Ruthenium(II) complex enantiomers as cellular probes for diastereomeric interactions in confocal and fluorescence lifetime imaging microscopy

Ruthenium dipyridophenazine (dppz) complexes are sensitive luminescent probes for hydrophobic environments. Here, we apply multi-frequency fluorescence lifetime imaging microscopy (FLIM) to Δ and Δ enantiomers of lipophilic ruthenium dppz complexes in live and fixed cells, and their different lifetime staining patterns are related to conventional intensity-based microscopy. Excited-state lifetimes of the enantiomers determined from FLIM measurements correspond well with spectroscopically measured emission decay curves in pure microenvironments of DNA, phospholipid membranes, or a model protein. We show that FLIM can be applied to monitor the long-lived excited states of ruthenium complex enantiomers and, combined with confocal microscopy, give new insight into their biomolecular binding and reveal differences in the microenvironment probed by the complexes.