In our laboratory research, we use laser spectroscopy to characterize the excited states of photochemically relevant molecules. We record vibronically resolved spectra using primarily the cavity ringdown absorption technique. Through this work, we are able to determine fundamental vibrational frequencies of test molecules in their S1 or T1 states. Such experimental information is valuable for testing predictions available from excited-state computational techniques. Our spectroscopic work has focused on low-lying excited states of α,β-unsaturated carbonyl molecules — for example, the prototype acrolein molecule (propenal, CH2=CH-CH=O) as well as cyclic enones such as 2-cyclopenten-1-one and 2-cyclohexen-1-one. Beyond their utility in benchmarking computational methods, the enone molecules we study demonstrate an interesting interplay between C=O and C=C moieties and can serve as models for investigating the photochemistry of larger conjugated systems. In this talk, I will present a case study of our spectroscopic and computational investigations of 2-cyclohexen-1-one (2CHO). We have recorded vibronically resolved CRD spectra of the S1(n,π*) ← S0 and T1(n,π*) ← S0 band systems of 2CHO. A high-sensitivity technique is required to measure these spectra because the electronic transitions are weak, particularly the T1(n,π*) ← S0 transition, which is spin-forbidden. Moreover, the enone excited states, especially in 2CHO, tend to have short lifetimes because of rapid nonradiative decay to highly vibrationally excited levels of the S0 ground state. This necessitates a spectroscopic technique based on absorption rather than luminescence to characterize the excited states. The CRD method nicely satisfies the requirements of high sensitivity and absorption-based detection. Through analysis of the CRD spectra, we have determined fundamental frequencies for several ring vibrational modes in the T1(n,π*) and S1(n,π*) states of 2CHO as well as for higher-frequency C=O/C=C stretch modes in the S1(n,π*) state. In the talk, I will compare the observed fundamentals with harmonic-frequency predictions obtained from several computational methods, including TD-DFT, CIS(D), and EOM-EE-CCSD. Predictions from EOM-EE-CCSD (the highest-level ab initio method considered) are most accurate overall, and are sensitive to subtle differences between the S1 and T1 states of the same n,π* configuration. These outcomes offer important caveats for the use of economical techniques such as TD-DFT.

Characterization of excited electronic states by cavity ringdown spectroscopy

254th National Meeting (2017)

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