N-alkyl-p-nitroanilines: Impact of alkyl chain length on crystal structures and optical SHG

Article abstract of DOI:10.1039/a901591d

N-Alkyl-p-nitroanilines from the propyl to the octyl derivative are synthesised and characterised. Powder SHG studies show that the butyl derivative alone is active, in agreement with earlier observations. Crystal structures of the propyl, butyl and penty

Full text of DOI:10.1039/a901591d

J O U R N A L O F
N-Alkyl-p-nitroanilines: impact of alkyl chain length on crystal
structures and optical SHG
P. Gangopadhyay,a S. Venugopal Rao,b D. Narayana Raob and T. P. Radhakrishnan*a
aSchool of Chemistry and bSchool of Physics, University of Hyderabad, Hyderabad 500 046, India.
C H E M I S T R Y
E-mail: tprsc@uohyd.ernet.in; Fax: 91-40-3010120, 3010145
Received 1st March 1999, Accepted 4th May 1999
N-Alkyl-p-nitroanilines from the propyl to the octyl derivative are synthesised and characterised. Powder SHG
studies show that the butyl derivative alone is active, in agreement with earlier observations. Crystal structures of
the propyl, butyl and pentyl derivatives are determined; the propyl and pentyl derivatives belong to the
centrosymmetric space groups P 19 and P2 /c respectively and the butyl derivative to the noncentrosymmetric
1
P2 2 2 space group. Detailed analysis of these structures provides insight into the critical role of the alkyl chain
1
1 1
length in the formation of centrosymmetric or noncentrosymmetric crystal lattices. The utility of alkyl chain length
as a crystal design element for quadratic NLO materials is noted.
dipoles present in the native crystal lattice is broken and SHG
activity is achieved. Moderate SHG has been observed in pNA
incorporated in poly(methyl methacrylate) film18 as well as in
pNA included to optimum levels in molecular sieves.19 An
intercalation compound of pNA in tetramethylammonium
saponite prepared in the presence of an electric field showed
great enhancement of SHG activity over the compound
prepared in the absence of the electric field.20
Introduction
Molecules possessing electron donating and accepting groups
connected by extended p-electron pathways generally possess
large hyperpolarisabilities, b. Noncentrosymmetric assemblies
of such molecules in the form of single crystals and thin films
with appropriate alignment of the b tensor components display
efficient quadratic nonlinear optical (NLO) properties.1 One
of the most popular NLO applications sought in such materials
is optical second harmonic generation (SHG). In the two stage
fabrication of molecular materials for SHG involving the
synthesis of the molecular unit followed by the assembly of
the noncentrosymmetric bulk material, the latter is the more
difficult step and a variety of chemical and physical routes
have been explored for this purpose. The chemical approaches
based on the modification of molecular structures exploit
features such as intermolecular H-bonding,2 steric factors,3
vanishing ground state dipole moments,4 molecular chirality5
and salt formation.6 The physical techniques used to obtain
acentric structures include electric field poling in thin films,7
formation of X and Z type Langmuir–Blodgett films8 and
formation of host–guest systems.9 Though lack of a centre of
inversion is a necessary prerequisite, suitable alignment of the
molecules is required to obtain efficient SHG active materials.10
Development of novel molecular and crystal design techniques
for assembling such materials is of great current interest.
p-Nitroaniline (pNA) is the prototypical push–pull
conjugated molecule with appreciable hyperpolarisability of
potential interest in NLO applications. Since pNA itself has a
centrosymmetric crystal lattice,11 several related molecules
have been investigated for SHG capability in the crystalline
form. The simplest successful modifications include m-nitro-
aniline (mNA)12 and 2-methyl-4-nitroaniline (MNA),3a,3b,13
both of which have noncentrosymmetric crystal lattices and
show powder SHG of 20 and 80 U respectively (1 U=SHG
of urea). The chiral derivative 2,4-dinitrophenyl-()-alanine
methyl ester (MAP)14 shows a moderate SHG of 10 U. N-4-
Nitrophenyl-(S)-prolinol (NPP)15 is a derivative designed to
exploit intermolecular H-bonding interactions and molecular
chirality simultaneously and has a near optimal orientation of
the dipoles in the crystal lattice leading to a high SHG
capability of 150 U. The l-shaped packing motif in 4-nitrophe-
nylhydrazones leads to noncentrosymmetric materials capable
of very strong SHG.16 A good sampling of the large number
of pNA derivatives that are SHG active may be found in
Ref. 17. pNA itself has been incorporated in a variety of host
lattices so that the antiparallel alignment of the molecular
Another interesting approach reported for the induction of
SHG activity in pNA derivatives involves the introduction of
an n-alkyl chain on the amino group of pNA.21 Powder SHG
studies were carried out on N-propyl, N-butyl, N-pentyl, N-
hexyl and N-octyl derivatives of pNA. Though the linear
optical properties of these compounds were very similar, only
the N-butyl derivative showed SHG activity. Crystal structures
of these derivatives have not been reported so far and the
origin of this interesting phenomenon has not been investi-
gated. Among the simple n-alkyl derivatives of pNA, structural
studies have been published only on three systems: the N-
methyl22 and N,N-diethyl23 derivatives form centrosymmetric
crystals belonging to the P2 /n space group and the N,N-
1
dimethyl24 derivative has an acentric structure (P2 ) resulting
1
in moderate SHG capability. An interesting observation of
SHG activity in powder mixtures of pNA with N-alkylated
pNA has also been reported.25 However, since these studies
were carried out on powders, no structural characterisations
are available to explain the observed NLO properties. We
have recently discovered chain length dependent SHG activity
in n-alkyl substituted diaminodicyanoquinodimethanes26 in a
manner similar to that observed in n-alkyl substituted pNA’s.
The potential utility of alkyl chain length as a crystal design
element for fabricating noncentrosymmetric crystal lattices of
interest in quadratic NLO applications prompted us to carry
out detailed structural investigations on the pNA derivatives.
The varying powder SHG reported21,25 for N-alkyl pNA’s
recrystallised under different conditions points to the possibil-
ity of polymorphism. We have synthesised and characterised
the N-propyl (1), N-butyl (2), N-pentyl (3), N-hexyl (4), N-
heptyl (5) and N-octyl (6) derivatives of pNA. We have also
J. Mater. Chem., 1999, 9, 1699–1705
1699

carried out a systematic investigation of these compounds by
recrystallising all of them from the same solvent system and
examining the crystal structures of three derivatives 1, 2 and
n:/cm−1=3348.7 (N–H stretch), 2961.0, 2934.0 (aliphatic C–H
stretch), 2856.8 (CH : C–H stretch), 1462.2, 1300.0 (NO
2
2
stretches); E. I. mass: m/z=222 (M+), 151, 105; 1H-NMR
3
. We present a detailed analysis of the crucial role of the
alkyl chain in steering the crystal packing which effectively
(CDCl , d from TMS): 1.01 (t, 3H), 1.39–1.46 (m, 2H),
3
1.50–1.69 (m, 2H), 1.82–1.88 (m, 4H), 3.16–3.26 (q, 2H),
controls the optical SHG capability of these materials.
4.50 (broad, 1H), 6.54 (d, 2H), 8.09 (d, 2H); 13C
(
CDCl , ppm): 13.88, 22.49, 26.62, 29.14, 30.77, 31.47, 43.48,
3
1
10.91, 126.39, 153.46; elemental analysis (calculated for
Experimental
C H N O ): %C=65.03 (64.84), %H=8.37 (8.16), %N=
1
2 18 2 2
Synthesis and characterisation
13.33 (12.89).
All the compounds were synthesised following the procedure
reported in Ref. 27. The detailed procedure is illustrated using
the case of 2 as an example. 1.000 g (0.8 ml, 7.0 mmol) of 4-
fluoronitrobenzene was dissolved in 8 ml of freshly dried and
distilled DMSO and stirred at 75 °C. 2.8 g (20 mmol) of freshly
ignited and subsequently cooled potassium bicarbonate was
added to the solution and stirred. 1.243 g (1.7 ml, 17 mmol)
of dry n-butylamine was added to the mixture and refluxed at
N-Heptyl-4-nitroaniline (5). Mp: 62–64 °C; UV–Vis (meth-
anol): l =387 nm, l
=480 nm; FT-IR (KBr pellet):
max
cut-off
n:/cm−1=3358.4 (N–H stretch), 2967.0, 2930.1 (aliphatic C–H
stretch), 2854.9 (CH : C–H stretch), 1464.1, 1300.0 (NO
2
2
stretches); E. I. mass: m/z=236 (M+), 151, 105; 1H-NMR
(
1
4
(
CDCl , d from TMS): 0.99 (t, 3H), 1.39–1.46 (m, 6H),
3
.50–1.69 (m, 2H), 1.82–1.88 (m, 2H), 3.16–3.26 (q, 2H),
.50 (broad, 1H), 6.54 (d, 2H), 8.09 (d, 2H); 13C
1
20 °C under dry nitrogen gas. The progress of the reaction
CDCl , ppm): 13.97, 22.53, 26.94, 28.95, 29.17, 31.70, 43.48,
was monitored by TLC; about 98% conversion was observed
after 6 h. The reaction mixture was cooled and poured over
3
1
10.92, 126.40, 153.48; elemental analysis (calculated for
C H N O ): %C=65.85 (66.07), %H=8.72 (8.53), %N=
2
00 g of crushed ice and stirred vigorously for 30 min. The
13 20 2 2
1
2.45 (11.85).
yellow precipitate formed was filtered and washed with water
thoroughly. The filter cake was dried in air to give 1.3 g (96%
yield) of 2. It was purified by eluting through a basic alumina
column using ethyl acetate–hexane (65535) mixture. Further
purification was carried out by sublimation at 80 °C under
reduced pressure. Crystals were grown from toluene–chloro-
form (70530) mixture. 1, 3, 4, 5 and 6 were synthesised
similarly and recrystallised using the same solvents. The yields
for the various compounds were 85 to 96%.
(
(
8
2
(
N-Propyl-4-nitroaniline (1). Mp: 72 °C; UV–Vis (methanol):
4
l
3
2
=387 nm, l
=490 nm; FT-IR (KBr pellet): n:/cm−1=
max
cut-off
346.8 (N–H stretch), 2957.1, 2926.0 (aliphatic C–H stretch),
817.2 (CH : C–H stretch), 1473.8, 1300.0 (NO stretches); E.
%
2
2
I. mass: m/z=180 (M+), 151, 105; 1H-NMR (CDCl , d from
3
TMS): 1.03 (t, 3H), 1.64–1.75 (m, 2H), 3.19 (q, 2H), 4.50
(
(
broad, 1H), 6.52 (q, 2H), 8.08 (q, 2H); 13C-NMR
CDCl , ppm): 11.40, 22.42, 45.20, 110.92, 126.39, 153.45;
3
(
6
elemental analysis (calculated for C H N O ): %C=59.93
(
9
12 2 2
59.99), %H=6.76 (6.71), %N=15.52 (15.55).
(
N-Butyl-4-nitroaniline (2). Mp: 68–70 °C; UV–Vis (meth-
crystalline powder samples were graded using standard sieves;
sizes ranging from 50–355 mm were studied. Urea was used
for calibrating the powder SHG. The samples were loaded in
glass capillaries having an inner diameter of 600 mm. The SHG
emitted from the powder samples was collected by a concave
mirror, collimated and focused onto the monochromator slit.
The errors in the measured SHG’s are typically about 10–15%.
All the compounds showed very good stability under laser
irradiation and no sign of decomposition, even on continued
irradiation with a laser power of 1 GW cm−2 (6 ns, 10 Hz),
was detected.
anol): l =386 nm; l
=487 nm; FT-IR (KBr pellet):
max
cut-off
n:/cm−1=3339.1 (N–H stretch), 2959.1, 2926.3 (aliphatic C–H
stretch), 2817.2 (CH : C–H stretch), 1462.2, 1319.4 (NO
2
2
stretches); E. I. mass: m/z=194 (M+), 151, 105; 1H-NMR
(
1
CDCl , d from TMS): 1.01 (t, 3H), 1.39–1.46 (m, 2H),
.50–1.69 (m, 2H), 3.16–3.26 (q, 2H), 4.5 (broad, 1H), 6.51
3
(
3
d, 2H), 8.08 (d, 2H); 13C-NMR (CDCl , ppm): 13.71, 20.13,
3
1.23, 43.14, 110.91, 126.41, 153.50; elemental analysis (calcu-
lated for C H N O ): %C=61.37 (61.84), %H=7.28 (7.27),
1
0 14 2 2
%
N=14.49 (14.42).
N-Pentyl-4-nitroaniline (3). Mp: 67–68 °C; UV–Vis (meth-
Crystal structure studies
anol): l =386 nm, l
=489 nm; FT-IR (KBr pellet):
max
cut-off
n:/cm−1=3350.7 (N–H stretch), 2957.1, 2957.1 (aliphatic C-H
X-Ray diffraction data were collected on an Enraf-Nonius
MACH3 diffractometer. Mo-Ka radiation with a graphite
crystal monochromator in the incident beam was used. Details
of data collection, solution and refinement, anisotropic thermal
parameters and full lists of bond lengths and angles are
presented in the Supporting Information.
stretch), 2858.8 (CH : C–H stretch), 1464.1, 1319.4 (NO
2
2
stretches); E. I. mass: m/z=208 (M+), 151, 105; 1H-NMR
(
1
(
1
CDCl , d from TMS): 1.01 (t, 3H), 1.39–1.46 (m, 2H),
3
.50–1.69 (m, 2H), 1.82–1.88 (m, 2H), 3.16–3.26 (q, 2H), 4.5
broad, 1H), 6.54 (d, 2H), 8.09 (d, 2H); 13C (CDCl , ppm):
3
3.85, 22.34, 28.34, 28.87, 29.12, 43.45, 110.92, 126.39, 153.50;
elemental analysis (calculated for C H N O ): %C=63.12
1
1 16 2 2
Results and discussion
(
63.44), %H=7.91 (7.74), %N=13.14 (13.45).
Chen et al.21 reported that 2 recrystallised from ether and
ethanol–cyclohexane shows powder SHG intensities of 14 and
0.5 U respectively, whereas 1, 3 and 4 are SHG inactive. They
N-Hexyl-4-nitroaniline (4). Mp: 62–64 °C; UV–Vis (meth-
anol): l =387 nm, l
=480 nm; FT-IR (KBr pellet):
max cut-off
1
700
J. Mater. Chem., 1999, 9, 1699–1705

Table 1 Powder SHG data for urea and 2 at various particle sizes
chain length increases. This indicates that the alkyl chain
exerts significant influence on the crystal packing. The high
solubility of the alkylated pNA’s caused some difficulty with
their crystallisation. Exploration of a variety of solvents and
solvent mixtures led to the choice of toluene–chloroform as a
suitable solvent system to grow crystals of most of the com-
pounds by slow evaporation. Since longer alkyl chain substi-
tuted compounds gave poor quality crystals, 1, 2 and 3 were
selected for detailed analysis.
SHG (arb. units)
Particle size/mm
Urea
2
5
1
1
2
2
3
0–100
0.4
1.0
1.4
1.0
1.3
1.3
1.0
—
00–150
50–200
00–250
50–300
00–355
11.1
15.8
12.7
15.8
23.8
19.8
1
, 2 and 3 form yellow, transparent crystals. The crystallo-
>
355
graphic data for these three compounds are collected in
Table 2. 1 belongs to the P 19 space group with two molecules
in the asymmetric unit. The molecular structure is shown in
Fig. 1 and the significant bond lengths and angles are collected
in Table 3. Intermolecular H-bonds are observed in this crystal,
did not study 5 but found that 6 recrystallised from ethanol–
cyclohexane or ether showed a weak SHG of 0.05 U or no
SHG activity respectively. Kurtz–Perry powder measurements
we have carried out on 1–6 recrystallised from choloroform–to-
luene are in general agreement with these observations; 2 alone
was found to be SHG active. The variation of SHG intensity
of 2 with particle size is compared with that of urea in Table 1.
The SHG of both urea and 2 are found to saturate above a
particle size of ~150 mm indicating phase-matching behaviour.
The average SHG of 2 at saturation is found to be about
connecting the nitro
O
atoms and amino
˚
=3.007 A). These H-bonds connect
H atoms
˚
(r
–
=3.012 A, r
–
N O
N
O
4
2∞
4∞ 2◊
the molecules into a polar zigzag chain extending approxi-
mately along the b axis (Fig. 2). Adjacent chains have opposite
polarity and are related by the centre of inversion. 2 crystallises
in the noncentrosymmetric space group P2 2 2 with two
1
1 1
molecules in the asymmetric unit. The molecular structure is
shown in Fig. 3 and the significant bond lengths and angles
are collected in Table 4. This structure also showed intermol-
1
7.6 U. Our investigation of a series of compounds, 7,7-
˚
˚
bis(dialkylamino)-8,8-dicyanoquinodimethanes26 has shown
that the butyl, pentyl and hexyl derivatives are SHG active
ecular H-bonds (r – =3.081 A,
r
–
4∞
=3.046 A) con-
N
O
N
O
4
2◊
2
necting molecules into zigzag chains running along the b axis
(
11–19 U) whereas the derivatives with shorter (propyl) as
(Fig. 4). However there are now pairs of chains running
well as longer (heptyl, octyl and dodecyl) alkyl chains are not.
Crystal structure studies on these compounds showed that the
alkyl chain length plays a crucial role in controlling the dipole
orientations. When the alkyl chain is short as in the propyl
derivative, the dipole–dipole interactions are dominant and
result in a centrosymmetric crystal lattice. When the chain is
long, a quasi-bilayer structure is formed with the dipoles
forming a polar layer and the alkyl chains forming a nonpolar
layer. Electrostatic interactions within and across the polar
regions lead once again to a centrosymmetric crystal lattice.
In the intermediate cases, the alkyl chain interactions interfere
with the dipole–dipole interactions and lead to noncentrosym-
metric crystal structures and appreciable SHG activity. The
SHG capability in the pNA series confined to the butyl
derivative alone also appears to be determined by the alkyl
chain length effect.
The pNA derivatives 1–6 possess lower melting points than
pNA indicating that the intermolecular interactions are weaker
in their crystals compared to the parent system. This may be
attributed partly to the reduction in the intermolecular H-
bonding due to the replacement of one of the amino H atoms.
More interestingly we find that these compounds exhibit a
smooth progression of melting points to lower values as the
Fig. 1 Molecular structure of 1 from single crystal X-ray analysis;
only one molecule in the asymmetric unit is shown; 25% probability
thermal ellipsoids are indicated; H atoms are omitted for clarity.
Table 2 Crystallographic data for 1, 2 and 3
1
2
3
Molecular formula
Space group
C H N O
C
H
N O
C
H N O
9
12
2
2
10 14
2
2
11 16 2 2
Triclinic, P19
5.076(2)
13.123(16)
15.190(5)
71.90(7)
81.91(4)
80.04(7)
943.1(13)
4
Orthorhombic, P2 2 2
Monoclinic, P2 /c
6.192(3)
15.61(7)
11.851(6)
90.0
97.52(4)
90.0
1
1 1
1
˚
a/A
7.468(18)
15.348(19)
18.528(4)
90.0
90.0
90.0
˚
b/A
˚
c/A
a/deg.
b/deg.
c/deg.
˚
V/A3
2123(3)
8
1136(5)
4
Z
r/g cm−3
No. unique reflections
No. of reflections
No. of parameters
1.269
0.91
3311
With I>2s 1873
238
1.215
0.86
3497
With I>2s 1415
254
1.218
0.85
2725
With I>3s 1593
137
m/cm−1
I
I
I
R (for I>2s )
0.0405
0.1564
0.0551
0.1232
0.0460
0.1338
I
wR2
J. Mater. Chem., 1999, 9, 1699–1705
1701

Table 3 Significant (a) bond lengths and (b) angles in 1 from single
crystal X-ray analysis (atom labelling is shown in Fig. 1)
(
a)
(b)
Distance/A Atom–atom–atom(–atom) Angle/deg.
˚
Atom–atom
N(3)–O(1)
N(3)–O(2)
N(3)–C(5)
N(4)–C(8)
N(4)–C(11) 1.455(3)
C(5)–C(10) 1.376(3)
C(5)–C(6)
C(6)–C(7)
C(7)–C(8)
C(8)–C(9)
1.222(3)
1.233(3)
1.432(4)
1.340(3)
O(1)–N(3)–O(2)
O(1)–N(3)–C(5)
O(2)–N(3)–C(5)
C(8)–N(4)–C(11)
C(10)–C(5)–C(6)
C(10)–C(5)–N(3)
C(6)–C(5)–N(3)
C(7)–C(6)–C(5)
122.6(2)
118.9(2)
118.5(2)
124.6(2)
120.6(2)
120.0(2)
119.3(2)
119.7(2)
121.0(2)
119.9(2)
122.4(2)
117.8(2)
121.0(2)
119.9(2)
111.4(2)
113.3(3)
−5.5(4)
−6.4(4)
−0.2(3)
1.381(4)
1.360(4)
1.408(3)
1.369(4)
C(6)–C(7)–C(8)
N(4)–C(8)–C(7)
N(4)–C(8)–C(9)
C(7)–C(8)–C(9)
C(10)–C(9)–C(8)
C(9)–C(10)–C(5)
N(4)–C(11)–C(12)
C(11)–C(12)–C(13)
O(1)–N(3)–C(5)–C(10)
O(2)–N(3)–C(5)–C(6)
C(11)–N(4)–C(8)–C(9)
C(9)–C(10) 1.366(4)
Fig. 3 Molecular structure of 2 from single crystal X-ray analysis;
only one molecule in the asymmetric unit is shown; 25% probability
thermal ellipsoids are indicated; H atoms are omitted for clarity.
together, the polarities within a pair being the same and the
polarities of adjacent pairs directed opposite to each other.
The chains within a pair are related by the screw rotation
about the b axis whereas the chains in the adjacent pair are
obtained by screw rotations about the a and c axes. Thus
there is clearly no centre of inversion relating the polar chains
oriented in opposite directions. Finally 3 is found to belong
to the P2 /c space group with one molecule in the asymmetric
1
unit. The molecular structure is shown in Fig. 5 and the
significant bond lengths and angles are collected in Table 5.
˚
Intermolecular H-bonds (r
=3.081 A) connect the mol-
N4–O2∞
ecules into chains as in 1 and 2; a centre of inversion is formed
as in the case of 1 (Fig. 6).
The crystal structures of 1, 2 and 3 clearly illustrate the
similarity in the packing motif and intermolecular interactions
present in these alkylated pNA’s. The singular behavior of 2
therefore appears to be a consequence of some subtle inter-
actions. We have collected in Table 6 several structural param-
eters of the three crystals in an attempt to identify the critical
features that lead to the observed dipole orientations and
crystal packing. As noted earlier, all the three possess well-
defined H-bonds. The values of r
in Table 6 indicate that
H-bond
their strengths are quite comparable, but show a marginal
decrease from 1 to 3; this appears to be a consequence of the
increasing alkyl chain length and contributes to the trend of
decreasing melting points observed in the series. The closest
Table 4 Significant (a) bond lengths and (b) angles in 2 from single
crystal X-ray analysis (atom labelling is shown in Fig. 3)
(
a)
(b)
Distance/A Atom–atom–atom(–atom) Angle/deg.
˚
Atom–atom
O(1)–N(3)
O(2)–N(3)
N(3)–C(5)
N(4)–C(8)
N(4)–C(11) 1.434(5)
C(5)–C(10) 1.374(5)
C(5)–C(6)
C(6)–C(7)
C(7)–C(8)
C(8)–C(9)
1.201(4)
1.225(4)
1.443(5)
1.345(4)
O(1)–N(3)–O(2)
O(1)–N(3)–C(5)
O(2)–N(3)–C(5)
C(8)–N(4)–C(11)
C(10)–C(5)–C(6)
C(10)–C(5)–N(3)
C(6)–C(5)–N(3)
C(7)–C(6)–C(5)
122.3(3)
119.5(3)
118.2(3)
125.4(3)
120.9(3)
119.8(3)
119.2(3)
119.6(3)
120.9(3)
122.7(3)
118.0(3)
118.0(3)
120.7(3)
119.9(3)
1.4(4)
1.384(5)
1.347(5)
1.416(5)
1.397(5)
C(6)–C(7)–C(8)
N(4)–C(8)–C(9)
N(4)–C(8)–C(7)
C(9)–C(8)–C(7)
C(10)–C(9)–C(8)
C(9)–C(10)–C(5)
O(1)–N(3)–C(5)–C(10)
O(2)–N(3)–C(5)–C(6)
C(11)–N(4)–C(8)–C(7)
C(9)–C(10) 1.357(5)
Fig. 2 Crystal packing in 1 viewed close to the a axis; H-bonds are
shown as broken lines; N atoms are shown as filled circles.
0.7(5)
−176.5(4)
1
702
J. Mater. Chem., 1999, 9, 1699–1705

Table 5 Significant (a) bond lengths and (b) angles in 3 from single
crystal X-ray analysis (atom labelling is shown in Fig. 5)
(
a)
(b)
Distance/A Atom–atom–atom(–atom) Angle/deg.
˚
Atom–atom
O(1)–N(3)
O(2)–N(3)
N(3)–C(5)
N(4)–C(8)
N(4)–C(11) 1.440(2)
C(5)–C(10) 1.374(2)
C(5)–C(6)
C(6)–C(7)
C(7)–C(8)
C(8)–C(9)
1.222(4)
1.2290(19) O(1)–N(3)–C(5)
1.436(6)
1.350(5)
O(1)–N(3)–O(2)
121.36(18)
119.3(2)
118.75(19)
125.37(17)
120.9(2)
119.95(18)
119.1(2)
119.2(2)
O(2)–N(3)–C(5)
C(8)–N(4)–C(11)
C(10)–C(5)–C(6)
C(10)–C(5)–N(3)
C(6)–C(5)–N(3)
C(7)–C(6)–C(5)
1.386(4)
1.361(5)
1.397(2)
1.410(5)
C(6)–C(7)–C(8)
121.36(18)
120.16(18)
121.6(2)
N(4)–C(8)–C(7)
N(4)–C(8)–C(9)
C(7)–C(8)–C(9)
C(9)–C(10) 1.367(5)
118.2(2)
C(10)–C(9)–C(8)
C(9)–C(10)–C(5)
O(2)–N(3)–C(5)–C(10)
O(1)–N(3)–C(5)–C(6)
C(11)–N(4)–C(8)–C(7)
120.0(2)
120.19(18)
178.0(3)
176.3(3)
179.3(3)
atoms of the nearest oppositely aligned molecular dipoles, r
reflects the extent of dipole–dipole interaction. Based on the
dip
values of r we infer that the electrostatic interaction decreases
dip
slightly from 1 to 2 but is enhanced in 3. In the packing
diagrams shown in Fig. 2, 4 and 6, it is seen that the alkyl
Fig. 4 Crystal packing in 2 viewed close to the a axis; H-bonds are
shown as broken lines; N atoms are shown as filled circles.
Fig. 5 Molecular structure of 3 from single crystal X-ray analysis;
2
5% probability thermal ellipsoids are indicated; H atoms are omitted
for clarity.
C–C distance between alkyl chains of adjacent molecules, r
C–C
shows an interesting trend. It decreases slightly from 1 to 2
and is considerably shorter in 3. This indicates that the alkyl
chain interactions set in with 3, but are perhaps already
incipient in 2. The distance between the nitro N and amino N
Fig. 6 Crystal packing in 3 viewed close to the a axis; H-bonds are
shown as broken lines; N atoms are shown as filled circles.
J. Mater. Chem., 1999, 9, 1699–1705
1703

Table 6 Structural parameters for 1, 2 and 3 derived from single
large b tensor components perpendicular to the dipole axis
will be of particular interest in this regard. We are currently
exploring this idea.
crystal X-ray analysis; r
: average of the H-bond distances; r
:
:
H-bond
C–C
dip
closest C–C distance between alkyl chains of adjacent molecules; r
average distance between the nitro N and amino N of nearest
oppositely aligned dipoles; r : average of the closest distances
between the end C of alkyl chain and benzene ring C atoms of
adjacent molecules; h: angle between the mean benzene planes of the
nearest oppositely aligned molecules
C–ring
Acknowledgements
Financial support from the Department of Science and
Technology (Swarnajayanti Fellowship) and the use of the
National Single Crystal Diffractometer Facility funded by the
DST at the School of Chemistry, University of Hyderabad are
gratefully acknowledged. PG thanks UGC for a Junior
Research Fellowship.
˚
˚
˚
˚
Crystal
r
/A
r
/A
r
/A
r
/A
h/ °
0.0
53.9
0.0
H-bond
C–C
dip
C-ring
1
2
3
3.010
3.063
3.081
4.181
4.105
3.625
5.081
5.147
4.149
5.120
3.949
4.338
Supporting information
chain of one molecule gets close to the benzene ring of a near
neighbour molecule leading to some van der Waals interaction.
The proximity and hence the interaction may be quantified by
taking an average of the distances between the end C atom of
the alkyl chain and the C atoms of the benzene ring. This
Details of crystal structure analysis and listing of atomic
coordinates, thermal parameters, bond distances and angles
from X-ray crystal structure analysis for 1, 2 and 3.†
value, denoted as r
is weak in 1, slightly larger in 3 and very strong in 2.
shows that this chain–ring interaction
C–ring
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1705