HOMO-LUMO Overlap Research Articles

Role of electron withdrawing moieties in phenoxazine–oxadiazole-based donor–acceptor compounds towards enriching TADF emission

Herein, we reported the effect of electron withdrawing units, such as trifluoromethyl (CF3) and cyanide (CN), substitution on a thermally activated delayed fluorescent (TADF) molecule (10-(4-(5-phenyl-1,3,4-oxadiazol-2-yl)phenyl)-10H-phenoxazine (PXZ-OXD)). The addition of electron withdrawing units has increased the acceptor strength and reduced the interaction between PXZ and OXD, thus reducing the gap between the singlet and triplet states (ΔEST) of excitons. To understand the variation in the acceptor strength of PXZ-OXD as a function of its molecular structure and density of states, density functional theory (DFT) and time-dependent DFT (TDDFT) were conducted. Transition density matrix investigations revealed that the addition of −CF3 and –CN to PXZ-OXD triggers CT excitons, while inducing lower ΔEST values. Particularly, the lowest ΔEST (0.05 eV) with a notable red shift in the emission spectrum was observed with 4′-CF3PXZOXD. The delayed component lifetime of PXZOXD is found to be reduced after the substitution of −CNwhm and −CF3. Despite the weak charge transfer transitions, the substitution of conjugative –CN group is found to be beneficial in improving the HOMO-LUMO overlap with a moderate decrease in reverse intersystem crossing (KRISC), which attained enhancement in the photoluminescent quantum yield. Additionally, the substitution of the above electron withdrawing units on PXZ-OXD yielded a red shift in the electroluminescence spectrum. Furthermore, the external quantum efficiency (at 100 cd/m2, i.e., EQE100) of the 4′-CNPXZOXD-based organic light-emitting diode is found to improve by 21.13 % against the PXZOXD (16.9 %).

Molecular interaction between N-butylpyridinium tetrafluoroborate and cyclic sulfur compounds: A DFT study

Extraction of sulfur compounds from crude oil is one of the problems of industries related to crude oil. The use of ionic liquids (ILs) as a green solvent to extract these sulfur compounds, especially cyclic sulfur compounds, can be considered a potential point in this regard. A functional theory comparison was made to investigate the interaction between N-butylpyridinium tetrafluoroborate as IL and several different cyclic sulfur compounds including Thiophene, benzothiophene, dibenzothiophene, and 4,6-dibenzothiophene. The difference between the solution phase and the gas phase has been investigated. Based on the results, the proper interaction between the components of IL and the cyclic sulfur compounds, the extraction of these compounds is theoretically possible. Natural bond orbital, atoms in molecules, HOMO-LUMO overlap integral, and electron density difference were analyzed. The analysis informs on the use of ionic liquids as solvents for the removal of sulfur compounds from crude oil. The hydrogen and vander Waals bonds formed between the anion and cation of ionic liquid and the sulfur compounds have been determined. The main bonds are formed between the anion of ionic liquid and the sulfur compounds. Between the compound [C4Py][BF4] and C4H4S, the hydrogen bonds are S…H13, F29.... H38, F26…H38, has been established. The hydrogen bonds between the anion of the ionic liquid and the sulfur compound are shorter than the other hydrogen bonds formed. Due to the intermolecular bonds, the transition state with the lowest energy has been obtained. In this way, we will have a proposed mechanism for the extraction of the cyclic compounds from oil with the help of ionic liquids.

2D-Graph of intermolecular interactions predicts radical character of anion-π* type charge-transfer complexes.

The molecular orbital (MO) theory is one of the most useful methods to describe the formation of a new chemical bond between two molecules. However, it is less often employed for modelling non-bonded intermolecular interactions because of the small charge-transfer contribution. Here we introduce two simple descriptors, the energy difference (EDA) of the HOMO of an electron donor and the LUMO of an acceptor against such HOMO-LUMO overlap integral (SDA), to show that the MO theory could give a unified charge-transfer picture of both bonding and non-bonding interactions for two molecules. It is found that similar types of interactions tend to be closer to each other in this 2D graph. Notably, in a transition region from strong bonding to single-electron transfer, the interacting molecular pairs appear to present a "hybrid" between chemical bonding and a radical pair, such as anion-π* interactions. It is concluded that the number of nodes in the HOMO and LUMO play a crucial role in determining the bonding character of the molecular pair.

Organic Photothermal Materials Obtained Using Thermally Activated Delayed Fluorescence Design Principles.

Organic small molecules with high photothermal conversion efficiencies that absorb near-infrared light are desirable for photothermal therapy due to their improved biocompatibility compared to inorganic materials and their ability to absorb light in the biological transparency window (650-1350 nm). Here we report three donor-acceptor organic materials DM-ANDI, O-ANDI, and S-ANDI that show high photothermal conversion efficiencies of 46-68 % with near-infrared absorption. The design of these molecules is based on the rational modification of a thermally activated delayed fluorescence material to favour a low photoluminescence quantum yield by reducing HOMO-LUMO overlap. Encapsulating these materials into either neat nanoparticles or aggregated organic dots modulates their photothermal conversion efficiencies, and also facilitates dispersion in water.

Design of highly stable thermally activated delayed fluorescence emitters via the overlap degree of HOMO-LUMO distributions

Two questions including the judgement of thermally activated delayed fluorescence (TADF) property and high stability are crucial for designing TADF emitters. In this work, based on the experimental TADF emitters, we investigated the essential reason of improving the stability of TADF device by introducing tert-butyl and phenyl into the peripheral position of carbazole, and suggested to judge the TADF property via the concrete HOMO-LUMO overlap calculation. To design highly stable TADF emitters, several pairs of positional isomers combining tert-Butyl-substituted carbazole as donor with different acceptors are designed by connecting the donor and acceptor groups through the orth-, meta- and para-position. Via the HOMO-LUMO overlap calculation, the potential TADF emitters have been point out. Our results find that the HOMO-LUMO overlap values are dependent on the linkage styles and acceptor species. For example, the designed molecules m-Cz-BF/BF-Cz/NOBF2-DTCz/CPS, o-CNImPy-Cz/PQM-Cz, and m-, p-Cz-NAI have the potential to perform better TADF performance than other isomers. These results indicate that the HOMO-LUMO overlap could be used to identify synthetic motifs for promising TADF candidates among structural isomers.

Controlling Symmetry Breaking Charge Transfer in BODIPY Pairs.

Symmetry breaking charge transfer (SBCT) is a process in which a pair of identical chromophores absorb a photon and use its energy to transfer an electron from one chromophore to the other, breaking the symmetry of the chromophore pair. This excited state phenomenon is observed in photosynthetic organisms where it enables efficient formation of separated charges that ultimately catalyze biosynthesis. SBCT has also been proposed as a means for developing photovoltaics and photocatalytic systems that operate with minimal energy loss. It is known that SBCT in both biological and artificial systems is in part made possible by the local environment in which it occurs, which can move to stabilize the asymmetric SBCT state. However, how environmental degrees of freedom act in concert with steric and structural constraints placed on a chromophore pair to dictate its ability to generate long-lived charge pairs via SBCT remain open topics of investigation.In this Account, we compare a broad series of dipyrrin dimers that are linked by distinct bridging groups to discern how the spatial separation and mutual orientation of linked chromophores and the structural flexibility of their linker each impact SBCT efficiency. Across this material set, we observe a general trend that SBCT is accelerated as the spatial separation between dimer chromophores decreases, consistent with the expectation that the electronic coupling between these units varies exponentially with their separation. However, one key observation is that the rate of charge recombination following SBCT was found to slow with decreasing interchromophore separation, rather than speed up. This stems from an enhancement of the dimer's structural rigidity due to increasing steric repulsion as the length of their linker shrinks. This rigidity further inhibits charge recombination in systems where symmetry has already enforced zero HOMO-LUMO overlap. Additionally, for the forward transfer, the active torsion is shown to increase LUMO-LUMO coupling, allowing for faster SBCT within bridging groups.By understanding trends for how rates of SBCT and charge recombination depend on a dimer's internal structure and its environment, we identify design guidelines for creating artificial systems for driving sustained light-induced charge separation. Such systems can find application in solar energy technologies and photocatalytic applications and can serve as a model for light-induced charge separation in biological systems.

Molecular and excited state properties of photostable anthraquinone red and violet dyes for hydrophobic fibers

The molecular, spectroscopic, and excited state properties of synthetic dyes for fiber-based outdoor materials continue to be of commercial interest. Early developments in this area were reported in the 1980s, when the need for dyes for polyester (PET)-based automobile interiors gave rise to commercially viable nitrodiphenylamine yellow, anthraquinone red and blue, and azo red dyes. To augment that initial knowledge base, the present study involved the use of experimental and theoretical methods to help establish the molecular structures and excited state properties of some more recent dyes for producing photostable colors on PET fibers. Having completed the characterization of present-day scarlet, blue, and yellow disperse dyes for PET-based fibers used outdoors, our attention turned to commercially available red and violet dyes. In this regard, HPLC analysis showed that the red product was a mixture containing four components, while the violet product contained only one component. Results from 1H NMR, HRMS, and single crystal X-ray diffraction analyses indicated that the principal components were dyes having a 1-amino-4-hydroxyanthraquinone base structure. The presence of an –OH group alpha to an anthraquinone C=O moiety provides for intramolecular H-bonding and a subsequent opportunity for intramolecular proton transfer in the excited state – as a photostabilizing mechanism. Further, for both dyes, results from the analysis of Frontier HOMO and LUMO isosurfaces indicated strong HOMO-LUMO overlap without molecular gaps and were consistent with strong excited state energy dissipation in a non-destructive way.

Synthesis and characterization of novel tetra anchoring A2-D-D-D-A2 architecture sensitizers for efficient dye-sensitized solar cells

Novel metal free organic dyes coded TET(RA)4, TET(CA)4, and TET(QA)4 were designed, synthesized, and characterized as effective sensitizers for dye sensitized solar cells (DSSCs). These new push-pull sensitizers used a strong electron donor consisting of 3,4-ethylenedioxythiophene and two triphenylamine molecules connected together to form a TPA-EDOT-TPA (TET) motif, which is directly connected to tetra anchoring groups (A) without any π-spacers to construct A2-D-D-D-A2 architecture, three different anchoring series, viz. rhodanine-3-acetic acid (RA), cyanoacetic acid (CA), and 2-methyl quinoline-6-carboxylic acid (QA) were employed to investigate the influence of anchoring moieties on the electrochemical, thermodynamic, kinetics, and photovoltaic efficiency of DSSCs. The DSSCs devices showed a maximum overall power conversion efficiency (PCE) = 5.13%, short-circuit current density (JSC) = 12.71 mA.cm−2, open circuit voltage (VOC) = 0.62 V, and fill factor (FF) = 65.36% with a maximum incident photon conversion efficiency (IPCE) = 75% for TET(QA)4. The optical and electrochemical studies showed that TET(QC)4 achieved higher electron injection free energy (ΔG°inj) into CB edge of TiO2 as well as high recombination resistance (Rrec) compared to TET(RA)4, and TET(CA)4, which explains the outstanding charge separation and superior power conversion efficiency (PCE) of TET(QC)4 possessing quinoline-6-carboxylic acid (QC) anchoring group. Molecular modeling calculations using DFT and TD-DFT showed effective charge separation, where HOMO is delocalized over the donor scaffold (TPA-EDOT-TPA), and the LUMO is delocalized over only two anchoring groups on the same side of the donor system, which provides strong HOMO-LUMO overlap as well as intimate electronic coupling with TiO2 nanoparticle surface for electron injection. Further, the calculated values of the energy gaps (E0-0) and ground/excited stated oxidation potentials were in perfect agreement with the experimental results.

Rhodium-Catalyzed Domino Hydroformylation/Double-Cyclization Reaction of Arylacetylenecarboxamides: Diastereoselectivity Studies and Application in the Synthesis of 1-Azabicyclo[x.y.0]alkanes.

A domino method for the rapid syntheses of 1-azabicyclo[x.y.0]alkane scaffolds, such as indolizidines, quinolizidines, decahydropyridoazepines, and their derivatives, has been developed. This strategy involved a rhodium-catalyzed hydroformylation of allyl-, 3-butenyl-, or homoallyl amides, followed by two spontaneous ring closures under mild conditions. The reaction scope and diastereoselectivity were fully explored by changing the substitution pattern on the amide and by altering the length of the carbon chain. This method was applied to the syntheses of natural alkaloids tashiromine and epilupinine. The clear differences between the reactivities of two isomeric amide substrates, 3-butenamide 1 j and homoallyl amide 1 i, could be attributed to better HOMO-LUMO overlap in the transition states that were derived from butenamides during cyclization. This explanation was supported by DFT calculations.

Key factors of exciplex emission: Exciton binding and intermolecular molecular orbital overlap

Key parameters of exciplex emission energy and efficiency were elucidated by synthesizing two electron donor type hosts derived from carbazole and diphenylamine. The two hosts were mixed with an electron acceptor type host to make exciplexes. Investigation of the exciplex emission characteristics revealed that the distance between the HOMO of the donor type host and the LUMO of the acceptor type host is critical to the emission energy and efficiency. The exciplex which has short intermolecular distance between HOMO and LUMO showed low emission energy and high device efficiency possibly due to strong exciton binding and extensive HOMO-LUMO overlap. Therefore, it was found that intermolecular HOMO and LUMO distance is also a key factor affecting the device performances of exciplexes.

Azaacenes Bearing Five-Membered Rings.

We report the synthesis of processible (dihydro)pyracyclene- and acenaphthylene-substituted azaacenes using condensation reactions in solution. The targets are characterized via cyclic voltammetry, X-ray crystallography, UV/Vis, fluorescence spectroscopy and DFT/NICS calculations. Formal hydrogenation of the annulated five-membered ring surprisingly alters emission in the solid-state as a consequence of modulation of aromaticity and HOMO-LUMO overlap. Five highly fluorescent, crystalline azaacenes were investigated as emitters in organic light-emitting diodes, and their performance with respect to luminance and efficiency was compared to that of structurally related azaacenes.

Molecular interactions between 1-butyl-3-methylimidazolium tetrafluoroborate and model naphthenic acids: A DFT study

A density functional theory study was performed to investigate the interactions between 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM][BF4]) and six different model naphthenic acids (NAs). Natural bond orbital, atoms in molecules, noncovalent interactions, HOMO-LUMO overlap integral, and electron density difference were analyzed. The analysis informs on the use of ionic liquids as solvents for the removal of NAs from crude oil. The main extraction mechanism for NAs without long alkyl chain is hydrogen bonding, whereas van der Waals interaction and hydrogen bonding are the dominant extraction mechanisms for NAs with long alkyl chain. F⋯H hydrogen bonding is the strongest hydrogen bond, and O⋯H is the second strongest for all the interactions. NAs with polycyclic hydrocarbons or multiple carboxylic groups have larger total interaction energies than those with monocyclic hydrocarbons and one carboxylic group. The results indicate that σ(CO)-σ*(CH), LP(O)-σ*(CH), σ*(CO)-σ*(CH), and σ(CH)-σ*(CO) interactions can occur between [BMIM][BF4] and model NAs without aromatic ring, whereas π(CO)-σ*(CH), LP(O)-σ*(CH), π*(CO)-σ*(CH), and σ(CH)-π*(CO) interactions take place for [BMIM][BF4]-benzoic acid.

Theoretical study on interactions between Trifluoromethanesulfonate (Triflate) based ionic liquid and thiophene

Density functional theory was employed to investigate the interactions between 1-ethyl-3-methylimidazolium ([EMIM]+)/1-ethylpyridinium ([EPY]+)/1-ethyl-1-methylpyrrolidinium ([EPYRO]+)/1-ethyl-1-methylpiperidinium ([EPIP]+) cations and trifluoromethanesulfonate ([OTf]−) anion, as well as the interactions between trifluoromethanesulfonate ([EMIM]+[OTf]−)/1-ethylpyridinium trifluoromethanesulfonate ([EPY]+[OTf]−)/1-ethyl-1-methylpyrrolidinium trifluoromethanesulfonate ([EPYRO]+[OTf]−)/1-ethyl-1-methylpiperidinium trifluoromethanesulfonate ([EPIP]+[OTf]−) and thiophene. The ionic liquids and complexes formed between [EMIM]+[OTf]−/[EPY]+[OTf]−/[EPYRO]+[OTf]−/[EPIP]+[OTf]− and thiophene were analyzed by natural bond orbital, atoms in molecules, and noncovalent interaction, HOMO-LUMO overlap integral analyses, and electron density difference analysis. The calculated results show that thiophene ring is parallel to [EMIM]+/[EPY]+ ring, beneficial to π-π interaction, while [EPYRO]+/[EPIP]+ branched chains tend to form C-H⋯π interactions with thiophene. The interactions between [EMIM]+/[EPY]+/[EPYRO]+/[EPIP]+ and thiophene are stronger than that of [OTf]− and thiophene proposed by interaction energy and electron density differences. The HOMO-LUMO overlap integral analyses demonstrate the different charge transfer direction and interacting nature of frontier orbitals.

Enhanced Electroluminescence from a Thermally Activated Delayed-Fluorescence Emitter by Suppressing Nonradiative Decay

Thermally activated delayed-fluorescence (TADF) emitters have attracted increasing attention as third-generation electroluminescent materials for organic light-emitting diodes. This study presents a design strategy for highly efficient TADF emitters, guided by quantum-chemistry calculations of radiative and nonradiative decay rates. By optimizing HOMO-LUMO overlap density and suppressing nonradiative decay, the authors develop a purely organic emitter with very high quantum yields from photoluminescence and electroluminescence.

Origin of inversion versus retention in the oxidative addition of 3-chloro-cyclopentene to Pd(0)L(n).

The preference for syn versus anti oxidative addition of 3-chloro-cyclopentene to Pd(0)L(n) was investigated using density functional theory (L = PH3, PMe3, PF3, ethylene, maleic anhydride, pyridine, imidazol-2-ylidene). Both mono- and bis-ligation modes were studied (n = 1 and 2). The pathways were analyzed at the B2PLYP-D3/def2-TZVPP//TPSS-D3/def2-TZVP level, and an interaction/distortion analysis was performed at the ZORA-TPSS-D3/TZ2P level for elucidating the origin of the selectivity preferences. Mechanistically, the anti addition follows an S(N)2 type mechanism, whereas the syn addition has partial S(N)1 and S(N)2' character. Contrary to the traditional rationale that orbital interactions are dominant in the anti pathway, analysis of the variation of the interaction components along the intrinsic reaction coordinate shows that the syn pathway exhibits stronger overall orbital interactions. This orbital preference for the syn pathway diminishes with increasing donor capacity of the ligand. It is caused by the donation of the isolated p orbitals on the migrating chlorine atom to the PdL(n) fragment, which is lacking in the anti pathway, whereas the HOMO-LUMO overlap between the fragments is greater for the anti pathway. Electrostatically, the syn pathway is preferred for weakly donating and withdrawing ligands, whereas the anti pathway is favored with strongly donating ligands.

Ene‐ene‐yne Reactions: Activation Strain Analysis and the Role of Aromaticity

The trend in reactivity of the thermal cycloisomerization reactions of 1,3-hexadien-5-ynes, A=B-C=D-E≡F, were explored and analyzed by using density functional theory at the M06-2X/def2-TZVPP level. These reactions proceed through formally aromatic transition states to form a bent-allene intermediate with relatively high activation barriers. Activation-strain analyses show that the major factor controlling this Hopf cyclization is the geometrical strain energy associated with the rotation of the terminal [A] group. This rotation is necessary for achieving a favorable HOMO-LUMO overlap with the yne-moiety [F] associated with the formation of the new A-F single bond. In addition, the relationship between the aromaticity of the corresponding cyclic transition states (all six-membered rings) and the computed activation barriers were analyzed. The calculations also indicate that the aromatization of the bent-allene structures takes place through two consecutive 1,2-hydrogen shifts, the second one exhibiting negligible energy barriers.

Bioheterojunction Effect on Fluorescence Origin and Efficiency Improvement of Firefly Chromophores

We propose the heterojunction effect in the analysis of the fluorescence mechanism of the firefly chromophore. Following this analysis, and with respect to the HOMO-LUMO gap alignment between the chromophore's functional fragments, three main heterojunction types (I, II, and I*) are identified. Time-dependent density-functional theory optical absorption calculations for the firefly chromophore show that the strongest excitation appears in the deprotonated anion state of the keto form. This can be explained by its high HOMO-LUMO overlap due to strong bio-heterojunction confinement. It is also found that the nitrogen atom in the thiazolyl rings, due to its larger electronegativity, plays a key role in the emission process, its importance growing when HOMO and LUMO overlap at its location. This principle is applied to enhance the chromophore's fluorescence efficiency and to guide the functionalization of molecular optoelectronic devices.

A Computational Study on the Stacking Interaction in Quinhydrone

The stability and electron density topology of quinhydrone complex was studied using multiple computational levels, including MPW1B95 Truhlar's density functional. The QTAIM analysis demonstrates that an electron population transfer from hydroquinone to quinone monomer accompanies the complex formation. The variations undergone by atomic populations indicate that the electron transfer through HOMO LUMO overlap is combined with a reorganization of the electron density within each monomer. Variations of two- and six-center delocalization indices show a small reduction of electron delocalization in the hydroquinone ring upon complex formation.

The Role of Binary and Many-centre Molecular Interactions in Spin Crossover in the Solid State. Part II. Non-ideality Parameters Defined via Binary Molecular Potentials

Parameters of the formalism [1–6] describing spin crossover in the solid state have been defined via molecular potentials in model systems of neutral and ionic complexes. In the first instance Lennard-Jones and electric dipole–dipole potentials have been used whereas in ionic systems Lennard-Jones and electric point-charge potentials have been used. Electric dipole–dipole interaction of neutral complexes brings about a positive excess energy controlled by the difference of electric dipole moments of HS and LS molecules. Differences of the order of Δμ = 1–2 D cause an abrupt spin crossover in systems with T1/2 = 100–150 K. Magnetic coupling contributes both to the excess energy and excess entropy, however the overall effect is equivalent to a modest positive excess energy. Ionic systems in the absence of specific interactions are characterised by very small excess energies corresponding to practically linear van’t Hoff plots. Detectable positive and negative excess energies in these systems may arise from interactions of ligands belonging to neighbouring complexes. The HOMO–LUMO overlap in HS–LS pairs can bring about a nontrivial variation of the shape of transition curves. Examples of regression analysis of experimental transition curves in terms of molecular potentials are given.

Intermolecular electron transfer reactivity determined from cross-rate studies.

Electron-transfer cross-reactions between neutral molecules and their radical cations spanning a wide range of structural type and intrinsic reactivity have been analyzed using classical Marcus theory. The principal factor found to govern intrinsic reactivity is the inner-shell bond reorganization energy. The HOMO-LUMO overlap of alkyl groups on reacting molecules is generally sufficient to provide facile electron transfer; however, a significant steric effect on this overlap is observed for hydrazines with alkyl groups larger than methyl.