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Showing 1 to 4 of 4 (0.008 seconds)| 1 |
Electroabsorption spectroscopy of quasi-one-dimensional organic molecular crystalsGuo, Wenge 13 March 2004 (has links) (PDF)
We have presented a thorough experimental investigation of electroabsorption spectroscopy on quasi-one-dimensional organic molecular crystals such as PTCDA and MePTCDI vapor deposited thin films to clarify the involvement of the charge-transfer exciton in the lowest excited state. By a self-built experimental setup, two kinds of electroabsorption measurements, called "perpendicular" and "parallel" measurements, were conducted at room temperature in ambient air. The crystalline texture of PTCDA and MePTCDI thin film samples are characterized by X-ray diffraction measurements. Current-voltage, capacitance-frequency and capacitance-voltage measurements are performed to clarify the electric field distribution inside organic layers. The results from electrical measurements show that only under certain conditions (electroabsorption measurements with proDC bias), the perpendicular and parallel electroabsorption meaurements can be directly compared. The electroabsorption spectra of MePTCDI and PTCDA thin films can be interpreted by neither pure Frenkel exciton nor pure charge-transfer exciton model. Essential features of electroabsorption spectra of MePTCDI and PTCDA thin films can be understood by the the mixture of Frenkel and charge-transfer exciton model. However, there is still a discrepancy in the directional properties of electroabsorption signals between experimental results and modle calculations. This small discrepancy suggests that a full interpretation of electroabsorption spectra of quasi-one-dimensional organic molecular crystals needs further experimental and theoretical investigations.
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Frenkel and Charge-Transfer Excitons in Quasi-One-Dimensional Molecular Crystals with Strong Intermolecular Orbital OverlapHoffmann, Michael 19 December 2000 (has links)
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.
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Electroabsorption spectroscopy of quasi-one-dimensional organic molecular crystalsGuo, Wenge 16 December 2003 (has links)
We have presented a thorough experimental investigation of electroabsorption spectroscopy on quasi-one-dimensional organic molecular crystals such as PTCDA and MePTCDI vapor deposited thin films to clarify the involvement of the charge-transfer exciton in the lowest excited state. By a self-built experimental setup, two kinds of electroabsorption measurements, called "perpendicular" and "parallel" measurements, were conducted at room temperature in ambient air. The crystalline texture of PTCDA and MePTCDI thin film samples are characterized by X-ray diffraction measurements. Current-voltage, capacitance-frequency and capacitance-voltage measurements are performed to clarify the electric field distribution inside organic layers. The results from electrical measurements show that only under certain conditions (electroabsorption measurements with proDC bias), the perpendicular and parallel electroabsorption meaurements can be directly compared. The electroabsorption spectra of MePTCDI and PTCDA thin films can be interpreted by neither pure Frenkel exciton nor pure charge-transfer exciton model. Essential features of electroabsorption spectra of MePTCDI and PTCDA thin films can be understood by the the mixture of Frenkel and charge-transfer exciton model. However, there is still a discrepancy in the directional properties of electroabsorption signals between experimental results and modle calculations. This small discrepancy suggests that a full interpretation of electroabsorption spectra of quasi-one-dimensional organic molecular crystals needs further experimental and theoretical investigations.
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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 OrbitalüberlappungHoffmann, 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.
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