First hyperpolarizabilities of hexa(ethynyl)benzene derivatives: Effect of conjugation length

Article abstract of DOI:10.1039/b601982j

A variety of dipolar and octupolar molecules containing CC bonds as the conjugation bridge have been synthesized and the linear and nonlinear optical properties (β) were studied. The β(0) value of the dipole increases with conjugation length, whereas that

Full text of DOI:10.1039/b601982j

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www.rsc.org/materials | Journal of Materials Chemistry
First hyperpolarizabilities of hexa(ethynyl)benzene derivatives: effect of
conjugation length
Ming Jun Piao,a Khalil Chajara,a Soo Jung Yoon,a Hwan Myung Kim,a Seung-Joon Jeon,*a Tae-Hyun Kim,b
Kai Song,c Inge Asselberghs,c Andre´ Persoons,*c Koen Clays*c and Bong Rae Cho*a
Received 2nd February 2006, Accepted 17th March 2006
First published as an Advance Article on the web 21st April 2006
DOI: 10.1039/b601982j
A variety of dipolar and octupolar molecules containing CMC bonds as the conjugation bridge
have been synthesized and the linear and nonlinear optical properties (b) were studied. The b(0)
value of the dipole increases with conjugation length, whereas that of the octupole decreases from
2a to 2b with concomitant decrease in lmax. A further increase in the conjugation length to 3a–c
increased b(0) because of the larger oscillator strength. Moreover, 3a with the most distorted
structure shows the largest b(0), probably because of the dipolar contribution. On the other
hand, byyy/bzzz ratios are smaller than unity, probably because bzzz is overestimated by the errors
involved in the resonance correction.
noncentrosymmetric crystals that showed one of the largest
1. Introduction
second harmonic generations (SHGs) known in the litera-
Much effort is being focused to develop octupolar nonlinear
ture.4b Very recently, we reported that 1,3,5-tricyano-2,4,6-
optical (NLO) materials for possible applications in electro-
tris(p-diethylaminophenylethynyl)benzene, which is the triple
optic devices.1–5 The advantages of octupoles in comparison
bond analogue of 1,3,5-tricyano-2,4,6-tris(p-diethylamino-
to the more conventional dipolar molecules include easier
styryl)benzene, shows
a similar first hyperpolarizability
noncentrosymmetric arrangements and the polarization-
independent second harmonic response.1d,4 Also, there is a
design strategy for the synthesis of octupoles with large first
hyperpolarizabilities.5 According to the VB-3CT model, the b
value of two-dimensional octupoles should increase with the
extent of charge transfer. Hence, the b values of such molecules
can be enhanced by simultaneously increasing the donor–
acceptor strength, conjugation length, and by using a more
efficient conjugation bridge.
to that of the latter. Also, 1,3,5-tris(p-diethylaminophenyl-
ethynyl)-2,4,6-tris(p-nitrophenylethynyl)benzene has been
shown to exhibit a large first hyperpolarizability.2g These
results seem to indicate that the poor conjugation ability of the
CMC bond is largely compensated by the more planar struc-
ture. However, there is no report on the b of the octupoles with
a more extended conjugation length.
Here, we have studied a series of dipolar and octupolar
molecules containing donor–acceptor groups and triple bonds
as the conjugation bridge. We were interested in learning the
effects of extending the conjugation length from 2a to 2b, to
3a–c (Scheme 1). Compounds 1a and 1b are dipolar references
to the octupolar analogues 2a and 2b. Compounds 3b and 3c
are essentially the same except for the para substituent in the
peripheral phenyl groups, whereas 3a may be distorted due to
the steric hindrance. Hence, the effects of planarity on the
octupoles could be assessed by comparing 3a with 3b,c. The
first hyperpolarizabilities were measured by the frequency-
resolved hyper-Rayleigh scattering (HRS) experiments.6 By
this method, the b values of 1–3 can be measured accurately
without the possibility of overestimation due to multi-photon
fluorescence (MPF).3jThe results of these studies are now
reported.
Earlier, we reported that 1,3,5-trinitro-2,4,6-tris(stryryl)ben-
zene derivatives show significant first hyperpolarizabilities that
increased with the conjugation length.3a However, the b values
were not much larger than the corresponding dipoles because
of the distorted structures that hampered the intramolecular
charge transfer (ICT). When sterically less hindered acceptor
groups were employed, the structure became more planar and
the b of the octupole showed significant enhancement in
comparison to the dipole, i.e. byyy of 1,3,5-tricyano-2,4,6-
tris(p-diethylaminostyryl)benzene was larger by a factor of 3.5
than bzzz of 4-cyano-49-diethylaminostilbene.3a,f The large
enhancement has been attributed to the effect of the sharing
of the central phenyl group by the three dipolar units that
induced efficient delocalization of the molecular orbital
and enhanced ICT. Furthermore, this compound produced
aMolecular Opto-Electronics Laboratory, Department of Chemistry and
Center for Electro- and Photo-Responsive Molecules, Korea University,
Seoul 136-701, Korea. E-mail: chobr@korea.ac.kr
2. Experimental
Synthesis
bDepartment of Chemistry, University of Incheon, Dowha-Dong,
Nam-Gu, Incheon 402-749, Korea
Synthesis of 1a, 1b, 2b, and 3a–c is described below. Except for
3b and 3c, whose solubility was too low to obtain 13C NMR
spectra, all new compounds were thoroughly characterized by
cCenter for Research on Molecular and Photonics, Laboratory of
Chemical and Biological Dynamics, University of Leuven,
Celestijnenlaan 200D, B-3001, Leuven, Belgium.
1
E-mail: andre.persoons@fys.kuleuven.ac.be
IR, H and 13C NMR, and elemental analysis.
This journal is ß The Royal Society of Chemistry 2006
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Scheme 1 Structures of 1–3.
4-[4-(Diphenylamino)phenylethynyl]iodobenzene (A1). Et3N
1,4-Bis(2-ethylhexyloxy)benzene (B1). Synthesized from
p-hydroquinone and 2-ethylhexyl bromide by known
(10 mL) was added to a stirred solution of Pd2Cl2(PPh3)2
(0.065 g, 0.093 mmol), CuI (0.020 g, 0.11 mmol), 4-(diphenyl-
amino)phenylacetylene7 (1.0 g, 3.7 mmol), and 1,4-diiodoben-
zene (2.5 g, 7.6 mmol) in anhydrous THF (20 mL) at RT under
nitrogen. The mixture was stirred for 12 h. After evaporating
off the THF and a standard work-up procedure, the crude
product was purified on a silica gel column using CH2Cl2–
hexane (1 : 17) as eluent. Yield: 1.1 g (63%); mp 166 uC; IR
a
method.9 Yield: 74%; H NMR (300 MHz, CDCl3): d = 6.82
(s, 4H), 3.78 (d, J = 5.7 Hz, 4H), 1.69 (m, 2H), 1.42 (m, 8H),
1.33 (m, 8H), 0.92 (t, J = 7.5 Hz, 6H), 0.90 ppm (t, J = 6.6 Hz,
6H); 13C NMR (75 MHz, CDCl3): d = 153.63, 115.56, 71.40,
39.67, 30.74, 29.31, 24.06, 23.30, 14.34, 11.34 ppm; Elemental
analysis: calcd (%) for C22H38O2: C 78.99, H 11.45; found: C
78.94, H 11.48.
1
1
(KBr, cm21): 2204; H NMR (300 MHz, CDCl3): d = 7.65 (d,
J = 8.3 Hz, 2H), 7.35 (d, J = 8.7 Hz, 2H), 7.27 (dd, J = 7.8,
7.2 Hz, 4H), 7.21 (d, J = 8.3 Hz, 2H), 7.11 (d, J = 7.8 Hz, 4H),
7.06 (t, J = 7.2 Hz, 2H), 6.99 ppm (d, J = 8.7 Hz, 2H); 13C
NMR (75 MHz, CDCl3): d = 148.37, 147.33, 137.68, 133.18,
132.76, 129.65, 125.29, 123.87, 123.40, 122.36, 115.77, 93.83,
91.41, 97.93 ppm; Elemental analysis: calcd (%) for C26H18IN:
C 66.25, H 3.85, N 2.97; found: C 66.30, H 3.64, N 2.90.
1,4-Bis(2-ethylhexyloxy)-2,5-diiodobenzene (B2). Synthesized
from B1 by a known method.9 Yield: 67%; 1H NMR
(300 MHz, CDCl3): d = 7.16 (s, 2H), 3.82 (d, J = 5.4 Hz,
4H), 1.73 (m, 2H), 1.49 (m, 8H), 1.33 (m, 8H), 0.94 (t, J =
7.5 Hz, 6H), 0.91 ppm (t, J = 6.6 Hz, 6H); 13C NMR (75 MHz,
CDCl3): d = 153.04, 122.52, 86.25, 72.51, 39.66, 30.73, 29.28,
24.16, 23.29, 14.38, 11.46 ppm; Elemental analysis: calcd (%)
for C22H36I2O2: C 45.07, H 6.19; found: C 45.12, H 6.19.
4-(4-Cyanophenylethynyl)iodobenzene (A2). Synthesized from
4-ethynylbenzonitrile8 by the same procedure as described for
2,5-Bis(2-ethylhexyloxy)-1-iodo-4-[(p-diphenylamino)phenyl-
ethynyl]benzene (B3). Synthesized from 4-(diphenylamino)phe-
nylacetylene7 and 2 equiv. of B2 by the same procedure as
described for A1. Yield: 51%; IR (KBr, cm21): 2206; 1H NMR
(300 MHz, CDCl3): d = 7.35 (d, J = 8.4 Hz, 2H), 7.28 (dd, J =
7.8, 7.5 Hz, 4H), 7.25 (s, 1H), 7.11 (d, J = 7.8 Hz, 4H), 7.06 (t,
J = 7.5 Hz, 2H), 7.00 (d, J = 8.4 Hz, 2H), 6.88 (s, 1H), 3.87 (d,
J = 5.1 Hz, 2H), 3.85 (d, J = 5.1 Hz, 2H), 1.75 (m, 2H), 1.53
1
A1. Yield: 50%; mp 182 uC; IR (KBr, cm21): 2224, 2214; H
NMR (300 MHz, CDCl3): d = 7.72 (d, J = 8.4 Hz, 2H), 7.64 (d,
J = 8.4 Hz, 2H), 7.59 (d, J = 8.4 Hz, 2H), 7.26 ppm (d, J =
8.4 Hz, 2H); 13C NMR (75 MHz, CDCl3): d = 137.95, 133.40,
132.32, 132.29, 128.07, 121.92, 118.67, 111.98, 95.54, 92.97,
89.24 ppm; Elemental analysis: calcd (%) for C15H8IN: C
54.74, H 2.45, N 4.26; found: C 54.76, H 2.81, N 4.12.
2274 | J. Mater. Chem., 2006, 16, 2273–2281
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(m, 8H), 1.35 (m, 8H), 0.92 (t, J = 7.5 Hz, 6H), 0.87 ppm (t, J =
6.9 Hz, 6H); 13C NMR (75 MHz, CDCl3): d = 154.62, 152.10,
148.09, 147.43, 132.66, 129.62, 125.22, 123.76, 123.74, 122.48,
116.57, 115.62, 114.18, 94.74, 86.91, 85.14, 72.39, 72.35, 39.85,
39.71, 30.87, 30.79, 29.40, 29.32, 24.21, 23.31, 14.39, 14.35,
11.52, 11.48 ppm; Elemental analysis: calcd (%) for
C42H50INO2: C 69.32, H 6.92, N 1.92; found: C 69.39, H
7.00, N 1.83.
N,N-Bis(4-decyloxyphenyl)-4-ethynylaniline (D2).
Synthesized from C2 by the same procedure described for
1
D1. Yield: 65% (brown oil); IR (KBr, cm21): 2104; H NMR
(300 MHz, CDCl3): d = 7.26 (d, J = 8.8 Hz, 2H), 7.04 (d, J =
8.7 Hz, 4H), 6.82 (d, J = 8.7 Hz, 4H), 6.80 (d, J = 8.8 Hz, 2H),
3.92 (t, J = 6.6 Hz, 4H), 2.98 (s, 1H), 1.77 (m, 4H), 1.30 (m,
28H), 0.88 ppm (t, J = 6.8 Hz, 6H); 13C NMR (75 MHz,
CDCl3): d = 156.20, 149.48, 140.11, 133.13, 127.38, 119.02,
115.58, 112.65, 84.58, 75.77, 68.46, 32.17, 29.87, 29.84, 29.68,
29.61, 29.60, 26.35, 22.96, 14.40 ppm; Elemental analysis: calcd
(%) for C40H55NO2: C 82.57, H 9.53, N 2.41; found: C 82.37,
H 9.72, N 2.52.
N,N-Bis(4-tert-butylphenyl)-4-(triisopropylsilylethynyl)ani-
line (C1). Sodium tert-butoxide (1.8 g, 19 mmol) and
4-(triisopropylsilylethynyl)aniline10 (1.0 g, 3.7 mmol) were
added to a stirred solution of Pd2(dba)3 (0.13 g, 0.14 mmol),
DPPF (0.12 g, 0.22 mmol) and 4-tert-butylbromobenzene
(2.0 g, 9.4 mmol) in dry toluene (80 mL) under nitrogen. The
mixture was stirred at RT for 30 min and then heated at 90 uC
for 24 h. The cooled mixture was filtered and the solvent
was evaporated. The crude product was taken up in CH2Cl2
(50 mL) and the organic layer was washed several times with
water. The solvent was evaporated, and the product was
purified on a silica gel column chromatography using CH2Cl2–
hexane (1 : 20) as the eluent. Yield: 1.4 g (71%); mp 133 uC;
N,N-Bis(4-tert-butylphenyl)-4-(p-iodophenylethynyl)aniline
(E1). Synthesized from D1 by the same procedure as described
for A1. Yield: 63%; mp 204 uC; IR (KBr, cm21): 2214; 1H
NMR (300 MHz, CDCl3): d = 7.66 (d, J = 8.7 Hz, 2H), 7.32 (d,
J = 8.7 Hz, 2H), 7.28 (d, J = 8.8 Hz, 4H), 7.22 (d, J = 8.7 Hz,
2H), 7.04 (d, J = 8.8 Hz, 4H), 6.96 (d, J = 8.7 Hz, 2H),
1.32 ppm (s, 18H); 13C NMR (75 MHz, CDCl3): d = 148.70,
146.79, 144.53, 137.65, 133.16, 132.63, 126.44, 124.96, 123.54,
121.34, 114.70, 93.67, 91.72, 87.63, 34.60, 31.67 ppm;
Elemental analysis: calcd (%) for C34H34IN: C 69.98, H 5.87,
N 2.40; found: C 69.92, H 5.88, N 2.38.
1
IR (KBr, cm21): 2149; H NMR (300 MHz, CDCl3): d = 7.30
(d, J = 8.9 Hz, 2H), 7.26 (d, J = 8.6 Hz, 4H), 7.00 (d, J =
8.6 Hz, 4H), 6.94 (d, J = 8.9 Hz, 2H), 1.31 (s, 18H), 1.11 ppm
(m, 21H); 13C NMR (75 MHz, CDCl3): d = 148.46, 146.49,
144.70, 133.09, 126.36, 124.63, 121.83, 115.90, 107.83, 89.13,
34.57, 31.66, 18.95, 11.61 ppm; Elemental analysis: calcd (%)
for C37H51NSi: C 82.62, H 9.56, N 2.60; found: C 82.49, H
9.86, N 2.73.
N,N-Bis(4-decyloxyphenyl)-4-(p-iodophenylethynyl)aniline
(E2). Synthesized from D2 by the same procedure as
described for A1. Yield: 61%; IR (KBr, cm21): 2210; 1H
NMR (300 MHz, CDCl3): d = 7.64 (d, J = 8.3 Hz, 2H), 7.28 (d,
J = 8.9 Hz, 2H), 7.20 (d, J = 8.3 Hz, 2H), 7.06 (d, J = 8.8 Hz,
4H), 6.83 (d, J = 8.8 Hz, 4H), 6.82 (d, J = 8.9 Hz, 2H), 3.93 (t,
J = 6.6 Hz, 4H), 1.77 (m, 4H), 1.30 (m, 28H), 0.88 ppm (t,
J = 6.8 Hz, 6H); 13C NMR (75 MHz, CDCl3): d = 156.20,
149.30, 140.07, 137.62, 133.10, 132.60, 127.38, 123.65, 119.13,
115.56, 113.40, 93.49, 91.88, 87.35, 68.48, 32.13, 29.82, 29.80,
29.64, 29.57, 29.56, 26.31, 22.92, 14.36 ppm; Elemental
analysis: calcd (%) for C46H58INO2: C 70.48, H 7.46, N 1.79;
found: C 70.48, H 7.53, N 1.80.
N,N-Bis(4-decyloxyphenyl)-4-(triisopropylsilylethynyl)aniline
(C2). Synthesized by the same procedure described for C1
using 4-(decyloxy)bromobenzene11 in place of 4-tert-butyl-
1
bromobenzene. Yield: 82%; IR (KBr, cm21): 2149; H NMR
(300 MHz, CDCl3): d = 7.24 (d, J = 8.9 Hz, 2H), 7.02 (d, J =
9.0 Hz, 4H), 6.80 (d, J = 9.0 Hz, 4H), 6.79 (d, J = 8.9 Hz, 2H),
3.91 (t, J = 6.5 Hz, 4H), 1.77 (m, 4H), 1.30 (m, 28H), 1.11 (m,
21H), 0.88 ppm (t, J = 6.8 Hz, 6H); 13C NMR (75 MHz,
CDCl3): d = 156.04, 149.08, 140.34, 133.08, 127.12, 119.54,
115.53, 114.61, 108.10, 88.60, 68.45, 32.20, 29.90, 29.87, 29.72,
29.65, 29.64, 26.38, 22.99, 18.99, 14.42, 11.67 ppm; Elemental
analysis: calcd (%) for C49H75NO2Si: C 79.72, H 10.24, N 1.90;
found: C 79.85, H 10.45, N 1.79.
4-(4-Diphenylaminophenylethynyl)benzonitrile (1a).
Synthesized from 4-ethynylbenzonitrile7 and 4-(diphenyl-
amino)iodobenzene12 by the same procedure as described for
1
A1. Yield: 59%; mp 159 uC; IR (KBr, cm21): 2226, 2214; H
NMR (300 MHz, CDCl3): d = 7.62 (d, J = 8.8 Hz, 2H), 7.56 (d,
J = 8.8 Hz, 2H), 7.37 (d, J = 9.0 Hz, 2H), 7.30 (dd, J = 8.8,
7.2 Hz, 4H), 7.12 (d, J = 8.8 Hz, 4H), 7.09 (t, J = 7.2 Hz, 2H),
7.00 ppm (d, J = 9.0 Hz, 2H); 13C NMR (75 MHz, CDCl3): d =
148.93, 147.15, 133.00, 132.23, 132.04, 129.71, 128.91, 125.51,
124.15, 121.93, 118.91, 114.69, 111.13, 94.74, 87.39 ppm;
Elemental analysis: calcd (%) for C27H18N2: C 87.54, H 4.90,
N 7.56; found: C 87.56, H 5.07, N 7.51.
N,N-Bis(4-tert-butylphenyl)-4-ethynylaniline (D1). Bu4N+F2
in THF (3.7 mL, 3.7 mmol) was added into the solution of C1
(1.0 g, 1.9 mmol) in THF dropwise. The mixture was stirred at
RT for 30 min. Solvent was evaporated and the mixture was
dissolved in CH2Cl2. After usual work-up, the product was
purified on a silica gel column chromatography using CH2Cl2–
hexane (1 : 5) as the eluent. Yield: 0.59 g (83%); mp 157 uC; IR
(KBr, cm21): 2104; 1H NMR (300 MHz, CDCl3): d = 7.36 (d,
J = 8.7 Hz, 2H), 7.34 (d, J = 8.8 Hz, 4H), 7.10 (d, J = 8.8 Hz,
4H), 7.00 (d, J = 8.7 Hz, 2H), 3.03 (s, 1H), 1.40 ppm (s, 18H);
13C NMR (75 MHz, CDCl3): d = 148.89, 146.77, 144.52,
133.13, 126.42, 124.94, 121.22, 113.86, 84.43, 76.04, 34.59,
31.65 ppm; Elemental analysis: calcd (%) for C28H31N: C
88.14, H 8.19, N 3.67; found: C 88.45, H 8.37, N 3.53.
4-{4-[4-(N,N-Diphenylamino)phenylethynyl]phenylethynyl}
benzonitrile (1b). Synthesized from 4-ethynylbenzonitrile8 and
A1 by the same procedure as described for A1. Yield: 49%; mp
202 uC; IR (KBr, cm21): 2228, 2206; 1H NMR (300 MHz,
CDCl3): d = 7.64 (d, J = 8.4 Hz, 2H), 7.60 (d, J = 8.4 Hz, 2H),
7.50 (s, 4H), 7.38 (d, J = 8.5 Hz, 2H), 7.29 (dd, J = 7.8, 7.2 Hz,
4H), 7.12 (d, J = 7.8 Hz, 4H), 7.08 (t, J = 7.2 Hz, 2H), 7.01 ppm
This journal is ß The Royal Society of Chemistry 2006
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(d, J = 8.5 Hz, 2H); 13C NMR (75 MHz, CDCl3): d = 148.47,
147.30, 132.82, 132.31, 132.28, 131.92, 131.68 129.66, 128.28,
125.34, 124.74, 123.94, 122.26, 121.67, 118.73, 115.63, 111.81,
93.80, 92.56, 89.52, 88.41 ppm; Elemental analysis: calcd (%)
for C35H22N2: C 89.33, H 4.71, N 5.95; found: C 89.30, H 4.60,
N 5.91.
129.63, 125.98, 125.26, 123.80, 122.41, 118.03, 116.71, 116.46,
115.83, 112.92, 97.05, 95.94, 91.49, 87.09, 85.60, 80.81, 72.25,
72.09, 39.83, 39.60, 30.88, 30.54, 29.94, 29.40, 29.27, 24.21,
23.95, 23.32, 23.31, 14.35, 11.52, 11.25; Elemental analysis:
calcd (%) for C144H153N3O6: C 85.55, H 7.63, N 2.08; found: C
85.62, H 7.83, N 2.06.
1,3,5-Trichloro-2,4,6-tris-[4-(diphenylamino)phenylethynyl]
benzene (I). Diisopropylamine (1.3 mL) was added to a
stirred solution of Pd2Cl2(PPh3)2 (0.088 g, 0.13 mmol), CuI
(0.024 g, 0.13 mmol), 1,3,5-trichloro-2,4,6-triiodobenzene13
(1.4 g, 2.5 mmol) and N,N-diphenyl-4-ethynylaniline14 (3.0 g,
11 mmol) in anhydrous THF (20 mL) under nitrogen. The
mixture was heated at 90 uC for 24 h. After cooling, the solvent
was evaporated and the mixture was purified on a silica gel
column using CH2Cl2–hexane (1 : 5) as eluent. Yield: 0.84 g
1,3,5-Tris-{2,5-bis-(ethylhexyloxy)-4-[4-(diphenylamino)phe-
nylethynyl]phenylethynyl}-2,4,6-tris-[4-(4-cyanophenylethynyl)
phenylethynyl]benzene (3a). Synthesized from II and 6 equiv. of
A2 by the same procedure as described for 2b. Yield: 41%; mp
96 uC; IR (KBr, cm21): 2226, 2195; 1H NMR (300 MHz,
CDCl3): d = 7.65 (d, J = 8.4 Hz, 6H), 7.62 (d, J = 8.1 Hz, 6H),
7.59 (d, J = 8.4 Hz, 6H), 7.46 (d, J = 8.1 Hz, 6H), 7.39 (d, J =
8.7, 6H), 7.30 (t, J = 7.8 Hz, 12H), 7.13 (d, J = 7.8 Hz, 12H),
7.09 (s, 3H), 7.08 (s, 3H), 7.07 (t, J = 7.8 Hz, 6H), 7.02 (d, J =
8.7 Hz, 6H), 3.86 (d, J = 5.7 Hz, 6H), 3.75 (d, J = 5.7 Hz, 6H),
1.75 (m, 6H), 1.52 (m, 24H), 1.26 (m, 24H), 0.92, (t, J = 7.2 Hz,
9H), 0.83 (t, J = 6.6 Hz, 9H), 0.78 (t, J = 6.6 Hz, 9H), 0.74 ppm
(t, J = 7.2 Hz, 9H); 13C NMR (125 MHz, CDCl3): d = 154.20,
153.77, 148.05, 147.13, 132.49, 132.25, 132.09, 132.04, 131.60,
129.42, 128.08, 127.93, 126.99, 125.08, 124.00, 123.66, 122.46,
122.09, 118.45, 117.39, 116.65, 116.08, 115.39, 113.25, 111.70,
98.91, 96.93, 95.90, 93.36, 91.86, 89.88, 89.76, 85.22, 73.00,
71.78, 39.59, 39.14, 30.64, 30.38, 29.69, 29.15, 29.06, 23.96,
23.69, 23.06, 23.02, 14.06, 11.27, 10.86 ppm; Elemental
analysis: calcd (%) for C189H174N6O6: C 86.46, H 6.68, N
3.20; found: C 86.42, H 6.65, N 3.02.
1
(34%); mp 230 uC; IR (KBr, cm21): 2208; H NMR (300 Hz,
CDCl3): d = 7.45 (d, J = 8.7 Hz, 6H), 7.29 (dd, J = 7.8, 7.2 Hz,
12H), 7.13 (d, J = 7.8 Hz, 12H), 7.08 (t, J = 7.2 Hz, 6H),
7.02 ppm (d, J = 8.7 Hz, 6H); 13C NMR (125 MHz, CDCl3):
d = 148.81, 146.95, 136.89, 132.93, 129.45, 125.19, 123.83,
123.11, 121.84, 114.70, 101.32, 82.78 ppm; Elemental analysis:
calcd (%) for C66H42Cl3N3: C 80.61, H 4.30, N 4.27; found: C
80.58, H 4.43, N 4.36.
1,3,5-Tris[4-(diphenylamino)phenylethynyl]-2,4,6-tris(4-cya-
nophenylethynyl)benzene (2b).
A mixture of I (0.10 g,
0.10 mmol), 4-ethynylbenzonitrile8 (64.0 mg, 0.50 mmol),
PdCl2(PPh3)2 (1.0 mg, 0.001 mmol), CuI (2.0 mg, 0.010 mmol),
PPh3 (8.0 mg, 0.31 mmol) and Et3N–DMF(1 : 3) (10 mL) in a
pressure tube was heated in a microwave oven for 10 s and
cooled to RT to avoid overheating. This procedure was
repeated 10 times. The mixture was poured into water and
extracted with CH2Cl2. The product was purified on a silica gel
column using CH2Cl2 as eluent. Yield: 0.030 g (24%); mp >
280 uC; IR (KBr, cm21): 2228, 2198; 1H NMR (300 MHz,
CDCl3): d = 7.62 (d, J = 8.1 Hz, 6H), 7.55 (d, J = 8.1 Hz, 6H),
7.35 (d, J = 8.4 Hz, 6H), 7.32 (dd, J = 7.8, 7.2 Hz, 12H), 7.13
(t, J = 7.2 Hz, 6H), 7.12 (d, J = 7.8 Hz, 12H), 6.96 ppm (d, J =
8.4 Hz, 6H); 13C NMR (125 MHz, CDCl3): d = 149.09, 146.71,
132.69, 132.15, 132.07, 129.60, 129.25, 127.90, 125.50, 125.46,
124.32, 121.28, 118.31, 114.46, 111.98, 101.59, 96.81, 91.48,
86.42 ppm; Elemental analysis: calcd (%) for C93H54N6: C
88.97, H 4.34, N 6.69; found: C 88.96, H 4.12, N 6.56.
1,3,5-Triethynyl-2,4,6-tris(4-{4-[N,N-bis(4-tert-butylphenyl)
amino]phenylethynyl}phenylethynyl)benzene (III). Synthesized
from
1,3,5-triethynyl-2,4,6-tris[(trimethylsilyl)ethynyl]ben-
zene15 and 4 equiv. of E1 by the same procedure as described
for II, followed by the deprotection of TMS with Bu4N+F2
(TBAF) in THF. Yield: 23%; mp > 280 uC; IR (KBr, cm21):
1
2204; H NMR (300 MHz, CDCl3): d = 7.58 (d, J = 8.5 Hz,
6H), 7.50 (d, J = 8.5 Hz, 6H), 7.36 (d, J = 8.8 Hz, 6H), 7.28 (d,
J = 8.7 Hz, 12H), 7.04 (d, J = 8.7 Hz, 12H), 6.98 (d, J = 8.8 Hz,
6H), 3.78 (s, 3H), 1.32 ppm (s, 54H); 13C NMR (75 MHz,
CDCl3): d = 148.76, 146.82, 144.53, 132.72, 132.19, 131.57,
129.73, 126.45, 126.28, 125.00, 124.89, 122.05, 121.31, 114.70,
99.89, 92.90, 88.41, 88.18, 80.49, 77.46, 34.61, 31.67; Elemental
analysis: calcd (%) for C120H105N3: C 90.70, H 6.66, N 2.64;
found: C 90.62, H 6.46, N 2.68.
1,3,5-Triethynyl-2,4,6-tris{2,5-bis(2-ethylhexyloxy)-4-[4-
(diphenylamino)phenyl-ethynyl]phenylethynyl}benzene (II).
Synthesized from 1,3,5-triethynyl-2,4,6-tris[(trimethylsilyl)
ethynyl]benzene15 and 4 equiv. of B3 by the same procedure
as described for A1, followed by the deprotection of TMS with
Bu4N+F2 (TBAF) in THF. Yield: 31;. mp 69 uC; IR (KBr,
1,3,5-Triethynyl-2,4,6-tris(4-{4-[N,N-bis(4-decyloxyphenyl)
amino]phenylethynyl}phenylethynyl)benzene (IV). Synthesized
from
1,3,5-triethynyl-2,4,6-tris[(trimethylsilyl)ethynyl]ben-
zene15 and E2 by the same procedure as described for II.
Yield: 27%; mp 54 uC; IR (KBr, cm21): 2203; 1H NMR
(300 MHz, CDCl3): d = 7.56 (d, J = 8.0 Hz, 6H), 7.48 (d, J =
8.0 Hz, 6H), 7.30 (d, J = 8.3 Hz, 6H), 7.06 (d, J = 8.3 Hz, 12H),
6.84 (d, J = 8.3 Hz, 12H), 6.83 (d, J = 8.3 Hz, 6H), 3.93 (t, J =
6.5 Hz, 12H), 3.77 (s, 3H), 1.77 (m, 12H), 1.28 (m, 84H),
0.89 ppm (t, J = 6.8 Hz, 18H); 13C NMR (75 MHz, CDCl3): d =
156.23, 149.36, 140.07, 132.72, 132.17, 131.51, 129.75, 127.42,
126.25, 125.02, 121.94, 119.11, 115.59, 113.42, 99.93, 93.11,
88.17, 87.46, 80.51, 77.46, 68.48, 32.15, 29.84, 29.82, 29.67,
29.59, 29.58, 26.34, 22.94, 14.38; Elemental analysis: calcd (%)
1
cm21): 2195; H NMR (300 MHz, CDCl3): d = 7.38 (d, J =
8.7 Hz, 6H), 7.28 (t, J = 7.8 Hz, 12H), 7.12 (d, J = 7.8 Hz,
12H), 7.09 (s, 3H), 7.06 (t, J = 7.8 Hz, 6H), 7.01 (d, J =
8.7 Hz, 6H), 7.00 (s, 3H), 3.94 (d, J = 6.0 Hz, 6H), 3.91 (d, J =
6.0 Hz, 6H), 3.65 (s, 3H), 1.79 (m, 6H), 1.53 (m, 24H), 1.32 (m,
24H), 0.96 (t, J = 7.8 Hz, 9H), 0.93 (t, J = 7.8 Hz, 9H), 0.90 (t,
J = 6.6 Hz, 9H), 0.87 ppm (t, J = 6.6 Hz, 9H); 13C (75 MHz,
CDCl3): d = 154.26, 153.74, 148.17, 147.40, 132.74, 130.15,
2276 | J. Mater. Chem., 2006, 16, 2273–2281
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for C156H177N3O6: C 85.55, H 8.15, N 1.92; found: C 85.38, H
8.35, N 1.82.
followed by the coupling of I with p-cyanophenylacetylene
under microwave conditions. To synthesize 3a, 1,3,5-tri(ethy-
nyl)-2,4,6-tris(trimethylsilylethynyl)benzene
was
reacted
1,3,5-Tris-(4-{4-[N,N-bis(4-tert-butylphenyl)amino]phenyl-
ethynyl}phenylethynyl)-2,4,6-tris-[4-(4-cyanophenylethynyl)phe-
nylethynyl]benzene (3b). Synthesized from III and 6 equiv. of
A2 by the same procedure as described for 2b. Yield: 33%;
mp > 280 uC; IR (KBr, cm21): 2228, 2212; 1H NMR (300 MHz,
CDCl3): d = 7.54 (d, J = 8.7 Hz, 6H), 7.48 (d, J = 8.7 Hz, 6H),
7.43 (d, J = 8.4 Hz, 6H), 7.38 (s, 12H), 7.36 (d, J = 8.7, 6H),
7.31 (d, J = 8.8 Hz, 6H), 7.30 (d, J = 8.8 Hz, 12H), 7.04 (d, J =
8.8 Hz, 12H), 6.92 (d, J = 8.8 Hz, 6H), 1.33 ppm (s, 54H);
Elemental analysis: calcd (%) for C165H126N6: C 90.38, H 5.79,
N 3.83; found: C 90.39, H 5.71, N 3.83.
with 4-[(p-diphenylaminophenyl)ethynyl]-2,5-bis(2-ethylhexyl-
oxy)iodobenzene and the trimethylsilyl groups were
removed by treatment with TBAF. The intermediate II was
then reacted with p-[(p-cyanophenyl)ethynyl]iodobenzene to
obtain the desired product. Compounds 3b and 3c were
synthesized by the same procedure as described for 3a, except
that p-[di(arylaminophenyl)ethynyl]iodobenzene was used in
place of 4-[(p-diphenylaminophenyl)ethynyl]-2,5-bis(2-ethyl-
hexyloxy)iodobenzene. Except for 3b and 3c, whose solubility
was too low to obtain the 13C NMR spectra, the structures of
the new compounds were unambiguously confirmed by 1H and
13C NMR, IR, and elemental analysis.
1,3,5-Tris-(4-{4-[N,N-bis(4-decyloxyphenyl)amino]phenyl-
ethynyl}phenylethynyl)-2,4,6-tris-[4-(4-cyanophenylethynyl)phe-
nylethynyl]benzene (3c). Synthesized from IV and 6 equiv. of
A2 by the same procedure as described for 2b. Yield: 47%; mp
170 uC; IR (KBr, cm21): 2226, 2204; 1H NMR (300 MHz,
CDCl3): d = 7.53 (d, J = 8.4 Hz, 6H), 7.44 (d, J = 8.4 Hz, 6H),
7.38 (d, J = 8.5 Hz, 6H), 7.35 (s, 12H), 7.30 (d, J = 8.5, 6H),
7.26 (d, J = 8.7 Hz, 6H), 7.08 (d, J = 9.0 Hz, 12H), 6.86 (d, J =
9.0 Hz, 12H), 6.80 (d, J = 8.7 Hz, 6H), 3.95 (t, J = 6.5 Hz,
12H), 1.79 (m, 12H), 1.29 (m, 84H), 0.89 ppm (t, J = 6.8 Hz,
18H); Elemental analysis: calcd (%) for C201H198N6O6: C
86.41, H 7.14, N 3.01; found: C 86.41, H 7.35, N 3.02.
Linear optical properties
Fig. 1 shows that the lmax values of 1a and 1b are nearly the
same. This indicates the poor conjugation ability of the CMC
bond for the intramolecular charge transfer (ICT). However,
the oscillator strength (f), which is proportional to the square
of the transition moment (m10), increases with the conjugation
length. When the molecular structure is changed from 1a to 2a,
lmax is red-shifted by more than 100 nm, indicating that ICT
increases significantly as the three dipolar units share the
central phenyl group as the conjugation path. Also, the
oscillator strength increases by 3-fold from 1a to 2a (Table 1).
Hence, 2a is almost the same as three molecules of 1a in terms
of transition dipole moment. On the other hand, 2b shows a
smaller red-shift (60 nm) and a modest increase in the
oscillator strength (2 fold) in comparison to 1b, indicating
that the effect of sharing the central phenyl group becomes less
important as the conjugation length increases. The net result of
these effects is the decrease in lmax from 2a to 2b without
changing the oscillator strength (Table 1). When the conjuga-
tion length is further increased to 3a–c, the lmax decreases
with concomitant increase in the localized bands (Fig. 1). The
poorly conjugating CMC bonds seem to have attenuated the
ICT to distort the structures and break the symmetry of
the molecules. Consistently, 3a with bulky substituents exhibits
more distinct localized bands (Fig. 1).
Spectroscopic measurements
All spectroscopic measurements were performed on THF
solutions. Absorption spectra were recorded on a Hewlett-
Packard 8453 diode array spectrophotometer, and the
fluorescence spectra were obtained with a Amico Bowman
series 2 luminescence spectrometer. The first hyperpolarizabi-
lities of 1–3 were measured in THF by the frequency-resolved
HRS experiments following a previously described protocol.6,7
3. Results and discussion
Synthesis
Compound 2a was available from
a
previous study.3j
The fluorescence spectra of 1–3 are displayed in Fig. 2.
f1
Compounds 1a and 1b were synthesized by the Sonogashira
coupling of p-diphenylaminoiodobenzene and 4-[4-(diphenyl-
amino)phenylethynyl]iodobenzene (A1) with p-cyanophenyl-
acetylene, respectively (Scheme 2).
Compared with the absorption spectra, l
of 1b shows a
max
greater bathochromic shift and hence a larger Stokes shift in
comparison to 1a, indicating that the ICT is more efficient in
the excited than in the ground state (Table 1). Interestingly, the
Stokes shifts of 1a, 2a, and 2b are nearly the same, that is, the
energy gap between the ground and the Franck–Condon states
is more or less the same. However, a further increase in the
conjugation length to 3a–c results in a large increase in the
Synthesis of the other compounds is summarized in
Scheme 3. Compound 2b was synthesized by the Sonogashira
coupling of 1,3,5-trichloro-2,4,6-triiodobenzene with p-diphe-
nylaminophenylacetylene to obtain the intermediate I,
Stokes shift, probably because the excited state is more planar
f1
than the ground state. Also, l
of 3a is shorter than 3b and
max
3c, indicating that the former is more distorted in the excited
state. Except for 3c, which emits little fluorescence, all
compounds emitted strong fluorescence with the fluorescence
lifetime ranging from 3 to 6 ns. The much weaker fluorescence
of 3c can be attributed to the long chain alkoxy group, which
may facilitate the nonradiative pathways.
Scheme 2 Reagents
and
conditions:
(a)
p-CNC6H4CMCH/
Pd(PPh3)2Cl2/CuI/Et3N/THF.
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Scheme 3 Reagents and conditions: (a) p-Ph2NC6H4CMCH/Cat(I)/THF, 90 uC; (b) p-CNC6H4CMCH/Cat(II)/PPh3/DMF, microwave; (c) i)
p-Ph2NC6H4CMCC6H2-2,5-(Oeh)2-4-I/Cat(II)/THF/RT, ii) TBAF/THF; (d) p-CNC6H4CMCC6H4-p-I/Cat(II)/PPh3/DMF, microwave; (e) i)
p-R2NC6H4CMCC6H4-p-I [R = C6H4-p-tert-Bu (III), C6H4-p-OC10H21 (IV)]/Cat(II)/THF, RT, ii) TBAF/THF. Oeh: 2-ethylhexyloxy; Cat(I):
Pd2Cl2(PPh3)2/CuI/(i-Pr)2NH; Cat(II): Pd2Cl2(PPh3)2/CuI/Et3N.
Nonlinear optical properties
long-lived and time-delayed MPF. We have implemented a
frequency-domain technique that relies on the demodulation
(decrease in amplitude) and phase delay between HRS and
MPF. By measuring the amplitude and phase as a function of
amplitude modulation frequency, it is possible to extract an
accurate value for the hyperpolarizability that is not over-
estimated due to MPF. By simultaneously analyzing the
frequency-dependent demodulation and phase data towards
the hyperpolarizability value, the fluorescence contribution
and the fluorescence lifetime, a relatively precise value can
also be obtained.
Since octupolar molecules do not possess a dipole moment,
hyper-Rayleigh scattering (HRS) is the only measurement
technique that is capable of determining the molecular second-
order nonlinear polarizability (or first hyperpolarizability).
HRS is the second-order analogue of the linear Rayleigh
scattering process.16 In HRS, the signal is comprised of
incoherently scattered photons at the second-harmonic wave-
length. Multi-photon fluorescence (MPF) can also result in
such photons at exactly the same wavelength. Different
techniques have been proposed to discriminate between HRS
and MPF. In the spectral domain, the narrow HRS peak
can be isolated from the broad MPF band by background
subtraction. In the time domain, a narrow and early temporal
gate around the infrared laser pulse can largely suppress the
We have performed frequency-resolved HRS experiments in
THF at two different near-infrared wavelengths. At 800 nm,
the reference is the octupolar byyy of crystal violet in methanol,
340 6 10230 esu.6a At 1300 nm, we used the dipolar bzzz of
Disperse Red 1 of 54 6 10230 esu in chloroform as the
2278 | J. Mater. Chem., 2006, 16, 2273–2281
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Fig. 1 Molar absorptivity spectra for 1–3.
Fig. 2 Fluorescence spectra for 1–3.
reference.6b At this wavelength, the difference in symmetry
between the reference and the octupolar compounds as well as
the local field corrections factors from the difference in
refractive index between THF and methanol or chloroform
were taken into account. The femtosecond pulses at 1300 nm
are derived from a femtosecond optical parametric oscillator
(Spectra-Physics, model OPAL), pumped by an 800 nm
femtosecond laser (Spectra-Physics, model Tsunami). Hence,
the pulses at 1300 nm have lower peak power. Also, the
resonance enhancement factors for the first hyperpolarizability
values at 800 nm are larger than at 1300 nm. Therefore, the
HRS measurements at 1300 nm are more difficult and result in
a less precise result. However, they do confirm the results
obtained at 800 nm. The results for the linear and nonlinear
optical properties are given in Table 1.
For the dipolar model compound 1a, we did observe
significant MPF at 400 nm. This is clearly seen from Fig. 3,
which shows the demodulation (decrease in amplitude for
higher frequencies) and the phase shift between the MPF
and HRS signals. At 480 MHz, the MPF contribution is
Table 1 Linear and nonlinear optical properties of dipolar and octupolar molecules (1–3)
lmaxa/nm
l
a/nm
DnST
fc
b800
b0,800
t/nsf
b1300
b0,1300
t/nsf
b
d
d,e
d
d,e
f1
max
1a
1b
2a
2b
3a
3b
3c
a
378
380
481
441
371
382
379
473
495
637
575
557
619
640
5313
6114
5135
5284
9001
0.46
0.73
1.4
1.5
3.7
1340 ¡ 70
2260 ¡ 100
450 ¡ 45
500 ¡ 50
1750 ¡ 175
1760 ¡ 175
1125 ¡ 110
b
111 ¡ 6g
170 ¡ 7g
128 ¡ 13i
76 ¡ 8i
192 ¡ 19
120 ¡ 12
90 ¡ 10
2.7 ¡ 0.4
3.0 ¡ 3.5
nmh
nmh
125 ¡ 90
55 ¡ 70
170 ¡ 70
90 ¡ 70
nmh
nmh
nmh
50 ¡ 36
27 ¡ 35
106 ¡ 40
60 ¡ 40
nmh
nmh
nmh
nfj
nfj
nfj
nfj
nfj
3 ¡ 3
6 ¡ 4
5 ¡ 2
5 ¡ 3
nmh
10 023
10 760
4.1
4.0
lmax of the one-photon absorption and emission spectra in nm. Stokes shift in cm21 (1/lmax 2 1/lmf1ax). Oscillator strength estimated by
c
the relationship: f = 4.319 6 1029?e?n1/2, where n1/2 is the band width at half-height in THF. 10230 esu. Static hyperpolarizability corrected
d
e
by using two- and three-level models for the dipolar and octupolar molecules, respectively.1a,18 f Fluorescence lifetime. bzzz values calculated
g
from the relationships, ,b2. = (6/35)bzzz are 268 6 10230 and 411 6 10230 esu for 1a and 1b, respectively. Not measurable. byyy values
2
h
i
calculated from the relationships, ,b2. = (8/21)byyy are 207 6 10230 and 123 6 10230 esu for 2a and 2b, respectively. No fluorescence
contribution to the HRS observed.
2
j
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the resonance energy is not shifting significantly, this enhance-
ment cannot be attributed to a more efficient conjugation but
to the large increase in oscillator strength (Table 1).
For the octupolar compounds 2a and 2b, no MPF was
observed at 400 nm. A constant hyperpolarizability value was
observed for increasing modulation frequencies. For these
compounds, we could perform additional HRS experiments at
1300 nm, but at two modulation frequencies only. Because of
the small hyperpolarizability values and the large MPF
contributions with nanosecond lifetime, the signal is already
demodulated at the second modulation frequency (80 MHz).
From the resulting very limited number of data, the precision
in hyperpolarizability and lifetime value is very low, but these
data confirm the results based on the much better data at
800 nm. While the dynamic hyperpolarizability values for 2a
and 2b at 800 nm are similar, the static values, after correction
for the resonance enhancement, show the opposite trend when
compared to 1a and 1b. The octupolar version 2b of the
dipolar model compound 1b has a smaller hyperpolarizability
value than 2a. This corresponds well with the smaller wave-
length of maximal absorption, pointing towards less efficient
conjugation for the molecularly more elaborated octupole 2b
[eqn. (1)]. For comparison, bzzz of 1,3,5-tris(p-diethylamino-
phenylethynyl)-2,4,6-tris(p-nitrophenylethynyl)benzene mea-
Fig. 3 Demodulation (bottom) and phase (top) for the femtosecond
HRS measurement at 800 nm for compound 1a as a function of
amplitude modulation frequency.
sured by the nanosecond pulses is 1670 6 10250 Cm3 V22 2g
.
completely demodulated and the fluorescence-free dynamic
hyperpolarizability value [(1340 ¡ 70) 6 10230 esu] is
obtained. Therefore, a precise value for the fluorescence
lifetime [(2.7 ¡ 0.4) ns] could be determined. From the much
smaller MPF contribution at 400 nm for 2a, it was not possible
to do so, but the resulting value for the dynamic hyperpolariz-
ability [(450 ¡ 100) 6 10230 esu] is more precise. Because of
the absence of MPF and the small resonance enhancement
at 1300 nm, no values could be obtained at this longer
fundamental wavelength. For comparison, b(0) values of 1b
and 2a measured by the nanosecond pulses are 95 6 10230 and
273 6 10230 esu, respectively. The larger value of 170 6
10230 esu for 1b measured by the femtosecond pulses may be
because of the shorter wavelength of the fundamemtal wave,17
or imperfect resonance correction (vide infra). On the other
hand, the larger value of 2a measured with nanosecond pulses
may be due to the MPF contribution (vide infra).
After conversion from SI to esu unit and considering the
difference in the b values of the reference,2e,2g,19 the b(0)zzz
value becomes 228 6 10250 esu, which is approximately 2-fold
larger than b(0)zzz = 123 6 10250 esu measured for 2b in this
work (vide infra). The larger value of b(0)zzz for the former is
consistent with the larger lmax (453 nm vs. 441 nm), but not
with the smaller e (10 100 vs. 99 300). To make a direct
comparison, a further study is needed to measure the first
hyperpolarizability by the femtosecond pulses.
For the most elaborated octupolar series 3a, 3b and 3c, no
MPF was observed at 400 nm, while for 3a and 3b, as shown
for 2a and 2b, low-precision data at 1300 nm confirm the
trend. For 3c at 1300 nm, no signal could be obtained because
the MPF was very weak. It is observed from linear (one-
photon) fluorescence experiments that 3c has a very weak
emission. Very large dynamic hyperpolarizabilities are
observed at 800 nm, largely due to the strong resonance
enhancement. The enhancement factors are very similar, as a
consequence of the similar wavelength of maximal absorption
for 3a, 3b and 3c. In terms of linear optical properties,
these three molecules behave very similarly, with the exception
of the quenching of the fluorescence in 3c. However, in
terms of second-order nonlinear optical properties, we
observe a significant improvement for 3a. Because this
compound is more distorted than others, the symmetry may
be broken to induce dipolar contribution and increase the first
hyperpolarizability.
The first hyperpolarizability of a D3 symmetric molecule can
be expressed by the three-level model [eqn. (1)], where m10 is the
transition moment between the ground and degenerate first
excited CT state, m21 is the transition moment connecting the
degenerate excited states, v10 is the CT energy and v is the
energy of the incident laser light.1a The equation is identical to
that of the two-level model of molecular nonlinearity for the
dipoles if m21 is substituted by (m2 2 m1), where m1 and m2 are
the dipole moments in the ground and in the first excited CT
state, respectively.18
1
B2
m210m21
v210
v410
byyy
~
|
|
(1)
The bzzz values of 1a and 1b, calculated from the relation-
ship ,b2> = (6/35)bzzz2, are 268 6 10230 and 411 6 10230 esu,
respectively.20 Similarly, byyy values calculated from ,b2> =
(v120{4v2)(v210{v2)
Table 1 shows that there is a sizeable enhancement of the
first hyperpolarizability with the extension of the conjugation
length from 2a to 3a. Owing to the similarity in wavelength
of maximal absorption, the static hyperpolarizability values
(without resonance enhancement) show the same trend. Since
2
(8/21)byyy are 207 6 10230 and 123 6 10230 esu for 2a and
2b, respectively (Table 1).20 Hence, byyy(2a)/bzzz(1a) = 0.77
and byyy(2b)/bzzz(1b) = 0.30. The smaller ratio in the latter is
apparently due to the large decrease in b from 2a to 2b
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S. Mukamel, G. P. Bartholomew and G. Baza, J. Am. Chem. Soc.,
2000, 122, 11 956; (g) J. Brunel, I. Ledoux, J. Zyss and
M. Blanchard-Dsce, Chem. Commun., 2001, 923; (h) K. Se´ne´chal,
O. Maury, H. L. Bozec, I. Ledoux and J. Zyss, J. Am. Chem. Soc.,
2002, 124, 4560.
(vide supra). However, it is not readily apparent why both
values are smaller than the reported value of byyy(2a)/bzzz(1a) =
3.9 based on the nanosecond experiment.3j Furthermore, this
ratio is consistent with the large bathochromic shift and 3-fold
increase in the oscillator strength from 1a to 2a (Table 1). The
dichotomy could be explained if bzzz(1a) and bzzz(1b) measured
by the femtosecond pulses are overestimated. As can be seen in
Table 1, b0,1300 of 2a and 2b are 50 6 10230 and 27 6
10230 esu, respectively, whereas those of 1a and 1b are not
measurable. This implies that the former may be a more
efficient NLO-phore than the latter. In sharp contrast, b0,800
values of 1a and 1b are either comparable to or much larger
than those of 2a and 2b. This raises a strong possibility that the
b0,800 values of 1a and 1b may be overestimated by resonance
enhancement, that is, the resonance correction based on the
three-level model may not be perfect. Note that the resonance
enhancement factor of 1a and 1b are 12 and 13, respectively.
Alternatively, it may also be possible that byyy(2a) determined
by the nanosecond experiment may be overestimated by MPF,
despite the base line correction,12 to give rise to a large
2 (a) R. Wortman, C. Glania, P. Kra¨mer, R. Mastchiner, J. J. Wolff,
S. Kraft, B. Treptow, E. Barbu, D. La¨ngle and G. Go¨rlitz, Chem.–
Eur. J., 1997, 3, 1756; (b) C. Lambert, G. No¨ll, E. Scha¨mlzlin,
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byyy(2a)/bzzz(1a) value (vide supra).
4. Conclusions
We have studied linear and nonlinear optical properties of a
variety of newly synthesized dipolar and octupolar molecules
containing CMC bonds as the conjugation bridge. The b(0)
value of dipole increased with conjugation length, whereas that
of the octupole decreased from 2a to 2b with concomitant
decrease in lmax. The lmax further decreased by the increase in
the conjugation length from 2b to 3a–c, but b(0) increased
because of the larger oscillator strength. Moreover, 3a, with
the most distorted structure, shows the largest b(0), probably
because of the dipolar contribution. Among the octupoles
studied in this work, b(0)/MW is optimized in 2a. Owing to
the similarity in structure to 1,3,5-tricyano-2,4,6-tris(p-diethyl-
aminostyryl)benzene, 2a may produce noncentrosymmetric
crystals. Therefore, a further study is needed to develop
octupolar materials with large SHG based on this molecule.
Furthermore, the smaller byyy/bzzz ratio than the reported
values indicate that the resonance correction based on the
three-level model in the femtosecond measurement and the
base line correction in the nanosecond measurement may not
be perfect.
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Acknowledgements
13 J. J. Wolff, F. Gredel, T. Oeser, H. Irngartinger and H. Pritzkow,
Chem.–Eur. J., 1999, 5, 29.
This work was supported by Korea Research Foundation
Grant funded by Korea Government (MOEHRD, Basic
Research Promotion Fund) (KRF-2005-015-C00258).
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