Design and synthesis of photoactive ionic liquids

Article abstract of DOI:10.1021/ol501111d

Two ionic liquids with photoisomerizable p-hydroxycinnamic acid moieties were synthesized and characterized by X-ray crystallography and DSC, and their photochemistry was studied in solution and neat conditions. Irradiation at absorption maxima led to trans-cis photoisomerization and resulted in significant reduction of melting temperatures of the ionic liquids. X-ray structures of both compounds show an intricate network of supramolecular interactions before irradiation. Physical and chemical transformations are completely reversible upon irradiation at lower wavelengths of ionic liquid solutions in acetonitrile.

Full text of DOI:10.1021/ol501111d

Letter
pubs.acs.org/OrgLett
Design and Synthesis of Photoactive Ionic Liquids
Joao Avo,*,† Luís Cunha-Silva,‡ Joao Carlos Lima,† and A. Jorge Parola†
́
̃
̃
†REQUIMTE, Departamento de Química, Faculdade de Cien
Portugal
‡REQUIMTE, Departamento de Química e Bioquímica, Faculdade de Cien
̂
cias e Tecnologia, Universidade Nova de Lisboa, 2829-516 Caparica,
̂
cias, Universidade do Porto, 4169-007 Porto, Portugal
S
* Supporting Information
ABSTRACT: Two ionic liquids with photoisomerizable p-hydroxycin-
namic acid moieties were synthesized and characterized by X-ray
crystallography and DSC, and their photochemistry was studied in
solution and neat conditions. Irradiation at absorption maxima led to
trans−cis photoisomerization and resulted in significant reduction of
melting temperatures of the ionic liquids. X-ray structures of both
compounds show an intricate network of supramolecular interactions
before irradiation. Physical and chemical transformations are completely
reversible upon irradiation at lower wavelengths of ionic liquid solutions in acetonitrile.
responsive moiety that exhibits reversible photochemistry that
esearch in ionic liquids (ILs) is one of the most rapidly
Rgrowing fields in recent years, focusing on the ultimate
aim of large-scale industrial applications. Because of their
unique physicochemical properties such as negligibly low vapor
pressure at ambient temperatures, high ionic conductivity, and
high thermal stability at elevated temperatures, ILs have
attracted considerable technological and scientific interest in
the past decade.1 By varying the length of the alkane chains of
the cationic core and the type or size of the anion, the physical
properties of ILs can be tailored to meet the requirements of
specific applications to create an almost infinite set of organic
salts.2−4 One of the main interests in ILs, in particular those
that are liquid at room temperature (RTILs), arises from their
applicability as an alternative to organic volatile solvents as
reaction and extraction media,5 where properties such as
viscosity and liquid range play an important role.6 Herein, we
report the synthesis of novel ionic liquids whose melting points
are reversibly tunable with light, leading to the formation of
RTIL after irradiation of a solid salt. Photoresponsive ionic
liquids (PRILs) have attracted considerable attention in the
past decade due to attractive features such as photoresponsive
magnetic moments or ionic conductivity and structure
dependent photochromism.7 Instead of relying on the photo-
chemistry of diarylethene or azobenzene derivatives, which are
usually expensive and difficult to prepare,7c−e the PRILs
reported herein are based on cinnamic acid (CA) derivatives.
CA is a naturally occurring and readily available photosensitive
moiety, with well-characterized photochemistry8,9 that has been
applied extensively in photoresponsive systems.10 In particular,
photosensitive ionic liquids bearing CA have been prepared and
have been shown to undergo photochemical trans-to-cis
isomerization.11 However, this reaction was used with the aim
of separating both isomers and was not characterized in terms
of quantum yields or reversibility. We report the first account of
a photoresponsive ionic liquid bearing a natural photo-
allows changes in their physical properties with light stimuli.
The syntheses of the compounds reported through this work
are summarized in Scheme 1. The esterification of p-coumaric
Scheme 1. Synthetic Pathway for the Preparation of
Cinnamate Ionic Liquids 5a and 5b
acid, 1, is readily achieved through acid catalysis with H2SO4 in
methanol. Alkylation of the phenol moiety is accomplished
through nucleophilic substitution assisted with potassium
carbonate in dry acetonitrile. The corresponding product, 3,
undergoes another nucleophilic substitution in the presence of
N-methylimidazole to yield the imidazolium bromides 4a and
4b. Melting points of these ionic species were high (142 and
153 °C, respectively), and thus, anion metathesis with lithium
bis(trifluoromethylsulfonyl)amide (Tf2N−) was performed to
lower the melting point of the organic salts and yield ionic
liquids 5a and 5b, with melting points at ca. 83 and 53 °C,
respectively.
Compounds 5a and 5b were dissolved in a mixture of CHCl3
and MeOH 1:1, and the solution was allowed to be in contact
Received: January 31, 2014
Published: May 2, 2014
© 2014 American Chemical Society
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Letter
with a slow diffusion of Et2O leading to the formation of
colorless single crystals with high quality for X-ray diffraction
analysis which confirmed the formation of the two desired ILs
(Figure 1). The crystal structures of the two compounds were
intermolecular interactions, eventually leading to observable
changes in physical properties of 5a and 5b.
The photochemical behavior of ionic species 5a and 5b was
initially investigated in acetonitrile. Both compounds under-
went photochemical reactions when irradiated at their
absorption maxima (Figure 2a). The decrease in absorbance
Figure 1. Crystal structures of the compounds 5a and 5b, showing the
label scheme for all non-H atoms which are represented as thermal
ellipsoids drawn at the 50% probability level, while H atoms are shown
as small spheres with arbitrary radius.
Figure 2. Spectral modifications of 5.4 × 10−5 M 5b in acetonitrile
upon irradiation at 300 nm (a) and 240 nm (b). Under the used
experimental setup, the PSS was reached after 10 min irradiation at
300 nm, while the recovery upon irradiation at 240 nm required ca.
600 min.
determined in the triclinic space group P-1 with both
asymmetric units revealing only one respective organic cation
and one Tf2N− anion (detailed information on the crystallo-
graphic data collection and structure refinement are provided in
the Supporting Information). The organic cation in compound
5a reveals considerable planarity with the phenyl and the
imidazolium rings practically in the same plane (the dihedral
angle between the average planes of these two- aromatic groups
is ca. 3.563°), while the structural arrangement of the organic
cation in 5b is significantly distinct, showing the imidazolium
ring almost perpendicular to the phenyl group (the dihedral
angle between the planes is ca. 79.341°). This structural
difference is certainly related with the size of the alkylic chain
between the cinnamate moieties and the imidazolium groups,
as well as with the numerous C−H···X (X = F, N or O)
intermolecular interactions involving adjacent organic cations
and Tf2N− anions.
In fact, the crystal packing arrangement in both 5a and 5b is
strongly influenced by an extensive network of C−H···O weak
hydrogen bonds involving neighboring organic cations, as well
as C−H···F, C−H···N, and C−H···O between the organic
moieties and the inorganic Tf2N− anions (see Figure S15 in the
Supporting Information). In compound 5a, the adjacent
organic cations interact via three C−H···O bonds, while the
inorganic anions establish various C−H···O and C−H···N weak
hydrogen bond interactions with contiguous organic cations,
leading to a 3D supramolecular structure (Supporting
Information, Figure S15b). The same type of interactions are
observed in the crystal packing of 5b, ultimately leading to a 3D
supramolecular network (Supporting Information, Figure
S15c). In principle, irradiation of these ionic liquids should
induce changes in the molecular structure, by isomerization
and/or dimerization of the cinnamate moieties, and result in
the disruption of the described intricate network of
around 300 nm and a slight increase in absorbance at lower
wavelengths are consistent with the expected photoreactivity of
cinnamic acid derivatives, i.e., photoisomerization and/or
photodimerization, leading to species with blue-shifted maxima.
In order to investigate the mechanism of the photochemical
reaction, quantum yields (ΦR) were calculated for a wide range
of concentrations, from 10−5 to 10−2 M in acetonitrile and are
summarized in Table 1. For both 5a and 5b, ΦR values did not
Table 1. Absorption Maxima (λmax), Photochemical
Quantum Yields (ΦR), and Cis Isomer Fraction before
Irradiation and at the Photostationary State (PSS) of Ionic
Liquids 5a and 5b in Acetonitrile
λmax
(nm)
concn
(M)
initial cis
fraction
PSS cis
fraction
compd
ΦR
5a
304
10−2
10−3
10−4
10−5
10−2
10−3
10−4
10−5
0.12
0.11
0.11
0.12
0.11
0.10
0.12
0.11
0.0
0.47
5b
304
0.0
0.48
vary within this concentration range. This result led to the
conclusion that an intramolecular reaction occurs upon
irradiation, since the reaction rate does not depend on the
concentration of the ionic liquid. Furthermore, the existence of
an isosbestic point at 265 nm indicates that no secondary
reactions occur during irradiation.12 To corroborate these
results, 1H and 13C NMR spectra were obtained for the
irradiated solutions of both ionic liquids, and it was evidenced
that photostationary states were composed only by both trans
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Letter
and cis isomers in similar proportions (see details in the
Supporting Information, Figures S22−S25). Therefore, the
occurrence of photodimerization of cinnamate moieties was
excluded for this concentration range, in accordance with
previously described results for cinnamate ILs.11 Moreover, the
presence of decarboxylation products (arylethenes) was not
detected. Further conversion of trans-5 into cis-5 was not
possible due to the similar absorbance of both isomers at the
excitation wavelength.11,13
The reversibility of the system was evaluated both thermally
and photochemically, followed by UV−vis and 1H NMR
spectroscopy. Irradiated solutions were allowed to stand
overnight at room temperature and thermal recovery was not
detected, as expected for room temperatures.14 In addition,
neat samples at the photostationary state were kept at 120 °C
for 8 h without giving rise to thermal recovery. On the other
hand, both ionic liquids exhibited photochemical reversibility
by irradiation at 240 nm. The spectral modifications observed
by UV−vis show full recovery of the initial spectrum, Figure 2b.
The degree of reversibility from isomer mixture to pure trans-5
was further corroborated by NMR data (see the Supporting
Information, Figures S26 and S27). Full conversion to the
trans-isomer was achieved after long irradiation times due to the
low absorption at the selected wavelength and lower cis-to-trans
isomerization quantum yield (0.015).
composed by both cis and trans isomers melted at 55 °C.
Moreover, its lower enthalpy of melting and larger difference
between onset (Tm) and peak (Tp) temperatures indicates that
the structural order is reduced after irradiation, due to the
presence of both isomers.15 For ionic liquid 5b, the changes in
physical properties were even more significant, Table 2. These
results can be explained by the different intermolecular
interactions established between the photoresponsive moieties
in both ionic liquids. Whereas the proximity to the ionic
domain in 5a gives rise to interactions between cinnamate and
the cationic and anionic cores (Supporting Information, Figure
S15b), in 5b these interactions are only established between
cinnamate moieties (Supporting Information, Figure S15c).
Upon isomerization, it is expected that loss of planarity in the
photoresponsive core leads to the disruption of interactions
between cinnamate moieties, while interactions with the ionic
domain can still be established.
Upon irradiation at 240 nm to achieve complete recovery of
trans isomers (5a,brec), both Tm and ΔHm were measured by
DSC. The values obtained are very close to those of
nonirradiated samples (5a,b), showing that chemical and
physical transformations are reversibly adjustable. After
characterization of the photochemical behavior of ionic liquids
5a and 5b in solution, their photochemistry and resulting
physical properties modifications were investigated in neat
conditions. Both compounds were irradiated at 300 nm at their
melting temperature as thin films, and the photochemical
transformations were followed by 1H NMR spectroscopy.
NMR data show that, although 5a and 5b also undergo trans-
to-cis isomerization upon irradiation under these conditions,
other photochemical reactions take place with significant
quantum yields, as indicated by the presence of several proton
peaks with high chemical shifts (see the Supporting
Information, Figure S28).
The melting point (Tm) and enthalpies of melting (ΔHm) of
the ionic liquids were determined by differential scanning
calorimetry (DSC). The obtained curves are depicted in Figure
3 and the corresponding values are summarized in Table 2.
Photostationary states were achieved after 1 h of irradiation
and were composed of a mixture of trans-5 (ca. 45%), cis-5 (ca.
30%), and other new compounds (ca. 25%). Although the
complexity of the obtained NMR spectra complicaes the
identification of the new species, it is expected that photo-
oxidative processes occurring in the cinnamate moieties may
give rise to different imidazolium salts. To test this hypothesis,
ionic liquids 5a and 5b were melted, deaerated in vacuo, and
irradiated under inert atmosphere.
NMR data show that, under these conditions, the extension
of the photochemical byproducts is significantly reduced (<3%)
(see the Supporting Information, Figure S29, for details). Since
these experiments had to be carried out in closed quartz tubes
to achieve inert atmosphere, ionic liquid samples had a low
surface-to-volume ratio which in turn led to heterogeneous
mixtures of isomers during irradiation.
Figure 3. DSC curves of ionic liquids 5a and 5b before irradiation (i),
at the PSS (PSS), and after recovery (rec).
Table 2. Melting (Tm) and Peak (Tp) Temperatures and
Melting Enthalpies (ΔHm) of ILs 5a and 5b before
Irradiation at 300 nm (i), after the PSS Has Been Reached
(PSS) and after Recovery upon Irradiation at 240 nm (rec)
Therefore, photochemical quantum yields could not be
determined, and photostationary states were not achieved.
However, melting points were still lowered after partial trans to
cis isomerization of compounds 5a and 5b (for details, see the
Supporting Information, Table S2). Moreover, it appears that
melting temperature depression (ΔTm) is directly proportional
to the fraction of cis-5 in the irradiated sample (Figure 4) and
extrapolates to the values of irradiated solution samples with
equal fractions of both isomers. These results suggest that
irradiation of thin films of 5a and 5b under inert atmosphere
would yield ΔTm values similar to those obtained for irradiated
solutions, if the same isomerization rate was achieved.
5a
5b
compd
5ai
5aPSS
5arec
5bi
5bPSS
5brec
Tm (°C)
Tp (°C)
ΔHm (J g−1
83.9
85.8
67.1
54.9
66.1
20.9
82.2
85.4
63.3
53.4
55.8
62.3
51.8
55.1
62.2
a
n.a.
)
a
Tg at −42.8 °C.
Both 5a and 5b showed significantly lower melting points
(onset temperatures) and ΔHm after irradiation (photosta-
tionary state). While the pure trans-5a (5ai) isomer had an
initial melting point of 83 °C, its photostationary state (5aPSS
)
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Figure 4. Isomer fraction dependency of melting temperature
depression of irradiated neat samples (black dots) and acetonitrile
solution (hollow square) of 5a.
In conclusion, a new class of photoresponsive ionic liquids
based on trans−cis isomerization of cinnamic acid derivatives
was obtained through rational design. Physical properties, such
as the melting point, of these compounds are addressable with
light stimuli, which could be related to changes in
intermolecular interactions in ionic clusters. In addition, these
changes are reversible upon irradiation in solution. This
behavior can be further explored for the tuning of properties
that are governed by intermolecular interactions, such as
viscosity or glass transition temperatures, to develop new
photoresponsive RTILs, expanding their technological applic-
ability as smart materials in fields ranging from microfluidics to
drug delivery.
ASSOCIATED CONTENT
* Supporting Information
■
S
Additional experimental data. This material is available free of
charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
■
*E-mail: jma17142@campus.fct.unl.pt.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
This study has been supported by the Fundaca
e Tecnologia through Grant No. Pest-C/EQB/LA0006/2011
and PhD Grant No. SFRH/BD/65127/2009) (J.A.).
■
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o para a Cien
̂
cia
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