Charge transport in columnar stacked triphenylenes: Effects of conformational fluctuations on charge transfer integrals and site energies

Probing the relation between charge transport and supramolecular organization down to ångström resolution in a benzothiadiazole-cyclopentadithiophene copolymer

Molecular modeling shows that longitudinal displacement of the backbones by a couple of ångströms has a profound impact on the electronic coupling mediating charge transport in a conjugated copolymer. These changes can be probed by monitoring the calculated X-ray scattering patterns and NMR chemical shifts as a function of sliding of the polymer chains and comparing them to experiment.

Understanding the Density Functional Dependence of DFT-Calculated Electronic Couplings in Organic Semiconductors

We present an analysis of the magnitude of density functional theory (DFT)-calculated intermolecular electronic couplings (transfer integrals) in organic semiconductors to give insight into the impact that the choice of functional has on the value of this parameter, which is particularly important in the context of charge transport. The major factor determining the magnitude of the calculated transfer integrals is the amount of nonlocal Hartree-Fock (HF) exchange within a given functional, with the transfer integrals increasing by up to a factor of 2 when going from 0 to 100% HF exchange for a series of conventional functionals. We underline that these variations in the transfer integrals are in fact to be expected, with the computed transfer integrals evolving linearly with the amount of HF exchange. We also use a long-range corrected functional to tune the contributions of (semi)local and nonlocal HF exchanges and highlight their respective roles as a function of intermolecular separation.

Efficient algorithms for the simulation of non-adiabatic electron transfer in complex molecular systems: application to DNA

In this work, a fragment-orbital density functional theory-based method is combined with two different non-adiabatic schemes for the propagation of the electronic degrees of freedom. This allows us to perform unbiased simulations of electron transfer processes in complex media, and the computational scheme is applied to the transfer of a hole in solvated DNA. It turns out that the mean-field approach, where the wave function of the hole is driven into a superposition of adiabatic states, leads to over-delocalization of the hole charge. This problem is avoided using a surface hopping scheme, resulting in a smaller rate of hole transfer. The method is highly efficient due to the on-the-fly computation of the coarse-grained DFT Hamiltonian for the nucleobases, which is coupled to the environment using a QM/MM approach. The computational efficiency and partial parallel character of the methodology make it possible to simulate electron transfer in systems of relevant biochemical size on a nanosecond time scale. Since standard non-polarizable force fields are applied in the molecular-mechanics part of the calculation, a simple scaling scheme was introduced into the electrostatic potential in order to simulate the effect of electronic polarization. It is shown that electronic polarization has an important effect on the features of charge transfer. The methodology is applied to two kinds of DNA sequences, illustrating the features of transfer along a flat energy landscape as well as over an energy barrier. The performance and relative merit of the mean-field scheme and the surface hopping for this application are discussed.

Exploring the energy landscape of the charge transport levels in organic semiconductors at the molecular scale

The extraordinary semiconducting properties of conjugated organic materials continue to attract attention across disciplines including materials science, engineering, chemistry, and physics, particularly with application to organic electronics. Such materials are used as active components in light-emitting diodes, field-effect transistors, or photovoltaic cells, as a substitute for (mostly Si-based) inorganic semiconducting materials. Many strategies developed for inorganic semiconductor device building (doping, p-n junctions, etc.) have been attempted, often successfully, with organics, even though the key electronic and photophysical properties of organic thin films are fundamentally different from those of their bulk inorganic counterparts. In particular, organic materials consist of individual units (molecules or conjugated segments) that are coupled by weak intermolecular forces. The flexibility of organic synthesis has allowed the development of more efficient opto-electronic devices including impressive improvements in quantum yields for charge generation in organic solar cells and in light emission in electroluminescent displays. Nonetheless, a number of fundamental questions regarding the working principles of these devices remain that preclude their full optimization. For example, the role of intermolecular interactions in driving the geometric and electronic structures of solid-state conjugated materials, though ubiquitous in organic electronic devices, has long been overlooked, especially when it comes to these interfaces with other (in)organic materials or metals. Because they are soft and in most cases disordered, conjugated organic materials support localized electrons or holes associated with local geometric distortions, also known as polarons, as primary charge carriers. The spatial localization of excess charges in organics together with low dielectric constant (ε) entails very large electrostatic effects. It is therefore not obvious how these strongly interacting electron-hole pairs can potentially escape from their Coulomb well, a process that is at the heart of photoconversion or molecular doping. Yet they do, with near-quantitative yield in some cases. Limited screening by the low dielectric medium in organic materials leads to subtle static and dynamic electronic polarization effects that strongly impact the energy landscape for charges, which offers a rationale for this apparent inconsistency. In this Account, we use different theoretical approaches to predict the energy landscape of charge carriers at the molecular level and review a few case studies highlighting the role of electrostatic interactions in conjugated organic molecules. We describe the pros and cons of different theoretical approaches that provide access to the energy landscape defining the motion of charge carriers. We illustrate the applications of these approaches through selected examples involving OFETs, OLEDs, and solar cells. The three selected examples collectively show that energetic disorder governs device performances and highlights the relevance of theoretical tools to probe energy landscapes in molecular assemblies.

Muoniated spin probes in the discotic liquid crystal HHTT: rapid electron spin relaxation in the hexagonal columnar and isotropic phases

Avoided level crossing muon spin resonance (ALC-μSR) spectroscopy was used to study radicals produced by the addition of the light hydrogen isotope muonium (Mu) to the discotic liquid crystal (DLC) 2,3,6,7,10,11-hexahexylthiotriphenylene (HHTT). Mu adds to the secondary carbon atoms of HHTT to produce a substituted cyclohexadienyl radical, whose identity was confirmed by comparing the measured hyperfine coupling constants with values obtained from DFT calculations. ALC-μSR spectra were obtained in the isotropic (I), hexagonal columnar (Col(h)), helical (H), and crystalline (Cr) phases. In the I and Col(h) phases the radicals, which are incorporated within the stacks of HHTT molecules as isolated paramagnetic defects, undergo extremely rapid electron spin relaxation, on the order of a hundredfold faster than in the H or Cr phases. The electron spin relaxation rate increases significantly with increasing temperature and appears to be caused by the liquidlike motion within the columns, which modulates the overlap between the π system of the radical and the π systems of the neighboring HHTT molecules, and hence, the hyperfine coupling constants. Rapid electron spin relaxation should occur for any π radical incorporated within the columns of a DLC, which may limit the utility of DLCs for future spin-based technologies.

Thieno analogues of RNA nucleosides: a detailed theoretical study

We use first-principles density functional theory calculations to investigate the structural, energetic, bonding aspects, and optical properties of recently synthesized thieno-analogues of RNA nucleosides. The results are compared against the findings obtained for both the natural nucleosides as well as available experimental data. We find that the modified nucleosides form the hydrogen bonded Watson-Crick (WC) base pairing with similar H-bonding energy as obtained for the natural nucleosides. We have calculated and compared the charge transfer integrals for the H-bonded natural and thieno-modified nucleosides. We find that the thieno modification of these nucleosides strongly affects the charge transfer integrals due to the difference in extent of orbital delocalization in these two types of nucleosides. We also find that the degree of reduction of charge transfer integrals is larger for the H-bonded A-U pair than in the G-C pair. We also focus on the optical absorption properties of these thieno-modified nucleosides and their WC H-bonded base pairs in gas phase as well as with implicit water. Our calculated results show that the low energy peaks in the absorption spectra mainly arise because of the π-π* electronic transition for both the nucleosides, and the observed red shift for thieno-nucleosides compared to natural nucleosides are consistent with the calculated decrease in electronic gaps. Our results demonstrate that the thieno modification of natural nucleosides significantly modifies their electronic and optical properties, although the basic structural and bonding aspects remained the same. It also gives a microscopic understanding of the experimentally observed optical behaviors.

Synthesis of BN-fused polycyclic aromatics via tandem intramolecular electrophilic arene borylation

A tandem intramolecular electrophilic arene borylation reaction has been developed for the synthesis of BN-fused polycyclic aromatic compounds such as 4b-aza-12b-boradibenzo[g,p]chrysene (A) and 8b,11b-diaza-19b,22b-diborahexabenzo[a,c,fg,j,l,op]tetracene. These compounds adopt a twisted conformation, which results in a tight and offset face-to-face stacking array in the solid state. Time-resolved microwave conductivity measurements prove that the intrinsic hole mobility of A is comparable to that of rubrene, one of the most commonly used organic semiconductors, indicating that BN-substituted PAHs are potential candidates for organic electronic materials.

Conformation, defects, and dynamics of a discotic liquid crystal and their influence on charge transport

Future applications of discotic liquid crystals (DLCs) in electronic devices depend on a marked improvement of their conductivity properties. We present a study of 2,3,6,7,10,11-hexakishexyloxytriphenylene (HAT6) and show how local conformation, structural defects, and thermal motions on the picosecond time scale strongly affect the efficient charge transport in DLCs. A direct and successful comparison of classical molecular dynamics (MD) simulations with both neutron powder diffraction and quasielastic neutron scattering (QENS) give a full insight into the structural and dynamical properties of HAT6. The local conformation of HAT6 molecules is characterized by a mutual rotation (twist) angle of about 37° and typically a mutual aromatic-core distance of 3.4 Å instead of the average distance of 3.65 Å usually quoted. We show that a considerable number of structural traps is present in HAT6, which persist at the picosecond time scale. We find that the high disorder in the mutual positions of the aromatic cores is an important factor contributing to the limited conductivity of HAT6 compared to larger DLCs.

Thienoacene-based organic semiconductors

Thienoacenes consist of fused thiophene rings in a ladder-type molecular structure and have been intensively studied as potential organic semiconductors for organic field-effect transistors (OFETs) in the last decade. They are reviewed here. Despite their simple and similar molecular structures, the hitherto reported properties of thienoacene-based OFETs are rather diverse. This Review focuses on four classes of thienoacenes, which are classified in terms of their chemical structures, and elucidates the molecular electronic structure of each class. The packing structures of thienoacenes and the thus-estimated solid-state electronic structures are correlated to their carrier transport properties in OFET devices. With this perspective of the molecular structures of thienoacenes and their carrier transport properties in OFET devices, the structure-property relationships in thienoacene-based organic semiconductors are discussed. The discussion provides insight into new molecular design strategies for the development of superior organic semiconductors.

Long-range corrected DFT calculations of charge-transfer integrals in model metal-free phthalocyanine complexes

An assessment of several widely used exchange-correlation potentials in computing charge-transfer integrals is performed. In particular, we employ the recently proposed Coulomb-attenuated model which was proven by other authors to improve upon conventional functionals in the case of charge-transfer excitations. For further validation, two distinct approaches to compute the property in question are compared for a phthalocyanine dimer.

Electronic coupling matrix elements from charge constrained density functional theory calculations using a plane wave basis set

We present a plane wave basis set implementation for the calculation of electronic coupling matrix elements of electron transfer reactions within the framework of constrained density functional theory (CDFT). Following the work of Wu and Van Voorhis [J. Chem. Phys. 125, 164105 (2006)], the diabatic wavefunctions are approximated by the Kohn-Sham determinants obtained from CDFT calculations, and the coupling matrix element calculated by an efficient integration scheme. Our results for intermolecular electron transfer in small systems agree very well with high-level ab initio calculations based on generalized Mulliken-Hush theory, and with previous local basis set CDFT calculations. The effect of thermal fluctuations on the coupling matrix element is demonstrated for intramolecular electron transfer in the tetrathiafulvalene-diquinone (Q-TTF-Q(-)) anion. Sampling the electronic coupling along density functional based molecular dynamics trajectories, we find that thermal fluctuations, in particular the slow bending motion of the molecule, can lead to changes in the instantaneous electron transfer rate by more than an order of magnitude. The thermal average, (<|H(ab)|(2)>)(1/2)=6.7 mH, is significantly higher than the value obtained for the minimum energy structure, |H(ab)|=3.8 mH. While CDFT in combination with generalized gradient approximation (GGA) functionals describes the intermolecular electron transfer in the studied systems well, exact exchange is required for Q-TTF-Q(-) in order to obtain coupling matrix elements in agreement with experiment (3.9 mH). The implementation presented opens up the possibility to compute electronic coupling matrix elements for extended systems where donor, acceptor, and the environment are treated at the quantum mechanical (QM) level.

Muon spin spectroscopy of the discotic liquid crystal HAT6

Avoided level crossing muon spin resonance (ALC-muSR) has been used to study the cyclohexadienyl-type radicals produced by the addition of muonium (Mu) to the discotic liquid crystal HAT6 (2,3,6,7,10,11-hexahexyloxytriphenylene) in the crystalline (Cr) phase, the hexagonal columnar mesophase (Col(h)) and isotropic (I) phase. In the Cr phase unpaired electron spin density can be transferred from the radical to neighboring HAT6 molecules depending on the overlap of their pi-systems and hence on the relative orientation of the triphenylene rings. The two Delta(1) resonances in the ALC-muSR spectra of the Cr phase indicate that the neighboring HAT6 molecules have two preferred orientations with respect to the radical: one which results in negligible spin density transfer and a second where 17% of the unpaired spin density is transferred. The ALC-muSR spectra in Col(h) and I phases are substantially different from those of the Cr phase in that there are two narrow resonances superimposed on an extremely broad and intense resonance. The narrow resonances are due to highly mobile radicals located in the aliphatic region between the columns and the broad resonance is due to radicals incorporated within the columns of HAT6 molecules. The large width and amplitude of this resonance indicates that the radicals within the columns are undergoing rapid electron spin relaxation but the mechanism that causes this relaxation is unknown.

Ultra-low resistance at TTF-TCNQ organic interfaces

We have investigated the conduction mechanism at the TTF-TCNQ organic hetero-interface by means of quantum mechanical (QM) calculations. The calculated resistances at the TTF-TCNQ interface are 39-64 kohms per square, which is in good agreement with the experimental values of 1-30 kohms per square.

Theoretical characterization of the structural and hole transport dynamics in liquid-crystalline phthalocyanine stacks

We present a joint molecular dynamics (MD)/kinetic Monte Carlo (KMC) study aimed at the atomistic description of charge transport in stacks of liquid-crystalline tetraalkoxy-substituted, metal-free phthalocyanines. The molecular dynamics simulations reproduce the major structural features of the mesophases, in particular, a phase transition around 340 K between the rectangular and hexagonal phases. Charge transport simulations based on a Monte Carlo algorithm show an increase by 2 orders of magnitude in the hole mobility when accounting for the rotational and translational dynamics. The results point to the formation of dynamical structural defects along the columns.