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Chapter 2 11
Figure 2.7. a, Three classes of host materials categorized according to attributes of softness,
hardness (rigidity) and regularity. The overlapping zone of the two stages indicates materials
belonging to either of the ends. b, Classification of porous CPs into three categories. Reprinted by
permission from Macmillan Publishers Ltd: Nature Chemistry, 1, 695–704, copyright 2009. [37]
Besides features mentioned above (structural versatility and permanently porosity),
MOFs might also be flexible. Kitagawa et. al classified porous CPs into three generations
(Figure 2.7). [37-38] The 1 st generation, which have been earlier mentioned in this chapter,
represents porous coordination networks, which are comparably labile and collapse upon
removal of the guests from the pores. The 2 nd generation includes robust frameworks
exhibiting permanent porosity (reversible adsorption/desorption of guest molecules)
with complete preservation of the crystalline order. The 3 rd generation, identified as
flexible or dynamic porous frameworks, is able to undergo defined (and reversible) phase
transitions as a respond to such external stimuli as thermal, [39] , guest molecule
adsorption/desorption, [40] mechanical stress, [41] or light. [42] However, such structural
transitions do not lead to breaking of coordination bonds and the internal connectivity of