Intermolecular exciton–exciton annihilation in phospholipid vesicles doped with [Ru(bpy)2dppz]2+

The ultrafast photophysics of [Ru(bpy)2dppz]2+ (dppz=dipyrido[3,2-a:2′,3′-c]-phenazine) embedded into the walls of phospholipid vesicles has been studied by femtosecond time-resolved pump–probe spectroscopy. While [Ru(bpy)2dppz]2+ has been studied intensively with respect to its intramolecular charge transfer processes, which are associated with the well known light-switch effect, this study focuses on intermolecular energy transfer processes taking place upon dense packing of the complexes into a phospholipid membrane composed of dipalmitoyl-l-α-phosphatidylglycerol, which can be thought of as a simplistic model of a cellular membrane. The data indicate additional quenching of excited [Ru(bpy)2dppz]2+ upon increasing the pump–pulse intensity. Hence, the observed photophysics, which is assigned to the presence of intermolecular exciton–exciton annihilation at high pump-intensities, might be related to the ultrafast photophysics of [Ru(bpy)2dppz]2+ when used as a chromophore to stain cells, an effect that may be taken into account during the employment of novel cellular markers based on Ru polypyridine complexes.