Synthesis and Investigation of Electro-Optical Properties of H-Shape Dibenzofulvene Derivatives.

We have synthetized two classes of dibenzofulvene-arylamino derivatives with an H-shape design, for a total of six different molecules. The molecular structures consist of two D-A-D units connected by a thiophene or bitiophene bridge, using diarylamino substituents as donor groups anchored to the 2,7- (Group A) and 3,6- (Group B) positions of the dibenzofulvene backbone. The donor units and the thiophene or bithiophene bridges were used as chemico-structural tools to modulate electro-optical and morphological-electrical properties. A combination of experiments, such as absorption measurements (UV-Vis spectroscopy), cyclic voltammetry, ellipsometry, Raman, atomic force microscopy, TD-DFT calculation and hole-mobility measurements, were carried out on the synthesized small organic molecules to investigate the differences between the two classes and therefore understand the relevance of the molecular design of the various properties. We found that the anchoring position on dibenzofulvene plays a crucial key for fine-tuning the optical, structural, and morphological properties of molecules. In particular, molecules with substituents in 2,7 positions (Group A) showed a lower structural disorder, a larger molecular planarity, and a lower roughness.

Control of Electron Transfer Processes in Multidimensional Arylamine-Based Mixed-Valence Compounds by Molecular Backbone Design.

Four trigonal topology compounds with three diarylamines redox centers and dibenzofulvene as core bridge have been synthesized. Their radical cations exhibit appealing intramolecular electron transfer pathways between three redox centers, depending on their position on the core bridge. By changing such positions (on either 2,7- or 3,6-), and the length of the bridge, the control of the intramolecular electron transfer pathways was achieved through the electron self-exchange route. These processes were investigated by absorption spectroscopy, electron paramagnetic resonance spectroscopy, and (time-dependent) density functional theory calculations. Hole mobility measurements were carried out as well, to correlate the intramolecular electron transfer with the hole-transporting ability for possible applications in optoelectronic devices.

Exploiting Photo- and Electroluminescence Properties of FIrpic Organic Crystals.

In this work, we investigate the optical and structural properties of the well-known triplet emitter bis(4',6'-difluorophenylpyridinato)-iridium(III) picolinate (FIrpic), showing that its ability to pack in two different ordered crystal structures promotes attractive photophysical properties that are useful for solid-state lighting applications. This approach allows the detrimental effects of the nonradiative pathways on the luminescence performance in highly concentrated organic active materials to be weakened. The remarkable electro-optical behavior of sky-blue phosphorescent organic light-emitting diodes incorporating crystal domains of FIrpic, dispersed into an appropriate matrix as an active layer, has also been reported as well as the X-ray diffraction, nuclear magnetic resonance, electro-ionization mass spectrometry, and scanning electron microscopy analyses of the crystalline samples. We consider this result as a crucial starting point for further research aimed at the use of a crystal triplet emitter in optoelectronic devices to overcome the long-standing issue of luminescence self-quenching.

Polymorphism, fluorescence, and optoelectronic properties of a borazine derivative.

We have prepared a new borazine derivative that bears mesityl substituents at the boron centers and displays exceptional chemical stability. Detailed crystallographic and solid-state fluorescence characterizations revealed the existence of several polymorphs, each of which showed different emission profiles. In particular, a bathochromic shift is observed when going from the lower- to the higher-density crystal. Computational investigations of the conformational dynamics of borazine 1 in both the gas phase and in the solid state using molecular dynamics (MD) simulations showed that the conformation of the peripheral aryl groups significantly varies when going from an isolated molecule (in which the rings are able to flip over the 90° barrier at RT) to the crystals (in which the rotation is locked by packing effects), thus generating specific nonsymmetric intermolecular interactions in the different polymorphs. To investigate the optoelectronic properties of these materials by fabrication and characterization of light-emitting diodes (LEDs) and light-emitting electrochemical cells (LECs), borazine 1 was incorporated as the active material in the emissive layer. The current and radiance versus voltage characteristics, as well as the electroluminescence spectra reported here for the first time are encouraging prospects for the engineering of future borazine-based devices.

Tuning photoinduced processes of covalently bound isoalloxazine and anthraquinone bichromophores.

We present here the synthesis of several new isoalloxazine cyclophanes containing electroactive anthraquinones linked by aliphatic chains of different lengths. Such structural changes provide different interchromophoric orientations leading to the tuning of the rate of the photoinduced electron transfer process from the anthraquinone unit towards the isoalloxazine singlet excited state. Molecular modelling studies were undertaken in order to determine the minimal energy of the proposed structures using Monte Carlo calculations (Amber, Macromodel v.8.1). The compounds have been fully characterised by NMR spectroscopy and the solid state structures of some of the macrocycles have been elucidated. The photophysical studies have been carried out in order to investigate the influence of π-π stacking on the optical properties of the macrocycles.

Charged bis-cyclometalated iridium(III) complexes with carbene-based ancillary ligands.

Charged cyclometalated (C(^)N) iridium(III) complexes with carbene-based ancillary ligands are a promising family of deep-blue phosphorescent compounds. Their emission properties are controlled primarily by the main C(^)N ligands, in contrast to the classical design of charged complexes where N(^)N ancillary ligands with low-energy π* orbitals, such as 2,2'-bipyridine, are generally used for this purpose. Herein we report two series of charged iridium complexes with various carbene-based ancillary ligands. In the first series the C(^)N ligand is 2-phenylpyridine, whereas in the second one it is 2-(2,4-difluorophenyl)-pyridine. One bis-carbene (:C(^)C:) and four different pyridine-carbene (N(^)C:) chelators are used as bidentate ancillary ligands in each series. Synthesis, X-ray crystal structures, and photophysical and electrochemical properties of the two series of complexes are described. At room temperature, the :C(^)C: complexes show much larger photoluminescence quantum yields (ΦPL) of ca. 30%, compared to the N(^)C: analogues (around 1%). On the contrary, all of the investigated complexes are bright emitters in the solid state both at room temperature (1% poly(methyl methacrylate) matrix, ΦPL 30-60%) and at 77 K. Density functional theory calculations are used to rationalize the differences in the photophysical behavior observed upon change of the ancillary ligands. The N(^)C:-type complexes possess a low-lying triplet metal-centered ((3)MC) state mainly deactivating the excited state through nonradiative processes; in contrast, no such state is present for the :C(^)C: analogues. This finding is supported by temperature-dependent excited-state lifetime measurements made on representative N(^)C: and :C(^)C: complexes.

Blue-emitting dinuclear N-heterocyclic dicarbene gold(I) complex featuring a nearly unit quantum yield.

Dinuclear N-heterocyclic dicarbene gold(I) complexes of general formula [Au(2)(RIm-Y-ImR)(2)](PF(6))(2) (R = Me, Cy; Y = (CH(2))(1-4), o-xylylene, m-xylylene) have been synthesized and screened for their luminescence properties. All the complexes are weakly emissive in solution whereas in the solid state some of them show significant luminescence intensities. In particular, crystals or powders of the complex with R = Me, Y = (CH(2))(3) exhibit an intense blue emission (λ(max) = 450 nm) with a high quantum yield (Φ(em) = 0.96). The X-ray crystal structure of this complex is characterized by a rather short intramolecular Au···Au distance (3.272 Ǻ). Time dependent density functional theory (TDDFT) calculations have been used to calculate the UV/vis properties of the ground state as well as of the first excited state of the complex, the latter featuring a significantly shorter Au···Au distance.

Influence of halogen atoms on a homologous series of bis-cyclometalated iridium(III) complexes.

A series of homologous bis-cyclometalated iridium(III) complexes Ir(2,4-di-X-phenyl-pyridine)(2)(picolinate) (X = H, F, Cl, Br) HIrPic, FIrPic, ClIrPic, and BrIrPic has been synthesized and characterized by NMR, X-ray crystallography, UV-vis absorption and emission spectroscopy, and electrochemical methods. The addition of halogen substituents results in the emission being localized on the main cyclometalated ligand. In addition, halogen substitution induces a blue shift of the emission maxima, especially in the case of the fluoro-based analogue but less pronounced for chlorine and bromine substituents. Supported by ground and excited state theoretical calculations, we rationalized this effect in a simple manner by taking into account the σp and σm Hammett constants on both the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy levels. Furthermore, in comparison with FIrPic and ClIrPic, the impact of the large bromine atom remarkably decreases the photoluminescence quantum yield of BrIrPic and switches the corresponding lifetime from mono to biexponential decay. We performed theoretical calculations based on linear-response time-dependent density functional theory (LR-TDDFT) including spin-orbit coupling (SOC), and unrestricted DFT (U-DFT) to obtain information about the absorption and emission processes and to gain insight into the reasons behind this remarkable change in photophysical properties along the homologous series of complexes. According to theoretical geometries for the lowest triplet state, the large halogen substituents contribute to sizable distortions of specific phenylpyridine ligands for ClIrPic and BrIrPic, which are likely to play a role in the emissive and nonradiative properties when coupled with the heavy-atom effect.

Bright blue phosphorescence from cationic bis-cyclometalated iridium(III) isocyanide complexes.

We report new bis-cyclometalated cationic iridium(III) complexes [(C(^)N)(2)Ir(CN-tert-Bu)(2)](CF(3)SO(3)) that have tert-butyl isocyanides as neutral auxiliary ligands and 2-phenylpyridine or 2-(4'-fluorophenyl)-R-pyridines (where R is 4-methoxy, 4-tert-butyl, or5-trifluoromethyl) as C(^)N ligands. The complexes are white or pale yellow solids that show irreversible reduction and oxidation processes and have a large electrochemical gap of 3.58-3.83 V. They emit blue or blue-green phosphorescence in liquid/solid solutions from a cyclometalating-ligand-centered excited state. Their emission spectra show vibronic structure with the highest-energy luminescence peak at 440-459 nm. The corresponding quantum yields and observed excited-state lifetimes are up to 76% and 46 µs, respectively, and the calculated radiative lifetimes are in the range of 46-82 µs. In solution, the photophysical properties of the complexes are solvent-independent, and their emission color is tuned by variation of the substituents in the cyclometalating ligand. For most of the complexes, an emission color red shift occurs in going from solution to neat solids. However, the shift is minimal for the complexes with bulky tert-butyl or trifluoromethyl groups on the cyclometalating ligands that prevent aggregation. We report the first example of an iridium(III) isocyanide complex that emits blue phosphorescence not only in solution but also as a neat solid.

Luminescent ionic transition-metal complexes for light-emitting electrochemical cells.

Higher efficiency in the end-use of energy requires substantial progress in lighting concepts. All the technologies under development are based on solid-state electroluminescent materials and belong to the general area of solid-state lighting (SSL). The two main technologies being developed in SSL are light-emitting diodes (LEDs) and organic light-emitting diodes (OLEDs), but in recent years, light-emitting electrochemical cells (LECs) have emerged as an alternative option. The luminescent materials in LECs are either luminescent polymers together with ionic salts or ionic species, such as ionic transition-metal complexes (iTMCs). Cyclometalated complexes of Ir(III) are by far the most utilized class of iTMCs in LECs. Herein, we show how these complexes can be prepared and discuss their unique electronic, photophysical, and photochemical properties. Finally, the progress in the performance of iTMCs based LECs, in terms of turn-on time, stability, efficiency, and color is presented.

Non-Linear Optical Properties of Biexciton in Ellipsoidal Quantum Dot.

We have presented a theoretical investigation of exciton and biexciton states for the ground and excited levels in a strongly oblate ellipsoidal quantum dot made from GaAs. The variational trial wave functions for the ground and excited states of the exciton and biexciton are constructed on the base of one-particle wave functions. The energies for the ground and excited levels, depending on the ellipsoidal quantum dot's geometrical parameters, are depicted in the framework of the variational method. The oscillator strength of the transition from exciton to biexciton states for ground and excited levels is investigated as a function of the ellipsoidal quantum dot's small and large semiaxes. The third-order optical susceptibilities of ground and excited biexcitons around one-photon and two-photon resonances are calculated as a function of the photon energy. The dependences of third-order optical susceptibilities for the ground and excited levels on the photon energy for different values of the ellipsoidal quantum dot's semiaxis are revealed. The absorption coefficients in the ellipsoidal quantum dot, both for ground and excited states of exciton and biexciton, are calculated. The absorption coefficients for the ground level of exciton and biexciton for the fixed value of the large semiaxis and for the different values of the small semiaxis are determined. Finally, the two-photon absorption coefficient of the biexciton in the GaAs ellipsoidal quantum dot is computed.

Pressure response of decylammonium-containing 2D iodide perovskites.

Manipulation by external pressure of the optical response of 2D Metal Halide Perovskites (MHPs) is a fascinating route to tune their properties and promote the emergence of novel features. We investigate here DA2PbI4 and DA2GeI4 (DA = decylammonium) perovskites in the pressure range up to ∼12 GPa by X-ray powder diffraction, absorption, and photoluminescence spectroscopy. Although the two systems share a similar structural evolution with pressure, the optical properties are rather different and influenced by Pb or Ge. DA2PbI4 shows a progressive red shift from 2.28 eV (P = 0 GPa) to 1.64 eV at 11.5 GPa, with a narrow PL emission, whereas DA2GeI4, changes from a non-PL system at ambient pressure to a clear broadband emitter centered around 730 nm with an intensity maximum at about 3.7 GPa. These results unveil the role of the central atom on the nature of emission under pressure in 2D MHPs containing a long alkyl chain.

Photophysical properties of charged cyclometalated Ir(III) complexes: a joint theoretical and experimental study.

The photophysical properties of a series of charged biscyclometalated [Ir(ppy)(2)(N^N)](1+) complexes, where ppyH is 2-phenylpyridine and N^N is 2,2'-bipyridine (bpy), 6-phenyl-2,2'-bipyridine (pbpy), and 6,6'-diphenyl-2,2'-bipyridine (dpbpy) for complexes 1, 2, and 3, respectively, have been investigated in detail. The photoluminescence performance in solution decreases from 1 to 3 upon attachment of phenyl groups to the ancillary ligand. The absorption spectra recorded over time suggest that complex 3 is less stable compared to complexes 1 and 2 likely due to a nucleophilic-assisted ancillary ligand-exchange reaction. To clarify this behavior, the temperature dependence of the experimental intrinsic deactivation rate constant, k(in) = 1/τ, has been investigated from 77 K to room temperature. Temperature-dependent studies show that nonemitting metal-centered (MC) states are accessible at room temperature for complex 3. The experimental results are interpreted with the help of theoretical calculations performed within the density functional theory (DFT) approach. Calculations suggest that attachment of a phenyl group to the ancillary ligand (2) promotes the temperature-independent deactivation pathways, whereas attachment of a second phenyl group (3) also makes the temperature-dependent ones accessible through population of nonradiative (3)MC excited states.

Dinuclear gold(I) complexes with N-phosphanyl, N-heterocyclic carbene ligands: synthetic strategies, luminescence properties and anticancer activity.

A small library of dinuclear gold(I) complexes with the title ligands has been prepared, encompassing neutral, mono- and dicationic complexes. The luminescence properties of the complexes in the solid state have been evaluated, and it turns out that neutral and monocationic complexes not presenting a rigid metallamacrocyclic structure can exhibit rather strong emissions that extend towards the red region of the visible spectrum. The in vitro anticancer activity of the complexes has been also preliminarly evaluated; cytotoxicity seems to correlate with complex lipophilicity, whereas selectivity towards cancer cells can be apparently enhanced upon a judicious choice of the ligands.

The exceptional near-infrared luminescence properties of cuprorivaite (Egyptian blue).

Cuprorivaite (CaCuSi(4)O(10), also known as Egyptian blue) exhibits an exceptionally high emission quantum efficiency in the near-infrared region (lambda(max) = 910 nm, Phi(EM) = 10.5%) and a long excited state lifetime (107 mus); these properties make it appealing for several applications in the fields of biomedical analysis, telecommunications and lasers.

Tunable photophysical properties of phenyleneethynylene based bipyridine ligands.

A bipyridine-based system with phenyleneethynylene at the 4,4' positions (1) and its p-methyl (2) and p-methoxy (3) substituted derivatives were synthesized via Sonogashira coupling reactions. The photophysical properties of 1-3 and their related H+ and Zn2+ adducts (1:H+-3:H+ and 1:Zn2+-3:Zn2+) were investigated, as a function of solvent polarity, by using steady-state and time-resolved spectroscopic techniques. Molecular systems 1-3 exhibit trans conformation, whereas adducts with H+ and Zn2+ are conformationally locked cis species. The unsubstituted compound 1 emits at 360 nm with low fluorescence quantum yield (phi(fl) = 0.2%) regardless of the solvent polarity. Fluorescence spectra of 2 and 3 are bathochromically shifted in polar solvents, and the p-methoxy (3) derivative possesses phi(fl) as high as 12%. Complexation of 1-3 with H+ or Zn2+ in acetonitrile causes red-shift of the lowest energy absorption bands, whereas dramatic changes of the emission properties are found as a function of the electron donating ability of the substituents on the phenyleneethynylene moiety (-CH3 or -OCH3), suggesting a charge-transfer character of the lowest electronic transition of 1-3. 1:H+, 1:Zn2+, 2:H+ and 2:Zn2+ exhibit intense fluorescence with phi(fl) up to 33% (1:Zn2+) whilst 3:H+ and 3:Zn2+ are found to be weakly emissive. The singlet radiative and non-radiative rate constants of compounds and complexes were determined, along with triplet parameters, via phosphorescence and transient absorption spectroscopy. More conclusive evidence regarding the protonation of bipyridine nitrogen atoms of compounds 1-3 were obtained through 1H NMR titration studies. These studies indicate that the conjugate molecular systems based on 2,2'-bipyridine and phenyleneethenylenes possess tunable optical properties which can be further utilized for preparing organic and inorganic luminophores with potential application in optoelectronic systems.

Luminescent complexes beyond the platinum group: the d10 avenue.

Luminescent metal complexes are key materials for several applications such as lighting, analytical probes, and lasers. In many cases compounds based on precious (i.e. platinum group) and rare earth metals are utilized, which are often rather expensive and environmentally problematic. In recent years, interest is growing in luminescent complexes based on less traditional but more abundant and cheaper metal elements. In this scenario compounds of metals with a d10 electronic configuration are playing a prominent role, also thanks to the versatility of their luminescent levels which can be of ligand centred, charge transfer or, in the case of polynuclear compounds, even metal-centred nature. Here we focus on some selected examples of Cu(I), Ag(I), Au(I), Zn(II) and Cd(II) luminescent complexes to suggest some possible routes towards promising and unprecedented emitting materials.

Heteroleptic copper(I) complexes coupled with methano[60]fullerene: synthesis, electrochemistry, and photophysics.

Heteroleptic copper(I) complexes CuPOP-F and CuFc-F have been prepared from a fullerene-substituted phenanthroline ligand and bis[2-(diphenylphosphino)phenyl] ether (POP) and 1,1'-bis(diphenylphosphino)ferrocene (dppFc), respectively. Electrochemical studies indicate that some ground-state electronic interaction between the fullerene subunit and the metal-complexed moiety are present in both CuPOP-F and CuFc-F. Their photophysical properties have been investigated by steady state and time-resolved UV-vis-NIR luminescence spectroscopy and nanosecond laser flash photolysis in a CH2Cl2 solution and compared to those of the corresponding model copper(I) complexes CuPOP and CuFc and of the fullerene model compound F. Selective excitation of the methanofullerene moiety in CuPOP-F results in regular deactivation of the lowest singlet and triplet states, indicating no intercomponent interactions. Conversely, excitation of the copper(I)-complexed unit (405 nm, 40% selectivity) shows that the strongly luminescent triplet metal-to-ligand charge-transfer ((3)MLCT) excited state located at 2.40 eV is quenched by the carbon sphere with a rate constant of 1.6 x 10(8) s(-1). Details on the mechanism of photodynamic processes in CuPOP-F via transient absorption are hampered by the rather unfavorable partition of light excitation between the two chromophores. By determination of the yield of formation of the lowest fullerene triplet level through sensitized singlet oxygen luminescence in the NIR region, it is shown that the final sink of photoinduced processes is always the fullerene triplet. This can be populated via a two-step charge-separation charge-recombination process and a less favored (3)MLCT --> (3)C60 triplet-triplet energy-transfer pathway. In CuFc-F, both of the photoexcited copper(I)-complexed and fullerene moieties are quenched by the presence of the ferrocene unit, most likely via ultrafast energy transfer.

Photophysical properties and tunable colour changes of silica single layers doped with lanthanide(III) complexes.

Highly stable Eu(III) and Tb(III) complexes, emitting in the red and green visible regions, respectively, have been anchored onto a single SiO(2) transparent layer, yielding ca. 40 nm thick films; this allows high loading of tailored proportions of the red and green emitters within the films and results in highly uniform and easily colour-tunable luminescent layers.

Electrophosphorescent homo- and heteroleptic copper(I) complexes prepared from various bis-phosphine ligands.

Homo- and heteroleptic copper(I) complexes obtained from various chelating bis-phosphine ligands and Cu(CH3CN)4BF4 have been used for the preparation of light emitting devices.