Highly efficient modulation of FRET in an orthogonally arranged BODIPY–DTE dyad

The photoswitchable boron-dipyrromethene–dithienylethene molecular dyad is introduced as a prototype for the efficient fluorescence intensity modulation on the molecular level. The functionality of the system is based on the photochromism of the dithienylethene, which facilitates an efficient on- and off-switching of a Förster-type intramolecular energy transfer between the photoexcited BODIPY donor and the dithienylethene acceptor moiety. The switching behavior and dynamics of the molecular dyad are monitored by steady state and time-resolved spectroscopic methods. A quenching efficiency of up to 96% in the off-state is observed and explained by a drastically accelerated decay of the boron-dipyrromethene excited state due to the efficient energy transfer despite the orthogonal arrangement of donor and acceptor. An energy transfer time orders of magnitude shorter than the lifetime of the boron-dipyrromethene in the open state is determined.