Previously unknown capabilities of one of the original squaraine dyes as a viscosity probe and dual-state emitter are revealed, with aggregated forms of the dye being responsible for the near-IR emission in the solid state.

Easily accessible small organic fluorophores with multiplexing capabilities are of interest for a variety of fields. Here, a combination of experimental and computational methods reveals previously unknown, yet potentially useful properties of one of the original squaraine dyes, i.e., bis-kryptopyrrol-2-yl squaraine. The rotational flexibility of the pyrrole units around the squaraine core allows for this dye to act as a viscosity probe. In addition to the previously known fluorescence emission in solution with λ_em in the 550–580 nm range, this dye exhibits solid-state emission with λ_em at ≈800 nm. Thus, this dye is a dual-state emitter. density functional theory/time-dependent density functional theory calculations and concentration-dependent studies, in both solution and solid-state, indicate aggregated form of these dyes is responsible for the solid-state emission in the near-IR range.