Photochromism emergence in N-salicylidene P-aminobenzenesulfonate diallylammonium salts

Article abstract of DOI:10.1002/chem.201406573

N-Salicylidene p-aminobenzenesulfonate salts were prepared by in situ condensation of p-aminobenzenesulfonate diallylammonium salt and salicylaldehyde. Modulation of thermo- and photochromism was achieved by varying the alkyl chain length of the diallylam

Full text of DOI:10.1002/chem.201406573

DOI: 10.1002/chem.201406573
Full Paper
&
Photochromism
Photochromism Emergence in N-Salicylidene
p-Aminobenzenesulfonate Diallylammonium Salts
[a]
¨
Pierre-Loıc Jacquemin, Koen Robeyns, Michel Devillers,* and Yann Garcia*
Abstract: N-Salicylidene p-aminobenzenesulfonate salts
were prepared by in situ condensation of p-aminobenzene-
sulfonate diallylammonium salt and salicylaldehyde. Modula-
tion of thermo- and photochromism was achieved by vary-
ing the alkyl chain length of the diallylammonium counter-
cation. A structural–optical properties investigation reveals
that both crystal packing and dihedral angle between aro-
matic rings of the N-salicylidene aniline switch are not suffi-
cient to predict the occurrence of photochromism in the
solid state. The available free space around the N-salicyli-
dene p-aminobenzenesulfonate, in addition to the flexibility
of the nearby environment, is shown to be of major impor-
tance for the cis!trans isomerisation to occur as well as for
the stabilisation of the trans-keto form. Emergence of photo-
chromic properties was determined from the diallylhexylam-
monium cation within the series of investigated counter-cat-
ions. High stability is observed for the trans-keto form of
one polymorph of N-salicylidene p-aminobenzenesulfonate
diallylhexylammonium salt (k=2.4”10À7 sÀ1).
Introduction
Photochromic molecules can find numerous applications of
various nature, for instance in optical data storage and display
devices,[1] photo-pharmacology,[2] sensors[3] and molecular
rotors.[4] Major efforts have focused on diarylethenes[1c] and spi-
ropyrazines,[1c] but little attention was devoted to N-salicyli-
dene aniline derivatives (anils).[5] These molecules are an impor-
tant substance class, which can present both solid-state
thermo- and photochromism with different colours due to the
presence of prototropic and isomeric forms.[5] Since their dis-
covery, special focus was given to the determination of rela-
tionships between structural and optical properties to under-
stand the mechanisms involved in their switching phenomen-
on occurring in the solid state.[6] Firstly, a thermal tautomeric
equilibrium was identified between the yellow cis-keto form at
room temperature and the white enol form at lower tempera-
ture to account for their thermochromism.[5] This equilibrium is
effective when the alcohol group of the enol form is hydrogen
bonded to the imine of the anil (Scheme 1). Secondly, a photo-
isomerisation between the yellow cis-keto form and the red
trans-keto form is induced by irradiating at l=365 or
450 nm.[5] The resulting trans-keto form usually relaxes back
Scheme 1. Thermochromism and photochromism pathways of N-salicylidene
aniline derivatives.
thermally to the cis-keto form but the reverse switching is also
possible by irradiating at l=545 nm (Scheme 1).
Two main structural factors to account for the cis–trans
photo-isomerisation were defined: the planar character of the
anil molecule and the crystal packing.[7] The planar character of
the anil molecule is evaluated by the determination of the di-
hedral angle F between phenolic and benzene rings, which is
effective when F<258. Twisted molecules (F>258) present
an intramolecular hydrogen bond weaker than in planar mole-
cules, which favours the photo-isomerisation. With a dense
crystal packing, thermochromism is the solely observed phe-
nomenon as the cis–trans isomerisation is prevented due to
steric hindrance when rotating half of the molecule. In con-
trast, photochromism is observed for “open” crystal pack-
ings.[5c,d,7] Recent studies have, however, shown the limit of
these structural considerations with the need to consider elec-
tronic effects.[8] In particular, the study of N-salicylidene amino-
pyridine derivatives revealed that molecules presenting
a dense crystal packing and a planar character can display
[a] P.-L. Jacquemin, Dr. K. Robeyns, Prof. Dr. M. Devillers, Prof. Dr. Y. Garcia
Institute of Condensed Matter and Nanosciences
Molecules, Solids and Reactivity (IMCN/MOST)
UniversitØ Catholique de Louvain
Place L. Pasteur 1, 1348 Louvain-la-Neuve (Belgium)
Fax: (+32)1047-2330
E-mail: michel.devillers@uclouvain.be
yann.garcia@uclouvain.be
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201406573.
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photochromism too, along with molecules in “open” structures
displaying exclusively thermochromism.[8a] Most interestingly,
the study of N-salicylidene aminomethylpyridines revealed the
presence of the trans-keto form in non-photochromic spe-
cies.[8b]
The modification of the direct environment of anil molecules
is another way to modify their optical properties, in particular
photochromism. Indeed, the insertion of neutral N-salicylidene
aniline derivatives inside various host matrices (mesoporous
silica,[9–11] polymers,[12,13] cyclodextrins)[14] could modulate the
photochromic property and even induce it, as observed inside
clathrates[15,16] and zeolites,[17,18] after modification of the dihe-
dral angle (F). The inclusion of anil inside zeolites also modi-
fied the polarity of the medium allowing the formation of zwit-
terions, which change the photochemical pathway of forma-
tion of the trans-keto form.[17]
Scheme 2. The anions and diallylammonium derivatives used in the present
work.
These methods suffer from lack of control of the number of
guest molecules included, contrary to the entrapment of anils
into molecular capsules,[19] as well as the functionalisation of
these molecules as ligands for coordination complexes.[8a,20–22]
Anionic derivatisation of anils constitutes an alternative for
benzenesulfonate-substituted diallylammonium salts (2–6) in
large amounts, to ease further characterisation of the optical
properties. The first synthetic step concerns the deprotonation
of sulfanilic acid by a substituted diallylamine. The resulting
salt is then added to a methanolic solution of salicylaldehyde
for the condensation into N-salicylidene para-aminobenzene-
sulfonate (Scheme 3). Diallylammonium salts were successfully
purified by recrystallisation, as could be concluded from ele-
a
controlled incorporation into positively charged matri-
ces,[13,23,24] and in particular coordination complexes.[23] A sulfo-
nate group was selected because of its permanent charge and
low coordination ability, and deprotonation of the enol form
was avoided to prevent any Schiff base coordination competi-
tion with transition metals. [M(phen)3]2+ (M=Cu, Ni; phen=
1,10-phenanthroline) was chosen as a template to successfully
crystallise N-salicylidene aminobenzenesulfonate as counter-
anions. These hybrid molecules present a tautomeric equilibri-
um after insertion, despite their exclusive thermochromic prop-
erties.[23b] A keto–enol equilibrium was also detected for N-sali-
cylidene aminobenzenesulfonate inserted in FeII-1,2,4-triazole
trinuclear complexes displaying thermo-induced spin crosso-
ver.[23a]
1
mental analysis and H NMR spectroscopy.
Slow evaporation of a methanolic solution of 1 at room tem-
perature afforded long yellow needles, after 6 months, which
were found suitable for X-ray data collection. Compounds 2, 4,
5 and 6 were crystallised from cold methanol at 253 K over
one night as yellow needles. Orange needles were obtained
for 3 by using the same conditions (3a), whereas yellow nee-
dles were crystallised by slow evaporation in darkness at room
temperature (3b).
Herein, we present the first examples of photochromic N-sal-
icylidene aminobenzenesulfonate anions, which were previous-
ly regarded as exclusively thermochromic.[13,23] Substituted di-
allylammonium derivatives as counter-cations were selected
after functionalisation of the amine moiety (Scheme 2), thus al-
lowing study of the effect of the alkyl chain length of the
counter-cation on the optical properties of N-salicylidene para-
aminobenzenesulfonate salts. The limit of the structural–optical
properties model to predict the occurrence of photochromism
in anil molecules is discussed with a focus on the available free
space around the molecular switch, in addition to the flexibility
of the nearby environment. A preliminary account of the opti-
cal properties of one salt (3a) is also communicated.[25]
Structural studies
Crystal structure of sodium N-salicylidene p-aminobenzene-
sulfonate (1): Compound 1·H2O·MeOH crystallises in a mono-
clinic space group P21/c (Table S1 in the Supporting Informa-
tion). The structure can be seen as a two-dimensional network
in which sodium atoms are linked by bridging water and
methanol molecules or by a bidentate bridging sulfonate
group (Na21ÀO2 and Na21ÀO3, respectively 2.356(2) and
2.399(2) Š; Figure 1). Each sodium centre sits in a slightly dis-
torted octahedral coordination sphere (distortion parameter
Results
Synthesis and crystallisation
The synthesis of N-salicylidene
p-aminobenzenesulfonate
sodium salt (1)[23b] was adapted
to yield N-salicylidene p-amino-
Scheme 3. Synthesis of the substituted diallylammonium salt of N-salicylidene para-aminobenzenesulfonate.
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not involved in intermolecular
hydrogen bonds and therefore
allow the thermal keto–enol
equilibrium.
Crystal structure of diallylam-
monium N-salicylidene p-ami-
nobenzenesulfonate (2): Com-
pound 2 crystallises in the or-
thorhombic space group Pbcn
(Table S1 in the Supporting In-
formation). The asymmetric unit
of the cell presents two dially-
lammonium cations and two
anil sulfonate anions in their
enol form (Tables S2 and S4 in
the Supporting Information;
Figure 3).
Both anils are twisted with
regard to the dihedral angles
between their aromatic rings.
The dihedral angle of anion 1 is
Figure 1. Left: ORTEP view of the asymmetric part of the unit cell of 1·H2O·MeOH, showing displacement ellip-
soids at the 50% probability level. Right: Crystal packing projection of 1 along the a axis showing the 2D coordi-
nation network.
S=97(1)8)[26] (Figure 2). The anil molecule is planar with a dihe-
dral angle between two aromatic rings lower than 258 (F=
9.59(14)8). The planarity of the molecule is confirmed by the
three torsion angles G1, G2 and G3 (respectively À10.6(4),
180.0(2) and À2.7(4)8; Table S5 in the Supporting Information).
The dense crystal packing results from three intermolecular
hydrogen bonds at 1.99(4), 2.08(3) and 2.12(3) Š (Table S3 in
the Supporting Information) and two additional T-shaped CÀ
H···p interactions that include aromatic rings at 2.83 and 2.88 Š
(CÀH(H) centroid distance). The enol and imine functions are
Figure 3. ORTEP view of the asymmetric part of the unit cell of 2, showing
displacement ellipsoids at the 50% probability level.
F1 =26.29(12)8. The twisting of this anion is also observed
through torsion angles G1, G2 and G3 (À30.3(3), 177.4(2),
3.5(4)8, respectively). Anion 2 is even more twisted than
anion 1 with a dihedral angle between aromatic rings of F2 =
34.95(12)8 (torsion angles: G4, G5 and G6 of 35.6(3), À178.8(2)
and À0.3(4)8, respectively).
The crystal structure of 2 is organised by intermolecular in-
teractions (Table S3 in the Supporting Information). Contrary to
1, there is no coordination of the cation to oxygen atoms.
Crystal structure of diallylhexylammonium N-salicylidene p-
aminobenzenesulfonate (3): The crystal structure of 3a pres-
ents one diallylhexylammonium cation and one N-salicylidene
para-aminobenzenesulfonate in the monoclinic P21 space
group (Figure 4). With its dihedral angle F of 17.69(12)8, N-sali-
cylidene p-aminobenzenesulfonate can be considered as nearly
planar. The planarity of the molecule is also confirmed through
the three torsion angles G1, G2 and G3 (respectively 17.8(4),
Figure 2. View of the octahedral coordination sphere of Na, formed by two
water molecules, two methanol molecules and two sulfonate groups of the
anions, within the crystal packing of 1·H2O·MeOH.
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Figure 4. Top: ORTEP view of the asymmetric part of the unit cell of 3a and 3b, showing displacement ellipsoids at the 50% probability level. Bottom: Com-
parison of the crystal packings along the b axis.
178.8(2) and À1.0(4)8). The crystal packing presents two inter-
molecular hydrogen bonds at 1.98(1) and 2.51(1) Š (Table S3 in
the Supporting Information), as well as various closed contacts
and can be regarded as “open” according to the Hadjoudis
classification.[7]
sured in anion 2 (F3 =22.85(13)8 and torsion angles: G7, G8 and
G9 of À23.7(4), 178(3) and 1.3(6)8, respectively). Anion 3 is
slightly more twisted with a dihedral angle between aromatic
rings F4 =25.71(13)8 (torsion angles G10, G11 and G12: À24.3(4),
178.1(3) and 1.0(5)8, respectively). Six intermolecular hydrogen
Compound 3b crystallises in the monoclinic space group
P21/c. One diallylhexylammonium cation and one anil anion
are observed in the asymmetric part of the unit cell (Figure 4).
The anion is twisted with regard to the dihedral angle be-
tween aromatic rings of 25.39(11)8, as well as torsion angles of
G1, G2 and G3 (respectively 21(1), À172(1) and À1 (1)8).
Compound 3b presents only one strong intermolecular hy-
drogen bond, contrary to 3a, which presents two intermolecu-
lar hydrogen bonds (Table S4 in the Supporting Information).
Crystal structure of diallyldecylammonium N-salicylidene p-
aminobenzenesulfonate (4): Compound 4 crystallises in the
monoclinic space group Cc. Three diallyldecylammonium cat-
ions in addition to three anil anions are observed in the asym-
metric part of the unit cell (Figure 5). Anion 1 is slightly twisted
with a dihedral angle between aromatic rings F1 =23.28(13)8,
which is confirmed by inspecting torsion angles G1, G2 and G3
(À23.9(6), À179.0(3) and 0.2(6)(1)8, respectively). The same di-
hedral angle between aromatic rings as for anion 1 is mea-
Figure 5. ORTEP view of the asymmetric part of the unit cell of 4, showing
50% probability displacement ellipsoids; carbon atom labels are omitted for
clarity.
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bonds are observed between the molecules (Table S3 in the
Supporting Information), which lead to a highly organised crys-
tal structure. The C18ÀO19 and N11ÀC12 distances are shorter
in the anions of 4 than in other compounds (Table S4 in the
Supporting Information). Nonetheless, the distances relative to
the intramolecular hydrogen bonds (Table S3 in the Supporting
Information) indicate that the enol form is mainly observed in
the crystal structure of 4.
Crystal structure of diallyldodecylammonium N-salicylidene
p-aminobenzenesulfonate (5): The crystal structure of 5 (mon-
oclinic space group P21/c) reveals one N-salicylidene p-amino-
benzenesulfonate anion, one diallyldodecylammonium cation
and one MeOH molecule in the asymmetric part (Figure 6). The
Figure 7. ORTEP view of the asymmetric part of the unit cell of 6, showing
displacement ellipsoids at the 50% probability level.
Figure 6. ORTEP view of the asymmetric part of the unit cell of 5, showing
displacement ellipsoids at the 50% probability level.
is almost planar with a dihedral angle between aromatic rings
F1 =14.66(16)8. The planar character is also observed through
torsion angles G1, G2 and G3 of À14.7(1), À179.6(6) and 2.3(1)8,
respectively. In addition, one strong intermolecular hydrogen
bond is noticed between the cations and anions (Table S3 in
the Supporting Information).
anion is planar with a dihedral angle between aromatic rings
F1 =4.80(15)8, the planar character being confirmed by torsion
angles G1, G2 and G3 of 6.3(1), À177.9(6) and À2.7(1)8, respec-
tively. Non-coordinated methanol is trapped in the crystal
packing through an intermolecular hydrogen bond and
a second intermolecular hydrogen bond is observed between
cations and anions (Table S3 in the Supporting Information).
Optical properties
Thermochromic and photochromic phenomena in 1–6: All
six compounds display thermochromism, from yellow at 298 K
(cis-keto form) to white on cooling at 77 K (enol form;
Figure 8). Exceptions were noticed for 3a and 3b for which
a light orange colour is observed, which calls for the presence
of the trans-keto form, which is interestingly stabilised without
light irradiation at room temperature. Considering the series,
Crystal structure of diallyloctadecylammonium N-salicyli-
dene p-aminobenzenesulfonate (6): Compound 6, which crys-
¯
tallises in the triclinic space group P1, presents one diallylocta-
decylammonium cation and one N-salicylidene p-aminobenze-
nesulfonate anion in the asymmetric unit (Figure 7). The anion
Figure 8. Photographs of tubes containing: top: 1–6 at 298 and 77 K; bottom: 3a to 6 before and after irradiation at l=450 nm.
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five compounds display photochromism in addition to their
thermochromic properties (3a to 6; Figure 8). Indeed, 1 and 2
do not show any modification of colour after irradiation, nei-
ther at l=365 nm nor at 450 nm. Photochromism is observed
from yellow (3b to 6) or yellow-orange (3a) before irradiation
to orange after irradiation at l=450 nm during 30 min. The
orange colour is due to the absorption of the trans-keto form.
The colour change is strong for 3a, 3b, 5 and 6, but weaker
for 4 (Figure 8).
Diffuse reflectance spectroscopy: All compounds were ana-
lysed by diffuse reflectance spectroscopy (DRS) to confirm the
presence of possible conformations of N-salicylidene aniline p-
aminobenzenesulfonate derivatives (Figure 9). Three areas can
be defined: the white enol form, which is usually observed
below about 400 nm, the yellow cis-keto form between about
400 and 525 nm, and the red trans-keto form above approxi-
mately 525 nm.
Figure 9. Comparison between diffuse reflectance spectra of 1–6.
of the keto form under the same experimental conditions as
for 2 (Figure 10), but a different relaxation behaviour for the
trans-keto form is noticed (Figure 10). Indeed, for 3a, only 12%
of the trans-keto form is reverted back to the cis-keto form
after 8000 min (k3a =2.4”10À7 sÀ1) whereas 50% of the trans-
keto form relaxed for 3b (k3b =2.2”10À6 sÀ1). The trans-keto
form is thus less stable in 3b than in 3a (k3b >k3a).
The photo-reversibility of the cis–trans isomerisation of the
keto form in 3a and 3b was confirmed by time-dependent
measurements (Figure 11). The reflectance was measured at
l=550 nm using irradiation cycles at l=450 nm during
30 min for the cis!trans isomerisation and at l=546 nm
during 30 min for the reversible photoreaction.
For 1, the enol form is the most representative but a weaker
band is noticed between 400 and 500 nm corresponding to
the cis-keto form, which explains the very light yellow coloura-
tion of 1 at 298 K (Figure 8).[23b] For 2, the contribution of the
cis-keto form is much more important, which is confirmed by
the pronounced yellow colour at 298 K (Figure 8). Both diffuse
reflectance spectra remain unchanged after irradiation at l=
450 nm during 20 min (Figure S1 in the Supporting Informa-
tion). The trans-keto form was detected for both 3a and 3b.
Interestingly, polymorphs 3a and 3b show remarkable photo-
chromic properties accompanied by a cis–trans isomerisation
Figure 10. Top: Diffuse reflectance spectra of 3a (left) and 3b (right) before (black) and after (grey) irradiation at l=450 nm during 20 min. Circles highlight
the increase of trans-keto contribution, as shown in the inset. Bottom: Time dependence of the Kubelka–Munk function of 3a (black) and 3b (grey) at
l=550 nm showing thermal relaxation after light irradiation.
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Figure 11. Time dependence of the Kubelka–Munk function of 3a (left) and 3b (right) at l=550 nm showing the reversibility of the cis–trans isomerisation
with irradiation cycles at l=450 and 545 nm.
Upon irradiation, 4 and 5 show a slight increase of a new
band above 520 nm corresponding to the contribution of the
trans-keto form (Figure 12). This form is much less stable than
for 3a and 3b as concluded from time-dependent measure-
ments (Figure 13). Indeed, the trans-keto form is completely re-
laxed after 100 min for 4 (k=6.7”10À4 sÀ1) and 120 min for 5
(k=4.7”10À4 sÀ1).
slowly, thus indicating that the trans-keto is re-formed. A simi-
lar phenomenon, although more pronounced, was observed
for
N-salicylidene-4-amino-3,5-bis(pyridine-2-yl)-1,2,4-triazole,
which was attributed to the formation of a cyclised intermedi-
ate predicted by computational means (spring-type effect[20]).
For 4 and 5, the back reaction may be explained by a resistance
of the surrounding of the anion to the isomerisation, packed
in the crystal lattice. The trans-keto form is then regenerated
after the forced formation of a cis-keto form.
Photo-reversibility was checked by time-dependent meas-
urements using photo-cycles at 450 nm during 30 min for the
cis!trans isomerisation and at 545 nm for the back formation
of the cis-keto form (Figure 13). Interestingly, after irradiation
at l=545 nm, the intensity of the band at 550 nm increases
The last compound 6 also displays photochromism, which
was observed in the DRS spectra, with the rising of a new
band above 520 nm as a result of the trans-keto form popula-
Figure 12. Top: Diffuse reflectance spectra of 4 (left) and 5 (right) before (black) and after (grey) irradiation at l=450 nm during 20 min. Circles highlight the
increasing trans-keto contribution. Bottom: Time dependence of the Kubelka–Munk function of 4 (left) and 5 (right) at 550 nm showing thermal relaxation
after irradiation at l=450 nm during 20 min.
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Figure 13. Time dependence of the Kubelka–Munk function of 4 (left) and 5 (right) at 550 nm showing the reversibility of the cis–trans isomerisation with irra-
diation cycles at 450 and 545 nm.
tion (Figure 14). The thermal stability of this form, recorded by
time-dependent DRS, shows its full disappearance after
7000 min (k=1.1”10À5 sÀ1). The photo-reversibility of the cis–
trans isomerisation was also demonstrated (Figure 14). As no-
ticed for 4 and 5, a slight increase of the Kubelka–Munk func-
tion, due to the spontaneous formation of the trans-keto form,
is observed.
tance spectrum corresponding to the band maximum of the
prototropic form of the N-salicylidene derivative to be ana-
lysed, and the emission wavelength is scanned.[8b] The spec-
trum contains all emissions driven from the generated excited
form (e.g., irradiating at lexc =450 nm leads to the enol* form
and emissions of enol*!enol (A) and cis-keto*!cis-keto (B), as
depicted in Scheme 4). In the excitation mode, the emission
wavelength is fixed at its maximum and an excitation spec-
trum is scanned. For example, we shall focus hereafter on the
cis-keto*!cis-keto emission at lem =540 nm and scan excita-
tion wavelengths. The excitation spectrum is expected to
reveal two bands corresponding to pathways C (enol!enol*!
cis-keto*) and D (cis-keto! cis-
Solid-state fluorimetry: Crystals of the six compounds were
analysed by solid-state fluorimetry to study the photochemical
processes involved. In emission measurements, the excitation
wavelength is locked on a selected area of the diffuse reflec-
keto*) leading to the formation
of the monitored emitting excit-
ed state (Scheme 4).
Molecules 1–6 were analysed
using an excitation wavelength
of lexc =450 nm (Figure 15). The
analysis of emission spectra
shows that the fluorescence of
N-salicylidene aniline sulfonate
derivatives is similar, with differ-
ences observed for the poly-
morphs of 3. As an example,
1 presents an emission maxi-
mum at 540 nm, which is due
to the enol!enol* and cis-
keto!cis-keto* formations, and
another band at 660 nm, which
is due to the radiative relaxation
of the trans-keto* form. Such
a trans-keto!trans-keto* transi-
tion was already observed in
non-photochromic molecules of
this substance class.[8b] For 2,
the emission maximum is re-
Figure 14. Top: Left: Diffuse reflectance spectra of 6 before (black) and after (grey) irradiation at 450 nm during
20 min. The circles highlight the increase of the trans-keto contribution. Right: Time dependence of the Kubelka–
Munk function of 6 at 550 nm showing thermal relaxation after irradiation at 450 nm during 20 min. Bottom:
Time dependence of the Kubelka–Munk function of 6 at l=550 nm showing the reversibility of the cis–trans iso-
merization with irradiation cycles at l=450 and 545 nm.
corded at 550 nm. Two major
contributions are observed at
475 and 520 nm in the excita-
tion spectrum at lexc =550 nm.
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Discussion
The quest for N-salicylidene aniline derivatives that crystallise
as single crystals is of major importance to determine optical–
structural property relationships that will favour the crystal en-
gineering of new materials with desired functional properties.
In this context, we have prepared a series of crystals with vari-
ous substituted diallylammonium counter-cations and N-salicy-
lidene aniline sulfonate counter-anions. This approach con-
trasts with classic studies in which modifications were made
either on the anil derivatives by chemical substitution[27,28] or
on their immediate surroundings, for instance by inserting
a switch into zeolites[17,18] or by dispersion into a given poly-
mer,[13] ways that are less controllable than a direct electrostat-
ic interaction as in the present case. Modifying the surround-
ings of N-salicylidene p-aminobenzenesulfonate is indeed ex-
pected to impact the optical properties of the molecular
switch.
Scheme 4. Photochemical processes evidencing two radiative relaxation
pathways in emission mode (A, B) and two absorption pathways in excita-
tion mode (C, D) for the N-salicylidene aniline derivatives under consider-
ation.
Compounds 1–6 present ther-
mochromism on cooling from
room temperature, which was
expected from the analysis of
their crystal structures, which
reveal an intramolecular hydro-
gen bond between the alcohol
function and the amine moiety
of the N-salicylidene p-amino-
benzenesulfonate anion in its
enol form (Tables S2 and S4 in
the Supporting Information).
This intramolecular hydrogen
Figure 15. A) Emission spectra of solid samples of 1 to 6 (lexc =450 nm) at 298 K. B) Excitation spectra of solid
samples of 1 (lem =540 nm), 2 (lem =550 nm), 3a, 3b, 4 and 6 (lem =600 nm), and 5 (lem =580 nm) at 298 K.
bond favours the tautomeric
equilibrium between enol and
cis-keto forms, and therefore the
These contributions are due to the cis-keto!cis-keto* transi-
tion for the two anions present in the asymmetric part of the
crystal structure (Figure 3). Two weak contributions are also no-
ticed at 445 and 350 nm, which are due to the enol!enol*
transitions for the same anions. The main emission band is ob-
served at 600 nm for 3a and 3b with two other bands at 550
and 660 nm. Both polymorphs present the same three contri-
butions in the excitation spectra (lexc =600 nm). The main con-
tribution observed at 400 nm corresponds to the enol!enol*
transition, the second (at 475 nm) is associated with the cis-
keto!cis-keto* transition and a weak band is observed at
560 nm, which is due to the trans-keto!trans-keto* transition.
For 3a and 3b, the presence of this particular transition may
be due to the stabilisation of the trans-keto form in the
ground state as observed in the DRS spectra (Figure 10). Com-
pounds 4 and 6 present an emission maximum at 600 nm,
which originates from the enol!enol* and cis-keto!cis-keto*
transitions that are found at 400 and 500 nm, respectively, in
the excitation spectra. No trans-keto!trans-keto* transition
was detected in the excitation spectra. Finally, 5 presents an
emission band at 580 nm. This emission is also due to the con-
tributions of the enol!enol* and cis-keto!cis-keto* transi-
tions (respectively at 400 and 500 nm).
colour change. Prediction of the photochromic properties of
anil derivatives is usually guaranteed by considering the dis-
torted character of the switching molecule (with a dihedral
angle between aromatic moieties F>258) and a crystal pack-
ing of “open”-type nature.[7] Several studies have, however,
pointed out the limit of this structural prediction.[8,21,29] A simi-
lar situation is found in the present work. Indeed, although the
prediction is confirmed for 2, 3b and 4, which present an aver-
age dihedral angle higher than F>258 (35(1), 25(1) and 26(1)8,
respectively; Figure 17), 2 is not photochromic. On the other
hand, out of the planar molecules 1, 3a, 5 and 6, all are photo-
chromic except 1. The latter situation is in agreement with the
crystal packing prediction because this material presents an in-
termolecular distance (3.4 Š) with five stabilising interactions
(three H-bonds and two CÀH···p interactions), which qualifies
this packing as “closed”. “Open”-type structures are found for
2–6 with an intermolecular distance higher than 3.5 Š, but 2 is
not photochromic. Considering such noticeable limitations to
predict the optical properties of anils, the search for a more re-
liable tool was undertaken in the present work.
A significant trend was identified by comparing the size of
the counter-cation of the anil derivatives containing a substitut-
ed diallylammonium group (2–6), evaluated through the pro-
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jected length of the chain, with their optical properties. As ex-
pected, an overall increase of the length of the cation from 7.3
to 27.2 Š with the increase of lateral alkyl chain length is no-
ticed, as a result of the linear character of diallylammonium
salts (Figure 16). These lengths were compared to that of the
N-salicylidene p-aminobenzenesulfonate anion with an average
size of 12.3 Š.
Figure 17. Comparison between the crystal packing of 2 (top), 4 and 5
(middle), and 6 (bottom). Anions are depicted in light grey and cations in
dark grey.
a bilayer lamellar structure is observed with an inter-planar dis-
tance of 34.1 Š. Noticeably, a lamellar structure with an inter-
chain distance of 32.5 Š was postulated from X-ray powder dif-
fraction analysis of the copolymer synthesised from substituted
diallylamine, that is, poly(N,N-diallyloctadecylamine-alt-maleic
acid) (CopoC18H).[30] We thus present the first indirect proof of
this organisation, which was discussed in earlier works.[31,32]
Considering, as discussed above, the limit of the structural
considerations to predict optical properties of N-salicylidene
derivatives, we introduce the determination of the average
free available space Vfree around each switching molecule in
the crystal lattice, as a complementary criterion. This free
space is important because it allows the local rearrangement
of molecules during the cis–trans isomerisation, thereby pro-
voking photochromism. The free volume Vfree was evaluated by
considering the unit cell volume, the anion and cations vol-
umes as well as Z, the number of molecules in the unit cell
[Eq. (1)]:
Figure 16. Top: Evolution of size of the diallylammonium cations relative to
the size of the anion in 2–6. All salts display thermochromism (depicted in
light grey for 2), whereas 3–6 display photochromism (dark grey bars).
Bottom: Comparison between the size of diallylammonium cations 2–6 and
the N-salicylidene p-aminobenzenesulfonate anion of 12.3 Š. The dotted line
indicates a cation/anion ratio of one. The cation size of 4–6 clearly exceeds
that of the anion.
Analysis of the crystal packing of 2 (Figure 17), 3a and 3b
(Figure 4) reveals that flexible substituted diallylammonium
cations are organised around the anions. A reverse situation is
met when the size of the cation exceeds that of the anion as
found for 4–6. In this case, the anions are inserted between
the cationic planes in a regular fashion. Actually, a transition is
observed from a monolayer (for 4) with a head-to-tail arrange-
ment of molecules, to an intermediate situation (5) up to a bi-
layer one for 6, with a head-to-head arrangement of molecules
(Figure 17). Indeed, in each crystal structure, N-salicylidene p-
aminobenzenesulfonate is parallel to the alkyl chain of the
substituted diallylammonium unit and has its sulfonate moiety
electrostatically linked to the ammonium part of the counter-
ion. In the crystal packing of 4–6, a segregation of the ionic
part of the molecules is noticed leading to the separation of
lipophilic domains. This separation is stronger for 6, in which
Vunit cell À Z Â ðVanion þ Vcation
Þ
ð1Þ
Vfree
¼
Z
The anion and cation volumes were evaluated using the
Molinspiration platform based on group contributions.[33] By
considering the free volume, we determined that photochrom-
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Figure 18. Evolution of several structural parameters for 1–6. A) Anil maximum dihedral angle [8]. The black line corresponds to the 258 limit as discussed in
the text. B) Minimum distance between molecules [Š]. The black line points out 3.5 Š intermolecular interactions. C) Number of interactions. D) Free
space [Š]. Compound 3a was taken as reference being the first photochromic molecule of the series with the lower free volume (1608). Light grey bars corre-
spond to compounds presenting only thermochromism, dark grey bars represent photochromic species.
3
ism is observed when Vfree exceeds 171 Š (for 3–6) whereas
this context, it is interesting to note that the trans-keto form is
stabilised in 3a and 3b right after synthesis without any light
irradiation. A stabilised trans-keto form was also observed in
blend materials obtained by the insertion of N-salicylidene ani-
line sulfonate derivatives inside a polymer matrix (CopoC6H),
but no photochromism was observed in such materials.[13]
3
thermochromism is exclusively observed when Vfree ꢀ153 Š
(for 1 and 2; Figure 18). This criterion is of interest for predict-
ing optical properties. Indeed, considering the case of an
“open” structure in which the anil molecules are packed in an
open crystal lattice and reveal a large intramolecular distortion,
photochromism is not necessarily observed if the new free
volume condition is not fulfilled (like for 2). This criterion is
thus of major importance for a valuable prediction.
The flexibility of the local surroundings around the switching
molecule may also explain the back reaction observed in
photo-relaxation experiments for 4 and 5. Indeed, during the
thermal relaxation, only a fraction of the trans-keto form fully
relaxes to the cis-keto form and the other fraction is trapped
by the surrounding environment. This is seen after irradiation
at l=545 nm of 5; the trans!cis isomerisation proceeds but
a fraction of the cis-keto form relaxes back to the trans-keto
form, as noticed in Figure 19 for t>100 min. Such a process,
which occurs without relaxation to the cis-keto form, contrary
to the spring-type effect,[20] is observed for the first time for an
N-salicylidene derivative.
This phenomenon may be the signature of a cooperative
mechanism that allows the stabilisation of the trans-keto form,
usually considered as metastable at room temperature.[7] In
Figure 19. Time dependence of F(R) for 5 at l=550 nm showing the reversi-
bility of the cis–trans isomerisation with irradiation cycle at l=450 and
545 nm, for 30min.
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Scheme 5. Photo-isomerisation mechanism of N-salicylidene p-aniline sulfonate.
Experimental Section
A different isomerisation mechanism is then proposed to ex-
plain the cis–trans isomerisation in a confined volume. Because
the sulfonate interacts electrostatically with the ammonium
cation, the ketone-bearing ring can rotate around with minimal
packing restraint. The presence of intermolecular interaction
and the alkyl surroundings may thus allow the stabilisation of
the trans-keto form (Scheme 5).
Starting materials
Solvents (HPLC grade methanol from Prolabo; [D6]DMSO
99.98% D) and reagents (analytical grade sodium hydroxide from
Fisher Scientific, salicylaldehyde 99%, 5-chlorosalicylaldehyde 98%
from Acros Organics, sulfanilic acid from Sigma–Aldrich, 3-amino-
benzenesulfonic acid from Fluka) were obtained commercially and
used as received. The diallylamine derivatives were synthesised fol-
lowing previously reported procedures.[31]
Another point of interest concerns the stability of these mol-
ecules. All these compounds present a high thermal stability
with a degradation temperature higher than 1008C (2838C for
3, 2848C for 1[23b]). Interestingly, water molecules are observed
inside the crystal structure of 1·H2O·MeOH, which shows that
water is important for the crystallisation of this class of mole-
cules, which are not that sensitive to hydrolysis compared with
N-salicylidene aniline derivatives.[10] The high thermal stability,
in addition to the resistance to hydrolysis, makes this sub-
stance class suitable for applications, for instance in optical
storage and photochromic coatings.
Instrumentation
1H and 13C NMR spectra were recorded on a Bruker AC 300 MHz
spectrometer with DMSO as internal standard. Infrared spectra
were obtained on KBr disks by using an Equinox 55 instrument
from Bruker. Diffuse reflectance spectra were recorded on a Varian
Cary 5E spectrophotometer using PTFE as reference. Spectra were
measured on pure solids to avoid matrix effects and presented as
normalised Kubelka–Munk functions to allow meaningful compari-
sons. The reversibility of the cis–trans isomerisation was checked
with a LOT-ORIEL 200 W high-pressure mercury arc lamp (LSN261).
Ex situ irradiations were carried out for 30 min within a home-
made chamber using appropriate filters (450 and 546 nm).
Conclusion
X-ray powder diffractograms were obtained using a Siemens
D5000 X-ray diffractometer, with Cu radiation (lKa =1.5418 Š).
Two approaches are usually followed to modify the optical
properties of anils: 1) the variation of the nature of the sub-
stituent on the anil; and 2) variation of the surroundings, that
is, by insertion in a given matrix. In this work, we carried out
a systematic study of a series of well-characterised salts and
even induced photochromism in a non-photochromic anion
(3) without modifying its chemical nature, simply by modifying
the counter-cation.
Single-crystal data were collected on a MAR345 image plate, with
MoKa radiation (Xenocs fox3D mirror) generated by a Rigaku
UltraX 18 rotating anode. The reflections on the images were in-
dexed and integrated using the CrysalisPro package (Agilent Tech-
nologies).[34] Data were scaled and corrected for absorption using
the integrated Scale3 Abspack procedure. The structures were
solved by direct methods (SHELXS-97)[35] and refined first isotropi-
cally and then anisotropically using SHELXL-97.[35] Hydrogen atoms
were placed at calculated positions and refined in riding mode
with respect to the parent atoms.
We have also shown that previous considerations on the im-
portance of crystal packing and dihedral angles between aro-
matic rings[6,7] are no longer sufficient to account for optical
properties, and that a new parameter considering the flexibility
of the surrounding environment, represented by the available
free space parameter in the crystal structure, needs to be
taken into account. The flexibility of the local surroundings has
also been considered to explain the spontaneous back reaction
observed in photo-relaxation experiments as a consequence of
the trapping of the anion in the trans-keto form.
CCDC 1040108, 1040109, 1040110, 1040111, 1040112 and 1040113
contain the supplementary crystallographic data for this paper.
These data can be obtained free of charge from The Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_
request/cif.
Calculations
The molecular volume of each compound was calculated using the
Molinspiration free toolkit (www.molinspiration.com).[36] The occu-
pied volume was obtained by multiplying the volume of the anion
and the cation by the number of monomers per aggregate.[37]
This study also demonstrates that increasing the length of
the lateral alkyl chain in diallylammonium derivatives favours
a lamellar organisation, which was earlier suggested for
poly(N,N-diallylamine-alt-maleic acid) copolymers.[31,32] The pos-
sibility of inducing photochromism by a slight change of the
surroundings of an anil molecule, in addition to the ionic char-
acter of such anil derivatives, is promising for the incorporation
of these charged derivatives into hybrid multifunctional materi-
als and in co-crystal synthesis.[33]
Synthesis
Synthesis of N-salicylidene p-aminobenzenesulfonate sodium
(1): Sulfanilic acid (2.89 mmol) and NaOH (0.116 g, 2.89 mmol) were
dissolved in MeOH (20 mL). Vigorous stirring was maintained until
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complete solubilisation of the reagents. Salicylaldehyde
(2.89 mmol) was then added dropwise to the solution. The temper-
ature was increased to reflux (808C) and stirring was maintained
overnight. A yellow solid was formed and then isolated by filtra-
tion. Purification was performed by recrystallisation from hot
MeOH, which afforded yellow crystals. Yield: 66%. 1H NMR
116.55, 54.05, 51.44, 44.30, 42.45, 41.65, 41.15, 31.27, 28.98, 28.89,
28.41, 25.90, 22.96, 22.07, 13.93 ppm; FTIR (KBr disk): n˜ =1617 (C=
N), 1117 (S=O), 714 (SÀOÀ), 641 cmÀ1 (CÀS); elemental analysis
calcd (%) for C31H46N2O4S: C 68.60, H 8.54, N 5.16; found: C 68.42,
H 8.90, N 5.02.
N-Salicylidene p-aminobenzenesulfonate diallyloctadecylammo-
3
1
([D6]DMSO): d=13.04 (s, 1H), 8.98 (s, 1H), 7.68 (d, JHÀH =8.43 Hz,
nium salt (6): M.p. 768C; H NMR ([D6]DMSO): d=8.97 (s, 1H), 7.66
3
3
3
3H), 7.40 (m, 3H), 6.98 ppm (t, JHÀH =7.38 Hz, 2H); FTIR (KBr disk):
(d, JHÀH =8.5 Hz, 3H), 7.37 (m, 3H), 6.97 (t, JHÀH =7.6 Hz, 2H), 5.93
n˜ =1625 (C=N), 1134 (S=O), 730 (SÀOÀ), 651 cmÀ1 (CÀS); elemental
analysis calcd (%) for C13H10NSO4Na: C 52.17, H 3.37, N 4.68; found:
C 51.95, H 3.50, N 4.80.
3
(m, 2H), 5.53 (m, 4H), 3.72 (d, JHÀH =6.7 Hz, 4H), 2.95 (m, 2H), 1.60
(m, 2H), 1.22 (s, 30H), 0.84 ppm (m, 3H); 13C NMR: d=163.67,
160.33, 146.96, 133.36, 132.63, 126.79, 120.67, 119.18, 116.60, 54.11,
31.31, 29.07, 28.95, 25.96, 22.12, 13.97, 7.54 ppm; FTIR (KBr disk):
n˜ =1617 (C=N), 1118 (S=O), 717 (SÀOÀ), 642 cmÀ1 (CÀS); elemental
analysis calcd (%) for C37H58N2O4S: C 70.89, H 9.32, N 4.47; found: C
71.00, H 9.80, N 4.21.
Synthesis of N-salicylidene p-aminobenzenesulfonate salts: In
a round flask, diallylamine derivative (5.52 mmol) was dissolved in
MeOH (50 mL). Vigorous stirring at room temperature was applied
and sulfanilic acid (6.07 equiv) was added when the diallylamine
had completely dissolved. The stirring was maintained until no
more acid was dissolved, which indicated the end of the reaction.
The solution was filtered to remove the excess of acid and the so-
lution was evaporated under vacuum. The product was then puri-
fied by recrystallisation from hot methanol. The sulfanilate of dially-
lammonium (1.41 mmol) was dissolved in MeOH (50 mL) in
a round flask at room temperature. Salicylaldehyde (1.41 mmol)
was added and vigorous stirring was maintained. The reaction was
followed by 1H NMR spectroscopy and then the solvent was evapo-
rated under vacuum. The product was finally purified by recrystalli-
sation in hot methanol.
Acknowledgements
This work was funded by the ARC AcadØmie Louvain program
(08/13-010), the Fonds National de la Recherche Scientifique-
FNRS and the COST MP1202. We also thank the Fonds pour la
Formation à la Recherche dans l’Industrie et dans l’Agriculture
for a doctoral scholarship allocated to P.-L.J.
N-Salicylidene p-aminobenzenesulfonate diallylammonium salt
Keywords: isomerization
photochromism · tautomerism · thermochromism
·
molecular
devices
·
1
(2): M.p. 1228C; H NMR ([D6]DMSO): d=8.97 (s, 1H), 8.74 (s, 2H),
3
3
7.66 (d, JHÀH =8.5 Hz, 3H), 7.37 (m, 3H), 6.97 (t, JHÀH =8 Hz, 2H),
5.87 (m, 2H), 5.42 (m, 4H), 3.56 ppm (d, 3JHÀH =5.9 Hz, 4H);
13C NMR ([D6]DMSO): d=163.66, 160.27, 147.97, 146.80, 133.34,
132.57, 128.86, 126.74, 122.61, 120.66, 119.28, 119.15, 116.57,
48.05 ppm; FTIR (KBr disk): n˜ =1620 (C=N), 1120 (S=O), 728 (SÀOÀ),
639 cmÀ1 (CÀS); elemental analysis calcd (%) for C19H22N2O4S: C
60.94, H 5.92, N 7.48; found: C 60.42, H 5.73, N 7.38.
[1] a) G. H. Brown, Photochromism, Wiley, New York, 1971; b) J. C. Crano,
R. J. Guglielmetti, Organic Photochromic and Thermochromic com-
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N-Salicylidene p-aminobenzenesulfonate diallylhexylammonium
salt (3): M.p. (TGA-TDA): 112 8C; degradation: 2838C; 1H NMR
([D6]DMSO): d=8.97 (s, 1H), 7.66 (d, 3JHÀH =8.5 Hz, 3H), 7.37 (m,
3
3H), 6.97 (t, JHÀH =8 Hz, 2H), 5.93 (m, 2H), 5.53 (m, 4H), 3.73 (d,
3JHÀH =7 Hz, 4H), 2.97 (m, 2H), 1.61 (m, 2H), 1.25 (s, 6H), 0.85 ppm
(m, 3H); 13C NMR: d=170.68, 163.64, 160.27, 147.92, 133.34,
132.57, 127.00, 126.74, 125.31, 120.63, 119.28, 119.14, 116.55, 54.05,
51.44, 30.58, 25.58, 22.92, 21.81, 13.78 ppm; FTIR (KBr disk): n˜ =
1628 (C=N), 1167 (S=O), 727 (SÀOÀ), 667 cmÀ1 (CÀS); elemental
analysis calcd (%) for C25H34N2O4S: C 65.47, H 7.47, N 6.11; found: C
65.21, H 7.37, N 5.89.
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N-Salicylidene p-aminobenzenesulfonate diallyldecylammonium
salt (4): M.p. 918C; 1H NMR ([D6]DMSO): d=8.97 (s, 1H), 7.66 (d,
3
3JHÀH =8.5 Hz, 3H), 7.37 (m, 3H), 6.97 (t, JHÀH =7.5 Hz, 2H), 5.93 (m,
3
2H), 5.53 (m, 4H), 3.73 (d, JHÀH =7 Hz, 4H), 2.96 (m, 2H), 1.61 (m,
2H), 1.23 (s, 14H), 0.84 ppm (m, 3H); 13C NMR: d=170.68, 163.66,
160.31, 147.93, 132.60, 132.60, 127.04, 127.04, 126.76, 125.33,
120.65, 119.32, 116.59, 114.59, 54.09, 51.48, 37.00, 31.29, 28.88,
25.93, 23.00, 22.09, 21.00, 17.82, 13.96 ppm; FTIR (KBr disk): n˜ =
1617 (C=N), 1118 (S=O), 715 (SÀOÀ), 639 cmÀ1 (CÀS); elemental
analysis calcd (%) for C29H42N2O4S: C 67.67, H 8.22, N 5.44; found: C
67.46, H 8.57, N 5.36.
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N-Salicylidene p-aminobenzenesulfonate diallyldodecylammoni-
1
um salt (5): M.p. 688C; H NMR ([D6]DMSO): d=8.97 (s, 1H), 7.66
3
3
(d, JHÀH =8.5 Hz, 3H), 7.37 (m, 3H), 6.97 (t, JHÀH =7.6 Hz, 2H), 5.93
3
(m, 2H), 5.53 (m, 4H), 3.73 (d, JHÀH =7 Hz, 4H), 2.96 (m, 2H), 1.60
(m, 2H), 1.22 (s, 18H), 0.84 ppm (m, 3H); 13C NMR: d=163.63,
160.27, 146.89, 133.33, 132.57, 126.74, 125.28, 120.63, 119.28,
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Received: December 19, 2014
Published online on March 12, 2015
Chem. Eur. J. 2015, 21, 6832 – 6845
www.chemeurj.org
6845
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