EXCITED-STATE DYNAMICS OF MICROCRYSTALLINE ACRIDINE BY FEMTOSECOND DIFFUSE-REFLECTANCE SPECTROSCOPY

The electronic structure of the excited states and triplet formation d ynamics in the acridine microcrystal have been studied by femtosecond diffuse reflectance absorption spectroscopy and picosecond fluorescenc e spectroscopy. It was confirmed that an excimer was formed in the mic rocrystal within a few picoseconds after excitation. The time constant of triplet formation from the lowest excited singlet (S-1) state was obtained to be about 900 ps at room temperature, which is about ten ti mes slower than that in an aprotic solvent. Also, triplet formation wa s not observed up to 6 ns at 77 K. Slow intersystem crossing in the cr ystal was explained in terms of a selective stabilization of the (1)pi pi electronic configuration due to excimer formation. An important r ole of intermolecular interactions on the intersystem crossing process of the crystal is discussed.