Computational Studies on Structural, Excitation, and Charge-​Transfer Properties of Ureidopeptidomimetics

Peptides with ureido group enclosing backbones are considered peptidomimetics and are known for their higher stabilities, biocompatibilities, antibiotic, inhibitor, and charge-​transduction activities. These peptidomimetics have some unique applications, which are quite different from those of natural peptides. Hence, it is imperative to appreciate their properties at a microscopic level. In this regard, this work outlines, in detail, the charge transfer (CT) properties, hole-​migration dynamics, and electronic structures of various exptl. comprehended ureidopeptidomimetic models using d. functional theory (DFT)​. Time-​dependent DFT and complete active space SCF computations on basic models provide the necessary evidence for the viability of CT from the end enfolding the ureido group to the other end with a carboxylate entity. This donor-​to-​acceptor CT has been reflected in excitation studies, in which the higher intensity band corresponds to CT from the π orbital of the ureido group to the π* orbital of the carboxylate entity. Further, hole-​migration studies have shown that charge can evolve from the ureido end, whereas the hole generated at the carboxylate end does not migrate. However, hole migration has been reported to occur from both ends (amino and carboxylate ends) in glycine oligopeptides, and our studies show that the ability to transfer and migrate charge can be tuned by modifying the donor and acceptor functional groups in both the neutral and cationic charge states. We have analyzed the possibility of hole migration following ionization using DFT-​based wave-​packet propagation and found its occurrence on a ∼2-​5 fs time scale, which reflects the charge-​transduction ability of peptidomimetics

Effect of Donor and Acceptor on Optoelectronic Properties of Benzo[1,2-B:4,5-B′]dithiophene

A series of acceptor and donor groups anchored to benzo[1,2-b:4,5-b′]dithiophene (BDT) molecule have been systematically investigated at the density functional theory (DFT) and time-dependent density functional theory (TDDFT) level to reveal structure–property relationships, charge transfer, and fluorescence lifetimes. The DFT optimization shows that the hetero atom in the ring induces the polarity from central ring to both ends of the thiophene ring, participating in the conjugation. The donor and acceptor groups were anchored at the terminals of the BDT at two different positions to fine-tune the properties according to the requirement and study the push–pull effect. All the models studied in this work retain their aromaticity as estimated from NICS(0) and NICS(1) aromaticity index in ground and excited states. The results show that the hardness, softness, HOMO–LUMO gaps, ionization potentials (IP), and electron affinities (EA) of the BDTs are significantly affected by the electron-withdrawing and electron-donating groups. The 1H and 13C NMR chemical shift values have been computed to quantify the push–pull effect. Further, the charge transfer properties in these BDTs were explored based on reorganization energies and diagnostic descriptors derived from hole–electron theory that present different electron excitation behavior. The relationship between the computed variables such as highest occupied molecular orbital, lowest unoccupied molecular orbital, oscillator strength, dipole moment, absorption, and fluorescence energy correlates the system with one another and also to extend the possible applications of the system in optical devices. Structure–property relationship of various BDTs reveal that, upon optical excitation, the resonance effect plays an important role changing the bonding character between the substituent and BDT unit, enabling efficient electron delocalization. The examination of TDDFT results indicates that among the various models studied in this work, nitro-substituted model is better candidate for optoelectronic properties with relatively large absorption wavelength and long fluorescence lifetime

DFT Studies on the Influence of Ligation on Optical and Redox Properties of Bimetallic [Au4M2] Clusters

Density functional theory based calculations have been employed to understand the lowest energy conformers of bare [Au4M2] and ligated [Au4M2(SCH3)6] and [Au4M2(PH3)6]2+ (where M = Au, Cu, Ag, Ni, Pd, Pt) clusters in the gas phase and in various implicit solvent media (water, DMSO and DCM). Computations predict that [Au4M2] clusters in all three charge states exist with a planar 2D-geometry, with distortion introduced by the hetero atoms. And the studies show that the ligation promotes a 2D to 3D geometrical conversion either through bridging coordination or single bond formation. The sulfur atom in the thiol ligand becomes a part of the cluster skeleton, while the PH3 forms a passivation layer around the cluster. Moreover, the presence of sulfur in the cluster skeleton increases the chemical stability of coinage metal containing clusters, while stability decreases for d8 metal containing clusters. And the PH3 passivation layer decreases the chemical stability of both coinage metal and d8 metal atom containing clusters. The computed redox behaviors show that the addition of an electron requires less energy compared to that needed for removal, and both occur with negligible geometrical reorganisation. The calculated blue shift in excitation energy values show a ligand to metal charge transfer in the -SCH3 ligated cluster. However, the red shift in wavelength is observed for the -PH3 passivated cluster, which corresponds to excitation from the HOMO to LUMO, where the orbitals have an equal contribution from the metals and ligands

Redox Executed Logic Operations through the Reversible Voltammetric Response Characteristics of Electroactive Self-Assembled Monolayers

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Computational Studies on Ground and Excited State Charge Transfer Properties of Peptidomimetics

Chemical modifications at various peptide positions result in peptidomimetics with unique physical and chemical properties that can be used for a range of applications. Among many peptidomimetics, ureidopeptides are interesting due to their ability to act as donor–bridge–acceptor systems through which charge transfer occurs in one direction and can be triggered by an electrochemical pulse without perturbing the nuclear position. In this regard, some UP mimetics with different chromophoric units are studied in this work to understand their role using DFT based methods. Computational results and natural charge analysis provide evidence for the extensive contribution of the substituents to the excitation and hole migration dynamics. Further, the results show that the UP backbone preserves its uni-directional charge transfer phenomenon from the ureido to carboxylate terminal irrespective of the terminal groups and position. However, the substituent affects the excitation energies and the time scales of the hole migration. Among the substituents studied here, fluorine migrates to the hole within a shorter time scale while phenyl groups take longer

Density functional theoretical investigation of the aromatic nature of BN substituted benzene and four ring polyaromatic hydrocarbons

We have studied the topological and local aromaticity of BN-substituted benzene, pyrene, chrysene, triphenylene and tetracene molecules. The nucleus-independent chemical shielding (NICS), harmonic oscillator model of aromaticity (HOMA), para-delocalization index (PDI) and aromatic fluctuation index (FLU) have been calculated to quantify aromaticity in terms of magnetic and structural criteria. We find that charge separations due to the introduction of heteroatoms largely affect both the local and topological aromaticity of these molecules. Our studies show that the presence of any kind of heteroatom in the ring not only reduces the local delocalization in the six membered ring, but also affects strongly the topological aromaticity. In fact, the relative orders of the topological and local aromaticity depend strongly on the position of the heteroatoms in the structure. In general, more ring shared BN containing molecules are less aromatic than the less ring shared BN molecules. In addition our results provide evidence that the structural stability of the molecule is dominated by the s bond rather than the π bond

computational studies on structural and excited-state properties of modified chlorophyll f with various axial ligands

Time-dependent density functional theory (TDDFT) calculations have been used to understand the excited-state properties of modified chlorophyll ƒ (Chlide ƒ), Chlide a, Chlide b, and axial ligated (with imidazole, H2O, CH3OH, CH3COOH, C6H5OH) Chlide ƒ molecules. The computed differences among the Qx, Qy, Bx, and By band absorbance wavelengths of Chlide a, b, and ƒ molecules are found to be comparable with the experimentally observed shifts for these bands in chlorophyll a (chl a), chl b, and chl ƒ molecules. Our computations provide evidence that the red shift in the Qy band of chl ƒ is due to the extended delocalization of macrocycle chlorin ring because of the presence of the -CHO group. The local contribution from the -CHO substituent to the macrocycle chlorin ring stabilizes the corresponding molecular orbitals (lowest unoccupied molecular orbital (LUMO) of the Chlide ƒ and LUMO-1 of the Chlide b). All the absorption bands of Chlide ƒ shift to higher wavelengths on the addition of axial ligands. Computed redox potentials show that, among the axial ligated Chlide f molecules, Chlide ƒ -imidazole acts as a good electron donor and Chlide ƒ -CH3COOH acts as a good electron acceptor