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-alpha). Its infinite rod-shaped building block features an alternation of octahedral Zn4O and paddle-wheel Zn-2 clusters bridged by 2,6-diaminopurines. The paddle-wheel Zn-2 cluster undergoes reversible transformation with half into quasi-paddle-wheel Zn-2 cluster and the other half into two tetrahedral mononuclear clusters upon release/uptake of guest molecules, resulting in a new phase 1-beta. 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-alpha and the porphyrin-stacking-caused non-photoactive 1-beta. Interestingly, 1-beta exhibits quick luminescence turn-on in alcohol vapor instead of other volatile organic solvents by transforming into an intermediate phase 1-gamma, which shows a partial luminescence enhancing likely due to the intermittent porphyrin pi-pi 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.