A harmonic Condon approach is used to calculate excitation and emission band shapes for the lowest dipole-allowed electronic transitions in conjugated oligomers: polyenes, oligorylenes, and para-phenylenevinylenes. Ground- and excited-state adiabatic energies, equilibrium structures, and vibrational modes are obtained within standard all-valence-electron molecular Hamiltonian incorporating extended configuration interaction. The interstate distortion is cast in normal coordinates and used to calculate transition probabilities from the zero-phonon initial state to the vibrational manifold of the final state. Spectral profiles are obtained as a superposition of Lorentzian line shapes. Theoretical band shapes reproduce prominent features in the absorption and fluorescence spectra of the oligomers in question. The strength of the bond-stretching vibronic progression increases with oligomeric length in polyenes, but decreases in para-phenylenevinylenes. In line with experiment, absorption and emission band shapes of para-phenylenevinylenes are obtained intrinsically nonsymmetric due to stiffening of the accepting vibrational modes in the excited state. The Stokes shifts of the apparent 0-0 features in the latter are reproduced and traced back to relaxations in slow, ring-torsional motions.

Ab initio study of the excited singlet states of all-trans α , ω -diphenylpolyenes with one to seven polyene double bonds: Simulation of the spectral data within Franck-Condon approximation Proceeding from quantum-chemical potential energy surfaces, we compute the absorption and fluorescence spectra of conventional and ladder-type para-phenylene oligomers ͑OPP and OLPP͒ with up to 7 benzene rings. Electronically excited states are addressed by means of extended configuration interaction within a standard molecular all-valence-electron semiempirical Hamiltonian. Adiabatic excitation energies, interstate distortions and normal modes are used to compute Franck-Condon band shapes with rigorous consideration of vibrational structure. Theoretical spectra agree with the experiment and rationalize the striking disparities in the linear optical response of OPP and OLPP. Whereas electron-phonon coupling in OLPP is essentially restricted to the carbon-carbon bond-stretching modes, photoexcitation, and emission processes in OPP are followed by significant relaxations in ring-torsional degrees of freedom. The broadening of spectra of OPP, especially pronounced in absorption, and the large Stokes shift between absorption and emission are traced back to the strong coupling of electronic excitations and low-frequency libration motions. The results highlight the importance of ring-torsional flexibility in conjugated polymers.

Estimating the toxicity of reactive xenobiotics to aquatic organisms requires physicochemical descriptors of passive transport and chemical reactions with nucleophilic biological ligands. Herein, electrophiles whose toxic action is attributed to nucleophilic substitution (SN), Michael-type addition and Schiff-base formation were examined. Training sets for each molecular mechanism were generated through substructure search applied to chemicals in a fathead minnow (Pimephules promelus) database. Based on a delineation of compounds by a presumed molecular mechanism, relationships between modes of toxic action, potency (96-hour LC,, values) and mechanistically-appropriate quantum-chemica1 descriptors were explored. Monohalo-C(sp3) function which may give rise to S, reactivity was encountered in 35 compounds.The inclusion of E L , , , , a nonspecific electrophilicity descriptor, to the generic LC,, -hydrophobicity relation increased the explained variance from r' = 36% to 69%. Eighteen potential Michael-type acceptors, mainly acrylates, were identified by the presence of a localized CC double bond at an E, position to a polar group. Due to different modes of action, the toxic potency of these chemicals varies almost independently of hydrophobicity (? = 0.12). Two additional electronic descriptors that are consistent with the likely molecular mechanism provide a multivariate QSAR with ? = 0.78. Forty-five aldehydes and 3 formamides comprised the training set associated with probable Schiff-base mechanism of toxicity. The results suggest a marginal increase of toxic potency from that expected due to narcosis for more electrophilic carbonyl groups. Overall, it was concluded that regressions based on data sets that combine reactive chemicals with narcotics typically require an electronic descriptor in addition to hydrophobicity, even if the compounds all contain a common electrophilic moiety related to the putative specific reaction mechanism. However, without the generation of additional toxicity data from chemical sets that incorporate a broader range of electronic and steric character, it will likely remain extremely difficult to develop a quantitative ability to predict the potency of electrophilic compounds.

The neutral excitations in poly(p-phenylenevinylene) are studied in conjunction with the vibronic structure of the lowest optical transitions. Combining the configuration interaction of Wannier-localized electron–hole pairs with an empirical description of electron–phonon coupling, we obtain the potential energy surfaces of monoexcited states and the Condon electron–vibrational spectra in absorption and emission. The S1→S0 luminescence band shape is found compatible with self-localization of S1 within about 10 monomers, driven exclusively by electron–phonon coupling. The singlet and triplet polaron–excitons are exchange–split by about 1 eV and differ substantially in terms of average electron–hole separation.

In this paper we consider the essential electronic excited states in parallel chains of semiconducting polymers that are currently being explored for photovoltaic and light-emitting diode applications. In particular, we focus upon various type II donor-acceptor heterojunctions and explore the relation between the exciton binding energy to the band offset in determining the device characteristic of a particular type II heterojunction material. As a general rule, when the exciton binding energy is greater than the band offset at the heterojunction, the exciton will remain the lowest-energy excited state and the junction will make an efficient light-emitting diode. On the other hand, if the offset is greater than the exciton binding energy, either the electron or hole can be transferred from one chain to the other. Here we use a two-band exciton to predict the vibronic absorption and emission spectra of model polymer heterojunctions. Our results underscore the role of vibrational relaxation and suggest that intersystem crossings may play some part in the formation of charge-transfer states following photoexcitation in certain cases.