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Metal clusters with appropriate molecular ligands have been shown to be suitable subnanometer building blocks for supramolecular architectures with controlled secondary interactions, providing access to physical regimes not achievable with conventional intermolecular motifs. An example is the excimer photophysics exhibited by individual cluster-based superstructures produced by top-down etching of gold nanoparticles. Now, a supramolecular architecture of copper clusters is presented with controlled optical properties and efficient non-resonant luminescence produced via a novel bottom-up synthesis using mild green reductants followed by a ligand exchange reaction and spontaneous supramolecular assembly. Spectroscopic experiments confirm the formation of the intercluster network and reveal the permanent nature of their excimer-like behavior, thus extending the potential impact and applicability of metal cluster superstructures as efficient and stable non-resonant single-particle emitters.

Small conjugated molecules (SM) are gaining momentum as an alternative to semiconducting polymers for the production of solution-processed bulk heterojunction (BHJ) solar cells. The major issue with SM-BHJs is the low carrier mobility due to the scarce control on the phase-segregation process and consequent lack of preferential percolative pathways for electrons and holes to the extraction electrodes. Here, a new paradigm for fi ne tuning the phase-segregation in SM-BHJs, based on the post-deposition exploitation of latent hydrogen bonding in binary mixtures of PCBM with suitably functionalized electron donor molecules, is demonstrated. The strategy consist in the chemical protection of the H-bond forming sites of the donor species with a thermo-labile functionality whose controlled thermal cleavage leads to the formation of stable, crystalline, phase-separated molecular aggregates. This approach allows the fi ne tuning of the nanoscale fi lm connectivity and thereby to simultaneously optimize the generation of geminate carriers at the donor‐acceptor interfaces and the extraction of free charges via ordered phase-separated domains. As a result, the PV effi ciency undergoes an over twenty-fold increase with respect to control devices. This strategy, demonstrated here with mixtures of diketopyrrolopyrrole derivatives with PCBM can be extended to other molecular systems for achieving highly effi cient SM-BHJ solar cells.

White electroluminescence and fine-tuning of the emission color from binary blends of a blue-emitting polymer and a green/yellow-emitting threaded molecular wire consisting of a conjugated polymer supramolecularly encapsulated by functionalized cyclodextrins are demonstrated. Encapsulation controls the minimum intermolecular distance on the nanoscale, resulting in suppressed energy-transfer between the blend constituents and reduced formation of interchain charge-transfer complexes. The use of a green-emitting polyrotaxane significantly improves the electrical properties with respect to blends of a blue electroluminescent polyrotaxane and leads to a significant reduction in the turn-on voltage required for achieving white electroluminescence (V ON = 3V), with only 20% by weight of the encapsulated material. Blends of a blue-emitting polymer and a green/yellow-emitting threaded molecular wire emit white luminescence due to control of the minimum intermolecular distance at the nanoscale, which results in suppressed energy transfer between the blend constituents and reduced formation of interchain charge-transfer complexes. Light-emitting diodes incorporating only 20% by weight of the encapsulated material display white electroluminescence with a turn-on voltage of just 3 V. Copyright © 2012 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim.

Highly oriented luminescent films are produced by stretching a 30-μm-thick polyvinyl alcohol matrix doped with water-soluble polyrotaxanes and their unthreaded analogues. Photoluminescence experiments reveal that over 95% of the emitted light is polarized along the orientation direction. A hybrid organic-inorganic light-emitting diode is built to investigate the possibility of using these films as polarizing filters for solid-state lighting and display technology. © 2011 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim.

A comparative study of the self-assembly at a variety of surfaces of a dithiophene rotaxane 1⊂β-CD and its corresponding dumbbell, 1, by means of atomic force microscopy (AFM) imaging and scanning tunneling microscopy (STM) imaging on the micrometer and nanometer scale, respectively. The dumbbell is found to have a greater propensity to form ordered supramolecular assemblies, as a result of π-π interactions between dithiophenes belonging to adjacent molecules, which are hindered in the rotaxane. The fine molecular structure determined by STM was compared to that obtained by molecular modelling. The optical properties of both rotaxane and dumbbell in the solid state were investigated by steady-state and time-resolved photoluminescence (PL) experiments on spin-cast films and diluted solutions. The comparison between the optical features of the threaded and unthreaded systems reveals an effective role of encapsulation in reducing aggregation and exciton migration for the rotaxanes with respect to the dumbbells, thus leading to higher PL quantum efficiency and preserved single-molecule photophysics for longer times after excitation in the threaded oligomers. © 2011 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim.

A systematic investigation is reported into the influence of the counter cations on the optical, electrical and electroluminescent properties of polyelectrolytic conjugated polymers and of their cyclodextrin-threaded rotaxanes. We compare conjugated polyelectrolytes with sulfonated side groups where the anionic charge is balanced by Li+, K+, Cs +, tetramethylammonium (Me4N+) and cryptate-encapsulated potassium (K+ at [2.2.2]). Narrowing (for the unthreaded analogues) and a slight red-shift of the absorption spectra (for the rotaxanes) are found upon exchange of Li+ for larger cations, together with a blue-shift and an efficiency enhancement of the luminescence. These effects are similar in nature to those induced by rotaxination, and are therefore assigned to a marked reduction of intermolecular interactions between the conjugated cores. Exchange of Li+ for K+, Cs +, or Me4N+ results in a higher electroluminescence external quantum efficiency (EQE) for both polyrotaxanes and unthreaded polymers. For polyrotaxane-based devices the EQE increased approximately 7 times upon substitution of Li+ with Cs+ or Me4N+, thereby demonstrating the importance of the selection of the counter-cations for optimizing the performance of polyelectrolytic conjugated polymers in light-emitting devices. © 2008 WILEY-VCH Verlag GmbH and Co. KGaA.

The energy levels belonging to the three lowest J multiplets of the Er3+ ion in Er2O3 and ErF3 were determined by means of high-resolution optical absorption spectroscopy. A crystal-field analysis of these data by means of Newman's Superposition Model was performed, allowing to determine intrinsic parameters which are quite consistent with other oxides and fluorides and to check the validity of the transferability postulate within these compounds. The possibility to use these data to analyze crystalline nanophases in glasses is briefly discussed. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Radiation-induced spectral modifications of the 5 eV excited photoluminescence (PL) of oxygen vacancies have been found in a nanostructured silica-based material. This system, made up of SnO2 nanocrystals embedded in amorphous SiO2, is generated by nano-clustering in oversaturated Sn-doped silica produced by sol-gel method. Treatment in oxygen-poor atmosphere introduces oxygen deficiency in the glassy host and a photo-reactive SnO-like interphase at the SnO2 nanocrystal surface. PL measurements initially show native PL activity at 2.7 eV excited at 5 eV, ascribable to oxygen vacancies in intrinsic two-fold coordinated Si sites. Prolonged UV irradiation at 4.7 eV (fourth harmonic of Nd-YAG pulsed laser) gives rise to extrinsic PL activity at 3.1 eV, ascribable to Sn-variant of oxygen vacancy. The results suggest that the UV irradiation causes a progressive release of Sn from the nanophase surface to the glassy network in oxygen vacancy sites. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim).

Intracellular pH is a key parameter in many biological mechanisms and cell metabolism and is used to detect and monitor cancer formation and brain or heart diseases. pH-sensing is typically performed by fluorescence microscopy using pH-responsive dyes. Accuracy is limited by the need for quantifying the absolute emission intensity in living biological samples. An alternative with a higher sensitivity and precision uses probes with a ratiometric response arising from the different pH-sensitivity of two emission channels of a single emitter. Current ratiometric probes are complex constructs suffering from instability and cross-readout due to their broad emission spectra. Here, we overcome such limitations using a single-particle ratiometric pH probe based on dot-in-bulk CdSe/CdS nanocrystals (NCs). These nanostructures feature two fully-separated narrow emissions with different pH sensitivity arising from radiative recombination of core- and shell-localized excitons. The core emission is nearly independent of the pH, whereas the shell luminescence increases in the 3–11 pH range, resulting in a cross-readout-free ratiometric response as strong as 600%. In vitro microscopy demonstrates that the ratiometric response in biologic media resembles the precalibralation curve obtained through far-field titration experiments. The NCs show good biocompatibility, enabling us to monitor in real-time the pH in living cells.

The design of nanostructured materials with specific physical properties is generally pursued by tuning nanoparticle size, concentration, or surface passivation. An important step forward is to realize “active” systems where nanoparticles are vehicles for controlling, in situ, some specific, tuneable features of a responsive functional material. In this perspective, this work focuses on the rational design of a nanostructured glass with electrically tuneable dielectric function obtained by injection and accumulation of charge on embedded conductive nanocrystals. This enables electrically controlled switching of semiconducting nanophases to charged polarisable states to be achieved, which could lead to smart, field-enhancement applications in nanophotonics and plasmonics. Here, it is shown that such response switching can be obtained if a percolating charge-transport mechanism is activated through a disordered tree-like network, as is demonstrated to be possible in SiO2 films where suitable dispersions of SnO2 nanocrystals, with conductive interfaces, are obtained as a result of a new synthesis strategy.

The cover picture shows the structure of 5-aza[5]helicene (top) and the crystal packing (single enatiomers, bottom). This is one of the aza[5]helicenes reported in the paper that were synthesized by photochemical ring closure of suitable ethenyl derivatives. The photophysical properties (very long triplet-state lifetime) and crystal packing (columnar arrangement) were studied and shown to be dependent on the nitrogen position. These molecules may be suitable for studies on π−π interactions and complexation with metal ions. The extremely long triplet lifetime that was observed (in the range of seconds) makes these molecules promising candidates for practical applications in photo- and optoelectronics. Details are discussed in the article by T. Caronna et al. on p. 1247 ff.