Monica Caselli

Dimerization of green sensitizing cyanines in solution. A spectroscopic and theoretical study of the bonding nature

Abstract The bonding nature in cyanine-dye aggregates has been investigated by studying dimerization in solution of 3,3′-disulfopropyl-4,5,4′,5′-dibenzo-9-ethyloxacarbocyanine (D) and three other oxacarbocyanine analogues (E, F, G) used as photographic sensitizers in the green spectral region. Quantitative information on the monomer–dimer equilibrium of dye D in different solvents and of its analogues (E, F, G) in water was obtained by measurement of the absorption spectrum as a function of dye concentration and of temperature. Dimerization was found to be generally driven by enthalpic factors traceable to strong attractive van der Waals interactions between the two large and highly polarizable dye molecules. Entropic contributions to ΔG0 usually favour dissociation but are smaller than the enthalpic ones. The visible absorption spectrum of the dimer consists in a classic two-branched exciton band with a marked splitting ( 1600 cm −1 in water). The experimental observations were the subject of a theoretical study including classical molecular dynamics (MD) and Monte Carlo (MC) calculations of the dimer structure and comparative analysis of monomer and dimer spectra by the CS INDO CI method. Computer simulations led to three similar H-type structures, the most stable of which is characterized by a distance of 4.7 A between the planes of the chromophores and an endo–endo configuration of the sulfopropyl substituents. The calculated dimer spectrum was clearly interpreted in terms of exciton model but a quantitative agreement with the two-maximum exciton band could be obtained only by assuming substantial deviation of the long molecular axes from parallelism. On the basis of normal coordinate calculations it is suggested that such configurations may occur with a high probability in virtue of twisting vibrational motions of extremely low frequency.

Consequences of H-dimerization on the photophysics and photochemistry of oxacarbocyanines

The 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.

Rotamerism in 2,2′-binaphthyl. A study based on fluorescence analysis and CS-INDO/CI calculations

The torsional isomerism of 2,2′-binaphthyl in the ground state and lowest excited singlet states has been investigated theoretically and experimentally. The combined results of a fluorescence steady-state and time-resolved analysis and of CS-INDO/CI calculations showed that this molecule exists in the ground state in two rotameric forms in which the naphthyls are rotated around the interconnecting quasi-single bond by different angles (ϕ≈ 35 and 145°). The absorption spectrum of 2,2′-binaphtyl was assigned and resolved into the individual spectra of the two rotamers. The fluorescence spectra, lifetimes and quantum yields of the two isomers at room temperature were also derived. The effects of solvent viscosity increase and of temperature decrease on the measured fluorescence lifetimes were found to be small. Lastly, a discussion of the rotational behaviour of this molecule as opposed to that of other biaryls is offered on the basis of the calculated torsional energy curves.

Irradiation-wavelength dependent photochemistry of the bichromophoric sulfonylurea chlorsulfuron

Abstract The photochemistry of the bichromophoric sulfonylurea chlorsulfuron has been investigated at different irradiation wavelengths and environmental conditions. The identified products indicate that different reaction paths are followed, depending whether the benzene or the triazine component chromophores are excited. In the first case, the chlorine atom is substituted by hydrogen or hydroxyl in water; in the second one three competitive photodegradation paths may be followed, the most efficient one being the cleavage of the S–N bond in the sulfonylurea bridge. Photodegradation quantum yield measurements, combined with laser flash photolysis experiments have shown that, while both singlet and triplet states contribute to all photolytic paths in deaerated solution, quenching by oxygen makes the triplet contribution negligible in samples equilibrated with air.

Dimer–monomer equilibrium of human thymidylate synthase monitored by fluorescence resonance energy transfer

An ad hoc bioconjugation/fluorescence resonance energy transfer (FRET) assay has been designed to spectroscopically monitor the quaternary state of human thymidylate synthase dimeric protein. The approach enables the chemoselective engineering of allosteric residues while preserving the native protein functions through reversible masking of residues within the catalytic site, and is therefore suitable for activity/oligomerization dual assay screenings. It is applied to tag the two subunits of human thymidylate synthase at cysteines 43 and 43′ with an excitation energy donor/acceptor pair. The dimer–monomer equilibrium of the enzyme is then characterized through steady‐state fluorescence determination of the intersubunit resonance energy transfer efficiency.

Modelling of the cis-trans partitioning in the photoisomerizations of cyanines and stilbene derivatives

Abstract In the course of photoisomerization, polymethine cyanines as well as stilbene and its derivates decay from the S1 potential energy minimum, corresponding to the perpendicular geometry, to yield either cis or trans ground-state molecules. The fraction of cis isomers obtained, α, spans a larger range of values for symmetric cyanines than for stilbene derivatives. It is argued that such different behaviour for the two classes of compounds should be traceable to the electronically different nature of their S1 perp species. Making use of radiationless transition theory results, it is shown the relative location of the S1 minimum and S0 maximum along the internal rotation coordinate is crucial to the evaluation of α: even small differences between these critical twisting angles, which are more reasonably expected for polymethine cyanines than for stilbene-like compounds, may cause strong deviations from equipartitioning (α=0.5).

Make sense of nanochemistry and nanotechnology

A class in a Scientific-Technological Lyceum (age 17) decided to produce a PowerPoint presentation to introduce nanochemistry and nanotechnology to the students in lower grades. Because the subject is very new, there was nothing in the School textbooks and, therefore, the students had to cooperate in order to find materials, to use ICT sources and to take decisions, such as selecting information and choosing slide layouts. Furthermore, the Cooperative Learning methodology was employed to solve the problem of setting up the presentation. To make nanochemistry and nanotechnology a reality for the students, they used a link between these new frontiers of Chemistry and subjects currently tackled at the Secondary School level. This was the quantitative determination of Ca2+ ions by using calcein, a luminescent chemosensor, in which well known concepts, such as chemical equilibrium and stability constants of coordination compounds, are involved. The educational aims of the project were to promote both content knowledge and social skills in Secondary School students. The activity created a good class atmosphere and also led to the retention of content knowledge.

Porphyrin-based electrostatically self-assembled multilayers as fluorescent probes for mercury(II) ions: a study of the adsorption kinetics of metal ions on ultrathin films for sensing applications

The electrostatic multilayer self-assembly technique was employed to prepare fluorescent probes for mercury(II) ions. Films showing an appreciable fluorescence emission were obtained by alternate adsorption of 5,10,15,20-tetrakis(4-sulfonatophenyl)porphyrin (TPPS) with polyelectrolytes. Fluorescence intensity quenching, arising from the coordinative interaction between the porphyrin and mercury ions, was monitored as a function of time. The response mechanism was studied on the basis of kinetic and equilibrium models for adsorption. The adsorption process of mercury ions on porphyrin-based films was described by the pseudo-second-order equation and the kinetic parameters were correlated with the initial concentrations of the metal ions in aqueous solution. The kinetic approach was evaluated as an alternative to monitoring the sensing responses at equilibrium.

Optical and photophysical properties of anisole- and cyanobenzene-substituted perylene diimides

One- and two-photon absorption cross-sections and spectra and the photophysical properties of eight perylenetetracarboxy-3,4:9,10-diimide (PDI) derivatives are reported and analyzed. The investigated compounds are characterized by direct binding of the phenyl rings of the substituents to the bay positions of the perylene core. They have been designed to test the effects of differences in the electronic nature - electron donating (anisole) or accepting (cyanobenzene) - and binding topology (cis or trans, meta or para disubstitution or tetrasubstitution) of the bay substituents on the above optical and photophysical observables. (TD)DFT and Hückel MO calculations have provided theoretical information on the ground-state geometries, the MOs and the electronic spectra of several model compounds. For tetrasubstituted and cis disubstituted derivatives, strong steric interactions in the bay area determined the preferred conformations, with perylene cores distorted near the substituted bay(s) and a 42-44° twisting of the substituent rings relative to the core, quite irrespective of the electronic nature of the substituents. On the other hand, in trans-disubstituted PDI steric hindrance in the bay areas was much weaker and similar in the cyanobenzene and the anisole derivatives. So, the large differences found in the conformational preferences were completely attributable to electronic effects. With electron-accepting cyanobenzene, the substituent rings were found normal to the central planar perylene core, thus enabling the assignment of the moderate spectroscopic effects to inductive interactions. The DFT analysis of the PDI trans-disubstituted with electron-donating anisoles gave quite strongly distorted perylene-core geometries and less twisted (59°) substituent rings. The corresponding increased substituent/core conjugative interactions resulted in new CT allowed electronic transitions and an extremely pronounced solvent-polarity dependence of the emission spectra and intensities. All anisole substituted PDI feature a very fast radiationless decay path in polar solvents, likely related to a relaxation to a charge-separated configuration in the lowest excited-state.

Theoretical and photophysical study of photoisomerism of cyanine dyes: bisphenylaminopentamethine cyanine (BPPC)

Abstract The trans-cis photoisomerism of BPPC was investigated as part of a comprehensive, both theoretical and experimental, study on the photophysical and photochemical properties of cyanines having pentamethine cyanine as a common chromophore. In this work the determination of the trans → cis photoisomerization and thermal back isomerizatkm kinetics in alcohols was combined with CS INDO CI calculations of S 0 and S 1 potential energy curves for the paths leading to mono-cis isomers, where the dielectric solvent effects were taken into account by the simple virtual charge model. In agreement with the indications of a previous steady-state spectroscopy study, it was concluded that irradiation of BPPC into the visible absorption region gives rise to formation of the C(3)–C(4) cis planar isomer, already observed with the parent chromophore (BMPC), and an additional isomer, N(1)-C(2) cis, peculiar to BPPC. The role of the solute-solvent interactions is discussed with reference to the purely intramolecular model previously proposed for the cyanine photoisomerization.