Effect of concentration and wavelength of excitation on the photophysics of salicylate anion in acetonitrile and water.

Sodium salicylate (NaSA) molecule exists as salicylate anion in acetonitrile (ACN) and water solvents, and exhibits large Stokes shifted fluorescence due to excited state intramolecular proton transfer (ESIPT), with decay times of âˆ¼5 ns in ACN and âˆ¼3.9 ns in water by 300 nm (absorption maxima) excitation. Previous studies report both ground and excited state enol-keto tautomerization in ACN, but only excited state tautomerization in water at 10-4 M. However, the current work explores the effect of concentration and excitation wavelengths on the photoinduced dynamics of ESIPT in the salicylate anion. On increasing the concentration of NaSA, no change in absorption spectra appears in both the solvents, and emission spectra of NaSA in water remain unaffected by changes in concentration or excitation wavelength, whereas, a slight red shift and decrease in FWHM appears in ACN. Time-domain fluorescence measurements show predominantly single-exponential decay throughout the emission profile by 300 nm excitation above the 10-5 M concentration in both the solvents, while by 375 nm excitation, multi-exponential fluorescence decay is observed at low concentrations, and as the concentration of NaSA increases, this decay behaviour tends to converge towards a single exponential decay. These results suggest that solute-solvent interactions stabilize the ground-state intermolecular hydrogen-bonded species at low concentrations, while higher concentrations weaken these interactions, leading to emission solely from the salicylate anion. Peak fit analysis of excitation spectra confirms enol-keto tautomerization in both the solvents, with the keto form being more stabilized in ACN. These findings underscore that in ACN, behaviour of NaSA is influenced by both concentration and excitation wavelength and contrary to previous reports, the keto form of the molecule is also present in water, though at a very low concentration and an increase in non-radiative transitions in water cause fluorescence quenching.

Anion-Driven Bandgap Tuning of AgIn(SxSe1-x)2 Quantum Dots.

Accurate tuning of the electronic and photophysical properties of quantum dots is required to maximize the light conversion efficiencies in semiconductor-assisted processes. Herein, we report a facile synthetic procedure for AgIn(SxSe1-x)2 quantum dots with S content (x) ranging from 1 to 0. This simple approach allowed us to tune the bandgap (2.6-1.9 eV) and extend the absorption of AgIn(SxSe1-x)2 quantum dots to lower photon energies (near-IR) while maintaining a small QD size (∼5 nm). Ultraviolet spectroscopy studies revealed that the change in the bandgap is modulated by the electronic shifts in both the valence band and the conduction band positions. The negative overall charge of the as-synthesized quantum dots enabled us to make films of quantum dots on mesoscopic TiO2. Excited state studies of the AgIn(SxSe1-x)2 quantum dot films demonstrated a fast charge injection to TiO2, and the electron transfer rate constant was found to be 1.5-3.5 × 1011 s-1. The results of this work present AgIn(SxSe1-x)2 quantum dots synthesized by the one-step method as a potential candidate for designing light-harvesting assemblies.

Molecular Design Engineering Regulates the Ground-State Passivation Ability of Benzophenone Derivatives in Perovskite Solar Cells.

Intramolecular hydrogen bonding (H-bonding) involved in the excited-state proton transfer (ESPT) process results in benzophenone derivatives (BPDs) with an excellent ability to passivate defects. However, the BPDs are in a continuing dynamic transition process between the ground state and the excited state under light radiation conditions. The ground-state BPDs may lose their ability to passivate defects, resulting in an increased defect density of the perovskite. Therefore, enhancing the passivation ability of the ground-state BPDs can help to achieve the full passivation ability of their ground state to excited state. Herein, we have researched the various BPDs by density functional theory and found that intramolecular H-bonding can weaken the passivation ability of ground-state BPDs, but intramolecular H-bonding is indispensable in the ESPT process. To address the issue, we investigated the influence of electron-donor properties and dipole moments of hydroxyl (-OH), methoxy (-OCH3), and n-octyloxy (-OC8H17) groups in BPD molecules on their coordination capacity through molecular design engineering. Ultimately, 2-hydroxy-4-n-octyloxy-benzophenone (UV5) with strong electron-donor n-octyloxy (-OC8H17) and elongated carbon-chain structure was selected as an additive, which enhances the passivate defect capability in both the ground and excited states. As a result, the UV5-based champion device achieved a power conversion efficiency (PCE) of 24.46% and remained at 75% of the initial PCE with exposure to UV light. This work focuses on the defect passivation capability of ground-state BPDs for the first time and opens a new concept for applying BPDs in PSCs.

The Interaction Of Homodimer Styrylcyanine Dyes With Sodium Dodecyl Sulfate And Triton X-100.

Solubilization of the styrylcyanine dye Sbt ((E)-2-(4-(dimethylamino)styryl)-3-methylbenzo[d]thiazol-3-ium iodide) and its homodimers Dbt-5 and Dbt-10 in aqueous solution of sodium dodecyl sulfate and Triton X-100 has been studied by steady state and picosecond time-resolved fluorescence spectroscopy. At low concentration of sodium dodecyl sulfate in solution, between Sbt, Dbt-5 dyes molecules and surfactant ion pairs are formed followed by the formation non-luminescent H-aggregates. The nature of the interaction between molecules of dyes and surfactants has been revealed. The binding constants Ks of the dyes to the surfactants, free energy changes (ΔG0), the number of dye molecules (n) included in a single micelle and photophysical parameters have been determined. The degree of solubilization of dyes in micellar solution of Triton X-100 is higher as compared to sodium dodecyl sulfate and depends on the molecular weight and size of both dye molecules and micelles.

Aromaticity and Antiaromaticity Reversals between the Electronic Ground State and the Two Lowest Triplet States of Thiophene.

It is shown, by examining the variations in off-nucleus isotropic magnetic shielding around a molecule, that thiophene which is aromatic in its electronic ground state (S0) becomes antiaromatic in its lowest triplet state (T1) and then reverts to being aromatic in T2. Geometry relaxation has an opposite effect on the aromaticities of the ππ* vertical T1 and T2: The antiaromaticity of T1 is reduced whereas the aromaticity of T2 is enhanced. The shielding picture around T2 is found to closely resemble those around certain second singlet ππ* excited states (S2), for example, those of benzene and cyclooctatetraene, thought to be "strongly aromatic" because of their very negative nucleus-independent chemical shift (NICS) values. It is argued that while NICS values correctly follow the changes in aromaticity along the potential energy surface of a single electronic state, the use of NICS values for the purpose of quantitative comparisons between the aromaticities of different electronic states cannot be justified theoretically and should be avoided. "Strongly aromatic" S2 and T2 states should be referred to simply as "aromatic" because detailed comparisons between the properties of these states and those of the corresponding S0 states do not suggest higher levels of aromaticity.

Theoretical study of excited state dynamics of a ratiometric fluorescent probe for detection of SO2 derivatives.

Sulfur dioxide (SO2), a toxic air pollutant, can have harmful effects on human health when inhaled or when it forms bisulfite in the body. In the present work, a ratiometric fluorescent probe, 2-(2'-hydroxyphenyl)benzothiazole-3-ethyl-1,1,2-trimethyl-1H-benzo[e]indolium (HBT-EMBI), was selected to study the mechanism of SO2 derivatives detection. This study provides insights into the attributions of ratiometric fluorescence through hydrogen bond dynamics, electronic excitation properties, radiation rates, and excited state intramolecular proton transfer (ESIPT) processes using the density functional theory (DFT) and the time-dependent density functional theory (TDDFT) level. The results confirm that the large Stokes shifts and broad emission spectra of the HBT-EMBI probe are associated with its intramolecular charge transfer (ICT) characteristics and hydrogen bonding-driven ESIPT processes, respectively. After the addition reaction between the probe and HSO3-/SO32-, the conformational populations of HBT-EMBI-HSO3- transfer abnormally from enol configurations to more stable keto configurations, which leads to a distinguished change in the visible color and the ratiometric fluorescence signal, and is not due to the blockage of the ICT process of HBT-EMBI-HSO3-, as previously reported. This work provides a new perspective on the mechanism of detection of SO2 derivatives by ESIPT fluorescent probes.

Time-Resolved Circular Dichroism in Molecules: Experimental and Theoretical Advances.

Following changes in chirality can give access to relevant information on the function or reactivity of molecular systems. Time-resolved circular dichroism (TRCD) spectroscopy proves to be a valid tool to achieve this goal. Depending on the class of molecules, different temporal ranges, spanning from seconds to femtoseconds, need to be investigated to observe such chiroptical changes. Therefore, over the years, several approaches have been adopted to cover the timescale of interest, especially based on pump-probe schemes. Moreover, various theoretical approaches have been proposed to simulate and explain TRCD spectra, including linear and non-linear response methods as well as non-adiabatic molecular dynamics. In this review, an overview on both experimental and theoretical advances in the TRCD field is provided, together with selected applications. A discussion on future theoretical developments for TRCD is also given.

Cellular damage photosensitized by dasatinib, radical-mediated mechanisms and photoprotection in reconstructed epidermis.

Dasatinib (DAS) is an anticancer drug employed in the treatment of certain hematological malignancies. Although DAS has been mainly developed for oral administration, it has recently garnered attention for its possible topical application. The use of topical drugs can cause photosensitivity, which is not listed as an adverse reaction for DAS. Since DAS absorbs UVA, it could potentially induce photosensitivity reactions and lead to oxidative damage to cellular targets. This study aims to investigate whether DAS exhibits phototoxic reactions on primary cellular targets in both solution and artificial skin, mimicking topical drug administration. It also examines the potential generation of highly reactive intermediates like organic radicals and ROS, which could trigger photosensitivity reactions. Upon DAS irradiation in the UVA region, the first transient species detected was the diradicaloid triplet excited state (3DAS∗) with an absorption maximum of around 490 nm, which was quenched by oxygen to produce singlet oxygen. Quenching experiments with linoleic acid and 3-methylindole indicated that radical-mediated (Type I) photosensitized damage to lipids and proteins is possible. However, the lack of triplet quenching with guanosine suggests that the Type II mechanism also plays a role in the photooxidation of biomolecules. Accordingly, the neutral red uptake phototoxicity test (photoirritation factor of 5) and the comet assay, revealed that this drug is photo(geno)toxic to cells. Moreover, investigations on lipid photoperoxidation, and protein and DNA photooxidation strongly support that different cellular compartments are potential targets for DAS-induced phototoxicity. Regarding its potential application in topical dermatological formulations, an O/W emulsion of DAS was prepared and tested in reconstructed human epidermis, and a significant phototoxicity was also demonstrated. Fortunately, this undesired side effect disappeared upon formulation of DAS along with a sunscreen. Thus, for topical treatments, the photosensitivity reactions induced by DAS can be prevented by using formulations including appropriate UVA filters.

On the Simulation of Photoreactions Using Restricted Open-Shell Kohn-Sham Theory.

It is a well-established standard to describe ground-state chemical reactions at an ab initio level of multi-electron theory. Fast reactions can be directly simulated. The most widely used approach is density functional theory for the electronic structure in combination with molecular dynamics for the nuclear motion. This approach is known as ab initio molecular dynamics. In contrast, the simulation of excited-state reactions at this level of theory is significantly more difficult. It turns out that the self-consistent solution of the Kohn-Sham equations is not easily reached in excited-state simulations. The first program that solved this problem was the Car-Parrinello molecular dynamics code, using restricted open-shell Kohn-Sham theory. Meanwhile, there are alternatives, most prominently the Q-Chem code, which widens the range of applications. The present study investigates the suitability of both codes for the molecular dynamics simulation of excited-state motion and presents applications to photoreactions.

Photomodulation of Charge Transfer through Excited-State Processes: Directing Donor-Acceptor Binding Dynamics.

Modulating charge transfer (CT) interactions between donor and acceptor molecules may give rise to unique dynamic changes in physicochemical properties, exhibiting great importance in supramolecular chemistry and materials science. In this work, we demonstrate the first instance of reversible photomodulation of donor-acceptor (D-A) CT interaction in the solid state.Pyridinium-based chromophore featuring π-conjugated D-A structures can not only function as a good electron acceptor to undergo photoinduced electron transfer (ET) or engage in intermolecular CT interaction, but also exhibit unique dual emission depending on the excitation wavelengths. The rotatable C-C single bonds within D-A pairs enhance the tunability of molecular structure. Through the synergy of a photoinduced ET and an excited-state conformational change, the intermolecular CT interaction can be switched on and off by alternate light irradiation to enables reversibly modulation of the affinity between donor and acceptor molecules, accompanied by visual color switching and fluorescence on-off as feedback signals.