Showing papers by "Sergei Tretiak published in 2008"

Enhanced two-photon absorption of organic chromophores: theoretical and experimental assessments ∗∗

Abstract: Functional organic materials with enhanced two-photon absorption lead to new technologies in the fields of chemistry, biology, and photonics. In this article we review experimental and theoretical methodologies allowing detailed investigation and analysis of two-photon absorption properties of organic chromophores. This includes femtosecond two-photon excited fluorescence experimental setups and quantum-chemical methodologies based on time-dependent density functional theory. We thoroughly analyze physical phenomena and trends leading to large twophoton absorption responses of a few series of model chromophores focusing on the effects of symmetric and asymmetric donor/acceptor substitution and branching.

How chromophore shape determines the spectroscopy of phenylene-vinylenes: origin of spectral broadening in the absence of aggregation.

Abstract: Single oligo(phenylene-vinylene) molecules constitute model systems of chromophores in disordered conjugated polymers and can elucidate how the actual conformation of an individual chromophore, rather than that of an overall polymer chain, controls its photophysics. Single oligomers and polymer chains display the same range of spectral properties. Even heptamers support π-electron conjugation across ∼80° curvature, as revealed by the polarization anisotropy in excitation and supported by quantum chemical calculations. As the chain becomes more deformed, the spectral linewidth at low temperatures, often interpreted as a sign of aggregation, increases up to 30-fold due to a reduction in photophysical stability of the molecule and an increase in random spectral fluctuations. The conclusions aid the interpretation of results from single-chain Stark spectroscopy in which large static dipoles were only observed in the case of narrow transition lines. These narrow transitions originate from extended chromophores...

Electronic structure of self-assembled amorphous polyfluorenes.

Abstract: We investigate the role of conformational disorder and intermolecular interactions on the electronic structure of amorphous clusters of polyfluorenes. Classical molecular dynamics simulations are used to determine probable molecular geometries and chain packing, and first-principles density functional theory calculations are employed to determine electronic structure and orbital localization properties. Intramolecular and intermolecular effects are disentangled by contrasting results for densely packed oligomer clusters and for ensembles of isolated oligomers with the same intramolecular geometries. Our simulations show that intermolecular disorder allows for nearly planar configurations of interacting fluorenes compared to the isolated molecules. This rationalizes the experimentally detected formation of the planar crystalline morphologies that frequently accompany twisted glassy configurations in fluorene films. The energy gap (HOMO-LUMO gap) significantly decreases for planar configurations. The electron and hole orbital energies are strongly dependent on both torsional angles and intermolecular interactions. This leads to strong localization of electronic states in amorphous polymer aggregates, which is analyzed by examining the respective orbital participation ratios. Notably, the energies of unoccupied levels show stronger dependence on the conformational disorder, compared to that of occupied levels. This results in the more probable formation of trap states near the edge of the conduction band than near the valence band.

Quantum chemistry of the minimal CdSe clusters.

Abstract: Colloidal quantum dots are semiconductor nanocrystals (NCs) which have stimulated a great deal of research and have attracted technical interest in recent years due to their chemical stability and the tunability of photophysical properties. While internal structure of large quantum dots is similar to bulk, their surface structure and passivating role of capping ligands (surfactants) are not fully understood to date. We apply ab initio wavefunction methods, density functional theory, and semiempirical approaches to study the passivation effects of substituted phosphine and amine ligands on the minimal cluster Cd2Se2, which is also used to benchmark different computational methods versus high level ab initio techniques. Full geometry optimization of Cd2Se2 at different theory levels and ligand coverage is used to understand the affinities of various ligands and the impact of ligands on cluster structure. Most possible bonding patterns between ligands and surface Cd∕Se atoms are considered, including a ligan...

Cross-polarized excitons in carbon nanotubes

Abstract: Polarization of low-lying excitonic bands in finite-size semiconducting single-walled carbon nanotubes (SWNTs) is studied by using quantum-chemical methodologies. Our calculations elucidate properties of cross-polarized excitons, which lead to the transverse optical absorption of nanotubes and presumably couple to intermediate-frequency modes recently observed in resonance Raman excitation spectroscopy. We identify up to 12 distinct excitonic transitions below the second fundamental band associated with the E22 van Hove singularity. Calculations for several chiral SWNTs distinguish the optically active “bright” excitonic band polarized parallel to the tube axis and several optically “weak” cross-polarized excitons. The rest are optically (near) forbidden “dark” transitions. An analysis of the transition density matrices related to excitonic bands provides detailed information about delocalization of excitonic wavefunction along the tube. Utilization of the natural helical coordinate system accounting for the tube chirality allows one to disentangle longitudinal and circumferential components. The distribution of the transition density matrix along a tube axis is similar for all excitons. However, four parallel-polarized excitons associated with the E11 transition are more localized along the circumference of a tube, compared with others related to the E12 and E21 cross-polarized transitions. Calculated splitting between optically active parallel- and cross-polarized transitions increases with tube diameter, which compares well with experimental spectroscopic data.

Linear optical response of current-carrying molecular junction: a nonequilibrium Green's function-time-dependent density functional theory approach.

Abstract: We propose a scheme for calculation of linear optical response of current-carrying molecular junctions for the case when electronic tunneling through the junction is much faster than characteristic time of external laser field. We discuss relationships between nonequilibrium Green’s function (NEGF) and time-dependent density functional theory (TDDFT) approaches and derive expressions for optical response and linear polarizability within NEGF-TDDFT scheme. Corresponding results for isolated molecule, derived within TDDFT approach previously, are reproduced when coupling to contacts is neglected.

Performance of a nonempirical meta-generalized gradient approximation density functional for excitation energies.

Abstract: It is known that the adiabatic approximation in time-dependent density functional theory usually provides a good description of low-lying excitations of molecules. In the present work, the capability of the adiabatic nonempirical meta–generalized gradient approximation (meta-GGA) of Tao, Perdew, Staroverov, and Scuseria (TPSS) to describe atomic and molecular excitations is tested. The adiabatic (one-parameter) hybrid version of the TPSS meta-GGA and the adiabatic GGA of Perdew, Burke, and Ernzerhof (PBE) are also included in the test. The results are compared to experiments and to those obtained with two well-established hybrid functionals PBE0 and B3LYP. Calculations show that both adiabatic TPSS and TPSSh functionals produce excitation energies in fairly good agreement with experiments, and improve upon the adiabatic local spin density approximation and, in particular, the adiabatic PBE GGA. This further confirms that TPSS is indeed a reliable nonhybrid universal functional which can serve as the starting point from which higher-level approximations can be constructed. The systematic underestimate of the low-lying vertical excitation energies of molecules with time-dependent density functionals within the adiabatic approximation suggests that further improvement can be made with nonadiabatic corrections.

Multiscale modeling of electronic excitations in branched conjugated molecules using an exciton scattering approach.

Abstract: The exciton scattering (ES) approach attributes excited electronic states in quasi-1D branched polymer molecules to standing waves of quantum quasiparticles (excitons) scattered at the molecular vertices. We extract their dispersion and frequency-dependent scattering matrices at termini, ortho, and meta joints for � -conjugated phenylacetylene-based molecules from atomistic time-dependent density-functional theory (TD DFT) calculations. This allows electronic spectra for any structure of arbitrary size within the considered molecular family to be obtained with negligible numerical effort. The agreement is within 10 – 20 meV for all test cases, when comparing the ES results with the reference TD DFT calculations.

Exciton scattering approach for branched conjugated molecules and complexes. I. Formalism.

Abstract: We develop a formalism for the exciton scattering (ES) approach to calculation of the excited state electronic structure of branched conjugated polymers with insignificant numerical expense. The ES approach attributes electronic excitations in quasi-one-dimensional molecules to standing waves formed by the scattering of quantum quasiparticles. We derive the phenomenology from the microscopic description in terms of many-electron excitations. The presented model can be used to compute both excited state frequencies and transition dipoles in large molecules after the ES ingredients are extracted from smaller molecular fragments.

Exciton scattering approach for branched conjugated molecules and complexes. III. Applications

Abstract: The exciton scattering (ES) approach is an efficient tool to calculate the excited states electronic structure in large branched polymeric molecules. Using the previously extracted parameters, we apply the ES approach to a number of phenylacetylene-based test molecules. Comparison of ES predictions with direct quantum chemistry results for the excitation energies shows an agreement within several meV. The ES framework provides powerful insights into photophysics of macromolecules by revealing the connections between the molecular structure and the properties of the collective electronic states, including spatial localization of excitations controlled by the energy.

Molecular-orbital-free algorithm for excited states in time-dependent perturbation theory.

Abstract: A nonlinear conjugate gradient optimization scheme is used to obtain excitation energies within the random phase approximation (RPA). The solutions to the RPA eigenvalue equation are located through a variational characterization using a modified Thouless functional, which is based upon an asymmetric Rayleigh quotient, in an orthogonalized atomic orbital representation. In this way, the computational bottleneck of calculating molecular orbitals is avoided. The variational space is reduced to the physically-relevant transitions by projections. The feasibility of an RPA implementation scaling linearly with system size N is investigated by monitoring convergence behavior with respect to the quality of initial guess and sensitivity to noise under thresholding, both for well- and ill-conditioned problems. The molecular-orbital-free algorithm is found to be robust and computationally efficient, providing a first step toward large-scale, reduced complexity calculations of time-dependent optical properties and linear response. The algorithm is extensible to other forms of time-dependent perturbation theory including, but not limited to, time-dependent density functional theory.

Exciton scattering approach for branched conjugated molecules and complexes. II. Extraction of the exciton scattering parameters from quantum-chemical calculations.

Abstract: We obtain the parameters of the exciton scattering (ES) model from the quantum-chemical calculations of the electronic excitations in simple phenylacetylene-based molecules. We determine the exciton dispersion and the frequency-dependent scattering matrices which describe scattering properties of the molecular ends as well as of meta- and orthoconjugated links. The extracted functions are smooth, which confirms the validity of the ES picture. We find a good agreement between the ES and quantum-chemical results for the excitation energies in simple test molecules.

Absorption spectra of blue-light-emitting oligoquinolines from time-dependent density functional theory.

Abstract: Recently, it has been discovered that a series of four conjugated oligomers, oligoquinolines, exhibits many desirable properties of organic materials for developing high-performance light-emitting diodes: good blue color purity, high brightness, high efficiency, and high glass-transition temperatures. In this work, we investigate the optical absorption of oligoquinolines in the gas phase and chloroform (CHCl3) solution, respectively, using time-dependent density functional theory with the adiabatic approximation for the dynamical exchange-correlation potential. Our calculations show that the first peak of optical absorption corresponds to the lowest singlet excited state, whereas several quasi-degenerate excited states contribute to the experimentally observed higher-frequency peak. We find that, compared with the gas phase, there is a moderate red shift in excitation energy in solution due to the solute-solvent interaction simulated using the polarizable continuum model. Our results show that the lowest singlet excitation energies of oligoquinolines in chloroform solution calculated with the adiabatic hybrid functional PBE0 are in a good agreement with experiments. Our simulated optical absorption agrees well with the experimental data. Finally, analysis of the natural transition orbitals corresponding to the excited states in question underscores the underlying electronic delocalization properties.