Altering the Photochromic Properties of N-Salicylideneanilines Using a Co-Crystal Engineering Approach

Article abstract of DOI:10.1021/acs.cgd.6b00108

In this work photochromic properties of N-salicylideneanilines were modified using a crystal engineering approach. Co-crystallization allowed modification of the structural arrangement and electronic properties of the target compounds altering its photoch

Full text of DOI:10.1021/acs.cgd.6b00108

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Article
Altering the photochromic properties of N-
Salicylideneanilines using a co-crystal engineering approach
Gabriel M. Mercier, Koen Robeyns, and Tom Leyssens
Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.6b00108 • Publication Date (Web): 11 May 2016
Downloaded from http://pubs.acs.org on May 14, 2016
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Altering the photochromic properties of N-Salicylideneanilines using a co-crystal
engineering approach
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Gabriel M. Mercier†, Koen Robeyns †, Tom Leyssens †*
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† Institute of Condensed Matter and Nanosciences, Université catholique de Louvain, 1348 LouvainꢀlaꢀNeuve,
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Belgium.
* Corresponding author: tom.leyssens@uclouvain.be.
Abstract
In this work photochromic properties of NꢀSalicylideneanilines were modified using a crystal
engineering approach. Coꢀcrystallization allowed to modify the structural arrangement and
electronic properties of the target compounds altering its photochromic behavior without having
to change the nature of the molecule. A coꢀcrystal of NꢀSalicylideneaniline and citraconic acid
characterized by a zwitterionic anil character and showing inverted photochromic behavior when
compared to the parent anil is reported here for the first time.
H
N
O
H
O
O
O
OH
ORTEP representation of the crystal structure of coꢀcrystal between anil NꢀSalicylidenaniline
and citraconic acid coꢀformer. Dashed blue line represent the ꢀꢁꢁ(4) synthon with two hydrogen
atoms shared between the three electronegative atoms. Right: 2ꢀd scheme of the structure
showing the zwitterionic character of anil moiety.
*Corresponding author:
Tom Leyssens
Institute of Condensed Matter and Nanosciences
Molecules, Solids and Reactivity
Université Catholique de Louvain
Place Louis Pasteur 1, bte L4.01.03
Bꢀ1348 LouvainꢀLaꢀNeuve
Tel: +32 10 47 2811 Fax: +32 10 47 27 07
tom.leyssens@uclouvain.be
http://www.uclouvain.be/leyssensꢀgroup
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Altering the photochromic properties of N-
Salicylideneanilines using a co-crystal
engineering approach
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Gabriel M. Mercier †, Koen Robeyns †, Tom Leyssens †*
† Institute of Condensed Matter and Nanosciences, Université catholique de Louvain, 1348
LouvainꢀlaꢀNeuve, Belgium
* Corresponding author: tom.leyssens@uclouvain.be
Keywords
NꢀSalicylideneaniline, photochromism, crystal engineering, coꢀcrystal.
Abstract
In this work photochromic properties of NꢀSalicylideneanilines were modified using a crystal
engineering approach. Coꢀcrystallization allowed to modify the structural arrangement and
electronic properties of the target compounds altering its photochromic behavior without having
to change the nature of the molecule. A coꢀcrystal of NꢀSalicylideneaniline and citraconic acid
characterized by a zwitterionic anil character and showing inverted photochromic behavior when
compared to the parent anil is reported here for the first time.
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Introduction
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Within the race to novel technologies, chromotropic materials have gained an increasing amount
of attention over the last few decades, from industrial as well as academic point of view [1].
Photochromism is likely the most sought chromotropic property, and is defined as a light
induced reversible color change of a material. Most often photochromism is associated to a
compound, which can occur in two specific forms absorbing at different wavelengths [2].
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Among photochromic materials, NꢀSalicylideneaniline (anils) derivatives have been widely
studied since 1939 [3] as they present multiple advantages: they are relatively straightforward to
synthesize [4,5], easy to handle and use in the solid state, and resistant to fatigue [6,7,8]. Nꢀ
Salicylideneanilines belong to the group of Schiff bases as they contain an azomethine (imine)
bond [9]. The orthoꢀhydroxyl (phenol) group can undergo a ketoꢀenol equilibrium through a
temperature induced proton transfer to the nitrogen of the azomethine bond [4,10ꢀ12] (Figure 1).
At room temperature in general both the enol and cisꢀketo forms are present in equilibrium. In
some cases, upon UVꢀvisible irradiation, both undergo transition to a third form, the transꢀketo
form [4ꢀ12] as presented in Figure 1.
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Figure 1: Photo and thermoꢀinduced transitions in NꢀSalicylideneaniline [13].
The enol form (usually the most stable form) is usually colorless whereas the cisꢀketo form,
which becomes more populated at high temperature, exhibits more often a deep yellow to orange
coloration. Finally, the transꢀketo form obtained after light irradiation is usually deep redꢀcolored
[3,4]. As mentioned, color changes can either be thermally or photoꢀinduced, allowing to classify
anil compounds as thermochromic and/or photochromic. Figure 2 shows the overall mechanisms
believed to occur upon photoꢀirradiation [6,14ꢀ16]. In the most simple scheme, excitation of the
enol form, will lead to an exited enol state, which either returns to the ground state by
fluorescence, or which undergoes nonꢀradiative transition to an exited cisꢀketo state. This latter
can undergo a radiative relaxation to the ground cisꢀketo state, or undergo a nonꢀradiative
transition to an excited transꢀketo state. Nonꢀradiative relaxation of this latter is believed to lead
to the redꢀcolored ground state transꢀketo species. Recent theoretical and ultraꢀfast laser
spectroscopy studies have shown this scheme to be over simplified, with a more complex
situation involving multiple transient species being more likely [17,18].
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Figure 2: Simplified scheme of transition pathways for anils upon light irradiation.
From a structural point of view, photochromic anils usually present a nonꢀplanar shape with a
dihedral angle between aromatic rings φ >25° [12,16,19,20]. When the molecule is planar the
anil tends to be only thermochromic. Recent work, however, suggest that planarity or nonꢀ
planarity of the anil molecules is not the determining factor for thermochromic or photochromic
behavior of those compounds [21]. The thermo and/or photochromic character of the anil
strongly depends on the overall structural environment, explaining why different polymorphs of
the same compound can exhibit different thermo/photochromic properties [4].
Although sought for, only a limited number of anils show photochromism at the solid state. In
general, identification of novel photochromic anils focuses on the synthesis of new compounds
by inserting or combining different functional groups. Alternatively, the photochromic
compound can be inserted in a different environment such as polymers and rigid glasses [19] or
porous materials [22] and zeolite [23]. In a final approach, a crystal engineering approach, the
crystal environment of the compound can be impacted. By adding a metal component, metal
organic frameworks (MOFs) can be obtained [13,24]. In a recent contribution, a novel
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methodology was used, consisting in coꢀcrystallizing the molecule of interest with another
organic molecule. This approach allows combining properties of different materials rendering
them for instance more stable or less reactive to external factors. More specifically, coꢀ
crystallization allows the creation of novel crystalline forms, even for those compounds for
which no salts or solvates can be formed [25,26].
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In the case of NꢀSalicylideneanilines, coꢀcrystallization could allow to increase the space
available in the crystal structure required for the “pedal motion” involved in the photochromic
mechanism [20]. Doing so nonꢀphotochromic materials have been shown to be transformable
into photochromic ones [27]. In their contribution, Johmoto et al. studied acidꢀbase coꢀ
crystallization [27], with a proton transfer occurring between the coꢀformer and the anil of
interest, hereby obtaining two charged species. Another recent research succeeded in obtaining a
photochromic coꢀcrystal coming from a nonꢀphotochromic anil [28]. Hutchins et al. used a
phenolic derivative which was hydrogen bonded with 4,4ꢀBipyridin [28]. We used a similar
approach but aimed not only to avoid acidꢀbase coꢀcrystallization, but also to impact the
photochromically active site directly by changing its hydrogen bonding pattern. To do so, we
focused on two model compounds (see experimental section) that do not contain any basic or
acidic group other than the ones at the photochromically active site.
By allowing hydrogen bonding to the photochromic site of the anil, the goal of this study was to
directly impact the photochromic behavior of these reference compounds.
Experimental Section
Starting Materials.
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Carboxylic acids used as coꢀformers, solvents and starting materials for synthesis were all
purchased and used as such. Product codes are given in the brackets. From Accros Organic: 1ꢀ
hydroxyꢀ2ꢀnaphtoic acid (149231000); 2,5ꢀdihydroxybenzoic acid (165200500); 4ꢀ
hydroxybenzoic acid (120995000); Acetylsalicylic acid (158185000); Citric acid (110450010);
Fumaric acid (119750010); Maleic acid (125230010); Mandelic acid R (101691137); Oxalic acid
(186432500); Succinic acid (158745000); Tartaric acid D (137871000); 4ꢀfluoroꢀaniline
(119290250). From Sigma Aldrich: 2,3ꢀdihydroxybenzoic acid (206ꢀ135ꢀ5); 2,4ꢀ
dihydroxybenzoic acid (101527441); 3,4ꢀdihydroxybenzoic acid (100986520); 3,5ꢀ
dihydroxybenzoic acid (202ꢀ730ꢀ7); Salicylaldehyde (101669521). From Alfa Aesar: 2,2ꢀ
dimethylsuccinic acid (209ꢀ899ꢀ6). From U.C.B. s.a.: Salicylic acid (4388); Aniline (1163ꢀ
4944). From Janssen Chimica: Acetic acid (14.893.52). From TCI: Citraconic acid (207ꢀ858ꢀ7).
From VWR Chemicals: Acetonitrile (200ꢀ835ꢀ2); Diethyl ether (200ꢀ467ꢀ2); Ethanol (200ꢀ578ꢀ
6); Ethyl acetate (205ꢀ500ꢀ4). From Fisher Chemicals: Acetone (200ꢀ662ꢀ2).
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Synthesis. Compound A (NꢀSalicylideneaniline) was prepared by hand grinding (ten minutes)
of salicylaldehyde (10 mmol) with aniline (10 mmol) in presence of a catalytic amount of acetic
acid (0.2 mmol) and dried for 48 h at 45 °C and used as such (yield: 98%, purity: 99%)1.
Compound B (4ꢀfluoroꢀSalicylideneaniline) was prepared by grinding (5 minutes) of
salicylaldehyde (10 mmol) with 4ꢀfluoroꢀaniline (10 mmol) and dried for 48 h at 45°C and used
as such (yield: 98%, purity: 99%).
Mechanochemical co-crystal screen. The experimental coꢀcrystal screening procedure was
performed as follows: in a first stage, solventꢀdrop grinding (10 ꢁl of acetonitrile) was performed
1 The photochromically active polymorph of this compound was obtained using this procedure.
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using an equimolar (10ꢀ3 mol) mixture of starting materials. Samples were ground in a RETSCH
Mixer Mill MM 400 for 90 min with a beating frequency of 30 Hz. The resulting powders were
characterized using Xꢀray Powder Diffraction. Upon coꢀcrystal identification, attempts were
made to obtain a single crystal of suitable size and quality for single crystal Xꢀray diffraction
measurement. As a chemical reaction can occur during solventꢀdrop grinding, 1HꢀNMR was used
to eliminate samples were degradation/reaction occurred.
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Single Crystal formation attempts. Single crystals of A-C10 were grown by MeCN vapor
diffusion in a furnace at 45 °C for 1 week after grinding. Standard slow solvent evaporation at
room temperature was tried for each positive hit with acetone, acetonitrile, diethylether, ethanol,
ethylacetate but did not yield successful results. MeCN vapor diffusion at 45 °C was tested for B
coꢀcrystals but did not yield single crystals suitable for analysis.
X-ray Powder Diffraction (XRPD). Xꢀray diffraction measurements were performed on a
Siemens D5000 diffractometer equipped with a Cu Xꢀray source operating at 40 kV and 40 mA
and a secondary monochromator allowing to select the Kα radiation of Cu (λ=1.5418 Å). A 2θ
scanning range from 2° to 50° at a scan rate of 0.6° minꢀ1 was applied. Simulated patterns of the
known starting compounds were calculated from their single crystal structures with Mercury 3.1
and plotted with WinPLOTR (version: September 2015). For each reported mixture diffraction
pattern (in blue) is superimposed onto simulated patterns of its components (in red for the
corresponding anil and green for the coꢀformer). All data are normalized at 100 for the highest
peak. We assume formation of new phases when at least 2 new peaks appear and at least 2
reference peaks are missing.
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Single crystal X-Ray diffraction. The structure of A-C10 was solved from singleꢀcrystal
synchrotron Xꢀray diffraction data, which were collected on the SwissꢀNorwegian Beam Line
BM01A at the European Synchrotron Radiation Facility (ESRF) (Grenoble, France), using a
PILATUS 2M hybrid pixel detector at a wavelength of 0.82103 Å. Prior to the data collection the
sample was flashꢀcooled to 100(2) K using an Oxford Cryostream 700 system. The data were
converted with the SNBL toolbox software [29] for integration with the CrysAlisPro software
[30].
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Structure was integrated with the CrysalisPro software and the implemented multiꢀscan
absorption correction procedure was applied. The structure was solved by direct methods,
SHELXSꢀ97 [30] and was refined by fullꢀmatrix least squares on |F|2 using SHELXLꢀ2014 [31]
Nonꢀhydrogen atoms were refined anisotropically and hydrogen atoms were either placed on
calculated positions in riding mode with temperature factors fixed at 1.2 times Ueq of the parent
atoms and 1.5 times Ueq for methyl groups, or located in the Fourier difference maps when
involved in Hꢀbonds.
NMR analysis. 1H NMR spectra were recorded on Brukerꢀ300 MHz. 1H NMR chemical shifts
are reported relative to CD3CN (1.94 ppm).
Compound A: δ (ppm): 13.26 (s, 1H); 8.82 (s, 1H); 7.56ꢀ7.31 (m, 7H); 7.03ꢀ6.98 (dt, 2H,
J1=7.8Hz, J2=1.2Hz).
Diffuse Reflectance Spectroscopy measurements. DRS spectra were obtained on a Varian
Cary 5E spectrometer using PTFE as a reference. Spectra were measured on pure solid. Diffuse
Reflectance spectra were normalized (value at λ=395 nm set at 1). Signal assignment was
supported by reported band range for Nꢀsalicylideneaniline derivatives [15]. Light irradiations
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were carried out with a LOTꢀORIEL 200 W highꢀpressure mercury arc lamp combined with
selected band pass filter. KubelkaꢀMunk spectra were taken before and after irradiation
(irradiation time is described in the text).
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CSD statiscal analysis. Reference bond lengths of interest were obtained by statistical
analysis of the data available in the CSD: we searched all reported organic structure (with R
factor ≤ 0.05) containing at least two bonds corresponding to critical bonds of the two forms
(enol and keto: Figure 3) studied. A search was performed in order to specifically describe bond
lengths in the case of anil derivatives and to be more accurate than average values already
reported by Frank H. Allen et al. [32]. Reference structures used to approximate enolꢀimine and
conjugatedꢀketone form are presented in Figure 4. Doing so, reference bond lengths were
obtained (noted as bond 2, 3, 4 and 5 in Figure 3) in various environments regardless of
temperature. Results of average bond lengths are reported in table 1 with standard deviation and
number of CSD hits. As shown by this table, there is a clear cut difference between enolꢀimine
and ketone bond lengths, with a slightly more narrow distribution in the case of the enolꢀimine
form. Bond lengths obtained for Conjugated Ketone 1 are different from bond lengths of
Conjugated Ketone 2 and Amine 1 (Figure 4). We considered Conjugated Ketone 1 already
partially displaced toward EnolꢀImine 1 and furthermore there are only a few hits (126) meaning
results on this form are less accurate. We therefore used both forms, ConjugatedꢀKetone 2 and
Amine 1 (for bond 5), to better describe the conjugated ketone character. To describe the enolꢀ
imine character we selected bond length values of EnolꢀImine 1 as it is the closest to the anil A
structure and they are in good agreement with the results observed in Phenol 1 and Imine 1.
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Figure 3: Common structure of all anils considered. Important bonds are noted from 1 to 6.
Figure 4: Structures with bond lengths researched through CSD to approach enolꢀimine form
(EnolꢀImine 1, Phenol 1, Imine 1) and ketone form (Conjugated Ketone 1, 2 and Amine 1) bond
lengths. Non described bonds/atoms were not defined during the search.
Table 1: bond lengths and standard deviations for each pattern researched with its number of hits
in the CSD
Pattern
EnolꢀImine 1
Hits = 2652
Phenol 1
Bond 2, σ (Å) Bond 3, σ (Å) Bond 4, σ (Å) Bond 5, σ (Å)
1.344, 0.022
1.406, 0.017
1.450, 0.021
1.286, 0.021
1.364, 0.020
/
1.399, 0.017
/
1.499, 0.310
1.443, 0.023
/
Hits = 10000
Imine 1
1.283, 0.021
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Hits = 10000
Conj. Ketone 1
Hits = 126
Conj. Ketone 2
Hits = 881
1.291, 0.015
1.431, 0.013
1.463,0.032
/
1.414, 0.014
1.303, 0.015
/
1.249, 0.034
/
1.375, 0.040
1.356, 0.026
Amine 1
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1.358, 0.027
Hits = 10000
Results
Co-crystal screening. Two very basic, wellꢀdescribed model anil derivatives were chosen as
well as 18 common carboxylic acid coꢀformer candidates (Figure 5). A first anil is Nꢀ
Salicylideneaniline (A, reported with crystal structures REFCOD SALCAN to SALCAN04 in
the CSD [34,35,36]) known to show strong photochromism; the second one is 4ꢀfluoroꢀNꢀ
Salicylideneaniline (B, reported as CAVQAR in the CSD [37]) known to show only
thermochromism in the solidꢀstate. Both compounds have been extensively studied
[4,38,39].With the principal functionality available on classical anils being the hydroxyl
function, carboxylic acids have been tested in order to form a hydrogen bridge as this type of
hydrogen bonding is strong and regularly encountered in coꢀcrystals [25,26]. A typical synthon
occurring between an alcohol and carboxylic acid group is the synthon ꢀꢁꢁ(4) (according to Etter
Nomenclature [33]) shown in Figure 6. Another common synthon is ꢂꢁꢁ(6) (which would extend
to ꢂꢁꢁ(10) in the case of anils) but wasn’t observed here.
A
B
C2
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C3
C4
C5
C6
C7
C8
C9
C10
C14
C18
C11
C12
C13
C15
C16
C17
Figure 5: Compounds studied (IUPAC name in Table 1).
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Figure 6: Expected ꢀꢁꢁ(4) supramolecular interaction [33] between NꢀSalicylideneaniline and a
carboxylic acid (R1 stands for alkyl group).
Coꢀcrystals were identified using a mechanochemical approach (see experimental details). All
possible combinations of 1:1 molar ratio between the coꢀformers and the anils were tested. A
change in XRPD pattern (Figure 7) is expected to be related to either a change in polymorphic
nature of one of the compounds, degradation (=reaction) or coꢀcrystal formation. When
degradation occurred (usually dissociation of anil in amine and aldehyde), the result was
discarded.
Table 2: XRPD and HꢀNMR comparison results between ground compounds and reference. X
implies a perfect superposition of XRPD data, XX implies reaction/degradation of products and
ѵ means a structural change without degradation of starting materials (coꢀcrystal or polymorph)2.
Ground with
A (Nꢀ
B (4ꢀfluoroꢀNꢀ
2 Screening was limited to grinding methods, so the possibility exists that some coꢀcrystals might
have been overlooked.
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Salicylideneaniline) Salicylideneaniline)
5
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8
C1 (1ꢀhydroxyꢀ2ꢀnaphtoic acid)
C2 (2,2ꢀdimethylsuccinic acid)
C3 (2,3ꢀdihydroxybenzoic acid)
C4 (2,4ꢀdihydroxybenzoic acid)
C5 (2,5ꢀdihydroxybenzoic acid)
C6 (3,4ꢀdihydroxybenzoic acid)
C7 (3,5ꢀdihydroxybenzoic acid)
C8 (4 hydroxybenzoic acid)
C9 (acetylsalicylic acid)
C10 (citraconic acid)
X
X
X
X
9
X
X
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XX
X
Ѵ
X
XX
X
Ѵ
X
X
X
X
X
Ѵ
XX
X
C11 (citric acid)
X
C12 (fumaric acid)
X
X
C13 (maleic acid)
XX
X
Ѵ
X
C14 (mandelic acid R)
C15 (oxalic acid)
XX
X
Ѵ
X
C16 (salicylic acid)
C17 (succinic acid)
X
X
C18 (tartaric acid D)
X
X
As shown in table 2, a positive hit was obtained when working with NꢀSalicylideneaniline
(entry A) and citraconic acid (Entry C10). Figure 7 shows that the XRPD patterns of the ground
material is different from any of the known polymorphic forms of A, as well as from the XRPD
pattern of citraconic acid. NMR analysis confirmed no degradation occurred, leaving coꢀcrystal
formation as the likely outcome, as confirmed by single crystal analysis. This result shows how
coꢀcrystallization can effectively occur even when a limited amount of functional groups is
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present on the target compound. For compound B, a total of 4 positive results were identified,
using 2,4ꢀdihydroxybenzoic acid, 3,4ꢀdihydroxybenzoic acid, maleic acid, or oxalic acid as a coꢀ
former.
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Co-crystals of 4-fluoro-N-Salicylideneaniline
Although 4 novel coꢀcrystals of 4ꢀfluoroꢀNꢀSalicylideneaniline are suspected based on the data
in Table 2, no single crystals of these have been obtained so far. One reason could be the
sensitivity of anil to hydrolysis catalyzed by acids (coꢀformers). As solvent used are all slightly
hygroscopic process like slow evaporation could lead to the degradation of anil and
recombination of amine with the coꢀformer to make a salt. Such a process should then be
performed under waterꢀfree atmosphere to avoid this.
Furthermore, DRS analyses on the ground coꢀcrystal powders did not show significant
differences with respect to the reference compound, nor did irradiation at different wavelengths
for prolonged periods. These coꢀcrystals, are thus, like the reference parent compound, expected
to be thermochromic, with no significant difference between the ketoꢀenol equilibrium found in
the parent compound as compared to that within the coꢀcrystals.
Co-Crystal between N-Salicylideneaniline (A) and citraconic acid (C10)
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Figure 7: XRPD pattern of A-C10 powder versus references: the ground mixture (blue spectra) is
clearly different from all references of A (four polymorphs) and C10 (black).
A single crystal was successfully grown and structural analysis shows both compounds to coꢀ
crystallize in the P21/c space group (structural details are presented in table 3), with the expected
ꢀꢁꢁ(4) synthon (which is a combination at the binary level of S(6) and D patterns) forming
between the carboxylic acid group and the phenolate function (Figure 8). To the best of our
knowledge, we are the first to report on a direct impact of coꢀcrystallization on the functional
groups that are involved in the thermochromic and photochromic nature of the anils by impacting
the hydrogen bonding pattern. The coꢀcrystal is not a salt as the carboxylic proton was clearly
visible in the density maps and the carbonꢀoxygen bond lengths in the carboxylic group are
significantly different, identifying them as a C=O double bond ( 1.223(2) Å ) and a single CꢀO
bond (1.298(2) Å ), opposed to the salt form where equal bond distances are expected.
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H
N
O
H
O
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O
O
OH
Figure 8 left: ORTEP projection of crystal structure of coꢀcrystal between anil A and coꢀformer
C10. Dashed blue line represent the ꢀꢁꢁ(4) synthon with two hydrogen atoms shared between the
three electronegative atoms. Right: schematic representation of the structure showing the
zwitterionic character of anil moiety.
To study the impact of coꢀcrystallization on the ketoꢀenol equilibrium, we compare the main
bond lengths involved in the A-C10 coꢀcrystal with those of the free anil (A), as well as with a
statistical analysis as specified in the experimental details of similar derivatives available in the
CSD (Figure 5). Results are given in table 3.
Table 3: Expected values with their standards deviations3 for bond lengths 2 to 5 of EnolꢀImine
and Ketoꢀform (regardless to the temperature) based on CSD statistical analysis and comparison
with A-C10 coꢀcrystal (bond lengths measured and their standard deviations) and A
(SALCAN01).
Bond 2, σ (Å) Bond 3, σ (Å) Bond 4, σ (Å) Bond 5, σ (Å)
EnolꢀImine
Conj. Ketone 1.249, 0.034
1.344, 0.022
1.406, 0.017
1.463, 0.032
1.419, 0.003
1.45, 0.021
1.375, 0.04
1.455, 0.003
1.286, 0.021
1.358, 0.027
1.280, 0.003
1.352, 0.003
A
3
The errors of the CSD search are one order of magnitude higher compared to the error on a
single structure, as they are derived from an ensemble of structure.
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(120K)
A-C10
(100K)
5
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7
1.316, 0.002
1.424, 0.003
1.407, 0.003
1.309, 0.002
8
9
Table 3 clearly shows A to exist in an enol/cisꢀketo equilibrium which is strongly displaced
towards the enol form. An important structural variation is observed when A coꢀcrystallizes with
C10 (crystal data and structure refinement are given in table 4), as bond lengths indicate the
equilibrium is shifted in favor of the keto form. As data were collected at 120 K for A and 100 K
for A-C10 we expect only a neglectible temperature effect on the bond lengths. Bonds 2 and 4
are shorter within the coꢀcrystal, whereas the length of bond 5 increases.
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Even though these bond lengths indicate a displacement towards the keto form, at this stage,
the use of the term ‘keto’ꢀform does seem inappropriate. The ‘keto’ꢀform can actually be
represented under two resonance structures, either using a charge neutral keto representation or
through a zwitterionic representation. This latter seems much more pertinent in the current case:
the hydrogen of the enol is clearly transferred to the nitrogen atom as shown by the Xray density
maps. Even though, this clear transfer of the hydrogen atom, bond 2 (1.316 Å) is intermediate
between a pure C=O double bond (1.344 Å) and a CꢀOH single bond (1.249 Å) as shown in table
3. Furthermore the CꢀC bond 3 (1.424 Å) is almost not affected whereas a clear increase up to
1.46 Å would be observed (according to table 3) if one would expect a more keto character.
Finally, the CꢀN bond 5 length (1.309 Å) is closer to that of a CꢀN (1.286 Å) single bond, than
that of a C=N double bond (1.358 Å). These combined observations suggest to consider the
‘keto’ꢀform rather as a ‘zwitterionic form’ in this case, and the ketoꢀenol equilibrium should
rather be presented as the “enolꢀzwitterionic” equilibrium shown in Figure 9. This zwitterionic
form was already reported for anils inserted into zeolites however in that case no photochromism
was observed [23].
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The displacement from the enol form towards the zwitterionic form upon coꢀcrystallization is
likely induced by the hydrogen donating effect of the coꢀformer (synthon ꢀꢁꢁ(4)). The
directionalized interaction of the coꢀformer hydrogen towards the alcohol function, could allow
for a better hydrogen transfer of this latter to the imine function, and stabilizes the negative
charge on the oxygen atom, create by doing so. This displacement also induces a change in
optical properties as illustrated in Figure 10 showing the color change between the original
compound and the coꢀcrystals, at room temperature and at 50 °C. At room temperature coꢀcrystal
A-C10 has a more intense yellow color compared to the reference A which can be explained by
an increased amount of zwitterionic form. When heated up to 50 °C anil A starts to melt and is
characterized by a more intense brown color whereas A-C10 remains solid, exhibiting a more
intense yellow color (melting occurs at 80 °C). When cooled to ꢀ80 °C anil A turns to pale
yellow whereas no change is observed for A-C10.
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O
O
1
5
2
1
5
2
H
N
H
N
6
6
3
3
4
4
R
R
EnolꢀImine
Zwitterionic
Figure 9: New equilibrium pattern based on structural analysis
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Figure 10: Comparison between A and A-C10 at room temperature (left) and upon heating at 50
5
6
°C (right).
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To study the effect of coꢀcrystallization on the photochromic properties of the parent anil,
KubelkaꢀMunk spectra of the anil A and coꢀcrystal A-C10 were measured under standard
conditions (room temperature and pressure) as shown in Figure 11. Data are normalized to 1 for
the peak at 395 nm. Spectra of the two compounds clearly differ. No such difference is found
when comparing the spectra of reference A (polymorph alpha 2) with that of a physical mixture
(see supporting information) between A and C10. Both the parent anil A as well as the coꢀcrystal
A-C10 show a strong absorption band at 375 nm, attributed to the absorption of the enol form
[4]. The strongest difference for the coꢀcrystal comes from the very strong absorption band
centered on 488 nm which is typically attributed to the keto form (or the zwitterionic form in our
case) of the product. This form appears to be strongly populated which is in line with the
structural analysis, pointing towards an increased amount of zwitterionic form in the equilibrium.
The weak absorption at 450 nm clearly shows why the parent anil, was labeled nonꢀ
thermochromic.
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When zooming into the 525ꢀ700 nm range both forms present a small shoulder centered on
600nm, attributed to the transꢀketo form.
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Figure 11: KubelkaꢀMunk spectra of anil A and coꢀcrystal A-C10 at room temperature.
The photochromic effect of both forms was studied by comparing variation of the absorption
spectra upon prolonged irradiation (30 minutes to 1 hour). Irradiation of A at 365 nm leads to an
increase in cisꢀketo and transꢀketo form, whereas an irradiation at 436 nm leads to a depopulation
of the already small amount of cisꢀketo form in favor of the transꢀketo form, in agreement with
the earlier work by Fujiwara et al. [15]. However in the case of A-C10 a surprisingly different
behavior is observed. Irradiation at 365 nm, leads to both a strong depletion of the cisꢀ
zwitterionic form (centered at 450 nm) and transꢀketo (600 nm) forms relatively to the enol form
(centered at 380 nm). Subsequent irradiation at 546 nm (+/ꢀ 20 minutes) restores the original
situation contrary to what is observed for the parent anil (Figure 12). Another difference
observed is irradiation at 436 nm which has no observable effect on the zwitterion/enol
equilibrium in this case. These results might indicate that the transꢀketo form is also impacted by
coꢀcrystallization, and might also show a strong zwitterionic character, although this remains to
be proven.
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After prolonged irradiation, the initial equilibrium can be restored thermally (without
irradiation), but at room temperature, one requires about 8 hours to return to original levels.
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Figure 12: KubelkaꢀMunk spectra of A-C10 after irradiation at 365 nm (blue) for 20 minutes
and reference (red).
From the results above, it is clear that coꢀcrystallization strongly affects the photochromic
behavior of anils, likely due to the impact on the hydrogen bonding pattern which is affected. To
fully understand the underlying mechanism, one would have to review the scheme as presented
in Figure 2. A future full investigation using e.g. femtosecond spectroscopy could be interesting
to elucidate this behavior.
Our results clearly show that tackling the structure of anils through coꢀcrystallization and more
specifically through alteration of the hydrogen bonding pattern directly involved in the keto
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(zwitterion)/enol equilibrium, offers new possibilities for varying the photochromic behavior of
anils, and might lead to promising results on future candidates.
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Table 4: Crystal data and structure refinement for coꢀcrystal A-C10
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Coꢀcrystal A-C10
Empirical formula
M
C18H17NO5
327.32
T [K]
Crystal system
100(2)
Monoclinic
Space group
a [Å]
b [Å]
c [Å]
P21/c (no.14)
10.7434(4)
8.3025(2)
19.0548(7)
113.813(3)
1554.94(10)
4
β [°]
V [ų]
Z
ρ
calcd [Mgmꢀ3]
1.398
F(000)
688
µ [mmꢀ1]
0.142
Crystal size [mm]
0.15x0.10x0.10
31.2129
7945/2471
0.0393
θmax [°]
Reflection collected/unique
Rint
R1 [I>2σ(I)]
wR2
0.0401
0.0926
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Largest diff. Peak/hole [e Åꢀ3] 0.152/ꢀ0.223
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Conclusion
We have crystallized a coꢀcrystal of NꢀSalicylideneaniline and Citraconic acid which shows
new and unexpected photochromic behavior. A careful structural analysis reveals coꢀ
crystallization affected bond lengths and electronic structure of the photochromic center in such
a manner to suggest viewing the classical ketoꢀenol equilibrium rather as an enolꢀzwitterionic
equilibrium. This effect is due to the coꢀformers we chose for synthon creation in order to build
the coꢀcrystal. By altering the hydrogen bonding pattern of the alcohol functionality, we are able
to impact the photochromic mechanism directly. To do so, we added a hydrogen donor to the
alcohol group, creating a pushꢀandꢀpull effect on the hydrogen involved in the enolꢀzwitterionic
equilibrium. For the parent NꢀSalicylideneaniline, coꢀcrystallization not only led to a substantial
change in the classical view of the keto/enol equilibrium, which is in our case described as an
enolꢀzwitterionic equilibrium, but furthermore to some unexpected photochromic behavior,
which up to now remains unexplained by the classical scheme. These findings open new ways to
fineꢀtune photochromism of NꢀSalicylideneaniline derivatives: one can not only increase the
space available in the crystal structure through coꢀcrystallization (allowing cisꢀtrans
isomerization), but one can also directly impact the electronic behavior of the centers involved in
the photochromic and thermochromic behavior.
Supporting Information. Structures described in this contribution have been deposited at the
Cambridge Crystallographic Data Centre and allocated the deposition numbers CCDC 1448718
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for A-C10. Supporting information also contains TGA, DSC, XRPD and DRS patterns of
discussed compounds. This material is available free of charge via the Internet at
http://pubs.acs.org.
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Corresponding Author
* Email: tom.leyssens@uclouvain.be.
Acknowledgments. The authors would like to thank the UCL for financial support (Gabriel
Mercier is a UCL teaching assistant). This work was published thanks to funding of "Actions de
Recherche Concertées (ARC) de la Direction générale de l’Enseignement non obligatoire et de la
Recherche scientifique – Direction de la Recherche scientifique – Communauté française de
Belgique. The authors would like to thank Johan Wouters (UNamur) for his review of the article.
References
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13, 269ꢀ286.
[2] G. H. Brown, Photochromism, WileyꢀInterscience, New York 1971.
[3] V. de Gaouck, R. J. W. Le Fevre, J. Chem. Soc. 1939, 1457.
[4] E. Hadjoudis, Molecular Engineering, 1995, 5, 301.
[5] B. Kaitner, M. Zbačnik, Acta Chim. Slov. 2012, 59, 670.
[6] Robert, F., 2010, Engineering Nꢀsalicylidene aniline derivatives: from solidꢀstate
molecular switches to multifunctional organicꢀinorganic architectures. PhD thesis,
Université Catholique de Louvain, Belgique.
[7] M. Sliwa, S. Létard, I. Malfant, M. Nierlich, P. G. Lacroix, T. Asahi, H. Masuhara, P. Yu,
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Altering the photochromic properties of N-Salicylideneanilines using a crystal engineering
approach
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Gabriel M. Mercier, Koen Robeyns, Tom Leyssens.
A new coꢀcrystal of NꢀSalicylideneaniline with citraconic acid was reported and studied through
Diffuse Reflectance Spectroscopy and XꢀRay Diffraction. Coꢀcrystallization strongly altered the
electronic structure as well as photochromic properties.
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