Pathways for Excited-State Nonradiative Decay of 5,6-Dihydroxyindole, a Building Block of Eumelanin

The photophysics of 5,6-dihydroxyindole (DHI) following excitation to its lowest two optically bright states was investigated using the complete active space self-consistent field method with second-order perturbative energy corrections. There is a barrierless pathway for the molecule to relax from the second-lowest bright state (2¹ππ*) to the lowest bright state (1¹ππ*). The 1¹ππ* state has a conical intersection with the optically dark 1¹πσ* state, which further intersects with the ground state along the NH and OH stretching coordinates. Moreover, the 1¹ππ* has out-of-plane conical intersections with the ground state. For accessing the conical intersections with the ground state, there are energy barriers, which are higher than the available energy following vertical excitation to the lowest bright state. The nature of the calculated deactivation pathways helps interpret the experimentally estimated lifetimes of the lowest two bright states of DHI. The relatively long lifetime of the lowest excited state suggests that isolated DHI in monomeric form cannot rationalize the ultrafast deactivation property of eumelanin.