Inversion of 4-methoxybenzophenone triplet in aqueous solutions

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,pi*) configuration with the typical triplet-triplet absorption spectrum of benzophenone (lambda(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 lambda(max) at 450 and 680 nm, which can be attributed to a T1 (pi,pi*) state. Further, transient absorption spectra due to T1 (n,pi*) and T2 (pi,pi*) being simultaneously populated are observed upon laser excitation of 4-MBP in polar solvents such as acetonitrile or methanol. The triplet state inversion (n,pi* to pi,pi*) is also detected by the measurement of triplet quenching rate constants by 1,4-cyclohexadiene (a good hydrogen donor) in acetonitrile and water (kH ca. 2 x 10(8) and 5 x 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 x 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 T2 (pi,pi*) contribution is observed in the triplet-triplet absorption spectrum of 4-ABP in cyclohexane.