Near-Infrared-Emitting Nitrogen-Doped Nanographenes.

The quantum-size effect, which enables nanographenes to emit photoluminescence (PL) in the UV to visible region, has inspired intense research. However, the control of the PL properties of nanographenes through manipulation of their π-system by post-modifications is not well developed. By utilizing a ring-closure reaction between an aromatic 1,2-dicarboxylic acid and a 1,8-naphthalenediamine derivative, which produces a perimidine framework, nitrogen-doped nanographenes were realized. Two nanographenes produced by a one-pot reaction of edge-oxidized nanographene (GQD-2) with 1,8-naphthalenediamine derivatives (GQD-1 a and GQD-1 b) displayed an absorption band extending to >1000 nm; furthermore, the PL wavelength of GQD-1 a was significantly red-shifted into the near-infrared (NIR) region in which it can be used for bioimaging. Time-dependent DFT calculations of model nanographenes showed that the functional groups narrow the HOMO-LUMO gap, realizing the NIR-emitting nanographenes.

A Supramolecular Polymer Network of Graphene Quantum Dots.

Graphene quantum dot (GQD)-organic hybrid compounds (GQD-2 b-e) were prepared by introducing 3,4,5-tri(hexadecyloxy)benzyl groups (C16) and linear chains terminated with a 2-ureido-4-[1H]-pyrimidinone (UPy) moiety onto the periphery of GQD-1. GQD-2 b-e formed supramolecular assemblies through hydrogen bonding between the UPy units. GPC analysis showed that GQDs with high loadings of the UPy group formed larger assemblies, and this trend was confirmed by DOSY and viscosity measurements. AFM images showed the polymeric network structures of GQD-2 e on mica with flat structures (ca. 1.1 nm in height), but no such structures were observed in GQD-2 a, which only carries the C16 group. GQD-2 c and GQD-2 d formed organogels in n-decanol, and the gelation properties can be altered by replacing the alkyl chains in the UPy group with ethylene glycol chains (GQD-3). GQD can thus be used as a platform for supramolecular polymers and organogelators by suitable chemical functionalization.

Photoluminescence responses of graphene quantum dots toward organic bases and an acid.

Edge-modified graphene quantum dots (GQD-1) showed base-dependent photoluminescence (PL) responses. DBN, DBU, and Et3N gradually enhanced the PL intensity and then caused quenching, whereas quenching was not observed when pyridine and pyrimidine were used. The quenching can be explained by the photoinduced electron transfer from the anchored bases at the periphery to the emissive sites of GQD-1.

Intrinsic Emission from Nanographenes.

Top-down approaches have been widely used as convenient methods for the production of nanographenes. To understand the photoemission properties of nanographenes, their separation and the optical properties of the individual fractions is important. By using a combination of size-exclusion and silica-gel-adsorption chromatography, we separated lipophilic nanographenes that contained para-methoxybenzyl groups. The mixture consisted of large (average 19.8 nm) and small (average 4.9 nm) nanographenes, whilst unreacted carboxy groups remained in the latter group. Optical measurements revealed that oxygen-containing functional groups had little influence on the photoemission of the nanographenes, thus indicating that the intrinsic emission, that is, emission from the sp2 surfaces, was responsible for the photoemission. Two photoemission bands were observed for all of the fractions, which likely originated from the edge and inner parts of nanographene.

Separation of Spectroscopically Uniform Nanographenes.

Excitation-dependent photoluminescence (PL) is a well-known property of graphene quantum dots (GQDs). For the development of carbon-based photofunctional materials, GQDs possessing uniform PL properties are in high demand. A protocol has been established to separate spectroscopically uniform lipophilic GQD-1 a from a mixture of GQD-1 mainly composed of GQD-1 a and GQD-1 b. The mixture of GQD-1 was synthesized through the reaction of p-methoxybenzylamine with GQD-2 prepared from graphite by common oxidative exfoliation. Size-exclusion chromatography gave rise to GQD-1 a and GQD-1 b, with diameters of 19.8 and 4.9 nm, respectively. Large GQD-1 a showed that the PL was fairly independent of the excitation wavelengths, whereas the PL of small GQD-1 b was dependent on excitation. The excitation-dependent nature is most likely to be associated with the structures of sp2 domains on the graphene surfaces. The large sp2 -conjugated surface of GQD-1 a is likely to possess well-developed and large sp2 domains, the band gaps of which do not significantly vary. The small sp2 -conjugated surface of GQD-1 b produces small sp2 -conjugated domains that generate band gaps differing with domain sizes.