Porphyrin-Based Triad with Exceptionally High Magnetic Field Effects (MFE) on Phosphorescence

Abstract

Magnetic control over molecular emissivity presents interests for various fields of technology and medicine. Here, we report experiments exploring magnetic field effects (MFE) on phosphorescence in a new triad, consisting of phosphorescent Pt porphyrin (PtP), rosamine B (RosB) and trialkoxybenzene (TAB), covalently linked together by conductive oligophenylene bridges. Upon photoexcitation the system undergoes two sequential reversible electron transfer processes (ET), generating two radical pairs (RP). The first rate-limiting ET originates in the local PtP triplet state, and it is slightly endergonic, competing with, but not entirely quenching the PtP phosphorescence. The second ET is exergonic, resulting in the formation of the final RP with large inter-radial distance. The RPs recombine either to the ground state (singlet channel) or back to the emissive PtP triplet state (triplet channel). The net distribution over the recombination channels, and hence the phosphorescence decay lifetime and intensity, are governed by the spin dynamics in the RP(s) and, therefore, are sensitive to external magnetic fields. The triad was found to exhibit remarkably strong positive MFE on phosphorescence at ambient temperatures with the magnitude of ~10% in fields as low as 200 mT. The observed features are characteristic of the hyperfine mechanism, although at higher fields a slight negative MFE was also observed, presumably due to the Δg mechanism. The system was characterized by static and time-resolved optical spectroscopies in the fs, ns and μs-to-ms domains as well as by time-resolved EPR. These studies revealed peculiar dynamics of the system, whereby the initial population of the PtP triplet rapidly equilibrates with the RosB triplet, forming a long-lived triplet reservoir, existing in equilibrium with the RP states. A kinetic model has been developed that allowed us to accurately reproduce the observed charge and energy dynamics of the triad. The model makes it possible to predict how the MFE and the performance of the system can be improved. Overall, this work constitutes an important step forward towards the design of magnetically sensitive luminescent materials.