Guest Exchange through Facilitated Transport in a Seemingly Impenetrable Hydrogen-Bonded Framework

A new inclusion compound consisting
of a guanidinium 1,3,5-tri­(4-sulfophenyl)­benzene
(G3TSPHB) host framework containing isophorone guests that
surround isolated and seemingly inaccessible pockets was amenable
to guest exchange with hexafluorobenzene (HFB) through a single crystal–single
crystal transformation (SCSCT). Single-crystal X-ray diffraction of
intermediate transformation states, from the parent compound G3TSPHB·(isophorone)3.7·(methanol)5.4 to the final state G3TSPHB·(isophorone)3.1·(HFB)2·(methanol)2, indicated
a crystal symmetry change from monoclinic to hexagonal prior to full
incorporation of HFB. Optical microscopy during the SCSCT revealed
the formation of lamellae, which expanded and then coalesced into
a single crystal when the phase transformation was complete. In situ
Raman microscopy revealed changes in the orientation of isophorone
guests during the transformation that suggested a pathway for HFB
entry into the host cavities. The SCSCT occurs more rapidly than expected
on the basis of simple diffusion, consistent with facilitated transport
along the lamellae interfaces and a reduction in the length scale
for guest exchange.