The diversity of photophysics and photochemistry of the lowlying excited triplet states of aromatic carbonyl compounds has attracted considerable interest in the field of organic photochemistry. For instance, the intersystem crossing rates, phosphorescence lifetimes, and photoreduction activities 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 alterations in the energy-level ordering of the states. The photoreduction proceeds via the T1 state, [5] which has approximate quantum yields that vary between 1 for the np* T1 state, 0.1 for the pp* state, and 0 for the CT state. 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. It is known that the photoreduction activity of aromatic carbonyl compounds varies gradually with substituents or with solvents. In particular, the pp* T1 states show varying reactivities that cannot be accounted for solely by energylevel ordering. There have been several arguments about this reactivity variation of the pp* T1 state. It is generally considered that the reactivity arises from mixing of the np* character into the pp* state. A mechanism that involves the thermal excitation to a closely lying np* state has also been suggested. These arguments are not based on direct experimental evidence on the ordering of the excited triplet states and therefore are not conclusive; conventional spectroscopic techniques have not been effective in observing close-lying excited triplet states of aromatic carbonyl compounds. Thus, it is highly important to experimentally clarify the energy-level ordering and the electronic configurations 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 themid-infrared region. We have focused on the substituent dependence of both the triplet–triplet absorption spectra and the photoreduction 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 , with a peak at 3500 cm . The other band starts from 7000 cm 1 and extends to the higher-wavenumber region above 12000 cm . The decay of these two bands is completely synchronous with the recovery of the ground-state
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