Syntheses, crystal structures, optical limiting properties, and DFT calculations of three thiophene-2-aldazine Schiff base derivatives

Article abstract of DOI:10.1039/b704009a

The syntheses, characterizations and structural determinations of N,N′-bis(thiophenyl-2-methylene)hydrazine 1, N,N′-bis(4- bromothiophenyl-2-methylene)hydrazine 2 and N,N′-bis(5-bromothiophenyl-2- methylene)hydrazine 3 are presented. The materials show third-order nonlinear behaviour with transmissions of 20, 22 and 18 μJ for an input energy of 150 μJ. The 4- and 5-bromothiophenic structures show Br...Br interactions of 3.562 and 3.626 A, respectively. Analysis of bromine containing aromatic compounds in the Cambridge Crystallographic Database (CSD) showed the expected angle dependences of the Br...Br interactions dividing these into "type I" and "type II". A semi-quantitative agreement was found between the CSD data and a model derived form calculated electrostatic potentials. The Royal Society of Chemistry and the Centre National de la Recherche Scientifique.

Full text of DOI:10.1039/b704009a

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PAPER
www.rsc.org/njc | New Journal of Chemistry
Syntheses, crystal structures, optical limiting properties, and DFT
calculations of three thiophene-2-aldazine Schiff base derivativesw
Mohamed Ghazzali,ad Vratislav Langer,a Cesar Lopes,b Anders Erikssonc and
a
¨
Lars Ohrstrom*
¨
Received (in Montpellier, France) 16th March 2007, Accepted 1st June 2007
First published as an Advance Article on the web 28th June 2007
DOI: 10.1039/b704009a
The syntheses, characterizations and structural determinations of N,N0-bis(thiophenyl-2-
methylene)hydrazine 1, N,N0-bis(4-bromothiophenyl-2-methylene)hydrazine 2 and N,N0-
bis(5-bromothiophenyl-2-methylene)hydrazine 3 are presented. The materials show third-order
nonlinear behaviour with transmissions of 20, 22 and 18 mJ for an input energy of 150 mJ. The
4- and 5-bromothiophenic structures show Brꢀ ꢀ ꢀBr interactions of 3.562 and 3.626 A, respectively.
Analysis of bromine containing aromatic compounds in the Cambridge Crystallographic
Database (CSD) showed the expected angle dependences of the Brꢀ ꢀ ꢀBr interactions dividing these
into ‘‘type I’’ and ‘‘type II’’. A semi-quantitative agreement was found between the CSD data and
a model derived form calculated electrostatic potentials.
materials and were not themselves expected to be highly active.
Introduction
However, a systematic study of the OL properties of thio-
phene-2-aldazines derivatives may increase the understanding
of this phenomenon and help in the development of a rational
approach to materials with optimized properties.
Bromine containing aromatic compounds have important
applications on their own, a current issue is for example
bromine containing flame retardants,1 but are also versatile
starting materials for the preparation of larger molecular units
via, for example, Suzuki or Sonogashira couplings. Moreover,
it has been realized that halogen substituents can form inter-
molecular bonds with various acceptor groups,2 or even with
other halogens,3,4 thus rendering these compounds useful to
the growing field of ‘‘crystal engineering’’.5
Results and discussion
Synthesis
This article deals with the structural chemistry and inter-
molecular interactions of the two bromo-derivatives (Scheme
1) N,N0-bis(4-bromothiophenyl-2-methylene)hydrazine 2 and
N,N0-bis(5-bromothiophenyl-2-methylene)hydrazine 3, in par-
ticular the differences to the parent N,N0-bis(thiophenyl-2-
methylene)hydrazine 1. Specifically we will discuss the role
of the Brꢀ ꢀ ꢀBr interaction in competition with other inter-
molecular forces.
Compounds 1–3 were synthesized adopting standard proce-
dures8 by condensation of thiophene-2-aldehyde, 4-bro-
mothiophene carbaldehyde and 5-bromothiophene car-
baldehyde, respectively, with hydrazine monohydrate giving
yields of 79–88%.
Compound 2 is described here for the first time, while the
preparation of compound 3 has been mentioned in preliminary
reports only9 and a complete characterization is given in the
current communication. The parent compound N,N0-bis(thio-
phenyl-2-methylene)hydrazine 1 was prepared for reference
purposes.
We also discuss the C–Brꢀ ꢀ ꢀBr–C interaction in general,
both by analysis of data in the Cambridge Crystallography
Database and through quantum chemistry and simple models.
Moreover, we have earlier reported on the optical limiting
(OL) properties for a number of thiophene–acetylene com-
pounds,6,7 and we include similar data for 1–3 in this study. In
fact, 2 and 3 were prepared as versatile building blocks for OL
a Department of Chemical and Biological Engineering, Chalmers
Tekniska Hogskola, SE-412 96 Goteborg, Sweden. E-mail:
¨
mghazz@chalmers.se
¨
b Department of Functional Materials, Swedish Defence Research
Agency, SE-581 11 Linkoping, Sweden
¨
c Department of Laser Systems, Swedish Defence Research Agency,
SE-581 11 Linkoping, Sweden
¨
d Department of Chemistry, Faculty of Science, Alexandria University,
P.O.426, 21321 Alexandria, Egypt
w Electronic supplementary information (ESI) available: Table S1 for
bond lengths and Table S2 for bond angles for 1, 2 and 3, and DFT
optimized coordinates for 1, 2 and 3. See DOI: 10.1039/b704009a
Scheme 1
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Table 1 Crystallographic, diffraction and structural refinement data for azine 1, 2 and 3, respectively
1
2
3
Empirical formula
Formula weight
Crystal system
Space group
C10H8N2S2
220.30
Monoclinic
P21/n
C10H6Br2N2S2
378.11
Monoclinic
P21/c
C10H6Br2N2S2
378.11
Monoclinic
P21/c
a/A
b/A
9.6504(7)
11.4015(8)
9.6510(7)
100.871(1)
1042.83(13)
4.5456(2)
26.9807(13)
10.0137(5)
97.484(1)
1217.65(10)
10.6766(6)
11.5253(6)
10.2713(6)
96.797(1)
1255.01(12)
4
c/A
b/1
V/A3
Z
F(000)
4
456
4
728
728
1.10 ꢂ 0.18 ꢂ 0.12
Crystal size/mm3
y range for data collection/1
Reflections collected
Independent reflections
Rint
0.28 ꢂ 0.18 ꢂ 0.16
0.92 ꢂ 0.19 ꢂ 0.02
2.15–33.04
18997
2.19–33.03
21672
2.61–25.37
10951
3775
0.0361
4395
0.0359
2298
0.0942
Max., min. transmission
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices [I 4 2s(I)]
R indices (all data)
0.9287, 0.8798
3775/0/136
0.995
R1 = 0.0346, wR2 = 0.0913
R1 = 0.0418, wR2 = 0.0974
0.8732, 0.0602
4395/0/151
0.988
R1 = 0.0304, wR2 = 0.0730
R1 = 0.0416, wR2 = 0.0779
0.4974, 0.0503
2298/0/151
1.037
R1 = 0.0490, wR2 = 0.1112
R1 = 0.0854, wR2 = 0.1259
Crystal structure analysis
treating the twining correctly artificially generated electron
density peaks disappeared, thus removing doubts about the
correctness, of the structure.
It should be noted that the room-temperature crystal structure
of 1 has been described earlier,10 although the twinning
phenomenon of the crystals was not recognized and conse-
quently the data in this report are of significantly higher
precision.
The intermolecular arrangements are not discussed in ref. 10
and are found to be a mixture of weak interactions, somewhat
surprisingly excluding any significant p–p stacking. Instead the
crystal is held together by numerous s–p C–H to p interac-
tions both from the rings and the chains (C–Hꢀ ꢀ ꢀC 2.75–2.86
A). As the ring–ring interactions (Fig. 2 right) are in the
c-direction and the ring–chain interactions are in the a-direc-
tion (and propagated along the molecules in the b-direction)
an intricate 3D pattern is formed, see Fig. 3. The C–Hꢀ ꢀ ꢀS
(3.27, 4.02 A and 1401) and C–Hꢀ ꢀ ꢀN (2.75, 3.50 A and 1381)
hydrogen bonds are very weak,11 if at all significant, and
sulfur–sulfur interactions are also absent. (In this and follow-
ing paragraphs hydrogen bond interactions are presented as
Hꢀ ꢀ ꢀA and Dꢀ ꢀ ꢀA distances and the D–Hꢀ ꢀ ꢀA angle.)
All three compounds crystallize in the same monoclinic
space group and crystallographic data are summarized in
Table 1.
The structure of azine 1 contains two crystallographic
independent molecules A and B, each situated on an inversion
centre and with almost no torsions in the chain (176–1801),
and the two thiophene rings in each molecule are parallel, see
Fig. 1. However, the mean planes defined by these two rings
have inter-planar distances of 0.277 and 0.207 A for A and B,
respectively, thus the entire molecule cannot be described as
truly planar.
In contrast to the molecular structure of 1, in azine 2 the
mean planes defined by the two thiophenes are no longer
parallel having a dihedral angle of 5.67(10)1 between them.
The major reason for this is a torsion of the central N–N bond
that now has a CQN–NQC angle of 175.88(17)1 compared to
180.01 (enforced by symmetry) in 1, see Fig. 4.
Neither in this structure is there any classical p–p stacking,
however, another p–p stacking motif may be found as the
thiophene rings and the CQN–NQC chain are stacked in a
The structure is twinned around a twofold axis and simulat-
ing a C-centred orthorhombic cell with a twinning ratio of
0.492(1)/0.508(1). The earlier report on this structure is qua-
litatively the same,10 but by taking the twinning into account
we have obtained a significantly higher precision, i.e. standard
deviations of 0.002 A, vs. 0.01 A in ref. 10. Moreover, by
Fig. 1 ORTEP type diagram of 1. Thermal ellipsoids are drawn
Fig. 2 The s–p C–H to p interactions in 1, left chain–ring and right
ring–ring.
at 50%.
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Fig. 5 Staircase like stacking of the thiophene rings and the
CQN–NQC chain in 2, giving closest Cꢀ ꢀ ꢀN and Sꢀ ꢀ ꢀN distances of
3.378(3) and 3.502(3) A, respectively.
Fig. 3 Packing diagram of 1.
staircase like way giving closest Cꢀ ꢀ ꢀN and Sꢀ ꢀ ꢀN distances of
3.378(3) and 3.502(3) A, respectively. This may be due to the
donor properties of the thiophene unit interacting with the
acceptor character of the carbon–nitrogen double bond, see
Fig. 5.
Fig. 6 Brꢀ ꢀ ꢀBr interactions in 2 shown perpendicular to the view in
Fig. 5. This results in a fish-bone like pattern of the stacks.
These stacks are interconnected by weak C–Hꢀ ꢀ ꢀS (2.88 A,
3.807 A and 166.61) hydrogen bonds but C–Hꢀ ꢀ ꢀN (2.83, 3.362
A and 1171) hydrogen bonds and sulfur–sulfur interactions are
absent in this structure. However, there are also Brꢀ ꢀ ꢀBr
interactions with a distance of 3.5618(4) A between the
bromine atoms, torsion angle (C–Brꢀ ꢀ ꢀBr–C) of 64.051 and
angles (Brꢀ ꢀ ꢀBr–C) of 99.831 and 176.701 giving raise to zigzag
chains along the b-axis, see Fig. 6.
The resulting structure has the staircase stacks propagating
in the c-direction with neighbouring stacks having the mole-
cular planes at 981 angles, see Fig. 7.
The molecular geometry of azine 3, see Fig. 8, is distinctively
none-planar with a angle of 20.1(3)1 between the planes
defined by the thiophene rings and a CQN–NQC torsion
angle of 170.8(6)1. This can be compared to molecules of N,N0-
bis(5-methylthiophenyl-2-methylene)hydrazine12 with the cor-
responding plane–plane angle of 20.21 and a CQN–NQC
torsion angle of 186.41. There is also a seemingly longer N–N
bond in 3 (1.410(8) A) compared to 1.406(2)–1.403(2) in 1 and
2 but this difference is not significant (the corresponding
distance in the methyl derivative is 1.408 A12).
Fig. 7 Packing diagram of 2, note that in neighbouring stacks the
molecular planes make 981 angles towards each other. C–Hꢀ ꢀ ꢀS
hydrogen bonds are indicated with dashed lines.
In compound 3, (Fig. 8) finally, there is classical p–p
stacking but only for one thiophene ring per molecule, see
Fig. 9. The shortest atom–atom distance, C4Aꢀ ꢀ ꢀS1A, is
3.707(8) A and the two rings are perfectly parallel. Compared
to 2 this gives a different packing pattern of the stacks
propagating in the same direction, in the former case the
Brꢀ ꢀ ꢀBr interaction gives a fish-bone pattern, in the case of 3
we get a 2D array of pair wise grouped azines. Also in this
Fig. 4 ORTEP type diagram of 2. Thermal ellipsoids are drawn
Fig. 8 ORTEP type diagram of 3. Thermal ellipsoids are drawn
at 50%.
at 50%.
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Fig. 9 Stacking interactions in 3.
case, however, we get Brꢀ ꢀ ꢀBr interactions with a Brꢀ ꢀ ꢀBr
distance of 3.626(1) A, (C–Brꢀ ꢀ ꢀBr–C) torsion angle of
48.581 and angles (Brꢀ ꢀ ꢀBr–C) of 94.581 and 152.751. This
gives zigzag chains this time running at approximately 201
angle with the a-axis. In this compound, again the C–Hꢀ ꢀ ꢀS
interactions are very weak, closest Hꢀ ꢀ ꢀS is 3.102 A.
Fig. 11 Packing diagram of 3.
Similar to compound 2 the stacks, or in this case better the
2D array, has neighbouring stacks turned about 901. Never-
theless, this similarity may not be significant as the two
systems have quite different intermolecular interactions, see
Fig. 10. The resulting packing diagram of 3 is found in Fig. 11.
In summary we can say that both substituents in 3- and
5-position will block the C–Hꢀ ꢀ ꢀp interactions found in 1,
whereas the C–Hꢀ ꢀ ꢀS interaction in 2 is blocked in 3 by
replacing this hydrogen with a bromine.
y2 = Br1ꢀ ꢀ ꢀBr2–C2) because of the polarization (anisotropy) of
the bromine electron density.14 The ‘‘type-I’’ interactions have
y1 = y2 and are usually 41101. This is illustrated by the
calculated electrostatic potentials (see DFT section for details)
for the 5-bromothiophene monomer in Fig. 12.
We have substantiated this by an analysis of the Brꢀ ꢀ ꢀBr
interactions in bromine substituted aromatic compounds in
the Cambridge Structural Database.15 1018 not-disordered
hits were found with 4049 Brꢀ ꢀ ꢀBr distances shorter than
5 A. These were divided into three groups: (1) interactions
smaller than twice the van der Waals radii (rvdW) (3.6 A); (2)
distances between 3.6 and 4.0 A (attractive interaction may
occur up to this value) and (3) Brꢀ ꢀ ꢀBr distances longer than
4 A.
Brꢀ ꢀ ꢀBr interactions
The nature of the interaction in the ‘‘halogen–halogen syn-
thon’’ has recently been reviewed and found to be weakly
attractive.3 Estimates of intermolecular forces are always
difficult and may vary largely from structure to structure,
but available figures put the C–Hꢀ ꢀ ꢀS hydrogen bond and
the Brꢀ ꢀ ꢀBr interaction in the same range as the p–p stacking.
This is not, however, uncontroversial as some studies indicate
that the directionality instead comes from minimizing ex-
change repulsion forces.13
A histogram showing the number of hits vs. distances is
displayed in Fig. 13 and a maximum can be seen at roughly
2rvdW + 0.4 A.16
In Fig. 14 y1 has been plotted against y2 and clear differences
can be seen between the three different Brꢀ ꢀ ꢀBr subsets.
For shorter interactions we have roughly as many hits on
the diagonal as off, and the diagonal hits, ‘‘type I’’, are centred
around 140–1601 in qualitative agreement with Awwadi et al.3
It may therefore be tempting to ascribe the difference in
structures between the parent azine, 1, and 2 and 3 as a
consequence of this, as the same ‘‘synthon’’ is found in both
structures. However, the 5-bromo derivative 3 is actually
isostructural with the 5-methyl derivative reported in ref. 10,
thus it is more likely that this interaction will only modulate
the overall structure. In this case the C–CMeꢀ ꢀ ꢀC–CMe angles
(153 and 1601) are changed to the 94.58 and 152.751 of the
bromo derivative.
This prompted us to make a closer investigation of the
C–Brꢀ ꢀ ꢀBr–C interactions of aromatic compounds. The type
of Brꢀ ꢀ ꢀBr interaction found in 2 and 3 has been described in
the literature as a ‘‘type-II interaction’’ and is predicted to
have angles of y1 = 1801 and y2 = 901 (y1 = C1–Br1ꢀ ꢀ ꢀBr2,
Fig. 12 Surfaces with isoelectrostatic potentials for 5-bromothio-
phene calculated by DFT. Solid surface has +10 eV and mesh surface
has ꢃ10 eV. The view has the bromine atom pointing outwards toward
the viewer and the molecular plane is horizontal.
Fig. 10 Comparing the perpendicular arrangements in 2 and 3.
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that it is only responsible for a minor modulation of the
overall structure.
Computational studies
Computing the intermolecular interactions accurately is a
difficult theoretical problem that needs to involve high levels
of theory such as MP2, and large basis sets, and such calcula-
tions are still relatively expensive. However, we expect the
workhorse of molecular computations, density functional
theory (DFT), to give a good picture of the electrostatic
potential around a molecule. This could then be used to gain
a qualitative understanding of the interaction.
The electrostatic iso-surfaces were presented in Fig. 12 and
based on this we can make a simple model. We reduce the
problem to two dimensions by cutting vertically through the
bromide atom in Fig. 12 and assume that the electron density
is still approximately spherical. We will then have a central
larger positive region with a width of about the radius of the
circle and peripheral regions of negative polarisation.
Fig. 13 Brꢀ ꢀ ꢀBr interactions in aromatic compounds in the CSD.
The off diagonal hits occur preferentially in the region y1:
150–1801 and y2: 80–1251 and are thus of ‘‘type II’’.
We can then discern the two ways of obtaining contacts
between negative and positive parts corresponding to the type
I and type II interactions mentioned before, see Fig. 15.
This picture explains the type I and type II interactions, but
there is nothing especially new in this. However, with this
model we can also compute the preferred angles y1 and y2.
Type I should thus have y1 = y2 = 1501 and type II should
have y1 = 1801 and y2 o1201. This is in agreement with the
CSD data analysis. Moreover, the observation of relatively
more type II interactions at shorter distances (about half the
hits) may perhaps be understood by considering what will
happen if the two spheres are pushed into each other. For type
I this will immediately give repulsion as same sign potentials
meet. On the contrary, for type II we can push the spheres a
considerable distance into each other before repulsion takes
place.
For the intermediate region the spread is larger, and half the
hits fall on the diagonal. This time these are centred around
130–1501 but with a prominent peak also at 70–1101. The off-
diagonal hits show a trend of y1 + y2 = 1801. These cases
correspond to p–p stacking with parallel C–Br bonds inter-
secting the skew-axis of the stacking (typically 100–1201)
giving the geometrical relation of y1 = 1801 ꢃ y2. The other
off-diagonal hits show a weak trend of a type II behaviour in
the region y1: 130–1801 and y2: 70–1251.
Finally, in the upper region of Brꢀ ꢀ ꢀBr distances where we
do not really expect any interactions the p–p stacking effect is
dominant.
While this study supports the proposed ‘‘type-II interac-
tion’’ model, it also indicates that other factors may be more
important for the overall structure. Indeed, it is highly likely
that a large proportion of the studied compounds contain
other important supramolecular ‘‘synthons’’ such as weak or
strong hydrogen bonds. For the present compounds, the
Brꢀ ꢀ ꢀBr interaction is at interplay with the s–p C–H to p
interactions found in 1 and the p-interactions and C–Hꢀ ꢀ ꢀS
hydrogen bonds in 2 and 3, and in the case of 3 it seems clear
A couple of other features of these systems can be tackled
theoretically: We observe a substantial torsion around the
N–N bond in 3 and a survey of the CSD indicates that this is
rare (5 out of 31 compounds have this angle 421). This
‘‘flatness’’ should be due to the partly double bond character
of this bond. However, we do not see a significant elongation
Fig. 14 The C–Brꢀ ꢀ ꢀBr–C angles y1 and y2 for the three different Brꢀ ꢀ ꢀBr criteria: left; interactions smaller than twice the van der Waals radii (3.6
A). 2. Middle; distances between 3.6 and 4.0 A (attractive interaction may occur up to this value) and right; Brꢀ ꢀ ꢀBr distances longer than 4 A.
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1.373 A found for the completely optimized planar structure,
and an energy difference to this conformer of +6 kJ molꢃ1
.
Indeed, this difference is within our experimental standard
deviations and too small to be detected by the most precise X-
ray measurements.
Fig. 16 shows the HOMOs and LUMOs for 1–3. Small but
significant differences are noted between the compounds; thus
the contribution of bromine lone pair orbitals to HOMO and
LUMO in 2 is much smaller than in 3, that also has completely
lost the sulfur character of the HOMO. This gives a stronger
antibonding nature to the HOMO orbital and, consequently, a
higher energy resulting in a lower HOMO–LUMO energy gap
(as the LUMOs are more alike). Moreover, this effect de-
pended on the strong polarizability of the bromine atom or a
better match of orbital energies, as is shown by complemen-
tary calculations on the 5-F and 5-Cl derivatives 3b and 3c
(Fig. S2, ESIw). These sets of results show an increasing
halogen contribution to the HOMO as the atomic number is
increased. (The major molecular orbital coefficients for the
halogens in the HOMO increase from 0.21 with F to 0.29
with Br.)
Fig. 15 Simple 2D model of the Brꢀ ꢀ ꢀBr interaction based on the
electrostatic potentials in Fig. 12.
of the N–N bond in the X-ray structure. Thus, we wanted to
predict the magnitude of this change.
We also wanted to know about the HOMO–LUMO struc-
ture of these compounds and how they are affected by the
substitution in different positions.
Full geometry optimizations were performed on all mole-
cules, and in addition a restricted optimization was run on 3
with three dihedral angles frozen so as to retain the deforma-
tion found in the crystal structure. For the 5-F and 5-Cl
derivatives 3b–3c of the 5-Br azine 3, only halogen–carbon
distances were optimized starting from the fully optimized
structure of 3.
Thus, if an electronic effect is desired on the HOMO–
LUMO system, a substitution in the 5-position will be more
efficient than in the 4-position.
Optical limiting measurements
As expected, all calculated structures are completely planar
and torsion free, thus indicating that the substantial torsions
in 2 and 3 are the effect of intermolecular interactions. The
torsion is primarily found around the N–N bond and thus
should be related to the bond-order of this bond, formally of
order one. However, substantial delocalization will increase
the double bond character of the N–N interaction, and indeed
for the crystal structure of thiophene 1 the N–N bond lengths
are in between the double bond (1.20 A) and the single bond
(1.50 A). As the experimental evidence of a bond length
increase was non-conclusive we also optimized a molecule 3a
with the three torsion angles along the chain frozen at the X-
ray values. This structure gave an N–N length of 1.377 A,
significantly (on the computational level) longer that the
The possibility to change the intensity of laser radiation in a
predetermined and predicted manner is one of the most
fundamental challenges in optics science. Since Maiman17
invented the first intense light beam based on a laser source,
the need for protection against this highly coherent radiation
has triggered research at different levels.18
An ideal optical power limiting (OPL) material, attenuates
the transmitted intensity of laser radiation to some specific safe
values. At the same time, the material should exhibit high
transmittance for normal light. Thus, such a material has non-
linear properties.19
As a part of our efforts to find such self-activating laser
radiation power limiters we have earlier reported results for a
number of thiophene–acetylene compounds.6,7 We are now
investigating thiophene-2-aldazines as these could easily be
Fig. 16 HOMO and LUMO DFT ‘‘molecular orbitals’’ and their
energies for 1, 2 and 3, respectively.
Fig. 17 Nonlinear transmission of 1–3 (50 mM in THF).
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conjugated with other classes of molecules (Pt–acetylene com-
plexes,20 porphyrines7,21) to prepare new OL materials.
Even though no great effect was expected for 2 and 3 a
systematic study of the OL properties of different derivatives
and building blocks may increase the understanding of this
phenomenon and help in the development of a rational
approach to materials with optimized properties.
Nd:YAG laser operating at 10 Hz, delivering 10-ns pulse
widths. The transmittances of 50 mM solutions of 1–3 in
THF were measured in 2 mm Quartz cells.
Search of the Cambridge Crystallography Database
Compounds in the Cambridge Crystallography Database,
CSD, version 5.27, November 2006 were searched.15 Only
organic structures were retrieved, and these were error and
disorder free with R-values lower than 10% and restricted to
bromides attached to a carbon having aromatic bonds to its
neighbours.
Fig. 17 shows the nonlinear optical behaviour of the com-
pounds in a logarithmic diagram. The input laser energy is
shown on the X-axis and the resulting transmittance (log(Iout
/
Iin)) of the solution has been plotted on the Y-axis. For an
ideal optical limiting material the transmittance should
approach zero for high input energies.
This is not the case for these compounds. However, the
materials show third-order nonlinear behaviour with trans-
mitted energies of 20, 22 and 18 mJ for an input laser energy of
150 mJ. These values are comparable to what has been
obtained for other, more elaborate, thiophene derivatives
(8–36 mJ), and thus encourage further studies of these com-
pounds.6,7
Materials
Hydrazine monohydrate and the thiophene carbaldehydes
corresponding to products 1–3 were purchased from Aldrich.
Solvents employed were of analytical reagent grade and were
used without further purifications. Analytical thin-layer chro-
matography (TLC) was carried out using Merck silica gel 60
plates. Single crystals of 1–3 suitable for X-ray diffraction were
obtained by slow evaporation of a methanol (99.5%) solution
of the respective compound over a period of 1 month.
It was also found that 3 has the highest linear absorption in
this study and this is in agreement with the frontier orbital
energy description as the presence of bromine atoms in the
position 5 implies an existence of a stronger electronic inter-
action at molecular level compared to 2 with bromine atoms at
the position 4.
Syntheses
N,N0-Bis(thiophenyl-2-methylene)hydrazine 1, N,N0-bis(4-
bromothiophenyl-2-methylene)hydrazine 2 and N,N0-bis(5-
bromothiophenyl-2-methylene)hydrazine 3 were synthesized
adopting standard procedures8,23 by condensation in an
95%-ethanolic solution of thiophene-2-aldehyde, 4-bro-
mothiophene carbaldehyde and 5-bromothiophene carbalde-
hyde, respectively (40 mmol) with hydrazine monohydrate (20
mmol). The resulting solutions were heated under reflux for
3–4 h and then a gradual cooling produced crystalline yellow
solids that were separated by filtration, washed by diethyl
ether and dried in vacuo giving yields of 81.5% (96% in ref.
10), 87.5 and 79%, respectively; mp 150 1C (148–149 1C in ref.
10 and 153 1C in ref. 16), 144 and 146 1C, respectively.
1H, 13C NMR (d/ppm, CDCl3) and IR (n/cmꢃ1, KBr) for 1
were found to be in good agreement with reported data.10 For
N,N0-bis(4-bromothiophenyl-2-methylene)hydrazine 2 we re-
port: IR (n, cmꢃ1, KBr): 509 (w), 590 (s, nCBr), 698 (w), 725
(m), 735 (m), 769 (w), 830 (m), 880 (m), 950 (m), 1180 (m),
1230 (m), 1280 (s), 1345 (m, nCQS), 1410 (s), 1500 (s), 1550 (w),
1605 (s, nCQN), 1660 (w), 1700 (w), 1740 (w), 1900 (w), 2330
(w), 2350 (w), 2950 (w), 3100 (s), 3743 (w). 1H NMR (d, ppm,
CDCl3): 8.66 (s, 2H), 7.40 (s, 2H), 7.30 (s, 2H). 13C NMR (d,
ppm, CDCl3): 111 (s, 2H), 127.6 (s, 2H), 134.5 (s, 2H), 140 (s,
2H), 155 (s, 2H). Correspondingly for N,N0-bis(5-bromothio-
phenyl-2-methylene)hydrazine 3: IR (n, cmꢃ1, KBr): 485 (m),
590 (s, nCBr), 678 (w), 745 (m), 785 (m), 1060 (m), 1200 (m),
1230 (m), 1265 (w), 1385 (s, nCQS), 1410 (s), 1500 (w), 1550
(m), 1605 (s, nCQN), 1660 (w), 1735 (w), 1876 (w), 2094 (w),
2455 (w), 2938 (w), 3060 (s), 3743 (w). 1H NMR (d, ppm,
CDCl3): 8.60 (s, 2H), 7.15 (s, 2H), 7.06 (s, 2H). 13C NMR
(d, ppm, CDCl3): 118.6 (s, 2H), 129.5 (s, 2H), 131 (s, 2H),
140.7 (s, 2H), 155.4 (s, 2H).
Conclusions
The structures of aldazines 1–3 were found to depend on many
weak intermolecular interactions with no single type being
common or decisive in the three structures. The ‘‘type II’’
Brꢀ ꢀ ꢀBr interactions were, however, found in both brominated
aldazines and play a structural role in these compounds.
Moreover, the Brꢀ ꢀ ꢀBr interactions in aromatic bromine
substituted compounds could be explained by a simple model
based on the calculated electrostatic potentials and semi-
quantitative agreement is found between this model and an
analysis of data from the CSD.
The optical power limiting properties of 2 and 3 indicate
that they may be used as versatile starting materials for more
extensive p-conjugations at the position of the peripheral
bromine atoms using Suzuki coupling conditions to improve
both chemical and optical properties.
Experimental
Methods and instrumentation
The IR spectra were recorded in the region 4000–400 cmꢃ1
using spectroscopic grade KBr-pellet method on Bruker IFS66
v/S with resolution of 8 cmꢃ1 and 64 scans, the aperture
opening was 12 mm with a double sided acquisition mode
function. 1H and 13C NMR spectra were recorded on a Varian
UNITY-400 at ambient temperature using the CDCl3 residual
CHCl3 peak as internal reference. Melting points were mea-
sured on Mettler FP-90 Hot Stage thermoanalyser and were
not corrected. The non-linear absorption experiments were
performed by an f/5 set up22 with a frequency-doubled
ꢁc
This journal is the Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2007
New J. Chem., 2007, 31, 1777–1784 | 1783

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Crystallography
(b) P. Metrangolo, T. Pilati and G. Resnati, CrystEngComm, 2006,
8, 946–947.
Diffraction data were collected using a Siemens SMART CCD
diffractometer with Mo-Ka radiation (l = 0.71073 A, gra-
phite monochromator). The crystals were cooled to 173(2) K
by a flow of nitrogen gas using the LT-2A device. Full spheres
of reciprocal space were scanned by 0.31 steps in o with a
crystal-to-detector distance of 3.97 cm. Preliminary orienta-
tion matrices were obtained from the first frames using
SMART.24 The collected frames were integrated using the
preliminary orientation matrices which were updated every
100 frames. Final cell parameters were obtained by refinement
of the positions of reflections with I 4 10s(I) after integration
of all the frames using SAINT software.24 The data were
empirically corrected for absorption and other effects using
the SADABS program.25 The structures were solved by direct
methods and refined by full-matrix least squares on all |F2|
data using SHELXTL software.26 For compound 1 [N,N0-
bis(thiophenyl-2-methylene)hydrazine] we initially obtained
unreasonably high residuals and residual electron density,
thus an investigation of possible twinning was made. As the
unit cell parameters a = 9.6504 A and c = 9.6510 A were very
close to each other, the twinning law (0 0 ꢃ1 0 ꢃ1 0 ꢃ1 0 0)
was successfully employed with resulting 0.5084(13) to
0.4916(13) ratio.
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CCDC reference numbers 609791–609793 for 1, 2 and 3,
respectively.
For crystallographic data in CIF or other electronic format
see DOI: 10.1039/b704009a
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Computational details
Calculations were made with the DFT module in Spartan
5.1.3a127 at using the exchange and correlation functionals of
Becke28 and Perdew,29 respectively (BP86) as a perturbation
on self consistent density. A DN** basis set including polar-
ization and d-functions for C, N, O, S and Br was used. This
basis set is roughly equivalent in ‘‘size’’ to the 6-31G** basis
set but is claimed to yield results closer to much larger
Gaussian basis sets.30
24 J. Tao, X. Yin, Y. B. Jiang, R. B. Huang and L. S. Zheng, Inorg.
Chem. Commun., 2003, 6, 1171–1174.
25 SADABS: Program for Empirical Absorption Correction of Area
Acknowledgements
This work was supported by the Swedish Research Council
and the Swedish International Development Agency-SIDA
through the Swedish Research Links program (VR-Grant
348-2002-6879).
Detectors, Version 2.10, University of Gottingen, Germany,
¨
2003.
26 SHELXTL: Structure Determination Programs, Version 6.12,
Bruker AXS Inc., Madison, WI, USA, 2001.
27 SPARTAN ‘04, Ver. 1.0.3e, Wavefunction Inc., 18401 Von Kar-
man Avenue, Suite 370. Irvine, CA 92612, USA, 2006.
28 A. D. Becke, Phys. Rev. A, 1988, 38, 3098–3100.
29 J. P. Perdew, Phys. Rev. B, 1986, 33, 8822–8824.
30 W. J. Hehre, J. Yu and P. E. Klunzinger, A Guide to Molecular
Mechanics and Molecular Orbital Calculations in Spartan, Wave-
function, Inc., 1997.
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This journal is the Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2007
1784 | New J. Chem., 2007, 31, 1777–1784