A new class of photoactivatable dyes provides the ability to study cell-cell communication in live animals non-invasively with high spatiotemporal resolution. The compositions are made up of a macromolecule, a caging group, and a coumarin dye. Upon photolysis, the coumarin dye is released from the macromolecule caging group complex and is freely diffusible in cells and between cells. The compositions are retained in cells very well, having no observable side effects, no susceptibility to metabolism, and the ability to generate bright fluorescence signals after photolysis. Because of their high loading capacity and long cellular retention, they can be selectively uncaged in specific cells.
