Organic nanocrystals of [2.2]paracyclophanes achieved via sonochemistry: Enhanced and red-shifted emission involving edge-to-face chromophores

Article abstract of DOI:10.1039/c2ce26000j

We have prepared organic nanocrystals of [2.2]paracyclophane (pCp) and tetrakis(4-pyridylcyclobutyl)[2.2]paracyclophane (tpcp) via sonochemistry. Both nanocrystals exhibit an enhanced fluorescence compared to dilute solution, while the tpcp nanocrystals also demonstrate a red-shifted fluorescence.

Full text of DOI:10.1039/c2ce26000j

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COMMUNICATION
Organic nanocrystals of [2.2]paracyclophanes achieved via
sonochemistry: enhanced and red-shifted emission involving edge-to-
face chromophores{
Elizabeth Elacqua,a Paul T. Jurgens,a Jonas Baltrusaitisac and Leonard R. MacGillivray*ab
Received 22nd June 2012, Accepted 22nd August 2012
DOI: 10.1039/c2ce26000j
laterally- substituted derivative tetrakis(4-pyridylcyclobutyl)[2.2]
paracyclophane (tpcp) (Fig. 1).18 Originally studied by Cram,19
and extensively developed by Hopf20 and others,21 pCp has
garnered much interest owing to unique properties21 (e.g. optical,
reactivity, chirality) conferred by the two co-facially stacked
benzene rings connected by aliphatic bridges. Moreover, while
both synthesis and materials aspects of pCp are of much continued
interest, the generation of nanostructured pCp is underdeveloped.
We demonstrate that while exclusive reprecipitation does not
afford nanostructured pCp, the use of sonochemistry produces
nanocrystals of sizes ,500 nm. Nanodispersions of the pCps are
also shown to exhibit enhanced emission compared to solution.
The emission is attributed to edge-to-face aggregation and packing
in the solid state that promotes intermolecular interactions able to
maximize interchromophore communication (Scheme 1).22
We have prepared organic nanocrystals of [2.2]paracyclophane
(pCp) and tetrakis(4-pyridylcyclobutyl)[2.2]paracyclophane
(tpcp) via sonochemistry. Both nanocrystals exhibit an
enhanced fluorescence compared to dilute solution, while the
tpcp nanocrystals also demonstrate a red-shifted fluorescence.
Introduction
Over the past decade, extensive research has been conducted on
the controllable synthesis of nanocrystals1 owing to correlations
between size,2 morphology,3 and optoelectronic properties.4
Inorganic and polymer-based nanocrystals have garnered much
interest, owing to emerging widespread applications in fields
ranging from diagnostic medicine5 to materials science.6 Organic
nanocrystals of small molecules remain relatively less studied
despite a potential to modify the structures and tune optical
properties of such solids7 using methods of organic synthesis.
Early studies on the fluorescence of organic nanomaterials
based on small molecules were conducted by Nakanishi8 and
Yao,9 which involved aromatics such as perylene, phthalocya-
nine, and pyrazoline. More recent studies by Park,10 Diau,11
and Yang12 have focused on conjugated stilbenoids that exhibit
strong emission, yet are weakly fluorescent in solution.
Enhancements of solid-state fluorescence are quite unusual
with organic materials owing to facile quenching of chromo-
phores13 in the condensed phase, with conjugated systems such
as poly(p-phenyleneethynylenes),14 pseudoisocyanines,15 and
pentaphenylsilols16 being exceptions.
Results and discussion
Our initial attempts to generate nanocrystals of pCp involved the
reprecipitation method wherein pCp is dissolved in a hot polar
solvent, which is followed by rapid injection into an antisolvent.
pCp (0.15 g) was, thus, dissolved in toluene (7.0 mL) and rapidly
injected into ethanol (100 mL). The resulting solid was analyzed
using powder X-ray diffraction (PXRD) and scanning electron
microscopy (SEM). SEM micrographs revealed large well-defined
crystals of micrometer-sized dimensions, wherein the smallest
crystals were on the order of 5 mm in both length and width
(Fig. 2a,b). The resulting microcrystals displayed block morphol-
ogies and were agglomerated as stacked crystals. An inspection of
a PXRD pattern confirmed the solid to match the reported
With this in mind, we report here the sonochemical prepara-
tion17 of nanocrystals of [2.2]paracyclophane (pCp) and the
aDepartment of Chemistry, University of Iowa, Iowa City, United
States. E-mail: len-macgillivray@uiowa.edu; Fax: +1-319-335-1270;
Tel: +1-319-335-3504
bOptical Science and Technology Center, University of Iowa, Iowa City,
USA
cPhotoCatalytic Synthesis Group, MESA+ Institute for
Nanotechnology, Faculty of Science and Technology,
University of Twente, Meander 229, P.O. Box 217, 7500 AE Enschede,
The Netherlands
{ Electronic Supplementary Information (ESI) available: Excitation and
emission spectra, PXRD characterization, and SEM micrographs. See
DOI: 10.1039/c2ce26000j
Fig. 1 pCp and tpcp: (a) schematics and (b) X-ray crystal structures.
CrystEngComm, 2012, 14, 7567–7571 | 7567
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Scheme 1 Solid-state packing motifs that influence emission.
structure of pCp,23 which was evidenced by prominent peaks at 2h
= 15.0u, 16.1u, 25.9u, and 27.7u (Fig. 2c).
To form nanocrystals of pCp, we turned to a sonochemical
approach. The method has been shown to generate crystals of
nanoscale dimensions wherein more standard reprecipitation24
fails.25 In our experiments, low-intensity ultrasonic radiation
using a sonication cleaning bath was applied in crystal growth of
pCp. In a typical experiment, pCp (0.15 g) was dissolved in
DMF (3.0 mL) and rapidly injected into water (100 mL)
subjected to ultrasonic radiation. After 5 min of sonication, the
suspension was vacuum filtered through an 8 mm membrane
filter (Whatman) and analyzed using PXRD. The resulting
diffractogram revealed the structure of the solid generated using
sonochemistry to match pure pCp (Fig. 3).
Fig. 3 PXRD diffractogram of pCp treated with sonochemistry
compared to calculated pattern of pure pCp.
light scattering (DLS). DLS measurements revealed average
particle sizes of ca. 477 nm with a polydispersity index (PDI) of
0.146 (Fig. 4c).
We next turned to influences of surfactant on nanocrystal
formation. The introduction of a surfactant to generate nanoma-
terials can promote a decrease in particle size via the formation of
micelles, where increases in nucleation rate are also realized.27
Smaller particles could, thus, be expected in the presence of a
surfactant.
SEM analysis of the solid obtained via sonochemistry confirmed
the generation of nanometer-sized crystals of pCp. The crystals
exhibited a spherical, or approximate cube, morphology,26 with
the smallest crystals displaying lengths and widths that range from
200 to 500 nm (Fig. 4a,b). An aliquot of the original suspension
generated using the sonochemistry was analyzed using dynamic
Anionic sodium dodecyl sulfate (SDS) was employed as the
surfactant, with water as antisolvent. In the experiment, pCp
(0.15 g) was dissolved in DMF (3.0 mL) and rapidly injected into
0.021 M aqueous SDS (100 mL) subjected to ultrasonic radiation
for 5 min. Following vacuum filtration through an 8 mm filter, the
solid was analyzed using PXRD and SEM, while an aliquot of the
suspension was analyzed using DLS. SEM micrographs demon-
strated the formation of approximately spherical particles that
range from 100 to 400 nm in diameter (Fig. 5a,b). DLS
measurements demonstrated particles with sizes of ca. 340 and a
PDI of 0.270. The incorporation of the SDS, thus, resulted in a
decrease in particle size of nanocrystalline pCp (Fig. 5c).
With the successful formation of nanocrystals of pCp achieved,
we extended our efforts to the laterally-substituted derivative tpcp.
The pCp is achieved via a double [2 + 2] photodimerization
conducted in the solid state.18,28 Tpcp was, thus, dissolved in hot
Fig. 2 (a,b) SEM micrographs of microcrystals of pCp from reprecipita-
tion and (c) PXRD diffractogram compared to calculated powder pattern
of pCp.
Fig. 4 (a, b) SEM micrographs of pCp nanocrystals generated using
sonochemistry and (c) particle size distribution.
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Fig. 5 (a,b) SEM micrographs of pCp nanocrystals prepared using
sonochemistry with the addition of SDS and (c) particle size distribution.
DMF and rapidly injected into water. Similar to initial experiments
involving pCp, SEM micrographs revealed large crystals of tpcp of
rectangular morphology with lengths and widths of 15 mm and
4 mm, respectively (Fig. 6a,b). PXRD (Fig. 6c) confirmed the solid
precipitate as pure crystalline tpcp.29
Fig. 7 PXRD diffractogram of tpcp nanocrystals compared to the
calculated powder pattern.
of 22.5 mV was determined, which is consistent with aggregation
of the small particles within the dispersion.30
Sonochemistry was next applied to promote the generation of
nanocrystals of tpcp. Tpcp (0.05 g) was dissolved in hot DMF
(0.7 mL) and rapidly injected into water (100 mL) subjected to
ultrasonic radiation. After 5 min of sonication, the suspension was
filtered through an 8 mm membrane filter and analyzed using
PXRD and SEM. An analysis of the PXRD pattern supported the
solid generated using sonochemistry to match tpcp (Fig. 7).
SEM analysis revealed the formation of tpcp nanocrystals. The
smallest particles were spherical in shape, displaying sizes of ca.
250 nm (Fig. 8a,b). Moreover, when SDS was used as surfactant,
particles on the order of 50 nm readily formed (Fig. 8c,d). Thus,
the incorporation of SDS resulted in an effective five-fold decrease
in particle size. DLS measurements were notably inconclusive
owing to rapid settling of the nanoparticles.30 Indeed, a f-potential
Optical properties of nanosized pCp and tpcp were investigated.
Samples of pCp and tpcp obtained from the sonochemistry
experiments were each examined as nanocrystalline suspensions in
either water or aqueous solutions of SDS31 and compared to dilute
solutions of the same concentration. As reported, tpcp displays
red-shifted excitation and emission in solution compared to pCp.
The red-shift occurs despite a lack of continuous p-orbital
conjugation. The fluorescence was attributed to the cyclobutane
rings acting as efficient electron donors that promote internal
charge transfer within the p-stacked molecule.28
From our experiments, both pCp and tpcp were determined to
exhibit more intense fluorescence as nanocrystal suspensions
compared to dilute solution. While nanocrystalline pCp exhibited
the same emission wavelength (356 nm) as pCp in solution, the
nanoparticles without and in the presence of SDS displayed
fluorescence ca. 40 times more intense (Fig. S-18{). For tpcp, the
nanocrystals were ca. 17 times more intense than dilute solution. In
contrast to pCp, the nanoparticles of tpcp also exhibited a
bathochromic emission at 490 nm in the presence of the SDS
(Fig. 9). The shift can likely be attributed to appreciable hydrogen
Fig. 6 (a,b) SEM micrographs of tpcp collected from reprecipitation and
Fig. 8 Nanocrystals of tpcp nanocrystals using (a,b) sonochemistry and
(c) PXRD diffractogram compared to the calculated powder pattern.
(c,d) sonochemistry with SDS.
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face-to-face geometry, forces between molecules are expected to
significantly impact fluorescence in the solid, owing to less
conformational freedom within the crystal. Both pCp23 and tpcp29
assemble edge-to-face, or in a herringbone fashion, in the
crystalline state (Fig. 10), which is a geometry known to support
enhanced fluorescence.37 Combinations of multiple hydrogen-
…
bonding also support the edge-to-face geometries (e.g. C–H p,
…
C–H N forces) by increasing conformational rigidity, as
particularly in the case of tpcp. For the nanocrystalline suspen-
sions, the edge-to-face aggregation and rigid stacking is presum-
ably maintained in the condensed environment.38
Conclusion
In this report, we have described a sonochemical approach to
prepare nanostructured pCps. The nanocrystals are on the order of
100–500 nm and exhibit optical properties that differ compared to
solution. Both nanostructured pCp and tpcp display more intense
fluorescence, which is ascribed to edge-to-face packing being
maintained in the nanocrystalline solid. We are currently focused
on preparing nanocrystals of additional pCps, where herringbone
stacking of the chromophores may also be preferred.
Fig. 9 Emission of tpcp nanocrystals compared to dilute solution
(nanoparticles on primary axis and solution on secondary axis).
bonding of water molecules associated with SDS at the N-atoms of
the pyridyl groups.32 The presence of hydrogen bonds may also
account for the significant decrease in particle size when both
sonochemistry and SDS were used to produce the nanocrystals. A
similar red shift has been observed for N-alkylated tpcp.28 We also
note that microsized crystals of pCp and tpcp exhibited
fluorescence more intense than solution yet significantly less (i.e.
ca. 10 times less) than the nanocrystal suspension. The enhance
fluorescence of the nanoparticles compared to the microsized
crystals may be related to surface effects and/or aggregation of the
nanoparticles.33 The microcrystals of tpcp in the presence of SDS,
in contrast to the nanocrystals, also did not exhibit an appreciable
red shift in fluorescence (Fig. S-19{). The lack of a red shift is also
supportive of influences of hydrogen bonding being appreciable
along the surfaces of the nanocrystals.
Experimental
Materials and instruments
pCp was purchased from Carbosynth (Compton, Berkshire, UK).
SDS was purchased from Sigma Aldrich Chemical Company (St.
Louis, MO, USA). N.N-dimethylformamide, toluene, and ethanol
were purchased from Fisher Scientific Company (Pittsburgh, PA,
USA). tpcp was prepared as reported.18a All chemicals were used
without further purification. PXRD data was collected using a
Bruker D-5000 diffractometer equipped with a Bruker SOL-X
˚
energy-sensitive detector using Cu-Ka radiation (l = 1.54056 A).
The enhanced fluorescence of nanocrystalline pCp and tpcp, as
well as the related microcrystals, compared to solution can be
attributed to aggregation that is maintained in the solid and arises
from minimal intermolecular p-overlap of the stacked p-faces.34
Fluorescence of organic chromophores is typically quenched by
either by co-planarization at the molecular level35 or intermole-
cular aggregation at the supramolecular level.36 Given that pCp
and tpcp possess two benzene rings covalently enforced in a
Particle size measurements were determined by a Zetasizer Nano
ZS (Malvern, Southborough, MA) instrument at 25 uC. The
reported particle size and PDI values are averages of three
measurements. SEM images were obtained using a Hitachi S-4800
with an accelerating voltage range of 2–5 kV. SEM samples were
prepared by depositing each sample on a Si wafer. Absorption and
emission measurements were obtained using a HORIBA Jobin
Yvon FluoroMax-4 (Edison, NJ, USA). All measurements
were made on the as-prepared suspensions with a scan rate of
5 mm sec21 and both slit widths set to 2 nm.
pCp nanocrystal synthesis. Nanocrystals of pCp were prepared
by dissolving 150 mg of pCp in 5 mL of DMF. The solution was
rapidly injected into 100 mL of distilled water at ambient
temperature and sonicated for 5 mins in a cleaning bath (Branson
2510R-DTM). After sonication, the sample was filtered through an
8 mm membrane filter (Whatman Grade 2) and dried. The surfactant
crystallization was performed with 0.021 M SDS as antisolvent.
tpcp nanocrystal synthesis. Nanocrystals of tpcp were prepared
by dissolving 50 mg of pcp in 0.7 mL of DMF. The solution was
rapidly injected into 100 mL of distilled water at ambient
temperature and sonicated for 5 mins in a cleaning bath
(Branson 2510R-DTM). After sonication, the sample was filtered
through an 8 mm membrane filter (Whatman Grade 2) and dried.
Fig. 10 Edge-to-face packing of crystalline: (a) pCp and (b) tpcp.
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G. C. Bazan, J. Am. Chem. Soc., 2002, 124, 5183; (d) Y. Morisaki and
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Acknowledgements
This work was supported by the National Science Foundation
(L.R.M., DMR-1104650). The authors acknowledge the
Central Microscopy Research Facility and the Office of the
Vice President of Research, University of Iowa.
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