Carlos A. Chesta

Salt‐induced Charge Separation in Photoinduced Electron Transfer Reactions. The Effect of Ion Size*

The effect of added salts on the efficiency for photoinduced charge separation in two typical electron acceptor (A)/electron donor (D) systems was studied by the technique of laser flash photolysis. We investigate the exciplex‐forming pyrene/p‐dicyanobenzene (Py/DCB) and pyrene/N,N‐dimethylaniline (Py/DMA) systems in ethyl acetate. The salts selected for this study are tetrabutylammonium chloride, tetrahexylammonium chloride, lithium perchlorate, sodium perchlorate, tetrabutylammonium perchlorate, sodium tetraphenylborate and tetrabutylammonium tetraphenyl‐borate. In most cases, the salts quench the emission of the exciplexes with rate constants near the diffusional rate limit in ethyl acetate. An apparent red shift of the fluorescence maximum of the exciplexes with increasing salt concentration is also generally observed. Laser flash photolysis experiments showed that in the absence of the salts both A/D systems yield exclusively the triplet excited state of the polyaromatic. However, in the presence of many of the electrolytes studied, induced free radical ion formation is observed. The experimental efficiencies for induced charge separation (n) depend on the A/D system and on the nature of the salt. The measured n values vary between 0 and 0.5. The most striking variation corresponds to the lithium and sodium perchlorates. These salts are almost totally inefficient in quenching the Py/DCB exciplex, while they quench and induce charge separation from the Py/DMA exciplex with a high yield. The effect of the different salts on both exciplexes may be rationalized by using the concept of the soft/hard character of the interacting ions.

Modulation of the Photophysical Properties of Pyrene by the Microstructures of Five Poly(alkyl methacrylate)s Over a Broad Temperature Range

Pyrene fluorescence spectra have been recorded in five poly(alkyl methacrylate)s (where alkyl is ethyl butyl, isobutyl, cyclohexyl and hexadecyl) over a 20–400 K temperature range. The changes in the position and the full width at half maximum (FWHM) of the 0–0 emission band (peak I) have been correlated with the structural characteristics of the alkyl groups in the different relaxation regimes of the polymers to assess the degree of coupling of the excited singlet states with the polymer cybotactic regions. Data treatment of the peak I positions using an electron–phonon model indicates that longitudinal optical modes are involved, and that the magnitude of coupling depends on the polymer structure and follows the same trend as the glass transition temperatures. The same spectral parameters have been correlated also with “hole” free volumes from positron annihilation spectroscopy over temperature ranges which span the glass or melting transitions of the polymers. Reasons why free volume and FWHM measurements follow the same trends, and other aspects of the systems, are discussed.

Hybrid films based on a bridged silsesquioxane doped with goethite and montmorillonite nanoparticles as sorbents of wastewater contaminants

The synthesis and characterization of silsesquioxane (SSO) films with pendant dodecyl groups and doped with goethite (Gt) or montmorillonite (MMT) nanoparticles were carried out and the new materials tested as sorbents of diverse contaminants. The synthetic method used yielded SSO films with the inorganic substrates homogeneously distributed within the polymeric matrices. The new materials were characterized by SEM, FTIR, XRD, and DSC and tested to evaluate their capability for adsorbing metallic cations, organic dyes, and phosphate, frequent contaminants of industrial effluents. All films were found suitable for removing metallic cations. Results also showed that the SSO films undoped and doped with Gt are primarily apt for anionic compounds removal. Although the SSO films doped with MMT are capable of removing cationic contaminants from aqueous samples, the stiffness of the SSO matrix hinders MMT properties as an adsorbent. The possibility of dispersing nanoparticulate systems in the stable and chemically inert SSO matrices simplifies their application for contaminant removal, particularly because it makes the separation process of the absorbed pollutant from the treated medium easier.

Crosslinked polymer nanoparticles containing single conjugated polymer chains

Conjugated polymer nanoparticles are widely used in fluorescent labeling and sensing, as they have mean radii between 5 and 100 nm, narrow size dispersion, high brightness, and are photochemically stable, allowing single particle detection with high spatial and temporal resolution. Highly crosslinked polymers formed by linking individual chains through covalent bonds yield high-strength rigid materials capable of withstanding dissolution by organic solvents. Hence, the combination of crosslinked polymers and conjugated polymers in a nanoparticulated material presents the possibility of interesting applications that require the combined properties of constituent polymers and nanosized dimension. In the present work, F8BT@pEGDMA nanoparticles composed of poly(ethylene glycol dimethacrylate) (pEGDMA; a crosslinked polymer) and containing the commercial conjugated polymer poly(9,9-dioctylfluorene-alt-benzothiadiazole) (F8BT) were synthesized and characterized. Microemulsion polymerization was applied to produce F8BT@pEDGMA particles with nanosized dimensions in a ∼25% yield. Photophysical and size distribution properties of F8BT@pEDGMA nanoparticles were evaluated by various methods, in particular single particle fluorescence microscopy techniques. The results demonstrate that the crosslinking/polymerization process imparts structural rigidity to the F8BT@pEDGMA particles by providing resistance against dissolution/disintegration in organic solvents. The synthesized fluorescent crosslinked nanoparticles contain (for the most part) single F8BT chains and can be detected at the single particle level, using fluorescence microscopy, which bodes well for their potential application as molecularly imprinted polymer fluorescent nanosensors with high spatial and temporal resolution.

SALT EFFECTS ON THE CHARGE SEPARATION YIELD IN PHOTOINDUCED ELECTRON TRANSFER REACTIONS: THE EFFECT OF EXCITED STATE MULTIPLICITY

The electron transfer quenching of the excited singlet and triplet states of 9-methylcarbazole (MCBz) by p-dicyanobenzene (DCNB) was investigated in several solvents. While the singlet quenching rate constant is diffusion controlled in all the solvents, the triplet quenching rate constants increase with the solvent polarity. In acetonitrile, free radical ions are produced in high yield from both excited states, while a very low yield is observed in low polarity solvents. The effect of two salts, tetrabutylammonium pechlorate (TBAP) and LiClO4 was investigated in THF. In the presence of the salts, an enhancement of the yield of free radical ions was observed. Two different concentrations of DCNB were employed such that the effect of the salts on either the singlet or the triplet state processes could be separated. A different efficiency was observed depending if the reaction proceeds by the singlet or triplet state.

On the Simultaneous Identification and Quantification of Microalgae Populations Based on Fluorometric Techniques

In this study, the phytoplankton structure of a freshwater reservoir located in central Argentina (Embalse Río Tercero) was analyzed using Beutlers method (Photosynthesis Research 72: 39–53, 2002), aiming to provide water quality control agencies with a reliable tool for early detection of algae blooms, particularly cyanobacteria. The method estimated the concentration of chlorophyll a (Chl a) contributed by individual algal groups in a real sample by fitting its fluorescence excitation spectrum to a linear combination of norm spectra of relevant algae groups. To this purpose, norm spectra for five algae genera usually found in Embalse Río Tercero, Microcystis, Chlorella, Cyclotella, Ceratium and Porphyridium, were constructed and posteriorly used to analyze samples collected in the reservoir in years 2014–2016. Results showed that the method worked well for the quick identification of the algae present in the samples, but it tended to overestimate its Chl a contents. This error was attributed to the large heterogeneity of the algal populations due to the aging of cells grown in environmental conditions.

Medium Effects on Zwitterionic-Biradicaloid Intermediates from Two Phenyl--α-oxoamides. Irradiations in Fluid and Solid Protic Media, Neat Solid Phases, and the Solid, Smectic and Isotropic Phases of a Completely Saturated Phosphonium Salt at Different Temperatures†

The photochemical processes of two N,N‐dialkyl phenyl‐α‐oxoamides, N,N‐diisopropyl phenyl‐α‐oxoamide (1) and N,N‐dibenzyl phenyl‐α‐oxoamide (2), are investigated at different temperatures in methanol and ethylene glycol (to probe the influences of H‐bonding and viscosity), in the solid phase of d‐sorbitol at room temperature (to compare with the results in the liquid alcohols and to assess the influence of a poorly organized “stiff” environment), in the neat solid phase (to probe the influence of well‐ordered, “stiff” matrices), and in the solid, smectic A2 and isotropic phases of methyl‐tris‐tetradecylphosphonium tetrafluoroborate (1P14BF4) (to assess the ability of the intermediates to respond to subtle changes in the order and polarity of their local environments). From differences between the activation parameters for product pathways from irradiations in methanol and in 1P14BF4, we conclude that the zwitterionic pre‐product intermediate from 1 is much more sensitive to the polarity, viscosity and order of its local environment than is the isomeric pre‐product biradicaloid intermediate or either of the pre‐product intermediates from 2. A very sensitive balance among the medium parameters, as well as internal steric and electronic factors of 1 and 2, controls the reactive pathways of the photochemically generated intermediates.