Theoretical Study on the Photoacidity of Hydroxypyrene Derivatives in DMSO Using ADC(2) and CC2

This work applies the thermodynamic Förster cycle to theoretically investigate the pK_a^*, i.e., excited-state pK_a values of pyranine-derived superphotoacids developed by Jung and co-workers. The latter photoacids are strong enough to transfer a proton to the aprotic solvent dimethyl sulfoxide (DMSO). The Förster cycle provides access to pK_a^* via the ground-state pK_a and the electronic excitation energies. We use the conductor-like screening model for real solvents (COSMO-RS) to compute the ground-state pK_a and the correlated wavefunction-based methods ADC(2) and CC2 with the continuum solvation model COSMO to calculate the pK_a change upon excitation. A comparison of the calculated UV/Vis absorption and fluorescence emission energies to the experimental results leads us to infer that this approach allows for a proper description of the electronic excitations. In particular, implicit solvation by means of the COSMO model appears to be sufficient for the treatment of these photoacids in DMSO. The calculations confirm the presumption that a charge redistribution from the hydroxy group to the aromatic ring and the electron-withdrawing substituents is the origin of photoacidity for these photoacids. Moreover, the calculations with the continuum solvation model predict that the pK_a jump upon excitation decreases with increasing solvent polarity, as rationalized based on the Förster cycle.

Detection of Chemical Weapon Agents Using Spectroscopic Probes: A Computational Study

Organophosphorus compounds are organic compounds widely employed in agriculture as well as in chemical weapons. The use in agriculture is due to their insecticidal properties. However, in chemical warfare, the use of organophosphorus is associated with acetylcholinesterase inhibition, which promotes the cholinergic syndromes. In this line, the fast detection of this class of compound is crucial for the determination of environmental exposure. This improved detection will naturally allow for more prompt courses of treatment depending on the contaminant findings. In this perspective, the dipyrrinone oxime (1) was employed for the detection of organophosphorus compounds that are employed as nerve agents, such as cyclosarin, sarin, soman, diethyl chlorophosphate, diisopropylfluorophosphate, 2-(dimethylamino)ethyl N,N-dimethylphosphoramidofluoridate, O-ethyl-S-[2-(diethylamino)ethyl]methylphosphonothioate, O-ethyl-S-[2(diisopropylamino)ethyl] methylphosphonothioate, and O,O-diethyl-S-[2-(diethylamino)ethyl] phosphorothioate, through fluorescent emission. The thermodynamics and kinetic parameters as well as spectroscopic properties of the complexes formed for 1 and all organophosphorus compounds previously cited were investigated by means of theoretical calculations. From our findings, only the diethyl chlorophosphate, 2-(dimethylamino)ethyl N,N-dimethylphosphoramidofluoridate, and O,O-diethyl-S-[2-(diethylamino)ethyl] phosphorothioate emitted fluorescence in the hexane, toluene, chloroform, dichloromethane, methanol, acetonitrile, water, and dimethyl sulfoxide solvents. The study of the absorption wavelength with the most polar solvent showed higher values compared to apolar solvents. In the same solvent, for instance, soman in hexane showed the lowest absorption wavelength value, 324.5 nm, and DCP the highest value, 330.8 nm. This behavior was observed in other tested solvents. The thermodynamic parameters indicate negative Gibbs free energy (ΔG) values for the O-ethyl-S-[2(diisopropylamino)ethyl] methylphosphonothioate with 1 reaction. On the other hand, the sarin and cyclosarin revealed the lowest Gibbs free energy (ΔG‡) values, being kinetically favorable and presenting more reactivity.

Photophysical properties for excited-state intramolecular proton transfer (ESIPT) reaction of N-salicylidene-o-aminophenol: Experimental and DFT based approaches

Photophysical properties for excited-state intramolecular proton transfer (ESIPT) reaction of N-salicylidene-o-aminophenol (SA) Schiff base were comprehensively studied based on experimental methods combined with theoretical calculations. The results revealed that the SA was mainly presented in enol form in acidic solutions while it was predominantly existed in keto form in basic solutions. From UV-vis absorption and fluorescence emission studies, it showed that the ESIPT could effectively take place in non-polar and aprotic polar solvents. By using the CAM-B3LYP/6-311G(d,p) level of theory, it was found that the intramolecular proton transfer could preferably occur through six-membered ring transition rather than through five-membered ring transition. The dynamics of the ESIPT reactions of enol and keto tautomers were studied using TD-CAM-B3LYP with 6-311G(d,p) basis set. The potential energy curves for the intramolecular proton transfer in the ground (GSIPT) and excited state (ESIPT) exhibited that the GSIPT could occur through a low activation barrier, whereas in the case of ESIPT, the process could arise via low energy barrier.

Synthesis, Structure, Properties, and Bioimaging of a Fluorescent Nitrogen-Linked Bisbenzothiadiazole

This paper describes the synthesis, structure, photophysical properties, and bioimaging application of a novel 2,1,3-benzothiadiazole (BTD)-based rationally designed fluorophore. The capability of undergoing efficient stabilizing processes from the excited state allowed the novel BTD derivative to be used as a stable probe for bioimaging applications. No notable photobleaching effect or degradation could be observed during the experimental time period. Before the synthesis, the molecular architecture of the novel BTD derivative was evaluated by means of DFT calculations to validate the chosen design. Single-crystal X-ray analysis revealed the nearly flat characteristics of the structure in a syn conformation. The fluorophore was successfully tested as a live-cell-imaging probe and efficiently stained MCF-7 breast cancer cell lineages.

Tuning ESIPT fluorophores into dual emitters

Using first-principle approaches, we show how ESIPT can be controlled by fine-tuning of substituents, hence leading to new dual emitters.

Dyes undergoing excited-state intramolecular proton transfer (ESIPT) are known to present large Stokes shifts as a result of the important geometrical reorganisation following photon absorption. When the ESIPT process is not quantitative, one can obtain dual emitters characterised by two distinct fluorescence bands, observed due to emissions from both the canonical and ESIPT isomers. However, dual emission generally requires to maintain a very specific balance, as the relative excited-state free energies of the two tautomers have to be within a narrow window to observe the phenomenon. Consequently, simple chemical intuition is insufficient to optimise dual emission. In the present contribution, we investigate, with the help of quantum-mechanical tools and more precisely, time-dependent density functional theory (TD-DFT) and algebraic diagrammatic construction (ADC), a wide panel of possible ESIPT/dual emitters with various substituents. The selected protocol is first shown to be very robust on a series of structures with known experimental behaviour, and next is applied to novel derivatives with various substituents located at different positions. This work encompasses the largest chemical library of potential ESIPT compounds studied to date. We pinpoint the most promising combinations for building dual emitters, highlight unexpected combination effects and rationalise the impact of the different auxochromes.

Benzothiadiazole Derivatives as Fluorescence Imaging Probes: Beyond Classical Scaffolds

This Account describes the origins, features, importance, and trends of the use of fluorescent small-molecule 2,1,3-benzothiadiazole (BTD) derivatives as a new class of bioprobes applied to bioimaging analyses of several (live and fixed) cell types. BTDs have been successfully used as probes for a plethora of biological analyses for only a few years, and the impressive responses obtained by using this important class of heterocycle are fostering the development of new fluorescent BTDs and expanding the biological applications of such derivatives. The first use of a fluorescent small-molecule BTD derivative as a selective cellular probe dates back to 2010, and since then impressive advances have been described by us and others. The well-known limitations of classical scaffolds urged the development of new classes of bioprobes. Although great developments have been achieved by using classical scaffolds such as coumarins, BODIPYs, fluoresceins, rhodamines, cyanines, and phenoxazines, there is still much to be done, and BTDs aim to succeed where these dyes have shown their limitations. Important organelles and cell components such as nuclear DNA, mitochondria, lipid droplets, and others have already been successfully labeled by fluorescent small-molecule BTD derivatives. New technological systems that use BTDs as the fluorophores for bioimaging experiments have been described in recent scientific literature. The successful application of BTDs as selective bioprobes has led some groups to explore their potential for use in studying membrane pores or tumor cells under hypoxic conditions. Finally, BTDs have also been used as fluorescent tags to investigate the action mechanism of some antitumor compounds. The attractive photophysical data typically observed for π-extended BTD derivatives is fostering interest in the use of this new class of bioprobes. Large Stokes shifts, large molar extinction coefficients, high quantum yields, high stability when stored in solution or as pure solids, no fading even after long periods of irradiation, bright emissions with no blinking, good signal-to-noise ratios, efficiency to transpose the cell membrane, and irradiation preferentially in the visible-light region are just some features noted by using BTDs. As the pioneering group in the use of fluorescent small-molecule BTDs for bioimaging purposes, we feel pleased to share our experience, results, advances, and personal perspectives with the readers of this Account. The readers will clearly note the huge advantages of using fluorescent BTDs over classical scaffolds, and hopefully they will be inspired and motivated to further BTD technology in the fields of molecular and cellular biology.