C O M M U N I C A T I O N S
generate large strain energies.16,17 Since the crystal layers at the
surface can relax more easily, the high surface-to-volume ratio of
the rods leads to a mechanism for strain relief that is absent in
larger crystals. This mechanism has been inferred from the epitaxial
growth of inorganic crystal layers18 but not in organic materials to
our knowledge.
In earlier work on spiropyran single crystals,19 it was determined
that 600 layers were required to undergo photoisomerization to
produce a reversible crystal expansion of 1 monolayer. Thus the
photoinduced change in the crystal dimensions was 0.16%, almost
2 orders of magnitude smaller than that in the 9-TBAE rods. We
have made preliminary attempts to examine the reversibility by
using short wavelength UV light in an attempt to photodissociate
the dimers. Irradiating the dimerized nanorods with 254 nm light
caused a slight ∼3% decrease in length, but even after extended
illumination (>1 h), the rods did not shrink back to their original
dimensions, and the yellow excimer fluorescence did not reappear.
Additional work is needed to determine the degree of reversibility.
We have shown how crystalline organic nanorods can undergo
large length changes upon photodimerization. While this photo-
chemistry typically leads to the fragmentation of 9-TBAE crystals,
the nanorods retain their shape and integrity. The ability of the
rods to survive the crystal-to-crystal transition is likely due to their
nanoscale dimensions, which maximize the surface-to-volume ratio
and provide a way to relieve the crystal strain induced by the
photoreaction. This phenomenon may provide a general way to
harness the power of organic solid-state photochemistry to do work
on the nanometer scale.
Figure 3. X-ray crystal structures of (a) monomer. The distance between
the peripheral hydrogens of the tert-butyl group is 12.6 Å, and the shortest
distance between the aromatic rings is 3.4 Å. (b) Dimer: Distance between
the peripheral hydrogens of the tert-butyl groups is 14.2 Å, and the distance
between the outer aromatic rings is 3.6 Å. The dimensions of the boxes in
(a) and (b) are the same.
herringbone arrangement of dimers.12 The face-to-face arrangement
of the anthracene moieties, with the tert-butyl groups of neighboring
anthracenes trans to each other, is known to facilitate efficient
photodimerization.13 The photodimer could not be crystallized
without inclusion of solvent molecules, but these fill the interstitial
spaces and probably have little effect on the molecular packing. A
more serious concern is whether the solvent-grown dimer crystals
accurately reflect the structures generated by the solid-to-solid
photodimerized nanorods. In general, solid-state photochemical
reactions require significant reconstructive phase transitions.14 The
powder X-ray diffraction patterns for the solution-grown crystals
and the photodimerized nanorods show the same number and
spacing of diffraction peaks, indicating that the solvent-grown
crystal reflects the photogenerated dimer structure in the rods. The
fact that the monomer and dimer crystals have different packing
motifs indicates that the photodimerization drives a phase transition
that alters the rod dimensions while preserving its morphology.
Figure 3b shows that, although the cycloaddition has drawn the
carbons on the central phenyl rings closer, the outer phenyl rings
are actually spaced farther apart due to a ring puckering induced
by loss of conjugation. Even more dramatic is the rotation of the
tert-butyl groups from overlapping the neighboring anthracene ring
to pointing in the opposite direction. The net effect is to increase
the volume occupied per anthracene unit. X-ray diffraction deter-
mination of the crystal parameters confirms that the volume per
anthracene moiety changes from 371 A3 in the monomer crystal to
407 A3 in the dimer crystal, an increase of 9.7%. Since the X-ray
analysis was done on pure crystals, this volume change should
provide an upper bound for what can be observed in the nanorods.
The AFM measurements on individual rods yield a 15% increase
in length accompanied by an average 3.5% decrease in radius,
resulting in an overall volume increase of 7% for a single rod. This
increase is slightly less than the 9.7% volume increase estimated
from the crystal structures, probably because we do not photo-
dimerize all of the monomers in a given rod. Evidence for
incomplete conversion is provided by the blue monomer fluores-
cence, which originates from isolated, unreacted monomers trapped
within the dimer crystal.15
Acknowledgment. This work was supported by the NSF, Grant
CHE-0517095, and the Central Facility for Advanced Microscopy
and Microanalysis (CFAMM) at UCR.
Supporting Information Available: Synthesis of 9-TBAE and the
photodimer, with the crystallographic information, CIF files, AFM,
SEM, and TEM experimental conditions are available. This material
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