ADVANCED MATERIALS FOR OPTICS AND ELECTRONICS, VOL 6,261-266 (1996)
Strong Hydrogen Bonds as a Design
Element for Developing New
Non-linear Optical Crystals: Cocrystals of Merocyanine Dyes and
Phenol Derivatives
Feng Pan, Man Shing Wong, Christian Bosshard and Peter Gunter
Nonlinear Optics Laboratory, Institute of Quantum Electronics, ETH-Honggerberg, CH-8093, Zurich, Switzerland
Volker Gramlich
Laboratorium fur Kristallographie, ETH-Zentrum, CH-8092, Zurich, Switzerland
In this work we show that with strong hydrogen bonds, including a ‘very strong’ hydrogen
bond and/or strong hydrogen-bonding networks, highly non-linear optically active
chromophores, merocyanine dyes and phenol derivatives, are assembled as salt or semi-salt
crystals, leading to improvement in crystallinity and interesting non-linear optical properties.
Our approach is discussed in connection with four co-crystal structures.
KEYWORDS
nonlinear optics; hydrogen bonds; merocyanine dye
INTRODUCTION
Organic materials based on extended n-electron
systems, both single crystals and poled polymers,
have been proposed for electro-optical and nonlinear optical (NLO) applications, especially for
fast electro-optical modulation, owing to the large
electronic contribution to the electro-optic effects
and the low dielectric constant of organic materials. 1*2 Compared with polymer materials, l S 3
organic crystals are of great interest, since a reliable and time-independent orientation of NLO
chromophores in the lattice can be imposed. In
addition, organic crystals usually contain a higher
density of NLO-active components for increasing
the macroscopic NLO susceptibilities. Recently,
the interest for further development has been
concentrated on crystals containing more active
NLO chromophores with highly extended
CCC 1057-9257/96/050261-06
0 1996 by John Wiley & Sons, Ltd.
x-electron conjugated systems. However, the
development of optimised NLO organic crystals
still presents severe challenges: (1) to grow active
NLO chromophores into crystals with optimised
orientation to maximise large macroscopic NLO
effects (there is no reliable approach for crystal
engineering to control the molecular packing); (2)
to grow high-quality crystals (optimised NLO
chromophores normally show poor crystallinity,
which limits the growth of high-quality crystals for
practical applications).
Crystal engineering, the rational design and
synthesis of specific structural aggregates in the
solid state, has been used to develop new organic
crystals for NLO applications. The use of hydrogen bonds as a steering force is now beginning to
emerge as one of the most important strategies in
crystal engineering. Compared with the strong
interaction of covalent bonds with a stabilisation
around 20- 100 kcal mol-’, a single hydrogen
Received 12 July 1996
Accepted 17 July 1996
262
F. PAN ETAL
bond with a bonding enthalpy of less than
5 kcalmol-' is not always perceived to be strong
enough to dominate intermolecular aggregates.
Instead, co-operative hydrogen bonds or hydrogen
bond networks are widely used in assemblies of
supramolecular aggregates, in which the enthalpy
of the network could be up to 24 kcalmol-'. In
addition, there is another class of hydrogen bonds,
so-called 'very short', 'very strong' or 'symmetric'
hydrogen bonds,4 in which the bonding energy
could be unusually strong, even comparable with
that of covalent bonds. Most of these kinds of
hydrogen bonds have been found in the aggregates
of small molecules or ions in crystals. In this work
we show that with strong hydrogen bonds, including a 'very strong' hydrogen bond and/or strong
hydrogen-bonding networks, highly NLO-active
chromophores, merocyanine dyes and phenol
derivatives, are assembled as salt or semi-salt
crystals, leading to improvement in crystallinity
and interesting NLO properties. We present our
approach with the example of four co-crystal
structures, among which the new NLO co-crystal
(M1.2A4NP*H2O)is presented for the first time.
CO-CRYSTALLISATIONAND
STRUCTURES OF CO-CRYSTALS OF
MEROCYANINE DYES AND PHENOL
DERIVATIVES
The merocyanine dye M (see Scheme 1) is one of
the best NLO chromophores, with a very large
molecular first-order hyperpolarisability, I p z I =
4200 x lo-''' m4V-' (measured by EFISH at
ill = 1.89 pm in DMSO),' and very good photostability and thermal stability. M shows a negative
solvatochromic behaviour6and a negative first-order
molecular hyperpolarisability, -:?I!t I ,7 which are
caused by the fact that the dipole moment of the
excited state (p! = 13.7 D) of M is smaller than that
of the ground state (pu,"= 22.6 D), giving rise to a
negative ApM (-8.9 D).7 This negative value
implies that
is in an antiparallel orientation to
the ground state dipole moment of M, p:. These
properties are different from those of most stilbenoid derivatives, such as the protonated form of
M , 4-hydroxy-4'-N'-methyl-stilbazolium cation
([H-MI +), which shows a positive first-order
hyperpolarisability value.7
Hence it is of considerable interest to develop
potentially useful non-linear optical crystals based
on merocyanine dyes. However, in addition to a
centrosymmetric packing, it was found that the
crystallinity of the compound M and its derivatives
was too poor to grow single crystals with good
optical quality. One of the strategies in crystal
engineering is to dissect and insulate different
types of intermolecular interactions from one
another* in order to induce a desired chromophoric
alignment and to improve crystallinity, which
motivated us to introduce a guest molecule to cocrystallise with M . For this purpose, aniline and
phenol derivatives with acceptors at 3- or 4-positions, which may interact with each other or/and
with M by means of hydrogen bonds, were chosen
as guest molecules. They are also traditional NLO
chromophores, which may even increase the
density of useful NLO elements.
Co-crystallisation of equimolar amounts of
aniline (or phenol) derivatives and M was carried
out by cooling of methanol or ethanol solutions or
slowly evaporating the solvents. Aniline derivatives seem not to interact with M but crystallise
independently. In contrast, M can co-crystallise
with most phenol derivatives, showing good
crystallinity and yielding red or black crystals in
the form of needles, plates or bulk structures.
These crystals can be grown large enough
(3 x 3 x 3 mm3) for optical devices.
Insight into the co-crystallisation can be
obtained from the single-crystal X-ray structures
of these co-crystals. Examples are shown in the
following.
pz
Co-crystal M I -2-amino-4-nitrophenol
(M I -2A4NP)'
M1: R = CH,
M2: R = CH2CH20H
Scheme 1
M1.2A4NP is an organic salt with the 'very short'
hydrogen-bonded dimeric cation [MlHMl] + and
anion [XHX] -. Note that in our notation X is an
anion of the phenol derivative which lost one
proton. In [MlHMl ] and [XHX] the hydrogen
bonds between the two phenolic oxygen atoms,
RO--H-o, are very short, 2.46(1) A , and the
+
263
STRONG HYDROGEN BONDS IN DEVELOPING NEW NLO CRYSTALS
protons stay nearly equidistant Jo the two phenolic
oxygen atoms (RHp0 = 1.2(1) A). According to an
empirical evaluation, lo the bonding energy of this
kind of short hydrogen bond could be unusually
strong (20 kcalmol-'), even comparable with the
lower limit of that of covalent bonds. In the cocrystal the dimeric cation [MlHMl]' and anion
[XHXI- are both almost coplanar, with an angle
between the two M- or X-planes of only 7". The
three-dimensional structure of the co-crystal
consists of alternating layers of cations and anions.
The dimeric cation [MlHMl]' lies in a plane at
an angle of about 65" to the plane of the dimeric
anions [XHXI-. The charge transfer axes (O-+N)
of the highly hyperpolarisable chromophores M
are almost completely antiparallel to one another,
with an angle of 178.8O between them, which is an
ideal packing for third-order NLO applications.
Co-crystal M I *2-arnino-5-nitrophenol
(MI -2A5NP)'
Like [MlHMl]' and [XHXI- in M1*2A4NP, the
neutral dimeric aggregate MlHX (see Fig. 1) in
the co-crystal M1-2A5NP is also assembled with
the 'very short' H bond, within which the chromophores M1 and X share one proton. Therefore M1
and X each possess half a negative charge, M1 -Osd
and X -0.56 . M1-2A5NP is a semi-salt with the
'symmetric'
hydrogen-bonded
aggregate
M1 -0.56H+IX-0.56 (see Fig. 1). In the three-dimensional structure of the co-crystal the M1- and Xplanes build alternating layers, in which the M1plane is nearly perpendicular (86") to the X-plane.
Note that the angle between the two charge
transfer axes (0+N of M1 and NO, 4 NH, of X)
is 36.8" and the electron donor groups (phenolic
oxygen atoms) of the chromophores M1 and X are
very close to each other in M1 -O."H +'X
It is
believed that the electron conjugation between M1
and X is enhanced in M1 -0.5dH+1X which can
enhance the molecular first-order ( p ) and secondorder ( 7 ) hyperpolarisabilities. By the semiempirical Austin Model 1 (AM1) the calculation of
molecular orbitals of the aggregate structure
~1 - 0 . 5 6 ~ + 1-0.56
~
shows that the highest occupied
molecular orbital (HOMO) is mainly composed of
n-electrons of 2A5NP, while for the lowest unoccupied molecular orbital (LUMO) the main
contribution comes from the n-electrons of M l , so
that the charge transfer may occur from 2A5NP to
M1. Furthermore, calculations by the finite field
method using the AM1 parametrisation in the
MOPAC Quantum Mechanical Calculation
Package" show that the vector part of the firstorder molecular hyperpolarisability ( p (static
value)) of the aggregate M1 -0"6H+1X-'."is twice
as large as that of the vectorical sum of pure M1
and 2A5NP. Hence the chromophore aggregate
with the short hydrogen bonding such as in the cocrystal is a potentially interesting active base for
NLO crystals.
-'"'.
-'"',
Co-crystal
M2.2,4-dihydroxybenzaldehyde.0.5H20
(M2-DBA)"
In this co-crystal there are two types of hydrogenbonded aggregates formed by DBA and M2
respectively. One is the dimeric DBA aggregate, a
short-hydrogen-bond dimeric anion [XHX] - (see
Fig. 2). The second is the M2 aggregate, formed
by four M2 molecules which accept two protons
to self -assemble as a multiple-hydrogen bond
network kept together by a four-centre (oxygen
atoms) three-hydrogen bond (see Fig. 2). The
interaction of these multiple-hydrogen bonds
leads to an increase in the length of the hydrogen
bond between two phenqlic oxygen atoms gf the
M2 (RO,-.HOd,
= 2.56(1) A, RH.-O,=1.6(1) A). It
also causes the proton not to stay at the centre but
to localise at one of the phenolic oxygen atoms of
M2, l 3 yielding a cationic protonated merocyanine, [H-M2] +.The three-dimensional strucFig. 1. Neutral dimeric aggregate MlHX in co-crystal ture of the co-crystal consists of alternating layers
M1.2A5NP. The chromophores M1 and 2A5NP are linkcd of the M2 and DBA aggregates with a positive
with the 'sympetric' hydrogen bond (RO--H-o=2.46(1) A,
and negative charge respectively, resulting in
R,-,= 1.2(1) A), yielding an aggregate MI -'1.5dH+1X
-0,56;
yM' and p2A5NP
are the dipole moments of the ground states of coulombic interactions between the alternating
dl and 2 h N P respectively.
layers (see Fig. 2).
264
F. PAN ETAL.
IXHXlFig. 2. M2 aggregate (a four-centre (oxygen atoms) three-hydrogen bond) and dimeric DBA aggregate [YHX] (a shorthydrogen-bond dimeric anion). The M2 aggregates (with a positive charge) and [XHX] - are in alternating layers with coulombic
interactions.
The M2 and [H-M2]' in the co-crystal are
almost completely antiparallel to one another, with
a dihedral angle of 189.4(5)" between them. In
contrast, the vector parts of the first-order hyperpolarisabilities of M2 and [H-M2]',
/3";c and
/3!,",-"2
1 , bearing opposite signs, are almost
completely parallel to one another, with a dihedral
angle of 0.6(5)" between them, which is the ideal
packing for an electro-optic crystal (see
Scheme 2). The highly active SHG efficiency of
the co-crystal in the Kurtz and Perry powder test
further convinced us that the vector parts of the
first-order molecular hyperpolarisabilities of the
chromophores in the co-crystal add up to contribute to the macroscopic susceptibilities.
+
Co-crystal M I ~2-amino-4-nitrophenol.H,O
(M I -2A4NP.H20)
Scheme 2
space group P2,. * Further crystal structure analysis reveals that an interesting motif of the
multiple-hydrogen bonds (see Fig. 3 ) is that the
phenolic oxygen (0,) of 2A4NP plays the role of
a hydrogen bond acceptor and bonds with three
hydrogen-bond-donating groups (HO group) with
three
diff ere? t
bonding
lengths,
ROAHODl
= 2.6361) A
(RH..OA 1.761) A), RCIA-.
HOD2=2-78(1)4 (RH..OA= 1.9(1)0A) imd ROA..
= 2.99(1) A (RH..OA
= 2.0(1) A) respectively.
Similarly to M2*DBA, the interaction of these
multiple-hydrogen bonds leads to an increase in
Co-crystallisation of equimolar 2-amino-4nitrophenol (2A4NP) and M1 was further investigated by slowly evaporating the solvent of the
methanol solution in the air. New deep red cocrystals in the form of prisms were obtained and
showed a strong second-harmonic generation
(SHG) signal in the powder test at 1.06 pm.
Single-crystal X-ray structure analysis shows that *Crystal data for M1.2A4NP.H20, C20H2,N,0,, M = 383.+,
the unit cell of the co-crystal contains molecules monoclinic, space group P2,: a = 10.116(3) A, b= $.761(2) A,
C = 13.608(2) A,
B = 101.55(2)", V=911.9(4) A', Z==2;
of M1 and 2A4NP together with one H,O mol- DCdc= 1.396 mgcm-'; R = 0.0262, wR = 0.0382; w ' = 0 2 ( F )+
ecule, in a non-centrosymmetric packing with 0.0020F2.
STRONG HYDROGEN BONDS IN DEVELOPING NEW NLO CRYSTALS
265
Fig. 3. Three-dimensional structure of co-crystal M1.2A4NP.H20 with alternating layers of [H-MI J and [XI ions and with
water molecules linking alternating layers with hydrogen bonds.
+
~
-
the length of the hydrogen bond between two
SUMMARY
phenolic oxygen atoms of the M1 and 2A4NP,
ROA-.OD,,
resulting in the localisation of the proton
at the phenolic oxygen atoms of M1. Hence in the The key parameters for the co-crystallisation of
three-dimensional structure of the co-crystal there merocyanine dyes (M) and phenol (X) derivaare anions [XI- (2A4NP lost one proton) and tives are that (1) the stable M or/and X
cations [H-Ml]
assembled together with H,O aggregates are assembled in the co-crystals by
molecules yielding a strong hydrogen bond net- strong hydrogen bonds, such as the 'very short'
work along the crystallographic b-axis (see hydrogen bonds and a multiple-hydrogen-bonding
Fig. 3). Hence the co-crystal is an organic salt network, and (2) alternating layers are isolated
assem- from each other in the three-dimensional strucwith anions [XI - and cations [H-Ml]
bled by a strong hydrogen-bonding network.
tures. For salt co-crystals, coulombic interactions
Note that the molecular optical non-linearity between the alternating layers increase the layer
of the anion [XI- should be increased interactions. The electron conjugation between M
significantly, since an electron donor of the and X is enhanced when the electron donor
group 0 - (one of the strongest electron donor groups (phenolic oxygen atoms) of the chromogroups) in the anion [XI- replaces the group phores M and X are very close to each other and
-OH in A4NP. Hence there are two highly are connected by 'very short' hydrogen bonds,
active NLO chromophores, the anion [XI - and which can enhance the molecular first-order ( B )
cation [H-Ml]',
in the co-crystal. We found and second-order ( y ) hyperpolarisabilities. Strong
from the crystal structure analysis of the co- hydrogen bonds are a design element for NLO
crystal M1*2A4NP-H2Othat the charge transfer crystal engineering in both framework and
(CT) axes of the anion [XI- and cation application-oriented considerations owing to their
[H-Ml]'
are oriented at angles of about 70" high bonding strength and high degree of
and 80" respectively with respect to the b-axis flexibility.
(the polar axis in the crystal). Therefore we have
two different chromophores with different orientations in the co-crystal that can contribute
ACKNOWLEDGEMENT
differently to NLO coefficients and show different dispersion effects. Hence co-crystals based
on bis-chromophores may be new interesting We thank the Swiss National Science Foundation
for financial support.
materials for NLO applications.
+
+
266
F.PAN ET AL.
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