Femtosecond-Resolved Excited State Relaxation Dynamics of Copper (II) Tetraphenylporphyrin (CuTPP) After Soret Band Excitation

Excited state relaxation dynamics of Copper (II) tetraphenylporphyrin (CuTPP) after Soret band excitation have been investigated in various solvents by femtosecond broadband transient absorption spectroscopy. Significant role of charge transfer state has been confirmed from fast relaxation of triplet CuTPP in pyridine, giving τ ~ 26.5 ps. In piperidine, the transient measured at 480 nm shows biexponential behavior with distinct time constants of 300 fs and 27.4 ps. The fast component with τ ~ 300 fs is attributed to relaxation of the CuTPP-piperidine adduct populated in the ground state, giving the intrinsic relaxation rate of the CuTPP exciplex for the first time. For CuTPP in O-coordinating solvents of 1,4-dioxane and tetrahydrofuran (THF), a completely new relaxation channel via the (2)[d(z2), d(x2−y2)] state is opened. As the exciplex formation is diffusion controlled, triplet CuTPP lifetimes in pure solvents employed here are all measured to be more or less same to give ~30 ps, whereas the (2)[d(z2), d(x2−y2)] exciplex formed by the ligation with O-coordinating solvents is found to relax much slowly to the ground state, giving lifetimes of ~360 and ~270 ps in 1,4-dioxane and THF, respectively.