Conjugated polymers offer potential for many diverse applications, but we still lack a fundamental microscopic understanding of their electronic structure. Elementary photoexcitations (excitons) span only a few nanometres of a molecule, which itself can extend over microns, and how their behaviour is affected by molecular dimensions is not immediately obvious. For example, where is the exciton formed within a conjugated segment and is it always situated on the same repeat units? Here, we introduce structurally rigid molecular spoked wheels, 6 nm in diameter, as a model of extended π conjugation. Single-molecule fluorescence reveals random exciton localization, which leads to temporally varying emission polarization. Initially, this random localization arises after every photon absorption event because of temperature-independent spontaneous symmetry breaking. These fast fluctuations are slowed to millisecond timescales after prolonged illumination. Intramolecular heterogeneity is revealed in cryogenic spectroscopy by jumps in transition energy, but emission polarization can also switch without a spectral jump occurring, which implies long-range homogeneity in the local dielectric environment.

Inter- or intramolecular coupling processes between chromophores such as excimer formation or H- and J-aggregation are crucial to describing the photophysics of closely packed films of conjugated polymers. Such coupling is highly distance dependent and should be sensitive to both fluctuations in the spacing between chromophores as well as the actual position on the chromophore where the exciton localizes. Single-molecule spectroscopy reveals these intrinsic fluctuations in well-defined bichromophoric model systems of cofacial oligomers. Signatures of interchromophoric interactions in the excited state--spectral red shifting and broadening and a slowing of photoluminescence decay--correlate with each other but scatter strongly between single molecules, implying an extraordinary distribution in coupling strengths. Furthermore, these excimer-like spectral fingerprints vary with time, revealing intrinsic dynamics in the coupling strength within one single dimer molecule, which constitutes the starting point for describing a molecular solid. Such spectral sensitivity to sub-Ångström molecular dynamics could prove complementary to conventional FRET-based molecular rulers.

Molecular polygons with three to six sides and binary mixtures thereof form long-range ordered patterns at the TCB/HOPG interface. This includes also the 2D crystallization of pentagons. The results provide an insight into how the symmetry of molecules is translated into periodic structures.

A new solid material has been created in ultra high vacuum by utilizing the aggregation process of C58 molecules deposited onto highly oriented pyrolytic graphite from a mass selected low-energy ion beam comprising C58+. Cluster fluxes of up to 3x10(11) ions s-1 cm-2 with impinging kinetic energies of 6+/-0.5 eV were typically applied. Growth of the solid C58 phase proceeds according to the cluster-aggregation-based Volmer-Weber scenario where initially ramified 2D islands transform into 3D pyramid-like structures at higher coverages. The C58 films created exhibit much higher thermal stability than the C60 solid phase. Sublimation of C58 sets in at a temperature of 700 K. Ultraviolet photoionization spectra (He I, 21.2 eV) yield a molecular ionization potential in the range between 6.6 and 7 eV. Density functional and Hartree-Fock theories suggest that the formation of C58 dimers and higher multimers upon deposition/aggregation gives rise to the high thermal stability and unique electronic properties of this material.

Bending the rules: Strained bicyclophanes (see structure) with highly bent biphenylene units and a central aromatic moiety (yellow) forced into a perpendicular position were accessible in high yields by cyclization of the appropriate bromides by Yamamoto condensation. They were able to bind to graphite cutouts in solution and were adsorbed at the liquid/highly oriented pyrolytic graphite (HOPG) interface to form extended 2D structures.

This perspective focuses on the cage size dependent properties of novel solid fullerene nanofilms grown by soft-landing of mass-selected C(n)(+) (48, 50, 52, 54, 56, 58, 62, 64, 66 and 68) onto room temperature graphite surfaces under ultra-high vacuum conditions. Such non-isolated-pentagon-ring (non-IPR) fullerene materials are not accessible to standard fullerene preparation methods. The component molecular building blocks of non-IPR films were generated by electron impact induced ionization/fragmentation of sublimed IPR-C(70)(D(5h)) (-->C(n) (n = 68, 66, 64, 62)) or IPR-C(60)(I(h)) (-->C(n) (n = 58, 56, 54, 52, 50)). Non-IPR fullerene films on graphite grow via formation of dendritic C(n) aggregates, whereas deposition of IPR fullerenes under analogous conditions (via deposition of unfragmented C(60)(+) and C(70)(+)) leads to compact islands. The latter are governed by weak van der Waals cage-cage interactions. In contrast, the former are stabilized by covalent intercage bonds as mediated by the non-IPR sites (primarily adjacent pentagon pairs, AP). A significant fraction of the deposited non-IPR C(n) cages can be intactly (re)sublimed by heating. The corresponding mean desorption activation energies, E(des), increase from 2.1 eV for C(68) up to 2.6 eV for C(50). The densities of states in the valence band regions (DOS), surface ionization potentials (sIP) and HOMO-LUMO gaps (Delta) of semiconducting non-IPR films were measured and found to vary strongly with cage size. Overall, the n-dependencies of these properties can be interpreted in terms of covalently interconnected oligomeric structures comprising the most stable (neutral) C(n) isomers-as determined from density functional theory (DFT) calculations. Non-IPR fullerene films are the first known examples of elemental cluster materials in which the cluster building blocks are covalently but reversibly interconnected.

Rotaxanes are interlocked molecular architectures that can be perceived as simple mechanical devices. [1] A macrocycle that is threaded onto an axle and is deterred from dethreading by bulky stoppers can move translationally along the vector of the axle as well as rotate around it. To ensure that these molecular assemblies can carry out directional mechanical motion, the respective components require sufficient dimensional stability, or stiffness, over the entire working space. [2] In case of rotaxanes, it is primarily the axle that needs to exist as a non-deformable unit to efficiently convert the microscopic movement of the macrocycle into mechanical energy and to employ it for power transmission, otherwise the momentum of the moving macrocycle simply leads to a deformation of the axle, and thus cannot be further employed. We have recently described a DNA rotaxane that has a translational amplitude of about 100 base pairs (bp). [3] In a double-stranded DNA, however, the length of persistence of approximately 130 bp is too short to meet the required mechanical stability along the dumbbell axle. [4] Many systematic studies have devised methods in structural DNA nanotechnology [5] that not only allow for the construction of topologically defined architectures by selfassembly of DNA sequences, but also lead to robust two-and three-dimensional objects. [6] Seminal work in this field was established by Seeman, who demonstrated that two DNA double strands that are interwoven by multiple reciprocal strand exchange can lead to molecular assemblies that exhibit increased stiffness. [4, 7] Among them, particularly the so-called paranemic crossover structures PX and JX [8] were often applied for mechanical switching in DNA nanotechnology. [9] PX elements are characterized by a strand exchange that occurs at each contact point of two antiparallel DNA double strands. In a JX element, however, the strand exchange is abrogated at two consecutive positions. What makes the PX and JX elements so special is that two independent DNA double strands can be held together by reciprocal base pairing. Consequently, a paranemic crossover structure always exists in equilibrium with the respective DNA double strands. In presence of Mg 2+ ions, the equilibrium is strongly shifted towards the crossover product. [10] For the assembly of dsDNA rotaxanes, we devised a threading strategy that relies on the formation of eight bp between the DNA axle and the macrocycle. [3] The hybridization of these two components occurs highly efficiently, leading to quantitative rotaxane formation. Owing to the highly flexible single-stranded region, the DNA axle is able to easily accommodate its conformation to the geometry inherent to the macrocycle, thus leading to quantitative threading of the axle. However, higher-order DNA architectures like paranemic crossover DNA or even DNA origami [11,12] do not permit this flexibility anymore. On the contrary: it is precisely their mechanical robustness that accounts for their importance in DNA nanotechnology. Conver...

The synthesis, purification, and structure characterization of a seven-ring interlocked DNA catenane is described. The design of the seven-ring catenane allows the dynamic reconfiguration of any of the four rings (R1, R3, R4, and R6) on the catenane scaffold, or the simultaneous switching of any combination of two, three, or all four rings to yield 16 different isomeric states of the catenane. The dynamic reconfiguration across the states is achieved by implementing the strand-displacement process in the presence of appropriate fuel/antifuel strands and is probed by fluorescence spectroscopy. Each of the 16 isomers of the catenane can be transformed into any of the other isomers, thus allowing for 240 dynamic transitions within the system.