Marcus Theory Of Electron Transfer Research Articles

Theoretical Study of Internal Vibrational Relaxation and Energy Transport in Polyatomic Molecules

We attempted to theoretically characterize internal vibrational relaxation and energy relaxation pathways due to anharmonicity in polyatomic molecules. Energy transport dynamics have been modeled based on a generalization of Marcus electron transfer theory. Modifications have been made to our previously developed theory in order to improve the description of internal vibrational dynamics. We applied our method to several molecules studied experimentally by relaxation-assisted two-dimensional infrared spectroscopy (RA 2DIR). The theoretical predictions were found to be consistent with the majority of the experimental data.

A study on photo-induced intramolecular electron-transfer in fullerene-benzothiadiazole-triphenylamine using time-dependent density functional theory

We employed first-principle calculations to study the photo-induced electron transfer (PIET) process of a fullerene-benzothiadiazole-triphenylamine (C60-BTD-TPA), fullerene-diphenylbenzothiadiazole-triphenylamine (C60-PBTD-TPA), and fullerene-triphenylamine (C60-TPA). The ground state geometries of C60-BTD-TPA, C60-PBTD-TPA, and C60-TPA were optimized using density functional theory (DFT). Their excited states were investigated using time-dependent HF, and time-dependent DFT (TDDFT) methods. The long-range corrected CAM-B3LYP functional was found to give the best agreements with the experimentally observed transition energies. CAM-B3LYP-based wave functions were also employed to calculate the charge transfer integrals using generalized Mulliken–Hush (GMH) approach, and the photo-induced charge separation (kCS) and charge recombination rate constants (kCR) were calculated using Marcus theory. The results showed the calculated kCS and observed kCS of C60-PBTD-TPA and C60-TPA correspond to each other, however, the both of C60-BTD-TPA differ by two orders of magnitude. For kCR of these compounds, the calculated and observed values were more contrary to each other due to improper application of Marcus electron-transfer theory.

Catalytic Electron‐Transfer Oxygenation of Substrates with Water as an Oxygen Source Using Manganese Porphyrins

Manganese(V)-oxo-porphyrins are produced by the electron-transfer oxidation of manganese-porphyrins with tris(2,2'-bipyridine)ruthenium(III) ([Ru(bpy)(3)](3+); 2 equiv) in acetonitrile (CH(3)CN) containing water. The rate constants of the electron-transfer oxidation of manganese-porphyrins have been determined and evaluated in light of the Marcus theory of electron transfer. Addition of [Ru(bpy)(3)](3+) to a solution of olefins (styrene and cyclohexene) in CH(3)CN containing water in the presence of a catalytic amount of manganese-porphyrins afforded epoxides, diols, and aldehydes efficiently. Epoxides were converted to the corresponding diols by hydrolysis, and were further oxidized to the corresponding aldehydes. The turnover numbers vary significantly depending on the type of manganese-porphyrin used owing to the difference in their oxidation potentials and the steric bulkiness of the ligand. Ethylbenzene was also oxidized to 1-phenylethanol using manganese-porphyrins as electron-transfer catalysts. The oxygen source in the substrate oxygenation was confirmed to be water by using (18)O-labeled water. The rate constant of the reaction of the manganese(V)-oxo species with cyclohexene was determined directly under single-turnover conditions by monitoring the increase in absorbance attributable to the manganese(III) species produced in the reaction with cyclohexene. It has been shown that the rate-determining step in the catalytic electron-transfer oxygenation of cyclohexene is electron transfer from [Ru(bpy)(3)](3+) to the manganese-porphyrins.

Electronic and Charge Transport Properties of peri-Xanthenoxanthene: The Effects of Heteroatoms and Phenyl Substitutions

The electronic and charge transport properties of peri-xanthenoxanthene (PXX) and its phenyl-substitued derivative (Ph-PXX) are explored via quantum chemical calculations. To gain a better understanding of the physical properties of PXX, a comparative study is performed for its analogue, that is, anthanthrene. By employing Marcus electron transfer theory coupled with an incoherent charge hopping and diffusion model, we estimate the charge mobilties of PXX and Ph-PXX. Our calculated results indicate that the introduction of a heteroatom (oxygen) at the reactive sites of anthanthrene can stabilize the extended π-system and improve the effiecient charge injection in electronic devices. The phenyl substitution of PXX makes a remarkable change of charge transport characteristics from a p-type semiconductor to an n-type semiconductor, which shed light on molecular design for an n-type semiconductor through simple chemical structural modification.

Quantitative Analysis of Intramolecular Exciplex and Electron Transfer in a Double-Linked Zinc Porphyrin–Fullerene Dyad

Photoinduced charge transfer in a double-linked zinc porphyrin-fullerene dyad is studied. When the dyad is excited at the absorption band of the charge-transfer complex (780 nm), an intramolecular exciplex is formed, followed by the complete charge separated (CCS) state. By analyzing the results obtained from time-resolved transient absorption and emission decay measurements in a range of solvents with different polarities, we derived a dependence between the observable lifetimes and internal parameters controlling the reaction rate constants based on the semiquantum Marcus electron-transfer theory. The critical value of the solvent polarity was found to be ε(r) ≈ 6.5: in solvents with higher dielectric constants, the energy of the CCS state is lower than that of the exciplex and the relaxation takes place via the CCS state predominantly, whereas in solvents with lower polarities the energy of the CCS state is higher and the exciplex relaxes directly to the ground state. In solvents with moderate polarities the exciplex and the CCS state are in equilibrium and cannot be separated spectroscopically. The degree of the charge shift in the exciplex relative to that in the CCS state was estimated to be 0.55 ± 0.02. The electronic coupling matrix elements for the charge recombination process and for the direct relaxation of the exciplex to the ground state were found to be 0.012 ± 0.001 and 0.245 ± 0.022 eV, respectively.

Effect of dihydropyrazine on structures and charge transport properties of N-heteropentacenes matters: A theoretical investigation

A family of N-heteropentacenes acted as promising candidates for organic semiconductor materials is of immense interest. It should be attributed to the following reasons that (1) the positions, numbers and valence-states of N atom in N-heteropentacenes can effectively tune their electronic structure, stability, solubility, and molecular stacking; (2) diverse intermolecular interaction and π-stacking motifs appear in their crystals. The effect of the position and number of the 6-π-pyrazine on their structures and charge-transport properties has been systematically investigated in our previous work (J. Phys. Chem. C 115 (2011) 21416). Therefore, in this work, the study on the role of 8-π-dihydropyrazine with another valence-state N atoms is our focus. Density functional theory, Marcus electron transfer theory and Brownian diffusion assumption coupled with kinetic Monte-Carlo simulation are applied into this investigation. Our theoretical results indicate that in contrast with pyrazine, dihydropyrazine introduced is more helpful for promoting p-type organic semiconductor materials. For molecule 4, hole mobility of its single crystal theoretically reach 0.71cm2V−1s−1, and coupled with its fine hole-injection ability, it should be a promising candidate for p-type organic semiconductor materials. Although the lowest triplet-state energies of the molecules studied are very small, introduction of dihydropyrazine is very helpful for increasing the energies.

Enhanced Photoinduced Electron-Transfer Reduction of Li+@C60in Comparison with C60

Kinetics of photoinduced electron transfer from a series of electron donors to the triplet excited state of lithium ion-encapsulated C60 (Li(+)@C60) was investigated in comparison with the corresponding kinetics of the photoinduced electron transfer to the triplet excited state of pristine C60. Femtosecond laser flash photolysis measurements of Li(+)@C60 revealed that singlet excited state of Li(+)@C60 (λmax = 960 nm) underwent intersystem crossing to the triplet excited state [(3)(Li(+)@C60)*: λmax = 750 nm] with a rate constant of 8.9 × 10(8) s(-1) in deaerated benzonitrile (PhCN). The lifetime of (3)(Li(+)@C60)* was determined by nanosecond laser flash photolysis measurements to be 48 μs, which is comparable to that of C60. Efficient photoinduced electron transfer from a series of electron donors to (3)(Li(+)@C60)* occurred to produce the radical cations and Li(+)@C60(•-). The rate constants of photoinduced electron transfer of Li(+)@C60(•-) are significantly larger than those of C60 when the rate constants are less than the diffusion-limited value in PhCN. The enhanced reactivity of (3)(Li(+)@C60)* as compared with (3)C60* results from the much higher one-electron reduction potential of Li(+)@C60 (0.14 V vs SCE) than that of C60 (-0.43 V vs SCE). The rate constants of photoinduced electron transfer reactions of Li(+)@C60 and C60 were evaluated in light of the Marcus theory of electron transfer to determine the reorganization energies of electron transfer. The reorganization energy of electron transfer of Li(+)@C60 was determined from the driving force dependence of electron transfer rate to be 1.01 eV, which is by 0.28 eV larger than that of C60 (0.73 eV), probably because of the change in electrostatic interaction of encapsulated Li(+) upon electron transfer in PhCN.

Theoretical study of the charge carrier mobilities of the molecular materials tetrathiafulvalene (TTF) and 2,5-bis(1,3-dithiolan-2-ylidene)-1,3,4,6-tetrathiapentalene (BDH-TTP)

Tetrathiafulvalene (TTF) is a kind of fused ring aromatic compound containing four sulfur atoms in one molecule, which is well known as a charge transport material. In order to calculate the charge mobility of this semiconductor, Marcus electron transfer theory and the embedded model, which can give small intramolecular reorganization energies, were employed. The calculated results were in good agreement with the experimental values, so the above computing model is appropriate to assess the electrical property of TTF. On this basis, we predicted the charge mobility of 2,5-bis(1,3-dithiolan-2-ylidene)-1,3,4,6- tetrathiapentalene (BDH-TTP) crystals, for which the molecular structure is similar to TTF. The calculated results indicated that BDH-TTP is a p-type material, which has a better performance than TTF in hole transfer due to larger hole coupling and the smaller hole injection barrier. In addition, the direct coupling (DC) and the site energy correction (SEC) methods were used to calculate the charge transfer integrals. Although the results were slightly different, the qualitative trends were the same. Furthermore we took into account the anisotropic transfer properties of TTF and BDH-TTF, since obviously the mobilities along one dimension are larger than those along three dimensions. Finally, natural bond orbital analysis was used to study the interactions in all of the dimers.

Electron Tunneling Pathways and Role of Adenine in Repair of Cyclobutane Pyrimidine Dimer by DNA Photolyase

Electron tunneling pathways in enzymes are critical to their catalytic efficiency. Through electron tunneling, photolyase, a photoenzyme, splits UV-induced cyclobutane pyrimidine dimer into two normal bases. Here, we report our systematic characterization and analyses of photoinitiated three electron transfer processes and cyclobutane ring splitting by following the entire dynamical evolution during enzymatic repair with femtosecond resolution. We observed the complete dynamics of the reactants, all intermediates and final products, and determined their reaction time scales. Using (deoxy)uracil and thymine as dimer substrates, we unambiguously determined the electron tunneling pathways for the forward electron transfer to initiate repair and for the final electron return to restore the active cofactor and complete the catalytic photocycle. Significantly, we found that the adenine moiety of the unusual bent flavin cofactor is essential to mediating all electron-transfer dynamics through a superexchange mechanism, leading to a delicate balance of time scales. The cyclobutane ring splitting takes tens of picoseconds, while electron-transfer dynamics all occur on a longer time scale. The active-site structural integrity, unique electron tunneling pathways, and the critical role of adenine ensure the synergy of these elementary steps in this complex photorepair machinery to achieve maximum repair efficiency which is close to unity. Finally, we used the Marcus electron-transfer theory to evaluate all three electron-transfer processes and thus obtained their reaction driving forces (free energies), reorganization energies, and electronic coupling constants, concluding that the forward and futile back-electron transfer is in the normal region and that the final electron return of the catalytic cycle is in the inverted region.

Fascinating effect of dehydrogenation on the transport properties of N-heteropentacenes: transformation from p- to n-type semiconductor

The fascinating effect of dehydrogenation on the charge transport properties of N-heteropentacene (N-PEN) derivatives was systematically investigated through Marcus electron transfer theory coupled with normal mode analysis distributed into internal coordinates for the reorganization energies and the random walk simulation of charge diffusion constants. The calculated results show that dehydrogenation of N-PEN derivatives will markedly increase the electron transfer integrals and simultaneously decrease the energy levels of the lowest unoccupied molecular orbitals (LUMOs) as well as the electron reorganization energies (λe), which provides a three-in-one advantage for improving the electron transfer. The lower LUMO levels detected in the dehydrogenated N-PENs compared to the hydrogenated ones can be inferred from their electronic structures, that is, the latter are formally antiaromatic, having 2 more electrons than the former (aromatic 22 π electron species). The reduced reorganization energies of the dehydrogenated N-PENs could be intimately connected to the bonding nature of the nitrogen atoms in the LUMOs. Upon the dehydrogenation of N-PENs, the effective intermolecular π-overlap in the LUMOs of the nearest-neighboring molecules is enhanced, and then increased the electron transfer integrals. Interestingly, the transport parameters evaluated from the band and hopping models both indicate intrinsic mobility for electron transfer in the dehydrogenated N-PENs. The dehydrogenation of N-PEN derivatives can, thus, be a useful strategy for preparing n-type organic semiconductors.

Linking the historical and chemical definitions of diabatic states for charge and excitation energy transfer reactions in condensed phase

Marcus theory of electron transfer (ET) and Förster theory of excitation energy transfer (EET) rely on the Condon approximation and the theoretical availability of initial and final states of ET and EET reactions, often called diabatic states. Recently [Subotnik et al., J. Chem. Phys. 130, 234102 (2009)], diabatic states for practical calculations of ET and EET reactions were defined in terms of their interactions with the surrounding environment. However, from a purely theoretical standpoint, the definition of diabatic states must arise from the minimization of the dynamic couplings between the trial diabatic states. In this work, we show that if the Condon approximation is valid, then a minimization of the derived dynamic couplings leads to corresponding diabatic states for ET reactions taking place in solution by diagonalization of the dipole moment matrix, which is equivalent to a Boys localization algorithm; while for EET reactions in solution, diabatic states are found through the Edmiston-Ruedenberg localization algorithm. In the derivation, we find interesting expressions for the environmental contribution to the dynamic coupling of the adiabatic states in condensed-phase processes. In one of the cases considered, we find that such a contribution is trivially evaluable as a scalar product of the transition dipole moment with a quantity directly derivable from the geometry arrangement of the nuclei in the molecular environment. Possibly, this has applications in the evaluation of dynamic couplings for large scale simulations.

Intermolecular and Intracomplex Photoinduced Electron Transfer from Planar and Nonplanar Metalloporphyrins to p-Quinones

The rate constants of intermolecular photoinduced electron transfer from triplet excited states of metalloporphyrins to a series of p-benzoquinone derivatives in benzonitrile were determined to examine the effects of the driving force, the metal, and the conformational distortion of the porphyrin ring on the reorganization energies (λ) of electron transfer by laser flash photolysis. The λ values were evaluated from the determined rate constants on the basis of the Marcus theory of electron transfer. The λ values of planar metalloporphyrins, [Al(TPP)(PhCOO)] and [Zn(TPP)] (TPP(2-)=tetraphenylporphyrin dianion), are approximately the same, but they are 0.27 eV smaller than those of the corresponding nonplanar (saddle-distorted) metalloporphyrins [Al(DPP)(PhCOO)] and [Zn(DPP)] (DPP(2-)=dodecaphenylporphyrin dianion) when they are compared for the same driving force of photoinduced electron transfer. The axial ligand PhCOO(-) of [Al(TPP)](+) and [Al(DPP)](+) was replaced by anthraquinone-2-carboxylate (AqCOO(-)) to afford the electron donor-acceptor complexes [Al(TPP)(AqCOO)] and [Al(DPP)(AqCOO)], respectively. The X-ray crystal structure of [Al(TPP)(AqCOO)] revealed strong coordination of AqCOO(-) to the Al(3+) ion of [Al(TPP)](+) and the existence of π-π interactions between AqCOO(-) and the porphyrin ring. In the case of the saddle-distorted [Al(DPP)(AqCOO)], however, the AqCOO(-) moiety is nearly perpendicular to the porphyrin ring. The photodynamics of intracomplex photoinduced electron transfer from the singlet excited state of [Al(TPP)](+) and [Al(DPP)](+) to the AqCOO(-) moiety were also examined in comparison with the intermolecular photoinduced electron-transfer reactions, and the determined rate constants were evaluated in light of the Marcus theory of electron transfer to reveal that the electron transfer is adiabatic in each case.

Nonlinear Charge Transport in Redox Molecular Junctions: A Marcus Perspective

Redox molecular junctions are molecular conduction junctions that involve more than one oxidation state of the molecular bridge. This property is derived from the ability of the molecule to transiently localize transmitting electrons, implying relatively weak molecule-leads coupling and, in many cases, the validity of the Marcus theory of electron transfer. Here we study the implications of this property on the nonlinear transport properties of such junctions. We obtain an analytical solution of the integral equations that describe molecular conduction in the Marcus kinetic regime and use it in different physical limits to predict some important features of nonlinear transport in metal-molecule-metal junctions. In particular, conduction, rectification, and negative differential resistance can be obtained in different regimes of interplay between two different conduction channels associated with different localization properties of the excess molecular charge, without specific assumptions about the electronic structure of the molecular bridge. The predicted behaviors show temperature dependences typically observed in the experiment. The validity of the proposed model and ways to test its predictions and implement the implied control strategies are discussed.

Theoretical calculation of the photo-induced electron transfer rate between a gold atom and a gold cation solvated in CCl 4

A theoretical calculation was performed to evaluate the photo-induced electron transfer (PIET) rate between a gold atom and a gold ion solvated in carbon tetrachloride (CCl 4) in the framework of Marcus electron transfer (ET) theory, including both solvent reorganization effects and electronic wavefunction coupling between the ET diabatic states. A novel component of this work involves calculation of the electronic coupling strength using a recently developed constrained real-time time-dependent density-functional-theory (CRT-TDDFT) method. It is found that the PIET rate reaches its maximum value at the electronic resonance wavelength regardless of the inter-particle separation, suggesting a strong correlation between PIET and light absorption. In comparison with thermally activated electron transfer (TAET) at room temperature, light irradiation is demonstrated to be much more efficient than thermal fluctuations in promoting long-range ET, at least for the most common situations, when the light travelling substantially exceeds thermal energy. This work is the first step towards a quantum theory of plasmon enhanced electron transfer, and the theory can also be used to calculate electron transfer rates quite generally for condensed phase problems.

Evidence for Concerted Electron Proton Transfer in Charge Recombination between FADH– and 306Trp• in Escherichia coli Photolyase

Proton-coupled electron-transfer (PCET) is a mechanism of great importance in protein electron transfer and enzyme catalysis, and the involvement of aromatic amino acids in this process is of much interest. The DNA repair enzyme photolyase provides a natural system that allows for the study of PCET using a neutral radical tryptophan (Trp(•)). In Escherichia coli photolyase, photoreduction of the flavin adenine dinucleotide (FAD) cofactor in its neutral radical semiquinone form (FADH(•)) results in the formation of FADH(-) and (306)Trp(•). Charge recombination between these two intermediates requires the uptake of a proton by (306)Trp(•). The rate constant of charge recombination has been measured as a function of temperature in the pH range from 5.5 to 10.0, and the data are analyzed with both classical Marcus and semi-classical Hopfield electron transfer theory. The reorganization energy associated with the charge recombination process shows a pH dependence ranging from 2.3 eV at pH ≤ 7 and 1.2 eV at pH(D) 10.0. These findings indicate that at least two mechanisms are involved in the charge recombination reaction. Global analysis of the data supports the hypothesis that PCET during charge recombination can follow two different mechanisms with an apparent switch around pH 6.5. At lower pH, concerted electron proton transfer (CEPT) is the favorable mechanism with a reorganization energy of 2.1-2.3 eV. At higher pH, a sequential mechanism becomes dominant with rate-limiting electron-transfer followed by proton uptake which has a reorganization energy of 1.0-1.3 eV. The observed 'inverse' deuterium isotope effect at pH < 8 can be explained by a solvent isotope effect that affects the free energy change of the reaction and masks the normal, mass-related kinetic isotope effect that is expected for a CEPT mechanism. To the best of our knowledge, this is the first time that a switch in PCET mechanism has been observed in a protein.

Theoretical investigation of charge injection and transport properties of novel organic semiconductor materials—cyclic oligothiophenes

By employing the Marcus electron transfer theory coupled with two-state model, the Brownian diffusion assumption and density functional theory (DFT), we have investigated the charge injection and transport properties for three types of the cyclic oligothiophenes (A, B and C in Fig. 1), and their corresponding linear molecules (linear-A, linear-B and linear-C in Fig. 1). By indentifying 13 distinct nearest-neighbor hopping pathways based on the crystal structures of the molecules reported (A and B), we predicted the electronic coupling matrix elements for a wide variety of charge transfer pathways using the “energy splitting in dimer” (ESD) method and their carrier mobility. The theoretical results indicate that they possess large hole carrier mobility, importantly, very outstanding properties of electron transport. The major reason should be that (1) the closed ring structure restricts the rotation of the thiophene rings in charge transfer process, (2) the introduction of the alkynyl and double bonds weakens the coulomb repulsion between lone electron pairs of sulfur atoms, the ring stain and the steric effect resulting from the butyls, more importantly, (3) their introduction stabilizes LUMO level, in result to decrease electronic organization energy ( λ e ) and thereby improve their ability of electron transport. The temperature dependences of the carrier mobility were theoretically investigated for molecules A and B and analyzed within the hopping mechanism.

Analysis of [formula omitted] charge recombination with the Arrhenius, Eyring and Marcus theories

The Q band of photosynthetic thermoluminescence, measured in the presence of a herbicide that blocks electron transfer from PSII, is associated with recombination of the S 2 Q A - charge pair. The same charge recombination reaction can be monitored with chlorophyll fluorescence. It has been shown that the recombination occurs via three competing routes of which one produces luminescence. In the present study, we measured the thermoluminescence Q band and the decay of chlorophyll fluorescence yield after a single turnover flash at different temperatures from spinach thylakoids. The data were analyzed using the commonly used Arrhenius theory, the Eyring rate theory and the Marcus theory of electron transfer. The fitting error was minimized for both thermoluminescence and fluorescence by adjusting the global, phenomenological constants obtained when the reaction rate theories were applied to the multi-step recombination reaction. For chlorophyll fluorescence, all three theories give decent fits. The peak position of the thermoluminescence Q band is correct by all theories but the form of the Q band is somewhat different in curves predicted by the three theories. The Eyring and Marcus theories give good fits for the decreasing part of the thermoluminescence curve and Marcus theory gives the closest fit for the rising part.

Transmission Coefficients for Chemical Reactions with Multiple States: Role of Quantum Decoherence

Transition-state theory (TST) is a widely accepted paradigm for rationalizing the kinetics of chemical reactions involving one potential energy surface (PES). Multiple PES reaction rate constants can also be estimated within semiclassical approaches provided the hopping probability between the quantum states is taken into account when determining the transmission coefficient. In the Marcus theory of electron transfer, this hopping probability was historically calculated with models such as Landau-Zener theory. Although the hopping probability is intimately related to the question of the transition from the fully quantum to the semiclassical description, this issue is not adequately handled in physicochemical models commonly in use. In particular, quantum nuclear effects such as decoherence or dephasing are not present in the rate constant expressions. Retaining the convenient semiclassical picture, we include these effects through the introduction of a phenomenological quantum decoherence function. A simple modification to the usual TST rate constant expression is proposed: in addition to the electronic coupling, a characteristic decoherence time τ(dec) now also appears as a key parameter of the rate constant. This new parameter captures the idea that molecular systems, although intrinsically obeying quantum mechanical laws, behave semiclassically after a finite but nonzero amount of time (τ(dec)). This new degree of freedom allows a fresh look at the underlying physics of chemical reactions involving more than one quantum state. The ability of the proposed formula to describe the main physical lines of the phenomenon is confirmed by comparison with results obtained from density functional theory molecular dynamics simulations for a triplet to singlet transition within a copper dioxygen adduct relevant to the question of dioxygen activation by copper monooxygenases.

Theoretical design study on photophysical property of the organoboron quinolate derivatives

The photophysical properties of organoboron quinolate derivatives can be modified readily by manipulating the coordination environment around the central boron atom. This class of compounds applied in organic light-emitting diodes (OLEDs) materials has been studied by quantum chemistry. To reveal the relationship between the structures and properties of these electroluminescent materials, the ground- and excited-state geometries were optimized at the B3LYP/6-31G(d) and CIS/6-31G(d) levels, respectively. The ionization potentials and electron affinities were computed. The mobilities of hole and electron in these compounds were studied computationally based on the Marcus electron transfer theory. The maximum absorption and emission wavelengths of these compounds were calculated using the time-dependent density functional theory method. The solvent effect on the absorption and emission wavelengths of these compounds was also considered by a polarizable continuum model. These results show that boron compounds which containing both the hydroxyquinoline/hydroxybenzoquinoline as ligand and O/S in position X follow the rule, that is, the emission shifts to longer wavelength as covalent nature of the boron–ligand bonding is increased. Meanwhile, the negative HOMO and IPs decrease but the negative LUMO and EAs increase by substitution of O with S in position X. It was deduced that both the hole- and electron-injection abilities are improved by substituting S in place of O in position X. After chemical modification in position R2 with electron-donating properties of NH2 or 1,4-diethynyl-2,5-dihexyloxybenzene, introduced 1,4-diethynyl-2,5-dihexyloxybenzene improves both the hole- and electron-transfer rate, which leads to better equilibrium property. It can be concluded that the better equilibrium property depends on the conjugated length of side chain in position R2. Moreover, exchanging the substituents R1 and R2 in BNO1a and BNO1’a can slightly change the hole-transfer rate by 0.04 eV. According to these calculations, series BNO and BNS can be applied as electron transport and hole transport materials at the same time. Specially, series BNO2 and BNS have better performance than Mes2B[p-4,4’-biphenyl-NPh(1-naphthyl)] (BNPB) in both the hole- and electron-injection ability.

Theoretical study on charge carrier mobilities of tetrathiafulvalene derivatives

We calculated the hole and electron mobilities of tetrathiafulvalene (TTF) derivative crystals using first-principles calculations and the Marcus theory of electron transfer. The hole and electron reorganization energies were found to decrease with the extension of π-conjugated orbitals. The calculated hole mobilities of TTF, dibenzo-tetrathiafulvalene (DB-TTF), and dinaphtho-tetrathiafulvalene (DN-TTF) agree well with the experimental results. In addition, with the increase of the number of benzene rings attached to the TTF skeleton, the hole mobilities decrease and the electron mobilities increase. The calculated electron mobility of dianthro-tetrathiafulvalene (DA-TTF) based on a virtual crystal structure is much larger than the hole one due to the small electron reorganization energy and large electron coupling. This suggests that the charge transfer properties of the TTF derivatives can be modified when the number of aromatic rings on TTF skeleton increases.