Lanthanide-Based Organic Salts: Synthesis, Characterization, and Cytotoxicity Studies.

The formulation of magnetic ionic liquids (MILs) or organic salts based on lanthanides as anions has been explored. In this work, a set of choline-family-based salts, and two other, different cation families, were combined with Gadolinium(III) and Terbium(III) anions. Synthetic methodologies were previously optimized, and all organic salts were obtained as solids with melting temperatures higher than 100 °C. The magnetic moments obtained for the Gd(III) salts were, as expected, smaller than those obtained for the Tb(III)-based compounds. The values for Gd(III) and Tb(III) magnetic salts are in the range of 6.55-7.30 MB and 8.22-9.34 MB, respectively. It is important to note a correlation between the magnetic moments obtained for lanthanides, and the structural features of the cation. The cytotoxicity of lanthanide-based salts was also evaluated using 3T3, 293T, Caco2, and HepG2 cells, and it was revealed that most of the prepared compounds are not toxic.

Hybrid Zn-ß-Aminoporphyrin-Carbon Nanotubes: Pyrrolidine and Direct Covalent Linkage Recognition, and Multiple-Photo Response.

To unveil and shape the molecular connectivity in (metallo)porphyrin-carbon nanotube hybrids are of main relevance for the multiple medicinal, photoelectronic, catalytic, and photocatalytic applications of these materials. Multi-walled carbon nanotubes (MWCNTs) were modified through 1,3-dipolar cycloaddition reactions with azomethine ylides generated in situ and carrying pentafluorophenyl groups, followed by immobilization of the ß-amino-tetraphenylporphyrinate Zn(II). The functionalities were confirmed via XPS and FTIR, whereas Raman spectroscopy showed disruptions on the graphitic carbon nanotube surface upon both steps. The functionalization extension, measured via TGA mass loss and corroborated via XPS, was 0.2 mmol·g-1. Photophysical studies attest to the presence of the different porphyrin-carbon nanotube connectivity in the nanohybrid. Significantly different emission spectra and fluorescence anisotropy of 0.15-0.3 were observed upon variation of excitation wavelength. Vis-NIR absorption and flash photolysis experiments showed energy/charge transfer in the photoactivated nanohybrid. Moreover, evidence was found for direct reaction of amino groups with a carbon nanotube surface in the presence of molecular dipoles such as the zwitterionic sarcosine amino acid.

Dinuclear Cu(I) Halides with Terphenyl Phosphines: Synthesis, Photophysical Studies, and Catalytic Applications in CuAAC Reactions.

Several dinuclear terphenyl phosphine copper(I) halide complexes of composition [CuX(PR2Ar')]2 (X = Cl, Br, I; R = hydrocarbyl, Ar' = 2,6-diarylterphenyl radical), 1-5, have been isolated from the reaction of CuX with 1 equiv of the phosphine ligand. Most of them have been characterized by X-ray diffraction studies in the solid state, thus allowing comparative discussions of different structural parameters, namely, Cu···Cu and Cu···Aryl separations, conformations adopted by coordinated phosphines, and planarity of the Cu2X2 cores. Centrosymmetric complexes [CuI(PMe2ArXyl2)]2, 1c, and [CuI(PEt2ArMes2)]2, 3c, despite their similar structures, show very distinct photoluminescence (PL) in powder form at room temperature. The photophysical behavior of these compounds in liquid solution, solid-solid Zeonex solution and powder samples at room temperature and 77 K have been investigated and supported by DFT calculation. Identification of vibronic coupling modes, done by group theory calculations and the technique of projection operators, shows that the manifestation of these modes is conditioned by crystal packing. Complexes [CuI(PMe2ArXyl2)]2, 1c, and [CuI(PEt2ArMes2)]2, 3c, display remarkable activity in copper-catalyzed azide-alkyne cycloaddition reactions involving preformed and in situ-made azides. Reactions are performed in H2O, under aerobic conditions, with low catalyst loadings and tolerate the use of iodoalkynes as substrates.

A Visible-Near-Infrared Light-Responsive Host-Guest Pair with Nanomolar Affinity in Water.

The discovery of stimuli-responsive high affinity host-guest pairs with potential applications under biologically relevant conditions is a challenging goal. This work reports a high-affinity 1:1 complex formed between cucurbit[8]uril and a water-soluble photochromic diarylethene derivative. It was found that, by confining the open isomer within the cavity of the receptor, a redshift in the absorption spectrum and an enhancement of the photocyclization quantum yield from Φ=0.04 to Φ=0.32 were induced. This improvement in the photochemical performance enables quantitative photocyclization with visible light that, together with the near-infrared light-induced ring-opening reaction and the 100-fold selectivity for the closed isomer, confirms this as an outstanding light-responsive affinity pair.

Hiding and unveiling trans-chalcone in a constrained derivative of 4',7-dihydroxyflavylium in water: a versatile photochromic system.

A structurally constrained derivative of 4',7-dihydroxyflavylium was studied in aqueous solution and in CTAB micelles by pH jumps, flash photolysis and continuous irradiation with spectroscopic details assessed as well by theoretical calculations. In water, up to pH = 8, the compound shows only acid base chemistry with deprotonation of the flavylium cation to form a quinoidal base that further deprotonates with pKas of 4.8 and 7.4. In the basic region, unprotonated trans-chalcones are formed. No neutral trans-chalcone (Ct) is formed in water preventing the establishment of the well-known photochromism involving photoisomerization of this species with subsequent formation of the flavylium cation. Addition of 0.02 M CTAB drastically changes the mole fraction distribution of species, leading to the formation of Ct (χCt = 1 at pH = 5) and unveiling a photochromic behavior with a pH-tunable colour contrast in a large pH range (2 < pH < 8). The Ct species can be hidden again (irreversibly) upon addition of α-cyclodextrin that disrupts the CTAB micelles, reverting the system to its initial mole fraction distribution of species. These supramolecular inputs work atop the molecular reaction networks by modifying their species' mole fraction distribution.

Characterization of a novel intrinsic luminescent room-temperature ionic liquid based on [P6,6,6,14 ][ANS].

Intrinsically luminescent room-temperature ionic liquids (RTILs) can be prepared by combining a luminescent anion (more common) or cation with appropriate counter ions, rendering new luminescent soft materials. These RTILs are still new, and many of their photochemical properties are not well known. A novel intrinsic luminescent RTIL based on the 8-anilinonaphthalene-1-sulfonate ([ANS]) anion combined with the trihexyltetradecylphosphonium ([P6,6,6,14 ]) cation was prepared and characterized by spectroscopic techniques. Detailed photophysical studies highlight the influence of the ionic liquid environment on the ANS fluorescence, which together with rheological and (1) H NMR experiments illustrate the effects of both the viscosity and electrostatic interactions between the ions. This material is liquid at room temperature and possesses a glass transition temperature (Tg ) of 230.4 K. The fluorescence is not highly sensitive to factors such as temperature, but owing to its high viscosity, dynamic Stokes shift measurements reveal very slow components for the IL relaxation.

Europium(III) tetrakis(ß-diketonate) complex as an ionic liquid: a calorimetric and spectroscopic study.

An intrinsic photoluminescent ionic liquid based on europium(III) tetrakis(ß-diketonate) complex with a tetraalkylphosphonium as counterion was synthesized. Calorimetric measurements showed a melting point at 63 °C, which allows the ionic liquid classification. When cooling the material from the liquid state, metastable supercooled ionic liquid is obtained, as seen from NMR spectroscopy as well. Eu(III) photoluminescence is clearly observed while the absorption spectra of the ligand is dominant, showing the antenna effect. This was confirmed with submicrosecond time scale luminescence spectroscopy, where a rise of Eu(III) emission is observed with the correspondent decay of the ligand excited state. Temperature effects in the photoluminescence are also shown, being prominent above the melting point where the intensity decreases with Arrhenius behavior. Eu(III) luminescence decays also show features characteristic of energy migration between homologue Eu(III) species. Solvent effects were also studied by NMR and Luminescence spectroscopies, highlighting that the nucleophilicity of organic solvents such as n-alcohols leads to a coordination with Eu(III), which ultimately compromises the stability of the complex.

Chemistry and applications of flavylium compounds: a handful of colours.

Flavylium compounds are versatile molecules that comprise anthocyanins, the ubiquitous colorants used by Nature to confer colour to most flowers and fruits. They have found a wide range of applications in human technology, from the millenary colour paints described by the Roman architect Vitruvius, to their use as food additives, combining colour and antioxidant effects, and even as light absorbers in solar cells aiming at a greener solar energy conversion. Their rich complexity derives in part from their ability to switch between a variety of species (flavylium cations, neutral quinoidal bases, hemiketals and chalcones, and negatively charged phenolates) by means of external stimuli, such as pH, temperature and light. This critical review describes (i) the historical advancements in the understanding of the equilibria of their chemical reaction networks; (ii) their thermodynamics and kinetics; (iii) the mechanisms underlying their colour development, such as co-pigmentation and host-guest interactions; (iv) the photophysics and photochemistry that lead to photochromism; and (v) applications in solar cells, models for optical memories, photochromic soft materials such as ionic liquids and gels, and their properties in solid state materials (274 references).

Parylene-Sealed Perovskite Nanocrystals Down-Shifting Layer for Luminescent Spectral Matching in Thin Film Photovoltaics.

The present contribution aims to enhance solar cells' performance via the development of advanced luminescent down-shifting based on encapsulated nanostructured perovskite materials. Here, thin films of inorganic lead halide (CsPbBr3) perovskite nanocrystal luminophores were synthetized, by hot-injection, deposited on glass substrates by spin-coating, and encapsulated with parylene type C, via chemical vapor deposition, to protect and stabilize the films. The optical properties of these thin films were characterized by absorption, emission and 2D contour spectra, their structure by X-ray diffraction and X-ray photoelectron spectroscopy, and the morphology by Scanning Transmission Electron microscopy. I-V curve and spectral response nanocrystalline silicon photovoltaic (nc-Si:H PV) cells were studied in the absence and presence of the perovskite and parylene luminescent down-shifting layers. The incorporation of the CsPbBr3 nanocrystals and their encapsulation with the parylene type C polymeric coating led to an increase in the current generated and the spectral response of the PV cells in the regime of the nanocrystals' fluorescence emission. A 3.1% increase in the short circuit current density and a 5.6% increase in the power conversion efficiency were observed.

Formation of a leuco spirolactone from 4-(2-carboxyphenyl)-7-diethylamino-4'-dimethylamino-1-benzopyrylium: design of a phase-change thermochromic system based on a flavylium dye.

A phase-change thermochromic system was designed through the reversible transformation of the 4-substituted flavylium dye 4-(2-carboxyphenyl)-7-diethylamino-4'-dimethylamino-1-benzopyrylium into its leuco form, in the presence of a developer (ethyldiisopropylamine) and a suitable solvent (e.g., acetonitrile, n-pentadecanonitrile). The leuco form of the flavylium-based dye is a spirolactone species whose ring opens at low temperature (below the solvent melting point) to form the blue flavylium cation. Decarboxylation of the lactone to give 4-phenyl-7-diethylamino-4'-dimethylamino-1-benzopyrylium was observed upon irradiation of the system with UV light, erasing the thermochromic effect.

A Europium(III) Complex with an Unusual Anion-Cation Interaction: A Luminescent Molecular Thermometer for Ratiometric Temperature Sensing.

An unusual thermally sensitive anion-cation interaction, which is characteristic of the anion [Eu(FOD)4 ]- , occurs in the complex [CHOL][Eu(FOD)4 ] (1; CHOL=choline; FOD=1,1,1,2,2,3,3-heptafluoro-7,7-dimethyl-4,6-octanedionate) and affects both quantum yield and thermochromic behavior. This prompted the design of an Eu3+ -based ratiometric thermometer that functions at temperatures up to 95 °C through a thermally excited state absorption of the Eu3+ ion. The reusable temperature-sensitive luminescent complex showed a range of relative sensitivity between 0.45 % C-1 at 25 °C, with an increase to 7.0 % C-1 at 95 °C. Confinement of compound 1 in a transparent film of polysulfone resulted in a higher thermal stability of 1 while its luminescence showed a strong temperature dependence.

Tunable white emission of silver-sulfur-zeolites as single-phase LED phosphors.

Metal clusters confined inside zeolite materials display remarkable luminescent properties, making them very suitable as potential alternative phosphors in white LED applications. However, up to date, only single-color emitters have been reported for luminescent metal-exchanged zeolites. In this study, we synthesized and characterized white emitting silver-sulfur zeolites, which show a remarkable color tunability upon the incorporation of silver species in highly luminescent sulfur-zeolites. Via a combined steady-state and time-resolved photoluminescence spectroscopy characterization, we suggest that the observed luminescence and tunability arise from the presence of two different species. The first associated to an orange-red emitting silver cluster (Ag-CL), whereas the second is related to a blue-white emitting S-Ag-species. The relative contribution of both luminescent species depends on the synthesis procedure. It was shown that the formation of the blue-white emitting S-Ag-species is favored upon a heat-treatment of the samples.

On the mechanism of photochromism of 4'-N,N-dimethylamino-7-hydroxyflavylium in pluronic F127.

The photochromism of the compound 4'-N,N-dimethylamino-7-hydroxyflavylium incorporated in Pluronic F127 micelles and gels was studied in great detail. The red flavylium ion (AH(+)) or the quinoidal base (A), depending on pH, are the irradiation products of the colorless trans-chalcone (Ct). Absorption and fluorescence (steady-state, time-resolved, and anisotropy), pH jumps, and flash photolysis were used to characterize the system. At moderately acidic to neutral pH values, the Ct species is distributed between the core and corona of the Pluronic micelle, as well as in the aqueous phase. At acidic pH values, AH(+) remains most probably in the water phase. The Ct maximum absorption wavelength constitutes a good sensor for the critical micelle concentration (CMC) or critical micelle temperature (CMT). The apparent acidity constant pK'(a) was found to be a relatively good sensor for CMC and also for detection of the sol-gel critical temperature. The Ct photochromic mechanism was analyzed by comparing the photophysics in pure solvents and the pluronic media. Solvatochromic effects show a lack of solvent polarity dependence of the Stokes shift, indicating a low dipolar moment change between the ground and the locally excited state. An internal charge transfer nonradiative process (ICT) competes with Ct photoisomerization and is the dominant process in highly polar solvents, preventing the appearance of photochromism, in contrast with lower polar environments, such as micelles and ethanol. In high viscous environments as those found in the core of the Pluronic F127 micelles or glycerol, both ICT and photoisomerization are reduced, enhancing the Ct fluorescence quantum yield. According to the data from fluorescence measurements and pH jumps, evidence for the Ct distribution among different sites within the pluronic aggregate was found, (i) a hydrophilic/fluid region where Ct has poor fluorescence and isomerization yields, bulk region; (ii) the corona of the micelle where photoisomerization is maximized; and (iii) the hydrophobic/viscous region where the fluorescence quantum yield is higher (and photoisomerization lower). This effect leads to a selective Ct photochemistry.

Photochemistry of 7-hydroxy-2-(4-hydroxystyryl)-1-benzopyrylium and related compounds.

2-Styryl-1-benzopyrylium derivatives exhibit deeper hues and absorption spectra that are substantially red-shifted when compared with their 2-phenyl-1-benzopyrylium analogues. They follow the same pH and light-dependent network of chemical reactions previously described for 2-phenyl-1-benzopyrylium compounds. In this work, the photochromic properties of 7-hydroxy-2-(4-hydroxystyryl)-1-benzopyrylium chloride are reported. This compound was fully characterized by UV-vis absorption, fluorescence emission, pH jumps, and flash photolysis, and its properties were compared with the analogue 7,4'-dihydroxyflavylium (7-hydroxy-2-(4-hydroxyphenyl)-1-benzopyrylium). The trans-chalcones of both compounds lacking the hydroxyl in position 2 were synthesized and used as model compounds since they exhibit cis-trans isomerization but cannot be involved in the other processes resulting from the ring closure. The transient absorption of two triplets attributed to the chalcones Ct/Ct(-), and a tautomer was detected by nanosecond flash photolysis, independent of the existence of the 2-hydroxyl substituent. The experimental results are compatible with the main formation of cis-chalcone from the singlet state. In the case of the styryl derivatives, the fraction of triplet formed from excitation of Ct is much higher, and the fraction of isomerization is much smaller. For this reason, the photochemistry of 7-hydroxy-2-(4-hydroxystyryl)-1-benzopyrylium in water is much less efficient than that of its parent 7,4'-dihydroxyflavylium; however, in the presence of CTAB micelles, intense red colors can be obtained upon irradiation, confirming the usefulness of this family of compounds as photochromic systems.

Extreme Enhancement of Single-Molecule Fluorescence from Porphyrins Induced by Gold Nanodimer Antennas.

Porphyrins are typically weak emitters, which presents challenges to their optical detection by single-molecule fluorescence microscopy. In this contribution, we explore the enhancement effect of gold nanodimer antennas on the fluorescence of porphyrins in order to enable their single-molecule optical detection. Four meso-substituted free-base porphyrins were evaluated: two cationic, one neutral, and one anionic porphyrin. The gold nanodimer antennas are able to enhance the emission from these porphyrins by a factor of 105-106 increase in the maximum detected photon rates. This extreme enhancement is due to the combination of an antenna effect on the excitation rate that is estimated to be above 104-fold and an emission efficiency that corresponds to an increase of 2-10 times in the porphyrin's fluorescence quantum yield.

Interaction of zinc tetrasulfonated phthalocyanine with cytochrome C in water and Triton-X 100 micelles.

The interaction of a zinc tetrasulfonated phthalocyanine with cytochrome c was studied using steady-state spectroscopic techniques and time-correlated single photon counting in water and Triton-X 100 micelles. The dye forms dimers in water with a high equilibrium constant (70 x 10(6) M(-1)). Because of a specific electrostatic interaction, the presence of cytochrome c does not lead to a dissociation of this dimer, but increases its formation, with an equilibrium constant of about 7.9 x 10(9) M(-1). Triton-X 100 micelles dissociate the dimer, creating two populations of dye molecules: one in a hydrophilic media, probably on the surface of the micelles, another on a hydrophobic environment, probably inside the micelles. However, when cytochrome c is added the dye aggregation is again induced leading to a strong fluorescence quenching. This fluorescence quenching may also be caused by a photoinduced electron-transfer due to the formation of a 1:1 complex between the dye and the protein, but the present work does not give direct evidence of such an effect because the fluorescence decays did not show the presence of an extra component. The results presented here are quite different from those reported for aluminum sulfonated phthalocyanines, where aggregation does not occur and the fluorescence quenching is solely due to photoinduced electron-transfer reactions.

Sulfur dioxide induced aggregation of wine thaumatin-like proteins: Role of disulfide bonds.

Aggregation of heat unstable wine proteins is responsible for the economically and technologically detrimental problem called wine protein haze. This is caused by the aggregation of thermally unfolded proteins that can precipitate in bottled wine. To study the influence of SO2 in this phenomenon, wine proteins were isolated and thaumatins were identified has the most prone to aggregate in the presence of this compound. Isolated wine thaumatins aggregation was followed by dynamic light scattering (DLS), circular dichroism (CD), fluorescence spectroscopy and size exclusion chromatography (SEC). Our experimental results demonstrate that protein thermal unfolding after exposure of the protein to 70 °C does not present differences whether SO2 is present or not. Conversely, when the protein solution is cooled to 15 °C (after heat stress) significant analytical changes can be observed between samples with and without SO2. A remarkable change of circular dichroism spectra in the region 220-230 nm is observed (which can be related to S-S torsion angles), as well as an increase in tryptophan fluorescence intensity (absence of fluorescence quenching by S-S bonds). Formation of covalently-linked dimeric and tetrameric protein species were also detected by SEC. The ability to dissolve the aggregates with 8 M urea seems to indicate that hydrophobic interactions are prevalent in the formed aggregates. Also, the reduction of these aggregates with tris (2-carboxyethyl) phosphine (TCEP) to only monomeric species reveals the presence of intermolecular S-S bonds.

Small phospho-donors phosphorylate MorR without inducing protein conformational changes.

Phosphorylation is an essential mechanism of protein control and plays an important role in biology. The two-component system (TCS) is a bacterial regulation mechanism mediated by a response regulator (RR) protein and a kinase protein, which synchronize the regulatory circuit according to the environment. Phosphorylation is a key element in TCS function as it controls RR activity. In the present study, we characterize the behavior of MorR, an RR associated with Mo homeostasis, upon acetylphosphate and phosphoramidate treatment in vitro. Our results show that MorR was phosphorylated by both phospho-donors. Fluorescence experiments showed that MorR tryptophan emission is quenched by phosphoramidate. Furthermore, theoretical and computational results demonstrate that phosphorylation by phosphoramidate is more favorable than that by acetylphosphate. In conclusion, phosphorylated MorR is a monomeric protein and phosphorylation does not appear to induce observable conformational changes in the protein structure.

Promoting photochromism on flavylium derived 2-hydroxychalcones in aqueous solutions by addition of CTAB micelles.

A strategy to obtain photochromism from the network of chemical reactions originated by flavylium compounds in solution is described. This strategy is particularly useful for flavylium salts bearing amino groups which give rise to a variety of beautiful colors but lack photochemistry in water. The trans-chalcone of 7-(N,N-diethylamino)-4'-hydroxyflavylium interacts strongly with CTAB micelles defining a yellow dark state. Upon irradiation, the system switches to a pink-red state emerging from the flavylium cation that is formed inside the micelle and ejected to the bulk aqueous phase. The photochemical product reverts back to the trans-chalcone adduct with the micelle in the dark. The thermodynamics as well as the kinetics of the photochromic system were studied in detail. The best color contrast is obtained at pH = 4.25 with Phi = 0.001 and a recovery lifetime of approximately 3 h. This photochromic system works with no need of changing the pH, which constitutes an important improvement over previously described systems dependent on pH jumps.

Photochromism of 7-(N,N-diethylamino)-4'-hydroxyflavylium in a water-ionic liquid biphasic system.

Photochromism of trans-4-(N,N-diethylamino)-2,4'-dihydroxychalcone, with formation of the photoproduct 7-(N,N-diethylamino)-4'-hydroxyflavylium, is promoted in the ionic liquid phase of a water/[bmim][PF6] biphasic system.