Heteroleptic Cu(I) bis-diimine complexes as sensitizers in dye-sensitized solar cells (DSSCs): on some factors affecting intramolecular charge transfer.

A set of eight heteroleptic bis-diimine copper dye complexes with two different ancillary ligands (functionalised 2,9-dimethyl-1,10-phenanthroline (dmp) and functionalised 6,6'-diphenyl-2,2'-bipyridine (dpbpy)) are investigated for their potential use as sensitizers in dye-sensitized solar cells (DSSCs), using first principles density functional theory (DFT) and time dependent DFT (TDDFT). A detailed analysis of the structural properties, projected density of electronic states and Kohn-Sham energy levels, and optical absorption spectra in the UV-visible region reveals that substituting the thiophene group in the ancillary ligand, and enhancing conjugation in the anchoring ligand, lead to increase in the light harvesting efficiency (LHE). However, a natural transition orbital (NTO) analysis, shows that the nature of charge transfer depends mainly on the nature of the parent ancillary group and is not significantly affected by the structural modifications. Importantly, the lower energy excitations lead to favourable mixed metal to ligand charge transfer (MLCT) and ligand to ligand charge transfer (LLCT), as well as good electron injection. The best charge transfer directionality is found in the dmp-based dyes, particularly thiophene substituted dyes, thus making these the more effective sensitizers in DSSCs.

Quantum dots as an electron or hole acceptor: on some factors affecting charge transfer in dye-quantum dot composites.

We report a density functional theory (DFT)/time dependent DFT (TDDFT) computational investigation on some factors affecting the nature of charge transfer in CdS quantum dots (QDs) of two different sizes attached to one or two units of dyes among three species viz., coumarine (C343), fluorescein (FLU) and NKX-2388 (NKX). The direction and nature of charge transfers have been ascertained from natural transition orbital analysis. Factors affecting the charge transfer mechanism include the interaction of dyes with QDs, the interaction of a dye with another dye and the effect of solvation. The strength of interaction of the dye and QD depends on the orientation of the dye unit(s) and the type of anchoring group of the dyes and even the direction of charge transfer reverses for different orientation of the dye with respect to the QD in some systems. In addition, hybridized energy levels of the dye-QD composites are formed which leads to direct charge transfer. We observe both direct and indirect charge transfers for different excited states, which is indeed an interesting feature. Interestingly, when two dye molecules are attached to a QD, the direct charge transfer exists between dyes of the same species only. The energy levels, as well as corresponding absorption peaks, exhibit pronounced energy shifts in implicit solvation models.

Density functional investigation and some optical experiments on dye-sensitized quantum dots.

Dye-sensitized quantum dots (QDs) are promising candidates for dye-sensitized solar cells (DSSCs). Here, we report steady state (absorption and photoluminescence) optical measurements on several sizes of CdS QDs ligated with Coumarin 343 dye (C-343) and two different solvents, viz., chloroform and toluene. We further report detailed first principles density functional theory and time-dependent density functional theory studies of the geometric, electronic and optical (absorption and emission) properties of three different sized capped QDs, ligated with C-343 dye. The absorption spectrum shows a QD-size-independent peak, and another peak which shifts to blue with decrease in QD size. The first peak is found to arise from the dye molecule and the second one from the QD. Charge transfer using natural transition orbitals (NTOs) is found to occur from dye-to-QDs and is solvent-dependent. In the emission spectra, the luminescence intensity of the dye is quenched by the addition of the QD indicating a strong interaction between the QD and the dye.

Understanding the antioxidant behavior of some vitamin molecules: a first-principles density functional approach.

The structures, energetics, vertical and adiabatic ionization potentials, electron affinities, and global reactivity descriptors of antioxidant vitamins (both water- and fat-soluble) in neutral, positively charged, and negatively charged states were investigated theoretically. We worked within the framework of first-principles density functional theory (DFT), using the B3LYP functional and both localized (6-311G+(d,p) and plane-wave basis sets. Solvent effects were modeled via the polarizable continuum model (PCM), using the integral equation formalism variant (IEFPCM). From the computed structural parameters, ionization potentials, electron affinities, and spin densities, we deduced that these vitamins prefer to lose electrons to neutral reactive oxygen species (·OH and ·OOH), making them good antioxidants. Conceptual DFT was used to determine global chemical reactivity parameters. The computed chemical hardnesses showed that these antioxidant vitamins are more reactive than neutral reactive oxygen species (ROS), thus supporting their antioxidant character towards these species. However, in the neutral state, these vitamins do not act as antioxidants for [Formula: see text]. The reactivity of vitamins towards ROS depends on the nature of the solvent. Amongst the ROS, ·OH has the greatest propensity to attract electrons from a generic donor. The reactivities of fat-soluble vitamins towards neutral reactive oxygen species were found to be larger than those of water-soluble vitamins towards these species, showing that the former are better antioxidants.

Chemical reactivity analysis of deoxyribonucleosides and deoxyribonucleoside analogues (NRTIs): a first-principles density functional approach.

The structures, energetics, as well as several important chemical parameters, of antiretroviral drugs - nucleoside reverse transcriptase inhibitors (NRTIs) - and natural deoxyribonucleosides in both neutral, and positively and negatively charged states, are investigated. These studies are carried out within the frame work of first-principles density-functional theory (DFT), using the Becke-Lee-Yang-Parr (BLYP) generalized gradient corrections to the local spin density approximation exchange and correlation energy, norm-conserving pseudopotentials and a plane-wave expansion of Kohn-Sham orbitals. Conceptual DFT is used to determine global and local chemical reactivity parameters. Our results are in good agreement with the best available experiments to date. The variation in the bond lengths and bond angles on cation formation indicates that the electron is lost from the base part of these molecules. Further, the presence of the deoxyribose sugar moiety lowers their ionization potential and increases their electron affinity, in comparison to the isolated DNA base. The effectiveness of the drug action in terminating the viral DNA chain, is explained using the global reactivity parameters, by comparing the reactivities of the drug molecules with those of the competing deoxyribonucleosides. The widely followed clinical practice, of avoiding the simultaneous administration of certain drugs, is also explained from the hardness and softness parameters. For most of the drug molecules, our study validates the generally accepted wisdom, that monophosphorylation is the crucial reaction step in the phosphorylation reaction in DNA nucleotide synthesis.

Characterisation and evaluation of pharmaceutical solvates of Atorvastatin calcium by thermoanalytical and spectroscopic studies.

BACKGROUND: Atorvastatin calcium (ATC), an anti-lipid biopharmaceutical class II drug, is widely prescribed as a cholesterol-lowering agent and is presently the world's best-selling medicine. A large number of crystalline forms of ATC have been published in patents. A variety of solid forms may give rise to different physical properties. Therefore, the discovery of new forms of this unusual molecule, ATC, may still provide an opportunity for further improvement of advantageous properties. RESULTS: In the present work, eight new solvates (Solvate I-VIII) have been discovered by recrystallization method. Thermal behaviour of ATC and its solvates studied by DSC and TGA indicate similar pattern suggesting similar mode of entrapment of solvent molecules. The type of solvent present in the crystal lattice of the solvates is identified by GC-MS analysis and the stoichiometric ratio of the solvents is confirmed by 1HNMR. The high positive value of binding energy determined from thermochemical parameters indicates deep inclusion of the solvent molecules into the host cavity. The XRPD patterns point towards the differences in their crystallanity, however, after desolvation solvate II, III, IV, V and VIII transform to isostructral amorphous desolvated solvates. The order of crystallinity was confirmed by solution calorimetric technique as the enthalpy of solution is an indirect measure of lattice energy. All the solvates behaved endothermically following the order solvate-VIII (1-butanol solvate) < solvate-I (isoproplyate) < solvate-V (methanol solvate) < solvate-III (ethonalate) < solvate-VI (acetone ethanol solvate) < solvate-IV (t-butanol solvate) < solvate-II (THF solvate) < solvate-VII (mixed hemi-ethanol hydrate). The positive value of the heat capacity of the solvate formation provides information about the state of solvent molecules in the host lattice. The solvents molecules incorporated in the crystal lattice induced local chemical environment changes in the drug molecules which are observed in 13CP/MAS NMR spectral changes. CONCLUSIONS: Aqueous solubility of solvate-VIII was found to be maximum, however, solvate-I and VIII showed better reduction in total cholesterol and triglyceride levels as compared to atorvastatin against triton-induced dyslipidemia.