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91	Frenkel and Charge-Transfer Excitons in Quasi-One-Dimensional Molecular Crystals with Strong Intermolecular Orbital Overlap / Frenkel und Charge-Transfer Exzitonen in Quasi-Eindimensionalen Molekülkristallen mit starker intermolekularer Orbitalüberlappung Hoffmann, Michael 04 December 2000 (has links) (PDF) We present a theoretical and experimental study on the lowest electronically excited states in quasi-one-dimensional molecular crystals. The specific calculations and the experiments are performed for the model compounds MePTCDI (N-N'-dimethylperylene-3,4:9,10-dicarboximide) and TCDA(3,4:9,10-perylenetetracarboxylic dianhydride). The intermolecular interactions between nearest neighbors are quantum chemically analyzed on the basis of semi-empirical (ZINDO/S) Hartree-Fock calculations and a singly excited configuration interaction scheme. Supermolecular dimer states are projected onto a basis set of localized excitations. The nature of the lowest states is then completely explained as a superposition of molecular and low lying charge-transfer excitations. The CT excitations show a significant intrinsic transition dipole, which is oriented approximately parallel to the molecular planes and has a large component along the molecular M-axis. The exciton states in the one-dimensional stacks are described by a model Hamiltonian that includes interactions between three vibronic levels of the lowest molecular excitation and nearest-neighbor CT excitations. The three-dimensional crystal structure is considered by Frenkel exciton transfer between arbitrary molecules. This model is compared to polarized absorption spectra. With a small set of parameters, we can describe the key features of the absorption spectra, the polarization behavior, and the Davydov splitting. The variation of the polarization ratio for the various exciton states is analyzed as a direct qualitative proof for the mixing between Frenkel and charge-transfer excitons. 3 4:9 10-Perylentetracarbonsäuredianhydrid Charge-Transfer Exzitonen Dünne Schichten Frenkel Exzitonen MePTCDI N N'-Dimethylperylen-3 4:9 10-bis-dicarboximid Organische Kristalle PTCDA Polarisierte Absorption 3 4:9 10-perylenetetracarboxylic dianhydride Frenkel excitons MePTCDI N-N'-dimethylperylene-3 4:9 10-dicarboximide PTCDA charge-transfer excitons organic crystals polarized absorption thin films ddc:29 rvk:UP 3710 Absorptionsspektrum Exziton Kristallstruktur Ladungstransfer Molekülkristall Organischer Kristall Perylentetracarbonsäurederivate
92	Optical Properties of Organic Semiconductors: from Submonolayers to Crystalline Films Nitsche, Robert 23 November 2005 (has links) We have measured the optical properties of films of the organic semiconductors PTCDA (3,4:9,10-perylene-tetracarboxylic dianhydride) and HBC (peri-hexabenzocoronene), prepared by Organic Molecular Beam Expitaxy (OMBE), on different substrates by means of Differential Reflectance Spectroscopy (DRS). The optical setup enables us to directly follow the thickness dependent optical properties of the organic films, starting from submonolayer coverage up to thicker films on the order of 20 monolayers (ML) film thickness. Due to the different optical nature of the different substrates used, i.e., mica, glass, Au(111), and HOPG, the direct interpretation of the DRS signal is not feasible. Therefore, we have proposed a method by which the calculation of the optical constants n (index of refraction) and k (absorption index) of thin films on arbitrary substrates from just one spectral measurement (in our case the DRS) becomes possible. The results fulfill a priori a Kramers-Kronig consistency and no specific model is needed to express the spectral behavior of the optical constants. Based on our method, we have successfully calculated the optical constants, and therefore the absorption behavior, of films of different thickness of PTCDA on mica, glass, Au(111), and HOPG, as well as of HBC on mica, glass, and HOPG. Extrinsic effects due to island growth or the presence of a polarizable substrate (screening) have been accounted for. We have introduced a finite dipole model which considers the extended geometry and anisotropy of the organic molecules. The calculated absorption behavior is discussed in great detail in terms of spectral changes with varying film thickness, different growth modes, degree of ordering of the films, interactions with the substrates and oscillator strength. A direct observation of a monomer-dimer transition in solid films could be observed for the first time. Our results indicate an exciton delocalization over about 4 molecules for both molecules. info:eu-repo/classification/ddc/530 ddc:530

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Frenkel and Charge-Transfer Excitons in Quasi-One-Dimensional Molecular Crystals with Strong Intermolecular Orbital Overlap / Frenkel und Charge-Transfer Exzitonen in Quasi-Eindimensionalen Molekülkristallen mit starker intermolekularer Orbitalüberlappung

Optical Properties of Organic Semiconductors: from Submonolayers to Crystalline Films