Electron Spin Polarization Transfer from a Nitroxide Incarcerated within a Nanocapsule to a Nitroxide in the Bulk Aqueous Solution

A thioxanthone derivative containing a covalently attached N-labeled nitroxide was incarcerated into an octaacid nanocapsule. Photoexcitation of the thioxanthone chromophore generated electron spin polarization of the nitroxide. This spin polarization of the N-labelednitroxidewas transferred through thewalls of the carcerand to a N-labeled nitroxide in external bulk solvent, a process that was directly observed by time-resolved EPR spectroscopy. The efficiency of the communication between the incarcerated guest andmolecules in the bulk solvent was shown tobe controlledbysupramolecular factors suchasCoulombic attraction and repulsion between the guest@host complex and charged molecules in the bulk solvent phase. SECTION Kinetics, Spectroscopy I n pioneering work Balzani, 1 Deshayes, and Piotrowiak demonstrated by studying electronic energy transfer and electron transfer processes that electronically excited states of guest molecules incarcerated in hemicarcerands can communicate through the walls of the host with molecules in bulk solvent. As expected, the interactions of the incarcerated guest with molecules in the bulk solvent outside the host occur with rate constants considerably smaller than those found for the free guest in solution. The fact that interactions occur at all is the result of overlap of the orbitals of the guest with the orbitals of externalmolecules in the bulk solvent through the orbitals of the carcerand (host). Orbital interactions that occur through bonds are termed “superexchange”. The superexchange phenomenon is well established in nuclear and electronic spin spectroscopy. Previously, we reported an investigation of the electron spin-spin “superexchange” between an incarcerated nitroxide and a nitroxide in the external bulk aqueous phase (Scheme 1). For this study, we employed a N-labeled incarcerated nitroxide (Bz-T, Chart 1) and a N-labeled free nitroxide (Tx) in the external phase (Chart 1). By using different nuclear spins for incarcerated and free nitroxides, the simultaneous observation of both radicals by EPR spectroscopy can be achieved. As host, we selected a cavitand termed “octa acid” (OA, Chart 1), which is known to incarceratenonpolarmolecules by forminga capsuleplexunderbasic conditions in aqueous solutions. It was established that the nonpolar benzophenone derivative containing a N-labeled nitroxide substituent forms a 2:1 capsuleplex with OA, where two OA molecules encapsulate Bz-T. In the presence of a positively charged N-labeled free nitroxide in solution, a broadening of Bz-T and Tx signals was observed by steady-state EPR spectroscopy. We interpreted this signal broadening to be spin-spin communication between the incarcerated nitroxide with the free nitroxide in solution caused by “superexchange” through the walls of the host. A more direct method to investigate spin-spin interactions between an incarcerated nitroxide with a free nitroxide in the bulk solution would be time-resolved observation of spin-polarization transfer between the twonitroxides by timeresolved EPR spectroscopy (TR-EPR). Spin-polarized nitroxides can be generated by chemically induced dynamic electron polarization (CIDEP) by quenching of triplet excited states with nitroxides. High triplet quantum yields, fast intersystem crossing, and high triplet energies favor aromatic ketones, such as benzophenone derivatives as triplet sensitizers in CIDEP experiments. If the aromatic ketone and the Scheme 1. Spin-Spin Communication between an Incarcerated Nitroxide (Blue) and a Positively Charged Nitroxide (red) through the Walls of a Carcerand (Green) Received Date: July 22, 2010 Accepted Date: August 17, 2010