The: Trans / cis photoisomerization in hydrogen bonded complexes with stability controlled by substituent effects: 3-(6-aminopyridin-3-yl)acrylate case study

Article abstract of DOI:10.1039/c8ra03042a

The association of aminopyridine-based acrylic acid and its salt was studied by NMR titration experiments. The AA (acceptor, acceptor) hydrogen-bonding pattern present in the salt forms a complex readily with a DD (donor, donor) hydrogen-bonding pattern of the substituted ureas even in polar and competitive environment. The double carbon-carbon bond in the acrylic acid derivative is subjected to photoisomerization. This is dependent on the association with substituted urea derivatives. The substituent in ureas influences the trans/cis isomerization kinetics and position of the photostationary state. Two mechanisms that influence the photoisomerization were proposed. To the best of our knowledge, the trans/cis photoisomerization influenced by the substituent in such a hydrogen-bonding pattern has not observed previously. It was shown that interaction with urea derivatives causes lowering of the trans-to-cis photoreaction rates.

Full text of DOI:10.1039/c8ra03042a

RSC Advances
View Article Online
View Journal | View Issue
PAPER
The trans/cis photoisomerization in hydrogen
bonded complexes with stability controlled by
substituent effects: 3-(6-aminopyridin-3-yl)
acrylate case study†
Cite this: RSC Adv., 2018, 8, 23698
Adam Kwiatkowski,a Beata Jedrzejewska,a Marek Jozefowicz,b Izabela Grelaa
˛
´
c
*
and Borys Osmiałowski
´
The association of aminopyridine-based acrylic acid and its salt was studied by NMR titration experiments.
The AA (acceptor, acceptor) hydrogen-bonding pattern present in the salt forms a complex readily with
a DD (donor, donor) hydrogen-bonding pattern of the substituted ureas even in polar and competitive
environment. The double carbon–carbon bond in the acrylic acid derivative is subjected to
photoisomerization. This is dependent on the association with substituted urea derivatives. The
substituent in ureas influences the trans/cis isomerization kinetics and position of the photostationary
state. Two mechanisms that influence the photoisomerization were proposed. To the best of our
knowledge, the trans/cis photoisomerization influenced by the substituent in such a hydrogen-bonding
pattern has not observed previously. It was shown that interaction with urea derivatives causes lowering
of the trans-to-cis photoreaction rates.
Received 9th April 2018
Accepted 19th June 2018
DOI: 10.1039/c8ra03042a
rsc.li/rsc-advances
also in crystals15). Taking the proton transfer process into
account some general remarks should be made. First, it is worth
Introduction
mentioning that carboxylic acids are self-complementary
(Fig. 1a) and the same is realized in 2-aminopyridine (Fig. 1b)
or its N-acylated (Fig. 1c) derivative.16 Secondly, the carboxylic
acid is complementary to the 2-acylaminopyridine (Fig. 1d). On
the other hand, its deprotonated form (carboxylate) may
interact with ureas (Fig. 1e) readily but not with 2-acylamino-
pyridine. Thus, the dissociation of the carboxylic acid causes
that its hydrogen-bonding pattern does not t the interaction
regime in 2-acylaminopyridine.
The urea moiety is one of the most popular supramolecular
synthons.1–3 It forms double hydrogen-bonding but this feature
is just a starting point in research on its properties. These
molecules may be used in crystal engineering, as a selective
sensor for nitro-compounds4 and are able to form intra-
molecular hydrogen bond that is broken upon association.5–7
Their main popularity in organic chemistry lies in organo-
catalytic properties of sulfur derivatives (thioureas).8 The
studies on complexation of ureas are quite popular especially in
light of interactions of these molecules with oxoanions or
carboxylates.9–11 Regarding other simple supramolecular syn-
thons able to form two intermolecular hydrogen bonds
carboxylic acids are good co-crystallizing agents that interact
with 2-acylaminopyridines forming co-crystals while the same
acids form salts with aminopyridine in the solid state.12–14 These
examples of doubly-hydrogen bonded associates show
a number of possibilities of inter-synthon interactions, which is
further extended due to the proton transfer reaction (probably
aFaculty of Chemical Technology and Engineering, UTP University of Science and
Technology, Seminaryjna 3, PL-85326 Bydgoszcz, Poland
b
´
Faculty of Mathematics, Physics and Informatics, University of Gdansk, Wita Stwosza
´
57, 80-308 Gdansk, Poland
c
´
Faculty of Chemistry, Nicolaus Copernicus University in Torun, 7 Gagarin Street, 87-
100 Torun, Poland. E-mail: borys.osmialowski@umk.pl
† Electronic supplementary information (ESI) available: NMR spectra, titration
curves, absorption spectra, Cartesian of structures. See DOI: 10.1039/c8ra03042a
Fig. 1 Inter-synthon interaction by double hydrogen bonding.
23698 | RSC Adv., 2018, 8, 23698–23710
This journal is © The Royal Society of Chemistry 2018

View Article Online
Paper
Over the plethora of possible interactions the most chal-
RSC Advances
lenging ones are those that may be present in the investigated
system simultaneously. One of examples may be the bis(urea)
that is, in theory, able to break two intramolecular hydrogen
bonds upon association.5,7 Thus, it is crucial to know the
interaction of the simple supramolecular synthons. This, for
sure, move the research towards more complex systems as it was
realized for ternary ones.17 This is important to keep in mind
that non-covalent interactions are geometry-dependent. This
includes the rotational movements of one part of molecule with
respect to another one. These rotations play a crucial role in
tting molecule to various guests as, for example, in 2,4-bis(a-
cylamino)pyrimidines when no intramolecular hydrogen bond
is broken18 but also when rotamerism must be present to form
complexes6 and the dissociation of the intramolecular hydrogen
bond is a condicio sine qua non.
Fig. 2 Structures of compounds used in photoisomerization study.
The isomerism of the double bonds is one of the most well
investigated elementary reaction in photochemistry.19
Although, the geometrical isomerization of alkenes can be
effected thermally, catalytically and photochemically, the light
induced process is crucial due to variety of applications.20 In
particular, the process was investigated from the theoretical,
mechanistic and synthetic point of view. It proceeds in about
200 fs with quantum efficiency reaching up to 67%.21–24 The
photoisomerization of alkenes is believed to take place by the
excited state (singlet or triplet) in which the two sp2 carbons are
twisted by about 90ꢀ with respect to one another leading to an
intermediate in a form of zwitterionic or biradical. The basic
features of the process is that the ground state thermodynamics
does not inuence the trans/cis ratio in the photostationary
state but is, instead, controlled by the excited state potential
surfaces.19
be inuenced by hydrogen bonding. The said thermal reaction
usually proceeds in cis-to-trans direction. It may also be guided
by irradiation of the cis isomer by light of higher energy than in
case of trans-to-cis photoisomerization. The initial idea to study
this particular structure and its photoisomerization was
inspired by the curiosity if quadruple hydrogen bonding in the
dimer of 1 (vide infra) is able to hinder cis-to-trans thermal
isomerization. For comparison purposes two simpler
compounds were used (3 and 4). Fig. 2 shows structures used in
the current study.
Results and discussion
A
numerous studies on the application of photo- Photoisomerization of 3-(6-aminopyridin-3-yl)acrylate
isomerization have been conducted. For instance, the C]C derivatives
double bond conversion plays a role in many molecules25 that
The NMR data conrm that aminopyridine-based acrylic acid
are called molecular machines.26,27 The photoisomerization is
responsible for reversible processes in crystals28–30 and in solu-
tion.31–33 It can be used in photoswitches34 or photoswitchable
polymers.35
(1) and its salt (2) exist as trans isomer in solution. This isomer
may be partially transformed into a cis-form upon irradiation.
To do so the solution of compounds 1 and 2 in DMSO-d6 were
irradiated with xenon lamp. The trans-to-cis conversion was
monitored by 1H NMR spectrometry. The spectra were recorded
at each step of irradiation (time intervals, see ESI, Fig. S1 and
S2†).
The obtained data revealed that both compounds (1 and 2)
isomerize upon irradiation but their photostationary states are
different. Because the proton spectra of the compounds show
considerable changes in chemical shis, the kinetics of the
photoisomerization was studied. This was monitored by the
changes in the area under corresponding trans and cis peaks as
a function of irradiation time (t). The observed rate constant (k)
was determined based on the analysis of the experimental data
applying the following equation:
The trans/cis isomerism changes the photophysical proper-
ties of dyes. Thus, the stabilization of one form over another
might be crucial to material chemistry and molecular design.
Joining the photoisomerization with intermolecular interac-
tions may deliver interesting results. This is even more inter-
esting and challenging in the light of mentioned proton transfer
reaction that may take place between hydrogen-bonded coun-
terparts. It is worth noting that excited state proton transfer is
the mechanism that causes non-radiative energy loss. Our
interests in this topic came from the idea that association and
energy loss within the supramolecular complex may have an
impact on the stabilization of the double bond. Our intention
was to use the hydrogen bonding in studies of stability of
double C]C bonds in compounds that are prone to photo-
isomerization. The aim of this work is to (a) study the effect of
irradiation on isomerization of 1 and 2, (b) study the isomeri-
zation together with interaction of 2 with substituted ureas, (c)
ln[% trans] ¼ ln[% trans]0 ꢁ kt
In Fig. 3 the comparison of the kinetic behavior for
check if the photoisomerization or thermal back-reaction might aminopyridine-based acrylic acid (1) and its salt (2) are shown.
This journal is © The Royal Society of Chemistry 2018
RSC Adv., 2018, 8, 23698–23710 | 23699

View Article Online
RSC Advances
Paper
Fig. 4 Two dimers of 1 and possible proton transfer that could lead to
the non-radiative energy loss.
changes in their absorbance (Fig. 5). The absorption spectra
in the near ultraviolet of 1 and 2 consist of two intense
absorption bands with maxima at ca. 290 nm and 340 nm. The
trans-1 absorbs more light at 340 nm, whereas for its salt (trans-
2), the more intense is the short-wavelength absorption band.
Upon irradiation the inversion in the intensity of the main
bands of the acid 1 is observed. The short-wavelength band is
more intense like in the trans form of the salt 2 whereas the
intensity of the absorption band with maximum at 340 nm does
not change signicantly. This effect may be due to the different
molecular conguration of trans and cis isomers. The short-
wavelength band would be assigned to the electron transition
Fig. 3 Top: The isomerization of 1 and 2 as a function of irradiation
time (H8 chemical shift based data, in DMSO-d6, c ¼ 10.6 mmol dmꢁ3);
Bottom: Kinetics of the photoisomerization process induced by
irradiation.
The corresponding rate constants36 are rst-order reaction:37
11.2 ꢂ 10ꢁ4 sꢁ1 (1) and 29.1 ꢂ 10ꢁ4 sꢁ1 (2). It is reasonable to
tell that the proton transfer in 1 is responsible for lower k value
than that in 2 (lack of OH proton, Fig. 4). That supports the
general conclusion on the proton-transfer assisted mechanism
of the stabilization (non-radiative energy loss) of trans form of 2
by hydrogen bonded urea counterparts discussed in next
sections. Except the proton transfer within dimers it is
reasonable to tell the –COOH group dissociation in polar envi-
ronment is important.
For comparison the behaviour of the cinnamic acid (3) and
its tetrabutylammonium salt (4) upon irradiation was also
determined (see ESI, Fig. S3 and S4†). The respective rate
constants of trans-to-cis isomerization are 2.9 ꢂ 10ꢁ4 sꢁ1 (3) and
6.8 ꢂ 10ꢁ4 sꢁ1 (4). The higher values for 1 and 2 than that for 3
and 4 suggest the aminopyridine moiety plays a role in this
process. Most probably the charge transfer by electron donating
NH2 group causes photoisomerization to proceed faster. The
charge transfer in 1 and 2 was studied by spectroscopic
methods and further described in ESI.†
Except NMR the photoisomerization process was also
studied by the UV-vis spectroscopy. Fig. 5 shows the absorption
spectra before and aer (time intervals) irradiation at the
330 nm with the xenon lamp equipped with monochromator.
Comparison of the structure–isomerization relationships for
1 and 2 suggests that upon UV irradiation (330 nm) the
absorption band position of the acid and its salt moves to the
higher energy level, which is accompanied by non-monotonic
Fig. 5 Electronic absorption spectra of compounds 1 (top) and 2
(bottom) recorded before and after irradiation ([s], 330 nm) in
acetonitrile.
23700 | RSC Adv., 2018, 8, 23698–23710
This journal is © The Royal Society of Chemistry 2018

View Article Online
Paper
RSC Advances
Table 1 The association constants [Kassoc, Mꢁ1] measured in DMSO-d6
and CIS values (in italics) [ppm] for anion of 2 associated with
substituted ureas and the linear correlation coefficient for Kassoc ¼ f
(substituent constant) (a–c, trans)
in 2-aminopyridine group whereas the peak at ca. 340 nm
should belong to the intramolecular charge transfer (ICT).
Assuming that ethylene group of cis-1 adopts more twisted
conformation, which hinders the ICT process, the incidence of
charge transition is reduced, so the main absorption band is
localized at the short-wavelength region.
The transformation between trans and cis forms is also re-
ected in the occurrence of the sharp isosbestic point at ca.
370 nm (ESI, Fig. S5†). Due to the overlapping of the electronic
absorption spectra of the two isomers, the isomerization
quantum yields were not studied quantitatively.
Guest
H4
H6
H7
H8
H12
a (NMe2) 200, 0.112 200, 0.108 150, 0.153 130, 0.270 170, 0.097
b (OMe) 240, 0.124 300, 0.120 220, 0.156 170, 0.289 280, 0.112
c (H)
430, 0.139 480, 0.133 300, 0.200 190, 0.359 400, 0.126
c (H, cis) 300, 0.187 260, 0.135 300, 0.369 250, 0.156 250, 0.169
Ra
0.97, 0.96 0.93, 0.97 0.97, 0.78 1.00, 0.87 0.97, 0.98
a
Correlation coefficient for the trans form.
Association studies
stabilized in its dimeric form by quadruple hydrogen bonding
(in CDCl3).38 The same hydrogen-bonding pattern is present in
the cis form of 1, but both, the mentioned naphthyridine
derivative and cis-1, do not dimerize in DMSO. Since anions39–43
are able to interact with urea moiety in polar and highly
competitive solvents,7 the tetrabutylammonium salt 2 was used
in association (vide infra) experiments.
The compounds 1 and 2 were studied for their self-association
in their trans and cis forms. It is worth mentioning that the
trans and cis forms of 1 have much different self-interacting
scheme, id est. the trans form should be able to form cyclic
hexamers or oligomers, while the cis form should be able to
form dimers stabilized by four hydrogen bonds (Fig. 6).
However, the results show that 1 does not form any associate
(trans or cis) in DMSO solution due to highly polar character of
this solvent and its solvation abilities towards compounds
carrying NH/OH groups. Unfortunately 1 is not soluble in
CDCl3. The fact that double hydrogen bonded neutral mole-
cules barely form dimers in polar solvent is in agreement with
the studies of 2-acylamino-1,8-naphthyridin-7-ones that are
As before for anionic species7 the anion of salt 2 interacts
readily with ureas in polar solution. The association was
monitored by the change in chemical shi of several protons in
2 (host molecule). Table 1 shows the complexation induced shi
(CIS) values and association constants (Kassoc) determined, as
before,7 with the use of various protons. The CIS value is the
difference between chemical shi of the chosen proton in a free
molecule and the chemical shi of the same proton with the
concentration of the guest molecule extrapolated to innity. Not
all data are collected in Table 1 because, most probably, proton
transfer accompanies association. This is further discussed in
next sections. Thus the ureas substituted with electron accept-
ing groups are not included in correlation (R in Table 1). In
Fig. 7 two titration curves are shown. The comparison is made
between ureas carrying extreme substituents (NMe2 and NO2).
It is seen that for ureas carrying electron-donating substit-
uents the association constants based on various protons are in
line with the character of the substituent.5,6 The mentioned
substituent effect in ureas is manifested by association but also
by the chemical shi of the NH protons. For d[ppm] ¼ f(s) where
s is a substituent constant the correlation coefficient R ¼ 0.97
Fig. 6 Probable arrangement of self-interacting trans and cis forms Fig. 7 Example of the titration curve based on H8 chemical shift of
of 1.
2+a (NMe2) and 2+f (NO2).
This journal is © The Royal Society of Chemistry 2018
RSC Adv., 2018, 8, 23698–23710 | 23701

View Article Online
RSC Advances
Paper
for pure a–f recorded in DMSO-d6. Thus it is safe to tell the
substituent effect is transmitted in a classical way.
For most of complexes the course of the titration curves are
regular. A serious deviation of the titration curve (Fig. 7) from
the ideal course of the Benesi–Hildebrand equation is observed
for NO2 substituted urea (compare also other electron-acceptor
substituted compounds used in titrations in ESI†). Previously
the sigmoidal shape of the titration curve was caused by the
coexistence of at least two forms in solution.5 Currently the
proton transfer may be observed in the ground state of the
complex.
Since the acidity of NH groups in ureas is tuned by substit-
uent the binding of those should also inuence the change in
electronic structure of the anion of 2 in complex in a regular
way. The same association-induced charge reorganization
inuences the photoisomerization. It is worth remembering
that trans and cis forms differ in their geometry and thus their
intermolecular interactions might be different to some extent
even if the hydrogen-bonding pattern of the carboxylate group is
preserved. Thus, to study these differences the cis form of 2 was
also titrated by c but the curve obtained by tting the data had
larger residuals than that for trans form (see ESI†). For the cis
form the data t linear function at the beginning of titration as
in case of triuret studied by us recently.5 That suggest two
geometrically different forms of cis-2 co-exist in solution. These
were further studied with the use of DFT methods (vide infra).
Fig. 8 The relation between substituent constant and the percentage
of the cis form (photostationary state) in 2.
stabilization of the C]C double bond. It is worth to note that
for 2+f (NO2) complex the amount of the cis form in photosta-
tionary state aer irradiation (18.7%) is even lower than that in
1 (42.8%). Thus the position of the photostationary state in 1 is
different from that for 1+fꢁ/2+f complex and may be interpreted
as the loss of excitation energy by intermolecular proton
transfer. The proton transfer may also be present in other ureas
but the most efficient reaction of this type is observed for NO2-
derivative due to the highest acidic character of NH protons.
Thus, lowering acidity of NHs and lowering the association
constants weakens the proton transfer in the ground state
giving relatively lowered stabilization of the trans C]C bond.
That, in turn, causes the photoisomerization proceeds faster.
The photoisomerization rate constants and half-life of reaction
are collected in Table 2.
Based on the data from Table 2 it is clear that k for 2 is ca.
three times higher than that in 2+f (R ¼ NO2) complex and only
ca. twice higher than that in 2+a (R ¼ NMe2). Still the double
hydrogen bonding of the anion carrying C]C double bond
causes photoisomerization to slow down. It is worth noting that
only in case of 2+c the value of k is comparable to that of 2
(Fig. 9). Substitution of urea with strong electron donor (a) or
strong electron acceptor (f) causes that the photoisomerization
rate constant of the 2 in these complex is similar to the 1. Thus,
the photoisomerization rate constants do not follow the
Photoisomerization of the complexes tested
The next step in experiments was to probe the photo-
isomerization (as before for 1–4 including the same concen-
trations) with the assistance of ureas as a hydrogen bonding
counterparts. The 1 : 2 ratio of 2 and a–f was used. Samples were
irradiated as before and the procedure of the trans : cis ratio
determination was repeated (see Experimental section). In
Fig. 8 the substituent effect on the position of photostationary
state was shown as a percentage of the cis form dependent from
the substituent constant. Two sets of data represent all ureas (a–
f, in blue) and “all except NO2 derivative” (in red).
It is easy to see that the data presented do not t well to the
linear function as dened by classical Hammett relation. The
reasons for that may be the mentioned proton transfer or
complicated association/dissociation equilibrium present in
the studied system. The proton transfer, for sure, causes addi-
tional loss of excitation energy so it decreases the amount of the
cis form in the mixture. This is especially important when one
realize that, most probably, in 2+f mixture most of the 2 is
transformed into 1 due to the proton transfer in the ground
state in DMSO. The data obtained suggest the association cau-
ses the decrease of photoisomerization rate constant (compare
irregular changes in 2+f titration curve). We have recently
shown that stronger electron donating group leading to the
intramolecular charge transfer causes easier photo-
isomerization of the double C]C bond.44 Currently the data
presented in Fig. 8 shows donor–acceptor properties of
substituent in ureas inuences charge transfer in 2 and thus its
photoisomerization. That leads to the association-assisted
substituent effect in the whole series. Instead,
a good
Table 2 The photoisomerization rate constants (k [ꢂ10ꢁ4
s
ꢁ1]), half-
times (t1/2 [s]) and percentage of cis form determined at 20 ꢀC
Compound/complex
k
t1/2
% cis
2+a
2+b
2+c
2+d
14.0
21.3
23.7
16.7
12.2
10.6
11.2
29.1
2.9
495
326
293
414
566
6540
620
238
2402
1023
79.1
76.7
76.6
66.5
68.3
18.7
42.8
82.2
42.8
64.5
2+e
2+f/1+fꢁ
1
2
3
4
6.8
23702 | RSC Adv., 2018, 8, 23698–23710
This journal is © The Royal Society of Chemistry 2018

View Article Online
Paper
RSC Advances
transformation. Upon prolonged irradiation, the changes
become smaller and a photostationary state is reached. The
absorption thus consists of two contributions, from the orig-
inal trans-2 and from the newly formed isomer cis-2 inuenced
by association with ureas.
Fig. 9 The rate constant of trans/cis photoisomerization in acids and
salts (top) and complexes of 2 with substituted ureas (bottom).
correlation was found separately for donors + unsubstituted
derivatives and the same for acceptors
derivatives.
+ unsubstituted
The unsubstituted derivative is a common point for both
series since c is a reference molecule (no substituent present).
Both correlations are very good (Rs are given in Fig. 9b) albeit it
is fair to note that the number of points is three and four for
donors and acceptors, respectively.
Fig. 9b shows that any electronic effect by substitution of
ureas causes change in value of k. Since the slopes of the tted
linear functions have opposite signs we assumed two different
mechanisms of photoisomerization reaction. The similar non-
linearity in the larger series have been observed for binding
barbital with triple hydrogen-bonding.45 It is worth noting that,
except for 2+f, photoisomerization rate constant for complexes
fall between values of k for 1 and 2. This shows the hydrogen
bonding with ureas slows down the photoisomerization of the
anion 2 and the changes in k are, most probably, related to the
existence of equilibrium between –COOꢁ and –COOH upon
irradiation. As already said, for comparison purposes, the cin-
namic acid (3) and its anion (4, salt) was used. These experi-
ments conrm that anions (2, 4) undergo faster isomerization
than acids (1, 3). Again, the loss of excitation energy by proton
transfer, acid dissociation or vibrations of OH groups may cause
the slower photoisomerization in acids.
The photoisomerization of 2 with the assistance of ureas as
a hydrogen bonding counterparts was also studied by UV-vis
spectroscopy (Fig. 10). Upon irradiation a decrease of the
main absorption band with a clear two isosbestic points
(270 nm and 350 nm) are observed. The existence of the iso-
sbestic points conrms that two parallel processes occur. The
rst isosbestic point indicates a transition between two
species, trans and cis form of the salt. The second one may be
connected with formation of the complex between 2 and urea
that affects the probability and potential of the
Fig. 10 (a) Changes in UV-vis absorption spectra of complexes upon
irradiation with xenon lamp equipped with monochromator (330 nm),
(b) area normalized absorption spectra of the complexes upon irra-
diation (time given).
This journal is © The Royal Society of Chemistry 2018
RSC Adv., 2018, 8, 23698–23710 | 23703

View Article Online
RSC Advances
Paper
To sum up, the rate constant of photoisomerization in the mechanism) and hydrogen-bonded 2 (minor part of mecha-
studied series is dependent on the following factors: (a) the nism) simultaneously. The effectiveness of acid formation
association constant of counterparts and (b) the proton transfer affects the rate of photoisomerisation and causes that the k
that can accompany the association in ground or excited state.46 value tends the photoisomerization rate constant close to k
The proton transfer takes place in case of NO2 substituted urea value for pure 1. The dissociation of two complexes in the same
similarly as during interaction with uoride anion.47 However, solution might also be responsible for lowered correlation
as opposite to previous works, it was impossible to study proton coefficient between the position of the photostationary state
transfer by UV-vis absorption spectroscopy because the and substituent constant as mentioned above (see Fig. 8). Since
absorption spectra of aromatic, interacting molecules overlap.
the photoisomerization rate constant for 2+a mixture is closer
As mentioned above the association of 2 with substituted by its value to k for 1 the photoisomerization in dissociated state
ureas causes the stabilization of the trans form. The lowest k was proposed as a major sub-mechanism (Fig. 11).
value was obtained for the complex with nitro-carrying urea. In
The second mechanism is proposed for complexes contain-
this case, the intermolecular proton transfer that is present in ing an electron acceptor substituent in series d–f. In this case,
the ground state causes the excited state relaxation so efficiently most probably, the tendency to intermolecular proton transfer
that most of energy is consumed for this reaction. This that is dependent from the substituent takes place. The NO2
conclusion is supported by a well-known observation telling group is an exception since it transfers proton in the ground
that the excited state proton transfer quenches uorescence state. This reaction was already noticed for similar urea deriv-
and/or shis its maximum towards red part of the spectrum.48,49 atives.50,51 The proton transfer in the current complexes takes
place in much more complicated system than simple anio-
n:urea associate. This is due to the fact that if the proton was
transferred from urea derivative to the carboxylate group the
Proposed mechanism of the reaction in tested complexes
We propose two mechanisms that inuence photo-
isomerization in studied complexes. However, before some
already observed conclusions are given. The position of the
photostationary state follows the general substituent effect
giving the lowest value of the percentage of the cis form for CF3
substituted urea (Fig. 8). In the a–c part of data (Fig. 9b) the
more electron-donating substituent is the lower k value is ob-
tained. The decrease of the k in 2+a and 2+b with respect to 2+c
is, most probably, attributed to the intramolecular charge
transfer in a and b (NMe2 and OMe substituted urea deriva-
tives). Thus, the negative charge transferred from electron
donating group (for example NMe2) inuences the complex
stability, which dissociate (Fig. 11) upon excitation – mecha-
nism no. 1. The said association/charge transfer/dissociation
might be the source of the energy loss by thermal vibrations
(geometry relaxation).
obtained acid 1 might be subjected to other reactions. For that
to happen, the dissociation of the complex must take place.
Taking into account the strongest acid in urea series (f) two
complexes of compounds 2 and f should be considered. These
are 2+f and 1+fꢁ(a) (complex aer proton transfer reaction,
Fig. 12) and 1+fꢁ(b) (rotameric form of 1+fꢁ(a)).
In the above discussion on proposed mechanisms it is
important to keep in mind that stability of complexes in the
ground state is variable thus some effects may vanish. Unfor-
tunately compounds used here do not exhibit efficient uores-
cence that could be used during additional experiments in very
systematic way. Still, it was possible to record uorescence
spectra and to conclude the origins of the variable photo-
isomerization reactions. Thus, the respective spectra were
recorded in chosen solvents to study the ICT within 1 and 2. Due
to the space limitations those results were placed in ESI† and
more detailed studies are planned with the use of highly uo-
rescent compounds.
Moreover, the said dissociation of the complex caused by
electronic repulsion between counterparts, most probably,
leads to the photoisomerization of the acid 1 (major part of
Fig. 11 The charge transfer in a and its influence on complex dissociation.
23704 | RSC Adv., 2018, 8, 23698–23710
This journal is © The Royal Society of Chemistry 2018

View Article Online
Paper
RSC Advances
Fig. 12 Structure of 2+f complex exhibiting intermolecular proton transfer.
The change in properties of the substituent is able to tune determination of intermolecular interaction energy, (c) QTAIM-
the photophysical properties of molecules as, for example, based54,55 calculation of the hydrogen bond energy,56,57 (d)
association constants but also the molar attenuation coefficient calculation of the barrier of proton transfer in the ground state
(3). Table 3 contains the 3 and l for 1–4 and a–f.
for NO2 derivative. The results are discussed below while
It is worth reminding here that the association in the ground geometries of the optimized structures are collected in ESI.†
state is substituent-dependent and the amount of the cis form The QTAIM data consist of Laplacian value calculated for
in the photostationary state also follows the substituent effect hydrogen bond critical point (H-BCP) and the energy of the
(Fig. 8). A correlation for k ¼ f(3) for the acceptor part of data is hydrogen bond (EHB) calculated using Espinosa's approach
equal to R ¼ 0.79 while for donors R ¼ 0.98. The similar applies (Table 4).56,57
for k ¼ f (Hammett substituent constant) where for acceptors R
¼ 0.98 and donors R ¼ 1.00 (n ¼ 4 and n ¼ 3, respectively). No
ꢀ
Table 4 The hydrogen bond (NH/O) lengths [A] (first row) and values
such tendency was observed for the whole set of substituents.
This suggests the light-initiated proton transfer in the
complexes takes place in case of acceptors and another (vide
supra) mechanism is present in donor part of series a–f. This
type of correlations (separate for acceptors and donors) has
been already described in a series of azobenzenes suggesting
two different mechanisms of photoreaction.37,52,53
of Laplacian (second row) at H-BCP and EHB [kJ molꢁ1] (third row)
trans
cis
Complex
H/O10
H/O11
H/O10
H/O11
2+a
1.865
1.866
1.880
1.886
0.0937
ꢁ31.7
1.869
0.0926
ꢁ31.4
1.852
0.0956
ꢁ32.9
1.833
0.0992
ꢁ34.8
1.830
0.0933
ꢁ31.5
1.855
0.0955
ꢁ32.7
1.847
0.0967
ꢁ33.3
1.841
0.0976
ꢁ33.9
1.817
0.0907
ꢁ30.2
1.879
0.0914
ꢁ30.7
1.870
0.0925
ꢁ31.4
1.847
0.0962
ꢁ33.2
1.836
0.0980
ꢁ34.2
1.806
0.1035
ꢁ37.4
0.0897
ꢁ29.8
1.867
0.0937
ꢁ31.6
1.860
0.0946
ꢁ32.1
1.854
0.0970
ꢁ33.3
1.832
2+b
2+c
2+d
2+e
2+f
Computations
In order to have a deeper insight into the non-covalent inter-
actions a series of computations were performed. These are: (a)
geometry optimization of 1, 2 and a–f and their complexes, (b)
Table
3 The molar attenuation coefficients (3), wavelength in
absorption maximum (l) for studied compounds in acetonitrile
0.0994
ꢁ35.1
1.791
0.1022
ꢁ36.4
1.800
0.0997
ꢁ34.9
1.801
3
3
[dm3 molꢁ1 cmꢁ1],
[dm3 molꢁ1 cmꢁ1],
Compound
l [nm]
Compound
l [nm]
0.1058
ꢁ38.8
1.721a
0.0881
ꢁ56.5
0.1064
ꢁ38.9
1.922
0.0853
ꢁ26.8
0.1050
ꢁ37.9
1
2
3
4
a
19 200, 326
22 700, 280
19 800, 271
22 500, 258
16 300, 276
b
c
d
e
f
14 200, 256
12 700, 252
14 100, 262
11 700, 266
19 700, 340
1+fꢁ
a
N/HO hydrogen bond.
This journal is © The Royal Society of Chemistry 2018
RSC Adv., 2018, 8, 23698–23710 | 23705

View Article Online
RSC Advances
Paper
Based on the data in Table 4 it is easy to note that the electron conjugation that stabilize the conjugated structure.
hydrogen bond length follows the character the substituent The said p-electron conjugation in cis form is not as efficient as
expressed as the Hammett sigma constant58 (s) giving the in trans conformation because steric interaction prohibits the
correlation coefficients (R) for NH/O ¼ f(s) linear function co-planarity of two parts of molecule. We have studied rota-
from 0.88 to 0.95 (NH/O10 in trans form and NH/O11 in the merism in cis form by DFT methods. Indeed, two rotamers of
cis form, respectively). The same is realized for values of Lap- the cis-2 (a and b, Fig. 13) are different in energy by ca.
lacian (R ¼ 0.87–097) and the EHB (R ¼ 0.88–0.95) in complexes. 12 kJ molꢁ1, but what is more important the transition state for
That shows the effect of the substituent on association is such process (cis(TS)) is located at the energy level higher than
regular. The last row in Table 4 refers to 1+fꢁ complex and 30 kJ molꢁ1 with respect to the most stable form (cis(a)).
shows the hydrogen bonding in the ground state aer proton
The variable stability of forms cis(a) and cis(b) may be
transfer is stronger than before such reaction (complex 2+f). attributed to the relatively weak CH/O interaction. This kind of
Also, the BSSE (Basis Set Superposition Error) corrected ener- interaction, although, weak, might be responsible for molecular
gies of intermolecular interaction (Eint) collected in Table 5 packing in crystal as shown by us59 and others.60,61 If so, the CH
show the character of the substituent in urea derivatives inu- moiety being in close proximity to the negatively charged oxygen
ences linearly the Eint values. As expected the strongest inter- may stabilize considered rotamers (both H6 and H4 protons
action was found for NO2 substituted derivative.
interact with oxygen carrying negative charge). The respective
˚
The computationally obtained data related to the energy of H/O distances are: 2.051 and 2.125 A for cis(a) and cis(b),
intermolecular interaction, in general, follow the effect of the respectively. As shown in Fig. 13 the energy of cis(b) form with
substituent in anion binding (both in trans and cis forms) as for respect to the cis(a) is ca. 12 kJ molꢁ1. The QTAIM-based data for
previously studied derivatives.5,6 Thus, it is reasonable to tell the CH/O interaction energy calculated by Espinosa's approach
same effects drive the association in the currently studied gives values of ꢁ20.6 kJ molꢁ1 and ꢁ17.2 kJ molꢁ1 for cis(a) and
compounds. Moreover, the proton transfer reaction suggested cis(b) forms, respectively. The Laplacian of electron density is
by the experimental data is supported by the strength of the equal to 0.0740 and 0.0668 conrming the interaction is of
hydrogen bonds in 1+fꢁ complex versus that in 2+f. In these hydrogen bond nature according to the recent denition of
complexes the sum of the energy of hydrogen bonds is higher in hydrogen bond.62 Thus, in a middle-complicated system studied
1+fꢁ than that in 2+f. Unfortunately the studied complexes and here the computations shed additional light on properties of
processes are too large to consider computations of reactions molecules.
taking place in their excited state. Instead, the calculations were
performed to inspect the proton transfer reaction in ground
state. The DFT calculations show the proton transfer in 2+f
Experimental
complex is a barrier-less process (or the real barrier is very low),
To keep the conditions of experiments constant procedures
while the energy of the 1+fꢁ complex is 26.9 kJ molꢁ1 higher
related to intermolecular interactions were the same as in our
than that for 2+f (relatively low comparing to Eint for both
recent publication on bisurea and association in polar solvent.7
complexes).
The synthesis of the main compound (1) was performed by the
It was mentioned that during the titration of the trans-2/cis-2
reaction of 2-amino-5-bromopyridine with acrylic acid under
mixture the data suggest two individuals are present in solution
the Heck coupling conditions, while substituted ureas were
for cis isomer. The only possibility is that two cis forms in
obtained by reaction of the respective anilines with 1,10-car-
various rotameric forms are present. This might be a bit
bonyldiimidazole.63 The salt 2 was obtained by reaction of 1
confusing since rotation around single bonds should be easy in
with tetrabutylammonium hydroxide, as before.6 All NMR
solution and, for sure, it is so in the trans form (no steric
intramolecular interaction is present in this isomer). However,
for cis-2 form it is easy to recognize the competition between two
opposite mechanisms responsible for rotamerism. These are (a)
steric hindrance that acts as a destabilizing force and (b) the p-
Table
complexes corrected to basis set superposition error
5
The energy of interaction (Eint [kJ molꢁ1]) in studied
trans form
cis form
Complex
2+a
2+b
2+c
2+d
Eint
Complex
2+a
2+b
2+c
2+d
Eint
ꢁ47.1
ꢁ50.0
ꢁ53.6
ꢁ57.1
ꢁ62.0
ꢁ69.7
ꢁ43.5
ꢁ45.7
ꢁ47.5
ꢁ52.8
ꢁ55.9
ꢁ59.3
ꢁ67.7
ꢁ37.4
2+e
2+e
2+f
2+f
1+fꢁ
1+fꢁ
Fig. 13 Two rotameric forms of anion of 2 in cis form and transition
state between those forms.
23706 | RSC Adv., 2018, 8, 23698–23710
This journal is © The Royal Society of Chemistry 2018

View Article Online
Paper
RSC Advances
spectra were recorded on Bruker Avance 3HD 400 MHz spec- Drying under vacuum gave 1.53 g (62%) as pale yellow solid, mp
trometer working at 400 MHz and 100 MHz for 1H and 13C, 224.6–228.2 ꢀC. 1H NMR (TMS, DMSO) d: 12.08 (bs, 1H), 8.12 (d,
respectively. Irradiation was performed with the use of xenon 3JHH ¼ 2.3 Hz, 1H), 7.76 (dd, 3JHH ¼ 8.7 Hz, 1H), 7.43 (d, 3JHH
¼
lamp with water lter to remove the heat ow from the lamp to 15.9 Hz, 1H), 6.53 (bs, 1H), 6.45 (d, 3JHH ¼ 8.7 Hz, 1H), 6.22 (d,
studied solution. The samples were kept in dark aer irradia- 3JHH ¼ 15.9 Hz, 1H). 13C NMR (TMS, DMSO) d: 168.54, 161.41,
tion. Degree of isomerization was calculated by the comparison 151.02, 142.47, 135.59, 118.97, 114.09, 108.77. Anal. calc. for
1
of the integrals of respective signals in H NMR spectra of the C8H8N2O2: C 58.53, H 4.91, N 17.06. Found C 58.46, H 5.02, N
irradiated sample. The signal at ca. 7.22 ppm (3JH,H ¼ ca. 16 Hz) 16.98.
was used to identify trans isomer and the one at ca. 6.15 ppm
(3JH,H ¼ ca. 12 Hz) for cis isomer. The back reaction (cis / trans)
Tetra-n-butylammonium 3-(6-aminopyridin-3-yl)acrylate (2)
was not detected aer keeping solution at 50 ꢀC for 4 h. The
3-(6-Aminopyridin-3-yl)acrylic acid (1 g, 0.006 mol) was dis-
photoisomerization rate constants were determined with the
solved in MeOH (30 cm3). Solution was adjusted to pH ¼ 7
use of integrals of three protons (H4, H6 and H7) in trans and cis
using tetrabutylammonium hydroxide. Then, MeOH was
forms of 2. The irradiation for UV-vis measurements of the
evaporated. Residue was washed with cold MeOH. Drying
trans–cis isomerization was performed with xenon lamp
under vacuum gave 1.84 g (74%) as brown yellow solid, mp
equipped with monochromator, lEM ¼ 330 nm in a quartz
114.6–117.2 ꢀC. 1H NMR (TMS, DMSO) d: 7.91 (d, 3JHH ¼ 2.2 Hz,
cuvette. Acetonitrile was used as a solvent. The samples were
3
3
1H), 7.54 (d, JHH ¼ 8.6 Hz, 1H), 6.90 (d, JHH ¼ 16.1 Hz, 1H),
stirred during irradiation. The steady-state absorption and
emission spectra were recorded at room temperature using
a Shimadzu UV-vis Multispec-1501 spectrophotometer and
a Hitachi F-7100 spectrouorimeter, respectively. The uores-
cence quantum yields for the acid (1), its salt (2) and complexes
(2+a, 2+c, 2+f) in acetonitrile were calculated using following
equation:
3
3
6.41 (d, JHH ¼ 8.6 Hz, 1H), 6.11 (d, JHH ¼ 16.1 Hz, 1H), 6.09
(bs, 1H), 3.17 (m, 8H), 1.57 (m, 8H), 1.3 (m, 8H), 0.94 (m, 12H).
13C NMR (TMS, DMSO) d: 169.81, 160.03, 148.26, 134.67,
133.06, 126.67, 121.34, 108.64, 57.97, 23.54, 19.67, 13.97. Anal.
calc. for C24H43N3O2: C 71.07, H 10.69, N 10.36. Found C 70.98,
H 10.83, N 10.27. trans-3-Phenylacrylic acid (3) was available
from commercial sources.
2
IsAref ns
fs ¼ fref
2
Iref As nref
Tetra-n-butylammonium (2E)-3-phenylprop-2-enoate (4)
Cinnamic acid was (1 g, 0.007 mol) dissolved in MeOH (30 cm3)
and the solution was adjusted to pH ¼ 7 using tetrabuty-
lammonium hydroxide. Then, MeOH was evaporated and
residue was washed with small portions of cold MeOH and
dried under vacuum. That 1.89 g (72.00%) of 4 as light-yellow
where: Is and Iref are the integrated emission intensity, ns and
nref are the refractive indexes of the solvents used for the sample
and the reference, respectively.
The absorbances (A) of both the sample tested and reference
solution at an excitation wavelength (325 nm) was ca. 0.1. 9,10-
Diphenylanthracene in cyclohexane (fref ¼ 0.90 (ref. 64)) was
used as reference. Computations were carried out with
Gaussian program65 (M05/6-311++G(2d,2p) level of computa-
tion) with the PCM66 model of solvation and the same solvent as
in experiments. The frequency calculations (the same level of
theory) showed all structures were optimized (zero imaginary
frequencies) except for the transition state investigated (one
imaginary frequency). The interaction energy (Eint.) was cor-
rected to the basis set superposition error (BSSE) in vacuum
with the use of counterpoise methods as implemented in
Gaussian with default settings. The zero-point energy correction
was applied.
1
3
oil. H NMR (TMS, DMSO) d: 7.46 (d, JHH ¼ 7.6 Hz, 2H), 7.32
3
3
(t, 2H), 7.25 (t, 1H), 7.11 (d, JHH ¼ 16 Hz, 1H), 6.42 (d, JHH
¼
16 Hz, 1H), 3.17 (m, 8H), 1.54 (m, 8H), 1.3 (m, 8H), 0.94 (m,
12H). 13C NMR (TMS, DMSO) d: 169.70, 137.24, 135.68, 130.72,
129.04, 128.32, 127.27, 57.98, 23.56, 19.65, 13.94. Anal. calc. for
C
25H43NO2: C 77.07, H 11.12, N 3.60. Found C 76.97, H 11.30, N
3.53.
General procedure for synthesis of substituted diphenylureas
(a–f)
In anhydrous THF (50 cm3) was dissolved 4-substituted aniline
(0.01 mol) and 1,10-carbonyldiimidazole (0.81 g, 0.005 mol).
Mixture was heated at reux during 24 h. Then, solvent was
evaporated, residue was recrystallized from MeOH and dried
under vacuum.
3-(6-Aminopyridin-3-yl)acrylic acid (1)67
Acrylic acid (2.38 g, 0.033 mol) was added carefully to a solution
of 2-amino-5-bromopyridine (2.59 g 0.015 mol) and Na2CO3,
(5.56 g 0.053 mol) in H2O (50 cm3). Next, PdCl2 (0.053 g, 0.0003
N,N0-Bis[4-(dimethylamino)phenyl]urea (a)
mol) was added and initiated the Heck reaction. Mixture was N,N-Dimethylbenzene-1,4-diamine (1.36 g) gave product (1.26 g,
heated (100 ꢀC) at reux for 24 h. Aer, the reaction was cooled 85%) as brown solid, mp 252.3–257.8 ꢀC (lit.68 250 ꢀC). 1H NMR
to room temperature and ltered. Filtrate was adjusted to pH 6 (TMS, DMSO) d: 8.13 (bs, 2H), 7.23 (d, 3JHH ¼ 9.2 Hz, 4H), 6.68
with aqueous HCl. Additional H2O (20 cm3) was added to (d, 3JHH ¼ 9.0 Hz, 4H), 2.82 (s, 12H). 13C NMR (TMS, DMSO) d:
improve mixing, and the mixture was stirred for 1 h. Then the 153.50, 146.72, 130.40, 120.41, 113.67, 41.23. Anal. calc. for
solid was collected by ltration. The lter pad was washed
C17H22N4O: C 68.43, H 7.43, N 18.78. Found C 68.52, H 7.51, N
sequentially with H2O (20 cm3), cold absolute EtOH (20 cm3). 18.63.
This journal is © The Royal Society of Chemistry 2018
RSC Adv., 2018, 8, 23698–23710 | 23707

View Article Online
RSC Advances
Paper
N,N0-Bis(4-methoxyphenyl)urea (b)
substituent effect might be used as a method for studying
photochemical behavior of supramolecular complexes.
4-Methoxyaniline (1.23 g) gave product (1.19 g, 87%) as white-
grey powder, mp 239.5–241.6 ꢀC (lit.69 239–240 ꢀC). 1H NMR
(TMS, DMSO) d: 8.37 (bs, 2H), 7.33 (d, 3JHH ¼ 9.2 Hz, 4H), 6.85
Conflicts of interest
3
(d, JHH ¼ 9.2 Hz, 4H), 3.71 (s, 6H). 13C NMR (TMS, DMSO) d:
There are no conicts to declare.
154.7, 153.4, 133.39, 120.35, 114.41, 55.62. Anal. calc. for
C
15H16N2O3: C 66.16, H 5.92, N 10.29. Found C 66.02, H 5.99, N
Acknowledgements
10.15.
This work was supported by the National Science Centre,
Poland under Grant 2013/09/B/ST4/02308. PL-Grid Infrastruc-
ture supported this research.
N,N0-Diphenylurea (c)
Aniline (0.93 g) gave product (0.81 g, 76%) as a white powder,
mp 239–241 ꢀC (lit.70 235–237 ꢀC). 1H NMR (TMS, DMSO) d: 8.65
(bs, 2H), 7.44 (d, 3JHH ¼ 8.4 Hz, 4H), 7.28 (t, 4H), 6.97 (t, 2H). 13
C
Notes and references
NMR (TMS, DMSO) d: 153.00, 140.17, 129.26, 122.27, 118.85.
Anal. calc. for C13H12N2O: C 73.56, H 5.70, N 13.20. Found C
73.49, H 5.79, N 13.01.
1 G. R. Desiraju, Angew. Chem., Int. Ed., 1995, 34, 2311–2327.
2 V. R. Hathwar, T. S. Thakur, T. N. G. Row and G. R. Desiraju,
Cryst. Growth Des., 2011, 11, 616–623.
3 J. D. Dunitz and A. Gavezzotti, Cryst. Growth Des., 2012, 12,
5873–5877.
4 A. Azhdari Tehrani, L. Esrali, S. Abedi, A. Morsali,
L. Carlucci, D. M. Proserpio, J. Wang, P. C. Junk and
T. Liu, Inorg. Chem., 2017, 56, 1446–1454.
N,N0-Bis(4-bromophenyl)urea (d)
4-Bromoaniline (1.7 g) gave product (1.29 g, 70%) as white
powder, mp 291–294 ꢀC (lit.71 292–293 ꢀC). 1H NMR (TMS,
DMSO) d: 8.85 (bs, 2H), 7.44 (m, 8H). 13C NMR (TMS, DMSO) d:
152.73, 139.42, 131.99, 120.99, 113.85. Anal. calc. for
´
5 K. Mroczynska, M. Kaczorowska, E. Kolehmainen,
C
13H10Br2N2O: C 42.20, H 2.72, N 7.57. Found C 42.09, H 2.80, N
´
I. Grubecki, M. Pietrzak and B. Osmiałowski, Beilstein J.
7.43.
Org. Chem., 2015, 11, 2105–2116.
´
´
N,N0-Bis[4-(triuoromethyl)phenyl]urea (e)
6 B. Osmiałowski, K. Mroczynska, E. Kolehmainen,
M. Kowalska, A. Valkonen, M. Pietrzak and K. Rissanen, J.
Org. Chem., 2013, 78, 7582–7593.
4-(Triuoromethyl)aniline (1.61 g) gave product (1.31 g, 75%) as
a pale yellow needles, mp 231.2–234.5 ꢀC (lit.72 228–230 ꢀC). 1H
NMR (TMS, DMSO) d: 9.25 (bs, 2H), 7.67 (m, 8H). 13C NMR
(TMS, DMSO) d: 152.58, 143.58, 126.65 (q, JC,F ¼ 3.6 Hz), 123.63
(q, JC,F ¼ 270.6 Hz), 122.10 (q, JC,F ¼ 31.7 Hz), 118.59. Anal. calc.
for C15H10F6N2O: C 51.73, H 2.89, N 8.04. Found C 51.80, H
2.94, N 7.98.
´
7 A. Kwiatkowski, I. Grela and B. Osmiałowski, New J. Chem.,
2017, 41, 1073–1081.
8 P. R. Schreiner and A. Wittkopp, Org. Lett., 2002, 4, 217–220.
9 A. Zafar, S. J. Geib, Y. Hamuro and A. D. Hamilton, New J.
Chem., 1998, 22, 137–141.
10 A. E. Hooper, S. R. Kennedy, C. D. Jones and J. W. Steed,
Chem. Commun., 2016, 52, 198–201.
N,N0-Bis(4-nitrophenyl)urea (f)
11 P. R. Schreiner, Chem. Soc. Rev., 2003, 32, 289–296.
4-Nitroaniline (1.38 g) gave product (0.94 g, 62%) as yellow solid, 12 S. Mohamed, D. A. Tocher, M. Vickers, P. G. Karamertzanis
mp 299–305 ꢀC (lit.73 299–305 ꢀC). 1H NMR (TMS, DMSO) d: 9.71
and S. L. Price, Cryst. Growth Des., 2009, 9, 2881–2889.
(bs, 2H), 8.22 (d, 3JHH ¼ 12 Hz, 4H), 7.73 (d, 3JHH ¼ 8.8 Hz, 4H). 13 C. B. Aakeroy, M. E. Fasulo and J. Desper, Mol. Pharm., 2007,
¨
13C NMR (TMS, DMSO) d: 152.12, 146.17, 141.97, 125.62, 118.45.
4, 317–322.
Anal. calc. for C13H10N4O5: C 51.66, H 3.33, N 18.54. Found C 14 S. L. Childs, G. P. Stahly and A. Park, Mol. Pharm., 2007, 4,
51.55, H 3.40, N 18.45.
323–338.
15 A. Parkin, C. C. Seaton, N. Blagden and C. C. Wilson, Cryst.
Growth Des., 2007, 7, 531–534.
Conclusions
¨
16 C. B. Aakeroy, I. Hussain and J. Desper, Cryst. Growth Des.,
The synthesized acid isomerizes but it does not form dimers in
2005, 6, 474–480.
polar solvent. However, we observed that the trans/cis photo- 17 D. A. Adsmond, A. S. Sinha, U. B. R. Khandavilli,
isomerization of its salt is inuenced by non-covalent interac-
tions with ureas. It was shown, in contrast to usual role of ureas
A. R. Maguire and S. E. Lawrence, Cryst. Growth Des., 2016,
16, 59–69.
´
(organocatalysis), that these compounds might act as organo- 18 B. Osmiałowski, E. Kolehmainen, S. Ikonen, A. Valkonen,
inhibitor in photoisomerization reaction. In this sense the
supramolecular counterpart carrying acidic group with acidity
A. Kwiatkowski, I. Grela and E. Haapaniemi, J. Org. Chem.,
2012, 77, 9609–9619.
tuned by the substituent character is able to inuence the 19 T. Mori and Y. Inoue, in Molecular and supramolecular
photochemical processes within the complexes. The said tuned
acidity results in variable association but also inuences the
tendency of intermolecular, excited state proton transfer. The
photochemistry, ed. A. G. Griesbeck and J. Mattay, CRC
Press Taylor Francis Group, 2004, ch. C]C
Photoinduced Isomerization Reactions, p. 417.
&
23708 | RSC Adv., 2018, 8, 23698–23710
This journal is © The Royal Society of Chemistry 2018

View Article Online
Paper
RSC Advances
´
20 W. M. Horspool, in Photochemistry, ed. I. Dunkin, RCS 47 D. Esteban-Gomez, L. Fabbrizzi and M. Licchelli, J. Org.
Publishing, 2007, vol. 36, pp. 55–90. Chem., 2005, 70, 5717–5720.
21 J. Herzfeld and J. C. Lansing, Annu. Rev. Biophys. Biomol. 48 Y.-A. Chen, F.-Y. Meng, Y.-H. Hsu, C.-H. Hung, C.-L. Chen,
Struct., 2002, 31, 73–95.
22 H. Kandori, Y. Shichida and T. Yoshizawa, Biochemistry,
2001, 66, 1197–1209.
23 T. Okada and K. Palczewski, Curr. Opin. Struct. Biol., 2001,
11, 420–426.
K.-Y. Chung, W.-F. Tang, W.-Y. Hung and P.-T. Chou,
Chem.–Eur. J., 2016, 22, 14688–14695.
49 S. Tasaki, A. Momotake, Y. Kanna, T. Sato, Y. Nishimura and
T. Arai, Photochem. Photobiol. Sci., 2015, 14, 1864–1871.
´
50 M. Boiocchi, L. Del Boca, D. Esteban-Gomez, L. Fabbrizzi,
24 J. L. Spudich, Science, 2000, 288, 1358–1359.
25 A. S. Lubbe, W. Szymanski and B. L. Feringa, Chem. Soc. Rev.,
2017, 46, 1052–1079.
M. Licchelli and E. Monzani, Chem.–Eur. J., 2005, 11, 3097–
3104.
´
51 M. Boiocchi, L. Del Boca, D. E. Gomez, L. Fabbrizzi,
26 M. Asakawa, P. R. Ashton, V. Balzani, C. L. Brown, A. Credi,
O. A. Matthews, S. P. Newton, F. M. Raymo, A. N. Shipway,
M. Licchelli and E. Monzani, J. Am. Chem. Soc., 2004, 126,
16507–16514.
N. Spencer, A. Quick, J. F. Stoddart, A. J. P. White and 52 R. Cimiraglia and H.-J. Hofmann, Chem. Phys. Lett., 1994,
D. J. Williams, Chem.–Eur. J., 1999, 5, 860–875. 217, 430–435.
27 D. Bleger and S. Hecht, Angew. Chem., Int. Ed., 2015, 54, 53 H. Rau and E. Lueddecke, J. Am. Chem. Soc., 1982, 104, 1616–
11338–11349. 1620.
28 V. V. Avdeeva, M. I. Buzin, E. A. Malinina, N. T. Kuznetsov 54 R. F. W. Bader, Atoms in Molecules: A Quantum Theory, Oxford
´
and A. V. Vologzhanina, CrystEngComm, 2015, 17, 8870–
University Press, New York, 1990.
8875.
55 R. F. W. Bader, Chem. Rev., 1991, 91, 893–928.
29 G. Kaupp and J. Schmeyers, J. Photochem. Photobiol., B, 2000, 56 E. Espinosa, E. Molins and C. Lecomte, Chem. Phys. Lett.,
59, 15–19. 1998, 285, 170–173.
30 Y. Hasegawa, K. Takahashi, S. Kume and H. Nishihara, 57 E. Espinosa, M. Souhassou, H. Lachekar and C. Lecomte,
Chem. Commun., 2011, 47, 6846–6848. Acta Crystallogr., Sect. B: Struct. Sci., 1999, 55, 563–572.
31 Y. Norikane, K. Kitamoto and N. Tamaoki, J. Org. Chem., 58 C. Hansch, A. Leo and R. W. Ta, Chem. Rev., 1991, 91, 165–
2003, 68, 8291–8304. 195.
32 D. Aronzon, E. P. Levy, P. J. Collings, A. Chanishvili, 59 B. Osmiałowski, E. Kolehmainen, A. Valkonen, M. Kowalska
G. Chilaya and G. Petriashvili, Liq. Cryst., 2007, 34, 707–718. and S. Ikonen, Struct. Chem., 2013, 24, 2203–2209.
33 E. Merino and M. Ribagorda, Beilstein J. Org. Chem., 2012, 8, 60 S. Scheiner, Phys. Chem. Chem. Phys., 2011, 13, 13860–13872.
´
1071–1090.
61 J. L. Dragelj, G. V. Janjic, D. Z. Veljkovic and S. D. Zaric,
CrystEngComm, 2013, 15, 10481–10489.
34 T.-T. Yin, Z.-X. Zhao and H.-X. Zhang, New J. Chem., 2017, 41,
1659–1669.
35 P. Weis, D. Wang and S. Wu, Macromolecules, 2016, 49, 6368–
6373.
36 E. Bernard, P. Britz-McKibbin and N. Gernigon, J. Chem.
Educ., 2007, 84, 1159–1161.
37 G. Angelini, N. Canilho, M. Emo, M. Kingsley and
C. Gasbarri, J. Org. Chem., 2015, 80, 7430–7434.
62 E. Arunan, G. R. Desiraju, R. A. Klein, J. Sadlej, S. Scheiner,
I. Alkorta, D. C. Clary, R. H. Crabtree, J. J. Dannenberg,
P. Hobza, H. G. Kjaergaard, A. C. Legon, B. Mennucci and
D. J. Nesbitt, Pure Appl. Chem., 2011, 83, 1637–1641.
63 W. C. Drewe, R. Nanjunda, M. Gunaratnam, M. Beltran,
G. N. Parkinson, A. P. Reszka, W. D. Wilson and S. Neidle,
J. Med. Chem., 2008, 51, 7751–7767.
´
38 B. Osmiałowski, E. Kolehmainen, E. Kalenius, B. Behera, 64 M. Brouwer Albert, Pure Appl. Chem., 2011, 83, 2213–2228.
¨
R. Kauppinen and E. Sievanen, Struct. Chem., 2011, 22, 1143. 65 M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria,
39 L. Cao, R. Jiang, Y. Zhu, X. Wang, Y. Li and Y. Li, Eur. J. Org.
Chem., 2014, 2687–2693.
40 C. R. Bondy and S. J. Loeb, Coord. Chem. Rev., 2003, 240, 77–
99.
41 N. A. Esipenko, P. Koutnik, T. Minami, L. Mosca,
V. M. Lynch, G. V. Zyryanov and P. Anzenbacher, Chem.
Sci., 2013, 4, 3617–3623.
42 A. J. Evans and P. D. Beer, J. Chem. Soc., Dalton Trans., 2003,
4451–4456, DOI: 10.1039/b308541d.
M. A. Robb, J. R. Cheeseman, G. Scalmani, V. Barone,
B. Mennucci, G. A. Petersson, H. Nakatsuji, M. Caricato,
X. Li, H. P. Hratchian, A. F. Izmaylov, J. Bloino, G. Zheng,
J. L. Sonnenberg, M. Hada, M. Ehara, K. Toyota,
R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda,
O. Kitao, H. Nakai, T. Vreven, J. A. Montgomery Jr,
J. E. Peralta, F. Ogliaro, M. Bearpark, J. J. Heyd,
E. Brothers, K. N. Kudin, V. N. Staroverov, R. Kobayashi,
J. Normand, K. Raghavachari, A. Rendell, J. C. Burant,
S. S. Iyengar, J. Tomasi, M. Cossi, N. Rega, J. M. Millam,
M. Klene, J. E. Knox, J. B. Cross, V. Bakken, C. Adamo,
J. Jaramillo, R. Gomperts, R. E. Stratmann, O. Yazyev,
A. J. Austin, R. Cammi, C. Pomelli, J. W. Ochterski,
R. L. Martin, K. Morokuma, V. G. Zakrzewski, G. A. Voth,
P. Salvador, J. J. Dannenberg, S. Dapprich, A. D. Daniels,
43 X. Huang, B. Wu, C. Jia, B. P. Hay, M. Li and X.-J. Yang,
Chem.–Eur. J., 2013, 19, 9034–9041.
´
´
44 B. J˛edrzejewska, B. Osmiałowski and R. Zalesny, Photochem.
Photobiol. Sci., 2016, 15, 117–128.
45 J. M. McGrath and M. D. Pluth, J. Org. Chem., 2014, 79,
11797–11801.
¨
46 L. G. Arnaut and S. J. Formosinho, J. Photochem. Photobiol., A,
1993, 75, 1–20.
O. Farkas, J. B. Foresman, J. V. Ortiz, J. Cioslowski and
This journal is © The Royal Society of Chemistry 2018
RSC Adv., 2018, 8, 23698–23710 | 23709

View Article Online
RSC Advances
Paper
D. J. Fox, Gaussian 09, Revision D.01, Gaussian, Inc., 69 S. Gavade, R. Balaskar, M. Mane, P. N. Pabrekar and
Pittsburgh PA, 2009. D. Mane, Synth. Commun., 2012, 42, 1704–1714.
66 M. Cossi, V. Barone, R. Cammi and J. Tomasi, Chem. Phys. 70 A. Krogul and G. Litwinienko, J. Mol. Catal. A: Chem., 2015,
Lett., 1996, 255, 327–335. 407, 204–211.
67 W. H. Miller, M. A. Seefeld, K. A. Newlander, I. N. Uzinskas, 71 R. Chutia and G. Das, Dalton Trans., 2014, 43, 15628–15637.
W. J. Burgess, D. A. Heerding, C. C. K. Yuan, M. S. Head, 72 N. Busschaert, I. L. Kirby, S. Young, S. J. Coles, P. N. Horton,
D. J. Payne, S. F. Rittenhouse, T. D. Moore, S. C. Pearson,
V. Berry, W. E. DeWolf, P. M. Keller, B. J. Polizzi, X. Qiu,
M. E. Light and P. A. Gale, Angew. Chem., Int. Ed., 2012, 51,
4426–4430.
C. A. Janson and W. F. Huffman, J. Med. Chem., 2002, 45, 73 R. P. McGeary, A. J. Bennett, Q. B. Tran, J. Prins and
3246–3256.
B. P. Ross, Tetrahedron, 2009, 65, 3990–3997.
68 D. Asthana, R. Pandey and P. Mukhopadhyay, Chem.
Commun., 2013, 49, 451–453.
23710 | RSC Adv., 2018, 8, 23698–23710
This journal is © The Royal Society of Chemistry 2018