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Electron paramagnetic resonance (EPR) spectroscopy was used to study the interactions between stable free radicals and gold nanoparticles. The nitroxyl free radicals used were TEMPO, TEMPAMINE, and TEMPONE. Two sizes of Au particles, 15 and 2.5 nm in diameter, were synthesized to investigate the interactions with the metallic particles. We find that the EPR signal is reduced upon adsorption of the radicals onto the 15 nm Au particle surface. Despite the strong adsorption of TEMPAMINE on the particles, the signal intensity recovers upon the introduction of a high concentration of ethanolamine to the solution. The signal reduction was proportional to the concentration of Au particles, and the signal totally disappeared at high concentrations of Au particles. Possible explanations of the signal reduction are discussed in this Article. We propose that the reduction in signal intensity arises from exchange interactions between the unpaired electrons of the adsorbed radicals and conduction-band electrons of the metallic particles. In addition, in the presence of oxygen, the adsorbed TEMPAMINE radicals are catalytically oxidized to the carbonyl derivative, TEMPONE. A mechanism for this unexpected catalytic reaction is proposed.

We report on an unusual intramolecular electron transfer following the photoexcitation of the ground, charge separated, state of the zwitterionic compound, tetraphenylhexaazaanthracene, which results in a neutral biradical product. The reaction, studied by time-resolved electron paramagnetic resonance (TREPR) spectroscopy, could be carried out only when liquid crystals were used as the orienting solvents. On the basis of the analysis of the magnetic, kinetic, and polarization parameters of the TREPR spectra, the radical pair genesis is attributed to an intramolecular electron transfer from the singlet excited state.

Three corroles, which differ by their cavity's core, namely, diamagnetic free-base tris(pentafluorophenyl)corrole and its gallium(III) complex and the paramagnetic oxo-chromium(V) complex, were studied by steady-state and time-resolved electron paramagnetic resonance (EPR) spectroscopy. The magnetic and orientational parameters of the corroles, oriented in a nematic liquid crystal, were determined and interpreted in terms of their structure, geometry, and excited states spin dynamics. It was shown that both diamagnetic corroles, photoexcited to their triplet states, exhibit similar EPR line shapes, which is characterized by a negative zero-field splitting parameter, D, whose origin is due to molecular "stretching". Photoexcited Cr(V)O-corrole exhibits polarized ground-state EPR spectrum in emission mode. This polarization stems from the sequence of photophysical and photochemical reactions, involving the formation of the trip-quartet/trip-doublet composite states and their selective quenching via a charge transfer state.