Publications

Abstract 6-Bromochromone-2-carboxylic acid (3) was synthesized by a microwave-assisted process. The optimization of the reaction was performed varying parameters, such as type of base/number of reagent equivalents, solvent, temperature and reaction time. The yield of the reaction was improved to 87%. The new synthetic route is versatile as several chromone-2-carboxylic acids (compounds 4B-10B) were obtained with good yields (54-93%). Only in the case of the nitro substituent (compound 11B), an ester was obtained instead of the desired carboxylic acid. Following this synthetic route chromone carboxylic acids can be attained with a high degree of purity, without the need of the tedious and expensive purification processes through column chromatography. The reaction is safe, cost-effective, fast and robust, and can be used in the development of concise and diversity-oriented libraries based on chromone scaffold. The overall study can be looked as a step forward to speed-up the discovery of chromone-based multitarget-directed ligands.

Abstract Carbon dots have attracted great attention from the research community given their very attractive luminescent properties. However, the recent discovery that some of these properties may result from fluorescent impurities originating from the synthesis process, and not from the carbon dots themselves, constitute a significant setback to our knowledge of these materials. Herein, we proceeded to the study of carbon dots generated from citric acid and urea via a microwave-assisted synthesis, focusing on their analysis by AFM, HR-TEM, XPS, FT-IR, ESI-MS, UV-Vis and fluorescence spectroscopy. We have found that this synthesis process does generate molecular fluorophores that can mask the luminescence of the carbon dots. More importantly, our data demonstrates that when present in the same solution, the carbon dots and these fluorophores do not behave as separated species with individual emission. Instead, they interact to produce a hybrid luminescence, which excited state properties and reactivity are different from the properties of the individual species. These results indicate the possibility for the development of hybrid materials composed by carbon dots and related molecular fluorophores with new and improved properties.

Abstract The ring-opening reaction of epoxides is an essential step in CO2 conversion into value-added heterocyclic carbonates. There has been an increasing focus on developing organocatalysts composed by a nucleophile and a hydrogen bond donor (HBD), so that CO2 conversion can occur at ambient conditions, with the rationale that HBD can aid epoxide's ring-opening. Nevertheless, it is not clear what is the real role of HBDs on this reaction. Herein, it was employed a combined spectroscopic and theoretical approach to obtain mechanistic insight into the ring-opening of a model epoxide, when catalyzed by pyridine and different HBDs. While the pyridine-HBD system decreased the activation barrier from (similar to)60 to (similar to)22-23 kcal mol(-1), by itself pyridine decreased it to (similar to)27 kcal mol(-1). This shows that HBDs present a small effect on the activation energy. Furthermore, it was found that water molecules can act as HBD with similar results to other molecules. This helps to explain why catalytic amounts of water have found to be advantageous in CO2 conversion. Further work found that the activation barrier is mainly controlled by steric contributions. Subsequent spectroscopic analysis showed that the obtained rate constants follow the same trend found in the theoretical calculations, while these reactions lead to the formation of the same products in similar ratios. It was also found that one pyridine molecule can catalyze the opening of two epoxides, resulting in a trimeric structure. Finally, the addition of electron-donating groups to pyridine increases the catalytic properties of the system.

Abstract While brown carbon is a strongly-light-absorbing type of organic aerosol that is capable of significant regional radiative forcing, it has been neglected from climate models, which results in differences between model predictions and measured data. This also results from uncertainty regarding the relationship between the chemical composition of brown carbon and its optical properties. Herein, here was utilized a time-dependent density functional theory (TD-DFT) approach to model the "real-world" absorption of thirty polycyclic aromatic hydrocarbons (PAHs) and twenty-five derivatives (ten nitro-PAHs and fifteen oxygenated-PAHs) present in the atmosphere over three Southern European cities (Porto, Florence and Athens). These data were corrected both for "real-world" experimental concentration of these molecules over these cities, and for their theoretical fluorescence yield. These results indicate that the absorption of the molecules more relevant for climate forcing are at similar to 330, similar to 360 and similar to 440 nm. Furthermore, the absorption is explained mainly by PAH and oxygenated-PAH molecules, while nitro-PAHs provide only negligible contributions. Porto should be the city to be most affected by radiative forcing induced by these molecules, while Florence and Athens appear to be similarly affected. Finally, these models also demonstrate that absorption at similar to 330 nm is explained by both PAH and oxygenated-PAH-molecules, while absorption at similar to 360 and similar to 440 nmis only attributed to oxygenated-PAHs. More specifically, from the fifty-five studied molecules, only coronene (a PAH), 1,8-naphthalic anhydride, 6-H-benzo[cd]pyrene-6-one and 7H-benz[de]anthracence-7-one (three oxygenated-PAHs) provide relevant contributions to radiative forcing.

Abstract Cypridina bioluminescence has been increasingly used in bioimaging, bioanalysis, and biomedicine, due to high quantum yield and high signal-to-noise ratio. However, there is still no consensus regarding different aspects of the chemiluminescent mechanism of this system, which impairs the development of new applications. Herein, we have used a theoretical DFT and TD-DFT approach to (i) determine the identity of the dioxetanone species responsible for efficient chemiexcitation and (ii) identify the bioluminescent emitter and determine if light-emission occurs from the fluorescent or chemiluminescent state. Our results demonstrate that upon oxygenation of the imidazopyrazinone scaffold, a dioxetanone with a neutral amide group and a cationic guanidinopropyl group is formed. This species is efficiently chemiexcited (with no obvious charge transfer step) to the corresponding oxyluciferin with a neutral amide and cationic guanidinopropyl groups. After the "dark" chemiluminescent state, this oxyluciferin species is converted into a bright blue-emitting fluorescent state.

Abstract <jats:p>Dissolved carbon dioxide (dCO2) evaluation is very important in many different fields. In this work, a new, integrated, colorimetric-optical fiber-based system for dCO2 monitoring in aquaculture industry was developed. The sensing chemistry is based on colorimetric changes of the used indicator—poly p-nitrophenol (pNPh)—in contact with CO2. Preliminary tests were done in a laboratory environment (calibration) and in a laboratory Recirculating Aquaculture System (RAS) with controlled CO2 injection. The results have shown the suitability of the new sensor for assessing dCO2 dynamics in RAS and its fast detection of low dCO2 concentrations in an appropriate operation range.</jats:p>

Abstract A sensing configuration for the real-time monitoring, detection, and quantification of dissolved carbon dioxide (dCO(2)) was developed for aquaculture and other applications in freshwater and saline water. A chemical sensing membrane, based on a colorimetric indicator, is combined with multimode optical fiber and a dual wavelength light-emitting diode (LED) to measure the dCO(2)-induced absorbance changes in a self-referenced ratiometric scheme. The detection and processing were achieved with an embeded solution having a mini spectrometer and microcontroller. For optrode calibration, chemical standard solutions using sodium carbonate in acid media were used. Preliminary results in a laboratory environment showed sensitivity for small added amounts of CO2 (0.25 mg.L-1). Accuracy and response time were not affected by the type of solution, while precision was affected by salinity. Calibration in freshwater showed a limit of detection (LOD) and a limit of quantification (LOQ) of 1.23 and 1.87 mg.L-1, respectively. Results in saline water (2.5%) showed a LOD and LOQ of 1.05 and 1.16 mg.L-1, respectively. Generally, performance was improved when moving from fresh to saline water. Studies on the dynamics of dissolved CO2 in a recirculating shallow raceway system (SRS+RAS) prototype showed higher precision than the tested commercial sensor. The new sensor is a compact and robust device, and unlike other sensors used in aquaculture, stirring is not required for correct and fast detection. Tests performed showed that this new sensor has a fast accurate detection as well as a strong potential for assessing dCO(2) dynamics in aquaculture applications.

Abstract The co-precipitation synthesis assisted YF3: Er3+/Yb3+ upconversion nanoparticles were examined as a function of pump power which showed a tuneable emission efficiency through 976 nm diode laser excitation. Different emission bands in the visible range corresponding to the F-4(7/2) -> I-4(15/2) (490 nm), H-2(11/2) -> I-4(15/2) (522 nm), S-4(3/ 2) -> I-4(15/2) (548 nm) and F-4(9/2) -> I-4(15/2) (659 nm) transitions were observed. The sample crystal structure, particle morphology and optical properties were characterized. The applications of the present sample in latent fingermarks detection and security ink demonstration were successfully examined through the optimized annealing temperature and pump power of excitation. This demonstration claimed the highly utilization of the YF3:Er3+/Yb3+ upconversion nanoparticles for possible crime scene and anti-counterfeiting practitioners.

Abstract Despite decades of research, it is still not clear what is the mechanism behind the efficient chemiexcitation of dioxetanones in chemiluminescent and bioluminescent reactions. In fact, long-standing theories (charge transfer-initiated luminescence and chemically induced electron-exchange luminescence) have been demonstrated to not be able to explain this phenomenon. Herein, a theoretical approach using reliable and up-to-date methodology was used to address this problem, by focusing on model dioxetanones. Time-dependent (TD)-Density functional theory (DFT) and multireference complete-active-space second-order perturbation theory (CASPT2) calculations provided evidence that points to efficient intramolecular chemiexcitation being the result of the reacting molecules having access to a long zone of the Potential energy surface (PES), within the biradicalar region, where S-0 and S-1 are degenerate. Molecules with inefficient chemiexcitation are unable to reach this zone of degeneracy. Our main finding is that access to the region of degeneracy appears to be given due to increased interaction between the keto and CO2 moieties, as supported by the use of the activation strain model and Born-Oppenheimer molecular dynamics, which extends the biradical region by delaying the rupture of the peroxide ring. Increased interaction derives from attractive electrostatic interactions between the moieties of dioxetanone. Thus, we hypothesize that efficient chemiexcitation results not only from electron/charge transfer and subsequent charge annihilation but is instead based on the degree of interaction between the keto and CO2 moieties, which controls the access to a region of degeneracy between the ground and excited states.

Abstract Herein we present a new synthesis methodology for the transprotection of the 1-phenylethyl protected tertiary amines to tert-butyloxycarbonyl (Boc) derivatives under exocyclic N-C hydrogenolysis catalyzed by Pd/C. We provide mechanistic insights into the N-dealkylation of hindered tertiary amines under the unrecognized role of tert-butyloxycarbonyl anhydride (Boc(2)O) as an additive to effectively promote the exocyclic N-C hydrogenolysis of tertiary amines by disclosing the role of Boc(2)O, which was not fully understood and studied until now. NMR and in silico experiments suggest the formation of a transient charged carbamate as the plausible intermediate. This selective transprotection enabled the development of a robust stereoselective methodology for the preparation of both enantiomers of 2-azanorbornane-3-exo-carboxylates by highly asymmetric aza-Diels-Alder reactions using (-)-8-phenylmenthol (8PM) and (+)-8-phenylneomenthol (8PnM) as chiral auxiliaries.