Reversible Interconversion between Luminescent Isomeric Metal-Organic Frameworks of [Cu4I4(DABCO)2] (DABCO = 1,4-Diazabicyclo[2.2.2]octane)

The metal-org. frameworks (MOF) of cluster [Cu4I4(DABCO)2] (DABCO = 1,4-diazabicyclo[2.2.2]octane) were prepd. and characterized as two different cryst. forms, I and II. Form I was obtained by reaction of DABCO and CuI in aq. soln. or by solvothermal reaction, while form II was obtained by reacting DABCO and CuI in acetonitrile. Their luminescence properties in the solid state were analyzed at room temp. and at 77 K. MOF II has bright emission with a max. at 556 nm that shifts bathochromically at low temp. in conjunction with a marked change in the color of the emission. The emission of MOF I has a max. at 580 nm and a less pronounced temp. dependence. The peculiar luminescence properties of the two isomers were interpreted by using current knowledge on the excited states properties of CuI cubane clusters. The two isomers exhibit a high degree of porosity and can release the disordered solvent mols. trapped in the channels, while preserving the crystal structure. Isomer I can be converted into II on exposure to acetonitrile or methanol vapor, whereas II reverts to I when heated in a closed pan at 250 Deg.