties. Recent studies with calix-resorcin[4]arene hydrogen-bonded hexameric capsules revealed that encapsulated metal complexes exhibit enhanced and blue-shifted photoluminescence compared to their unencapsulated forms. The photophysical change has been hypothetically attributed to encapsulation-induced confinement, which isolates the metal complex from the solvent, suppressing stabilization of the excited state of the guest by solvent reorganization and structural relaxation, and altering the local environment, such as solvent polarity and viscosity, around the guest. In this study, density-functional theory calculations were conducted to explore how encapsulation affects the photophysical properties of a cationic iridium complex within a hydrogen-bonded hexameric capsule. The encapsulation-induced emission shift was analyzed by separating it into three factors: suppression of solvent reorganization, suppression of structural relaxation of the complex, and electronic interactions between the complex and the capsule. The findings indicate that the photoluminescence modulation is driven by the electronic interaction between the host and guest, which affects the energy levels of the molecular orbitals involved in the T1 excited state and the suppression of excited-state structural relaxation of the Ir complex due to the presence of the host. This study advances our understanding of the photophysical dynamics of coordination complexes within the confined spaces of hexameric capsules, providing a valuable approach for tuning the excited state properties of guest molecules.

A luminescent binuclear Cu(I) complex constructed by bipyridine, TD-DFT calculation, and its fluorescent sensing properties for VOCs

The design and research of materials with strong luminescence and fluorescence sensing for the detection of volatile organic compounds (VOCs) are of great significance and challenge. Herein, we report a binuclear Cu(I) complex, [Cu2(POP)2(MeCN)2(4,4′-bipy)](PF6)2 (2), which is synthesized rationally through a substitution reaction between complex [Cu(POP)(4-PBO)(MeCN)](PF6) (1) and 4,4′-bipy lignand (POP is bis[2-(diphenylphosphino)phenyl]ether, 4,4′-bipy is bipyridine, 4-PBO is 2-(4′-pyridyl)-benzoxazole). The complex 2 was characterized by single-crystal X-ray diffraction as an isolated binuclear structure constructed by a bridged 4,4′-bipy ligand. Under UV irradiation, both the powder and doped thin film of 1 and 2 exhibit broadband photoluminescence emission. The TD-DFT calculation reveals the details of the electronic transitions corresponding to the light absorption and emission of the two complexes, and also illustrates that the improved luminescence of 2 compared to 1 is due to the T1 excited state transforms from a localized transition to a charge transfer transition similar to the S1 excited state and the extremely small energy gap ΔE(S1-T1) value of 0.09 eV (2). Based on these complexes, fluorescent test strips were developed. It is gratifying to find that the test strip based 2 shows a distinguishable fluorescence response behavior to a variety of VOCs. Especially, the test strip showed a rapid quenching fluorescence response to acetophenone, and a rapid quenching followed by rapid enhancement fluorescence response to acetonitrile. Therefore, it can be used for rapid and highly selective fluorescence sensing detection of acetonitrile and acetophenone.

Kinetic Study and Reaction Mechanism of the Gas-Phase Thermolysis Reaction of Methyl Derivatives of 1,2,4,5-Tetroxane.

Tetroxane derivatives are interesting drugs for antileishmaniasis and antimalaric treatments. The gas-phase thermal decomposition of 3,6,-dimethyl-1,2,4,5-tetroxane (DMT) and 3,3,6,6,-tetramethyl-1,2,4,5-tetroxane (acetone diperoxide (ACDP)) was studied at 493-543 K by direct gas chromatography by means of a flow reactor. The reaction is produced in the injector chamber at different temperatures. The resulting kinetics Arrhenius equations were calculated for both tetroxanes. Including the parent compound of the series 1,2,4,5-tetroxane (formaldehyde diperoxide (FDP)), the activation energy and frequency factors decrease linearly with the number of methyl groups. The reaction mechanisms of ACDP and 3,6,6-trimethyl-1,2,4,5-tetroxane (TMT) decomposition have been studied by means of the DFT method with the BHANDHLYP functional. Our calculations confirm that the concerted mechanism should be discarded and that only the stepwise mechanism occurs. The critical points of the singlet and triplet state potential energy surfaces (S- and T-PES) of the thermolysis reaction of both compounds have been determined. The calculated activation energies of the different steps vary linearly with the number of methyl groups of the methyl-tetroxanes series. The mechanism for the S-PES leads to a diradical O···O open structure, which leads to a C···O dissociation in the second step and the production of the first acetaldehyde/acetone molecule. This last one yields a second C···O dissociation, producing O2 and another acetone/acetaldehyde molecule. The O2 molecule is in the singlet state. A quasi-parallel mechanism for the T-PES from the open diradical to products is also found. Most of the critical points of both PES are linear with the number of methyl groups. Reaction in the triplet state is much more exothermic than the singlet state mechanism. Transitions from the singlet ground state, S0 and low-lying singlet states S1-3, to the low-lying triplet excited states, T1-4, (chemical excitation) in the family of methyl tetroxanes are also studied at the CASSCF/CASPT2 level. Two possible mechanisms are possible here: (i) from S0 to T3 by strong spin orbit coupling (SOC) and subsequent fast internal conversion to the excited T1 state and (ii) from S0 to S2 from internal conversion and subsequent S2 to T1 by SOC. From these experimental and theoretical results, the additivity effect of the methyl groups in the thermolysis reaction of the methyl tetroxane derivatives is clearly highlighted. This information will have a great impact for controlling these processes in the laboratory and chemical industries.

What is TADF (thermally activated delayed fluorescence) compared to the mechanisms of FL (fluorescence), PH (phosphorescence), and TTA (triplet–triplet annihilation) based on a novel naphthalimide sulfonylphenyl derivative as a host?

One of the biggest challenges in the field of optoelectronics is to easily identify the mechanisms involved in light-emitting materials. There are several different mechanisms of emission, and the best known are fluorescence (FL), phosphorescence (PH), thermally activated delayed fluorescence (TADF), and triplet–triplet annihilation (TTA). In this study, simple ways to diagnose these mechanisms are presented with an example. N,N-di(benzenesulfonylphenyl)-N-(N-2-aminomethylpyridine-1,8-naphthalimide)amine is synthesized as a host compound due to its high HOMO-LUMO and ionization potential (IP) values and three emitters consisting of 50 wt% Bis (1phenylisoquinoline))(acetylacetonate)iridium(III) (Ir(piq)2(acac)), 10-(4-(4,6-Diphenyl-1,3,5-triazin-2-yl)phenyl)-9,9-dimethyl-9,10-dihydroacridine (DMAC-TRZ) and 1,1′-(2,5-Dimethyl-1,4-phenylene)dipyrene (DMPPP) were considered. Overall, the host compound, doped Ir(piq)2(acac) and doped DMPPP follow the mechanisms of FL, the combination of FL and PH, and the combination of FL and TTA, as the photoluminescence (PL) decay life time and the PL intensity increased with decreasing temperature. However, DMAC-TRZ doped with the host compound showed the TADF mechanism, as the PL decay lifetime and the PL intensity increase simultaneously with increasing temperature. In addition, the difference between the singlet excited state (S1) and the triplet excited state T1 (ΔEST) from the PL and PH spectra at 77 K for the spin-coated DMAC-TRZ-doped host show the lowest value in between the samples at 0.02 eV, and the reverse intersystem crossing (rISC) effect is activated. Moreover, the linear slopes of the power dependence analysis of the DMPPP-doped host were calculated to be 2.04 and 0.91, indicating that there are two components (photons) for this sample which is strong evidence for the TTA mechanism. Taking into account the synthesized host compound, (Ir(piq)2(acac):host), (DMAC-TRZ:host) and (DMPPP:host) as emitter layers, the organic light emitting diodes (OLEDs) were fabricated via solution processing, and the TADF OLED exhibited the highest efficiency among the other OLEDs.

Tailoring Carbene-Metal-Amides for Thermally Activated Delayed Fluorescence: A Computationally Guided Study on the Effect of Cyclic (Alkyl)(amino)carbenes.

Gold-centered carbene-metal-amides (CMAs) containing cyclic (alkyl)(amino)carbenes (CAACs) are promising emitters for thermally activated delayed fluorescence (TADF). Aiming at the design and optimization of new TADF emitters, we report a density functional theory study of over 60 CMAs with various CAAC ligands, systematically evaluating computed parameters in relation to photoluminescence properties. The CMA structures were primarily selected based on experimental synthesis prospects. We demonstrate that TADF efficiency of the CMA materials originates from a compromise between oscillator strength coefficients and exchange energy (ΔEST). The latter is governed by the overlap of HOMO and LUMO orbitals, where HOMO is localized on the amide and LUMO over the Au-carbene bond. The S0 ground and excited T1 states of the CMAs adopt approximately coplanar geometry of carbene and amide ligands, but rotate perpendicular in the excited S1 states, resulting in degeneracy or near-degeneracy of S1 and T1, accompanied by a decrease in the S1-S0 oscillator strength from its maximum at coplanar geometries to near zero at rotated geometries. Based on the computations, promising new TADF emitters are proposed and synthesized. Bright CMA complex (Et2CAAC)Au(carbazolide) is obtained and fully characterized in order to demonstrate that excellent stability and high radiative rates up to 106 s-1 can be obtained for the gold-CMA complexes with small CAAC-carbene ligands.

Synthesis of Bis(amino acids) Containing the Styryl-cyclobutane Core by Photosensitized [2+2]-Cross-cycloaddition of Allylidene-5(4H)-oxazolones.

The irradiation of 2-aryl-4-(E-3'-aryl-allylidene)-5(4H)-oxazolones 1 with blue light (456 nm) in the presence of [Ru(bpy)3](BF4)2 (bpy = 2,2'-bipyridine, 5% mol) gives the unstable cyclobutane-bis(oxazolones) 2 by [2+2]-photocycloaddition of two oxazolones 1. Each oxazolone contributes to the formation of 2 with a different C=C bond, one of them reacting through the exocyclic C=C bond, while the other does so through the styryl group. Treatment of unstable cyclobutanes 2 with NaOMe/MeOH produces the oxazolone ring opening reaction, affording stable styryl-cyclobutane bis(amino acids) 3. The reaction starts with formation of the T1 excited state of the photosensitizer 3[Ru*(bpy)3]2+, which reacts with S0 of oxazolones 1 through energy transfer to give the oxazolone T1 state 3(oxa*)-1, which is the reactive species and was characterized by transient absorption spectroscopy. Measurement of the half-life of 3(oxa*)-1 for 1a, 1b and 1d shows large values for 1a and 1b (10-12 μs), while that of 1d is shorter (726 ns). Density functional theory (DFT) modeling displays strong structural differences in the T1 states of the three oxazolones. Moreover, study of the spin density of T1 state 3(oxa*)-1 provides clues to understanding the different reactivity of 4-allylidene-oxazolones described here with respect to the previously reported 4-arylidene-oxazolones.

Homoleptic Alkynylphosphonium Au(I) Complexes as Push-Pull Phosphorescent Emitters.

A series of compounds P1-P4 bearing terminal alkynyl sites connected with a phosphonium group via different π-conjugated linkers have been synthesized. The compounds themselves are efficient push-pull emitters and exhibit bright fluorescence in blue and near-UV regions. P1-P4 were used as alkynyl ligands to obtain a series of homoleptic bis-alkynyl Au(I) complexes 1-4. The complexes demonstrate bright phosphorescence and dual emission with dominating phosphorescence (2-4). Terphenyl derivative complex 3 exhibits warm white emission in DMSO solution and pure white emission in PMMA films. Time-dependent density functional theory calculations have shown that the T1 excited state has a hybrid MLCT/ILCT nature with a dominant contribution of charge transfer across a ligand-centered "D-π-A" system. The variation of linker allows tuning the effect of intermolecular charge transfer and thus changing the electronic and photophysical properties of the organogold "D-π-A" system. The results presented unambiguously display the advances of the conception of organometallic "D-π-A" construction.

Conjugation-Modulated Excitonic Coupling Brightens Multiple Triplet Excited States.

The design and regulation of multiple room-temperature phosphorescence (RTP) processes are formidably challenging due to the restrictions imposed by Kasha's rule. Here, we report a general design principle for materials that show multiple RTP processes, which is informed by our study of four compounds where there is modulation of the linker hybridization between donor (D) and acceptor (A) groups. Theoretical modeling and photophysical experiments demonstrate that multiple RTP processes can be achieved in sp3 C-linked D-A compounds due to the arrest of intramolecular electronic communication between two triplet states (T1H and T1L) localized on the donor and acceptor or between two triplet states, one localized on the donor and one delocalized across aggregated acceptors. However, for the sp2 C-linked D-A counterparts, RTP from one locally excited T1 state is observed because of enhanced excitonic coupling between the two triplet states of molecular subunits. Single-crystal and reduced density gradient analyses reveal the influence of molecular packing on the coincident phosphorescence processes and the origin of the observed aggregate phosphorescence. These findings provide insights into higher-lying triplet excited-state dynamics and into a fundamental design principle for designing compounds that show multiple RTP.

Tuning the Photophysical Properties of Homoleptic Tris-Cyclometalated Ir(III) Complexes by Facile Modification of the Imidazo-Phenanthridine and Their Application to Phosphorescent Organic Light-Emitting Diodes.

To explore the excited-state electronic structure of the blue-emitting Ir(dmp)3 dopant material (dmp = 3-(2,6-dimethylphenyl)-7-methylimidazo[1,2-f]phenanthridine), which is notable for durable blue phosphorescent organic light-emitting diode (PhOLED), a series of homoleptic dmp-based Ir(III) complexes (DMP–R, tris[3-(2,6-dimethylphenyl)-7-R-imidazo[1,2-f]phenanthridin-12-yl-κC12,κN1]iridium, R = H, CH3, F, and CF3) were prepared by introducing an electron-donating group (EDG; −CH3) or an electron-withdrawing group (EWG; −F and −CF3) at the 7-position of the imidazo-phenanthridine ligand. The photophysical analysis demonstrated that the alteration from EDG to EWGs led to redshifted structureless emission profiles, which were correlated with variations in the 3MLCT/3ILCT ratio in the T1 excited state. From electrochemical studies and density functional theory calculations, it turned out that the excited-state nature of the dmp-based Ir(III) complexes was significantly affected by the inductive effect of the 7-substituent of the cyclometalating dmp ligand. As a result of the lowest unoccupied molecular orbital energy stabilization by the EWGs that suppressed the non-radiative pathway from the emissive triplet excited state to the 3d–d state, the F- and CF3-modified Ir(dmp)3 complexes (DMP–F and DMP–CF3) showed quantum yields of 27–30% in the solution state, which were at least 4- or 5-fold higher than those shown by DMP–H and DMP–CH3. A PhOLED device based on DMP–CF3 [CIE chromaticity (0.17, 0.39)], which demonstrated a distinct 3MLCT characteristic, exhibited better electroluminescent efficiencies with an external quantum efficiency of 13.5% than that based on DMP–CH3.

Design, synthesis, and properties of polystyrene‐based through‐space charge transfer polymers: Effect of triplet energy level of electron donor moiety on delayed fluorescence and electroluminescence performance

AbstractTwo kinds of polystyrene‐based through‐space charge transfer (TSCT) polymers consisting of spatially‐separated acridan donor moieties bearing phenyl or naphthyl substituents and triazine acceptor moieties are designed and synthesized. It is found that TSCT polymers containing phenyl‐substituted acridan donors exhibit high‐lying singlet (S1) and triplet (T1) states with small singlet‐triplet energy splitting (∆EST) of 0.04–0.05 eV, resulting in thermally activated delayed fluorescence (TADF) with reverse intersystem crossing rate constants of 1.1–1.2 × 106 s−1. In contrast, polymers bearing naphthyl‐substituted acridan donors, although still having TSCT emission, exhibit no TADF effect because of the large ∆EST of 0.30–0.33 eV induced by low‐lying locally excited T1 state of naphthyl donor moiety. Solution‐processed organic light‐emitting diodes using TSCT polymers containing phenyl‐substituted acridan donors reveal sky‐blue emission at 483 nm together with maximum external quantum efficiency (EQE) of 11.3%, which is about 30 times that of naphthyl‐substituted counterpart with maximum EQE of 0.38%, shedding light on the importance of high triplet energy level of donor moiety on realizing TADF effect and high device efficiency for through‐space charge transfer polymer.

The structure and spectroscopic properties of the metallophilic Pt/Pd complexes based on pyridine/pyrazol ligands: A computational investigation

DFT/TD-DFT, NBO and AIM methods were applied to explore the synthetic homo-metallophilic Pt⋅⋅⋅Pt (d8⋅⋅⋅d8) complex (self-assembled by [Pt(C^N^Npyr)(CNPh)]+, HC^N^Npyr = 2-phenyl-6-(1H-pyrazol-3-yl)-pyridine)]) recently reported with good photophysical properties. The calculated geometries and the spectroscopies of the complexes are consistent with the experiment. The strong phosphorescence of experimental observation for Pt-dimer was evidenced by its bigger order of μS1/ΔES1-T1 values (μS1: S0 → S1 transition dipole, ΔES1-T1: splitting energy between the lowest-lying singlet S1 and the triplet excited state T1 states). The phosphorescence spectra are predicted to be blue-shifted under polar solvent water comparing those in gas. The calculated maximal absorption is predominantly characteristic with MLCT/ILCT/LLCT whereas the photophysical spectrum was found to be responsible for the 3MLCT characters in the Pt-dimer. In addition, on the basis of complex Pt-monomer and Pt-dimer, Pd-monomer, Pd-dimer and Pt,Pd-dimer were constructed to investigate the modulation of Pd-substitued on the structure and spectroscopic properties of monomer and dimer [M(C^N^Npyr)(CNPh)]+ complex.

An Europium (III) Luminophore with Pressure-Sensing Units: Effective Back Energy Transfer in Coordination Polymers with Hexadentate Porous Stable Networks.

A luminescent EuIII coordination polymer with O2 -sensing units under air, EuIII -hcpt (hcpt: 2,3,6,7,10,11-hexakis(4-carboxy-phenyl)triphenylene), is reported. The hexadentate carboxylic acids in hcpt ligands play an important role in the formation of tight-packed three-dimensional networks in EuIII -hcpt, giving hyper thermo-stable structures (decomposition temperature=420 °C). The three-dimensional porous network promotes bright luminescence (4f-4f emission quantum yield=70 %). The emission lifetime of EuIII -hcpt under vacuum (0.86 ms) was twice as large as that under O2 (0.48 ms, 1 atom:101.3 kPa). The Arrhenius analysis of the emission decay profile indicates that the back energy transfer (BEnT) from the emitting level of the europium(III) ion to the excited T1 state of the hcpt ligand should be activated at room temperature. The gradual decrease of emission lifetime is caused by the BEnT process in EuIII -hcpt. Finally, an advanced pressure-sensitive luminophore is demonstrated.

Intersystem crossing in tunneling regime: T1 → S0 relaxation in thiophosgene.

The T1 excited state relaxation in thiophosgene has attracted much attention as a relatively simple model for the intersystem crossing (ISC) transitions in polyatomic molecules. The very short (20-40 ps) T1 lifetime predicted in several theoretical studies strongly disagrees with the experimental values (∼20 ns) indicating that the kinetics of T1 → S0 ISC is not well understood. We use the nonadiabatic transition state theory (NA-TST) with the Zhu-Nakamura transition probability and the multireference perturbation theory (CASPT2) to show that the T1 → S0 ISC occurs in the quantum tunneling regime. We also introduce a new zero-point vibrational energy correction scheme that improves the accuracy of the predicted ISC rate constants at low internal energies. The predicted lifetimes of the T1 vibrational states are between one and two orders of magnitude larger than the experimental values. This overestimation is attributed to the multidimensional nature of quantum tunneling that facilitates ISC transitions along the non-minimum energy path and is not accounted for in the one-dimensional NA-TST.

Converging Fate of the Oxidation and Reduction of 8-Thioguanosine.

Thione-containing nucleobases have attracted the attention of the scientific community for their application in oncology, virology, and transplantology. The detailed understanding of the reactivity of the purine derivative 8-thioguanosine (8-TG) with reactive oxygen species (ROS) and free radicals is crucial for its biological relevance. An extensive investigation on the fate of 8-TG under both reductive and oxidative conditions is here reported, and it was tested by employing steady-state photooxidation, laser flash photolysis, as well as γ-radiolysis in aqueous solutions. The characterization of the 8-TG T1 excited state by laser flash photolysis and the photooxidation experiments confirmed that singlet oxygen is a crucial intermediate in the formation of the unexpected reduced product guanosine, without the formation of the usual oxygenated sulfinic or sulfonic acids. Furthermore, a thorough screening of different radiolytic conditions upon γ-radiation afforded the reduced product. These results were rationalized by performing control experiments in the predominant presence of each reactive species formed by radiolysis of water, and the mechanistic pathway scenario was postulated on these bases.

Realization of Highly Efficient Red Phosphorescence from Bis-Tridentate Iridium(III) Phosphors.

Bis-tridentate Ir(III) metal complexes bring forth interesting photophysical properties, among which the orthogonal arranged, planar tridentate chelates could increase the emission efficiency due to the greater rigidity and, in the meantime, allow strong interligand stacking that could deteriorate the emission efficiency. We bypassed this hurdle by design of five bis-tridentate Ir(III) complexes (1-5), to which both of their monoanionic ancillary and dianionic chromophoric chelate were functionalized derivative of 2-pyrazolyl-6-phenylpyridine, i.e. pzpyphH2 parent chelate. Hence, addition of phenyl substituent to the pyrazolyl fragment of pzpyphH2 gave rise to the precursors of monoanionic chelate (A1H-A3H), on which the additional tert-butyl and/or methoxy groups were introduced at the selected positions for tuning their steric and electronic properties, while precursors of dianionic chelates was judiciously prepared with an isoquniolinyl central unit on pziqphH2 in giving the red-shifted emission (cf. L1H2 and L2H2). Factors affected their photophysical properties were discussed by theoretical methods based on DFT and TD-DFT calculation, confirming that the T1 excited state of all investigated Ir(III) complexes shows a mixed metal-to-ligand charge transfer (MLCT), intraligand charge transfer (ILCT), ligand-to-ligand charge transfer (LLCT), and ligand-centered (LC) transition character. In contrast, the poor quantum yield of 3 is due to the facilitation of the nonradiative decay in comparison to the radiative process. As for potential OLED applications, Ir(III) complex 2 gives superior performance with max. efficiencies of 28.17%, 41.25 cd·A-1 and 37.03 lm·W-1, CIEx,y = 0.63, 0.37 at 50 mA cm-2, and small efficiency roll-off.

Theoretical study on photophysical properties of a series of functional pyrimidine-based organic light-emitting diodes emitters presenting thermally activated delayed fluorescence.

Issue concerning accurate prediction of the reverse intersystem crossing rate (kRISC ) is critical for developing novel efficient thermally activated delayed fluorescence (TADF) materials. In this contribution, the kRISC rates from the lowest excited triplet T1 state to the lowest excited singlet S1 state were evaluated for five donor-π-acceptor-type pyrimidine-based TADF emitters using the semiclassical Marcus theory. Both the singlet-triplet energy difference (ΔEST ) and spin-orbit coupling (V) between the S1 and T1 states were investigated by performing the density functional theory (DFT) and time-dependent density functional theory (TD-DFT) calculations. In addition, their fluorescence emission wavelengths (λem ) were also calculated at the TD-DFT level. The predicted kRISC and λem values are found to reproduce well the available experimental findings. The present results reveal that the kRISC rates of molecules possessing the unsymmetrical diphenyl pyrimidine acceptor core are calculated to be slightly larger than those of their analogues with the symmetrical diphenyl pyrimidine. In addition, introducing two tert-butyl groups into the 2,7-positions of the donor moiety of the latter is also an effective method for increasing kRISC when designing TADF emitters. Such a difference is related to the nature of the T1 excited state. A more remarkable charge-transfer (CT) contribution to the state can achieve a smaller ΔEST , leading to a more efficient RISC process, and consequently a shorter delayed fluorescence lifetime as observed experimentally. © 2019 Wiley Periodicals, Inc.

Phosphorescence enhancement by close metal-metal interaction in T1 excited state in a dinuclear copper(i) complex.

The dinuclear copper(i) complex [Cu2(μ-dppm)2(lact)(μ-lact)] (1) (dppm = bis(diphenylphosphino)methane; lact = l-(+)-lactate) was synthesized and fully characterized both in solution and solid state. Variable temperature NMR experiments (1H and 31P), conductivity measurements and infrared spectroscopy, suggest the occurrence of a fluxional behavior in solution involving the lactate anion. The crystal structure shows the presence of both monodentate and bridged lactate in the complex. In the solid state, 1 shows green phosphorescent emission characterized by a very large Stokes shift (161 nm, 1.09 eV) and a good absolute quantum yield (0.43). Calculations performed at the Density Functional Theory level demonstrate that the electronic transition responsible for the emission originates from a triplet excited state where the shortening of the CuCu distance plays a crucial role.

Synthesis and structural, redox and photophysical properties of tris-(2,5-di(2-pyridyl)pyrrolide) lanthanide complexes.

A first series of lanthanide complexes of tris(dipyridyl)pyrrolide ligands has been prepared. The [Ln(dppR1,R2)3] complexes (Ln = La(iii), Sm(iii), Eu(iii), Gd(iii) and Yb(iii); and dppR1,R2 = 2,5-di(2-pyridyl-3-(R1)-4-(R2)) pyrrolide) have been isolated and their structures and photophysical and redox properties characterised, both in the solid-state and in solution. In the complexes, the three dpp- ligands form a distorted tricapped trigonal prismatic coordination geometry about the lanthanide ions, with the antiparallel isomer observed in the solid state for non-symmetric (dppCO2Me,Me)-. However, 1H NMR spectroscopy of the diamagnetic and paramagnetic [Ln(dppR1,R2)3] complexes in d6-benzene solution reveal evidence for a statistical distribution of all possible isomers. Time-resolved luminescence studies suggest that the dpp- ligand (with triplet excited state T1 energy at 18 622 cm-1) sensitises red emission from [Eu(dppCO2Me,Me)3] and near-infrared emission from [Yb(dppCO2Me,Me)3] through the antenna effect. Cyclic voltammetry reveals three consecutive, reversible, one-electron oxidation processes for each [Ln(dppR1,R2)3] complex, corresponding to oxidations of each dpp- ligand between 0.3-0.8 V vs. E1/2 (Fc+/0), and for [Eu(dppCO2Me,Me)3] the EuIII/II couple was -2.099 V vs. E1/2 (Fc+/0).

Realization of Thermally Stimulated Delayed Phosphorescence in Arylgold(III) Complexes and Efficient Gold(III) Based Blue-Emitting Organic Light-Emitting Devices.

To address and overcome the difficulties associated with the increased reactivity and susceptibility of blue emitters to deactivation pathways arising from the high-lying triplet excited states, we have successfully demonstrated an innovative strategy of harvesting triplet emission via the "thermally stimulated delayed phosphorescence" mechanism, where thermal up-conversion of excitons from the lower-energy triplet excited states (T1) to higher-energy triplet excited states (T1') are observed to generate blue emission. The lower-lying T1 excited state could serve as a mediator to populate the emissive T1' state by up-conversion via reverse internal conversion, which could enhance the photoluminescence quantum yield by over 20-folds. Organic light-emitting devices with respectable external quantum efficiencies of up to 7.7% and sky-blue emission with CIE coordinates of (0.17, 0.37) have been realized. The operational stability for the device based on complex 1 has also been explored, and the device is found to show fairly respectable lifetime. This work opens up a new avenue to the design and synthesis of blue phosphorescent emitters.

Phosphorescent Modulation of Metallophilic Clusters and Recognition of Solvents through a Flexible Host-Guest Assembly: A Theoretical Investigation.

MP2 (Second order approximation of Møller–Plesset perturbation theory) and DFT/TD-DFT (Density functional theory/Time-dependent_density_functional_theory) investigations have been performed on metallophilic nanomaterials of host clusters [Au(NHC)2]+⋅⋅⋅[M(CN)2]−⋅⋅⋅[Au(NHC)2]+ (NHC = N-heterocyclic carbene, M = Au, Ag) with high phosphorescence. The phosphorescence quantum yield order of clusters in the experiments was evidenced by their order of μS1/ΔES1−T1 values (: S0 → S1 transition dipole, : splitting energy between the lowest-lying singlet S1 and the triplet excited state T1 states). The systematic variation of the guest solvents (S1: CH3OH, S2: CH3CH2OH, S3: H2O) are employed not only to illuminate their effect on the metallophilic interaction and phosphorescence but also as the probes to investigate the recognized capacity of the hosts. The simulations revealed that the metallophilic interactions are mainly electrostatic and the guests can subtly modulate the geometries, especially metallophilic Au⋅⋅⋅M distances of the hosts through mutual hydrogen bond interactions. The phosphorescence spectra of hosts are predicted to be blue-shifted under polar solvent and the excitation from HOMO (highest occupied molecular orbital) to LUMO (lowest unoccupied molecular orbital) was found to be responsible for the 3MLCT (triplet metal-to-ligand charge transfer) characters in the hosts and host-guest complexes. The results of investigation can be introduced as the clues for the design of promising blue-emitting phosphorescent and functional materials.