Person:

Brédas, Jean-Luc

Permanent Link

https://hdl.handle.net/1853/71119

Associated Organization(s)

Organizational Unit

School of Chemistry and Biochemistry

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Now showing 1 - 10 of 10

First-principles study of the geometric and electronic structure of Au₁₃ clusters: Importance of the prism motif

(Georgia Institute of Technology, 2008-04) Gruber, Mathis ; Heimel, Georg ; Romaner, Lorenz ; Brédas, Jean-Luc ; Zojer, Egbert

The geometric structure, symmetry, and spin of Au13 clusters are investigated in the framework of densityfunctional theory, with particular attention paid to the correlation among these properties. Several computational approaches are carefully tested on previously proposed cluster configurations. Complications and possible pitfalls in electronic-structure calculations on these systems are highlighted. Using molecular dynamics with quantum mechanically calculated forces, a set of favored high binding energy geometric structures, where a trigonal prism acts as the central building block, is discussed.
Impact of bidirectional charge transfer and molecular distortions on the electronic structure of a metal-organic interface

(Georgia Institute of Technology, 2007-12) Romaner, Lorenz ; Heimel, Georg ; Gerlach, Alexander ; Schreiber, Frank ; Johnson, Robert L. ; Zegenhagen, Joerg ; Duhm, Steffen ; Koch, Norbert ; Zojer, Egbert ; Brédas, Jean-Luc

Interface energetics are of fundamental importance in organic and molecular electronics. By combining complementary experimental techniques and first-principles calculations, we resolve the complex interplay among several interfacial phenomena that collectively determine the electronic structure of the strong electron acceptor tetrafluoro-tetracyanoquinodimethane chemisorbed on copper. The combination of adsorption-induced geometric distortion of the molecules, metal-to-molecule charge transfer, and molecule-to-metal back transfer leads to a net increase of the metal work function.
Structure-property relationships for three-photon absorption in stilbene-based dipolar and quadrupolar chromophores

(Georgia Institute of Technology, 2006-07) Zhu, Lingyun ; Yi, Yuanping ; Shuai, Zhigang ; Brédas, Jean-Luc ; Beljonne, David ; Zojer, Egbert

Based on essential-state models for three-photon absorption (3PA), we have investigated the structure-property relationships for stilbene-based dipolar and quadrupolar chromophores. The emphasis lies on the evolution of the 3PA cross section with the degree of ground-state polarization. For dipolar systems, we find a dominant role played by Δμ, which expresses the change in dipole moment between the ground state and the 3PA active excited state. Thus, the strategies usually applied to maximize the second-order polarizability β are also applicable to optimize the 3PA cross section. For quadrupolar systems, the 3PA response is dominated by contributions from channels including various low-lying two-photon allowed states, which limits the applicability of essential-state models. Optimization strategies can be proposed but vary for different ranges of ground-state polarization.
Interface energetics and level alignment at covalent metal-molecule junctions: pi-conjugated thiols on gold

(Georgia Institute of Technology, 2006-05) Heimel, Georg ; Romaner, Lorenz ; Zojer, Egbert ; Brédas, Jean-Luc

The energetics at the interfaces between metal and monolayers of covalently bound organic molecules is studied theoretically. Despite the molecules under consideration displaying very different frontier orbital energies, the highest occupied molecular levels are found to be pinned at a constant energy offset with respect to the metal Fermi level. In contrast, the molecular properties strongly impact the metal work function. These interfacial phenomena are rationalized in terms of charge fluctuations and electrostatics at the atomic length scale as determined by first-principles calculations.
Quantum-chemical investigation of second-order nonlinear optical chromophores: Comparison of strong nitrile-based acceptor end groups and role of auxiliary donors and acceptors

(Georgia Institute of Technology, 2006-01) Leclercq, A. ; Zojer, Egbert ; Jang, S.-H. ; Barlow, Stephen ; Geskin, V. ; Jen, A. K.-Y. ; Marder, Seth R. ; Brédas, Jean-Luc

We report a detailed quantum-chemical investigation of donor-acceptor substituted dipolar nonlinear optical chromophores incorporating the 4-(dimethylamino)phenyl donor end group and a variety of strong heterocyclic acceptor end groups, including tricyanofurans and tricyanopyrroles. In particular, we study the variation of the molecular second-order polarizability (β) with the acceptor end group and when inserting auxiliary donors (thiophene) and acceptors (thiazole) into the π bridge. Both finite-field calculations (in the context of local contributions) and sum-over-states calculations were carried out in order to probe the relationship between β and the chemical structure of the various chromophores. The trends obtained with these two methods are fully consistent. The large β values (up to 700×10 ⁻³⁰ esu) as well as the observed tunability of the optical absorption maximum (λmax) make the chromophores investigated here interesting candidates for use in electro-optic applications at telecommunications wavelengths.
Effective conjugation and Raman intensities in oligo(para-phenylene)s: A microscopic view from first-principles calculations

(Georgia Institute of Technology, 2005-03) Heimel, Georg ; Somitsch, Dieter ; Knoll, Peter ; Brédas, Jean-Luc ; Zojer, Egbert

Electron-phonon coupling in oligo(para-phenylene)s is addressed in terms of the off-resonance Raman intensities of two distinct modes at 1220 and 1280 cm−1. On the basis of Albrecht’s theory, vibrational coupling and Raman intensities are calculated from first-principles quantum-chemical methods. A few-state model is used to evaluate the dependence of the mode intensities on oligomer length, planarity, and excitation wavelength. The link between electron delocalization∕conjugation and Raman intensities is highlighted. Extending on prior studies, the present work focuses on providing an in-depth understanding of the origin of this correlation in addition to reproducing experimental findings. The model applied here allows us to interpret the results on a microscopic, quantum-mechanical basis and to relate the observed trends to the molecular orbital structure and nature of the excited states in this class of materials. We find quantitative agreement between the results of the calculations and those of measurements performed on oligo(para-phenylene)s of various chain lengths in the solid state and in solution.
Breakdown of the mirror image symmetry in the optical absorption/emission spectra of oligo(para-phenylene)s

(Georgia Institute of Technology, 2005-01) Heimel, Georg ; Daghofer, Maria ; Gierschner, Johannes ; List, Emil J. W. ; Grimsdale, Andrew C. ; Müllen, Klaus ; Beljonne, David ; Brédas, Jean-Luc ; Zojer, Egbert

The absorption and emission spectra of most luminescent, π-conjugated, organic molecules are the mirror image of each other. In some cases, however, this symmetry is severely broken. In the present work, the asymmetry between the absorption and fluorescence spectra in molecular systems consisting of para-linked phenyl rings is studied. The vibronic structure of the emission and absorption bands is calculated from ab initio quantum chemical methods and a subsequent, rigorous Franck-Condon treatment. Good agreement with experiment is achieved. A clear relation can be established between the strongly anharmonic double-well potential for the phenylene ring librations around the long molecular axis and the observed deviation from the mirror image symmetry. Consequences for related compounds and temperature dependent optical measurements are also discussed
Three-photon absorption in anthracene-porphyrin-anthracene triads: A quantum-chemical study

(Georgia Institute of Technology, 2004-12) Zhu, Lingyun ; Yang, Xia ; Yi, Yuanping ; Xuan, Pengfei ; Shuai, Zhigang ; Chen, Dezhan ; Zojer, Egbert ; Brédas, Jean-Luc ; Beljonne, David

We have applied correlated quantum-chemical methods to investigate the three-photon absorption (3PA) response of a porphyrin triad derivative, where the central macrocycle is linked in mesopositions to two anthracene units via acetylenic bridges. The 3PA frequency-dependent spectrum of this derivative is dominated by a single resonance feature in the transparent region, associated with charge-transfer states between porphyrin and anthracene. The calculations indicate a two order of magnitude enhancement in the 3PA cross section in the triad molecule with respect to the individual entities, which is attributed to close one-, two-, and three-photon resonances together with strong electronic couplings among the units.
Resonant enhancement of two-photon absorption in substituted fluorene molecules

(Georgia Institute of Technology, 2004-08) Hales, Joel M. ; Hagan, David J. ; Van Stryland, Eric W. ; Schafer, K. J. ; Morales, A. R. ; Belfield, Kevin D. ; Pacher, P. ; Kwon, O. ; Zojer, Egbert ; Brédas, Jean-Luc

The degenerate and nondegenerate two-photon absorption (2PA) spectra for a symmetric and an asymmetric fluorene derivative were experimentally measured in order to determine the effect of intermediate state resonance enhancement (ISRE) on the 2PA cross section δ. The ability to tune the individual photon energies in the nondegenerate 2PA (ND-2PA) process afforded a quantitative study of the ISRE without modifying the chemical structure of the investigated chromophores. Both molecules exhibited resonant enhancement of the nonlinearity with the asymmetric compound showing as much as a twentyfold increase in δ. Furthermore, the possibility of achieving over a one order of magnitude enhancement of the nonlinearity reveals the potential benefits of utilizing ND-2PA for certain applications. To model ISRE, we have used correlated quantum-chemical methods together with the perturbative sum-over-states (SOS) expression. We find strong qualitative and quantitative correlation between the experimental and theoretical results. Finally, using a simplified three-level model for the SOS expression, we provide intuitive insight into the process of ISRE for ND-2PA.
On the polarization of the green emission of polyfluorenes

(Georgia Institute of Technology, 2003-10) Yang, X. H. ; Neher, D. ; Spitz, C. ; Zojer, Egbert ; Brédas, Jean-Luc ; Guntner, R. ; Scherf, U

An experimental and theoretical study of the anisotropic optical properties of polyfluorenes (PFs) bearing ketonic defects is presented. Polarized emission experiments performed on photooxidized aligned PF layers indicate that the transition dipole of the ‘‘green’’ CT π-π * transition of the keto-defect is oriented parallel to the chain direction. It is further observed that the polarization ratio of the green emission is slightly smaller than that of the blue emission component originating from undisturbed chains. Quantum mechanical calculations have been performed to support these observations. It is shown that the transition dipole moment of the CT π-π * transition of the defect is slightly misaligned with respect to the π-π* transition of the undisturbed PF chain, and that the angle between both depends on the chain conformation. For the most probably 5/2 helical conformation, this angle is, however, smaller than 5°. Further, polarized PL spectroscopy with polarized excitation has been performed to determine the extent of energy migration prior to emission from the keto-defect. For excitation at 380 nm, the polarization ratio of the green emission is essentially independent of the excitation polarization, indicating almost complete depolarization of the excitation before it is captured at a defect site. In contrast to this, energy migration after direct excitation of the keto-defect is inefficient or even absent.