Characterization of Excited States of Quinones and Identification of Their Deactivation Pathways

Abstract

Femtosecond time-resolved transient absorption was used to investigate the properties of the excited states of p-benzoquinone derivatives: ubiquinone 0 (UQ(0)), thermoquinone (TQ), and tetramethylbenzoquinone (TMBQ). To elucidate details of the deactivation pathways of quinones, experiments were carried out in solvents with different polarity by exciting the samples at 400 and 266 nm. The measurements carried out upon excitation centered at 400 nm pointed out that the S(1) --> T(1) intersystem crossing (ISC) is operative only for TMBQ, where the T(2)(pi,pi*) state is likely below S(1). For UQ(0) and TQ, this decay process is not efficient because of the high energy of T(2) located above S(1). Instead, the lowest triplet state of UQ(0) was detected upon excitation at 266 nm, showing the involvement of an upper excited singlet state (S(n)) in the ISC process.