A photochemical on-off switch for tuning the equilibrium mixture of H 2 nuclear spin isomers as a function of temperature

Abstract

The photochemical interconversion of the two allotropes of the hydrogen molecule [para-H2 (pH2) and ortho-H2 (oH 2)] incarcerated inside the fullerene C70 (pH 2@C70 and oH2@C70, respectively) is reported. Photoexcitation of H2@C70 generates a fullerene triplet state that serves as a spin catalyst for pH2/oH2 conversion. This method provides a means of changing the pH2/oH 2 ratio inside C70 by simply irradiating H 2@C70 at different temperatures, since the equilibrium ratio is temperature-dependent and the electronic triplet state of the fullerene produced by absorption of the photon serves as an "on-off" spin catalyst. However, under comparable conditions, no photolytic pH 2/oH2 interconversion was observed for H 2@C60, which was rationalized by the significantly shorter triplet lifetime of H2@C60 relative to H 2@C70. © 2011 American Chemical Society.