Magnetic Field Effects on Molecular Emissivity in Solutions

Control over dynamics of excited states of molecules is fundamental to utilization of these states in all areas of technologies, including optical microscopy and tomography. We explored the possibility of magnetically controlling molecular emissivity by influencing spin dynamics in radical pairs (RPs) and triplet-triplet pair. We envisioned that by including RPs into a pathway leading to the formation (or decay) of an emissive triplet state, magnetic influence on phosphorescence could be realized via modulation of the RP's spin dynamics. RPs can initially be produced in their singlet or triplet spin state. These two cases were explored in the studies of electron and energy dynamics in series of donor-acceptor systems, comprising phosphorescent platinum (II) porphyrins (PtP) and rhodamine B (RhB+) derivatives. In one series, the phosphorescent triplet state of PtP is generated by way of photo-excitation of RhB+, followed by photoinduced electron transfer with formation of a singlet-born RP, RP intersystem crossing, and subsequent recombination of the triplet RP. Similarly, singlet-born RPs were included into a pathway leading to PtP triplet state in a triad system comprising PtP, anthracene, and boron dipyrromethene. Using another series, we have demonstrated that visible room-temperature phosphorescence can be modulated by weak magnetic fields (<1T). In this case, the RP is initially born in its triplet state upon direct excitation of PtP, followed by electron transfer originating in the PtP triplet state. External magnetic field modulates spin dynamics in the RP, affecting contribution of the singlet charge recombination channel and thereby influencing the phosphorescence. Spin dynamics of triplet-triplet pair is also susceptible to magnetic field. Triplet-triplet pair can undergo triplet-triplet annihilation (TTA) process leading to photon upconversion, which is typically observed as p-type delayed fluorescence. We have demonstrated TTA-sensitized delayed fluorescence and delayed phosphorescence, mediated by near-infrared absorbing metalloporhyrins as sensitizers in solution at room temperature, can be magnetically modulated. These studies present unusual and interesting examples of magnetic field effects on molecular emission and set the stage for rational design of optical imaging probes with magnetically controlled emissivity.