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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Publication Search Results

Now showing 1 - 10 of 38

Long-range corrected hybrid functionals for pi-conjugated systems: Dependence of the range-separation parameter on conjugation length

(Georgia Institute of Technology, 2011-11) Koerzdoerfer, Thomas ; Sears, John S. ; Sutton, Christopher ; Brédas, Jean-Luc

Long-range corrected hybrids represent an increasingly popular class of functionals for density functional theory (DFT) that have proven to be very successful for a wide range of chemical applications. In this Communication, we examine the performance of these functionals for time-dependent (TD)DFT descriptions of triplet excited states. Our results reveal that the triplet energies are particularly sensitive to the range-separation parameter; this sensitivity can be traced back to triplet instabilities in the ground state coming from the large effective amounts of Hartree-Fock exchange included in these functionals. As such, the use of standard long-range corrected functionals for the description of triplet states at the TDDFT level is not recommended.
Communication: Orbital instabilities and triplet states from time-dependent density functional theory and long-range corrected functionals

(Georgia Institute of Technology, 2011-10) Sears, John S. ; Koerzdoerfer, Thomas ; Zhang, Cai-Rong ; Brédas, Jean-Luc

Long-range corrected hybrids represent an increasingly popular class of functionals for density functional theory (DFT) that have proven to be very successful for a wide range of chemical applications. In this Communication, we examine the performance of these functionals for time-dependent (TD)DFT descriptions of triplet excited states. Our results reveal that the triplet energies are particularly sensitive to the range-separation parameter; this sensitivity can be traced back to triplet instabilities in the ground state coming from the large effective amounts of Hartree-Fock exchange included in these functionals. As such, the use of standard long-range corrected functionals for the description of triplet states at the TDDFT level is not recommended.
Theoretical study of substitution effects on molecular reorganization energy in organic semiconductors

(Georgia Institute of Technology, 2011-09) Geng, Hua ; Niu, Yingli ; Peng, Qian ; Shuai, Zhigang ; Coropceanu, Veaceslav ; Brédas, Jean-Luc

Chemical substitutions are powerful molecular design tools to enhance the performance of organic semiconductors, for instance, to improve solubility, intermolecular stacking, or film quality. However, at the microscopic level, substitutions in general tend to increase the molecular reorganization energy and thus decrease the intrinsic charge-carrier mobility. Through density functional theory calculations, we elucidate strategies that could be followed to reduce the reorganization energy upon chemical substitution. Specific examples are given here for hole-transport materials including indolo-carbazoles and several triarylamine derivatives. Through decomposition of the total reorganization energy into the internal coordinate space, we are able to identify the molecular segment that provides the most important contributions to the reorganization energy. It is found that when substitution reduces (enhances) the amplitude of the relevant frontier molecular orbital in that segment, the total reorganization energy decreases (increases). In particular, chlorination at appropriate positions can significantly reduce the reorganization energy. Several other substituents are shown to play a similar role, to a greater or lesser extent.
An improved dynamic Monte Carlo model coupled with Poisson equation to simulate the performance of organic photovoltaic devices

(Georgia Institute of Technology, 2011-03) Meng, Lingyi ; Wang, Dong ; Li, Qikai ; Yi, Yuanping ; Brédas, Jean-Luc ; Shuai, Zhigang

We describe a new dynamic Monte Carlo model to simulate the operation of a polymer-blend solar cell; this model provides major improvements with respect to the one we developed earlier [J. Phys. Chem. B 114, 36 (2010)] by incorporating the Poisson equation and a charge thermoactivation mechanism. The advantage of the present approach is its capacity to deal with a nonuniform electrostatic potential that dynamically depends on the charge distribution. In this way, the unbalance in electron and hole mobilities and the space-charge induced potential distribution can be treated explicitly. Simulations reproduce well the experimental I-V curve in the dark and the open-circuit voltage under illumination of a polymer-blend solar cell. The dependence of the photovoltaic performance on the difference in electron and hole mobilities is discussed.
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.
Quantum chemical modeling of electron transfer in amorphous organic films

(Georgia Institute of Technology, 2010-06-01) Brédas, Jean-Luc ; Li, Hong
Electronic, optical and interfacial properties of pi-conjugated materials

(Georgia Institute of Technology, 2009-06-05) Brédas, Jean-Luc
Simulations of the emission spectra of fac-tris(2-phenylpyridine) iridium and Duschinsky rotation effects using the Herman-Kluk semiclassical initial value representation method

(Georgia Institute of Technology, 2008-12-07) Wu, Yinghua ; Brédas, Jean-Luc

The phosphorescent emission spectra of fac-tris(2-phenylpyridine) iridium [fac-Ir(ppy)₃] due to the lowest triplet T1 and T2 states are simulated using the harmonic oscillator approximation for the S0, T₁, and T₂potential energy surfaces (PESs) and taking the Duschinsky rotation into account. The simulations involve the propagation of 177-dimensional wave packets on the coupled PES according to the Herman–Kluk (HK) semiclassical (SC) initial value representation (IVR) method. The HK SC-IVR method is employed because of its accuracy for the PES with mode mixing and its efficiency in dealing with coupled degrees of freedom for large systems. The simulated emission spectrum due to T₁ reproduces the structures of the emission spectra observed experimentally, while T₂ is found very unlikely to participate in the phosphorescent emission. Although the effect of the Duschinsky mode mixing is small for the T₁ state, neglecting it blueshifts the spectrum due to the T₂ state by 800 cm−1 and changes the relative intensities, indicating that the importance of the Duschinsky rotation is rather unpredictable and should not be overlooked. The present simulations demonstrate that the simple harmonic oscillator approximation combined with the Duschinsky rotation can adequately describe the photophysics of fac-Ir(ppy)₃ and that the HK SC-IVR method is a powerful tool in studies of this kind.
Electronic and vibronic interactions at weakly bound organic interfaces: The case of pentacene on graphite

(Georgia Institute of Technology, 2008-07) Paramonov, Pavel B. ; Coropceanu, Veaceslav ; Brédas, Jean-Luc

The electronic and vibronic processes at the interface between a pentacene monolayer and a graphite surface were characterized using a combination of density-functional theory (DFT) and dynamic vibronic coupling simulations. The electronic interactions were evaluated at the DFT level first between the highest occupied states of pentacene and the graphite surface, as well as among the pentacene molecules within a monolayer. The former are found to be ca. four times stronger than the latter for a parallel molecule/surface geometry. A dynamic vibronic model was used to analyze the interplay between the electronic and electron-vibration couplings and their effects on spectroscopic characteristics. The agreement between the simulated and experimental photoelectron spectra underlines the importance of weak electronic interactions on the vibronic coupling at the interface.
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.