A multimode analysis of the gas-phase photoelectron spectra in oligoacenes

Molecular fingerprints in the electronic properties of crystalline organic semiconductors: from experiment to theory

By comparing photoemission spectroscopy with a nonperturbative dynamical mean field theory extension to many-body ab initio calculations, we show in the prominent case of pentacene crystals that an excellent agreement with experiment for the bandwidth, dispersion, and lifetime of the hole carrier bands can be achieved in organic semiconductors, provided that one properly accounts for the coupling to molecular vibrational modes and the presence of disorder. Our findings rationalize the growing experimental evidence that even the best band structure theories based on a many-body treatment of electronic interactions cannot reproduce the experimental photoemission data in this important class of materials.

Interaction of charge carriers with lattice and molecular phonons in crystalline pentacene

The computational protocol we have developed for the calculation of local (Holstein) and non-local (Peierls) carrier-phonon coupling in molecular organic semiconductors is applied to both the low temperature and high temperature bulk crystalline phases of pentacene. The electronic structure is calculated by the semimpirical INDO/S (Intermediate Neglect of Differential Overlap with Spectroscopic parametrization) method. In the phonon description, the rigid molecule approximation is removed, allowing mixing of low-frequency intra-molecular modes with inter-molecular (lattice) phonons. A clear distinction remains between the low-frequency phonons, which essentially modulate the transfer integral from a molecule to another (Peierls coupling), and the high-frequency intra-molecular phonons, which modulate the on-site energy (Holstein coupling). The results of calculation agree well with the values extracted from experiment. The comparison with similar calculations made for rubrene allows us to discuss the implications for the current models of mobility.

Structural variability and dynamics of the P3HT/PCBM interface and its effects on the electronic structure and the charge-transfer rates in solar cells

Using a range of realistic interface geometries obtained from a molecular dynamics simulation we study the effects of different microscopic atomic arrangements on the electronic structure and charge transfer rates of the prototypical photovoltaic interface between P3HT (poly(3-hexylthiophene)) and PCBM ([6,6]-phenyl-C(61)-butyric acid methyl ester). The electronic structures of charge-transfer (CT) states belong to two groups that can be denoted as "charge-separated" and "charge-bridging" states. For the former group of structures, which may lead to fully separated charges, the ranges and the average values of internal reorganization energy, the electronic coupling and the charge separated states energy are evaluated. A range and distribution of absolute charge separation (CS) and recombination (CR) rates are computed using the Marcus-Levich-Jortner rate equation. Due to the variety of P3HT/PCBM interface structures, a very broad range of CS (7.7 × 10(9)-1.8 × 10(12) s(-1)) and CR (2.5 × 10(5)-1.1 × 10(10) s(-1)) "instantaneous" rates are computed. However, the energetic parameters affecting the rate evolve in time due to the dynamic nature of the interface with a characteristic timescale of about 10 ns. For this reason the slowest CR instantaneous rates are not observed and the minimum CR rate observed is determined by the rate of conformational rearrangement at the interface. The combination of these observations provides a more general framework for the interpretation of experimental spectroscopic data, suggesting that the analysis based on simple first order rates may be insufficient to describe charge transfer in organic solar cell interfaces.

Hole localization in molecular crystals from hybrid density functional theory

We use first-principles computational methods to examine hole trapping in organic molecular crystals. We present a computational scheme based on the tuning of the fraction of exact exchange in hybrid density functional theory to eliminate the many-electron self-interaction error. With small organic molecules, we show that this scheme gives accurate descriptions of ionization and dimer dissociation. We demonstrate that the excess hole in perfect molecular crystals forms self-trapped molecular polarons. The predicted absolute ionization potentials of both localized and delocalized holes are consistent with experimental values.

Charge transport in high mobility molecular semiconductors: classical models and new theories

The theories developed since the fifties to describe charge transport in molecular crystals proved to be inadequate for the most promising classes of high mobility molecular semiconductors identified in the recent years, including for example pentacene and rubrene. After reviewing at an elementary level the classical theories, which still provide the language for the understanding of charge transport in these systems, this tutorial review outlines the recent experimental and computational evidence that prompted the development of new theories of charge transport in molecular crystals. A critical discussion will illustrate how very rarely it is possible to assume a charge hopping mechanism for high mobility organic crystals at any temperature. Recent models based on the effect of non-local electron-phonon coupling, dynamic disorder, coexistence of localized and delocalized states are reviewed. Additionally, a few more recent avenues of theoretical investigation, including the study of defect states, are discussed.

The interface bonding and orientation of a quinonoid zwitterion

We have investigated the interaction and orientation of a strongly dipolar zwitterionic p-benzoquinonemonoimine-type molecule, with a large intrinsic dipole of 10 Debye, on both conducting and on polar insulating substrates. Specifically, we deposited (6Z)-4-(butylamino)-6-(butyliminio)-3-oxocyclohexa-1,4-dien-1-olate C(6)H(2)([horiz bar, triple dot above]NHR)(2)([horiz bar, triple dot above]O)(2) where R = n-C(4)H(9), on both gold and ferroelectric lithium niobate surfaces. An influence of both transient and static electric dipoles on the zwitterionic adsorbate has been observed. For adsorption on gold, we find that the molecule bonds to the surface through the nitrogen atoms, forming films that remain fairly uniform down to thicknesses in the 1 nm range. Adsorption of this zwitterionic compound from solution on insulating, periodically poled ferroelectric lithium niobate substrates, showed preferential adsorption on one type of ferroelectric domain. For both gold and the lithium niobate substrates, the zwitterion adopts a preferential orientation with the plane of its "C(6) core" along the surface normal. This simplified geometry of strong dipole alignment provides a symmetry simplification allowing better identification of the vibrational modes responsible for Frank-Condon scattering revealed in the fine spectroscopic signature in the photoemission spectrum.

Resistive molecular memories: influence of molecular parameters on the electrical bistability

The electrical bistability behavior of 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) along with two additional benzoquinone derivatives (TCQ and TCN) and pentacene (PNT) is investigated by computing intra- and intermolecular charge transfer parameters and by comparing the efficiency of bulk charge transport and charge injection at the electrode/organic interface in the presence of neutral and charged molecular species. The bulk charge transport is modeled assuming a charge hopping regime and by computing hopping rates and mobilities. Molecular dynamics simulations are carried out to estimate the effect of thermal disorder on charge transfer integrals. The efficiency of the interface transport is estimated by comparing the electron affinities of benzoquinone derivatives and the ionization potential of pentacene with the work function of commonly employed electrodes. It is shown that the observed memory effect can be rationalized in terms of an interplay of the two transport mechanisms by showing that the OFF state is dominated by interface limited phenomena and the ON state may be determined also by bulk transport limited phenomena. While the contribution of collective effects cannot be ruled out for the macroscopic memory phenomenon, we show that, at a molecular level, sizable intramolecular reorganization energies are fundamental for the efficiency of the device, provided their magnitude does not hamper the charge transport across the device. It is suggested that control over molecular parameters might be exploited to design more efficient resistive molecular memories.

Modelling charge transport in organic semiconductors: from quantum dynamics to soft matter

The charge carrier dynamics in organic semiconductors has been traditionally discussed with the models used in inorganic crystalline and amorphous solids but this analogy has severe limitations because of the more complicated role of nuclear motions in organic materials. In this perspective, we discuss how a new approach to the modelling of charge transport is emerging from the alliance between the conventional quantum chemical methods and the methods more traditionally used in soft-matter modelling. After describing the conventional limit cases of charge transport we discuss the problems arising from the comparison of the theory with the experimental and computational results. Several recent applications of numerical methods based on the propagation of the wavefunction or kinetic Monte Carlo methods on soft semiconducting materials are reviewed.

Theoretical characterization of titanyl phthalocyanine as a p-type organic semiconductor: short intermolecular pi-pi interactions yield large electronic couplings and hole transport bandwidths

The charge-transport properties of the triclinic phase II crystal of titanyl phthalocyanine (alpha-TiOPc) are explored within both a hopping and bandlike regime. Electronic coupling elements in convex- and concave-type dimers are calculated using density functional theory, and the relationship between molecular structure and crystal packing structure in model dimer configurations is considered. Hole transport bandwidths derived from crystal structure dimers are compared to those obtained from electronic band structure calculations; very good agreement between the two approaches is found. The calculations predict large hole bandwidths, on the order of 0.4 eV, and correspondingly very low hole reorganization energies.

Hole-vibronic coupling in oligothiophenes: impact of backbone torsional flexibility on relaxation energies

Density functional theory calculations together with highly resolved gas-phase ultraviolet photoelectron spectroscopy have been applied to oligothiophene chains with up to eight thiophene rings. One of the important parameters governing the charge transport properties in the condensed phase is the amount of energy relaxation upon ionization. Here, we investigate the impact on this parameter of the backbone flexibility present in oligothiophenes as a result of inter-ring torsional motions. With respect to oligoacenes that are characterized by a coplanar and rigid backbone, the torsional flexibility in oligothiophenes adds to the relaxation energy and leads to the broadening of the first ionization peak, making its analysis more complex.

Excited-State Properties and Emission Spectra of Nonplanar Heterocyclic Helicenes

We discuss the electron-vibration coupling in mono-aza-[5]helicenes on the basis of a Franck-Condon analysis and density functional theory (DFT) calculations of the fluorescence and phosphorescence spectra measured in ethanol. The geometries of the initial states were obtained from time-dependent DFT (S(1)) and unrestricted DFT (T(1)) excited-state optimizations. In general, the position of the nitrogen atom has only a minor impact on the vibronic fine-structure in both absorption and emission. The shapes of the emission spectra from the lowest singlet and triplet states are found to be determined by contributions from multiple normal modes. The results of the calculations demonstrate how the interplay among these normal modes results in qualitatively and quantitatively different spectra for fluorescence and phosphorescence.

Vibronic Coupling in the Ground and Excited States of Oligoacene Cations

The vibrational coupling in the ground and excited states of positively charged naphthalene, anthracene, tetracene, and pentacene molecules is studied on the basis of a joint experimental and theoretical study of ionization spectra using high-resolution gas-phase photoelectron spectroscopy and first-principles correlated quantum-mechanical calculations. Our theoretical and experimental results reveal that, while the main contribution to relaxation energy in the ground state of oligoacene systems comes from high-energy vibrations, the excited-state relaxation energies show a significant redistribution toward lower-frequency vibrations. A direct correlation is found between the nature of the vibronic interaction and the pattern of the electronic state structure.

Probing Charge Transport in π-Stacked Fluorene-Based Systems

The molecular parameters governing charge transport along a pi-stacked fluorene chain in poly(dibenzofulvene) are studied by a joint experimental and theoretical approach involving high-resolution gas-phase photoelectron spectroscopy and quantum-mechanical methods. We specifically investigate the electronic couplings between fluorene moieties as well as the intramolecular reorganization energies, for both holes and electrons. Our results indicate that a pi-stacked fluorene chain favors hole transport over electron transport. The values for electronic couplings and reorganization energies estimated here are compared with those derived recently for pentacene.

Studies of the Inner Reorganization Energies of the Cation Radicals of 1,4-Bis(dimethylamino)benzene, 9,10-Bis(dimethylamino)anthracene, and 3,6-Bis(dimethylamino)durene by Photoelectron Spectroscopy and Reinterpretation of the Mechanism of the Electrochemical Oxidation of the Parent Diamines

The inner reorganization energy of the cation radical of 1,4-bis(dimethylamino)benzene, 1, has been determined to be 0.72 +/- 0.02 eV by means of gas-phase photoelectron spectroscopy (PES). PES studies of 9,10-bis(dimethylamino)anthracene, 2, and 3,6-bis(dimethylamino)durene, 3, demonstrate that their reorganization energies are smaller than that of 1. The effect of lowering the inner reorganization energy on the rate constant for an electrochemical electron-transfer reaction is to increase the electron-transfer rate constant, k(s). However, voltammetric studies of the two-electron oxidation of 2 and 3 indicate that the values of k(s) for each step are smaller than those for 1, in contradistinction to the measured differences in reorganization energies. The voltammetric studies of 2 and 3 were reinterpreted according to a mechanism in which each step of oxidation was written as a two-step process, electron transfer with a small inner reorganization energy plus a chemical step of structural change. The agreement of simulations according to this mechanism with the experimental data was excellent. The new reaction scheme eliminated some suspicious features previously obtained with an analysis where electron transfer and structural change were considered to be concerted. In particular, all electron-transfer coefficients (alpha) were close to one-half, whereas the earlier treatment produced values of alpha much larger or smaller than one-half.

Vibronic Coupling in Organic Semiconductors: The Case of Fused Polycyclic Benzene–Thiophene Structures

The nature of vibronic coupling in fused polycyclic benzene-thiophene structures has been studied using an approach that combines high-resolution gas-phase photoelectron spectroscopy measurements with first-principles quantum-mechanical calculations. The results indicate that in general the electron-vibrational coupling is stronger than the hole-vibrational coupling. In acenedithiophenes, the main contributions to the hole-vibrational coupling arise from medium- and high-frequency vibrations. In thienobisbenzothiophenes, however, the interaction of holes with low-frequency vibrations becomes significant and is larger than the corresponding electron-vibrational interaction. This finding is in striking contrast with the characteristic pattern in oligoacenes and acenedithiophenes in which the low-frequency vibrations contribute substantially only to the electron-vibrational coupling. The impact of isomerism has been studied as well.

Vibronic interactions and possible electron pairing in positively charged cyanodienes

The conditions under which the attractive electron-electron interactions are realized in the monocations of sigma-conjugated cyanodienes such as C(6)N(4)H(4), C(8)N(6)H(4), and C(10)N(8)H(4) and of pi-conjugated acenes are discussed. The total electron-phonon coupling constants for the monocations l(HOMO) of cyanodienes are much larger than those for the monocations of acenes. The strong sigma orbital interactions between two neighboring atoms in the highest occupied molecular orbitals (HOMO) of sigma-conjugated cyanodienes are the main reason for the calculated results. Furthermore, we discuss how the conditions under which the monocation crystals become good conductor are related to the molecular size. Both the l(HOMO) values and the reorganization energies between the neutral molecules and the monocations decrease with an increase in molecular size in cyanodienes. The calculated results for the sigma-conjugated cyanodienes are compared with those for the pi-conjugated acenes in order to investigate how the CH-N substitutions in cyanodienes are closely related to the l(HOMO) values and the reorganization energies. Both the l(HOMO) and the reorganization energies in the positively charged sigma-conjugated cyanodienes are much larger than those in the positively charged pi-conjugated acenes. This means that in order to become good conductors, the positively charged sigma-conjugated cyanodienes need larger overlap integral between two adjacent molecules than the positively charged pi-conjugated acenes. On the other hand, since the l(HOMO) values for cyanodienes are much larger than those for acenes, the condition of attractive electron-electron interactions is more easily to be realized in the monocations of cyanodienes than in the monocations of acenes. It is suggested that the positively charged sigma-conjugated cyanodienes cannot easily become good conductors, but the conditions under which the electron-electron interactions become attractive are realized more easily in the positively charged sigma-conjugated cyanodienes than in the positively charged pi-conjugated acenes.

Assessment of Recently Developed Multicoefficient Strategies for the Treatment of π-Conjugated Molecules

Newly developed hybrid functionals (MPW1k and BB1k) have been systematically applied for the description of conjugation effects in organic molecules. These functionals are also used as part of the recently developed general-purpose multicoefficient methods MC3MPW and MC3BB. The performance of the various approaches is compared not only for relative energies but also through the calculation of torsion energy profiles for critical comparison with available reference data; thus, a numerical criterion depending on local behavior could be correspondingly defined. The results show that MC3-based methods are very accurate when faced to other approaches having comparable computational cost; thus, paving the way toward new applications and achievements in the field of conjugated materials.

Design of π-Conjugated Organic Materials for One-Dimensional Energy Transport in Nanochannels

Various end-substituted distyrylbenzenes have been synthesized to serve as guest molecules in inclusion compounds to promote efficient energy transport along one-dimensional channels. Their optical and photophysical properties have been characterized at both experimental and theoretical levels. All molecules display a large transition dipole moment between the ground state and lowest excited state and hence a short radiative lifetime (on the order of 1-2 ns). They also exhibit a large spectral overlap between the emission and absorption spectra, which enables efficient energy transport between molecules arranged in a head-to-tail configuration in nanochannels. Hopping rates on the order of 10(12) s(-1) are calculated at a full quantum-chemical level; this is much larger than the radiative lifetimes and opens the way for energy migration over large distances. Changes in the nature of the terminal substituents are found to modulate the optical properties weakly but to impact significantly the energy transfer rates.

Inner-Sphere Electron-Transfer Reorganization Energies of Zinc Porphyrins

Inner-sphere electron-transfer reorganization energies of Zn(protoporphyrin IX) and Zn(octaethylporphyrin) are determined from band-shape analyses of the first ionization obtained by gas-phase valence photoelectron spectroscopy. The experimentally determined total inner-sphere reorganization energies for self-exchange (120-140 meV) indicate that structural changes upon oxidation are largely confined to the porphyrin ring, and substituents on the ring or solvent and other environmental factors make smaller contributions. Computational estimates by different models vary over a wide range and are sensitive to numerical precision factors for these low reorganization energies. Of current computational models that are widely available and practical for molecules of this size, functionals that contain a mixture of Hartree-Fock exchange and DFT exchange-correlation appear to be the most applicable.

Vibronic coupling in the ground and excited states of the naphthalene cation

The hole-vibrational coupling in naphthalene is studied using high-resolution gas-phase photoelectron spectroscopy and density functional theory calculations (DFT), and a remarkable increase of the coupling with low-frequency vibrations is observed in the excited states.