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).

A. CRD Spectroscopy of 2-Cyclohexen-1-one (2CH)

A primary goal of our work on the 2CH molecule is to characterize potential-energy functions for the lowest-frequency ring-bending (or inversion) mode, ν₃₉, in the S₁(n,π*) and T₂(n,π*) excited states. During year 2 of the PRF grant we made further assignments in the previously reported[1] S₁(n,π*) ← S₀ CRD spectrum, and thereby determined frequency intervals of the ν₃₉ vibrational mode in the S₁ state (see Figure 2). These new results are reported in manuscript submitted in July 2007 to The Journal of Physical Chemistry A. In collaboration with Prof. Jaan Laane (Texas A&M) we determined a one-dimensional effective potential for ν₃₉ in the S₁ state by using the assigned frequency intervals (~ 120 cm^-1) in the CRD spectrum. These intervals differ from those of the ground state (~ 99 cm^-1) and stem from a potential function that has a significantly higher barrier to planarity.

We have also continued our analysis of the CRD spectrum of 2CH in the T₂(n,π*) ← S₀ absorption band region. This band system is exceedingly weak, with ε_max on the order of 0.1 M^-1 cm^-1. Under the present PRF grant we have increased the signal-to-noise ratio in this spectrum by improving the spatial conditioning of the laser. The T₂(n,π*) ← S₀ spectrum of 2CH in Figure 3 was presented at the National ACS meeting in March 2007. A notable finding in this work is that the ν₃₉ fundamental frequency drops to 99 cm^-1 in the T₂(n,π*) excited state, compared to 122 cm^-1 in the S₁(n,π*) state.

We plan to record a spectrum of a deuterated derivative, 2CH-2,6,6-d₃, in order to confirm the vibronic assignments in Figure 3. We recently developed and implemented a protocol for preparing the deuterated derivative.

Figure 2: CRD absorption spectrum of 2CH recorded in a static gas cell at room temperature. The region shown includes the S₁(n,π*) ← S₀ origin band at 26,081 cm^-1, and includes a progression in the ring-inversion mode, ν₃₉, indicated by the blue tie lines. The same progression, shown by the purple tie lines, is attached to the ν₃₆ (ring torsion) fundamental, beginning at 344 cm^-1. The ν₃₉ progression is also repeated (red tie lines) as a series of bands attached to the 38₁¹ (ring twist) sequence band.

Figure 3: Room-temperature CRD absorption spectrum of 2CH in the T₂(n,π*)←S₀ region. Key to symbols: ● triplet-state origin or fundamental; ♦ combination band. Blue tie lines indicate progressions in the triplet-state ring-inversion mode; red tie lines indicate progressions in the ground-state.

B. CRD Spectroscopy of 4-Cyclopenten-1,3-dione (4CPD)

In 2CH, as well as the 2-cyclopenten-1-one (CP) molecule studied previously,[2] we found significant differences when comparing ring vibrational frequencies in the singlet vs. triplet (n,π*) excited states. To investigate whether this is a general property of the conjugated cyclic enones, we recorded the first T₁(n,π*) ← S₀ CRD absorption spectrum of the 4CPD molecule. The spectrum is shown below along with vibronic assignments. Analysis of combination differences shows that the ν₁₉ (ring pucker) fundamental frequency in the T₁(n,π*) and S₁(n,π*)[3] states is nearly the same, whereas the ν₁₄ (ring twist) frequency is 20% lower in T₁ than in S₁.

Figure 4: Room-temperature CRD absorption spectrum of 4CPD vapor in the T₁(n,π*)←S₀ region.  The origin band is observed at 20,540 cm^-1.

C. Construction of a Free-Jet Expansion Chamber

In 2007 we published the jet-cooled phosphorescence excitation (PE) spectrum of 2CP,[4] complementing the room-temperature CRD study we had previously reported (Ref. 2). The jet-cooled study was conducted at Purdue University  in collaboration with Prof. Timothy Zwier. With the present PRF support we have constructed a free-jet expansion chamber in our own laboratory at UW-Eau Claire. The chamber, equipped with a pulsed nozzle source, will be used to record PE spectra of 4-pyrone (see last year's progress report) and 4CPD. The jet cooling will minimize rovibronic congestion observed in the room-temperature singlet-triplet spectra.