The impact of solid solution composition on kinetics and mechanism of [2 + 2] photodimerization of cinnamic acid derivatives

Article abstract of DOI:10.1039/d0ce01718c

We propose the use of solid solutions for the modulation of the kinetics in [2 + 2] photocycloaddition. Solid solutions with general formula [1H]Br_xCl_1?x(1= 4-amino-cinnamic acid; 0

Full text of DOI:10.1039/d0ce01718c

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DOI: 10.1039/D0CE01718C
ARTICLE
The impact of solid solution composition on kinetics and
mechanism of [2+2] photodimerizations of cinnamic acid
derivatives
Received 00th January 20xx,
Accepted 00th January 20xx
Lorenzo Pandolfi,†a Andrea Giunchi,†a Tommaso Salzillo,*ab Aldo Brillante, Raffaele G. Della Valle,
a
a
a
c
c
DOI: 10.1039/x0xx00000x
Elisabetta Venuti,* Fabrizia Grepioni, and Simone D’Agostino*
We propose the use of solid solutions for the modulation of the kinetic in [2+2] photocycloaddition. Solid solutions with
general formula [1H]Br Cl1‒x (1 = 4-amino-cinnamic acid; 0 < x < 1) were prepared via mechanochemical methods, structurally
x
characterized, and found to undergo a single-crystal-to-single-crystal [2+2] photodimerization. The effect of the solid
solutions composition on the kinetics was successfully investigated via a combination of FTIR and Principal Component
Analysis (PCA), and the results were compared with those obtained for the parent materials [1H]Cl and [1H]Br. All systems
follow the same nearly first-order law, and the overall reaction kinetics is influenced by a subtle interplay of structural and
electronic factors as well as other processes regulating the formation and lifetime of the reacting excimer.
reports is scarce, and concerning mainly the possibility of
2
9-35
Introduction
obtaining specific regioproducts.
3
6
According to Kitaigorodsky crystalline solid solutions can be
defined as nonstoichiometric multicomponent crystals in which
at least two components combine homogeneously in a unique
phase with random occupancy of the crystallographic sites. The
as a tool for modifying the
physicochemical properties of materials has been recently
pointed out in several studies, including modulation of the
enantioselectivity
melting point,44,45 and plastic phase transition,46
among others. We have recently reported on the activation of
the solid-state photoreactivity in 4-aminocinnamic acid (1)
through formation of molecular salts of general formula [1H]A,
[
2+2] photoreactions in crystalline materials are paradigmatic in
1
,2
3,4
solid-state photochemistry and crystal engineering studies.
5
Since the pioneering work of Schmidt and Cohen, who
formulated the well-known topochemical postulate, i.e.,
maximum bond separation of 4.2 Å is a necessary condition for
3
7,38
potential of solid solutions
[
2+2] cycloaddition to occur, these reactions have constantly
attracted the curiosity of scientists, and have been at the core
of intense debate concerning the mechanistic
3
9
,40,41
thermosalient effect,
transition,
polymorphic
6
-8
and
4
2,43
9
geometrical factors affecting these kinds of transformations.
Over the last decades the focus has been mainly on the search
of templating units able to pre-organize molecular double-
bonds in the solid-state, via non-covalent interactions such as
4
7-49
obtained by reacting 1 with a series of inorganic acids.
Interestingly, we have found that in the salts [1H]Cl and [1H]Br,
obtained by reaction of 1 with the respective hydrohalic acid,
1
0-13
14-16
17-20
hydrogen
and halogen bonds,
metal-coordination,
and also B-N bonds.21 On the other hand, only a few works are
devoted to kinetic studies of such reactions, leaving, therefore,
[
2+2] photoreactions are quantitative and occur in a single-
2
2-25
this field rather unexplored.
cycloaddition reactions have been studied for applications such
as conversion of light energy into mechanical work, and
synthesis of polymeric materials from natural molecules or
recovered from waste. Among the widespread interest for
cycloaddition reactions, the study of the [2+2] photoreactions
involving solid solutions has remained rather unexplored; to the
best of these authors’ knowledge the number of literature
More recently, [2+2]
crystal-to-single-crystal (SCSC) fashion, albeit at different rates
4
7,48
(
8 h for [1H]Cl vs. 4 h for [1H]Br).
2
6
Intrigued by this feature, and taking advantage of the
interchangeability in the solid-state between the chloride and
bromide anions, we explored the preparation of binary solid
2
7
2
8
x
solutions with general formula [1H] Br Cl1‒x (0 < x < 1), to
investigate the effect of composition on the kinetics of the [2+2]
photoreaction with respect to the parent materials. The
crystalline solid solutions have been prepared via
mechanochemical methods and investigated by a combination
of solid-state techniques, including single-crystal and powder X-
ray diffraction, micro-Raman, and FTIR-ATR spectroscopy. The
combined use of these methods has made possible the
correlation between the kinetics of the [2+2] SCSC
a. Dipartimento di Chimica Industriale “Toso Montanari”, Università di Bologna,
viale del Risorgimento, 4, 40136, Bologna.Address here.
b.
Department of Materials and interfaces, Weizmann Institute of Science, Herzl
street 234, 76100 Rehovot, Israel.
c Dipartimento di Chimica “Giacomo Ciamician”, Università di Bologna, Via
c.
Francesco Selmi, 2, 40126, Bologna, Italy.
†
These authors contributed equally to this work.
Electronic Supplementary Information (ESI) available: SCXRD data and refinement photoreaction and the composition of the solid solutions.
details, PXRD patterns, FTIR and Raman spectra, representation of the PCA method,
and kinetic scheme. See DOI: 10.1039/x0xx00000x
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Journal Name
Cambridge CB21EZ, UK; fax: (+44)1223-336-033; or e-mail:
deposit@ccdc.cam.ac.uk). CCDC numbers 2038954-2038962.
Experimental
View Article Online
DOI: 10.1039/D0CE01718C
Synthesis. All reactants and reagents were purchased from
Sigma-Aldrich, and used without further purification except 4-
amino cinnamic acid (1) which was recrystallized from ethanol
prior use. Reagent grade solvents and bi-distilled water were
used.
Powder X-ray diffraction. For phase identification purposes
powder X-ray diffractograms in the 2θ range 5-30° (step size,
0
.02°; time/step, 20 s; 0.04 rad soller; 40 mA x 40 kV) were
collected on a Panalytical X’Pert PRO automated diffractometer
equipped with an X'Celerator detector and in Bragg-Brentano
geometry, using Cu Kα radiation without a monochromator. The
[
1H]Cl and [1H]Br:
according to previously reported
4
7,48
procedures
200 mg (1.23 mmol) of 4-aminocinnamic acid (1)
5
3
program Mercury was used for simulation of X-ray powder
patterns on the basis of single-crystal data determined in this
work or retrieved from the CCDC database: PACPOB and for
were suspended in ca. 5 mL of water and neutralized with the
respective HX acid, undissolved material was filtered off, and
the resulting solutions were left to slowly evaporate in the dark.
[
1H]Cl and KEHFAH02 for [1H]Br. In all cases, the identity
[
x
1H]Br Cl1‒x (binary solid solutions): the reactants were weighed
between polycrystalline samples and single-crystals was always
verified by comparing experimental and simulated powder
diffraction patterns (see Fig. ESI-2).
in the proper stoichiometric ratio (see Table 1) and manually
ground in presence of a catalytic amount of ethanol (kneading).
Subsequently, the so-obtained crystalline materials were
suspended in ca. 5 mL of water, sonicated up to complete
dissolution and left to slowly evaporate in the dark. Single-
crystal suitable for XRD were obtained upon complete
evaporation.
Solid-state UV-irradiation. Single-crystals samples were
irradiated using a UV-LED (Led EnginLZ1-10UV00-0000) with λ =
3
65 nm and placed at a distance of 1 cm. The same light source
and distance were used to irradiate powders for FTIR-ATR and
single-crystals for micro-Raman measurements. Powders were
obtained by finely grinding in a mortar the crystalline material
from the syntheses. A sufficient amount (about 1 mg) of powder
was spread on a support and irradiated, taking care of
frequently mixing it during the process to expose new surface
and prevent the occurrence of surface phenomena only.
Constant quantities of powder were analyzed by FTIR-ATR at
intervals of 60 min until the spectrum either coincided with that
of the pure dimer or did not show any further change.
Table 1 Amounts of reagents used in the synthesis of binary
solid solutions.
[
1H]Br
[1H]Cl
[
[
[
1H]Br0.25Cl0.75
1H]Br0.5Cl0.5
1H]Br0.75Cl0.25
20.8 mg (0.09 mmol)
51.6 mg (0.21 mmol)
61.0 mg (0.25 mmol)
48.6 mg (0.24 mmol)
41.2 mg (0.21 mmol)
17.7 mg (0.09 mmol)
Single-crystal X-ray diffraction. Single-crystal data for
1H]Br Cl1‒x,before and after UV irradiation (see below), were
collected at RT on an Oxford XCalibur S CCD diffractometer
equipped with a graphite monochromator (Mo-K radiation, λ =
.71073 Å). The structures were solved by intrinsic phasing with
[
x
FTIR-ATR spectroscopy. FTIR-ATR measurements in the
α
‒1
‒1
wavenumber range 400-4000 cm with a resolution of 0.5 cm
were carried out on Perkin Elmer Spectrum Two
0
a
5
0
SHELXT and refined on F2 by full-matrix least squares
spectrophotometer, equipped with a Universal ATR accessory.
In Fig. ESI-3 the spectra of the [1H]Br monomer and of its dimer
are compared, as a representative example of the spectra of the
5
1
52
refinement with SHELXL implemented in Olex2 software.
OH and HNH atoms were either directly located or, when not
H
possible, added in calculated positions, HCH atoms for all
compounds were added in calculated positions and refined
riding on their respective carbon atoms. In the crystal structure
of the binary solid solutions the site occupancy factors (SOFs) of
the anions were fixed according to those of the water solutions
from which the mixed crystals were grown. Crystal structures of
partially reacted crystal were treated as follows: (i) upon
irradiation, new electron density peaks, consistent with the
emergence of the photoproduct, appeared in the Delta Fourier
Maps. The atoms of the disappearing reactant and appearing
photoproduct were treated as disordered over two positions
and refined anisotropically, refinement of the SOFs by adding a
second free variable in the FVAR command line allowed the
extraction of the dimer and reactant contents.
[
x
1H]Br Cl1‒x series. In Fig. ESI-3 the bands corresponding to
vibrational modes which disappear with the progress of the
reaction are indicated.
Micro-Raman spectroscopy. Raman spectra of the crystalline
samples in the lattice phonon region (10-150 cm ) were
collected with
monochromator spectrometer interfaced with the optical stage
of an Olympus BX40 microscope. The excitation wavelength was
‒1
a
Horiba Jobin Yvon T64000 triple
+
a Kr laser tuned at 647.1 nm set at very low power to avoid
sample damage.
Principal Component Analysis (PCA) and the Rate Law. The PCA
method employs an eigenvalue diagonalization procedure
which, by analyzing the correlation of the data, decomposes the
signal into a weighted sum of independent components
Data collection and refinement details are listed in Table ESI-1.
5
3
The program Mercury was used to calculate intermolecular
interactions and for molecular graphics. Crystal data can be
(orthogonal to each other and therefore uncorrelated), ordered
obtained
free
of
charge
via
according to their contribution to the total variance. Important
contributions are contained in the first few components. For
our specific application to the analysis of the spectra during
www.ccdc.cam.ac.uk/conts/retrieving.html (or from the
Cambridge Crystallographic Data Centre, 12 Union Road,
54
2
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ARTICLE
the dimerization reaction, the first component mostly
represents the contribution common to the spectra of both
reactant and product and thus its weight remains nearly
constant over time. The second component, which picks up the
most important differences between the spectra, has a weight
wPCA(t) which rapidly changes with time t and in fact describes
the progress of the reaction. All further components collect the
noise and can be ignored. A graphical representation of the PCA
method is shown in Fig. ESI-5.
Results and Discussion
Crystalline [1H]Cl and [1H]Br are isomorphous,47,48 therefore their
crystal packings feature the same pattern of multiple charge-assisted
hydrogen bonds (Fig. ESI-1). In both crystals the [1H] cations are
properly aligned in an head-to-tail arrangement, with the double
bonds at a distance of 3.657(5) Å and 3.618(6) Å for [1H]Cl and
View Article Online
DOI: 10.1039/D0CE01718C
+
[
1H]Br, respectively. Upon UV irradiation both crystals undergo an
SCSC [2+2] photodimerization leading to the respective head-to-tail
dimers [1 ]Cl and [1 ]Br (see Fig. 1). The occurrence of the
H
2 2
2
H
2 2
2
We have obtained the rate constants k reported in the results
and discussion section by least square fitting wPCA(t) with a
model wFIT(t) assuming a linear relationship with the unreacted
fraction α(t) of the monomer, that is wFIT(t) = 훼(푡) + 푏, where,
SCSC transformation in these systems can also be followed by
analysis of the features displayed, upon irradiation, in the lattice
‒
1
phonon region (10-150 cm ) of the crystal vibrational spectrum, as
determined also by the micro-Raman measurements (see Below).
‒
kt
for the first order reaction which gave the best fit, α(t) = e .
The fitting parameters a, b and k where thus determined by
2
2
2
minimizing Χ = Σ
t w
[wPCA(t) – wFIT(t)] / σ , with standard errors σw
∝
a, i.e. constant standard errors σα on α(t).
The standard errors on the fitting parameter k reported in the
Fig. 1 Representation of the [2+2] photodimerization occurring upon UV
irradiation of single-crystals of [1H]Cl and [1H]Br. HCH omitted for clarity.
result and discussion section have been estimated by jackknife
5
5
“
leave-one-out” resampling, because the usual propagation of
incertitudes (based on deviation from the fit, linearity,
independence and normality assumptions) is unreliable for
highly non-linear procedures such as the PCA.
5
6
The differences in the reaction rate as a response to the different
anion, and the isomorphism of [1H]Cl and [1H]Br prompted us to
investigate (i) the possibility of obtaining solid solutions, via
mechanochemistry and/or crystallization from solution, over the
entire range of composition, and (ii) the effect of solid solution
formation on the [2+2] photoreactivity.
Polycrystalline powders of [1H]Br and [1H]Cl were thus weighed in
different stoichiometric ratios (see Table 1), subjected to
mechanochemistry, recrystallized, and further analyzed with powder
XRD to assess solid solution formation. Fig. 2 shows a comparison
between the XRD patterns of the parent materials and that of the
In addition to a first order law, the curves were tentatively fit
푛
in the form 훼(푡) = 푒― , which
푘푡
5
7-59
also to an Avrami equation
coincides with the first order exponential decay when the
additional parameter n takes the value 1. On releasing n, a
2
smaller χ is obtained as expected, but the lack of an improved
quality of the fit can be judged by applying the Akaike
6
0
Information Criterion AIC. In fact, the choice of a model
cannot be based solely on the agreement between
measurement and fit; for instance, even though the
experimental data could reproduced exactly by using a
polynomial with number of terms (i.e. parameters) equal to the
number of measurements, such a fit would contain no useful
information beyond that already contained in the data. In some
sense, we aim to maximize the additional information provided
by the fit. A reasonable measure of the amount of information
in a fit is the AIC (Akaike Information Criterion), defined as:
5
0:50 solid solution, [1H]Br0.5Cl0.5, as an example. In this and in all the
other cases the grinding process resulted in the physical mixture of
the two starting phases, while kneading with a few drops of ethanol
yielded solid solutions isomorphous with pure [1H]Cl and [1H]Br, as
shown by powder diffraction patterns recorded on the solid
products (see Fig. 2).
2
퐴퐼퐶 = 푛푙푛휒 + 2퐾
where n is he number of experimental points (i.e. observations)
and K is the number of adjustable parameters in the model. The
model with the minimum AIC is regarded as the best
representation of the experimental data. Thus, if different
2
models yield similar χ deviations, the smaller the number of
parameters the lower the AIC. By choosing the model with the
minimum AIC, one effectively combines a best fit criterion with
a “principle of parsimony”.
Fig. 2 Detail of the experimental powder patterns for the [1H]Br0.5Cl0.5 solid
solution obtained via kneading with ethanol, the physical mixture resulting
from the grinding process, and the simulated patterns for the parent
materials [1H]Br and [1H]Cl. The (002) peaks at 18.5° and 18.1° for [1H]Cl and
[
1H]Br, respectively, are still present in the physical mixture (black line), while
they merge into one intermediate peak (blue line) upon formation of the
1H]Br0.5Cl0.5 solid solution.
[
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Monitoring the position of the (002) reflection makes it possible to To test their photoreactivity, the same single-crysVtieawlsAruticsleedOnflioner
clearly appreciate how the patterns progressively shift to those of structure determination were irradiated sDiOmI:u1l0ta.1n0e39o/uDs0lCyEe0x1-7s1i8tCu
the pure components, thus reflecting a change in the unit cell volume for 2 h at 365 nm. Subsequent analysis on the irradiated
as a function of the composition (Fig. 3 and ESI-2). Additionally, solid specimens showed that the cyclization reaction had partially
solutions with 25:75, 50:50 and 75:25 stoichiometric ratios were occurred via SCSC in all cases, but unexpectedly with a different
‒
‒
obtained as single-crystals from slow evaporation of aqueous extent depending on the Br /Cl ratio. In these conditions, the
solutions containing the two reactants [1H]Cl and [1H]Br. Based on a reaction extent was estimated to be of ca 50% for [1H]Br0.5Cl0.5
,
comparison of the unit cell constants (Table ESI-1), the two while the remaining solid solutions [1H]Br0.25Cl0.75 and
supramolecular salts are indeed miscible, leading to crystalline [1H]Br0.75Cl0.25 showed ca 25% and 30%, respectively. Further
6
1
phases whose lattice parameters follow the Vegard's rule in the irradiation resulted in the quantitative conversion and afforded
whole composition range (Fig. 3) and can be formulated as the crystal structure of the corresponding photodimers
[
1H]Br
x
Cl1‒x (with 0 < x < 1).
[1
2
H
H
2
]BrCl (ca 4 h more), [1
2 2
H ]Br1.5Cl0.5 (ca 6 h more),
[
1
2
2
]Br0.5Cl1.5 (overnight).
This prompted us to perform
a
detailed solid-state
spectroscopic study to gain more insights into the kinetics of the
pure parent salts and of the solid solutions. We expected in this
way to fully answer the second question, that is how solid
solution composition impacts, in terms of time and mechanism,
on the photoreactivity.
FTIR-ATR measurements were performed on the salt powders upon
ex-situ irradiation. In Fig. ESI-3 we compare the spectra of [1H]Br and
2 2 2
its photodimer [1 H ]Br , obtained with an irradiation time long
enough to assure a complete transformation of the reactant. The
occurrence of the transformation is demonstrated by: (i) the shift of
–1
–1
the stretching C=O band from 1693 cm to 1699 cm as a result of
the loss of conjugation in the formation of the four member cycle; (ii)
the disappearance of the vinyl C=C stretching band lying at about
–1
–1
62-64
1
640 cm and of the =C-H bending band at 1277 cm [1H]Br.
Of
course, the very crowded product and reactant spectra differ in a
number of other features. As a typical example of the outcome of the
kinetic study, Fig. 4 shows a full set of spectra recorded as function
‒
1
of the irradiation time over the wavenumber range 1700-400 cm
for polycrystalline [1H]Br. Similar sequences have been collected for
[
1H]Br0.75Cl0.25, [1H]Br0.5Cl0.5, [1H]Br0.25Cl0.75 and [1H]Cl
.
Fig. 3 Top, detail of the PXRD patterns showing the shift of the (002) peak
‒
towards higher angles upon increasing the Br content. Bottom, relationship
‒
between the cell metrics and the Br content in [1H]Br
x
Cl1‒x (error bars are
smaller than the markers).
For the crystalline solid solutions [1H]Br
x
Cl1‒x with x = 0.25, 0.5, and
0
.75, we determined the structure via single-crystal XRD. The
hydrogen bonding interaction patterns and the distances (d)
between the double bonds present in the head-to-tail arrangements
have remained approximately the same (Fig. ESI-1) with respect to
the starting materials [1H]Br and [1H]Cl.
The strong similarity among the structures of the series is also
corroborated by the observed trend of their lattice phonon Raman
spectra. Such data reflect structural changes by probing variations in Fig. 4 FTIR-ATR spectra of [1H]Br (orange-trace) as a function of the irradiation
lattice dynamics (Fig. SI-4). Indeed, all the salts display nearly time (recorded at intervals of 60’) in the wavenumber interval considered for
identical spectral features, and the cell volume contraction of [1H]Cl the kinetics analysis (see text). The brown-trace identifies the pure dimer
with respect to [1H]Br merely produces the blue shift of a couple of [1 H ]Br Vertical dashed lines indicate features mentioned in the text.
2
2
2.
wavenumbers of the main peaks, which becomes hardly detectable
in the solid solutions. Therefore, the solid solutions should still
undergo [2+2] photoreactions.
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The most intuitive way to study the time progress of a chemical
reaction from a series of spectra is to compare the spectra of the
pure reactant and product, identify characteristic peaks of each
species, and then use their intensity ratios as a function of time in
the reactant/product mixtures to obtain concentration profiles that
can be fitted to a kinetic law.65 This method potentially discards the
information contained in the overall differences among the spectra.
For the FTIR data of this work, such a problem was solved by
obtaining the unreacted monomer fraction 훼(푡) as a function of the
irradiation time 푡 by means of the principal component analysis (PCA)
View Article Online
DOI: 10.1039/D0CE01718C
5
4
approach, as reported in detail in the ESI. Fig. 5 reports the fractions
훼(푡) vs 푡 for the salts under investigation. All of them are well
described by a first order kinetics 훼(푡) = 푒 ―푘푡. The curves resulting
from the fits are also shown in the Fig. while the rate constants 푘 are
given in the inset of Fig. 5. As reported in the ESI, the data do not
justify kinetics different from a first order model.
Fig. 6 Representation of the overall [2+2] photodimerization
occurring upon UV irradiation of crystalline samples of [1H] Br
CH omitted for clarity. 퐼푣 is the light beam intensity.
x
Cl1‒x.
H
The full set of rate equations governing steps (1)-(4) are reported in
ESI. Under steady state conditions for the excited species M* and
MM*, the kinetic law yielding the decay of the monomer M is found
to be:
―2푘1퐼푣푘3푘5[푀]2
푘4 + 푘5 + 푘 푘 [푀]
[푀]2
ⅆ[푀]
=
= ― 2푘1퐼푣
####(1)
ⅆ푡
푘2
(
)
퐾 + [푀]
3
5
#
#
푘2
(
푘4 + 푘5
)
x
Fig. 5 Unreacted fraction α(t) of the [1H]Br Cl(1‒x) monomers as a function of
where 퐾 =
푘3푘5
irradiation time t; dots: estimates from the PCA of the experimental spectra;
curves: exponential decays from the fits with a first order kinetic law 훼(푡) =
―푘푡
푒
.
Inset, first order kinetic constants k (h ) as deduced by fitting the PCA
results for the FTIR-ATR spectra of [1H]Br Cl1‒x in the wavenumber interval
‒1
The reaction appears of pseudo second order in [푀] for [푀]
[푀] ≫ 퐾
≪
퐾
. Instead for
this corresponds to a very small 푘 and/or a large 푘 . Clearly
a pseudo first order is obtained, and
x
‒
1
55
4
00-1750 cm , with standard deviations estimated by jackknife resampling.
2
3
the introduction of a spontaneous decay of the excited
Thus, in all cases we can assume the monomer concentration to monomer (i.e. k > 0) is needed to have a non-trivial kinetics,
2
decay with a kinetics law close to a first order. To understand the that could be described as a process in which the absorption of
origin of this, we can consider the overall reaction mechanism, as every photon produces and exciton reacting with a molecule M.
illustrated in Fig. 6. The steps are: (1) upon light absorption, the The experimental differences detected in the reactivity of the
monomer M transforms into the excited monomer M* (rate constant various salts must result from the different rates of the single
푘 퐼 , where 퐼 is the light beam intensity); (2) spontaneous de- steps, which contribute to the effective rate constants, even
1
푣
푣
excitation from M* back to M (rate 푘2); (3) reversible reaction of M* though the result of a photoinduced transformation, especially
and M molecules which combine to form the excimer (MM)*(rates one in the solid-state, can only be understood as the interplay
푘 , 푘 ): note that such a process may involve the excitonic transport of several processes.
3
4
of the activation energy to sites far from that initially excited. Finally The lower reactivity of [1H]Cl might be rationalized in terms of
(
4), the excimer can decay to form the dimer D (rate 푘5).
packing "compactness", which is linked to the reaction cavity
size, as well as number and strength of the hydrogen-bonds
+
between the -NH
3
and -COOH groups, located on the reactive
molecules, and the halides. In this sense, a strong hydrogen-
‒
66,67
bond acceptor, such as the Cl ,
could offer more resistance
to the molecular rearrangements occurring during the
photocyclization, thus making the reaction hindered. On the
‒
other hand, an anion such as Br , which is a weaker hydrogen-
6
6,67
bond acceptor,
should favor molecular migration while
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Journal Name
maintaining the hydrogen bonding interactions during the
monomer to dimer transformation, thus speeding up the overall
process. Similarly, a more “compact” packing should reduce the
reaction cavity volume and could, in principle, offer more
resistance to such rearrangements, making the reaction
disfavored again.27
View Article Online
DOI: 10.1039/D0CE01718C
Indeed, with a probe radius of the algorithm implemented in
Mercury53 set to 0.6, a void occupying 0.8% of the unit cell
volume can be found in [1H]Br, while in the solid solutions the
same quantity ranges between 0.7 and 1.4%. With the same
settings, the [1H]Cl structure displays a smaller void, accounting
for the 0.4% of the unit cell volume, which makes this phase the
most efficiently packed in the series, and consequently also with
a smaller reaction cavity. The lower reactivity of chloride salt
also reflects in the lower reactivity of the solid solutions, for
none of them shows kinetics as fast as [1H]Br’s. Noteworthy,
the trend is not monotonic, as [1H]Br0.5Cl0.5 has, unexpectedly,
the highest kinetic constant among the series. Because of the
1
:1 ratio between the chloride and bromide anions, such a
phase might be regarded as a “pseudo” stoichiometric
multicomponent crystal, hence possessing slightly different
features compared to the other solid solutions. However, the
halide positions result to be indistinguishable by XRD, even by
lowering the symmetry down to the triclinic system (P1) and
refining the halides SOFs. This leaves this phase peculiar
behavior not fully explained.
Fig. 7 Raman spectra in the lattice phonon interval of a single-crystal of [1H]Br
under irradiation. The spectra have been recorded at time intervals of 10’.
The orange trace identifies the pure monomer [1H]Br; the brown trace the
FTIR-ATR spectroscopy, holding the advantage of analysis
easiness and speed, is a useful technique for identifying the
species in the course of the chemical reaction but provides no
information about the lattice transformation that takes place
with it. On the other hand, micro-Raman spectroscopy in the
lattice phonon range may provide information about the
2 2
pure dimer [1 H ]Br2. The vertical line marks the shift of the lattice phonon
band at 20 cm‒1 in the monomer with the advancement of the reaction.
Conclusions
In this paper we have reported our findings on the solid-state
6
5-70
related lattice transformation.
As an example, Fig. 7 shows
[
2+2] photoreactivity of the 4-aminocinnamic acid solid
the evolution of the [1H]Br lattice phonon Raman spectrum
solutions with general formula [1H]Br Cl1‒x (0 < x < 1). These
x
–1
under irradiation. The band at 21 cm loses intensity and
salts were obtained as crystalline materials through
mechanochemistry by mixing the parent compounds, [1H]Br
and [1H]Cl in different molar ratios. The unit cell constants
showed a linear response with composition; this behavior is
typical of solid solutions and of crystals constructed by almost
isostructural species. For all the solid solutions, single-crystals
were also proved to undergo an SCSC photoreaction upon UV
exposure.
To investigate the effect of the solid solutions composition on
the kinetics of the [2+2] photoreaction, FTIR-ATR data collected
upon irradiation were treated using the PCA method. Monomer
and dimer vibrational spectra are very crowded and similar, so
that the maximum information on the reaction progress is
obtained from the entirety of the spectral features, rather than
a limited number of bands assumed as diagnostic. The analysis
has allowed us to derive a kinetic law in agreement with a very
plausible reaction scheme and to quantify the rate differences
displayed by the salts. All systems obey to the same kinetics,
which is satisfactorily described as a nearly first order. However,
in treating solid-state reactions, one should consider that the
true system is often too complex and is not adequately
described in terms of elementary steps. Structural features and
–1
–1
gradually shifts up to 26 cm , while that at 38 cm disappears
‒
1
and those at higher wavenumbers (in the range 75-100 cm )
merge together into a broad envelope. The spectrum of lattice
vibrations changes continuously, evolving from that of the
reagent to that of the product and therefore the spectra
sequence cannot be solved as the sum of spectra of the two.
This behavior is at variance from what observed in solid-state
reactions that take place with a phase reconstruction
mechanism,6 where the lattice phonons of both reactant and
product appear in the spectrum as long as the host (the
reactant) and the guest (the product) phases coexist. Thus, this
evidence agrees with a picture in which at each moment of the
SCSC transformation the system consists of a solid solution of
monomer and dimer.
5-70
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and G. Resnati, J. Am. Chem. Soc., 2004, 126, 4500–4501.
material properties justify the varying reactivity along the
series. [1H]Cl very low reactivity links with the highest packing
View Article Online
1
1
5 J. Quentin and L. R. MacGillivray, ChemPhysChem, 2020, 21, 154–
DOI: 10.1039/D0CE01718C
1
63.
“compactness”, and the maximum number of the stronger
6 J. Quentin and L. R. MacGillivray, Cryst. Growth Des., 2020, 20,
501-7515.
hydrogen-bonding interactions, namely those between the
7
+
between the -NH
3
and -COOH groups, located on the 17 I. H. Park, K. Sasaki, H. S. Quah, E. Lee, M. Ohba, S. S. Lee and J. J.
Vittal, Cryst. Growth Des., 2019, 19, 1996–2000.
monomers, and the chloride. The photoreactive behavior of
1H]Br0.5Cl0.5, which has proved to be the fastest compared to
the other two solid solutions, namely [1H]Br0.25Cl0.75 and
1H]Br0.75Cl0.25, represents an exception.
1
1
8 K. Yadava and J. J. Vittal, Chem. - A Eur. J., 2019, 25, 10394–
[
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2
In conclusion, such results suggest that the overall reaction 20 C. Li, G. Campillo-Alvarado, D. C. Swenson and L. R. Macgillivray,
Inorg. Chem., 2019, 58, 12497–12500.
kinetics is in fact governed by a subtle interplay involving
structural and electronic factors such as the small voids left
within the structure, the type and strength of the charge
assisted hydrogen bonding interactions as well as other
processes which regulate the formation and the lifetime of the
reacting excited state, i.e. the excimer. From these authors'
knowledge, this research represents the first attempt to tuning
the speed of the [2+2] photoreactivity by using solid solutions
expanding, thus, the study of such reactions in non-
2
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Conflicts of interest
There are no conflicts to declare.
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Table of Content Entry (Graphical Abstract)
We propose the use of crystalline solid-solutions for the modulation of the kinetic in solid-state
2+2] photocycloaddition. The effect of the solid-solutions composition on the kinetics was
[
evaluated via a combination of Single Crystal XRD, FTIR, Raman, and Principal Component
Analysis (PCA).