Electronic relaxation in photoexcited graphenes is central to their photoreactivity and their optoelectrical applications such as photodetectors and solar cells. Herein we report on the first ensemble studies of electronic energy relaxation pathways in colloidal graphene quantum dots with uniform size. We show that the photoexcited graphene quantum dots have a significant probability of relaxing into triplet states and emit both phosphorescence and fluorescence at room temperature, with relative intensities depending on the excitation energy. Because of the long lifetime and reactivity of triplet electronic states, our results could have significant implications for applications of graphenes.
Inspired by biology, contemporary chemistry is challenged with the synthesis of architecturally defined functional structures that are much larger than ordinary molecules. [1] One emerging strategy is the use of noncovalent interactions in polymolecular assemblies to craft the shapes, dimensions, and functions of nanostructures. The scientific goal is to gain access to unknown static and dynamic functions with supramolecular systems which could be designed with the same ease that small molecules are now synthesized. Supramolecular pores, [2] tubes, [3] cylinders, [4] helices, [5] and mushroomlike noncentrosymmetric clusters [6] have been among the targets of recent work in this area. We report here a torsional strain mechanism to tune the pitch of micrometer-long helical assemblies in the range of tens to hundreds of nanometers.Helices in biology, such as double-helical DNA, protein coiled coils, and twisted b sheets, [7] have inspired an extensive amount of work on synthetic helical nanostructures.[5] Artificial structures containing peptide b sheets have been of particular interest [8,9] since they only require short amino acid sequences to form and also because of their relevance in diseases linked to amyloid fibrils. During our recent work on the functionalization of cylindrical nanofibers formed by tripeptide amphiphiles in organic solvents (such as cyclohexyl chloride), [9,10] we found that simple modification of the compounds led to nanostructures with dramatically different morphologies.Compounds 1 and 2 were synthesized with an acetate and a dimethyl acetate end group, respectively, and we used atomic force microscopy (AFM) to examine the morphology of the supramolecular aggregates they form. At a concentration of 1 % (by weight) both compounds dissolve in cyclohexyl chloride at about 80 8C and form translucent selfsupporting gels upon cooling to room temperature. AFM studies of the diluted gels dried on silicon substrates revealed straight cylindrical fibers for compound 1 (Figure 1 a). In contrast, AFM images of the aggregates formed by molecules of compound 2 show helices with a regular pitch (Figure 1 b) of 22(AE2) nm (Figure 1 c), and the orientation of the height contour clearly indicates these helices have a left-handed sense.The significant change in the morphology of the assembly when the acetate end group in 1 was replaced by a dimethyl acetate substituent in 2 suggested that a bulkier substituent at the terminus of the alkyl segment causes twisting of the initially cylindrical assemblies. To gain a mechanistic view of this process, we considered first the driving force for aggregation in these systems. In a low polarity solvent, the amphiphilic molecules studied here assemble as a result of solvophobic interactions as well as the formation of an intermolecular b sheet between the peptide segments.[9] The charged tetraalkylammonium head groups are buried inside the nanofibers as a result of their low affinity for the organic solvent, and the less polar tails are present on the surfaces of t...
Good as gold: Nanofibers with surface hydrogen‐bonding motifs co‐assembled from peptide‐based amphiphilic molecules were used to template one‐dimensional assemblies of preformed lipophilic inorganic nanoparticles in apolar organic solvents (see figure).
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