Separation of Spectroscopically Uniform Nanographenes.

Excitation-dependent photoluminescence (PL) is well-known property of graphene quantum dots (GQDs). For the development of carbon-based photo-functional materials, GQDs possessing uniform PL properties are highly demanded. We established a protocol to separate spectroscopically uniform lipophilic GQD-1a from a GQD-1 mixture mainly composed of GQD-1a and GQD-1b. The GQD-1 mixture was synthesized by the reaction of p-methoxybenzyl amine with GQD-2 prepared from graphite by common oxidative exfoliations. Size exclusion chromatography gave raise to GQD-1a and GQD-1b with diameters of 19.8 and 4.9 nm, respectively. The large GQD-1a showed the PL that was fairly independent on the excitation wavelengths, whereas the PL of the small GQD-1b was excitation-dependent. The excitation-dependent nature is most likely associated with the structures of sp2 domains on the graphene surfaces. The large sp2 conjugated surface of GQD-1a likely possesses well-developed and large sp2 domains which the band gaps don't significantly vary for. The small sp2 conjugated surface of GQD-1b produces small sp2 conjugated domains that generate the band gaps, associating with the domain sizes.