First hyperpolarizability orientation in asymmetric pyrrole-based polyene chromophores

Article abstract of DOI:10.1016/j.dyepig.2009.10.019

Novel, asymmetrically shaped, pyrrole-based polyene chromophores were designed to obtain a large angle θ(μ,β_max) between the directions of the maximum first hyperpolarizability β_max and the dipole moment μ, by introducing an asymmetric core pyrrole into the π-conjugated polyene bridge of the chromophore. Asymmetric N-substituted pyrrolic nonlinear optical chromophores based on configurationally locked polyene (CLP) were synthesized and their first hyperpolarizabilities were theoretically determined using density functional theory (DFT). High asymmetry with an angle θ(μ,β_max) between the dipole moment μ and the main direction of the charge-transfer transition β_max of up to 48° was achieved.

Full text of DOI:10.1016/j.dyepig.2009.10.019

Dyes and Pigments 85 (2010) 162e170
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
First hyperpolarizability orientation in asymmetric pyrrole-based
polyene chromophores
O-Pil Kwon a , Mojca Jazbinsek , Jung-In Seo , Pil-Joo Kim , Eun-Young Choi ,
,
b
c
a
a
*
c
b
Yoon Sup Lee , Peter Günter
Department of Molecular Science and Technology, Ajou University, Suwon 443-749, South Korea
Nonlinear Optics Laboratory, ETH Zurich, CH-8093 Zurich, Switzerland
Department of Chemistry and School of Molecular Science (BK 21), Korea Advanced Institute of Science and Technology (KAIST), Daejeon 305-701, South Korea
a
b
c
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 26 May 2009
Received in revised form
Novel, asymmetrically shaped, pyrrole-based polyene chromophores were designed to obtain a large
angle
q(
m
,b
max) between the directions of the maximum first hyperpolarizability
bmax and the dipole
moment
m
, by introducing an asymmetric core pyrrole into the -conjugated polyene bridge of the
p
30 October 2009
chromophore. Asymmetric N-substituted pyrrolic nonlinear optical chromophores based on configura-
tionally locked polyene (CLP) were synthesized and their first hyperpolarizabilities were theoretically
Accepted 31 October 2009
Available online 11 November 2009
determined using density functional theory (DFT). High asymmetry with an angle
dipole moment and the main direction of the charge-transfer transition
q
(
m
,
b
max) between the
ꢀ
m
b
max of up to 48 was achieved.
Keywords:
Ó 2009 Elsevier Ltd. All rights reserved.
Nonlinear optics
Molecular nonlinearity
Asymmetric chromophores
Density functional theory
Isomerization
Polyene
1. Introduction
[3,4,16e20], achieving optimal acentric molecular ordering in both
polymer and crystal systems remains a challenge.
Organic materials exhibiting second-order optical nonlinearity
Various strategies have been suggested to reduce the tendency
have been considered very attractive for numerous applications
during the last 20 years, such as high-speed electro-optic modu-
lation, field detection, frequency conversion [1e8] and terahertz
towards antiparallel alignment of the first hyperpolarizabilities b in
bulk materials: for example, by using spherically shaped chromo-
phores and dendritic chromophores with different shape and size
of substituents [21,22], or by antiparallel dipoleedipole aggregation
of two chromophores bearing opposite sign of the first hyper-
polarizability for polymers [23] and crystals [24], as well as by using
non-polar octupolar molecules [25e27].
(
THz) wave generation and detection [9e12]. Conventional organic
nonlinear optical chromophores are based on a long -conjugated
p
bridge that connects electron donor and electron acceptor groups
and which, therefore, display a large dipole moment [1,2]. To ach-
ieve a macroscopic second-order nonlinearity, the polar chromo-
phores are aligned by additional external electric-field poling in
polymers and by self-assembly based on specific intermolecular
interactions in crystals. However, the tendency for highly polar
chromophores towards antiparallel dipoleedipole aggregation
often leads to reduced poling efficiency in a polymer system [13]
and to centrosymmetric arrangement of molecules in the crystal-
line state [14,15]. Whilst many experimental and theoretical studies
have been devoted to the enhancement of molecular nonlinearity
Another interesting approach for achieving acentric alignment
of the first hyperpolarizabilities
having a large angle max) between the direction of the dipole
moment and the main direction of the first hyperpolarizability
max [28]. For example, 2-(5-methyl-3-(4-(pyrrolidin-1-yl)styryl)
b is to use polar chromophores
q(m,b
m
b
cyclohex-2-enylidene)malononitrile (MH2) chromophore based on
configurationally locked polyene (CLP) [29,30] exhibits a relatively
ꢀ
large angle
q(
m,b
max) of about 22 [28]. The dipole moments
m
of the
molecular pair in the crystalline state are almost antiparallel and
therefore the crystal almost non-polar. However, due to the large
angle q(m,bmax), the first hyperpolarizabilities b still add construc-
tively and MH2 crystals exhibit a large macroscopic second-order
nonlinearity [28].
*
Corresponding author. Tel.: þ82 31 219 2462; fax: þ82 31 219 1610.
E-mail address: opilkwon@ajou.ac.kr (O.-P. Kwon).
0143-7208/$ e see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.dyepig.2009.10.019

O.-P. Kwon et al. / Dyes and Pigments 85 (2010) 162e170
163
In this work, we investigate new chromophores that are
designed to have a large angle max) between the dipole
moment and the main direction of the charge-transfer transition
max. In order to achieve this, 2-dimensional chromophores with
a strongly asymmetric shape are considered, in contrast to the
commonly studied 1-dimensional chromophores and 2-dimen-
sional chromophores with L- or X-shape that are in general
symmetric [25e27,31e33] and show a high tendency of antiparallel
aggregation of the chromophores. To design asymmetric-shaped
2.1.2. Synthesis of pyrrolic CLP chromophores
q(m,
b
The equimolar amounts of corresponding aldehyde and 2-(3,5-
dimethylcyclohex-2-enylidene)malononitrile were mixed with the
catalyst piperidine, and was stirred for 24 h at room temperature
[28]. A crystalline solid was obtained by filtration. The materials
were purified by recrystallization in methylene chloride/methanol
and by column chromatography (methylene chloride:meth-
anol¼300:1) (yield >19%).
m
b
chromophores having a large angle
asymmetric core pyrrole into the
phores. We have designed and synthesized new asymmetric-sha-
ped N-substituted pyrrolic chromophores based on
q
(
m
,
b
max), we here introduce an
2.1.3. 2-(3-(2-(1H-Pyrrol-2-yl)vinyl)-5,5-dimethylcyclohex-2-
p-conjugated bridge of chromo-
enylidene)malononitrile (HP)
1
3 3 2
H NMR (CDCl , d): 1.07 (6H, s, eCH ), 2.42 (2H, s, eCH e), 2.58
(2H, s, eCH
2
e), 6.32 (1H, m, eC]CHe), 6.52 (1H, m, Py-H), 6.73
configurationally locked polyene (CLP) bridge (see Fig. 1) and
investigated the first hyperpolarizabilities theoretically using
quantum chemical calculations with density functional theory
(1H, m, Py-H), 6.96 (1H, m, Py-H), 6.52e6.56 (1H, d, J ¼ 16 Hz,
eCH]CHe), 6.92e6.96 (1H, d, J ¼ 16 Hz, eCH]CHe), 8.5 (1H, s,
17 3
NH). Elemental analysis for C17H N : (%) Calcd. C 77.54, H 6.51, N
(
DFT). High asymmetry with the angle
q(m,
bmax) between the dipole
15.96; Found C 77.42, H 6.61, N 15.85.
moment
m
and the main direction of the charge-transfer transition
ꢀ
b
max of up to 48 has been achieved.
2.1.4. 2-(5,5-Dimethyl-3-(2-(1-methyl-1H-pyrrol-2-yl)vinyl)
cyclohex-2-enylidene)malononitrile (MP)
1
H NMR (CDCl
e), 3.72 (3H, s, eNCH
.66e6.78 (3H, m, Py-H), 6.72e6.76 (1H, d, J ¼ 16 Hz, eCH]CHe),
.90e6.94 (1H, d, J ¼ 16 Hz, eCH]CHe). Elemental analysis for
: (%) Calcd. C 77.95, H 6.90, N 15.15; Found C 77.67, H 7.08,
3
,
d
): 1.08 (6H, s, eCH
3 2
), 2.42 (2H, s, eCH e), 2.58
(
2H, s, eCH
2
3
), 6.21 (1H, m, eC]CHe),
2
. Experimental
6
6
C
2.1. Synthesis and characterization
18 19 3
H N
N 14.96.
All chemicals were obtained from commercial suppliers (mainly
from Aldrich) and used without further purification. H NMR
spectra were recorded on a Varian 400 MHz. The chemical shifts are
3 4
reported in ppm (d) relative to (CH ) Si. UV/vis. absorption spectra
were recorded by a Jasco V-570 spectrometer. Elemental analysis
was performed by LECO CHN-900 for determination of elements C,
H, N and LECO RO-478 for determination of element O. The chro-
mophores were prepared by the following synthetic routes as
shown in Fig. 1.
1
2.1.5. 2-(5,5-Dimethyl-3-(2-(1-(4-nitrophenyl)-1H-pyrrol-2-yl)
vinyl)cyclohex-2-enylidene)malononitrile (NP)
1
H NMR (CDCl
2H, s, eCH e), 6.46 (1H, m, eC]CHe), 6.75 (1H, m, Py-H), 6.87
1H, m, Py-H), 7.01 (1H, m, Py-H), 6.68e6.72 (1H, d, J ¼ 16 Hz,
3 3 2
, d): 1.03 (6H, s, eCH ), 2.23 (2H, s, eCH e), 2.56
(
(
2
eCH]CHe), 6.79e6.83 (1H, d, J ¼ 16 Hz, eCH]CHe), 7.47e7.49
2H, d, J ¼ 9.2 Hz, Ar-H), 8.38e8.41 (2H, d, J ¼ 9.2 Hz, Ar-H).
Elemental analysis for C23 : (%) Calcd. C 71.86, H 5.24, N
4.57, O 8.32; Found C 71.57, H 5.40, N 14.34.
(
20 4 2
H N O
1
2
.1.1. 1-(4-Nitrophenyl)-1H-pyrrole-2-carbaldehyde
Pyrrole-2-carbaldehyde (52.6 mmol, g) and 4-fluoroni-
trobenzene (52.6 mmol, 7.42 g) were dissolved in dry DMF (30 mL).
5
3. Results and discussion
K
2
CO
3
(78.9 mmol, 10.9 g) was added to this solution and the
ꢀ
mixture was stirred for 5 days at 90 C. The resulting mixture was
3.1. Design of new pyrrolic chromophores
cooled to room temperature. The crystalline solid and K
filtered and then washed with water to remove K CO . The crys-
talline solid was purified by recrystallization in methanol/methy-
2 3
CO were
2
3
The chemical structures of newly designed asymmetric chro-
mophores are shown in Fig. 1, together with their abbreviations.
Asymmetric chromophores consist of the configurationally locked
1
3
lene chloride solution. H NMR (CDCl , d): 6.49 (1H, m, Py-H), 7.12
(
1H, m, Py-H), 7.20 (1H, m, Py-H), 7.51e7.53 (2H, d, J ¼ 9.2 Hz, Ar-H),
p-conjugated hexatriene bridge linked between heteroaromatic
N-substituted pyrrole electron donor and dicyanomethylidene
electron acceptor. We employ the configurationally locked
8
.31e8.33 (2H, d, J ¼ 9.2 Hz, Ar-H), 9.61 (1H, s, eCHO). Elemental
: (%) Calcd. C 61.11, H 3.73, N 12.96, O 22.20;
Found C 61.10, H 3.70, N 12.85.
8 2 3
analysis for C11H N O
p-conjugated hexatriene bridge, since it exhibits efficient charge
Fig. 1. Synthetic route for the investigated chromophores.

164
O.-P. Kwon et al. / Dyes and Pigments 85 (2010) 162e170
delocalization without loss of the aromaticity [34], high thermal
stability [28e30] and large photochemical stability [35].
The investigated chromophores incorporate the heteroaromatic
N-substituted pyrrole group on the electron-donor site. Hetero-
aromatic pyrrole is a more electron-excessive bridge than the well-
known thiophene bridge [36,37]. However, pyrrole-based
nonlinear optical chromophores have been rarely investigated
[
38,39]. The electron-rich heteroaromatic pyrrole-2-carbaldehyde
is an asymmetric core due to its asymmetric shape and asymmetric
electron density with respect to the direction of the -conjugated
p
bridge (i.e. the long axis of the chromophores). Moreover, there is
a variety of possibilities of chemical modification of pyrrole by N-
substitution to increase the asymmetry of both shape and electron
density. Introduction of pyrrole as an asymmetric core into chro-
mophores may therefore lead to a large angle
the direction of the dipole moment and the main direction of the
first hyperpolarizability max. The investigated chromophores have
different N-substituted groups (X: eH, eCH , ePheNO ) on pyrrole
see Fig. 1). “HP” stands for the hydrogen N-substituent on the N
q(m,bmax) between
m
b
3
2
(
atom in the pyrrole ring, while “MP” and “NP” stand for methyl and
nitrophenyl N-substituents, respectively.
The pyrrole-based chromophores were synthesized by Knoe-
venagel condensations (see Fig. 1) [28,40,41]. The aldehyde, 1-(4-
nitrophenyl)-pyrrole-2-carbaldehyde for NP chromophore was
prepared from pyrrole-2-carbaldehyde and 4-fluoronitrobenzene.
For the synthesis of the chromophores, 2-(3,5,5-trimethylcyclohex-
2-enylidene)malononitrile was mixed with an equimolar amount
of the corresponding pyrrole-based aldehyde. The materials were
purified by recrystallization in methylene chloride/methanol
solution and by column chromatography (methylene chloride:
methanol ¼ 300:1).
In order to check the chemical structure and purity of the
synthesized materials, we investigated them by thin layer chro-
1
matography, elemental analysis, and H NMR measurement. In all
these measurements, the synthesized materials exhibited a high
1
purity. Interestingly, in the H NMR spectra we observed the exis-
tence of two structural features for all chromophores (see Fig. 2).
The observed two structural features are not due to decomposition
or photo- and thermal-isomerization of the materials. Moreover,
the ratio of these structural features changes as a function of the
solution concentration: when the concentration is increased, the
occurrence of the major structural feature is increased as shown in
Fig. 2c. These features are possibility due to different conformations
of the molecules in the solution. Two structural features are
attributed to rotational isomers (rotamers) [42e44]. The investi-
gated pyrrole-based chromophores possess two rotatable single
bonds: one single bond that links the pyrrole and the hexatriene
bridge and the other single bond within the hexatriene bridge,
which links the eC]Ce double bond and the cyclohexene ring. In
the CLP chromophores having N-dialkylaminophenyl electron
donors studied previously [28e30], we did not observe any rota-
tional isomerization in the hexatriene bridge. Moreover, the single
bond between the pyrrole and the hexatriene bridge presents
Fig. 2. The presence of rotational isomers: for example, 1H NMR spectrum of MP
chromophore in CDCl solution with different concentration of 0.613 mol/L (a) and
.064 mol/L (b). (c) The relative NMR ratio is the ratio between the NMR intensity of
3
0
the minor rotamer (Iminor) and the major rotamer (Imajor) at around 2.6 ppm.
microscopic nonlinear optical properties, which is discussed in
Section 3.3.
3.2. Linear optical properties
The new pyrrole-based chromophores are soluble in polar
solvents such as methanol, acetone, acetonitrile and dime-
thylformamide, as well as in less polar solvents methylene chloride,
chloroform and THF. Fig. 3 shows the absorption spectra of
the molecules in the polar solvent methanol and the less polar
solvent chloroform with low concentration in the range of
a bond linking different types of
p-conjugation (heteroaromatic
ring and polyene), while the bond between the eC]Ce double
bond and the cyclohexene ring presents a bond linking the same
ꢂ5
1.3e1.9 ꢁ 10 mol/L. The wavelength of the maximum absorption
type of
p
-conjugation in the polyene bridge. As a result, the single
lmax is considerably shifted depending on different N-substituents
bond between the pyrrole and the hexatriene bridge is easier to
rotate than the other. Therefore, the observed two structural
features are attributed to rotational isomers of the single bond
linking the pyrrole and the hexatriene bridge. We also observed
many complex polymorphs of asymmetric pyrrole-based molecules
in solid crystalline state, which will be described elsewhere. These
include both R- and L-rotamers, which supports the hypothesis
about rotational isomers. The two rotamers may exhibit different
in the pyrrole ring:
methanol for HP,
methanol for MP, and
in methanol for NP.
The NP chromophore having nitrophenyl N-substituent group
shows a hypsochromic shift of the wavelength of maximum
l
max ¼ 461 nm in chloroform and 470 nm in
l
max ¼ 471 nm in chloroform and 472 nm in
l
max ¼ 450 nm in chloroform and 447 nm
absorption
lmax with respect to HP and MP chromophores, owing
to the influence of the nitrophenyl substituent, which has

O.-P. Kwon et al. / Dyes and Pigments 85 (2010) 162e170
165
addition, when the chromophores possess rotatable single bonds in
the -conjugated bridge, the influence of the rotation of these
bonds on nonlinearity is significant and the rotational isomers
rotamers) exhibit different hyperpolarizabilities [47]. We addi-
tionally consider here that rotamers of highly asymmetric mole-
cules may also exhibit different angles max) between and
p
(
q(m,
b
m
b .
max
In the investigated pyrrole-based chromophores, the pyrrole
and the hexatriene bridge are linked by a rotatable single bond and
therefore can possess rotational isomers. The representative
isomers, R- and L-isomers are depicted in Fig. 4. Since the pyrrole-
based chromophores have an asymmetric-shape, the rotation of the
single bond changes the shape of the chromophores. Therefore, the
influence of the rotational isomerism in asymmetric-shaped
pyrrole-based chromophores is expected to be more crucial than
for linear symmetric-shape chromophores.
In an effort to rationalize the nonlinear optical properties of the
rotational isomers of new asymmetric pyrrolic chromophores, we
performed quantum chemical calculations using Gaussian 03
Program [48] by using the hybrid functional B3LYP [49,50] with the
6e311 þ G(d) basis set. Optimized (OPT) molecular structure was
analyzed by time-dependent density functional theory (TD-DFT)
and finite-field (FF) methods. The resulting hyperpolarizabilities
are given in Tables 1 and 2. For the TD-DFT calculation, we used the
conventional two-level model [51,52], in which the static first
Fig. 3. Absorption spectra of molecules in different solvents: HP (
chloroform and 470 nm in methanol, Dlmax ¼ 9 nm), MP ( max ¼ 471 nm in chloroform
and 472 nm in methanol, Dlmax ¼ 1 nm), and NP ( max ¼ 450 nm in chloroform and
47 nm in methanol, Dlmax ¼ 3 nm).
l
max ¼ 461 nm in
l
l
4
a character of electron acceptor, on the pyrrole ring, which acts as
electron donor. The first hyperpolarizability usually increases
with increasing the wavelength of the maximum absorption
according to the so-called nonlinearity-transparency tradeoff [1,2].
Therefore, the values of the first hyperpolarizability of the NP
b
l
max
0
hyperpolarizability b is given by
ꢀ
ꢁ
2
b
3
Dmge
m
ge
b
¼
;
(1)
molecules are expected to be lower than that of the HP and MP
molecules.
0
2
2ðEmax
Þ
The difference Dlmax between the wavelength of maximum
absorption in the polar solvent methanol and the less polar solvent
chloroform for HP (Dlmax ¼ 9 nm) is larger than for MP and NP
where
m
is the transition dipole moment between the excited e
ge
and the ground-state g, Dmge is the dipole moment difference
between both states, and Emax is the energy at the maximal charge-
transfer absorption [53].
In order to determine the probability of the R- and L-isomer
states, the total energies of the optimized structure of rotamers
were calculated. The total energy differences between two rotam-
(
Dlmax < 3 nm). This can be attributed to the intermolecular
hydrogen bond formation between the pyrrole group and the
solvent [45]. In HP molecules, the hydrogen atom connected to the
N atom in pyrrole ring can act as a strong hydrogen bond donor site
and form strong hydrogen bonds with the polar OH group of
methanol. In MP and NP molecules the hydrogen atom was elimi-
nated by N-substitution on pyrrole, which evidently decreases the
chromophore sensitivity to external environment compared to HP
molecules.
ers,
D
E ¼ E (R-isomer) ꢂ E (L-isomer) are relatively small:
ꢂ4.9 kJ/mol for HP isomers, 1.9 kJ/mol for MP isomers, and
7.1 kJ/mol for NP isomers, which leads to relative energy differences
ꢂ6
D
E/E of only in the order of 10 . Therefore, the representative R-
and L-rotamers are all reasonably probable from the energy
consideration point of view.
3.3. Molecular nonlinearity
The TD-DFT calculations indicate that the main transition of the
HP and MP chromophores corresponds to a HOMO / LUMO
transition, while of the NP chromophore to a HOMO / LUMOþ1
The hyperpolarizabilities of nonlinear optical chromophores
depend on their geometrical and cisetrans isomerism [46]. In
3 2
Fig. 4. Rotational isomers of the investigated chromophores (X: eH, eCH , ePheNO ).

166
O.-P. Kwon et al. / Dyes and Pigments 85 (2010) 162e170
Table 1
Results of the TD-DFT calculations for chromophores in vacuum and in methanol using a PCM model: the wavelength
lmax (nm) and the energy Emax (eV) of the maximal ICT
absorption, the oscillator strength coefficients fos, the transition dipole moment
mge (D), the dipole moment difference between ground and excited electronic states Dmge, the
ꢂ30
esu) and the major molecular orbital contribution to the ICT transition.
static first hyperpolarizability
b
0
(ꢁ10
Dmgea
b
0
b
Major contribution
l
max
E
max
f
os
m
ge
HP-R
HP-L
MP-R
MP-L
NP-R
Vacuum
CH OH
424
468
2.93
2.65
1.26
1.4
10.67
11.81
10.22
10.05
79.23
116.34
HOMO / LUMO
HOMO / LUMO
3
Vacuum
CH OH
431
475
2.88
2.61
1.24
1.38
10.66
11.83
10.6
10.12
84.98
121.19
HOMO / LUMO
HOMO / LUMO
3
Vacuum
CH OH
425
465
2.91
2.67
1.27
1.42
10.72
11.85
10.2
9.85
80.6
113.63
HOMO / LUMO
HOMO / LUMO
3
Vacuum
CH OH
438
479
2.83
2.59
1.25
1.4
10.81
11.92
9.94
9.48
84.63
117.23
HOMO / LUMO
HOMO / LUMO
3
Vacuum
558
2.44
2.93
3.71
2.21
2.68
3.51
0.02
1.02
0.02
0.02
1.15
0.35
1.45
9.59
1.29
1.34
10.62
5.11
23.41
10.02
22.93
24.63
10.49
19.9
4.82
62.58
1.62
5.26
95.91
24.55
HOMO / LUMO
4
3
23
34
HOMO / LUMOþ1
HOMO-1 / LUMO
HOMO / LUMO
3
CH OH
561
4
3
62
53
HOMO / LUMOþ1
HOMO-1 / LUMO
NP-L
Vacuum
543
2.28
2.88
3.58
2.21
2.63
3.46
0.02
1.24
0.02
0.03
1.34
0.25
1.52
10.66
1.28
1.91
11.61
4.36
31.84
9.87
35.05
31.91
10.05
16.37
8.28
78.95
2.63
13.9
114.78
15.11
HOMO / LUMO
HOMO / LUMOþ1
HOMO-1 / LUMO
HOMO / LUMO
4
3
31
46
3
CH OH
561
4
3
72
58
HOMO / LUMOþ1
HOMO-1 / LUMO
a
Excited-state dipole moments were calculated by using one particle RhoCI density [59].
Calculated by Equation (1).
b
transition (see Table 1). In order to understand the electronic
structure of the rotamers, the calculated HOMO and LUMO orbitals
are shown in Fig. 5. The HOMO orbital distribution is localized
mainly at the pyrrole electron donor group. The LUMO orbital
distribution is localized at the dicyanomethylidene electron
acceptor groups for HP and MP chromophores and at the dicya-
nomethylidene and nitrophenyl electron acceptor groups for NP
chromophores. The localizations indicate the intramolecular
charge-transfer (ICT) characteristics of these molecules, such that
the electron density moves from the electron donor group to the
electron acceptor group. On the other hand, the HOMOeLUMO
overlap appears along the hexatriene bridge for HP and MP chro-
mophores and along the hexatriene and pyrrole bridges for NP
chromophores. Therefore, an efficient long-range charge transfer
occurs between electron donor and acceptor of all chromophores.
When comparing R- and L-isomers for all three chromophores,
ring, which leads to a lower HOMO energy of R-isomers compared
to L-isomers (see Fig. 5). However, for MP and NP chromophores,
L-isomers exhibits a lower excited state (LUMO and/or LUMOþ1)
energy compared to R-isomers. This is attributed to higher steric
hindrance in R-isomers, which occurs in the excited state
between the N-substituted methyl or nitrophenyl groups and the
hydrogens on the first double bond from pyrrole in the hexatriene
bridge.
As expected from the wavelength of maximum absorption l
max
measurements of the chromophores in solution (see Fig. 3), the HP
and MP chromophores exhibit a higher hyperpolarizability than the
NP chromophores (see Table 1). However, the calculated values of
lmax in vacuum show large differences to experimental results in
solution (see Table 1). To include the solvent effects, we have
carried out calculations by using the optimized molecular geome-
tries obtained via the DFT method, additionally incorporating
methanol solvent by using the polarizable continuum model (PCM)
[54,55]. When the solvent effect is taken into account, the calcu-
R-isomers have a more favorable
pep conjugated angle (anti
form) between the central hexatriene and carbones of the pyrrole
lated wavelengths of maximum absorption lmax show a reasonable
agreement with the experimental results, and for HP and MP
chromophores match almost perfectly. For NP chromophores, the
calculated wavelength of maximum absorption lmax of the main
transition (462 nm for NP-R and 472 nm for NR-L) with PCM model
shows a small red shift with respect to the experimental value
Table 2
Results of the finite-field (FF) method: dipole moments
hyperpolarizability tensor
dipole moment direction, the maximal component of the first hyperpolarizability
m
g
(D), the zero-frequency
along the
ꢂ30
b
ijk (ꢁ10
esu), the vector component
b
z
ꢂ30
bmax (ꢁ10
esu) and the angle q(m,bmax) (deg.) between the dipole moment m and
g
the main direction of the charge-transfer transition
HP-R HP-L MP-R MP-L
9.62 11.80 9.93
max
b .
(447 nm). These results can be interpreted as follows. In contrast to
NP-R
12.56
NP-L
8.81
HP and MP chromophores, NP chromophore exhibits rather
complex transitions, one main transition and two additional weak
transitions. These weak bands result from HOMO / LUMO and
HOMO-1 / LUMO transitions, for which ICT also involves the
movement of the electron density from the electron donor group to
the electron acceptor group. Note that these transitions are not
observed in the calculations of HP and MP chromophores. In
particular, in PCM model, the oscillator strength fos of HOMO-
m
b
b
b
b
b
b
b
b
b
b
b
b
g
(¼ ꢂ
m
z
)
12.43
ꢂ0.01
0.57
ꢂ1.82
ꢂ20.43
ꢂ1.30
2.56
29.11
ꢂ2.90
ꢂ35.21
34.42
62.23
88.81
40.8
xxx
xxy
xyy
yyy
xxz
xyz
yyz
xzz
yzz
zzz
z
ꢂ0.14
ꢂ0.01
ꢂ0.22
0.26
0.19
ꢂ4.61
ꢂ22.19
ꢂ0.84
2.64
31.32
ꢂ5.22
ꢂ21.15
21.53
52.01
72.53
48.0
0.07
ꢂ0.25
ꢂ3.27
ꢂ15.22
0.78
ꢂ4.42
ꢂ8.42
11.82
ꢂ21.44
70.79
63.14
81.31
15.9
0.25
0.51
0.09
ꢂ0.63
ꢂ7.57
ꢂ1.41
1.62
18.24
ꢂ2.80
ꢂ34.46
51.12
67.95
85.02
30.9
ꢂ2.06
ꢂ21.54
ꢂ1.20
2.81
29.86
ꢂ3.14
ꢂ35.61
35.25
63.92
90.73
41.0
ꢂ0.16
ꢂ3.58
ꢂ1.55
0.90
13.04
ꢂ1.99
ꢂ32.49
59.76
71.25
85.06
26.0
1
/ LUMO transition exhibits a considerable increment compared
with the calculation in vacuum (i.e. 0.02 / 0.35 for NP-R and
0.02 / 0.25 for NP-L). Therefore, for NP chromophores having
complex ICT bands [56e58], the contribution of the HOMO-
max
max
q(m,b )
1
/ LUMO transition with considerably lower lmax values

O.-P. Kwon et al. / Dyes and Pigments 85 (2010) 162e170
167
Fig. 5. KohneSham molecular orbitals (isovalue ¼ 0.04 a.u.) involved in the ICT transition obtained at the B3LYP/6e311 þ G(d) level for the investigated chromophores.
compared to the main transition may explain the observed blue
The components of the first hyperpolarizability tensor bijk given
shift of the experimental
the main transition.
lmax compared to the calculated of
lmax of
in Table 2 were calculated in the Cartesian coordinate system (x,y,z),
in which the dipole moment points along the z direction; large off-
The results of the finite-field (FF) calculations for the dipole
moment , the static hyperpolarizability tensor ijk, and the vector
component zzz along the dipole moment
direction are listed in Table 2. From these data we calculated the
maximal component of the first hyperpolarizability max as well
as the angle max) between the dipole moment and the
direction of
directions
diagonal components
angle max), also resulting in a large deviation of the
component from the maximal component max. To additionally
determine to which extent the charge-transfer transition in our
molecules is multipolar, we transformed the calculated ijk tensor
into a more appropriate Cartesian system (x ,x ,x ) that we here
refer to as the hyperpolarizability coordinate system. We chose the
3
x axis along the main direction of the charge-transfer transition
bijk may be only due to the mentioned large
m
b
q(
m,b
b
zzz
b
z
¼
b
xxz
þ
b
yyz
þ
b
b
b
b
q
(
m,
b
m
g
1 2 3
b
max, which are also listed in Table 2. In Fig. 6, the
max of the maximal first hyperpolarizability and
b
the dipole moment
m
of the investigated molecules as determined
b
max and the x
1
axis so that the off-diagonal component j
b
113j is
by finite-field calculations are presented by the solid and dotted
vectors, respectively. Due to the asymmetry, all rotamers exhibit
a different amplitude and also a different direction of the dipole
maximal (and j
is
components, out of which
b
223j minimal). In this system, the main component
b
333
¼
bmax and other components are referred to as minor
b
ij3 h 0 if i s j; all other minor
moment
transfer transition
maximum absorption
tions, in FF calculation the HP and MP chromophores exhibit
a higher maximal hyperpolarizability component max than the
m
g
with respect to the main direction of the charge-
max. As expected from the wavelength of
max measurements and TD-DFT calcula-
components are close to zero only if the charge-transfer transition
is mainly one-dimensional. The results are given in Table 3.
HP and MP chromophores exhibit only marginal minor hyper-
polarizability components due to the essentially one-directional
charge-transfer transition. For HP and MP chromophores we also
see practically no difference between R- and L-isomers in terms of
b
l
b
NP chromophores. All pyrrole-based chromophores exhibit large
ꢀ
angles
q(m,
b
max) up to 48 , which is larger than for previously
the bijk tensor; these isomers only differ by the value and the
investigated CLP chromophores having N-dialkylaminophenyl
group [28], confirming the strongly asymmetric character of the
designed chromophores.
orientation of the dipole moment with respect to the hyper-
polarizability tensor, as also illustrated in Fig. 6. Strong molecular
asymmetry can therefore lead to very large values of q(m,bmax), in

168
O.-P. Kwon et al. / Dyes and Pigments 85 (2010) 162e170
Fig. 6. Optimized (OPT) molecular structures obtained by the DFT calculations projected along the yz molecular plane. The dashed vectors present the directions of the ground-state
P
dipole moments
m
g
jjz. The shaded areas present the calculated hyperpolarizability tensor in the yz plane according to j
b
j ¼
b
ijkxixjxk with
b
ijk from Table 2 and a different color
i;j;k
for positive and negative values of j
b
j. The solid vectors present the directions of the maximum first hyperpolarizabilities
b
maxjjx
3
. Note that the direction of max also makes a small
b
ꢀ
angle to the yz plane of this figure, the largest angle being 9.1 for the NP-L chromophore.
ꢀ
our case of up to 41 , even though the charge-transfer transition
is also the reason for the substantial difference between the
tensor elements for R- and L-isomers, in contrast to HP and MP
chromophores. Relatively large minor diagonal components of
tensor are obtained in the hyperpolarizability coordinate system for
NP chromophores, with 111 reaching 30% of the maximal value
max. This presents an interesting possibility for materials
engineering with a more isotropic nonlinear optical response. In
macroscopic systems, substantial minor diagonal values of
b
ijk
occurs along only one direction.
For NP chromophores with a bulky N-substituent, the results of
the finite-field calculations indicate a strong deviation from simple
one-dimensional charge-transfer typical for conventional dipolar
chromophores, as expected from the molecular orbital calculations
b
ijk
b
b
333
¼
b
(
see Fig. 5). Multiple charge-transfer transition in NP chromophores
b
ijk
tensor in the macroscopic dielectric eigensystem can be for dipolar
molecules only achieved for very specific and rare crystalline
packing, in which one molecule orients perpendicularly to the
other and then such molecular pair packs non-centrosymmetrically
with point group symmetry 1 or m [58]. For poled polymers, all
minor diagonal components are zero for symmetry reasons.
Molecules with a strongly asymmetric and multi-directional
charge-transfer transition as NP are expected to result in such
macroscopic systems with a much greater probability.
Table 3
ꢂ30
The zero-frequency hyperpolarizability tensor
hyperpolarizability system (x
other non-zero components reflect the deviation from the strictly one-dimensional
charge-transfer transition.
b
ijk (ꢁ10
esu) in the Cartesian
ij3 h 0 if i s j. All
1
,x
,x
2 3
), in which
b
333
¼
b
max and b
HP-R
HP-L
0.52
0.03
0.43
ꢂ0.20
ꢂ3.64
0
MP-R
MP-L
NP-R
NP-L
b
b
b
b
b
b
b
b
b
b
111
112
122
222
113
123
223
133
233
333
0.54
0.03
0.55
ꢂ0.20
ꢂ3.71
0
0.53
0.05
0.63
ꢂ0.21
ꢂ3.74
0
0.68
0.03
0.45
ꢂ0.19
ꢂ3.98
0
ꢂ7.46
ꢂ4.63
ꢂ1.11
ꢂ0.24
ꢂ3.64
0
22.34
ꢂ1.16
ꢂ0.49
0.27
ꢂ9.76
0
4. Conclusion
ꢂ1.47
ꢂ1.55
ꢂ1.48
ꢂ1.61
ꢂ1.36
ꢂ1.22
0
0
0
0
0
0
0
0
0
0
We have investigated new asymmetric-shaped N-substituted
0
0
pyrrolic nonlinear optical chromophores in order to achieve a large
angle max) between the dipole moment and the main
85.02
90.73
85.06
88.81
72.53
81.31
q(m,b
m

O.-P. Kwon et al. / Dyes and Pigments 85 (2010) 162e170
169
direction of the charge-transfer transition
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