Search Results (20)

Understanding electron density migration along excited-state pathways in photochemical systems is critical for optimizing solar energy conversion processes. In this study, we investigate photoinduced electron transfer (PET) in a covalently linked donor–bridge–acceptor (D-B-A) system, where [Cu(I)-bis(1,10-phenanthroline)]⁺ acts as an electron donor, and anthraquinone, tethered to one of the phenanthroline ligands via a vibrationally active ethyne bridge, behaves as an electron acceptor. Visible transient absorption spectroscopy revealed the dynamic processes occurring in the excited state, including PET to the acceptor species. This was indicated by the spectral features of the anthraquinone radical anion that appeared on a timescale of 30 ps in polar solvents. Time-resolved infrared (TRIR) spectroscopy of the alkyne vibration (CC stretch) of the ethyne bridge provided insight into electronic structural changes in the metal-to-ligand charge transfer (MLCT) state and along the PET reaction coordinate. The observed spectral shift and enhanced transition dipole moment of the CC stretch demonstrated that there was already partial delocalization to the anthraquinone acceptor following MLCT excitation, verified by DFT calculations. An additional excited-state TRIR signal unrelated to the vibrational mode highlighted delocalization between the phenanthroline ligands in the MLCT state. This signal decayed and the CC stretch narrowed and shifted towards the ground-state frequency following PET, indicating a degree of localization onto the acceptor species. This study experimentally elucidates charge redistribution during PET in a Cu(I) diimine D-B-A system, yielding important information on the ligand design for optimizing PET reactions. Full article

Cancer has a threatening impact on human health, and it is one of the primary causes of fatalities worldwide. Different conventional treatments have been employed to treat cancer, but their non-specific nature reduces their therapeutic efficacy. This study employs a C₅N₂-based targeted drug carrier to study the delivery mechanism of anticancer drugs, particularly cisplatin, carmustine, and mechlorethamine, using density functional theory (DFT). The geometries of the drugs, the C₅N₂ substrate, and the drug@C₅N₂ complexes were optimized at the PBE0-D3BJ/def2SVP level of theory. Interaction energy was computed for the complexes which follow the trend, i.e., cisplatin@C₅N₂ (−27.60 kcal mol⁻¹) > carmustine@C₅N₂ (−19.69 kcal mol⁻¹) > mechlorethamine@C₅N₂ (−17.79 kcal mol⁻¹). The non-covalent interaction (NCI) and quantum theory of atoms in molecules (QTAIM) analyses confirmed the presence of van der Waals forces between the carmustine@C₅N₂ and mechlorethamine@C₅N₂ complexes, while weak hydrogen bonding has also been observed between the cisplatin@C₅N₂ complex. Electron localization function (ELF) analysis was performed to analyze the degree of delocalization of electrons within the complexes. The electronic properties of the analytes and the C₅N₂ substrate confirmed the enhanced reactivity of the complexes and illustrated electron density shift between the drugs and the C₅N₂ sheet. Recovery time was determined to assess the biocompatibility and the desorption behavior of the drugs. Moreover, negative solvation energies and increased dipole moments in a solvent phase manifested enhanced solubility and easy circulation of the drugs in biological media. Subsequently, this study illustrates that cisplatin@C₅N₂, carmustine@C₅N₂, and mechlorethamine@C₅N₂ complexes can be utilized as efficient drug delivery systems. Full article

This study reports the synthesis, characterization, and property analysis of four novel multilayer 3D polymers (1A to 1D) with 1,3-phenyl bridge architectures spanning 248 to 320 layers. High-molecular-weight polymers were successfully synthesized via catalytic Suzuki–Miyaura cross-coupling over a four-day reaction period. Structures, thermal, and optical properties were examined using multiple analytical techniques. Fourier transform-infrared (FT-IR) spectroscopy was used to study the hydrogen bonding within the polymer system, suggesting the formation of the polymer through the Suzuki–Miyaura coupling reaction. Ultraviolet–visible (UV-vis) spectroscopy indicated strong electronic delocalization, with maximum absorbance peaks between 257 and 280 nm. Thermal characterization, using differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA), was used to investigate the thermal stability. TGA results showed that all four polymers retained more than 20% of their initial mass at 1000 °C, indicating good thermal stability across the series. DSC analysis revealed that polymer 1A exhibited a glass transition temperature (Tg) of 167 °C, indicating the presence of a network formed by aromatic conjugation and hydrogen bonding. Furthermore, the subtle Tg step observed for 1A suggests a degree of crystallinity within the polymer matrix, which was further supported by X-ray diffraction (XRD) analysis. Aggregation-induced emission (AIE) experiments provided further insights into intermolecular packing, and scanning electron microscopy (SEM) contributed to a better understanding of the morphology of the obtained polymers. These results highlight the potential of these polymers as thermally stable and conductive materials for biomedical and industrial applications. Full article

Based on density functional theory (DFT) and wave function analysis, the ultraviolet and visible spectrophotometry (UV-Vis) spectra and Raman spectra of 1-meso and 1-rac obtained by the chiral separation of chiral nanographenes are theoretically investigated. The electron excitation properties of 1-meso and 1-rac are studied by means of transition density matrix (TDM) and charge density difference (CDD) diagrams. The intermolecular interaction is discussed based on an independent gradient model based on Hirshfeld partition (IGMH). The interaction of 1-meso and 1-rac with the external environment is studied using the electrostatic potential (ESP), and the electron delocalization degree of 1-meso and 1-rac is studied based on the magnetically induced current under the external magnetic field. Through the chiral separation of 1-rac, two enantiomers, 1-(P, P) and 1-(M, M), were obtained. The electrical–magnetic interaction of the molecule is revealed by analyzing the electron circular dichroism (ECD) spectra of 1-meso, 1-(P, P) and 1-(M, M), the transition electric dipole moment (TEDM) and the transition magnetic dipole moment (TMDM). It is found that 1-(P, P) and 1-(M, M) have opposite chiral properties due to the inversion of the structure. Full article

H-bonding has achieved massive advancements by utilizing an H-bond donor (HBD) to interact with the electron-rich site of the substrate, and an H-bond acceptor (HBA) to coordinate with the electron-deficient site. Rapid transformation is often correlated with the acidity of HBD, namely the degree of charge deficiency of the hydrogen proton. In addition, the positive cations were employed to enhance the HBD; the electron-withdrawing groups were also a dissimilar approach for increasing the capability of the H-bond donor. We first introduced the H-bonding organic ion pair tris(phenylamino)cyclopropenium (TPAC·Cl) into the Friedel–Crafts alkylation of indoles with nitroalkenes, which was implemented via vicinal positive charges on the cyclopropenium core. The counter ion chloride anion became a potential HBA to activate the electron-deficient part of the substrate. X-ray analyses of a single crystal of TPAC·Cl described the 3D architecture and the delocalized cationic charge in the solid state. The aromatic cyclopropenium endowed the N–H moieties with the ability of the H-bond donor to activate the nitroalkene; meanwhile, the chloride anion acted as the H-bond acceptor to activate the indole. The amino-cyclopropenium-offered HBD and HBA displayed cooperative organocatalysis in the Friedel–Crafts alkylation of indole with nitroalkene. A new class of hydrogen bonding catalysis and a working mechanism were proposed. Full article

Metal chalcogenides are primarily used for thermoelectric applications due to their enormous potential to convert waste heat into valuable energy. Several studies focused on single or dual aliovalent doping techniques to enhance thermoelectric properties in semiconductor materials; however, these dopants enhance one property while deteriorating others due to the interdependency of these properties or may render the host material toxic. Therefore, a strategic doping approach is vital to harness the full potential of doping to improve the efficiency of thermoelectric generation while restoring the base material eco-friendly. Here, we report a well-designed counter-doped eco-friendly nanomaterial system (~70 nm) using both isovalent (cerium) and aliovalent (cobalt) in a Bi₂Se₃ system for enhancing energy conversion efficiency. Substituting cerium for bismuth simultaneously enhances the Seebeck coefficient and electrical conductivity via ionized impurity minimization. The boost in the average electronegativity offered by the self-doped transitional metal cobalt leads to an improvement in the degree of delocalization of the valence electrons. Hence, the new energy state around the Fermi energy serving as electron feed to the conduction band coherently improves the density of the state of conducting electrons. The resulting high power factor and low thermal conductivity contributed to the remarkable improvement in the figure of merit (zT = 0.55) at 473 K for an optimized doping concentration of 0.01 at. %. sample, and a significant nanoparticle size reduction from 400 nm to ~70 nm, making the highly performing materials in this study (${Bi}_{2-x}{Ce}_x{Co}_{\raisebox{1ex}{$2x$}\!\left/ \!\raisebox{-1ex}{$3$}\right.}{Se}_3$) an excellent thermoelectric generator. The results presented here are higher than several Bi₂Se₃-based materials already reported. Full article

Frenkel excitons are responsible for the transport of light energy in many molecular systems. Coherent electron dynamics govern the initial stage of Frenkel-exciton transfer. Capability to follow coherent exciton dynamics in real time will help to reveal their actual contribution to the efficiency of light-harvesting. Attosecond X-ray pulses are the tool with the necessary temporal resolution to resolve pure electronic processes with atomic sensitivity. We describe how attosecond X-ray pulses can probe coherent electronic processes during Frenkel-exciton transport in molecular aggregates. We analyze time-resolved absorption cross section taking broad spectral bandwidth of an attosecond pulse into account. We demonstrate that attosecond X-ray absorption spectra can reveal delocalization degree of coherent exciton transfer dynamics. Full article

Multilayered van der Waals heterostructures based on transition metal dichalcogenides are suitable platforms on which to study interlayer (dipolar) excitons, in which electrons and holes are localized in different layers. Interestingly, these excitonic complexes exhibit pronounced valley Zeeman signatures, but how their spin-valley physics can be further altered due to external parameters—such as electric field and interlayer separation—remains largely unexplored. Here, we perform a systematic analysis of the spin-valley physics in MoSe${}_2$/WSe${}_2$ heterobilayers under the influence of an external electric field and changes of the interlayer separation. In particular, we analyze the spin ($S_z$) and orbital ($L_z$) degrees of freedom, and the symmetry properties of the relevant band edges (at K, Q, and $\mathsf{\Gamma }$ points) of high-symmetry stackings at 0° (R-type) and 60° (H-type) angles—the important building blocks present in moiré or atomically reconstructed structures. We reveal distinct hybridization signatures on the spin and the orbital degrees of freedom of low-energy bands, due to the wave function mixing between the layers, which are stacking-dependent, and can be further modified by electric field and interlayer distance variation. We find that H-type stackings favor large changes in the g-factors as a function of the electric field, e.g., from $-5$ to 3 in the valence bands of the H${}_{\mathrm{h}}^{\mathrm{h}}$ stacking, because of the opposite orientation of $S_z$ and $L_z$ of the individual monolayers. For the low-energy dipolar excitons (direct and indirect in k-space), we quantify the electric dipole moments and polarizabilities, reflecting the layer delocalization of the constituent bands. Furthermore, our results show that direct dipolar excitons carry a robust valley Zeeman effect nearly independent of the electric field, but tunable by the interlayer distance, which can be rendered experimentally accessible via applied external pressure. For the momentum-indirect dipolar excitons, our symmetry analysis indicates that phonon-mediated optical processes can easily take place. In particular, for the indirect excitons with conduction bands at the Q point for H-type stackings, we find marked variations of the valley Zeeman (∼4) as a function of the electric field, which notably stands out from the other dipolar exciton species. Our analysis suggests that stronger signatures of the coupled spin-valley physics are favored in H-type stackings, which can be experimentally investigated in samples with twist angle close to 60°. In summary, our study provides fundamental microscopic insights into the spin-valley physics of van der Waals heterostructures, which are relevant to understanding the valley Zeeman splitting of dipolar excitonic complexes, and also intralayer excitons. Full article

The experimental and theoretical study of the influence of metal complexing on geometry, aromaticity, chirality, and the ability to twist the nematic phase by complexes based on modified natural chlorin e6 was carried out. The geometry optimization of the chlorin e6 13(N)-methylamide-15,17-dimethyl ester (MADMECl) and its Zn, Cu, and Ni complexes by DFT (CAM-B3LYP/6–31 G(d,p) functional) method was performed. Based on these calculations, the acoplanarity degree of the macrocyclic ligand and the distortion energy of its dianion were estimated, which allowed the arrangement of the MADMECl complexes in the series Ni > Cu > Zn. Aromaticity was evaluated using the NICS criterion (nuclear independent chemical shift). An increase in the degree of aromaticity of the macrocycle upon complex formation was established. At the same time, the aromaticity of the inner conjugation contour corresponds to the same series as the acoplanarity, while the outer π-delocalization is characterized by the reverse sequence. An experimental evaluation of the electron circular dichroism of the Soret and the Q-bands, as well as the g-factor of dissymmetry, was carried out. The growth of these parameters with an increase in the degree of acoplanarity and aromaticity of the internal conjugation contour was determined. The induction of helical phases in mixtures of nematic liquid crystals (LCs) based on cyanobiphenyls and MADMECl macrocyclic metal complexes was studied by polarization microscopy, and the clearance temperatures and helix pitch of the mesophases were measured. A strong effect of the metal on the phase transition temperature and helical twisting power was established. Full article

A novel two-branched twistacene (PyDN) has been designed and synthesized for application on ultrafast optical limiting. This twistacene exhibits excellent two photon absorption and two photon absorption-induced excited singlet state absorption, which was systematically investigated with a femtosecond Z-scan experiment, transient absorption spectrum, and two-photon excited fluorescence experiments. The admirable two photon absorption is attributed to the high degree of π electron delocalization in twistacene which is caused by introduction of two strong donors. The excited singlet state absorption cooperates with two-photon absorption to provide an excellent ultrafast optical limiting behavior with high linear transmittance, where the thresholds are 2.3–5.3 mJ/cm² in the spectral region of 532–800 nm of femtosecond laser and 133 mJ/cm² for picosecond pulse at 532 nm. These thresholds are lower than that of most of the optical limiters reported previously, which indicates PyDN is a promising candidate for ultrafast optical limiting. Full article

Keto-enol prototropic conversions for carbonyl compounds and phenols have been extensively studied, and many interesting review articles and even books appeared in the last 50 years. Quite a different situation takes place for derivatives of biologically active azulene, for which only scanty information on this phenomenon can be found in the literature. In this work, quantum-chemical studies have been undertaken for symmetrically and unsymmetrically substituted azulenols (constitutional isomers of naphthols). Stabilities of two enol (OH) rotamers and all possible keto (CH) tautomers have been analyzed in the gas phase {DFT(B3LYP)/6-311+G(d,p)} and also in aqueous solution {PCM(water)//DFT(B3LYP)/6-311+G(d,p)}. Contrary to naphthols, for which the keto forms can be neglected, at least one keto isomer (C1H, C2H, and/or C3H) contributes significantly to the tautomeric mixture of each azulenol to a higher degree in vacuo (non-polar environment) than in water (polar amphoteric solvent). The highest amounts of the CH forms have been found for 2- and 5-hydroxyazulenes, and the smallest ones for 1- and 6-hydroxy derivatives. The keto tautomer(s), together with the enol rotamers, can also participate in deprotonation reaction leading to a common anion and influence its acid-base properties. The strongest acidity in vacuo exhibits 6-hydroxyazulene, and the weakest one displays 1-hydroxyazulene, but all azulenols are stronger acids than phenol and naphthols. Bond length alternation in all DFT-optimized structures has been measured using the harmonic oscillator model of electron delocalization (HOMED) index. Generally, the HOMED values decrease for the keto tautomers, particularly for the ring containing the labile proton. Even for the keto tautomers possessing energetic parameters close to those of the enol isomers, the HOMED indices are low. However, some kind of parallelism exists for the keto forms between their relative energies and HOMEDs estimated for the entire molecules. Full article

The standard D-G-2D pattern of Raman spectra of sp² amorphous carbons is considered from the viewpoint of graphene domains presenting their basic structure units (BSUs) in terms of molecular spectroscopy. The molecular approximation allows connecting the characteristic D-G doublet spectra image of one-phonon spectra with a considerable dispersion of the C=C bond lengths within graphene domains, governed by size, heteroatom necklace of BSUs as well as BSUs packing. The interpretation of 2D two-phonon spectra reveals a particular role of electrical anharmonicity in the spectra formation and attributes this effect to a high degree of the electron density delocalization in graphene domains. A size-stimulated transition from molecular to quasi-particle phonon consideration of Raman spectra was experimentally traced, which allowed evaluation of a free path of optical phonons in graphene crystal. Full article

The esterification of malic acid using traditional homogenous catalysts suffers from the difficulty in reuse of the catalyst and undesirable side reactions. In this work, Zr(SO₄)₂/SiO₂ and Zr(SO₄)₂/activated carbon (AC) as solid acid catalysts were prepared for malic acid esterification with methanol. The conversion of malic acid over these two catalysts is comparable to that over H₂SO₄ and unsupported Zr(SO₄)₂∙4H₂O catalysts; however; a 99% selectivity of dimethyl malate can be realized on these two supported catalysts, which is much higher than that of conventional H₂SO₄ (75%) and unsupported Zr(SO₄)₂∙4H₂O (80%) catalysts, highlighting the critical role of AC and SiO₂ supports in tuning the selectivity. We suggest that the surface hydroxyls of AC or lattice O²⁻ ions from SiO₂ donate electrons to Zr⁴⁺ in Zr(SO₄)₂/AC and Zr(SO₄)₂/SiO₂ catalysts, which results in the increase in electron density on Zr⁴⁺. The enhanced electron density on Zr⁴⁺ reduces the degree of H delocalization from crystal water and then decreases the Brønsted acid strength. Consequently, the reduced Brønsted acid strength of Zr(SO₄)₂/AC and Zr(SO₄)₂/SiO₂ catalysts suppresses the intermolecular dehydration side reaction. In addition, these two supported catalysts can be easily separated from the reaction system by simple filtration with almost no loss of activity. Full article

Our study of tunnelling in proton-coupled electron transfer (PCET) oxidation of ascorbate with hexacyanoferrate(III) follows the insights obtained from ultrafast 2D IR spectroscopy and theoretical studies of the vibrational water dynamics that led to the proposal of the involvement of collective intermolecular excitonic vibrational water dynamics in aqueous chemistry. To test the proposal, the hydrogen tunnelling modulation observed in the PCET reaction studied in the presence of low concentrations of various partial hydrophobic solutes in the water reaction system has been analyzed in terms of the proposed involvement of the collective intermolecular vibrational water dynamics in activation process in the case. The strongly linear correlation between common tunnelling signatures, isotopic values of Arrhenius prefactor ratios ln A_H/A_D and isotopic differences in activation enthalpies ΔΔH^‡ (H,D) observed in the process in fairly diluted water solutions containing various partial hydrophobic solutes (such as dioxane, acetonitrile, ethanol, and quaternary ammonium ions) points to the common physical origin of the phenomenon in all the cases. It is suggested that the phenomenon can be rooted in an interplay of delocalized collective intermolecular vibrational dynamics of water correlated with vibrations of the coupled transition configuration, where the donor-acceptor oscillations, the motions being to some degree along the reaction coordinate, lead to modulation of hydrogen tunnelling in the reaction. Full article

Tetrel bonds, the purportedly non-covalent interaction between a molecule that contains an atom of group 14 and an anion or (more generally) an atom or molecule with lone electron pairs, are under intense scrutiny. In this work, we perform an interacting quantum atoms (IQA) analysis of several simple complexes formed between an electrophilic fragment (A) (CH₃F, CH₄, CO₂, CS₂, SiO₂, SiH₃F, SiH₄, GeH₃F, GeO₂, and GeH₄) and an electron-pair-rich system (B) (NCH, NCO-, OCN-, F-, Br-, CN-, CO, CS, Kr, NC-, NH₃, OC, OH₂, SH-, and N₃-) at the aug-cc-pvtz coupled cluster singles and doubles (CCSD) level of calculation. The binding energy ( $E_{\mathrm{bind}}^{\mathrm{AB}}$ ) is separated into intrafragment and inter-fragment components, and the latter in turn split into classical and covalent contributions. It is shown that the three terms are important in determining $E_{\mathrm{bind}}^{\mathrm{AB}}$ , with absolute values that increase in passing from electrophilic fragments containing C, Ge, and Si. The degree of covalency between A and B is measured through the real space bond order known as the delocalization index ( ${\delta }^{\mathrm{AB}}$ ). Finally, a good linear correlation is found between ${\delta }^{\mathrm{AB}}$ and $E_{\mathrm{xc}}^{\mathrm{AB}}$ , the exchange correlation (xc) or covalent contribution to $E_{\mathrm{bind}}^{\mathrm{AB}}$ . Full article