2,6-Bis(styryl)anthracene derivatives with large two-photon cross-sections

Article abstract of DOI:10.1039/b309124d

The first synthesis of 2,6-bis(styryl)anthrance derivatives with very large two-photon cross sections is reported.

Full text of DOI:10.1039/b309124d

2,6-Bis(styryl)anthracene derivatives with large two-photon
cross-sections†
Wen Jun Yang, Dae Young Kim, Mi-Yun Jeong, Hwan Myung Kim, Seung-Joon Jeon and Bong Rae
Cho*
Molecular Opto-Electronics Laboratory, Department of Chemistry and Center for Electro- and
Photo-Responsive Molecules, Korea University, Anam-Dong, Sungbuk-Ku, Seoul 136-701, Korea.
E-mail: chobr@korea.ac.kr
Received (in Cambridge, UK) 31st July 2003, Accepted 12th September 2003
First published as an Advance Article on the web 23rd September 2003
The first synthesis of 2,6-bis(styryl)anthrance derivatives
with very large two-photon cross sections is reported.
The normalized one-photon absorption and fluorescence
spectra as well as the two-photon induced fluorescence
excitation spectra for 3a are shown in Fig. 1. The corresponding
spectroscopic parameters are reported in Table 1.
Organic nonlinear optical materials exhibiting large two-photon
absorption (TPA) cross sections (dTPA) are of considerable
interest for applications in a number of new areas including
three dimensional fluorescence imaging, optical limiting,
photodynamic therapy, and three dimensional optical data
storage and microfabrication.1–14 A variety of compounds
including donor–bridge–acceptor (D–p–A) dipoles, donor–
bridge–donor (D–p–D) quadrupoles, multi-branched com-
pounds, dendrimers, and octupoles have been synthesized and
their structure–property relationships established.15–22 The
results reveal that that dTPA increases with the donor/acceptor
strength, conjugation length, and planarity of the p-center.
Among the most extensively utilized p-centers are benzene,
biphenyl, fluorene, dithieniothiophene, and dihydrophenan-
threne.
The lowest energy one-photon absorption maxima (l(1)
)
max
for 1–3 are red shifted by approximately 80–210 nm from that
max
of anthracene (l(1) = 378 nm). Moreover, l(1)max gradually
increases with a stronger donor/acceptor, i.e., NAr2 < NHex2
and 1 < 3 < 2 (Table 1). Interestingly, the lowest energy bands
for 2a,b are virtually separated from the higher energy bands.
This suggests a significant intramolecular charge transfer (ICT)
We thought that anthrancene might be a useful p-center not
only because it is an excellent fluorophore but also because its
derivatives have been extensively used as fluorescence sensors.
Hence, if such derivatives with large dTPA are synthesized, it
might be possible to develop two-photon sensors for biological
applications. However, there has been no report regarding the
TPA properties of such compounds. In this work, we have
synthesized a series of 2,6-bis(styryl)anthracence derivatives
(1–3) and measured their dTPA by two-photon induced fluores-
cence. We now report that these compounds show among the
largest TPA cross-sections ever reported.
Synthesis of 1–3 is shown in Scheme 1. The reaction between
2,6-dimethylanthraquinone (A) and p-cyanophenyllithium at
260 °C produced 9,10-bis(p-cyanophenyl)-2,6-dimethylan-
thracene (B) in 56% yield. Reduction of A afforded 2,6-dime-
thylanthracene (C) in 73% yield. Bromination of C followed by
cyanation produced 9,10-dicyano-2,6-dimethylanthracene (D)
in 75% yield. To prepare E, B–D were reacted with NBS and
then treated with P(OEt)3. Compounds 1–3 were synthesized by
the condensation between the phosphonate ylids and benzalde-
hyde derivatives as shown in Scheme 1. The structures of 1–3
Scheme 1 Reagents and conditions: (a) i: p-CNC6H4Br/BuLi/THF/278
°C–RT/5 h; ii: Zn/AcOH/120 °C/3 h; (b) HgCl2/Al/CCl4/cyclohexanol/85
°C/48 h; (c) Br2/CCl4/0 °C/5 h; (d) CuCN/pyridine/220 °C/4 h; (e) i: NBS/
BPO/benzene/reflux/3 h; ii: P(OEt)3/reflux/12 h; (f) LDA/THF/p-YC6H-
4CHO/278 °C–RT/6 h.
1
were unambiguously characterized by H and 13C NMR, IR,
and elemental analysis.
The two-photon cross section d was measured by using the
two-photon-induced fluorescence measurement technique with
eqn. (1), where the subscripts s and r stand for the sample and
reference molecules.8b The intensity of the signal collected by a
PMT detector was denoted as S. F is the fluorescence quantum
yield. f is the overall fluorescence collection efficiency of the
experimental apparatus. The number density of the molecules in
solution was denoted as c. dr is the TPA cross section of the
reference molecule.
(1)
Fig. 1 Normalized one-photon emission (:, excitation at 488 nm),
absorption (-) spectra and two-photon excitation spectrum (5) for 3a in
toluene. The two-photon spectrum is plotted as a function of total transition
energy (twice the photon energy).
† Electronic supplementary information (ESI) available: synthesis details;
measurement of two-photon cross-sections. See http://www.rsc.org/supp-
data/cc/b3/b309124d/
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CHEM. COMMUN., 2003, 2618–2619
This journal is © The Royal Society of Chemistry 2003

Table 1 One- and two-photon properties of 1–3
Notes and references
a
b
c
e
f
g
l(1)
lfl
DnST
Fd
l(2)
max
dmax
Fdmax
‡ The most efficient two-photon chromophores with very large two-photon
cross-sections measured by the two-photon fluorescence method are
max
max
2,5-dicyano-1,4-bis(p-diphenylaminostyryl)benzene (l(2)
=
835 nm,
1a
1b
2a
2b
3a
3bA
455
454
587
566
488
474
487
479
656
635
535
509
1444
1450
1792
1920
1800
1451
0.78
0.82
0.11
0.15
0.64
0.84
800
820
990
990
840
820
1100
1340
2290
2490
1570
1760
850
1100
250
370
1000
1480
max
dmax = 1940 GM),8a dihydrophenanthrene derivatives (l(2)
= 740 nm,
max
dmax = 3760 GM),9a and 4,4A,4B-tris{[2,5,-dicyano-4-(p-diphenylamino-
styryl)]styryl}triphenylamine (l(2) = 840 nm, dmax = 5030 GM).12e
max
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max
d Fluorescence quantum yield. e lmax of the two-photon absorption spectra
in nm. f The peak two-photon absorptivity in GM. g TPF action cross
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fluorescence spectra, i.e., larger bathochromic shift with a
stronger donor/acceptor. This indicates a significant stabiliza-
tion of the emitting states by the substituents, and provides
additional evidence for the ICT. Also, the Stokes shift increases
with a stronger acceptor, although it is relatively insensitive to
the variation of the donors. As expected, the fluorescence
quantum yields are always higher when NAr2 is the donor.
Fig. 1 shows that the two-photon allowed states for 3a are
located at somewhat higher energy than the ICT bands.
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withdrawing ability of the 9,10-substituents (Table 1). This
indicates an interesting possibility that the wavelength of the
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appropriate substituents. Interestingly, l(2)
for 1 and 3
max
appear near 800 nm. This turns out to be very important for
practical applications, because most TPF microscopy uses a
visible beam with a wavelength around 800 nm.
The dmax value of 1a is 1100 GM (1 GM = 10250 cm4 s
photon21), which is somewhat larger than those for the closely-
related 1,4-bis(p-dibutylaminostryl)benzene and 4,4A-bis[4-(p-
dihexylaminophenylethynyl)styryl]biphenyl. The value in-
creases approximately two-fold when CN groups are bound to
the 9,10-position (2a,b).8b,9b Note that 2a,b show large
bathochromic shifts in the absorption spectra and large Stokes
shifts (Table 1). The acceptor groups seem to stabilize the
excited state more than the ground state to diminish the energy
gap between the ground- and two-photon-allowed states. This
would predict a larger two-photon cross section, because the
smaller the energy gap is, the higher the probability of the
excitation will be. This result underlines the importance of ICT
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cyanophenyl groups at the 9,10-positions. Hence, dmax values of
3a,bA are 1570–1760 GM, which are among the largest values
reported in the literature.‡ Also, the fluorescence quantum
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yields are relatively high, hence large Fdmax. Finally, l(2)
max
values of 3a,bA are 840 and 820 nm, respectively, which are
close to 800 nm. Therefore, 3a,b may be useful for applications
that use two-photon excited fluorescence. It should be noted that
dmax and Fdmax are always larger when NAr2 is used as the
donor. The special effect of NAr2 donor has been previously
reported.8
In conclusion, we have synthesized a series of 2,6-bis(styr-
yl)anthracence derivatives with large dmax. l(2)
and dmax
max
increase with stronger acceptors at the 9,10-positions. Both
l(2)
and Fdmax have been optimized by introducing donor-
max
substituted styryl and p-cyanophenyl groups at the 2,6- and
9,10-positions, respectively. These molecules may ultimately
find useful applications as two-photon materials.
This work was supported by NRL-MOST and CRM-KOSEF.
DYK and HMK were supported by BK21 program.
CHEM. COMMUN., 2003, 2618–2619
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