Synthesis and Electron Accepting Properties of Two Di(benz[f]indenone)-Fused Tetraazaanthracene Isomers.

We designed and synthesized a novel di(benz[f]indenone)-fused tetraazaanthracene derivative and isolated its two isomers, 1a and 1s, having anti and syn configurations, respectively. Their structure and that of the condensation reaction intermediates, anti-2a and syn-2s, were fully characterized using one- and two-dimensional nuclear magnetic resonance spectroscopy and single-crystal X-ray diffraction. The optical and electronic properties of 1a and 1s were investigated using ultraviolet-visible absorption and fluorescence spectroscopies, cyclic voltammetry, and time-dependent density functional theory calculations. The presence of the carbonyl and ethynyltris(isopropyl)silane groups endows the di(benzoindenone)-fused azaacene derivatives with a strong electron accepting character. With an electron affinity of approximately -3.7 eV, the two isomers represent attractive electron-deficient molecular systems for the generation of n-channel semiconducting materials. Organic field effect transistors of 1a and 1s showed electron transport, and organic solar cells gave a proof of concept of the potential of the two compounds as electron acceptor materials when they are paired with an electron donor polymer.

Exciton Interactions, Excimer Formation, and [2π+2π] Photodimerization in Nonconjugated Curcuminoid-BF2 Dimers.

We describe the synthesis of a series of covalently linked dimers of quadrupolar curcuminoid-BF2 dyes and the detailed investigation of their solvent-dependent spectroscopic and photophysical properties. In solvents of low polarity, intramolecular folding induces the formation of aggregated chromophores, the UV/Vis absorption spectra of which display the optical signature characteristic of weakly-coupled H-aggregates. The extent of folding and, in turn, of ground-state aggregation is strongly dependent on the nature of the flexible linker. Steady-state and time-resolved fluorescence emission spectroscopies show that the Frenkel exciton relaxes into a fluorescent symmetrical excimer state with a long lifetime. Furthermore, our in-depth studies show that a weakly emitting excimer lies on the pathway toward a photocyclomer. Two-dimensional 1 H NMR spectroscopy and density functional theory (DFT) allowed the structure of the photoproduct to be established. To our knowledge, this represents the first example of a [2π+2π] photodimerization of the curcuminoid chromophore.

Blue-Shifting Intramolecular Charge Transfer Emission by Nonlocal Effect of Hyperbolic Metamaterials.

Metallic nanostructures permit controlling various photophysical processes by coupling photons with plasmonic oscillation of electrons confined in the tailored nanostructures. One example is hyperbolic metamaterial (HMM) leading to an enhanced spontaneous emission rate of emitters located nearby. Noting that emission in organic molecules is from either π-π* or intramolecular charge-transfer (ICT) states, we address here how HMM modifies ICT emission spectral features by comparing them with a spectral shift dependent on the local polarity of the medium. The 7.0 nm blue shift is observed in ICT emission from 4-dicyanomethylene-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran dispersed into a polymer matrix prepared on HMM multilayered structure, while no spectral shift is observed in π-π* emission from perylene diimide. In the frame of the Lippert-Mataga formalism, the blue shift is explained by the HMM nonlocal effects resulting from 8% decrease in refractive index and 18% reduction in dielectric permittivity. This phenomenon was also shown in a hemicurcuminoid borondifluoride dye yielding 15.0 nm blue shift. Such a capability of spectral shift control in films by HMM structure opens new prospects for engineering organic light-emitting devices.

Optical properties of quadrupolar and bi-quadrupolar dyes: intra and inter chromophoric interactions.

In this work we present the synthesis, characterization and theoretical investigation of three boron-difluoride-curcuminoid derivatives and their covalent homodimers chemically linked through a polymethylenic chain. Low-lying electronic excited states and photophysical properties of the monomeric species have been described as the convolution of different donor-acceptor intramolecular excitations. Covalent dimers in solution can present open or folded structural conformations. Analysis of absorption profiles and computational results allow to identify the factors that control the relative stability of the two forms and rationalize its dependence with the solvent polarity. Interestingly, the strong electronic coupling in the folded forms results in low-lying excitations with sizable mixings of intra- and inter-chromophoric contributions, which cannot be described by means of the Kasha model of interacting chromophores. Our study demonstrates how decomposition of the computed excitations in terms of diabatic states can be extremely valuable in order to identify and quantify the nature of electronic transitions in the presence of several electron donor and acceptor fragments.

Boron Difluoride Curcuminoid Fluorophores with Enhanced Two-Photon Excited Fluorescence Emission and Versatile Living-Cell Imaging Properties.

The synthesis of boron difluoride complexes of a series of curcuminoid derivatives containing various donor end groups is described. Time-dependent (TD)-DFT calculations confirm the charge-transfer character of the second lowest-energy transition band and ascribe the lowest energy band to a "cyanine-like" transition. Photophysical studies reveal that tuning the donor strength of the end groups allows covering a broad spectral range, from the visible to the NIR region, of the UV-visible absorption and fluorescence spectra. Two-photon-excited fluorescence and Z-scan techniques prove that an increase in the donor strength or in the rigidity of the backbone results in a considerable increase in the two-photon cross section, reaching 5000 GM, with predominant two-photon absorption from the S0-S2 charge-transfer transition. Direct comparisons with the hemicurcuminoid derivatives show that the two-photon active band for the curcuminoid derivatives has the same intramolecular charge-transfer character and therefore arises from a dipolar structure. Overall, this structure-relationship study allows the optimization of the two-photon brightness (i.e., 400-900 GM) with one dye that emits in the NIR region of the spectrum. In addition, these dyes demonstrate high intracellular uptake efficiency in Cos7 cells with emission in the visible region, which is further improved by using porous silica nanoparticles as dye vehicles for the imaging of two mammalian carcinoma cells type based on NIR fluorescence emission.

Theoretical and Experimental Study on Boron ß-Diketonate Complexes with Intense Two-Photon-Induced Fluorescence in Solution and in the Solid State.

Three boron diketonate chromophores with extended π-conjugated backbone were prepared and their spectroscopic features were investigated through a combined theoretical/experimental study. It was shown that these complexes, which undergo very large electronic reorganization upon photoexcitation, combine large two-photon absorption cross section with an emission energy and quantum efficiency in solution that is strongly dependent on solvent polarity. The strong positive influence of boron complexation on the magnitude of the two-photon absorption was clearly established, and it was shown that the two-photon absorption properties were dominated by the quadrupolar term. For one of the synthesized compounds, intense one- and two-photon-induced solid-state emission (fluorescence quantum yield of 0.65 with maximum wavelength of 610 nm) was obtained as a result of antiparallel J-aggregate crystal packing.

Borondifluoride complexes of hemicurcuminoids as bio-inspired push-pull dyes for bioimaging.

Hemicurcuminoids are based on half of the π-conjugated backbone of curcuminoids. The synthesis of a series of such systems and their borondifluoride complexes is described. The electrochemical and photophysical properties of difluorodioxaborine species were investigated as a function of the nature of electron donor and acceptor groups appended at either terminal positions of the molecular backbone. The emissive character of these dipolar dyes was attributed to an intraligand charge transfer process, leading to fluorescence emission that is strongly dependent on solvent polarity. Quasi-quantitative quenching of fluorescence in high polarity solvents was attributed to photoinduced electron transfer. These dyes were shown to behave as versatile fluorophores. Indeed, they display efficient two-photon excited fluorescence emission leading to high two-photon brightness values. Furthermore, they form nanoparticles in water whose fluorescence emission quantum yield is less than that of the dye in solution, owing to aggregation-induced fluorescence quenching. When cos7 living cells were exposed to these weakly-emitting nanoparticles, one- and two-photon excited fluorescence spectra showed a strong emission within the cytoplasm that originated from the individual molecules. Dye uptake thus involved a disaggregation mechanism at the cell membrane which restored fluorescence emission. This off-on fluorescence switching allows a selective optical monitoring of those molecules that do enter the cell, which offers improved sensitivity and selectivity of detection for bioimaging purposes.

Optical and morphological properties of thin films of bis-pyrenyl π-conjugated molecules.

1,4-Di-n-octyloxy-2,5-bis(pyren-1-ylethenyl)benzene (bis-pyrene) has been studied by the means of surface cavity ring-down (s-CRD) spectroscopy on an amorphous BK7 glass substrate and scanning tunnelling microscopy (STM) on Au(111). Absorption spectra show a modification of the optical properties as a function of coverage, i.e. appearance of a shoulder around 505 nm followed by a saturation of the intensity of this signal observed at higher coverages. We attribute this shoulder to the change of the molecular orientation between the first and the second monolayer and thus to an interfacial effect. These results are confirmed by scanning tunnelling microscopy (STM) measurements where the bis-pyrene molecules have been deposited on Au(111) at room temperature (RT) and onto a cold substrate. Independently of the temperature in the range from 210 K to RT, the first monolayer is always highly organized. At low temperature bis-pyrene molecules constituting the second monolayer are randomly distributed, suggesting that self-organisation is kinetically hindered. Deposited at room temperature, the molecular diffusion is enhanced and the formation of an organized second layer takes place after storing the sample for 150 minutes at room temperature. A HOMO-LUMO gap of 2.85 eV has been determined by scanning tunnelling spectroscopy, which is in very good agreement with the observed optical transition at 434 nm (2.86 eV) in s-CRD spectroscopy.

Tuning the Direction of Intramolecular Charge Transfer and the Nature of the Fluorescent State in a T-Shaped Molecular Dyad.

Controlling photoinduced intramolecular charge transfer at the molecular scale is key to the development of molecular devices for nanooptoelectronics. Here, we describe the design, synthesis, electronic characterization, and photophysical properties of two electron donor-acceptor molecular systems that consist of tolane and BF2-containing curcuminoid chromophoric subunits connected in a T-shaped arrangement. The two π-conjugated segments intersect at the electron acceptor dioxaborine core. From steady-state electronic absorption and fluorescence emission, we find that the photophysics of the dialkylamino-substituted analogue is governed by the occurrence of two closely lying excited states. From DFT calculations, we show that excitation in either of these two states results in a distinct shift of the electron density, whether it occurs along the curcuminoid or tolane moiety. Femtosecond transient absorption spectroscopy confirmed these findings. As a consequence, the nature of the emitting state and the photophysical properties are strongly dependent on solvent polarity. Moreover, these characteristics can also be switched by protonation or complexation at the nitrogen atom of the amino group. These features set new approaches toward the construction of a three-terminal molecular system in which the lateral branch would transduce a change of electronic state and ultimately control charge transport in a molecular-scale device.

Synthesis and photophysical properties of difluoroboron complexes of curcuminoid derivatives bearing different terminal aromatic units and a meso-aryl ring.

The synthesis of nine curcuminoids and their difluoroboron complexes is described, with seven of them containing a meso-phenyl ring. Dynamic (19)F NMR confirmed the fact that rotation of that meso-aryl fragment is restricted in the latter systems at room temperature and become allowed at higher temperature (>45 °C). The molecular structure of a meso-substituted derivative in the solid state showed that the phenyl ring lies in a highly twisted plane with respect to the mean curcuminoid plane. The photophysical properties of the nine compounds were investigated in solvents of different polarity. Meso-substitution with a phenyl ring has little influence on fluorescence emission properties in solution, radiative and nonradiative kinetic constants being similar for meso- and nonsubstituted compounds, which is in contrast to the case of BODIPY derivatives. However, introduction of an electron donor p-methoxy group at the meso-phenyl ring leads to small perturbation of the curcuminoid π-system fluorescence emission. We also report the influence of the meso-phenyl group on the emission properties of the aggregated solids.

Boron difluoride complexes of 3-hydroxyflavone derivatives: efficient bioinspired dyes for solution and solid-state emission.

A series of six boron difluoride complexes of 3-hydroxyflavone derivatives is described. These dyes were synthesized very simply and exhibited bright visible emission in solution (up to unity) as well as emission in the solid state. Complexation to boron difluoride was shown to impart donor-acceptor character to the excited state of these dyes, which further shifted the emission towards the visible part of the spectrum. Furthermore, differences were noticed following the strength of the donor and the acceptor. Light emission from the π-stacked molecules was quenched relative to the solution behaviour which was interpreted in terms of both packing and electronic properties of the molecular dyes. The packing arrangement was found to be a function of the substituent on the 3-hydroxyflavone derivatives.

Efficient NIR-light emission from solid-state complexes of boron difluoride with 2'-hydroxychalcone derivatives.

This article describes a series of nine complexes of boron difluoride with 2'-hydroxychacone derivatives. These dyes were synthesized very simply and exhibited intense NIR emission in the solid state. Complexation with boron was shown to impart very strong donor-acceptor character into the excited state of these dyes, which further shifted their emission towards the NIR region (up to 855 nm for dye 5 b, which contained the strongly donating triphenylamine group). Strikingly, these optical features were obtained for crystalline solids, which are characterized by high molecular order and tight packing, two features that are conventionally believed to be detrimental to luminescence in organic crystals. Remarkably, the emission of light from the π-stacked molecules did not occur at the expense of the emission quantum yield. Indeed, in the case of pyrene-containing dye 4, for example, a fluorescence quantum yield of about 15 % with a fluorescence emission maximum at 755 nm were obtained in the solid state. Moreover, dye 3 a and acetonaphthone-based compounds 1 b, 2 b, and 3 b showed no evidence of degradation as solutions in CH(2) Cl(2) that contained EtOH. In particular, solutions of brightly fluorescent compound 3 a (brightness: ε×Φ(f) =45,000 M(-1) cm(-1)) could be stored for long periods without any detectable changes in its optical properties. All together, these new dyes possess a set of very interesting properties that make them promising solid-state NIR fluorophores for applications in materials science.

Effect of the electron donating group on the excited-state electronic nature and epsilon-near-zero properties of curcuminoid-borondifluoride dyes.

Epsilon-near-zero (ENZ) properties have been reported in organic molecular films. In particular, cyanine and squaraine films have been shown to exhibit ENZ properties in the visible spectral region with a strong 3rd order nonlinear optical response near the ENZ spectral region. Noting both cyanine and squaraine belong to the polymethine family, a series of six curcuminoid borondifluoride (Curc) derivatives were developed to examine whether such a polymethine character is positively correlated with the ENZ property of the organic films. Those Curc derivatives possess a Donor-Acceptor-Donor (D-A-D) architecture with acceptor, AcacBF2, located at the molecular center. The backbone of Curc is designed such that the donor strength can be tuned to transit between charge transfer (CT) and polymethine character. This balance between CT and polymethine character of the Curc series is examined based on the Lippert-Mataga plot. As donor strength in the D-A-D structure increases, CT character is less marked resulting in a more dominant polymethine character. The structural and optical properties of the Curc films with a thickness in the order of 30 nm were examined to correlate the polymethine character with the ENZ response. The results obtained in isotropic Curc thin films demonstrate that an increase of polymethine character associated with a stronger donor strength leads to an appearance/enhancement of the ENZ property in the visible spectrum range from 500 to 670 nm. Overall, this study provides useful guidelines to engineer new organic materials showing ENZ properties in a desired spectral range.

Spontaneous exciton dissociation enables spin state interconversion in delayed fluorescence organic semiconductors.

Engineering a low singlet-triplet energy gap (ΔEST) is necessary for efficient reverse intersystem crossing (rISC) in delayed fluorescence (DF) organic semiconductors but results in a small radiative rate that limits performance in LEDs. Here, we study a model DF material, BF2, that exhibits a strong optical absorption (absorption coefficient = 3.8 × 105 cm-1) and a relatively large ΔEST of 0.2 eV. In isolated BF2 molecules, intramolecular rISC is slow (delayed lifetime = 260 µs), but in aggregated films, BF2 generates intermolecular charge transfer (inter-CT) states on picosecond timescales. In contrast to the microsecond intramolecular rISC that is promoted by spin-orbit interactions in most isolated DF molecules, photoluminescence-detected magnetic resonance shows that these inter-CT states undergo rISC mediated by hyperfine interactions on a ~24 ns timescale and have an average electron-hole separation of ≥1.5 nm. Transfer back to the emissive singlet exciton then enables efficient DF and LED operation. Thus, access to these inter-CT states, which is possible even at low BF2 doping concentrations of 4 wt%, resolves the conflicting requirements of fast radiative emission and low ΔEST in organic DF emitters.

Metal ion modulated torsion angle in a ditopic oligothiophene ligand: toward supramolecular control of π conjugation.

A π-conjugated oligomer bearing two 15-crown-5-containing styryl moieties connected at the inner ß positions of the terminal thiophene nuclei can adopt either a U or a Z shape depending on the structures of its complexes with magnesium and barium ions. We show that barium cations lead to the formation of a mononuclear complex in solution, which causes the system to fold into the U shape. Magnesium ions lead to the same effect at low concentration, but force the ligand to adopt the Z-shaped geometry at high concentrations favoring formation of a binuclear complex. These geometrical reorganizations in solution are accompanied by profound changes in spectroscopic properties, which can be rationalized in terms of variations in the extent of electron delocalization along the oligothiophene backbone. The effects are analyzed by mass spectrometry and (1)H NMR, UV/Vis absorption, and fluorescence spectroscopy in the steady-state and time-resolved regimes. The experimental results are compared to data calculated by using MOPAC2007 with the PM6 Hamiltonian including the COSMO solvation model.

Acetylenic spacers in phenylene end-substituted oligothiophene core for highly air-stable organic field-effect transistors.

Two thiophene-phenylene semiconductors, bis(2-phenylethynyl) end-substituted oligothiophenes (diPhAc-nTs, n = 2, 3), were synthesized and studied with respect to their optical, electrochemical, structural and electrical properties. The optical and electrochemical properties of the oligomers in solution were investigated by UV-vis absorption and photoluminescence spectroscopies, and cyclic voltammetry. High vacuum evaporated thin films were investigated by optical absorption, X-ray diffraction and AFM, and implemented as p-type semiconducting layers into organic thin-film transistors (OTFTs). A comparative study in solution and in the solid state with distyryl-oligothiophenes (DSnTs, n = 2, 3) reveals the great influence of acetylenic (-C[triple bond]C-) vs. olefinic (-C=C-) spacers in thiophene-phenylene derivatives on electronic structure, physical properties, and device efficiencies. Substituting olefinic for acetylenic pi-spacers in terthiophene-based conjugated semiconductors leads to one of incontrovertible attributes of OTFTs for low cost applications, a high mobility at low substrate temperature (T(sub)) i.e. typically 25 degrees C. Fine-tuning in the HOMO/LUMO levels by reducing the HOMO level introduces increased air-oxidation strength of thin films where OTFTs provide exactly the same hole mobility value after 100 days in air. All the results suggested that introduction of carbon-carbon triple bonds provided an efficient route to highly air-stable organic thin film transistors.

Access to Fluorenones Using Benzocyclopentynone Surrogate as Partner for the [2 + 2 + 2] Cycloaddition Reaction.

A convenient and versatile procedure for the straightforward synthesis of substituted fluorenones as valuable scaffolds is described under rhodium catalysis. The present [2 + 2 + 2] cycloaddition reaction of diynes with 3-acetoxy or-3-alkoxyindenones as surrogates of the highly reactive benzocyclopentynone 2π partner allows the preparation of various fluorenone-type derivatives in good yields and provides an additional and tunable process for the generation of more challenging molecules with application in pharmaceutical, polymer, and material sciences.

Oligothiophene self-assembly on the surface of ZnO nanorods: toward coaxial p-n hybrid heterojunctions.

We describe herein the design, synthesis and detailed structural characterization of hybrid 1D nanostructures. They are prepared by supramolecular self-assembly of oligothiophene molecules on the surface of zinc oxide nanorods in solution at room temperature. Electronic absorption spectroscopy and X-ray diffraction show that both organic and inorganic components in the coaxial p-n heterojunctions are crystalline. Especially, it is demonstrated that the organic compounds form a self-assembled monolayer at the surface of the nanorods, which is not the case when zinc oxide quantum dots are instead used. As a result of their hybrid nature, the 1D nanostructures lead to ambipolar semiconducting nanostructured materials as active layers in field-effect transistors.

Dye-adsorption-induced gelation of suspensions of spherical and rodlike zinc oxide nanoparticles in organic solvents.

The adsorption of amphiphilic Ru(II) complex Z907 onto the surface of ZnO nanospheres and nanorods causes the gelation of organic solvents, such as THF and acetone. The gels are thermally stable at very low concentration (nanoparticle volume fraction phi = 0.009) but mechanically fragile, with the behavior being dependent on the nature of the solvent, nanoparticle concentration, and the Z907/ZnO mole/weight ratio. Rheological experiments confirmed that the solid component built up a network to give a viscoelastic gel-phase material with a weak value of storage modulus G'. However, TEM and SEM experiments did not give evidence that nanoparticle long-range ordering occurred under the experimental conditions investigated. Moreover, time-dependent SAXS measurements pointed to a decrease in the nanoparticle aggregate size upon gelation. All together, the data obtained might be rationalized in terms of the aggregate-to-aggregate transition in solution, with the primitive large aggregates giving rise to smaller ones upon reaction with Z907. The resulting smaller hybrid aggregates could be the active species that act as self-assembling components in the gelation process. Given the interesting electronic and photonic properties of zinc oxide nanoparticles, such hybrid organic-inorganic gels could open new directions in materials science, low-cost electronics, and photovoltaics.

Internally referenced analysis of charge-transfer reactions in a new ferrocenyl bithiophenic conducting polymer through cyclic voltammetry.

A new ferrocenyl bithiophenic polymer has enabled the first demonstration of internally referenced quantitative analysis of charge-transfer reactions, which may enable better understanding and applications of conducting polymers in e.g. supercapacitors.