Fine-Tuning of the Optical and Electrochemical Properties of Ruthenium(II) Complexes with 2-Arylbenzimidazoles and 4,4'-Dimethoxycarbonyl-2,2'-bipyridine.

A series of cyclometalated complexes of ruthenium (II) with four different substituents in the aryl fragment of benzimidazole was synthesized in order to study the effect of substituent donation on the electronic structure of the substances. The resulting complexes were studied using X-ray diffraction, NMR spectroscopy, MALDI mass spectrometry, electron absorption spectroscopy, luminescence spectroscopy, and cyclic voltammetry as well as DFT/TDDFT was also used to interpret the results. All the complexes have intense absorption in the range of up to 700 nm, the triplet nature of the excited state was confirmed by measurement of luminescence decay. With an increase in substituent donation, a red shift of the absorption and emission bands occurs, and the lifetime of the excited state and the redox potential of the complex decrease. The combination of these properties shows that the complexes are excellent dyes and can be used as photosensitizers.

Influence of Ligand Environment Stoichiometry on NIR-Luminescence Efficiency of Sm3+, Pr3+ and Nd3+ Ions Coordination Compounds.

Six new complexes of the ligand HQcy (-4-(cyclohexanecarbonyl)-5-methyl-2-phenyl-2,4-dihydro-3H-pyrazol-3-one) and Ln3+ ions with emission in the near-infrared (Nd3+) or visible and near-infrared (Sm3+, Pr3+) spectral regions were synthesized and characterized using various methods, including single crystal X-ray diffraction. The study demonstrated that both tris complexes [LnQcy3(H2O)(EtOH)] and tetrakis-acids [H3O][LnQcy4] can be synthesized by varying the synthetic conditions. The photochemical properties of the complexes were investigated experimentally and theoretically using various molecular spectroscopy techniques and Judd-Ofelt theory. The objective was to quantitatively and qualitatively disclose the influence of complex stoichiometry on its luminescence properties. The study showed that the addition of an extra ligand molecule (in the tetrakis species) increased molar extinction by up to 2 times, affected the shape of photoluminescence spectra, especially of the Pr3+ complex, and increased the quantum yield of the Sm3+ complex by up to 2 times. The results obtained from this study provide insights into the luminescent properties of lanthanide coordination compounds, which are crucial for the design and development of novel photonic materials with tailored photophysical properties.

Dynamics of the Ligand Excited States Relaxation in Novel ß-Diketonates of Non-Luminescent Trivalent Metal Ions.

Complexes emitting in the blue spectral region are attractive materials for developing white-colored light sources. Here, we report the luminescence properties of novel coordination compounds based on the trivalent group 3, 13 metals, and the 1-phenyl-3-methyl-4-cyclohexylcarbonyl-pyrazol-5-onate (QCH) ligand. [M(QCH)3] (M = Al, Ga, and In), [M(QCH)3(H2O)] (M = Sc, Gd, and Lu), [Lu(QCH)3(DMSO)], and [La(QCH)3(H2O)(EtOH)] complexes were synthesized and structurally characterized by a single-crystal X-ray diffraction study. It has been found that the luminescence quantum yields of the ligand increase by one order of magnitude upon metal coordination. A significant correspondence between the energies of the ligand's excited states and the luminescence quantum yields to the metal ion's atomic numbers was found using molecular spectroscopy techniques. The replacement of the central ion with the heavier one leads to a monotonic increase in singlet state energy, while the energy of the triplet state is similar for all the complexes. Time-resolved measurements allowed us to estimate the intersystem crossing (ISC) rate constants. It was shown that replacing the Al3+ ion with the heavier diamagnetic Ga3+ and In3+ ions decreased the ISC rate, while the replacement with the paramagnetic Gd3+ ion increased the ISC rate, which resulted in a remarkably bright and room-temperature phosphorescence of [Gd(QCH)3(H2O)].

OLED Structure Optimization for Pure and Efficient NIR Electroluminescence of Nd3+ Complexes Bearing Fluorinated 1,3-Diketones.

NIR emitting OLEDs (organic light-emitting diodes) with high photoluminescence quantum yields were developed on the basis of fluorinated 1,3-diketonate coordination compounds of the Nd3+ ion. Both thermal evaporation and spin-coating techniques were successfully employed for active layer deposition resulting in electroluminescence quantum yields up to 1.38·10-2%. Blueish-green emission from exciplex and electroplax formations was almost suppressed with the topology optimization of the cell.

Yellow to blue switching of fluorescence by the tuning of the pentaphenylphosphole structure: phosphorus electronic state vs. ring conjugation.

The interaction between diphenylacetylene and dichlorophenylphosphine under various conditions is a simple method for the preparation of pentaphenylphosphole derivatives exhibiting fluorescence properties. Depending on the electronic state of the various centers of the phospholic structure, it was possible to obtain molecules with fluorescence, as in the blue area for 1,2,3,4,5-pentaphenyl-2,5-dihydro-phosphole-1-oxide (H2PPPO), in the yellow area for 1,2,3,4,5-pentaphenylphosphole-1-oxide (PPPO) and in the cyan area for 1,2,3,4,5-pentaphenylphosphole (PPP). The effect of the structure and π-conjugation on the optical properties of these compounds was studied using PPP derivatives as examples. Unusual changes in the optical properties of PPP derivatives in solution and in the crystalline state are explained. In the case of agglomeration of PPPO and PPP molecules, the effect of aggregation-induced emission (AIE) was observed to have weak fluorescence in solution and strong fluorescence in the aggregated state. However, for H2PPPO, the AIE effect remains mild. With the help of experimental studies, supported by theoretical calculations, the main mechanism of the optical properties of pentaphenylphosphole derivatives has been revealed. It was observed that the intramolecular motions of PPPO and PPP are more limited in the solid state than the motions of H2PPPO, which is associated with less conjugation of the phenyl rotors of H2PPPO. The analysis of the structure and distribution of electron density showed why hydrogenation of the phosphole ring leads to a sharp change in the optical properties of pentaphenylphosphole derivatives, while the oxidation of phosphorus does not lead to the disappearance of the AIE effect and to a lesser extent affects the change in the fluorescence wavelength. Thus, it was shown how the regulation of various structural features of the phospholic ring helps to control the optical properties of such compounds.

Tuning the luminescence efficiency by perfluorination of side chains in Eu3+ complexes with ß-diketones of the thiophene series.

A series of europium and gadolinium complexes comprising a ß-diketone moiety modified with a fluorinated side-group and thiophene ring have been designed and synthesized and a comparative study of their luminescence properties has been carried out. In this study, when the methyl side group was modified by sequential addition of fluorine substituents and then the perfluorinated carbon chain was extended up to n-C8F17 by adding CF2 fragments, it transpired that the non-radiative energy processes are significantly suppressed in structurally more rigid polyfluorinated ß-diketonate compounds of the series as C-H oscillators are replaced with low-energy C-F oscillators. The impact of other electron-withdrawing and electron-donating substituents on the spectroscopic and photophysical properties of the complexes in the present study has also been observed. Despite the presence of low-lying ligand-to-metal charge transfer states, the fluorinated Eu3+ complexes proved to be bright luminophores capable of delivering ca. 50% quantum yield under UV radiation. The role of fluorination and carbon chain length was examined by using experimental spectroscopic methods and the results obtained were largely in good agreement with theoretical calculations (Judd-Ofelt theory analysis, and semiempirical quantum chemistry calculations) supporting our key experimental findings.

Mono- and Mixed Metal Complexes of Eu3+, Gd3+, and Tb3+ with a Diketone, Bearing Pyrazole Moiety and CHF2-Group: Structure, Color Tuning, and Kinetics of Energy Transfer between Lanthanide Ions.

Three novel lanthanide complexes with the ligand 4,4-difluoro-1-(1,5-dimethyl-1H-pyrazol-4-yl)butane-1,3-dione (HL), namely [LnL3(H2O)2], Ln = Eu, Gd and Tb, were synthesized, and, according to single-crystal X-ray diffraction, are isostructural. The photoluminescent properties of these compounds, as well as of three series of mixed metal complexes [EuxTb1-xL3(H2O)2] (EuxTb1-xL3), [EuxGd1-xL3(H2O)2] (EuxGd1-xL3), and [GdxTb1-xL3(H2O)2] (GdxTb1-xL3), were studied. The EuxTb1-xL3 complexes exhibit the simultaneous emission of both Eu3+ and Tb3+ ions, and the luminescence color rapidly changes from green to red upon introducing even a small fraction of Eu3+. A detailed analysis of the luminescence decay made it possible to determine the observed radiative lifetimes of Tb3+ and Eu3+ and estimate the rate of excitation energy transfer between these ions. For this task, a simple approximation function was proposed. The values of the energy transfer rates determined independently from the luminescence decays of terbium(III) and europium(III) ions show a good correlation.

Novel D-A-D Fluorescent Dyes Based on 9-(p-Tolyl)-2,3,4,4a,9,9a-hexahydro-1H-carbazole as a Donor Unit for Solution-Processed Organic Light-Emitting-Diodes.

New fluorescent D-A-D dyes containing 9-(p-tolyl)-2,3,4,4a,9,9a-hexahydro-1H-carbazole as a donor unit and 2,1,3-benzochalcogenadiazoles as an electron-withdrawing group were synthesized. The photoluminescent and electroluminescent properties of novel dyes for fluorescent OLED application were investigated. It was demonstrated that the replacement of lightweight heteroatoms by heavier ones enables the fine tuning of the maximum emission without significantly reducing the luminescence quantum yield. The maximum quantum yield value of 62.6% for derivatives based on 2,1,3-benzoxadiazole (1a) in cyclohexane was achieved. Two devices with the architecture of glass/ITO/PEDOT-PSS/poly-TPD/EML/TPBi/LiF/Al (EML = emitting layer) were fabricated to check the suitability of the synthesized compounds as a single active emission layer in OLED. These OLEDs exhibited clear red electroluminescence of the dyes with the maximum current efficiency of 0.85 Cd/A.

Sterically hindered phenanthroimidazole ligands drive the structural flexibility and facile ligand exchange in cyclometalated iridium(III) complexes.

A series of bis-cyclometalated iridium(iii) complexes with 2-arylphenanthroimidazole "antenna" ligands containing electron-donor or withdrawing substituents and a more flexible ancillary aromatic ß-diketone bearing the "anchoring" carboxymethyl function has been prepared. Thorough X-ray study of the complexes revealed significant structural strains caused by bulky cyclometalated 2-arylphenanthroimidazoles resulting in dramatic distortions of the iridium octahedron and even in twist of the phenanthrene fragment. The crystal data were corroborated by gas-phase DFT calculations whereby the geometry of the complexes was distorted in the same way. While redox potentials, absorption and emission maxima of the complexes displayed expected change upon the variation of the electron-donating ability of the cyclometalated ligands, the complexes readily exchanged the bidentate ancillary ligand in the presence of a negligible amount of protons that was inspected in solution by UV-Vis spectroscopy. Moreover, after hydrolysis of the carboxymethyl group the resulting complexes readily react with the surface of titanium dioxide giving unique binuclear structures in which the deprotonated carboxy group of the coordinated ß-diketonate binds the second bis-cyclometalated unit by forming a four-membered metallacycle. Though the enhanced reactivity of the complexes is contrary to the common idea of the high inertness of iridium(iii) compounds it can be seen as a consequence of the interplay between the steric hindrance induced by the ligands and the strong preference of the iridium(iii) ion for octahedral geometry. This study demonstrates that the use of bulky ligands provides access to light-harvesting iridium(iii) complexes with required extent of lability which may be promising as photocatalysts and biologically active molecules.

Molecular and Polymer Ln2M2 (Ln = Eu, Gd, Tb, Dy; M = Zn, Cd) Complexes with Pentafluorobenzoate Anions: The Role of Temperature and Stacking Effects in the Structure; Magnetic and Luminescent Properties.

Varying the temperature of the reaction of [{Cd(pfb)(H2O)4}+n·n(pfb)-], [Ln2(pfb)6(H2O)8]·H2O (Hpfb = pentafluorobenzoic acid), and 1,10-phenanthroline (phen) in MeCN followed by crystallization resulted in the isolation of two type of products: 1D-polymers [LnCd(pfb)5(phen)]n·1.5nMeCN (Ln = Eu (I), Gd (II), Tb (III), Dy (IV)) which were isolated at 25 °C, and molecular compounds [Tb2Cd2(pfb)10(phen)2] (V) formed at 75 °C. The transition from a molecular to a polymer structure becomes possible because of intra- and intermolecular interactions between the aromatic cycles of phen and pfb from neighboring tetranuclear Ln2Cd2 fragments. Replacement of cadmium with zinc in the reaction resulted in molecular compounds Ln2Zn2 [Ln2Zn2(pfb)10(phen)2]·4MeCN (Ln = Eu (VI), Tb (VIII), Dy (IX)) and [Gd2Zn2(pfb)10(H2O)2(phen)2]·4MeCN (VII). A new molecular EuCd complex [Eu2Cd2(pfb)10(phen)4]·4MeCN (X)] was isolated from a mixture of cadmium, zinc, and europium pentafluorobenzoates (Cd:Zn:Ln = 1:1:2). Complexes II-IV, VII and IX exhibit magnetic relaxation at liquid helium temperatures in nonzero magnetic fields. Luminescent studies revealed a bright luminescence of complexes with europium(III) and terbium(III) ions.

Near-infrared single-photon emitters based on colloidal CdSe/CdS/ZnS nanocrystals and Nd(III) 1,3-diketonate.

A novel, to the best of our knowledge, approach to the design of the single-photon sources emitting in the spectral regions of 1060 and 1337 nm was developed. A unique hybrid structure based on colloidal CdSe/CdS/ZnS nanocrystals and neodymium(III) 1,3-diketonate was created. Direct energy transfer from the CdSe/CdS/ZnS single nanocrystal to the near-infrared (NIR) luminescent Nd(III) complex was observed and investigated by spectroscopic methods. The single-photon emission mode was confirmed for the novel hybrid source by Hanbury Brown and Twiss experiments.

A novel candle light-style OLED with a record low colour temperature.

The possibility of using a single light-emitting layer consisting of newly synthesized fluorescent small organic molecules of D-A-π-A type incorporated into a conductive matrix together with an electron conductive Alq3 layer in order to form radiation in candle light-style OLEDs was shown for the first time. A record low color temperature of 1722 K OLED radiation was achieved, which is by 80 K lower than that of the best devices reported previously.