Acenaphtho[1,2-c]phosphole p-oxide: a phosphole-naphthalene π-conjugated system with high electron mobility

Article abstract of DOI:10.1002/chem.200901378

The first comparative study on the optical and electrochemical properties of naphthalene, acenaphthene, and acenaphthylene fused phosphole derivatives, was reported. Naphthalene-, acenaphthene-, and acenaphthylene-fused phosphole derivatives 3-5 were succ

Full text of DOI:10.1002/chem.200901378

DOI: 10.1002/chem.200901378
AcenaphthoACTHNUTRGNEUG[N 1,2-c]phosphole P-Oxide: A Phosphole–Naphthalene
p-Conjugated System with High Electron Mobility
Arihiro Saito,[a] Tooru Miyajima,[a] Makoto Nakashima,[a] Tatsuya Fukushima,[b]
Hironori Kaji,[b] Yoshihiro Matano,*[a] and Hiroshi Imahori[a, c, d]
Phosphole is known to be a poorly aromatic heterole with
a low-lying LUMO due to the effective s*(P-R)–p*(1,3-
fields. This finding encouraged us to systematically investi-
gate the ring-fusion effects on the electron-accepting and
electron-transporting properties of [c]-fused phosphole P-
oxides. In this context, we chose naphthalene as the fused
arene framework because its p-system could be easily modu-
lated at peripheral positions. Herein, we report the first
comparative study on the optical and electrochemical prop-
erties of naphthalene-, acenaphthene-, and acenaphthylene-
fused phoshole derivatives. Remarkably, the naphthalene-
AHCTUNGTRENNUNG
diene) hyperconjugation.[1] It is well known that chemical
functionalizations at the phosphorus center further reduce
the LUMO level considerably. In this regard, phosphole-
based p-systems are promising building blocks of p-conju-
gated materials with both high electron-accepting and elec-
tron-transporting abilities.[2] Recently, [b]- and [b,d]-fused
phospholes, such as benzo[b]phospholes,[2a,g,3a] dibenzo-
A
N
fused phosphole P-oxide (acenaphthoACTHNGUTERNNU[G 1,2-c]phosphole P-
phosphoryl-bridged stilbenes[3h,i] have received growing in-
terest, as they exhibit unique optical and electrochemical
properties endowed by the flat, rigid, and elongated p-net-
works. On the contrary, benzo[c]-fused phospholes have
been scarcely explored due to their intrinsic instability.[4] We
focused on potentially high electron-accepting ability of
benzo[c]phosphole skeleton[5] and reported the first exam-
ples of thermally stable derivatives wherein the bithiophene
subunit is [c]-fused at the b positions of the phosphole
ring.[2b,c] More importantly, the electron mobility of P-oxo
oxide) exhibits the highest electron mobility ever reported
for phosphole P-oxides.
derivative
hydroxyquinoline)aluminumAHCNUTGTRENNUNG
1
(Figure 1) is higher than that of tris(8-
(III) (Alq3) at low electric
Figure 1. Bithiophene-fused benzo[c]phosphole 1, trimethylene-fused
phosphole 6, and phosphole models 4m and 5m. l.p.=lone pair.
[a] A. Saito, T. Miyajima, M. Nakashima, Prof. Dr. Y. Matano,
Prof. Dr. H. Imahori
Naphthalene-, acenaphthene-, and acenaphthylene-fused
phosphole derivatives 3–5 were successfully prepared by uti-
lizing a TiII-mediated cyclization[6] of dialkynylated naphtha-
lene (2a),[7] acenaphthene (2b), and acenaphthylene (2c),
respectively, as a key step (Scheme 1). Treatment of gold(I)
Department of Molecular Engineering
Graduate School of Engineering, Kyoto University
Nishikyo-ku, Kyoto 615-8510 (Japan)
Fax : (+81)75-383-2571
E-mail: matano@scl.kyoto-u.ac.jp
complexes 3b,c with excess PACHTNUGRTNEUNG
(NMe2)3 afforded s3-P com-
[b] T. Fukushima, Prof. Dr. H. Kaji
Institute for Chemical Research, Kyoto University
Uji, Kyoto 611-0011 (Japan)
pounds 4b,c, which were subsequently oxidized by m-chloro-
perbenzoic acid (mCPBA) to the corresponding P-oxides
5b,c. Compound 5c was alternatively prepared in 39% yield
by dehydrogenation of the bridging ethylene moiety of 5b
with 2,3-dichloro-5,6-dicyanobenzoquinone (DDQ). The P-
oxides 5a and 5b could be directly prepared from 2a and
2b in 44 and 48% yield, respectively, without isolating the
corresponding s3-P intermediates. Compound 4a was ob-
tained by reduction of 5a with excess HSiCl3.
[c] Prof. Dr. H. Imahori
Institute for Integrated Cell-Material Sciences (iCeMS)
Kyoto University, Nishikyo-ku, Kyoto 615-8510 (Japan)
[d] Prof. Dr. H. Imahori
Fukui Institute for Fundamental Chemistry, Kyoto University
Sakyo-ku, Kyoto 606-8103 (Japan)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200901378.
10000
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Chem. Eur. J. 2009, 15, 10000 – 10004

COMMUNICATION
Figure 2. Top views of a) 3b and b) 5a. Hydrogen atoms are omitted for
ꢀ
ꢀ
ꢀ
clarity. Selected bond length [ꢁ]: 3b: P C1 1.798(4); P C4 1.810(4); P
ꢀ
ꢀ
ꢀ
ꢀ
Au 2.2122(15); Au Cl 2.2743(15); C1 C2 1.341(5) C2 C3 1.491(5); C3
Scheme 1. Synthesis of naphthalene-, acenaphthene-, and acenaphthy-
lene-fused phosphole derivatives 3–5.
ꢀ
ꢀ
ꢀ
ꢀ
C4 1.347(5). 5a: P C1 1.8174(5); P C4 1.8184(16); P O 1.4780(13); C1
C2 1.342(2); C2 C3 1.508(2); C3 C4 1.347(2).
ꢀ
ꢀ
Compounds 3–5 were fully characterized by conventional
spectroscopic techniques (1H, 13C and 31P NMR, MS, and
IR). The 31P peaks of 3, 4, and 5 appeared at d=62.7–65.7,
40.5–43.9, and 54.5–57.1 ppm, respectively. The structures of
3b and 5a were further elucidated by X-ray crystallography
(Figure 2).[8] In each compound, the phosphorus center
adopts a distorted tetrahedral geometry, and the p-conjugat-
ed phosphole and naphthalene rings are nearly on the same
plane. On the contrary, the two a-phenyl groups are largely
twisted from the phosphole ring with dihedral angels of
ties of the naphthalene- and acenaphthene-fused derivatives
(4a,b and 5a,b) are quite different from those of the ace-
naphthylene-fused derivatives (4c and 5c). As shown in
Figure 3, 5a,b display similar spectral shapes and emit weak
ꢀ
37.4–49.08 for 3b and 40.9–46.78 for 5a. The Ca Cb bond
lengths [1.341(5)–1.347(5) ꢁ] of the phosphole ring are con-
ꢀ
siderably shorter than the Cb Cb bond lengths [1.491(5)–
1.508(2) ꢁ], reflecting the cis-dienic character of the phosp-
hole ring in 3b and 5a. The packing structure of 3b differs
from that of 5a with respect to the mutual orientation of the
neighboring phosphole–naphthalene p-systems (Figure S1 in
Supporting Information).
Figure 3. UV/Vis absorption (c) and fluorescence (a) spectra of 5a–
c in CH2Cl2.
To disclose electronic effects of the peripheral and phos-
phorus substituents on the optical and electrochemical prop-
erties of the newly prepared [c]-fused phosphole derivatives,
we compared the UV/Vis absorption/fluorescence spectra
and redox potentials of 4, 5, and 6. The observed data are
summarized in Table 1, and the absorption/fluorescence
spectra of 4, 5, and 6 are depicted in Figure 3 and S2 in the
Supporting Information. The absorption and fluorescence
maxima (labs and lem) of 5a–c are remarkably red-shifted
relative to those of trimethylene-fused reference 6, indicat-
ing that the p-conjugation is effectively extended to the [c]-
fused p systems. It has been found that the optical proper-
fluorescence (FF =0.07–0.08). The lowest transitions of 5a,b
are observed at longer wavelength than the corresponding
transitions of 4a,b, implying that P-oxidation from 4a,b to
5a,b narrows the HOMO–LUMO gaps of their fused p sys-
tems. In sharp contrast, 5c shows a characteristic intense
band at labs =350 nm, and is non-fluorescent. It should be
noted that P-oxidation from 4c to 5c makes little impact on
the lowest transition, namely the HOMO–LUMO gap of its
fused p system (see below).
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Y. Matano et al.
Table 1. Optical[a] and electrochemical[b] data for 4a–c, 5a–c, and 6.
and energies. The opposite is noted for 5c-m; the LUMO
has a character of the acenaphthylene subunit, whereas the
LUMO+1 possesses a character of phosphole. A similar
trend was found for the s3-P models (4a-m, 4b-m versus 4c-
m). It is evident that the peripheral functionalizations dra-
matically alter the characters of LUMO and LUMO+1 of
the [c]-fused phosphole-naphthalene p-systems.
[c]
[c]
[d]
[e]
[e]
Compd
labs (log e)
lem (FF
)
Eox
Ered
DE[f]
4a
4b
4c
5a
5b
5c
6
399 (4.12)
404 (4.23)
464 (3.89)
439 (3.62)
459 (3.58)
414 (3.96)
386 (4.15)
480 (0.38)
482 (0.23)
–
+0.64
+0.52
+0.74
+0.94
+0.77
+0.98
+1.19
ꢀ2.38
ꢀ2.42
ꢀ1.76
ꢀ1.82
ꢀ1.89
ꢀ1.61
ꢀ2.02
3.02
2.94
2.50
2.76
2.66
2.59
3.21
[g]
552 (0.08)
583 (0.07)
[g]
–
To reveal the nature of the observed excitations, we also
performed time-dependent (TD)-DFT calculations on the
models (Table S1 in the Supporting Information). Qualita-
tively, the theoretically calculated results well explain the
experimentally observed absorption spectra. In a series of
the s3-P derivatives, HOMO–LUMO and HOMO–
LUMO+1 transitions of 4a,b are overlapping at around
400–410 nm, whereas the respective transitions of 4c are ob-
served separately at 464 and 380 (shoulder) nm. In a series
of the P-oxo derivatives, HOMO–LUMO and HOMO–
LUMO+1 transitions of 5a,b appear separately at 460–490
and 380–400 nm, respectively, whereas those of 5c are over-
lapping at around 410–480 nm. Specifically, the P-oxidation
considerably stabilizes the phosphole-based p* orbitals
(DELUMO =0.59 and 0.55 eV for the naphthalene- and ace-
naphthene-fused derivatives; DELUMO+1 =0.53 eV for the
acenaphthylene-fused derivative) as compared to the respec-
tive HOMO (DEHOMO =0.20–0.21 eV), which results in large
red-shifts of the HOMO–LUMO transitions for the naph-
thalene- and acenaphthene-fused derivatives (Dl=63 and
60 nm) and the HOMO–LUMO+1 transition for the ace-
naphthylene-fused derivative (Dl=49 nm). It should be
noted that the P-oxidation from 4c to 5c makes a relatively
small impact on the transition from the HOMO to the ace-
naphthylene-based LUMO (Dl=22 nm).
491 (0.19)
[a] UV/Vis absorption and fluorescence measurements were made in
CH2Cl2. [b] Redox potentials were determined by DPV in CH2Cl2 with
ꢀ
0.1m nBu4N+PF6 (Ag/AgNO3). [c] Absorption and emission maxima are
given in nm. [d] Fluorescence quantum yield relative to Rꢂauꢃs 3,4-C4-
bridged 1-phenyl-2,5-bis(2-thienyl)phosphole–AuCl Complex (FF =0.129;
ref. [3c]). [e] First oxidation (Eox) and reduction (Ered) potentials are
given in V versus Fc/Fc+ couple. [f] EoxꢀEred (in V). [g] Non-fluorescent.
To gain a deep insight into the electronic structures of the
naphthalene-, acenaphthene-, and acenaphthylene-fused
phosphole derivatives, we carried out density functional
theory (DFT) calculations on model compounds 4x-m and
5x-m (x=a, b, c) without the phenyl group on the P atom
(Figure 1). The Cs symmetric structures of these compounds
were optimized at the B3LYP/6-31G* level. As visualized in
Figure 4 and S3 in the Supporting Information, the HOMO
Redox potentials of 4, 5, and 6 were measured by cyclic
voltammetry (CV) and differential pulse voltammetry
(DPV) (Table 1 and Figure S4 in the Supporting Informa-
tion). It was found that the electrochemical reduction pro-
cesses of P-oxo derivatives 5a–c and 6 occurred reversibly.
The first reduction potentials (Ered) of 5a–c are more posi-
tive than Ered of 6, indicating that the electron-accepting
ability of [c]-fused phospholes is appreciably improved by
the introduction of the naphthalene, acenaphthene, or ace-
naphthylene subunit. Among 5a–c, the difference (DE) in
the first oxidation potential (Eox) and Ered decreases in the
order: 5a (2.76 V) > 5b (2.66 V) > 5c (2.59 V). In the
cases of the naphthalene- and acenaphthene-fused deriva-
tives, P-oxidation (from 4a,b to 5a,b) shifts Ered to the posi-
tive side more considerably than Eox. As a result, the DE
values of 5a,b are smaller than those of 4a,b, which is in
good accordance with the results obtained from the absorp-
tion spectra and DFT calculations (see above). Most impor-
tantly, the above electrochemical data suggested that 5a–c
would be possible candidates for the phosphole-based elec-
tron-transporting materials in terms of the electric potential
and the reversibility of their reduction processes.[9]
Figure 4. HOMO (lower), LUMO (middle), and LUMO+1 (upper) of a)
5a-m, b) 5b-m, and c) 5c-m. The calculated orbital energies (in eV) are
shown in parentheses.
of each model is spread over the entire p-conjugated plane
with anti-bonding character between the phosphole and
naphthalene rings. On the other hand, the LUMO and
LUMO+1 display the character of each subunit. In the case
of 5a-m, the LUMO basically consists of a typical LUMO
of phosphole, and the LUMO+1 displays a character of the
naphthalene backbone. The LUMO and LUMO+1 of 5b-m
resemble those of 5a-m with respect to the orbital diagrams
With this in mind, we evaluated the thermal stability of
5a–c. Compounds 5a and 5b were found to be stable up to
2508C, indicating that the thermal stability of the naphtha-
10002
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Chem. Eur. J. 2009, 15, 10000 – 10004

A Phosphole–Naphthalene p-Conjugated System
COMMUNICATION
lene- and acenaphthene-fused phosphole P-oxides is reason-
ably high for device fabrication using high-temperature
vacuum deposition.[10] By contrast, 5c decomposed at 1558C,
implying that the acenaphthylene-fusion does not provide
sufficient thermal stability.
Experimental Section
Typical procedure for the synthesis of 5a (Scheme 1): To a mixture of 2a
(1.92 g, 5.9 mmol), Ti
ACHTUNGTRENNUNG
added an ether solution of iPrMgCl (2.0m
ꢄ
ꢀ508C, and the resulting mixture was stirred for 4 h at ꢀ408C. PhPCl2
(0.80 mL, 5.9 mmol) was then added to the mixture at ꢀ408C, and the re-
sulting suspension was stirred for 1 h at 08C and for an additional 5 h at
room temperature. The reaction mixture was then evaporated, and the
residue was passed through a short silica gel column (CH2Cl2). To the
eluent containing the crude product (Rf =0.7) was added mCPBA (77%
max, 1.35 g). After stirring for 15 min at room temperature, the reaction
mixture was evaporated under reduced pressure. The residue was subject-
At the last stage of this study, we examined the electron
mobility of vacuum-deposited films of 5a and 5b by the
time-of-flight (TOF) technique (for experimental details, see
the Supporting Information). Figure 5 shows the field de-
pendency of the logarithmic electron mobilities of 5a, 5b,
and Alq3.[2c] Both 5a and 5b exhibit the positive field de-
pendency of the electron mobility (mE), which increases with
increasing the electric field (E). It is remarkable that, at any
given electric field, the electron mobility of 5a is about one-
order higher than that of Alq3.[11] The mE value of 8ꢄ
10ꢀ5 cm2 Vꢀ1 sꢀ1 observed for 5a (at E=1.0ꢄ106 Vcmꢀ1) is
the highest electron mobility ever reported for the phosp-
hole P-oxides.[2a,c,12] These results unambiguously exemplify
ed to silica-gel column chromatography (CH2Cl2
30:1). The yellow fraction was collected, evaporated, and precipitated
from hexane/CH2Cl2 to give 5a as a yellow solid (1.17 g, 44%).
!
CH2Cl2/acetone
Acknowledgements
the high potential of acenaphthoACTHNUGTRNEUNG[1,2-c]phosphole skeleton
for use in electron-transporting organic materials.
This work was partially supported by a research grant from The Murata
Science Foundation. We thank Dr. Yoshihide Nakao for his helpful sug-
gestions on DFT calculations.
Keywords: electron mobility · electronic structure · fused-
ring systems · metallacycles · phospholes
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ples of naphthalene-, acenaphthene-, and acenaphthylene-
fused phosphole derivatives via TiII-mediated cyclization of
the corresponding dialkynylated arenes. The p-conjugation
mode of the arene backbone has a large impact on the opti-
cal properties of the [c]-fused phosphole derivatives. Among
three types of P-oxides, the naphthalene- and acenaph-
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is prerequisite for the device fabrication using high-tempera-
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the naphthalene-fused phosphole P-oxide is one-order
higher than that of Alq3 at any given electric field. The pres-
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protocol to the development of a new class of phosphole-
based p-systems with high electron-transporting ability.
Chem. Eur. J. 2009, 15, 10000 – 10004
ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
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Y. Matano et al.
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5a: C32H21OP, MW =452.46, monoclinic, 0.40ꢄ0.20ꢄ0.20 mm, P21,
a=11.262(3), b=8.145(2), c=12.226(3) ꢁ, b=93.820(4)8, V=
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, , collected
m=1.471 cmꢀ1
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¯
[8] 3b: C34H23AuClP, MW =694.91, triclinic, 0.40ꢄ0.20ꢄ0.10 mm, P1,
a=10.491(7), b=10.545(7), c=12.615(8) ꢁ, a=94.548(4), b=
104.538(11), g=107.284(4)8, V=1272.0(15) ꢁ3, Z=2, 1calcd
=
1.814 gcmꢀ3, m=59.74 cmꢀ1, collected 10288, independent 5697, pa-
rameters 335, Rw =0.0509, R=0.0310 (I>2.00s(I)), GOF=1.052.
Received: May 23, 2009
Published online: August 26, 2009
10004
www.chemeurj.org
ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2009, 15, 10000 – 10004