A standard halide metathesis reaction between fluorescein and trihexyltetradecyl phosphonium chloride resulted in partial exchange (40.8%) of chloride for dianionic fluorescein, thereby creating a photochromic binary ionic liquid (IL) salt. Spectroscopic behavior of fluorescein incorporated into the IL network differed from that observed thus far for fluorescein solvated by pure ILs or mixtures containing IL as a co-solvent. The UV-vis spectrum of the fluorescein-containing IL was characterized by a progression of 3-4 strong absorption bands. The fluorescence excitation spectrum of the material also contained a series of bands, though the positions and relative intensities of the bands differed from that observed with the absorption spectrum, while emission from the material occurred as a single band with a maximum at 555 nm. The observed behavior of the system points toward physical restraint/confinement of fluorescein motion, as well as the significant presence of various aggregated fluorescein microdomains in the IL material. (C) 2011 Elsevier B.V. All rights reserved.