Pedro F. Aramendía

Crystal structures and luminescent properties of terbium(III) carboxylates

Abstract Single crystals of three terbium(III) carboxylates of formulae [Tb2(CH3COO)6(H2O)4]·4H2O (1), [Tb2(CF3COO)6(H2O)6] (2) and [Tb(Hoda)3]·H2oda·H2O (3) (H2oda=2,2′-oxydiacetic acid) were obtained and their structures determined by X-ray crystallography. Compounds 1 and 2 are dimeric, in the former the terbium atoms are bound by two tridentate carboxylates in the μ2-bridging mode, whereas in the latter the bridging is fourfold with all carboxylates in the syn–syn coordination mode. Compound 3 is mononuclear containing three tridentate Hoda anions, and consecutive units are linked by a network of H-bonds involving the interstitial molecules. The luminescence spectra of the carboxylates were analyzed in the solid state and in aqueous solution. Comparison of the emission lifetimes in H2O and D2O allowed the determination of the average value for q, the number of coordinated water molecules, being 9.2 for 1 and 2 and 3.6 for 3, respectively. The quenching effect of Cu(II) on the luminescence of the terbium(III) carboxylates was evaluated through the emission decay constants. From the addition of Cu(II) to an aqueous solution of 3, single crystals of polymeric [{Cu3Tb2(oda)6(H2O)6}·12H2O]n (4) were isolated with completely quenched luminescence. Compound 4 exhibits an overall antiferromagnetic interaction.

CARBOXYLATED ZINC‐PHTHALOCYANINES–II* DIMERIZATION AND SINGLET MOLECULAR OXYGEN SENSITIZATION IN HEXADECYLTRIMETHYLAMMONIUM BROMIDE MICELLES

The dimerization of the diamide of zinc‐tetracarboxyphthalocyanine was studied spectro‐scopically in hexadecyltrimethylammonium bromide (CTAB) micelles at surfactant concentrations from 0.026 to 0.1 M and dye concentrations between 0.1 and 10 μM. The apparent dimerization constant in CTAB 0.1 M is 8.6 × 105M−1, while the intramicellar dimerization constant is 1.8 × 103. The dimer absorption spectrum was also obtained. Singlet molecular oxygen sensitization was studied by steady state photolysis using 1,3‐diphenylisobenzofurane as scavenger in 0.1 M CTAB. The usual sensitization mechanism is extended to include dimer reactions. Singlet molecular oxygen sensitization yields for monomer and dimer in the micelles are 0.7 and 0.1, respectively. With the reported values it is possible to calculate the average yield of singlet molecular oxygen production at any surfactant and dye concentrations.

CARBOXYLATED ZINC-PHTHALOCYANINE, INFLUENCE OF DIMERIZATION ON THE SPECTROSCOPIC PROPERTIES. AN ABSORPTION, EMISSION, AND THERMAL LENSING STUDY

Monomer and dimer absorption and emission spectra, and dimerization constants are reported for the diamide of the zinc‐tetracarboxyphthalocyanine in pure dimethylformamide (DMF) and in H2O/DMF mixtures at room temperature. The dimerization constant increases steadily with the water content. The monomer absorption Q‐band is insensitive to the solvent composition, whereas dimer spectra show great variations with the water content. Stationary emission measurements show that fluorescence originates exclusively from the monomers. The fluorescence spectrum as well as its absolute fluorescence quantum yield, measured by steady‐state thermal lensing, are also insensitive to the solvent composition. The thermal lensing method is discussed for the case of two absorbing species in equilibrium.

DIRECT MEASUREMENT OF THE DIPOLE MOMENT OF A METASTABLE MEROCYANINE BY ELECTROMECHANICAL INTERFEROMETRY

Abstract The permanent dipole moments of an indolinespirobenzopyran and its merocyanine from in a poly(isobutyl methacrylate) host were determined by electromechanical interferometry. This technique allows the determination of the piezoelectric and electrostrictive properties of thin films for which the polarization of the sample can be determined. The sample was exposed to an external electric field and the polarization was measured as a function of time before and after UV irradiation. Results show that the orientation of spiropyran and merocyanine molecules in the direction of the field is fast compared with the time scale of the photoisomer decay kinetics, allowing the determination of dipole moments from polarization values in the limit of thermal equilibrium. The experiments reveala pronounced increase in the dipole moment upon ring opening with μ0=6.4 D for the spirobenzopyran and μ0=14.2 D for the merocyanine form.

Europium(III) and terbium(III) trans-2-butenoates: syntheses, crystal structures, and properties

Abstract Homometallic europium(III) and terbium(III) trans -2-butenoate polymers, chemical formulas [{Eu 3 (MeCHue605CHCO 2 ) 9 (H 2 O) 4 }·H 2 O·EtOH] n ( 1 ) and [{Tb(MeCHue605CHCO 2 ) 3 (H 2 O)}·MeCHue605CHCO 2 H] n ( 2 ) and their dimeric 1,10-phenanthroline derivatives [Ln 2 (MeCHue605CHCO 2 ) 6 (phen) 2 ]·2H 2 O, with Ln=Eu ( 3 ) and Tb ( 4 ), were prepared and characterized by single-crystal X-ray diffraction. The coordination polymers 1 and 2 adopt different carboxylate-bridged infinite chain structures. While the europium compound 1 shows three independent europium centers linked to each other by three different types and number of carboxylate bridges, the terbium polymer 2 shows one independent metal center bridged by the same type of double carboxylates along the chains. Compounds 3 and 4 are isomorphous dinuclear structures with quadruply bridged carboxylate groups, two in the η 1 :η 1 :μ 2 and two in the η 2 :η 1 :μ 2 modes. The magnetic and photophysical behaviors of the four compounds are reported in the solid state.

The mechanism of the photochromic transformation of spirorhodamines

We investigate the equilibrium, kinetics, and mechanism of the photochromic transformation of a series of amido spirorhodamine compounds-differing in the nature of the substituents of the amido group and in the rhodamine chromophore-in ethanol at room temperature in the presence of trifluoroacetic acid. A proton participates in the equilibrium between the spiro form and the open rhodamine form. The relaxation times in the dark or under continuous irradiation show a linear dependence on the proton concentration. The slopes of these plots show a linear free energy relation with the equilibrium constant of the transformation. A mechanism involving reversible reaction steps between four states: the two thermodynamically stable isomers, a protonated spiro form, and a deprotonated open form, can account for the kinetic observations in the dark and under irradiation.

Luminescence quenching of Ru(II) complexes in polydimethylsiloxane sensors for oxygen

Abstract The luminescence quenching of ruthenium(II)-tris(1,10-phenanthroline) and ruthenium(II)-tris(4,7-diphenyl-1,10-phenanthroline) by oxygen in films of commercial polydimethylsiloxane was studied by steady state and time-resolved methods. The decay kinetics is not monoexponential in the absence and in the presence of oxygen. It can be accurately described in all cases by the function exp(− Bt 1/2 ), a particular case of the stretched exponential behaviour often found in solid media. The steady state and integrated time-resolved quenching efficiency coincide and show a downward curvature as a function of oxygen partial pressure. This dependence is explained by assuming a Freundlich type adsorption of oxygen in the dopants of the polymer. Both complexes render the same dependence of adsorbed oxygen on its partial pressure.

Tryptophan Quenching of Zinc‐Phthalocyanine and Porphycene Fluorescence in Micellar CTAC

Singlet excited state deactivation of a zinc phthalocyanine (ZnPc), porphycene (Po) and tetrapropyl‐porphycene (PrPo) by anionic tryptophan (Trp−) were investigated in cetyltrimethylammonium chloride (CTAC) micelles at pH 9.2 ± 0.1, regulated by a Tris buffer. Data obtained from steady‐state experiments over a wide range of detergent and added NaCl concentrations were analyzed by using a pseudophase ion‐exchange model (Abuin et al., J. Phys. Chem. 87, 5166–5172, 1983). The model was applied to derive singlet quenching rate constants for ZnPc and the porphycenes by Trp− and the selectivity coefficient for Trp/Cl exchange at the micellar surface. The results point to an electron transfer quenching. Neutral tryptophan also quenches efficiently ZnPc fluorescence in CTAC without added buffer and Trp− does not deactivate the triplet state of these dyes. By flash photolysis, only the absorption of the triplet species was detected.

Mechanistic insight into the Z–E isomerization catalysis of azobenzenes mediated by bare and core–shell gold nanoparticles

We explored the catalytic effect of 15 nm diameter gold nanoparticles (AuNPs) upon the thermal Z–E isomerization reaction of azobenzene and nine 4 and 4-4′ substituted azobenzenes (ABs). The kinetics follows a first order rate in ranges of [ABs] = 5 to 50 μM and [AuNPs] = 50 pM to 1 nM. A kinetic analysis of this compartmentalized system renders the thermal Z–E isomerization rate constant associated with each AuNP. Enhancements of 10- to 106-fold were measured for this rate constant in comparison to the same free ABs in solution. Experiments with selective Au facet coverage, as well as the kinetics studied in gold–silica core–shell nanoparticles (AuNP@SiO2) of different thicknesses, demonstrate the surface nature of the catalysis and allow one to evaluate the diffusion coefficient of azobenzene in the silica layer.

Fluorescence quenching kinetic model for a bound and partitioned quencher in micelles

Abstract A kinetic model for quenching of a fluorescent probe in micelles is analysed. The probe is totally incorporated into the micelles. The quencher distributes between water and the micellar pseudophase in two different ways; i.e. by binding and by partition. The model is analytically solved under the assumption that quenching of the probe is much faster than the quencher exchange either between the two phases or between partition and binding sites within the same micelle. Equations for the time-resolved and steady-state fluorescence intensities are derived. The model is applied to the quenching of a carboxylated zinc phthalocyanine fluorescence by diethyl-aniline in CTAC micelles.