Solid-state photochemical and photomechanical properties of molecular crystal nanorods composed of anthracene ester derivatives

Article abstract of DOI:10.1039/c1jm10228a

A series of 9-anthroate esters that can form photoresponsive molecular crystal nanorods is prepared and their properties are investigated. All crystal structures that can support a [4 + 4] photodimerization reaction lead to nanorods that undergo photomechanical deformations without fragmentation. In order to determine the molecular-level motions that give rise to the nanorod photomechanical response, the reaction of anthracene-9-carboxylic acid tert-butyl ester is studied in detail using X-ray diffraction and solid-state NMR techniques. The monomer crystal is well-aligned within the nanorod and reacts to form the photodimer crystal according to first-order kinetics. The solid-state reacted dimer crystal is a metastable intermediate that slowly converts into the low energy dimer crystal structure over the course of weeks. Based on single crystal X-ray diffraction studies and solid-state NMR data, this intermediate structure is likely composed of the [4 + 4] photodimer that has not yet undergone the ester group rotations and repacking is necessary to form the lower energy crystal polymorph that is produced directly by crystallization from solution. Our results show that the photomechanical response of these molecular crystal nanostructures is determined by nonequilibrium intermediate states and cannot be predicted based solely on knowledge of the equilibrium reactant and product crystal structures.

Full text of DOI:10.1039/c1jm10228a

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PAPER
Solid-state photochemical and photomechanical properties of molecular
crystal nanorods composed of anthracene ester derivatives†
Lingyan Zhu,a Arun Agarwal,a Jinfeng Lai,a Rabih O. Al-Kaysi,b Fook S. Tham,a Tarek Ghaddar,c
a
Leonard Mueller and Christopher J. Bardeen
a
*
Received 15th January 2011, Accepted 21st February 2011
DOI: 10.1039/c1jm10228a
A series of 9-anthroate esters that can form photoresponsive molecular crystal nanorods is prepared
and their properties are investigated. All crystal structures that can support a [4 + 4] photodimerization
reaction lead to nanorods that undergo photomechanical deformations without fragmentation. In
order to determine the molecular-level motions that give rise to the nanorod photomechanical response,
the reaction of anthracene-9-carboxylic acid tert-butyl ester is studied in detail using X-ray diffraction
and solid-state NMR techniques. The monomer crystal is well-aligned within the nanorod and reacts to
form the photodimer crystal according to first-order kinetics. The solid-state reacted dimer crystal is
a metastable intermediate that slowly converts into the low energy dimer crystal structure over the
course of weeks. Based on single crystal X-ray diffraction studies and solid-state NMR data, this
intermediate structure is likely composed of the [4 + 4] photodimer that has not yet undergone the ester
group rotations and repacking is necessary to form the lower energy crystal polymorph that is produced
directly by crystallization from solution. Our results show that the photomechanical response of these
molecular crystal nanostructures is determined by nonequilibrium intermediate states and cannot be
predicted based solely on knowledge of the equilibrium reactant and product crystal structures.
remain relatively rare. But interest in such photoreactive systems
Introduction
is increasing because of their possible use as photomechanical
transducers,8 providing an alternative type of material to
complement well-known polymer based photomechanical
materials.9–14 Reversible photomechanical changes were first
observed in charge-transfer crystals15 and later workers have
exploited photoinduced charge transfer dynamics to drive rapid
mechanical motions in microcrystals.16,17 Recently, Irie and
others have demonstrated that the reversible ring opening/
closing reaction in diarylethene derivatives18 can be used to drive
shape changes and induced motions in single crystals.19–22 Ther-
mally reversible photoinduced motion has also been observed in
single crystals of an azobenzene derivative,23 while dramatic
bending motions have been observed in crystals composed of the
photodimerized analog of the Green Fluorescence Protein
chromophore.24 These examples show that it is possible to find
molecules that can undergo physical rearrangements large
enough to induce crystal deformation, but not so large that they
disrupt the overall crystal integrity. Our group has taken
a different approach, where we use nanostructured molecular
crystals as a general way to avoid the fracture problem.25,26
We have previously shown that crystalline nanorods
composed of anthracene-9-carboxylic acid tert-butyl ester
(9TBAE) remain intact after undergoing an expansion of up to
15%.27 Under identical irradiation conditions, macroscopic
crystals of 9TBAE quickly fragment and disintegrate. We later
showed that a reversible [4 + 4] photodimerization reaction in
Organic solid-state photoreactions have long served as a testbed
for theories about how spatial constraints influence chemical
reactivity.1–3 Reactions in the crystal provide an environment
where the positions and orientations of both the reactant and
product molecules can be determined to atomic precision using
X-ray diffraction (XRD) analysis. But the study of photochem-
ical reactions in organic crystals is hampered by the tendency of
such crystals to shatter under illumination. This fragmentation is
believed to result from the build-up of interfacial strain between
reacted and unreacted regions of the crystal, and the resulting
phase reconstruction leads to fracture and decomposition.4
Attempts to circumvent this problem using irradiation in the
long-wavelength tail of the reactant absorption,5,6 or using two-
photon absorption,7 have been successful only in a few select
cases. On the whole, observations of single crystal-to-crystal
phototransformations of macroscopic organic molecular crystals
aDepartment of Chemistry, University of California, Riverside, Riverside,
CA 92521, USA. E-mail: christopher.bardeen@ucr.edu
bDepartment of Basic Sciences, King Saud bin Abdulaziz University for
Health Sciences, Riyadh, 11426, Kingdom of Saudi Arabia
cDepartment of Chemistry, American University of Beirut, Beirut, 11-0236,
Lebanon
† Electronic supplementary information (ESI) available. CCDC
reference numbers 808561–808575. For ESI and crystallographic data
in CIF or other electronic format see DOI: 10.1039/c1jm10228a
6258 | J. Mater. Chem., 2011, 21, 6258–6268
This journal is ª The Royal Society of Chemistry 2011

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anthracene-9-carboxylic acid (9AC) crystalline nanorods could
lead to reversible bending28 and that using spatially localized
two-photon excitation could induce controllable motion in these
nanoscale structures.29
benzene (Aldrich, 99.9%, 1.5 mL) and dried by adding activated
˚
molecular sieves (4 A). The alcohol/benzene solution was added
to the carboxylic acid solution and stirred for 1 hour at room
temperature. The progress of reaction was monitored using thin
layer chromatography (silica, 50 : 50 CH2Cl2 : hexane). After the
9AC was consumed, the organic layer was first washed with 20
mL of distilled water, and then with 15 mL of a K2CO3 (w/w
20%) water solution to hydrolyze excess trifluoroacetic anhy-
dride and remove the generated trifluoroacetic acid. The organic
layer was then washed with enough distilled water to get rid of
excess K2CO3 and dried with anhydrous MgSO4. The organic
solvents were removed under reduced pressure and the crude
product recrystallized using a minimum amount of hot ethanol
(20 mL). The product was obtained as yellow grey crystals 0.94 g
The use of nanostructured molecular crystals provides
a general way to harness organic solid-state photochemical
reactions to generate larger motions and perhaps do useful work
on submicron length scales. But there are still several outstanding
scientific questions in our work. First, how general is this
phenomenon—can it be extended beyond the two molecules
(9TBAE and 9AC) studied thus far? Second, what is the mech-
anism that underlies the observed photomechanical response?
The role of molecular alignment within the nanorods, the nature
of the crystal-to-crystal transformation, and the molecular-level
conformational changes in conformation are all open questions
raised by our previous work. In this paper, we attempt to address
these questions in a systematic way. We first show that there
exists a large family of anthracene ester derivatives, crystallizing
in a variety of packing motifs, that can be formed into nanorods
and that exhibit a significant photomechanical response. In order
to better understand the molecular level origins of the photo-
mechanical response, we study the 9TBAE system in detail.
Using XRD and solid-state nuclear magnetic resonance
(ssNMR), we find that the monomer nanorods consist of ordered
single crystal domains, and that irradiation leads to a continuous
transition to a second crystalline form. Surprisingly, XRD and
ssNMR data show that the crystal formed immediately after
photoreaction of the monomer solid is not the same as the crystal
obtained by crystallization of the photodimer directly from
solution. The photomechanical response is actually determined
by a crystalline intermediate that converts into the low-energy
polymorph only after a period of months. For 9TBAE, the XRD
and ssNMR data suggest that this intermediate structure
involves a photodimer whose ester sidegroups have not yet
rotated to their outward-facing equilibrium positions. The
creation of metastable intermediate crystal structures via
the solid-state photoreaction appears to be a general feature of
the anthracene esters. An important conclusion is that the
photomechanical response of this class of materials cannot be
predicted based on equilibrium structures of the reactant and
product molecular crystals, but instead arises from nonequilib-
rium crystal structures whose characterization is quite chal-
lenging.
ꢀ
1
(65% yield). Melting point: uncorrected 124–125 C. H-NMR
(400 MHz, CDCl3): d 1.58 (m, 8H), 1.90 (s, 3H), 2.38 (d, 2H),
7.49 (m, 4H), 8.00 (d, 2H), 8.12 (d, 2H), 8.47 (s, 1H). The other
anthracene-9-carboxylic acid esters were synthesized by the
same method, but with different alcohols and reaction times
(30 min–2 h).
The anthracene-9-carboxylic acid esters (3–6) were synthesized
by adding the anthracene-9-carboxylic acid to a solution of
N,N-dimethylmethanamide/K2CO3 to produce the anthracene-
9-carboxylate anion which is then reacted with excess of the
corresponding n-alkyl bromide or iodide. The average product
yield was around 55% after recrystallization from ethanol.
2
Preparation of nanorods and crystals
All the nanorods were prepared using similar procedures.27 As an
example, we describe the preparation of nanorods composed of
molecule 12. Approximately 5 mg of 12 was dissolved in 0.1 mL
of dry spectroscopic grade tetrahydrofuran (THF). The solution
was then deposited on top of anodized aluminium oxide (AAO)
template (Whatman Anodisc-13, 200 nm pore diameter, with
average membrane thickness of 60 microns, and 13 mm diam-
eter). This AAO template, loaded with the molecule of interest, is
then placed on top of a homemade Teflon support ring. The
AAO/Teflon holder was placed on a piece of Kimwipe (saturated
with 2 mL of THF) on a piece of thick ground glass (Chemglass,
4 inch diameter, 0.25 inch thickness). The whole setup was
covered by a 5 oz glass bell-jar. At room temperature, the solvent
in the bell jar evaporates over the course of 8–24 hours. During
this time, the solid 12 in the AAO dissolves in the solvent vapors
and slowly recrystallizes in the AAO nanochannels to form single
crystalline nanorods. After the solvent had evaporated, the AAO
template surface was polished using 1500 grit sandpaper to
remove excess 12 from both faces. Nanorods were released by
dissolving the AAO template in 50% aqueous phosphoric acid,
forming an aqueous suspension.
Experimental
1
Synthesis of materials
The anthracene-9-carboxylic acid esters (1, 2, 7–13) were
synthesized by the method of Parish and Stock.30 As an example,
we describe the synthesis of anthracene-9-carboxylic acid 1-
methyl-cyclohexyl ester (12) in detail. Anthracene-9-carboxylic
acid (TCI, 97% pure, 1.0 g, 4.5 mmoles) was suspended in dry
benzene (30 mL). Trifluoroacetic anhydride (Aldrich, 99% pure,
2.5 mL, 17.8 mmoles) was added via syringe to the stirring
suspension of anthracene-9-carboxylic acid in benzene. The
mixture was allowed to stir until the carboxylic acid dissolved
and the mixture became a clear yellow solution. 1-Methyl-
cyclohexanol (TCI, 96% pure, 1.1 g, 9.6 mmoles) was dissolved in
For single crystal X-ray analysis, single crystals were grown
using the slow evaporation method. 5 mg of the anthracene-9-
carboxylic acid ester was dissolved in 0.1 mL THF in a 2 mL
glass vial, which was then placed in a larger glass vial with 1 mL
methanol in the bottom. The larger glass vial is sealed and the
THF diffuses into the methanol over the course of several days.
After all the THF has partitioned into the methanol, single
crystals of the anthracene esters were collected from the bottom
of the 2 mL vial.
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In order to photoreact a single crystal of 9TBAE, a monomer
single crystal was mounted onto a glass fiber with epoxy resin,
inserted into a hollow copper pin (glued with epoxy) and
attached to a goniometer head. The goniometer head was placed
on a three dimensional translation stage to center the single
crystal for uniform irradiation using a frequency-doubled Ti:
Sapphire Laser at 400 nm. The laser power and exposure time
were controlled to convert the monomer single crystal homoge-
neously into the photodimer single crystal without breaking the
crystal.
Calculated 13C chemical shifts were referenced to benzene at
128.0 ppm from TMS.
Results and discussion
1
Characterization of monomer crystal nanorods
Fig. 1 shows the molecular structures of the compounds studied
in this paper. We are interested in the anthracene esters because
they comprise a family of compounds whose molecular structure
and crystal packing can be tuned by changing the ester group
attached at the 9-anthracene position. They are highly soluble
and, as shown below, amenable to nanorod growth. This family
of compounds was previously studied in the context of solid-state
triboluminescence,39 but here we are concerned with their solid-
state photochemistry. The ester substituents can be divided into
three broad classes: n-alkyl chains (1–6), branched alkyl chains
(7–10), and cyclohexyl derivatives (11–13). Using single crystal
XRD, we have determined the crystal structures for all the
molecules in Fig. 1. The photoreactive molecules crystallized into
one of the two basic crystal classes summarized in Fig. 2:
herringbone pair (HBP) and layered pair (LP). The other mole-
cules crystallized into structures where either neighboring
anthracene rings were not cofacial, for example herringbone
packing (2) or where the anthracene rings were too far away from
each other to react (10, 13). Note that the distance limit for
3
X-Ray diffraction measurements
Single-crystal X-ray diffraction data were collected on a Bruker
APEX2 platform CCD X-ray diffractometer system (Mo-radi-
˚
ation, l ¼ 0.71073 A, 50 KV/40 mA power) at 296 K and 100 K.
The frames were integrated using the Bruker SAINT software
package and a narrow-frame integration algorithm. Absorption
corrections were applied (absorption coefficient m ¼ 0.081 mmꢁ1
max/min transmission ¼ 0.9839/0.9603) to the raw intensity data
using the SADABS program. The Bruker SHELXTL software
package was used for phase determination and structure refine-
ment.31–34 Powder X-ray diffraction data were collected on
a Bruker D8 Advance X-ray powder diffractometer (CuK radi-
˚
ation, l ¼ 1.5418 A, 40 KV/40 mA power) at 296 K.
40
˚
photodimerization is estimated to be 4.7 A. One sign that
a solid would be reactive was a yellow color and yellow-green
fluorescence, indicating excimer formation due to interacting
anthracene p systems. In one case (13), the room temperature
crystal was unreactive, but the growth of a different polymorph
inside the AAO could be induced by modifying the nanorod
growth conditions. For 13, this entailed simply placing a micro-
scope coverslip underneath the AAO template during the solvent
annealing step. This polymorph was found to be photoreactive,
but its structure remains unknown since we were unable to grow
bulk crystals. A second point is that while molecules 4 and 5 were
photoreactive in the solid, their very low melting points pre-
vented observation of a mechanical response. The n-butyl and
n-pentyl groups appear to lead to a high amount of packing
disorder, making it difficult to grow single crystals and impos-
sible to grow nanorods. All the photoreactive molecules (1–3,
6–12 and the photoreactive polymorph of 13) could be formed
into 200 nm diameter nanorods using our solvent annealing
4
Scanning Electron Microscopy (SEM) measurements
For SEM measurements, a drop of the nanorod sample in
distilled water was placed on a piece of microscope coverslip and
stuck to a piece of conducting copper tape which was then fixed
on a SEM stub. The water was gently evaporated under vacuum
leaving behind dispersed bundles of nanorods. The SEM stub
was placed in a Sputter coater (Cressington 108 Auto) and coated
with Pt/Au for 40 seconds. The SEM stub was placed inside
a Scanning Electron Microscope (XL30-FEG) and the data were
collected.
5
13C Solid-State Nuclear Magnetic Resonance (ssNMR)
measurements
Cross-polarization (CP) magic-angle-spinning (MAS) solid-state
NMR experiments were performed at 14.1T (1H frequency
600 MHz) on a Bruker AVANCE spectrometer equipped with
a double-resonance 2.5 mm MAS probe, spinning at a MAS rate
of 27 kHz. 83 kHz 1H p/2 and decoupling pulses were used along
with a 2 ms CP spin-lock. During CP the 13C nutation rate was set
1
to 40.5 kHz and the H nutation rate ramped from 58–77 kHz.
For each spectrum, 4096 complex data points with a dwell of
12.5 ms (spectral width 40 kHz, total acquisition time 51.26 ms)
were acquired with a recycle delay of 5 s for a total experiment
time of 5 hours and 45 minutes.
6
Chemical shift calculations
Ab initio calculations were performed at the density functional
(DFT) level of theory using the B3LY functional35–37 within the
Gaussian 03 software package.38 Molecules were optimized to
their ground state geometries using the 6–31G(d,p) basis and
NMR chemical shifts calculated with the 6–311++G(d,p) basis.
Fig. 1 Molecular structures of anthracene esters studied in this paper.
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6260 | J. Mater. Chem., 2011, 21, 6258–6268

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properties of the molecules studied in this paper. One surprising
aspect of Table 1 is that the nanorod photomechanical response
is not specific to a single crystal type.
With the photomechanical response of the anthracene ester
nanorods established as a general phenomenon, the next ques-
tion concerns the physical basis of this response. We have
concentrated on the case of 9TBAE 8, as a prototypical ester
with a sizable photomechanical response. The room tempera-
ture single crystal XRD structure reveals rotational disorder in
the tert-butyl groups that disappears at 100 K. The details of the
XRD monomer crystal structure are summarized in Table 2. An
earlier 9TBAE crystal structure39 could not resolve this disorder
and had larger standard deviations in the unit cell dimensions as
well as a larger R1 value in comparison to our tert-butyl
disordered model. Both structures give the same overall picture
of the herringbone pair packing motif in the bulk and thus the
first question concerns the form of the monomer crystal within
the nanorods: is it the same as the bulk monomer crystal, and
does it have a preferred orientation? The powder XRD data in
Fig. 4 answer both questions in the affirmative. The powder
XRD pattern calculated from the bulk single crystal (Fig. 4a)
structure matches up well with that obtained experimentally
from the powdered sample (Fig. 4b)—all predicted diffraction
peaks are observed. If the nanorods are left in the amorphous
AAO template, oriented vertically, only a single diffraction peak
appears, corresponding to the (011) plane as assigned from the
calculated XRD pattern (Fig. 4c). This plane is shown in
Fig. 4d. The oriented, crystalline nature of individual nanorods
prepared by the solvent annealing method has been previously
established by TEM observations on individual rods.41 The data
in Fig. 4 are consistent with previous powder XRD measure-
ments on 9AC nanorods42 and confirm that the preferential
crystal orientation is a property of the entire sample and does
not vary from rod to rod.
Fig. 2 Crystal packing motifs of the two classes of photoreactive
anthracene esters: (a) layered pair (6); (b) herringbone pair (1).
methods in AAO templates. SEM images of some representative
examples are shown in Fig. 3. In these samples, where the rods
are clustered together, we noticed that over the course of several
weeks, individual rods would sometimes merge with neighboring
rods to form larger diameter crystals. This indicates that the
nanorods are not the equilibrium structure, and that the mole-
cules retain some mobility and the ability to merge into larger
crystals in the absence of the template constraints.
When nanorods composed of photoreactive molecules were
exposed to 365 nm light in a fluorescence microscope, they
typically underwent movement and expanded in length. In bulk
crystals, the coexistence of the two different crystalline phases
during the course of the photodimerization reaction causes the
crystal to build up internal stress and shatter.4 Apparently, the
nanorods’ large surface-to-volume ratio allows them to accom-
modate the volume changes during photodimerization and
remain intact. Bundles tend to separate under the influence of the
light (see Fig. S2 and S3 in the ESI†). The rods retained their
shape and did not break. We found that the amount of length
change for a given molecular crystal nanorod varied within
a sample. For example, for nanorods composed of 12, expan-
sions ranged from 5% to 25%. This variability has also been
observed in rods composed of 9AC that undergo reversible
bending.29 Variations in crystallinity or the degree of surface
adhesion have been identified as possible factors that contribute
to the observed variability. Nevertheless, we can say that the
anthracene-9-carboxylic acid esters form a large family of
compounds that can be solvent annealed into crystalline nano-
rods manifesting measurable photomechanical motions in
response to near-UV irradiation. Table 1 summarizes the
2
X-Ray diffraction characterization of photodimer crystal
structures: SSRD, SGD and PRD
After exposure to UV light, the monomer photodimerizes and
a new crystal structure is formed, which we will refer to as the
solid-state reacted dimer (SSRD). In an earlier paper, we char-
acterized the crystal structure of the dimer grown from a solution
of CH2Cl2, which resulted in solvent molecules being incorpo-
rated into the lattice.27 After some trial-and-error, we found that
slow evaporation of a monomer solution in n-hexane during UV
irradiation could produce large photodimer crystals without
inclusion of solvent molecules. This crystal structure, which we
Fig. 3 SEM images of 200 nm nanorods composed of different anthracene esters: (a) 7, (b) 11 and (c) 12 (scale bar is 5 mm).
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Table 1 Properties of anthracene esters
Anthracene esters
Crystal packinga
Photoreactivity of crystal
Nanorods growth
Photomechanical response
1
2
3
4
5
6
7
8
HBP
HB
LP
HBP
LP
LP
HBP
HBP
HBP
DP
HBP
LP
OHBP
Shatter
No response
Shatter
Melt
Yes
Not attempted
Yes
No
No
Yes
Yes
Yes
Yes
Not attempted
Yes
Yes
Yes
Yes
—
Yes
—
Melt
—
Shatter
Shatter
Shatter
Shatter
No response
Shatter
Shatter
Shatterb
Yes
Yes
Yes
Yes
—
Yes
Yes
Yes
9
10
11
12
13
a
HB ¼ herringbone, HBP ¼ herringbone pair, LP ¼ layered pair, DP ¼ distant pair where anthracenes are too far apart to react, OHBP ¼ offset
b
herringbone pair where anthracenes are not aligned for reaction. Photoreactivity and photomechanical response for 13 reported for polymorph
with the unknown crystal structure.
Table 2 Crystal data and structure refinement for monomer, PRD and SGD
Empirical formula
C19H18O2 (monomer)
C19H18O2 (PRD)
C38H36O4 (SGD)
Formula weight
Temperature
Wavelength
278.33
296(2) K
˚
0.71073 A
278.33
296(2) K
˚
0.71073 A
556.67
100(2) K
˚
0.71073 A
Crystal system
Space group
Unit cell dimensions
Monoclinic
P2(1)/n (#14)
Monoclinic
P2(1)/n (#14)
Triclinic
P-1
ꢀ
ꢀ
ꢀ
ꢀ
˚
˚
˚
˚
a ¼ 9.1313(7) A, a ¼ 90
a ¼ 9.1419(7) A, a ¼ 90
a ¼ 9.1605(2) A, a ¼ 67.1901(3)
ꢀ
ꢀ
ꢀ
˚
b ¼ 17.5205(14) A, b ¼ 99.9029(14)
b ¼ 17.5398(14) A, b ¼ 99.8192(13)
b ¼ 9.6923(2) A, b ¼ 85.3952(4)
ꢀ
ꢀ
ꢀ
˚
˚
˚
c ¼ 9.7613(8) A, g ¼ 90
c ¼ 9.7526(8) A, g ¼ 90
c ¼ 10.2180(3) A, g ¼ 62.2311(3)
3
3
3
˚
˚
˚
734.34(3) A
Volume
Z
Density (calculated)
Absorption coefficient
F(000)
1538.4(2) A
4
1540.9(2) A
4
1
1.202 Mg mꢁ3
0.077 mmꢁ1
592
1.200 Mg mꢁ3
0.077 mmꢁ1
592
1.259 Mg mꢁ3
0.080 mmꢁ1
296
Crystal size
q Range
Index ranges
0.20 ꢃ 0.17 ꢃ 0.10 mm3
2.32 to 24.71ꢀ
ꢁ10 # h # 10
ꢁ20 # k # 20
ꢁ11 # l # 11
23 942
0.47 ꢃ 0.39 ꢃ 0.18 mm3
2.32 to 29.57ꢀ
12 # h # 12
ꢁ24 # k # 24
ꢁ13 # l # 13
22 802
0.37 ꢃ 0.25 ꢃ 0.09 mm3
2.32 to 30.50ꢀ
ꢁ13 # h # 13
ꢁ13 # k # 13
ꢁ14 # l # 14
17 538
Reflections collected
Independent reflections
Completeness to q ¼ 24.71ꢀ
Absorption correction
Max. and min. transmission
Refinement method
2630[R(int) ¼ 0.0352]
4329[R(int) ¼ 0.0192]
4448[R(int) ¼ 0.0224]
100.0%
100.0%
99.7%
Semi-empirical from equivalents
0.9925 and 0.9845
F2
Semi-empirical from equivalents
0.9864 and 0.9649
F2
Semi-empirical from equivalents
0.9926 and 0.9710
F2
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices [I > 2s(I)]
R indices (all data)
2630/36/224
1.038
4329/230/260
1.021
4448/0/193
1.056
R1 ¼ 0.0470, wR2 ¼ 0.1144
R1 ¼ 0.0391, wR2 ¼ 0.1062
R R1 ¼ 0.0415, wR2 ¼ 0.1110
R1 ¼ 0.0688, wR2 ¼ 0.1303
R1 ¼ 0.0644, wR2 ¼ 0.1254
R1 ¼ 0.0467, wR2 ¼ 0.1155
ꢁ3
ꢁ3
ꢁ3
˚
˚
˚
Largest diff. peak and hole
0.361 and ꢁ0.274 e A
0.176 and ꢁ0.122 e A
0.460 and ꢁ0.203 e A
will refer to as the solution grown dimer (SGD), is shown in
Fig. 5a, and its characteristics are summarized in Table 2. In
addition to the cyclohexyl bridge structure seen in all anthracene
dimers, there are two other important changes in the SGD
structure when compared to the monomer crystal in Fig. 4d.
First, the ester groups are now rotated ꢂ180ꢀ so that the
tert-butyl group on one anthracene points outward, away from
the neighboring anthracene. Second, the anthracene dimers have
rearranged themselves from the HBP structure into a layered
crystal where the anthracene rings are parallel with each other.
As can be seen from Table 2, the volume per anthracene of the
SGD is significantly smaller than that of the monomer crystal. At
first glance, this result is surprising given our previous results that
indicated a volume increase after irradiation.27 But the powder
XRD data make it clear that the crystal structure of the SGD is
different from that of the SSRD crystal formed immediately after
irradiation of the crystalline monomer. Fig. 5b and c compare the
calculated powder XRD pattern for the SGD to the experimental
data for the powdered SGD. The good agreement between the
two patterns shows that there is no systematic error in our
powder XRD measurements. But when the SGD diffraction
pattern is compared to that of the SSRD, obtained either from an
irradiated powder (Fig. 5d) or from the nanorods (Fig. 5e), there
are clear differences in peak positions and relative intensities.
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Fig. 4 Powder X-ray diffraction patterns of 9TBAE 8 monomer:
(a) pattern of 9TBAE monomer calculated based on the single crystal
structure; (b) experimental pattern of 9TBAE monomer bulk; (c) oriented
9TBAE monomer nanorods; and (d) oriented 9TBAE monomer
nanorods in AAO template. No peaks were observed below 2q ¼ 10ꢀ.
Fig. 6 Powder X-ray diffraction patterns of single-crystal-to-single-
crystal transition from orientated 9TBAE monomer nanorods to dimer:
(a) (011) peak of 9TBAE monomer nanorods before irradiation; (b) after
1 min irradiation; (c) after 5 min irradiation; (d) after 10 min irradiation;
(e) after 20 min irradiation; (f) after 1 hour irradiation; and (g) expo-
nential fit of the decay of the monomer peak (solid line) and the growth of
the dimer peak (dashed line).
Fig. 5f shows the powder XRD of the reacted nanorods in the
AAO template, which exhibit only a single peak from the SSRD
powder pattern, providing evidence that the crystal orientation
in the nanorod is retained after photoreaction.
The transformation from monomer to the SSRD appears to be
a simple first-order kinetic process. We can monitor these
kinetics using powder XRD peak areas. Fig. 6 shows the trans-
formation of the templated nanorods after exposure to 365 nm
light. The diffraction peak at 10.5ꢀ disappears and a new peak
grows in at 10.8ꢀ. A series of powder XRD patterns showing the
crystal-to-crystal transformation in this region is shown in
Fig. 6a–f. Analysis of the peak areas in Fig. 6a–f indicates that
the reaction is a simple two-state process, since the exponential
decay of the reactant peak is mirrored by the growth of the
product peak, as shown in Fig. 6g. The temporal evolution of
the peaks in the ssNMR spectrum can also be monitored during
the course of the reaction. Again, no peaks other than the
monomer and SSRD are observed during the evolution of the
spectrum. Thus both powder XRD and ssNMR data are
consistent with formation of a single product during the reaction.
On longer timescales, we have found that the SSRD crystal
structure is metastable. Over the course of several months,
powder XRD and ssNMR measurements indicate that the SSRD
slowly converts into the SGD structure, which is stable on this
timescale, and also partially back to the monomer. Fig. 7 shows
the transformation of the SSRD powder XRD pattern into
a mixture with both monomer and SGD crystal peaks. A similar
transformation is observed in the ssNMR spectra of samples that
have been stored for a period of 2 year at room temperature
(see ESI†, Fig. S6). This same phenomenon has also been
observed in molecules 1 and 12 (ESI†, Fig. S7), and the existence
of a metastable SSRD structure that slowly relaxes to the more
stable SGD structure appears to be a general feature of the
anthracene esters.
In an attempt to determine the structure of the SSRD, we used
the long wavelength tail irradiation method to photoreact a mil-
limetre-scale crystal without fragmenting it. Many workers have
used single crystal XRD to follow the progress of solid-state
photodimerization reactions, with trans-cinnamic acid deriva-
tives comprising the most commonly studied system43–47 in
Fig. 5 (a) Crystal structure of solution grown dimer (SGD) from hexane; powder X-ray diffraction patterns: (b) calculated pattern of SGD; (c)
experimental pattern of SGD; (d) experimental pattern of dimer nanorods; (e) experimental pattern of SSRD bulk; and (f) experimental pattern of dimer
nanorods in AAO template.
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Fig. 7 Powder X-ray diffraction patterns: (a) calculated pattern of SGD
8; (b) black-experimental pattern of fresh SSRD 8, red-experimental
pattern of old SSRD 8; (c) calculated pattern of monomer 8. After several
months, SSRD 8 slowly turn into monomer 8 (new peaks of SSRD 8 at
10.48ꢀ, 11.04ꢀ and 12.26ꢀ are features of monomer 8) and SGD 8 (new
peaks at 9.36ꢀ, 11.3ꢀ and 12.64ꢀ are features of SGD 8).
addition to other molecules.48–51 After reacting a 9TBAE crystal
under 400 nm illumination for 25 min, ꢂ20% of the crystal was
converted into the dimer. The conversion was limited to about
20% due to crystal disintegration, so the reaction could not be
followed to completion, similar to what has been observed for the
solid-state [4 + 4] photodimerization of 9-methylanthracene.50,51
The structure was solved assuming a mixture of the known
monomer crystal structure and the unknown dimer, and the
crystal parameters are summarized in Table 2. The resulting
dimer structure, which we refer to as the partially reacted dimer
(PRD) is shown in Fig. 8, along with the monomer and SGD
molecular structures for comparison. The main difference
between the PRD and SGD molecular structures is that the ester
groups are inward-facing for the PRD but have rotated
approximately 180ꢀ to face outward in the SGD. The question is
whether this inward-facing ester conformation is a hallmark of
the metastable SSRD species that determines the photome-
chanical response of the nanorods.
Fig. 8 Three different views of the reactive molecular pairs obtained
from the crystal structures of (a) monomeric 8; (b) the PRD; (c) the SGD.
of the methyl carbons (17–19) from 31.5 ppm in the monomer
and 31.8 ppm in the SSRD to 30.4 ppm in the SGD. Also, carbon
16 shifts from 87.6 ppm in the SSRD to 84.8 ppm in the SGD.
Thus the environment of the tert-butyl group seems to be
different between the SSRD and SGD. The most obvious
difference between the two spectra is a symmetric pairing of the
3
NMR chemical shift analysis of SSRD molecular
conformation
In order to address the conformational structure of the dimer in
the SSRD crystal, we turn to ssNMR. First, we compare the 13
C
spectra of the different species. The inset in Fig. 9 shows the
carbon numbering of the carbons in 8, while Fig. 9a shows the
13C spectrum of a monomer crystal powder of 8. When this
powder is irradiated, the spectrum smoothly transforms into that
in Fig. 9b. If, on the other hand, we irradiate the monomer in
solution and grow the dimer crystal after reaction, this solid
yields the SGD 13C spectrum in Fig. 9c. Obvious changes in both
the SSRD and SGD spectra include the shift of four aromatic
carbons (5, 7, 12 and 14) from 130 ppm to 145 ppm due to the
creation of neighboring sp3 carbons, the appearance of 2 new sp3
(6, 13) carbon resonances at 58 ppm and 70 ppm, and the shift of
the carbonyl carbon (15) from 172.7 ppm to ꢂ176 ppm in the
dimer. In addition, some clear differences between the spectra of
the SSRD and SGD should be highlighted. First, there is a shift
Fig. 9 (Inset) Carbon numbering for 9TBAE monomer; (a) the 13C solid
state NMR spectra for the monomer 8; (b) SSRD; and (c) SGD. The
SSRD chemical shifts for C6 (58.6 ppm); C15 (175.8 ppm); C5, C7, C12
and C14 (144.5/147.5 ppm); C16 (87.6 ppm); and C18 (31.8 ppm) are
marked with red dashed lines in order to facilitate evaluations of chemical
shift changes between the three species as per the discussion in the text.
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sp2 carbon peaks (5, 7, 12, 14) at 145 ppm in the SSRD, whereas
these peaks overlap each other and appear as a single broadened
peak in the SGD.
(except methyls) in Table 3, we obtain a RMSD for the SSRD/
0ꢀ chemical shift comparison of 1.9 ppm, as opposed to a RMSD
for the SSRD/180ꢀ chemical shifts of 2.6 ppm. This lower RMSD
value indicates that the 0ꢀ calculation provides a better fit for the
SSRD data, although the two values are not sufficiently different
to conclusively prove that the 0ꢀ PRD structure provides a better
fit to the SSRD.
Detailed analysis of the 13C chemical shifts supports the
assignment of the SSRD to the inward-facing ester group species
suggested by the PRD structure. ssNMR experiments have been
used successfully to analyze both geometry changes and reaction
dynamics, particularly in the trans-cinnamic acid [2 + 2] photo-
dimerization,47,52–55 but also for 9-methylanthracene.56 In
conjunction with ab initio calculations of the chemical shifts,
these experiments can provide constraints on the molecular
geometry.57 Table 3 summarizes the experimental chemical shifts
for the assigned carbons, along with the theoretically calculated
values for the ester group rotated 0ꢀ (inward facing, corre-
sponding to the PRD structure) and 180ꢀ (outward facing, cor-
responding to the SGD structure). We found that the calculated
shifts for the methyl carbons systematically underestimate the
experimental values by 5 ppm or more, even for solution values.
From calculations, it appears that rotation of the tert-butyl
group, which we know is present from the XRD crystal structure,
can increase the chemical shift values for these peaks by 3 ppm or
more, which provides at least a partial explanation for the
discrepancy. To simplify the analysis, we do not consider these
peaks, centered around 30 ppm, when comparing the experi-
mental and theoretical chemical shift values. We also have not
tried to resolve individual shifts in the crowded aromatic region
around 130 ppm. When we compare the chemical shifts calcu-
lated for the 0ꢀ and 180ꢀ structures to the experimental chemical
shifts for the SSRD, we can calculate root-mean-square devia-
tion (RMSD) values that reflect the sum of various 13C chemical
shifts that are better reproduced by the 0ꢀ calculation than by the
180ꢀ calculation. Using the values for the assigned carbons
The pronounced symmetric pair at ꢂ145 ppm in the SSRD
deserves special mention. This chemical shift corresponds to the
sp2 carbons on both sides of the newly formed sp3 carbons in the
photodimer (carbons 5 and 7, and carbons 12 and 14). For any
stationary structure where the esters are asymmetrically tilted
(e.g. the SGD), these four carbons are inequivalent and each will
have distinct but similar chemical shifts, appearing as a single
broad peak in the ssNMR spectrum. If these four carbons see
a symmetric environment, however, carbons 5 and 7 will be
equivalent and carbons 12 and 14 will also be equivalent. The
four individual peaks then collapse into a symmetric pair, with
one peak due to carbons 5 and 7, and the other due to carbons 12
and 14. The question then becomes whether it is possible to
distinguish between the outward-facing ester groups (180ꢀ rota-
tion from the monomer, similar to the ꢂ160ꢀ rotation in the
SGD) and the inward-facing ester groups (0ꢀ rotation, corre-
sponding to the PRD). The advantage of using the relative shift
to analyze the geometry is that any systematic error in the shift
calculations will cancel out. From the theoretical values, we see
that the difference between carbons 5/7 and 12/14 is predicted to
be much larger for the 0ꢀ ester (4.3 ppm) than for the 180ꢀ ester
(2.0 ppm), similar to what is observed for the SSRD (3.0 ppm)
versus the SGD (<1.0 ppm). The smaller separation between the
two resonances for the outward facing ester group is confirmed
by comparison to the solution phase 13C NMR spectrum of the
SGD dimer, where the ester group is expected to be freely moving
and facing outward. In solution, the separation between the two
groups of peaks can be resolved at 1.1 ppm (ESI†, Fig. S5). The
large change in the 5/7 and 12/14 separation predicted for
outward versus inward-facing ester groups is consistent with the
experimentally observed difference between the SGD and the
SSRD. Both the RMSD value and the value of the separation
between the 5/7 and 12/14 peak pair are consistent with the
inward-facing ester structure suggested by the PRD structure.
The last question is whether the symmetric environment experi-
enced by these carbons is due to the ester perfectly oriented at
180ꢀ, or whether it is due to the dynamic averaging by ester
rotation. Previous workers have shown how chemical reactions
within a crystal lattice can generate strain and change the elastic
properties of the crystal, which could in turn change dynamic
properties like the ester rotation.2,58,59 Since dynamic ester
rotation would be most likely in the SSRD, we measured the
spectrum of the SSRD at various temperatures and found that
Table 3 Experimental chemical Shifts (ppm) for SSRD and SGD, and
theoretical chemical shifts calculated for 0ꢀ rotation (inward-facing) ester
group, and 180ꢀ rotation (outward-facing) ester group. The theoretical
chemical shift of carbon C18 actually is the average chemical shift of
carbons 17, 18, and 19, which are not resolved experimentally; ‘A’ refers
to the unassigned aromatic atoms in the region of 125–135 ppm
Experimental
Monomer
Theoretical
0 degree
SGD
A
SSRD
A
180 degree
C1
C2
C3
C4
C5
C6
C7
C8
127.3
125
125.4
127.8
145.7
57.1
145.7
127.8
125.4
125
127.3
150
70.3
128.3
124.7
125.5
126.7
147.5
55.6
147.5
126.7
125.5
124.7
128.3
149.5
70.5
146.3
58
146.3
144.5
58.6
144.5
A
C9
A
A
ꢀ
it does not change even after cooling to ꢁ30 C. Thus there are
C10
C11
C12
C13
C14
C15
C16
C17
C18
C19
no obvious signs of dynamic rotation of the ester group,
although it is possible that the barrier is low enough that the
temperature decrease to ꢁ30 ꢀC was not enough to affect the
dynamics. We have also looked for changes in the dynamics
by comparing the T1 relaxation times of the various carbons in
the monomer, SSRD and SGD spectra. All the carbon peaks
exhibit at most a 10% change in the relaxation times (ESI†,
Table S3).
146.3
69.6
147.5
70.6
146.3
176.2
84.8
147.5
175.8
87.6
150
149.5
177.6
83.8
172.7
86
31.5
176.2
84.1
23.1
30.4
31.8
22.8
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4
Changes in conformations and packing
analysis also failed to converge on structures with acceptable
figures of merit, at least when compared to previous literature
results. We are currently pursuing a multi-pronged effort to
understand the structure of this intermediate crystal using
computational methods subject to the constraints provided by
our XRD and ssNMR measurements.
In order to illustrate the sequence of steps that occur after
photoexcitation of 8 in the crystal, we now refer back to Fig. 8.
Fig. 8 shows several views of the structures involved in the
transformation, from the unreacted monomer pair (Fig. 8a), to
the PRD (Fig. 8b), and finally to the fully converted equilibrium
dimer structure of the SGD (Fig. 8c). Upon irradiation, the two
molecules that compose the dimer pair in Fig. 8a must undergo
two movements. First, the two anthracene rings must shift during
the [4 + 4] dimerization reaction so that the new bonds can form.
Second, the newly formed sp3 carbons force the anthracene rings
into a nonplanar ‘‘butterfly’’ conformation, as seen in Fig. 8b. We
hypothesize that both of these processes are rapid (ꢄ1 s) at room
temperature and drive the formation of the SSRD immediately
after the photoreaction. After the formation of the bicyclic ring
between the anthracenes, the newly formed dimer molecules
must execute two additional motions in order to eventually reach
the low energy SGD conformation. First, the tert-butyl ester
groups must rotate ꢂ180ꢀ from inward-facing with respect to the
anthracene sandwich pair to an outward-facing position with
the alkyl sidechains going diagonally up and away from the
anthracene pair. Our ab initio calculations have determined the
barrier for this rotation is approximately 8 kcal molꢁ1, suggesting
that this motion would be slow at room temperature in the
isolated molecule, and even slower in the dense crystal environ-
ment. Second, the dimer molecules must reassemble into the
layered structure shown in Fig. 5a. We hypothesize that both
processes are very slow compared to the formation of the
photodimer, which leaves the solid trapped in the metastable
SSRD structure for a period of months.
Conclusions
This paper extends our earlier study of the photophysics of
photomechanically responsive molecular crystal nanostructures
based on anthracene esters.27 These molecules form a versatile
family of compounds that can be formed into photoresponsive
nanorods despite variations in molecular structure and crystal
packing motifs. A detailed study of 9TBAE 8 provides significant
insight into the molecular-level dynamics that give rise to the
micron-scale response of the nanorods. The photomechanical
response of the nanorods is determined by a metastable crystal-
line intermediate (SSRD) that slowly converts into the low
energy solution grown dimer (SGD) crystal structure over the
course of weeks. Thus the photomechanical response of the
molecular crystal nanostructures arises from nonequilibrium
crystal forms and cannot be predicted using only knowledge of
the equilibrium reactant and product crystals. We have used
a variety of experimental measurements to establish the struc-
tural characteristics of the SSRD, which appears to involve the
photodimer in a higher energy conformation with the ester
sidegroups pointing inward prior to rotating to the lower energy
outward-facing position. The results in this paper indicate that
a deeper understanding of solid-state reactivity is required to
gain a predictive understanding of how solid-state photochem-
istry gives rise to micron-scale motions.
The micron-scale photomechanical response of the nanorods
is determined by the change in going between the monomer and
metastable SSRD crystal structures. To predict the overall
response, we need to know both how the local molecular struc-
ture changes and how the intermolecular packing changes, i.e.
how the unit cell changes in the SSRD. Unfortunately, the PRD
crystal parameters in Table 2 are obtained by solving the crystal
structure under the assumption that the crystal is a homogeneous
mixture of the monomer and a disordered component whose
structure is identical to the PRD. Thus the solution provides only
a single set of unit cell parameters which end up being close to
those of the majority species, in this case the monomer. While
this approach permits us to ascertain the molecular structure of
the PRD, it does not provide a reliable way to predict the packing
and thus the powder XRD pattern of the SSRD. Although there
are qualitative similarities between the calculated and experi-
mental powder XRD patterns (ESI†), such similarities are not in
general sufficient to infer crystalline structure.60 To determine the
packing motif of the PRD that generates the SSRD, we
attempted to work backward from the SSRD powder diffraction
pattern. Using the program DICVOL, we can index identifiable
peaks in the pattern shown in Fig. 5d to obtain the unit cell
parameters a, b, c and angles a, b, g. But while this approach
works very well for the monomer crystal, it failed for the SSRD,
where we obtained a large number of solutions, often with quite
different unit cell parameters, but with similarly poor figures of
merit. Attempts to use these unit cell parameters as inputs for
programs that optimize crystal structures for powder XRD
Acknowledgements
This research is supported by the National Science Foundation,
grants DMR-0907310 to CJB and CHE-0848607 to LJM. The
electron microscopy measurements were done at the Center for
Advanced Materials and Microscopy (CFAMM) at UC River-
side. RO Al-Kaysi acknowledges KSAU-HS/KAIMRC through
grant RC08/093 and Dr Bilal Kaafarani of the American
University of Beirut for providing the alkyl bromides.
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