Femtosecond laser studies of excited state intramolecular proton transfer in an ultraviolet-filter molecule

Following excitation by a femtosecond laser pulse at 346 nm, phenol,2-(2H-benzotriazol-2-yl)-4-methyl-6-[2-methyl-3-[1,3,3,3-tetramethyl-1-[(trimethylsilyl)oxy]disiloxanyl]propyl] in n-heptane relaxes towards its ground state in a four-step mechanism. By excited state intramolecular proton transfer (50 fs) the enol S1 state produces the keto S1′ state responsible for a small gain spectrum above 600 nm. The latter relaxes to its keto ground state S0′ (130 fs) and the keto→enol back proton transfer (500 fs) yields the vibrationally hot enol ground state, the cooling of which is a relatively slow process (≫2 ps). We present kinetics and detailed transient absorption and gain spectra between 380 and 720 nm.