Photochemistry DOI: 10.1002/anie.201004571 Ordering, Interaction, and Reactivity of the Low-Lying np* and pp* Excited Triplet States of Acetophenone Derivatives** Sohshi Yabumoto, Shinsuke Shigeto,* Yuan-Pern Lee, and Hiro-o Hamaguchi* The diversity of photophysics and photochemistry of the low- lying excited triplet states of aromatic carbonyl compounds has attracted considerable interest in the field of organic photochemistry. [1] For instance, the intersystem crossing rates, [2] phosphorescence lifetimes, [3] and photoreduction activities [4] of these compounds show a marked dependence on both substituents and solvents. Depending on their electronic configurations, the energy of the low-lying triplet states, namely np*, pp*, and charge transfer (CT), can be influenced by substituents and solvents, with possible alter- ations in the energy-level ordering of the states. The photo- reduction proceeds via the T 1 state, [5] which has approximate quantum yields that vary between 1 for the np*T 1 state, 0.1 for the pp* state, and 0 for the CT state. [6] The strong substituent and solvent dependence of the photophysics and photochemistry of aromatic carbonyl compounds has thus been discussed in terms of the energy-level ordering of the np*, pp*, and CT excited triplet states. [7–10] It is known that the photoreduction activity of aromatic carbonyl compounds varies gradually with substituents or with solvents. In particular, the pp*T 1 states show varying reactivities that cannot be accounted for solely by energy- level ordering. There have been several arguments about this reactivity variation of the pp*T 1 state. It is generally considered that the reactivity arises from mixing of the np* character into the pp* state. [1] A mechanism that involves the thermal excitation to a closely lying np* state has also been suggested. [10–12] These arguments are not based on direct experimental evidence on the ordering of the excited triplet states and therefore are not conclusive; conventional spec- troscopic techniques have not been effective in observing close-lying excited triplet states of aromatic carbonyl com- pounds. [13] Thus, it is highly important to experimentally clarify the energy-level ordering and the electronic config- urations of the low-lying excited triplet states of aromatic carbonyl compounds. We have constructed a nanosecond time-resolved absorption spectrometer that is suitable for observing the triplet–triplet transitions in the near-infrared region as well as the vibrational transitions in the mid-infrared region. [13] We have focused on the substituent dependence of both the triplet–triplet absorption spectra and the photo- reduction activity of a series of acetophenone derivatives. The time-resolved near-infrared spectra of acetophenone (AP) excited at 325 nm in benzene are shown in Figure 1. Upon photoexcitation, two broad transient absorption bands arise instantaneously within the time resolution of the apparatus, and decay synchronously. One band spans from 2000 to 7000 cm 1 , with a peak at 3500 cm 1 . The other band starts from 7000 cm 1 and extends to the higher-wavenumber region above 12 000 cm 1 . The decay of these two bands is completely synchronous with the recovery of the ground-state Figure 1. Time-resolved near-infrared spectra of photoexcited acetophe- none in benzene. The region around 3000 cm 1 is blocked by the solvent absorption signal. [*] Dr. S. Yabumoto, Prof. S. Shigeto, Prof. Y.-P. Lee, Prof. H. Hamaguchi Institute of Molecular Science and Department of Applied Chemistry National Chiao Tung University 1001 Ta Hsueh Rd., Hsinchu (Taiwan) Fax: (+ 886) 3-572-3764 E-mail: shigeto@mail.nctu.edu.tw Homepage: http://140.113.224.181/ Prof. H. Hamaguchi Department of Chemistry, School of Science the University of Tokyo 7-3-1 Hongo, Bukyo-ku, Tokyo (Japan) Fax: (+ 81) 3-3818-4621 E-mail: hhama@chem.s.u-tokyo.ac.jp Homepage: http://www.chem.s.u-tokyo.ac.jp/users/struct/ [**] This work was supported by the “Aim for the Top” University Plan of the National Chiao Tung University and the Ministry of Education, Taiwan, and in part by the National Science Council of Taiwan (grant nos. NSC-97-2811M-009-011, NSC-97-2811M-009-024, and NSC-98- 2113M-009-011-MY2). Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/anie.201004571. A ngewandte Chemi e 9201 Angew. Chem. Int. Ed. 2010, 49, 9201 –9205  2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim