(Figure ). The deepening of the purple color of the dichroic crystal is visualized by polarized light microscopy. Fluorescence measurements of bulk samples show large changes upon increasing the amount of sodium resorufin (SR1  SR4; when exciting the donor at  = 480 nm) (Figure ). The emission of the donor ( = 541 nm) becomes weaker relative to emission of the acceptor ( = 608 nm) upon increasing the concentration of the latter in the crystals. A similar effect can be seen when exciting the donor in the range  = 375-625 nm (Figure ). These changes are strongly indicative for an energy-transfer process . We also observed the appearance of a new emission band at higher wavelengths ( = 648 nm), whereas the initial band of the donor becomes weaker. This effect may be indicative of different stacking of sodium resorufin at higher concentrations.