NLO Activity in some non-conjugated 3D triazine derivatives: A non-centrosymmetric crystal through conformational flexibility

Article abstract of DOI:10.1039/b512362c

Four derivatives of 2,4,6-tris(benzyloxy)-1,3,5-triazine are synthesized and detailed computational and non-linear optical investigations are carried out. Computations indicate four conformations with different energies in all the systems in the gas phase; and the individual dipole moments are also of different magnitude. HRS measurements of these molecules in solution reveal moderately large β values; structure-property relations are analyzed through computations. These molecules have one added advantage of nearly 100% optical transmission through the visible range, due to the non-conjugated structure. Molecule 1 [2,4,6-tris(benzyloxy)-1,3,5-triazine] crystallizes in a non-centrosymmetric space group by adopting the conformation with the lowest dipole moment but not the lowest energy. It also shows SHG activity in the solid state. The Royal Society of Chemistry 2006.

Full text of DOI:10.1039/b512362c

View Article Online / Journal Homepage / Table of Contents for this issue
PAPER
www.rsc.org/materials | Journal of Materials Chemistry
NLO Activity in some non-conjugated 3D triazine derivatives: a
non-centrosymmetric crystal through conformational flexibility{{
K. Srinivas,ab Sanyasi Sitha,a V. Jayathirtha Rao,*b K. Bhanuprakash*a and K. Ravikumar*c
Received 31st August 2005, Accepted 7th November 2005
First published as an Advance Article on the web 25th November 2005
DOI: 10.1039/b512362c
Four derivatives of 2,4,6-tris(benzyloxy)-1,3,5-triazine are synthesized and detailed computational
and non-linear optical investigations are carried out. Computations indicate four conformations
with different energies in all the systems in the gas phase; and the individual dipole moments are
also of different magnitude. HRS measurements of these molecules in solution reveal moderately
large b values; structure–property relations are analyzed through computations. These molecules
have one added advantage of nearly 100% optical transmission through the visible range, due to
the non-conjugated structure. Molecule 1 [2,4,6-tris(benzyloxy)-1,3,5-triazine] crystallizes in a
non-centrosymmetric space group by adopting the conformation with the lowest dipole moment
but not the lowest energy. It also shows SHG activity in the solid state.
space groups.8,16,28,42–49 Though octupolar moments are not
Introduction
restricted to planar molecules, much of the NLO work in this
direction has been on planar conjugated molecules with only a
few studies on three dimensional molecules (3D) bearing Td or
D3 symmetry.28,49,50
Non-linear optics (NLO) deals with the interaction of applied
electromagnetic fields (like laser light) in various materials to
generate new electromagnetic fields altered in frequency, phase
or other physical properties.1–10 This field has attracted lot
of interest in the past two decades not only because of the
possible numerous applications in telecommunications, optical
data storage etc., but also because of the fundamental science
connected to issues like charge transfer, conjugation, polariza-
tion and crystallization into non-centrosymmetric lattices.11–23
A variety of systems including inorganic materials, organo-
metallic materials, organic molecules and polymers have been
studied for NLO activity.8,24–39 Owing to the combination of
chemical tunability and choice of synthetic strategies, organic
molecules in particular have received much attention. The
majority of the work in the literature has been confined to
D–p–A type dipolar molecules; they can be designated as one
dimensional (1D) type of systems, since they lack significant
contributions from the off-diagonal components of the NLO
coefficient and the main contribution (known as bvec4b) arises
along the dipole axis. A major drawback of these systems is
that they often tend to crystallize into centrosymmetric space
groups to satisfy electrostatic interactions leading to the
cancellation of quadratic and higher even order NLO
responses in the bulk state.40,41 On the other hand, the concept
of molecules having no dipole moment but only octupolar
moments attracted a lot of attention after it was shown that
Though btotal, the measure of the NLO response of a
molecule is a sum of the two components bvec and boct, in
many of the molecules one component is dominant. However
in some cases, especially those of 3D type molecules, both
components could be substantial for symmetry reasons;
reports of such type of molecules, especially with solid state
NLO activity are rare.42,51 For poling it is advantageous that
the molecule shows a dipole moment even if the octopolar
component is dominant.45c Here our understanding of the
NLO activity of these type of molecules, particularly in the
crystalline state is limited. In this paper we report detailed
studies on a group of 3D molecules and all of them show
measurable SHG activity in the solid state.
The 3D type of molecules chosen for this study consist of
a central acceptor triazine ring with donor groups symme-
trically substituted in 1,3,5 positions (Scheme 1); they are
cyanuric acid derivatives with benzyl as well as p-substituted
(methyl, methoxy and fluoro) benzyl groups. The choice of
the triazine ring as acceptor (due to its p-deficient nature, it
can act as an auxiliary acceptor in NLO chromophores18) is
due to the current focus on NLO chromophores containing
these systems also have a substantial NLO component, boct
,
and that they tend to crystallize into non-centrosymmetric
aInorganic Chemistry Division, Hyderabad 500 007, India.
E-mail: bhanu2505@yahoo.co.in
bOrganic Chemistry Division (II), Hyderabad 500 007, India
cLaboratory of X-Ray Crystallography, Indian Institute of Chemical
Technology, Hyderabad 500 007, India
{ Electronic supplementary information (ESI) available: UV–vis
spectrum of 1, I2v/Iv2 vs. number density for 1–4 and molecular
geometries for all four conformations of 1. See DOI: 10.1039/b512362c
{ IICT communication no: 051118/CMM-0022
Scheme 1
496 | J. Mater. Chem., 2006, 16, 496–504
This journal is ß The Royal Society of Chemistry 2006

View Article Online
heteroaromatics.52 In this context especially, s-triazine deriva-
tives due to their easily polarizable nature leads to high first
hyperpolarizabilities and better transparency, and have drawn
much attention recently.47,51–55 We report the synthesis,
characterization, HRS measurements and solid state studies
of these molecules. Ab initio calculations show that these
molecules, irrespective of the substituents, have four con-
formational minima in the gas phase with C3 and C1 symmetry
arising as a result of the flexible s linkage on the side groups
(vide infra). Interestingly though the energy difference between
the four different conformations is small for the different
derivatives, the calculated NLO response and dipole moment
is quite large. One molecule adopts the conformation with the
lowest dipole moment in the non-centrosymmetric crystal
though it is not the lowest energy conformer.54,55 The current
study provides insight into the molecular engineering of novel
molecules with a view to achieving quadratic NLO activity in
the bulk state. Since these molecules involve charge transfer
over non-conjugated bonds this study also provides some
understanding of the mechanistic aspects of this phenomenon,
a topic which has been explored little so far.20,56
extracted, concentrated and dried. The residue was purified
by column chromatography and followed by recrystallization
afforded the desired product in 55–70% yield. Spectral
characterization data is given below for all the compounds
(1–4).
2,4,6-Tris(benzyloxy)-1,3,5-triazine (molecule 1). (2.80g,
70%) mp 96–98 uC (Found: C, 72.11; H, 5.25; N, 10.49%;
M+, 399. C24H21N3O3 requires C, 72.16; H, 5.30; N, 10.52%,
M+, 399); dH (200 MHz; CDCl3; Me4Si) 5.5 (6H, s, 3 6
CH2O), 7.5 (15H, m, 3 6 aromatic); dC (300 MHz; CDCl3 ;
Me4Si) 173.1, 135.3, 128.5, 128.3, 128.2, 69.8.
2,4,6-Tris(4-methylbenzyloxy)-1,3,5-triazine (molecule 2).
(2.77g, 63%) mp 105–106 uC (Found: C, 73.41; H, 6.13; N,
9.54%; M+, 441. C27H27N3O3 requires C, 73.45; H, 6.16; N,
9.52%; M+, 441); dH (200 MHz; CDCl3; Me4Si) 2.35 (9H, s,
3 6 CH3); 5.45 (6H, s, 3 6 CH2O); 7.1 (6H, d, J = 9.0, 3 6
aromatic); 7.45 (6H, d, J = 8.9, 3 6 aromatic); dC (300 MHz;
CDCl3 ; Me4Si): 169.5, 132.3, 127.1, 126.8, 125.2, 67.1,22.3.
2,4,6-Tris(4-methoxybenzyloxy)-1,3,5-triazine (molecule 3).
(2.69g, 55%) mp 118–120 uC (Found: C, 66.21; H, 5.52; N,
8.55%; M+, 489. C27H27N3O6 requires C, 66.25; H, 5.56; N,
8.58%; M+, 489); dH (200 MHz; CDCl3; Me4Si) 3.85 (9H, s,
3 6 CH3O) 5.3 (6H, s, CH2O) 6.95 (6H, d, J = 9.6, 3 6
aromatic) 7.4 (6H, d, J = 9.0, 3 6 aromatic); dC (300 MHz;
CDCl3 ; Me4Si) 167.1, 130.7, 125.9, 126.1, 124.6, 66.5, 56.2.
Experimental
Measurements and Instruments
All the reagents were A. R. grade and used without further
purification. 1H-NMR spectra were recorded on Gemini
(200 MHz) spectrometer in CDCl3 and 13C-NMR spectra on
Bruker Avance (300 MHz) spectrometer both with TMS as
standard. Mass spectra were obtained on VG-AUTOSPEC
spectrometer. Elemental analyses were performed using
2,4,6-Tris(4-fluorobenzyloxy)-1,3,5-triazine (molecule 4).
(2.72g, 60%) mp 111–112 uC (Found: C, 63.51; H, 3.96; N,
9.22%; M+, 453. C24H18N3O3F3 requires C, 63.57; H, 4.0; N,
9.26%; M+, 453.); dH (200 MHz; CDCl3; Me4Si) 5.6 (6H, s, 3 6
CH2O) 7.25 (6H, d, J = 9.2, 3 6 aromatic) 7.65 (6H, d, J = 9.7,
3 6 aromatic); dC (300 MHz; CDCl3; Me4Si) 169.6, 133.1,
126.3, 125.5, 124.7, 68.9.
a
Vario-EL elemental analyzer. UV–visible absorption
spectra were measured on a Jasco V-550 spectrophotometer.
Thermogravimetric analysis (TGA) was performed using
TGA/SDTA 851e in the temperature range of 33–500 uC in a
nitrogen atmosphere at a heating rate of 10 uC min21
.
Second harmonic generation (SHG) of the crystal was
measured using the Kurtz–Perry technique.57 Molecular first
hyperpolarizability values were measured at 1064 nm by the
hyper-Rayleigh scattering (HRS) technique using an Nd :
YAG laser.58 For the measurement, solutions of individual
compounds were prepared using chloroform as solvent and the
results were analyzed taking p-nitroaniline (PNA) as standard.
All molecular orbital calculations have been carried out
using the ab initio Gaussian G03W package and G03 gauss-
view for viewing.59
Results and discussion
Molecular structure
For molecules 1–4, due to the flexible nature of the bridging
unit, we were able to locate four potential minima with four
different patterns of structural orientations for each of the
molecules at the ab initio HF/6–31G level of theory. The
conformational interconversion diagram for all the four
conformational isomers of molecules 1–4 is shown in Fig. 1.
The conformers are in the order of stability A . B . C . D
and the details of the relative stabilities are given in Table 1.
Conformer A has a chair-like conformation in which two of
phenyl rings lie below the plane of central moiety and the third
one above the plane (C1-symmetric). Conformer B is a tripod
stand like conformation with three rings positioned in a
symmetric fashion (C3-symmetric). Though in conformation C
and D all the three bridging units are in one plane, but the
major difference is the direction of their orientations i.e., in
conformation C all the benzyl groups are symmetrically
oriented in plane where as in conformation D, this symmetrical
orientation is disturbed at one substituent. Analysis of the
substituent effects, due to the para substitution in the phenyl
Synthesis and characterization
General procedure for the preparation of 2,4,6-tris(benzyl-
oxy)-1,3,5-triazine derivatives (molecules 1–4). In dry THF
(50 ml) 60 mmol of sodium hydride (60%) was taken in a
round bottom flask. Corresponding benzyl alcohol (30 mmol)
in THF (10 ml) is added drop wise at 0 uC and stirred for 2–4 h
till the effervescence is damped. Then a solution of 2,4,6-
trichloro-1,3,5-triazine (10 mmol) dissolved in dry THF (20 ml)
is added drop-wise at 0–5 uC and the reaction is monitored by
TLC. After completing the reaction, the solvent is evaporated
under vacuum to dryness and water is carefully added (50 ml),
This journal is ß The Royal Society of Chemistry 2006
J. Mater. Chem., 2006, 16, 496–504 | 497

View Article Online
than that of least stable conformation D. This difference in the
relative stabilities of these four conformations indicates that
there could be some steady equilibrium between these
conformations in the solvent. Analyzing the dipole moments
(m), shown in Table 1, it can be seen that in molecules 1 and 2,
the magnitude decreases in the order D . B . A . C, where as
for molecule 3 and 4, the magnitudes decreases in the order
D . C . A . B and B . D . A . C respectively. Along with
these symmetric considerations, planarity and non-planarity in
the bridging segment also plays a role in the relative values of
dipole moments in these conformations. In case of molecule 3,
due to the orientation directional property of the –OMe group
(due to the differential orientations of the –CH3 group present
in the –OMe group, the directional property arises), the
observed trend is deviated from that of the molecule 1 and 2
and further due to the same reason the m values for this
molecule are comparatively higher than that of all other
molecules. Again for molecule 4, where fluorine is the
substituent, due to the combined inductive effect and electron
withdrawing nature of the substituent, a change in order is
observed compared to molecule 1 and 2. Comparing the results
of these four molecules with that of our earlier work,60 it can
be seen that in respective conformations the dipole moment
values are smaller for –NH–CH2– than the –O–CH2– bridge
considered here.
Fig. 1 Four conformational minima of molecule 1–4, obtained from
the minimum energy conformational search carried out at HF/6–31G
level of theory.
ring, shows that the relative orientations of the rings in these
molecules are not affected very much (only minor deviations in
the twist angles along the bridging segment and also at the
phenyl rings at the side wings are observed). It is quite
interesting to analyze that, in our previous report60 of similar
kind of 3D molecules, only conformations of type A and B
were observed and here, we were able to get two more
conformations, namely C and D. This is because of the nature
of the bridge (–NH–CH2– in our previous work and –O–CH2–
Electronic absorption spectrum (UV–visible)
UV absorption analysis of molecules 1–4 was carried out in
methanol and other organic solvents and the corresponding
experimental lmax, extinction coefficients for respective mole-
cules are shown in Table 2 while the corresponding spectra is
depicted in Fig. 2. The analysis shows that all the molecules
absorb in the UV region and the absorption intensities are
relatively weak when compared with their analogues with a
conjugated bridge instead of –O–CH2– bridge considered here
for the study.47b This kind of weak absorption intensity is a
typical characteristic of a break in conjugation occurring in
such type of molecules11,12,56,61 and in our earlier work 60 with
–NH–CH2– bridges we have also observed that molecules of
these type show weak absorption maxima in the UV region of
the electromagnetic spectrum (but the observed oscillator
strengths are relatively higher compared to these systems
considered here for the study). Further experimental investiga-
tions by using different solvents like methanol, acetonitrile and
hexane (corresponding spectra of only molecule 1 is provided
in the electronic supplementary information (ESI){) reveal
bridge in present work). Though both
N and O are
sp3-hybridized still the tendency of the nitrogen to maintain
the pyramidality (involving three bonds and lone pair of
electron) results in only two stable minima, where as in the case
of oxygen along with the two minima similar to –NH–CH2–
bridge systems (because of the lone pair effect), the two more
stable minima is due to the tendency to maintain planarity (as
it involves two bonds only). Due to the sterical interactions,
the two extra minima observed in these systems are relatively
less stable compared to A and B. If we analyze the relative
stabilities of all four conformations for each molecule, then
it can be seen that the most stable conformation A is around
3.1–4.5 kcal (varying for different molecules 1–4) more stable
Table 1 Relative energies (in kcal), dipole moments m (in Debye), of four conformations for each of molecule 1–4, calculated at HF/6–31G level
of theory
Molecule
Confo-rmation
Relative energya
m
Molecule
Confor-mation
Relative energya
m
1: R = H
1A
1B
1C
1D
2A
2B
2C
2D
0.00
0.11
1.26
3.11
0.00
0.12
1.64
3.51
0.44
1.32
0.25
3.79
0.77
2.35
0.39
3.94
3: R = OCH3
3A
3B
3C
3D
4A
4B
4C
4D
0.00
0.24
2.20
4.46
0.00
0.18
1.91
3.89
3.55
2.53
3.88
6.28
1.89
5.68
0.34
4.89
2: R = CH3
4: R = F
a
Relative energies for each conformation are calculated wrt the most stable conformation type A.
498 | J. Mater. Chem., 2006, 16, 496–504
This journal is ß The Royal Society of Chemistry 2006

View Article Online
Table 2 Experimentally measured lmax (in nm), e (in mol L21 cm21
for molecule 1–4 and theoretically determined lmax (in nm) and f for all
four conformations for each of molecule 1–4
)
Table 3 Vertical excitation analysis of molecule 1 (conformations C)
obtained from ZINDO analysis at HF/6–31G geometry
Transition DE/eV l/nm f (61023) Major transitions
Ci
Theoretical
Experimental
1 A 4
4.6677 265.6 1.4
HOMO22 A LUMO+2 0.242
HOMO21 A LUM0+3 0.232
HOMO
A LUMO+4 0.232
a
Molecule
lmax
f (61023
)
lmax
ea
1: R = H
1A
1B
1C
1D
2A
2B
2C
2D
3A
3B
3C
3D
4A
4B
4C
4D
264
264
265
265
269
269
270
271
273
273
273
274
267
267
268
268
0.4
2.2
1.4
4.8
0.6
0.2
18.5
21.0
24.3
40.5
14.5
33.1
0.4
0.8
4.4
13.3
264
1341
HOMO25 A LUMO+5 20.206
HOMO24 A LUMO+6 20.198
HOMO23 A LUMO+7 20.198
HOMO22 A LUMO+5 20.193
HOMO21 A LUMO+6 20.184
2: R = CH3
3: R = OCH3
4: R = F
a
274
280
270
1709
3420
1770
HOMO
A LUMO+7 20.184
HOMO25 A LUMO+2 20.169
HOMO24 A LUMO+3 20.161
HOMO23 A LUMO+4 20.161
HOMO22 A LUMO+4 0.248
1 A 5
4.6679 265.6 6.1
HOMO
A LUM0+2
0.230
HOMO23 A LUMO+5 20.206
HOMO25 A LUMO+7 20.199
HOMO
A LUMO+5 20.182
HOMO22 A LUMO+7 20.174
HOMO23 A LUMO+2 20.169
HOMO25 A LUMO+4 20.162
HOMO21 A LUMO+4 20.148
Here the lmax, e are obtained from the solution phase measurement
from the methanol solution of the respective molecules.
HOMO
A LUMO+3 20.148
HOMO24 A LUMO+7 0.122
HOMO23 A LUMO+6 0.122
HOMO24 A LUMO+4 0.111
HOMO23 A LUM0+3
HOMO21 A LUMO+6 0.106
HOMO
A LUMO+7 20.106
HOMO22 A LUMO+3 0.248
HOMO21 A LUM0+2 0.230
0.111
1 A 6
4.6679 265.6 6.1
HOMO24 A LUMO+5 20.206
HOMO25 A LUMO+6 20.199
HOMO21 A LUMO+5 20.182
HOMO22 A LUMO+6 20.173
HOMO24 A LUMO+2 20.169
HOMO25 A LUMO+3 20.162
HOMO21 A LUMO+3 0.148
HOMO
A LUMO+4 20.148
HOMO24 A LUMO+6 20.122
HOMO23 A LUMO+7 0.122
HOMO24 A LUMO+3 20.111
HOMO23 A LUM0+4
HOMO21 A LUMO+7 0.106
HOMO
A LUMO+6 0.106
0.111
Fig. 2 Experimental UV–vis absorption spectra of molecules 1–4
in methanol.
measurements) and this can be explained by considering the
donating strengths of the individual chromophores. The
computed oscillator strength is weak which is again in
accordance with the observed extinction coefficients. The less
intense absorption can be attributed to the lack of conjugation
between the charge transfer directions.47b So in order to
analyze the nature of the charge transfer occurring in
these molecules, the detailed vertical excitation analysis for
molecule 1 is shown in Table 3 and the corresponding orbitals
participating in the charge transfer transitions are shown in
Fig. 3. From Table 3, it is clear that there are three near-
degenerate minor charge transfer transitions, which are
simultaneously leading to the lmax. This type of typical
behavior was also observed for octupolar molecules62 and this
indicates that though these molecules are structurally deviated
from the planar octupolar type of molecules, but the behavior
is still retained. The orbital analysis shows that the charge
transfer occurring in these molecules are inter-ring type and
occurring in the side wings and also some times the charge
negligible solvatochromism. The negligible solvatochromic
effect in the absorption maximum in solution phase can be
clearly identified that the charge transfer occurring in such
type of molecular systems are not of dipolar nature.
In order to further analyze the nature of charge transfer and
for comparison with the experimental absorption properties,
we have also carried out theoretical calculations of vertical
excitations at HF/6–31G optimized geometries of all the
conformations for each molecule using ZINDO implemented
in G03w59 and the results are shown in Table 2. Comparison of
the vertical excitation for all the four conformations for the
respective molecules, it can be seen that the lmax and f
values are almost same irrespective of the conformations, i.e.,
the absorption in these molecules are not conformational
dependent. Further it can also be seen from Table 2, that
substitution has also little effect in shifting the absorption
maximum (up to 10 nm red shift is observed in the theoretical
calculation and up to 16 nm red shift in the experimental
This journal is ß The Royal Society of Chemistry 2006
J. Mater. Chem., 2006, 16, 496–504 | 499

View Article Online
Earlier we have also observed a similar type of charge
transfer process occurring through –NH–CH2– bridges (which
is similar to a –O–CH2– type of bridge) and such type of
restricted charge flows occurring are important characteristics
for these types of 3D molecular systems.60
Molecular NLO response
The molecular first hyperpolarizability (b) values for all the
compounds were measured using the HRS technique in
chloroform solution at 1064 nm. The HRS signals at 532 nm
from dilute solutions (3 6 1025–6 6 1024 M) of molecules 1–4
were collected. The intensity of the HRS signal I2v produced
by the incident beam of intensity, Iv is given by:
2
2
2
I2v = G[N1<b1 > + N2<b2 >] Iv
(1)
Where G is the instrumental factor incorporating local field
factor, N1 and N2 are the number densities of the solvent and
solute respectively and b1 and b2 are the hyperpolarizabilities
of the solvent and solute respectively. At low solute con-
centrations, the solvent concentration does not change
2
significantly. Therefore I2v/Iv is a linear function of the
solute concentration. The plots (I2v/Iv2 vs. number density) for
all the molecules are provided in the ESI{. <b> values for each
molecule are obtained from the respective plots and the
corresponding plot for PNA used as the reference58 the
estimation is based on the following relation:
SlopeðreferenceÞ b2ðreferenceÞ
~
(2)
b2ðsampleÞ
SlopeðsampleÞ
The plot of the SHG signal (I2v) versus wavelength for each
of the molecules (see inset in the respective HRS signal plots in
the ESI{) show that all of them produce sharp SHG
exclusively at 532 nm indicating clearly the absence of two-
photon fluorescence interferences in these systems (the data
reported are over 150 pulses at each wavelength). In this work
the main aim being the SHG in the solid state, no attempt was
made to carry out the depolarized HRS for estimation of the
8
ratio of the dipolar to octopolar bHRS
.
The experimental bHRS are shown in Table 4 and it can be
seen that for all the four derivatives, the observed hyper-
polarizability values are slightly different. The minor deviation
and the trend for all the molecules can be established in
considering the donating strengths of the substituents, as the
triazine is the common acceptor for all. The analysis shows
that bHRS value is highest for molecules 3 and lowest for
molecule 1 and this can be attributed to the presence of strong
Fig. 3 Frontier molecular orbitals of molecule 1 (conformation C)
participating in the charge transfer transition leading to lmax, from the
HF/6–31G optimized structure.
Table 4 Experimental first hyperpolarizability (610230 esu) values
for molecule 1–4, obtained from HRS measurement in chloroform
solutiona
transfer is assisted through the central triazine ring. The
mixture of transitions constituting each charge transfer band
shows that some transitions are assisted through the bridge
and central triazine ring (remaining transitions are more or less
are due to a through-space type of interaction). Thus the
reason behind the small absorption intensities can be explained
as due to the charge transfer through the non-conjugated
bridges and/or due to through-space type interactions (TSI).
Molecule
bHRS
1
2
3
17.8
19.6
20.8
19.2
17.0
4
PNA
a
PNA as reference.
500 | J. Mater. Chem., 2006, 16, 496–504
This journal is ß The Royal Society of Chemistry 2006

View Article Online
donor (–OMe group) in the former case and absence of any
donor in the latter case. Further for molecule 2, where methyl
is the donor, smaller bHRS value compared to that in molecule
3 can be explained as due to the less donating capacity of –CH3
compared to –OMe. Similarly for molecule 4, where fluoro is
the substituent, due to the combined inductive effect and
electron withdrawing nature of the substituent, the bHRS is
having an intermediate value. Comparing the results of bHRS
values of all the four derivatives respectively for –NH–CH2–
and –O–CH2– bridges, it can be seen that for respective
derivatives, bHRS for –NH–CH2– is higher than –O–CH2–
bridge case.60 In an earlier work by McMahon et al., the
factors affecting the enhancement of b for N-bridged and the
O-bridged donor acceptor systems have been dealt with, hence
we do not go into the details here.63
In analyzing the btotal values (Table 5) for all the
conformations of each molecule it can be visualized that,
for all the molecules, conformation A is having least value
compared to all other conformations. This can be explained in
considering the structural orientation of this isomer, where due
to the position of two substituents opposite to other one results
a minor cancellation of some tensorial components and thus
the overall btotal is least. For the conformations C and D it can
be observed that these are nearly equal and are the highest
values compared to all other conformations. This can be
explained as due to the planar type of arrangements of the
substituents around the central triazine ring, which favors
constructive addition of the tensorial components of b
resulting in a larger btotal. Now for conformation B, which is
the tripod-stand-like conformation, for all the molecules
moderate btotal and almost equal values are obtained.
We have also computed b values for all the four conforma-
tions of each molecule at HF/6–31G optimized geometries,
using the CPHF/6–31G method implemented in the
Gaussian03 package.59 btotal and bvec were computed from
the tensorial components of first hyperpolarizability and as
the molecules are 3D nature, we have calculated the btotal
and bvec by taking into account the Kleinman symmetry
relations and the squared norm of the Cartesian expression
for the b tensor.4,8c The relevant expressions are shown below
and the calculated b values for all the molecules are given
in Table 5.
Now analyzing the bvec values of all the conformations
for each molecule, it can be seen that, for all the molecules,
conformation C is having the least value. As discussed earlier,
this conformation is symmetric type and the structural features
tending towards planarity (i.e., tending towards true octupolar
type) and resulting in a much lower value of bvec. Similar
planar conformations D, where due to the unsymmetrical
position of one substituent, is more of a dipolar type and as a
result of which the bvec values for these conformations are
comparatively higher than that of C. Now, if conformation A
and B (both are deviated from planarity) are compared, then
it can be observed for conformations of type B, bvec is
comparatively higher than that of conformations type A. This
can be explained in considering the structural orientation of
the two isomers, where in type A, due to the position of two
substituents opposite to the other one, results a minor
cancellation of some tensorial components and at the same
time for conformations of type B, due to symmetrical positions
of the substituents favors constructive addition path to have
higher bvec than that of conformations type A. Reasonably
good values of btotal for all the conformations of each molecule
and significant contributions of bvec and boct to the btotal for
these molecules is seen. This type of typical behavior we have
also experienced in our earlier report for similar type of
molecules with –NH–CH2– type of bridges.60 This kind of
characteristic of substantial contributions of bvec and boct to
the btotal is a clear indication of the 3D nature of the molecules.
To draw a correlation between the experimental and
theoretical b values, it should be stated here that the absolute
values of bHRS determined from the experiment are the
dynamic values and they cannot be directly compared to the
theoretical estimated static values.64 However, the trend in
these two can be compared and from the comparison it can be
seen that the trends in the variations of b values in both the
cases follows the same order.
!
1=2
X
btotal
~
b2ijk
(3)
(4)
ijk
ꢀ
ꢁ
1=2
3
bvec
~
b2xzb2yzb2z
5
Where,
bx = bxxx + bxyy + bxzz
by = byyy + byxx + byzz
bz = bzzz + bzxx + bzyy
(4(a))
(4(b))
(4(c))
Table 5 Theoretical (calculated using CPHF method) btotal
(610230 esu) and bvec (610230 esu) for all four conformations for
molecule 1–4
Theoretical
Molecule
btotal
bvec
1: R = H
1A
1B
1C
1D
2A
2B
2C
2D
3A
3B
3C
3D
4A
4B
4C
4D
3.80
4.80
4.64
4.65
4.10
4.60
6.90
6.45
3.80
4.60
9.90
9.85
3.70
4.40
6.57
6.60
0.10
2.12
0.01
0.81
0.30
0.74
0.06
2.51
0.90
2.30
0.50
1.54
0.70
2.00
0.01
1.13
2: R = CH3
3: R = OCH3
4: R = F
Crystal structure and SHG
At room temperature, all the molecules obtained from the
synthesis are solids. Hence the solid-state properties of these
molecules such as melting point, solid-state absorption
maximum (UV-DRS), thermal stability (TGA analysis)
measurements were carried out and the corresponding data
This journal is ß The Royal Society of Chemistry 2006
J. Mater. Chem., 2006, 16, 496–504 | 501

View Article Online
Table 6 Solid-state properties of molecules 1–4
lmax/nm
(UV-DRS)
Decomposition
temperature/uCa
Molecule
M.pt/uC
SHGb
0.4 6
,0.1 6
,0.1 6
,0.1 6
1
2
3
4
a
96–98
269
278
283
274
325
295
310
305
105–106
118–120
111–112
b
Obtained from TGA. With reference to urea as standard.
can be found in Table 6. The low melting point of all the
molecules is indicative of the weak intermolecular interactions
playing role in the solid-state packing. Further the TGA
analysis shows that, for these molecules the thermal decom-
position range is 295–325 uC. This high thermal stability of
these molecules indicates that these will be quite stable in the
incident laser environment, i.e., there will not be any damage
to the material by the impact of the laser light.
Among the four derivatives, we were successful in obtaining
a good quality crystal suitable for X-ray structure analysis of
molecule 1 only. This crystal was obtained by slow evapora-
tion of the solution of the compound in THF. The X-ray
diffraction data has been collected on a Bruker SMART
APEX (CCD area detector) diffractometer with Mo Ka
radiation and the structure was solved and refined by
SHELXS97 and SHELXL97 suite of programs. The detailed
crystal structure data for the molecule 1 is shown in the
footnote§ and the crystallographic information file (CIF) is
provided in the ESI".
Fig. 4 Crystal structure of molecule 1 obtained from single crystal
X-ray analysis (For clarity hydrogens are not shown).
around 1.26 kcal mol21 more stable, but its dipole moment is
slightly on the higher side (0.44 D compared to 0.25 D). There
is another stable isomer, B that is only 0.1 kcal mol21 less
stable than the lowest conformer but has a much larger dipole
moment of 1.32 D and it could be possible that this
conformation is not preferred due to the large dipole moment.
This indicates that, in this case the lower dipole moment
criteria are preferred in the structure to the lowest energy
criteria for the crystallization. This observation is in line with
our earlier work on 3D molecules60 where again the crystal
structure adopted in the cell is the one with the least dipole
The unit cell structural analysis of molecule 1 shows that, the
molecule crystallizes into non-centrosymmetric orthorhombic
space group, Pca21. The structure of the molecule obtained
from X-ray analysis is shown in Fig. 4. From Fig. 4, it can be
seen that O–CH2– bridges are in the plane of the central
triazine moiety and are arranged in a symmetrical fashion and
further the end phenyl rings in the side wings are little bit
twisted out of plane. This is similar to conformation C
minimized by computational methods. It is quite similar in
view of the symmetric arrangements of the bridging units but a
small deviation in the spatial orientations of the phenyl rings
between the computed and experimental structure is also
observed. In the conformation C, due to symmetrical
arrangements of the side wings (with symmetric twisting of
the phenyl groups), the computed conformation has adopted
C3 symmetry, where as in the crystal structure, though the
bridges are arranged in a symmetric fashion, but due to the
disturbed arrangement of phenyl rings in all three wings, it is
no longer symmetric. An interesting observation here is that
the conformer C as per our calculations is not the one with
lowest energy but with the lowest dipole moment. In fact from
Table 1 it is clearly seen that the lowest energy conformer, A, is
moment. We have carried out
a Cambridge Structural
Database (CSD) search of earlier reported crystal structures
of similar conformationally flexible structures.65,66 It is
interesting to find that there are not too many reports. One
study however reports the presence of two low-lying energy
conformers in the same unit cell.66a
All four derivatives showed detectable SHG activity in the
solid state and the results are presented in Table 6. Molecule 1
has an activity of 0.4 times to standard urea where as the other
three showed activity of less than 0.1 times of urea.
Conclusion
The solid state properties of conformationally flexible mole-
cules of symmetrically substituted benzyloxy derivatives of
triazine based 3D chromophores show a very interesting
trend. At the molecular level the tri-substituted derivatives
are predicted by computational methodologies to have four
different minima. When compared to our earlier work60 on
–NH–CH2 bridges, we find that there are two extra minima,
which we assign to the flexibility of –O–CH2 bonds. While one
conformer has two aryl rings in the plane above and one
below, another one has all three aryl rings below the plane of
the molecule. Two other minima have the aryl rings in the
§ Crystal data for molecule 1: C24H21N3O3, M = 399.44, orthorhom-
˚
˚
bic, a = 23.4804(13) A, b = 11.7367(7) A, c = 7.3650(4) A, a = b = c =
˚
3
˚
90u, V = 2029.7(2) A , T = 273(2) K, space group = Pca21, Z = 4,
m(Mo Ka)
=
0.088 mm21
,
11 286 reflections collected, 2591
independent reflections (Rint = 0.0306), R1 = 0.0483 (I . 2s(I), 2382
reflections), vR2 = 0.1150, s = 1.139.
" CCDC reference numbers 282826. For crystallographic data in CIF
or other electronic format see DOI: 10.1039/b512362c
502 | J. Mater. Chem., 2006, 16, 496–504
This journal is ß The Royal Society of Chemistry 2006

View Article Online
21 I. Ledoux, J. Zyss, J. S. Siegel, J. Brienne and J.-M. Lehn, Chem.
Phys. Lett., 1990, 172, 440.
same plane but with different symmetries. The importance of
these molecules is that these show first hyperpolarizability in
the molecular level and its possible retention in the bulk level is
of interest. We were able to determine the crystal structure of
only one molecule and this molecule we find has crystallized
into a non-centrosymmetric space group; it also shows SHG
activity of around 0.4 times that of urea. One interesting
observation here and in our earlier work is that the
conformation adopted by these flexible 3D molecules in the
crystal is similar to the conformation with lowest dipole
moment predicted by computational methods and not the one
with the lowest energy.
22 M. C. Etter and K.-S. Huang, Chem. Mater., 1992, 4, 824.
23 J. Zyss and J. L. Oudar, Phys. Rev. A, 1982, 26, 2028.
24 D. G. Allis and J. T. Spencer, Inorg. Chem., 2001, 40, 3373.
25 H. L. Bozec and T. Renouard, Eur. J. Inorg. Chem., 2000, 229.
26 N. J. Long, Angew. Chem., Int. Ed. Engl., 1995, 34, 21.
27 (a) S. P. Anthony and T. P. Radhakrishnan, Chem. Commun.,
2001, 931; (b) S. D. Bella, Chem. Soc. Rev., 2001, 30, 355.
28 W. Lin, Z. Wang and L. Ma, J. Am. Chem. Soc., 1999, 121, 11249.
29 C. C. Evans, R. Masse, J.-F. Nicoud and M. B. Beucher, J. Mater.
Chem., 2000, 10, 1419.
30 Poled Polymers and Their Applications to SHG and EO Devices,
advances of Nonlinear Optics, ed. S. Miyata and H. Sasabe, Gordon
and Breach, Amsterdam, 1997.
31 K. Meerholz, Y. D. Nardin, R. Bittner, R. Wortman and
F. Wurthner, Appl. Phys. Lett., 1998, 73, 4.
Acknowledgements
32 M. S. Paley and J. M. Harris, J. Org. Chem., 1991, 56, 568.
33 A. J. Kay, A. D. Woolhouse, G. J. Gainsford, T. G. Haskell,
T. H. Barnes, I. T. Mckinnie and C. P. Wyss, J. Mater. Chem.,
2001, 11, 996.
34 A. Boeglin, A. Fort, L. Mager, C. Combellas, A. Thiebault and
V. Rodriguez, Chem. Phys., 2002, 282, 353.
We are very much thankful to Prof. P. K. Das for helpful
discussions and providing the facility for NLO measurements.
KS and SS thank CSIR for fellowship.
35 G. J. Ashwell, K. Skjonnemand, M. P. S. Roberts, D. W. Allen,
X. Li, J. Sworakowski, A. Chyla and M. Bienkowski, Colloids
Surf., A, 1999, 155, 43.
References
36 C. Lambert, S. Stadler, G. Bourhill and C. Brauchle, Angew.
Chem., Int. Ed. Engl., 1996, 35, 644.
1 Introduction to Nonlinear Optical Effects in Molecules and
Polymers, ed. P. N. Prasad and D. J. Williams, Wiley, New York,
1991.
37 (a) C. R. Moylan, S. Ermer, S. M. Lovejoy, I.-H. McComb,
D. S. Leung, R. Wortmann, P. Krdmer and R. J. Twieg, J. Am.
Chem. Soc., 1996, 118, 12950; (b) R. Wortmann, C. Glania,
P. Kramer, R. Mastschiner, J. J. Wolff, S. Kraft, B. Treptow,
E. Barbu, D. Langle and G. Gorlitz, Chem.—Eur. J, 1997, 3, 1765.
38 M. B-. Desce, V. Alain, P. V. Bedworth, S. R. Marder, A. Fort,
C. Runser, M. Barzoukas, S. Lebus and R. Wortmann, Chem.–
Eur. J., 1997, 3, 1091.
39 I. D. L. Albert, T. J. Marks and M. A. Ratner, J. Am. Chem. Soc.,
1997, 119, 6575.
40 J. Abe, Y. Shirai, N. Nemoto and Y. Nagase, J. Phys. Chem. B,
1997, 101, 1910.
41 H. Pan, X. Gao, Y. Zhang, P. N. Prasad, B. Reinhardt and
R. Kannan, Chem. Mater., 1995, 7, 816.
42 G. Alcaraz, L. Euzenat, O. Mongin, C. Katan, I. Leudox, J. Zyss,
M. B. Desce and M. Vaultier, Chem. Commun., 2003, 2766.
43 Y. K. Lee, S. J. Jeon and M. Cho, J. Am. Chem. Soc., 1998, 120,
10921.
44 H. Lee, S. Y. An and M. Cho, J. Phys. Chem. B, 1999, 103, 4992.
45 (a) S. Brasslet, F. Cherioux, P. Audebert and J. Zyss, Chem.
Mater., 1999, 11, 1915; (b) J. Zyss, S. Brasselet, V. R. Thalladi and
G. R. Desiraju, J. Chem. Phys., 1998, 109, 658; (c) Y. Tu, Y. Luo
and H. Agren, J. Phys. Chem., 2005, DOI: 10.1021/jp051267t.
46 S. Stadler, F. Feiner, Ch. Brauchle, S. Brandl and R. A. Gompper,
Chem. Phys. Lett., 1995, 245, 292.
2 Conjugated Polymers: The Novel Science and Technology of Highly
Conducting and Nonlinear Optically Active materials, ed. J. L. Bredas
and R. J. Silbey, Kluwer, Dordrecht, Netherlands, 1991.
3 D. J. Williams, Angew. Chem., Int. Ed. Engl., 1984, 23, 690.
4 (a) D. R. Kanis, M. A. Ratner and T. J. Marks, Chem. Rev., 1994,
94, 195; (b) J. Zyss and I. Ledoux, Chem. Rev., 1994, 94, 77.
5 Nonlinear Optical Materials. Theory and Modeling, ed. S. P. Karna
and A. T. Yeates, ACS Symposium Series 628, American Chemical
Society, Washington, DC, 1996.
6 G. A. Lindsay and K. D. Singer, in Polymers for Second-Order
Nonlinear Optics,, ACS Symposium Series 601, Amerian Chemical
Society, Washington, DC, 1995.
7 Molecular Nonlinear Optics: Materials, Physics, and Devices, ed.
J. Zyss, Academic Press, Boston, 1993.
8 (a) T. Verbiest, S. Houbrechts, M. Kauranen, K. Clays and
A. Persoons, J. Mater. Chem., 1997, 7, 2175; (b) T. Verbiest,
K. Clays, C. Samyn, J. Wolff, D. Reinhoudt and A. Persoons,
J. Am. Chem. Soc., 1994, 116, 9320; (c) J. L. Bredas, F. Meyers,
B. M. Pierce and J. Zyss, J. Am. Chem. Soc., 1992, 114, 4928; (d)
T. Verbiest, K. Clays, A. Persoons, F. Meyers and J. L. Bredas,
Opt. Lett., 1993, 18, 525.
9 Nonlinear Optical Properties of Organic Molecules and Crystal, ed.
D. S. Chemla and J. Zyss, Academic Press, Orlando, FL, 1987,
vols. 1 and 2.
47 (a) P. C. Ray and P. K. Das, Chem. Phys. Lett., 1995, 244, 153; (b)
Y. Z. Cui, Q. Fang, H. Lei, G. Xue and W. T. Yu, Chem. Phys.
Lett., 2003, 377, 507.
48 J. Zyss, I. Ledoux, S. Volkov, V. Chernyak, S. Mukamel,
G. P. Bartholomew and G. C. Bazan, J. Am. Chem. Soc., 2000,
122, 11956.
10 Nonlinear Optics, ed. R. W. Boyd, Academic Press, New York,
1992.
11 W. Schuddeboom, B. Krijnen, J. W. Verhoeven, E. C. J. Staring,
G. L. J. A. Rikken and H. Oevering, Chem. Phys. Lett., 1991, 179,
73.
12 B. Krijnen, H. B. Beverloo, J. W. Verhoeven, C. A. Reiss,
K. Goubitz and D. Heijdenrijk, J. Am. Chem. Soc., 1989, 111,
4433.
49 C. Bourgogone, Y. L. Fur, P. Juen, P. Masson, J. F. Nicoud and
R. Masse, Chem. Mater., 2000, 12, 1025.
50 (a) V. Ostroverkhov, O. Ostroverkhova, R. G. Petschek,
K. D. Singer, L. Sukhomlinova, R. J. Twieg, S.-X. Wang and
L. C. Chien, Chem. Phys., 2000, 257, 263; (b) C. Lambert,
E. Schmalzlin, K. Meerholz and C. Brauchle, Chem.–Eur. J., 1998,
4, 512; (c) M. B. Desce, J.-B. Baudin, L. Jullien, R. Lorne,
O. Ruel, S. Brasselet and J. Zyss, Opt. Mater., 1999, 12, 333; (d)
G. P. Bartholomew, I. Ledoux, S. Mukamel, G. C. Bazan and
J. Zyss, J. Am. Chem. Soc., 2002, 124, 13480.
13 J. O. Morley and M. Naji, J. Phys. Chem. A, 1997, 101, 2681.
14 E. Brouyere, A. Persoons and J. L. Bredas, J. Phys. Chem. A, 1997,
101, 4142.
15 S. R. Marder, J. W. Perry, G. Bourhill, C. B. Gorman,
B. G. Tiemann and K. Mansour, Science, 1993, 261, 186.
16 F. Meyers, S. R. Marder, B. M. Pierce and J. L. Bredas, J. Am.
Chem. Soc., 1994, 116, 10703.
17 C. W. Spangler, J. Mater. Chem., 1999, 9, 2013.
18 I. D. L. Albert, T. J. Marks and M. A. Ratner, J. Am. Chem. Soc.,
1997, 119, 3155.
51 J. J. Wolff, F. Siegler, R. Mastchiner and R. Wortmann, Angew.
Chem., Int. Ed., 2000, 39, 1436.
52 V. P. Rao, A. K.-Y. Jen, J. Chandrasekhar, I. N. N. Namboothiri
and A. Rathna, J. Am. Chem. Soc., 1996, 118, 12443.
53 (a) L. Han, M.-Z. Xue, L. Xiang, H.-B. Quin and Y.-G. Liu,
Opt. Mater., 2004, 27, 235; (b) H. Lee, D. Kim, H.-K. Lee, W. Qiu,
19 I. D. L. Albert, T. J. Marks and M. A. Ratner, J. Am. Chem. Soc.,
1998, 120, 11174.
20 S. Sitha, J. L. Rao, K. Bhanuprakash and B. M. Choudary, J. Phys.
Chem. A, 2001, 105, 8727.
This journal is ß The Royal Society of Chemistry 2006
J. Mater. Chem., 2006, 16, 496–504 | 503

View Article Online
N. K. Oh, W. C. Zin and K. Kim, Tetrahedron Lett., 2004, 45,
1019; (c) V. R. Thalladi, R. Boese, S. Brasselet, I. Ledoux, J. Zyss,
R. K. R. Jetti and G. R. Desiraju, Chem. Commun., 1999, 1639; (d)
R. Boese, G. R. Desiraju, R. K. R. Jetti, M. T. Kirchner, I. Ledoux,
V. R. Thalladi and J. Zyss, Struct. Chem., 2002, 13, 321; (e)
F. Cherioux, H. Maillotte, P. Audebert and J. Zyss, Chem.
Commun., 1999, 2083; (f) L. Xiang, Y. G. Liu, A.-G. Jiang and
D.-Y. Huang, Chem. Phys. Lett., 2001, 338, 167; (g) Y. Sui,
X.-X. Guo, J. Yin, Y.-G. Liu, J. Gao, Z. K. Zhu, D. Y. Huang and
Z. G. Wang, J. Appl. Polym. Sci., 2000, 76, 1619; (h) Y. Liu,
A. Jiang, L. Xiang, J. Gao and D. Huang, Dyes Pigm., 2000, 45,
189.
J. Jaramillo, R. Gomperts, R. E. Stratmann, O. Yazyev,
A. J. Austin, R. Cammi, C. Pomelli, J. W. Ochterski, P. Y. Ayala,
K. Morokuma, G. A. Voth, P. Salvador, J. J. Dannenberg,
V. G. Zakrzewski, S. Dapprich, A. D. Daniels, M. C. Strain,
O. Farkas, D. K. Malick, A. D. Rabuck, K. Raghavachari,
J. B. Foresman, J. V. Ortiz, Q. Cui, A. G. Baboul, S. Clifford,
J. Cioslowski, B. B. Stefanov, G. Liu, A. Liashenko, P. Piskorz,
I. Komaromi, R. L. Martin, D. J. Fox, T. Keith, M. A. Al-Laham,
C. Y. Peng, A. Nanayakkara, M. Challacombe, P. M. W. Gill,
B. Johnson, W. Chen, M. W. Wong, C. Gonzalez, and J. A. Pople,
Gaussian, Inc., Wallingford CT, 2004.
60 K. Srinivas, S. Sitha, V. Jayathirtha Rao, K. Bhanuprakash,
K. Ravikumar, S. P. Anthony and T. P. Radhakrishnan, J. Mater.
Chem., 2005, 15, 965.
61 C. K. Prout and E. E. Castellano, J. Chem. Soc. A, 1970, 2775.
62 (a) A. Kiss and P. Fazekas, arXiv, cond-mat/0404099; (b) M. Cho,
H.-S. Kim and S.-J. Jeon, J. Chem. Phys., 1998, 108, 7114.
63 C. M. Whitaker, E. V. Patterson, K. L. Kott and R. J. Mcmahon,
J. Am. Chem. Soc., 1996, 118, 9966.
64 (a) K. Clays and B. J. Coe, Chem. Mater., 2003, 15, 642; (b)
S. P. Karna and P. N. Prasad, J. Chem. Phys., 1991, 94, 1171; (c)
M. G. Kuzyk, Phys. Rev. Lett., 2000, 85, 1218; (d) M. G. Kuzyk,
Opt. Lett., 2000, 25, 1183.
54 M. Y. Antipin, T. V. Timofeeva, R. D. Clark, V. N. Nesterov,
F. M. Dolgushin, J. Wu and A. Leyderman, J. Mater. Chem., 2001,
11, 351.
55 J. Bernstien, R. J. Davey and J. O. Henck, Angew. Chem., Int. Ed.,
1999, 38, 3440.
56 (a) K. Bhanuprakash and J. L. Rao, Chem. Phys. Lett., 1999, 314,
282; (b) J. L. Rao and K. Bhanuprakash, Indian J. Chem., Sect. A:
Inorg., Bio-inorg., Phys., Theor. Anal. Chem., 2000, 39, 114; (c)
J. L. Rao and K. Bhanuprakash, J. Mol. Struct. (THEOCHEM),
1999, 458, 269; (d) J. L. Rao and K. Bhanuprakash, Synth. Met.,
2003, 132, 315.
57 S. K. Kurtz and T. T. Perry, J. Appl. Phys., 1968, 39, 3798.
58 K. Clays and A. Persoons, Phys. Rev. Lett., 1991, 66, 2980.
59 Gaussian 03, Revision C.01, M. J. Frisch, G. W. Trucks,
H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman,
J. A. Montgomery, Jr., T. Vreven, K. N. Kudin, J. C. Burant,
J. M. Millam, S. S. Iyengar, J. Tomasi, V. Barone, B. Mennucci,
M. Cossi, G. Scalmani, N. Rega, G. A. Petersson, H. Nakatsuji,
M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa,
M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, M. Klene,
X. Li, J. E. Knox, H. P. Hratchian, J. B. Cross, C. Adamo,
65 (a) Cambridge Crystallographic Database, Graphic QUEST 3D
software, X-blackboard version 2.3.8, 1999; (b) B. Aurivillius and
R. E. Carter, J. Chem. Soc., Perkin Trans. 2, 1978, 10, 1033; (c)
R. E. Carter, B. Nilsson and K. Olsson, J. Am. Chem. Soc., 1975,
97, 6155; (d) R. E. Carter and P. Stilbs, J. Am. Chem. Soc., 1976,
98, 7515.
66 (a) B. J. Coe, T. A. Hamor, C. J. Jones, J. A. McCleverty, D. Bloor,
G. H. Cross and T. L. Axon, J. Chem. Soc., Dalton Trans., 1995,
673; (b) G. G. A. Balavoine, J.-C. Darav, G. Iftime, P. G. Lacroix
and E. Manoury, Org. Metallics, 1999, 18, 21.
504 | J. Mater. Chem., 2006, 16, 496–504
This journal is ß The Royal Society of Chemistry 2006