Bringing together compounds of intrinsically different functionality, such as inorganic nanostructures and organic molecules, constitutes a particularly powerful route to creating novel functional devices with synergetic properties found in neither of the constituents. We introduce nanophotonic functional elements combining two classes of materials, semiconductor nanocrystals and dyes, whose physical nature arises as a superposition of the properties of the individual components. The strongly absorbing rod-like nanocrystals focus the incident radiation by photopumping the weakly absorbing dye via energy transfer. The CdSe/CdS nanorods exhibit a large quantum-confined Stark effect on the single-particle level, which enables direct control of the spectral resonance between donor and acceptor required for nanoscopic Förster-type energy transfer in single nanorod-dye couples. With this far-field manipulation of a near-field phenomenon, the emission from single dye molecules can be controlled electrically. We propose that this effect could lead to the design of single-molecule optoelectronic switches providing building blocks for more complex nanophotonic circuitry.

The emission of semiconducting polyfluorenes is often accompanied by an undesired feature in the green spectral region. Whereas a number of previous investigations have argued in favor of a monomolecular origin of the emission species based on ketonic defects, recent experimental results suggested the necessity of excimer formation between individual fluorenone units. We provide a range of new evidence supporting the monomolecular origin of green band emission in polyfluorenes. Most importantly, we succeed in performing single‐molecule spectroscopy on fluorenone‐containing polyfluorene model compounds. Whereas most fluorenone‐containing molecules exhibit both blue backbone and green fluorenone emission independent of fluorenone concentration, it is the relative intensities of the two species which correlate strongly with the fluorenone concentration on the single‐molecule level. Furthermore, we consider a novel model compound with a bifacial arrangement of two fluorenone units. This compound does not provide any signatures of enhanced intramolecular excimer formation but does strongly indicate that concentration quenching effects occur once fluorenone units can interact electronically. The ability to detect on‐chain defect emission in a single polymer molecule demonstrates that photochemical reactions in conjugated polymers can be monitored by fluorescence spectroscopy down to the level of a few atoms, constituting an unprecedented degree of materials characterization.

Single oligo(phenylene-vinylene) molecules constitute model systems of chromophores in disordered conjugated polymers and can elucidate how the actual conformation of an individual chromophore, rather than that of an overall polymer chain, controls its photophysics. Single oligomers and polymer chains display the same range of spectral properties. Even heptamers support π-electron conjugation across ∼80°curvature, as revealed by the polarization anisotropy in excitation and supported by quantum chemical calculations. As the chain becomes more deformed, the spectral linewidth at low temperatures, often interpreted as a sign of aggregation, increases up to 30-fold due to a reduction in photophysical stability of the molecule and an increase in random spectral fluctuations. The conclusions aid the interpretation of results from single-chain Stark spectroscopy in which large static dipoles were only observed in the case of narrow transition lines. These narrow transitions originate from extended chromophores in which the dipoles induced by backbone substituents do not cancel out. Chromophores in conjugated polymers are often thought of as individual linear transition dipoles, the sum of which make up the polymer's optical properties. Our results demonstrate that, at least for phenylene-vinylenes, it is the actual shape of the individual chromophore rather than the overall chromophoric arrangement and form of the polymer chain that dominates the spectroscopic properties.Molecular-level engineering in plastic electronics requires a precise understanding of how a particular physical or chemical structure impacts on the physical properties of the material. Disorder effects on the ensemble level can often mask the subtle interplay between function and structure. Large macromolecules such as conjugated polymers are particularly prone to energetic disorder, which gives rise to substantial spectral broadening and is generally attributed to a "particle in a box"-like picture of varying chromophore lengths. 1 Disorder effects have commonly been investigated in matrix isolated materials, such as polyenes, where subtle interplays between molecular shape and electronic structure have been identified. 2-5 However, matrix isolation on its own is not sufficient to overcome disorder but merely helps to screen intermolecular effects. The intrinsic molecular properties themselves are accessible with single-molecule spectroscopy. Although this technique helps to overcome the ensemble limitations, a single polymer chain can still contain many chromophores. [6][7][8][9][10][11][12] Energy transfer between these chromophores can mask the true photophysics of the individual spectroscopic unit. Although polarization-resolved spectroscopy has yielded detailed insight into the conformation of the polymer chain, 7,8,11,[13][14][15] very little is known about the shape of the indiVidual chromophore. Because a physical bend can potentially interrupt the π-electron conjugation, the chromophore is generally thought to be linearly extended in space...

Single chains of the conjugated polymer polyfluorene are shown to exist in two distinct conformational arrangements. Planarization of the chain to the single molecule beta-phase leads to a red shift in the emission and a strong modification of the vibronic progression. Most importantly, this structural rearrangement dramatically affects the photophysical stability on the single molecule level. Single molecule beta-phase emission displays a vastly improved lifetime and much less noise on both the emission intensity and the spectral position. In the absence of signatures of multichromophoric emission on the single molecule level, we propose that the effective conjugation length accounts for most of the physical chain length of the beta-phase.

We explore the spontaneous formation of an excitonic insulator state at the semimetalsemiconductor transition of mixed-valence materials in the framework of the spinless Falicov-Kimball model with direct f -f electron hopping. Adapting the projector-based renormalization method, we obtain a set of renormalization differential equations for the extended Falicov-Kimball model parameters and finally derive analytical expressions for the order parameter, as well as for the renormalized c-and f -electron dispersions, momentum distributions, and wave-vector resolved single-particle spectral functions. Our numerical results proved the valence transition picture, related to the appearance of the excitonic insulator phase, in the case of overlapping c and f bands. Thereby the photoemission spectra show significant differences between the weak-to-intermediate and intermediateto-strong Coulomb attraction regimes, indicating a BCS-BEC transition of the excitonic condensate.

The development of sophisticated microscopic models of energy transfer in linear multichromophoric systems such as conjugated polymers is rarely matched by suitable experimental studies on the microscopic level. To assess the roles of structural, temporal, and energetic disorder in energy transfer, single molecule spectroscopic investigations of the elementary processes leading to energetic relaxation in conjugated polymers are desirable. We present a detailed study of energy transfer processes occurring in dye-endcapped conjugated polymer molecules on the single molecule level. These processes are mostly masked in ensemble investigations. Highly efficient intramolecular energy transfer along a single polyindenofluorene chain to a perylene endcap occurs in many instances and is resolved in real time. We further consider the spectral emission characteristics of the single molecule, the polarization anisotropy which reveals the chain conformation, the fluorescence intermittency, and the temperature dependence and conclude that the efficiency of energy transfer in the ensemble is controlled by the statistics of the individual molecules. The weak thermal activation of energy transfer indicates the involvement of vibrational modes in interchromophoric coupling. Whereas backbone-endcap coupling is strong, the rate-limiting step for intramolecular energy transfer is the migration along the backbone. The results are particularly relevant to understanding undesired exciton trapping on fluorenone defects in polyfluorenes.

The two-layer square lattice quantum antiferromagnet with spins 1 2 shows a zero-field magnetic order-disorder transition at a critical ratio of the inter-plane to intra-plane couplings. Adding a uniform magnetic field tunes the system to canted antiferromagnetism and eventually to a fully polarized state; similar behavior occurs for ferromagnetic intra-plane coupling. Based on a bond operator spin representation, we propose an approximate ground state wavefunction which consistently covers all phases by means of a unitary transformation. The excitations can be efficiently described as independent bosons; in the antiferromagnetic phase these reduce to the well-known spin waves, whereas they describe gapped spin-1 excitations in the singlet phase. We compute the spectra of these excitations as well as the magnetizations throughout the whole phase diagram.

Thermodynamic experiments as well as Raman scattering have been used to study the magnetic instabilities in the spin-tetrahedra systems Cu2Te2O5X2, X=Cl and Br. While the phase transition observed in the Cl system at To=18.2 K is consistent with 3D AF ordering, the phase transition at To=11.3 K in the Br system has several unusual features. We propose an explanation in terms of weakly coupled tetrahedra with a singlet-triplet gap and low lying singlets. 75.40.Gb, 75.40.Cx, 75.10.Jm, Reduced dimensionality of a quantum spin system in combination with frustration leads in many cases to unconventional and interesting ground states or magnetic phase diagrams. Prominent examples are the frustrated and dimerized spin-1/2 chain, represented by the lowtemperature phase of CuGeO 3 , or the two-dimensional Shastry-Sutherland lattice with orthogonally arranged spin dimers and a frustrating inter-dimer coupling, realized in SrCu 2 (BO 3 ) 2 . These systems show a spin liquid ground state with a singlet-triplet gap. Frustration is evident in the latter system as dispersionless elementary triplets and multi-particle bound states of triplet and singlet character [1][2][3].Spin triangles and tetrahedra that are strongly coupled into Kagomé or pyrochlore structures are at the origin of another important class of frustrated spin systems [4,5]. Although the consequences of the classical ground-state degeneracy for the quantum case have not been fully elucidated theoretically, there are good reasons to believe that such models do not possess magnetic long-range order (LRO) but low-lying singlets [6,7]. Scenarios leading to the development of LRO within this non-magnetic manifold have been put forward [8,9].The limit of weakly-coupled tetrahedra with spin S=1/2 has been studied in some detail in 3D and 1D, and the physics is expected to be very interesting [10][11][12][13]. However, they have not been investigated experimentally so far due to the lack of appropriate materials. The recently found spin system Cu 2 Te 2 O 5 X 2 , with X=Cl, Br, contains tetrahedral clusters of Cu 2+ with S = 1/2 in a distorted square planar CuO 3 X-coordination [14]. These tetrahedra align to tubes or chains along the [001] direction, as they are separated along [100] and [010] direction by different Te-O coordinations (see Fig. 1a). Substituting Br for Cl in this system widens up the unit cell and increases its volume from 367 to 391Å 3 , by 7%, while bond angles or anisotropies do not change essentially. Therefore, this system allows in a unique way to study the interplay of frustration and coupling in a tetrahedra quantum spin system. Preliminary measurements of the magnetic susceptibility χ(T) of both compounds showed a maximum at T χmax = 23 K and 30 K for X=Cl, Br and a strong reduction at low temperatures, typical for a spin gap system. Assuming that the compounds consist of weakly-coupled units of 4 spins with couplings J 1 and J 2 (see Fig. 1b), the best fit of the susceptibility -quite a good one actually -was obtained for J 1 = J 2 = 38.5 K ...