Multichromophoric calix[4]arenes: Effect of interchromophore distances on linear and nonlinear optical properties

Article abstract of DOI:10.1002/cphc.201200203

Multichromophoric calix[4]arenes with two or four disperse red one (DR1) moieties linked to the lower rim have been synthesized. The second-order nonlinear optical activity was measured by using the electric-field-induced second-harmonic generation technique and there was a nearly linear increase of the μβ value with the number of chromophores in the molecule without affecting the charge-transfer absorption wavelength. The effect that the number of DR1 units plays on the hyperpolarizability, the dipole moment, and the absorption maxima has been also studied by using quantum chemical calculations. It was found that it was necessary to synthesize multichromophores with distant chromophores to obtain large nonlinear optical responses. Copyright

Full text of DOI:10.1002/cphc.201200203

DOI: 10.1002/cphc.201200203
Multichromophoric Calix[4]arenes: Effect of
Interchromophore Distances on Linear and Nonlinear
Optical Properties
Raquel Andreu,[a] Santiago Franco,[a] Javier Garꢀn,[a] Judith Romero,[a] Belꢁn Villacampa,[b]
Marꢀa Jesffls Blesa,*[a] and Jesffls Orduna*[a]
Multichromophoric calix[4]arenes with two or four disperse red
one (DR1) moieties linked to the lower rim have been synthe-
sized. The second-order nonlinear optical activity was mea-
sured by using the electric-field-induced second-harmonic gen-
eration technique and there was a nearly linear increase of the
mb value with the number of chromophores in the molecule
without affecting the charge-transfer absorption wavelength.
The effect that the number of DR1 units plays on the hyperpo-
larizability, the dipole moment, and the absorption maxima
has been also studied by using quantum chemical calculations.
It was found that it was necessary to synthesize multichromo-
phores with distant chromophores to obtain large nonlinear
optical responses.
1. Introduction
Organic molecules with second-order nonlinear optical (NLO)
properties have generated great interest in the development
of organic materials.[1] In general, such chromophores have an
extended conjugated system, a low-energy transition with
a high extinction coefficient, and a large dipole moment
change between their ground and excited states.[2] It is well
known that the extension of the conjugated system increases
the hyperpolarizability, b, but unfortunately this is usually ac-
companied by a shift of the charge-transfer (CT) absorption
band to a longer wavelength,[3] thereby restricting the use of
these materials for certain applications, such as in frequency
doubling.[4] This undesirable effect constitutes the transpar-
ence–efficiency trade-off.[5]
benzenic rings have been used as spacers in multichromophor-
ic donor–p–acceptor assemblies; thus leading to low interchro-
mophore distances that allow through-space interactions.[9,12,13]
Since it seems desirable to keep NLO-active units as isolated
as possible to reach higher hyperpolarizabilities, we herein pro-
pose a different approach to multichromophore calix[4]arene
derivatives in which the calixarene ring is not part of the chro-
mophoric system. By using calix[4]arene as a template, multi-
chromophoric systems are constructed by functionalization of
the lower rim, giving rise to assemblies with more distant chro-
mophores with the aim of avoiding (or at least minimizing) in-
terchromophoric interactions.
Multichromophore assemblies have been proposed[6] as
a convenient way to increase the figure of merit, mb, overcom-
ing the transparence–efficiency trade-off and this approach
has led to a number of multichromophoric systems with en-
hanced NLO properties, such as V-shaped chromophores[7] or
spirofluorene compounds.[8] Calix[4]arene was soon recognized
as a suitable template to synthesize multichromophore assem-
blies with a controlled orientation of dipolar units. The NLO
properties of the four conformers of tetranitrotetrapropoxyca-
lix[4]arene were studied[9] and it was observed that the in-
crease in molecular hyperpolarizability was sublinear with the
number of chromophores, probably due to interchromophore
interactions. Similarly, a joint experimental–theoretical study
on tetrachromophores based on a cyclotetrasiloxane architec-
ture[10] concluded that most of the increased mb value, relative
to monochromophores, was due to an increased dipole
moment and the hyperpolarizability remained almost un-
changed as a consequence of nonbonding electronic interac-
tions between NLO chromophores.
2. Results and Discussion
2.1. Synthesis
Calix[4]arene is a versatile scaffold that can be readily function-
alized.[14] Acids 1 and 3 were prepared by procedures de-
scribed in the literature, involving the reaction of calix[4]arene
with ethylbromoacetate under basic conditions[15] and subse-
quent hydrolysis of the resulting esters.[16]
[a] Dr. R. Andreu, Dr. S. Franco, Prof. J. Garꢀn, J. Romero, Dr. M. J. Blesa,
Dr. J. Orduna
Departamento de Quꢀmica Orgꢁnica, ICMA
Universidad de Zaragoza-CSIC
50009, Zaragoza (Spain)
Fax: (+34)976761194
E-mail: jorduna@unizar.es
mjblesa@unizar.es
[b] Dr. B. Villacampa
Departamento de Fꢀsica de la Materia Condensada, ICMA
Universidad de Zaragoza-CSIC, 50009, Zaragoza (Spain)
The calix[4]arene structure has been used to preorganize
chromophores in one single molecule,[11] but, up to now, its
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cphc.201200203.
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M. J. Blesa, J. Orduna et al.
Following the experimental
procedure described by Hud-
homme et al.,[17] esterification of
acetic acid, 1,[18] and 3[19] with
disperse red one (DR1) in the
presence of dicyclohexylcarbodii-
mide (DCC), 4-(dimethylamino)-
pyridine (DMAP), and 1-hydroxy-
benzotriazole (HOBT) afforded
the corresponding Acetyl-DR1,
diester 2, and tetraester 4 in 58–
80% yield (Scheme 1). It is worth
noting that, although Acetyl-DR1
has been previously prepared,[20]
it was not completely character-
ized.
2.2. NMR Spectra
The calix[4]arene assemblies 1, 2,
3, and 4 were characterized by
NMR spectroscopy. The diaste-
reotopic protons ArꢀCHexoHendo
ꢀ
Ar of the aromatic cavity of
these calix[4]arenes were stud-
ied. It is known that Dd between
the Hexo and Hendo in calix[4]ar-
enes serves as a measure of the
flattening: Dd is generally
0.9 ppm for a system in the cone
conformation, whereas in the
flattened conformation Dd is sig-
nificantly decreased.[21] Thus, the
calix[4]arene diacid 1 has Dd=
1.23[18a] and this value decreases
to Dd=1.06 for calix[4]arene
diester 2, which shows that the
phenol units are more flattened
because of steric crowding be-
Scheme 1. Synthesis of Acetyl-DR1 and calixarenes 2 and 4.
tween the DR1 groups on the narrow, lower rim. An analogous
variation in the Dd values has been observed between tetra-
substituted calix[4]arenes 3 and 4 (with Dd values of 1.66 and
1.55, respectively).
2.3. Electronic Absorption Spectra
The linear optical properties were studied by means of UV/Vis
spectroscopy in dichloromethane and DMSO to follow the sol-
vatochromic effect. Figure 1 shows the UV/Vis spectra of DR1
and calix[4]arenes 2 and 4. The experimental parameters for
the lowest energy absorptions are gathered in Table 1 along
with the results of theoretical calculations. It can be seen that,
in every case, there is a redshift of these absorptions on pass-
ing from CH2Cl2 to DMSO. This positive solvatochromic shift
serves as an indicator of the increased dipole moment upon
excitation;[22] thus confirming the CT character of these transi-
tions[23] and indicating positive hyperpolarizability. The most
Figure 1. UV/Vis spectra of Acetyl-DR1, DR1, calixarene 2, and calixarene 4 in
CH2Cl2.
noticeable feature of the UV/Vis spectra of these compounds is
the blueshift of the CT band observed when linking the DR1
moiety to the calix[4]arene scaffold.
Considering that chromophores in molecules 2 and 4 are
isolated, in other words, there is no electronic interaction
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Multichromophoric Calix[4]arenes
Table 1. Experimental and theoretically calculated UV/Vis absorption data.
Experimental
Theoretical
Major
contributions[d]
CH2Cl2
E01
DMSO
B3LYP/6-31G*
[a]
[b]
[a]
[b]
[a]
[b]
[c]
lmax
loge
lmax
E01
loge
Symmetry
lmax
E01
Dm01
F
DR1
Acetyl-DR1
2
480
476
469
2.59
2.61
2.40
4.50
4.26
4.71
508
498
493
2.44
2.49
2.52
4.52
4.13
4.71
–
–
B
445
441
438
2.78
2.81
2.83
11.85
11.94
9.39
0.95
0.98
0.84
H!L
H!L
H-3!L+1
H-2!L
A
A
B
B
435
442
441
440
2.85
2.81
2.81
2.82
9.45
1.26
0.16
0.79
0.54
H-3!L
H-2!L+1
H!L+2
H-1!L+3
H!L+3
H-1!L+2
H-2!L+3
4
470
2.64
5.03
491
2.53
4.99
16.04
18.89
31.53
[a] In nm. [b] In eV. [c] In Debye. [d] H=HOMO, L=LUMO.
among chromophores within the same molecule, these multi-
chromophore systems can be studied in the framework of the
exciton model.[24,25] If we consider molecule 2, for composite
double molecules with the two chromophores in an oblique
arrangement, the exciton model predicts splitting of the
lowest energy absorption into two perpendicularly polarized
absorptions with that polarized perpendicularly to the symme-
try axis (and hence, to the total dipole moment) shifted to
lower energy and that polarized parallel to it shifted to higher
energy. The absence of an experimentally observable energy
splitting may be attributed to the large separation between
the two chromophores in 2, but the hypsochromic shift of 2
with respect to DR1 remains unexplained by this simple ap-
proach. To get a better understanding of the optical behavior
of these chromophores, we performed DFT calculations using
the hybrid B3LYP functional. The optimized geometry of 2
(Figure 2) reveals that the two amino nitrogen atoms are sepa-
The gas-phase excitation energies calculated by using the
time-dependent (TD) DFT approach are approximately 0.2 and
0.3 eV higher than those measured in CH2Cl2 and DMSO, re-
spectively, and are therefore in reasonable agreement with ex-
perimental results, taking into account the positive solvato-
chromism shown by these molecules. Furthermore, theoretical
calculations reproduce the hypsochromic shift of 2 and 4 rela-
tive to that of DR1. According to these calculations, molecule
2 follows the exciton model and presents two orthogonally
polarized excited states. However, the calculated energy sepa-
ration for these excited states is only 0.02 eV; thus explaining
the lack of an experimentally observable band splitting. The
cause of the higher excitation energy of 2 relative to DR1 can
be found in the increased energy gap from the occupied to
the unoccupied MOs involved in these electronic transitions.
Acylation of the OH group in DR1 causes the stabilization of
both occupied and unoccupied MOs, but this stabilization is
higher for occupied MOs, since they are closer to the acyl
group, and therefore, the energy gap increases from 2.89 to
2.98 eV and the excitation energy from 2.78 to 2.83 eV on pass-
ing from DR1 to 2. For the sake of comparison, we synthesized
Acetyl-DR1, which seemed to be a more reliable monochromo-
phoric model for 2 and 4, and its measured absorption band
was hypsochromically shifted with respect to DR1. Also, the
calculated HOMO–LUMO gap (2.94 eV) and excitation energy
(2.81 eV) for Acetyl-DR1 are similar to the values calculated for
2.
The optimized B3LYP/6-31G* geometry of 4 (Figure 2) shows
that, to accommodate the four DR1 units, this molecule adopts
a pinched-cone conformation with interchromophore distances
of about 5 Š between cone-cone and pinched-cone DR1 sub-
stituents. While we can expect some through-space electronic
interactions between cone chromophores, giving rise to
a modified UV/Vis spectrum, it has been described that energy
changes in the lowest CT transitions are only observed at very
low interchromophore distances (less than 3.3 Š).[26] TD-DFT
calculations on 4 predict a HOMO–LUMO gap of 2.95 eV and
a lowest excitation energy of 2.81 eV; this parallels the results
obtained for Acetyl-DR1 and 2.
Figure 2. B3LYP/6-31G*-optimized geometries of 2 (left) and 4 (right).
rated by 10.2 Š and the center-to-center distance between
chromophores is more than 16 Š. These distances are large
enough to allow any through-space electronic interactions to
be neglected, since it has been reported[26] that above 7 Š
chromophores behave as isolated moieties and molecular orbi-
tals (MOs) of the dimeric chromophores are sums and differen-
ces of nearly unperturbed monomer MOs.
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2.4. NLO Properties
dipole moment and the vector component of the hyperpolariz-
ability (bvec).
The hyperpolarizabilities of multichromophores 2 and 4 and
the reference compound DR1 were determined by electric-
field-induced second harmonic (EFISH) with a fundamental
wavelength of 1907 nm. Table 2 gives the mb values obtained
by EFISH measurements. The solvent used in these measure-
ments was dichloromethane. It is observed that the mb value is
almost doubled when two DR1 units are linked to calix[4]arene
1 and almost quadrupled when four DR1 units are linked to
calix[4]arene 3.
For a V-shaped molecule displaying C2 symmetry, bvec can be
calculated from Equation (3):
bvec ¼ bz ¼ bzzz þ bzyy¼ 2b1ðcos3q þ cosqsin2qÞ
ð3Þ
¼ 2b1cosq
The angles were defined by the symmetry axis and the
ground-state dipole moment of each subchromophore based
on the calculated dipole moment of Acetyl-DR1. The angle (q)
formed between each chromophore and the z axis in the
B3LYP/6-31G*-optimized geometry of 2 is 36.38, and therefore,
this simple additive model predicts mz =2m1cos(36.3)=1.61m1
and bvec =2b1cos(36.3)=1.61b1, giving rise to a measurable
mzbvec =2.60m1b1.
Table 2. NLO properties of DR1 and compounds 2 and 4.
Compound
Experimental[a]
mb(0)[b]
CPHF/6-31G*[c]
mb[b]
720(70)
m(D)
bvec(0)[d]
mb(0)[b]
By comparing the mb(0) values of DR1 and 2 obtained both
experimentally and theoretically, it can be seen that there is
almost a twofold increase, which is not far from the estimated
value obtained with the simple addition model. Furthermore,
a comparison with the calculated dipole moment and hyper-
polarizability of Acetyl-DR1 (m0 =8.93 D, bvec(0)=71”10ꢀ30 esu,
mb(0)=590”10ꢀ48 esu) shows a much better fit to this model,
indicating that Acetyl-DR1 is a better model for the chromo-
phore units in 2 and that interchromophore interactions have
a negligible influence on the molecular hyperpolarizability.
DR1
2
4
490(50)
11.19
12.73
19.15
73
105
112
795
1335
2141
1270(130)
2700(300)
900(90)
1900(200)
[a] In CH2Cl2 at 1907 nm. [b] 10ꢀ48 esu. [c] Calculated for the B3LYP/6-
31G*-optimized geometry. [d] 10ꢀ30 esu.
To get further insight into the NLO properties of these com-
pounds, we have also calculated the molecular hyperpolariza-
bilities of these compounds using the coupled perturbed Har-
tree–Fock (CPHF) method, which yields the hyperpolarizability
as the analytical second derivative of the dipole moment with
respect to an electrical field. The CPHF method calculated on
B3LYP/6-31G* geometries gave rise to the mb(0) values shown
in Table 2 and, while they were somewhat overestimated, they
reproduced the experimental trend reasonably well, bearing in
mind that conformational distribution and solvent effects were
not considered in the calculations.
The B3LYP/6-31G*-optimized geometry of
4
showed
a pinched-cone conformation with the two pinched chromo-
phore groups forming an angle of q=54.18 with the symmetry
axis, while the cone ones form an angle of f=19.08. By adding
the contribution of each of the Acetyl-DR1 moieties in 4, the
resulting dipole moment and bvec can be determined from
Equations (4) and (5):
To rationalize the NLO behavior of 2, this molecule must be
considered as a V-shaped (or L-shaped) chromophore.[27] As
discussed above, compound 2 presents two nearly degenerate
excited states polarized orthogonally and these must contrib-
ute to the two major components of the hyperpolarizability
tensor bzzz and bzyy, respectively. Assuming that there is no elec-
tronic interaction between chromophores, the total dipole
moment and the hyperpolarizability[28] of 2 can be calculated
by adding the contribution of each of the chromophores in
the molecule [Eqs. (1) and (2)]:
m¼ 2m1cosq þ 2m1cosꢀ
¼ 2m1ðcosq þ cosꢀÞ ¼ 3:06m1
ð4Þ
bvec¼ 2b1cosq þ 2b1cosꢀ
ð5Þ
¼ 2b1ðcosq þ cosꢀÞ ¼ 3:06b1
Thus, by simply adding the contribution of the four chromo-
phores in 4, the measured mbvec should be around 9.36
(=3.062) times the m1b1 value of a single chromophore.
When comparing the experimental measurements, it can be
seen that there is only a fourfold increase in the mb(0) value
when going from DR1 to 4; this is well below the estimated in-
crease obtained by adding the contribution of the four chro-
mophores in the molecule.
mz ¼ 2m1cosq
ð1Þ
ð2Þ
bzzz ¼ 2b1cos3q
bzyy ¼ 2b1cosqsin2q
It should be noted that to fit the “oriented gas” model to ex-
perimental results, a large angle between the symmetry axis
and subchromophores must be assumed to explain the four-
fold increase in mb(0) measured by EFISH. It is well known that
EFISH conditions with a large electric field favor conformations
that have a larger dipole moment,[29] and therefore, such large
angles between chromophores and the symmetry axis are not
expected to contribute to the experimental results.
in which m1 and b1 are the dipole moment and the hyperpolar-
izability of a single chromophore, respectively, and q is the
angle formed between the chromophore (defined herein by its
ground-state dipole moment) and the symmetry conforma-
tions with axis (z). The electric-field-induced second harmonic
generation (EFISHG) technique samples the product of the
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Multichromophoric Calix[4]arenes
d values are given in ppm [relative to tetramethylsilane (TMS)] and
J values are in Hz. High-resolution electrospray (ESI) mass spectra
were acquired on a Q-TOF instrument by using sodium formate
clusters as a reference for accurate mass measurements. MALDI-
TOF mass spectra were recorded by using dithranol as matrix and
the sodium adducts of polyethylene glycol (PEG) as an internal ref-
erence for accurate mass measurements.
Results from CPHF calculations parallel the experimental
trend, predicting that the NLO response of 4 is less than three
times that of DR1 and less than four times that of Acetyl-DR1.
As mentioned in the previous section, the interchromophore
distance between DR1 substituents in calixarene 4 is 5 Š. Ac-
cording to Di Bella et al.,[26] below 7 Š, hyperpolarizability de-
creases with respect to the oriented gas estimation. Further-
more, Terenziani et al. demonstrated that, for oblique dimers at
interchromophore distances between 5 and 9 Š, the hyperpo-
larizability never attained values expected in the oriented gas
model, irrespective of the angle between chromophores.[12d]
Analogous behavior has also been observed in tetrachromo-
phoric calix[4]arenes.[12c]
NLO Measurements
EFISHG measurements were performed by using the fundamental
radiation at 1.9 mm obtained from an H2 Raman cell excited by
a Q-switched Nd:YAG laser (1064 nm). The repetition rate was
10 Hz and the pulse width was 8 ns. A computer-controlled NLO
spectrometer completed the experimental setup. The 1.9 mm polar-
ized light was split into two beams. The less intense one was di-
rected to a N-(4-nitrophenyl)-l-prolinol (NPP) powder sample, the
SH signal of which was used as a reference to correct laser fluctua-
tions. The other beam was passed through a linear polarizer and
focused into the wedge-shaped EFISHG liquid cell. The SH light
from both the cell and the reference were measured with two pho-
tomultipliers. Interference filters were used to remove the residual
excitation light beyond the sample and the NPP reference. Static
mb(0) values were deduced from the experimental values by using
a two-level dispersion model. These experimental measurements
were performed in CH2Cl2. Several solutions of each compound,
with concentrations in the range 10ꢀ3-–10ꢀ4 m, were used in these
measurements and gave rise to the same mb values (within experi-
mental uncertainties) and indicated that there were no noticeable
aggregation effects.
When comparing the results obtained for 2 and 4, increasing
the number of chromophores in the molecules causes a large
increase in the dipole moment, but has little influence on the
hyperpolarizability, and therefore, increased mb(0) is mainly
a consequence of the larger dipole moment. Note that, while
the interchromophore distances (Figure 2) in 4 are not short
enough to modify the energy of the lowest electronic transi-
tions, through space interactions give rise to lower transition
dipoles and oscillator strengths with respect to 2; this causes
a decreased contribution of these transitions to the hyperpo-
larizability.
3. Conclusions
Lower-rim functionalization of calix[4]arene constitutes a new
approach to the synthesis of multichromophoric, NLO-active
molecules. The NLO activity of these new chromophores in-
creases with the number of NLO-active units in the molecule
without affecting the energy of the lowest absorption; thus
overcoming the transparency–NLO efficiency trade-off. The un-
desirable nonbonding interchromophore interactions that
cause a lowering of the molecular hyperpolarizability are
avoided in the disubstituted derivative 2 by keeping large in-
terchromophore distances, and therefore, the molecular hyper-
polarizability results from the addition of the hyperpolarizabili-
ties of single chromophores. However, due to its flexibility,
tetrasubstituted 4 adopts a pinched-cone conformation, the
cone chromophores of which are close enough to allow these
interactions, and its hyperpolarizability remains almost un-
changed compared to 2. These results reinforce our initial ap-
proach of synthesizing multichromophores with distant chro-
mophores to obtain large NLO responses. To minimize inter-
chromophore interactions in tetrasubstituted calix[4]arenes, we
propose the use of larger and more rigid spacers in the cal-
ix[4]arene scaffold and the chromophores linked to it to avoid
conformations that could bring the chromophores together.
Computational methods
All theoretical calculations were performed by using the Gaussi-
an 09[30] program. Geometry optimizations started from structures
with the alkyl groups in an all-staged anti arrangement and the C2
symmetry of 2 and 4 was the only imposed restriction. The hybrid
B3LYP[31] functional and the 6-31G(d)[32] basis set were used in both
geometry optimizations and TD-DFT calculations and the excited-
state dipole moments were calculated by using the CI density. Mo-
lecular hyperpolarizabilities at zero frequency were calculated by
the CPHF method using HF/6-31G(d) model chemistry and the de-
fault parameters provided by the “polar” keyword on the B3LYP/6-
31G*-optimized geometry. The default Gaussian 09 parameters
were used in each case.
Acknowledgements
Financial support from MEC-FEDER (CTQ2008-02942, CTQ2011-
22727, MAT2008-06522-C02-02) and Gobierno de Aragꢂn-Fondo
Social Europeo (E39 and E04) is gratefully acknowledged. We
thank Jesffls Clemente-Gallardo and the anonymous referees for
helpful discussions.
Keywords: calixarenes · chromophores · nonlinear optics ·
quantum chemistry · transparency–efficiency tradeoff
Experimental Section
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Received: March 8, 2012
Revised: May 11, 2012
Published online on && &&, 2012
&6
&
www.chemphyschem.org
ꢂ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemPhysChem 0000, 00, 1 – 7
ÝÝ
These are not the final page numbers!

ARTICLES
NLO response and effect: Multichro-
mophoric calix[4]arenes with two or
four disperse red one moieties linked to
the lower rim have been synthesized.
The second-order nonlinear optical ac-
tivity was measured and there was
a nearly linear increase of the mb value
with the number of chromophores in
the molecule without affecting the
charge-transfer absorption wavelength
(see picture).
R. Andreu, S. Franco, J. Garꢀn, J. Romero,
B. Villacampa, M. J. Blesa,* J. Orduna*
&& – &&
Multichromophoric Calix[4]arenes:
Effect of Interchromophore Distances
on Linear and Nonlinear Optical
Properties
ChemPhysChem 0000, 00, 1 – 7
ꢂ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemphyschem.org
&
7
&
ÞÞ
These are not the final page numbers!