ESR Studies of Phosphorescent Corannulene; Evidence for Pseudorotation

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Abstract
Electron spin resonance (ESR) spectra have been recorded for the lowest triplet state of corannulene. The experiments were performed by ultraviolet irradiation of the molecule in rigid glass solutions at 77 and 15°K. In the high-field (Δm = 1) region the line shape and line positions of the absorptions associated with the X and Y magnetic axes were strongly temperature dependent. From an analysis of the spectra the following conclusions were drawn: Corannulene in its first triplet state has a configuration of lowest energy which is not fivefold symmetric; at 15°K corannulene gives in the high-field region the spectrum of a molecule “frozen” in conformations of less than threefold symmetry; tunneling between equivalent conformations at 77°K is believed to occur at a rate of about 109 Hz.