Synthesis, redox properties, and electronic spectra of sterically crowded triarylphosphines conjugated with electron acceptors

Article abstract of DOI:10.1080/10426507.2012.741163

Synthesis, redox properties, and electronic spectra of the sterically crowded triarylphosphines conjugated with π-electron systems, especially electron acceptors such as carbonyl group, are briefly reviewed. The sterically crowded triarylphosphines conjug

Full text of DOI:10.1080/10426507.2012.741163

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Phosphorus, Sulfur, and Silicon and the
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Synthesis, Redox Properties, and
Electronic Spectra of Sterically Crowded
Triarylphosphines Conjugated with
Electron Acceptors
Shigeru Sasaki a , Kohji Sasaki a , Mariko Watanabe a , Kazunobu
Ogawa a , Noboru Morita a & Masaaki Yoshifuji a
a Department of Chemistry, Graduate School of Science , Tohoku
University , Aoba-ku , Sendai , Japan
Published online: 08 May 2013.
To cite this article: Shigeru Sasaki , Kohji Sasaki , Mariko Watanabe , Kazunobu Ogawa , Noboru
Morita & Masaaki Yoshifuji (2013) Synthesis, Redox Properties, and Electronic Spectra of Sterically
Crowded Triarylphosphines Conjugated with Electron Acceptors, Phosphorus, Sulfur, and Silicon and
the Related Elements, 188:1-3, 256-261, DOI: 10.1080/10426507.2012.741163
To link to this article: http://dx.doi.org/10.1080/10426507.2012.741163
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Phosphorus, Sulfur, and Silicon, 188:256–261, 2013
Copyright ꢀC Taylor & Francis Group, LLC
ISSN: 1042-6507 print / 1563-5325 online
DOI: 10.1080/10426507.2012.741163
SYNTHESIS, REDOX PROPERTIES, AND ELECTRONIC
SPECTRA OF STERICALLY CROWDED
TRIARYLPHOSPHINES CONJUGATED WITH ELECTRON
ACCEPTORS
Shigeru Sasaki, Kohji Sasaki, Mariko Watanabe,
Kazunobu Ogawa, Noboru Morita, and Masaaki Yoshifuji
Department of Chemistry, Graduate School of Science, Tohoku University,
Aoba-ku, Sendai, Japan
GRAPHICAL ABSTRACT
Abstract Synthesis, redox properties, and electronic spectra of the sterically crowded tri-
arylphosphines conjugated with π-electron systems, especially electron acceptors such as
carbonyl group, are briefly reviewed. The sterically crowded triarylphosphines conjugated
with various π-electron systems were synthesized from the common synthetic intermediate,
(bromoaryl)phosphine, by conventional manner. The sterically crowded triarylphosphines con-
jugated with the electron acceptors exhibit visible absorption and fluorescence with large Stokes
shift. Large solvent effect, redshift in polar solvent, and good correlation with the difference
of the oxidation potential of the phosphine moiety and the reduction potential of the acceptor
moiety suggest polar excited state resulting from HOMO-LUMO transition.
Keywords Triarylphosphine; π-electron systems; redox potential; electronic spectrum
Received 16 September 2012; accepted 15 October 2012.
We thank Grants-in-Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science and
Technology, Japan (Nos. 22605001, 17310063, 15550025) for financial support, Research and Analytical Center
for Giant Molecules, Graduate School of Science, Tohoku University, for taking mass spectra and elemental
analysis.
Address correspondence to Shigeru Sasaki, Department of Chemistry, Graduate School of Science, Tohoku
University, 6-3 Aramaki-Aoba, Aoba-ku, Sendai 980-8578, Japan. E-mail: ssasaki@m.tohoku.ac.jp
256

STERICALLY CROWDED TRIARYLPHOSPHINES
257
INTRODUCTION
Sterically crowded triarylphosphines such as 1 are reversibly oxidized at low potential
to the stable radical cations because of the synergetic effect of the high HOMO resulting
from the structural change around the phosphorus and the steric protection by the bulky
substituents (Chart 1).1 In addition, they exhibit unique properties such as red-shifted
absorption and emission spectra, helicity of the propeller composed of the three aromatic
rings, and reversible formation of the phosphonium salts.1 We have synthesized sterically
crowded triarylphosphines bearing various functional sites such as donors, acceptors, and
radicals, and revealed their redox properties.2 Among the compounds we surveyed, the
sterically crowded triarylphosphines bearing quinones such as 2 worth mentioning. They
exhibit purple to blue color resulting from the intramolecular charge transfer from the
phosphine to the quinone moieties, and allow construction of the extended structures.2e
Very recently, we found that the sterically crowded triarylphosphines conjugated with rather
simple electron-accepting substituents such as formyl and carbonyl groups exhibit intense
color.2g In this paper, we briefly review the sterically crowded triarylphosphines conjugated
with electron-accepting substituents especially focusing on the electronic spectra and the
redox properties.
RESULTS AND DISCUSSION
Synthesis and Structures
We synthesized the sterically crowded triarylphosphines bearing quinones such as 2
by Suzuki coupling of the arylboronic acid derivatives prepared from (bromoaryl)phosphine
such as 7 with haloquinones such as dichloronaphthoquinone (Scheme 1).2e The sterically
crowded triarylphosphines bearing electron acceptors such as 3 and 4 were also synthesized
from (bromoaryl)phosphine 7 by conventional manner.2 Triarylphosphines 3 and 4 can be
used as key synthetic intermediates for the synthesis of the extended π-conjugated systems
such as 6 and 5.2g
Molecular structures around the phosphorus of 3 and 4 obtained by X-ray crystallog-
raphy show no significant difference from that of 1 (C–P–C angles, 1: 111.5◦, 3: 111.9◦,
4: 111.6◦ and 111.2◦). As far as coplanarity of the aromatic ring and the carbonyl group in
Chart 1 Sterically crowded triarylphosphines conjugated with π-electron systems.

258
S. SASAKI ET AL.
Scheme 1 Synthesis of the sterically crowded triarylphosphines conjugated with π-electron systems.
the crystal is concerned, the formyl group of 3 is coplanar with the aromatic ring (dihedral
angle as 1.585◦) while benzoyl group of 4 shows considerable torsion (dihedral angle as
24.28◦ and 29.42◦). Introduction of the electron-withdrawing substituent in place of i-Pr
group leads to downfield shift of the 31P NMR chemical shifts.
Redox Properties and Electronic Spectra
Redox properties of the sterically crowded triarylphosphines bearing the electron-
withdrawing group were studied by cyclic voltammetry and the redox and spectral properties
are summarized in Table 1. All compounds shown in Chart 1 show reversible redox waves
corresponding to the oxidation of the phosphine moiety. Introduction of the electron-
accepting substituents raises oxidation potential of the phosphine moiety. They, except
for 6, also show redox waves corresponding to the reduction of the electron-acceptor
moiety although the shape of the waves depends on the redox stability of the acceptor
group.
Triarylphosphines 3, 4, and 5 exhibit intense orange to red color and the correspond-
ing absorptions are observed in visible region. They also exhibit weak fluorescence with
large Stokes shift around the complementary colors. The visible absorption and the emis-
sion show considerable redshift in dichloromethane as compared with those in n-hexane
indicating polar excited state, and especially large solvent effect in the fluorescence spectra
suggests emission from the highly polar excited state formed by considerable structural
change and stabilization by polar solvent such as twisted intramolecular charge transfer

STERICALLY CROWDED TRIARYLPHOSPHINES
259
Scheme 2 Excited state of 3. (Color figure available online).
(TICT) (Scheme 2).3 Preliminary TD-DFT calculation of UV–Vis spectrum of 3 (RB3LYP
6–311+G(2d,p)) showed good agreement with the experiment, where contribution of the
two transitions (λmax = 433.14 nm, f = 0.1487, HOMO to LUMO; λmax = 332.03 nm, f =
0.1372, HOMO to LUMO+1) were predicted.4 HOMO mainly consists of the phosphorus
lone pair, while LUMO and LUMO+1 consist of the π-orbitals of the 2,6-i-Pr2-4-CHO-
C6H2 group and the remaining two aromatic rings (Tip groups), respectively (Figure 1).
The experimental results show the absorption sensitive to the polarity of the solvent around
400 nm and the other around 330 nm, which is insensitive to the solvent similarly to 1. Since
both the longest absorption and the difference of the oxidation and the reduction potentials
(ꢀE) of the compounds shown in Table 1 have close relation with the HOMO-LUMO gap,
the longest absorption and ꢀE have a good correlation. The crowded triarylphosphines with
Figure 1 HOMO and LUMO of 3. (Color figure available online).

260
S. SASAKI ET AL.
Table 1 Redox and spectral properties of the sterically crowded triarylphosphines conjugated with electron
acceptors and the related compounds
1E1/2ox/Va
1E1/2red/Va
ꢀE/Vb
λ
max
(ε)/nm
λ
/nmc
max
Stokes shift/nmd
abs
em
Compound
1
0.33
n-Hexane
326 (14,300)
Dichloromethane
327 (13,500)
n-Hexane
332 (17,000)
534 (2100)
Dichloromethane
333 (16,600)
557 (1700)
n-Hexane
318 (7900)
388 (7900)
Dichloromethane
318 (8600)
407 (7800)
n-Hexane
319 (8400)
384 (8100)
Dichloromethane
318 (8700)
401 (7200)
502
516
176
189
2
3
4
5
6
0.26e
0.47
0.47
0.44
0.22e
0.89e
1.15
f
—
f
—
–2.42g
–2.43g
–1.85g
2.89
635
712
639
701
713
247
305
255
309
280
2.89
2.29
n-Hexane
361 (22,600)
433 (12,200)
Dichloromethane
375 (28,400)
447 (13,100)
n-Hexane
h
—
312 (20,700)
380 (16,200)
Dichloromethane
309 (27,000)
391 (23,000)
604
653
224
262
aV vs. Ag/Ag+. Conditions: ca. 10−3 mol/L in tetrahydrofuran with 0.1 mol/L n-Bu4NClO4; working elec-
trode: glassy carbon, counter electrode: Pt wire, reference electrode: Ag/0.01 mol/L AgNO3 in acetonitrile
1
ox
with 0.1 mol/L n-Bu4NClO4; E1/2(ferrocene/ferrocenium) = 0.15 V; scan rate: 30 mV/s; 293 K. bꢀE = E1/2
–
1E1/2 or pred. cExcited at absλmax
.
λ
max
–
λ
max
. eV vs. Ag/Ag+. Conditions: ca. 10−3 mol/L in dichloromethane
dem
abs
with 0.1 mol/L n-Bu4NClO4; working electrode: glassy carbon, counter electrode: Pt wire, reference electrode:
Ag/0.01 mol/L AgNO3 in acetonitrile with 0.1 mol/L n-Bu4NClO4; E1/2(ferrocene/ferrocenium) = 0.17 V; scan
rate: 30 mV/s; 293 K. fNot measured. gIrreversible, peak potential. hNo fluorescence.
smaller ꢀE exhibit longer absorption. As far as the fluorescence is concerned, triarylphos-
phines are known to exhibit considerable Stokes shift because of the structural change
from the pyramidalized ground state to the planarized excited state.5 Introduction of the
π-conjugated systems, especially electron-accepting group, leads to enhanced Stokes shift
at the cost of quantum yield because of the additional structural change discussed above.
Chloronaphthoquinone derivative 2 shows more red-shifted and weaker absorption with
similar solvent effect. Spatial separation of HOMO and LUMO leads to lower intensity,

STERICALLY CROWDED TRIARYLPHOSPHINES
261
while lower reduction potential of the chloronaphthoquinone moiety or smaller ꢀE results
in absorption in longer wavelength.
The sterically crowded triarylphosphines conjugated with or connected to the
electron-accepting groups can be regarded as good model compounds for the functional
dyes bearing main group elements as key structures. They exhibit inherent properties of
phosphines such as large Stokes shift as well as those of conventional π-conjugated systems
such as formation of the polar excited state. Further investigation toward detailed under-
standing of the sterically crowded triarylphosphines and construction of the functional dyes
bearing the sterically crowded triarylphosphines as key structures are in progress.
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