60th Annual Report on Research 2015

Acrolein (CH₂=CH–CH=O) is the smallest alpha,beta-unsaturated carbonyl molecule and serves as a prototype for investigating the photochemical properties of larger analogs. Much of this photochemistry is mediated by triplet excited species, including the T₁(n,pi*) state. The goal of this project is to obtain experimental information about the structure and dynamics of acrolein in its T₁(n,pi*) state. To accomplish this goal, we are planning to record the T₁←S₀ cavity ringdown spectrum of acrolein and deuterated isotopologues in a jet-cooled planar expansion. We will analyze the 0−0 band contours to obtain sets of inertial constants which will in turn establish the equilibrium geometry of the T₁(n,pi*) state.

In the first year of the grant, the PI recruited two sophomore-level undergraduate students (one physics major and one chemistry major) who plan to participate in the project throughout the course of their degree programs. With support from the grant, the PI has engaged the students in a comprehensive training program covering the theory and practice of laser spectroscopy.

As a prelude to the planned jet-cooled studies of acrolein, student Michael McDonnell conducted a spectroscopic investigation of the 2-cyclopenten-1-one (2CP) molecule, a monocyclic enone that has the same chomophore as acrolein. The T₁(n,pi*) vibronically resolved spectrum of 2CP is easier to analyze than that of acrolein because of the conformational rigidity imparted by 2CP ring structure. Previous work in our group utilized cavity ringdown (CRD) and phosphorescence excitation (PE) techniques to measure vibronically resolved T₁ ←S₀ spectra of 2CP near the origin band. [J. Phys. Chem. A 107, 10648 (2003); J. Phys. Chem A 111, 8357 (2007).] Our current work on 2CP culminated the training program for our newly recruited students and has begun to address unanswered questions about higher-frequency vibrational modes in the T₁(n,pi*) state.

We conducted our previous CRD study of 2CP using a bulk gas-phase sample at room temperature. This yielded fundamental frequencies for the lowest-energy ring modes in the T₁(n,pi*) state. We supplemented that investigation by recording a jet-cooled PE spectrum, which clarified room-temperature vibronic assignments and extended the spectral coverage into the C=O and C=C stretch regions, about 1500 cm⁻¹ above the T₁←S₀ origin. However, the PE spectrum in this region did not afford new vibrational assignments because the T₁←S₀ vibronic bands become weak at higher frequency. The Franck-Condon factors for the C=O and C=C stretch fundamentals are expected to be large, and so our inability to detect these vibronic features are attributable to a diminishing phosphorescence quantum yield at higher vibrational energies within the T₁←S₀ band system.

We carried out our most recent work on 2CP in 2015 under support from the current PRF grant. We recorded the room-temperature CRD spectrum of 2CP in the C=O / C=C stretch region of the T₁←S₀ band system. We had not surveyed this spectral region in our earlier CRD study of 2CP, because at the time, the wavelength coverage of our dye laser system did not extend far enough into the UV. Recently we built a second-harmonic generator that operates synchronously with the scanning of the dye laser. This new system extends the coverage of the dye laser into the UV by frequency-doubling the output of red laser dyes.

Figure 1 shows the room-temperature CRD spectrum of 2CP we obtained under current PRF support. Superimposed is the jet-cooled PE spectrum we measured previously. The spectral region shown in Figure 1 is expected to contain the C=O and/or C=C stretch fundamentals of the T₁ state. These T₁ vibronic levels are roughly isoenergetic with the v=0 level of the S₁ state. Thus the intense S₁←S₀ origin band, as well as nearby vibronic hot bands of the S₁←S₀ system, interfere with T₁←S₀ vibronic features we wish to assign. Established S₁←S₀ assignments are indicated in Figure 1. The jet-cooled PE spectrum is much less congested than the room-temperature CRD spectrum, so the interference from S₁←S₀ bands is less severe. Nonetheless, in this region of the PE spectrum, we do not observe bands assignable to the T₁←S₀ system, presumably because of a diminished quantum phosphorescence quantum yield, coupled with the nominal spin-forbiddenness of the T₁←S₀ transition.

Figure 1: Cavity Ringdown (red) and Phosphorescence Excitation (blue) spectra of 2-cyclopenten-1-one. Assignments within the S₁←S₀ band system [Cheatham and Laane, J. Chem. Phys. 94, 2234 (1991)] are shown. Features marked with dark-red dots are previously unassigned and attributed to the T₁←S₀ system.

Because CRD is an absorption-based technique, a low quantum yield for emission should not significantly degrade the CRD intensity of T₁←S₀ vibronic features. Thus it is likely that the triplet-state C=O and/or C=C fundamentals appear in the CRD spectrum of Figure 1, though these transitions may be partially submerged by intense (spin-allowed) S₁←S₀ hot bands. The present CRD spectrum contains several features, not assigned in previous work, that are likely T₁←S₀ vibronic bands, either fundamentals or attached sequences. These features are marked with dark-red dots in Figure 1.

In the upcoming second year of the PRF grant, we will record the CRD spectrum of 2CP in the same spectral region as shown in Figure 1, but under the cooling conditions of a planar jet expansion. The jet cooling will suppress hot bands assigned to the S₁←S₀ system and will reveal which of the unassigned features marked in the figure belong to the T₁←S₀ system. In the process, we will be able to determine fundamental frequencies for the C=O and/or C=C mode in the triplet state. These experimental values will be compared to predictions available from high-level ab initio computational methods. By completing this work on the 2CP molecule, undergraduate student collaborators will acquire crucial experience in operating our pulsed-nozzle molecular source in conjunction with the acquisition of CRD spectra. They will also bring this final component of our 2CP project to the stage of publication. These activities will offer the students excellent preparation for our planned studies of acrolein described above.