Pi-conjugated Oligomers Research Articles

Linear π-conjugated systems derivatized with C60-fullerene as molecular heterojunctions for organic photovoltaics

This tutorial review covers recent contributions in the area of linear pi-conjugated systems bound to fullerenes in view of their application as active materials in photovoltaic devices. The first part discusses the concepts of double-cable polymer and molecular hetero-junction and presents several examples of chemically or electrochemically synthesized C60-derivatized conjugated polymers. The second and main part of the article concerns the various classes of C60-derivatized pi-conjugated oligomers designed in view of their utilization in single-component photovoltaic devices. Thus, C60-containing pi-conjugated systems such as oligoarylenevinylenes, oligoaryleneethynylenes and oligothiophenes are discussed on the basis of the relationships between molecular structure, photophysical properties and performances of the derived photovoltaic devices. A brief last section presents some recent examples of surface-attached molecular hetero-junctions based on self-assembled monolayers and discusses possible routes for future research.

Excitation energies of π-conjugated oligomers within time-dependent current-density-functional theory

We study the pi*<--pi singlet excitations of the pi-conjugated oligomers of polyacetylene, polydiacetylene, polybutatriene, polythiophene, poly(para-phenylene vinylene), and the lowest singlet excitations of the hydrogen chain. For this we used time-dependent current-density-functional theory within the Vignale-Kohn and adiabatic local density approximations. By studying the dependence of the excitation spectrum on the chain length we conclude that the reduction of the static polarizability when using the Vignale-Kohn functional has two origins. First, the excitation energies of transitions with a large transition dipole are shifted upward. Second, the character of the transition between the lowest occupied and highest unoccupied molecular orbitals and the oscillator strength of the lowest transition within the adiabatic local density approximation is transferred to higher transitions. The lowest transitions that have a considerable oscillator strength obtained with the Vignale-Kohn functional have excitation energies that are in most cases in better agreement with available reference data than the adiabatic local density approximation.

Oscillation of Conductance in Molecular Junctions of Carbon Ladder Compounds

The electrical conductances of dithiolates of polyacene (PA(n)DTs) and polyphenanthrene (PPh(n)DTs), which are typical carbon ladder compounds, are calculated by means of the Landauer formulation combined with density functional theory, where n is the number of benzene rings involved. Surface Green function used in the Landauer formulation is calculated with the Slater-Koster parameters. Attention is turned to the wire-length dependence of the conductances of PA(n)DTs and PPh(n)DTs. The damping of conductance of PA(n)DTs is much smaller than that of PPh(n)DTs because of the small HOMO-LUMO gaps of PA(n)DTs. PA(n)DTs are thus good molecular wires for nanosized electronic devices. Conductance oscillation is found for both molecular wires when n is less than 7. The electrical conductance is enhanced in PA(n)DTs with even-numbered benzene rings, whereas it is enhanced in PPh(n)DTs with odd-numbered benzene rings. The observed conductance oscillation of PA(n)DTs and PPh(n)DTs is due to the oscillation of orbital energy and electron population. Other pi-conjugated oligomers (polyacetylene-DT, oligo(thiophene)-DT, oligo(meso-meso-linked zinc(II) porphyrin-butadiynylene)-DT, oligo(p-phenylethynylene)-DT, and oligo(p-phenylene)-DT) are also studied. In contrast to PA(n)DTs and PPh(n)DTs, the five molecular wires show ordinary exponential decays of conductance.

Temperature dependent exciton emission from herringbone aggregates of conjugated oligomers.

In this work, the effect of temperature, exciton bandwidth, and size on the photoluminescence spectra of defect-free two-dimensional herringbone aggregates of pi-conjugated oligomers such as oligophenylene vinylene and oligothiophene is investigated theoretically. The model is based on exciton-phonon coupling in two-dimensional herringbone lattices with the exciton deriving from the lowest optical (1Ag-->1Bu) transition and the phonon from the most strongly coupled intramolecular vibrational mode with frequency omega0. Simple analytical expressions are obtained for the line strengths of the emission origin (0-0) and first replica (0-1) as a function of the number of molecules comprising the aggregate, N, the free exciton bandwidth, WD, and the temperature, T. At a given temperature, the 0-0 emission intensity initially scales as N/Nth, where Nth is the superradiant threshold number, but eventually converges to NT/Nth, where NT is the size independent thermal coherence number. NT is inversely proportional to temperature and proportional to the exciton band curvature (omegac) near the band bottom; NT=1+4piomegac/kbT. In striking contrast, the 0-1 line strength is relatively insensitive to temperature and size, but scales as the inverse square of WD+omega0. The insensitivity of the first replica to the exciton coherence number makes the ratio of the 0-0 to 0-1 line strengths a measure of the exciton coherence number. The ratio can be used to test for crystal purity. Comparison to experiments on thin films of quaterthiophene shows that the thermal coherence size is given by NT approximately 1+450/T (K) and that superradiance, which requires NT>Nth, can only be observed at temperatures less than 1 K.

Highly emissive supramolecular oligo(p-phenylene vinylene) dendrimers.

pi-Conjugated oligo(p-phenylene vinylene) (OPV) guest molecules for interaction with dendritic hosts were synthesized and fully characterized by NMR spectroscopy, MALDI-TOF-MS, elemental analysis and optical measurements. The binding properties of the five different OPV guests to a N,N-bis[(3-adamantyl ureido) propyl] methylamine host have been investigated. The guests that contained an aryl urea glycine spacer were bound with the highest association constant. Subsequently, an adamantyl urea modified fifth generation poly(propylene imine) dendrimer was synthesized as a multivalent host which contains 32 N,N-bis[(3-adamantyl ureido) propyl] amine binding sites. Size exclusion chromatography showed that 32 of the OPV guests strongly bind to the fifth generation adamantyl functionalized dendritic host. In the case of the supramolecular dendritic host/guest system smooth homogeneous thin films could be obtained by spin coating. The dendritic guest-host complexes showed a significantly higher emission upon binding then that of the individual molecules due to the three-dimensional orientation of the OPV guest molecules. In the solid state, this enhancement in luminescence was a factor of 10. The pi-conjugated oligomers are less aggregated in the supramolecular assemblies presumably because of a shielding effect of the bulky adamantyl units present in the hosts.

Effect of local molecular structure on the chain-length dependence of the electronic properties of thiophene-based pi-conjugated systems.

Three series of thiophene-based pi-conjugated oligomers built with different combinations of thiophene cycles and double bonds have been synthesized and characterized. The analysis of the chain length dependence of the electronic, electrochemical, and vibrational properties of the three series of oligomers has been carried out using cyclic voltammetry, UV-vis, IR, and Raman spectroscopies. These various investigations provide consistent results showing that incorporation of ethylenic linkages in an oligothiophene structure leads to a faster decrease of the HOMO-LUMO gap with chain extension due to the combined effects of enhanced planarity and lower overall aromatic character of the system. Although the incorporation of two consecutive double bonds in the system leads to a stabilization of the dicationic state, this structural modification does not produce the expected further decrease of the HOMO-LUMO gap at large chain extension. This phenomenon is discussed on the basis of an interplay between aromaticity and bond length alternation.

Acetylenic scaffolding on solid support: poly(triacetylene)-derived oligomers by Sonogashira and Cadiot-Chodkiewicz-type cross-coupling reactions.

We report the first synthesis of poly(triacetylene)-derived oligomers by Pd(0)-catalysed Sonogashira and Cadiot-Chodkiewicz-type cross-coupling reactions on solid support. Oligo(phenylene triacetylene)s, members of a new class of linearly pi-conjugated oligomers with all-carbon backbones, feature very high fluorescence intensities.

Design and synthesis of a 256-membered pi-conjugated oligomer library of regioregular head-to-tail coupled quater(3-arylthiophene)s.

The rapid solid-phase synthesis of pi-conjugated oligomers is demonstrated by utilizing the parallel and the "mix-and-split" methods for the preparation of a library of 256 regioregular head-to-tail coupled oligo(3-arylthiophene)s. Chemical diversity was introduced to the growing oligomer starting from four resin-bound 3-(p-X-phenyl)-2-silylthiophenes via an iterative sequence of iodinations and Suzuki cross-coupling reactions with four 3-(p-X-phenyl)thiophene boronic esters (X = CF(3), H, CH(3), OCH(3)). Liberation from the solid support with TFA and subsequent chromatographic purification by normal-phase LC-MS provided all 256 regioregular head-to-tail coupled quater(3-arylthiophene)s.

Conjugation-length dependence of spin-dependent exciton formation rates in pi-conjugated oligomers and polymers.

We have measured the ratio, r = sigma(S)/sigma(T) of the formation cross section, sigma of singlet and triplet excitons from polarons in pi-conjugated oligomer and polymer films, using a spectroscopic technique we developed recently. We discovered a universal relation between r and the conjugation length (CL): r(-1) depends linearly on CL-1, irrespective of the chain structure. Since r is directly related to the maximum possible electroluminescence quantum efficiency in organic light emitting diodes (OLED), our results indicate that polymers have an advantage over small molecules in OLED applications.

From solid-phase synthesis of pi-conjugated oligomers to combinatorial library construction and screening.

A feature article describing concepts for the solid-phase synthesis of pi-conjugated oligomers as material-related structures and the translation of the synthetic routes into combinatorial protocols.

Molecular engineering of C60-based conjugated oligomer ensembles: modulating the competition between photoinduced energy and electron transfer processes.

A series of novel and soluble C60-(pi-conjugated oligomer) dyads were synthesized, starting from suitably functionalized oligomer precursors (i.e., dihexyloxynaphthalene, dihexyloxynaphthalene-thiophene, and dihexyloxybenzene-thiophene). A systematic change in the nature of the oligomeric component allowed (i) tailoring the light absorption of the chromophore by shifting the ground-state absorption from the ultraviolet to the visible region and (ii) varying the oxidation potential of the donor. The resulting electro- and photoactive dyads were examined by electrochemical and photophysical means. In general, both singlet-singlet energy transfer and intramolecular electron transfer were found to take place and, most importantly, to compete with each other in the overall deactivation of the photoexcited oligomer. The selection of polar solvents in combination with the dihexyloxybenzene-thiophene donor shifted the reactivity from an all energy (1a; dihexyloxynaphthalene) to an all electron-transfer scenario (1d, dihexyloxybenzene-thiophene). Encouraged by the favorable electron-transfer properties of dyad 1d, we prepared photodiodes by embedding 1d between asymmetric metal contacts, which showed external monochromatic efficiencies (IPCE) close to 10% at the maximum absorption of the molecule.

Photophysics of pi-conjugated metal-organic oligomers: aryleneethynylenes that contain the (bpy)Re(CO)(3)Cl chromophore.

A comprehensive study of a series of four monodisperse, metal-organic pi-conjugated oligomers of varying length is reported. The oligomers are based on the aryleneethynylene architecture, and they contain a 2,2'-bipyridine-5,5'-diyl (bpy) metal binding unit. The photophysical properties of the free oligomers and their complexes with the (L)Re(I)(CO)(3)X chromophore (where L = the bpy-oligomer and X = Cl or NCCH(3)) were explored by a variety of methods including electrochemistry, UV-visible absorption, variable temperature photoluminescence (PL), transient absorption (TA), and time-resolved electron paramagnetic spectroscopy (TREPR). The absorption of the free oligomers and the metal complexes is dominated by the pi,pi* transitions of the pi-conjugated oligomers. The free oligomers feature a strong blue fluorescence that is quenched entirely in the (L)Re(I)(CO)(3)X complexes. The metal-oligomers feature a weak, relatively long-lived red photoluminescence that is assigned to emission from both the (3)pi,pi* manifold of the pi-conjugated system and the dpi Re --> pi* bpy-oligomer metal-to-ligand charge transfer ((3)MLCT) state. On the basis of a detailed analysis of the PL, TA, and TREPR results an excited-state model is developed which indicates that the oligomer-based (3)pi,pi* state and the (3)MLCT states are in close energetic proximity. Consequently the photophysical properties reflect a composite of the properties of the two excited-state manifolds.

ESR Characterization of Oligomeric Thiophene Materials

Abstract : The ESR of electro-oxidation products of orthogonally bridged dimers of alpha,alpha-coupled thiophene oligomers and spiro-fused phenylene-thiophene mixed oligomers was studied as a function of temperature. ESR measurements on the one electron oxidation products indicate that the charge remains on a single oligomer unit and does not hop between the two bridged oligomer chains for temperatures up to 300K. This is evident from the presence of hyperfine structure in ESR spectra for the dimers of short chain oligomers and the small temperature dependent line broadening for longer chain spiro dimers, characteristic of a low energy process such as the paramagnetic site moving along the oligomer chain length. The activation energy for a paramagnetic center moving along the oligomer chain length has been estimated at <100/cm. The incorporation of phenylene into the oligomer unit shifts oxidation potentials anodic and also inhibits the mobility of the polaron along the pi-conjugated oligomer segment. Evidence for formation of pi-pi dimers in solutions of 7-mT has also been obtained.