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Photoluminescence studies of the charge transfer exciton emission from a narrow-bandgap polymer-based bulk heterojunction are reported. The quantum yield of this emission is as high as 0.03%. Low temperature measurements reveal that while the dynamics of the singlet exciton is slower at low temperature, the dynamics of the charge transfer exciton emission is temperature independent. This behavior rules out any diffusion process of the charge transfer excitons and energy transfer from these interfacial states toward lower lying states. Photoluminescence measurements performed on the device under bias show a reduction (but not the total suppression) of the charge transfer exciton recombination. Finally, based on the low temperature results the role of the charge transfer excitons and the possible pathways to populate them are identified.

Blends of poly(3-hexylthiophene) (P3HT) and the bis-adduct of [6,61-pheuyl-C\61-butyric acid methyl ester (bisPCBM) show enhanced performances in bulk-heterojunction solar cells compared to P3HT:PCBM thin films due to their higher open-circuit voltage. However, it is not clear whether the decrease of the shortcircuit current observed in P3HT-bisPCBM blends originates from the 100 mV reduction of the offset between the lowest unoccupied molecular orbitals of the donor and the acceptor or from, a change in the morphology. The analysis of the photoluminescence dynamics of the various bulk heterojunctions provides information on the dependence of the electron transfer process on their microstructure. We find that in solution, where the donor-acceptor distribution is homogeneous, the photoluminescence dynamics is the same for the bis- and PCBM-based blends, while in thin films the first shows a slower dynamics than the second. This result indicates that the reduction of the LUMO offset of ∼100 meV does not influence the electron transfer efficiency but that the diversity between the photoluminescence dynamics in thin films should be ascribed to the different microstructure of the bulk heterojunctions fabricated with the two acceptors. © 2009 American Chemical Society.

Systematic control of 3D energy transfer (ET) dynamics is achieved in supramolecular nanostructured host-guest systems using spacer-functionalized guest chromophores. Quantum chemistry-based Monte Carlo simulations reveal the strong impact of the spacer length on the ET dynamics, efficiency, and dimensionality. Remarkably high exciton diffusion lengths demonstrate that there is ample scope for optimizing oligomeric or polymeric optoelectronic devices.

Polymer-fullerene bilayer heterostructures are suited to study excitonic processes in conjugated polymers. Excitons are efficiently quenched at the polymer-fullerene interface, whereas the polymer-vacuum interface is often considered as an exciton-reflecting interface. Here, we report about efficient exciton quenching close to the polymer-vacuum interface of spin-coated MDMO-PPV (poly[2-methoxy-5-(2′-ethyl-hexyloxy)-p-phenylenevinylene]) films. The quenching efficiency is estimated to be as high as that of the polymer-fullerene interface. This efficient quenching is consistent with enhanced intermolecular interactions close to the polymer-vacuum interface due to the formation of a "skin layer" during the spin-coating procedure. In the skin layer, the polymer density is higher; that is, the intermolecular distances are shorter than in the rest of the film. The effect of exciton quenching at the polymer-vacuum interface should be taken into account when the thickness of the polymer film is on the order of the exciton diffusion length; in particular, in the determination of the exciton diffusion length. © 2009 American Chemical Society.

The synthesis and characterization of novel fluorinated Er3+ complexes emitting at the telecommunications wavelengths are presented. These new knowledge-based materials have been designed and optimized on the basis of an extensive photophysical investigation on the processes controlling the overall quantum yield, namely (i) the energy transfer process from the light harvesting conjugated antenna to the emitting ion and (ii) the nonradiative quenching of the lanthanide emission. The study of their photophysical properties has been used for the assessment of their application as active materials in a new generation of low-cost, optical communication amplifiers, based on solution-processed planar waveguides pumped by light emitting diodes (LEDs). Suitable ligands possessing a high-absorption cross-section (where commercially available LEDs operate), good energy level matching those of the ion, and high solubility in fluorinated polymers have been developed. © 2009 American Chemical Society.

The temperature dependence of the exciton dynamics in a conjugated polymer is studied using time-resolved spectroscopy. Photoluminescence decays were measured in heterostructured samples containing a sharp polymer-fullerene interface, which acts as an exciton quenching wall. Using a 1D diffusion model, the exciton diffusion length and diffusion coefficient were extracted in the temperature range of 4-293 K. The exciton dynamics reveal two temperature regimes: in the range of 4-150 K, the exciton diffusion length (coefficient) of approximately 3 nm (approximately 1.5 x 10 (-4) cm2/s) is nearly temperature independent. Increasing the temperature up to 293 K leads to a gradual growth up to 4.5 nm (approximately 3.2 x 10 (-4) cm2/ s). This demonstrates that exciton diffusion in conjugated polymers is governed by two processes: an initial downhill migration toward lower energy states in the inhomogenously broadened density of states, followed by temperature activated hopping. The latter process is switched off below 150 K.

Organic semiconductorshave longbeen investigated for their potential of enhancing laser technology owing to their large conversion efficiency, gain cross-section, and bandwidth. Oligophenylene-based p-conjugated materials, having high carrier mobilities (up to 14 cm V 1 s ) and emission yields (ca. 30%), represent an important class of organic semiconductors for which amplified spontaneous emission (ASE) and low-threshold lasing action have been achieved in single crystals and ordered thin films. The last decade has also witnessed great advances in the growth of ordered oligophenylenes thin films with enhanced optoelectronic properties, for example, through surface dipole-assisted heteroepitaxy. The resulting needle-shaped aggregates, usually referred to as nanofibers, feature charge-carrier mobilities that range from ca. 10 2 to ca. 10 cm V 1 s , depending on fabrication technique, self-guiding of optical emission for light amplification, and lasing under femtosecond excitation. In particular para-sexiphenyl (p-6P) nanofibers have been highlighted as prototypical material system for self-assembled organic lasers, with large stimulated-emission cross-section

The great progress in the growth of ordered thin films of small linear molecules achieved in the past decade has made control of supramolecular arrangement and its effect on the physical properties frontline challenges for materials science. During film growth, multiple nucleation results in small, randomly oriented crystal domains. It has been recently shown that dipole-assisted, heteroepitaxial techniques allow deposition of thin films with unusual long-range order, consisting of crystalline aggregates that self-assemble to form nanofibers with sub-micrometer cross section, lengths of up to millimeters, orientational alignment extending to the centimeter size range, and controllable interaggregate distance. [1–3] These nanofibers have shown remarkable optical functionalities: photo- and electroluminescence, [2,4] optical waveguiding, [5,6] frequency doubling, [7] and optical gain. [8,9] Very recently, we have also shown that, depending on the growth condition, a single nanofiber behaves as a low-threshold self-assembled laser resonator, with randomly distributed optical feedback provided by 100 nm wide fiber breaks. [10] The high sensitivity of

Erbium quinolinolates, commonly assumed to be mononuclear species with octahedral co-ordination geometry, have been proposed as promising materials for photonic devices but difficulties in obtaining well defined products have so far limited their use. We report here the conditions to obtain in high yields three different kinds of pure neutral erbium quinolinolates by mixing an erbium salt with 8-quinolinol (HQ) and 5,7-dihalo-8-quinolinol (H5,7XQ: X=Cl and Br): i) the trinuclear complex Er 3 Q 9 (1) which is obtained with HQ deprotonated by NH 3 in water or ethanol/water mixtures; ii) the already known dimeric complexes based on the unit [Er(5,7XQ) 3 (H 2 O) 2 ] [X=Cl (2) and Br (3)]; iii) the mononuclear [Er(5,7XQ)2(H5,7XQ) 2 Cl] [X= Cl (4) and Br (5)] complexes, obtained in organic solvents without base addition, where the ion results coordinated to four ligands, two deprotonated chelating, and two as zwitterionic monodentate oxygen donors. These results represent a further progress with respect to a recent reinvestigation on this reaction, which has shown that obtaining pure and anhydrous octahedral ErQ 3 , the expected reaction product, is virtually impossible, but failed in the isolation of 1 and of the neutral tetrakis species based on H5,7XQ ligands. Structural data provide a detailed description of the molecules and of their packing which involves short contacts between quinoxaline ligands, due to π-π interactions. Electronic and vibrational studies allow to select the fingerprints to distinguish the different products and to identify the presence of water. The structure/property relationship furnishes a satisfactory interpretation of the photo-physical properties. Experimental evidence confirms that the most important quenchers for the erbium emission are the coordinated water molecules and shows that the ligand emission is significantly affected by the π-π interactions.

One-dimensional light amplification in individual p-sexiphenyl nanofibers is investigated. The influence of fiber morphology on light propagation properties is studied via optical and atomic force microscopy. Isolated nanofibers are shown to yield low-threshold random laser emission in the deep blue. Model calculations of coherent light propagation in one-dimensional random media qualitatively reproduce the experimental results. Implications for photonic nanosensors are briefly discussed.

We report the first combined optical and structural investigation of the water free Er-quinolinolate complex, an organo-lanthanide system of interest for 1.5-mum telecom applications. Structural data demonstrate that the complex has a trinuclear structure (Er(3)Q(9)) which provides the Er metals with an octa-coordination by the organic ligand and prevents solvent and water molecules from entering the lanthanide coordination sphere. The results of the structural analysis allow us to infer that the strong Er luminescence quenching exhibited by the Er(3)Q(9) complex is due uniquely to resonant energy transfer to the aromatic C-H vibrations of the ligand, providing the correct tools to design more efficient emitters.

We investigate sensitization efficiency of near-infrared emission and population saturation of trivalent erbium in erbium-quinolinolato complexes photoexcited into the absorption band of the organic sensitizer. At low-excitation levels, we find high (∼80%) sensitization efficiencies. We observe excited-state populationsaturationatinversionthresholdundersubnanosecondpumpingatthe level of one injected photoexcitation per complex. SECTION Kinetics, Spectroscopy

The photophysical characterization of three soluble derivatives of single-wall carbon nanotubes (SWCNTs), functionalized with poly(ethylene glycol) (PEG), co-functionalized with PEG and aminofluorene and co-functionalized with PEG and aminoanthracene is reported. The peculiar excellent solubility of these derivatives allows, for the first time in covalently functionalized SWCNTs, the study of their excitation dynamics by monitoring the near-infra-red emission. Moreover, the aminoanthracene derivative shows higher photoluminescence efficiency in the visible range than the aminofluorene derivative, demonstrating the possibility to tune extensively the photophysical properties of these functionalized SWCNTs. (C) 2009 Elsevier Ltd. All rights reserved.

In this letter we compare the transistor performances of two solution-processed perylene derivatives: N,N'-bis (n-octyl)- dicyanoperylene-3,4:9,10-bis(dicarboximide) (PDI8-CN(2)) and N,N'-1H,1H-perfluorobutyl dicyanoperylenediimide (PDIF-CN(2)). Perylenediimide nitrogen functionalization with perfluoroalkyl vs. alkyl chains improves the electron mobility of solution-processed organic field effect transistors (OFETs) by one order of magnitude. Time resolved spectroscopy allows attributing this increment to a higher degree of co-facial arrangement of the fluorinated molecules. This supramolecular arrangement enhances the pi-pi overlap leading to more efficient electron transport.

The interaction dynamics between excitons and intramolecular vibrations is investigated in alpha-sexithiophene crystals using transient pump-probe spectroscopy. The ultrafast fission of vibrons, i.e., exciton states bound to intramolecular vibrations, predicted in the 1960s by Davydov and Rasbha, is observed. The fission process is monitored by stimulating the optical transitions from the outgoing scattered states, the hot ground state C-C stretching vibrations, to the lowest exciton bands. The vibrational energy is dissipated in 1.5+/-0.3 ps.

We investigate the quenching of the near infrared light emission in Er3+ complexes induced by the resonant dipolar interaction between the rare-earth ion and high frequency vibrations of the organic ligand. The nonradiative decay rate of the lanthanide ion is discussed in terms of a continuous medium approximation, which depends only on a few, easily accessible spectroscopic and structural data. The model accounts well for the available experimental results in Er3+ complexes, and predicts an ∼100% light emission quantum yield in fully halogenated systems.

We report on the morphological and nonlinear optical properties of highly ordered para-sexiphenyl nanofibers grown by hot-wall epitaxy on muscovite mica. For suitable substrate temperatures and deposition times, needle-shaped aggregates are obtained. These nanoaggregates, which are formed by regrouping of small islands of the material, can support optical waveguiding and amplification of the deep-blue emission of para-sexiphenyl. We show evidence of low-threshold coherent random lasing in ensembles of nanofibers and study the dependence of lasing on the nanofibers’ film thickness. We also present spatially resolved lasing measurements on individual nanofiber segments. The achievement of isolated nanofibers with suitable morphology and low optical losses could open the route to unprecedented photonic and optoelectronic devices at the submicrometric scale.