LUMO Excitation Research Articles

Structure-aromaticity-spectroscopy relationship in conjugated polymers

In this study, an effort has been made to analyze the aromaticity of oligomers of phenylenes and thiophenes, with the presence and absence of linkers using Nucleus-Independent Chemical Shift (NICS) as a descriptor. The relation between HOMO–LUMO gaps, reorganization and excitation energies with respective NICS values has been employed to develop a structure-aromaticity-conjugation spectroscopy relationship (SACSR). Results show that HOMO–LUMO gaps/excitation energies of various model systems exhibit linear relationships with the inverse of the NICS values, indicating the possible existence of the SACSR.

The mechanism of photoconversion of cyclic dinitrone to oxaziridine and dioxaziridine: A computational investigation of an experimentally reported photochemical reaction

AbstractThe unexplored photochemistry behind the conversion of cyclic dinitrone to oxaziridine and dioxaziridine has been investigated computationally. The mechanism behind the experimentally reported photo‐conversion reaction of 2,2,4,5‐tetramethyl‐2H‐imidazole‐1,3‐dioxide has been analyzed. The allowed S0‐S1 transition in this cyclic dinitrone system is characterized by a transition moment value of 3 Debye. The photo‐excitation to the vertically excited S1 state is subsequently followed by a passage through low‐lying conical intersection(S0/S1). This photoproduct oxaziridine can undergo reverse transformation to the parent dinitrone through a transition state (barrier height 30–40 kcal/mol) with imaginary frequency value close to 1000i cm−1. Under photo‐irradiation, oxaziridine of this dioxide system can undergo strongly allowed singlet‐singlet transition, characterized by HOMO LUMO excitation with transition moment value close to 2.70 Debye. Subsequently, formations of the trans‐ and cis‐dioxaziridines take place through S0/S1 conical intersections.

New Charge Transfer Complexes of K+-Channel-Blocker Drug (Amifampridine; AMFP) for Sensitive Detection; Solution Investigations and DFT Studies.

UV–Vis spectroscopy was used to investigate two new charge transfer (CT) complexes formed between the K+-channel-blocker amifampridine (AMFP) drug and the two π-acceptors 2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ) and tetracyanoethylene (TCNE) in different solvents. The molecular composition of the new CT complexes was estimated using the continuous variations method and found to be 1:1 for both complexes. The formed CT complexes’ electronic spectra data were further employed for calculating the formation constants (KCT), molar extinction coefficients (εCT), and physical parameters at various temperatures, and the results demonstrated the high stability of both complexes. In addition, sensitive spectrophotometric methods for quantifying AMFP in its pure form were proposed and statistically validated. Furthermore, DFT calculations were used to predict the molecular structures of AMFP–DDQ and AMFP–TCNE complexes in CHCl3. TD-DFT calculations were also used to predict the electronic spectra of both complexes. A CT-based transition band (exp. 399 and 417 nm) for the AMFP–TCNE complex was calculated at 411.5 nm (f = 0.105, HOMO-1 → LUMO). The two absorption bands at 459 nm (calc. 426.9 nm, f = 0.054) and 584 nm (calc. 628.1 nm, f = 0.111) of the AMFP–DDQ complex were theoretically assigned to HOMO-1 → LUMO and HOMO → LUMO excitations, respectively.

Nucleophilic reactivity of the gold atom in a diarylborylgold(i) complex toward polar multiple bonds.

A di(o-tolyl)borylgold complex was synthesized via the metathesis reaction of a gold alkoxide with tetra(o-tolyl)diborane(4). The resulting diarylborylgold complex exhibited a Lewis acidic boron center and a characteristic visible absorption that arises from its HOMO–LUMO excitation, which is narrower than that of a previously reported dioxyborylgold complex. The diarylborylgold complex reacted with isocyanide in a stepwise fashion to afford single- and double-insertion products and a C–C coupled product. Reactions of this diarylborylgold complex with C Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> O/N double bond species furnished addition products under concomitant formation of Au–C and B–O/N bonds, which suggests nucleophilic reactivity of the gold metal center. DFT calculations provided details of the underlying reaction mechanism, which involves an initial coordination of the CO/N bond to the boron vacant p-orbital of the diarylboryl ligand followed by a migration of the gold atom from the tetracoordinate sp3-hybridized boron center, which is analogous to the reactivity of the conventional sp3-hybridized borate species. The DFT calculations also suggested a stepwise mechanism for the reaction of this diarylborylgold complex with isocyanide, which afforded three different reaction products depending on the applied reaction conditions.

Synthesis, and spectroscopic, nanostructure, surface morphology, and density functional theory studies of new charge-transfer complexes of amifampridine with π-acceptors

The charge transfer (CT) complexes formed between the potassium-channel-blocking drug amifampridine (AMFP) and the π-acceptors; chloranilic acid (CLA) and 2,5-dihydroxy-p-benzoquinone (DHBQ) in both MeOH solutions, and the solid-state were synthesized and fully characterized. The stoichiometry of the complexes formed in MeOH is 1:1, while those of the solid complexes are 2:1 and 1:1 (donor: acceptor) for CLA and DHBQ, respectively. The CT complexes were studied in terms of their formation constants (KCT), molar extinction coefficients (εCT), and spectrophysical and thermodynamic properties, and the results demonstrated the high stability of the complexes in MeOH. The limits of detection and quantification for the complexes by UV–Vis spectrophotometry were also determined. The surface morphologies, particle sizes, and elemental compositions of the complexes were determined, utilizing X-ray powder diffraction, scanning electron microscope, and energy-dispersive X-ray techniques. The molecular structures, and the electronic properties of the complexes were investigated using B3LYP and WB97XD DFT calculations employing 6–31++G(d,p) basis sets, revealing that the magnitude of the CT is 0.357 e in AMFP-CLA and 0.317 e in AMFP-DHBQ, indicating that CLA is a better electron acceptor than DHBQ. The UV–Vis spectral bands observed for AMFP-CLA and AMFP-DHBQ experimentally in MeOH at 488 and 535 nm, respectively, while these bands were calculated at 493.0 at 535.6 nm, respectively, in the same solvent (MeOH). Both bands were found to be due to HOMO→LUMO excitation, where that in the AMFP-CLA complex is an internal electronic transition and that in AMFP-DHBQ complex is a charge transfer based transition from AMFP to DHBQ. Natural bond orbital calculations indicated that the CT processes LP(2)O11→LP*(1)H12 (127.62 kcal mol−1) and LP(1)N20→LP*(1)H12 (317.46 kcal mol−1) in AMFP-DHBQ, and LP(2)O9→LP*(1)H10 (94.84 kcal mol−1) and LP(1)N23→ LP*(1)H10 (363.42 kcal mol−1) in AMFP-CLA provide extra stability to the complexes.

Tetradentate N^N°N^N-type luminophores for Pt(II) complexes: Synthesis, photophysical and quantum-chemical investigation

Herein we report on dianionic tetradentate, pyridin-azole-based ligands for luminescent Pt(II) complexes. To improve their solubility in common organic solvents, a hexyl-chain was inserted in the periphery of the luminophoric ligand and different substituents were introduced at the 1H-pyrazole moieties including 4-hexylphenyl- (L1), adamantyl- (L2) and 3,5-dimethoxyphenyl (L3) substituents. For the new ligand precursors L1 and L2, the corresponding Pt(II) complexes Pt1 and Pt2 were synthesized and characterized by exact mass spectrometry as well as by 1H NMR. L3 was also successfully synthesized and isolated, but due to the limited solubility of the resulting coordination compound, the purification and structural characterization of the corresponding Pt(II) complex Pt3 was not achieved. The absorption and steady-state as well as time-resolved photoluminescence spectra of Pt1 and Pt2 were investigated and interpreted employing TD–DFT. We found that the S1 states for Pt1 and Pt2 can be described mainly as single electron HOMO→LUMO excitations with a 1MP-ILCT character comprising the diazole units and the pyridine rings. At room temperature, Pt1 and Pt2 show phosphorescence in the green region of the electromagnetic spectrum peaking at 506 nm and 497 nm with photoluminescence quantum yields reaching 17% and 25%, respectively. DFT calculations show that the T1 states of Pt1 and Pt2 can be mainly described as monoelectronic HOMO→LUMO excitations with predominant 3MP-ILCT character.

Diboron- and Diaza-Doped Anthracenes and Phenanthrenes: Their Electronic Structures for Being Singlet Fission Chromophores.

We used quantum chemistry methods at the levels of mixed-reference spin-flip time-dependent density functional theory and multireference perturbation theory to study diboron- and diaza-doped anthracenes and phenanthrenes. This class of structures recently surged as potential singlet fission chromophores. We studied electronic structures of their excited states and clarified the reasons why they satisfy or fail to satisfy the energy criteria for singlet fission chromophores. Many studied structures have their S1 states not dominated by HOMO → LUMO excitation, so they cannot be described using the conventional two site model. This is attributed to frontier orbital energy shifts induced by the doping and different charge-transfer energies in different one-electron singlet excitations or, in other words, different polarizations of hole and/or particle orbitals in their S1 and T1 states. There is a mirror relation between the orbital energy shifts induced by diboron- and diaza-dopings, which together with alternant hydrocarbon pairings of occupied and unoccupied orbitals, leads to more mirror relations between the excited states of the two types of doped structures.

Optimization, Spectroscopic (Excited States, UV/Vis, Polarization) Studies, FMO, ELF, LOL, QTAIM, NBO Analysis and Electronic Properties of Two New Azomethine Derivatives: A Theoretical and Experimental Investigations

In the current study, the molecular structures of the two new azomethine dyes such as AAFOH-1 and AAFOH-2 have been investigated with the use of density functional theory (DFT/B3LYP/6-311+G(d)) in a DMSO solvent for the first time. The electronic spectra of the new dyes in a DMSO solvent were carried out by temporally dependent density functional theory (TD-DFT) method. The computed absorption spectral data of the investigated molecules are in good agreement with the experimental data, thus allowing an assignment of the UV/Vis spectra. The equilibrium geometry, the HOMO and LUMO molecular orbitals, excitation energies, oscillator strengths and natural bond orbital (NBO) analysis for the molecules have also been calculated and presented. The electron location function (ELF), localized orbital locator (LOL), and quantum theory of atoms in molecules (QTAIM) analyses were also carried out. On the basis of polyvinyl alcohol (PVA) and the new synthesized azomethine dyes polarizing films were developed. The main optical parameters of polarizing PVA-films (transmittance, polarization efficiency, and dichroic ratio) have been measured and discussed. Anizotropy of thermal conductivity of the PVA-films have been studied.

In Silico Investigation for the Design of Redox Based Molecular Switch

Molecular simulation based on density functional theory was performed on the structure of anthraquinone, pentacenequinone, heptacenequinone and their dianions. The HOMO–LUMO gap, excitation energy and absorption wavelength were calculated to examine the switching action of molecules via two electron OX/RED reaction. Drastic change has been found in HOMO–LUMO gap, excitation energy and absorption wavelength of neutral molecules and their dianions. It has been observed that these three poly-acenes based organic molecules are very auspicious for the design and manufacturing of redox-active molecular switches.

Existence and nonexistence of HOMO–LUMO excitations in Kohn–Sham density functional theory

In numerical computations of response properties of electronic systems, the standard model is Kohn–Sham density functional theory (KS-DFT). Here we investigate the mathematical status of the simplest class of excitations in KS-DFT, HOMO–LUMO excitations. We show that such excitations, i.e. excited states of the Kohn–Sham Hamiltonian, exist for Z>N, where Z is the total nuclear charge and N is the number of electrons. The result applies under realistic assumptions on the exchange–correlation functional, which we verify explicitly for the widely used PZ81 and PW92 functionals. By contrast, and somewhat surprisingly, we find using a method of Glaser, Martin, Grosse, and Thirring (Glaser et al., 1976) that in case of the hydrogen and helium atoms, excited states do not exist in the neutral case Z=N when the self-consistent KS ground state density is replaced by a realistic but easier to analyze approximation (in case of hydrogen, the true Schrödinger ground state density). Implications for interpreting minus the HOMO eigenvalue as an approximation to the ionization potential are indicated.

Can Anti‐Aufbau DFT Calculations Estimate Singlet Excited State Aromaticity? Correspondence on “Dibenzoarsepins: Planarization of 8π‐Electron System in the Lowest Singlet Excited State”

AbstractThe simple anti‐aufbau DFT approach for estimating singlet excited state aromaticity suggested in a recent Communication published in this journal is shown to produce incorrect results because it targets a linear combination of the singlet and triplet configurations involving the HOMO and LUMO rather than the first singlet excited state. If the S1 state of a molecule is dominated by the HOMO→LUMO excitation, a comparably simple but theoretically consistent and qualitatively correct approximation to the S1 wavefunction can be achieved by performing a small “two electrons in two orbitals” CASSCF(2,2) calculation which can be followed by the evaluation of magnetic aromaticity criteria such as NICS.

Synthesis of spiroindolone analogue via three components reaction of olefin with isatin and sarcosine: Anti-proliferative activity and computational studies

Considerable attention has been focused on the [1,3] dipolar cycloaddition reaction approach of olefin with amino acid (sarcosine), and isatin which underwent smoothly, and afforded a highly functionalized complex molecule. The target spiroindolone analogue 4 was synthesized in excellent yield. The desired compound was elucidated based on X-ray single crystal diffraction technique. Compound 4 was examined against three different cancer cell lines for liver, breast and colorectal cancer (HepG2, MCF-7 and HCT-116, respectively). It showed high selectivity against colon cancer (HCT-116). Hirshfeld molecular packing analysis of 4 showed that the H⋯H, Cl⋯H and C⋯H contacts are the most abundant while the N⋯H and O⋯H hydrogen bonding interactions are the strongest. Molecular and electronic structures as well as the Uv–Vis and NMR spectra of 4 were discussed based on DFT calculations. The longest wavelength band observed at 298 nm was assigned for the HOMO-2/HOMO-1→LUMO excitations. The 1H and 13C NMR chemical shifts of 4 were calculated and compared with the experimental data (0.934–0.954).

Quantum-chemical modeling, spectroscopic (FT-IR, excited states, UV/Vis, polarization, and Dichroism) studies of two new benzo[d]oxazole derivatives

Abstract In the given research, the molecular structures of the two new benzo[d]oxazole derivatives including H-1 and H-2: have been studied with the use of density functional theory (DFT/B3LYP and M062X) in dimethylformamide (DMF) for the first time. The electronic spectra of the new compounds in a DMF solvent were carried out by temporally dependent density functional theory (TD-DFT) method. The computed absorption spectral data of the title compounds are in good agreement with the experimental data, thus allowing an assignment of the UV/Vis spectra. The equilibrium geometry, the HOMO and LUMO molecular orbitals, excitation energies, oscillator strengths and Natural Bond Orbital (NBO) analysis for the two new molecules have also been calculated and presented. Based on polyvinyl alcohol (PVA) and synthesized molecules polarizers for UV/Vis region of the spectrum have been developed.

Blue LED Irradiation of Iodonium Ylides Gives Diradical Intermediates for Efficient Metal‐free Cyclopropanation with Alkenes

AbstractA facile and highly chemoselective synthesis of doubly activated cyclopropanes is reported where mixtures of alkenes and β‐dicarbonyl‐derived iodonium ylides are irradiated with light from blue LEDs. This metal‐free synthesis gives cyclopropanes in yields up to 96 %, is operative with cyclic and acyclic ylides, and proceeds with a variety of electronically‐diverse alkenes. Computational analysis explains the high selectivity observed, which derives from exclusive HOMO to LUMO excitation, instead of free carbene generation. The procedure is operationally simple, uses no photocatalyst, and provides access in one step to important building blocks for complex molecule synthesis.

Blue LED Irradiation of Iodonium Ylides Gives Diradical Intermediates for Efficient Metal-free Cyclopropanation with Alkenes.

A facile and highly chemoselective synthesis of doubly activated cyclopropanes is reported where mixtures of alkenes and β-dicarbonyl-derived iodonium ylides are irradiated with light from blue LEDs. This metal-free synthesis gives cyclopropanes in yields up to 96 %, is operative with cyclic and acyclic ylides, and proceeds with a variety of electronically-diverse alkenes. Computational analysis explains the high selectivity observed, which derives from exclusive HOMO to LUMO excitation, instead of free carbene generation. The procedure is operationally simple, uses no photocatalyst, and provides access in one step to important building blocks for complex molecule synthesis.

Electronic structure and adsorption geometry of Pt and Pd metal complexes with 1,3-dithiole-2-thione-4,5-dithiolate ligand on TiO2(101) surface from first-principles calculations

Metal complexes based on 1,3-dithiole-2-thione-4,5-dithiolate (dmit) ligand have been intensively studied for more than 40 years due to their unusual chemical and physical properties. Besides, the highly delocalized frontier orbitals, that allow direct electron transfer through the ligand π orbitals, make these class of complexes promising candidates for photochemical devices as well as sensitizer for dye-sensitized solar cells. In this work, we investigated the electronic and geometric properties of Pd and Pt (CH3)2[M(dmit)2] complexes isolated on TiO2(101) surface by means of first-principles calculations using plane-wave basis sets and DFT calculation. Adsorption energies of metal complexes supported on a TiO2(101) surface are calculated for three different configurations, linked by the sulfur atom of Sthione, Sthiole–Sthiolate, and planar. The studies found that the most stable adsorption molecular configuration mode for the palladium(II) and platinum(II) complexes is the planar mode. TD-DFT molecular calculations revealed that the lowest energy transition in the ultraviolet visible near-infrared range mainly corresponds to the HOMO–LUMO excitation for the (CH3)2[M(dmit)2] complexes. Theoretical calculations of optical absorption spectra of (CH3)2[M(dmit)2] complexes adsorbed on the anatase (101) surface show that the interaction induces a slightly redshift of electronic absorption bands. Density of states for the metal complexes/TiO2(101) system revealed that the LUMO of the metal complexes lies at below the edge of the TiO2 conduction band. The adsorption of the (CH3)2[Pd(dmit)2] complex on the anatase (101) surface results in the emergence of new transitions below 1 eV that can be ascribed to the presence of a favorable overlap between the LUMO of the complex and the conduction band of the TiO2 semiconductor.

Effects of MoOx modification on photocatalytic activity of hydroxyapatite and Ti-doped hydroxyapatite

Titanium-doped hydroxyapatite (Ti-HAp, (Ca10-2x, Tix, □x) (PO4)6(OH)2, □: defect in Ca site) and hydroxyapatite (HAp, Ca10(PO4)6(OH)2) powders were modified with an ethanol solution of molybdenyl acetylacetonate (MoO2(C5H8O2)2) using chemisorption calcination cycle (CCC) technique, which provides metal oxide clusters. Their photocatalytic activity under UV illumination was evaluated by the decomposition of gaseous 2-propanol. The photocatalytic activity of the Mo-modified Ti-HAp samples increased concomitantly with increasing Mo concentration up to 0.72% against P. The highest photocatalytic activity of Mo-modified Ti-HAp was about 13 times higher than that of Ti-HAp. The signal appearance of Mo(V) in electron spin resonance spectra and the decrease of photoluminescence intensity suggest electron transfer from the Ti-hybridized band to the MoOx cluster, which suppresses recombination of the photoinduced electron and hole pairs. The photocatalytic activity of Mo-modified HAp samples was attributed to HOMO–LUMO excitation of MoOx cluster.

Thioethyl‐Porphyrazine/Nanocarbon Hybrids for Photoinduced Electron Transfer

AbstractA novel pyrene‐substituted thioethyl‐porphyrazine (PzPy) and the formation of supramolecular assembly with nanocarbons demonstrating photoinduced electron transfer ability are designed. As revealed by spectroscopic and electrochemical studies, PzPy displays wide spectral absorption in the visible range, charge separation upon photoexcitation, as well as bandgap and highest occupied/lowest unoccupied molecular orbital (HOMO/LUMO) energy values, matching the key requirements of organic optoelectronic. Moreover, the presence of a pyrene moiety promotes attractive interactions with π‐conjugated systems. In particular, theoretical calculations show that in the PzPy the HOMO and LUMO are localized on different positions of the molecule, i.e., the HOMO on the pyrene moiety and the LUMO on the macrocycle. Therefore, HOMO–LUMO excitation gives rise to a charge separation, preventing excitons recombination. Two kinds of noncovalent hybrid composites are prepared by mixing the PzPy with single‐wall carbon nanotubes (SWNTs) and graphene nanoflakes (GNFs), respectively, and used for photocurrent generation through charge transfer processes between PzPy and nanocarbons. Photoconduction experiments show photocurrent generation upon visible light irradiation of both PzPy/SWNT and PzPy/GNF composites (0.78 and 0.71 mA W−1 at 500 nm, respectively), demonstrating their suitability for optoelectronic applications and light harvesting systems.

The highly excited-state manifold of guanine: calibration for nonlinear electronic spectroscopy simulations

A computational protocol based on the complete and restricted active space self-consistent field (CASSCF/RASSCF) methods and their second-order perturbation theory extensions (CASPT2/RASPT2) is employed to benchmark the highly excited-state manifold of the DNA/RNA canonical purine nucleobase guanine in vacuo. Several RASPT2 schemes are tested, displaying a steady convergence of electronic transition energies and dipole moments upon active space enlargement toward the reference values. The outcome allows calibrating and optimizing computational efforts by considering cheaper and more approximate RAS schemes that could enable the characterization of the excited-state manifolds of multi-chromophoric systems, such as DNA/RNA nucleobase dimers or multimers. Simulations of two-dimensional electronic spectra show similar trends to those observed on the other purine nucleobase adenine, deviating from this and other pyrimidine nucleobases in featuring its main excited-state absorption signal, embodied by sizable double HOMO to LUMO excitation contributions, in the UV probing window.

Tuning Stilbene Photochemistry by Fluorination: State Reordering Leads to Sudden Polarization near the Franck–Condon Region

Spontaneous polarization of a nonpolar molecule upon photoexcitation (the sudden polarization effect) earlier discussed for 90°-twisted alkenes is observed and calculated for planar ring-fluorinated stilbenes, trans-2,3,5,6,2',3',5',6'-octofluorostilbene (tF2356) and trans-2,3,4,5,6,2',3',4',5',6'-decafluorostilbene (tF23456). Due to the fluorination, Franck-Condon states S1FC and S2FC are dominated by the quasi-degenerate HOMO-1 → LUMO and HOMO-2 → LUMO excitations, while their interaction gives rise to a symmetry-broken zwitterionic S1 state. After optical excitation of tF2356, one observes an ultrafast (∼0.06 ps) evolution that reflects relaxation from initial nonpolar S3FC to long-lived (1.3 ns in n-hexane and 3.4 ns in acetonitrile) polar S1. The polarity of S1 is evidenced by a solvatochromic shift of its fluorescence band. The experimental results provide a sensitive test for quantum-chemical calculations. In particular, our calculations agree with the experiment, and raise concerns about the applicability of the common TDDFT approach to relatively simple stilbenic systems.