By applying an external electric field, we have studied the de Stark shifts at the two states of a low-temperature single-molecule optical switch. We observe a reversible change from a mainly quadratic field dependence to a linear field dependence. The data analysis provides the S-1 -S-0 dipole moment and polarizability differences of both the original and photoproduct state. On the basis of these data, a detailed microscopic scenario for the underlying nonphotochemical hole burning mechanism is discussed.