Template-stereocontrolled [2 + 2] photoreactions directed by surface recognition on hydrophilic functionalized carbon materials

Article abstract of DOI:10.1039/c7ce01090g

Supramolecular assistance of the regioselective synthesis of single dimers from [2 + 2] photoreactions surface-directed by multivalent H-bonding exo-templates based on hydrophilic carbon materials (HCMs) is highlighted for the first time. Supramolecular p

Full text of DOI:10.1039/c7ce01090g

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Template-stereocontrolled [2 + 2] photoreactions
directed by surface recognition on hydrophilic
functionalized carbon materials†‡
Cite this: DOI: 10.1039/c7ce01090g
*
Gabriela Ortega and Alexander Briceño
Supramolecular assistance of the regioselective synthesis of single dimers from [2 + 2] photoreactions
surface-directed by multivalent H-bonding exo-templates based on hydrophilic carbon materials (HCMs) is
highlighted for the first time. Supramolecular photochemistry of stilbenes assisted by either functionalized
carbon nanotubes or sub-microspheres afforded a single rtct-isomer after two consecutive steps in solu-
tion (UV-irradiation and hydrothermal isomerization). Carbon spheres display the ability to mediate the di-
merization of 4-cinammic acid derivatives with moderate to quantitative yields. This result is obtained ei-
ther in solution or via mechanochemical grinding assistance of carbon sphere/cinnamic derivative
assemblies. The structure-templating features of such nanomaterials are discussed as a function of surface
recognition processes due to the presence of –COOH and –OH groups on the surface and the effective
carbon template : substrate ratio.
Received 10th June 2017,
Accepted 25th April 2018
DOI: 10.1039/c7ce01090g
rsc.li/crystengcomm
for crystal engineers. The difficulty in controlling the organi-
zation of the molecules in crystalline assemblies, together
Introduction
The control of nanomaterials with well-defined shape and
size and tunable chemical surface combined with deliberate
molecular recognition-directed processes, has emerged as a
promising alternative for programmed supramolecular assis-
tance in multi-component nanomaterials engineering.1 The
coupling of supramolecular chemistry with interesting physi-
cochemical properties of nanoarrays (supramolecular nano-
chemistry) provides new possibilities for the design of better
catalysts and selective binding receptors, sensors or tem-
plates.1,2 In this respect, supramolecular assistance to cova-
lent synthesis constitutes a reliable alternative as the catalyst
and/or template in the preparation of stereo- and regio-
controlled compounds either in solution or in solid state.3,4
Particularly, solid state reactions directed by topochemical
factors5 and assisted by auxiliary molecules as templates have
been recognised as an interesting supramolecular strategy for
the controlled synthesis of single products combined with
high yields.4 In spite of the well-known advantages of these
reactions over those in isotropic media, the development of
strategies to design photoreactive arrays remains a challenge
with the intrinsic limitations imposed by topochemical princi-
ples, does not allow the generalized application of a unique
strategy tolerant of structural factors such as the shape, size
and chemical group functionality of photoactive molecules.
In the past decades, chemists, inspired by biological sys-
tems, have devoted great effort to the tailor-made design of
nanocontainers capable of driving stereoselective chemical
transformations in their cavities.3 A number of constrained
and organized media have been explored as hydrophobic su-
pramolecular hosts in solution with moderate success either
as catalysts or endo-templates. This has been achieved by
employing confinement and supramolecular interactions to
enhance and control the photochemical reactivity and selec-
tivity of different guests in host–guest systems.3,6,7 Macrocy-
clic compounds such as cyclodextrins,8 cavitands,9
cucurbiturils,10 hollow coordination cages,11 crystal liquids12
and inorganic solids,13 among others, have been widely used
as supramolecular hosts to overcome such limitations. Two
recent reviews about supramolecular photochemistry in such
systems have been provided by Vallavoju, Sivaguru,14a and
Ramamurthy.14b More recently, strong non-covalent interac-
tions such as cation⋯π interactions have also been explored
as alternatives for the synthesis of head-to-tail dimers from
stilbazole derivatives in solution.15 These approaches play a
critical role in improving either the conversion percentage or
product distribution and selectivity of the resulting photo-
products. In this sense, efforts have been made in the study
of the reactivity of olefins in the solid state based on
Laboratorio de Síntesis y Caracterización de Nuevos Materiales, Centro de
Química, Instituto Venezolano de Investigaciones Científicas, San Antonio de Los
Altos, Miranda, Venezuela. E-mail: abriceno@ivic.gob.ve; Fax: +58 212 5041350;
Tel: +58 212 5041320
† Dedicated to Dr. Gabriel Chuchani on the occasion of his 59th anniversary in
chemical research. Deceased April 30 2017.
‡ Electronic supplementary information (ESI) available: Experimental details,
FT-IR, 1H-NMR spectra and SEM images. See DOI: 10.1039/c7ce01090g
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multivalent templates.16 The present work outlines an oppo-
site alternative that employs divergent exo-templates1a bear-
ing multiple hydrogen binding sites. Herein, we have envis-
aged the use of hydrophilic surfaces based on carbon nano
and submicro-materials such as functionalized multiwall car-
bon nanotubes (MWCNT–COOH) and carbon spheres (CS–
COOH), which bear –COOH and –OH groups as potential
multivalent exo-templates to direct [2 + 2] photoreactions ei-
ther from solution or from solid state. These materials are
susceptible to be covalently functionalized and the acid
group density of the surface can be modulated by means of
functionalization performed post-synthesis17 or in situ during
the preparation, which occurs in the hydrothermal carboniza-
tion of nano/microspheres from carbohydrates.18 Such tem-
plates can be easily prepared and be effective hydrophilic
nanoplatforms for the selective supramolecular recognition
of different molecular targets on the surface via complemen-
tary H-bonding homo- or heterosynthons (e.g. carboxyl acids
or pyridyl derivatives).19 Multivalent exo-templates might be
capable of geometrically positioning the building blocks by
templating and self-assembly in close proximity, allowing
flexible distances <4.2 Å so that the photoreaction can effec-
tively occur according to Schmidt's criteria.5 Therefore, subse-
quent reactions on the preassembled supramolecular array
can be deliberately performed on the surface and the cova-
lent product can be easily removed from the surface by sim-
ple acid–base control. Besides, such interactions might mod-
ify the photophysical and photochemical properties of the
substrate. In this contribution, we report the first examples
of HCMs as multivalent H-bonding templates to control [2 +
2] photoreactions in solution. The potential of these tem-
plates is evaluated in the supramolecular photochemistry of
pyridyl or carboxylic compounds to yield a single dimer. In
particular, the ability of CS–COOH to template different ole-
fins, such as bisIJpyridyl)ethylenes and cinnamic derivatives,
either in aqueous solution or in the solid state, is assessed.
(pHpzc), which defines the ionizing or ion exchange capacity,
it is evident that the carbon spheres have a higher surface
amount of COOH groups than MWCNT–COOH. FT-IR also re-
vealed the presence of absorption bands related to the pres-
ence of carboxylic and –OH groups (see ESI,‡ Fig. S1–S3).
These observations confirm the presence of surface hydro-
philic groups capable of interacting with complementary
chemical groups via the formation of supramolecular
synthons. Typical SEM images of MWCNT–COOH and CS–
COOH prepared with the addition of a POM are shown in
Fig. S1 and S2.‡
Photochemistry in solution
Symmetric stilbenes such as trans-1,2-bisIJn-pyridyl)ethylene
(where n = 2 or 4; 2,2′-bpe or 4,4′-bpe) were used as initial tar-
gets in order to evaluate the effect of both HCMs as exo-
templates. MWCNT–COOH or CS–COOH in a water/ethanol-
dispersion was mixed with a solution of stilbenes in H2O/eth-
anol, with a HCM/stilbene ratio of 1 : 3. The mixtures were
kept under magnetic agitation overnight to ensure surface/
molecule interaction and after, these dispersions were irradi-
ated at 302 nm for 48 h. After the irradiation, the products
were extracted with CHCl3 and characterized by 1H-NMR
spectroscopy (Fig. S4–S7‡). The corresponding spectra show
the formation of a mixture of dimers as unique products
(rctt-, rtct- and rcct-isomers) with a high to moderate conver-
sion of the dimerization reaction (100% and 72% for 2,2′-bpe
and 4,4′-bpe, respectively). In both cases the rctt-dimer was
the major product (Fig. 1 and Table 1).10a,21 Neither the
cis-isomer nor its by-products were observed in these photo-
reactions in any case when CS–COOH was used as the tem-
plate, in contrast to similar works reported in solution.21a
This result suggests the anchoring of the molecules on the
surface via supramolecular heterosynthon interactions with
the chemical groups available, thus avoiding the process of
cis–trans isomerisation and the formation of additional by-
products. Similar results were obtained using MWCNT–
COOH as the template, but the conversion yields of both re-
actions were lower than those obtained with CS–COOH (73%
and 64% for 2,2′-bpe and 4,4′-bpe, respectively). Such a
Results and discussion
Carbon material preparation and characterization
In previous studies on the synthesis and characterization of
new composites bearing polyoxometalates (POMs),20 an alter-
native route was found to generate hydrophilic groups based
on oxygenated groups such as carboxylic, epoxide, hydroxyl,
quinone or other carbonyl functional groups via chemical ox-
idation under mild conditions, using POMs with H2O2. This
process is greener than those involving the use of strong and
hazardous oxidants such as inorganic acids, bases and chlo-
rates, among others.17 In this case, functionalization of pris-
tine MWCNTs or CSs was carried out via oxidation with
H3PW12O40 and H2O2 under hydrothermal conditions at 180
°C for 24 h. The chemical changes to the surface were evalu-
ated by FT-IR spectroscopy and point of zero charge (pzc) de-
termination. The pzc values were acidic for both MWCNT–
COOH and CS–COOH (3.84 and 1.87, respectively). Since the
pzc value is directly proportional to the superficial pH
Fig.
1 (a) Schematic representation of functionalized carbon
nanotubes (MWCNT–COOH) and carbon spheres (CS–COOH). (b)
Possible supramolecular interactions between the surface and olefins.
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Table 1 Product distributiona (%) upon irradiation of trans-1,2-bisIJn-pyridyl)ethylenes in solution
Products (%)
Mediumb
% conv.
1
2
3
4
CS–COOH
2,2′-bpe
4,4′-bpe/
100
72
—
28
74
51
4
11
22
10
MWCNT–COOH
2,2′-bpe/
4,4′-bpe/
82
64
18
36
62
43
11
15
9
6
Isomerisation under HC 150 °Cc
CS–COOH
2,2′-bpe
4,4′-bpe
100
88
—
—
100
100
—
—
MWCNT–COOH
2,2′-bpe
4,4′-bpe
82
64
—
—
100
100
—
—
a
b
The percentage of conversion (%) was calculated based on integration of 1H NMR signals. Photochemistry assisted by HCMs in solution.
c
Isomerisation assisted under hydrothermal conditions. Quantitative conversion into the rtct-isomer is based on initial relative yield from the
mixture of isomers.
difference might be associated with the lower density of sur-
face acid groups on the MWCNT–COOH as compared to that
on the spheres, in agreement with the pHpzc values reported
above. In further photoreactions with stilbenes, the resulting
mixture was heated under hydrothermal conditions at 150 °C
for 2 days. The extraction with CHCl3 and re-crystallization re-
vealed the regioselective and quantitative isomerisation of the
rctt- and rcct-isomers to the most stable rtct-isomer for both
stilbenes (Fig. 2 and S9†). This isomerisation process is self-
catalyzed by the presence of acid groups on the template in
both cases22 (Fig. 3). The combination of supramolecular
photochemistry in solution with the isomerisation assisted by
HCMs under hydrothermal conditions provides a novel and
reliable synthetic route for the regioselective synthesis of a
single isomer from two sequential steps in solution.
photoreactions of other olefins with different functional
groups and substituents. Twelve asymmetric olefins based on
cinnamic acid derivatives were tested. The photochemical be-
haviour of these compounds has been widely explored both
in solution and in solid state.23,24 It is well-known that such
molecules do not dimerise in solution, and only the cis–trans
isomerisation process is observed. Similar irradiations of
cinnamic and 4-Cl–cinnamic acids were first carried out using
MWCNT–COOH or CS–COOH. Surprisingly, the preliminary
characterization by 1H NMR of the extracted solids with
CHCl3 after the irradiation revealed that the dimerization
took place cleanly on the hydrophilic surface when CS–COOH
was used as the template. This suggests that CS–COOH can
be used for the regioselective formation of a single isomer
with a quantitative yield from cinnamic derivatives in solu-
tion (Fig. S10 and S11‡). Other 4-cinnamic derivatives such
as 5f: 4-Br– and 5h: 4-CF3–cinnamic acid showed moderate
yield (Table 2, Fig. S12–S21‡), which is in contrast with the
formation of the mixture of dimers found during the stilbene
photoreactions. In some cases such dimers were obtained in
moderate to quantitative yield. On the other hand, low con-
version to the photodimer was obtained when MWCNT–
The above results prompted us to evaluate the general ap-
plicability of MWCNT–COOH and CS–COOH as templates in
Fig.
2
Comparative 1H-NMR spectra in CDCl3, showing the
quantitative conversion of the mixture of photodimers obtained from
the irradiation in solution (rctt-, rcct- and rtct-2,2′-tpcb isomers) (bot-
tom) into the rtct-isomer by heating under hydrothermal conditions in
the presence of CS–COOH (top).
Fig. 3 Photodimerization of stilbenes in the presence of MWCNT–
COOH or CS–COOH and isomerization of the stereoisomer mixture to
the thermodynamically more stable rtct-isomer.
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Table 2 Product distribution (%) upon irradiation of trans-cinnamic acid derivatives either in solution or in the solid state (runs 1–3). Photoproducts
identified: α-truxillic (6) or β-truxinic (8)/cis-isomer (7)
Solution producta
6 : 8/7
Run 1
6 : 8/7
Solid state run 2
6 : 8/7
Run 3
6 : 8/7
Cinnamic derivatives
5a
5b
5c
5d
5e
5f
5g
5h
5i
100
—
—
—
—
—
49
—
—
—
—
—
—
—
100
—
—
33
—
—
—
—
100
—
—
—
—
—
—
—
—
—
—
—
—
—
100
—
—
8
—
7
—
—
—
—
100
—
—
—
—
—
69
—
—
—
—
—
—
100
—
—
—
—
—
93
—
—
—
—
—
—
90/4
—
—
—
85
—
95
—
—
—
48/3
100
—
—
52
—
80
—
—
—
25
5j
5k
5l
—
10/8
80/7
a
The conversion percentages (%) were calculated based on integration of 1H-NMR signals corresponding to the photoproducts: α-truxillic (6)
or β-truxinic (8)/cis-isomer (7). Run 1: Photochemistry in the solid state from direct mechanochemical self-assembly of CS–COOH/olefin. Run 2:
Photochemistry in the solid state of impregnated and dried CS–COOH with an excess of olefin in a mass ratio of 1 : 20. Run 3: Photochemistry
of impregnated and irradiated CS–COOH/olefin mixtures combined with two additional grinding–irradiation cycles.
COOH was used as the template with cinnamic acid in solu-
tion. In this case the percentage yield of the dimer ranged
from very low to moderate (<30%), depending on the
MWCNT–COOH/cinnamic acid ratio (Fig. S22‡). These results
may be also attributed to the lower concentration of surface
acid groups on the MWCNT–COOH, as compared to that
found on the spheres.
head configurations, respectively) based on the shift of the
signals in the 1H NMR and the coupling constant of the
photoproducts together with the determination of their melt-
ing points.23,24 The formation of an α-truxillic-isomer was
favoured when the substituents were hydrophobic groups
(Fig. S10 and S14‡), while 4-halogen-substituted cinnamic de-
rivatives afforded exclusively the formation of a β-truxinic-iso-
mer (Fig. 4). These results are in sharp contrast with those of
common photodimerization of these molecules into hydro-
phobic endo-templates.24c
Both the nature of the substituents and the substitution at
positions 2- and 3- on the aryl ring may significantly affect
the relative orientation of the molecules on the surface. This
could be a consequence of possible secondary supramolecu-
lar cooperative interactions with the substituents, whether on
the surface or by solvation effects. Multivalent interactions
are stronger than the equivalent monovalent ones and may
lead to a greater and more stable affinity with the surface,
preventing the molecules from being oriented in favourable
photoreactive pairs. These results suggest that the reaction
occurs between pairs adsorbed on the surface, probably
In order to ascertain the effect of the template
functionalization degree on the photoreaction, a new assay
was conducted using pristine MWCNTs as a template model,
given their hydrophobic nature (MWCNT-blank). Irradiation
of cinnamic acid using MWCNT-blank in an ethanol-
dispersion was performed under the same conditions in a
mass ratio of 1 : 3. Interestingly, no dimerisation occurred af-
ter 48 h of irradiation. In contrast to the results obtained
1
with functionalized carbon templates, the H NMR spectrum
of the extract resulted in a 67% conversion to the cis-isomer
as the unique product. This conversion is higher than that
obtained in solution in other reports.10,23 Apparently, the lack
of hydrogen anchoring sites on MWCNT-blank and the ex-
tended π–surface and π–π interactions between the template
and substrate favour the stabilisation of the cis-isomer rather
than the photodimerisation reaction. These results strongly
suggest that the adsorption of the different substrates di-
rected by supramolecular hydrogen bonding interactions
plays a fundamental role in the control of the resulting
photoproducts by preventing the process of geometrical iso-
merisation and the formation of additional by-products.
A surprising result was that 2- or 3-substituted cinnamic
derivatives such as 2-Cl– or 3-Cl– e and 4-substituted ones
such as 5i: 4-NO2–, 5j: 4-CH3O–, and 5k: 4-OH– (see Table 2)
were photoinert upon irradiation using CS–COOH as the tem-
plate (Fig. S12–S21‡). On the other hand, in every case, the
regioselective formation of a single dimer-type was observed,
which was identified to be either an α-truxillic- or β-truxinic-
isomer (α-dimer: rctt-head-to-tail and β-dimer: rctt-head-to-
Fig. 4 Photodimerization of cinnamic acid derivatives in the presence
of CS–COOH in solution and/or in the solid state via
mechanochemical-assisted self-assembly.
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assisted by the formation of homosynthons between the acid
groups on the surface and the cinnamic derivatives. This re-
markable feature of the obtained dimer also suggests that
CS–COOH acts as an effective exo-template.
carried out for three CS–COOH/olefin mass ratios (30 : 50,
30 : 100 and 30 : 150), with the best conversion obtained for
30 : 100. These preliminary observations can be interpreted as
a function of the adsorption of the molecules and the density
of the location and preorientation of the substrate on the sur-
face. Thus, a plausible explanation for the observed sensitiv-
ity may be that the yield significantly depends on the sub-
strate loading. At low loading, there is a small probability
that the molecules are adsorbed on the surface by pairs in
close proximity, which decreases the possibility of favourable
intermolecular contact for the photoreaction to occur. On the
other hand, at high loading the molecules adsorbed on the
surface can react. However, the molecules remaining in solu-
tion can affect the relative orientation of adsorbed molecules
due to potential hydrophobic interactions between these two
groups of molecules, reducing the yield of the reaction. Like-
wise, it is possible that geometrical photoisomerization is
also favoured. It was therefore hypothesized that complete
surface saturation by the substrate is the ideal condition, giv-
ing the maximum surface coverage of CS–COOH. For such
cases the yield of the reaction should be close to quantitative
conversion (Fig. 5).
Mechanochemical self-assembly and photochemistry in the
solid state
To gain information about the photoreactivity on the spheres,
a new set of reactions were evaluated in the solid state.
Liquid-assisted mechanical co-grinding25 of CS–COOH and
the respective photoactive olefin in solution was performed
for 30 min with some drops of ethanol (Fig. 5). The resulting
mixtures were irradiated using the same wavelength and irra-
diation time. The characterization by 1H NMR of the
extracted solids revealed that the dimerization also occurred
in several 4-cinnamic derivatives in the absence of the solvent
(5a: 4-H–, 5c: 4-Cl–, 5f: 4-Br–, and 5h: 4-CF3–). However, the
yield of the photoreactions was lower or zero as compared to
the same reactions in solution, except for 5a: cinnamic and
5c: 4-Cl–cinnamic acid, wherein it was quantitative for both
compounds. These results confirm our hypothesis that the
photoreactivity is favoured by the presence of hydrophilic
groups on the surface. The stilbenes were also photoinert,
suggesting that the interaction of the stilbenes with the sur-
face may occur in a dynamic way in solution, probably due to
the presence of different interactions, such as in carboxylic–
pyridine and/or –OH⋯pyridine heterosynthons, among
others.4c,d This could explain the formation of a mixture of
dimers characteristic of the photoreaction in solution of
these compounds, in contrast to that observed for cinnamic
derivatives.
To test this hypothesis, a new set of solid state reactions
with photoactive 4-cinnamic molecules were performed. An
excess of substrate was used in relation to the template in so-
lution (600 mg of olefin and 30 mg of CS–COOH). The mix-
tures were maintained overnight under magnetic agitation to
ensure their interaction, and then centrifuged to separate the
loaded CS–COOH. The excess liquid was separated by decan-
tation. Mass balance between the amount added in excess
during the impregnation and the residue recovered from the
solution after centrifugation revealed a variable quantity of
the adsorbed amount in grams, depending on the substrate
studied. However, the normalized values by molecular weight
and amount of carbon spheres (g) tended to an average of ap-
proximately ca. 70 mmol g−1. This appears to be the surface
coverage saturation value, except for cinnamic acid, which
was much lower (33 mmol g−1). These results are consistent
with the proposed model (Fig. 5). The impregnated spheres
were dried and irradiated for 48 h and then split into two
portions. The first portion was treated with CHCl3 and a
small amount of DMSO to isolate the products of the reaction
The yield of the product in these reactions is very sensitive
to the carbon material/olefin ratio. Initial optimization was
1
in the solid state, and characterized by H-NMR spectroscopy.
The yield obtained was always higher than that in similar re-
actions in solution or by direct mechanochemical self-
assembly (Table 2, run 2). It is noteworthy that 5b: 4-F–
cinnamic acid was only reactive under these conditions,
which indicates that this approach can be also used for the
study of supramolecular catalysts in the solid state.27
Taking advantage of previous studies on mechano-
chemistry to improve the yield of any photoreaction via mul-
tiple grinding–irradiation cycles, we applied two additional
grinding–irradiation cycles to the second portion.26 These
mixtures were mechanically re-ground for 15–30 min with
some drops of ethanol and again irradiated at the same
wavelength and irradiation period. The characterization by
Fig. 5 (a) Cartoon representation of the self-assembly via mechano-
chemistry and the combination of irradiation and grinding cycles in
order to improve the yield of the [2 + 2] photoreactions in the solid
state, showing the possibility to obtain a β-isomer. (b) Different possi-
bilities of the amount of substrate loaded on a carbon sphere directed
by surface recognition, depending on the CS–COOH/olefin mass ratio
used.
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1H-NMR of the extracted solids revealed that the yield of the
products was always higher than the first set of reactions
with one single irradiation cycle (Table 2, run 3), which led
to a remarkable improvement in the reaction yield with high
conversion values for some cases (for example, 48–90%, 52–
85% and 25–80% for 5b: 4-F–, 5f: 4-Br– and 5l: 3,4-
dichlorocinnamic derivatives, respectively). In contrast, the 2-,
3- or 4-derivatives such as 5d: 3-Cl–, 5e: 2-Cl–, 5i: 4-O2N–, 5j:
4-CH3O–, and 5k: 4-OH–cinnamic acids were photoinert un-
der all of the experimental conditions studied. A representa-
tive example of the photoreactivity of one of the cinnamic de-
rivatives under the different experimental conditions above
described is shown in Fig. 6. These results suggest the con-
comitant disassembly of the photoproduct from the surface
followed by recombination of unreactive molecules into new
pairs on the anchored groups available on the surface. Thus,
a mechanochemically assisted re-assembly occurs that en-
ables the reactivity of the remaining unsaturated molecules.
At the present stage and based on the above observations,
some explanations can be put forward for the intriguing
regioselective formation of single α- or β-isomers. The
obtaining of such isomers can be speculated to have occurred
by three different ways of substrate adsorption on the sur-
face: a) perpendicular to the surface via hydrogen homo-
synthons between acid–acid groups. In this case, π–π inter-
digitation might occur between adsorbed molecules on
neighbouring carbon spheres2c (Fig. 7(a)); b) simple absorp-
tion by surface saturation of a single sphere due to substrate
excess. These two adsorption modes could lead to the forma-
tion of head-to-tail photodimers. This configuration would be
especially favourable for the derivatives bearing hydrophobic
or steric substituents; and c) edgewise or flat adsorption of
cinnamic molecules on the surface. In this case, multiple in-
teractions such as H-bonding, π⋯π and halogen⋯halogen,
Fig. 7 Possible supramolecular interactions directed by H-bonding in-
teractions between hydrophilic groups on the surfaces: a) stacking
head-to-tail via π–π interdigitalization between adsorbed molecules on
different spheres. b) Lateral adsorption of molecules on the surface.
and stacking of adjacent molecules in a head-to-head manner
can generate the necessary geometrical conditions for dimer-
ization. In particular, halogen⋯halogen contacts are attrac-
tive interactions well-known for favouring the molecular
stacking into β-arrangements in the solid state28 (Fig. 7(b)).
In the three absorption ways, contacts can be easily regene-
rated via mechanochemical assistance in the solid state,
which accounts for the yield improvement in all cases.
On the other hand, to provide some direct evidence of the
preferential adsorption and interaction sites on carbon
sphere templates, SEM analysis of saturated CS–COOH with
5a: cinnamic acid was performed. The image shows that the
molecules are homogenously distributed on the surface of
the spheres, with no formation of segregated crystals or pow-
der (Fig. 8). Additionally, the FT-IR spectrum of these spheres
shows the absorption bands corresponding to the
overlapping of both components (CS–COOH/cinnamic acid).
These results provide strong evidence that supports the full
coverage of CS–COOH by unsaturated molecules (Fig. S22‡).
Likewise, some FT-IR band shifts are observed when impreg-
nating the carbon spheres with the olefins. In particular, the
absorption band corresponding to CO groups associated to
the carboxylic groups of cinnamic derivatives on the surface
exhibits a shift in the range of 10–12 cm−1 in comparison to
free cinnamic derivatives. This suggests an additional
Fig. 6 Representative 1H NMR spectra of a cinnamic derivative (5h:
4-CF3–) in solution (bottom) and the solid state reactivity under
different experimental conditions (run 1: irradiation in the solid state
from mechanochemical self-assembly. Run 2: Irradiation of
impregnated CS–COOH with an excess of olefin in the solid state. Run
3: Impregnation–irradiation in the solid state + two additional grinding–
irradiation cycles).
Fig. 8 SEM image of impregnated CS–COOH with cinnamic acid.
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property of these spheres that make them potential adsor-
bents for preferential sequestering of carboxylic acid mole-
cules from solution.
2 (a) K. Ariga, H. Ito, J. P. Hillab and H. Tsukube, Chem. Soc.
Rev., 2012, 41, 5800–5835; (b) J. W. Steed and J. L. Atwood,
Supramolecular Chemistry, Wiley, New York, 2000; (c) H. He,
M. Feng, Q. Chen, X. Zhang and H. Zhan, Angew. Chem., Int.
Ed., 2016, 55, 936–940.
Conclusions
3 B. Bibal, C. Monging and D. M. Bassani, Chem. Soc. Rev.,
2014, 43, 4179–4198.
In summary, we have here reported the unprecedented ability
of HCMs as exo-templates to direct stereocontrolled [2 + 2]
photoreactions via surface recognition of different types of
olefins either in solution or in the solid state. The combina-
tion of supramolecular photochemistry assisted by HCMs in
solution with hydrothermal isomerisation can efficiently be
used for the regioselective synthesis of single rtct-dimers from
stilbenes via two sequential steps in solution. On the other
hand, CS–COOH spheres can efficiently drive the dimeriza-
tion of 4-cinammic acid derivatives to afford single α- or
β-dimers. The combination of self-assembly with sequential
grinding–UV irradiation cycles provides an efficient route to
obtain single dimers with high to quantitative yields in the
solid state. Both HCMs constitute new multivalent H-bonding
templates which can be easily prepared and may emerge as
novel alternative exo-templates versus endo-templates. Thus,
supramolecular nanochemistry opens up endless opportuni-
ties for designing a new class of multivalent nanotemplates
capable of favouring the regioselective preparation and/or im-
proving the yield of new and conventional cyclobutane-like
derivatives, which are difficult to access in the liquid phase or
by established solid state protocols. This supramolecular ap-
proach can be very helpful to overcome the limitations im-
posed by the topochemical postulates in the regioselective
obtention of dimers from crystalline assemblies. Additionally,
this alternative provides an efficient green route to scale these
reactions to gram quantities. The widespread potential use of
HCMs and other functionalized surfaces as templates is cur-
rently being explored for the dimerization of other stilbenes,
stilbazoles and unsaturated acids. Likewise, the optimization
of the HCMs/olefin mass ratio remains under investigation in
order to evaluate the efficiency of HCM templates in solution.
4 (a) For recent reviews on solid state reactivity: A. Briceño and
A. M. Escalona, Photochemistry, RSC, Cambridge, 2016, vol. 43,
pp. 286–320; (b) K. Biradha and R. Santra, Chem. Soc. Rev.,
2013, 42, 950–967; (c) Y. Sonoda, Molecules, 2011, 16, 119–148;
(d) I. Weissbuch and M. Lahav, Chem. Rev., 2011, 111,
3236–3267; (e) L. R. MacGillivray, J. Org. Chem., 2008, 73,
3311–3317; ( f ) L. R. MacGillivray, G. S. Papaefstathiou, T.
Friščić, T. D. Hamilton, D.-K. Bučar, Q. Chu, D. B. Varshney
and I. G. Georgiev, Acc. Chem. Res., 2008, 41, 280–291; (g) M.
Nagarathinam, A. M. P. Peedikakkal and J. J. Vittal, Chem.
Commun., 2008, 5277–5288.
5 (a) G. M. J. Schmidt, Pure Appl. Chem., 1971, 27, 647–678; (b)
S. K. Kearsley, The Prediction of Chemical Reactivity Within
Organic Crystals Using Geometric Criteria, in Organic Solid
State Chemistry, ed. G. R. Desiraju, Elsevier, New York, 1987,
pp. 69–115.
6 (a) S. R. Salpage, Y. Xu, B. Som, A. J. Sindt, M. D. Smith and
L. S. Shimizu, RSC Adv., 2016, 6, 18350–18355; (b)
Photochemistry in Organized and Constrained Media, ed. V.
Ramamurthy, VCH, New York, 1991.
7 M. Yoshizawa, J. K. Klosterman and M. Fujita, Angew. Chem.,
Int. Ed., 2009, 48, 3418–3438.
8 H. S. Banu, A. Lalitha, C. Srinivasan, K. Pitchumani and C.
Srinivasan, Chem. Commun., 1999, 607–608.
9 L. S. Kaanumalle and V. Ramamurthy, Chem. Commun.,
2007, 1062–1064.
10 (a) M. Pattabiraman, A. Natarajan, R. Kaliappan, J. T. Mague
and V. Ramamurthy, Chem. Commun., 2005, 4542–4544; (b)
M. Pattabiraman, A. Natarajan, L. S. Kaanumalle and V.
Ramamurthy, Org. Lett., 2005, 7, 529–532; (c) S. Y. Jon, Y. H.
Ko, S. H. Park, H. J. Kim and K. Kim, Chem. Commun.,
2001, 1938–1939.
Conflicts of interest
11 (a) Y. Nishioka, T. Yamaguchi, M. Yoshizawa and M. Fujita,
J. Am. Chem. Soc., 2007, 129, 7000–7001; (b) K. Takaoka, M.
Kawano, T. Ozeki and M. Fujita, Chem. Commun.,
2006, 1625–1627; (c) M. Yoshizawa, Y. Takeyama, T. Okano
and M. Fujita, J. Am. Chem. Soc., 2003, 125, 3243–3247.
12 (a) Y. Ishida, Y. Kai, S.-Y. Kato, A. Misawa, S. Amano, Y.
Matsuoka and K. Saigo, Angew. Chem., Int. Ed., 2008, 47,
8241–8245; (b) Y. Ishida, S. Amano, N. Iwahashi and K.
Saigo, J. Am. Chem. Soc., 2006, 128, 13068–13069; (c) S.
Amano, Y. Ishida and K. Saigo, Chem. – Eur. J., 2007, 13,
5186–5196.
13 (a) K. Tagaki, T. Scichi, T. Usami and Y. Sawaki, J. Am. Chem.
Soc., 1993, 115, 4339–4344; (b) H. Usami, K. Tagaki and Y.
Sawaki, J. Chem. Soc., Faraday Trans., 1992, 88, 77–81.
14 (a) N. Vallavoju and J. Sivaguru, Chem. Soc. Rev., 2014, 43,
4084–4101; (b) V. Ramamurthy, Acc. Chem. Res., 2015, 48,
2904–2917.
There are no conflicts to declare.
Acknowledgements
We thank FONACIT (grant LAB-97000821) for financial sup-
port. A. Briceño for technical assistance and the Language In-
stitute of Universidad Nacional de San Luis for language revi-
sion of the manuscript.
Notes and references
1 (a) J.-M. Lehn, Supramolecular Chemistry: Concepts and
Perspectives, VCH, Weinheim, 1995, pp. 139–197; (b) J.-M.
Lehn, Chem. Soc. Rev., 2007, 36, 151–160; (c) C. P. Pradeep,
D.-L. Long, G. N. Newton, Y.-F. Song and L. Cronin, Angew.
Chem., Int. Ed., 2008, 47, 4388–4391.
This journal is © The Royal Society of Chemistry 2018
CrystEngComm

View Article Online
Paper
CrystEngComm
15 (a) S. Yamada and Y. Tokugawa, J. Am. Chem. Soc.,
2009, 131, 2098–2099; (b) S. Yamada, N. Uematsu and K.
Yamashita, J. Am. Chem. Soc., 2007, 129, 12100–12101.
16 (a) A. Briceño, D. Leal, R. Atencio and G. Díaz de Delgado,
Chem. Commun., 2006, 3534–3536; (b) Y. Hill and A. Briceño,
Chem. Commun., 2007, 3930–3932; (c) A. Briceño, Y. Hill, T.
González and G. Díaz de Delgado, Dalton Trans.,
2009, 1602–1610.
23 (a) M. D. Cohen and G. M. J. Schmidt, J. Chem. Soc.,
1964, 1996–2000; (b) M. D. Cohen, G. M. J. Schmidt and F. I.
Sonntag, J. Chem. Soc., 1964, 2000–2013; (c) G. M. J.
Schmidt, J. Chem. Soc., 1964, 2014–2021.
24 (a) R. Telmesani, S. H. Park, T. Lynch-Colameta and A. B.
Beeler, Angew. Chem., Int. Ed., 2015, 54, 597–601; (b) Y. V.
Pol, R. Suau, E. Perez-Inestrosa and D. M. Bassani, Chem.
Commun., 2004, 1270–1271; (c) M. Pattabiraman, A.
Natarajan, L. S. Kaanumalle and V. Ramamurthy, Org. Lett.,
2005, 7, 529–532.
17 E. G. Rakov, Chemistry of Carbon Nanotubes, ed. Y. Gogotsi,
CRC Press, Boca Raton, 2006, pp. 77–147.
18 (a) K. Pan, H. Ming, Y. Liu and Z. Kang, New J. Chem.,
2012, 36, 113–118; (b) X. Sun and Y. Li, Angew. Chem., Int.
Ed., 2004, 43, 597–601.
19 G. R. Desiraju, Angew. Chem., Int. Ed. Engl., 1995, 34,
2311–2327.
20 C. Avendaño, A. Briceño, F. J. Méndez, J. L. Brito, G.
González, E. Cañizales, R. Atencio and P. Dieudonne, Dalton
Trans., 2013, 42, 2822–2830.
21 (a) J. Vansant, S. Toppet, G. Smets, J. P. Declercq, G.
Germain and M. V. Meerssche, J. Org. Chem., 1980, 45,
1565–1573; (b) S. Yamada, M. Kusafuka and M. Sugawara,
Tetrahedron Lett., 2013, 54, 3997–4000.
25 (a) N. Shan and W. Jones, Green Chem., 2003, 5, 728–730; (b)
T. Friščić, A. V. Trask, W. Jones and W. D. S. Motherwell,
Angew. Chem., Int. Ed., 2006, 45, 7546–7550; (c) M. B. J.
Atkinson, D.-K. Bučar, A. N. Sokolov, T. Friščić, C. N.
Robinson, M. Y. Bilal, N. G. Sinada, A. Chevannes and L. R.
MacGillivray, Chem. Commun., 2008, 5713–5715.
26 (a) A. Briceño, D. Leal, G. Ortega, G. Díaz de Delgado, E.
Ocando and L. Cubillan, CrystEngComm, 2013, 15,
2795–2799; (b) A. Briceño, D. Leal and G. Díaz de Delgado,
New J. Chem., 2015, 39, 4965–4971.
27 A. N. Sokolov, D.-K. Bučar, J. Baltrusaitis, S. X. Gu and L. R.
MacGillivray, Angew. Chem., Int. Ed., 2010, 49, 4273–4277.
28 (a) M. Capdevila-Cortada and J. J. Novoa, CrystEngComm,
2015, 17, 3354–3365; (b) A. Priimagi, G. Cavallo, P. Metrangolo
and G. Resnati, Acc. Chem. Res., 2013, 46, 2686–2695.
22 (a) Y. Hill, M. Linares and A. Briceño, New J. Chem.,
2012, 36, 554–557; (b) A. Briceño, A. Fulgence, Y. Hill and R.
Atencio, Dalton Trans., 2008, 3275–3278.
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