Reorganization Energies in the Transports of Holes and Electrons in Organic Amines in Organic Electroluminescence Studied by Density Functional Theory

Non-negligible Outer-Shell Reorganization Energy for Charge Transfer in Nonpolar Systems

Many charge-transporting molecular systems function as ordered or disordered arrays of solid state materials composed of nonpolar (or weakly polar) molecules. Due to low dielectric constants for nonpolar systems, it is common to ignore the effects of outer-shell reorganization energy (λout). However, ignoring λout has not been properly supported and it can severely impact predictions and insights derived. Here, we estimate λout by two means: from experimental ultraviolet photoelectron spectra, in which vibronic progression in these spectra can be fitted with the widths of peaks determining the low-frequency component in reorganization energy, regarded to be closely associated with λout, and from molecular dynamic (MD) simulation of nonpolar molecules, in which disorder or fluctuation statistics for energies of charged molecules are calculated. An upper bound for λout was obtained as 505 and 549 meV for crystalline anthracene (140 K) and pentacene (50 K), respectively, by fitting of experimental data, and 212 and 170 meV, respectively, from MD simulations. These values are comparable to the inner-sphere reorganization energy (λin) arising from intramolecular vibration. With corresponding spectral density functions calculated, we found that λout is influenced both by low- and high-frequency dynamics, in which the former arises from constrained translational and rotational motions of surrounding molecules. In an amorphous state, about half of the λout's obtained are from high-frequency components, which is quite different from the conventional polar solvation. Moreover, crystalline systems exhibit super-Ohmic spectral density, whereas amorphous systems are sub-Ohmic.

Designing Thieno[3,4-c]pyrrole-4,6-dione Core-Based, A2-D-A1-D-A2-Type Acceptor Molecules for Promising Photovoltaic Parameters in Organic Photovoltaic Cells

Nonfullerene-based organic solar cells can be utilized as favorable photovoltaic and optoelectronic devices due to their enhanced life span and efficiency. In this research, seven new molecules were designed to improve the working efficiency of organic solar cells by utilizing a terminal acceptor modification approach. The perceived A2-D-A1-D-A2 configuration-based molecules possess a lower band gap ranging from 1.95 to 2.21 eV compared to the pre-existing reference molecule (RW), which has a band gap of 2.23 eV. The modified molecules also exhibit higher λmax values ranging from 672 to 768 nm in the gaseous and 715-839 nm in solvent phases, respectively, as compared to the (RW) molecule, which has λmax values at 673 and 719 nm in gas and chloroform medium, respectively. The ground state geometries, molecular planarity parameter, and span of deviation from the plane were analyzed to study the planarity of all of the molecules. The natural transition orbitals, the density of state, molecular electrostatic potential, noncovalent interactions, frontier molecular orbitals, and transition density matrix analysis of all studied molecules were executed to validate the optoelectronic properties of these molecules. Improved charge mobilities and dipole moments were observed, as newly designed molecules possessed lower internal reorganization energies. The open circuit voltage (Voc) of W4, W5, W6, and W7 among newly designed molecules was improved as compared to the reference molecule. These results elaborate on the superiority of these novel-designed molecules over the pre-existing (RW) molecule as potential blocks for better organic solar cell applications.

Density Functional Calculation and Evaluation of the Spectroscopic Properties and Luminescent Material Application Potential of the N-Heterocyclic Platinum(II) Tetracarbene Complexes

A series of reported Pt(II) carbene complexes possibly have the ability to serve as the new generation of blue emitters in luminescent devices because of their narrow emission spectra, high photoluminescence quantum yields (PLQYs), and rigid molecular skeleton. However, the combination of all carbene ligands with different multidentate structures will affect the overall planarity and horizontal dipole ratio to varying degrees, but the specific extent of this effect has not previously been analyzed in detail. In this work, density functional computation is used to study a class of platinum tetracarbene bidentate complexes with similar absorption and emission band characteristics, which is the main reason for the remarkable difference in quantum efficiency due to subtle differences in electronic states caused by different ligands. From the calculation results, the major reason, which results in significantly decrease in quantum efficiency for [Pt(cyim)2]2+, is that [Pt(cyim)2]2+ can reach the non-radiative deactivation metal-centered d-d excited state through an easier pathway compared with [Pt(meim)2]2+. The result, based on changes in the dihedral angle between ligands, can achieve the goal of improving and designing materials by adjusting the degree of the dihedral angle. (meim: bis(1,1'-dimethyl-3,3'-methylene-diimidazoline-2,2'-diylidene); cyim: bis(1,1'-dicyclohexyl-3,3'-methylene-diimidazoline-2,2'-diylidene).

New bithiophene-based molecules as hole transporting materials for perovskite solar cells and or as donor for organic solar cells

DFT and TDDFT approaches were used to design three (T16,17,18) molecules based on 4,4'-dimethoxy-2,2'-bithiophene core to explore the influence of substitution of triphenylamine (TPA) fragment by methoxy groups, and introduction of azomethine π-bridges on the optoelectronic properties of hole transporting materials for perovskite solar cells (PSCs) or as donor for organic solar cells (OSCs). To shed light on the efficiency, stability, and solubility several physicochemical parameters were computed in dichloromethane solvent. All designed molecules show appropriate frontier molecular orbital levels, which facilitates effective hole transfer from the perovskite materials to the HTMs in the hole-transporting layer in PSC devices. They all show good efficiency and pore-fillings and are stable and soluble in dichloromethane. Electron-hole pairs can easily dissociate into free charge carriers, especially for T16 and T17; consequently, improve short-circuit current densities and facilitate hole transport. It is also advised to use T18 which includes azomethine bridges as a donor with a non-fullerene Y6 acceptor to create effective OSCs because it exhibits high open circuit voltage, fill factor and low gap energy.

A computational insight into enhancement of photovoltaic properties of non-fullerene acceptors by end-group modulations in the structural framework of INPIC molecule

Improving the open circuit voltage is a major challenge for enhancing the overall efficiency of organic solar cells. Current work has concentrated on improving open-circuit voltage by designing new molecular frameworks from an INPIC molecule having a conjugated fused core. We modulated the structure by changing the terminal groups of the reference molecule (INPIC) with seven strong electron-withdrawing units. We investigated various optoelectronic attributes, charge transfer, and photovoltaic and geometrical parameters by compiling the B3LYP/6-31G(d,p) functional of the DFT approach. The optical absorption for modulated molecules ranges from 748.51 nm to 845.96 nm while showing higher oscillation strength than INPIC. At the same time, their impressive charge transport is attributed to their smaller excitation and exciton binding energy, higher electron/hole mobility, narrower band gap, and a more than 99 % intramolecular charge transfer. The larger dipole moments help in the dense interaction of acceptors with employed donor J61 which, in turn, improves charge transfer at the donor-acceptor interface. One of the triumphs that are difficult to get in organic molecules is success in achieving a higher open circuit voltage (VOC). Our conceptualized molecular frameworks of acceptors are featured with a notable VOC improvement in the range of 1.84-2.05 eV. Thus, the results of the current investigation pave the root for architecting the acceptor molecules with impressive optoelectrical properties that may be capable of providing high photovoltaic output. Thus these acceptors can be utilized for the development of advanced organic solar cells in future.

The Evolution of Classical Spiro-OMeTAD: Synthesis of Arylamine Endcapped Indenone Spirofluorene

Spiro-OMeTAD is the well-known hole transporting material (HTM) in perovskite solar cells. In this work, its derivatives, namely four D-A shaped triphenylamine or biphenylamine endcapped indenone spirofluorene (SFD-TPA, SFD-OMeTPA, SFD-TAD, and SFD-OMeTAD), were designed and synthesized. With the introduction of electron-donating moieties and the extension of conjugation length, a series of changes in photophysical and electrochemical properties could be detected. Notably, in comparison with the optical gap (2.96 eV) of the reported spiro-OMeTAD, SFD-OMeTAD presents an optical gap as low as 1.87 eV. Moreover, density functional theory simulations were employed to further investigate their geometric and electronic structures. Finally, steady-state photoluminescence measurements proved the efficient charge separation and collection processes at the perovskite/HTM interface. It can be predicted that all four compounds with enhanced sunlight absorption capability and suitable frontier energy levels can be used as hole-transporting materials for perovskite solar cells.

A Phenanthrocarbazole-Based Dopant-Free Hole-Transport Polymer with Noncovalent Conformational Locking for Efficient Perovskite Solar Cells

Adequate hole mobility is the prerequisite for dopant-free polymeric hole-transport materials (HTMs). Constraining the configurational variation of polymer chains to afford a rigid and planar backbone can reduce unfavorable reorganization energy and improve hole mobility. Herein, a noncovalent conformational locking via S-O secondary interaction is exploited in a phenanthrocarbazole (PC) based polymeric HTM, PC6, to fix the molecular geometry and significantly reduce reorganization energy. Systematic studies on structurally explicit repeats to targeted polymers reveals that the broad and planar backbone of PC remarkably enhances π-π stacking of adjacent polymers, facilitating intermolecular charge transfer greatly. The inserted "Lewis soft" oxygen atoms passivate the trap sites efficiently at the perovskite/HTM interface and further suppress interfacial recombination. Consequently, a PSC employing PC6 as a dopant-free HTM offers an excellent power conversion efficiency of 22.2 % and significantly improved longevity, rendering it as one of the best PSCs based on dopant-free HTMs.

Ambipolar Charge Transport Properties of Naphthophenanthridine Discotic Liquid Crystals

A series of novel naphthophenanthridine derivatives are synthesized via N-annulation of hexabutoxytriphenylene-1-amine with various aliphatic aldehydes through the Pictet-Spengler reaction. The synthesized derivatives have been found to self-assemble into a columnar hexagonal mesophase over a wide temperature range, as validated through polarized optical microscopy, differential scanning calorimetry and X-ray diffraction experiments. The photophysical properties of these compounds were studied using UV-visible and emission spectroscopy. The synthesized compounds exhibit ambipolar charge transport, showing temperature-independent electron and hole mobility on the order of 3 × 10^-4 cm²/V s, as evaluated by the time-of-flight technique. These novel N-annulated derivatives can be of immense potential toward semiconducting applications of self-assembling supramolecular systems.

Loading Linear Arrays of CuII Inside Aromatic Amide Helices

The very stable helices of 8-amino-2-quinolinecarboxylic acid oligoamides are shown to uptake CuII ions in their cavity through deprotonation of their amide functions with minimal alteration of their shape, unlike most metallo-organic structures which generally differ from their organic precursors. The outcome is the formation of intramolecular linear arrays of a defined number of CuII centers (up to sixteen in this study) at a 3 Å distance, forming a molecular mimic of a metal wire completely surrounded by an organic sheath. The helices pack in the solid state so that the arrays of CuII extend intermolecularly. Conductive-AFM and cyclic voltammetry suggest that electrons are transported throughout the metal-loaded helices in contrast with hole transport observed for analogous foldamers devoid of metal ions.

Comparison of charge transport and opto-electronic properties of pyrene and anthracene derivatives for OLED applications

In this paper, three organic semiconductors such as 9-[(5-nitropyridin-2-aminoethyl)iminiomethyl]-anthracene (a), N'-((pyren-4-yl)methylene)isonicotinohydrazide (b), and novel organic semiconductor N-(2-((pyren-4-yl)methyleneamino)ethyl)-5-nitropyridin-2-amine (c) were prepared. Their structures were assessed using NMR and elemental analysis techniques. While compound (a) and compound (c) have the same wing unit ([(5-nitropyridin-2-aminoethyl) iminiomethyl]), compounds (b) and (c) have the same core unit (5-nitropyridin-2-amine). Based on TD-DFT and Marcus theories, we have explored the effects of molecular structure on the opto-electronic properties for OLED applications. Our results show that wing units of molecules impact more on the opto-electronics properties than on core units. The compounds (a) and (c) with the same wing unit have exhibited quite similar behaviors in terms of both structural and opto-electronic parameters. However, a similar situation has not been observed for compounds (b) and (c) with the same core unit. In conclusion, our results indicate that compounds (a) and (c) exhibit obvious advantages for OLEDs in terms of calculated opto-electronic and charge transport properties such as better absorption and emission parameters, lower energy gaps and reorganisation energies and higher charge mobility.

In silico modeling: electronic properties of phosphorene monoflakes and biflakes substituted with Al, Si, and S heteroatoms

This contribution explores the systematic substitution of phosphorene monoflakes (Mfs) and biflakes (Bfs) with aluminum, silicon, and sulfur. These systems were investigated using density functional theory employing the TPSS exchange-correlation functional and complete active space self-consistent field (CASSCF) calculations. Al and Si substitution produces significant structural changes in both Mfs and Bfs compared to S-substituted and pristine systems. However, in Mfs, all heteroatoms generate a decrease in band gap and the ionization potentials (IP), and an increase in electron affinity (EA) in comparison with pristine phosphorene. Al doping improves the hole mobility in the phosphorene monoflake, while Si and S substitutions exhibit a similar behavior on EAs and reorganization energies. For Bfs, the presence of Si-Si and Al-P interlaminar interactions causes structural changes and higher binding energies for Si-Bfs and Al-Bfs. Regarding the electronic properties of Bfs, substitution with Si does not produce significant variations in the band gap. Nevertheless, it conduces the formation of hole transport materials, which does not occur in Si-Mfs. The same is observed for Al systems, whereas no correlation was identified between the doping level and reorganization energies for S complexes. The substitution with Al and S leads to an opposite behavior of the band gap and IP values, while the EA variation is similar. In summary, the nature of heteroatom and the doping degree can modify the semiconductor character and electronic properties of phosphorene mono- and biflakes, whose trends are closely related to the atomic properties considered. Overall, these computational calculations provide significant insights into the study of doped phosphorene materials.

Computational study of the effect of π-spacers on the optoelectronic properties of carbazole-based organic dyes

In this article, we studied a series of dye-sensitized solar cells (DSSCs) type Donor-π-Acceptor involving carbazole as donors and cyanoacrylic acid as acceptors of the electrons. These cells are linked by different π-spacer unit's, with the aim to develop new organic dyes with high-performance optoelectronic properties. Different units have been introduced in the π-bridge in order to investigate their effects on the structural and optoelectronic properties of the studied compounds, as well as their adsorbed compounds-titanium dioxide (TiO₂) semi-conductor. We evaluated and assessed the important relevant parameters that influence the performance of photovoltaic cell to measure their involvement in the short-circuit photocurrent density (J_sc). Using Density Functional Theory (DFT) and Time-Dependent-BHandHLYP, the geometrical and optoelectronics properties have been predicted theoretically. The results obtained indicate that introducing the oxazole (S5) and thiazole (S6) molecules in the π-spacer have significant impact on the geometric properties for D5-D6 dyes. This results in the fact that dye D5 has a planar structure. Also, the insertion of the thiophene, oxazole and thiazole units improves the energies of the HOMO and LUMO molecular orbitals of D1, D5, and D6 dyes. Moreover, these results show the ability of electron transfer and regeneration from the studied sensitizers (D1-D6). Also, it can be noted that the application of the pyrrole group in the π-spacer moiety of the dye (D2) improves the electron's transfer performance with a lower regeneration motive force ΔG^reg, a more negative injection driving forces (ΔG^inject), and a higher values of open circuit-voltage (Voc).

Hole-mediated photoredox catalysis: tris(p-substituted)biarylaminium radical cations as tunable, precomplexing and potent photooxidants

Triarylamines are demonstrated as novel, tunable electroactivated photocatalysts that use dispersion precomplexation to harness the full potential of the visible photon (>4.0 V vs. SCE) in anti-Kasha photo(electro)chemical super-oxidations of arenes.

Ambipolar pentacyclic diamides with interesting electrochemical and optoelectronic properties

Developing organic semiconductors for organic thin film transistors (OTFT) and optoelectronic applications is a challenge. We developed highly crystalline pentacyclic diimides (3) and (4) which showed good OTFT and OLED potential and energy gaps of 2.60 eV and 2.54 eV. They exhibited interesting photo and eletroluminescence activity. Both compounds showed good quantum yields (0.56 for (3) and 0.60 for (4)).

Charge transfer and opto-electronic properties of some newly designed polycatenar discotic liquid crystal derivatives: a DFT study

Herein, we demonstrate effect of substituents on optoelectronic properties of discotic liquid crystals (DLCs) by using density functional theory (DFT) calculations at B3LYP/Lanl2Z level of theory. Three parent DLCs, namely, (1) benzene-1,3,5-triyl tris(3,5-dialkoxybenzoate), (2) N¹, N³, N⁵-tris(3-alkoxyphenyl)benzene-1,3,5-tricarboxamide, and (3) trialkyl 4, 4', 4″-(benzenetricarbonyltris (azanediyl)) tribenzoate benzoate and their -N and -S group derivatives of 1, 2, and 3, were investigated to observe the change in optoelectronic response of these systems. The frontier molecular orbital studies and electron affinity values indicate that the studied compounds are stable against the oxygen and moisture present in air. The calculated charge transfer integrals, electron, and hole mobility values revealed that parent DLCs and their derivatives can be employed as an effective n-type material for OLEDs; however, derivatives have enhanced charge transfer values compared with their parents. For better understanding of the thermochemistry and effect of substituents, frequency calculations were carried out. P₁-D₄ derivative having R = -NH-CO-CH₃ terminal group came out to be theoretically the most favored having the lowest ΔG value. Computed UV/visible spectroscopic analysis showed minimum absorbance and maximum transmittance for derivative P₂-D₁ having -S-NH₂ substituent. Molecular electrostatic potential surfaces mapped at potential range, i.e., - 8.531e-3esu to + 8.531e-3esu, describe electrophilic and nucleophilic characteristics. Introduction of electron donor groups enhanced electrical conductivity, excitation energy, and charge transfer integral, thus increasing optoelectronic properties of DLCs. However, these claims require further experimental verification.

Kinetic Evidence That the Solvent Barrier for Electron Transfer Is Absent in the Electric Double Layer

Classical capacitance studies have revealed that the first layer of water present at an aqueous metal-electrolyte interface has a dielectric constant less than 1/10th of that of bulk water. Modern theory indicates that the barrier for electron transfer will decrease substantially in this layer; yet, this important prediction has not been tested experimentally. Here, we report the interfacial electron transfer kinetics for molecules positioned at variable distances within the electric double layer of a transparent conductive oxide as a function of the Gibbs free energy change. The data indicate that the solvent reorganization is indeed near zero and increases to bulk values only when the molecules are positioned greater than 15 Å from the conductive electrode. Consistent with this conclusion, lateral intermolecular electron transfer, parallel to a semiconducting oxide electrode, was shown to be more rapid when the molecules were within the electric double layer. The results provide much needed feedback for theoretical studies and also indicate a huge kinetic advantage for aqueous electron transfer and redox catalysis that takes place proximate to a solid interface.

Theoretical perspective on the electronic structure and photophysical properties for a series of mixed-carbene tris-cyclometalated iridium(iii) complexes

The electronic structure and photophysical properties of four mixed-carbene tris-cyclometalated iridium(iii) complexes have been theoretically investigated by the density functional theory (DFT) and time-dependent density functional theory (TDDFT) methods. The effect of varying the main ligand by introducing different ring structures on the photophysical properties of the studied complexes has been explored. All studied complexes have slightly distorted octahedral geometries. The complex with a rigid skeletal structural main ligand possesses the smallest difference between the recombination energy of hole transport and recombination energy of electron transport among these complexes, enhancing the charge transfer balance. The lowest energy emission wavelength calculated is in very good agreement with the available experimental value. This study will provide useful information for the design of new phosphorescent organic light-emitting diode (OLED) materials.

Electron Transfer Reorganization Energies in the Electrode-Electrolyte Double Layer

The total reorganization energy, λ, for interfacial electron transfer, ET, from a conductive electrode to redox-active molecules at fixed positions within the electric double layer, EDL, has been determined experimentally. Conductive indium-tin-oxide (ITO, In₂O₃:Sn) mesoporous films were functionalized with 4-[N,N-di(p-tolyl)-amino]benzylphosphonic acid (TPA) and/or [Ru^II(bpy)₂(4,4'-(PO₃H₂)₂-bpy)]²⁺ (RuP), where bpy is 2,2'-bipyridine. The small inner-sphere reorganizations, λ_i, for Ru^III/IIP and TPA^+/0 make them excellent probes of outer-sphere reorganization energy, λ_o, as λ_i ≪ λ_o such that λ = λ_i + λ_o ≈ λ_o. Consecutive layer-by-layer addition of Zr^IV-bridged methylenediphosphonic acid enabled positioning at distances from 4 to 27 Å from the ITO. Excited-state injection into the ITO by RuP* generated ITO(e^-)|Ru^IIIP. For ITO cofunctionalized with TPA and RuP, subnanosecond lateral ET yielded ITO(e^-)|TPA⁺. The kinetics for ET from ITO to Ru^IIIP or TPA⁺ were quantified spectroscopically as a function of applied potential (E_app) and hence driving force, -ΔG°. Marcus-Gerischer analysis of this data provided λ. Significantly, λ_o was near zero at close electrode proximity, λ = 0.11 eV at a distance of ∼4 Å, as manifest by kinetics largely insensitive to E_app. In agreement with dielectric continuum theory, λ increased to values expected in CH₃CN solution when the molecule was positioned at a distance of ∼27 Å (λ = 0.94 eV). The data reveal small intrinsic barriers for electron transfer proximate to conductive interfaces, which is an exploitable behavior in solar energy conversion and other applications that utilize transparent conductive oxides to accept or deliver electrons.

Twisting the TAPPs: Bay-Substituted Non-planar Tetraazapero-pyrenes and their Reduced Anions

A new synthesis of tetraazaperopyrenes (TAPPs) starting from a halogenated perylene derivative 3,4,9,10- tetrabromo-1,6,7,12-tetrachloroperylene (1) gave access to bay-substituted TAPPs for the first time. Selective lithiation of the bromine-positions and subsequent addition of tosyl azide led to the formation of the tetraazidotetrachloroperylene (2), which was subsequently reduced by addition of sodium borohydride to the corresponding tetraaminotetrachloroperylene (3). Oxidation to its semiquinoidal form 4 and subsequent cyclization with acid chlorides gave rise to a series of bay-chlorinated TAPPs. Whereas the aromatic core of the previously studied ortho-substituted TAPPs was found to be planar, the steric pressure of the two chlorine substituents on each side leads to the twist of the peropyrene core of approximately 30 degrees, a structural feature also observed in other bay-substituted perylene derivatives. An experimental and computational analysis reveals that introducing chloride substituents at these positions leads to slightly increased electron affinities (EA) enabling the selective generation and characterization of the reduced mono-anionic radicals and closed shell di-anionic species. These anions were isolated and characterized by UV/Vis spectroscopy and EPR or NMR, respectively. Processing of the bay-chlorinated TAPPs in n-channel organic TFTs revealed electron mobilities of 0.001 to 0.003 cm2  V-1  s-1 . These reduced electron mobilities compared to the ortho-halogenated TAPPs are thought to be rooted in the less densely packed solid-state structures.

Theoretical insight into the photophysical properties of six heteroleptic Ir(iii) phosphorescent complexes bearing ppy-type ligands

By using density functional theory and time-dependent density functional theory, the geometrical, electronic and photophysical properties of six complexes with two ppy-type ligands and one acetylacetone anion around the Ir center have been explored. The lowest energy absorption wavelengths are located at 414 nm for 1, 434 nm for 2, 434 nm for 3, 421 nm for 4, 436 nm for 5, and 425 nm for 6, respectively. The lowest energy emissions of these complexes are localized at 617, 492, 633, 634, 491 and 491 nm, respectively, for complexes 1-6, simulated in CH2Cl2 medium at the M062X level. The calculated lowest lying absorption wavelength and the lowest energy emission wavelength for complex 3 are very close to the available experimental values. The position and number of the incorporated electron-withdrawing fluorine substituents have some effect on the electronic and photophysical properties of these studied complexes.

Optimizing electron-rich arylamine derivatives in thiophene-fused derivatives as π bridge-based hole transporting materials for perovskite solar cells

Based on the observations of thienothiophene derivatives as π-bridged small molecule hole transporting materials (HTMs), adjusting their electron-rich arylamine derivatives is an effective approach to obtain the alternative HTMs for perovskite solar cells (PSCs). In this work, starting from a new electron-rich arylamine derivative and different π-bridged units of thienothiophene derivatives, a series of arylamine derivative-based HTMs were designed, and their properties were investigated using density functional theory combined with the Marcus charge transfer theory. Compared with the parental Z26 material, the designed H01-H04 exhibit appropriate frontier molecular orbitals, good optical properties, better solubility, good stability and higher hole mobilities. H01-H04 materials with high hole mobility (∼× 10-2) can serve as promising HTMs for improving the efficiency of PSCs. The results confirm that the design strategy of adjusting the electron-rich arylamine derivatives in thienothiophene derivatives as π-bridged HTMs is a reliable approach to obtain the promising HTMs for PSC applications.

Deciphering the Multifarious Charge-Transport Behaviour of Crystalline Propeller-Shaped Triphenylamine Analogues

A collection of para-substituted propeller-shaped triphenylamine (TPA) derivatives have been computationally investigated for charge-transport characteristics exhibited by the derivatives by using the Marcus-Hush formalism. The various substituents chosen herein, with features that range from electron withdrawing to electron donating in nature, play a key role in defining the reorganisation energy and electronic coupling properties of the TPA derivatives. The TPA moiety is expected to possess weak electronic coupling on the basis of poor orbital overlap upon aggregation, owing to the restriction imposed by the propeller shape of the TPA core. However, the substituent groups attached to the TPA core can significantly dictate the crystal-packing motif of the TPA derivatives, wherein the variety of noncovalent intermolecular interactions subsequently generated drive the packing arrangement and influence electronic coupling between the neighbouring orbitals. Intermolecular interactions in the crystalline architecture of TPA derivatives were probed by using Hirshfeld and quantum theory of atoms-in-molecules techniques. Furthermore, symmetry-adapted perturbation theory analysis of the TPA analogues has revealed that a periodic arrangement of energetically stable dimers with significant electronic coupling is essential to contribute high charge-carrier mobility to the overall crystal.

The photophysical properties and electronic structures of bis[1]benzothieno[6,7-d:6′,7′-d′]benzo[1,2-b:4,5-b′]dithiophene (BBTBDT) derivatives as hole-transporting materials for organic light-emitting diodes (OLEDs)

The compounds with the D–π–A structure are found to be good candidates for blue-emitting materials.

Structures of the neutral and positively charged forms of the 4,4',4″-tris(N,N-phenyl-3-methylphenylamino)triphenylamine (m-MTDATA) molecule and its dimer, and charge localization in the corresponding cationic species

The structures of the 4,4',4″-tris(N,N-phenyl-3-methylphenylamino)triphenylamine (m-MTDATA) molecule and its dimer in their neutral and positively charged forms were studied by performing quantum-chemical calculations at the Hartree-Fock (HF) and density functional theory (DFT) levels of theory using several exchange-correlation functionals (PBE, PBE0, BHANDHLYP, and M06-HF) with different percentages of HF exchange. It was found that there are at least four possible isomeric structures of m-MTDATA with different (planar or perpendicular) arrangements of the peripheral diphenylamino groups. The charge localization in the monomeric and dimeric cationic species was also determined. The results indicated that the charge on the dimeric cation is localized on the central region or on the side fragment of the cationic part of the dimer, depending on the dimer structure. DFT calculations showed a tendency to overestimate the charge delocalization over the molecule, irrespective of the percentage of HF exchange applied. Graphical abstract Structure of an m-MTDATA dimer cation.

Role of Electron-Donating and Electron-Withdrawing Groups in Tuning the Optoelectronic Properties of Difluoroboron-Napthyridine Analogues

Five napthyridine-based fluorine-boron (BF₂-napthyridine) conjugated compounds have been theoretically designed, and subsequently, their photophysical properties are investigated. The influence of electron-donating and electron-withdrawing groups attached with the N^∧C^∧O moiety of BF₂-napthyridine molecule has been interpreted. The optoelectronic properties, including absorption spectra and emission spectra of the BF₂-napthyridine derivatives are studied using density functional theory (DFT) and time-dependent density functional theory (TD-DFT) based methods. Different characteristics, such as HOMO-LUMO gap, molecular orbital density, ionization potential, electron affinity, and reorganization energy for hole and electron, are calculated. All these molecules show excellent π-electron delocalization. TD-DFT results illustrate that the amine-substituted BF₂-napthyridine derivative has the highest absorption and emission maxima; it also shows a maximum Stoke shift. These results are well-correlated with the structural parameters and calculated HOMO-LUMO gap. Moreover, it is found that introduction of an electron-donating group into the BF₂-napthyridine complex improves the hole transport properties and provides useful clues in designing new materials for organic light emitting diodes (OLED). As a whole, this work demonstrates that electron-donating and electron-withdrawing groups in BF₂ derivatives can extend their effectiveness toward designing of OLED materials, vitro cellular studies, ex vivo assays, and in vivo imaging agents.