Trans-keto* form detection in non photochromic N-salicylidene aminomethylpyridines

Article abstract of DOI:10.1039/c2ce00006g

Five N-(5-chloro)salicylidene aminomethylpyridine derivatives were successfully synthesized and their structural and solid state optical properties analyzed. Yellow crystalline powders of CH₂L³ and CH ₂L^3-4Cl pr

Full text of DOI:10.1039/c2ce00006g

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PAPER
Trans-keto* form detection in non photochromic N-salicylidene
aminomethylpyridines{{
Fran c¸ ois Robert, Pierre-Lo ¨ı c Jacquemin, Bernard Tinant and Yann Garcia*
Received 3rd January 2012, Accepted 5th April 2012
DOI: 10.1039/c2ce00006g
Five N-(5-chloro)salicylidene aminomethylpyridine derivatives were successfully synthesized and their
3
structural and solid state optical properties analyzed. Yellow crystalline powders of CH L and
2
3
L
–4
2
4
CH
2
2 2
Cl present both thermo- and photochromism whereas CH L and CH L do not show any
switching properties, as probed by diffuse reflectance spectroscopy. Contrary to structural–optical
2
expectations, the non-photochromic CH L molecule (P2 /n) shows an open crystal structure, and
2
1
3
–4
photochromic molecules CH
2
L
1
Cl (P2 /c) present a closed crystal packing, revealed by single
crystal X-ray diffraction, which is typical of exclusively thermochromic molecules. After UV
2
4
2 2
irradiation, trans-keto* emission observation in CH L and CH L indicates the unexpected
formation of the trans-keto form in these non-photochromic anil molecules. Radiative relaxation of
*
the trans-keto form is in addition detected, for the first time, by fluorimetry for all N-salicylidene
molecules of the series, whatever their switchable chromic properties.
metastable trans-keto form thanks to a cis/trans photo-isomerisa-
Introduction
tion of the C–N bond (Scheme 1).
Solid state photochromism corresponds to a reversible photo-
induced equilibrium between two species presenting different
The photochemical process associated with photochromism
24–26
has been described.
UV irradiation of the enol form leads to
1
absorption spectra. This switching phenomenon, which is
an excited enol form (enol*) that can fluoresce or rapidly
undergo a fast tautomerisation to produce the excited cis-keto*
form. The cis-keto* form can also fluoresce or undergo photo-
isomerisation to produce the excited trans-keto* form by photo-
isomerisation, which is described to be non-fluorescent. Non-
radiative relaxations enol* A enol and cis-keto* A cis-keto and
potentially encountered in inorganic/organic species, can have
2
several origins such as a redox reaction, electron–hole pair
3
generation, spin crossover, photocyclization, coordination
4
5
6
change, photo-isomerisation, or both a tautomerisation and
7
8
9
bond cleavage, which can be associated to a reversible modifica-
tion of the geometric structure, dielectric constant, magnetic
8
susceptibility,
radiative relaxation trans-keto* A trans-keto have been con-
,10,11
5
and refractive index. Organic photochro-
24–26
sidered up to now as negligible.
Back reaction from trans-
5
mism, which is rarely observed in the crystalline state, has been
keto to cis-keto and enol forms is either thermally or
photochemically induced.
1
2,13
14
optimized for information storage,
1
photo optical switches, displays, sensors, and non-linear
molecular machines,
17
5
16
N-salicylidene aniline derivatives have been distinguished, on a
1
8
optics, and therefore still attracts considerable attention. Among
the rare classes of molecules that exhibit both thermo and
photochromism in the crystalline state, N-salicylidene aniline can
first approximation, by comparing their crystal structure and
27
optical properties at room temperature.
Thermochromic
molecules present a planar conformation, with a low dihedral
1
9–22
be highlighted.
Thermochromism, which corresponds to a
2
3
temperature-induced reversible colour change, results from a
tautomeric thermal equilibrium between the uncoloured enol form
and the yellow cis-keto form. Irradiation of the enol and cis-keto
forms at l = 365 nm and 436 nm, respectively, produces the red
Institute of Condensed Matter and Nanosciences, MOST – Inorganic
Chemistry, Universit e´ Catholique de Louvain, Place L. Pasteur 1, 1348,
Louvain-la-Neuve, Belgium. E-mail: yann.garcia@uclouvain.be;
Fax: +32(0) 1047 2330; Tel: +32(0) 1047 2831
{ This work is dedicated to the memory of our colleague, Prof. Jean-
Louis Habib Jiwan.
{
diffractograms of CH
Electronic supplementary information (ESI) available: X-ray powder
3
L Cl and CH
4
L Cl. CCDC reference numbers
2
2
2
L ) and 854916 (CH
3
L Cl). For ESI and crystallographic
Scheme 1 Thermochromism and photochromism pathways of N-salicylidene
854915 (CH
data in CIF or other electronic format see DOI: 10.1039/c2ce00006g
2
3
aniline.
4
396 | CrystEngComm, 2012, 14, 4396–4406
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View Online
angle between aromatic planes below 25u, and are closely packed
in their crystal lattice. Such structures are stabilized by p–p and/
to residual water (provoking hydrolysis leading to salicylalde-
hyde impurities) or high temperature (leading to a very fast
degradation) and a modified synthetic procedure had then to be
developed. A Dean–Stark set-up was used with benzene (instead
of toluene in order to reduce the reflux temperature) and the
resulting crude yellow oil was purified by several sonication
cycles in hexane. Target molecules were obtained in large
2
8
or CH–p interactions. In contrast, photochromic molecules are
defined by their non-planar character, with a dihedral angle
greater than 25u, and an open crystal packing which is
favourable for a conformation change, allowing the stabilization
2
9
of the trans-keto form. Nevertheless, N-salicylidene aminopyr-
3
idine derivatives have revealed intriguing features.
0
amounts with excellent purity as yellow microcrystalline
4
L , which was amorphous as concluded
3
N-salicylidene-3-aminopyridine (L ) first displays a close packing
powders, except for CH
2
and a planar geometry, but presents both thermochromism and
from powder X-ray diffraction (PXRD). 5-Chlorosalicylidene-2-
2
photochromism at room temperature. Secondly, N-salicylidene-
4
-aminopyridine (L ), which is highly twisted and packed in an
aminomethylpyridine (CH L Cl) was prepared similarly but
2
4
could not be isolated as a pure compound.
3
0
open structure, is exclusively thermochromic.
Theoretical
Single crystals were obtained after one week, as yellow blocks
2
3
3
studies of those model systems allowed to discuss the photo-
isomerization mechanism inside the crystal lattice and to
compute energies for cis and trans-keto forms in both ground
state and excited states, allowing to predict the best conditions
for photochromism observation when the activation barrier
(for CH L , CH L ) and yellow plates (for CH L Cl), by
2 2 2
recrystallization from a hot hexane mother solution followed
by slow evaporation, and analyzed by X-ray diffraction (see next
section).
2
8,30
between cis-keto* and trans-keto* forms is low.
Structural aspects
In this work, we have studied the influence of the increase of
flexibility and electronic conjugation loss among N-salicylidene
aminopyridine derivatives on the optical properties after the
introduction of a methylene spacer between salicyl and pyridine
rings (Scheme 2), as first postulated by Hadjoudis et al. for
Crystal structure of N-salicylidene 2-amino-methylpyridine
2
2
(
CH L ). The CH L molecule crystallizes in the monoclinic
2 2
space group P2 /n at 125(2) K (Table 1). The bond lengths C2–
1
˚
O16 (1.352(2) A) and C1–C7 (1.458(2) A) indicate single bonds
˚
˚
whereas the bond length C7–N8 of 1.278(2) A (Table 2) is typical
2
7
N-salicylidenebenzylamines. More precisely, N-salicylidene-2-
2
aminomethylpyridine (CH L ), N-salicylidene-3-aminomethylpyri-
of a double bond which identifies the enol form (Fig. 1a). The
dihedral angle between the two aromatic rings (W = 81(1)u) is
much higher than the usual range of angles observed in
2
3
4
dine (CH
2
L ), N-salicylidene-4-aminomethylpyridine (CH
2
L ),
L Cl) and 5-chlor-
2
L Cl) were prepared,
3
5
-chlorosalicylidene-3-aminomethylpyridine (CH
2
3
0
4
N-salicylidene aniline aminopyridine derivatives (5–49u). It
could take its origin from the increase in molecular flexibility
induced by the methylene spacer between aromatic rings, which
implies a large torsion angle C of 114(1)u (Table 2). An
intramolecular hydrogen bond is observed between imine and
osalicylidene-4-aminomethylpyridine (CH
characterized and investigated by using complementary methods
such as X-ray diffraction, UV-visible diffuse reflectance spectroscopy
and fluorimetry. Increasing flexibility of these molecules allows the
unexpected fluorescence of the trans-keto form of a functional Schiff
base derived from aniline, to be detected, for the first time, at room
temperature.
…
˚
alcohol functions (O16–H16 N8) with a distance of 1.74(3) A
…
(
Table 3). Two intermolecular C–H p interactions are also
2
L , CH
3
L and
Table 1 Crystal data and structure refinement for CH
3
L Cl
2
2
Results and discussion
CH
2
2
3 31
3
CH L Cl
Syntheses and crystallization
CH L
2
CH L
2
2
The synthesis of N-salicylidene aminomethylpyridine derivatives
Empirical
formula
M
C13H12N O
2
C13H12N O
2
C13H11N OCl
2
was initially carried out following reported procedures (e. g.
3
1–33
r
212.25
125(2)
212.25
297(1)
246.69
120(2)
Monoclinic
reflux in ethanol followed by recrystallization
and solvent
T/K
3
free synthesis ). The products appeared however to be sensitive
4
Crystal system Monoclinic
Space group P2 /n
Monoclinic
/n
1
P2
1
1
P2 /c
˚
a/A
9.863(3)
10.363(3)
11.563(3)
113.28(2)
1085.6(5)
4
1.299
448
0.084
10.49(1)
9.002(4)
12.42(1)
109.10(4)
1109(2)
4
1.271
448
0.08
13.864(4)
6.004(2)
14.041(4)
103.95(2)
1134.3(6)
4
1.445
512
0.32
˚
b/A
˚
c/A
b (u)
˚
˚
V/A
Z
2
3
)
rcalcd/Mg m
F(000)
m/mm
21
Crystal size/mm 0.4 6 0.3 6 0.3 0.1 6 0.3 6 0.4 0.5 6 0.4 6 0.1
h
max (u)
26.37
8564/2120
25.00
2411/886
26.39
19 898/2257
Reflections
collected/unique
R
R
int
0.045
0.053
0.049 [156]
—
0.053
1
[I . 2s(I)]
0.0475 [1931]
0.123
0.353 and 20.214 —
0.0409 [2167]
0.113
0.251 and 20.287
wR
Largest diff.
peak/hole
2
Scheme 2 Molecular schemes of N-salicylidene aminopyridines and
N-salicylidene aminomethylpyridines where
a methylene spacer is
introduced between the two aromatic rings.
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2
L , CH
3
…
intramolecular hydrogen bond (O16–H16 N8) with a distance
Table 2 Selected bond lengths and angles for CH
3
L Cl
2
2
L
and
CH
2
˚
of 1.81(3) A (Table 3) that can be considered similar to the
2
L
3 31
2
and CH L . Several
2
3 31
3
L Cl
hydrogen bond observed in CH
2
2
CH L
2
CH L
CH
2
intermolecular interactions contribute to the generation of a
dense supramolecular network (Fig. 2b). One intermolecular
W (u)
C (u)
81(1)
114(1)
5.99
1.509(2)
1.459(2)
1.278(2)
1.458(2)
1.410(3)
1.352(2)
90(1)
112(1)
5.51
1.507(7)
1.459(5)
1.269(5)
1.450(7)
1.403(6)
1.348(5)
72(1)
117(1)
5.54
1.512(2)
1.469(2)
1.279(2)
1.463(2)
1.406(2)
1.354(2)
a
hydrogen bond between C13H13 and N112 of two pyridine rings
…
p
˚
Dist. (A)
b
˚
C2–C1 (A)
N–C2 (A)
˚
C3–N (A)
C4–C3 (A)
˚
C5–C4 (A)
˚
C5–O (A)
thus forming supramolecular dimers (Table 3), two C–H
b
˚
interactions between C7–H7 and the benzene ring and between
b
b
b
C15–H15 and the pyridine group, and one p–p stacking (3.826(6)
˚
˚
A) between two benzene moieties (Table 4) lead to a closed
27
b
structure, typical of thermochromic molecules.
3
a
b
Average distance between molecule centroids.
respect to Scheme 2.
Distances with
Single crystals of CH
2
L
were identical to the known
polymorph, which shows an open structure with an intermole-
˚
cular distance between centroids of 5.5 A, as confirmed by X-ray
3
1
4
3
2 2
diffraction. Isostructurality of CH L Cl with CH L Cl was
established by powder X-ray diffraction (Fig. S1, ESI{).
Optical properties
Diffuse reflectance spectroscopy. Yellow crystalline powders of
3
3
4
CH
2 2 2
L , CH L Cl and CH L Cl turn white on cooling to 77 K
(
Fig. 3). On warming back to 298 K, their yellow colour is
recovered. This reversible colour change is due to a thermal
tautomerisation from a yellow cis-keto form (at 298 K) towards
2
3
a white enol form at lower temperature. Irradiation under
visible light (l = 450 nm) for 10 min leads to an orange
coloration for these three molecules, which is best marked for
3
2
4
CH
2 2 2
L Cl (Fig. 4). CH L and CH L however do not present
any thermo/photochromic properties.
Diffuse reflectance spectra were recorded on pure powder
samples and a Kubelka–Munk (KM) treatment was applied
(Fig. 3). It was not relevant dilute these samples, as is routinely
done for other compounds, because matrix effects can drama-
tically modify the keto–enol equilibrium that is effective for
2
L ,
Fig. 1 (a) ORTEP view of the asymmetric part of the unit cell of CH
2
showing 50% probability displacement ellipsoids. (b) View of the crystal
2
2
packing of CH L along the c axis. H atoms were omitted for clarity.
9
,35–38
N-salicylidene aniline molecules,
thus preventing any
present between H14 and centroids of salicyl ring (C1–C2–C3–
˚
C4–C5–C6, d = 2.88 A), and between H7 and pyridine (N11–
˚
C10–C11–C12–C13–C14–C15, d = 2.65 A) (Table 4). The large
reliable comparative study. In each spectrum, a large intense
band, attributed to the enol form, is observed in the UV range. A
bathochromic shift (of about 35 nm) of the right limit of this
2
4
3
˚
2 2 2
band is noted between CH L , CH L Cl and CH L Cl.
intermolecular distance between molecules (y6 A, Table 2)
qualifies the crystal packing as an open structure (Fig. 1b), which
Another band, corresponding to the cis-keto form, is located
2
7
is typical of photochromic molecules.
in the visible region. Although this signal is observed in all
2
L ,
spectra, its relative intensity is different, increasing from (CH
3
L . For CH
2
3–4
4
L Cl and CH
4
L ) , CH
3
L Cl , CH
Crystal structure of 5-chloro-N-salicylidene 3-aminomethylpyr-
3
L Cl was solved at
CH
2
2
2
2
2
L
Cl
3
L Cl). The crystal structure of CH
idine (CH
2
2
two contributions are noted for the cis-keto band (with maxima
at 420 and 460 nm) which are attributed to two different
conformations of the cis-keto form (Fig. 3).
1
20(2) K. The Schiff base crystallizes in the monoclinic space
group P21/c (Table 1) and the enol form is observed in the
3
3
4
asymmetric unit as deduced from bond lengths N8–C7 (1.279(2)
Irradiation at 450 nm of CH L , CH L Cl and CH L Cl leads
2 2 2
2
L ,
˚ ˚
A) and C2–O16 (1.354(2) A) (Table 2). As observed for CH
to a new band around 500 nm attributed to the population of the
metastable trans-keto form resulting from photo-isomerisation
(Fig. 4).
2
the molecule flexibility is responsible for the large distortion
3
2
angle (W = 72(1)u) (Table 2 and Fig. 2a). CH L Cl presents an
2
L , CH
3
L and CH
3
L Cl
Table 3 Intra- and inter-molecular hydrogen bonds in CH
2
2
2
…
˚
D–H (A)
…
˚
A (A)
…
˚
A (A)
D–H…A (u)
Molecule
D–H
A
H
D
Ref.
2
…
O16H16 N8
CH
CH
CH
2
2
2
L
L
0.96(3)
1.02(5)
0.87(2)
0.98(2)
1.74(3)
1.73(5)
1.81(3)
2.56(2)
2.602(2)
2.571(5)
2.616(2)
3.459(2)
149(2)
137(4)
153(2)
152(2)
This work
31
This work
3
…
O16H16 N8
3
…
…
L Cl
O16H16 N8
a
C13H13 N112
a
N112 is generated by the symmetry operation 3 2 x, 1 2 y, 2z.
398 | CrystEngComm, 2012, 14, 4396–4406
4
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…
2
3
3
2 2 2
Table 4 Inter-molecular p–p stacking and C–H p interactions in CH L , CH L and CH L Cl
a
b
˚
d (A)
Atoms involved 1
Atoms involved 2
Angle (u)
2
CH
2
L
N11–C10–C15–C14–C13–C12
C1–C2–C3–C4–C5–C6
C1–C2–C3–C4–C5–C6
C1–C2–C3–C4–C5–C6
C1–C2–C3–C4–C5–C6
N12–C11–C10–C15–C14–C13
C1–C2–C3–C4–C5–C6
C7–H7
C14–H14
C11–H8
C1–C2–C3–C4–C5–C6
C7–H7
C15–H15
2.65(2)
2.88(2)
2.85(1)
3.83(1)
2.81(2)
2.78(1)
3.826(2)
2(1)
8(1)
3(1)
0(1)
5(1)
6(1)
0(1)
3
3
31
CH
CH
2
L
2
L Cl
C1–C2–C3–C4–C5–C6
a
…
b
p–p stacking: distances between ring centroids; C–H p: distances between H atoms and ring centroids. p–p stacking: angles between ring
…
planes; C–H p: angles between C–H axis and ring planes.
Fig. 2 (a) ORTEP view of the asymmetric part of the unit cell of
3
L Cl, showing 50% probability displacement ellipsoids. (b) View of
CH
2
3
2
the crystal packing of CH L Cl along the b axis. H atoms were omitted
for clarity.
3
Fig. 4 Kubelka–Munk spectra at 298 K of (a) CH
2
L before (black line)
and after irradiation at 365 nm (black dotted line) and at 450 nm (red
3
L Cl before
dotted line), for 15 min and 5 min, respectively; (b) CH
2
(
black line) and after irradiation at 450 nm for 5 min (red dotted line); (c)
4
2
CH L Cl before (black line) and after irradiation at 450 nm for 10 min
(
red dotted line). The circles highlight the increase of the intensity of the
trans-keto band.
by the methylene spacer. The Kubelka–Munk spectrum of
4
3
2 2
CH L Cl reveal the same profile as for CH L Cl, with two weak
bands located at 505 and 530 nm, arising from the trans-keto
form (Fig. 4c).
The reversibility of the cis/trans isomerisation was investigated
by time dependent diffuse reflectance spectroscopy. Irradiation
3
L Cl at 450 nm for 20 min leads to a large increase of the
of CH
2
2
–4
3–4
Fig. 3 Left: Kubelka–Munk spectra of CH
2
2
L
and CH
2
L
Cl at 298
intensity of the signal of the trans-keto form (at 546 nm).
Thermal relaxation to the cis-keto form is complete after 3600 s,
as shown in Fig. 5a. The relaxation appears to be elaborated and
contains more than one chemical relaxation pathway. Attempts
to fit this curve with a single exponential function failed. Reverse
photo-switching of the trans-keto A cis-keto form can be
achieved by irradiating the trans-keto band at 546 nm, thus
demonstrating the reproducibility of the switch. The cycles were
repeated several times without significant alteration as shown
in Fig. 5b for two cycles. The thermal and photochemical
4
L (a) and CH L (c) over the
K. Right: Photographs of powders of CH
2 2
3
3
temperature range 298–77 K and of CH
2
L
(b), CH
L Cl (e) at 298 and 77 K. Thermochromism is clearly identified for
b, d and e.
2
L Cl (d) and
4
CH
2
3
For CH L , the band maximum is slightly shifted to 520 nm.
2
Photo-isomerisation is also efficient at 365 nm as shown in
3
L Cl, two unresolved bands with maxima at 510
Fig. 4a. For CH
2
and 540 nm are detected (Fig. 4b). Their contribution may be
related to the formation of two different conformations of the
trans-keto form which could arise from the large flexibility given
3
4
2 2
relaxation of the trans-keto form of CH L and CH L Cl could
not be analyzed because of a too fast thermal relaxation.
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Fig. 5 Time dependence of the Kubelka–Munk function (F) of
3
CH L Cl at 546 nm showing: (a) a fast thermal relaxation of the
2
metastable trans-keto form after irradiation; (b) reversibility of the cis/
trans isomerisation with irradiation cycles at 450 and 546 nm,
respectively.
Solid state fluorimetry. All molecules were analyzed by solid
state fluorimetry at room temperature in order to study
photochemical processes involved in photochromism observa-
tion. The results are presented in two different groups based on
3
3
4
the presence of photochromism (CH
2
L , CH
2
L Cl, CH
2
L Cl))
2
L and CH
4
L ) (Fig. 8).
(
Fig. 6 and 7) or its absence (CH
2
2
In emission measurements, the excitation wavelength is locked
3
Fig. 7 Excitation spectra of solid samples of CH
3
2
L
(black line) and
on a selected area of the diffuse reflectance spectrum corre-
sponding to the band maximum of the prototropic form of the
N-salicylidene aminomethylpyridine molecule to be analyzed,
4
CH
2 2
L Cl (green line) and CH L Cl (pink line) at 298 K at: (a) lem =
5
40 nm; (b) lem = 650 nm. The blue curve represents the diffuse
3
reflectance spectrum of CH L after irradiation at 450 nm during 5 min.
2
The enol, cis- and trans-keto ranges are observed from UV to y400 nm,
from 400 to 500 nm and between 500 and 550 nm, respectively.
and the emission wavelength is scanned. The spectrum contains
all emissions driven from the generated excited form (e.g.
3
irradiating CH
2
L
at 370 nm leads to the enol* form and
emissions of enol* A enol (A) and cis-keto* A cis-keto (B), as
depicted on Scheme 3). In the excitation mode, the emission
wavelength is fixed at its maximum and an excitation spectrum is
scanned. For example, we shall focus hereafter on the cis-
keto*A cis-keto emission at 540 nm and scan excitation
wavelengths. Excitation spectrum is expected to reveal two
bands corresponding to pathways C (enolAenol*Acis-keto*)
and D (cis-ketoAcis-keto*) leading to the formation of the
monitored emitting excited state (Scheme 3).
3
3–4
Solid state emission spectra of CH
2
L and CH
2
L
Cl were
3
thus recorded at lexc = 370 nm and 430 nm which corresponds to
the absorption bands of enol and cis-keto forms, respectively
(Fig. 3), and at lexc = 520 nm corresponding to the trans-keto
form, obtained after irradiation (Fig. 4).
Fig. 6 Emission spectra of solid samples of CH
3
2
L
(bold black line),
4
and CH
2 2
L Cl (dotted green line) and CH L Cl (bold green line) at 298 K
(
l
exc = 430 nm). The arrows indicate the three emission contributions
3–4
L
discussed in the text for CH
2
Cl.
4
400 | CrystEngComm, 2012, 14, 4396–4406
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its high resolution (Fig. 6). Three bands are detected with
emission maxima at approximately 520, 540 and 610 nm for
3
L , and 530, 570 and 660 nm for CH
3–4
CH
2
2
L
Cl, whose origin
shall be addressed below.
3
3–4
2 2
Excitation spectra of CH L and CH L
Cl at lem = 540 nm
(
or 520 nm) indicate two contributions (Fig. 7a). A very high
intensity is detected from UV to y450 nm which can be
attributed to the enol form absorption (Pathway C in Scheme 3).
This signal presents several unresolved contributions which are
probably due to different electronic transitions present in this
3
9
form. Another excitation band is observed between 450 and
5
00 nm, which is attributed to the cis-keto form, by comparison
9
with diffuse reflectance spectroscopy (Pathway D in Scheme 3).
3
Thus, the emission bands centred at 520 and 540 nm in Fig. 6
originate from the radiative de-excitation of the cis-keto* form
(
pathway B in Scheme 3), which is produced from the absorption
of the enol form (which undergoes then an excited state fast
tautomerisation from the enol* form) and from the cis-keto form
in the ground state.
Excitation spectra recorded at lem = 650 nm (Fig. 7b) reveal the
same two contributions with a third intense band centred at lmax
y500 nm, which can be clearly assigned to the absorption of the
trans-keto form by comparison with diffuse reflectance spectrum
also recalled on Fig. 7b. The emission at 610 and 660 nm (Fig. 6)
can thus be assigned to the radiative relaxation of the trans-keto*
form, which can be formed through the absorption of enol, cis-
keto and trans-keto forms in their ground state. It is worthwhile to
note that the emission of the trans-keto form of a functional Schiff
base derived from aniline is observed for the first time.
Similar solid state fluorimetry experiments were carried out on
2
4
non-photochromic molecules (CH
00 nm, which corresponds to the cis-keto form absorption
Fig. 3), both emission spectra are very similar and present a
broad band (Fig. 8a) which is composed of two contributions at
2 2
L and CH L ). At lexc =
4
(
2
4
y580 and 660 nm for CH
The origin of these contributions could be elucidated by
excitation spectra recorded at l = 655 and 660 nm for
2
2
L , and y550 and 650 nm for CH L .
2
4
Fig. 8 (a) Emission spectra of solid samples of CH
2
L
and CH
2
L
at
L (bold line) and
2
L (dotted line) at 298 K (lem = 655 nm and 660 nm, respectively).
2
2
98 K (lexc = 400 nm). (b) Excitation spectra of CH
2
4
em
CH
The blue curve represents the diffuse reflectance spectrum of CH
cis- and trans-keto ranges are observed from 400 to 450 nm and between
50 and 650 nm, respectively.
2
4
2
2 2
CH L and CH L , respectively (Fig. 8b). Interestingly, although
2
L . The
the most intense emission peak (around 600 nm) comes from the
radiative relaxation of the cis-keto* form, as observed for
photochromic molecules described above, the excitation spectra
at 655–660 nm show high intensity in the whole visible range
studied.
4
Indeed, in addition to a signal between 350 nm and 450 nm
coming from enol and cis-keto absorptions, an intense band is
noted between 450 and 650 nm, centred at 560 and 530 nm for
2
4
CH
2
L
2
and CH L , respectively. As this additional band
corresponds to the trans-keto form absorption, we can conclude
that the shoulder observed at lowest energy in the emission
2
4
2 2
spectra of CH L and CH L (Fig. 8a) originates from the
radiative de-excitation of the trans-keto* form coming from the
Scheme 3 Photochemical processes evidencing two radiative relaxation
pathways in emission mode (A, B) and two absorption pathways in
excitation mode (C, D) for N-salicylidene aniline derivatives.
absorption of the enol, cis-keto and trans-keto forms, as
3
previously pointed out for photochromic molecules CH L and
3–4
2
CH
2
L
Cl.
These emission spectra were also recorded on pure samples in
order to avoid matrix effect which can strongly modify the
Discussion
9
,35–38
population of keto/enol forms.
In the following, we shall
Five N-salicylidene aminomethylpyridine derivatives were suc-
only discuss emission spectra related to lexc = 430 nm because of
This journal is ß The Royal Society of Chemistry 2012
cessfully synthesized and their structural and solid state optical
CrystEngComm, 2012, 14, 4396–4406 | 4401

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3
3–4
properties analyzed. Three molecules, CH
2
L
and CH
2
L
Cl,
2
present both thermo- and photochromism in contrast to CH
4
2
L
and CH
2
L , which are not switchable. Surprisingly, the absence
2
L contradicts a previous claim,
40
of chromic properties for CH
2
as its crystal packing shows molecules packed with long
intermolecular distances, and nevertheless the molecule is not
photochromic.
Solid state fluorimetry was thought in order to probe radiative
relaxations and afford a better understanding of the energy
scheme involved during thermo- and photochromic processes.
This approach is rather unusual taking into account that the
literature is relatively silent in systematic fluorescence studies for
this class of molecules. It was claimed for instance that
N-salicylidene aniline derivatives showed exclusively one emis-
sion band attributed to the de-excitation of the cis-keto*
Scheme 4 Photochemical process leading to the formation of the trans-
keto form from the cis-keto absorption, and to the accumulation of this
form leading to photochromism observation. The activation barriers 1
and 2 are highlighted in the formation and relaxation chemical pathways
of the trans-keto form.
2
9,41,42
form.
In this work, we have successfully completed the
emission scheme by finding two other emissions coming from the
enol* and trans-keto* forms, for the first time. Solid state
fluorimetry thus provides new insights into the formation and
switchable properties of the trans-keto form, which was system-
atically associated, on a first basis, to its observation by diffuse
reflectance spectroscopy.
43
considered. The presence of the methylene group also induces a
large change in the energy of electronic levels which leads to a
modification of the shape of diffuse reflectance spectra (shifts of
the bands, modification of the enol/cis-keto population ratio,
etc). Moreover, the consequence of the addition of an electronic
substituent onto the phenol group on the optical properties was
also studied. Introduction of a chloride in N-salicylidene
aminopyridine derivatives leads to a bathochromic shift (as
Indeed, all N-salicylidene aminomethylpyridines present a low
energy emission which can be attributed to the radiative de-
excitation of the trans-keto* form whatever, whether the
molecules are originally photochromic or not. The non-detection
of the trans-keto* form emission in earlier studies could be either
due to: (i) the low resolution power of solid state fluorimetry
preventing distinguishing between cis-keto* and trans-keto*
emission bands or (ii) the absence of radiative relaxation of the
trans-keto* form. Surprisingly, trans-keto form emission is more
3
3
observed from L to L Cl, Fig. 9a) whereas no major effect is
3
3
2 2
observed from CH L to CH L Cl (Fig. 9b). A change in the
enol/cis-keto signals ratio by adding the chloride substituent and
the methyl group is also noted. (Fig. 9b). It could be explained
considering electronic conjugation in the molecule. The proton
transfer from enol to cis-keto form induces formation of an
amine (an electron donor group) which is stabilized by the
acceptor effect of the ketone and, for fully conjugated molecules
2
4
intense for non-photochromic molecules CH L and CH L ,
2
2
suggesting that formation of trans-keto* form from the cis-keto*
one is highly favourable. Therefore the trans-keto form is
produced by light irradiation in both photochromic and non-
photochromic molecules but is too unstable to be accumulated,
and therefore to be detected by diffuse reflectance spectroscopy
only. Indeed, even in photochromic molecules, the trans-keto
photoproduct appears less stabilized as demonstrated by the very
3
3
like L and L Cl, by the acceptor effect of the pyridine ring.
Introducing a chloride induces competition between the meso-
meric donor effect of the chloride and amines moieties, which
leads to a destabilization of the cis-keto form. The cis-keto
destabilization is much more important in derivatives incorpor-
3
3–4
L
fast thermal relaxation of CH
2
L and CH
2
Cl.
The absence of photochromism in this class of molecules can
be explained by considering the activation barriers heights for
the keto form being either too large between cis-keto* and trans-
keto* forms (barrier 1) or too low between cis-keto and trans-
keto forms (barrier 2) as shown in Scheme 4. It suggests that
although the trans-keto form is revealed in excitation spectra
3
3
ating a methyl spacer (from CH L to CH L Cl) than in
2
2
3
3
aminopyridine derivatives (from L to L Cl) which are com-
pletely conjugated. The loss of stabilization can be also observed
by solid state fluorimetry where important shifts of emission and
excitation bands occur when aminomethylpyridines derivatives
3
0
(
Fig. 8b), it could be present as traces in non-photochromic
are compared to aminopyridine molecules.
molecules (being stabilized by crystal defects), their formation
mechanism yet to be established. In this respect, a theoretical
study of the distribution of these species into the crystal could be
Conclusions
2
6,28
useful.
The impact of the methyl spacer on photochromism, which
Increasing the flexibility of N-salicylidene aminopyridine deri-
vatives allow to gain further insights into the understanding and
predictions of the solid state optical properties of N-salicylidene
2
7
was considered too in this work, is difficult to predict because
in addition to the loss of electronic conjugation, the CH group
3
0
2
derivatives, after our discussion about the discrepancy of
provides more flexibility to the molecules, which leads to new
molecular conformations, supramolecular interaction networks
and crystal packing, as described in the crystal structures section.
This study is particularly relevant taking into account that the
role of flexibility on this class of molecules was only briefly
structure–optical-properties relationship, previously widely
1
,29,44
quoted in the literature,
and which is no longer self
sufficient to predict solid state photochromism. In addition, the
study of N-salicylidene aminomethylpyridines by fluorescence
spectroscopy improves the understanding of photochemical
4
402 | CrystEngComm, 2012, 14, 4396–4406
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switchable organic molecules, which have recently shown useful
for proper insertion, for instance, in multifunctional spin
4
6,47
transition materials.
Experimental
General procedures
Sonication was performed with a Branson 3510 ultrasonic bath.
13
1
H and C NMR spectra were recorded with a Bruker AC
00 MHz instrument with the DMSO proton peak as internal
3
standard. Mass spectroscopy data were obtained with a
Thermofinnigan LCQ ion trap spectrometer by using an ESI
ionization mode. HRMS were carried out on a Micromass
Q-TOF 2 spectrometer using ESI mode, detecting positive ions.
Infrared spectra were recorded with a Shimadzu FTIR-8400S
spectrometer with KBr disks. Elementary analyses were
performed at University College London. The TGA/DTA
instrument used was a TA instrument SDT2960 Simultaneous.
X-ray powder diffractograms were performed with a Siemens
D5000 X-ray diffractometer, with a Cu anticathode (lKa =
˚
.5418 A) in Bragg–Brentano geometry (h/h mode). Samples
1
were deposited on silicon after calibration with a quartz
standard. Diffuse reflectance spectra were obtained with a
Varian Cary 5E spectrometer by using PTFE as a reference.
Spectra were measured on pure solids to avoid matrix effects.
Eventual distortions in the Kubelka–Munk spectra that could
result from the study of pure compounds have not been
considered because no comparison with absorption spectra was
necessary. The reversibility of the cis–trans isomerization was
checked with a LOT-ORIEL 200 W high-pressure mercury Arc
lamp (LSN261). In situ irradiations were carried out for 10 min
with appropriate filters (365, 450 and 546 nm). Data that were
accumulated each 30 s were normalized. Solid-state emission
spectra were obtained with a Fluorolog-3 (Jobin–Yvon-Spex
Company) spectrometer. Kubelka–Munk and emissions spectra
were normalized to allow meaningful comparisons.
Fig. 9 Effect of the chloride substitution on the diffuse reflectance
3
3
spectra of: (a) L (dotted line) and L Cl (bold line). (b) CH
3
2
L
(dotted
3
L Cl (bold line). The arrows indicate the bathochromic
line) and CH
2
shift and the modification of the enol/cis-keto signals ratio induced by the
adding of the chloride substituent.
Starting materials
processes involved in photochromism, by considering energies of
the different excited forms and activation barriers for a better
prediction.
Solvents (benzene for gas chromatography from Fluka, hexane
6
for analysis from Sigma-Aldrich, [d ]-DMSO from Aldrich) and
reagents (2-, 3- and 4-aminomethylpridine from Sigma-Aldrich;
salicyladehyde and 5-chlorosalicyladehyde from Acros Organics)
were obtained commercially and used as received.
The radiative relaxation of the trans-keto* form has been, for
the first time, detected by fluorimetry in photochromic and,
surprisingly, in non-photochromic molecules as well. The trans-
keto form is thus obtained in both systems but a sufficient
activation barrier between trans- and cis-keto forms allows the
stabilization of the trans-keto form in its ground state. Increasing
the flexibility in the different molecules seems to favour
photochromism but the loss of conjugation destabilizes the keto
forms. The absence of photochromism in diffuse reflectance
spectroscopy after light irradiation thus does not imply the
impossibility of trans-keto formation. The ongoing crystal
engineering efforts on this class of molecules combined to the
energy optimization of different forms in their excited and
ground states may help towards a better optical properties
control in view of potential applications (e.g. for sensors or
Syntheses
2
L ), N-salicylidene-
N-salicylidene-2-aminomethylpyridine (CH
2
3
L ), N-salicylidene-4-aminomethyl-
3
-aminomethylpyridine (CH
2
4
L ), 5-chlorosalicylidene-3-aminomethylpyridine
pyridine (CH
2
3
L Cl) and 5-chlorosalicylidene-4-aminomethylpyridine
(
(
CH
CH
2
4
L Cl) were prepared in benzene using a Dean–Stark setup.
2
2
N-Salicylidene 2-aminomethylpyridine (CH L ): 2-Amino-
2
methylpyridine (0.61 mL, 1 equiv., 5.88 mmol) was dissolved in
benzene (25 mL) in a 100 mL round-bottom flask with a Dean–
Stark setup. Salicylaldehyde (0.62 mL, 1 equiv., 5.88 mmol) was
added to the solution. The coloration immediately turned to light
yellow. After refluxing for 24 h, the dark yellow solution was
concentrated to obtain an orange oil. The crude product was
triturated in hexane affording a pure light yellow microcrystalline
1
3
displays ). In addition, N-salicylidene aminomethylpyridine
4
5
derivatives can be inserted into inorganic complexes, thus
offering promising prospects for this class of solid state
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1
solid which was filtered (0.40 g, 32%). H NMR (300 MHz,
6
d -DMSO, 298 K, d in ppm, numbering following Fig. 1): 13.41
6.93 (m, 2 H, HO–C–CH and HO–C–CH–CH–CH), 4.91 (s, 2 H,
13
N–CH2). C NMR (300 MHz, d -DMSO, 298 K, d in ppm):
6
(s, 1H, OH), 8.74 (s, 1H, C7–H), 8.55 (d, J = 4.83 Hz, 1 H, C12–H),
1
167.75 (NLCH), 160.31 (HO–C), 149.83 (N–CH –C–CH–CH),
2
7
.80 (m, 1 H, C14–H), 7.50 (dd, J
1
= 7.60 Hz, J
2
= 1.59 Hz, 1 H,
147.59 (HO–C–C), 132.66 (N–CH
CH), 122.67 (HO–C–CH–CH), 118.84 (HO–C–CH–CH–CH),
118.69 (N–CH –C), 116.47 (HO–C–CH), 60.68 (N–CH ). MS:
m/z = 213 (M + H ), 196 (C H N ). Purity checked in HRMS
2
–C–CH), 131.86 (NLCH–C–
C3–H), 7.35 (m, 2 H, C15–H, C13–H, C6–H), 6.91 (m, 2 H, C4–H,
13
6
C5–H), 4.91 (s, 2 H, C9–H). C NMR (300 MHz, d -DMSO,
98 K, in ppm): 167.42 (C7), 160.52 (C2), 157.78 (C10), 149.26
C12), 136.99 (C14), 132.50 (C13), 131.79 (C3), 122.49 (C6), 122.09
C15), 118.72 (C1), 118.64 (C4), 116.47 (C5), 63.86 (C9).
O (212.25) calcd: C 73.57, N 13.20, H 5.70; found C
2
2
+
2
1
3
10
2
(
(TOF MS ES+): m/z = 213.1025 (calculated: 213.1028). FTIR
2
1
(
(KBr, cm ): 3049 (m, br, OH), 2868 (m), 2725 (m, br), 1630 (s),
1602 (s, CLN), 1580 (m), 1489 (m), 1458 (s), 1413 (m), 1389 (w),
1279 (m), 1250 (m), 1219 (w), 1151 (w, C–O), 1116 (w), 1065 (w),
13 12 2
C H N
21
72.97, N 11.44, H 5.45. FTIR (KBr, cm ): 3057 (m, br, OH), 2867
(
m) 1630 (s, CLN), 1589 (m), 1491 (m), 1474 (m), 1458 (m), 1435
1034 (m), 1009 (w), 800 (m), 754 (s).
3
5-Chloro-N-salicylidene 3-aminomethylpyridine (CH L Cl):
(m), 1389 (w), 1335 (w), 1279 (m), 1259 (w), 1220 (w), 1151 (w),
2
1
116 (w), 1099 (w), 1047 (m), 1007 (w), 893 (w), 839 (w), 802 (m),
54 (s). Yellow single crystals (plates) were obtained by recrystalli-
3-Aminomethylpyridine (2.40 mL, 1 equiv., 23.56 mmol) was
dissolved in benzene (70 mL) in a 250 mL round bottom flask
with a Dean–Stark setup. 5-Chloro-salicylaldehyde (3.69 g,
1 equiv., 23.56 mmol) was added to the mixture which
immediately turned to dark yellow. After refluxing for 24 h,
7
zation in hot hexane after slow evaporation.
3
2
N-Salicylidene 3-aminomethylpyridine (CH L ): 3-Amino-
methylpyridine (2.40 mL, 1 equiv., 23.56 mmol) was dissolved
in benzene (100 mL) in a 250 mL round-bottom flask with
a Dean–Stark setup. Salicylaldehyde (2.48 mL, 1 equiv.,
the solution was concentrated leading to a dark crude oil which
3
L Cl as a
was triturated in hexane in order to obtain pure CH
2
1
yellow microcrystalline powder (5.44 g, 93%). H NMR
2
3.56 mmol) was added to the solution giving rise to an
6
(300 MHz, d -DMSO, 298 K, d in ppm, numbering following
immediate coloration change to yellow. After refluxing the
mixture for 24 h, the dark yellow solution was evaporated to give
1
Fig. 2): 8.76 (s, 1 H, C8–H), 8.59 (d, J = 1.80 Hz, 1 H, C13–H),
a crude solid which was triturated in hexane affording pure
3
CH L as a strongly yellow microcrystalline powder (4.99 g,
8.51 (dd, J = 4.80 Hz, J = 1.60 Hz, 1 H, C11–H), 7.76 (dt, J =
1
2
1
7.83 Hz, J = 1.83 Hz, 1 H, C14–H), 7.61 (d, J = 2.67 Hz, 1 H,
2 1
2
1
6
9
9%). H NMR (300 MHz, d -DMSO, 298 K, d in ppm,
numbering following cif files available in ref. 20): 13.22 (s, 1 H,
OH), 8.76 (s, 1 H, C7–H), 8.60 (d, J = 1.90 Hz, 1 H, C11–H),
.51 (dd, J = 4.8 Hz, J = 1.6 Hz, 1 H, C10–H), 7.76 (d, J =
1
C6–H), 7.40 (m, 2 H, C15–H, C4–H), 6.92 (d, J = 8.85 Hz, 1 H,
1
3
6
C3–H), 4.85 (s, 2 H, C9–H). C NMR (300 MHz, d -DMSO,
298 K, d in ppm): 165.67 (C7), 159.12 (C5), 149.13 (C13), 148.59
(C11), 135.66 (C14), 134.04 (C2), 132.15 (C4), 130.56 (C6),
123.72 (C15), 121.01 (C1), 119.84 (C10), 118.48 (C3), 61.58 (C9).
1
8
1
2
1
7
.9 Hz, 1 H, C12–H), 7.49 (dd, J = 7.65 Hz, J = 1.68 Hz, 1 H,
1
2
C12–H), 7.42 (dd, 1 H, J
1
= 6.36 Hz, J
2
= 0.48 Hz, C5–H), 7.35
13 12 2
C H N O (212.25) calcd: C 63.29, N 11.36, H 4.43; found: C
(
m, 2H, C3–H and C13–H), 6.92 (m, 2H, C2–H and C4–H), 4.85
63.29, N 11.49, H 4.43. X-ray powder diffraction (u): 6.6 (s),
13.12 (w), 16.14 (m), 16.84 (w), 19.62 (m), 20.5 (m), 20.84 (w),
21.62 (s), 24.00 (s), 24.10 (m), 25.02 (m), 25.46 (m), 25.74 (m),
26.12 (s), 28.16 (m), 29.16 (m), 30.08 (m), 30.16 (m), 30.54 (w),
32.26 (m), 35.64 (w), 36.46 (w), 39.56 (w), 41.84 (w), 44.34 (w),
1
s, 2 H, C8–H). C NMR (300 MHz, d -DMSO, 298 K, d in
3
6
(
ppm): 168.36 (C7), 161.56 (C4), 150.34 (C11), 149.78 (C10),
36.84 (C12), 135.52 (C1), 133.79 (C3), 133.06 (C5), 124.97
C13), 120.01 (C9), 119.93 (C6), 117.68 (C2), 60.67 (C8).
O (212.25) calcd: C 73.57, N 13.20, H, 5.70; found: C
1
(
2
1
C
H
13 12
N
2
45.04 (w), 48.20 (w), 49.48 (w). FTIR (KBr, cm ): 3057 (w, br,
OH), 2995 (w), 2914 (w), 2850 (w), 1633 (s, CLN), 1576 (m), 1558
(w), 1448 (w), 1450 (m), 1416 (m), 1394 (w), 1369 (m), 1338 (m),
1308 (w), 1277 (s), 1232 (w), 1203 (w), 1184 (m), 1107 (m), 1092
(w), 1036 (m), 1022 (s), 955 (w), 910 (w), 891 (w), 833 (w), 820 (s),
802 (m), 777 (w), 716 (m), 710 (s). Yellow single crystals (long
needles) were obtained by recrystallization in hot hexane after
slow solvent evaporation.
7
1
2
3
1
3.12, N 12.87, H 5.59. X-ray powder diffraction (u): 9.66 (m),
5.18 (w), 17.92 (m), 19.34 (m), 19.70 (m), 20.50 (m), 21.18 (w),
2
1
1.62 (m), 23.86 (s), 27.42 (m), 32.98 (m). FTIR (KBr, cm ):
030 (w, br, OH), 2891 (w), 1630 (s, CLN), 1575 (m), 1491 (m),
458 (m), 1421 (m), 1400 (m), 1350 (w), 1317 (w), 1275 (m), 1213
(
(
w), 1198 (w), 1147 (w), 1113 (w), 1057 (m, C–O), 1028 (m), 993
m), 970 (w), 924 (w), 876 (m), 856 (m), 795 (m), 770 (s), 712 (m).
4
2
Yellow single crystals (plates) were obtained by recrystallization
in hot hexane after slow evaporation. An X-ray investigation has
5-Chloro-N-salicylidene 4-aminomethylpyridine (CH L Cl):
4-Aminomethylpyridine (2.38 mL, 1 equiv., 23.56 mmol) was
dissolved in benzene (70 mL) in a 250 mL round bottom flask
with a Dean–Stark setup. 5-Chloro-salicylaldehyde (3.69 g,
1 equiv., 23.56 mmol) was added to the mixture which
immediately turned to yellow. After refluxing for 24 h, the
revealed the same cell parameters as those in ref. 31.
4
L ): 4-Amino-
N-Salicylidene 4-aminomethylpyridine (CH
2
methylpyridine (2.38 mL, 1 equiv., 23.56 mmol) was dissolved
in benzene (100 mL) in a 250 mL round bottom flask with
a Dean–Stark setup. Salicylaldehyde (2.48 mL, 1 equiv.,
solution was concentrated giving crude orange oil which was
4
triturated in hexane affording pure CH L Cl as a yellow
2
3.56 mmol) was added in the mixture which immediately
2
1
microcrystalline powder (5.48 g, 94%). H NMR (300 MHz,
turned to yellow. After refluxing for 24 h, the solution was
6
d -DMSO, 298 K, d in ppm): 13.11 (s, 1 H, OH), 8.73 (s, 1 H,
concentrated leading to a yellow solid which was triturated in
4
L as a pale yellow powder
hexane in order to obtain pure CH
2
NLCH), 8.56 (m, 2 H, N–CH
2 1
–C–CH–CH), 7.63 (d, J = 3 Hz,
1
4.90 g, 98%). H NMR (300 MHz, d -DMSO, 298 K, d in ppm):
6
(
1 H, NLCH–C–CH), 7.36 (m, 3 H, N–CH –C–CH and HO–C–
2
1
J
3.20 (s, 1H, OH), 8.73 (s, 1 H, NLCH), 8.56 (dd, J = 4.41 Hz,
CH–CH), 6.95 (d, J = 8.85 Hz, 1 H, HO–C–CH), 4.85 (s, 2 H,
1
1
1
3
6
2
= 1.62 Hz, 2 H, N–CH
2
–C–CH–CH), 7.49 (m, 1 H, NLCH–
–C–CH–CH),
N–CH
2
). C NMR (300 MHz, d -DMSO, 298 K, d in ppm):
–C–CH–CH),
C–CH), 7.35 (m, 3 H, HO–C–CH–CH and N–CH
2
167.22 (NLCH), 160.05 (HO–C), 150.79 (N–CH
2
4
404 | CrystEngComm, 2012, 14, 4396–4406
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1
1
1 Y. Garcia, V. Ksenofontov, R. Lapouyade, A. D. Naik, F. Robert
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1
47.30 (HO–C–C), 133.21 (N–CH
CH), 123.64 (HO–C–CH–CH), 121.79 (Cl–C), 120.86 (N–CH
C), 119.49 (HO–C–CH), 61.58 (N–CH ). MS: m/z = 247 (M 2
O). C13 O (212.25)
2
–C–CH), 131.50 (NLCH–C–
2
–
2
8
267.
13 S. Kawata and Y. Kawata, Chem. Rev., 2000, 100, 1777.
+
H ), 230 (C13
H
N
9 2
Cl), 210 (C13
H
10
N
2
12 2
H N
1
1
4 B. L. Feringa, Acc. Chem. Res., 2001, 34, 504.
calcd: C 63.29, N 11.36, H 4.43; found: C 63.81, N 12.15, H 4.70.
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X-ray powder diffraction (u): 6.74 (w), 16.06 (m), 16.78 (w), 19.60
(
(
(
(
m), 20.60 (w), 21.60 (w), 21.64 (s), 23.98 (m), 25.10 (m), 25.22
m), 26.16 (m), 26.22 (m), 28.18 (w), 29.14 (m), 30.08 (m), 30.16
m), 30.24 (w), 32.36 (m), 35.64 (w), 39.62 (w), 41.96 (w), 44.34
1
2
1
w), 44.44 (m), 46.74 (w), 49.48 (w). FTIR (KBr, cm ): 3069 (w,
br, OH), 2922 (w), 2899 (w), 2856 (w), 1633 (s, CLN), 1597 (m),
575 (w), 1560 (m), 1481 (s), 1450 (m), 1416 (m), 1389 (m), 1367
m), 1337 (m), 1327 (w), 1308 (w), 1277 (s), 1219 (m), 1182 (m),
1
7 V. A. Bren, A. D. Dubonosov, V. I. Minkin, T. N. Gribanova, V. P.
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1
(
1
8
117 (w), 1092 (m), 1068 (w), 1032 (m), 991 (m), 960 (m), 889 (w),
27 (m), 808 (m), 777 (w), 706 (m).
1
8 (a) K. Nakatani and J. A. Delaire, Chem. Mater., 1997, 9, 2682; (b)
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1
2
442; (b) M. D. Cohen, G. M. J. Schmidt and S. Flavian, J. Chem.
X-Ray crystallography
Soc., 1964, 2041; (c) M. D. Cohen, Y. Hirshberg and G. M. J.
Schmidt, J. Chem. Soc., 1964, 2051; (d) M. D. Cohen, Y. Hirshberg
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M. D. Cohen and S. Flavian, J. Chem. Soc. B, 1967, 317; (g) M. D.
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2
X-ray intensity data were collected at 125 K for CH
2
L
and
3
L Cl with a MAR345 image plate using Mo-Ka
1
20 K for CH
3
˚
l = 0.71069 A) radiation. The crystal was chosen, mounted in
(
inert oil and transferred to the cold gas stream for flash cooling.
The crystal data and the data collection parameters are
summarized in Table 1. The unit cell parameters were refined
using all collected spots after the integration process. The
structures were solved by direct methods and refined by full-
2
2
2
2
48
matrix least-squares on F using SHELXL97.
All non-
hydrogen atoms were refined anisotropically. Hydrogen atoms
were localized by Fourier difference and included in the
refinement with a common isotropic temperature factor. The
details of the refinements and the final R indices are presented in
Table 1.
23 T. Fujiwara, J. Hadara and O. Keiichiro, J. Phys. Chem. B, 2004,
08, 4035.
2
J. Photochem. Photobiol., A, 1997, 110, 267.
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1
4 M. E. Kletskii, A. A. Millov, A. V. Metelitsa and M. I. Knyazhansky,
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7 E. Hadjoudis, M. Vittorakis and I. Moustakali-Mavridis,
Tetrahedron, 1987, 43, 1345.
2
2
Acknowledgements
2
2
8 F. Robert, A. D. Naik, F. Hidara, B. Tinant, R. Robiette, J. Wouters
and Y. Garcia, Eur. J. Org. Chem., 2010, 621.
9 (a) E. Hadjoudis and I. M. Mavridis, Chem. Soc. Rev., 2004, 33, 579;
This work was partly funded by the Fonds de la Recherche
Scientifique-FNRS (FRFC No 2.4508.08, No 2.4537.12, IISN
(b) E. Hadjoudis, S. D. Chatziefthimiou and I. M. Mavridis, Curr.
Org. Chem., 2009, 13, 269.
4
.4507.10) and ARC-Louvain. We also thank the Fonds pour la
Formation a` la Recherche dans l9Industrie et dans l9Agriculture
30 F. Robert, A. D. Naik, B. Tinant, R. Robiette and Y. Garcia, Chem.–
Eur. J., 2009, 15, 4327.
for doctoral scholarships allocated to F.R and P.-L. J.
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