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

The nature of singlet excitons in oligoacene molecular crystals

(Georgia Institute of Technology, 2011-03) Yamagata, H. ; Norton, J. ; Hontz, E. ; Olivier, Y. ; Beljonne, David ; Brédas, Jean-Luc ; Silbey, R. J. ; Spano, F. C.

A theory for polarized absorption in crystalline oligoacenes is presented, which includes Frenkel exciton coupling, the coupling between Frenkel and charge-transfer (CT) excitons, and the coupling of all neutral and ionic excited states to the dominant ring-breathing vibrational mode. For tetracene, spectra calculated using all Frenkel couplings among the five lowest energy molecular singlet states predict a Davydov splitting (DS) of the lowest energy (0–0) vibronic band of only −32 cm ⁻¹, far smaller than the measured value of 631 cm ⁻¹ and of the wrong sign—a negative sign indicating that the polarizations of the lower and upper Davydov components are reversed from experiment. Inclusion of Frenkel-CT coupling dramatically improves the agreement with experiment, yielding a 0–0 DS of 601 cm ⁻¹ and a nearly quantitative reproduction of the relative spectral intensities of the 0–n vibronic components. Our analysis also shows that CT mixing increases with the size of the oligoacenes. We discuss the implications of these results on exciton dissociation and transport.
Experimental and theoretical study of temperature dependent exciton delocalization and relaxation in anthracene thin films

(Georgia Institute of Technology, 2008-02) Ahn, Tai-Sang ; Mueller, Astrid M. ; Al-Kaysi, Rabih O. ; Spano, Frank C. ; Norton, Joseph E. ; Beljonne, David ; Brédas, Jean-Luc ; Bardeen, Christopher J.

The spectroscopy of solid anthracene is examined both experimentally and theoretically. To avoid experimental complications such as self-absorption and polariton effects, ultrathin polycrystalline films deposited on transparent substrates are studied. To To separate the contributions from different emitting species, the emission is resolved in both time and wavelength. The spectroscopic data are interpreted in terms of a three-state kinetic model, where two excited states, a high energy state 1 and a low energy state 2, both contribute to the luminescence and are kinetically coupled. Using this model, we analyze the spectral lineshape, relative quantum yield, and relaxation rates as a function of temperature. For state 1, we find that the ratio of the 0-0 vibronic peak to the 0-1 peak is enhanced by roughly a factor of 3.5 at low temperature, while the quantum yield and decay rates also increase by a similar factor. These observations are explained using a theoretical model previously developed for herringbone polyacene crystals. The early-time emission lineshape is consistent with that expected for a linear aggregate corresponding to an edge-dislocation defect. The results of experiment and theory are quantitatively compared at different temperatures in order to estimate that the singlet exciton in our polycrystalline films is delocalized over about ten molecules. Within these domains, the exciton’s coherence length steadily increases as the temperature drops, until it reaches the limits of the domain, whereupon it saturates and remains constant as the temperature is lowered further. While the theoretical modeling correctly reproduces the temperature dependence of the fluorescence spectral lineshape, the decay of the singlet exciton appears to be determined by a trapping process that becomes more rapid as the temperature is lowered. This more rapid decay is consistent with accelerated trapping due to increased delocalization of the exciton at lower temperatures. These observations suggest that exciton coherence can play an important role in both radiative and nonradiative decay channels in these materials. Our results show that the spectroscopy of polyacene solids can be analyzed in a self-consistent fashion to obtain information about electronic delocalization and domain sizes.
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.
Chromophores in phenylenevinylene-based conjugated polymers: Role of conformational kinks and chemical defects

(Georgia Institute of Technology, 2006-07) Hennebicq, Emmanuelle ; Deleener, Caroline ; Scholes, Gregory D. ; Beljonne, David ; Brédas, Jean-Luc

The influence of chemical defects and conformational kinks on the nature of the lowest electronic excitations in phenylenevinylene-based polymers is assessed at the semiempirical quantum-chemical level. The amount of excited-state localization and the amplitude of through-space (Coulomb-like) versus through-bond (charge-transfer-like) interactions have been quantified by comparing the results provided by excitonic and supermolecular models. While excitation delocalization among conjugated segments delineated by the defects occurs in the acceptor configuration, self-confinement on individual chromophores follows from geometric relaxation in the excited-state donor configuration. The extent of excited-state localization is found to be sensitive to both the nature of the defect and the length of the conjugated chains. Implications for resonant energy transfer along conjugated polymer chains are discussed.
Transient absorption spectroscopy and quantum-chemical studies of matrix-isolated perylene derivatives

(Georgia Institute of Technology, 2006-06) Engel, E. ; Scholes, Gregory D. ; Beljonne, David ; Assa, J. ; Froeb, H. ; Leo, K. ; Hoffmann, M. ; Brédas, Jean-Luc

We present a comprehensive experimental and theoretical study of the optical properties of matrix-isolated molecules of the two perylene derivatives N,N′-dimethylperylene-3,4,9,10-dicarboximide (MePTCDI) and 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA). A solid solution of the dyes in an SiO2 matrix exhibits monomer-like behavior. Transient absorption pump-probe spectroscopy in the range 1.2–2.6 eV has been performed on an ultrafast time scale. The differential transmittance reveals contributions from ground-state bleaching, stimulated emission, and excited-state absorption. Both systems exhibit broad excited-state absorption features below 2.0 eV with a clear peak around 1.8 eV. The spectra can be consistently explained by the results of quantum-chemical calculations. We have applied both the coupled cluster singles and doubles (CCSD) model and the multireference-determinant single and double configuration-interaction (MRD-CI) technique on the basis of the intermediate neglect of differential overlap (INDO) Hamiltonian. The results are insensitive to whether the geometry is optimized for the electronic ground state or first excited state. The experimental polarization anisotropies for the two major transitions are in agreement with the calculated polarizations.
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.
Optical properties of singly charged conjugated oligomers: A coupled-cluster equation of motion study

(Georgia Institute of Technology, 2004-09) Ye, A. ; Shuai, Zhigang ; Kwon, O. ; Brédas, Jean-Luc ; Beljonne, David

We have implemented a coupled-cluster equation of motion approach combined with the intermediate neglect of differential overlap parametrization and applied it to study the excited states and optical absorptions in positively and negatively charged conjugated oligomers. The method is found to be both reliable and efficient. The theoretical results are in very good agreement with experiments and confirm that there appear two subgap absorption peaks upon polaron formation. Interestingly, the relative intensities of the polaron-induced subgap absorptions can be related to the extent of the lattice geometry relaxations.
Singlet-triplet splitting of geminate electron-hole pairs in conjugated polymers

(Georgia Institute of Technology, 2004-08) Kadashchuk, A. ; Vakhnin, A. ; Blonski, I. ; Beljonne, David ; Shuai, Zhigang ; Arkhipov, V. I. ; Heremans, P. ; Emelianova, E. V. ; Bassler, H. ; Brédas, Jean-Luc

The singlet-triplet splitting of geminate polaron pairs in a ladder-type conjugated polymer has been studied by the thermally stimulated luminescence technique. The energy gap separating the singlet and triplet states of the geminate pairs is measured to be in the range of 3–6 meV, depending on the polymer morphology. The results of correlated quantum-chemical calculations on a long ladder-type oligomer are fully consistent with the observed values of the geminate polaron pair singlet-triplet gap. Such low splitting values have important implications for the spin-dependent exciton formation in conjugated polymers.
Electronic and optical properties of polyfluorene and fluorene-based copolymers: A quantum-chemical characterization

(Georgia Institute of Technology, 2003-04) Cornil, Jerome ; Gueli, I. ; Dkhissi, A. ; Sancho-García, J. C. ; Hennebicq, Emmanuelle ; Calbert, J. P. ; Lemaur, V. ; Beljonne, David ; Brédas, Jean-Luc

We report a detailed quantum-chemical characterization of the electronic and optical properties of polyfluorene chains and compare them to those in copolymers containing alternating fluorene and benzothiadiazole or ethylenedioxythiophene units. The introduction of the comonomer can strongly modify the excitonic properties as well as the efficiency of charge- and energy-transfer processes. The choice of the comonomer is thus critical in targeting specific optical properties while maintaining good transport properties.