A water-soluble stilbene derivative with cationic charges enables tuning of the dispersibility of the single-walled carbon nanotubes (SWNTs) in aqueous solution by photo irradiation. The nearly coplanar core of the stilbene dispersant interacts with the SWNTs via pi-pi interactions, resulting in a stable dispersion of the SWNTs. Photoinduced cyclization for the dispersant-triggered reprecipitation of the SWNTs is due to detachment of the dispersant from SWNTs surfaces.