Magnetic Field Effects on Triplet Emissivity of Metalloporphyrin-Containing Multichromophoric Systems

Control over dynamics of excited states of molecules is of fundamental importance for utilization of these states in all areas of technology, including optical imaging. We are exploring the possibility of imposing magnetic control over triplet emissivity of metalloporphyrins in multichromophoric donor-acceptor systems by influencing spin dynamics in intermediates preceding formation of the phosphorescent triplet states. Studies of electron and energy transfer dynamics in systems, comprising phosphorescent platinum (II) porphyrins (PtP) and rhodamine B (RhB) derivatives will be discussed. We demonstrate that visible room-temperature phosphorescence, resulting from direct spin-allowed transition of PtP, can be modulated by weak magnetic fields (<1T). Furthermore, similar effect has been observed upon diffusional encounters of phosphorescent metalloporphyrins with dark electron transfer quenchers. In another example, metalloporhyrins serve as triplet sensitizers, giving rise to long-lived triplet states of aromatic chromophores (perylene, rubrene), which in turn undergo triplet-triplet annihilation (TTA) and fluorescence emission via upconversion. Magnetic modulation of TTA-sensitized delayed fluorescence and delayed phosphorescence in solutions at room temperature has been demonstrated. These studies present unusual and interesting examples of magnetic field effects on molecular emissivity and set the stage for rational design of magnetically controlled optical probes.