Bis-phosphoryl-bridged stilbenes synthesized by an intramolecular cascade cyclization

Article abstract of DOI:10.1021/ol7030608

Bis-phosphoryl-bridged stilbenes have been synthesized using an intramolecular cascade cyclization. They show intense blue fluorescences at longer wavelengths with higher quantum yields compared to those of the known element-bridged stilbenes. In addition

Full text of DOI:10.1021/ol7030608

ORGANIC
LETTERS
2008
Vol. 10, No. 5
913-916
Bis-Phosphoryl-Bridged Stilbenes
Synthesized by an Intramolecular
Cascade Cyclization
Aiko Fukazawa,† Masanao Hara,† Toshihiro Okamoto,† Eun-Cheol Son,‡
|
Caihong Xu,§ Kohei Tamao,‡, and Shigehiro Yamaguchi*,†,§
Department of Chemistry, Graduate School of Science, Nagoya UniVersity, and
SORST, Japan Science and Technology Agency (JST), Chikusa,
Nagoya 464-8602, Japan, and Institute for Chemical Research,
Kyoto UniVersity, Uji, Kyoto 611-0011, Japan
yamaguchi@chem.nagoya-u.ac.jp
Received December 19, 2007
ABSTRACT
Bis-phosphoryl-bridged stilbenes have been synthesized using an intramolecular cascade cyclization. They show intense blue fluorescences
at longer wavelengths with higher quantum yields compared to those of the known element-bridged stilbenes. In addition, they have much
lower reduction potentials due to the inductive effect of phosphoryl groups. The incorporation of the phosphoryl moiety is an effective way
for the construction of highly electron-accepting
π-conjugated systems.
Ladder-type π-conjugated molecules having fully ring-fused
structure represent an important class of fundamental materi-
als in the current organic electronics.1 Their rigid coplanar
frameworks endow them with a set of superior properties,
such as an intense luminescence and a high charge carrier
mobility. For further design to modulate their inherent
electronic and structural characteristics, one promising way
is the incorporation of main group elements into the ladder
framework.2 A series of ladder molecules with various
elements, such as B,3 Si,4 N,5 and S,6,7 have been synthesized,
some of which indeed show a high performance in organic
light-emitting diodes and transistors. Among the elements,
the incorporation of phosphorus atoms is of particular
interest, because of the facile functionalization of the
phosphorus atom, that is, oxidation to phosphine oxides or
sulfides, or complexation with Lewis acids or transition
(3) (a) Agou, T.; Kobayashi, J.; Kawashima, T. Org. Lett. 2006, 8, 2241.
(b) Agou, T.; Kobayashi, J.; Kawashima, T. Chem. Commun. 2007, 3204.
(c) Agou, T.; Kobayashi, J.; Kawashima, T. Chem.-Eur. J. 2007, 13, 8051.
(4) (a) Yamaguchi, S.; Xu, C.; Tamao, K. J. Am. Chem. Soc. 2003, 125,
13662. (b) Xu, C.; Yamada, H.; Wakamiya, A.; Yamaguchi, S.; Tamao, K.
Macromolecules 2004, 37, 8978. (c) Xu, C.; Wakamiya, A.; Yamaguchi,
S. J. Am. Chem. Soc. 2005, 127, 1638. (d) Mouri, K.; Wakamiya, A.;
Yamada, H.; Kajiwara, T.; Yamaguchi, S. Org. Lett. 2007, 9, 93.
(5) (a) Kaszynski, P.; Dougherty, D. A. J. Org. Chem. 1993, 58, 5209.
(b) Patil, S. A.; Scherf, U.; Kadashchuk, A. AdV. Funct. Mater. 2003, 13,
609. (c) Wakim, S.; Bouchard, J.; Blouin, N.; Michaud, A.; Leclerc, M.
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Org. Chem. 2007, 72, 5119.
† Nagoya University.
‡ Kyoto University.
§ JST.
| Present address: RIKEN, 2-1 Hirosawa, Wako, Saitama 351-0198,
Japan.
(1) (a) Scherf, U. J. Mater. Chem. 1999, 9, 1853. (b) Watson, M. D.;
Fechtenko¨tter, A.; Mu¨llen, K. Chem. ReV. 2001, 101, 1267. (c) Anthony, J.
E. Chem. ReV. 2006, 106, 5028.
(2) Yamaguchi, S.; Xu, C.; Okamoto, T. Pure Appl. Chem. 2006, 78,
721.
(6) (a) Haryono, A.; Miyatake, K.; Natori, J.; Tsuchida, E. Macromol-
ecules 1999, 32, 3146-3149. (b) Sirringhaus, H.; Friend, R. H.; Wang, C.;
Leuninger, J.; Mu¨llen, K. J. Mater. Chem. 1999, 9, 2095.
10.1021/ol7030608 CCC: $40.75
© 2008 American Chemical Society
Published on Web 02/09/2008

metals, which allows the electronic and thus properties
tuning. This advantage has been well demonstrated in various
π-conjugated systems consisting of phosphole,8 dibenzo-
phosphole,9 or dithienophosphole10 as a building unit. Among
the phosphole skeletons, we now focus our attention on the
oxidized form, phosphole oxide, and designed bis-phosphoryl
(P(dO)R)-bridged stilbenes 1 (Figure 1) as a new entity of
in terms of dipole moment. Third, the high chemical stability
of the phosphoryl functionality affords facile handling as a
stable material. Herein we report the efficient synthesis of
the bis-phosphoryl-bridged stilbenes based on a new intra-
molecular cascade cyclization. Throughout the study of their
fundamental properties, we also show the potential utility
of this skeleton as a building unit for π-electron materials.
Our new cyclization employs a simple bis(2-bromo-
phenyl)acetylene 5 as the starting material and comprises of
four-step sequential procedures, as shown in Scheme 1. Thus,
Scheme 1. Intramolecular Cascade Cyclization
Figure 1. Kohn-Sham HOMO and LUMO energy levels of
bridged stilbenes based on the calculations at the B3LYP/6-31G(d)
level.
compound 5 was first dilithiated with t-BuLi in THF,
followed by treatment with PhP(NEt2)Cl to produce bis-
(aminophosphanyl) derivative 6 in situ. An excess amount
of PCl3 (6 mol amounts) was then added to the reaction
mixture at room temperature. The final step was the oxidation
with H2O2, which afforded the bis-phosphoryl-bridged stil-
benes 1 as a mixture of cis and trans isomers. Notably, these
isomers could be easily separated from each other by silica
gel column chromatography, due to the significant difference
in their polarity, and were obtained in 34 and 24% yields,
respectively. Their structures were verified by NMR spec-
troscopy as well as mass spectrometry and finally by X-ray
crystallographic analyses.11 For this cyclization, the addition
of excess PCl3 is crucial. Decreasing the amount of PCl3 as
well as the replacement with HCl resulted in much lower
yields.
A plausible mechanism for the cyclization is shown in
Scheme 2. Because the bis(aminophosphanyl) intermediate
6 does not undergo cyclization at ambient temperature, the
initial step is likely the chlorination of one or both amino
groups with PCl3. The 31P NMR spectrum of the reaction
mixture indeed confirms the formation of (Et2N)PCl2. In the
produced 7a or 7b, one phosphanyl group acts as a nucleo-
phile12 and the other acts as an electrophile, and a nucleo-
philic cascade cyclization proceeds to produce a doubly
cyclized phosphonium intermediate 8a or 8b. Butters and
Winter reported a similar monocyclization from 2-(amino-
the ladder materials. In this skeleton, a fused phospholo-
[3,2-b]phosphole dioxide substructure would provide several
attractive features. First, the double substitution with the
strong electron-withdrawing phosphoryl group would lower
the LUMO level and provides a highly electron-accepting
nature. According to the molecular orbital calculations
(Figure 1), the bis-phosphoryl-bridged stilbene 1 has the
lowest-lying LUMO among a series of bridged stilbenes
having carbon (2), silicon (3), and phosphorus (4) bridges.
Second, the incorporation of two phosphoryl bridges gives
rise to two geometrical isomers with respect to the directions
of the PdO bonds, which would be different from each other
(7) (a) Sashida, H.; Yasuike, S. J. Heterocycl. Chem. 1998, 35, 725. (b)
Takimiya, K.; Kunugi, Y.; Konda, Y.; Ebata, H.; Toyoshima, Y.; Otsubo,
T. J. Am. Chem. Soc. 2006, 128, 3044. (c) Takimiya, K.; Ebata, H.;
Sakamoto, K.; Izawa, T.; Otsubo, T.; Kunugi, Y. J. Am. Chem. Soc. 2006,
128, 12604. (d) Yamamoto, T.; Takimiya, K. J. Am. Chem. Soc. 2007, 129,
2224. (e) Wang, C.-H.; Hu, R.-R.; Liang, S.; Chen, J.-H.; Yang, Z.; Pei, J.
Tetrahedron Lett. 2005, 46, 8153. (f) Wong, K. T.; Chao, T. C.; Chi, L.
C.; Chu, Y. Y.; Balaiah, A.; Chiu, S. F.; Liu, Y. H.; Wang, Y. Org. Lett.
2006, 8, 5033.
(8) Recent reviews: (a) Hobbs, M. G.; Baumgartner, T. Eur. J. Inorg.
Chem. 2007, 3611. (b) Baumgartner, T.; Re´au, R. Chem. ReV. 2006, 106,
4681. (c) Mathey, F. Angew. Chem., Int. Ed. 2003, 42, 1578. (d) Hissler,
M.; Dyer, P. W.; Re´au, R. Coord. Chem. ReV. 2003, 244, 1.
(9) (a) Makioka, Y.; Hayashi, T.; Tanaka, M. Chem. Lett. 2004, 33, 44.
(b) Su, H.-C.; Fadhel, O.; Yang, C.-J.; Cho, T.-Y.; Fave, C.; Hissler, M.;
Wu, C.-C.; Re´au, R. J. Am. Chem. Soc. 2006, 128, 983.
(10) (a) Baumgartner, T.; Neumann, T.; Wirges, B. Angew. Chem., Int.
Ed. 2004, 43, 6197. (b) Baumgartner, T.; Bergmans, W.; Ka´rpa´ti, T.;
Neumann, T.; Nieger, M.; Nyula´szi, L. Chem.-Eur. J. 2005, 11, 4687. (c)
Dienes, Y.; Durben, S.; Ka´rpa´ti, T.; Neumann, T.; Englert, U.; Nyula´szi,
L.; Baumgartner, T. Chem.-Eur. J. 2007, 13, 7487.
(11) See Supporting Information for detail.
(12) (a) Butters, T.; Haller-Pauls, I.; Winter, W. Chem. Ber. 1982, 115,
578. (b) Vedejs, E.; Steck, P. L. Angew. Chem., Int. Ed. 1999, 38, 2788.
914
Org. Lett., Vol. 10, No. 5, 2008

Scheme 2. Plausible Mechanism
Figure 2. UV-vis absorption and fluorescence spectra of cis-1
(blue broken line), trans-1 (blue solid line), and 4 (black solid line)
in CH2Cl2 at rt. The photographs of 4 and trans-1 under irradiation
of a black light at 365 nm are given in inset.
phosphanyl)phenylacetylene mediated by treatment with HCl,
which gives the corresponding benzophosphole.13 Notably,
whereas their monocyclization required harsh condition
(heating at 90 °C in neat condition), our double cyclization
proceeds smoothly even at room temperature. These results
demonstrate that the presence of two phosphanyl groups
facilitates the cyclization and reflects the ambiphilic nature
of the chloro-substituted phosphanyl group. Finally, the direct
oxidation of 8 with H2O2 or a stepwise elimination-oxidation
process via 414 gives the bis-phosphoryl-bridged stilbenes
1.
as follows: (1) In the UV-visible absorption spectra, both
cis- and trans-1 have absorption maxima at 395 nm, which
are the longest wavelengths among those of the bridged
stilbenes. Notably, the absorption spectra of the cis and trans
isomers are almost identical to each other. This indicates
that the configuration of the phosphoryl group does not affect
the absorption properties.16 In addition, λabs of the bis-
phosphoryl-bridged stilbenes 1 are significantly red-shifted
compared to those of the known diaryl-fused phosphole
oxides 9 and 10 due to the effective extension of the
π-conjugation. (2) The bis-phosphoryl-bridged stilbenes 1
exhibit intense blue emissions with the fluorescence quantum
yield ΦF of almost unity, whereas they have large Stokes
shifts of 85 nm (4500 cm-1). This is in contrast to the faint
fluorescence of the phosphanyl counterpart 4 (ΦF ) 0.07).
It is also noteworthy that the λem of 1 is more than 110 nm
longer than that of the carbon analogue 2, demonstrating the
considerable effect of the phosphoryl groups. The ΦF values
of cis- and trans-1 are extremely higher even compared to
Bis-phosphanyl-bridged stilbene 4 was also prepared for
the sake of comparison with 1, as shown in Scheme 3. Thus,
Scheme 3. Synthesis of Bis-Phosphanyl-Bridged Stilbene 4
Table 1. Photophysical Data for the Bridged Stilbenes and
Related Compounds
the reduction of the pure trans isomer of 1 with an excess
amount of HSiCl3 afforded 4 in 58% yield as a mixture of
cis and trans isomers.15 All attempts to separate these isomers
from each other by chromatography failed due to their
negligible difference in polarity.
UV-vis absorption and fluorescence spectra of the bis-
phosphoryl-bridged stilbenes cis- and trans-1 are shown in
Figure 2. Their photophysical data are summarized in Table
1, together with those of carbon- (2), silicon- (3), and
phosphanyl-bridged stilbenes (4),16 and fused phosphole
oxides (dibenzophosphole oxides 9 and dithienophosphole
oxide 10) for comparison. Several features are summarized
UV-vis absorption
fluorescence
a
b
c
λabs
ꢀ/103
λem
τs
(ns)
cmpd
(nm)
(M-1cm-1
)
(nm)
ΦF
cis-1e
trans-1e
2f,g
395
395
322
360
351
332
366
6.60
6.92
28.2
12.0
13
480
480
367
426
415
366
453
0.99 d
0.98 d
0.92
15.7
15.7
1.6
3 f,g
0.58
5.5
4e,h
0.07 d
0.042
0.57
1.4, 9.8
-
-
9e,i
3.01
2.14
10e,j
a Only the longest absorption maxima are shown. b Emission maxima
upon excitation at the absorption maximum wavelengths. c Fluorescence
lifetimes within (0.5 ns error. d Absolute fluorescence quantum yields
determined by a calibrated integrating sphere system within (3% errors.
e In CH2Cl2. f In THF. g Ref 4a. h Mixture of cis and trans isomers. i Ref
9b. j Ref 10a.
(13) Butters, T.; Winter, W. Chem. Ber. 1984, 117, 990.
(14) The formation of cis- and trans-4 as the intermediates was confirmed
by the 31P NMR spectrum of the reaction mixture.
(15) The ratio of two isomers was found to be 1/1.1, which was estimated
1
by the integral ratio of the H NMR and 31P NMR spectra.
Org. Lett., Vol. 10, No. 5, 2008
915

those of the hitherto known phosphole oxide derivatives.8-10
(3) Both cis- and trans-1 have fluorescence lifetime τs of
15.7 ns, which is significantly longer than those of 2-4.
Figure 3 shows a comparison of the radiative rate constant
Figure 4. Cyclic voltammograms of cis-1 (solid line), trans-1
(broken line), and 2 (dotted line) in THF (1 mM) containing
-
Bu4N+PF6 (0.1 M) with a scan rate of 50 mV s-1
.
1 show the first reversible reduction waves with reduction
potentials of -1.63 and -1.67 V (vs Fc/Fc+) and the second
irreversible reduction waves with peak potentials of -2.14
and -2.35 V, respectively. The first reduction potentials of
the bis-phosphoryl-bridged stilbenes 1 are approximately 1.5
V more positive compared to that of the carbon-bridged one
2 (-3.17 V). These observations demonstrate that the
phosphoryl-bridges substantially alter the nature of the
stilbene to be highly electron-accepting. These results are
consistent with the results of the molecular orbital calcula-
tions as shown in Figure 1.
In summary, the bis-phosphoryl-bridged stilbenes 1, ef-
ficiently synthesized by a newly developed intramolecular
cascade cyclization, show unusual photophysical properties
as well as a high electron-accepting ability, which are totally
different from the known bridged stilbenes. These results
demonstrate not only the potential utility of this skeleton as
a building unit but also the effectiveness of the incorporation
of the phosphoryl moiety for construction of highly electron-
accepting π-conjugated systems. Further study of the syn-
thesis of a variety of phosphoryl-bridged ladder molecules
based on the present methodology is now in progress.
Figure 3. Comparison of radiative rate constant (kr) and non-
radiative rate constant (knr) among various element-bridged stilbenes
in solutions. Rate constants were calculated with ΦF and τs
according to the formula of kr ) ΦF/τs and knr ) (1-ΦF)/τs.
kr and nonradiative rate constant knr, calculated from ΦF and
τs, for the series of bridged stilbenes. Two trends are noted.
First, the replacement of the bridging element from carbon
to the heavier main group element, such as silicon or
phosphorus, causes a dramatic decrease in the kr value (5.8
× 108, 1.1 × 108, and 6.2 × 107 s-1 for 2, 3, and trans-1,
respectively). This trend is consistent with a decrease in the
oscillator strength for the lowest-energy transition, which is
confirmed by the TD-DFT calculations.11 Second, the bis-
phosphoryl-bridged derivatives 1 also have extremely smaller
nonradiative rate constant knr (1.3 × 106 s-1) compared to
other derivatives such as 2 (5.0 × 107 s-1) and 3 (7.6 × 107
s-1). The phosphoryl-bridge significantly impedes the non-
radiative decay process, despite the large Stokes shift.
Acknowledgment. This work was partially supported by
a Grant-in-Aid for Scientific Research on Priority Areas (No.
17069011) and SORST, Japan Science and Technology
Agency.
We also evaluated the electrochemical properties of the
bis-phosphoryl-bridged stilbenes 1 by means of cyclic
voltammetry. The cyclic voltammograms of cis- and trans-1
and carbon-bridged stilbene 2 are shown in Figure 4. The
cis- and trans-isomers of bis-phosphoryl-bridged stilbenes
Supporting Information Available: Experimental de-
tails, spectral data for all new compounds, crystallographic
data, and ORTEP drawings of cis- and trans-1, photophysical
data, electrochemical data, and results of theoretical calcula-
tions. This material is available free of charge via the Internet
at http://pubs.acs.org.
(16) TD-DFT calculations for compounds 1 and 4 demonstrated that the
transition energies for the lowest-energy excited state are nearly identical
to each other between the cis and trans isomers. See Supporting Information
for detail.
OL7030608
916
Org. Lett., Vol. 10, No. 5, 2008