energy‐transfer process between the chromophores. Herein, we thoroughly describe the influence of the attached spacer group, as well as the distance and orientation between the donor–acceptor pair, on the excitation energy transfer. The observed fast dynamics and efficiency suggest that the process mainly takes place “through‐bond”, although the “through‐space” mechanism also contributes to the whole
delivered fluxional fluorophores that blink spontaneously upon photoisomerization. The compatibility of these probes with the self-labeling protein HaloTag enabled single-molecule imaging of specific molecular targets in livecells for exceptionally long periods of time.