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

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

Influence of Pyrene Grafting on PMMA Nanosecond Laser Ablation at 248 nm

In this work, we investigate the effects of KrF nanosecond laser ablation on poly­(methyl methacrylate) (PMMA) in combination with pyrene. Three materials containing PMMA were studied: (1) one doped with pure pyrene, (2) one doped with methyl 3-(1-pyrenyl)­propanoate (so called alkylpyrene derivative thereafter), and (3) one grafted with pyrene. This last new material was developed by covalently bonding pyrene molecules to PMMA side-chains. A comparative study was undertaken to determine and compare the respective properties of the PMMA dye containing pyrene during nanosecond laser ablation at 248 nm. Cavities were etched for each material with up to 20 pulses for fluences between 0.03 and 1.7 J/cm² in samples containing 1, 2, and 4 mol % chromophore. The threshold fluences and the effective absorption coefficients were obtained. It was observed that effective absorption coefficients increased and threshold fluences decreased with the chromophore percentages in each kind of sample. Ablation parameters were not significantly modified when the dopant was changed from pyrene to the alkylpyrene derivative. On the other hand, when pyrene molecules were grafted on the polymer, the threshold fluences decreased, whereas the effective absorption coefficients became similar at fluences above 0.6 J/cm²

Effects of Ligand Geometry on the Photophysical Properties of Photoluminescent Eu(III) and Sm(III) 1‑Hydroxypyridin-2-one Complexes in Aqueous Solution

A series of 10 tetradentate 1-hydroxy-pyridin-2-one (1,2-HOPO) ligands and corresponding eight-coordinated photoluminescent Eu­(III) and Sm­(III) complexes were prepared. Generally, the ligands differ by the linear (nLI) aliphatic linker length, from 2 to 8 methylene units between the bidentate 1,2-HOPO chelator units. The photoluminescent quantum yields (Φ_tot) were found to vary with the linker length, and the same trend was observed for the Eu­(III) and Sm­(III) complexes. The 2LI and 5LI bridged complexes are the brightest (Φ_totxε). The change in ligand wrapping pattern between 2LI and 5LI complexes observed by X-ray diffraction (XRD) is further supported by density functional theory (DFT) calculations. The bimodal Φ_tot trends of the Eu­(III) and Sm­(III) complexes are rationalized by the change in ligand wrapping pattern as the bridge (<i>n</i>LI) is increased in length

Effects of Ligand Geometry on the Photophysical Properties of Photoluminescent Eu(III) and Sm(III) 1‑Hydroxypyridin-2-one Complexes in Aqueous Solution

A series of 10 tetradentate 1-hydroxy-pyridin-2-one (1,2-HOPO) ligands and corresponding eight-coordinated photoluminescent Eu­(III) and Sm­(III) complexes were prepared. Generally, the ligands differ by the linear (nLI) aliphatic linker length, from 2 to 8 methylene units between the bidentate 1,2-HOPO chelator units. The photoluminescent quantum yields (Φ_tot) were found to vary with the linker length, and the same trend was observed for the Eu­(III) and Sm­(III) complexes. The 2LI and 5LI bridged complexes are the brightest (Φ_totxε). The change in ligand wrapping pattern between 2LI and 5LI complexes observed by X-ray diffraction (XRD) is further supported by density functional theory (DFT) calculations. The bimodal Φ_tot trends of the Eu­(III) and Sm­(III) complexes are rationalized by the change in ligand wrapping pattern as the bridge (<i>n</i>LI) is increased in length