Spectroscopic Studies of Cyclic Enones in Triplet Excited States Stephen Drucker • Department of Chemistry • University of Wisconsin-Eau Claire The main goal of this grant is to characterize the structure and dynamics of low-lying triplet excited states of monocyclic enones. We have used the highly sensitive cavity ringdown (CRD) and phosphorescence excitation (PE) spectroscopic techniques to contend with the spin-forbiddenness of the singlet-triplet transitions originating in the electronic ground state. During year 3 of the grant we completed our analyses of the spectra of 4H-pyran-4-one (4PN) and 4-cyclopentene-1,3-dione (4CPD). The T1(n,π*) ← S0 spectra (PE and CRD, respectively) of these molecules are shown below. We found significant differences when comparing ring vibrational frequencies in the T1(n,π*) excited states with those available in the literature for the isoconfigurational singlet states. To summarize our results on the three planar molecules 2-cyclopenten-1-one (studied earlier in the grant period), as well as 4PN and 4CPD, the antibonding effects of the π* ← n excitation extend to the ring atoms in cases of both singlet and triplet excitation. However, the triplet state, in its own specific way for each molecule, shows a more extensive weakening of the forces responsible for ring planarity.   27,292 cm-1 20,541 cm-1