Dirk Veldman

Compositional and electric field dependence of the dissociation of charge transfer excitons in alternating polyfluorene copolymer/fullerene blends

The electro-optical properties of thin films of electron donor-acceptor blends of a fluorene copolymer (PF10TBT) and a fullerene derivative (PCBM) were studied. Transmission electron microscopy shows that in these films nanocrystalline PCBM clusters are formed at high PCBM content. For all concentrations, a charge transfer (CT) transition is observed with absorption spectroscopy, photoluminescence, and electroluminescence. The CT emission is used as a probe to investigate the dissociation of CT excited states at the donor-acceptor interface in photovoltaic devices, as a function of an applied external electric field and PCBM concentration. We find that the maximum of the CT emission shifts to lower energy and decreases in intensity with higher PCBM content. We explain the red shift of the emission and the lowering of the open-circuit voltage (V(OC)) of photovoltaic devices prepared from these blends with the higher relative permittivity of PCBM (epsilon(r) = 4.0) compared to that of the polymer (epsilon(r) = 3.4), stabilizing the energy (E(CT)) of CT states and of the free charge carriers in blends with higher PCBM concentration. We show that the CT state has a short decay time (tau = ca. 4 ns) that is reduced by the application of an external electric field or with increasing PCBM content. The field-induced quenching can be explained quantitatively with the Onsager-Braun model for the dissociation of the CT states when including a high electron mobility in nanocrystalline PCBM clusters. Furthermore, photoinduced absorption spectroscopy shows that increasing the PCBM concentration reduces the yield of neutral triplet excitons forming via electron-hole recombination, and increases the lifetime of radical cations. The presence of nanocrystalline domains with high local carrier mobility of at least one of the two components in an organic heterojunction may explain efficient dissociation of CT states into free charge carriers.

Triplet formation involving a polar transition state in a well-defined intramolecular perylenediimide dimeric aggregate

A cofacially stacked perylenediimide (PDI) dimer with a xanthene linker was studied under a variety of conditions (solvent, temperature) and serves as a model for the molecular interactions occurring in solid films. Intrinsically, the PDI units have a fluorescence quantum yield (Phi F) close to unity, but Phi F is lowered by a factor of 6-50 at room temperature when two PDI moieties are held in a cofacial arrangement, while the decay time of the most emissive state is increased significantly (tau F = 27 ns in toluene) compared to a monomeric PDI molecule (tau F = 4 ns). Fluorescence measurements show a strong solvent and temperature dependence of the characteristics of the emissive excited state. In a glassy matrix of toluene (TOL) or 2-methyltetrahydrofuran (2-MeTHF), Phi F is high, and the decay time is long (tau F = approximately 50 ns). At higher temperature, both Phi F and tau F are reduced. Interestingly, at room temperature, Phi F and tau F are also reduced with increasing solvent polarity, revealing the presence of a polar transition state. Photoinduced absorption of the stacked molecules from the picosecond to the microsecond time scale shows that after photoexcitation reorganization occurs in the first nanoseconds, followed by intersystem crossing (ISC), producing the triplet excited state. Using singlet oxygen ( (1)Delta g) luminescence as a probe, a triplet quantum yield (Phi T) greater than 50% was obtained in air-saturated 2-Me-THF. Triplet formation is exceptional for PDI chromophores, and the enhanced ISC is explained by a decay involving a highly polar transition state.

Helical aromatic oligoamide foldamers as organizational scaffolds for photoinduced charge transfer

Here we report the synthesis and characterization of four quinoline-derived foldamers with increasing oligomeric length; dimer O2P, tetramer O4P, pentamer O5P, and nonamer O9P functionalized with on one end an oligo(p-phenylene vinylene) (OPV) and on the other end a perylene bisimide (PB) chromophore. (1)H NMR confirms the formation of the expected folded structures in both toluene and chloroform solution. The structural predictability and rigidity of the oligomeric series enabled us to investigate the effect of a helical bridge and chromophore position on the photoinduced processes in the electron OPV-PB donor-acceptor pair in chloroform and toluene. The helical properties of the bridge ensured that the chromophore separation distance through space is different from the separation distance through the bridge. For all foldamer-solvent combinations studied, excitation of either OPV or PB results in nearly quantitative quenching of the fluorescence indicating a fast charge separation reaction between the OPV and PB. Femtosecond photoinduced absorption measurements confirmed the fast formation of a charge-separated state. The recombination reaction involves a combination of direct decay to the ground state and the formation of an intermediate triplet state, with their balance depending on the foldamer-solvent combination. Molecular orbital calculations rationalize the fast photoinduced charge separation, by revealing that the bridging foldamer mediates the charge transfer from donor to acceptor via the superexchange mechanism. Remarkably low attenuation factors (beta(CS) approximately 10(-2) A(-1)) were obtained using either through space or through bridge separation distance. However, in these calculations only three of the four foldamers show the expected linear behavior between the logarithm of the charge separation rate constant and the distance between the chromophores. The combined results show when a helical bridge is separating the charge transfer couple, hampering the usefulness of a uniform description of the charge-separation phenomena.

PbSe Nanocrystal Network Formation during Pyridine Ligand Displacement

Solution-phase pyridine treatment displaced oleic acid capping ligands from the surface of PbSe nanocrystals. During ligand displacement the nanostructure morphology dramatically changed from well-stabilized, individual nanocrystals to form crystallographically connected nanocrystal networks. The network morphology was governed by the diameter of the constituent nanocrystals. Larger nanocrystals showed dipolar alignment but maintained individual nanocrystal character, while smaller nanocrystals crystallographically fused along the <100> axis. Optical studies of nanocrystal thin films showed that pyridine ligand displacement quenches the nanocrystal photoluminescence. Blends of nanocrystals and conjugated polymer showed photoluminescence quenching of the polymer with increasing nanocrystal content. The extent of photoluminescence quenching was only weakly dependent on the nanocrystal size or surface chemistry, suggesting that the interaction between nanocrystal and polymer is mostly in the form of energy transfer rather than charge transfer.

Enhanced Intersystem Crossing via a High Energy Charge Transfer State in a Perylenediimide−Perylenemonoimide Dyad

The electronic relaxation processes of a photoexcited linear perylenediimide-perylenemonoimide (PDI-PMI) acceptor-donor dyad were studied. PDI-PMI serves as a model compound for donor-acceptor systems in photovoltaic devices and has been designed to have a high-energy PDI (-*)-PMI (+*) charge transfer (CT) state. Our study focuses on the minimal Gibbs free energy (Delta G ET) required to achieve quantitative CT and on establishing the role of charge recombination to a triplet state. We used time-resolved photoluminescence and picosecond photoinduced absorption (PIA) to investigate excited singlet (S 1) and CT states and complemented these experiments with singlet oxygen ( (1)Delta g) luminescence and PIA measurements on longer timescales to study the population of triplet excited states (T 1). In an apolar solvent like cyclohexene (CHX), photoinduced electron transfer does not occur, but in more polar solvents such as toluene (TOL) and chlorobenzene (CB), photoexcitation is followed by a fast electron transfer, populating the PDI (-*)-PMI (+*) CT state. We extract rate constants for electron transfer (ET; S 1-->CT), back electron transfer (BET; S 1<--CT), and charge recombination (CR) to lower-energy states (CT-->S 0 and CT-->T 1). Temperature-dependent measurements yield the barriers for the transfer reactions. For ET and BET, these correspond to predictions from Marcus-Jortner theory and show that efficient, near quantitative electron transfer ( k ET/ k BET >or= 100) can be obtained when Delta G ET approximately -120 meV. With respect to triplet state formation, we find a relatively low triplet quantum yield (Phi T < 25%) in CHX but much higher values (Phi T = 30-98%) in TOL and CB. We identify the PDI (-*)-PMI (+*) state as a precursor to the T 1 state. Recombination to T 1, rather than to the ground-state S 0, is required to rationalize the experimental barrier for CR. Finally, we discuss the relevance of these results for electron donor-acceptor films in photovoltaic devices.

Synthesis and photophysical properties of conjugated polymers with pendant 9,10-anthraquinone units

We have synthesized and investigated the photophysical properties of a series of electron-donor conjugated copolymers with pendant electron-acceptor units. The copolymers consist of diethynyl-1,4-phenylene, fluorene, or phenylene rings alternating with a phenylene unit bearing a pendant 9,10-anthraquinone moiety. The pendant donor-acceptor polymers were designed to have different optical pi-pi* band gaps, while the oxidation potential of the polymer backbone remains approximately constant in the series. The reduction potential of the donor-acceptor polymers is associated with the pendant acceptor units. This leads to the special situation that the electrochemical gap between oxidation and reduction potentials is constant, while the optical band gap decreases, going from PPP, via PPF, to PPE. This design is used to study the effect of the optical gap on the photoinduced electron-transfer reaction that occurs between the main chain electron donor and the pendant acceptor, while the same polymer architecture and energy of the charge separated state are maintained. Fluorescence and photoinduced absorption spectroscopy are used to study the electron transfer following photoexcitation in relation to solvent polarity and in thin solid films. For the fluorene-phenylene alternating copolymer, intramolecular photoinduced electron transfer occurs in the Marcus optimal region.

A convergent synthesis of (diphenylvinyl)benzene (DPVB) star-shaped compounds with tunable redox, photo- and electroluminescent properties

In this paper we describe a convenient and convergent approach to the synthesis of novel and highly luminescent (diphenylvinyl)benzene (DPVB)-based conjugated star-shaped molecules, which can be used as emitting dyes in doped OLEDs. The versatility of this synthetic route allows tuning of the HOMO–LUMO energy levels of the materials by means of the introduction of different electroactive peripheral groups. Two related star-shaped dyes have been prepared, one with amine and one with alkoxy end groups, and have been characterized using optical spectroscopy and cyclic voltammetry. The energy levels of the dyes are found to lie within the band gap of a polyfluorene (PF) derivative such that energy transfer from the photoexcited PF to the dyes takes place. In OLEDs that contain 1 wt% of dye in PF, the emission is predominantly from the dye. Although the EL spectra are similar for the two dyes, other device characteristics differ greatly. OLEDs using the amine-terminated dye have a luminance that is a few orders of magnitude lower than that based on the alkoxy-terminated dye. The differences are explained by the positioning of the HOMO and LUMO levels of the two dyes compared to those of PF. The dye with the amine end group can act as a deep trap for holes, reducing charge transport in the film.