pH-Dependent Optical Properties of Nanographenes

Nanographenes (NGs) produced by oxidative cleavage of carbons exhibit pH-dependent optical properties. We performed UV-vis and PL measurements on size-separated NGs across various pH levels and observed their halochromic behaviors in both the visible and near-infrared (NIR) regions. The absorption spectra exhibited a two-step change. The initial change is attributed to the deprotonation and protonation of the carboxy groups, and the latter is likely due to the aggregation and disaggregation of NGs. Notably, no significant shifts in the absorption and PL bands were observed, indicating that the deprotonation of the carboxy groups has a minimal impact on the electronic structures of NGs in both their ground and excited states. This implied that the effect of the deprotonation was limited to the edge moieties. Computational calculations suggested edge-to-edge and edge-to-surface π-π* transitions in the visible and NIR regions for a deprotonated model NG. Thus, a plausible explanation for the halochromic behavior of NGs involves the disappearance and appearance of these transitions upon the protonation and deprotonation of the carboxy groups. These changes affect the absorption in the visible and NIR regions, resulting in the halochromic behavior of NGs.