Synthesis and Photophysical Properties of Difluoroboron Complexes of Curcuminoid Derivatives Bearing Different Terminal Aromatic Units and a <i>meso</i>-Aryl Ring

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

Synthesis and Photophysical Properties of Difluoroboron Complexes of Curcuminoid Derivatives Bearing Different Terminal Aromatic Units and a <i>meso</i>-Aryl Ring

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

Design of Dipicolinic Acid Ligands for the Two-Photon Sensitized Luminescence of Europium Complexes with Optimized Cross-Sections

International audienceThe multistep synthesis of an extensive series of push−pull donor-π-conjugated dipicolinic acid ligands is described. The charge transfer character of the ligand can be tuned by changing the donor group (CH2R, OR, SR, or NR2) or the nature of the conjugated backbone (phenyl, phenylethynyl, naphtylethynyl, bis(phenylethynyl), or chalcone). The photophysical properties of related D3 symmetric europium complexes (absorption and luminescence) were measured. Experiments using two-photon sensitized luminescence of a EuIII complex reveal large two-photon absorption (TPA) cross-section values (775 GM at 740 nm) in dichloromethane. Furthermore, some structure−property relationships can be derived from this systematic study, allowing an optimization of TPA properties of lanthanide complexes

Efficient Sensitization of Europium, Ytterbium, and Neodymium Functionalized Tris-Dipicolinate Lanthanide Complexes through Tunable Charge-Transfer Excited States

International audienceA series of push−pull donor-π-conjugated dipicolinic acid ligands and related tris-dipicolinate europium and lutetium complexes have been prepared. The ligands present broad absorption and emission transitions in the visible spectral range unambiguously assigned to charge-transfer transitions (CT) by means of time-dependent density functional theory calculations. The photophysical properties (absorption, emission, luminescence quantum yield, and lifetime) of the corresponding europium complexes were thoroughly investigated. Solvatochromism and temperature effects clearly confirm that Eu(III) sensitization directly occurs from the ligand CT state. In addition, modulation of the energy of the CT donating state by changing the nature of the donor fragment allows the optimal energy of the antennae for europium sensitization to be determined, and this optimal energy was found to be close to the 5D1 accepting state. Finally, this CT sensitization process has been successfully extended to near-infrared emitters (neodymium and ytterbium)

C–H···BF₂O₂ Interactions in Crystals: A Case for Boron Hydrogen Bonding?

Molecules bearing the borondifluoride unit represent an important class of dyes. We report the intriguing occurrence of short H···B contacts in the crystal structure of compounds featuring the BF₂O₂ fragment. Using density functional theory-based calculations, we discovered that, in contrast to what is suggested by the structural observation, the driving force behind these short H···B contacts is not a direct H···B interaction but it relies on the existence of a combination of single, bifurcated, or trifurcated hydrogen bond paths that are practically isoenergetic within a broad range of C–H···B interaction orientations. The BF₂O₂ group as a whole displays a very isotropic hydrogen bonding ability and, in turn, a strong adaptability to structural requirements in the solid state

Adsorption and Growth of Bis-pyrene Molecular Layers on Au(111) Studied by STM

The adsorption and growth of 1,4-di-<i>n</i>-octyloxy-2,5-bis­(pyren-1-ylethenyl)­benzene (bis-pyrene) thermo-evaporated onto a Au(111) single crystal surface has been investigated using low temperature scanning tunneling microscopy (STM) under ultrahigh vacuum (UHV) conditions. At low coverage, bis-pyrene molecules arrange into molecular chains, which exclusively grow on the fcc domains of Au(111) herringbone reconstruction. In these molecular chains, the individual molecules are connected by a van der Waals interaction between the alkyl chains of the molecules. As the coverage is increased the hcp domains of Au(111) herringbone reconstruction are also decorated by molecular chains. Finally, for coverages above 0.5 ML, four distinct ordered commensurate phases are observed, which suggests a delicate balance between the intermolecular and the molecule–surface interactions. A detailed analysis of the STM images suggests that distinct molecular configurations of the bis-pyrene compound in these ordered phases are at the origin of their formation

NIR Emission in Borondifluoride Complexes of 2′-Hydroxychalcone Derivatives Containing an Acetonaphthone Ring

We report the synthesis of borondifluoride complexes of a series of 2′-hydroxychalcone derivatives containing the hydroxynaphthone ring. TD-DFT calculations confirm the charge transfer character of the lowest-energy transition band and reveal large values for the ground-state dipole moment. The fluorescence emission spectra in the solid state are considerably red-shifted relative to those in solution, leading to maximum emission wavelengths spanning from the far red to the NIR. The emission properties are clearly characteristic of tightly packed chromophores with strong excitonic coupling, as confirmed by X-ray structure analysis and electronic absorption spectroscopy. The optical properties of the six dyes <b>1-BF</b>_<b>2</b>–<b>6-BF</b>_<b>2</b> are discussed in terms of interchromophoric interactions in the crystal and dipole moment values. Dyes <b>3-BF</b>_<b>2</b> and <b>4-BF</b>_<b>2</b> emit light at 690 and 751 nm with 13% and 6.5% efficiencies, respectively, and as such they represent very appealing materials. Compound <b>6-BF</b>_<b>2</b> presents an emission maximum wavelength at 804 nm with a fluorescence quantum yield value of 2%. This class of solid-state fluorophores may lead to exciting prospects for the generation of advanced materials

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<i>–</i>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