Fausto Elisei

Photophysical Properties and Antibacterial Activity of Meso-substituted Cationic Porphyrins¶

A series of derivatives of 5,10,15,20‐tetrakis‐(4‐N‐methylpyridyl)‐porphine, where one N‐methyl group was replaced by a hydrocarbon chain ranging from C6 to C22, were characterized for their photophysical and photosensitizing properties. The absorption and fluorescence features of the various compounds in neutral aqueous solutions were typical of largely monomeric porphyrins, with the exception of the C22 derivative, which appeared to be extensively aggregated. This was confirmed by the very low triplet quantum yield and lifetime of the C22 derivative as compared with 0.2–0.7 quantum yields and 88–167 μs lifetimes for the other porphyrins. The photophysical properties and photosensitizing activity toward N‐acetyl‐l‐tryptophanamide of the C22 porphyrin became comparable to those typical of the other derivatives in 2% aqueous sodium dodecyl sulfate, where the C22 compound is fully monomerized. All the porphyrin derivatives exhibited at micromolar concentrations photoinactivation activity against both Staphylococcus aureus and Escherichia coli, even though the gram‐negative bacteria were markedly less photosensitive. The photosensitizing efficiency was influenced by (1) the amount of cell‐bound porphyrin, which increased with increasing length of the hydrocarbon chain; and (2) the tendency to undergo partial aggregation in the cell, which seems to be especially important for the C22 derivative.

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.

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.

Intramolecular Charge Transfer of Push–Pull Pyridinium Salts in the Singlet Manifold

The solvent effect on the photophysical and photochemical properties of the iodides of three trans (E) isomers of 2-D-vinyl,1-methylpyridinium, where D is a donor group (4-dimethylaminophenyl, 3,4,5-trimethoxyphenyl and 1-pyrenyl), was studied by stationary and transient absorption techniques. The results obtained allowed the negative solvatochromism and relaxation pathways of the excited states in the singlet manifold to be reasonably interpreted. Resorting to ultrafast absorption techniques and DFT calculations allowed information on the excited state dynamics and the role of the solvent-controlled intramolecular charge transfer (ICT) processes to be obtained. The structure-dependent excited state dynamics in nonpolar solvents, where the ICT is slower than solvent rearrangement, and in polar solvents, where an opposite situation is operative, was thus explained. The push-pull character of the three compounds, particularly the anilino-derivative, suggests their potential application in optoelectronics.

Ultrafast photoinduced intramolecular charge transfer in push–pull distyryl furan and benzofuran: solvent and molecular structure effect

The excited state deactivation pathways of push-pull distyryl furan and benzofuran derivatives in several organic solvents were investigated in detail by using time-resolved transient absorption and fluorescence spectroscopies, with nano- and femto-second time resolution. Solvent polarity was found to play a key role in determining the efficiencies of fluorescence, intersystem crossing and internal conversion. The triplet yield gradually decreased, while the internal conversion increased upon increasing the solvent dielectric constant. However the fluorescence showed a different solvent polarity effect in the low and high solvent polarity region, with a reversal of the trend of fluorescence properties (quantum yield and lifetime). This fact points to an emitting state of a different nature (smaller and larger dipole moments) in the two cases, as also suggested by the huge fluorosolvatochromism. In fact the ultrafast spectroscopic investigation evidenced the presence of two transients characterized by peculiar spectral shapes assigned to a locally excited (LE) and a charge transfer (CT) state. In the more polar solvents the CT state was the longer lived, fluorescent one and an intramolecular charge transfer process was found to be operative and to become faster (up to ∼200-250 fs) in the higher polarity media. On the contrary, distyrylfuran, which exhibits the same molecular skeleton without the push-pull character showed a similar excited state dynamics in solvents of different polarities.

Excited state behaviour of some trans-stilbene analogues bearing thiophene rings

Abstract The effect of replacing the phenyl group(s) of trans -stilbene with thienyl groups, or polycyclic groups containing a thienyl moiety, on the relaxation properties of the lowest excited states was studied by fluorometric, photochemical and laser flash photolysis techniques, as well as by theoretical calculations, for four trans compounds in non-polar and polar solvents. In some cases, a larger contribution of intersystem crossing and, consequently, a triplet mechanism to trans → cis photoisomerization, with respect to the parent hydrocarbons, was found. Although the compound with a single thienyl group, 2-styrylthiophene, shows reactive relaxation in the singlet manifold as in the case of stilbene, the presence of two heteroaromatic rings in di(2-thienyl)ethene enhances the spin-orbit coupling, thus leading to a mixed singlettriplet mechanism in non-polar solvents. The presence of polycondensed rings in dibenzothienylethene and thienyl-naphthothienylethene reduces the isomerization yield due to an increase in the torsional barrier for twisting in the singlet manifold. Therefore these compounds deactivate mainly through fluorescence emission and intersystem crossing, which leads to a predominant triplet mechanism for trans → cis photoisomerization. Polar solvents reduce the activation barrier to twisting, thus favouring isomerization in the singlet manifold.

Ion-forming processes on 248 nm laser excitation of benzophenone in aqueous solution: a time-resolved absorption and conductivity study

Abstract The laser flash photolysis (excitation at 248 nm) of benzophenone in aqueous solution was investigated using absorption and conductivity measurements. In addition to intersystem crossing to the ground state, benzophenone triplets undergo triplet—triplet (T—T) annihilation partly by an electron transfer reaction leading to cation and anion radicals. A second ion-forming process is two-photon photoionization which leads to radical cations and hydrated electrons. Secondary reactions, involving the protolytic equilibrium between ketyl radicals and anion radicals, are observed and discussed.

Comprehensive Photophysical Behaviour of Ethynyl Fluorenes and Ethynyl Anthracenes Investigated by Fast and Ultrafast Time-Resolved Spectroscopy

Detailed investigations by time-resolved transient absorption and fluorescence spectroscopies with nano- and femtosecond time resolutions are carried out with the aim of characterising the lowest excited singlet and triplet states of three ethynyl fluorenes (1-3) and three ethynyl anthracenes (4-6) in solvents of different polarity. The solvent is found to modify the deactivation pathways of the lowest excited singlet state of compounds 1-4, thus changing their fluorescence, intersystem crossing and internal conversion efficiencies. The fluorescence and triplet yields gradually decrease, while the internal conversion quantum yield increases upon increasing the solvent dielectric constant. These experimental results, coupled with the marked fluorosolvatochromic effect, point to the involvement of an emitting state with a charge-transfer (CT) character, strongly stabilised by polar solvents. This is proved by ultrafast spectroscopic studies in which two transients, distinguished by characteristic spectral shapes assigned to locally excited (LE) and CT states, are detected, the CT state being the longer lived and fluorescent one in highly polar solvents. The intramolecular LE→CT process, operative in highly polar media, becomes particularly fast (up to ≈300 fs) in the case of the NO(2) derivative 1. No push-pull character is found for 5 and 6, which exhibit different photophysical behaviour; indeed, the solvent polarity does not modify significantly the dynamics of the lowest excited singlet states. Quantum mechanical calculations at the TDDFT level are also used to determine the state order and nature of the lowest excited singlet and triplet states and to rationalise the different photophysical behaviour of fluorine and anthracene derivatives, particularly concerning the intersystem crossing process.

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.