Spectroscopic Studies of Cyclic Enones in Triplet Excited States Stephen Drucker • Department of Chemistry • University of Wisconsin-Eau Claire Wavelength 475
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O
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2CH
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Absorption
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The main goal of this grant is to characterize the structure and dynamics of low-lying triplet excited states of monocyclic enones. We use the highly sensitive cavity ringdown (CRD) spectroscopic technique to contend with the spin-forbiddenness of the singlet-triplet transitions originating in the electronic ground state. During year 2 of the grant we focused on the molecules 2-cyclohexen-1-one (2CH) and 4-cyclopenten-1,3-dione (4CPD).
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-1
Frequency Shift from Origin, cm
Cavity Ringdown Spectrum of the T1(n,π π*) S0 band system of 4-cyclopenten-1,3-dione. The origin band is observed at 20,540 cm-1.
4CPD
In 2CH, as well as the 2-cyclopenten-1-one (CP) molecule studied previously, we found significant differences when comparing ring vibrational frequencies in the singlet vs. triplet (n, *) excited states. To investigate whether this is a general property of the conjugated cyclic enones, we recorded the first T1(n, *) S0 CRD absorption spectrum of the 4CPD molecule. The spectrum is shown at left along with vibronic assignments. Analysis of combination differences shows that the 19 (ring pucker) fundamental frequency in the T1(n, *) and S1(n, *) states is nearly the same, whereas the 14 (ring twist) frequency is 20% lower in T1 than in S1. The 13 (carbonyl wag) frequency is 15% lower in T1 than in S1.
