Gian Gaetano Aloisi

Preparation and photo-physical characterisation of nanocomposites obtained by intercalation and co-intercalation of organic chromophores into hydrotalcite-like compounds

Several chromophores with donor–acceptor properties and containing a carboxylic or sulfonic group [coumarin-3-carboxylic acid (3-CCA), 9-anthracenecarboxylic acid (9-ACA), 4-benzoylbenzoic acid (4-BBA) and 2-naphthalenesulfonic acid (2-NSA)] have been intercalated into Mg–Al hydrotalcite-like compounds, both via anion exchange procedures and by using the “memory effect” of the hydrotalcites. The obtained intercalation compounds have been characterised by X-ray diffractometry, thermal analysis and chemical composition. Data obtained indicate that the guest species are accommodated in the interlayer region as a monofilm of interdigitaded anions with the C–COO− or C–SO3− bond almost perpendicular to the layer plane. The pairs of chromophores investigated, 3-CCA–9-ACA and 4-BBA–2-NSA, can give rise to energy transfer processes because of the characteristics of their excited states. Co-intercalation of the above-mentioned pairs of chromophores has been achieved with different synthetic procedures. The mono-intercalated and co-intercalated systems were investigated by absorption and emission spectroscopy and by laser flash photolysis. The absorption spectrum of MgAl-9-ACA shows an unexpected band around 490 nm that was attributed to an aggregate. The fluorescence characteristics of the various co-intercalated samples depend on the excitation wavelength and on the preparation methods, which control the relative amounts of the two fluorophores in the interlayer region. In all the samples containing 9-ACA excitation around 500 nm generates an emission attributed to the anthracene aggregate. In mono-intercalated and co-intercalated samples, the fluorescence decays satisfactorily fit bi-exponential and tri-exponential functions, respectively. Laser flash photolysis experiments showed that excitation of the intercalated chromophores produces transients which can be attributed to the guest triplets and, in the case of co-intercalated 4-BBA–2-NSA composite, the probable occurrence of an energy transfer process.

Photoinduced electron transfer between styrylanthracenes and electron donors and acceptors in acetonitrile

The decay pathways of the lowest singlet and triplet excited states (1trans* and 3trans*) of trans-n-styrylanthracenes (n-StA, where n= 1, 2 or 9 on the anthracene) have been studied in acetonitrile at room temperature. Fluorescence lifetimes (τF) and quantum yields (ϕF), as well as the yield and spectral and kinetic properties of the lowest triplet state of the three StAs, were determined by steady-state and transient techniques. The formation and the decay of the respective StA radical cations (trans˙+) were observed by laser flash photolysis; the yield of photoionization is ca. 0.07 on 353 nm excitation and is enhanced by fluorescence quenching with 1,4-dicyanobenzene. The formation and decay of the StA radical anions (trans˙–) in the presence of diethylaniline (DEA) is concluded from transient conductivity (Δκ) and optical results. DEA significantly enhances the yield of trans˙– and the initial amplitude of Δκ and correspondingly quenches ϕF. The bimolecular interaction between 1trans* and 4-bromodimethylaniline enhances the triplet population.

Excited state properties and in vitro phototoxicity studies of three phenothiazine derivatives

Abstract This work concerns a combined photophysical, photochemical and photobiological study of three drugs (psychotherapeutic agents) of the phenothiazine series: perphenazine, fluphenazine hydrochloride and thioridazine hydrochloride. The excited-state properties were first investigated by stationary and time-resolved fluorimetry and by laser flash photolysis. The spectral description was assisted by quantum-mechanical calculations with the INDO/1-CI method. In organic media the lowest excited singlet state was found to decay by fluorescence (small quantum yield) and mainly by intersystem crossing to the lowest triplet state, which is responsible for oxygen photosensitization (high yields of singlet oxygen production) and photodegradation. A further decay pathway in aqueous solutions was the photoionization process, which led to the formation of the phenothiazine radical cations and the solvated electron. After the preliminary study of the photobehavior in organic solvents and in water, the phototoxicity of the three drugs was investigated on various biological substrates through a series of in vitro assays under UVA irradiation. Photohemolysis of mouse erythrocytes and phototoxicity on cultured murine fibroblasts were observed for all three compounds. Lipid photoperoxidation was then investigated using linoleic acid as the unsaturated lipid model and isolated red blood cell membranes. The drug-induced photodamage was also evaluated on proteins by measuring the photosensitizing cross-linking in erythrocyte ghosts. The combined approach proved to be useful in understanding the mechanism by which these phenothiazine derivatives induce skin photosensitization. In particular, the photophysical properties of the compounds under investigation and the results of the study on their phototoxicity are in agreement with a mechanism that involves the radical cation of the drugs as a main intermediate.

Space-resolved fluorescence properties of phenolphthalein-hydrotalcite nanocomposites

Phenolphthalein has been included into Zn–Al hydrotalcite-like compounds by taking advantage of the “memory effect” of hydrotalcites. In particular [Zn0.65Al0.35(OH)2] (CO3)0.1750.5H2O was calcined at 500 °C and the obtained mixture of Zn and Al oxides was dispersed in an aqueous alcohol solution containing 7 × 10−2 mol dm−3 phenolphthalein. The reconstruction of the layered hydrotalcite structure in the presence of the dye yielded formation of the composites in which phenolphthalein is intercalated in the interlayer region and/or adsorbed on the layer surface. Two samples were studied, the first, obtained at 25 °C, contained 0.17 mmol g−1 of phenolphthalein, mainly adsorbed on the surface, the second, obtained at 60 °C, contained 0.3 mmol g−1, in part intercalated. The samples were characterized by their X-ray diffraction patterns, specific surface areas and scanning electron micrographs. The photophysical characterisation of the bulk samples was based on the determination of their reflectance absorption and fluorescence spectra, the fluorescence lifetimes and fluorescence anisotropy. The results indicated different properties of the lowest singlet excited state of the adsorbed and intercalated dye. The space-resolved fluorescence images and fluorescence spectra obtained by confocal fluorescence spectroscopy of the two samples gave valuable information on the dye distribution and on the nature of the interactions between the dye and the inorganic matrix.

Photophysical properties of N-alkylated azahelicene derivatives as DNA intercalators: counterion effects.

In this work, three compounds having the same organic moiety (N-methyl-5-azahelicenium salts) but different counterions (I-, NO3- and COOCF3-) have been investigated in buffered aqueous solutions and in the presence of DNA to give information on the counterion effects on the binding. In particular, the absorption spectra, fluorescence quantum yields and fluorescence lifetimes in aqueous solution for free organic molecules have been determined by steady-state and time-resolved spectrofluorimetric measurements. The obtained values are compared with those of the chromophores in the presence of increasing concentrations of DNA. The results allow determination of the association constants (K(a)) and the number of base couples per chromophore molecule (n) by means of the McGhee Von Hippel model. The binding parameters are strongly affected by the nature of counterions since the highest K(a) value was determined for the compound having COOCF3-; on the other hand the NO3- derivative is able to interact with the highest number of binding sites. The morphology and structural properties of the DNA-chromophore complexes were investigated by circular dichroism (CD) and atomic force microscopy (AFM). The data revealed that I- and COOCF3- derivatives preferentially form intercalation complexes, while the NO3- salt is able to form intercalation and grove binding complexes at the same time.

Preparation and characterization of zirconium phosphonate–azobenzene intercalation compounds. A structural, photophysical and photochemical study

The physical and chemical photo-responses of azobenzene were employed to investigate the ‘nanoscopic’ properties of a layered zirconium phosphonate of formula ZrF(O3PCH2)2NHC8H17 (ZrC8, interlayer distance 1.82 nm), a solid belonging to a new family of zirconium diphosphonates with a poorly hindered interlayer region. Two samples were prepared: an intercalation compound containing 0.8 mol of dye per mol of zirconium, having an interlayer distance of 2.82 nm; and a compound in which the azobenzene was adsorbed on the surface of the host. The photobehaviour of the samples was examined by absorption, fluorescence and photochemical techniques and compared with those of the dye in the solid state and in solution. Intercalated or adsorbed azobenzene is able to fluoresce not only from the lowest excited singlet state 1(n,π*) but also from the upper one 1(π,π*), as observed in the solid dye. However, differently from what happens in the pure solid dye, the trans→cis photoisomerization of azo-groups occurred in both intercalated and adsorbed samples upon UV irradiation. The structural changes associated with the photoisomerization process in the interlamellar region induced an irreversible deintercalation of the dye, which was detected as adsorbed on the host surface or present in an amorphous phase. Space-resolved properties of the ZrC8–azobenzene materials were investigated by confocal fluorescence microscopy.

Fluorescence of conformational isomers of trans 2-styryl-naphthalene. Further evidence for different radiative decay parameters of the two rotamers

Abstract The fluorescence properties of the conformer mixture of trans 2-styrylnaphthalene in inert solvents have been reinvestigated. The temperature, excitation wavelength and oxygen quenching effects on fluorescence spectra, quantum yields and decay profiles have been carefully examined. The results obtained give further confirmation to our previous fluorescence analysis thus supporting the assignment of different radiative decay parameters to the two conformers.

Photophysical Properties of Quinolizinium Salts and Their Interactions with DNA in Aqueous Solution

The photophysical properties of three quinolizinium salts (naphto[2,1-b]quinolizinium bromide (Q2), naphto[1,2-b]quinolizinium bromide (Q3), and indolo[2,3-b]quinolizinium tetrafluoroborate (HI)) in fluid media and their interactions with DNA were investigated by steady-state and by nanosecond and femtosecond time-resolved techniques. The main decay pathways of the excited singlet state S(1), fluorescence, intersystem crossing, and internal conversion, were characterized in terms of quantum yields and rate constants. The lowest triplet state of the quinolizinium salts is able to sensitize singlet oxygen in rather high efficiency (phi(Delta) = 0.4 to 0.5 in MeCN). The complexes between quinolizinium salts and DNA formed in the ground state were characterized in terms of lifetimes and decay channels to give more details of the mechanism of photoinduced DNA strand break.

Photophysical and photobiological behavior of antimalarial drugs in aqueous solutions.

This article describes the results of a combined photophysical and photobiological study aimed at understanding the phototoxicity mechanism of the antimalarial drugs quinine (Q), quinacrine (QC) and mefloquine (MQ). Photophysical experiments were carried out in aqueous solutions by stationary and time‐resolved fluorimetry and by laser flash pbotolysis to obtain information on the various decay pathways of the excited states of the drugs and on transient species formed on irradiation. The results obtained showed that fluorescence and intersystem crossing account for all the adsorbed quanta for Q and MQ (quantum yield of about 0.1 and 0.9, respectively) and only for 24% in the case of QC, which has a negligible fluorescence quantum yield (0.001). Laser flash photolysis experiments evidenced, for QC and, MQ, the occurrence of photoionization processes leading to the formation of the radical cations of the drugs. The effects of tryptophan and histidine on the excited states and transient species of the three drugs were also investigated. In parallel, the photoactivity of the antimalarial drugs was investigated under UV irradiation on various biological targets through a series of in vitro assays in the presence and in the absence of oxygen. Phototoxicity on 3T3 cultured fibroblasts and lipid photoperoxidation were observed for all the drugs. The photodamage produced by the drugs was also evaluated on proteins by measuring the photosensitized cross‐linking of spectrin. The combined approaches were proven to be usefulfor understanding the mechanism of phototoxicity induced by the antimalarial drugs.

Time-resolved fluorescence photophysics of trans-stilbene in a DPPC lipid bilayer: evidence for a free rotation, location within two sites and a pre-liquid crystalline phase transition

Abstract The photophysical properties of the first two members of the diphenylpolyene series are reported in a phospholipid bilayer membrane using time-resolved fluorescence anisotropy. A contrasting photophysical behaviour is noted for the time-resolved fluorescence anisotropy where a free rotation with a long rotational correlation time of ∼ 10 ns for trans-stilbene in the gel phase of the bilayer at 35°C is in sharp contrast to the restricted motion observed for trans,trans-diphenylbutadiene (DPB) and other diphenyl-polyenes. For trans-stilbene bi-exponential fluorescence decay components in the gel phase (e.g. 0.40 ns and 1.12 ns at 30°C) indicate two sites within the bilayer. The mean rotational correlation time and fluorescence decay components are found to decrease with increasing temperature prior to the main phase transition. Together these provide valuable information on the changing “microviscosity” of a lipid bilayer membrane in the pre-transition region leading to the melting of the main chain. Traditional fluorescence probes offer no information on this region.