921
solvatochromism of absorption and fluorescence spectra of 3e, 3f,
4a, and 4e using eight solvents with different polarity (Table S5 in
SI16). With increasing the solvent polarity from cyclohexane
(³* = 0) to DMF (³* = 0.88), -ab/-em of 3f and 4a shifted to
longer wavelength by 670/1810 and 1090/3230 cm¹1, respective-
ly, showing a good linear correlation with ³*.13 On the other hand,
-ab/-em of 3e and 4e were insensitive to the solvent polarity and
almost constant. These results suggest that the combination of
·4-P=O/NMe2 (pull/push) or ·3-P/NO2 (push/pull) is effective
for enhancing the polarizability of the benzene-phosphole-
benzene ³-systems. The larger solvent effect on -em than on -ab
in the cases of 3f and 4a is indicative of the larger structural
change in the excited state due to the CT character.
Except for 3f and 6, the present phosphole derivatives are
moderately fluorescent with fluorescent quantum yields (Φf) of
0.04-0.66. To evaluate the substituent effects on the photophysical
processes from excited singlet (S1) states, fluorescence lifetimes
(¸f) were measured in CH2Cl2 at 22 °C. The fluorescence decays
were well-fitted by the single-exponential component. As listed in
Table 1, ¸f of 3a-3g varied from 0.12 to 2.29 ns depending on the
para-substituents. With Φf and ¸f values in hand, we calculated
rate constants for radiative decay (kr) and nonradiative decay
(internal conversion and intersystem crossing, knr) from the S1
states (Table S6 in SI16). In comparison with kr and knr of 3e
(1.4 © 108 and 6.2 © 108 s¹1, respectively), the para-substituent
effects on kr are small (kr = 0.8-1.7 © 108 s¹1), while those on knr
are relatively large (knr = 3.1-83 © 108 s¹1). It is likely that the
substituent effects on Φf depend on the change in knr rather than the
change in kr. In particular, knr of 3f and 3g are 3-14 times as large
as knr of 3e, which suggests that the rotation at the NMe2 and OMe
groups in 3f and 3g accelerates the vibronic relaxation from their
S1 states in solution.14
This paper is in celebration of the 2010 Nobel Prize awarded
to Professors Richard F. Heck, Akira Suzuki, and Ei-ichi Negishi.
References and Notes
1
Dyer, in Comprehensive Heterocyclic Chemistry III, ed. by A. R.
Katritzky, C. A. Ramsden, E. F. V. Scriven, R. J. K. Taylor, Elsevier,
Oxford, 2008, Vol. 3, Chap. 3.15, pp. 1029-1048. doi:10.1016/
2
3
For synthetic methods until the ’90s, see: F. Mathey, in Science of
Synthesis, ed. by E. J. Thomas, Georg Thieme Verlag, Stuttgart,
2000, Vol. 9, Chap. 9.14, pp. 553-600. See also recent papers cited
in refs. 1d, 1g, and 1h.
For example, see: a) Metal-Catalyzed Cross-Coupling Reactions, ed.
by A. de Meijere, F. Diederich, Wiley-VCH, Weinheim, 2004. b)
Handbook of Organopalladium Chemistry for Organic Synthesis,
ed. by E. Negishi, Wiley-Interscience, New York, 2002, Vol. 1,
Part III. d) E. Negishi, Q. Hu, Z. Huang, M. Qian, G. Wang,
Aldrichimica Acta 2005, 38, 71.
4
For transition-metal-catalyzed cross- and homo-coupling of ¡-
halophospholes, see: a) E. Deschamps, L. Ricard, F. Mathey, Angew.
Ricard, F. Mathey, Bull. Soc. Chim. Fr. 1996, 133, 33. c) T.-A.
5
6
For Sonogashira reactions of ¡-ethynylphospholes, see: Y. Matano,
Pd-catalyzed cross-coupling reactions have been applied to the
synthesis of ¢-arylated benzo[b]phosphole derivatives. a) H. Tsuji,
2263. b) A. Fukazawa, Y. Ichihashi, Y. Kosaka, S. Yamaguchi,
The redox behavior of 3-8 was studied by cyclic voltammetry
and differential pulse voltammetry (DPV). In most cases, electro-
chemical oxidations occurred irreversibly, whereas reductions
occurred reversibly or quasi-reversibly. As summarized in Table 1,
the introduction of the electron-withdrawing and electron-donating
groups at para-positions causes anodic (for 3a-3d) and cathodic
(for 3f and 3g) shifts of the reduction potentials (Ered) over the
range from ¦Ered = +0.39 to ¹0.22 V (vs. 3e).15 As expected, the
nitro groups made the most significant impact on the electron-
accepting ability of the ³-systems. The P-oxidation state (4a and
4e) and the other ¡-substituents (5-8) also exhibited their own
electronic effects on the redox properties.
7
V. Devreux, J. Wiesner, H. Jomaa, J. Rozenski, J. V. der Eycken,
8
9
We recently succeeded in preparing three kinds of phosphole-
heterole-phosphole ³-systems (heterole: pyrrole, furan, and thio-
phene) by using the Stille-type reactions of 2-tributylstannyl-5-
phenylphosphole and 2-iodo-5-phenylphosphole. Y. Matano, A.
Saito, M. Fujita, H. Imahori, Heteroat. Chem., in press.
In summary, we have established convenient methods for the
divergent synthesis of 2,5-diarylphosphole derivatives based on
cross-coupling reactions starting from the common phosphole
synthons. The electronic effects of the para-substituents attached
to the terminal ¡-phenyl groups on the optical and redox properties
of the benzene-phosphole-benzene ³-systems were evaluated for
the first time. Notably, the chemical functionalization at the
phosphorus center from ·4-P=O to ·3-P has proven to change the
electron-donating/-accepting abilities as well as the polarizability
of the whole ³-systems. The present results provide valuable
information about the synthesis and fundamental properties of
phosphole-containing hybrid ³-conjugates.
10 In the absence of CuI, the formation of biphosphole was suppressed,
but the conversion rate from 1 to 3 was slowed down significantly.
11 Pd-catalyzed homo-coupling of arylstannanes was reported. For
example, see: a) L. S. Liebeskind, S. W. Riesinger, J. Org. Chem.
1993, 58, 408. b) V. Farina, B. Krishnan, D. R. Marshall, G. P. Roth,
12 A. Saito, T. Miyajima, M. Nakashima, T. Fukushima, H. Kaji, Y.
13 Kamlet-Taft plots (M. J. Kamlet, J. L. M. Abboud, M. H. Abraham,
³* parameters showed linear solvation energy relationship with
large slopes (absolute values: 2.3 © 103 cm¹1 for 3f; 4.1 © 103 cm
¹1
for 4a). See Figure S3 in SI.16
14 Although we cannot exclude the contribution of intersystem
crossing, no phosphorescence of 3g was observed even at ¹196 °C.
15 In 3a and 4a, electrochemical reductions of the nitro groups took
place at Ered = ca. ¹1.4 V.
16 Supporting Information is available electronically on the CSJ-
We thank Dr. Hirohiko Watanabe (Hamamatsu Photonics K.K.)
for some photophysical measurements. This work was partially
supported by Grants-in-Aid (Nos. 21108511 and 22350016) from
the Ministry of Education, Culture, Sports, Science and Technology
(MEXT), Japan and Asahi glass foundation.
Chem. Lett. 2011, 40, 919-921
© 2011 The Chemical Society of Japan