Triarylethene-based extended π-systems: Programmable synthesis and photophysical properties

Article abstract of DOI:10.1021/jo0477401

(Chemical Equation Presented) On the basis of an efficient Pd-catalyzed triarylation to a vinylsilane platform, four types of structurally well-defined triarylethene-based extended π-systems have been prepared very rapidly. From a compound library constructed by the present method, it was possible to find a number of interesting fluorescent materials, as well as interesting fluorescent properties such as aggregation-induced enhanced emission. A useful method for the rapid synthesis and property evaluation has also been developed.

Full text of DOI:10.1021/jo0477401

Triarylethene-Based Extended π-Systems: Programmable
Synthesis and Photophysical Properties
Kenichiro Itami,* Youichi Ohashi, and Jun-ichi Yoshida*
Department of Synthetic Chemistry and Biological Chemistry, Graduate School of Engineering,
Kyoto University, Nishikyo-ku, Kyoto 615-8510, Japan
itami@sbchem.kyoto-u.ac.jp; yoshida@sbchem.kyoto-u.ac.jp
Received December 24, 2004
On the basis of an efficient Pd-catalyzed triarylation to a vinylsilane platform, four types of
structurally well-defined triarylethene-based extended π-systems have been prepared very rapidly.
From a compound library constructed by the present method, it was possible to find a number of
interesting fluorescent materials, as well as interesting fluorescent properties such as aggregation-
induced enhanced emission. A useful method for the rapid synthesis and property evaluation has
also been developed.
Introduction
The carbon-carbon double bond (CdC) is a minimal
unit in π-systems. Therefore, an extended π-system
equipped with π-systems such as aryl groups on the
minimal π-unit CdC (arylethene-based extended π-sys-
tem) would be an interesting target as a functional
material (Figure 1). Although some of the π-systems
within this family, such as stilbene derivatives and poly-
(arylene vinylene)s (1,2-diarylethene-based π-systems),
have already established their utility as functional
materials,1 we felt that an important object still remains
in this class of extended π-systems, namely, exploring
the chemistry (synthesis and properties) of π-systems
based on a highly substituted CdC core (tri- and tetra-
arylethene-based extended π-systems). Since such an
investigation is subject to the establishment of a general
synthetic scheme for multisubstituted olefins, which has
been a formidable challenge for chemists for years, the
chemistry is extremely challenging as a whole.
FIGURE 1. Arylethene-based extended π-systems.
We herein describe a simple but powerful strategy that
enables rapid and systematic assembly of small π-sys-
tems into structurally well-defined triarylethene-based
extended π-systems in a programmable and diversity-
oriented format. By following this strategy, we succeeded
in rapidly making chemical libraries of extended π-sys-
tems, in which a number of interesting fluorescent
organic materials as well as interesting fluorescence
properties were found.
Recently, we have initiated a program directed toward
the development of a programmable and diversity-
oriented synthesis of multisubstituted olefins2 and re-
ported such a synthetic scheme for diarylethenes and
triarylethenes using vinyl(2-pyridyl)silane (1a) as a
platform (eq 1).2c We first reported that the otherwise
difficult Mizoroki-Heck reaction (MHR) of vinylsilanes3,4
was efficiently promoted by appending a catalyst-direct-
ing 2-pyridyl group on silicon.2a,5-8 The suitably posi-
tioned nitrogen atom of the pyridyl group might accel-
erate the rate-determining CdC π-complexation and
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10.1021/jo0477401 CCC: $30.25 © 2005 American Chemical Society
Published on Web 03/11/2005
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J. Org. Chem. 2005, 70, 2778-2792

Triarylethene-Based Extended π-Systems
successive carbopalladation (insertion) events in MHR.2c
Moreover, because of the strong directing effect of the
that (i) all aryl groups assembled stem from readily
available aryl iodides (Ar-I), (ii) the installation at the
desired position can be achieved by the addition of aryl
iodides in the appropriate order, and (iii) simple alter-
ation of addition order in the sequence results in the
production of all possible regio- and stereoisomers of
multisubstituted olefins.
This represents a new synthetic strategy that permits
assembly of π-systems, such as aryl groups, onto a CdC
core in a programmable and diversity-oriented format.11
Clearly, the programmability of the synthesis is at-
tributed to the inherent stereo-electronic bias of the
vinylsilane platform (differentiated reactivity of C-H and
C-Si bonds) as well as the aid of catalyst-directing
(coordinating) groups. It should be noted that this
2-pyridyl group, a hard-to-achieve double-MHR has been
accomplished (1 f 2 f 3), which allows us to install two
aryl groups at two â-C-H bonds in one pot.2c,9 We also
found that the resultant alkenyl(2-pyridyl)silanes (2 and
3) undergo an efficient Hiyama-type silicon-based cross-
coupling reaction (CCR)10 with aryl iodides producing
diarylethenes 4 and triarylethenes 5 in a regio- and
stereoselective manner (eq 1).2b,c Noteworthy features are
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Itami et al.
SCHEME 1. Programmable and
Diversity-Oriented Construction of
Triarylethene-Based Extended π-Systems
FIGURE 2. Aryl groups (π-units) used in this study.
Results and Discussion
1. Assignment of Aryl Groups. In this study, we
used the following 18 aryl groups (or π-units) for the
construction of the triarylethene-based extended π-sys-
tems. For simplicity, we assigned them alphabetically as
described in Figure 2 and used them in compound
assignment.
2. Improved Conditions for Installing Two Aryl
Groups at â-C-H Bonds of Vinylsilanes. Before
initiating this study, we were aware of the need for a
reliable method of installing two aryl groups at â-C-H
bonds of vinylsilanes. Although we have already reported
in our previous paper that the Mizoroki-Heck reaction
(MHR) of vinyl(2-pyridyl)silanes is useful for such a
purpose, this method suffers from limited reaction scope.
For example, by using our previous catalytic conditions
[Pd2(dba)3, P(2-furyl)3, Et3N, THF, 60 °C], the efficient
double-MHR (1 f 2 f 3) of dimethyl(2-pyridyl)vinyl-
silane (1a) occurs only with iodobenzene, 2-iodothiophene,
and 3-iodothiophene.2c
platform strategy differs from the classical olefin syn-
thesis as exemplified by the Wittig reaction and its
analogues, which connect two components with the
creation of a CdC bond, where the selectivity (stereo-
selectivity) depends inevitably on the existing substitu-
ent(s).
Because π-assembling sites are programmed in our
synthesis, we envisaged that the strategic use of bifunc-
tional aryl units (ArX2) in place of a monofunctional unit
(ArX) in the reaction sequence described in eq 1 would
result in the selective production of various types of
triarylethene-based extended π-systems, which are oth-
erwise difficult to construct. The programmable and
diversity-oriented synthetic scheme for triarylethene-
based extended π-systems is shown in Scheme 1. For
example, the CCR of â,â-diarylvinylsilane 3 (double-MHR
product of 1) with aryl dihalides (ArX2) under the
influence of a Pd catalyst and Bu4NF results in the
production of the interesting extended π-system 10. When
the double-MHR/CCR sequence is performed with the
addition order of ArX/ArX2/ArX (1 f 2 f 6 f 11) and
ArX2/ArX/ArX (1 f 7 f 8 f 12), extended π-systems of
interesting structure (11 and 12) are selectively produced.
Moreover, when double-MHR is performed with ArX2
alone, an unprecedented type of polymerization takes
place (1 f 7 f 9). The successive CCR with ArX then
affords the novel cross-conjugated polymer 13, which is
otherwise difficult to construct. The power of this syn-
thetic strategy is apparent, as all of these extended
π-systems can be selectively prepared at will by using
common platforms (1),12 common reactions (MHR and
CCR), and common reagents (ArX and ArX2).
After extensive reinvestigation of catalysts as well as
substrates, we established more general and reliable
procedures for the double-MHR of vinylsilanes. We found
13
14
that the use of P(t-Bu)3 or P[OC6H3(t-Bu)2-2,4]3 as
supporting ligand to palladium substantially broadens
the scope of double-MHR of 1a (eq 2).15,16 Not only aryl
iodides but also aryl bromides as well as heteroaryl
halides and alkenyl halides can be used as substrates
when the Pd/P(t-Bu)3 system is used.17 When aryl iodides
were used as arylating agents, the Pd2(dba)3/P[OC6H3(t-
Bu)2-2,4]3 catalyst was as effective as the Pd/P(t-Bu)3
catalyst in many cases.15 Moreover, we found dimethyl-
(13) Pd/P(t-Bu)3 catalyst for Mizoroki-Heck reactions: (a) Littke,
A. F.; Fu, G. C. J. Am. Chem. Soc. 2001, 123, 6989. (b) Itami, K.;
Yamazaki, D.; Yoshida, J. Org. Lett. 2003, 5, 2161. (c) Reference 2e.
(14) Albisson, D. A.; Bedford, R. B.; Scully, P. N. Tetrahedron Lett.
1998, 39, 9793.
(12) Dimethyl(2-pyridyl)vinylsilane (1a) is now commercially avail-
able from Tokyo Kasei Kogyo Co., Ltd (TCI), catalog no. D2935.
2780 J. Org. Chem., Vol. 70, No. 7, 2005

Triarylethene-Based Extended π-Systems
(2-pyrimidyl)vinylsilane (1b) to be another excellent
substrate for MHR.15 Depending on the reaction se-
quence, one can choose either 1a or 1b as a platform in
our synthesis.
Moreover, it is also possible to install different aryl
groups selectively onto 1b by adding two different aryl
iodides in a stepwise fashion. Thus, a solution of 1b (1.0
equiv), first aryl iodide (1.0 equiv), Et3N (2.5 equiv), and
Pd[P(t-Bu)3]2 (5 mol %) was stirred at 80 °C to afford
monoarylated vinylsilane in situ. Thereafter, the second
aryl iodide (1.2 equiv) was added to the mixture and
further heating of the mixture at the same temperature
afforded the unsymmetrical â,â-diarylvinylsilane. For
example, the employment of 4-iodoacetophenone/4-io-
doanisole (first/second) afforded 3b-bc in quantitative
yield. The stereochemistry was determined to be 98% E
by NMR analysis of the crude product. Importantly, by
simply changing the addition order of the two aryl iodides
in the reaction sequence, the stereoisomer 3b-cb could
be obtained in quantitative yield with 97% stereoselec-
tivity.
3. Synthesis of Triarylethene-Based Extended
3.1.2. Synthesis of 10 through Hiyama-Type Cou-
pling of 3 with Aryl Diiodides (3 f 10). With an
efficient method of installing two aryl groups at â-C-H
bonds of vinylsilanes established, we subsequently ex-
amined the Hiyama-type CCR of the resultant alkenyl-
silanes 3 (Scheme 3). Gratifyingly, under our standard
conditions [5% PdCl2(PhCN)2, 1.5 equiv Bu4NF, THF, 60
°C],2b 3 underwent efficient CCR with aryl diiodides
affording an interesting triarylethene-based extended
π-system 10 in moderate to excellent yields (Scheme 3).18
For property comparison (vide infra), unsymmetrically
substituted analogues 10′, which bear two different
diarylvinyl groups on the central aryl units, were needed.
We found that these π-systems can be prepared by one-
pot sequential CCR of aryl dihalides with two different
â,â-diarylvinylsilanes 3. For example, the selective mono-
CCR of 3b-bb with 1-bromo-4-iodobenzene (at the Ar-I
bond) afforded mono-alkenylated bromobenzene in situ.
The subsequent addition of 3b-cc and Bu4NF (1.5 equiv
each) to the mixture then gave the desired π-system
10′bcA in 45% yield (Scheme 4).
π-Systems.
3.1. Synthesis of Extended π-System 10.
3.1.1. Synthesis of 3 through Double Mizoroki-
Heck Reaction of 1 with Aryl Halides (1 f 2 f 3).
Having established the effective catalysts and substrates,
we investigated the one-pot double-MHR of dimethyl(2-
pyrimidyl)vinylsilane (1b) with various aryl halides
(Scheme 2). Under the influence of Pd[P(t-Bu)3]2 catalyst
(5 mol %) and Et3N (2.5 equiv), the double-MHR of 1b
proceeded with various electronically and structurally
diverse aryl halides (2.5 equiv) at 80 °C in dioxane to
give â,â-diarylvinylsilanes 3 in excellent yields. It was
found that not only aryl iodides but also aryl bromides
can be used as substrates (3b-ee and 3b-ff). Most of these
aryl groups cannot be introduced (doubly) by our previous
method using the Pd2(dba)3/P(2-furyl)3 catalyst.2c Het-
eroaryl iodides (h and i) were again found to be ap-
plicable. Moreover, it should also be mentioned that the
yields of 3 were greater than 95%, except for the case of
4-iodoanisole (86%). Use of the Pd2(dba)3/P[OC6H3(t-Bu)2-
2,4]3 catalyst instead of Pd[P(t-Bu)3]2 was beneficial for
this case, giving the desired â,â-diarylvinylsilane 3b-cc
in 93% yield.
3.2. Synthesis of Extended π-System 11.
3.2.1. Synthesis of 6 through Mizoroki-Heck
Reaction of 2 with Aryl Diiodides (2 f 6). The use
of aryl diiodides in the second â-C-H arylation results
in the production of aryl-linked bis-alkenylsilanes 6. For
these reactions, we found that both Pd[P(t-Bu)3]2 and Pd2-
(dba)3/P[OC6H3(t-Bu)2-2,4]3 are effective as catalyst. The
results using the Pd2(dba)3/P[OC6H3(t-Bu)2-2,4]3 catalyst
in combination with Et3N are shown in Scheme 5. The
stereoselectivities were virtually complete, as in the cases
of using simple aryl halides (Scheme 2).
(15) Representative results using P(2-furyl)3, P(t-Bu)3, and P[OC6H3-
(t-Bu)2-2,4]3 ligands in the Mizoroki-Heck reaction of styryl(2-pyridyl)-
silane and styryl(2-pyrimidyl)silane are shown below.
3.2.2. Synthesis of 11 through Hiyama-type Cou-
pling of 6 with Aryl Iodides (6 f 11). The resultant
bis-alkenylsilanes 6 underwent CCR with aryl iodides to
furnish another type of triarylethene-based π-system 11
(Scheme 6). However, under our standard conditions
(Scheme 3), substantial amounts of protodesilylation
products were obtained. After reinvestigation of CCR
(16) Currently, we assume that the exceptionally high reactivity
exerted by P(t-Bu)3 and P[OC6H3(t-Bu)2-2,4]3 might be attributed to
their propensity to dissociate from Pd to form highly reactive mono-
or nonligated Pd complexes in the olefin coordination step and/or
carbopalladation (insertion) step. (a) Stambuli, J. P.; Bu¨hl, M.; Hartwig,
J. F. J. Am. Chem. Soc. 2002, 124, 9346. (b) Galardon, E.; Ramdeehul,
S.; Brown, J. M.; Cowley, A.; Hii, K. K.; Jutand, A. Angew. Chem., Int.
Ed. 2002, 41, 1760.
(18) Very recently, we found that vinylboronate pinacol ester can
also be used as a platform for triarylethene-based extended π-systems
such as 10. However, because the double-MHR step is not stereo-
selective, the boron-based method can only be used in the synthesis of
“symmetrical” 10 where all four terminal aryl groups (Ar1 and Ar2)
are same. Therefore, the selective synthesis of “unsymmetrical” 10
(such as 10bcA and 10cbA) as well as the structural isomers of 10
(11 and 12) is not possible with the boron-based method. Itami, K.;
Tonogaki, K.; Ohashi, Y.; Yoshida, J. Org. Lett. 2004, 6, 4093.
(17) Unfortunately, aryl chlorides were found not to be applicable
in our system even using highly reactive Pd/P(t-Bu)3 catalyst.
J. Org. Chem, Vol. 70, No. 7, 2005 2781

Itami et al.
SCHEME 2. Synthesis of 3 through Double Mizoroki-Heck Reaction of 1b with Aryl Halidesa
a Employing 2.5 equiv of aryl iodides. bThe reaction was carried out with Pd2(dba)3 (2.5%) and P[OC6H3(t-Bu)2-2,4]3 (5%) instead of
Pd[P(t-Bu)3]2. cAryl bromides were used. dThe stereochemistry was determined to be 98% E by NMR analysis. eThe stereochemistry was
determined to be 97% Z by NMR analysis. f The stereochemistry was determined to be 96% Z by NMR analysis.
conditions, we found that the slow addition of Bu4NF as
well as lowering the reaction temperature (60 f 40 °C)
had a beneficial effect on cross-coupling. Under these
conditions, 11 was obtained in reasonable yield (Scheme
6).
MHR of the resultant 7 with aryl iodides then gave
another type of aryl-linked bis-alkenylsilanes 8 (Scheme
8). The Pd2(dba)3/P[OC6H3(t-Bu)2-2,4]3 system was found
to be suitable for these substrates. Even using equimolar
quantities of each substrate, 8 could be obtained in high
yields with virtually complete stereoselectivities under
these conditions (Scheme 8).
3.3.3. Synthesis of 12 through Hiyama-type Cou-
pling of 8 with Aryl Iodides (8 f 12). The final end-
capping CCR of 8 with aryl iodides then furnished the
extended π-system 12 (Scheme 9). As in the cases using
6, the slow addition of Bu4NF as well as lowering the
reaction temperature (40 °C) had a beneficial effect on
cross-coupling efficiency, giving 12 in good yields except
for the sterically congested 12Alb (25%).
3.3. Synthesis of Extended π-System 12.
3.3.1. Synthesis of 7 through Mizoroki-Heck
Reaction of 1 with Aryl Diiodides (1 f 7). The use
of aryl diiodides in the first â-C-H arylation results in
the production of aryl-linked bis-alkenylsilanes 7 (Scheme
7). However, when highly reactive Pd[P(t-Bu)3]2 or Pd2-
(dba)3/P[OC6H3(t-Bu)2-2,4]3 catalyst was used for the
reaction of 1a and 1,4-diiodobenzene, an undesirable
double-Mizoroki-Heck-type polymerization took place
(vide infra). After careful reinvestigation of MHR condi-
tions for this particular reaction, we found that the use
of our original Pd2(dba)3/P(2-furyl)3 catalyst is most
effective, furnishing the desired 7a-A in 94% yield
(Scheme 7).
3.4. Synthesis of Extended π-System 13.
3.4.1. Synthesis of 9 through Double Mizoroki-
Heck Reaction of 1 with Aryl Diiodides (1 f 9).
When double-MHR of 1 was performed with ArX2 alone,
double Mizoroki-Heck-type polymerization took place (1
f 7 f 9) to afford novel polymers 9 in high yields
3.3.2. Synthesis of 8 through Mizoroki-Heck
Reaction of 7 with Aryl Iodides (7 f 8). The second
2782 J. Org. Chem., Vol. 70, No. 7, 2005

Triarylethene-Based Extended π-Systems
SCHEME 3. Synthesis of 10 through Hiyama-Type Coupling of 3 with Aryl Diiodides
(Scheme 10). It should be noted that such a type of double
arylative polymerization is unprecedented.19
3.4.2. Synthesis of 9 through Mizoroki-Heck
Reaction of 7 with Aryl Diiodides (7 f 9). Although
a new type of polymer can be prepared by the above-
mentioned procedure, a similar type of polymer having
different aryl units on the backbone should also be
intriguing to prepare. It was found that such copolymers
could be obtained when two different ArX2 were sequen-
tially employed in the double-MHR of 1. Although one-
J. Org. Chem, Vol. 70, No. 7, 2005 2783

Itami et al.
SCHEME 4. Synthesis of π-System 10′ through Sequential Cross-Coupling of Two Different 3 with Aryl
Dihalide
SCHEME 5. Mizoroki-Heck Reaction of 2 with Aryl Dihalides
a Reaction carried out using styryl(2-pyrimidyl)silane 2b-a.
pot reactions of this type were not possible because of
the difficulty in the selective production of mono-arylated
7 with Pd[P(t-Bu)3]2 catalyst, the stepwise MHR of 7a-A
with 1,3-diiodobenzene yielded the desired copolymer 9a-
AD in 85% yield (Scheme 11).
3.4.3. Synthesis of 13 through Hiyama-Type Cou-
pling of 9 with Aryl Iodides (9 f 13). The successive
CCR of the resultant polymers 9 with aryl iodides in the
presence of PdCl2(PhCN)2 catalyst and Bu4NF finally
produced novel cross-conjugated polymers 13 in good
yields (Scheme 12). To our knowledge, this type of
polymer has not been prepared previously. The molecular
weight (Mn) and polydispersity (PD) of the polymers 13,
determined by size exclusion chromatography using
polystyrene standards, are as follows: 13AAm (Mn
)
9235, PD ) 2.06), 13DDb (Mn ) 7148, PD ) 1.86),
13DDm (Mn ) 7804, PD ) 2.03), 13ADb (Mn ) 7575,
PD ) 2.14), 13ADn (Mn ) 7661, PD ) 3.03).
4. Photophysical Properties of Triarylethene-
Based Extended π-Systems 10-13.
4.1. Photophysical Properties in CHCl3. The pho-
tophysical properties (UV-vis and fluorescence) of the
triarylethene-based extended π-systems 10-13 con-
structed (30 compounds) were first measured as their
solution in degassed CHCl3 (Table 1).
In the UV-vis absorption spectra, the absorption
maxima (λabs) ascribed to the π-π* transitions depend
on the nature of the aryl groups installed, as expected.
For example, the λabs values increase as the phenyl group
(a) is replaced by the p-acetylphenyl group (b) in the
π-systems 10, 11, and 12: 10aaA (354 nm) f 10bbA (377
nm); 10aaB (353 nm) f 10bbB (372 nm); 11aAa (338
nm) f 11aAb (360 nm); 12Aaa (299 nm) f 12Aab (327
(19) Selected examples of Pd-catalyzed Mizoroki-Heck-type poly-
condensation of diolefins and dihalides: (a) Suzuki, M.; Lim, J.-C.;
Saegusa, T. Macromolecules 1990, 23, 1574. (b) Bao, Z.; Chen, Y.; Cai,
R.; Yu, L. Macromolecules 1993, 26, 5281. (c) Pan, M.; Bao, Z.; Yu, L.
Macromolecules 1995, 28, 5151. (d) Pasco, S. T.; Lahti, P. M.; Karasz,
F. E. Macromolecules 1999, 32, 6933. (e) Jung, S.-H.; Kim, H. K.; Kim,
S.-H.; Kim, Y. H.; Jeoung, S. C.; Kim, D. Macromolecules 2000, 33,
9277. (f) Mikroyannidis, J. A.; Spiliopoulos, I. K.; Kulkarni, A. P.;
Jenekhe, S. A. Synth. Met. 2004, 142, 113. (g) Kim, J. H.; Lee, H. Synth.
Met. 2004, 144, 169. (h) Mikroyannidis, J. A.; Vellis, P. D.; Karastatiris,
P. I.; Spiliopoulos, I. K. Synth. Met. 2004, 145, 87. (i) Mikroyannidis,
J. A. Synth. Met. 2004, 145, 271. (j) Takagi, K.; Mori, K.; Kunisada,
H.; Yuki, Y. Polym. Bull. 2004, 52, 125. (k) Jin, S.-H.; Kim, M.-K.;
Koo, D.-S.; Kim, Y.-I.; Park, S.-H.; Lee, K.; Gal, Y.-S. Chem. Mater.
2004, 16, 3299. (l) Jin, S.-H.; Hwang, C.-K.; Gal, Y.-S.; Park, D.-K.;
Cho, S.-J.; Shin, D.-M.; Lee, J.-W. Eur. Polym. J. 2004, 40, 1975. (m)
Morisaki, Y.; Chujo, Y. Macromolecules 2004, 37, 4099.
2784 J. Org. Chem., Vol. 70, No. 7, 2005

Triarylethene-Based Extended π-Systems
SCHEME 6. Synthesis of 11 through Hiyama-Type Coupling of 6 with Aryl Iodides
a Reaction carried out using 2-pyrimidylsilane 6b-aC.
SCHEME 7. Synthesis of 7 through Mizoroki-Heck Reaction of 1 with Aryl Diiodides
nm). Presumably this is a reflection of the effective
extension of π-conjugation with terminal carbonyl groups.
The olefinic isomers in π-system 10 (10bcA/10cbA and
10bcB/10cbB) were found to possess similar absorption
bands.
In the fluorescence spectra, many of these π-systems
were found to have their emission bands in the visible
region (blue to yellow). In addition to the emission color,
the fluorescence quantum yields were also found to
depend significantly on the attaching sites, as well as the
nature of the aryl groups installed.
For example, the π-systems 10 emit light in the blue
to yellow region, but their emission maxima (λem) and
quantum yields (ΦF) depend heavily on the nature and
position of the aryl groups (vide infra). In this class of
π-system, we detected an interesting substituent effect
on the Stokes shifts. For example, when two terminal
aryl groups (Ar1 and Ar2) are in a donor/acceptor (push/
pull) relationship, larger Stokes shifts were observed
(10bcA 130 nm, 10cbA 122 nm) compared with those of
the parent “symmetrical” π-systems (10bbA 109 nm,
10ccA 101 nm). Such a donor/acceptor substitution can
bring about the green emission (10bcA and 10cbA) from
otherwise blue-fluorescent 10aaA. In comparison with
the well-known blue-fluorescent DPVBi (10aaB; ΦF ) 6.0
× 10-2), which has been used in organic electrolumines-
cence devices,20,21 10aaC was found to possess greater
quantum yield (ΦF ) 0.30). The unsymmetrically sub-
stituted (at terminal olefinic carbons) π-systems 10bcA,
(20) Tokailin, H.; Hosokawa, C.; Kusumoto, T. Eur. Pat. Appl.
EP387715, 1990; Chem. Abstr. 1990, 115, 123507. (b) Hosokawa, C.;
Higashi, H.; Nakamura, H.; Kusumoto, T. Appl. Phys. Lett. 1995, 67,
3853. (c) Hosokawa, C.; Eida, M.; Matsuura, M.; Fukuoka, K.; Naka-
mura, H.; Kusumoto, T. Synth. Met. 1997, 91, 3. (d) Yang, J. P.; Jin,
Y. D.; Heremans, P. L.; Hoefnagels, R.; Dieltiens, P.; Blockhuys, F.;
Geise, H. J.; Van der Auweraer, M.; Borghs, G. Chem. Phys. Lett. 2000,
325, 251. (e) Savvate’ev, V.; Friedl, J. H.; Zou, L.; Shinar, J.; Chris-
tensen, K.; Oldham, W.; Rothberg, L. J.; Chen-Esterlit, Z.; Kopelman,
R. Appl. Phys. Lett. 2000, 76, 1501. (f) Huang, Y.-S.; Jou, J.-H.; Weng,
W.-K.; Liu, J.-M. Appl. Phys. Lett. 2002, 80, 2782.
(21) Shinar, J., Ed. Organic Light-Emitting Devices: A Survey;
Springer-Verlag: New York, 2004.
J. Org. Chem, Vol. 70, No. 7, 2005 2785

Itami et al.
SCHEME 8. Synthesis of 8 through Mizoroki-Heck Reaction of 7 with Aryl Iodides
SCHEME 9. Synthesis of 12 through Hiyama-Type Coupling of 8 with Aryl Iodides
10cbA, 10bcB, and 10cbB also emit relatively intense
light (ΦF ) 0.16-0.17) in green region.
π-system 10, the substitution of all terminal aryl groups
with p-MeCOC6H4 or p-MeOC6H4 groups only had det-
rimental effects on emission efficiency (ΦF 2.1 × 10-2 for
10bbA, 3.5 × 10-3 for 10ccA) compared with the parent
phenyl group (10aaA ΦF ) 5.9 × 10-2). The π-system
Interestingly, it was found that the π-systems 11 and
12 (the structural isomers of 10) are almost nonfluores-
cent in solution (Table 1 and Figure 3). This is in a sharp
contrast to the high fluorescence efficiencies obtained in
10, which clearly indicates the importance of the attach-
ing sites of aryl groups on photophysical properties. The
polymeric π-systems 13 are also intriguing, not only
because of their structural novelty but also because they
are also fluorescent (green to yellow region).
4.2. Effect of Substituents on Fluorescence Ef-
ficiency. While constructing such fluorescent libraries,
we were able to find interesting effects of substituents
on fluorescence efficiency (Figure 4). For example, in the
10′bcA was also found to be weakly fluorescent (ΦF
)
5.4 × 10-3). However, the unsymmetrical substitutions
with these groups at terminal olefinic carbons resulted
in a substantial increase in fluorescence quantum yield
(ΦF 0.16 for 10bcA and 10cbA) (Figure 4). These
structure-property relationship analyses clearly showed
the beneficial effect of the unsymmetrical substitutions
at terminal olefinic carbons with an electron-accepting
p-MeCOC6H4 group and an electron-releasing p-MeOC6H4
group on fluorescence efficiency.
2786 J. Org. Chem., Vol. 70, No. 7, 2005

Triarylethene-Based Extended π-Systems
SCHEME 10. Synthesis of 9 through Double Mizoroki-Heck Reaction of 1 with Aryl Diiodides
SCHEME 11. Synthesis of 9 through
Mizoroki-Heck Reaction of 7 with Aryl Diiodides
state. The λmax values of the solid-state emission were
found to be similar to those obtained in chloroform
solution (Table 1).
Because it has been assumed in many cases that the
fluorescence efficiency of organic chromophores generally
decreases in the solid state, mainly attributed to the
intermolecular vibronic interactions that trigger non-
radiative deactivation processes (fluorescence quench-
ing),23 we became interested in the enhanced emission
of triarylethene-based extended π-systems in the solid
state.
Currently, we assume this interesting fluorescence
property of triarylethene-based extended π-systems is
mainly attributed to their unique structure. On the basis
of the X-ray crystal structure analysis2e and quantum
calculations, we found that triarylethenes and tri-
arylethene-based extended π-systems adopt a nonplanar
structure as the result of a sterically congested environ-
ment around the CdC core. We suppose that such
structural features of triarylethene-based extended π-sys-
tems would reduce the probability of effective inter-
molecular fluorescence-quenching interactions even in
the solid state. If this is the case, then the aggregation
(solidification) should lead to the reduced probability and
amplitude of molecular motions such as twisting and out-
of-plane bending motion, which might trigger radiation-
less transitions, thereby enhancing the fluorescence
In addition to the increased chance of discovering new
materials, the increased chance of discovering such
interesting photophysical properties is clearly an advan-
tage of programmable and diversity-oriented synthesis
in fluorescent materials science, where the properties are
often not predictable.22 Currently, we are trying to clarify
this interesting substituent effect on fluorescence ef-
ficiency.
4.3. Enhanced Emission in the Solid State. During
the course of our investigation aimed at clarifying the
controlling factors determining the fluorescence effi-
ciency, we found that the fluorescence efficiency of these
triarylethene-based extended π-systems 10-13 generally
(and dramatically in many cases) increases in the solid
state. As well as 10, otherwise nonfluorescent π-systems
such as 11 and 12 also became fluorescent in the solid
SCHEME 12. Synthesis of 13 through Hiyama-Type Coupling of 9 with Aryl Iodides
J. Org. Chem, Vol. 70, No. 7, 2005 2787

Itami et al.
FIGURE 3. Effect of aryl-group attachment site on fluorescence efficiency.
TABLE 1. Photophysical Properties of
Triarylethene-Based Extended π-Systems 10-13
UV-visa
fluorescence
a
a,b
c
compd
λabs (log ꢀ)
λem
ΦF
λem(solid)
10aaA
10bbA
10ccA
10ddA
10bcA
10cbA
10aaB
10bbB
10ccB
10bcB
10cbB
10aaC
10ccC
10ddD
10′bcA
11aAa
11aAb
11cAb
11jAk
11aCb
11cCb
12Aaa
12Aab
12Acb
12Alb
354 nm (4.54)
377 nm (4.63)
369 nm (4.59)
407 nm (4.63)
378 nm (4.49)
383 nm (4.42)
353 nm (4.71)
372 nm (4.63)
364 nm (4.71)
372 nm (4.67)
375 nm (4.74)
370 nm (4.74)
380 nm (4.71)
354 nm (4.53)
381 nm (4.53)
338 nm (4.57)
360 nm (4.58)
367 nm (4.56)
339 nm (4.53)
374 nm (4.68)
373 nm (4.65)
299 nm (4.53)
327 nm (4.57)
343 nm (4.60)
302 nm (4.53)
335 nm (4.18)
330 nm (4.16)
322 nm (4.22)
330 nm (4.42)
303 nm (4.34)
461 nm
486 nm
470 nm
477 nm
508 nm
505 nm
442 nm
469 nm
453 nm
484 nm
482 nm
449 nm
460 nm
nd
525 nm
415 nm
441 nm
490 nm
nd
466 nm
nd
nd
nd
nd
450 nm
492 nm
463 nm
452 nm
498 nm
469 nm
5.9 × 10-2
2.1 × 10-2
3.5 × 10-3
<1.0 × 10-3
0.16
480 nm
486 nm
478 nm
541 nm
515 nm
504 nm
455 nm
495 nm
468 nm
481 nm
493 nm
460 nm
479 nm
nd
519 nm
433 nm
486 nm
485 nm
433 nm
530 nm
551 nm
451 nm
469 nm
463 nm
481 nm
nd
0.16
6.0 × 10-2
2.0 × 10-2
5.6 × 10-3
0.17
FIGURE 4. Effect of substituents on fluorescence efficiency.
0.17
0.30
efficiency as a whole.24,25 Alternatively, an aggregation-
induced planarization of the π-system might also be the
reason for enhanced emission.
4.4. Aggregation-Induced Enhanced Emission.
While investigating the enhanced emission of the tri-
arylethene-based extended π-systems in the solid state,
we became aware of the existence of several interesting
organic fluorophores that show unique enhanced emis-
sion rather than a fluorescence quenching in the solid
(aggregated) state.26-34 For example, very recently, Tang
<1.0 × 10-3
<1.0 × 10-3
5.4 × 10-3
4.1 × 10-3
3.9 × 10-3
2.7 × 10-3
7.7 × 10-4
6.1 × 10-3
1.1 × 10-3
3.3 × 10-3
<1.0 × 10-3
<1.0 × 10-3
9.8 × 10-3
nd
(22) (a) Valeur, B. Molecular Fluorescence: Principles and Applica-
tions; Wiley-VCH: Weinheim, 2002. (b) Lakowicz, J. R. Principles of
Fluorescence Spectroscopy: 2nd ed.; Kluwer Academic: New York, 1999.
(23) Birks, J. B. Photophysics of Aromatic Molecules; Wiley-Inter-
science: London, 1970.
(24) Turro, N. J. Modern Molecular Photochemistry; University
Science Books: Mill Valley, CA, 1991; pp 170-172.
(25) Go¨rner, H.; Kuhn, H. J. In Advances in Photochemitry; Neckers,
D. C., Volman, D. H., von Bu¨nau, G., Eds.; Wiley: New York, 1995;
Vol. 19, pp 1-117.
(26) Deans, R.; Kim, J.; Machacek, M. R.; Swager, T. M. J. Am.
Chem. Soc. 2000, 122, 8565.
13AAm
13DDb
13DDm
13ADb
13ADn
nd
nd
nd
nd
nd
nd
nd
nd
a Measured in degassed CHCl3. nd ) could not be detected.
b Determined with reference to 9,10-diphenylanthracene (excited
at 350 nm). nd ) could not be detected. c Solid-state fluorescence.
nd ) could not be detected.
2788 J. Org. Chem., Vol. 70, No. 7, 2005

Triarylethene-Based Extended π-Systems
FIGURE 5. UV-vis spectra of 10ccB in dioxane/water
mixtures (10 µM).
and co-workers reported that a series of siloles exhibit a
strongly enhanced fluorescence emission in the solid state
as well as in nanoparticles (nanoaggregates).29,35 Thus,
we decided to see whether such an aggregation-induced
enhanced emission is also operating in our π-systems.
First, we measured the UV-vis absorption spectra of
10ccB in various dioxane/water mixtures (Figure 5).
Because 10ccB does not dissolve in water, 10ccB was
expected to aggregate in dioxane/water mixtures with
high water content. From 60% volume fraction of water,
the maximum peaks in the absorption spectra are red-
shifted with obvious level-off tailing (Figure 5). These
spectral changes are commonly observed in nanoparticle
suspensions.29,30,35
Next, we measured the fluorescence spectra of 10ccB
in various dioxane/water mixtures (Figure 6). As in
chloroform (Table 1), 10ccB exhibited an extremely low
fluorescence quantum yield (ΦF ) 9.3 × 10-3) in pure
dioxane. Although 10ccB showed similar ΦF values up
to 50% volume fraction of water, we observed an obvious
and drastic increase of the ΦF values from 60% volume
fraction of water. In particular, the ΦF value at this
composition (dioxane/water ) 40/60) is 46 and 70 times
higher (ΦF ) 0.42) than that in pure dioxane and
FIGURE 6. Aggregation-induced enhanced emission. (A)
Fluorescence quantum yields (ΦF) of 10ccB depending on
water fractions in dioxane. (B) Emissive behaviors of 10ccB
in dioxane/water mixtures under irradiation of light at 365
nm.
chloroform, respectively. The aggregation seems to be
responsible for this dramatic increase in fluorescence
efficiency. The formation of molecular aggregates was
also indicated from the observation that the mixtures
containing greater than 60% volume fraction of water
exhibited an off-white turbidity due to light scattering.
Interestingly, the ΦF values were found to drop at higher
water content (though they were still greater than that
in pure dioxane). Although more in-depth experiments
must be conducted, these results indicate the existence
of additional aggregation effects, such as a size effect,
on fluorescence properties.
To see the generality of aggregation-induced enhanced
emission in the triarylethene-based extended π-systems,
we measured the fluorescence spectra of representative
compounds in a 20/80 dioxane/water mixture (Table 2).
In all cases examined, the fluorescence quantum yields
in such aggregated species (ΦFagg) were found to be
greater than those of molecularly dissolved species in
dioxane (ΦFsol). In particular, otherwise nonemissive
π-systems such as 10′bcA, 11aAa, and 12Aaa were
found to fluoresce in the aggregation state (ΦFagg 0.15 for
10′bcA, 0.18 for 11aAa, 4.0 × 10-2 for 12Aaa). The
increase (ΦFagg/ΦFsol) observed was 21, 49, and 33 times,
respectively. This aggregation-induced enhanced emis-
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2001, 1740. (b) Freemantle, M. Chem. Eng. News 2001, 79 (41), 29. (c)
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(35) Organic nanoparticles and microcrystals bearing interesting
photophysical properties: (a) Kasai, H.; Yoshikawa, Y.; Seko, T.;
Okada, S.; Oikawa, H.; Matsuda, H.; Watanabe, A.; Itoh, O.; Toyotama,
H.; Nakanishi, H. Mol. Cryst. Liq. Cryst. 1997, 294, 173. (b) Kasai, H.;
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Y.; Okada, S.; Oikawa, H.; Watanabe, A.; Itoh, O.; Nakanishi, H. Chem.
Lett. 1997, 1181. (d) Komai, Y.; Kasai, H.; Hirakoso, H.; Hakuta, Y.;
Okada, S.; Oikawa, H.; Adschiri, T.; Inomata, H.; Arai, K.; Nakanishi,
H. Mol. Cryst. Liq. Cryst. 1998, 322, 167. (e) Nakanishi, H.; Katagi,
H. Supramol. Sci. 1998, 5, 289. (f) Fu, H.-B.; Yao, J.-N. J. Am. Chem.
Soc. 2001, 123, 1434. (g) Fu, H.; Loo, B. H.; Xiao, D.; Xie, R.; Ji, X.;
Yao, J.; Zhang, B.; Zhang, L. Angew. Chem., Int. Ed. 2002, 41, 962.
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(30) (a) An, B.-K.; Kwon, S.-K.; Jung, S.-D.; Park, S. Y. J. Am. Chem.
Soc. 2002, 124, 14410. Also see: (b) Ryu, S. Y.; Kim, S.; Seo, J.; Kim,
Y.-W.; Kwon, O.-H.; Jang, D.-J.; Park, S.-Y. Chem. Commun. 2004,
70. (c) An, B.-K.; Lee, D.-S.; Lee, J.-S.; Park, Y.-S.; Song, H.-S.; Park,
S. Y. J. Am. Chem. Soc. 2004, 126, 10232. (d) Lim, S.-J.; An, B.-K.;
Jung, S. D.; Chung, M.-A.; Park, S. Y. Angew. Chem., Int. Ed. 2004,
43, 6346.
(31) Manimaran, B.; Thanasekaran, P.; Rajendran, T.; Lin, R.-J.;
Chang, I.-J.; Lee, G.-H.; Peng, S.-M.; Rajagopal, S.; Lu, K.-L. Inorg.
Chem. 2002, 41, 5323.
(32) Jayanty, S.; Radhakrishnan, T. P. Chem. Eur. J. 2004, 10, 791.
(33) Li, S.; He, L.; Xiong, F.; Li, Y.; Yang, G. J. Phys. Chem. B 2004,
108, 10887.
(34) Wilson, J. N.; Smith, M. D.; Enkelmann, V.; Bunz, U. H. F.
Chem. Commun. 2004, 1700.
J. Org. Chem, Vol. 70, No. 7, 2005 2789

Itami et al.
TABLE 2. Fluorescence Efficiency of Representative
Compounds in Dioxane/Water (20/80) Mixturea
agg
sol
sol
compd
ΦF
0.44
0.31
0.19
0.61
0.15
0.18
ΦF
ΦFagg/ΦF
10aaA
10aaB
10ccB
10bcB
10′bcA
11aAa
12Aaa
8.7 × 10-2
5.1
1.5
20
2.9
21
49
33
0.21
9.3 × 10-3
0.21
7.0 × 10-3
3.7 × 10-3
1.2 × 10-3
4.0 × 10-2
agg
a The ΦF
values were determined in dioxane/water (20/80)
sol
mixture with reference to 9,10-diphenylanthracene. The ΦF
values were determined in dioxane with reference to 9,10-
diphenylanthracene.
FIGURE 8. Rapid synthesis and property evaluation in the
extended π-system 10.
now in a position to take full advantage of the program-
mable and diversity-oriented nature of the present
synthetic strategy for conducting rapid synthesis/evalu-
ation process.37 Although the execution of such experi-
ments in a truly meaningful manner requires automated
synthesis as well as high-throughput screening, we
herein provide “proof-of-principle” (manual) experiments
using our synthesis (Figure 8).
FIGURE 7. FE-SEM images of nanoparticles obtained from
suspension of 10ccB in 20/80 dioxane/water mixture (bar ) 1
µm). Inset shows the magnified FE-SEM image (bar ) 100
nm).
This was done as follows. The 20 compounds (10) with
varied five aryl groups were synthesized in a parallel
fashion and then placed in a microtiter plate directly from
crude reaction solutions. Irradiation of this plate (at 365
nm) allowed us quickly to detect several fluorescent
compounds from this crude library, as can be clearly seen
from the picture in Figure 8. The emission color as well
as relative emission efficiency was found to correlate with
those of isolated samples. It should be noted that five
highly fluorescent π-systems found in this study (10aaC,
10bcA, 10cbA, 10bcB, and 10cbB) were correctly de-
tected by this method (Figure 8). We believe that this
process not only serves as a first step toward our next
goal (fully automated synthesis/evaluation of functional
π-systems) but also is useful in its own right as a first-
round qualitative manual screening method.
sion is also operative in highly fluorescent π-systems. For
example, the ΦF value of 10bcB was found to rise to 0.61
from 0.21 obtained in pure dioxane. Overall, the forma-
tion of molecular aggregates in dioxane/water mixtures
generally and dramatically boosts the emission efficiency
of triarylethene-based extended π-systems.
The formation of molecular aggregates under these
conditions (dioxane/water mixture) was indicated from
many aspects, but direct evidence for it was lacking.
Gratifyingly, dynamic light scattering (DLS) experiments
on the suspension of 10ccB in 20/80 dioxane/water
mixture revealed the presence of molecular aggregates
with average diameter of 240 nm. The shape of nano-
particles (nanoaggregates) was observed by field emission
scanning electron microscopy (FE-SEM).36 The FE-SEM
images in Figure 7 show that the diameter of the
nanoparticles is in a range of 100-250 nm, which agrees
roughly with that determined by DLS experiments.
5. Rapid Synthesis/Evaluation Process. Through
this study, we mainly focused our efforts on the conven-
tional “one-at-a-time” synthesis/purification/evaluation
process because the properties of the triarylethene-based
extended π-systems that we dealt with were largely
unknown. However, after obtaining a clearer picture of
the photophysical properties of these π-systems, we are
(37) Combinatorial approaches in materials science: (a) Jandeleit,
B.; Schaefer, D. J.; Powers, T. S.; Turner, H. W.; Weinberg, W. H.
Angew. Chem., Int. Ed. 1999, 38, 2494. (b) Schiedel, M.-S.; Briehn, C.
A.; Ba¨uerle, P. Angew. Chem., Int. Ed. 2001, 40, 4677. (c) Briehn, C.
A.; Schiedel, M.-S.; Bonsen, E. M.; Schuhmann, W.; Ba¨uerle, P. Angew.
Chem., Int. Ed. 2001, 40, 4680. (d) Anderson, S. Chem. Eur. J. 2001,
7, 4706. (e) Merrington, J.; James, M.; Bradley, M. Chem. Commun.
2002, 140. (f) Lavastre, O.; Illitchev, I.; Jegou, G.; Dixneuf, P. H. J.
Am. Chem. Soc. 2002, 124, 5278. (g) Zhu, Q.; Yoon, H.-S.; Parikh, P.
B.; Chang, Y.-T.; Yao, S. Q. Tetrahedron Lett. 2002, 43, 5083. (h)
Rosania, G. R.; Lee, J. W.; Ding, L.; Yoon, H.-S.; Chang, Y.-T. J. Am.
Chem. Soc. 2003, 125, 1130. (i) Deeg, O.; Ba¨uerle, P. Org. Biomol.
Chem. 2003, 1, 1609. See also: (j) Hoogenboom, R.; Meier, M. A. R.;
Schubert, U. S. Macromol. Rapid Commun. 2003, 24, 15. (k) Meier,
M. A. R.; Hoogenboom, R.; Schubert, U. S. Macromol. Rapid Commun.
2004, 25, 21.
(36) FE-SEM images were acquired by dropping nanoparticle
suspension on copper tapes.
2790 J. Org. Chem., Vol. 70, No. 7, 2005

Triarylethene-Based Extended π-Systems
and the resultant mixture was further stirred at 80 °C for 19
h. After cooling the reaction mixture to room temperature, the
catalyst and salts were removed by filtration through a short
silica gel pad (EtOAc). The yield of 3b-bc was estimated to be
>99% (3b-bc/3b-cb ) 98/2) by the 1H NMR of the crude
mixture. The subjection of the crude mixture to gel permeation
chromatography (CHCl3) afforded 3b-bc (398.8 mg, 99%) as
pale yellow oil: 1H NMR (300 MHz, CDCl3) δ 0.20 (s, 6H), 2.59
(s, 3H), 3.76 (s, 3H), 6.51 (s, 1H), 6.77 (d, J ) 9.3 Hz, 2H),
7.11 (t, J ) 5.4 Hz, 1H), 7.18 (d, J ) 9.3 Hz, 2H), 7.22 (d, J )
8.1 Hz, 2H), 7.81 (d, J ) 8.1 Hz, 2H), 8.66 (d, J ) 5.4 Hz, 2H);
13C NMR (75 MHz, CDCl3) δ -2.1, 26.5, 55.2, 113.3, 119.4,
124.2, 127.9, 128.6, 129.8, 134.7, 136.0, 147.5, 154.9, 157.2,
159.6, 180.4, 197.6; IR (neat) 1682, 1252 cm-1; HRMS (EI) m/z
calcd for C23H24N2O2Si 388.1607, found 388.1604.
Typical Procedure for Synthesis of 10 (Scheme 3). To
a mixture of 3b-aa (95.6 mg, 0.30 mmol), 4,4′-diiodobiphenyl
(41.1 mg, 0.10 mmol), and PdCl2(PhCN)2 (3.9 mg, 10.2 µmol)
in dry THF (0.8 mL) was added a solution of Bu4NF (0.30
mmol, 1.0 M) in THF at room temperature. The mixture was
stirred at 60 °C for 4 h under argon. After cooling the reaction
mixture to room temperature, the catalyst and salts were
removed by filtration through a short silica gel pad (EtOAc).
The subjection of the crude mixture to gel permeation chro-
matography (CHCl3) afforded 10aaB (47.6 mg, 92%) as pale
yellow solid: 1H NMR (300 MHz, CDCl3) δ 6.98 (s, 2H), 7.03-
7.06 (m, 4H), 7.21-7.24 (m, 4H), 7.29-7.35 (m, 20H); 13C NMR
(125 MHz, CDCl3) δ 126.2, 127.45, 127.50, 127.6, 127.7, 128.2,
128.7, 129.9, 130.3, 136.4, 138.6, 140.4, 142.6, 143.3; IR (KBr)
1495, 1441 cm-1; HRMS (EI) m/z calcd for C40H30 510.2348,
found 510.2349.
Conclusions
In conclusion, we have developed a programmable and
diversity-oriented synthetic method for the structurally
well-defined triarylethene-based extended π-systems. By
following the basic synthetic scheme developed, we
succeeded in preparing a number of interesting tri-
arylethene-based extended π-systems (four types, 30
compounds) very efficiently. The measurement of photo-
physical properties showed that many of these π-systems
have their emission bands in the visible region (blue to
yellow) and that the fluorescence quantum yields as well
as emission color depend significantly on the attaching
sites and the nature of the aryl groups attached. It was
also found that many of the triarylethene-based extended
π-systems prepared emit more intense light (do not
quench their fluorescence) in the solid state. Moreover,
such an aggregation-induced enhanced emission was also
observed in dioxane/water mixtures with high water
contents, where the formation of molecular aggregates
such as nanoparticles has been indicated. A useful
method for the rapid synthesis and property evaluation
has also been developed.
The simple but powerful synthetic strategy described
herein clearly benefits from the inherent stereo-electronic
bias of the vinylsilane platform (differentiated reactivity
of C-H and C-Si bonds) with the aid of catalyst-
directing (coordinating) groups. The present successful
application to functional extended π-systems speaks well
for the potential of such an orchestrated interplay of
organo-element chemistry (organosilicon chemistry herein,
but not to be limited to silicon)2e and coordination
chemistry in the development of other platforms38 to
construct versatile chemical libraries to face more chal-
lenging quest in materials science.
Procedure for Synthesis of 10′bcA (Scheme 4). To a
mixture of 1-bromo-4-iodobenzene (28.3 mg, 0.10 mmol), 3b-
bb (40.1 mg, 0.10 mmol), and PdCl2(PhCN)2 (3.9 mg, 10.2
µmol) in dry THF (0.5 mL) were added a solution of Bu4NF
(0.10 mmol, 1.0 M) in THF and additional THF (0.5 mL) at
room temperature under argon. After stirring the mixture at
60 °C for 1 h, a solution of 3b-cc (57.8 mg, 0.15 mmol) in THF
(1.0 mL), a solution of Bu4NF (0.15 mmol, 1.0 M) in THF, and
additional THF (0.5 mL) were added to the mixture. The
resultant mixture was further stirred at 60 °C for 3 h. After
cooling the reaction mixture to room temperature, the catalyst
and salts were removed by filtration through a short silica gel
pad (EtOAc). The subjection of the crude mixture to gel
permeation chromatography (CHCl3) afforded 10′bcA (26.1
mg, 45%) as yellow solid: 1H NMR (300 MHz, CDCl3) δ 2.59
(s, 3H), 2.64 (s, 3H), 3.81 (s, 3H), 3.82 (s, 3H), 6.72 (s, 1H),
6.81-6.85 (m, 8H), 7.01 (s, 1H), 7.07 (d, J ) 8.7 Hz, 2H), 7.21
(d, J ) 8.7 Hz, 2H), 7.28 (d, J ) 8.4 Hz, 2H), 7.32 (d, J ) 8.4
Experimental Section
Typical Procedure for Synthesis of 3 Where Two Aryl
Groups Are the Same (Scheme 2). A mixture of dimethyl-
(2-pyrimidyl)vinylsilane (1b; 173.5 mg, 1.06 mmol), iodoben-
zene (421.6 mg, 2.07 mmol), triethylamine (253.0 mg, 2.50
mmol), and Pd[P(t-Bu)3]2 (25.6 mg, 50.1 µmol) in dry dioxane
(3.0 mL) was stirred at 80 °C for 8 h under argon. After cooling
the reaction mixture to room temperature, the catalyst and
salts were removed by filtration through a short silica gel pad
(EtOAc). The subjection of the crude mixture to gel permeation
chromatography (CHCl3) afforded 3b-aa (330.6 mg, 99%) as a
pale yellow oil: 1H NMR (300 MHz, CDCl3) δ 0.21 (s, 6H), 6.59
(s, 1H), 7.12-7.16 (m, 3H), 7.23-7.34 (m, 8H), 8.72 (d, J )
5.1 Hz, 2H); 13C NMR (125 MHz, CDCl3) δ -2.2, 119.4, 125.6,
127.40, 127.44, 127.81, 127.83, 127.9, 129.5, 142.2, 142.8,
154.9, 159.0, 180.8; IR (neat) 1736, 1559, 1393 cm-1; HRMS
(EI) m/z calcd for C20H20N2Si 316.1396, found 316.1398.
Note: 3b-cc was prepared using Pd2(dba)3 (2.5%) and
P[OC6H3(t-Bu)2-2,4]3 (5%) instead of Pd[P(t-Bu)3]2.
Hz, 2H), 7.89 (d, J ) 7.8 Hz, 2H), 7.93 (d, J ) 7.8 Hz, 2H); 13
C
NMR (100 MHz, CDCl3) δ 26.6 (two carbons), 55.2, 55.3, 113.5,
113.9, 125.5, 127.6, 128.4, 128.8, 128.9, 129.2, 129.3, 130.7,
131.0, 131.5, 132.5, 134.2, 136.0, 136.18, 136.25, 137.4, 139.9,
142.5, 145.1, 147.4, 159.0, 159.3, 197.5, 197.7; IR (KBr) 1682,
1603, 1510 cm-1; HRMS (FAB) m/z calcd for C40H34O4 578.2457,
found 578.2452.
Typical Procedure for Synthesis of 6 (Scheme 5). A
mixture of 2a-a (1.15 g, 4.96 mmol), 1,4-diiodobenzene (824.4
mg, 2.50 mmol), triethylamine (1.01 g, 10.0 mmol), Pd2(dba)3‚
CHCl3 (128.0 mg, 0.12 mmol), and tris(2,4-di-tert-butylphenyl)-
phosphite (161.0 mg, 0.25 mmol) in dry dioxane (5.0 mL) was
stirred at 80 °C for 18 h under argon. After cooling the reaction
mixture to room temperature, the catalyst and salts were
removed by filtration through a short silica gel pad (EtOAc).
The filtrate was evaporated and the residue was chromato-
graphed on silica gel (hexane/EtOAc) to afford 6a-aA (797 mg,
58%): 1H NMR (400 MHz, CDCl3) δ 0.18 (s, 12H), 6.58 (s, 2H),
7.07-7.12 (m, 6H), 7.17-7.24 (m, 6H), 7.22 (s, 4H), 7.37 (d, J
) 7.6 Hz, 2H), 7.45 (td, J ) 7.6, 1.6 Hz, 2H), 8.74 (d, J ) 4.8
Hz, 2H); 13C NMR (100 MHz, CDCl3) δ -1.9, 122.1, 126.1,
126.7, 127.1, 127.5, 128.8, 129.2, 133.4, 141.7, 141.8, 149.6,
Typical Procedure for Synthesis of 3 Where Two Aryl
Groups Are Different (Scheme 2). A mixture of dimethyl-
(2-pyrimidyl)vinylsilane (1b; 171.4 mg, 1.04 mmol), 4-iodo-
acetophenone (246.4 mg, 1.00 mmol), triethylamine (253.0 mg,
2.50 mmol), and Pd[P(t-Bu)3]2 (25.6 mg, 50.1 µmol) in dry
dioxane (2.0 mL) was stirred at 80 °C for 5 h under argon. To
this mixture was added 4-iodoanisole (286.1 mg, 1.22 mmol)
(38) For a recent other example from our laboratory using such a
platform strategy for the construction of pyrimidine-core extended
π-systems, see: Itami, K.; Yamazaki, D.; Yoshida, J. J. Am. Chem.
Soc. 2004, 126, 15396.
J. Org. Chem, Vol. 70, No. 7, 2005 2791

Itami et al.
157.8, 167.4; IR (KBr) 1565, 1242 cm-1; HRMS (EI) m/z calcd
for C36H36N2Si2 552.2417, found 552.2418.
7.21 (s, 4H), 7.30-7.43 (m, 10H); 13C NMR (100 MHz, CDCl3)
δ 126.8, 127.5, 127.6, 127.9, 128.2 (two carbons), 129.6, 130.8,
137.4, 139.5, 142.4, 143.1; IR (KBr) 762, 695 cm-1; HRMS (EI)
m/z calcd for C34H26 434.2034, found 434.2033.
Typical Procedure for Synthesis of 11 (Scheme 6). To
a mixture of 6a-aA (53.6 mg, 0.10 mmol), iodobenzene (80.2
mg, 0.39 mmol), and PdCl2(PhCN)2 (3.7 mg, 9.6 µmol) in dry
THF (1.0 mL) was slowly added a solution of Bu4NF (0.25
mmol, 1.0 M) in THF by syringe pump (0.0167 mmol/min) at
40 °C under argon, and the resultant mixture was further
stirred at 40 °C for 2 h. After cooling the reaction mixture to
room temperature, the catalyst and salts were removed by
filtration through a short silica gel pad (EtOAc). The subjection
of the crude mixture to gel permeation chromatography
(CHCl3) afforded 11aAa (21.9 mg, 52%) as yellow solid: 1H
NMR (400 MHz, CDCl3) δ 7.02 (s, 2H), 7.03-7.06 (m, 4H),
7.12-7.18 (m, 6H), 7.23-7.26 (m, 4H), 7.29 (s, 4H), 7.33-7.37
(m, 6H); 13C NMR (100 MHz, CDCl3) δ 126.6, 127.2, 127.3,
127.8, 127.9, 128.5, 129.4, 130.2, 137.2, 140.1, 142.0, 142.3;
IR (KBr) 1445, 695 cm-1; HRMS (EI) m/z calcd for C34H26
434.2034, found 434.2034.
Procedure for Synthesis of 7a-A (Scheme 7). To a
solution of Pd2(dba)3‚CHCl3 (41.2 mg, 0.04 mmol) and tri-2-
furylphosphine (37.5 mg, 0.16 mmol) in THF (24 mL) were
added 1,4-diiodobenzene (1.32 g, 4.00 mmol), dimethyl(2-
pyridyl)vinylsilane 1a (1.30 g, 8.00 mmol), and triethylamine
(871.1 mg, 8.40 mmol) at room temperature under argon and
the reaction mixture was stirred at 60 °C for 18 h. After cooling
to room temperature, toluene (5 mL) was added to the reaction
mixture. This mixture was extracted with 1 N aqueous HCl
(6 × 10 mL). The combined aqueous phase was neutralized
by adding NaHCO3 and then was extracted with EtOAc (3 ×
30 mL). Drying over Na2SO4 and removal of the solvents under
reduced pressure afforded 7a-A (1.51 g, 94%): 1H NMR (300
MHz, CDCl3) δ 0.49 (s, 12H), 6.64 (d, J ) 19.2 Hz, 2H), 6.99
(d, J ) 19.2 Hz, 2H), 7.17-7.24 (m, 2H), 7.43 (s, 4H), 7.52-
7.63 (m, 4H), 8.80 (dm, J ) 5.1 Hz, 2H); 13C NMR (75 MHz,
CDCl3) δ -3.4, 122.9, 126.3, 126.8, 129.5, 134.1, 138.0, 145.4,
150.3, 166.9; IR (KBr) 1603, 1574, 1557, 1509, 1451, 1420, 1248
cm-1; HRMS (EI) m/z calcd for C24H28N2Si2: 400.1791, found
400.1793.
Typical Procedure for Synthesis of 8 (Scheme 8). A
mixture of 7a-A (398.7 mg, 1.00 mmol), iodobenzene (408.5
mg, 2.00 mmol), triethylamine (253.0 mg, 2.50 mmol), Pd2-
(dba)3‚CHCl3 (23.4 mg, 0.025 mmol), and tris(2,4-di-tert-
butylphenyl)phosphite (31.6 mg, 0.05 mmol) in dry dioxane
(3.0 mL) was stirred at 80 °C for 20 h under argon. After
cooling the reaction mixture to room temperature, the catalyst
and salts were removed by filtration through a short silica gel
pad (EtOAc). The filtrate was evaporated and the residue was
chromatographed on silica gel (hexane/EtOAc) to afford 8a-
Aa (429 mg, 78%): 1H NMR (400 MHz, CDCl3) δ 0.25 (s, 12H),
6.54 (s, 2H), 7.10 (s, 4H), 7.17 (ddd, J ) 7.6, 4.8, 1.6 Hz, 2H),
7.27-7.34 (m, 10H), 7.49 (dt, J ) 7.2, 1.6 Hz, 2H), 7.54 (td, J
) 7.6, 1.6 Hz, 2H), 8.78 (dt, J ) 4.8, 1.6 Hz, 2H); 13C NMR
(100 MHz, CDCl3) δ -1.7, 122.3, 126.2, 127.2, 127.6, 127.7,
128.9, 129.0, 133.6, 141.3, 142.8, 149.8, 158.2, 167.5; IR (KBr)
1586, 1244, 857 cm-1; HRMS (EI) m/z calcd for C36H36N2Si2
552.2417, found 552.2418.
Typical Procedure for Synthesis of 12 (Scheme 9). To
a mixture of 8a-Aa (56.3 mg, 0.10 mmol), iodobenzene (81.9
mg, 0.40 mmol), and PdCl2(PhCN)2 (3.7 mg, 9.6 µmol) in dry
THF (1.0 mL) was slowly added a solution of Bu4NF (0.25
mmol, 1.0 M) in THF by syringe pump (0.0167 mmol/min) at
40 °C under argon, and the resultant mixture was further
stirred at 40 °C for 3 h. After cooling the reaction mixture to
room temperature, the catalyst and salts were removed by
filtration through a short silica gel pad (EtOAc). The subjection
of the crude mixture to gel permeation chromatography
(CHCl3) afforded 12Aaa (26.7 mg, 60%) as yellow solid: 1H
NMR (400 MHz, CDCl3) δ 7.02 (s, 2H), 7.15-7.24 (m, 10H),
Typical Procedure for Synthesis of 9 Where Two Aryl
Units in the Polymer Backbone are the Same (Scheme
10). A mixture of 1a (164.3 mg, 1.01 mmol), 1,4-diiodobenzene
(329.6 mg, 1.00 mmol), triethylamine (253.0 mg, 2.50 mmol),
and Pd[P(t-Bu)3]2 (25.0 mg, 50.0 µmol) in dry dioxane (2.0 mL)
was stirred at 80 °C for 27 h under argon. After cooling the
reaction mixture to room temperature, H2O (2 mL) was added
to the mixture. The mixture was extracted with CHCl3. The
combined organic phase was washed with brine and dried over
Na2SO4. The subjection of the crude mixture to gel permeation
chromatography (CHCl3) afforded 9a-AA (222.5 mg, 93%): 1H
NMR (400 MHz, CDCl3) δ 0.20-0.25 (br, 6H), 6.50-6.60 (br,
1H), 7.00-7.30 (m, 5H), 7.30-7.60 (m, 2H), 8.73 (br, 1H); 13
C
NMR (75 MHz, CDCl3) δ -1.93, -1.88, 122.4, 122.5, 126.6,
126.8, 127.0, 129.1, 133.7, 133.8, 141.4, 141.5, 141.8, 142.1,
142.3, 149.9, 150.0, 157.8, 157.9, 167.6, 167.7; IR (CHCl3) 1576,
1248, 1215 cm-1
.
Typical Procedure for Synthesis of 9 Where Two Aryl
Units in the Polymer Backbone Are Different (Scheme
11). A mixture of 7a-A (405.3 mg, 1.01 mmol), 1,3-diiodoben-
zene (333.6 mg, 1.01 mmol), triethylamine (98.0 mg, 2.50
mmol), and Pd[P(t-Bu)3]2 (25.7 mg, 50.0 µmol) in dry dioxane
(3.0 mL) was stirred at 80 °C for 12 h under argon. After
cooling the reaction mixture to room temperature, H2O (2 mL)
was added to the mixture. The mixture was extracted with
CHCl3. The combined organic phase was washed with brine
and dried over MgSO4. The subjection of the crude mixture to
gel permeation chromatography (CHCl3) afforded 9a-AD (405.7
mg, 85%): 1H NMR (400 MHz, CDCl3) δ 0.19 (br, 12H), 6.49
(br, 2H), 6.90-7.20 (br, 9H), 7.30-7.60 (br, 5H), 8.73 (br, 2H);
13C NMR (100 MHz, CDCl3) δ -1.7, 122.3, 125.5, 126.8, 127.2,
128.89, 128.95, 133.6, 141.3, 143.0, 149.8, 158.3, 167.5; IR
(CHCl3) 1217 cm-1
.
Typical Procedure for Synthesis of 13 (Scheme 12).
To a mixture of 9a-AA (92.8 mg, 0.39 mmol for unit),
4-iodobenzoic acid ethyl ester (214.0 mg, 0.78 mmol), and
PdCl2(PhCN)2 (14.9 mg, 38.8 µmol) in dry THF (1.5 mL) was
added a solution of Bu4NF (0.78 mmol, 1.0 M) in THF at room
temperature, and the resulting mixture was stirred at 60 °C
for 5 h under argon. After cooling the reaction mixture to room
temperature, the mixture was evaporated. The subjection of
the crude mixture to gel permeation chromatography (CHCl3)
afforded 13AAm (69.6 mg, 79%): 1H NMR (400 MHz, CDCl3)
δ 1.32 (br, 3H), 4.30 (br, 2H), 6.90-7.50 (m, 7H), 7.60-7.90
(m, 2H); IR (CHCl3) 1715, 1605 cm-1. Mn ) 9235. PD ) 2.06.
Acknowledgment. This work was supported in part
by the Grant-in-Aid for Scientific Research and Project
of Micro-Chemical Technology for Production, Analysis
and Measurement Systems of NEDO, Japan. We greatly
appreciate Prof. Shigehiro Yamaguchi (Nagoya Univer-
sity, Japan) and Prof. Kohei Tamao (Kyoto University,
Japan) for making us aware of the existence of interest-
ing organic fluorophores, including their own. We thank
Mr. Hideyuki Murata and Mr. Hideyuki Nomura (Micro-
Chemical Process Technology) for their help in DLS and
FE-SEM experiments.
Supporting Information Available: Experimental pro-
cedures and analytical and spectroscopic data of all compounds
prepared in this study. This material is available free of charge
via the Internet at http://pubs.acs.org.
JO0477401
2792 J. Org. Chem., Vol. 70, No. 7, 2005