Inversion of 4-methoxybenzophenone triplet in aqueous solutions

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,π*) configuration with the typical triplet–triplet absorption spectrum of benzophenone (λ_maxca. 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 λ_max at 450 and 680 nm, which can be attributed to a T₁ (π,π*) state. Further, transient absorption spectra due to T₁ (n,π*) and T₂ (π,π*) being simultaneously populated are observed upon laser excitation of 4-MBP in polar solvents such as acetonitrile or methanol. The triplet state inversion (n,π* to π,π*) is also detected by the measurement of triplet quenching rate constants by 1,4-cyclohexadiene (a good hydrogen donor) in acetonitrile and water (k_Hca. 2 × 10⁸ and 5 × 10⁵ 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 T₂ (π,π*) contribution is observed in the triplet–triplet absorption spectrum of 4-ABP in cyclohexane.