A novel organic-inorganic hybrid with two polyhedral
oligosilsesquioxane (POSS) nanoparticles
covalently attached to perylene diimide (PDI) via a rigid 1,4-phenylene linkage
(POSS-PDI-POSS)
was designed and synthesized to examine the effect of bulky and well-defined nanoparticle
side chains on the self-assembly behavior of PDI derivatives. The molecules were self-assembled directly by
slow evaporation of a cast drop from solution in tetrahydrofuran to give rise
to uniform crystalline nanobelts with dimensions typically of0.2 mm1 μm 50 nm. The
phase behavior and crystal structure of the sample were then elucidated via a
combination of different experimental techniques such as differential scanning
calorimetry (DSC), wide angle X-ray diffraction (WAXD), selected area electron
diffraction (SAED) in transmission electron microscopy (TEM), polarized light
microscopy and atomic force microscopy. One-dimensional (1D) WAXD and DSC revealed
that only one crystalline phase exists. Based on the 2D WAXD fiber pattern obtained
from the orientedPOSS-PDI-POSSsamples, the crystalline structure was
determined to be a triclinic unit cell with dimensions of a = 6.577 nm, b = 5.213 nm, c = 1.107 nm, a = 93.26o, b = 94.85o, and g = 92.73o, which was confirmed by SAED experiments on the
single crystals with different crystal zone orientations. The detailed molecular conformational analysis
indicated that the steric hindrance of thePOSSnanoparticles covalently attached to PDI via a rigid 1,4-phenylene linkage makes
it difficult to achieve a continuous stacking of PDIs. Instead, the molecules dimerized to maximize
the π-π interaction. The dimers then became the building blocks and
packed themselves into the unit cell. This strong tendency for dimerization was proven by concentration-dependent
ultraviolet/visible absorption spectra and tandem mass spectroscopy with traveling wave ion mobility
separation. The combined SAED and TEM results showed that the c*-axis of the crystal is along the
elongated direction of the single crystal nanobelt and the normal direction of
the π-π stacking is along the a*-axis. A crystal structure with six dimers as one
supra-molecular motif in one unit cell was proposed to account for the
unusually large unit cell dimensions. The
complex structure could be attributed to the longitudinal, transverse, and
slightly rotational offsets between the PDIs in the dimers and interdigitated
neighboring dimers due probably to both electrostatic interactions and steric
demands. The molecular packing scheme in
the crystal was simulated using Cerius2 software, and the resulting
diffraction data agreed well with the experimental results. The rationale for such 1D nanostructured morphology
formation is also discussed.
