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 sp 2 domains on the graphene surfaces. The large sp 2 ‐conjugated surface of GQD‐ 1 a is likely to possess well‐developed and large sp 2 domains, the band gaps of which do not significantly vary. The small sp 2 ‐conjugated surface of GQD‐ 1 b produces small sp 2 ‐conjugated domains that generate band gaps differing with domain sizes.