A bridging double bond as an electron acceptor for optical nonlinearity of furan-containing [n.2]cyclophenes

Article abstract of DOI:10.1002/anie.200603557

All in the twist: Unusually large Stokes shifts and nonlinear optical properties are exhibited by furan-containing teraryl cyclophene derivatives (see picture). These cyclophenes have neither particularly strong electron-donating nor electron-withdrawing

Full text of DOI:10.1002/anie.200603557

Angewandte
Chemie
DOI: 10.1002/anie.200603557
Nonlinear Optics
A Bridging Double Bond as an Electron Acceptor for Optical
Nonlinearity of Furan-Containing [n.2]Cyclophenes**
Hsin-Chieh Lin, Wei-Yu Lin, Hao-Tien Bai, Jia-Hong Chen, Bih-Yaw Jin, and Tien-Yau Luh*
Dedicated to Professor Sunney I.Chan on the occasion of his 70th birthday
Studies on the relationship between the structures of organic
conjugated systems and their molecular hyperpolarizabilities
have been extensive because they may provide useful
information for the application
relative to those of the saturated analogue 2.[5] The emission
band of 2 appears at a similar wavelength to that of the
unbridged teraryl 3 (ca. 380 nm). The observation of similar
of nonlinear optical (NLO)
properties in electrooptical
devices.[1] The criterion for an
organic molecule to exhibit
second-order optical nonline-
arity is the lack of a center of
symmetry. The presence of
donor and acceptor moieties
conjugated with the linking
p system is known to facilitate
the formation of intramolecu-
lar charge transfer, which will
induce second-order NLO
properties.[1] Recently, a series
of [2.2]paracyclophane deriva-
tives has been shown to dem-
onstrate three-dimensional dipolar and octupolar NLO
properties as a result of through-space delocalization of the
chromophores.[2] [2.2]Metacyclophenes and metacyclophane-
dienes can readily undergo electrocyclization to yield the
corresponding bridged annulene derivatives.[3] Presumably,
interactions between the arene moieties and the bridging
double bond(s) may take place. Five-membered hetero-
phanes can be viewed as analogues of metaphanes; however,
the chemistry of five-membered [2.2]heterophenes and het-
erophanedienes has been only sporadically explored,[4,5] even
though the five-membered heteroaromatic rings are electron
rich.[6,7] It is envisioned that interactions between the hetero-
arene rings and the bridging double bond(s) in [2.2]hetero-
phenes may lead to a dipolar resonance structure.
Recently, we reported that furan-containing [2.2]cyclo-
phene 1a exhibits extraordinary photophysical properties
vibronic structures in 2 and 3 suggests that there is no
delocalization between the two teraryl moieties in 2. In the
absorption spectrum of 1a there is a low-energy band at about
380 nm, as well as an absorption band at about 330 nm arising
from the teraryl chromophore. The emission spectrum of 1a is
striking, with a large Stokes shift (178 nm) being observed. It
seems likely that interactions between the teraryl sections and
the double bond in 1a may be different in the ground and the
excited states. The teraryl moieties and the double bond in 1a
are clearly not in the same plane. Recently, it has been shown
that chromophores with twisted p-electron systems in a biaryl
system having charged donor and acceptor moieties exhibit
ultralarge molecular hyperpolarizability with exceptionally
high mb values.[8] It is therefore envisaged that 1a may have
second-order NLO activity even in the absence of any
apparent electron-withdrawing substituents. We now report
a systematic investigation of the NLO properties of a series of
[n.2]cyclophenes 1 with tethering chains of different lengths.
Furan-containing cyclophenes 1a–f were synthesized
according to Scheme 1.[9] Thiophene analogue 1g was pre-
pared in a similar manner[10] (see the Supporting Informa-
tion).
[*] Dr. H.-C. Lin, Dr. W.-Y. Lin, H.-T. Bai, J.-H. Chen, Prof. B.-Y. Jin,
Prof. T.-Y. Luh
Department of Chemistry
National Taiwan University
Taipei 106 (Taiwan)
Electric-field-induced
second-harmonic
generation
Fax: (+886)223-644-971
(EFISH) measurements at 1.91 mm were employed for the
NLO investigations.[11] The photophysical properties and the
mb values for 1 as well as the reference compound 9 are
summarized in Table 1. The absorption and emission spectra
of selected [n.2]cyclophenes (1a, 1c, 1e, and 1 f) are shown in
E-mail: tyluh@ntu.edu.tw
[**] This work was supported by the National Science Council of the
Republic of China.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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Communications
tailing at longer wavelengths.
They all showed large Stokes
shifts (157–179 nm). These
results suggested that there
might be significant difference
in the delocalization in the
ground and excited states in
these substrates. The quantum
yields of 1e and 1 f were higher
than those of 1a–d. Apparently,
the relief of the strain arising
from the increase in the length
of the tethering chain may
result in better conjugation
between the teraryl chromo-
phores and the bridged double
Scheme 1. a) 1. nBuLi, THF, À788C, 50 min; 2. 5a–f, THF, À788C, 1 h; RT, 30 min, 3. TFA, RT, 12 h,
40–55%; b) DIBAL-H, THF, 08C, 3 h, 89–98%; c) MnO2, CH2Cl2, RT, 6 h, 90–97%; d) TiCl4, Zn, Py, reflux,
16 h, 25–75%. TFA=trifluoroacetic acid, DIBAL-H=diisobutylaluminum hydride, Py=pyridine.
Figure 1 (the spectra of the other compounds can be found in
the Supporting Information).
The spectra of all the cyclophenes 1 exhibited, in addition
to the absorptions arising from the teraryl chromophores,
bond. This effect can also be evidenced by slight changes in
the lmax values with different lengths of the tethering chain.
It is noteworthy that cyclophenes 1a–g exhibited second-
order optical nonlinearity. The mb values for 1a–e are two
orders of magnitude higher than that of 1 f, and that of 1d is
fivefold larger than that of 1e. Surprisingly, the mb values for
1b–d are comparable with that of 9. Compound 9 has both a
strong electron-donating substituent (Me2N) and a strong
electron-withdrawing substituent (NO2) linked through a
stilbene moiety. The dipole moment for 9 is 6.6 D.[12] Cyclo-
phenes 1 are much less polar. The calculated dipole moments
are given in Table 1. Consequently, it is surprising that 1a–e
exhibited such unusually high mb values. Furan (sp = À0.39)
and thiophene rings (sp = À0.43) may serve as electron-
donating substituents,[6,7] and replacement of the furan ring by
the thiophene ring in 1g essentially did not alter the NLO
properties of the cyclophenes. Thus, the mb value for 1g is
203 ” 10À48 esu, which is comparable to that of 1a. The
contribution of the double bond to the NLO properties in
1a–g is striking. It is noteworthy that 2 does not exhibit any
nonlinear optical behavior and is essentially nonpolar (m =
0.27 D). These results suggest that the bridging double bond
may be viewed as an electron acceptor.
DFT calculations on 1a showed that the distribution of
electron density in the highest occupied molecular orbital
(HOMO) was highest on the two furan moieties, whereas the
electron density shifted to the bridging double bond in the
lowest unoccupied molecular orbital (LUMO, Figure 2). Such
electron distributions in the HOMO and LUMO of 1a might
explain their p-donor and p-acceptor behaviors, respec-
tively.[1] Similar behavior was also found in 1b–f and
thiophene derivative 1g (see the Supporting Information).
On the other hand, the electron distributions in both the
HOMO and LUMO of 2 were superimposed, and thus no
charge transfer would occur (Figure 2).
Table 1: Photophysical and mb values of 1 and related compounds in
CHCl3.
Compound lmax [nm] lem [nm] F[a] m [D][b] w[b,c] c[b,d]
mb1.91
[e]
1a
1b
1c
1d
1e
1 f
1g
2
325
327
330
330
331
337
314
315
430
501
506
496
498
498
494
491
380
0.28 1.12
0.32 1.23
0.36 1.26
0.39 1.29
0.51 1.48
0.57 1.05
0.22 1.14
0.59 0.27
40.1 20.7 232
39.8[f] 18.5[f] 370
38.7 16.3 530
38.5[f] 15.5[f] 502
31.9 14.5 110
24.6 1.2
3
51.6 30.3 203
n.d.
450
9
6.60[g]
[a] Measured in EtOAc using coumarin as a reference (F=0.99)
[b] Calculated by DFT at the 6-31G** level. [c] Calculated dihedral angle
À
À
(8) between the C1 C2 and C3 C4 bonds in 1. [d] Calculated dihedral
angle (8)between the C5 C6 and C7 C8 bonds in 1. [e] In 10À48 esu.
[f] Compounds 1b and 1d contain an odd number of carbon atoms in
the tethering chain and do not have a C2 symmetry axis, which led to two
different w and c values being obtained (1b: w=38.5 and 41.18, c=12.5
and 24.78; 1d: w=37.7 and 39.48, c=12.2 and 18.98). The average
values are shown in the Table. [g] Ref. [11].
À
À
À
The dihedral angles w formed between the C1 C2 and
À
C3 C4 bonds in 1 also decrease with the length of the
tethering chain. A plot of the mb values of 1a–f against w is
shown in Figure 3. It is interesting to note that the mb value
reaches a maximum at w = 38.78 (tethering chain length = 4)
and drops abruptly when the value of w increases to around
408 (tethering chain length = 2, 3). Presumably, the strain may
be relieved significantly when the tethering chain length
Figure 1. Absorption and emission spectra of 1a (c), 1c (a),
1e (g), and 1 f (d) in CHCl3.
898
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Angewandte
Chemie
In summary, we have detailed a new series of furan-
containing teraryl cyclophene derivatives 1 which exhibit
unusually large Stokes shifts and NLO properties. These
cyclophenes 1 have neither particularly strong electron-
donating moieties nor electron-withdrawing groups and
have relatively low polarity. Yet they exhibit exceptionally
high mb values, which are even comparable with that of the
highly polar compound 9. Structurally, the strained cyclo-
phenes 1 furnish a unique feature that dictates these unusual
photophysical properties: strain results in the p systems of the
teraryl system and the bridging double bond being twisted.
Such a twisted system may thus induce significant enhance-
ment in hyperpolarizability.[8] The five-membered heteroar-
omatic rings in 1 may not only serve as electron donors, but
may also accommodate the appropriate geometry to enable
the interactions to occur between the oligoaryl systems and
the double bond that lead to unusual photophysical and NLO
properties.
Figure 2. Contour plots of the frontier molecular orbitals obtained by
DFT calculations at the B3LYP/6-31G** level (upper: LUMO, lower:
HOMO) of 1a (left) and 2 (right). Butyl substituents and hydrogen
atoms have been omitted for clarity.
Received: August 31, 2006
Revised: January 12, 2006
Keywords: cyclophanes · density functional calculations ·
.
hyperpolarizability · nonlinear optics · pi interactions
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