Influence of Flexible Side-Chains on the Breathing Phase Transition of Pillared Layer MOFs: A Force Field Investigation

The prototypical pillared layer MOFs, formed by a square lattice of paddle-
wheel units and connected by dinitrogen pillars, can undergo a breathing phase
transition by a “wine-rack” type motion of the square lattice. We studied this not
yet fully understood behavior using an accurate first principles parameterized force
field (MOF-FF) for larger nanocrystallites on the example of Zn 2 (bdc) 2 (dabco) [bdc:
benzenedicarboxylate, dabco: (1,4-diazabicyclo[2.2.2]octane)] and found clear indi-
cations for an interface between a closed and an open pore phase traveling through
the system during the phase transformation [Adv. Theory Simul. 2019, 2, 11]. In
conventional simulations in small supercells this mechanism is prevented by periodic
boundary conditions (PBC), enforcing a synchronous transformation of the entire
crystal. Here, we extend this investigation to pillared layer MOFs with flexible
side-chains, attached to the linker. Such functionalized (fu-)MOFs are experimen-
tally known to have different properties with the side-chains acting as fixed guest
molecules. First, in order to extend the parameterization for such flexible groups,
1a new parametrization strategy for MOF-FF had to be developed, using a multi-
structure force based fit method. The resulting parametrization for a library of
fu-MOFs is then validated with respect to a set of reference systems and shows very
good accuracy. In the second step, a series of fu-MOFs with increasing side-chain
length is studied with respect to the influence of the side-chains on the breathing
behavior. For small supercells in PBC a systematic trend of the closed pore volume
with the chain length is observed. However, for a nanocrystallite model a distinct
interface between a closed and an open pore phase is visible only for the short chain
length, whereas for longer chains the interface broadens and a nearly concerted trans-
formation is observed. Only by molecular dynamics simulations using accurate force
fields such complex phenomena can be studied on a molecular level.