Resonant third-order optical nonlinearity of the neutral and the dication molecules of a triphenodithiazine model compound

Third order nonlinear optical susceptibility of solutions of a triphenodithiazine derivative in dichloromethane has been measured at several different wavelengths utilizing sub-picosecond degenerate four-wave mixing. The values of the second hyperpolarizability, γ, for the neutral and for the oxidized form (dication) are reported. The measurements performed at the wavelengths corresponding to the absorption maxima of the neutral and the dication forms (578 nm and 800 nm, respectively) indicate a twofold increase of the γ/σ figure of merit (where σ is the absorption cross section) for the dication molecule.Time resolved experiments reveal different dynamics of photoexcitation relaxation in the neutral and the oxidized molecules. The photoexcited dication returns to the ground state following a single-exponential decay law, whereas the photoexcited neutral molecule exhibits a more complex dynamics. Analysis of the heterodyned optical Kerr effect as well as the transient absorption experiment performed with a 90 fs resolution indicates an intermediate state generated during the fast decay of the primary photoexcited neutral molecules.