Experimental and theoretical evaluations have proven that very low fluorescence quantum yields of azlactones in solution are not caused by an efficient inter system crossing from S~*(xn) to T*(nn) states, but rather from solvation and steric effects, that result in non-planarity of the molecular system. High fluorescence quantum yields in the solid state are attributed to the planarity of the azlactone molecule upon packing into the crystal lattice. Supporting evidence was found upon observation ofthe excited state proton transfer (ESIPT) bands of fluorescence emissions of o-hydroxyarylidene derivatives. The photoinstability of azlac-553 554 E L I ET AL.tones in liquid states are attributed to photochemical E-Z isomerization and cleavage of the hetero ring a to the carbonyl group.

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