Irradiating H2 encaged in a C70 fullerene (H2@C70) quickly converts H2 molecules from the nuclear-spin-aligned triplet state to the spin-paired singlet state, reports a research group led by Columbia University chemistry professor Nicholas J. Turro (J. Am. Chem. Soc., DOI: 10.1021/ja206383n). Although the same effect can be achieved by allowing H2 to equilibrate thermally, such equilibration can take months, hampering investigations into spin control of chemical reactions. Turro and colleagues chilled toluene solutions of H2@C70 to various temperatures, irradiated the frozen solutions with a xenon arc lamp for several hours, and used NMR spectroscopy to track the amount of triplet ortho-H2. Irradiating the samples produces an electronically excited state in the fullerene shells that induces a spin flip in the hydrogen nuclei within. That process converts ortho-H2 to singlet para-H2, achieving the same amounts of para-H2 as the radiation-free equilibration, but in much less time. The reverse conversion—para-H2 to ortho-H2—also works at low temperatures. Similar experiments with H2@C60 failed, however, likely because the C60 excited state decays too quickly, the authors say.