2.2. Materials 929 2.3. Factors Influencing Charge Mobility 931 2.3.1. Molecular Packing 931 2.3.2. Disorder 932 2.3.3. Temperature 933 2.3.4. Electric Field 934 2.3.5. Impurities 934 2.3.6. Pressure 934 2.3.7. Charge-Carrier Density 934 2.3.8. Size/molecular Weight 935 3. The Charge-Transport Parameters 935 3.1. Electronic Coupling 936 3.1.1. The Energy-Splitting-in-Dimer Method 936 3.1.2. The Orthogonality Issue 937 3.1.3. Impact of the Site Energy 937 3.1.4. Electronic Coupling in Oligoacene Derivatives 938

Charge transport in organic semiconductors.

Charge-Transfer and Energy-Transfer Processes in π-Conjugated Oligomers and Polymers: A Molecular Picture

日本物理学会誌及びJournal of the Physical Society of Japanの月刊について

Nano-optics of surface plasmon polaritons

Acenes have long been the subject of intense study because of the unique electronic properties associated with their pi-bond topology. Recent reports of impressive semiconductor properties of larger homologues have reinvigorated research in this field, leading to new methods for their synthesis, functionalization, and purification, as well as for fabricating organic electronic components. Studies performed on high-purity acene single crystals revealed their intrinsic electronic properties and provide useful benchmarks for thin film device research. New approaches to add functionality were developed to improve the processability of these materials in solution. These new functionalization strategies have recently allowed the synthesis of acenes larger than pentacene, which have hitherto been largely unavailable and poorly studied, as well as investigation of their associated structure/property relationships.

The larger acenes: versatile organic semiconductors.

A phase transition starting at 84 K and indicated by a marked color change of the sample has been found in the organic insulator tetrathiafulvalene-chloranil. Optical, infrared, and Raman measurements indicate that this is a reversible transition from a nominally neutral (N) solid to a nominally ionic (I) salt. Surprisingly, the N-I transition is not first order, but occurs over a broad temperature region (\ensuremath{\sim}30 K), in which there is a coexistence of N and I molecules.

Anomalous Nature of Neutral-to-Ionic Phase Transition in Tetrathiafulvalene-Chloranil

An extensive vibrational assignment of TTF and TTF‐d4 is achieved, improving the previously reported one through the use of polarized infrared spectra of single crystals of the monoclinic form. Infrared spectra of the monoclinic and triclinic forms are compared and the different crystal field effects discussed. Powder Raman and infrared spectra of (TTF)Br1.0 and (TTF‐d4)Br1.0, Raman depolarization ratios and infrared spectra of (TTF)ClO4 and (TTF‐d4)ClO4 solutions are reported. The assignment of the ag, b1u, b2u, and b3u fundamental modes of (TTF)+ and (TTF‐d4)+ radicals allows the identification of most of the relevant frequency shifts following the ionization of the TTF structure. The possible use of the ionization shifts for the study of the electronic charge distribution in the conducting TTF systems is considered. The parallel investigation of the concentration effects on the visible and infrared absorption spectra of TTF+ in solution let us to identify anomalous infrared absorptions associated with ...

Vibrational spectroscopy of molecular constituents of one‐dimensional organic conductors. Tetrathiofulvalene (TTF), TTF+, and (TTF+)2 dimer

The infrared and Raman spectra for the room temperature, quasineutral, and the low temperature, quasi‐ionic, phases of the mixed stack charge transfer complex tetrathiafulvalene–chloranil (TTF–CA) are reported. The analysis of the analogous data for a newly synthesized room temperature phase point to a dimerized segregated stack structure. All the vibrational data are interpreted and exploited through a clear identification of the differences, for the two types of stacks, in the spectroscopic effects due to the vibronic interaction, i.e., the coupling between electron and molecular vibration (e‐mv). It is shown that for distorted mixed stack complexes both Raman and infrared spectra can be substantially influenced by the vibronic interaction, whereas the dimerized segregated stack complexes, as already known, display striking vibronic effects only in infrared. The theoretical model which explains the origin of these effects is briefly summarized and its extension to mixed stack structures successfully use...

Vibrational spectroscopy of mixed stack organic semiconductors: Neutral and ionic phases of tetrathiafulvalene–chloranil (TTF–CA) charge transfer complex

A general framework for the theory of electron–intramolecular vibration (e–mv) interaction in charge‐transfer (CT) compounds is presented, particular attention being devoted to the effect of this interaction on the optical spectra of quasi‐one‐dimensional organic semiconductors. The common physical basis of different theoretical models of e–mv coupling is evidenced by taking into consideration an isolated CT pair. In particular, it is shown that a complete and substantially equivalent description of the phenomena is provided by the approaches based on the linear response theory or on the Herzberg–Teller coupling scheme, the latter method being developed here to its full extent. The comparison with experimental data shows that a collection of isolated CT pairs can satisfactorily model the consequences of e–mv interaction on the optical spectra of dimerized stack organic semiconductors, either of segregated or mixed type. In addition, we develop here for the first time a model also able to take into account the effect of a nonzero interdimer CT integral. Finally, the case of regular stack CT crystals is analyzed, in such a way, a complete picture of the e–mv (vibronic) perturbation on the optical spectra is obtained, putting in evidence how the spectra can be used to obtain detailed information on the structure and properties of quasi‐one‐dimensional organic semiconductors. The consequences of the coupling between molecular vibrations and lattice modes are also briefly investigated.

Electron–molecular vibration (e–mv) coupling in charge‐transfer compounds and its consequences on the optical spectra: A theoretical framework

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.

Alberto Girlando

Papers

Anomalous Nature of Neutral-to-Ionic Phase Transition in Tetrathiafulvalene-Chloranil

Vibrational spectroscopy of molecular constituents of one‐dimensional organic conductors. Tetrathiofulvalene (TTF), TTF+, and (TTF+)2 dimer

Vibrational spectroscopy of mixed stack organic semiconductors: Neutral and ionic phases of tetrathiafulvalene–chloranil (TTF–CA) charge transfer complex

Electron–molecular vibration (e–mv) coupling in charge‐transfer compounds and its consequences on the optical spectra: A theoretical framework

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