HOMO-LUMO Transition Research Articles

Resonance Effects from Substituents on L-Type Ligands Mediate Synthetic Control of Gold Nanocluster Frontier Orbital Energies.

The ligands of metal nanoclusters can be used to control their properties and reactivity, but a framework guiding their use remains elusive. Hammett studies of Au8(PPh3)72+ and Au9(PPh3)83+ nanoclusters with para- and meta-methyl and -methoxy groups indicate that resonance effects, not inductive effects, yield quantitative shifts of the HOMO-LUMO transitions involving orbitals local to the cluster core. Individual ligand exchanges reveal that these shifts are caused by only four of seven ligands, inconsistent with inductive effects. Quantum chemical calculations predict no trend in Au atom charges with respect to Hammett parameter but do predict bond length trends expected for a resonance structure that includes the Au atoms. Computed orbitals show contributions from specific para-OMe oxygen lone pairs to the HOMO, indicating delocalization from the core to specific ligands. These results suggest that resonance structures could be drawn including Au and ligands, guiding efforts to modulate nanocluster electronic structure and energy transfer.

Molecular Engineering for UV-Vis to NIR Absorption/Emission Bands of Pyrazine-based A-π-D- π-A Switches to Design TiO2 Tuned Dyes: DFT Insights.

This study investigates the tuning of the UV-Vis/NIR absorption bands of pyrazine-based A-D-A switches for designing efficient UV retardancy over TiO2 surfaces. The electronic properties and optical characteristics of seven dyes (DP1-DP7) were analyzed using computational methods. The results indicate that the dyes possessed distinct UV-Vis/NIR absorption properties. Their absorption wavelengths ranged from 389 to 477nm, with corresponding energies ranging from 2.59 to 3.19eV. The major contributions to the absorption were found to be the HOMO-LUMO transitions, varying from 86 to 96%. The dyes exhibited different donor (D) and acceptor (A) groups, influencing their electronic properties and absorption characteristics. The tunable electronic and optical properties of these dyes make them promising candidates for applications requiring UV protection for TiO2-based materials. The results contribute to understand the structure-property relationships in the design of UV-Vis/NIR absorbers and provide a foundation for further experimental investigations in the field of UV retardancy.

20π-Electron Antiaromatic Benziphthalocyanines with Absorption Reaching the Near-Infrared-II Region.

Although second near-infrared (NIR-II, 1000-1500 nm) light has attracted considerable attention, especially for life sciences applications, the development of organic dyes with NIR-II absorption remains a formidable challenge. Herein we report the design, synthesis, and electronic properties of 20π-electron antiaromatic benziphthalocyanines (BPcs) that exhibit intense absorption bands in the NIR region. The strong, low-energy absorption of the antiaromatic BPcs is attributed to electric-dipole-allowed HOMO-LUMO transitions with narrow band gaps, enabled by the reduced structural symmetry of BPc compared with regular porphyrins and phthalocyanines. The combination of peripheral substituents and a central metal decreases the HOMO-LUMO energy gaps, leading to the extension of the absorption bands into the NIR-II region (reaching 1100 nm) under reductive conditions.

Color modulation in organometallic dyes. Purple-colored acyclic carbenes derived from 2-isocyanoazulene gold(I) complexes.

This study reports new carbene azulene gold(I) complexes [AuCl{C(NHAz)(NR2)}] (R = Me (1) and nBu (2)) and [Au(C6F4OC10H21){C(NHAz)(NR2)}] (R = Me (3) and nBu (4)) prepared by reaction of 2-isocyanoazulene gold(I) complexes [AuX(CNAz)] (X = Cl and C6F4OC10H21) with the corresponding secondary amines. Their photophysical properties were investigated by absorption and emission spectroscopies, and by TD-DFT calculations. All the compounds display an intense coloration based on HOMO-LUMO transitions, dominated by the azulene core. Gold-isocyanide complexes show a slight bathochromic shift related to azulene, while a hypsochromic shift was observed after formation of carbene complexes. Thus, the transformation of the gold-isocyanide group into the gold-carbene functionality produces a substantial color change from blue to deep purple. This different electronic behavior is mainly due to the stabilization of the LUMO orbital in the isocyanide complexes, and to the stabilization of the HOMO in the carbene derivatives. These carbene gold complexes show fluorescence in solution associated with the azulene core. This work illustrates how the synthetically easy isocyanide-carbene transformation open new perspectives to a fine color modulation in organometallic azulene dyes.

Synthesis, spectral, quantum chemical, biological, molecular docking and mathematical investigations of piperidin-4-one oxime picrates

By using spectroscopic techniques, the 3,5-diethyl-2r,6c-diarylpiperidin-4-one oxime picrates (DDPPOP and DDMPPOP) were characterised by elemental analysis, FT-IR, 1H, 13C and 2D NMR and mass spectral techniques. The compounds follow a normal chair conformation with all substituents orientated equatorially, as indicated by the reported chemical shifts and coupling constants from the NMR spectral data. The optimised molecular structures of the title compounds were done by using DFT method with B3LYP/6-311++G(d,p) theory. According to the calculated first hyperpolarizability (β0) values of the compounds display non-linear optical (NLO) behaviour. The molecule may have undergone intra-molecular charge transfer, according to the HOMO–LUMO transition. The reactive sites of the title molecule was analysed by using the Molecular Electrostatic Potential (MEP) surface. The Mulliken atomic charges were calculated by using same basis set. The antibacterial and antifungal activity of the title compounds was examined by biological study. Docking studies were thoroughly conducted to establish knowledge regarding the molecular interactions between protein and this compound. This study is further developed and uses the idea of graph colouring in mathematical computation to support the molecular structure we created using NMR spectroscopy.

Cyclohexene-Embedded Dicyanomethylene Merocyanines - Consecutive Three-Component Coupling-Addition Synthesis and Chromophore Characteristics.

A concise and efficient consecutive three-component alkynylation-addition synthesis of cyclohexene-embedded dicyanomethylene merocyanines furnishes a small library of dyes in moderate to excellent yield. The dyes possess strong absorption coefficients of the longest wavelength absorption bands. According to the crystal structure, the small bond length alternations account for a highly delocalized electronic ground state. The electronic structure of the absorption bands is qualitatively rationalized by TDDFT calculations, which explain that intense HOMO-LUMO transitions along the merocyanine axis lead to cyanine similar Stokes shifts.

Vibrationally Resolved Absorption and Fluorescence Cross Sections of Adamantane in the Far-ultraviolet Spectral Range on an Absolute Scale

High-resolution absorption, dispersed fluorescence emission, and photoionization cross sections are presented for gas-phase adamantane excited by synchrotron radiation in the exciting-photon energy range of 6–30 eV. Relative and absolute absorption cross sections of so-far unmatched resolution of down to 0.27 cm−1 line width in the region from 6.4–28 eV are shown along with newly discovered vibronic substructures around the HOMO–LUMO transition. Absorption line positions are provided with very high accuracy and listed in tabular form to be used as spectral fingerprints for the detection of adamantane in interstellar media, where its column density may be determined via the absolute cross sections. The fluorescence emission lies in the ultraviolet range from 190–250 nm and is excited starting at the HOMO–LUMO transition at 6.49 eV, which corresponds to the highest fluorescence emission energy. Hitherto unreported fluorescence in the same spectral range and relative photoionization cross sections in the exciting-photon energy range up to 30 eV are also presented along with lifetime measurements for differentiation of the involved electronic states.

Nature of Pigments in Orange and Purple Coloured Chinese Freshwater Cultured Pearls: Insights from Experimental Raman Spectroscopy and DFT Calculations

Pearls, a well-known organic gemstone, are popular for their attractive lustre and rich colour. The pigmentation and colour of pearls have never been clearly explained. Understanding the pigments and colour origin of pearls can be a guide for artificial cultivation and rational conservation. In this study, Chinese freshwater cultured pearls were collected as research samples. The appearance and colour characteristics of pearls were characterised using D65 standard light source photography and UV–Vis spectroscopy, the molecular structure of the pigments in the pearls was characterised using Raman spectroscopy, and Density Functional Theory (DFT) calculations were used to reveal the characteristics of the pigments in the pearls in terms of molecular structure and electronic excitation. It was proposed that freshwater pearls are coloured with polyene pigments, with the chain length of the polyene determining the type of colour and the concentration of the polyene determining the colour intensity of the pearl. The HOMO–LUMO transition of conjugated polyenes is intrinsically responsible for the colour of pearls. Many colour-rich biominerals also have similar Raman spectral features to pearls, and this study has wider implications for understanding the nature of pigments and their colour origins.

Spectroscopic and optical properties of 1,2,4-triazolo[4,3-a]pyridin-3(2H)-one as a component of herbicides

The herbicides azafenidin [(2-(2,4-dichloro-5-prop-2-ynoxyphenyl)-5,6,7,8-tetrahydro-[1,2,4]triazolo[4,3-a]pyridin-3-one)] and flumetsulam [(N-(2,6-difluorophenyl)-5-methyl-[1,2,4]triazolo[1,5-a]pyrimidine-2-sulfonamide)] were subjected to IR, Raman, UV–Vis and emission studies. As triazolopyridine is the most prominent and active component of these herbicides, this molecule was characterised by XRD studies, FTIR, Raman, UV–Vis and emission spectra. The experimental data were compared to the results of the DFT quantum chemical calculations carried out for its optimised structure, IR intensities and Raman activities, HOMO-LUMO transitions, and energies of the singlet and triplet states. The characteristics for triazolopyridine quantities were used in the analysis of the studied herbicides.

Insight into Tyrosine-Containing Pharmaceuticals as Potential Inhibitors of SARS-CoV-2 3CLpro and NSP16: Structural Analysis, Docking Studies, Molecular Dynamics Simulations, and Density Functional Theory Investigations

Tyrosine-containing pharmaceuticals’ (TPh) potential to inhibit SARS CoV-2 3-chymotrypsin-like proteases (3CLpro) and nonstructural protein 16 (NSP16) has been explored using docking studies, molecular dynamics simulations, and density functional theory. The TPh with FDA approval showed excellent contact with the active site pockets of 3CLpro and NSP16. Their binding affinity scores ranged from −5.8 to −4.9 kcal/mol and −6.3 to −4.8 for 3CLpro and NSP16, respectively. A 100-ns molecular dynamics simulation confirmed the stability of the carbidopa/NSP16 complex and N-acetyl tyrosine with both target enzymes. Further, the HOMO-LUMO transitions, molecular orbitals, and dipole moments of carbidopa, droxidopa, and N-acetyl tyrosine were computed using density functional theory (DFT). Considering N-acetyl tyrosine and carbidopa’s substantial inhibitory activity, it is recommended to investigate them further in order to explore their application for the treatment of COVID-19 or any other coronaviruses in the future.

Theoretical modelling of structure, vibrational and UV–vis absorbance spectra of rubrofusarin molecule

The structure of the rubrofusarin molecule (CAS: 3567-00-8, IUPAC name 5,6-dihydroxy-8-methoxy-2-methyl-4H-benzo[g]chromen-4-one, molecular formula C15H12O5) and its possible rotational conformers and tautomer were investigated within DFT approach. It was noted that for a stable molecules the group symmetry is close to Cs. The smallest potential barrier for rotational conformers is associated with the methoxy group rotation. The rotation of hydroxyl groups leads to a stable states that are substantially higher in energy than the ground state. Modeling and interpretation of vibrational spectra for the case of the ground state molecule in the gas phase and methanol solution was carried out, the influence of the solvent is discussed. The modelling of electronic singlet transition within the TD-DFT approach and the interpretation of obtained UV–vis absorbance spectra were carried out. A relatively small shift in the two most active absorption bands wavelength takes place for methoxy group rotation conformer. At the same time the redshift of the HOMO-LUMO transition takes place for this conformer. Much larger long wavelength shift of the absorption bands was noted for the tautomer.

A Switchable Near‐Infrared‐Absorbing Dye Based on Redox‐Bistable Benzitetraazaporphyrin

AbstractActivatable near‐infrared (NIR) dyes responsive to external stimuli are used in medical and other applications. Here, we describe the design and synthesis of bench‐stable 18π‐ and 20π‐electron benzitetraazaporphyrins (BzTAPs) possessing redox‐switchable NIR properties. X‐Ray, NMR, and UV/Visible‐NIR analyses revealed that 20π‐electron BzTAP 1 exhibits NIR absorption and antiaromaticity with a paratropic ring‐current, while 18π‐electron BzTAP 2 shows weakly aromatic character with NIR inertness. Notably, the NIR‐silent BzTAP 2 was readily converted to the NIR‐active BzTAP 1 in the presence of mild reducing agents such as amine. The intense NIR absorption band of BzTAP 1 is in sharp contrast to the very weak absorption bands of previously reported antiaromatic porphyrinoids. Molecular orbital analysis revealed that symmetry‐lowering perturbation of the 20π‐electron porphyrinoid skeleton enables the HOMO–LUMO transition of 1 to be electric‐dipole‐allowed. BzTAPs are expected to be useful for constructing activatable NIR probes working in reductive environments.

A Switchable Near-Infrared-Absorbing Dye Based on Redox-Bistable Benzitetraazaporphyrin.

Activatable near-infrared (NIR) dyes responsive to external stimuli are used in medical and other applications. Here, we describe the design and synthesis of bench-stable 18π- and 20π-electron benzitetraazaporphyrins (BzTAPs) possessing redox-switchable NIR properties. X-Ray, NMR, and UV/Visible-NIR analyses revealed that 20π-electron BzTAP 1 exhibits NIR absorption and antiaromaticity with a paratropic ring-current, while 18π-electron BzTAP 2 shows weakly aromatic character with NIR inertness. Notably, the NIR-silent BzTAP 2 was readily converted to the NIR-active BzTAP 1 in the presence of mild reducing agents such as amine. The intense NIR absorption band of BzTAP 1 is in sharp contrast to the very weak absorption bands of previously reported antiaromatic porphyrinoids. Molecular orbital analysis revealed that symmetry-lowering perturbation of the 20π-electron porphyrinoid skeleton enables the HOMO-LUMO transition of 1 to be electric-dipole-allowed. BzTAPs are expected to be useful for constructing activatable NIR probes working in reductive environments.

The impact of π-π stacking interactions on photo-physical properties of hydroxyanthraquinones

The impact of π-π stacking interactions on photo-physical properties of hydroxyanthraquinone (HA) has been investigated using the density functional (DFT) and time-dependent density functional theory (TD-DFT) calculations in the gas phase and solution media. The vertical transition is characterized with strong HOMO-LUMO transition in the complexes. The intramolecular hydrogen bond (IHB) made in the HA and π-π complexes is strengthened after S0 → S1 excitation, such that the proton transfers is facilitated in the first excited state. The complexes exhibit an exothermic excited state intramolecular proton transfer (ESIPT) in the solution media, which is a barrierless process for some complexes. The π-π stacking interaction affects the absorption and emission bands of HA, and provides a large Stokes shift. This indicates the desirable fluorescence properties of π-π complexes, which are cross-validated by geometries, potential energy curve scannings, electronic and vibrational spectra, and frontier molecular orbital analyses.

Intense absorption of azulene realized by molecular orbital inversion.

The introduction of diarylamino groups at the 2- and 6-positions of azulene was found to invert the order of the orbital energy levels and allowed the HOMO-LUMO transition, resulting in a substantial increase in absorbance in the visible region. In addition, the stability of their one-electron oxidised species was improved by introducing bromine or methoxy groups at the 1- and 3-positions.

Excited-state singlet-triplet inversion in hexagonal aromatic and heteroaromatic compounds.

The inversion of the energies of the lowest singlet (S1) and lowest triplet (T1) excited states in violation of Hund's multiplicity rule is a rare phenomenon in stable organic molecules. S1-T1 inversion has significant consequences for the photophysics and photochemistry of organic chromophores. In this work, wave-function based ab initio computational methods were employed to explore the possibility of S1-T1 inversion in hexagonal polycyclic aromatic and heteroaromatic compounds. In these molecules, the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) are two-fold degenerate. The HOMO-LUMO transition gives rise to three singlet and three triplet excited states. While the singlet-triplet energy gap ΔST, defined as the energy difference between the S1 state and the T1 state, is clearly positive for benzene, it is predicted to be close to zero for borazine, the boron nitride analogue of benzene. Although ΔST decreases with increasing size of hexagonal polycyclic aromatics, it remains positive up to circumcoronene (19 rings). However, symmetry-preserving substitution of C-C pairs by B-N groups in the interior, keeping the conjugation of the outer rim intact, results in compounds with robustly negative ΔST. These findings establish the existence of a new family of boron carbon nitrides with inverted singlet-triplet gaps.

Absorption spectra of calix[3]pyrrole analogs as probes for contracted macrocycles

Calix[3]pyrrole and its furan derivatives exhibited bathochromically shifted lowest-energy electronic transition bands in their absorption spectra compared with their calix[4]- and calix[6]-pyrrole analogues, despite the repeating units being the same. We also observed fluorescence emission for calix[3]-type macrocycles. The Stokes shift of the furan/pyrrole hybrid macrocycles, calix[2]furan[1]-pyrrole, and calix[1]furan[2]pyrrole, substantially varied depending on the solvent polarity. Non-covalent interaction (NCI) analyses indicated enhanced interactions between neighboring aromatic rings in the calix[3]pyrrole macrocycle, whereas such interactions were weak in calix[4]pyrrole, in which the interchromophore distance is remarkably longer than in calix[3]pyrrole. Theoretical analyses indicated that the red-shifted, lowest-energy bands of calix[3]pyrrole correspond to HOMO–LUMO transitions, the energy gap of which was narrow compared with calix[4]pyrroles. The characteristic absorption bands for calix[3]pyrrole and its related macrocycles are useful as probes for distinguishing such macrocycles from higher calix[[Formula: see text]]pyrrole ([Formula: see text] 4) analogues that exhibit almost identical absorption spectra.

Stacked antiaromaticity in the π‐congested space between the aromatic rings in the anthracene dimer

AbstractSubstances containing two π‐congested aromatic systems are attractive targets in synthetic studies as well as efforts designed to explore the unique properties that originate from π‐congestion. Since the time of the computational studies by Schleyer and Warner, the concept of stacked aromaticity created by the encounter of two antiaromatic rings has received much attention. A structure containing π‐congestion between two antiaromatic norcorrole rings was prepared by Shinokubo et al. In contrast, questions about what happens when two aromatic rings are closely stacked have remained unanswered. Specifically, the electronic consequences of stacking of two aromatic π‐conjugated systems have not been fully evaluated. To the best of our knowledge, only one computational study has been performed by Herges, the results of which suggest that [2.2]paracyclophane, possessing short distance (<3.00 Å) between two aromatic ring planes, has a paratropic ring current between two the benzene rings. This observation is consistent with the conclusion that stacked antiaromaticity exists within the π‐congested space between aromatic rings. Herein, we investigated the stacked antiaromaticity of highly π‐congested anthracene dimers using anthracenophane, which possesses a short interplane carbon–carbon distance of ca. 2.80 Å. The absorption spectrum of this substance contains a weak broad band from 450 to 550 nm that is attributed to the HOMO–LUMO transition. These properties exist in planar cyclooctatetraene derivatives that have 8π‐electron antiaromaticity. The results of nucleus‐independent chemical shift and anisotropy of the induced current density calculations indicate that a relatively weak antiaromatic character exists between the central six‐membered rings of the two anthracene moieties in anthracenophane. In addition, an attempt to enhance the stacked antiaromaticity of anthracenophane using pressure to enhance π‐congestion was unsuccessful.

Mechanisms for the Deactivation of the Electronic Excited States of α-(2-Hydroxyphenyl)-N-phenylnitrone: From Intramolecular Proton and Charge Transfer to Structure Twisting and Aggregation.

The search for new prominent chemosensors is significantly related to the rationalization of possible multiple pathways of excited-state deactivation. We have prepared and studied compound α-(2-hydroxyphenyl)-N-phenylnitrone (Nit-OH), observing that Nit-OH is stable in acetonitrile solution under UV-vis light. The experimentally observed 540 nm fluorescence for Nit-OH was shown to be related to excitation at 360 nm from the highest occupied molecular orbital to the lowest unoccupied molecular orbital (HOMO-LUMO transition). Potential energy curves (PECs) for the S1 state of Nit-OH did show that there are structures associated with excited-state intramolecular proton transfer (ESIPT), and the existence of an intramolecular H-bonding was confirmed using X-ray powder diffraction (XRD). Twisted intramolecular charge transfer (TICT) took place following ESIPT, and a nonradiative deactivation at the S1/S0 conical intersection occurred; aggregation-induced emission was observed at 540 nm associated with the formation of a stacked dimer. Anti-Kasha emission from the S2 was proposed based on the dependence of the fluorescence excitation wavelength on Nit-OH concentration. From the calculation of the PEC for the S2 state, we obtained radiative transitions at 379 and 432 nm, similar to the obtained experimental values of 383 and 453 nm. We proposed a Jablonski-like diagram that depicts all experimental and theoretical electronic transitions for Nit-OH, summarizing the unique intricate photophysical behavior of this nitrone derivative.

Synthesis, spectral characterisations of 3t-pentyl-2r,6c-diarylpiperidin-4-one oxime picrates: DFT studies and potent anti-microbial agents

In this paper, various substituted 3t-pentyl-2r,6c-diarylpiperidin-4-one oxime picrates were synthesised and characterised by elemental analysis, FT-IR, NMR, UV–Visible and Fluorescence spectral techniques. For compound 1, NMR spectral assignments were made unambiguously by their one-dimensional (1H and 13C NMR) and two-dimensional (1H–1H COSY, NOESY, HSQC and HMBC) spectra. The observed chemical shifts and coupling constants suggest that compounds 1–7 adopt normal chair conformation with the equatorial orientation of all the substituents like aryl group at C(2) and C(6) and pentyl group at C(3). Compounds 2 and 7 exhibited potent anti-bacterial activity and fungal activity against all the tested strains while all the other compounds showed moderate activity. The molecular structure of compounds (1–7) was optimised at the density functional theory (DFT)/B3LYP/6-311++G(d,p) level of basis set. The calculated first hyperpolarisability (β 0) values show that all the molecules might have non-linear optical (NLO) behaviour. The HOMO–LUMO transition implies that intra-molecular charge transfer takes place within the molecule. The molecular electrostatic potential (MEP) surface is used to understand the reactive sites of a molecule.