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

The metal-organic frameworks (MOF) of cluster [Cu4I4-(DABCO)(2)] (DABCO=1,4-diazabicyclo[2.2.2]octane) have been prepared and characterized as two different crystalline forms, I and II. Form I is obtained by reaction of DABCO and Cut in aqueous solution or by solvothermal reaction, while form II is obtained by reacting DABCO and Cut in acetonitrile. Their luminescence properties in the solid state have been analyzed at room temperature and at 77 K. MOF II has bright emission with a maximum at 556 nm that shifts bathochromically at low temperature in conjunction with a marked change in the colour of the emission. The emission of MOF I has a maximum at 580 nm and a less pronounced temperature dependence. The peculiar luminescence properties of the two isomers have been interpreted by utilising current knowledge on the excited states properties of Cut cubane clusters. The two isomers exhibit it high degree of porosity and can release the disordered solvent molecules trapped in the channels, whilst preserving the crystal structure. Isomer I can be converted into II on exposure to acetonitrile or methanol vapour, whereas Ill reverts to I when heated in a closed pan at 250 degrees C.