Two-photon absorption properties of 2,6-bis(styryl)anthracene derivatives: Effects of donor-acceptor substituents and the π center

Article abstract of DOI:10.1002/chem.200401134

A series of 2,6- and 2,7-bis-(styryl)anthracene derivatives with the donors at the styryl group and acceptors at the 9,10-positions have been synthesized, and their two-photon cross sections (φδ_max) were determined. These compounds exhibit a peak two-photon absorptivity (δ_max) in the range of 700-2500 GM at 780-1030 nm. Values of λ_max and Stokes shifts increase as the acceptor is changed to a stronger one. There is also a parallel increase in λ_max ⁽²⁾ and δ_max with the same variation of the chromophore structure. Both λ_max⁽²⁾ and φδ_max have been optimized by introducing donor-substituted styryl groups at the 2,6-positions and p-cyanophenyl groups at the 9,10-positions, respectively. The effect of a π center on the two-photon absorption properties has been assessed by comparing the existing data for a variety of D-π-D derivatives.

Full text of DOI:10.1002/chem.200401134

FULL PAPER
Two-Photon Absorption Properties of 2,6-Bis(styryl)anthracene Derivatives:
Effects of Donor–Acceptor Substituents and the p Center
Wen Jun Yang, Dae Young Kim, Mi-Yun Jeong, Hwan Myung Kim, Yun Kyoung Lee,
Xingzhong Fang, Seung-Joon Jeon,* and Bong Rae Cho*[a]
ð2Þ
Abstract: A series of 2,6- and 2,7-bis-
(styryl)anthracene derivatives with the
donors at the styryl group and accept-
ors at the 9,10-positions have been syn-
thesized, and their two-photon cross
crease as the acceptor is changed to a
Both l
and Fdmax have been opti-
max
stronger one. There is also a parallel
mized by introducing donor-substituted
styryl groups at the 2,6-positions and p-
cyanophenyl groups at the 9,10-posi-
tions, respectively. The effect of a p
center on the two-photon absorption
properties has been assessed by com-
paring the existing data for a variety of
D–p–D derivatives.
ð2Þ
increase in l and dmax with the same
max
variation of the chromophore structure.
sections (Fdmax
)
were determined.
Keywords: anthracene · donor–ac-
ceptor systems · excited states ·
fluorescence · two-photon absorp-
tion
These compounds exhibit a peak two-
photon absorptivity (dmax) in the range
of 700–2500 GM at 780–1030 nm.
Values of lmax and Stokes shifts in-
Introduction
(styryl)benzene, dihydrophenanthrene, tricyanobenzene, and
dendrimers have been synthesized and their structure–prop-
erty relationship has been investigated.[16–20] The results of
these studies reveal that the TPA cross section increases
with increasing donor strength, conjugation length, and pla-
narity of the p center.
The synthesis of organic materials with a large two-photon
cross section is of considerable research interest owing to
their potential applications in a number of new areas, in-
cluding biological imaging, optical power limiting, two-
photon upconversion lasing, two-photon fluorescence excita-
tion microscopy, 3D microfabrication, and photodynamic
therapy.[1–15] The most frequently investigated structural
motifs for efficient two-photon absorption (TPA) chromo-
phores are donor–bridge–acceptor (D–p–A) dipoles, donor–
bridge–donor (D–p–D) quadruples, and octupoles. Such de-
rivatives of fluorene, triphenylamine, dithienothiophene, bis-
Among the most attractive p centers is the anthracene
moiety, not only because it is planar, but it is also an excel-
lent fluorophore. Moreover, anthracene derivatives have
been extensively utilized as fluorescence sensors for metal
ions and biological molecules.[21] Therefore, there seems to
be a great potential for their development as two-photon
sensors for biological applications. We recently reported
that 2,6-bis(styryl)anthracene and 9,10-bis(arylethynyl)an-
thracene derivatives exhibit large two-photon cross sections,
demonstrating the usefulness of the anthracene moiety as
the p center.[20f,h] In this work, we have synthesized a series
of 2,6- and 2,7-bis(styryl)anthracene derivatives (1–4;
Scheme 1) and studied their one- and two-photon absorp-
tion properties (dTPA). We were interested in studying the ef-
fects of 1) changing the electron-withdrawing abilities of the
9,10-substituents, 2) changing the donors from N(Hex)2 to
N(i-amyl)tol to NPh2, and 3) quadrupolar (1–4) and dipolar
(5) symmetry on the one- and two-photon spectroscopic
properties. Finally, the effect of changing the p center on
the two-photon absorption properties was assessed by a
comparison with the existing data for a variety of D–p–D
derivatives.[17,19]
[a] Dr. W. J. Yang,+ D. Y. Kim, Dr. M.-Y. Jeong, H. M. Kim,
Dr. Y. K. Lee, Dr. X. Fang, Prof. S.-J. Jeon, Prof. B. R. Cho
Molecular Opto-Electronics Laboratory
Department of Chemistry and Center for Electro-
and Photo-Responsive Molecules, Korea University
1-Anamdong, Seoul 136–701 (Korea)
Fax : (+82)2-3290-3121
E-mail: sjjeon@korea.ac.kr
chobr@korea.ac.kr
[+] On leave from Key Lab of Rubber-Plastics, Ministry of Education
(Qingdao University of Science and Technology), 53- Zhengzou
Road, Qingdao 266042 (China)
Supporting information for this article is available on the WWW
under http://www.chemeurj.org/ or from the author. This information
includes one-photon absorption, fluorescence, and two-photon excita-
tion spectra for 1b, 1c, 2a, 2b, 2c’, 3a, 3b, 3c’, 4a–c, and 5a in tolu-
ene.
Chem. Eur. J. 2005, 11, 4191 – 4198
DOI: 10.1002/chem.200401134
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4191

were unambiguously character-
1
ized by H and 13C NMR spec-
troscopic analysis and elemen-
tal analysis.
One-photon absorption and
Scheme 1. The 2,6- and 2,7-bis(styryl)anthracene derivatives used in this study: X=H (1), Ph (2), p-CNC6H4
(3), CN (4); Y=N(Hex)2 (a), NH2 (a’), N(i-amyl)tol (b), N(C6H4-p-tBu)2 (c), NPh2 (c’).
emission
shows the one-photon absorp-
spectra:
Figure 1
Results and Discussion
Synthesis: All of the 2,6-bis(styryl)anthracene derivatives
(1–4) were synthesized from 2,6-dimethylanthraquinone (6)
(Scheme 2).[22] Reduction of 6 with aluminum powder af-
forded 1,6-dimethylanthracene (7) in 73% yield. Bromina-
tion of 7 followed by cyanation produced 9,10-dicyano-2,6-
dimethylanthracene in overall 75% yield. 2,6-Dimethyl-
9,10-diphenylanthracene (8’) and 2,6-dimethyl-9,10-bis(p-cy-
anophenyl)anthracene (8’’) were prepared by the reaction
between 6 and PhMgBr and p-CNC6H4Li, respectively, fol-
lowed by the reduction with Zn powder.
Figure 1. Molar absorption spectra for 1a–5a in toluene.
tion spectra of solutions of 1a–5a in toluene. The shapes of
the spectra are similar to that of anthracene, except that the
absorption maxima (lmax) are shifted to a longer wavelength
by the extended conjugation. For a given donor, lmax in-
creases monotonically with the electron-accepting ability of
the 9,10-substitutent, that is, 1a<2a<3a<4a. This indi-
cates a gradual increase in the intramolecular charge trans-
fer (ICT) between the donor and acceptor. Similarly, lmax of
5a is shorter than 4a, probably because the acceptor
strength is attenuated in the former by the fact that only the
9-CN group is in direct resonance with the N(Hex)2 donors.
When the donor is changed from N(Hex)2 to N(i-amyl)tol
to NAr2, lmax of 1a–c remains nearly the same, despite the
decrease in the donor strength (Table 1). It is well establish-
ed that the substitution of the NPh group in a simple D–p–
A conjugate system increases the p conjugation in a “non-
traditional” manner, where the additional p-electron unit
lies outside the donor–acceptor framework.[23] Hence, this
result could be understood if the weaker donor strength is
compensated for by the “nontraditional” conjugation effect
of the NPh group. On the other hand, lmax values of 2–4 de-
crease with a weaker donor, indicating that the special
effect of the NPh group becomes less important as the ICT
from the donor to the acceptor becomes more important.
Moreover, the longer the lmax is, the larger the blueshift
caused by the change to a weaker donor is (Table 1).
Scheme 2. Reaction conditions: a) 1. PhMgBr/THF/RT/4 h (8’) or p-
CNC6H4Br/BuLi/THF/ꢀ788C to RT/5 h (8’’); 2. Zn/AcOH/1208C/3 h;
b) HgCl2/Al/CCl4/cyclohexanol/858C/48 h; c) Br2/CCl4/08C/5 h; d) CuCN/
pyridine/2208C/4 h; e) 1. NBS/BPO/benzene/reflux/3 h; 2. P(OEt)3/reflux/
12 h; f) LDA/THF/p-YC6H4CHO/ꢀ788C to RT/6 h.
To prepare 10, compounds 7–9 were reacted with NBS
and then treated with P(OEt)3. Compounds 1–4 were syn-
thesized by a condensation reaction between the phospho-
nate ylides and the appropriate benzaldehyde derivatives
(Scheme 2). The 2,7-bis(p-dihexylaminostyryl)anthracene
derivative (5a) was synthesized by the same procedure as
described for 4a except that 2,7-dimethylanthraquinone was
used as the starting material. The structures of 1–4 and 5a
The fluorescence spectra of 1a–5a are shown in Figure 2.
None of these spectra have multiple peaks, which indicates
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2,6-Bis(styryl)anthracene Derivatives
FULL PAPER
Table 1. One- and two-photon properties of 2,6- and 2,7-bis-
(styryl)anthracene derivatives (1–5).
Stokes shift for 5a is much larger than that of 4a. This is be-
fl
cause the l value is almost the same for both compounds,
max
[a]
fl [b]
max
ð2Þ [e]
[f]
[g]
lmax
l
Dn˜[c]
F[d]
l
dmax
Fdmax
whereas lmax of 5a is significantly blueshifted. Because the
dipole moment of 5a is larger than that of 4a, the emitting
state of the former would be more stabilized by the ICT
than expected from the acceptor strength. Except for 2c’
and 3c’, the Stokes shifts are relatively insensitive to the
donors. This indicates that the effect of donors on the
Franck–Condon and emitting states are similar. Most of the
compounds are strongly fluorescent with high fluorescence
quantum yields. The much smaller quantum yields for 4a–c
and 5a may be attributable to the lower energy of the emit-
ting states, which may facilitate nonradiative pathways. As
expected, the fluorescence quantum yields are always higher
when N(i-amyl)tol or NAr2 is the donor.[24]
max
1a
1b
1c
2a
2b
2c’
3a
3b
3c’
4a
4b
4c
5a
455
454
454
466
463
458
488
482
474
587
575
566
562
487
483
479
501
495
481
535
530
509
656
643
635
650
1444
1322
1450
1499
1396
1044
1800
1879
1451
1792
1839
1920
2409
0.78
0.84
0.82
1.0
1.0
1.0
0.64
0.88
0.84
0.11
0.15
0.15
0.088
800
780
780
780
780
780
840
840
820
990
990
990
1100
1080
1340
770
700
720
1570
1180
1760
2290
2210
2490
720
860
910
1100
770
700
720
1000
1040
1480
250
330
370
1030
60
[a] lmax of the one-photon absorption spectra in nm. [b] lmax of the one-
photon fluorescence spectra in nm. [c] Stokes shift in cmꢀ1. [d] Fluores-
cence quantum yield. [e] lmax of the two-photon absorption spectra in
nm. [f] The peak two-photon absorptivity in GM, where 1 GM (Goppert–
Two-photon cross sections: The two-photon cross section
(dTPA) was measured by means of the two-photon-induced-
fluorescence measurement technique with nanosecond laser
pulses as previously reported.[17b,20b]
Mayer)=1ꢁ10ꢀ50 cm4 sphotonꢀ1. [g] Two-photon action cross section in
ð2Þ
GM at l
.
max
Figure 3 shows the two-photon excited spectra for 1a–5a
in toluene. All compounds show large two-photon cross sec-
tions in the range of dmax =700–2300 GM at lðm2aÞx =780–
1030 nm. The normalized one-photon absorption, emission,
and the two-photon excitation spectra for 1a are depicted in
Figure 4. The corresponding spectra for other compounds
are shown in Figure S1 in the Supporting Information. The
results are summarized in Table 1.
ð2Þ
Figure 4 shows that l /2 of 1a is located at a shorter
max
wavelength than the lowest energy absorption maximum
(lmax). This is consistent with the prediction that the two-
photon allowed states for quadrupoles are at a higher
energy than the Franck–Condon states.[25] Nevertheless,
there is a significant overlap between the one- and two-
photon spectra in terms of the total absorption energy. Per-
haps this may be the reason why dmax values for 1–4 are so
large. In addition, compounds 3a, 3b, and 3c’ show appreci-
Figure 2. Normalized one-photon fluorescence spectra for 1a–5a in tolu-
ene.
that the emission occurs from the lowest excited state with
the largest oscillator strength. The peak positions in the ab-
sorption and fluorescence spectra are summarized in
Table 1. As already noted in the absorption spectra, the
fluorescence spectra exhibit a monotonous bathochromic
shift with increasing acceptor strength. All compounds show
large Stokes shifts ranging from 1044 to 2409 cmꢀ1 for 2c’
and 5a, respectively (Table 1). As expected, the fluorescence
Stokes shift increases as the acceptor strength increases.
This means that the energy gap between the ground and the
excited states decreases monotonically in the order, 1a>
2a>3a>4a (Table 1). As the acceptor strength increases,
the charge-transfer character of the excited electronic state
increases. This leads to the prediction that the solvation
energy of the excited electronic state becomes large and the
fluorescence Stokes shift will increase too. Moreover, the
Figure 3. Two-photon spectra for 1a–5a in toluene.
Chem. Eur. J. 2005, 11, 4191 – 4198
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B. R. Cho, S.-J. Jeon et al.
As stated above, the acceptor group is expected to stabilize
the excited state more than the ground state. This is expect-
ed to diminish the energy gap between the ground and two-
photon allowed states, which would in turn increase the two-
photon cross section, because the smaller the energy gap,
the higher will be the excitation probability. This result un-
derlines the importance of ICT in obtaining a TPA chromo-
phore with a large dmax. On the other hand, the values of
dmax for 2a–c are smaller than those of 1a–c, despite similar
lmax. When the donor is changed from N(Hex)2 to N(i-
amyl)tol to NAr2, dmax of 1a–c increases gradually. However,
no clear trend is observed for 2–4 with the same variation of
the donors. Moreover, the dmax value of 5a is much smaller
than that of 4a. This result underlines the importance of the
symmetry consideration in the design of efficient two-
photon materials. A similar result has been previously repor-
ted.[17f]
~
&
Figure 4. One-photon absorption ( ), fluorescence ( ), and two-photon
*
spectra ( ) for 1a in toluene. The two-photon spectrum is plotted against
The values of Fdmax, the two-photon excited fluorescence
(TPEF) action cross section, increase in the order 5<4<2<
1<3. The much smaller Fdmax values for 5a and 4a–c rela-
tive to those of the others is attributed to the smaller dmax
and/or fluorescence quantum yields. Fdmax values were opti-
mized by the use of p-cyanophenyl groups at the 9,10-posi-
tions. Hence, Fdmax values for 3a–c are in the range of
half the wavelength (twice the photon energy).
able two-photon absorption into the lowest excited states
(Figure S1). Moreover, the values of l
with the electron-withdrawing ability of the 9,10-substituents
(Table 2). This indicates an interesting possibility that the
ð2Þ
for 1–4 increase
max
1000–1480 GM, which are among the largest values reported
ð2Þ
in the literature. Also, l
values of 3a–c are close to
max
Table 2. Effect of the p center on the two-photon absorption properties
of D–p–D derivatives.
800 nm. Therefore, 3a–c may be useful for applications that
use a Ti/sapphire laser as the excitation source. Finally,
Fdmax is usually larger when NAr2 is used as the donor. The
special effect of the NAr2 donor has been previously report-
ed.[23,24]
[a]
fl [b]
max
ð2Þ [d]
[e]
[f]
lmax
l
F[c]
l
dmax
Fdmax
max
A1a[g,h]
1a[i]
410
455
465
410
411
454
490
587
475
566
455
487
582
456
457
483
536
656
528
643
0.88
0.78
0.63
0.86
0.93
0.82
0.69
0.11
0.87
0.15
730
800
795
740
745
780
830
990
830
990
995
1100
340
1730
805
1340
1750
2290
1640
2490
880
860
530
1490
750
1100
1210
250
1430
370
B1a[j,k]
C1a[l,m]
A1c[g,n]
1c[i]
Effect of the p center: The effects of the p center on the
one- and two-photon absorption properties of a variety of
D–p–D derivatives are summarized in Table 2. Comparison
of the data for 1 and 4 with those for the closely related 1,4-
A4a[g,o]
4a[i]
A4c[g,p]
4c[i]
bis(styryl)benzene derivatives (A1 and A4) reveals that lmax
,
fl
max
ð2Þ
l
, l , and dmax values increase significantly as the p
[a–f] See Table 1 for the footnotes. [g] Ref. [17c]. [h] 1,4-Bis(p-dibutyl-
aminostyryl)benzene. [i] This work. [j] Ref. [19a]. [k] 2,6-Bis(p-dibutyl-
aminostyryl)dithienothiophene. [l] Ref. [19b]. [m] 2,7-Bis(p-dioctylami-
nostyryl)-9,10-dihydrophenanthrene. [n] [p-(N-phenyl-N-toly)aminostyr-
yl]benzene. [o] 1,4-Bis(p-dibutylaminostyryl)-2,5-dicyanobenzene. [p] 1,4-
Bis(p-diphenylamino)-2,5-dicyanobenzene.
max
center is changed from a phenyl to an anthryl group
(Table 2).[17d] For instance, the values of lmax, lmfl ax, l , and
ð2Þ
max
dmax for 1a are 455, 487, 800 nm, and 1100 GM, respectively,
which are significantly larger than lmax =410, lfml ax =455,
ð2Þ
l
=730 nm, and dmax =995 GM reported for 1,4-bis(p-di-
max
butylaminostyryl)benzene (A1a). Essentially, the same
changes can be seen if the data for 1c, 4a, and 4c are com-
pared with A1c, A4a, and A4c (Table 2). Whereas, the fluo-
rescence quantum yields always decrease for the same varia-
tion of the fluorophore structure. The results can be readily
attributed to the more extended conjugation by the anthryl
than the phenyl group. As the conjugation length increases,
the energy gap between the ground state and the one- and
two-photon allowed states is expected to decrease in order
wavelength of the maximum two-photon cross section could
be tuned by the use of an appropriate substituent. Further-
ð2Þ
more, the longest l
is noted for 5a at 2lmax, as predicted
max
for a dipolar molecule, although we cannot rule out the pos-
ð2Þ
sibility that the real l
for 5a may exist at a longer wave-
max
length (Figure S1).[25] Interestingly, l
for 1–3 appear near
ð2Þ
max
800 nm (Table 2). This turns out to be important for practi-
cal applications because most of the two-photon fluores-
cence (TPF) microscopy uses an excitation laser with a
to increase the lmax, lðm2aÞx, and dmax values (vide supra). It
fl
would also stabilize the emitting states to increase l
and
max
wavelength of ꢁ800 nm.
decrease the fluorescence quantum yield (vide supra).
ð2Þ
The dmax value of 1a is 1100 GM at l =800 nm. Except
The values of lmax, lfml ax, and fluorescence quantum yields
of phenyl (A) and dihydrophenanthryl (C1a) derivatives
max
for 2a–c, the dmax value increases with the acceptor strength.
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2,6-Bis(styryl)anthracene Derivatives
FULL PAPER
J=9.0 Hz, 2H), 7.40 (s, 2H), 7.14 (d, J=9.0 Hz, 2H), 2.38 ppm (s, 6H;
CH3).
are almost the same, although dmax values are much larger
for C1a than A1a. This may be caused by the increased mo-
lecular weight. As the molecular size increases, the density
of states will increase, providing more effective coupling
channels between the ground and two-photon allowed
states, which would in turn increase the TPA cross section.
2,6-Dimethyl-9,10-di(p-cyanophenyl)anthracene (8’’): BuLi (1.6m in hex-
anes, 14 mL, 22 mmol) was added to a two-necked flask containing anhy-
drous THF (40 mL) under Ar. After cooling the solution to ꢀ788C, a so-
lution of p-cyanobromobenzene (4.6 g, 25 mmol) in THF (20 mL) was
added by means of a syringe. The solution was stirred for 1 h, and 6
(2.0 g, 8.5 mmol in 40 mL THF) was slowly added. The mixture was stir-
red (1. 1 h at ꢀ788C, 2. 4 h at RT), poured into water, and extracted with
CH2Cl2. The solvent was evaporated and acetic acid (80 mL) was added.
The resulting solution was heated to 1208C, and zinc powder (6.0 g) was
added in small portions over a period of 30 min. After continuous stirring
for 3 h at 1208C, the mixture was cooled to RT, poured into water, and
extracted with CH2Cl2. The solvent was evaporated and the residue was
purified by column chromatography (silica gel, hexane/CH2Cl2 1:1).
Yield: 1.9 g (56%); m.p. >3008C; 1H NMR (300 MHz, CDCl3, 258C,
TMS): d=7.93 (d, J=9.0 Hz, 4H), 7.58 (d, J=9.0 Hz, 4H), 7.42 (d, J=
9.0 Hz, 2H), 7.25 (s, 2H), 7.20 (d, J=9.0 Hz, 2H), 2.40 ppm (s, 6H, CH3).
On the other hand, when anthryl and dithienothienyl groups
fl
max
are used as the p center, both lmax and l increase signifi-
cantly and the fluorescence quantum yields decrease. This
indicates the efficiency of these groups in facilitating intra-
molecular change transfer from the donor to the p center.
From a practical perspective, the anthracene derivatives
(1–4) are advantageous for applications that utilize the two-
ð2Þ
photon absorption because l
are in the range of 780–
max
990 nm and dmax values are larger. On the other hand, 1,4-
bis(styryl)benzene derivatives (A1 and A4) may be more
useful for applications where the two-photon excited fluo-
rescence (Fdmax) is important.
Dimethyl-9,10-dibromoanthracene: Bromine (1.1 mL, 21 mmol) was
slowly added through a dropping funnel into a solution of 2,6-dimethy-
lanthracene (2.0 g, 9.7 mmol) in CCl4 (200 mL) at 08C. The solution was
stirred for 5 h at 08C, and aqueous sodium hydrosulfite (20%, 50 mL)
was added. The organic layer was separated and washed with water, and
the solvent was evaporated to obtain 2,6-dimethyl-9,10-dibromoanthra-
cene. Yield: 3.2 g (91%); m.p. 188–1908C; 1H NMR (300 MHz, CDCl3,
258C, TMS): d=8.44 (d, J=9.0 Hz, 2H), 8.29 (s, 2H), 7.42 (d, J=9.0 Hz,
2H), 2.61 ppm (s, 6H; CH3).
Conclusion
Herein, the TPA properties of a series of 2,6- and 2,7-bis-
2,6-Dimethyl-9,10-dicyanoanthracene (9): 2,6-Dimethyl-9,10-dibromoan-
thracene (1.8 g, 5.0 mmol), pyridine (4.0 mL), and CuCN (0.98 g,
11 mmol) were added to a pressure tube with a plunger valve and a mag-
netic bar under Ar. The tube was heated at 2208C for 4 h. The mixture
was cooled to room temperature and then poured with vigorous stirring
into methanol. The precipitate that formed was collected on a filter
paper and washed with methanol. The crude product was separated by
column chromatography (silica gel, CH2Cl2). Yield: 1.1 g (82%); m.p.
292–2948C; 1H NMR (300 MHz, CDCl3, 258C, TMS): d=8.36 (d, J=
9.0 Hz, 2H), 8.22 (s, 2H), 7.64 (d, J=9.0 Hz, 2H), 2.67 ppm (s, 6H;
CH3).
(styryl)anthracene derivatives have been investigated. Over-
ð2Þ
all, a parallel increase in lmax, lfml ax, l , and dmax values with
max
the donor–acceptor strength is observed, although 2,7-deriv-
atives have significantly smaller dmax values. All compounds
exhibit large two-photon cross sections that are useful for a
variety of applications mentioned in the Introduction. More-
ð2Þ
over, the values of lmax, lfml ax, l , and dmax increased signifi-
max
cantly on changing the p center from a phenyl to an anthryl
group. This result underlines the usefulness of an anthryl
group as the p center for the design of efficient two-photon
fluorophores.
2,6-Bis[(diethoxyphosphoryl)methyl]anthracene (10’):
A mixture of 7
(1.5 g, 7.3 mmol), NBS (2.7 g, 15 mmol), and benzoyl peroxide (71 mg,
0.29 mmol) in benzene (100 mL) was refluxed for 3 h. The mixture was
poured into methanol, and the precipitate that collected was dried by
vacuum. This intermediate was added to P(OEt)3 (50 mL), and the result-
ing solution was refluxed for 12 h. The solvent was removed in vacuo,
and the residue was purified by column chromatography (silica gel, ethyl
acetate/CH2Cl2 2:1). Yield: 1.9 g (53%); m.p. 140–1428C; 1H NMR
(300 MHz, CDCl3, 258C, TMS): d=8.34 (s, 2H), 7.96 (d, J=9.0 Hz, 2H),
7.88 (s, 2H), 7.44 (d, J=9.0 Hz, 2H), 4.04 (m, 8H; OCH2), 3.36 (d, J=
21 Hz, 4H; ArCH2P), 1.25 ppm (t, J=7.5 Hz, 12H; CH3).
Experimental Section
Synthesis of 2,6-bis(styryl)anthracene derivatives
2,6-Dimethylanthracene (7): A mixture of Al powder (8.5 g, 0.31 mol),
HgCl2 (0.17 g, 0.63 mmol), cyclohexanol (250 mL), and CCl4 (1.7 mL)
was heated under Ar for 4 h at 858C and then for 12 h at 1608C. To this
flask, 2,6-dimethylanthraquinone (6, 6.0 g, 25 mmol) was added and the
mixture was refluxed for 48 h. The solvent was removed in vacuo, and
the product was purified by column chromatography (silica gel, hexane/
dichloromethane 20:1). Yield: 3.8 g (73%); m.p. 235–2378C; 1H NMR
(300 MHz, CDCl3, 258C, TMS): d=8.24 (s, 2H), 7.86 (d, J=9.0 Hz, 2H),
7.71 (s, 2H), 7.25 (d, J=9.0 Hz, 2H), 2.52 ppm (s, 6H; CH3).
Synthesis of phosphonate derivatives 10’’–10’’’’: These derivatives were
synthesized from 8’, 8’’, and 9 by the same procedure as described for 10’.
2,6-Bis[(diethoxyphosphoryl)methyl]-9,10-diphenylanthracene
(10’’):
Yield: 62%; m.p. 180–1828C; 1H NMR (300 MHz, CDCl3, 258C, TMS):
d=7.62 (d, J=9.0 Hz, 4H), 7.57 (d, J=9.0 Hz, 4H), 7.53 (s, 2H), 7.46 (d,
J=9.0 Hz, 2H), 7.45 (d, J=9.0 Hz, 2H), 7.30 (d, J=9 Hz, 2H), 3.97 (m,
8H; OCH2), 3.19 (d, J=21 Hz, 4H; ArCH2P), 1.19 ppm (t, J=7.5 Hz,
12H; CH3).
2,6-Dimethyl-9,10-diphenylanthracene (8’): PhMgBr (1.0m in THF,
51 mL) was added to a solution of 6 (4.0 g, 17 mmol) in THF (150 mL) at
RT. After stirring for 4 h at RT, the solution was poured into water. The
solvent was evaporated, and the product was extracted with CH2Cl2. The
resulting solution was evaporated, and acetic acid (80 mL) was added.
The residue in acetic acid was heated to 1208C, and zinc powder (6.0 g)
was added in small portions over a period of 30 min. After continuous
stirring for 3 h at 1208C, the mixture was cooled to RT, poured into
water, and extracted with CH2Cl2. The solvent was evaporated and the
product was purified by column chromatography (silica gel, hexane).
Yield: 2.2 g (30%); m.p. 234–2368C; 1H NMR (300 MHz, CDCl3, 258C,
TMS): d=7.59 (m, 4H), 7.56 (m, 4H), 7.46 (d, J=9.0 Hz, 2H), 7.45 (d,
2,6-Bis[(diethoxyphosphoryl)methyl]-9,10-bis(p-cyanophenyl)anthracene
(10’’’): Yield: 49%; m.p. >3008C; 1H NMR (300 MHz, CDCl3, 258C,
TMS): d=7.94 (d, J=9.0 Hz, 4H), 7.59 (d, J=8.5 Hz, 4H), 7.50 (d, J=
9.0 Hz, 2H), 7.40 (s, 2H), 7.35 (d, J=9 Hz, 2H), 3.99 (m, 8H; OCH2),
3.19 (d, J=24 Hz, 4H; ArCH2P), 1.21 ppm (t, J=7.5 Hz, 12H; CH3).
2,6-Bis[(diethoxyphosphoryl)methyl]-9,10-dicyanoanthracene
(10’’’’):
1
Yield: 44%; m.p. >3008C; H NMR (300 MHz, CDCl3, 258C, TMS): d=
8.46 (d, J=9.0 Hz, 2H), 8.35 (s, 2H), 7.83 (d, J=9.0 Hz, 2H), 4.10 (m,
8H; OCH2), 3.45 (d, J=24 Hz, 4H; ArCH2P), 1.30 ppm (t, J=7.5 Hz,
12H; CH3).
Chem. Eur. J. 2005, 11, 4191 – 4198
www.chemeurj.org
ꢀ 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
4195

B. R. Cho, S.-J. Jeon et al.
2,6-Bis(p-dihexylaminostyryl)anthracene (1a): LDA (1.5m in cyclohex-
ane, 0.34 mL, 0.51 mmol) was added dropwise to a stirred solution of 10’
(100 mg, 0.21 mmol) in anhydrous THF (15 mL) at ꢀ788C under Ar. The
mixture was stirred for 1 h, and then 4-(N,N-dihexylamino)benzaldehyde
(150 mg, 0.52 mmol) in THF (5 mL) was added dropwise over a period of
10 min. After the mixture was stirred for 2 h at ꢀ788C and for 6 h at RT,
water (1 mL) was added and the solvent was evaporated. The residue
was dissolved in CH2Cl2 and washed several times with water. The sol-
vent was evaporated, and the crude product was separated by column
chromatography (silica gel, hexane/CH2Cl2 3:1–1:1). Yield: 58 mg (37%);
m.p. 182–1848C; 1H NMR (300 MHz, CDCl3, 258C, TMS): d=8.27 (s,
2H), 7.92 (d, J=9.0 Hz, 2H), 7.85 (s, 2H), 7.74 (d, J=9.0 Hz, 2H), 7.44
(d, J=7.5 Hz, 4H), 7.18 (d, J=16.5 Hz, 2H), 7.08 (d, J=16.5 Hz, 2H),
6.65 (d, J=7.5 Hz, 4H), 3.30 (t, J=7.5 Hz, 8H; NCH2), 1.61 (m, 8H),
1.31 (m, 24H), 0.92 ppm (t, J=6.0 Hz, 12H; CH3); 13C NMR (75 MHz,
CDCl3): d=148.09, 135.31, 132.23, 131.65, 129.40, 128.52, 128.09, 125.86,
124.79, 124.12, 123.42, 111.88, 100.17, 51.33, 31.98, 27.55, 27.10, 22.94,
14.30 ppm; elemental analysis calcd (%) for C54H72N2: C 86.57, H 9.69, N
3.74; found: C 86.59, H 9.68, N 3.68.
2,6-Bis(4-diphenylaminostyryl)-9,10-diphenylanthracene (2c’): Synthe-
sized by the same procedure as described for 1a from 10’’ and the appro-
priate benzaldehyde. Yield: 51%; m.p. >3008C; 1H NMR (300 MHz,
CDCl3, 258C, TMS): d=7.62 (m, 10H), 7.55 (s, 2H), 7.51 (d, J=9.0, 2H),
7.50 (d, J=9.0 Hz, 2H), 7.36 (d, J=9.0 Hz, 4H), 7.25 (m, 8H), 7.10 (m,
4H), 7.07 (d, J=15 Hz, 2H), 7.05 (d, J=15 Hz, 2H), 7.03 (d, J=9 Hz,
8H), 7.01 ppm (d, J=9 Hz, 4H); 13C NMR (75 MHz, CDCl3): d=147.72,
147.58, 139.10, 137.21, 134.53, 134.37, 131.75, 131.58, 130.48, 130.00,
129.48, 128.73, 128.65, 127.79, 127.69, 127.63, 126.20, 124.70, 123.75,
123.25, 122.48 ppm; elemental analysis calcd (%) for C66H48N2: C 91.21,
H 5.57, N 3.22; found: C 91.22, H 5.60, N 3.20.
2,6-Bis(4-dihexylaminostyryl)-9,10-bis(p-cyanophenyl)anthracene
(3a):
Synthesized by the same procedure as described for 1a from 10’’’ and the
appropriate benzaldehyde. Yield: 53%; m.p. 244–2428C; 1H NMR
(300 MHz, CDCl3, 258C, TMS): d=7.96 (d, J=9.0 Hz, 4H), 7.65 (d, J=
9.0 Hz, 2H), 7.62 (d, J=9.0 Hz, 4H), 7.44 (d, J=9.0 Hz, 2H), 7.36 (d, J=
9.0 Hz, 4H), 7.30 (s, 2H), 7.02 (d, J=16.5 Hz, 2H), 6.86 (d, J=16.5 Hz,
2H), 6.58 (d, J=9.0 Hz, 4H), 3.27 (t, J=6.5 Hz, 8H; NCH2), 1.58 (m,
8H), 1.31 (m, 24H), 0.90 ppm (t, J=6.0 Hz, 12H; CH3); 13C NMR
(75 MHz, CDCl3): d=148.27, 144.48, 135.77, 135.05, 132.67, 132.49,
130.35, 129.86, 129.18, 128.23, 126.62, 124.19, 123.99, 123.56, 123.34,
119.21, 111.96, 111.75, 51.27, 31.95, 27.49, 27.05, 22.92, 14.30 ppm; ele-
mental analysis calcd (%) for C72H70N4: C 87.23, H 7.12, N 5.65; found:
C 87.31, H 7.13, N 5.56.
2,6-Bis{4-[N-isoamyl-N-(p-tolyl)amino]styryl}anthracene (1b): Synthe-
sized by the same procedure as described for 1a from 10’ and the appro-
priate benzaldehyde. Yield: 42%; m.p. 210–2128C; 1H NMR (300 MHz,
CDCl3, 258C, TMS): d=8.28 (s, 2H), 7.92 (d, J=9.0 Hz, 2H), 7.87 (s,
2H), 7.74 (d, J=9.0 Hz, 2H), 7.42 (d, J=9.0 Hz, 4H), 7.20 (d, J=16 Hz,
2H), 7.16 (d, J=9.0 Hz, 4H), 7.12 (d, J=16 Hz, 2H), 7.06 (d, J=9.0 Hz,
4H), 6.82 (d, J=9.0 Hz, 4H), 3.71 (t, J=7.5 Hz, 4H; NCH2), 2.36 (s, 6H;
tolyl-CH3), 1.62 (m, 6H), 0.94 ppm (d, J=6.0 Hz, 12H; CH(CH3)2);
13C NMR (75 MHz, CDCl3): d=148.43, 145.09, 135.08, 133.51, 132.25,
131.74, 130.41, 129.11, 128.64, 128.05, 127.74, 126.38, 126.06, 125.63,
125.06, 123.37, 116.97, 51.10, 36.36, 26.59, 22.91, 21.15 ppm; elemental
analysis calcd (%) for C54H56N2: C 88.48, H 7.70, N 3.82; found: C 88.49,
H 7.80, N 3.77.
2,6-Bis{4-[N-isoamyl-N-(p-tolyl)amino]styryl}-9,10-bis(p-cyanophenyl)an-
thracene (3b): Synthesized by the same procedure as described for 1a
from 10’’’ and the appropriate benzaldehyde. Yield: 49%; m.p. >3008C;
1H NMR (300 MHz, CDCl3, 258C, TMS): d=7.96 (d, J=9.0 Hz, 4H),
7.66 (d, J=9.0 Hz, 2H), 7.62 (d, J=9.0, 4H), 7.46 (d, J=9.0 Hz, 2H),
7.33 (s, 2H), 7.32 (d, J=9.0 Hz, 4H), 7.15 (d, J=7.5 Hz, 4H), 7.04 (d, J=
15 Hz, 2H), 7.02 (d, J=9.0 Hz, 4H), 6.90 (d, J=15 Hz, 2H), 6.74 (d, J=
9.0 Hz, 4H), 3.68 (t, J=9.0 Hz, 4H; NCH2), 2.35 (s, 6H; tolyl-CH3), 1.58
(m, 6H), 0.92 ppm (t, J=6.0 Hz, 12H; CH(CH3)2); 13C NMR (75 MHz,
CDCl3): d=148.65, 144.83, 144.34, 135.57, 135.24, 133.89, 132.70, 132.46,
130.43, 130.10, 129.88, 129.27, 127.84, 127.22, 126.73, 125.39, 124.78,
124.45, 123.54, 119.15, 116.40, 112.05, 51.06, 36.27, 26.53, 22.85,
21.14 ppm; elemental analysis calcd (%) for C72H70N4: C 87.23, H 7.12, N
5.65; found: C 87.384, H 6.89, N 5.72.
2,6-Bis{[4-N,N-di(p-tert-butylphenyl)amino]styryl}anthracene (1c): Syn-
thesized by the same procedure as described for 1a from 10’ and the ap-
propriate benzaldehyde. Yield: 56%; m.p. >3008C; 1H NMR (300 MHz,
CDCl3, 258C, TMS): d=8.28 (s, 2H), 7.92 (d, J=9.0 Hz, 2H), 7.88 (s,
2H), 7.73 (d, J=9.0 Hz, 2H), 7.42 (d, J=9.0 Hz, 4H), 7.28 (d, J=9.0 Hz,
8H), 7.18 (m, 4H), 7.06 (d, J=9.0 Hz, 12H), 1.32 ppm (s, 36H; tBu);
13C NMR (75 MHz, CDCl3): d=148.04, 146.16, 145.00, 134.89, 132.24,
131.78, 130.88, 128.88, 128.71, 127.52, 126.91, 126.77, 126.31, 126.19,
124.41, 123.34, 122.85, 34.56, 31.68 ppm; elemental analysis calcd (%) for
C70H72N2: C 89.31, H 7.71, N 2.98; found: C 89.24, H 7.82, N 2.94.
2,6-Bis(4-diphenylaminostyryl)-9,10-bis(p-cyanophenyl)anthracene (3c’):
Synthesized by the same procedure as described for 1a from 10’’’ and the
appropriate benzaldehyde. Yield: 38%; m.p. >3008C; 1H NMR
(300 MHz, CDCl3, 258C, TMS): d=7.94 (d, J=9.0 Hz, 6H), 7.80 (d, J=
9.0 Hz, 6H), 7.74 (d, J=9.0 Hz, 6H), 7.62 (d, J=9.0 Hz, 6H), 7.56 (s,
3H), 7.48 (d, J=9.0 Hz, 6H), 7.22 (d, J=15 Hz, 3H), 7.14 (d, J=9.0 Hz,
6H), 7.06 ppm (d, J=15 Hz, 3H); elemental analysis calcd (%) for
C72H70N4: C 87.23, H 7.12, N 5.65; found: C 87.38, H 7.04, N 5.58.
2,6-Bis(4-dihexylaminostyryl)-9,10-diphenylanthracene (2a): Synthesized
by the same procedure as described for 1a from 10’’ and the appropriate
benzaldehyde. Yield: 46%; m.p. 140–1428C; 1H NMR (300 MHz, CDCl3,
258C, TMS): d=7.60 (m, 10H), 7.51 (d, J=9.0 Hz, 2H), 7.50 (d, J=
9.0 Hz, 2H), 7.49 (s, 2H), 7.33 (d, J=9.0 Hz, 4H), 7.00 (d, J=15 Hz,
2H), 6.89 (d, J=15 Hz, 2H), 6.57 (d, J=9 Hz, 4H), 3.26 (t, J=7.5 Hz,
8H; NCH2), 1.57 (m, 8H), 1.31 (m, 24H), 0.90 ppm (t, J=6.0 Hz, 12H;
CH3); 13C NMR (75 MHz, CDCl3): d=148.02, 139.38, 136.77, 134.86,
131.66, 130.42, 129.79, 129.30, 128.66, 128.05, 127.62, 127.40, 125.14,
124.73, 124.30, 122.54, 111.80, 51.29, 31.96, 27.51, 27.06, 22.92, 14.29 ppm;
elemental analysis calcd (%) for C66H80N2: C 87.95, H 8.95, N 3.11;
found: C 87.99, H 8.85, N 3.03.
2,6-Bis(4-dihexylaminostyryl)-9,10-dicyanoanthracene (4a): Synthesized
by the same procedure as described for 1a from 10’’’’ and the appropriate
benzaldehyde. Yield: 51%; m.p. 208–2108C; 1H NMR (300 MHz, CDCl3,
258C, TMS): d=8.26 (d, J=9.0 Hz, 2H), 8.10 (s, 2H), 7.94 (d, J=9.0 Hz,
2H), 7.42 (d, J=9.0 Hz, 4H), 7.24 (d, J=15 Hz, 2H), 7.00 (d, J=15 Hz,
2H), 6.62 (d, J=9.0 Hz, 4H), 3.29 (t, J=7.5 Hz, 8H; NCH2), 1.61 (m,
8H), 1.34 (m, 24H), 0.92 ppm (t, J=6.0 Hz, 12H; CH3); 13C NMR
(75 MHz, CDCl3): d=149.18, 145.65, 136.15, 132.69, 132.12, 130.56,
129.38, 128.76, 126.21, 125.04, 124.65, 123.77, 112.65, 101.45, 51.25, 31.93,
27.44, 27.03, 22.91, 14.30 ppm; elemental analysis calcd (%) for C56H70N4:
C 84.16, H 8.83, N 7.01; found: C 83.94, H 8.66, N 6.90.
2,6-Bis{4-[N-isoamyl-N-(p-tolyl)amino]styryl}-9,10-diphenylanthracene
(2b): Synthesized by the same procedure as described for 1a from 10’’
and the appropriate benzaldehyde. Yield: 38%; m.p. 260–2628C;
1H NMR (300 MHz, CDCl3, 258C, TMS): d=7.61 (m, 10H), 7.51 (d, J=
9.0 Hz, 2H), 7.50 (d, J=9.0, 2H), 7.49 (s, 2H), 7.32 (d, J=9.0 Hz, 4H),
7.12 (d, J=9.0 Hz, 4H), 7.03 (d, J=15 Hz, 2H), 7.01 (d, J=9.0 Hz, 4H),
6.94 (d, J=15 Hz, 2H), 6.76 (d, J=9.0 Hz, 4H), 3.68 (t, J=7.5 Hz, 4H;
NCH2), 2.34 (s, 6H; tolyl-CH3), 1.54 (m, 6H), 0.92 ppm (d, J=9.0 Hz,
12H; CH(CH3)2); 13C NMR (75 MHz, CDCl3): d=152.27, 148.27, 145.04,
139.23, 136.97, 134.62, 133.36, 131.62, 130.43, 130.35, 129.87, 129.00,
128.70, 128.01, 127.68, 127.44, 125.81, 125.65, 124.85, 122.48, 116.96, 51.05,
36.27, 26.54, 22.86, 21.11 ppm; elemental analysis calcd (%) for C66H64N2:
C 89.55, H 7.29, N 3.16; found: C 89.58, H 7.21, N 3.15.
2,6-Bis{4-[N-isoamyl-N-(p-tolyl)amino]styryl}-9,10-dicyanoanthracene
(4b): Synthesized by the same procedure as described for 1a from 10’’’’
and the appropriate benzaldehyde. Yield: 47%; m.p. >3008C; 1H NMR
(300 MHz, CDCl3, 258C, TMS): d=8.25 (d, J=9.0 Hz, 2H), 8.08 (s, 2H),
7.90 (d, J=9.0, 2H), 7.37 (d, J=7.5 Hz, 4H), 7.23 (d, J=16 Hz, 2H), 7.20
(d, J=9.0 Hz, 4H), 7.07 (d, J=9.0 Hz, 4H), 7.00 (d, J=16 Hz, 2H), 6.74
(d, J=9.0 Hz, 4H), 3.70 (t, J=7.5 Hz, 4H; NCH2), 2.38 (s, 6H; tolyl-
CH3), 1.62 (m, 6H), 0.94 ppm (d, J=9.0 Hz, 12H; CH(CH3)2); 13C NMR
(75 MHz, CDCl3): d=149.25, 144.54, 139.16, 134.55, 132.74, 132.13,
132.01, 130.57, 128.40, 127.69, 126.22, 126.04, 122.98, 122.46, 116.46,
115.64, 109.65, 106.90, 51.13, 36.26, 26.57, 22.88, 21.23 ppm; elemental
4196
ꢀ 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
Chem. Eur. J. 2005, 11, 4191 – 4198

2,6-Bis(styryl)anthracene Derivatives
FULL PAPER
analysis calcd (%) for C56H54N4: C 85.89, H 6.95, N 7.15; found: C 86.14,
H 6.82, N 7.09.
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10’ and the appropriate benzaldehyde. Yield: 44%; m.p. >3008C;
1H NMR (300 MHz, CDCl3, 258C, TMS): d=8.34 (d, J=9.0 Hz, 2H),
8.21 (s, 2H), 8.00 (d, J=9.0 Hz, 2H), 7.43 (d, J=7.5 Hz, 4H), 7.30 (d, J=
7.5 Hz, 8H), 7.16 (m, 4H), 7.06 (d, J=7.5 Hz, 12H), 1.34 ppm (s, 36H;
tBu); 13C NMR (75 MHz, CDCl3): d=149.05, 146.69, 139.16, 132.40,
132.23, 129.34, 128.13, 127.94, 126.43, 126.34, 124.87, 124.53, 123.04,
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7.15, N 5.58.
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2,7-Bis(4-dihexylaminostyryl)-9,10-dicyanoanthracene (5a): The 2,7-
bis(p-dihexylaminostyryl)anthracene derivative (5a) was synthesized by
the same procedure as described for 4a except that 2,7-dimethylanthra-
quinone was used as the starting material. Yield: 55%; m.p. 161–1628C;
1H NMR (300 MHz, CDCl3, 258C, TMS): d=8.30 (d, J=9.0 Hz, 2H),
8.21 (s, 2H), 7.96 (d, J=9.0 Hz, 2H), 7.45 (d, J=6.0 Hz, 4H), 7.28 (d, J=
18 Hz, 2H), 7.05 (d, J=15 Hz, 2H), 6.63 (d, J=9.0 Hz, 4H), 3.29 (t, J=
7.5 Hz, 8H; NCH2), 1.61 (m, 8H), 1.35 (m, 24H), 0.92 ppm (t, J=6.0 Hz,
12H; CH3); 13C NMR (75 MHz, CDCl3): d=152.28, 148.81, 140.09,
139.99, 133.66, 128.83, 127.37, 126.23, 124.74, 123.59, 121.87, 111.76,
106.40, 51.31, 31.97, 27.55, 27.08, 22.95, 14.32 ppm; elemental analysis
calcd (%) for C56H70N4: C 84.16, H 8.83, N 7.01; found: C 83.78, H 8.91,
N 6.83.
Spectroscopic measurements: All spectroscopic measurements were per-
formed with sample solutions in toluene (spectroscopic grade, Aldrich).
Absorption spectra were recorded on a Hewlett–Packard8453 diode
array spectrophotometer, and the fluorescence spectra were obtained
with a Amico Bowman series2 luminescence spectrometer. The fluores-
cence quantum yield was determined with fluorescein, coumarine307 or
RhodamineB as the reference by the literature method.[26]
The two-photon absorption cross section of 1–5 was measured with the
two-photon-induced fluorescence method that employed nanosecond
laser pulses and Equation (1):
SSFrꢀrcr
SrFSꢀScS
ð1Þ
d ¼
dr
in which the subscripts s and r represent the sample and reference mole-
cules.[17b,20b] The intensity of the signal collected by a PMT detector was
denoted as S. F is the fluorescence quantum yield. f is the overall fluo-
rescence 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. Samples were dissolved in toluene at
concentrations of 1.0ꢁ10ꢀ5 m and the two-photon-induced-fluorescence
intensity was measured at 750–1050 nm. The reference standards (r) used
were fluorescein (8ꢁ10ꢀ5 m in water, pH 11), coumarin307 (1ꢁ10ꢀ5 m in
MeOH), and RhodamineB (1ꢁ10ꢀ5 m in MeOH), whose two-photon
properties have been well characterized in the literature.[27] The intensi-
ties of the two-photon-induced fluorescence spectra of the reference and
sample emitted at the same excitation wavelength were determined. The
TPA cross section was calculated according to Equation (1).
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Published online: April 29, 2005
4198
ꢀ 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
Chem. Eur. J. 2005, 11, 4191 – 4198