F. Bosca, G. Cosa, M. Miranda
Sep 1, 2002
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0
Influential Citations
21
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Journal
Photochemical & Photobiological Sciences
Abstract
The triplet state of 4-methoxybenzophenone (4-MBP) has been investigated by laser flash photolysis and emission techniques in several solvents. In non-polar cyclohexane, 4-MBP triplet has an (n,p) configuration with the typical triplet–triplet absorption spectrum of benzophenone (?_max ca. 525 nm). However, due to the proximity of the two lowest triplet states of different configuration, some unusual features are observed in polar solvents. Thus, 4-MBP shows in aqueous solutions a transient absorption spectrum with?_max at 450 and 680 nm, which can be attributed to a T_1 (p,p) state. Further, transient absorption spectra due to T_1 (n,p) and T_2 (p,p) being simultaneously populated are observed upon laser xcitation of 4-MBP in polar solvents such as acetonitrile or methanol. The triplet state inversion (n,p to p,p) is also detected by the measurement of triplet quenching rate constants by 1,4-cyclohexadiene (a good hydrogen donor) in acetonitrile and water ( k _H ca. 2 × 10^8 and 5 × 10^5 M^−1 s^−1, respectively) and by the determination of room-temperature phosphorescence (the emission quantum yield at room-temperature decreases from 0.004 in acetonitrile to less than 1 × 10^−6 in water). Further, the energy of the 4-MBP triplet state is ca. 288 kJ mol^−1 both in polar and non-polar organic solvents, while in water it drops to 275 kJ mol^−1. The photophysical properties of 4-MBP are compared with those of 4-aminobenzophenone (4-ABP), which also possesses an electron-donating group. In polar organic solvents such as acetonitrile, the transient absorption spectrum and the quenching rate constant of hydrogen abstraction for triplet 4-ABP are practically the same as those obtained for 4-MBP in aqueous solutions. On the other hand, a small T_2 (p,p) contribution is observed in the triplet–triplet absorption spectrum of 4-ABP in cyclohexane.