Graphene nanoribbons (GNRs) are excellent candidates for next-generation electronic materials. Unlike GNRs produced by "top-down" methods such as lithographical patterning of graphene and unzipping of carbon nanotubes that cannot reach structural perfection, the fabrication of structurally well-defined GNRs has been achieved by a "bottom-up" organic synthesis via solution-mediated or surface-assisted cyclodehydrogenation. Specifically, non-planar polyphenylene precursors were first "build up" from small molecules, and then "graphitized" and "planarized" to yield GNRs. However, fabrication of processable and longitudinally well-extended GNRs has remained a major challenge. Here we report a "bottom-up" solution synthesis of long (>200 nm), liquid-phase processable GNRs with well-defined structure and a large optical bandgap of 1.88 eV. Scanning probe microscopy demonstrates self-assembled monolayers of GNRs, and non-contact, time-resolved Terahertz conductivity measurements reveal excellent charge-carrier mobility within individual GNRs. Such structurally well-defined GNRs offer great opportunities for fundamental studies into graphene nanostructures, as well as development of GNR-based nanoelectronics.DOI: 10.1038/NCHEM.1819 http://www.nature.com/nchem/journal/v6/n2/abs/nchem.1819.html 2 Graphene nanoribbons (GNRs), defined as nanometre-wide strips of graphene, are attracting increasing attention as highly promising candidates for next generation semiconductor materials 1,2,3,4 . Quantum confinement effects impart GNRs with semiconducting properties, i.e. with a finite bandgap, which critically depends on the ribbon width and its edge structure 1,3 . Fabrication of GNRs has been primarily carried out by "top-down" approaches such as lithographical patterning of graphene 5,6 and unzipping of carbon nanotubes 7,8 , revealing their semiconducting nature and excellent transport properties 1 . However, these methods are generally limited by low yields and lack of structural precision, leading to GNRs with uncontrolled edge structures.In contrast, a "bottom-up" chemical synthetic approach based on solution-mediated 9,10,11,12,13 or surface-assisted 14 cyclodehydrogenation, namely "graphitization" and "planarization", of tailor-made three-dimensional polyphenylene precursors offers an appealing strategy for making structurally well-defined and homogeneous GNRs. The polyphenylene precursors are built up from small molecules, and thus their structures can be tailored within the capabilities of modern synthetic chemistry 15 . However, GNRs (>30 nm) produced by solution-mediated methods have been precluded from unambiguous structural characterization, i.e. microscopic visualization, due to their limited processability 9,12 . On the other hand, GNRs produced by the surface-assisted protocol have been characterized to be atomically precise using scanning tunnelling microscopy (STM) 14 . Nevertheless, this method can only provide a limited amount of GNR material, which is further bound to a metal surface, impeding wide...

The past few years have witnessed significant progress toward the design of supramolecular architectures of nano-to micrometer dimensions with intriguing properties. [1,2] Of particular interest to chemists is the self-assembly of rigid pconjugated molecules with controlled size and shape, as active components of organic electronic devices. [3, 4] In this context, self-assembly of oligo(p-phenyleneethynylene)s (OPEs) has received considerable attention.[5] Control of the morphology of self-assembled structures requires rational design of molecular components.[6] For example, spherical to tubular or cylindrical assemblies are obtained by structural variation of the self-assembling units, [7] whereas nonhelical to helical transformation can be achieved by attaching chiral handles [8][9][10][11] or by the "sergeant-and-soldiers" coassembly approach. [12][13][14] However, transitions of linear p-conjugated molecules from vesicles to helical tubules or vice versa are rare.[15] Herein we reveal a spectacular chirality-amplification effect and an unprecedented transition from vesicles to helical tubules during the sergeant-and-soldiers coassembly of chiral and achiral OPEs.We chose OPE1 and its chiral analogue OPE2 for our studies. These compounds were synthesized by palladiumcatalyzed Sonogashira-Hagihara cross-coupling reactions [16] and characterized by 1 H NMR, 13 C NMR, and FAB-MS techniques.[17] Compound OPE1 self-assembles in nonpolar hydrocarbon solvents to form nanoparticles, microspheres, giant superstructures, and blue-light-emitting organogels. [18] Surprisingly, OPE2 failed to form aggregates in nonpolar hydrocarbon solvents, as evident from absorption, emission, and CD spectral features. This behavior of OPE2 is different from that of an analogous chiral oligo(p-phenylenevinylene), OPV, for which formation of helical structures in decane leads to gelation. [19] These observations reveal the differences in the self-organization propensities of OPEs and OPVs, which reiterate that a subtle difference in p interaction is sufficient to induce a large difference in the morphological features.The UV/Vis absorption spectrum of OPE1 in decane (1 10 À5 m) at 258C exhibited a maximum at 384 nm with a shoulder at 420 nm. At this concentration, a decrease in the intensity of the p-p* transition band with first-order kinetics (k = 47.41 min À1 ) is observed over 6 h (Figure 1 a). Interestingly, addition of OPE2 (5-30 mol %) to this solution at a total concentration of 1 10 À5 m resulted in reversal of the absorption, which indicates probable destruction of the initial OPE1 aggregates to form coassembled or molecularly dissolved species. On the other hand, addition of OPE2 (5-30 mol %) to a fresh solution of OPE1 in decane (without aging) at a total concentration of 0.7 10 À5 m led to a decrease in the intensity of the aggregate band at 420 nm.[17] After heating to 80 8C and cooling to room temperature, this solution did not show any considerable change in the absorption spectrum, that is, the time-dependent process is ...

The reversible shift of emission in fluorescent molecular gelators has been explored for the preparation of a composite polymer film useful for erasable thermal imaging and secret documentation. A gelation-assisted photopolymerization of styrene allowed the entrapment of the fluorescent gelator molecules within a polystyrene matrix with a weak green fluorescence, which upon heating above the T(g) of the polymer resulted in high-contrast fluorescence images due to the strong blue fluorescence of the individual molecules. The blue emission from the disassembled oligo(p-phenylenevinylene) molecules (OPVs) could be reversed to the green emission of the self-assembled OPVs by exposing the polymer film to chloroform vapors. The thermally written images are visible only under UV light and cannot be photocopied. A solvent-vapor-controlled recreation of the self-assembly of a fluorescent organogelator within a polymer matrix and its application in erasable secret documentation has not been reported previously.

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The self-assembly of linear p-conjugated molecules to form nanoarchitectures of different morphologies is important in various fields of nanoscience and technology, particularly in the emerging field of supramolecular electronics. [1,2] In this context, oligo(p-phenyleneethynylene)s (OPEs), a class of conjugated molecules often referred as "molecular wires" has attracted much attention, particularly for the design of molecular electronic devices.[3] Although meta-and paralinked oligo-and poly(p-phenyleneethynylene)s (PPEs) are known to form solvophobically induced assemblies in solvent mixtures of different polarities, [4][5][6] the creation of stable supramolecular selfassemblies of short OPEs with different shapes and sizes from a single solvent of choice continues to be challenging. Herein, we report an unprecedented hierarchical selfassembly of a short OPE into nanoparticles, microspheres, and superstructures of different size and morphology that eventually result in stable blue-light emitting organogels.Alhough the design of organic nanoparticles of small molecules [7, 8] and polymers [9] are known, linearly conjugated oligomers are only rarely exploited for this purpose; however, a few reports have been published recently.[10] p-Extended molecules preferably self-assemble, with a few exceptions, [10a,d,g] to form elongated structures; for example, the self-assembly of oligo(p-phenylenevinylene)s (OPVs) results in the formation of supramolecular tapes and helices. [11,12] We have now targeted OPEs, the dehydrogenated analogues of OPVs in an attempt to understand the self-assembly processes of conjugated systems. For this purpose, OPE1 and OPE2 were synthesized by using multistep synthetic strategies involving palladium-catalyzed Sonogashira-Hagihara crosscoupling reactions (Scheme 1).[13] These molecules were characterized by 1 H NMR and 13 C NMR spectroscopy as well as by MALDI-TOF mass spectrometry. [14] The UV/Vis spectrum of OPE1 in chloroform (1 10 À5 m) at 25 8C showed a strong p-p* absorption band at 377 nm. In decane, the absorption maximum is red-shifted to 385 nm and the formation of an additional band at 419 nm was observed. The emission maximum in chloroform was at 414 nm, which was significantly red-shifted towards 443 nm in decane but with decreased intensity. [14] These observations are attributed to the J-type aggregation of OPE1 in decane. Variabletemperature UV/Vis absorption spectra recorded in decane (1.1 10 À5 m) showed there was a reversible transition of the aggregates to isotropic species between 20 and 70 8C (Figure 1 a). A plot of the fraction of the aggregates (a) versus temperature revealed a sharp transition between 30 and 40 8C as well as a broad transition in the range of 45-55 8C, thus indicating the presence of aggregates of different thermal stabilities (Figure 1 a, inset). Furthermore, the absorption spectrum of OPE1 (1.1 10 À5 m) in decane at room temperature showed a time-dependent decrease in the absorption maximum, which reached a stable state after 4 h (Figure 1 b)....