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42824-B6
Spectroscopic Studies of Cyclic Enones in Triplet Excited States

Stephen Drucker, University of Wisconsin (Eau Claire)

1. Measurement and analyses of singlet-triplet spectra of 4H-pyran-4-one (4PN) and 4-cyclopentene-1,3-dione (4CPD). We have recorded the T₁ (n,π*) ← S₀ spectra of these molecules to examine the delocalized effects of π* ← n excitation. We measured the 4PN and 4CPD spectra at room temperature, using phosphorescence excitation (PE) and cavity ringdown (CRD) detection, respectively. During the third year of the grant period (2007-08), we completed the vibronic analyses of the 4PN and 4CPD spectra, published our results on 4PN, and reported the 4CPD spectrum at the 63rd International Symposium on Molecular Spectroscopy.

Figure 1 shows the PE spectrum of 4PN in the region of the T₁← S₀ origin band. Most of the Δv=0 sequence bands are assigned to ν₁₈ (ring bending) and ν₁₃ (ring twisting). The positions of the sequence bands imply that the frequencies for ring bending and twisting are 126 cm^-1 and 269 cm^-1, respectively, in the T₁ state. It is interesting to compare changes in the 4PN ring frequencies for S₁ (n,π*) ← S₀ vs. T₁ (n,π*) ← S₀ excitation. The drop in ring-twisting frequency is 32% for the S₁ excitation (R. D. Gordon and W. K. C. Park, Can. J. Chem., 71: 1672, 1993), which is very similar to the 34% reduction observed for T₁. So for both of the excited states, the effects of π* ← n promotion clearly extend to the highly conjugated system of ring atoms. However, these effects are not exactly the same in the two excited states: the S₁ ring-bending frequency is 145 cm^-1, barely reduced from its ground-state value of 149 cm^-1. This contrasts with a drop of 15% in the T₁ state. Thus in 4PN, as we found for 2-cyclopenten-1-one (2CP) previously, the chromophore differs in subtle but discernable ways for singlet vs. triplet excitation. For both molecules, delocalization into the ring appears to be more extensive in the triplet case, as evidenced by changes in the ring-bending potentials.

Figure 1: T₁ (n,π*) ←S₀ spectra of 4PN (left) and 4CPD (right). Frequencies are relative to the origin bands at 27,292 and 20,541 cm^-1, respectively.

We have also studied the π* ← n excitation in 4CPD, a constitutional isomer of 4PN. Figure 1 shows the CRD spectrum of 4CPD near the T₁ (n,π*) ←S₀ origin band. The assigned sequence structure indicates that the ν₁₉ (ring bending) fundamental frequency is 60 cm^-1 higher in the T₁ excited state than in the ground state. Each of the ν₁₉ sequence bands has a satellite observed 7 cm^-1 higher in frequency. We assign the satellites as combinations of the main bands with the ring-twisting transition 14₁¹. As with the other cyclic enones, the S₁ and T₁ states of 4CPD show differing effects of π* ← n excitation. Whereas S₁ and T₁ show the same 60% increase in ring-bending frequency compared to the ground state, only the S₁ state has a substantially increased ring-twisting frequency: the ground-state frequency is 239 cm^-1 (R. A. Back and R. D. Gordon, J. Mol. Spectrosc., 204: 85, 2000), which changes to 304 cm^-1 in S₁ (ibid.) (a 28% increase), but only 246 cm^-1 in T₁ (a 3% increase).

To summarize our results on the three planar molecules 2CP, 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.

2. Construction of jet-cooling apparatus. In grant year 3, we also completed construction of an apparatus that will allow us to record electronic spectra under jet-cooled conditions. This approach will eliminate nearly all the vibronic hot bands and thereby improve the precision in the vibronic band assignments of the current and future spectra. The jet cooling will also permit rotational contour analyses. The new apparatus consists of a vacuum chamber (six-way cross) pumped by a diffusion pump that contains an internal water-cooled baffle. The chamber is equipped with a current loop-actuated valve (Jordan TOF Products, Inc.) that can produce very short (60 µs) pulses under choke flow conditions. Shown in Figure 2 is the intense jet-cooled fluorescence excitation spectrum of the test molecule CS₂, coexpanded with 2 atm helium carrier gas. The relative line intensities indicate an effective rotational temperature around 5 K.

Figure 2: Jet-cooled fluorescence excitation spectrum of CS₂. The dye laser resolution was approximately 0.2 cm^-1. The spectrum shows the 000 (K′=0) ← 00⁰0 band of the ¹B₂ (¹Δ_u) ←¹Σ_g⁺ system. The band is strongly perturbed by numerous triplet levels, giving rise to several sets of P and R branches. Tie lies near resolved branches indicate J″ assignments determined previously (K. Nishizawa et al., J. Chem. Phys., 100: 3394 (1994)). Odd J″ levels are missing due to nuclear spin statistics.

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