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Novel optical pump-push – photocurrent probe ultrafast spectroscopy experiments on organic photoconversion systems show that excessive excitation energy in such systems is not lost but used to reach delocalised states that act as the gateway for long-range charge separation. We also show that the developed experimental approach can be generalised to inorganic and hybrid photoconversion systems.

The interaction between 2,20-bithiophene-5-carboxylic acid (PT2) sublimed under ultra-high vacuum conditions and anatase (101) and rutile (110) TiO2 single crystal surfaces is investigated by studying the electronic spectral density near the Fermi level with synchrotron-based spectroscopy. The experimental results are compared to density functional theory calculations of the isolated PT2 molecule and of the molecule adsorbed on an anatase TiO2 (101) cluster. The relative concentrations of Ti, C, and S atoms indicate that the adsorbed molecule remains intact upon deposition, which is typical of a Stranski-Krastanov growth mode. The analysis of the O1s spectrum suggests a predominant bidentate geometry of the adsorption with both rutile and anatase surfaces, as supported by previous theoretical simulations. It is also theoretically and experimentally demonstrated that the PT2 adsorption causes the appearance of new electronic states in the gap near the TiO2 valence band. A pinning effect of the LUMO level of the dye is also theoretically predicted. [ABSTRACT FROM AUTHOR]

The quantum-chemical study of the electronic and optical properties of unsubstituted oligo(phenylene vinylene) radical cations was reported. The goal is to distinguish the impact of the choice of molecular geometry from the impact of the choice of quantum-chemical method, on the calculated optical transition energies and the optical excitation is localized with HF-CI and delocalized with TD-DFT, almost irrespective of the molecular geometry.

The chain-length dependence of the electronic coupling for photoinduced charge separation in DNA hairpins associated to conjugated linkers, by means of correlated quantum-chemical calculations are explored. Pathways for charge transfer from the linker chromophore to a guanine site located at a well-defined distance along the DNA strand are identified.

Research has been conducted on the photoinduced charge transfer. The quantum-chemical techniques used in investigating the photoinduced charge transfer mechanism and rates are described.

Research is done on the configuration-interaction calculations which are performed to describe the singlet and triplet excited states of oligothiophene and oligo(phenylene ethynylene) conjugated chains. In conclusion, the roles of chain length, heavy-atom derivatization, and ring twists are evaluated.

Conjugated polymers are used in the low-cost fabrication of thin semiconducting films. Over recent years, many polymeric optoelectronic devices have been developed, although it is unclear whether the description of electronic excitations in such materials is more appropriately formed in a molecular or semiconductor picture. Electronic states associated with optical excitations in the conjugated polymer poly(2-methoxy-5-(2'-ethylhexyloxy)-p-phenylene vinylene) were studied, using photocurrent measurements and quantum-chemical calculations, and several photocurrent spectral features were found to be coupled with bands in the absorption spectrum.

Configuration interaction calculations on the lowest singlet and triplet excitations in oligothiophenes and their substituted derivatives reveal a decrease in singlet-singlet excitation energies with chain length, and an intersystem crossing between the singlet and triplet forms. This crossing populates the lowest triplet state and creates an absorption red-shift. An extended pi-conjugated path decreases the singlet-singlet excitation energies whereas exchange between two like-spins creates only a marginal effect in the lowest triplet excited state. Geometry relaxation phenomena are also noted.

Here, we present a general approach to treating vibronic coupling in molecular crystals based on atomistic simulations of large clusters. Such clusters comprise model aggregates treated at the quantum chemical level embedded within a realistic environment treated at the molecular mechanics level. As we calculate ground and excited state equilibrium geometries and vibrational modes of model aggregates, our approach is able to capture effects arising from coupling to intermolecular degrees of freedom, absent from existing models relying on geometries and normal modes of single molecules. Using the geometries and vibrational modes of clusters, we are able to simulate the fluorescence spectra of aggregates for which the lowest excited state bears negligible oscillator strength (as is the case, e.g., ideal H-aggregates) by including both Franck-Condon (FC) and Herzberg-Teller (HT) vibronic transitions. The latter terms allow the adiabatic excited state of the cluster to couple with vibrations in a perturbative fashion via derivatives of the transition dipole moment along nuclear coordinates. While vibronic coupling simulations employing FC and HT terms are well established for single-molecules, to our knowledge this is the first time they are applied to molecular aggregates. Here, we apply this approach to the simulation of the low-temperature fluorescence spectrum of para-distyrylbenzene single-crystal H-aggregates and draw comparisons with coarse-grained Frenkel-Holstein approaches previously extensively applied to such systems. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Recent work has suggested that correlations in the environments of chromophores can lead to a change in the dynamics of excitation transfer in both the coherent and incoherent limits. An example of this effect that is relevant to many single molecule experiments occurs in the standard Förster model for resonant energy transfer (RET). The standard formula for the FRET rate breaks down when the electronic excitations on weakly interacting donor and acceptor couple to the same vibrational modes. The transfer rate can then no longer be factored into donor emission and acceptor absorption lineshapes, but must be recast in terms of a renormalized phonon reorganization energy accounting for the magnitude and sign of the excitation-vibration couplings. In this paper, we derive theoretically how the FRET rate depends on the shared mode structure and coupling, examine the simplified case of Gaussian lineshapes and then provide a quantitative calculation for a system of current interest. [ABSTRACT FROM AUTHOR]

Single molecule fluorescence experiments have been performed on a BODIPY-based dye embedded in oligo(styrene) matrices to probe the density fluctuations and the relaxation dynamics of chain segments surrounding the dye molecules. The time-dependent fluorescence lifetime of the BODIPY probe was recorded as an observable for the local density fluctuations. At room temperature, the mean fraction of holes surrounding the probes is shown to be unaffected by the molecular weight in the glassy state. In contrast, the free volume increases significantly in the supercooled regime. These observations are discussed in the framework of the entropic theories of the glass transition. [ABSTRACT FROM AUTHOR]

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. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

We apply correlated quantum-chemical techniques to study the origin of the large two-photon absorption (TPA) cross sections, δ, in stilbene derivatives in which electron-donating and electron-withdrawing substituents provide a quadrupolar charge-transfer arrangement. An additional field created by a set of point charges is used to systematically modify the ground-state polarization to determine its consequences for the TPA response. The effect on the molecular structure can be quantified by the evolution of the π-bond-order alternation (π-BOA) of the conjugated backbone. For moderate ground-state polarizations, a marked increase of the TPA response occurs; for large polarizations, δ peaks and then drops dramatically. Insight into the origin of this evolution is gained by comparing the values of δ obtained via the converged sum-over-states approach to the results of a simple three-state model. [ABSTRACT FROM AUTHOR]

Semiempirical Hartree-Fock techniques are applied to determine the molecular parameters governing the efficiency of photoinduced charge generation and recombination processes in donor/acceptor complexes involving a three-ring oligophenylenevinylene as donor and perylene bisimide as acceptor. The results have shown that dissociation pathways involving the lowest two Dexcited states contribute to the dynamics of the whole process and the rates are sensitive to the relative position of the donor and acceptor units.

Atomistic models based on quantum-chemical calculations are combined with time-resolved spectroscopic investigations to explore the migration of electronic excitations along oligophenylenevinylene-based chiral stacks. It is found that the usual Pauli master equation (PME) approach relying on uncoherent transport between individual chromophores underestimates the excitation diffusion dynamics, monitored by the time decay of the transient polarization anisotropy.

Reports on correlated quantum-chemical calculations on phenylenevinylene oligomers with up to eleven repeat units to characterize the nature of electronic excitations relevant for the photophysical properties of the corresponding polymer. Nonlinear optical response of model conjugated chains; Effect of intrachain and interchain polarization on exciton binding energy.

We present a joint experimental and theoretical investigation of the electronic excitations in transition metal-containing phenylene ethynylenes. The influence of the metal on the nature of the lowest singlet and triplet excited states is characterized. We find that π conjugation occurs through the metal sites, which deeply modifies the optical properties of the conjugated chains. We also analyze the chain-length dependence of the singlet–singlet, S0→S1, singlet–triplet, S0→T1, and triplet–triplet, T1→Tn, transition energies; both experimental data and theoretical results indicate that the lowest triplet exciton, T1, is strongly localized on a single phenylene ring while the S1 and Tn states extend over a few repeating units. Finally, we estimate the geometric relaxation phenomena occurring in the lowest excited states and perform a Huang–Rhys analysis of the triplet emission spectrum in model systems. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We propose a two-band model for the description of the electronic structure of push–pull polyenes in order to analyze in simple ways their potentially useful electronic and optical features. The polyene part is described by two (valence and conduction) bands, which are coupled with two tight-binding frontier orbitals representing the donor and acceptor end groups. In this model, the ground state consists of the one-electron states of the (π) valence band hybridized with the donor orbitals, while the charge-transfer excited state is described as an excitation from the highest occupied molecular orbital to the lowest unoccupied molecular orbital, the latter being a hybridized one-electron state between the (π*) conduction band and the acceptor orbital. It is shown, by the Green’s function method, that the electron localizations (the partial density of states) at the end groups are determined by three factors; (1) the unperturbed energy levels of the frontier orbitals, (2) the density of states of the unperturbed polyene bands, and (3) the coupling constants between the π (π*) band and the donor (acceptor) orbital. Based on the results, a simple description is provided for the characteristic nonlinear optical responses and the intramolecular adiabatic charge-transfer mechanism of push–pull polyenes. In order to estimate the magnitude of the coupling constants, we compare the analytical results from the model with numerical calculations based on an established semiempirical method. This kind of modeling provides guidance for the design of functional push–pull polyenes. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The one- and two-photon optical absorption spectra as well as the frequency dependent third-harmonic generation response of a side-chain polymer containing 4-dialkylamino- 4′-nitro-stilbene (DANS) as its side group have been measured over a wide range of frequency. A three-state model, based on an intermediate neglect of differential overlap/multireference double-configuration interaction description of the excited states, provides a coherent picture of the dynamic response of DANS, but fails in reproducing the red shift observed when going from the linear to the nonlinear absorption spectra; including vibronic coupling within the Franck–Condon approximation improves the fit. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The optical transitions in α-conjugated oligothiophenes are investigated on the basis of a theoretical approach, including correlation effects; the authors consider both the neutral and doped states. The results are able to provide a coherent interpretation of the sometimes contradictory experimental data. For the neutral oligomers, the authors report the evolution of the lowest energy, dipole-allowed triplet–triplet transition with increasing chain length. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Using the semiempirical intermediate neglect of differential overlap (INDO) Hamiltonian in combination with configuration interaction techniques, we calculate the optical and photoinduced absorption spectra of poly(paraphenylene vinylene) oligomers containing from two to five phenyl rings; the evolutions with chain length of the singlet–singlet and triplet–triplet excitation energies as well as the values extrapolated for the polymer are in good agreement with experiment. The geometry relaxation phenomena in the first one-photon allowed singlet excited state and in the lowest triplet state are modeled on the basis of either bond-order/bond-length relationships or the formation of (bi)polaron-type defects; the results are compared to those of direct geometry optimizations in the excited state. The different methods consistently lead to more pronounced bond-length modifications in the triplet state than in the singlet state. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We present a theoretical investigation by the sum-over-states formalism of the static first- and third-order polarizabilities of thienylenemethylidene oligomers; following the incorporation of a single conjugated carbon between the rings, these compounds present a partially quinoid geometry. We use a multireference determinant single–double configuration interaction approach to calculate excited-state energies, state dipole moments, and transition dipole moments which are introduced in the sum-over-states expressions. All the results are discussed in comparison to those obtained in thiophene oligomers of similar sizes; they indicate that the third-order polarizabilities are smaller for the methine-bridged compounds than for the corresponding thiophene oligomers. This feature is rationalized by analyzing the main optical (virtual excitation) channels that contribute to the nonlinear response. [ABSTRACT FROM AUTHOR]

Using Hartree–Fock ab initio or semiempirical Austin model 1 (AM1) techniques to estimate geometries and an intermediate neglect of differential overlap/configuration interaction (INDO/CI) technique to calculate electronic transition energies, we examine the nature of the lowest singlet excited states in thiophene oligomers containing up to eight rings. The evolution of the ordering of the lowest two (1B and 2A) excited states is investigated as a function of chain length. The geometry relaxation phenomena in the 1B and 2A states are modeled on the basis of either bond-order/bond-length relationships or the formation of bipolaron-type defects. The chain-length dependence of the third-order polarizability γ is evaluated from a sum-over-states approach. All the results are discussed in comparison to those obtained in polyenes of similar sizes. [ABSTRACT FROM AUTHOR]

We use a multireference determinant single–double configuration interaction approach within a Pariser–Parr–Pople Hamiltonian (based on long-range hopping integrals and the use of the Ohno formula) to investigate linear polyenes containing from four to sixteen carbons. We calculate the low-lying excited states, the two-photon absorption spectrum, and the third-harmonic generation (THG) response. The mAg state, essential to the cubic nonlinear optical response, is found to saturate as the 6Ag state when chain length increases; for the longer polyenes, another high-lying Ag state also becomes important. We analyze the length dependence of the static third-order susceptibility χ(3); indication of the beginning of saturation behavior is found. Focusing our attention to the two-photon resonance peak present in the free-electron laser THG measurements on polyacetylene, we conclude that the experimental data can be explained within the strongly correlated electron one-dimensional model used in this work, in addition to the weakly interacting model successfully exploited in a previous study. [ABSTRACT FROM AUTHOR]

Estimates the second-order electro-optic response of chiral helicene molecules in isotropic media. Use of correlated quantum-chemical calculations; Derivation of the sum-over-states perturbation formalism from the density-matrix expression; Association of resonances with intrinsic electronic excitations.

Singlet fission, the spin-allowed photophysical process converting an excited singlet state into two triplet states, has attracted significant attention for device applications. Research so far has focused mainly on the understanding of singlet fission in pure materials, yet blends offer the promise of a controlled tuning of intermolecular interactions, impacting singlet fission efficiencies. Here we report a study of singlet fission in mixtures of pentacene with weakly interacting spacer molecules. Comparison of experimentally determined stationary optical properties and theoretical calculations indicates a reduction of charge-transfer interactions between pentacene molecules with increasing spacer molecule fraction. Theory predicts that the reduced interactions slow down singlet fission in these blends, but surprisingly we find that singlet fission occurs on a timescale comparable to that in pure crystalline pentacene. We explain the observed robustness of singlet fission in such mixed films by a mechanism of exciton diffusion to hot spots with closer intermolecular spacings. [ABSTRACT FROM AUTHOR]

We address the question of charge delocalization in amorphous and crystalline fullerene solids by performing state of the art calculations encompassing force-field molecular dynamics, microelectrostatic and quantum-chemical methods. The solution of a tight-binding model built from spatially (down to atomistic scale) and time (down to fs) resolved calculations yields the density of electronic states for the charge carriers and their energy-dependent intermolecular delocalization. Both pristine C60 and the soluble PC61BM/PC71BM acceptors may sustain high-energy states that spread over a few tens of molecules irrespective of morphology, yet electrostatic disorder (mostly dipolar and static in nature) makes the thermally available electron states collapse to hardly more than one molecule in PC61BM/PC71BM, while it has a much more limited impact in the case of the bare C60. Implications of these results for charge transport and exciton dissociation at donor–fullerene interfaces are discussed. [ABSTRACT FROM AUTHOR]

We have investigated at the theoretical Density Functional Theory level the way the work function of zinc oxide layers is affected upon deposition of self-assembled monolayers (SAMs). 4-tert-Butylpyridine (4TBP) and various benzoic acids (BA) were adsorbed on the apolar (101̅0) ZnO and used as probe systems to assess the influence of several molecular parameters. For the benzoid acids, we have investigated the impact of changing the nature of the terminal group (H, CN, OCH3) and the binding mode of the carboxylic acid (monodentate versus bidentate) on the apolar (101̅0) surface. For each system, we have quantified the contribution from the molecular core and the anchoring group as well as of the degree of surface reconstruction on the work function shift. For the benzoic acids, the structural reorganization of the surface induces a negative shift of the work function by about 0.3 ± 0.15 eV depending on the nature of the binding mode, irrespective of the nature of the terminal function. The bond-dipole potential strongly contributes to the modification of the work function, with values in the range +1.2 to +2.0 eV. In the case of 4TBP, we further characterized the influence of the degree of coverage and of co-adsorbed species (H, OH, and water molecules) on the ZnO/SAM electronic properties as well as the influence of the ZnO surface polarity by considering several models of the polar (0001) ZnO surface. The introduction of water molecules in the (un)dissociated form at full coverage on the non-polar surface only reduces the work function by 0.3–0.4 eV compared to a reference system without co-adsorbed species. Regarding the polar surface, the work function is also significantly reduced upon deposition of a single 4BTP molecule (from −1.44 eV to −1.73 eV for our model structures), with a shift similar in direction and magnitude compared to the non-polar surfaces. [ABSTRACT FROM AUTHOR]

Quantum-chemical calculations are combined to a model Frenkel-Holstein Hamiltonian to assess the nature of the lowest electronic excitations in the pentacene crystal. We show that an admixture of charge-transfer excitations into the lowest singlet excited states form the origin of the Davydov splitting and mediate instantaneous singlet exciton fission by direct optical excitation of coherently coupled single and double exciton states, in agreement with recent experiments. [ABSTRACT FROM AUTHOR]