Journal of the American Chemical Society
COMMUNICATION
Table 1. Photophysical and Electrochemical Properties of 1 and 2a
λabs/nma (ε/MÀ1 cmÀ1
)
λem/nma (Φ)
Ered1/2/Vd
Eox1/2/Vd
Eg/eVf
c
1
12À
769, 850 (5.37),b 980, 1100
459, 485 (4.75)
1179 (3.2 Â 10À5
)
À0.42, À0.47
0.52, 1.11
1.02 (0.94)g
2.36
528 (0.91)c
2
22À
578, 627 (5.12),b 683, 750
792 (1.4 Â 10À3 c
)
À0.42, À0.61
1.12(IR)e
1.58 (1.54)g
420, 436 (4.38)
507 (0.60)c
2.64
a Measured in dichloromethane. b The absorption coefficients are given in logarithmic units. c Quantum yield using ICG as a reference. d Cyclic
voltammetry on a carbon electrode with Bu4NClO4 in CH2Cl2 (0.1 M), corrected for ferrocene. e Irreversible. f Band gap determined from
absorption onset. g Band gap determined from cyclic voltammetry.
Scheme 3. Reduction of Tri-p-QM 1 and Di-p-QM 2
In view of the availability of methods to synthesize similar carbon
networks bridged by heteroatoms connected with a different
connectivity,10,11 we expect that this new molecular design will
allow us to study systematically the fundamental and applied
aspects ofbiradicaloidmoleculesfor researchonnonlinearoptics12
and organic electronics.13
’ ASSOCIATED CONTENT
S
Supporting Information. Experimental details. This ma-
b
’ AUTHOR INFORMATION
Corresponding Author
tsuji@chem.s.u-tokyo.ac.jp; nakamura@chem.s.u-tokyo.ac.jp
’ ACKNOWLEDGMENT
Second, they are air-stable. For instance, the tri-p-QM 1
remained unchanged under air and ambient light for 6 months
as a solid and over 2 months in solution, and it showed a half-life
of 2 days, even in air-saturated dichloromethane under contin-
uous UV irradiation (360 nm, 4 W).
Third, tri-p-QM 1 undergoes reversible, stepwise two-electron
reduction and oxidation (Table 1), while 2 can be two-electron-
reduced reversibly but oxidized only once and irreversibly. The
HOMO levels of 1 and 2 estimated from the oxidation potential
obtained by differential pulse voltammetry were quite deep:
À5.32 eV (1) and À5.92 eV (2), partly accounting for the
resistance to oxidation.
This Communication is dedicated to Prof. Christian Bruneau
on the occasion of his 60th birthday. We thank MEXT for
financial support (KAKENHI for E.N., No. 22000008; H.T., No.
20685005; and the Global COE Program for Chemistry In-
novation). This work was partly supported by the Strategic
Promotion of Innovative R&D, JST. Z.X. thanks the JSPS
Postdoctoral Fellowship for Foreign Researchers (No. P 09046).
’ REFERENCES
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Fourth, chemical reduction of 1 and 2 gave a stable dianionic
product. Comparison of the reduction potentials of 1 (À0.42 and
À0.47 V, Table 1) and 2 (À0.42 and À0.61 V) suggests that 1
may be reduced directly to a dianion, while 2 may be reduced in a
stepwise manner, which was indeed the case. Thus, we reduced 2
with a mild reducing agent, tetrabutylammonium borohydride
(TBABH4, 2 equiv) in anhydrous dichloromethane (Scheme 3).9
Upon addition of TBABH4 at room temperature, the blue color
of 2 changed to dark green, suggestive of a radical anion 23 À
(λmax 683 nm; spectrum in Supporting Information), and
gradually to yellow (436 nm) of a dianion 22À, which exhibited
green luminescence (507 nm, Φ = 0.60; excitation at 420 nm).
On the other hand, a green solution of 1 immediately changed to
the orange-red (485 nm) of dianion 12À, which exhibited orange
emission (528 nm, Φ = 0.91; excitation at 459 nm). Such high-
efficiency emission is characteristic of the parent planar pheny-
lenevinylene motif.6
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In summary, we have synthesized carbon-bridged tri-p-QM
compound 1, which exhibits distinctive biradical characteristics,
and its di-p-QM analogue 2, which exhibits similar characteristics.
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dx.doi.org/10.1021/ja206060n |J. Am. Chem. Soc. 2011, 133, 16342–16345