Ultrafast excited state dynamics of 5,6-dihydroxyindole, a key eumelanin building block: nonradiative decay mechanism

As part of a program designed to elucidate the excited state properties of key eumelanin building blocks, we report herein a study of 5,6-dihydroxyindole (DHI) in phosphate buffer at pH 3 and pH 7 using femtosecond transient absorption spectroscopy. The transient absorption changes following excitation at 266 nm were used to directly monitor relaxation of the excited states. It was found that the initially generated excited state of DHI, exhibiting two main absorption bands at approximately 450 and approximately 550 nm, decays with a time constant of 5-10 ps to the equilibrated singlet excited state characterized by a very similar spectrum. This latter state then decays to the ground state and the triplet state with a characteristic time of approximately 140-180 ps. Concomitant with the singlet excited state decay of DHI, spectral features characteristic of the DHI cation radical (band at approximately 575 nm) and the triplet state (band at 440-450 nm) are detected. These species do not decay further since geminate recombination of the solvated electron and the DHI radical cation, as well as deprotonation of the cation to form the neutral semiquinone radical, occur on a time scale longer than that covered by the present experiments. These results offer novel insights into the mechanisms of nonradiative decay of eumelanin building blocks of possible relevance to the putative photoprotective and phototoxic roles of these biopolymers.