Reversible Multiphase Transition in a BioMOF and Its Distinctive Luminescence Turn-On in Alcohol Vapor

Solvothermal
reaction of zinc meso-tetra­(4-carboxyphenyl)­porphyrin
and 2,6-diaminopurine with zinc salt in DMF affords a three-dimensional
bioMOF (1-α). Its infinite rod-shaped
building block features an alternation of octahedral Zn4O and paddle-wheel Zn2 clusters bridged by 2,6-diaminopurines.
The paddle-wheel Zn2 cluster undergoes reversible transformation
with half into quasi-paddle-wheel Zn2 cluster and the other
half into two tetrahedral mononuclear clusters upon release/uptake
of guest molecules, resulting in a new phase 1-β. This single-crystal to single-crystal transformation is
accompanied by luminescence on/off switching, possibly associated
with the structural conversion between the porphyrin-ligand-based
photoactive 1-α and the porphyrin-stacking-caused
non-photoactive 1-β. Interestingly, 1-β exhibits quick luminescence turn-on
in alcohol vapor instead of other volatile organic solvents by transforming
into an intermediate phase 1-γ, which shows a partial luminescence enhancing likely due to the
intermittent porphyrin π–π stacking. In view of
experimental results and theoretical calculations, this distinctive
alcohol-vapor-induced luminescence turn-on is attributed to the coordination
ability to porphyrin-bound zinc ion, molecular size, and vapor pressure,
in which methanol and ethanol are particularly favored.