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@INPROCEEDINGS{Dey:906821,
      author       = {Dey, Somnath and Berg, Angelika von and Hakala, Viliam B.
                      and Shahed, Hend and Paulmann, Carsten and Tolkiehn, Martin
                      and Grzechnik, Andrzej and Peters, Lars},
      title        = {{I}n-situ recrystallization of elastically bendable
                      cocrystals of caffeine post partialdesolvation},
      reportid     = {FZJ-2022-01715},
      year         = {2022},
      abstract     = {The system of 1:1:1 cocrystal solvate of caffeine,
                      4-chloro-3-nitrobenzoic acid and methanol isthe first
                      reported example of molecular crystal that exhibits
                      reversible elastic flexibility [1]. Thecompound crystallizes
                      in orthorhombic space group symmetry Fdd2 [T = 100 K: a =
                      32.784(9) Å,b = 55.541(15) Å, c = 3.9564(12) Å, V =
                      7191(4) Å3]. Elastic bending in these crystals has
                      beendetermined to be governed by changing distances between
                      molecules within stacks andmolecular rotations [2]. The
                      crystals have been demonstrated to lose flexibility at
                      hightemperatures which is attributed to partial loss of
                      solvent from their structure (irreversiblephase
                      transformation at Tc = 388 K) [1]. While combination of weak
                      dispersive interactions viz.C–H ···O hydrogen bonds,
                      π-stacking and van der Waals forces between pseudo
                      sphericalfunctional groups aids flexibility, permanent
                      plastic deformation in these crystals has beenargued to be
                      prevented by “interlocking”/ steric barriers in the
                      supramolecular architecture[1,2].High temperature in-situ
                      powder X-ray diffraction studies confirm the phase
                      transformationaround Tc. Upon cooling down to room
                      temperature, long acicular crystals were observed afterfew
                      days. These crystals demonstrate excellent elastic
                      flexibility. Single crystal X-ray diffractionexperiments
                      employing synchrotron radiation revealed that the in-situ
                      recrystallized crystalshave identical space group symmetry
                      but a unit cell volume significantly larger than as
                      growncrystals [T = 100 K: Vin-situ = 7444.7(3) Å3].
                      Interestingly, the large a and b axes are found to
                      beslightly shorter as compared to the as grown crystals
                      while the significantly larger c-axis [=4.15160(10) Å]
                      accounts for such increased unit cell volume of the regrown
                      crystals.Here we present the atomic level studies of the
                      regrown crystals in comparison to the as growncrystals. The
                      reversibility post partial desolvation is governed by the
                      re-inclusion of methanolalbeit different electron counts at
                      the site of the solvent that is suggested to be ~ 1.3-1.4
                      timeslarger than the as synthesized crystals from structure
                      refinements. Increased content in thestructure enhances the
                      solvent channels that concomitantly result in larger
                      distances betweenmolecules within stacks thus stabilizing
                      the lattice with a larger c-axis. The apparentreconstruction
                      of the molecular stacks is accompanied by tilting of the
                      molecules andreorientation between hydrogen bonded tapes
                      different to as synthesized crystals. While in thepresent
                      case, inherent disorder of the solvent possibly aids in such
                      inclusion of molecularentities, such in-situ
                      recrystallization could be employed to manipulate overall
                      structure to tunein variable and/or revive mechanical
                      response in molecular crystals.},
      month         = {Mar},
      date          = {2022-03-14},
      organization  = {30th annual meeting of the German
                       Crystallographic Society (DGK), Ludwig
                       Maximilians Universität München +
                       online (Germany + online), 14 Mar 2022
                       - 17 Mar 2022},
      cin          = {JCNS-2 / PGI-4 / JARA-FIT},
      cid          = {I:(DE-Juel1)JCNS-2-20110106 / I:(DE-Juel1)PGI-4-20110106 /
                      $I:(DE-82)080009_20140620$},
      pnm          = {632 - Materials – Quantum, Complex and Functional
                      Materials (POF4-632) / 6G4 - Jülich Centre for Neutron
                      Research (JCNS) (FZJ) (POF4-6G4)},
      pid          = {G:(DE-HGF)POF4-632 / G:(DE-HGF)POF4-6G4},
      typ          = {PUB:(DE-HGF)1},
      url          = {https://juser.fz-juelich.de/record/906821},
}