Picosecond flash photolysis studies on phenothiazine in organic and micellar solution

Abstract

Picosecond laser flash photolysis studies are carried out on phenothiazine (PTH) in cyclohexane, alcoholic solvents such as methanol and decanol, and aqueous micellar solutions like Triton X-100, Brij-58, and sodium dodecyl sulfate (SDS) on excitation at different wavelengths, viz., 266 and 355 nm. In cyclohexane solution, S₁ → S_n transient absorption has been characterized and the intersystem crossing process has been studied. In alcoholic and micellar media, transient absorptions due to PTH cation radical and solvated electron (e_s^-) have also been observed along with the absorption of singlet and triplet states of PTH. Quantum yields of the different transient species in different solvents have been determined using both 266 and 355 nm excitation. Dependence of the yields of photoionization and singlet excited state on light intensity has also been investigated at these two excitation wavelengths. It is observed that the photoionization of PTH in methanol at 355 nm is a stepwise biphotoionic process. In aqueous SDS micellar medium, the photoionization is found to be monophotonic at 266 nm, but both mono- and biphotonic processes seem to operate at 355 nm. Direct evidence of the involvement of the S₁ state of PTH in the biphotonic processes is presented.