Photoblinking of Rhodamine 6G in Poly(vinyl alcohol):  Radical Dark State Formed through the Triplet

We investigate the fluorescence intensity of rhodamine 6G in poly(vinyl alcohol) as a function of excitation intensity, illumination time, the presence of oxygen, and temperature. The variations in emissivity (or fluorescence brightness) are attributed to a dark state, which shows populating kinetics resembling those of the triplet state, but a much longer lifetime. We simulate the observed kinetics by a four-level model, in which a long-lived dark state is formed through the triplet as an intermediate state. The weak temperature dependence of the lifetime of the dark state points to electron tunneling as the main recovery process. This intermolecular mechanism also explains the observed broad distribution of lifetimes. An electron-spin-resonance experiment confirms the assignment of the dark state to a radical. For the first time, photoinduced charge transfer is identified as a source of blinking in single-molecule measurements.