71 research outputs found
Real-Time Acquisition and Preprocessing of Kinetic and Spectroscopic Data in Laser Flash Photolysis
No full textAn automatic flash photolysis system for the measurement of absorption decays and transient difference spectra is described. The apparatus performs a digital measurement of the light transmitted by the sample and a few on-line preprocessing operations on the measured data, such as the digital averaging of the signal and the computation of the change in absorbance. A boxcar averager, used as a gated integrator, constitutes the sampling head of the apparatus. A microprocessor-based acquisition and control unit performs a digital conversion of the value sampled by the boxcar, provides an on-line preprocessing of the data, and supervises every operation required by the measurement process. This unit is interfaced to a personal computer which allows a friendly interaction with the operator as well as the on-line display of the data. The electronic instrumentation features a time resolution of a few nanoseconds, with a boxcar averager as input head, and of a few hundreds of picoseconds, with a sampling oscilloscope as input hea
Aggregation modes of anionic oxacarbocyanines with polycations in solution and in ESAMs
No full textInteraction of oxacarbocyanines D-G with three polycations in aqueous solutions results in the formation of two types of likely small, distorted aggregates rather than the classical J aggregates. On the contrary, the latter are extensively and almost exclusively obtained in electrostatically self-assembled multilayers (ESAMs) prepared by alternate polycation/dye adsorption on quartz substrates. The J-aggregate growth on supported polycations is qualitatively shared by the four cyanines, a fact that reveals the crucial role of the double anionic substitutions on the dyes. On the other hand, films with D and E, which are known to have a stronger tendency to give dimers in water, exhibit higher J-band intensities and stability upon drying relative to those with F and G. Based on these observations, we suggest that energetic factors associated with cofacial dye/dye van der Waals interactions, ultimately related with the degree of planarity of the conjugated chromophores, may still play a major role in controlling aggregation equilibria in these complex systems
GENERATION OF SINGLET OXYGEN BY 2,2'/5',2''-TERTHIOPHENE AND SOME OF ITS DERIVATIVES
No full textThe kinetics of oxygen quenching of the lowest triplet states of 2,2':5',2#-terthiophene and several of its substituted derivatives in 95% ethanol were analysed as part of an investigation of the chemical and physicochemical basis of the phototoxicity of these compounds. Overall bimolecular rate constants and singlet oxygen (O-2((1) Delta(g))) formation quantum yields were measured by laser flash photolysis and time-resolved phosphorescence detection. With the exception of one compound, both parameters proved to be almost independent of the chemical nature, position and number of substituents, thus failing to indicate an explanation for the very large differences in phototoxicity observed between the various derivatives during in vivo experiments. On the basis of the measured kinetic parameters, a schematic description of the mechanism of interaction with oxygen of these compounds in their lowest triplet states is proposed
Theoretical conformational analysis of p-, m-, and o-terphenyl
No full textA ground state conformational analysis has been carried out on p-, m-, and o-terphenyl using a recently developed semiempirical calculation method (CS-INDO). The potential energy surface has been calculated as a function of the torsion angles of the two terminal phenyls relative to the central one. Two nearly isoenergetic rotamers are found to exist for both p- and m-terphenyl, at geometries corresponding to D2, and C2h symmetries for p-terphenyl, C2, and Cs symmetries for m-terphenyl. The torsion angles are ca. 35° in all cases. The barriers separating the two minima of each potential surface are low enough to allow a rapid interconversion of the rotamers at room temperature in the gas phase (they are in “dynamic” equilibrium). o-Terphenyl presents only one equilibrium geometry. These findings are compared with the results of previous experimental and theoretical studies, which are extensively reviewed
SINGLET TRIPLET INTERSYSTEM CROSSING IN 2,2' 5',2''-TERTHIOPHENE AND SOME OF ITS DERIVATIVES
No full textDuring the course of a study devoted to the investigation of the existence of a relationship between the phototoxic and photophysical properties of 2,2':5',2''-terthiophene (1a) and related compounds, the singlet-to-triplet intersystem crossing quantum yields of compound la and six of its monosubstituted derivatives (1b-1g) were determined in 95% ethanol solution. The variations in the phototoxicity of these compounds against some insect and mite species are large in comparison with changes in the quantum yields of triplet state formation. The latter were very similar in the compounds studied (range, 0.88-0.99), with the exception of 5-formyl-2,2':5',2''-terthiophene. (1c) (PHI(T)=0.50). Moreover, a complete characterization of the S1 decays of compounds la-1g was obtained by combining these data with the corresponding fluorescence quantum yields and lifetimes. Finally, the consequences of the nature and position of the substituents on the measured triplet-triplet absorption and rate constants of the two S1 decay paths, i.e. fluorescence emission and intersystem crossing, are qualitatively discussed
Intrinsic Fluorometric Reporters of Pteridine Reductase 1, a Target for Antiparasitic Agents
No full textThe intrinsic steady-state and time-resolved fluorescence of Leishmania major pteridine reductase 1, a tetrameric protein target for anti-infective agents, is investigated and deciphered in terms of the contributions from populations of the two tryptophans included in each protein monomer. Signals from these local fluorometric reporters contain molecular-level information on the conformational landscape of this protein and on its interaction with a nanomolar pteridinic inhibitor
Consequences of H-dimerization on the photophysics and photochemistry of oxacarbocyanines
No full textThe photophysical/photochemical behaviour of the monomers and the H dimers of four oxacarbocyanines (dyes D-G in the scheme) was investigated in water. In contrast with the usually observed effect of H dimerization, the dimers of dyes D-G were found to fluoresce with efficiencies comparable to or larger than those of the corresponding monomers. Analysis of the decay paths of the lowest excited singlet state showed, however, that dimerization causes a decrease of the radiative rate constants and an enhancement of intersystem crossing to the triplet manifold, as expected from application of exciton theory to a model H dimer. Twisting about one of the polymethine bonds contributes to the decay from the spectroscopic minimum of monomers, though yielding a rather small amount of a distorted cis isomer. The process is inhibited in dimers, likely due to a pronounced increase of activation energy connected with a loss of van der Waals attractive energy at the twisted geometry
Linear and nonlinear optical properties of V-shaped D-π-A-π-D chromophores: Effects of the incorporation of aromatic rings in the polyenic π-bridges of open-chain ketocyanines
No full textFollowing previous studies on a and b polarizabilities of ketocyanines, a subgroup of D–p–A–p–D quadrupolar
chromophores with moderately V-shaped structure, the present work analyses the effects of modifying the
p-bridges connecting the D (NMe2) and A (CO) groups. This aim is pursued through a detailed comparison
between the previously studied ketocyanines (KC2, KC3) and a Michler’s ketone analogue (KM1) bearing
styrenic (in the place of polyenic) p-bridges. First, we report a spectroscopic study, including absorption
and fluorescence anisotropy spectra, aimed to probe the electronic peculiarities of KM1 as well as to
derive consistent three-state model (TSM) parameters for the three compounds. The paper goes on with
an extensive theoretical study, carried out in the framework of the density functional theory (DFT),
encompassing the structure, the electronic spectrum, a and b polarizabilities and two-photon absorption
(TPA) cross-sections (sTP). Calculations performed according to the sum-over-states (SOS) approach are
discussed with reference to the performances of few-state descriptions, it is shown that such
descriptions (including TSM), which have been proved to be quite reliable in the case of KC2 and KC3,
lose their effectiveness with KM1 because of the electronic characteristics related to the styrenic
p-bridges. As to the TPA cross-sections, the results of TSM and SOS approaches concerning the TSM
g - c and g - e transitions are supplemented by those obtained using the quadratic response theory.
A common qualitative conclusion, traceable to the degree of bending of the V-shaped structure, is that
in the case of KM1 the allowed (g - e) and the ‘‘forbidden’’ (g - c) transitions both should be
observable in the TPA spectrum, as confirmed by experiment
Spectroscopic study of chromophores in ancient HIMT glass
No full textWe have studied a series of Late Roman glass fragments belonging to the peculiar group called HIMT (High Iron Magnesium Titanium glass), characterized by different color nuances, and previously chemically characterized [1-3]. XANES, UV-VIS and luminescence spectroscopies [4-6] are exploited for determining the distribution of the oxidation states of the two chromophores Fe and Mn in the ancient finds and in two synthetic glass specifically prepared with selected chemically compositions. Fe and Mn K-edge XANES spectra were collected in fluorescence mode at the GILDA-CRG beamline (ESRF, Grenoble, France). A dynamically and sagittally focussing monochromator with Si (311) crystals was used. Energy calibrations were achieved using Fe and Mn foils as references and the position of the first inflection point was taken at 7112.0 and 6538.0 eV respectively. The pre-edge region was extracted from the normalized spectra and analysed by least-square fitting of pseudo-Voight functions of equal width to the pre-edge spectral envelope. For each sample, the pre-edge centroid was calculated from the average position of the pseudo-Voigt functions, weighted by their respective integrated areas. UV-VIS absorption spectra were measured on a Varian Cary 100 double-beam spectrophotometer operated between 900 and 200 nm. Luminescence emission and excitation spectra were collected in a Spex Jobin-Yvon FluoroMax3 spectrofluorometer. On the basis of both XANES and UV-VIS results, in all the ancient glass samples Fe results to be mostly in its oxidate state 3+. XANES data indicate that the prevalent Mn oxidation state is 2+, however the presence of minor amounts of Mn3+ has been proved by UV-VIS and luminescence spectroscopy. The different spectroscopic techniques, used in a combined approach, are able to interpret the apparent anomalous colour of some of the ancient glass samples
Large third-order nonlinear optical response of porphyrin J-aggregates oriented in self-assembled thin films
No full textThe preparation and characterization of a self-assembled material showing a high nonlinear response and good photostability to ultrashort laser pulses is presented. The material is built by alternate deposition of tetrakis(4-sulfonatophenyl) porphyrin diacid (H4TPPS2-) and poly(diallyldimethylammonium chloride) (PDDA) forming electrostatically self-assembled multilayers (ESAMs). UV-visible absorption and emission experiments show that in this matrix H4TPPS2- is present mainly in its J-aggregated form. Furthermore, linear dichroism experiments on a 3 bilayer film show a preferential alignment of the porphyrin aggregate with the J-band transition dipole moment parallel to the film surface. The two photon absorption (TPA) properties of these films are investigated with the Z-scan technique at 806 nm, employing 130 fs pulses. The samples exhibit strong nonlinearities with a very large two-photon absorption coefficient beta(TPA) of 50 cm GW(-1). The origin of this large response is investigated. It has been already demonstrated that aggregation enhances the molecular TPA cross section of H4TPPS2- from 30 to 1000 GM in water solution thanks to cooperative effects. In a 20 bilayer film a further increase by a factor of 1.7 is observed and explained in terms of preferential alignment of J-aggregates in the multilayer
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