Experimental, Spectroscopic, and Computational Insights into the Reactivity of "Methanal" with 2-Naphthylamines.

The reactions of 2-naphthylamine and methyl 6-amino-2-naphthoate with formalin and paraformaldehyde were studied experimentally, spectrally, and by quantum chemical calculations. It was found that neither the corresponding aminals nor imines were formed under the described conditions but could be prepared and spectrally characterized at least in situ under modified conditions. Several of the previously undescribed intermediates and by-products were isolated or at least spectrally characterized. First principle density functional theory (DFT) calculations were performed to shed light on the key aspects of the thermochemistry of decomposition and further condensation of the corresponding aminals and imines. The calculations also revealed that the electrophilicity of methanal was significantly greater than that of ordinary oxo-compounds, except for perfluorinated ones. In summary, methanal was not behaving as the simplest aldehyde but as a very electron-deficient oxo-compound.

Conjugated Polymer Nanoparticles as Unique Coinitiator-Free, Water-Soluble, Visible-Light Photoinitiators of Vinyl Polymerization.

The use of conjugated polymer nanoparticles (CP NPs) of poly(9,9-dioctylfluorene-alt-benzothiadiazole) and poly(9,9-di-n-octylfluorenyl-2,7-diyl) as efficient photoinitiator systems (PIS) of vinyl polymerization in water is reported herein. CP NPs are biocompatible, excitable with blue commercial LEDs and, unlike visible light Type II PIS, do not need co-initiators to trigger a monomer chain reaction. CP NPs photoinitiate polymerization of a variety of acrylic monomers with initiation rates comparable to those observed for well-known Type II PIS. Given the extraordinarily large molar absorption coefficients of CP NPs (≈108 m-1 cm-1 ) very low particle concentration is required for effective polymerization. Additionally, CP NPs behave as conventional macrophotoinitiators significantly reducing contamination risks due to leaching of low molecular weight byproducts. These combined features make CP NPs PIS suitable to synthesize polymeric materials for many healthcare and biomedical applications including drug delivery, tissue engineering, prosthetic implants, and food/medicine packaging. These CP NPs PIS are also used to synthesize nano-hydrogels with a relatively narrow and controlled size distribution in the absence of surfactants. It is proposed that polymerization is initiated at the CP NPs surface by photogenerated free polarons, in close analogy to the mechanism previously described for PIS based on inorganic semiconductor NPs.

Light-activated conjugated polymer nanoparticles to defeat pathogens associated with bovine mastitis.

Bovine mastitis (BM) represents a significant challenge in the dairy industry. Limitations of conventional treatments have prompted the exploration of alternative approaches, such as photodynamic inactivation (PDI). In this study, we developed a PDI protocol to eliminate BM-associated pathogens using porphyrin-doped conjugated polymer nanoparticles (CPN). The PDI-CPN protocol was evaluated in four mastitis isolates of Staphylococcus and in a hyper-biofilm-forming reference strain. The results in planktonic cultures demonstrated that PDI-CPN exhibited a bactericidal profile upon relatively low light doses (∼9.6 J/cm2). Furthermore, following a seven-hour incubation period, no evidence of cellular reactivation was observed, indicating a highly efficient post-photodynamic inactivation effect. The successful elimination of bacterial suspensions encouraged us to test the PDI-CPN protocol on mature biofilms. Treatment using moderate light dose (∼64.8 J/cm2) reduced biofilm biomass and metabolic activity by up to 74% and 88%, respectively. The impact of PDI-CPN therapy on biofilms was investigated using scanning electron microscopy (SEM), which revealed nearly complete removal of the extracellular matrix and cocci. Moreover, ex vivo studies conducted on bovine udder skin demonstrated the efficacy of the therapy in eliminating bacteria from these scaffolds and its potential as a prophylactic method. Notably, the histological analysis of skin revealed no signs of cellular degeneration, suggesting that the protocol is safe and effective for BM treatment. Overall, this study demonstrates the potential of PDI-CPN in treating and preventing BM pathogens. It also provides insights into the effects of PDI-CPN on bacterial growth, metabolism, and survival over extended periods, aiding the development of effective control strategies and the optimization of future treatments.

Reusable antimicrobial antibiotic-free dressings obtained by photopolymerization.

In recent years significant efforts have been made to develop new materials for wound dressing with improved healing properties. However, the synthesis methods usually employed to this end are often complex or require several steps. We describe here the synthesis and characterization of antimicrobial reusable dermatological wound dressings based on N-isopropylacrylamide co-polymerized with [2-(Methacryloyloxy) ethyl] trimethylammonium chloride hydrogels (NIPAM-co-METAC). The dressings were obtained with a very efficient single-step synthesis procedure based on visible light (455 nm) by photopolymerization. To this end, F8BT nanoparticles of the conjugated polymer (poly(9,9-dioctylfluorene-alt-benzothiadiazole) - F8BT) were used as macro-photoinitiators, and a modified silsesquioxane was employed as crosslinker. Dressings obtained by this simple and gentle method show antimicrobial and wound healing properties, without the incorporation of antibiotics or any other additives. The physical and mechanical properties of these hydrogel-based dressings were evaluated, as well as their microbiological properties, through in vitro experiments. Results show that dressings with a molar ratio of METAC of 0.5 or higher exhibit high swelling capacity, appropriate water vapor transmission rate values, stability and thermal response, high ductility and adhesiveness. In addition, biological tests showed that the dressings have significant antimicrobial capacity. The best inactivation performance was found for hydrogels synthesized with the highest METAC content. The dressings were tested several times with fresh bacterial cultures, showing a bacterial kill efficiency of 99.99 % even after three repetitions in a row, employing the same dressing, demonstrating the intrinsic bactericidal property of the materials and their reusability. In addition, the gels show low hemolytic effect, high dermal biocompatibility and noticeable wound healing effects. Overall results demonstrate that some specific hydrogel formulations have potential application as dermatological dressings for wound healing and disinfection.

Quenching of the triplet state of Safranine-O by aliphatic amines in AOT reverse micelles studied by transient absorption spectroscopy.

The photophysics of Safranine-O (3,6-diamino-2,7-dimethyl-5 phenyl phenazinium chloride) (SfH(+)Cl(-)) was investigated in reverse micelles (RMs) of AOT (sodium bis(2-ethylhexyl)sulfosuccinate) with special emphasis on the triplet state processes. The triplet is formed in its monoprotonated form, independently of the pH of the water used to prepare the RMs. While the intersystem crossing quantum yields in RMs are similar to those in organic solvents, the triplet lifetime is much longer. Since the pH in the water pool of AOT RMs is close to 5 and the triplet state of the dye is subjected to proton quenching, the long lifetime indicates that the dye resides in a region where it cannot be reached by protons during its lifetime. All the measurements indicate that the dye is localized in the interface, sensing a medium of micropolarity similar to EtOH : water (3:1) mixtures. The quenching by aliphatic amines was also investigated. While the quenching by the hydrophobic tributylamine is similar to that in methanol, the hydro-soluble triethanolamine is one order of magnitude more effective in RMs than in homogeneous solution. In the latter case the quenching process is interpreted by a very fast intramicellar quenching, the overall kinetics being controlled by the exchange of amine molecules between RMs. Semireduced dye is formed in the quenching process in RMs in the di-protonated state with a comparable quantum yield to the monoprotonated state formed in homogeneous solvents. The results point to the advantage of the reverse micellar system for the generation of active radicals for the initiation of vinyl polymerization, since a much lower concentration of amine can be employed with similar quantum yields.

Sweet light o' mine: Photothermal and photodynamic inactivation of tenacious pathogens using conjugated polymers.

Each year a rising number of infections can not be successfully treated owing to the increasing pandemic of antibiotic resistant pathogens. The global shortage of innovative antibiotics fuels the emergence and spread of drug resistant microbes. Basic research, development, and applications of alternative therapies are urgently needed. Since the 90´s, light-mediated therapies have promised to be the next frontier combating multidrug-resistance microbes. These platforms have demonstrated to be a reliable, rapid, and efficient alternative to eliminate tenacious pathogens while avoiding the emergence of resistance mechanisms. Among the materials showing antimicrobial activity triggered by light, conjugated polymers (CPs) have risen as the most promising option to tackle this complex situation. These materials present outstanding characteristics such as high absorption coefficients, great photostability, easy processability, low cytotoxicity, among others, turning them into a powerful class of photosensitizer (PS)/photothermal agent (PTA) materials. Herein, we summarize and discuss the advances in the field of CPs with applications in photodynamic inactivation and photothermal therapy towards bacteria elimination. Additionally, a section of current challenges and needs in terms of well-defined benchmark experiments and conditions to evaluate the efficiency of phototherapies is presented.

Trojan horse monocyte-mediated delivery of conjugated polymer nanoparticles for improved photodynamic therapy of glioblastoma.

Aim: To assess monocyte-based delivery of conjugated polymer nanoparticles (CPNs) for improved photodynamic therapy (PDT) in glioblastoma (GBM). Materials & methods: Human monocyte cells (THP-1) and murine monocytes isolated from bone marrow (mBMDMs) were employed as stealth CPN carriers to penetrate into GBM spheroids and an orthotopic model of the tumor. The success of PDT, using this cell-mediated targeting strategy, was determined by its effect on the spheroids. Results: CPNs did not affect monocyte viability in the absence of light and did not show nonspecific release after cell loading. Activated monocytes incorporated CPNs in a higher proportion than monocytes in their naive state, without a loss of cellular functionality. In vitro PDT efficacy using cell-mediated delivery was superior to that using non vehiculized CPNs. Conclusion: CPN-loaded monocytes could efficiently deliver CPNs into GBM spheroids and the orthotopic model. Improved PDT in spheroids was confirmed using this delivery strategy.

Photodynamic Inactivation of ESKAPE Group Bacterial Pathogens in Planktonic and Biofilm Cultures Using Metallated Porphyrin-Doped Conjugated Polymer Nanoparticles.

Photodynamic inactivation (PDI) protocols using photoactive metallated porphyrin-doped conjugated polymer nanoparticles (CPNs) and blue light were developed to eliminate multidrug-resistant pathogens. CPNs-PDI protocols using varying particle concentrations and irradiation doses were tested against nine pathogenic bacterial strains including antibiotic-resistant bacteria of the ESKAPE (Enterococcus faecium, Staphylococcus aureus, Klebsiella pneumoniae, Acinetobacter baumannii, Pseudomonas aeruginosa, and Enterobacter species) pathogens group. The bactericidal effect was achieved in methicillin-resistant Staphylococus aureus (S. aureus) strains using low light doses (9.6-14.4 J/cm2), while Gram-negative bacteria required a higher light dose (28.8 J/cm2). The bacteria-CPN interaction was studied through flow cytometry, taking advantage of the intrinsic CPN fluorescence, demonstrating that CPNs efficiently bind to the bacterial envelope. Finally, the performance of CPNs-PDI was explored in biofilms; good antibiofilm ability and almost complete eradication were observed for S. aureus and Escherichia coli biofilms, respectively, using confocal microscopy. Overall, we demonstrated that CPNs-PDI is an efficient tool not only to kill superbugs as sessile cells but also to disrupt and eradicate biofilms of highly relevant pathogenic bacterial species.

Cage effect in supercritical fluids and compressed gases in the photolysis of an asymmetrically substituted diazene.

We studied the photolysis of (1-biphenyl-4-yl-1-methyl-ethyl)-tert-butyl diazene in supercritical CO(2) and Xe, as well as in compressed Kr. The compound has good solubility in the mentioned fluids, allowing the photolysis measurements to be performed in CO(2) at 1.4 K above T(c) and at pressures as low as 70 bar. We monitored relative cage effect after nanosecond laser pulses by measuring the absorbance at 320 nm (DeltaA(t-->0)) corresponding to the total amount of out-of-cage 1-biphenyl-4-yl-1-methyl-ethyl radical (BME.) produced after nitrogen loss of the diazene. In supercritical CO(2) and Xe, isothermal values of DeltaA(t-->0) showed an increase-decrease behavior with increasing pressure at constant temperature, a typical feature of the transition from the solvent energy transfer to the friction controlled regimes. The comparison of the behavior of DeltaA(t-->0) in CO(2) at reduced temperatures between 1.004 and 1.027, in Xe, and in Kr points to an absence of enhanced cage effect near the critical point. Compatibility with spectroscopic data is analyzed.

Photophysics and photochemistry of an asymmetrically substituted diazene: a suitable cage effect probe.

The photophysics and photochemistry of (1-biphenyl-4-yl-1-methyl-ethyl)-tert-butyl diazene were thoroughly studied by laser flash photolysis from the picosecond to the microsecond time domain. The compound has favorable features as a radical photoinitiator and as a probe for cage effect studies in liquids, supercritical fluids, and compressed gases. The biphenyl moiety acts as an antenna efficiently transferring electronic energy to the dissociative (1)n,pi* state centered on the azo moiety. By picosecond experiments irradiating at the biphenyl- and at the azo-centered transitions, we were able to demonstrate this fact as well as determine a lifetime of 0.7 ps for the buildup of 1-biphenyl-4-yl-1-methyl-ethyl radicals (BME*). The sum of in-cage reaction rate constants of BME* radicals by combination and disproportionation is 5 x 10(10) s(-1). The free radical quantum yield in solution is 0.21 (phi(BME*)) in n-hexane at room temperature, whereas the dissociation quantum yield approaches 50%. The symmetric ketone, 2,4-bis-biphenyl-4-yl-2,4-dimethyl-pentan-2-one, was used as a reference compound for the production and reaction of BME* radicals. Transient IR measurements show CO stretching bands of the excited (3)pi,pi* and (1)n,pi* states but no dissociation up to 0.5 ns. A fluorescence lifetime of 1 ns for this ketone is consistent with this observation. By transient actinometry and kinetic decays in the microsecond time range, we measured epsilon(BME*) = (2.3 +/- 0.2) x 10(4) M(-1) cm(-1) at 325 nm and a second-order rate constant of 5.8 x 10(9) M(-1) s(-1) for the consumption of BME* radicals.

Controlled release of antibiotics from photopolymerized hydrogels: Kinetics and microbiological studies.

The development of convenient synthetic methods and improved materials for the production of high load-capacity and biocompatible drug delivery systems is a challenging task with important implications in health sciences. In this work, acrylamide/2-hydroxyethylmethacrylate and N-isopropylacrylamide/2-hydroxyethylmethacrylate hydrogels were synthesized by photopolymerization using energy-efficient green-LEDs. A functionalized silsesquioxane was used as both crosslinker and co-initiator for the photopolymerization. The hybrid organic-inorganic nature of the silsesquioxane improved the resulting hydrogels' properties increasing their swelling capacity and biocompatibility. Additionally, the mild conditions used during the photopolymerization allowed the synthesis of hydrogels in the presence of antibiotics yielding high load-capacity materials in which the drug preserves its molecular structure and antimicrobial activity (as confirmed by HPLC and microbiological assays). The materials were characterized by FTIR, DSC and SEM. Additionally, the kinetics of gels´ swelling and drug release were studied under physiological conditions (pH 7.4 and 37 °C). The results demonstrate how hydrogel composition affects the antibiotics-release kinetics. The final drug release percentage increased with increasing molar fraction of acrylamide or N-isopropylacrylamide and in most cases exceeded 85%. Finally, the antibacterial effect of loaded gels was characterized using a number of assays against Gram negative and Gram positive bacteria. The observed antibacterial effect correlated well with swelling and drug release results. Furthermore, gels are not toxic for isolated erythrocytes as demonstrated by haemolytic tests. Overall, our results indicate that the produced hydrogels are promising materials to develop controlled drug-delivery devices such as capsules, dermatological patches and others.

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 Beutler's 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-alpha-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-alpha-oxoamides, N,N-diisopropyl phenyl-alpha-oxoamide (1) and N,N-dibenzyl phenyl-alpha-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.

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.

Charge-Transfer Dynamics of Fluorescent Dye-Sensitized Electrodes under Applied Biases.

The development of dye-sensitized solar cells requires an in-depth understanding of the interfacial charge-transfer dynamics that take place between dye sensitizers and semiconductors. Here, we describe a prototype system to probe these dynamics by monitoring in real time the fluorescence of two organic sensitizers, a perylene and a squaraine, bound to a SnO2 semiconductor thin film as a function of potentiostatic control of the Fermi level. The two different sensitizer fluorophores characterized by vastly different redox potentials undergo similar fluorescence modulation with applied bias, an indication that the density of states of the semiconductor largely influences the charge-transfer dynamics while energetics play a minimal role. We further show that the rate of photodegradation of the perylene sensitizer with applied bias provides a suitable marker to study the rate of charge injection and charge recombination. Taken together, our results demonstrate a suitable platform to visualize and study charge-transfer dynamics on films and constitute a step toward achieving single-molecule resolution in our quest to decipher the static and dynamic heterogeneity of charge-transfer dynamics in dye-sensitized photoanodes.

Fluorescent polymer coatings with tuneable sensitive range for remote temperature sensing.

Polymer films of poly(vinyl alcohol) containing the fluorescent dyes 4-aminophthalimide (AP) or 6-propionyl-2-dimethylamino-naphthalene (Prodan) are used as temperature-sensitive fluorescent coatings for remote temperature sensing. Temperature can be obtained by a two-wavelength ratiometric-based emission intensity measurement. The coatings are sensitive in a 100K temperature range that can be tuned by polymer-solute interactions. The usable range is 200-300 K for AP and 280-380 K for Prodan.

Spectral characteristics and photosensitization of TiO2 nanoparticles in reverse micelles by perylenes.

We report on the photosensitization of titanium dioxide nanoparticles (TiO2 NPs) synthesized inside AOT (bis(2-ethylhexyl) sulfosuccinate sodium salt) reverse micelles following photoexcitation of perylene derivatives with dicarboxylate anchoring groups. The dyes, 1,7-dibromoperylene-3,4,9,10-tetracarboxy dianhydride (1), 1,7-dipyrrolidinylperylene-3,4,9,10-tetracarboxy dianhydride (2), and 1,7-bis(4-tert-butylphenyloxy)perylene-3,4,9,10-tetracarboxy dianhydride (3), have considerably different driving forces for photoinduced electron injection into the TiO2 conduction band, as estimated by electrochemical measurements and quantum mechanical calculations. Fluorescence anisotropy measurements indicate that dyes 1 and 2 are preferentially solubilized in the micellar structure, creating a relatively large local concentration that favors the attachment of the dye to the TiO2 surface. The binding process was followed by monitoring the hypsochromic shift of the dye absorption spectra over time for 1 and 2. Photoinduced electron transfer from the singlet excited state of 1 and 2 to the TiO2 conduction band (CB) is indicated by emission quenching of the TiO2-bound form of the dyes and confirmed by transient absorption measurements of the radical cation of the dyes and free carriers (injected electrons) in the TiO2 semiconductor. Steady state and transient spectroscopy indicate that dye 3 does not bind to the TiO2 NPs and does not photosensitize the semiconductor. This observation was rationalized as a consequence of the bulky t-butylphenyloxy groups which create a strong steric impediment for deep access of the dye within the micelle structure to reach the semiconductor oxide surface.

Experimental and theoretical studies on the enantioselectivity of molecularly imprinted polymers prepared with a chiral functional monomer.

A comprehensive study on the enantioseparation of racemic bis[1-phenylethyl]amine (PEA) on a series of molecularly imprinted polymers (MIPs) prepared using the chiral functional monomer (S)-2-(2-methyl-acryloylamino)-3-phenyl propionic acid (MAPP) is reported. MIP-R, MIP-S and MIP-RS, were synthesized separately by imprinting the pure enantiomers (R-, S-PEA) and racemic PEA, respectively, MAPP, EDGMA as crosslinker and chloroform as the porogen. It was found that all MIPs prepared were able to resolve the PEA racemate. Residence times (t(r)) and enantioselectivity factors (α) were estimated from typical elution chromatography experiments. Frontal chromatography experiments were conducted to acquire the adsorption isotherms for both enantiomers on the different MIPs (and on the non-imprinted polymer, NIP). The adsorption isotherms were analyzed using the affinity spectrum (AS) and the expectation-maximization (EM) methods. The study also involved the theoretical evaluation of the MAPP/enantiomers interactions in the pre-polymer mixture. The EM method predicts mono- and bimodal distribution of affinity binding sites depending upon the polymer analyzed. Apparently, the enantioseparation process depends on relatively small differences in the stabilization of the diasteroisomeric ion-pairs PEA/MAPP complexes on the surface of the polymers.

Free energy dependence of the diffusion-limited quenching rate constants in photoinduced electron transfer processes.

Electron-transfer rate constants were determined by means of lifetime measurements for the fluorescence quenching of 9,10-dicyanoanthracene by aromatic amines and methoxybenzenes as electron donors, and for the quenching of the synthetic dyes eosin Y and phenosafranine by a series of p-benzoquinones as electron acceptors. All determinations were carried out in acetonitrile at 298 K. The quenching rate constants (k(q)) in the region of -1.9 eV < DeltaG(et) < -0.2 eV do not decrease as predicted by Marcus theory, but they show a small increase with decreasing DeltaG(et). Although this behaviour is in qualitative agreement with the current theories for reactive systems in the diffusion limit region, a closer analysis of the experimental data showed that several aspects of the dependence of the k(q) on DeltaG(et) are not entirely explained, suggesting that new, refined theoretical models may be required.

Proton-transfer mediated quenching of pyrene/indole charge-transfer states in isooctane solutions.

The fluorescence quenching of pyrene (Py) by a series of N-methyl and N-H substituted indoles was studied in isooctane at 298 K. The fluorescence quenching rate constants were evaluated by mean of steady-state and time-resolved measurements. In all cases, the quenching process involves a charge-transfer (CT) mechanism. The I(o)/I and tau(o)/tau Stern-Volmer plots obtained for the N-H indoles show a very unusual upward deviation with increasing concentration of the quenchers. This behavior is attributed to the self-quenching of the CT intermediates by the free indoles in solution. The efficiency of quenching of the polyaromatic by the N-H indoles increases abruptly in the presence of small amount of added pyridine (or propanol). A detailed analysis of the experimental data obtained in the presence of pyridine provides unambiguous evidence that the self-quenching process involves proton transfer from the CT states to indoles.