Jet-Cooled Phosphorescence Excitation Spectrum of the T 1 (n,π*) r S 0 Transition of 2-Cyclopenten-1-one Nathan R. Pillsbury, ² Timothy S. Zwier, ² Richard H. Judge, ‡ and Stephen Drucker* ,§ Department of Chemistry, Purdue UniVersity, 560 OVal DriVe, West Lafayette, Indiana 47907-2084, Department of Chemistry, UniVersity of WisconsinsParkside, Kenosha, Wisconsin 53141-2000, and Department of Chemistry, UniVersity of WisconsinsEau Claire, Eau Claire, Wisconsin 54702-2002 ReceiVed: March 25, 2007; In Final Form: May 30, 2007 The T 1 (n,π*) r S 0 transition of 2-cyclopenten-1-one (2CP) was investigated by using phosphorescence excitation (PE) spectroscopy in a free-jet expansion. The origin band, near 385 nm, is the most intense feature in the T 1 (n,π*) r S 0 PE spectrum. A short progression in the ring-bending mode (ν′ 30 ) is also observed. The effective vibrational temperature in the jet is estimated at 50 K. The spectral simplification arising from jet cooling helps confirm assignments made previously in the room-temperature cavity ringdown (CRD) absorption spectrum, which is congested by vibrational hot bands. In addition to the origin and ν′ 30 assignments, the jet-cooled PE spectrum also confirms the 28 0 1 (CdO out-of-plane wag), 29 0 1 (CdC twist), and 19 0 1 (CdO in-plane wag) band assignments that were made in the T 1 (n,π*) r S 0 room-temperature CRD spectrum. The temporal decay of the T 1 state of 2CP was investigated as a function of vibronic excitation. Phosphorescence from the V′ ) 0 level persists the entire time the molecules traverse the emission detection zone. Thus the phosphorescence lifetime of the V′ ) 0 level is significantly longer than the 2 μs transit time through the viewing zone. Higher vibrational levels in the T 1 state have shorter phosphorescence lifetimes, on the order of 2 μs or less. The concomitant reduction in emission quantum yield causes the higher vibronic bands (above 200 cm -1 ) in the PE spectrum to be weak. It is proposed that intersystem crossing to highly vibrationally excited levels of the ground state is responsible for the faster decay and diminished quantum yield. The jet cooling affords partial rotational resolution in the T 1 (n,π*) r S 0 spectrum of 2CP. The rotational structure of the origin band was simulated by using inertial constants available from a previously reported density functional (DFT) calculation of the T 1 (n,π*) state, along with spin constants obtained via a fitting procedure. Intensity parameters were also systematically varied. The optimized intensity factors support a model that identifies the S 2 (π,π*) r S 0 transition in 2CP as the sole source of oscillator strength for the T 1 (n,π*) r S 0 transition. Introduction Triplet excited states often play a central role in molecular photochemistry. Low-energy triplet states may be readily populated in a solution-phase environment, via rapid nonradia- tive relaxation (intersystem crossing) from an initially photo- excited S 1 state. 1 Once the excited system reaches the triplet surface, it is especially prone to chemical reaction, due to a diradical electronic structure and slow radiative decay rate. The return to the ground state typically occurs via T 1 f S 0 surface hopping that is enhanced in regions where the two states are in close proximity with one another. Thus the T 1 excitation energy, as well as the shape of the triplet potential surface, can strongly influence ground-state product formation. In such cases, char- acterization of the triplet potential surface is an important step toward understanding or predicting photochemical outcomes. The computational chemistry community is making signifi- cant progress in characterizing excited-state potential surfaces. Improved ab initio 2 and density functional 3,4 methods are making it possible to calculate, with increasing accuracy, the properties of triplet and singlet excited states of medium-sized molecules. In some cases, the accuracy of these excited-state calculations is approaching that of the ground state. Vibronically resolved spectra provide a rigorous test of computed potential surfaces, via comparison of experimental versus calculated vibrational frequencies, electronic excitation energies, and geometry changes associated with electronic excitation. Comparisons with experiment are critical for refining the computational methods used to treat excited states. Spec- troscopic data are routinely obtained for singlet excited states, but the experimental database for triplet excited states is far less well developedsessentially because singlet-triplet transi- tions originating from the ground state are nominally spin- forbidden. To obtain the crucial experimental benchmarks for testing triplet-state calculations, we have employed the cavity ringdown (CRD) 5 spectroscopic technique in prior studies. 6,7 Here we report continued work, this time using phosphorescence excitation (PE). Both PE and CRD are very sensitive and permit detection of T n r S 0 transitions in the gas phase. We have recently used CRD spectroscopy to investigate T(n,π*) r S 0 transitions of simple cyclic enones. 6,7 One of these compounds, shown in Figure 1, is 2-cyclopenten-1-one (2CP). Figure 2 shows a CRD spectrum of 2CP recorded at room temperature. This spectrum was obtained previously 7 and is * To whom correspondence should be addressed. E-mail: druckers@ uwec.edu. ² Purdue University. ‡ University of WisconsinsParkside. § University of WisconsinsEau Claire. 8357 J. Phys. Chem. A 2007, 111, 8357-8366 10.1021/jp072353r CCC: $37.00 © 2007 American Chemical Society Published on Web 08/04/2007