Zinc-Porphyrin Based Dyes for Dye-Sensitized Solar Cells

We have designed seven efficient sensitizers based on the zinc-porphyrin structure for dye sensitized solar cells (DSSCs). The geometries, electronic properties, light harvesting efficiency (LHE), and electronic absorption spectra of these sensitizers are studied using density functional theory (DFT) and time-dependent density functional theory (TD-DFT) calculations. We found that the designed sensitizers have smaller HOMO–LUMO energy with broadened and red-shifted absorption bands (300–1100 nm) having high molar extinction coefficient compared to the so far known best sensitizer (YD2-o-C8). The position of HOMO–LUMO energy level of these sensitizers ensures a positive effect on the process of electron injection and dye regeneration. Our theoretical calculations reveal that the new sensitizer can be used as a potential sensitizer for DSSCs compared to YD2-o-C8

Structural Stability and Electronic Properties of CdS Condensed Clusters

First-principles calculations are carried out to understand the structural stability and electronic properties of 1-D condensed clusters and their fundamental building blocks, Cd_<i>n</i>S_<i>n</i> (<i>n</i> = 1–6) small clusters. By linear stacking of these stable isomers, the condensed clusters, (Cd_<i>n</i>S_<i>n</i>)_<i>m</i>, where <i>n</i> = 1–4 and <i>m</i> = 1–9, are modeled. The structural stability of condensed clusters and their building blocks are obtained from the electronic density of states, and it infers that s–p hybridizations play a crucial role in stabilizing these clusters. Electronic properties of all condensed clusters, with <i>m</i> > 4, are interesting in photocatalytic applications as they have a lesser energy gap than that of bulk. Our calculations also show that the (Cd₃S₃)_<i>m</i> clusters are energetically more stable as compared with other-sized condensed clusters, but such clusters can fragment into two smaller clusters by an application of an external temperature, on the order of 450 K

How Different Are Aromatic π Interactions from Aliphatic π Interactions and Non-π Stacking Interactions?

We compare aromatic π interactions with aliphatic π interactions of double- and triple-bonded π systems and non-π stacking interactions of single-bonded σ systems. The model dimer systems of acetylene (C₂H₂)₂, ethylene (C₂H₄)₂, ethane (C₂H₆)₂, benzene (C₆H₆)₂, and cyclohexane (C₆H₁₂)₂ are investigated. The ethylene dimer has large dispersion energy, while the acetylene dimer has strong electrostatic energy. The aromatic π interactions are strong with particularly large dispersion and electrostatic energies, which would explain why aromatic compounds are frequently found in crystal packing and molecular self-engineering. It should be noted that the difference in binding energy between the benzene dimer (aromatic–aromatic interactions) and the cyclohexane dimer (aliphatic–aliphatic interactions) is not properly described in most density functionals

Influence of the Substituents on the CH...π Interaction: Benzene–Methane Complex

Recently we showed that the binding energy of the benzene...acetylene complex could be tuned up to 5 kcal/mol by substituting the hydrogen atoms of the benzene molecule with multiple electron-donating/electron-withdrawing groups (<i>J. Chem. Theory Comput.</i> <b>2012</b>, <i>8</i>, 1935). In continuation, we have here examined the influence of various substituents on the CH...π interaction present in the benzene...methane complex using the CCSD­(T) method at the complete basis set limit. The influence of multiple fluoro substituents on the interaction strength of the benzene...methane complex was found to be insignificant, while the interaction strength linearly increases with successive addition of methyl groups. The influence of other substituents such as CN, NO₂, COOH, Cl, and OH was found to be negligible. The NH₂ group enhances the binding strength similarly to the methyl group. Energy decomposition analysis predicts the dispersion energy component to be on an average three times larger than the electrostatic energy component. Multidimensional correlation analysis shows that the exchange-repulsion and dispersion terms are correlated very well with the interaction distance (<i>r</i>) and with a combination of the interaction distance (<i>r</i>) and molar refractivity (MR), while the electrostatic component correlates well when the Hammett constant is used in combination with the interaction distance (<i>r</i>). Various recently developed DFT methods were used to assess their ability to predict the binding energy of various substituted benzene...methane complexes, and the M06-2X, B97-D, and B3LYP-D3 methods were found to be the best performers, giving a mean absolute deviation of ∼0.15 kcal/mol

Tuning the C–H···π Interaction by Different Substitutions in Benzene–Acetylene Complexes

The influence of substitutions in aromatic moieties on the binding strength of their complexes is a subject of broad importance. Using a set of various substituted benzenes, Sherrill and co-workers (J. Am. Chem. Soc. 2011, 133, 13244; J. Phys. Chem. A 2003, 107, 8377) recently showed that the strength of a stacking interaction (π···π interaction) is enhanced by adding substituents regardless of their nature. Although the binding strength of an activated C–H···π interaction is comparable to that of a stacking interaction, a similar systematic study is hitherto unknown in the literature. We have computed the stabilization energies of the C–H···π complex of acetylene and multiple fluoro-/methyl-substituted benzenes at the coupled-cluster single and double (triple) excitation [CCSD­(T)]/complete basis set (CBS) limit. The trend for interaction energies was found to be hexafluorobenzene–acetylene < <i>sym</i>-tetrafluorobenzene–acetylene < <i>sym</i>-trifluorobenzene–acetylene < <i>sym</i>-difluorobenzene–acetylene < benzene–acetylene < <i>sym</i>-dimethylbenzene–acetylene < <i>sym</i>-trimethylbenzene–acetylene < <i>sym</i>-tetramethylbenzene–acetylene < hexamethylbenzene–acetylene. Therefore, contrary to the case of stacking interaction (Hohenstein et al. J. Am. Chem. Soc. 2011, 133, 13244), we show here that electron-withdrawing groups weaken the dimer while electron-donating groups strengthen the interaction energy of the dimer. Various recently developed density functional theoretic (DFT) methods were assessed for their performance and the M05-2X, M06-2X, and ωB97X-D methods were found to be the best performers. These best DFT performers were employed in determining the influence of other representative substituents (-NO₂, -CN, -COOH, -Br, -Cl, -OH, and -NH₂) as an extension to the above work. The results for the complex of acetylene and various para-disubstituted benzenes revealed a trend in binding energies that is in accordance with the ring-activating/deactivating capacity of each of these groups. The stabilization energy was partitioned via the DFT symmetry-adapted perturbation theory (SAPT) method, and both dispersion and electrostatic interactions were seen to be major driving forces for the complex stabilization. Interestingly, the sum of the energy contributors such as dispersion, exchange, induction, etc., is close to zero and the total energy follows the trend of the electrostatic energy. We observe an excellent linear correlation between the optimized intermolecular separation of the different complexes and the exchange/dispersion interaction

Self-Assembly of Manganese(I)-Based Molecular Squares: Synthesis and Spectroscopic and Structural Characterization

Syntheses of manganese­(I)-based molecular squares have been accomplished in facile one-pot reaction conditions at room temperature. Self-assembly of eight components has resulted in the formation of M₄L₄-type metallacyclophanes [Mn­(CO)₃Br­(μ-L)]₄ (<b>1</b>–<b>3</b>) using pentacarbonylbromomanganese as metal precursor and rigid azine ligands such as pyrazine, 4,4′-bipyridine, and <i>trans</i>-1,2-bis­(4-pyridyl)­ethylene, respectively, as bridging ligands. The metallacyclophanes have been characterized on the basis of IR, NMR, and UV–vis spectroscopic techniques and single-crystal X-ray diffraction methods