3P109
Theoretical investigation of the structures and dynamics of moleculargyroscopes encapsulating halogen-substituted phenylene rotators(1Department of Chemistry, Graduate School of Science, Tohoku University,
2
Faculty of Pharmaceutical Sciences, Tokushima Bunri University)
Anant Babu Marahatta1, Manabu Kanno1, Wataru Setaka2 and Hirohiko Kono1

I. Introduction: The crystalline molecular
Statorgyroscopes encapsulating phenylene rotators have
Rotatorreceived much attention in terms of their uniquestructures, dynamics and functions. Especiallyhalogen-substituted phenylene-bridged molecularFFgyroscopes are expected to have dipolar units thatF
can be reoriented under the influence of electricFfields [1]. Recently, Setaka et al. have reported theSpinsynthesis and structure of the novel molecularaxis
gyroscopes having halogen-substituted phenylene
rotators encased in three long siloxaalkane spokes Fig.1. An X-ray crystallography of the(Stator) [2]. They observed two different stable siloxaalkane molecular gyroscope having a
positions for the difluorophenylene rotator at difluorophenylene rotator at 273 K. Two273 K (Fig. 1) and only one position for the stable positions of the rotator differ by πdichlorophenylene rotator at 153 K. Similarly, a flipping are shown.
strong deformation of the siloxaalkane spokes caused by the steric interaction with the halogen
atoms on the central phenylene ring has also been reported. During the course of theoreticallyinvestigating crystal structures and rotational dynamics of experimentally synthesized crystalline
molecular gyroscopes, we are interested in a crystalline molecular gyroscope having a
difluorophenylene rotator (ROT-F). The notable results explored currently are reported here.
II. Method: Recently, we applied the Density Functional based Tight Binding (DFTB) method[3] to investigate the molecular packing structure and rotary dynamics of the crystalline
siloxaalkane molecular gyroscope having a phenylene rotator (ROT-H) [4]. We achieved good
agreements with the experimental results. Therefore, in our present study, we have also applied
the same theoretical method for characterizing crystalline ROT-F. In accordance with theexperimental condition, a periodic boundary condition (PBC) is imposed in the calculations. By
using the force constants computed by DFTB, the motions of all nuclei are evaluated based on
the classical mechanics with constraints on the lattice constants of the unit cell.
III. Results and Discussions: The X-ray geometries of theunit cells of two degenerate molecular structures of ROT-F are
semiquantitatively reproduced by the DFTB method except
the Si-O-Si angles of the highly flexible siloxaalkane spokes(see Table 1). These angles are more acute in ROT-F than in
ROT-H derivative. It indicates that the siloxaalkane-cage ofthe ROT-F expands outward with respect to the oxygen atom
for accommodating more bulky rotator. In order to confirm it,
we measured the intervening space that exists between anFig.2. A schematic representation
oxygen atom of each siloxaalkane arm and the nearest carbonof the siloxaalkane molecularatom of the phenylene ring (O-C distances: d1, d2 and d3 in Fig. gyroscope. The encircled atoms
2) for ROT-F and ROT-H and found that ROT-F possesses
define the dihedral angle. X = H
for ROT-H and X = F for ROT-F.

Table 1. X-ray vs. DFTB optimized parameterscomparatively large amount ofMolecular
Si-O-SiO-C distance (Å)
Phenylene dihedral
clearance around the rotating gyroscopes
angle (π)
angle ( π)
X-ray DFTB
X-ray
DFTB
X-ray
DFTB
unit (see Table 1). It assuresthat the shape of the
7.7
0.96
0.78
d1
8.4
siloxaalkane-cage in ROT-F
ROT-F
0.84
0.75
d2
5.9
5.5
0.56
0.44
5.9
5.8
0.84
0.74
d3
deforms from that of the ROT5.3
0.96
0.81
d1 5.2H derivative to avoid steric
4.1
0.55
0.35
ROT-H
0.90
0.77
d2 4.3effects with the fluorine on
4.1
0.96
0.81
d3 3.9phenylene. To estimate arotational energy barrier (Ea) ofthe ROT-F, we scanned thepotential energy surface (PES)
under PBC (Fig. 3) and found thatEa
the difluorophenylene rotator–1experiences as high as 1700 cm
α′energy barrier (Ea of theαphenylene rotator in ROT-H
250 cm-1 [4]). The PES clearly
displays two stable degenerate
molecular structures, α and α′ andFig.3. Potential energy as a function of the difluorophenylene angle
theirrelationbyπobtained by DFTB under PBC.
difluorophenylene flipping. It
confirms the two-fold rotational axis of the ROT-F molecule with respect to thedifluorophenylene rotator. These theoretical findings are in reasonable agreements with theexperimental observations. Moreover, we performed DFTB / molecular dynamics (MD)simulations of the ROT-F at 1200 K and 300 K, and calculated the concerned dihedral angle thatdesignates the difluorophenylene orientation at each MD step. At each kinetic temperature, theinitial angle of the difluorophenylene (~0.44π) corresponds to its equilibrium position α asshown in Fig. 4 and from this position; difluorophenylene ring is observed to flip by π andreaches to its degenerate position α′ (~1.44 π). The timescale of the π flipping at hightemperature is found to be shorter than at moderate temperature, as expected.
At 1200 K

At 300 K

α′α′π flipping

π flippingααTime (ps)Fig.4. Rotational angle of difluorophenylene as a function of time at 1200 K and 300 K.

IV. Conclusion: We successfully located two degenerate molecular structures of the ROT-F
under crystal condition. They are found to be related to each other by π difluorophenylene
flipping and have to cross ~1700 cm-l energy barrier to undergo structural exchange between
them. The flipping motion of the difluorophenylene inside a cage of the siloxaalkane spokes athigh and moderate temperatures indicates a promising function as a molecular gyroscope.
References : [1] Garcia-Garibay et al. J. Am. Chem. Soc., 124 (2002) 7719. [2] W. Setaka et al. Chem.
Lett. 39 (2010) 468. [3] D. Porezag et al. Phys. Rev. B, 51 (1995) 12947. [4] A. B. Marahatta et al. (to besubmitted).