Modulation of Visible Room Temperature Phosphorescence by Weak Magnetic Fields.

Magnetic control over excited states of molecules presents interest for many applications. Here we show for the first time that visible room temperature phosphorescence in multichromophoric donor-acceptor systems can be modulated by weak magnetic fields (<1 T) via magnetic field effects (MFE) on the spin dynamics in photogenerated radical pairs (RPs). The studied compounds comprise Pt porphyrin (PtP)-Rosamine B (RosB) dyads, which possess strong visible absorption bands and phosphoresce at room temperature. The observed MFE is unique in that it occurs upon direct excitation of the PtP in the dyads, whereby ultrafast quantitative formation of the local PtP triplet state precedes the occurrence of radical intermediates. A model explaining the effect is proposed, which is based on reversible electron transfer between the local triplet state and a long-lived RP. External magnetic field modulates spin dynamics in the RP, affecting contribution of the singlet RP recombination channel and thereby influencing phosphorescence.