We have designed and studied the photophysics of a class of organic fluorophores termed "DCDHFs," which were
originally used as push-pull chromophores for nonlinear optical applications. In this paper, we describe the general
photophysics of many realizations of the DCDHF class of single-molecule emitters. Moreover, we have reengineered a
red-emitting DCDHF fluorophore so that it is dark until photoactivated with a short burst of low-intensity violet light.
Photoactivation of the dark fluorogen leads to conversion of an azide to an amine, which shifts the absorption to long
wavelengths. After photoactivation, the fluorophore is bright and photostable enough to be imaged on the singlemolecule
level in living cells. This molecule and its relatives will provide a new class of bright photoactivatable
fluorophores, as are needed for super-resolution imaging schemes that require active control of single-molecule