Crystal polymorphism and crystalline-state photochromism of a rhodium dithionite complex with n-methoxypropyl moieties

A rhodium dinuclear complex having n-methoxypropyltetramethylcyclopentadienyl (Cp^MPro = η⁵-C₅Me₄nC₃H₆OCH₃) and photoresponsive dithionite (μ-O₂SSO₂) ligands, [(Cp^MProRh)₂(μ-CH₂)₂(μ-O₂SSO₂)] (1^MPro), was found to form two polymorphic crystals (a previously reported block-like α-crystal and a newly obtained plate-like β-crystal). The crystalline-state photochromism of 1^MPro in the β-crystal, which involves photoisomerization from the μ-O₂SSO₂ complex (1^MPro) to the corresponding μ-O₂SOSO complex (2^MPro) and a thermal back-reaction from 2^MPro to 1^MPro, was successfully studied by performing time-dependent X-ray diffraction experiments. The results obtained from the β-crystal were compared with the previous results obtained from the α-crystal: the thermal back-reaction in the β-crystal is eight times longer than that in the α-crystal.