Sub-picosecond absorption spectroscopy is used to examine the dynamics of calcium-azacrown interactions in the excited state of 4-(N-monoaza-15-crown-5)-4’-phenylstilbene, in butyronitrile. This intrinsic fluorescence probe leads (in 8 +/- 2 ps) to a new emitting state which is assigned to a twisted intramolecular charge transfer state. When the probe is complexed with Ca2+, the kinetic evolution of the absorption and gain bands is interpreted by a reversible breaking (in 8 +/- 2 ps) of the nitrogen-calcium bond followed by the reversible formation (in 50 +/- 10 ps) of a solvent-separated cation-probe pair.