Z. G. Soos

Charge-separation energy in films of π-conjugated organic molecules

Abstract We use inverse photoelectron spectroscopy (IPES) and ultraviolet photoelectron spectroscopy (UPS) to investigate unoccupied and occupied electronic states of five organic semiconductor materials: CuPc (copper phthalocyanine), PTCDA (3,4,9,10-perylenetetracarboxylic dianhydride), α-6T (α-sexithiophene), α-NPD (N,N′-diphenyl-N,N′-bis(l-naphthyl)-l,l′ biphenyl-4,4′′ diamine), and Alq3 (tris(8-hydroxy-quinoline)aluminum). The transport gap, Et, is the difference between the highest occupied and lowest unoccupied molecular orbitals, measured via UPS and IPES. The charge separation energy, or exciton binding energy, is the difference between Et and the optical gap, Eopt, measured via absorption. Et-Eopt in these correlated materials ranges from 0.4 to1.4 eV.

Singlet exciton energy transfer in organic solids

Abstract A comprehensive review is presented of the experimental data which have been obtained on singlet exciton energy transfer in organic crystals. The usefulness of different types of experimental techniques in elucidating the characteristics of both exciton migration and trapping is discussed, as are the effects of radiative reabsorption on the results obtained by different types of measurements. The importance of activator-induced host traps is stressed and previous data are reinterpreted in terms of such traps.

Electronic polarization at surfaces and thin films of organic molecular crystals: PTCDA

Abstract The electronic polarization energies, P = P + + P − , of a perylenetetracarboxylic acid dianhydride (PTCDA) cation and anion in a crystalline thin film on a metallic substrate are computed and compared with measurements of the PTCDA transport gap on gold and silver. Both experiments and theory show that P is 500 meV larger in a PTCDA monolayer than in 50 A films. Electronic polarization in systems with surfaces and interfaces are obtained self-consistently in terms of charge redistribution within molecules.

Valence bond approach to exact nonlinear optical properties of conjugated systems

Diagrammatic valence‐bond (DVB) theory is extended to dynamic nonlinear susceptibilities of interacting π electrons in Pariser–Parr–Pople (PPP) or other quantum cell models whose correlated ground state ‖G〉 is known. Corrections φ(1)(ω) and φ(2)(ω2,ω1) to ‖G〉 due to oscillating electric fields are found directly as linear combinations of VB diagrams. Any nonlinear optical coefficient is reduced to matrix elements that implicitly include all excited states, as verified for shorter polyenes. Static and dynamic χ(3) coefficients for cis and trans polyenes to N=12 carbons illustrate the importance of retaining the full spectrum. The coefficients βijk(ω,ω) for second harmonic generation are found for polar molecules like aniline and nitroaniline. Divergent responses are treated by lifetimes Γ for the resonant states, as shown for third harmonic generation in hexatriene with Γ=0 and in octatetraene with Γ>0. Electron–electron interactions reverse the sign of γijkl(ω,ω,ω) in linear polyenes, except for the large...

Vibronic structure of PTCDA stacks: the exciton–phonon-charge-transfer dimer

Abstract Perylenetetracarboxylic acid dianhydride (PTCDA) stacks face-to-face in crystals and multiple quantum wells (MQWs). Excitations of PTCDA stacks are mixed molecular (Frenkel) and charge-transfer (CT) states coupled to a molecular vibration. Eclipsed stacks and molecular conjugation imply strong Frenkel–CT mixing in absorption and electroabsorption, with k=0 at the top of the exciton band, and negligible mixing at k=π for emission from the bottom. The exciton–phonon-CT dimer developed for k=0 processes is a nonadiabatic approximation for narrow CT bands. Quantitative dimer spectra are obtained in the vibronic basis of displaced harmonic oscillators for excited PTCDA and radical ions. We present a joint analysis of absorption and emission in PTCDA stacks and MQWs using parameters from solution, molecular calculations, and related conjugated systems. Polarized single-crystal absorption decisively relates the entire 2–3 eV system to molecular π–π* transitions, while electroabsorption with field along the stack implicates adjacent ions in the stack. The simple structure and extensive PTCDA spectra make possible detailed modelling of mixed Frenkel–CT vibronics that were far less accessible in previous organic molecular crystals. Since the coupled mode is closely related to polyenes and conjugated polymers, PTCDA provides a bridge between molecular insulators and extended systems capable of charge transport.

Electronic polarization in pentacene crystals and thin films

Electronic polarization is evaluated in pentacene crystals and in thin films on a metallic substrate using a self-consistent method for computing charge redistribution in nonoverlapping molecules. The optical dielectric constant and its principal axes are reported for a neutral crystal The polarization energies P + and P - of a cation and anion at infinite separation are found for both molecules in the crystals unit cell in the bulk, at the surface, and at the organic-metal interface of a film of N molecular layers. We find that a single pentacene layer with herring-bone packing provides a screening environment approaching the bulk. The polarization contribution to the transport gap P=P + + P - , which is 2.01 eV in the bulk, decreases and increases by only ∼ 10% at surfaces and interfaces, respectively. We also compute the polarization energy of charge-transfer states with fixed separation between anion and cation, and compare to electroabsorption data and to submolecular calculations. Electronic polarization of ∼ I eV per charge has a major role for transport in organic molecular systems with limited overlap.

Correlated states in linear polyenes, radicals, and ions: Exact PPP transition moments and spin densities

The low‐lying eigenstates ψn of the Pariser–Parr–Pople (PPP) model for polyenes with N≤11 carbons are found exactly as linear combinations of nonorthogonal covalent and ionic valence‐bond (VB) diagrams. We extend diagrammatic VB theory to normalize ψn efficiently and to evaluate exactly transition moments, spin densities, and other matrix elements within subspaces of fixed total spin S. Charge orthogonality in the VB basis results in a block‐diagonal overlap matrix whose evaluation is rapid for linear combinations of over 105 diagrams. We obtain S=0 and 1 states of all‐trans decapentaene (N=Ne =10), S=1/2 states of nonatetraenyl (N=Ne =9), S=0 states of its anion and cation, and spin densities through undecapentaenyl (N=Ne =11), all with standard molecular PPP parameters. Correlation effects on excitation energies, on transition moments, and on spin densities are contrasted with one‐electron Huckel results and compared with data on finite polyenes and polyacetylene. Standard PPP parameters successfully de...

Charge redistribution and polarization energy of organic molecular crystals

We present an approach to electronic polarization in molecular solids treated as a set of quantum systems interacting classically. Individual molecules are dealt with rigorously as quantum-mechanical systems subject to classical external fields created by all other molecules and, possibly, external sources. Self-consistent equations are derived for induced dipoles and for atomic charges whose redistribution in external fields is given explicitly by an atom-atom polarizability tensor. Electronic polarization is studied in two representative organic molecular crystals, anthracene and perylenetetracarboxylic acid dianhydride, and contrasted to previous results for systems of polarizable points. The stabilization energies of the neutral lattice, of isolated anions and cations, and of cation-anion pairs are found. Charge redistribution on ions is included. The dielectric tensors of crystals are successfully related to gas-phase properties and provide consistency checks on polarization energies. The procedure is generally applicable to organic crystals in the limit of no intermolecular overlap.

Weak exchange in the Heisenberg linear chain: Structure and EPR of [N(n‐Bu)4]2[Cu(mnt)2]

We report magnetic and x‐ray diffraction studies on a single crystal of [N(n‐Bu)4]2[Cu(mnt)2], where mnt = maleonitriledithiolato (C4N2S2)−2. The planar paramagnetic (S=1/2) [Cu(mnt)2]−2 anions crystallize in stacks with crystallographically equivalent sites, and the stacks are isolated by the cations. The compound crystallizes in space group C1i‐P1 of the triclinic system, with unit cell dimensions a=11.149(5), b=13.201(8), c=9.403(6) A, α=105.44(5) °, β=114.92(5) °, and γ=72.09(9) °, with Z=1. Static susceptibility measurements show exchange to be weak. Single crystal EPR studies yield a rich variety of multilined spectra, in spite of the magnetically concentrated nature of [N(n−Bu)4]2[Cu(mnt)2]. Analysis of the EPR spectrum in terms of a one‐dimensional exchange, J0=0.0107 cm−1, at 4.2 °K between successive [Cu(mnt)2]−2 ions along the c axis yields a surprisingly detailed picture of one‐dimensional exchange in a hitherto unobserved, weak coupling regime. The temperature dependence of J0 and the angula...

Theory of Charge Transfer in Aromatic Donor–Acceptor Crystals

A narrow‐band, localized model is developed for charge transfer (CT) interactions in 1:1 stacks of planar, π‐electron donors (D) and acceptors (A). The inequivalence of D and A sites and the Madelung constant for the ionic lattice are treated phenomenologically. The electronic states, for arbitrary CT in the aromatic D–A stacks, are found self‐consistently by an equation‐of‐motion method. The ground‐state charge density and the band structures correspond to primarily neutral, ···DADA···, diamagnetic or to primarily ionic, ···D+A−D+A−···, paramagnetic one‐dimensional semiconductors. The activation energies for CT and for semiconduction, together with the CT stabilization, are found self‐consistently. In ionic lattices, CT is shown to provide an activation energy for paramagnetism of the proper magnitude. When the sites are equivalent, the theory reduces to a half‐filled Hubbard model, and the self‐consistent solutions are in good agreement with exact results.