First principles theoretical spectroscopy of methylene blue: Between limitations of time-dependent density functional theory approximations and its realistic description in the solvent.

Methylene blue [3,7-Bis(di-methylamino) phenothiazin-5-ium chloride] is a phenothiazine dye with applications as a sensitizer for photodynamic therapy, photoantimicrobials, and dye-sensitized solar cells. Time-dependent density functional theory (TDDFT), based on (semi)local and global hybrid exchange-correlation functionals, fails to correctly describe its spectral features due to known limitations for describing optical excitations of π-conjugated systems. Here, we use TDDFT with a non-empirical optimally tuned range-separated hybrid functional to explore the optical excitations of gas phase and solvated methylene blue. We compute solvated configurations using molecular dynamics and an iterative procedure to account for explicit solute polarization. We rationalize and validate that by extrapolating the optimized range separation parameter to an infinite amount of solvating molecules, the optical gap of methylene blue is well described. Moreover, this method allows us to resolve contributions from solvent-solute intermolecular interactions and dielectric screening. We validate our results by comparing them to first-principles calculations based on the GW+Bethe-Salpeter equation approach and experiment. Vibronic calculations using TDDFT and the generating function method account for the spectra's subbands and bring the computed transition energies to within 0.15 eV of the experimental data. This methodology is expected to perform equivalently well for describing solvated spectra of π-conjugated systems.

Morphological dependence of silver electrodeposits investigated by changing the ionic liquid solvent and the deposition parameters.

The low toxicity and environmentally compatible ionic liquids (ILs) are alternatives to the toxic and harmful cyanide-based baths used in industrial silver electrodeposition. Here, we report the successful galvanostatic electrodeposition of silver films using the air and water stable ILs 1-ethyl-3-methylimidazolium trifluoromethylsulfonate ([EMIM]TfO) and 1-H-3-methylimidazolium hydrogen sulphate ([HMIM(+)][HSO4(-)]) as solvents and AgTfO as the source of silver. The electrochemical deposition parameters were thoughtfully studied by cyclic voltammetry before deposition. The electrodeposits were characterized by scanning electron microscopy coupled with X-ray energy dispersive spectroscopy and X-ray diffraction. Molecular dynamics (MD) simulations were used to investigate the structural dynamic and energetic properties of AgTfO in both ILs. Cyclic voltammetry experiments revealed that the reduction of silver is a diffusion-controlled process. The morphology of the silver coatings obtained in [EMIM]TfO is independent of the applied current density, resulting in nodular electrodeposits grouped as crystalline clusters. However, the current density significantly influences the morphology of silver electrodeposits obtained in [HMIM(+)][HSO4(-)], thus evolving from dendrites at 15 mA cm(-2) to the coexistence of dendrites and columnar shapes at 30 mA cm(-2). These differences are probably due to the greater interaction of Ag(+) with [HSO4(-)] than with TfO(-), as indicated by the MD simulations. The morphology of Ag deposits is independent of the electrodeposition temperature for both ILs, but higher values of temperature promoted increased cluster sizes. Pure face-centred cubic polycrystalline Ag was deposited on the films with crystallite sizes on the nanometre scale. The morphological dependence of Ag electrodeposits obtained in the [HMIM(+)][HSO4(-)] IL on the current density applied opens up the opportunity to produce different and predetermined Ag deposits.

Urea enhances the photodynamic efficiency of methylene blue.

Methylene blue (MB) is a well-known photosensitizer used mostly for antimicrobial photodynamic therapy (APDT). MB tends to aggregate, interfering negatively with its singlet oxygen generation, because MB aggregates lean towards electron transfer reactions, instead of energy transfer with oxygen. In order to avoid MB aggregation we tested the effect of urea, which destabilizes solute-solute interactions. The antimicrobial efficiency of MB (30 µM) either in water or in 2M aqueous urea solution was tested against a fungus (Candida albicans). Samples were kept in the dark and irradiation was performed with a light emitting diode (λ = 645 nm). Without urea, 9 min of irradiation was needed to achieve complete microbial eradication. In urea solution, complete eradication was obtained with 6 min illumination (light energy of 14.4 J). The higher efficiency of MB/urea solution was correlated with a smaller concentration of dimers, even in the presence of the microorganisms. Monomer to dimer concentration ratios were extracted from the absorption spectra of MB solutions measured as a function of MB concentration at different temperatures and at different concentrations of sodium chloride and urea. Dimerization equilibrium decreased by 3 and 6 times in 1 and 2M urea, respectively, and increased by a factor of 6 in 1M sodium chloride. The destabilization of aggregates by urea seems to be applied to other photosensitizers, since urea also destabilized aggregation of Meso-tetra(4-n-methyl-pyridyl)porphyrin, which is a positively charged porphyrin. We showed that urea destabilizes MB aggregates mainly by causing a decrease in the enthalpic gain of dimerization, which was exactly the opposite of the effect of sodium chloride. In order to understand this phenomenon at the molecular level, we computed the free energy for the dimer association process (ΔG(dimer)) in aqueous solution as well as its enthalpic component in aqueous and in aqueous/urea solutions by molecular dynamics simulations. In 2M-urea solution the atomistic picture revealed a preferential solvation of MB by urea compared with MB dimers while changes in ΔH(dimer) values demonstrated a clear shift favoring MB monomers. Therefore, MB monomers are more stable in urea solutions, which have significantly better photophysics and higher antimicrobial activity. This information can be of use for dental and medical professionals that are using MB based APDT protocols.

Solvent polarity considerations are unable to describe fullerene solvation behavior.

Atomistic molecular dynamics simulations were employed to investigate the solvation properties of the fullerene C60 in binary water/dimethyl sulfoxide (DMSO) mixtures. Structural analysis indicates a preferential solvation with the predominance of DMSO molecules in the first solvation shell for the solutions with low concentrations of DMSO. PMF calculations indicate a maximization of the hydrophobic interaction at low concentrations of DMSO. The contact minima indicate a tendency of aggregation of these nanostructures in water/DMSO mixtures and in the both pure solvents. The free energy of solvation suggests that the hydrophobicity of the fullerene increases monotonically with the increase of DMSO concentration. This result is incompatible with the polarity of DMSO, since it was expected that increasing the concentration of DMSO entailed an increase of C60 solubility.

Prediction of the hydration properties of diamondoids from free energy and potential of mean force calculations.

Molecular dynamics simulations were used to predict the thermodynamical properties of the hydration process of the adamantane, diamantane, and trimantane, the first three members of the series of diamondoids. Free-energy results suggest that the water solubility of these molecules is low. The hydration free energy increases with size of the diamondoid. As for the alkane hydrocarbons, hydration free energy correlates linearly with the surface accessible solvent area; however, here it has been shown that small diamondoids present hydration free energy significantly lower than the n-alkanes of similar molecular weights. The decomposition of the hydration free energy in enthalpic and entropic terms revealed that the hydration process of the small diamondoids is entropic driven. The potential of mean-force calculations indicates that the aggregation of these species in the aqueous medium should occur spontaneously and that the contribution of the solvent is greater the larger the diamondoid.

Note on the free energy of transfer of fullerene C60 simulated by using classical potentials.

Diverse atomistic parameters of C60 have been developed and utilized to simulate fullerene solutions in biological environments. However, no thermodynamic assessment and validation of these parameters have been so far realized. Here, we employ extensive molecular dynamics simulations with the thermodynamic integration method in the isothermal-isobaric ensemble to investigate the transfer of a single fullerene C60 between different solvent environments using different potential models. A detailed analysis is performed on the structure and standard Gibbs free energy of transfer of C60 from benzene to ethanol. All of the interactions concerned in the transfer process are included via atomistic models. We notice that having only structural and dynamical properties is not decisive to validate reliable atomic parameters capable of describing a more realistic thermodynamic process. Thus, we employ the calculated free energy of transfer to validate more accurate atomic parameters for the solvation thermodynamics of fullerenes by direct comparison with the solubility experimental data.