C-H?Br-C: Vs. C-Br?Br-C vs. C-Br?N bonding in molecular self-assembly of pyridine-containing dyes

Article abstract of DOI:10.1039/c6ra10094e

We have studied a series of closely related N-(5-bromosalicylidene)-x-aminopyridine compounds (x = 2, 1; 3, 2; 4, 3), obtained by condensation of 5-bromosalicylaldehyde with the corresponding aminopyridine. ¹H NMR spectroscopy in solution revea

Full text of DOI:10.1039/c6ra10094e

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C–H/Br–C vs. C–Br/Br–C vs. C–Br/N bonding
in molecular self-assembly of pyridine-containing
dyes†
Cite this: RSC Adv., 2016, 6, 53669
*
Damir A. Safin, Maria G. Babashkina, Koen Robeyns and Yann Garcia
We have studied a series of closely related N-(5-bromosalicylidene)-x-aminopyridine compounds (x ¼ 2, 1;
3, 2; 4, 3), obtained by condensation of 5-bromosalicylaldehyde with the corresponding aminopyridine. 1H
NMR spectroscopy in solution revealed a single structure, at least in CDCl3. According to single crystal X-ray
diffraction it was established that the crystal structures of 1–3 each are stabilized by a linear intramolecular
hydrogen bond of the O–H/N type, formed between the hydroxyl hydrogen atom of the phenolic ring and
the imine nitrogen atom. The dihalogen C–Br/Br–C and halogen C–Br/N(Py) interactions play a crucial
role for the formation of supramolecular architectures in the structures of 2 and 3, respectively. The overall
geometry of each molecule in the structures of 1 and 2 was found to be almost planar, while a significantly
twisted structure was found for 3. Hirshfeld surface analysis showed that the structures of all compounds
are mainly characterized by H/X contacts as well as by a remarkable contribution from C/C and C/N
contacts which is clearly observed for 1 and 2. Diffuse reflectance spectra of 1–3 each exhibit a mixture
of enol, cis-keto and trans-keto forms. A major contribution of the trans-keto form is found for 1
whereas a moderate fraction is detected for 2, and only traces of the trans-keto form were observed for
3. Contrary to expectations based on dihedral angle
F
considerations,
1
exhibits negative
photochromism, although it was expected to be only thermochromic. Both
2
and are not
3
Received 19th April 2016
Accepted 23rd May 2016
photochromic, whereas 3 was expected to be photochromic (F > 25ꢀ). Highly favoured C/C, C/N and
O/Br intermolecular contacts as well as the absence of Br/Br dihalogen interactions in the structure of
1, are presumably mainly responsible for the photochromic behaviour. Furthermore, significantly
impoverished H/C and H/N contacts further support the observed negative photochromism of 1.
DOI: 10.1039/c6ra10094e
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interactions for constructing supramolecular assemblies.4
These interactions include halogen/halogen (X/X) and
Introduction
Crystal engineering1 is a key strategy for the creation and design
of new materials with desirable structures and properties,
which depend on intermolecular interactions. Understanding
of intermolecular interactions and packing in crystals is of the
utmost importance for the ne tuning of a number of useful
properties. Noncovalent interactions are considered as the most
effective tool for the creation of molecular aggregates and
assemblies.1,2 Among these interactions, hydrogen bonds and
p/p stacking are the most frequently used and legitimately
take front rank due to their ability for rational design.3
halogen/heteroatom (X/B) interactions. An IUPAC recom-
mendation dening these interactions as halogen bonds was
issued in 2013 and states that “A halogen bond occurs when there
is evidence of a net attractive interaction between an electrophilic
region associated with a halogen atom in a molecular entity and
a nucleophilic region in another, or the same, molecular entity.”5
This denition highlights the qualitative analogy between
halogen bonding and hydrogen bonding.
More than 50 years ago, Sakurai et al. noted that R–X/X–R
contacts occur in two preferential geometries.6 Much later, G. R.
Desiraju and R. Parthasarathy classied these geometries as
type I (symmetrical interactions where q1 ¼ q2) and type II (bent
interactions where q1 z 180ꢀ and q2 z 90ꢀ) (Chart 1).7 This
classication is still valid nowadays. There is a clear geometric
and chemical distinction between type I and type II X/X
interactions. Type I interactions which are geometry-based
contacts that arise from close-packing requirements, are
found for all halogens, but are however not halogen bonds
according to the IUPAC denition. Type II interactions arise
from the pairing between the electrophilic area on one halogen
On the other hand, short halogen atom contacts in crystals
have grown to one of the most interesting noncovalent
Institute of Condensed Matter and Nanosciences, Molecules, Solids and Reactivity
´
(IMCN/MOST), Universite Catholique de Louvain, Place L. Pasteur 1, 1348
Louvain-la-Neuve, Belgium. E-mail: yann.garcia@uclouvain.be; Fax: +32 1047 2330;
Tel: +32 1047 2831
† Electronic supplementary information (ESI) available: Fig. S1–S3. CCDC
1470798 and 1470799. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/c6ra10094e
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N-salicylidene aniline derivatives solely based on their crystal
structures, but energy differences between ground and excited
states need also to be considered.15b Furthermore, a complete
and detailed crystal structure knowledge, including F, crystal
packing and the available free space around the switching unit
in addition to the exibility of the nearby environment, is
requested.17f All this is obviously dictated by a diversity of non-
covalent intermolecular interactions responsible for the overall
crystal packing of molecules.18
Chart 1 Structural scheme for type I and type II halogen/halogen
short contacts.
In continuation of our comprehensive studies of N-salicyli-
atom and the electrophilic area on the other atom and are true dene aniline derivatives and with the aim to understand their
halogen bonds.8 Type II contacts are most favoured in iodinated structural features, which inuence on their optical properties,
derivatives, much less in brominated derivatives, and the least we have directed our attention to a series of three closely related
in chlorinated derivatives.8b
N-(5-bromosalicylidene)-x-aminopyridines (x ¼ 2, 1; 3, 2; 4, 3).
Halogen bonds, being selective and directional with their Although both crystal structure and thermochromic properties
energy comparable to that of hydrogen bonds,9 have extensively of 1 were described,19 it is also discussed herein for a better
been used for designing molecular systems of desired proper- comparison with the two other compounds.
ties,10 such as molecular conductors, optoelectronic materials11
and other functional materials.12
Due to the fact that the structures of molecules 1–3 could be
efficiently stabilized by hydrogen bonding and halogen inter-
N-Salicylidene aniline derivatives (Scheme 1), as well as their actions, it was interesting to study the contribution and inu-
N-heterocyclic analogues, dominate over other classes of ence of intermolecular interactions in their crystal structures.
molecules, exhibiting thermo- and photochromism in the Towards this aim, Hirshfeld surface analysis20 and associated
crystalline state.13 This is explained by both their colour panel 2D ngerprint plots,21 obtained using the CrystalExplorer 3.1
and accessible forms (Scheme 1) as well as ease of synthesis, soware,22 as well as the enrichment ratios,23 derived as the
thanks to Schiff base condensation.14 Another advantage of N- decomposition of the crystal contact surface between pairs of
salicylidene aniline derivatives, making them attractive from interacting chemical species, have been performed for the listed
a synthetic point of view, is the possibility to be included into compounds.
various matrices to form hybrid materials,15 and blends.16
The solid state thermochromic properties of N-salicylidene
aniline derivatives were rstly considered to result from the
Results and discussion
planarity of the molecule and the formation of a “close-packed
crystal structure” (dihedral angle between the aromatic rings F
< 25ꢀ), whereas photochromic behaviour is caused by the
signicant rotation of aromatic rings and the formation of an
“open structure” (F > 25ꢀ).14 Thermo- and photochromic prop-
erties were even stated over the years to be mutually exclusive.14b
A number of contradicting reports showing both thermo- and
photochromic properties, have however been recently dis-
closed.15b,17 All these newly obtained examples demonstrated
that it is not possible to explain thermo- and photochromism of
Compounds 1–3 were synthesized by reacting 5-bromosalicy-
laldehyde with the corresponding aminopyridine in ethanol
(Scheme 2). The as-synthesized molecules form orange or yellow
plate-like crystals, which are soluble in most polar solvents and
are insoluble in n-hexane and diethylether.
1H NMR spectra of 1–3 in CDCl3 each reveal a single set of
signals, which testies to the presence of a single structure in
solution. The signals for the benzene protons were found as two
Scheme 1
Scheme 2
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Fig. 1 View on the C–H/O–H and C–H/Br–C hydrogen bonded 1D polymeric chains, linked through p(Py)/p(C6H3) stacking interactions, in
the structure of 1. Colour code: C ¼ black, H ¼ light grey, N ¼ blue, O ¼ red, Br ¼ brown.
Table 2 Hydrogen and halogen bond lengths (A˚) and angles (ꢀ) for 1–3
doublets at 6.92–6.95 and 7.52–7.59 ppm and one doublet of
doublets at 7.46–7.50 ppm, respectively. Two singlet signals for
D–X/A
d(D–X)
d(X/A)
d(D/A) :(DXA)
the arylCHN and OH protons were observed at 8.54–9.37 and
12.57–13.48 ppm, respectively. The pyridine protons in the
spectra were observed as a number of signals with different
multiplicities at 7.12–8.66 ppm.
The crystal structures of 2 and 3 were elucidated by single
crystal X-ray diffraction. Crystals of 1 correspond to the reported
structure.19 Their structural parameters were extracted and used
herein to compare with those of 2 and 3. Compounds 1 and 2
each crystallize in the monoclinic space groups P21/n, while the
structure of 3 was rened in the orthorhombic space group
Pca21.
1a O(1)–H(1)/N(1)
0.74(5)
1.95(5)
2.54(4)
2.93(4)
1.91
2.619(5) 151(4)
3.383(6) 144(4)
C(10)–H(7)/O(1)#1 0.98(4)
C(7)–H(5)/Br(1)#2
1.08(5)
0.82
3.911
2.632(3) 147
131.05(9)
2.602(5) 148
5.036 170.82(13)
151(3)
2b O(8)–H(8)/N(10)
C(2)–Br(1)/Br(1)#1 1.898(3) 3.5747(6) 5.029
3c O(8)–H(8)/N(10)
C(2)–Br(1)/N(14)#1 1.901(4) 3.150(4)
0.82
1.87
a
Symmetry transformations used to generate equivalent atoms: #1 ꢁ1
b
ꢁ x, ꢁy, ꢁ2 ꢁ z; #2 ꢁx, ꢁy, ꢁz. Symmetry transformations used to
c
generate equivalent atoms: #1
2
ꢁ
x,
1
ꢁ y, ꢁz. Symmetry
transformations used to generate equivalent atoms: #1 5/2 ꢁ x, y,
1/2 + z.
Molecules in all the three structures were found in the eno-
limine form (Fig. 1–3). The bond lengths of C–O, with respect to
˚
the moieties marked in bold in Scheme 2, are about 1.35 A and
Table 3 p/p interaction distances (A˚) and angles (ꢀ) for 1 and 2a
˚
those of C–C are about 1.45 A (Table 1), which indicate single
˚
bonds, whereas a double bond of about 1.28 A is revealed for
Cg(I) Cg(J)
d[Cg(I)–Cg(J)]
a
b
g
Slippage
C]N (Table 1). The bond angles C–C]N and C–N]C of
119.2(4)–122.9(4)ꢀ indicate an sp2-hybridization of both carbon
and nitrogen atoms of the imine fragment, further supporting
the enolimine form (Table 1). The crucial difference between
the structures 1–3 consists in the dihedral angle F between
1 b Cg(1) Cg(2)#1 3.805(3)
Cg(2) Cg(1)#1 3.805(3)
Cg(1) Cg(2)#2 4.478(3)
Cg(2) Cg(1)#2 4.477(3)
2 c Cg(1) Cg(1)#1 4.5102(19)
Cg(1) Cg(1)#2 4.5102(19)
Cg(2) Cg(2)#1 4.5102(17)
Cg(2) Cg(2)#2 4.5104(17)
5.2(3)
5.2(3)
5.2(3)
5.2(3)
22.3 21.4 1.443
21.4 22.3 1.389
38.7 39.4 2.801
39.4 38.7 2.842
0.03(16) 39.8 39.8 2.890
0.03(16) 39.8 39.8 2.890
0.00(14) 41.2 41.2 2.971
0.00(14) 41.2 41.2 2.972
Table 1 Selected bond lengths (A˚) and angles (ꢀ) for 1–3a
a
Cg(I)–Cg(J): distance between ring centroids; a: dihedral angle
between planes Cg(I) and Cg(J); b: angle Cg(I) / Cg(J) vector and
normal to plane I; g: angle Cg(I) / Cg(J) vector and normal to plane
1
2
3
Bond lengths
C]N
C–C
C–O
C–N
J; slippage: distance between Cg(I) and perpendicular projection of
b
Cg(J) on ring I.
Symmetry transformations used to generate
1.281(6)
1.467(7)
1.346(6)
1.424(6)
1.268(4)
1.453(4)
1.350(4)
1.424(4)
1.281(5)
1.433(6)
1.358(5)
1.416(5)
equivalent atoms: #1 ꢁ1 ꢁ x, ꢁy, ꢁ1 ꢁ z; #1 ꢁx, ꢁy, ꢁ1 ꢁ z. Cg(1):
N(2)–C(8)–C(9)–C(10)–C(11)–C(12); Cg(2): C(1)–C(2)–C(3)–C(4)–C(5)–
c
C(6). Symmetry transformations used to generate equivalent atoms:
#1 ꢁ1 + x, y, z; #1 1 + x, y, z. Cg(1): N(15)–C(14)–C(13)–C(12)–C(11)–
C(16); Cg(2): C(2)–C(3)–C(4)–C(5)–C(6)–C(7).
Bond angles
C–C]N
C–N]C
121.6(4)
119.2(4)
122.5(2)
121.3(2)
122.9(4)
120.8(3)
benzene and pyridine rings. While in 1 and 2, the two rings are
at about 5.5ꢀ, the dihedral angle is remarkably larger in 3 and of
about 42.4ꢀ (Table 1).
All molecules are stabilized by a typical intramolecular
hydrogen bond between the hydroxyl hydrogen and the imine
nitrogen atoms (Fig. 1–3 and Table 2). The crystal structure of 1
Torsion angles
C–C]N–C
ꢁ179.2(4)
179.0(3)
173.1(3)
Dihedral angles (F)
C6H3–Py
5.2(3)
5.58(15)
42.44(19)
a
Values with respect to the moieties marked by bold in Scheme 2.
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contains linear intermolecular hydrogen bonds of the C–H/O– respectively (Fig. 1 and 2). Distances between the least squares
˚
H type, formed between the hydroxyl oxygen atom and the planes formed by adjacent parallel molecules is about 3.40 A
pyridine 3-H atom (Table 2) with the formation of R2 (16) and 3.54 A in the structure of 1 and 3.42 A in the structure of 2.
˚
˚
2
centrosymmetric dimers (Fig. 1). These dimers are linked to
Thanks to the C–Br/N type I intermolecular halogen inter-
each other, yielding 1D ribbons, through the C–H/Br–C actions, formed between bromine and pyridine nitrogen atoms,
hydrogen bonds formed between the bromine atom and the molecules of 3 are linked into 1D polymeric chains (Fig. 3 and
imine hydrogen atom with the formation of R22(12) centro- Table 2). The Br/N distances are 3.150(4) A and are substan-
˚
symmetric cycles (Fig. 1 and Table 2). These ribbons are further tially shorter than the sum of the van der Waals radii for
linked in 2D sheets through p/p stacking interactions nitrogen (1.55 A) and bromine (1.85 A).24 The shortening of the
˚
˚
between the pyridine and benzene rings (Fig. 1 and Table 3).
Br/N van der Waals distance is about 8% and also proves the
In the crystal structure of 2, centrosymmetric dimers are also key relevance of the Br/N halogen bonding in driving the self-
formed but linked through C–Br/Br–C intermolecular type I assembly of 3 to give the 1D polymeric chain architecture
dihalogen interactions (Fig. 2 and Table 2). The Br/Br (Fig. 3). These chains are further linked into 2D sheets through
˚
distances 3.5747(6) A are notably shorter than the sum of two C–H/p interactions (Fig. 3 and Table 4).
24
˚
van der Waals radii for bromine (1.85 A). This proves the key
According to the Hirshfeld surface analysis, for the molecule
relevance of the Br/Br halogen interaction in driving the self- of 1, intermolecular H/H contacts, comprising 35.6% of the
assembly of 2 to give the dimeric architecture (Fig. 2). These total number of contacts, are major contributors to the crystal
dimers are further linked into 1D ribbons due to p/p stacking packing (Table 5). A proportion of the same contacts notably
interactions between the same type of aromatic rings (Fig. 2 and decreases in the structure of 2 and further decreases in the
Table 3).
structure of 3, comprising 29.9% and 26.1%, respectively (Table
In general, p/p stacking interactions in the structures of 1 5). The shortest H/H contacts are shown in the ngerprint
˚
and 2 are formed between almost perfectly at adjacent mole- plots of 1–3 at de + di z 2.2–2.4 A (Fig. S1–S3 in the ESI†).
cules, situated on top of each other, with a slippage distance of Furthermore, a subtle feature is evident in the ngerprint plot
about one-half or one benzene ring diameter (Table 3). “Head- of 1. There is a splitting of the short H/H ngerprint. This
to-tail” or “head-to-head” stackings are adopted in 1 and 2, splitting occurs when the shortest contact is between three
Fig. 2 View on the C–Br/Br–C dihalogen bonded dimers, linked through p(Py)/p(Py) and p(C6H3)/p(C6H3) stacking interactions, in the
structure of 2. Colour code: C ¼ black, H ¼ light grey, N ¼ blue, O ¼ red, Br ¼ brown.
Fig. 3 View on the C–Br/N halogen bonded 1D polymeric chains, linked through C–H(Py)/p(Py) and C–H(C6H3)/p(C6H3) interactions, in the
structure of 3. Colour code: C ¼ black, H ¼ light grey, N ¼ blue, O ¼ red, Br ¼ brown.
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Table 4 C–H/p interaction distances (A˚) and angles (ꢀ) for 3a
C–H
Cg(J)
d[H/Cg(J)]
d[C/Cg(J)]
:[C–H/Cg(J)]
H-Perp
C(7)–H(7)
C(12)–H(12)
Cg(2)#1
Cg(1)#2
2.98
2.76
3.616(4)
3.450(5)
127
132
ꢁ2.95
ꢁ2.76
a
H-Perp: perpendicular distance of H to ring plane J. Symmetry transformations used to generate equivalent atoms: #1 1/2 + x, 1 ꢁ y, z; #2 ꢁ1/2 + x, 1
ꢁ y, z. Cg(1): N(14)–C(13)–C(12)–C(11)–C(16)–C(15); Cg(2): C(2)–C(3)–C(4)–C(5)–C(6)–C(7).
atoms, rather than for a direct two-atom contact.20 The struc- H/C contacts in the ngerprint plot of 3 are shown in the form
tures of 1–3 are also dominated by H/C and H/Br contacts. of clearly pronounced “wings” (Fig. S3 in the ESI†), with the
˚
While the latter contacts occupy a similar proportion of the shortest de + di z 2.8 A. These contacts are recognized as
molecular surfaces of 1–3 (17.0–19.5%), the former contacts characteristic of C–H/p nature,20 and correspond to the
reveal an opposite trend, compared to the H/H contacts, and abovementioned interactions, which link 1D polymeric chains
comprise 13.8%, 17.8% and 29.5%, of the total Hirshfeld into 2D sheets in the structure of 3 (Fig. 3 and Table 4). The H/
surface areas, respectively. Notably, a proportion of H/C Br contacts in the ngerprint plots of 1–3 are shown in the form
˚
contacts is the most dominant one in the structure of 3. The of a pair of broad “horns” with the shortest de + di z 2.9–3.1 A
Table 5 2D fingerprint plots, Hirshfeld contact surfaces and derived “random contacts” and “enrichment ratios” for 1–3
H
C
N
O
Br
H
C
N
O
Br
H
C
N
O
Br
Contacts (C, %)a
H
C
N
O
Br
35.6
13.8
4.3
8.4
19.3
—
—
—
1.0
0.2
1.4
—
—
—
0.0
0.9
—
—
—
—
0.0
29.9
17.8
7.4
8.6
19.5
—
—
—
0.8
0.2
0.0
—
—
—
0.0
0.4
—
—
—
—
1.3
26.1
29.5
10.4
6.1
—
—
—
0.0
0.1
3.5
—
—
—
0.0
0.0
—
—
—
—
0.3
10.7
3.8
0.0
0.6
8.2
5.1
0.2
0.7
2.0
0.0
3.2
1.6
17.0
Surface (S, %)
58.5
19.8
5.9
4.8
11.1
56.6
20.1
7.2
4.7
11.6
57.6
19.2
7.0
4.7
11.4
Random contacts (R, %)
H
C
N
O
Br
34.2
23.2
6.9
5.6
13.0
—
—
—
0.3
0.6
1.3
—
—
—
0.2
1.0
—
—
—
—
1.2
32.0
22.8
8.2
5.3
13.1
—
—
—
0.5
0.7
1.7
—
—
—
0.2
1.1
—
—
—
—
1.3
33.2
22.1
8.1
5.4
13.1
—
—
—
0.5
0.7
1.6
—
—
—
0.2
1.1
—
—
—
—
1.3
3.9
2.3
1.9
4.4
4.0
2.9
1.9
4.7
3.7
2.7
1.8
4.4
Enrichment (E)b
H
C
N
O
Br
1.04
0.59
0.62
1.50
1.48
—
—
—
—
—
—
—
—
—
—
—
—
—
0.93
—
—
—
—
—
—
—
—
—
—
—
—
—
0.79
—
—
—
—
—
—
—
—
—
—
—
—
—
2.74
1.65
0.00
0.14
0.78
0.90
1.62
1.49
2.05
1.76
0.11
0.15
1.33
1.28
1.13
1.30
0.54
0.00
1.78
0.36
1.08
0.90
0.00
0.00
0.36
1.00
2.19
0.00
0.23
a
b
Values are obtained from CrystalExplorer 3.1.22 The enrichment ratios were not computed when the “random contacts” were lower than 0.9%, as
they are not meaningful.23
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(Fig. S1–S3 in the ESI†). These shortest H/Br contacts in the
The enrichment ratios (E)23 of the intermolecular contacts
structure of 1 correspond to the above mentioned C–H/Br–C for 1–3 were also determined to study the propensity of two
hydrogen bonds responsible for the formation of 1D ribbons chemical species to be in contact. The enrichment ratio, derived
from R2 (16) centrosymmetric dimers (Fig. 1 and Table 2).
from the Hirshfeld surface analysis, is dened as the ratio
2
Further contributions into molecular surfaces of 1–3 arise between the proportion of actual contacts in the crystal and the
from H/N and H/O contacts (Table 5). While the latter theoretical proportion of random contacts. E is larger than unity
contacts occupy a similar proportion of the total Hirshfeld for pair of elements with a higher propensity to form contacts,
surface in all the structures (6.1–8.6%), the fraction of the while pairs which tend to avoid contacts yield a E value lower
former contacts proportionally increases from 1 (4.3%) to 2 than unity.
(7.4%) to 3 (10.4%). The H/O contacts in the ngerprint plot of
H/H contacts are favoured in the structure of 1 since the
1 are shown as a pair of relatively sharp spikes with the shortest enrichment ratio EHH ¼ 1.04 and generate a majority (35.6%) of
˚
de + di z 2.5 A (Fig. S1 in the ESI†), and correspond to the above the interaction surface (Table 5). Contrarily, H/H contacts are
mentioned C–H/O–H hydrogen bonds responsible for the less favoured in the structure of 2 (EHH ¼ 0.93) and even much
formation of R22(16) centrosymmetric dimers (Fig. 1 and less favoured in the structure of 3 (EHH ¼ 0.79). This is explained
Table 2).
by a signicantly lower proportion (29.9% and 26.1%, respec-
The molecular surface of 1 and 2 are further characterized by tively) of the H/H contacts of the total Hirshfeld surface area in
C/C and C/N contacts, comprising 8.2–10.7% and 3.8–5.1%, 2 and 3, although their structures contain almost the same
respectively (Table 5). While the former contacts are signi- amount of random contacts RHH as well as each are character-
cantly impoverished (2.0%) in the structure of 3, the latter ized by a high SH proportion as in the structure of 1 (Table 5).
contacts are completely absent. The C/C contacts are shown The opposite trend is observed for H/C and H/N contacts,
on the ngerprint plots of 1 and 2 as the characteristic pale which show an increased propensity to form (EHC ¼ 1.33, EHN
¼
¼
¼
blue/green, mixed with yellow points, area on the diagonal at de 1.28) in the structure of 3, and only slightly favoured (EHC
˚
¼ di z 1.7–2.0 A (Fig. S1 and S2 in the ESI†), and attributed to 0.78, EHN ¼ 0.90) in 2, and impoverished (EHC ¼ 0.59, EHN
the formation of the above mentioned strong p/p stacking 0.62) in 1. This is explained by comparable higher amounts of
interactions (Fig. 1, 2 and Table 3). The C/N contacts are H/C and H/N contacts of the total Hirshfeld surface area
shown on the ngerprint plots of 1 and 2 as the characteristic compared to those in 2 and 1, despite both structures are
˚
symmetric area on the same diagonal at de ¼ di z 1.8–2.0 A characterised by almost the same values of the SH, SC and SN
(Fig. S1 and S2 in the ESI†), and also correspond to strong p/p proportions, and random contacts RHC and RHN (Table 5). The
stacking interactions (Fig. 1, 2 and Table 3). It should be noted, EHO and EHBr values are larger than unity (1.13–1.62) for all
that the difference in the nature of p/p stacking interactions molecules, indicating that H/O and H/Br contacts have an
in the structures of 1 and 2 is clearly reected in a higher increased propensity to form, with extremely close random
proportion of C/C contacts and a lower proportion of C/N contacts (RHO ¼ 5.3–5.6, RHBr ¼ 13.0–13.1).
contacts in the former structure compared with those in the
Remarkably, the structure of 2 is further characterized by
latter one (Table 5). Notably, Br/Br and N/Br contacts, highly favoured C/C contacts (ECC ¼ 2.05), while the same
responsible for the halogen interactions in the structures 2 and contacts in the structure of 1 are even more favoured (ECC
¼
3, respectively, comprise 1.3% and 3.5% of the total Hirshfeld 2.74). This is due to a negligible amount of random contacts RCC
surface areas. The Br/Br contacts are shown on the ngerprint (3.9–4.0%). Despite the molecule of 3 is characterised by almost
plot of 2 as the pale blue/green, mixed with yellow points, sharp the same proportion of random contacts RCC (3.7%), the C/C
˚
spike on the diagonal at de ¼ di z 1.8–2.2 A (Fig. S2 in the ESI†). contacts are signicantly impoverished (ECC ¼ 0.54), which is
The N/Br contacts in the ngerprint plot of 3 are shown as explained by a negligible proportion of these contacts (2.0%) on
˚
a pair of sharp spikes with the shortest de + di z 3.1 A (Fig. S3 in the total Hirshfeld surface of 3. Furthermore, the C/N contacts
the ESI†). While the Br/Br contacts are completely absent, are highly favoured in the structures of 1 and 2 since the cor-
a minor proportion of the N/Br contacts (1.4%) were also responding enrichment ratios ECN are higher than unity (1.65
found on the molecular surface of 1 (Table 5). However, these and 1.76, respectively). Interestingly, while the ENBr value is
contacts are shown on the ngerprint plot of 1 as a pair of small slightly higher than unity for the structure of 1, the same
˚
spikes with the shortest de + di z 3.6 A (Fig. S1 in the ESI†), contacts are much more favoured in the structure of 3 (ENBr
¼
which is signicantly longer than the sum of van der Waals radii 2.19), which is explained by a signicantly higher amount of
24
˚
˚
for nitrogen (1.55 A) and bromine (1.85 A).
N/Br contacts on the molecular surface of 3 compared to that
It is worth to mention, that the molecular surface of 3 is also of 1 (Table 5). Moreover, a similar trend can be applied to
populated by C/O and C/Br contacts, comprising 3.2% and explain highly favoured C/O contacts in the structure of 3 (ECO
1.6%, respectively. Close inspection of other intermolecular ¼ 1.78), while the same contacts are very impoverished in the
contacts in the structures of 1 and 2 also revealed a negligible structure of 2 (ECO ¼ 0.11) and completely absent in 1 (Table 5).
proportion of C/Br (0.6–0.7%), N/N (0.8–1.1%), N/O (0.2%) The opposite trend is found for the O/Br contacts. In partic-
and O/Br (0.4–0.9%), as well as C/O (0.2%) in 2, contacts ular, while these contacts are almost favoured in the structure of
(Table 5, Fig. S1 and S2 in the ESI†). The molecular surface of 3 1 since the enrichment ratio EOBr is close to unity (0.90), the
also exhibits a negligible proportion of N/O (0.1%) and Br/Br same contacts are signicantly impoverished (0.36) in 2 and
(0.3%) contacts.
completely absent in 3. This is obviously due to a remarkable
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difference in the proportion of the O/Br contacts on the intense bands centred at about 570 and 595 nm are observed for
molecular surfaces of 1–3 (0.0%, 0.4% and 0.9% respectively), 3 and, most likely, for 2 with a single broad band at 570 nm,
while very similar values for SO (4.7–4.8%), SBr (11.1–11.6%) and while only one relatively narrow band centred at 585 nm is
ROBr (1.0–1.1%) were found (Table 5). Finally, the C/Br (ECO
¼
observed in the spectrum of 1. The uorescence has been
0.14–0.36) contacts in all the structures, as well as Br/Br assigned to the emission from the cis-keto form, which is
contacts in 3 (EBrBr ¼ 0.23), are signicantly impoverished, produced through proton transfer in the excited-state of the
while the structure of 2 is enriched by Br/Br contacts as evi- enol form. Therefore, an anomalously large Stokes shi is
denced from the enrichment ratio EBrBr equal to unity.
observed. The longer wavelength band originates from the
Compounds 1–3 were analyzed by diffuse reectance spec- emission of the more stable and relaxed conformation of the cis-
troscopy (DRS) as pure solid powders to avoid matrix and keto form and the band at the shorter wavelength is due to the
environment effects that are known to intensively modify the emission from the less stable planar conformation.27 A third
optical properties of N-salicylidene aniline derivatives.15b,c
weak emission band is also noted at about 650 nm for 1–3
Diffuse reectance spectra of 1–3, for which a Kubelka– (Fig. 5), which is much more pronounced in the spectrum of 1,
Munk (KM) treatment was applied, each exhibit three band in the region of the trans-keto form, as deduced from diffuse
regions: a broad band range in the UV region, corresponding to reectance data (Fig. 4).
the enol form, a second range in the visible region from about
Excitation spectra of 1–3 recorded at lem ¼ 620 nm each reveal
375 to 450 nm, originating from the cis-keto form, and a third two high-energy contributions centred at about 420 and 500 nm
range above 450 nm, originating from the trans-keto form (Fig. 5), with the latter band being signicantly less pronounced
(Fig. 4). A major contribution of the trans-keto form is found for in the spectrum of 1. These bands are assigned to the emission of
1 whereas a moderate fraction is detected for 2, and only traces two different conformers of the cis-keto* forms in the excited
of the trans-keto form were observed for 3 (Fig. 4). All state. The low-energy band is due to the emission from the more
compounds were also analyzed by DRS upon irradiation at stable and relaxed conformation, whereas the high-energy band
selected wavelengths (l ¼ 254, 365, 450 and 546 nm) in order to is due to the emission from the less stable planar conformation.27
photo-address enol and keto forms.
The spectra of 1 and 2 also exhibit a third low-energy band, which
Contrary to expectations based on dihedral angle F consid- is barely observed in the spectrum of 2, centred at about 590 nm
erations, 1 is photochromic upon irradiation at l ¼ 546 nm (Fig. 5). This band can be assigned to the absorption of the trans-
(Fig. 4), although it was expected to be only thermochromic (F < keto form by comparison with diffuse reectance spectroscopy
25ꢀ).19 A similar negative photochromism was observed for N- (Fig. 4). The emission at about 650 nm can thus be assigned to
(3,5-dichlorosalicylidene)-1-aminopyrene25 and 15-crown-5 the radiative relaxation of the trans-keto* form, which can be
ether-containing N-salicylidene aniline.26 Furthermore, both 2 formed through the absorption of enol, cis-keto and trans-keto
and 3 are not photochromic regardless of the irradiation forms in their ground state.28
wavelength and time, whereas 3 was expected to be also
photochromic (F > 25ꢀ).
Optical properties of N-salicylidene aniline derivatives were
rst systematically studied by Cohen and Schmidt in the sixties
Solid state uorimetric studies of 1–3 crystals were under- of the past century,29 and are still under debate being in the
taken to examine energy levels responsible for the photo- limelight of many researches.30 UV irradiation of the enol form
switchable optical properties. The emission spectra at lexc ¼ 400 leads to the formation of the excited enol* form (Fig. 6). This
nm, which corresponds to the band of the cis-keto form in the species can uoresce in the visible region in some rare cases,15b
diffuse reectance spectra (Fig. 4), are shown in Fig. 5. Two or complete a fast tautomerization to the excited cis-keto* form.
Fig. 5 Normalized solid-state emission and excitation spectra of 1 (Em
Fig. 4 Normalized KM spectra of 1 before (black) and after (red) irra- – black, Ex – purple), 2 (Em – red, Ex – olive) and 3 (Em – blue,
diation at lexc ¼ 546 nm for 30 min, 2 (blue) and 3 (purple) at 23 ꢀC.
Ex – brown) at 23 ꢀC (lexc ¼ 400 nm, lem ¼ 620 nm).
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intramolecular hydrogen bond of the O–H/N type, formed
between the hydroxyl hydrogen atom of the phenolic ring and
the imine nitrogen atom. In addition, the dihalogen C–Br/Br–
C and halogen C–Br/N(Py) interactions play a crucial role for
the formation of supramolecular architectures in the crystal
structures of 2 and 3, respectively. The overall geometry of each
molecule in the structures of 1 and 2 was found to be almost
planar, while a signicantly twisted structure was established
for 3.
Hirshfeld surface analysis showed that the structures of all
compounds are mainly characterized by H/X contacts as well
as a remarkable contribution from C/C and C/N contacts
which is clearly observed for 1 and 2.
Diffuse reectance spectra of 1–3 each reveal a mixture of the
enol, cis-keto and trans-keto forms. A major contribution is
found for 1 whereas a moderate fraction is detected for 2, and
only traces of the trans-keto form were observed for 3. Contrary
to expectations based on dihedral angle F considerations, 1
exhibits negative photochromism, although it was expected to
only be thermochromic. Both 2 and 3 are not photochromic,
whereas 3 was expected to be also photochromic (F > 25ꢀ). We
have postulated that highly favoured C/C, C/N and O/Br
intermolecular contacts as well as the absence of Br/Br
dihalogen interactions in the structure of 1, are mainly
responsible for the photochromic behaviour. Furthermore,
signicantly impoverished H/C and H/N contacts further
support the observed negative photochromism of 1. These
pyridine containing dyes bear potential interest in iron(II)
coordination chemistry since one could expect to get functional
materials whose spin state could be modied by ne tuning of
optical properties.15b
Fig. 6 Photochemical process leading to the formation of the trans-
keto form from the enol form for N-salicylideneaniline aniline deriv-
atives. The solid arrows indicate absorption phenomena or emission
relaxation and the dotted arrows represent non-radiative relaxation.
The cis-keto* to cis-keto relaxation is mainly radiative and is
always detected by uorescence spectroscopy. The cis-keto*
form can also undergo photoisomerization to produce the
excited trans-keto* form, which is described to be non-
uorescent. It is assumed that the trans-keto* to trans-keto
relaxation is mostly non-radiative. Back reaction from trans-keto
to cis-keto and enol forms is either thermally or photochemi-
cally induced. The latter reaction, namely negative photochro-
mism in the present case, was observed for 1 (Fig. 4).
Among the reported structures, only 1 exhibits pronounced
photochromic properties, although of negative nature. This is
surprising since 1, as well as 2, were expected to be exclusively
thermochromic, while 3 was expected exclusively photochromic
based on the widely shared structural perceptions for N-salicy-
lidene aniline derivatives.14 Elucidated contradictions within 1–
3, most likely, can be explained by the ne combination of
favoured and even highly favoured C/C, C/N and O/Br
contacts as well as the absence of Br/Br dihalogen interactions
(Table 5). Furthermore, signicantly impoverished H/C and
H/N contacts (Table 5) further support the observed negative
photochromism of 1. Moreover, these supramolecular factors
can also inuence on the energy differences between ground
and excited states, which, in turn, can favour the formation of
the keto form. Detailed theoretical studies which are in progress
in our laboratory are required to check this latter hypothesis.
Experimental
Physical measurements
1H NMR spectra in CDCl3 were obtained on a Bruker AC 300
MHz spectrometer at 25 ꢀC. Diffuse reectance spectra were
obtained with a Varian Cary 5E spectrometer using polytetra-
uoroethylene (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. Light irradiations were
carried out with a LOT-ORIEL 200 W high-pressure mercury Arc
lamp (LSN261). Elemental analyses were performed on a CHNS
HEKAtech EuroEA 3000 analyzer.
Conclusions
Hirshfeld surface analysis
In summary, a series of three closely related N-(5-bromosalicyli- The Hirshfeld molecular surfaces20 and their associated 2D
dene)-x-aminopyridine compounds 1–3 has been successfully ngerprint plots21 were generated using the CrystalExplorer 3.1
prepared by the condensation reaction of 5-bromosalicylaldehyde soware22 on the basis of crystal structures. The dnorm
with the corresponding aminopyridine. All the compounds were (normalized contact distance) surface and the breakdown of the
studied by means of 1H NMR spectroscopy in solution, revealing 2D ngerprint plots were used for decoding and quantifying the
the presence of a single structure at least in CDCl3.
intermolecular interactions in the crystal lattice. The dnorm is
According to single crystal X-ray diffraction, it was estab- a symmetric function of distances to the surface from the nuclei
lished, that the crystal structures of 1–3 are stabilized by a linear inside (di) and outside (de) the Hirshfeld surface, relative to
53676 | RSC Adv., 2016, 6, 53669–53678
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their respective van der Waals radii. 2D ngerprint plots were hydrogen atoms were placed on calculated positions in riding
˚
generated using di and de in the translated 0.4–3.0 A range and mode with temperature factors xed at 1.2 times Ueq of the
including reciprocal contacts as a pair of coordinates in 2D parent atoms. Figures were generated using the program
histograms. A colour gradient in the ngerprint plots ranging Mercury.33
from blue to red is used to visualize the proportional contri-
bution of contact pairs in the global surface.
Crystal data for 2. C12H9BrN2O, Mr ¼ 277.12 g molꢁ1
,
monoclinic, space group P21/n, a ¼ 4.5103(3), b ¼ 19.8005(15), c
3
ꢀ
˚
˚
¼ 12.1942(8) A, b ¼ 95.592(7) , V ¼ 1083.84(13) A , T ¼ 297(2) K,
Z ¼ 4, r ¼ 1.698 g cmꢁ3, m(Mo-Ka) ¼ 3.770 mmꢁ1, reections:
10 312 collected, 1934 unique, Rint ¼ 0.043, R1(all) ¼ 0.0426,
wR2(all) ¼ 0.0979.
Enrichment ratio
The enrichment ratio (E)23 of a pair of elements (X, Y) is the ratio
between the proportion of actual contacts in the crystal and the
theoretical proportion of random contacts. E is larger than unity
for pairs of elements which have a high propensity to form
contacts in crystals, while pairs which tend to avoid contacts
with each other yield an E value lower than unity. E values are
calculated from the percentage of contacts, which, in turn, are
given by the CrystalExplorer 3.1 soware,21 between one type or
two types of chemical elements in a crystal packing.
Crystal data for 3. C12H9BrN2O, Mr ¼ 277.12 g molꢁ1
,
orthorhombic, space group Pca21, a ¼ 6.1946(6), b ¼ 7.0152(7), c
3
˚
˚
¼ 25.100(2) A, V ¼ 1090.76(17) A , T ¼ 293(2) K, Z ¼ 4, r ¼ 1.688
g cmꢁ3, m(Mo-Ka) ¼ 3.746 mmꢁ1, reections: 9101 collected,
2076 unique, Rint ¼ 0.047, R1(all) ¼ 0.0540, wR2(all) ¼ 0.1185.
Acknowledgements
This work was funded by the Fonds National de la Recherche
Scientique (FNRS) (PDR T.0102.15) and COST actions MP1202
and CA15128.
Synthesis of 1–3
A solution of 5-bromosalicylaldehyde (10 mmol, 2.010 g) dis-
solved in ethanol (20 mL) was added to a solution of 2-, 3- or 4-
aminopyridine (10 mmol, 0.941 g) in the same solvent (20 mL).
The mixture was stirred under reux for 0.5 h. The resulting
solution was allowed to cool to room temperature to give X-ray
suitable crystals of 1–3.
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1. 1H NMR: d ¼ 6.92 (d, 3JH,H ¼ 8.8 Hz, 1H, 3-H, C6H3), 7.24 (t.
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€
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