An efficient synthesis of 9,9-bis(2-ethylhexyl)fluorene oligomers up to the heptamer is reported, with repetitive Suzuki and Yamamoto coupling reactions employed in the synthesis. The key steps for preparation of the essential intermediates include Pd-catalyzed transformation of aryl bromides to aryl boronic esters (Miyaura reaction) and the application of the much higher reactivity of aryl boronic esters over aryl bromides in the Pd-catalyzed cross-coupling reaction with aryl diazonium salts. Variation of the UV/Vis absorption and photoluminescence characteristics with chain length is reported. Moreover, glass transition and liquid-crystal characteristics of the oligomers are described and compared with those of the polymer.

The aggregation of conjugated polymers and electronic coupling of chromophores play a central role in the fundamental understanding of light and charge generation processes. Here we report that the predominant coupling in isolated aggregates of conjugated polymers can be switched reversibly between H-type and J-type coupling by partially swelling and drying the aggregates. Aggregation is identified by shifts in photoluminescence energy, changes in vibronic peak ratio, and photoluminescence lifetime. This experiment unravels the internal electronic structure of the aggregate and highlights the importance of the drying process in the final spectroscopic properties. The electronic coupling after drying is tuned between H-type and J-type by changing the side chains of the conjugated polymer, but can also be entirely suppressed. The types of electronic coupling correlate with chain morphology, which is quantified by excitation polarization spectroscopy and the efficiency of interchromophoric energy transfer that is revealed by the degree of single-photon emission.

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.

The synthesis of shape-persistent macrocycles based on the phenyl-ethynyl backbone containing various extraannular alkyl side chains is described. Although compound solubility increases with increasing size of the side groups, decreasing the solvent polarity induces aggregation of the rings by nonspecific interactions. This was investigated by proton NMR spectroscopy. The magnitude of aggregation can be varied by using solvent mixtures of different hexane content, supporting the model of a solvophobic effect. From 1,2,4-trichlorobenzene solution the macrocycle 1c adsorbs at the surface of highly oriented pyrolitic graphite (HOPG). The two-dimensional order of the structure was investigated by scanning tunneling microscopy (STM) revealing the formation of a two-dimensional lattice of p1(2)mm symmetry with lattice parameters A = 3.6 nm, B = 5.7 nm, and Gamma = 74 degrees.

With organic light-emitting diodes (OLEDs) emerging in ever more applications, such as smart phones, televisions, and lighting, it is easy to forget that the present technology is based on a brilliantly simple patch to an inherent problem of fluorescent hydrocarbons: three quarters of the electrically generated energy is dissipated as heat by triplet excitons. Radiative decay from the triplet state via phosphorescence is generally very weak, and has only been resolved in transient spectroscopy at low temperatures in select organic semiconductors. [1] The solution to this problem has been to incorporate metal-organic emitters in OLEDs, [2] which mix spin by enhancing intersystem crossing through spin-orbit coupling: the heavy-atom effect. As this approach relies on the longevity of triplet excitons and the associated diffusion lengths, it is highly effective: in a suitably homogeneous environment even ppm concentrations of covalently bound metal atoms are sufficient to activate electrophosphorescence. [3] The second conceivable approach to harvesting energy from triplets is based on endothermic conversion [4] to a fluorescent singlet by reverse intersystem crossing. [5] This method necessitates control not only over spin-orbit coupling, requiring a heavy atom or a carefully engineered charge-transfer state, but also over the singlet-triplet exchange gap, which can be tuned by excitonic confinement. [6] Although progress has been made recently, conceptually it parallels the former approach: all excitations are converted to either triplets or singlets, thereby losing information on the underlying spin correlations of charge carriers. Evidence is emerging, however, that spin correlations in excitonic electron-hole precursor pairs can be used for exquisitely sensitive measurements of magnetic fields [7] and possibly even for quantum coherence phenomenology, [7b] with analogies to avian radical-pair photomagnetosensory processes. [8] To quantify such spin correlations, it is desirable to develop materials without heavy-atom spin mixing that show both intrinsic fluorescence and phosphorescence.The third approach to triplet harvesting has not been explored previously: tuning spin-orbit coupling without heavy atoms such that non-radiative internal conversion from the triplet excited state to the singlet ground state is suppressed and phosphorescence is the only remaining relaxation mechanism. Even in low-atomic-order-number compounds such as hydrocarbons, the orbital component of the wavefunction can give rise to substantial magnetic moments, leading to non-negligible spin-orbit energy terms. The effect is well-studied in carbon nanotubes and graphene, where zero-field splitting correlates with nanoscale curva

Two 2D supramolecular structures of macrocycle 1 and 1/C60 have been obtained on HOPG by self-assembly under ambient conditions and investigated by high-resolution STM. The monolayers of 1 are characterized by structures with perfect ordering over relatively large areas. In the case of 1/C60, the size of the macrocycle 1 and the presence of two individual bithiophene units per ring lead in the final superstructure to a 1:2 stoichiometry. The fullerenes are not trapped at the graphite surface inside the macrocyclic holes but are located around the periphery of the bithiophene units. This clearly shows that the donor-acceptor interaction between C60 and the electron-rich units of the ring is the dominant factor for the structure formation.

Shape-persistent macrocycles with an interior in the nanometer regime allow the attachment of (functional) side groups at defined positions at the ring. These side groups can have either a fixed orientation relative to the molecular backbone or they can adapt their orientation according to an external stimulus. The properties and applications of the compounds depend strongly on the orientation of these side groups. Macrocycles with intraannular or adaptable long alkyl groups display a new design principle for discotic liquid crystals. Macrocycles with extraannular (oligo)alkyl groups can be used for surface patterning in the nanometer regime and rings with extraannular oligostyryl groups are able to aggregate to supramolecular hollow polymer brushes.

Proton-transfer-mediated electron transfer in organic materials is a way to regulate the electronic properties of solids. ['] Intramolecular proton transfers are responsible for the photoand thermochromic properties of the crystal forms of N-salicylidene anilides.t21 Proton transfers under hydrostatic pressure in quinhydrones are known to be accompanied by changes in the absorptionIn the phase change of dimethyl 3,6-dichloro-2,5-dihydroxyterephthalate, the color change is associated with a proton tran~fer.1~1 For this reason, investigations on crystal structures of self-assembling, hydrogen-bonded aggregates are gaining in importance.[51 We report here on a new hydrogen-bonded system which undergoes a thermally induced reversible single-crystal-single-crystal phase change. This process is associated with a proton transfer and a concomitant change in color.The reaction of 4,4'-bipyridine (1) (colorless) and squaric acid (2) (colorless) in water leads to a pale yellow-brown precipitate 3 ( The (extended) asymmetric units of salts 4 (monoclinic) and 5 (triclinic) are shown in Figure They consist of infinite chains connected by hydrogen bonds and are arranged in stacks. These are aligned with the crystallographic c axis. The position of the ordered protons in these chains can be explained by the acidity constants of both components (squaric acid: pK, = 1.2-1.7, pK, = 3.2-3.5; 4,4'-bipyridinium dihydrochloride: pK, = 2.73, pK, = 4.87).I9] Each asymmetric unit in salt 4 contains one bipyridine unit (torsion angle 24"), while two different bipyridine units are contained in 5 (C301 to C310, torsion angle 8 '; C201 to C210, torsion angle 12"). [**I This work was supported by the Deutsche Forschungsgemeinschaft (Leibnizatomic distances within the anions (Table 2) show that the geometry of the anions in salts 4 and 5 is strongly influenced by these hydrogen bonds. The formal mirror plane of an uncomplexed squaric acid monoanion does not occur. Such an ideal geometry is found in oxamide-dioxonium-dihydrosquarate (monodeprotonated squaric acid anion).["] Our structures are "intermediates" between unperturbed monohydrogen squarate and free squaric acid. Thus, they are examples of the correlation