Showing papers by "Sergei Tretiak published in 1998"

Stilbenoid Dimers: Dissection of a Paracyclophane Chromophore

Abstract: A series of paracyclophane derivatives that hold chromophores of varying conjugation lengths has been synthesized using palladium-mediated coupling reactions. These molecules mimic solid-state interactions in main-chain polychromophores and conjugated emissive polymers such as poly(p-phenylenevinylene) (PPV). Their optical properties give insight into the energetics of photoexcitations localized in a discrete chromophore relative to a state containing the through-space delocalized paracyclophane core. Thus, 4-vinyl[2.2]paracyclophane (5) is obtained by reaction of 4-bromo[2.2]paracyclophane (3) and ethylene using Pd(OAc)2 and P(o-tol)3. Similar reactions starting with pseudo-o- or pseudo-p-dibromo[2.2]paracyclophane (4a and 4b, respectively) give the pseudo-o- and pseudo-p-divinyl products (6a and 6b, respectively). Using styrene instead of ethylene provides the styryl-substituted products. Thus, 4-styryl[2.2]paracyclophane (7) is obtained from 3 while pseudo-p- and pseudo-o-distyryl[2.2]paracyclophane (1...

Localized Electronic Excitations in Phenylacetylene Dendrimers

Abstract: Electron-hole pairs created upon optical excitation of conjugated dendrimers (fractal antenna macromolecules) are shown to be localized within segments connected by benzene rings substituted at the meta-position. The absorption spectra of two families of dendrimers are analyzed using collective electronic normal modes representing the changes in charge and bond-order distributions induced by the optical field. The present approach may be used in the design of artificial light-harvesting antennae with controlled energy-funneling pathways.

Excited electronic states of carotenoids: Time-dependent density-matrix-response algorithm

Abstract: The response of the single-electron density matrix of a many-electron . system to an external field is calculated using the time-dependent Hartree)Fock TDHF technique. A procedure for inverting the resulting nonlinear response functions to obtain an effective quantum multilevel system that has the same response is developed. The number of effective states is gradually increased as higher-order nonlinearities are computed. The complete set of intrastate and interstate density matrices and excited-state energies can be calculated. A favorable N-scaling of computational effort with size can be obtained making use of the localization of the optical transitions in real space. Q 1998

Two-Dimensional Real-Space Analysis of Optical Excitations in Acceptor-Substituted Carotenoids

Abstract: Absorption spectra of substituted carotenoids with varying acceptor strength are analyzed using collective electronic normal modes obtained using the time-dependent Hartree−Fock (TDHF) technique combined with the INDO/S semiempirical Hamiltonian. Two-dimensional plots of the collective excitations in real space show an off-diagonal size associated with relative motion of electron-hole pairs created upon optical excitation and a diagonal size representing the pair's center of mass motion. By varying the polyene chain length we show that the response of symmetric molecules is controlled by “bulk” delocalized excitations with coherence size ∼12 double bonds, whereas the response of short polar molecules is dominated by a localized “charge-transfer” excitation created at the acceptor end with coherence and diagonal lengths ∼12 and ∼17 double bonds, respectively.

Localized Optical Excitations and Two-Exciton Spectroscopy of Phenylacetylene Dendrimers

Abstract: The one- and two-exciton manifolds of conjugated dendrimers possessing fractal geometries are studied using the Frenkel exciton model. Two-photon spectra can be used to determine both the magnitude and the sign of short-range coupling among segments. Self-similarity and the high degree of symmetry make it possible to compute the one-exciton states and the optical response with reduced numerical effort that scales linearly rather than exponentially with the number of generations. The third-order optical response and exciton scattering matrix are expressed in compact forms using irreducible representation of optical excitations, totally avoiding the expensive explicit calculation of two-exciton eigenstates.