Gianna Favaro

Static and dynamic interaction of a naturally occurring photochromic molecule with bovine serum albumin studied by UV-visible absorption and fluorescence spectroscopy.

In this work, the interaction of a naturally occurring chromene, flindersine (FL), and bovine serum albumin (BSA) has been investigated by UV-vis absorption and fluorescence spectroscopy, time-resolved lifetime measurements, steady state photochemistry, and semiempirical calculations. The interplay of FL with tryptophan (Trp) has been studied in parallel. The interaction of FL with BSA causes fluorescence quenching of BSA through both static and dynamic quenching mechanisms. FL binds BSA with a stoichiometry that varies from 1.09:1 to 0.80:1 as the temperature increases from 293 to 308 K. The reaction is characterized by negative enthalpy (deltaH degrees = -193 kJ mol(-1)) and negative entropy (deltaS degrees = -550 J K(-1) mol(-1)), indicating that the predominant forces in the FL-BSA complex are hydrogen bonding and van der Waals forces. The binding distance between the protein and the photochrome was calculated as 2.5 nm, according to the Foerster theory on resonance energy transfer. The effect of FL concentration on the BSA fluorescence was analyzed according to the maximum entropy method. FL also quenches the emission of Trp with a mechanism that, based on the experimental evidence, excludes both static and dynamic effects. An alternative relaxation pathway, consisting in an electron transfer from a prefluorescent state of Trp to FL, is put forward. The photobehavior of FL is affected by the interplay with BSA but not with Trp. When FL is complexed with BSA, it becomes a more fluorescent and more reactive species. Semiempirical calculations of the lowest optically active electronic transitions of hypothetical FL photoproducts suggest the most likely structure for the photoproduct.

A spectrophotometric and fluorimetric study of some anthraquinoid and indigoid colorants used in artistic paintings

Abstract The present study was undertaken to investigate the potential of spectrophotometric and fluorimetric techniques for identifying the materials used in artistic paintings. Two classes of organic colorants were examined, anthraquinoid and indigoid dyes, in their naturally occurring and synthetic forms. Absorption and fluorescence spectra were recorded in both solution and solid layer using linseed oil as a binder. Fluorescence quantum yields were determined. A non-destructive, instrumental set-up to record fluorescence spectra on painted surfaces was successfully tested. The solvent effects on the spectra of the dyes are interpreted in terms of intra- and inter-molecular hydrogen-bonding interactions. Molecular aggregation of indigo was investigated in dichloroethane solution and assigned to a dimer. The spectra recorded from the painted surfaces are broader than in solution. However, the emission spectra are still suitable to identify the colorants.

Acidichromic effects in 1,2‐di‐ and 1,2,4‐tri‐ hydroxyanthraquinones. A spectrophotometric and fluorimetric study

Absorption and fluorescence spectra of alizarin (1,2-dihydroxyanthraquinone) and purpurin (1,2,4-trihydroxyanthraquinone) were investigated as a function of pH in water–dioxane (2:1, v/v) in the pH range 2–14. Absorbance changes with pH are accompanied by color changes; by increasing the pH, the spectra shift to the red. These molecules exhibit fluorescence emissions over the whole pH range explored; fluorescence quantum yields increase with increasing pH and are greater for purpurin (ΦF = 10−3–10−2) than for alizarin (ΦF = 10−4–10−3). The fluorescence lifetimes were measured by the phase-shift technique for the three species derived from purpurin (neutral, τ = 150 ps; mono-anion, τ = 2600 ps and di-anion, τ = 380 ps), and only for the di-anion of alizarin (τ = 80 ps). Both spectrophotometric and fluorimetric titrations gave pKs of the first and second dissociation steps in the ground state. Excited-state pK*s were calculated by the Forster cycle. In the ground state, purpurin (pK1 = 4.7; pK2 = 9.5) is a stronger acid than alizarin (pK1 = 6.6; pK2 = 12.4) in both the first and second dissociation steps; in the excited state, purpurin is a weaker acid in both dissociation steps. Copyright

Photochromism, thermochromism and solvatochromism of some spiro[indolinoxazine]-photomerocyanine systems: effects of structure and solvent

Three spiro[indoline-naphthoxazines] and a spiro[indoline-phenanthroxazine], which exhibit photochromic and thermochromic properties, have been investigated. Solvent and structure effects on the absorption spectra of the merocyanines produced under UV irradiation and kinetic parameters for the ring-closure and ring-opening reactions were studied. Positive solvatochromism was found, indicating that the opened form is a weakly polar species. Equilibrium constants and rate constants for the forward and back reactions spiroxazine ⇄ merocyanine increase with increasing the solvent polarity and with electron-donating groups in the oxazine moiety. The reaction is endothermic by 10–20 kJ mol–1 and almost isoentropic. The activation entropy is generally negative, while the activation Gibbs energy is approximately independent of solvent and structure.

Spectrokinetic study of 2,2-diphenyl-5,6-benzo(2H)chromene: a thermoreversible and photoreversible photochromic system

Abstract In this paper the photochromic reaction of the 2,2-diphenyl-5,6-benzo(2H)chromene is investigated by spectrophotometric methods using monochromatic irradiation wavelengths in the UV and visible ranges. Two coloured species are produced upon irradiation, one is thermoreversible, the other is photoreversible. The colour-forming kinetics were followed under steady irradiation using UV light (357 nm). The kinetic parameters (rate coefficient and activation energy) of the thermal ring-closure reaction were determined. The photochemical bleaching was induced by exciting with visible light (422 nm). To determine reaction quantum yields and molar absorption coefficients of the metastable species, both photostationary and photokinetic methods were employed. Based on the results obtained, a plausible reaction mechanism is proposed

pH-Induced effects on the photophysics of dipyridyl ketones

The photophysics of dipyridyl ketones (DPKs) has been investigated in aqueous solutions as a function of pH using pulsed and steady-state emission spectroscopy and nanosecond laser flash photolysis. All six DPK isomers were found to emit phosphorescence in aqueous frozen solutions at 77 K, but only three of them exhibited phosphorescence emissions and showed triplet transient absorption at room temperature. From both phosphorescence and triplet–triplet absorption titrations, they revealed proton-transfer reactions in the excited state occurring at different pH/H0 values than the ground-state acid–base equilibrations. Comparing results from different techniques gave evidence that the carbonyl oxygen is involved in the triplet state protonation (pK*≈ 3.5–6) for the isomers where it conjugates with the ring nitrogen atom, while proton addition prevalently occurs on the nitrogen for the unconjugated meta–meta isomer. A second protolytic equilibrium (pK*≈–2 to –1.6) implies formation of nitrogen diprotonated ions for all the isomers. The presence of two positive charges in the molecules prevents further protonation at the carbonyl group in the ground states as well as in the excited states.

Photochromic, Thermochromic, and Fluorescent Spirooxazines and Naphthopyrans: A Spectrokinetic and Thermodynamic Study

The photochromic and thermochromic behavior of four commercially available Reversacol dyes are presented. The compounds studied belong to the class of spirooxazines and naphthopyrans, which are typically thermoreversible photochromic molecules. On stimulation with UV light, these compounds become colored and exhibit spectra which extend over the whole visible region. Increasing the temperature causes spontaneous coloration (thermochromism). Herein, absorption and fluorescence spectra, molar absorption coefficients of the colorless and colored forms, fluorescence and photochemical quantum yields, and kinetic parameters of thermal bleaching (rate constant, frequency factor, and activation energy) are determined in acetonitrile solution. The thermal ground-state reaction is exhaustively described in terms of thermodynamic parameters (equilibrium constant, free energy, enthalpy, and entropy). Temperature effects on photochemical and thermal colorabilities are evaluated. The results indicate that the two spirooxazines are good photochromes below room temperature, whereas they are efficient thermochromic compounds above room temperature. Naphthopyrans are better photochromes but worse thermochromic compounds than spirooxazines.

Photochromic Behavior of 2,2-Spiro-adamantylidene-2H-naphtho[1,2-b]pyran: A New Thermoreversible and Photoreversible Photochromic System¶

The photokinetic behavior of a photochromic compound, 2,2‐spiro‐adamantylidene‐2H‐naphtho[1,2‐b]pyran (Py), has been investigated by using monochromatic irradiation in the UV and visible ranges. An unusually complex photochemistry occurs whereby two colored forms (o‐quinone–allides) are produced, one of which is thermoreversible whereas the other is not thermoreversible but photoreversible. These are the result of ring opening of the pyran C–O bond to a short lived open‐form intermediate which converts to the two different colored forms by twisting around different C–C bonds. Their spectra and molar absorption coefficients were obtained in acetonitrile solution. The kinetic parameters of the thermal bleaching (rate constant and activation energy) and photobleaching (quantum yield) were measured. A plausible reaction mechanism is proposed. Based on this mechanism, mathematical methods were devised which were capable of analyzing the system during its dynamic evolution and evaluating the quantum yields of the color‐forming photoreactions.

Photokinetic methods: A mathematical analysis of the rate equations in photochromic systems

In this article the analytical integration of kinetic equations describing the dynamic behavior of reversible photoreactions has been undertaken. Photochemical systems giving rise to competing thermal and photochemical steps are kinetically analyzed; the rate law is set up and analytical solutions are obtained under precisely defined boundary conditions. More complicated kinetic systems, where several species interact both thermally and photochemically, are also investigated. The kinetic treatment leads to a system of coupled differential equations which are amenable to analytical solutions, under the appropriate experimental and boundary conditions. The usefulness of the equations developed is illustrated by their application to some spirooxazine and chromene photochromic systems: two examples are described in detail.

Unexpected chromogenic properties of 1,3,3-trimethylspiro(indoline-2,3′-[3H]naphtho [2,1-b][1,4]oxazine) in the solid phase: photochromism, piezochromism and acidichromism

New chromogenic properties of a spirooxazine {1,3,3-trimethylspiro(indoline-2,3′-[3H]naphtho[2,1-b][1,4]oxazine), SO} are presented in this work. This molecule was found to be photochromic and piezochromic in the pure microcrystalline solid phase and also exhibited acidichromism when kept in contact with a hydrogen-donating solid phase. Photochromism was observed under continuous UV irradiation. Biexponential thermal bleaching followed the interruption of irradiation. Piezochromism was detected by applying a mechanical force to the solid powder, using either a hydraulic press or manual crushing. After discontinuing pressure, biexponential bleaching kinetics were observed. The biexponential nature of the fading processes is interpreted in terms of merocyanine (M) micro-spot formation inside the crystalline phase. The third chromogenic property of the solid spirooxazine, acidichromism, is due to the extraction of protons from an acidic solid matrix, alpha zirconium phosphate [α-Zr(HPO4)2·H2O]. Proton transfer to SO in the solid phase causes the appearance of a purple colour. The spectral evolution is qualitatively similar to that observed in CH3CN solution when HClO4 is added at room temperature and in the dark. Temporal evolution of the colour bands, in both solid and liquid phases, is interpreted in terms of an initial protonation, inducing ring-opening, and a second protonation, leading to a colourless product.