Fusion of phosphole and 1,1′-biacenaphthene: Phosphorus(V)-containing extended π-systems with high electron affinity and electron mobility

Article abstract of DOI:10.1002/anie.201102782

A profusion of phospholes: Diacenaphtho[1,2-b:1′,2′-d] phospholes, a new class of arene-fused phosphole π-systems, were synthesized and their structural and electrochemical properties studied. The P-sulfide derivative (see picture) has a high electron-transporting ability (μ_E=2.4× 10^-3 cm² V^-1 s^-1) in a vacuum-deposited film.

Full text of DOI:10.1002/anie.201102782

Communications
DOI: 10.1002/anie.201102782
Extended p-Systems
Fusion of Phosphole and 1,1’-Biacenaphthene:
Phosphorus(V)-Containing Extended p-Systems with
High Electron Affinity and Electron Mobility**
Yoshihiro Matano,* Arihiro Saito, Tatsuya Fukushima, Yasuaki Tokudome,
Furitsu Suzuki, Daisuke Sakamaki, Hironori Kaji, Akihiro Ito, Kazuyoshi Tanaka,
and Hiroshi Imahori
Angewandte
Chemie
8016
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Angew. Chem. Int. Ed. 2011, 50, 8016 –8020

Recent developments in the chemistry of arene-fused phos-
phole p systems have shed light on their characteristic
properties for use in optoelectronic applications.[1] For
example, many derivatives of benzo[b]phospholes,[2] dibenzo-
[b,d]phospholes,[3] dithieno[b,d]phospholes,[4] and P,X-
bridged stilbenes (X = P, B, S)[5] have been reported to
possess highly fluorescent nature partly due to their rigid p
skeletons. It is well established that phosphole has a low-lying
LUMO derived from the effective s*–p* orbital interac-
tion.[1d] This implies that arene-fused phosphole p-systems,
especially those having the phosphorus(V) center, are also
potential candidates as n-type semiconducting materials.
Indeed, several research groups have designed and synthe-
sized their own phosphole derivatives with high electron
affinity.[2a,c,6–9] Furthermore, Tsuji, Nakamura, and co-workers
reported very high electron drift mobility for para-phenylene-
linked benzo[b]phosphole P-sulfide 1 and demonstrated its
utility in organic devices such as light emitting diode and
photovoltaic cell.[6] We independently reported a few phos-
phole-based n-type materials,[9] among which acenaphtho[1,2-
c]phosphole P-oxide 2 exhibited one-order higher electron
mobility than did tris(8-hydroxyquinoline)aluminum(III)
(Alq3).[9b] Despite these intriguing results, however, the
semiconducting ability of arene-fused phosphole derivatives
has not been fully explored due to the limited number of
promising candidates.
following concepts: 1) the intrinsically electron-accepting
nature of phosphole would be enhanced by the fusion with
a 1,1’-biacenaphthene moiety, 2) the electron affinity of the p-
system would be tunable by P-functionalizations, and 3) the
extended p-plane would be beneficial to p–p stacking and
electron-spin delocalization. In this study, we aimed to reveal
the effects of P-substituents as well as P-oxidation states on
the structural and electrochemical properties of the
diacenaphtho[b,d]phosphole p-systems. A high electron drift
mobility of 3b (R = cyclohexyl) in a vacuum-deposited film is
also reported.
Scheme 1 depicts the synthesis of target compounds.
Treatment of 2,2’-dibromo-1,1’-biacenaphthene[10] with two
equiv of nBuLi followed by the addition of PhPCl2 gave
Herein, we report the first examples of diacenaphtho[1,2-
b:1’,2’-d]phospholes including 3, which were designed as a
new class of n-type phosphole derivatives based on the
Scheme 1. Synthesis of diacenaphtho[1,2-b:1’,2’-d]phospholes.
mCPBA=m-chloroperoxybenzoic acid.
[*] Prof. Dr. Y. Matano, A. Saito, D. Sakamaki, Prof. Dr. A. Ito,
Prof. Dr. K. Tanaka, Prof. Dr. H. Imahori
Department of Molecular Engineering
diacenaphtho[1,2-b:1’,2’-d]phosphole 4a. As 4a was rather
difficult to isolate from the reaction mixture, crude 4a was
subsequently converted into gold(I) complex 5a, which was
then purified by column chromatography. The yield of
isolated 5a was 77% based on the dibromide. Demetallation
of 5a with P(NMe2)3 reproduced the s3-P derivative 4a
quantitatively. The P-thioxidation (with S8) and the P-
oxidation (with mCPBA) of 4a afforded P-sulfide 3a and P-
oxide 6a, respectively; however, 6a slowly decomposed in
solution at room temperature.[11] According to similar proce-
dures, another series of diacenaphtho[b,d]phosphole deriva-
tives 3b, 4b, 5b, and 6b bearing a P-cyclohexyl group were
prepared by using c-C6H11PCl2 instead of PhPCl2. The thermal
stability of 6b was found to be higher than that of 6a.
Methylation of the in-situ generated 4b with excess iodo-
methane, followed by treatment with an aqueous NaPF6
solution produced phosphonium salt 7b.
Graduate School of Engineering
Kyoto University, Nishikyo-ku, Kyoto 615-8510 (Japan)
E-mail: matano@scl.kyoto-u.ac.jp
T. Fukushima, Y. Tokudome, F. Suzuki, Prof. Dr. H. Kaji
Institute for Chemical Research
Kyoto University, Uji, Kyoto 611-0011 (Japan)
Prof. Dr. H. Imahori
Institute for Integrated Cell-Material Sciences (iCeMS)
Kyoto University, Nishikyo-ku, Kyoto 615-8510 (Japan)
Prof. Dr. K. Tanaka, Prof. Dr. H. Imahori
Fukui Institute for Fundamental Chemistry
Kyoto University, Sakyo-ku, Kyoto 606-8103 (Japan)
[**] We thank Dr. Hirohiko Watanabe (Hamamatsu Photonics K.K.) and
Dr. Seiji Akiyama (Mistubishi Chemical K.K.) for the measurements
of FF values and DSC, respectively. This work was supported from
MEXT (Japan) by Grants-in-Aid (Nos. 21108511 and 22350016) and
Asahi glass foundation. The computation time was provided by the
Academic Center for Computing and Media Studies, Kyoto
University.
Compounds 3–7 were characterized by conventional
spectroscopic methods. The 31P NMR spectra of the P-
phenyl derivatives 3a, 4a, 5a, and 6a showed a sharp singlet
peak at dP = 19.2, À27.7, À4.4, and 18.5 ppm, respectively,
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201102782.
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Communications
Table 1: Optical and electrochemical data for 2–8.[a]
whereas those of the P-cyclohexyl derivatives 3b, 4b, 5b, and
[b]
[c]
[c]
6b showed the corresponding peaks at more downfield (dP =
35.4, À8.41, 7.0, and 35.0 ppm). The onium salt 7b displayed
Compd
lab (loge)
lem
Eox
Ered
3a
3b
4a
4b
5a
5b
6b
7b
8
587 (3.54)
579 (3.57)
539 (3.72)
537 (3.68)
561 (3.62)
555 (3.68)
582 (3.53)
583 (3.71)
457 (3.76)
439 (3.62)
678
670
622
626
655
653
680
684
561
552
+0.66 (ir)
+0.64 (ir)
+0.28 (ir)
+0.30 (ir)
N.d.
N.d.
N.d.
N.d.
+0.59 (ir)
+0.94 (ir)
À1.36 (r)
À1.40 (r)
À1.76 (r)
À1.88 (r)
À1.36 (qr)
À1.44 (ir)
À1.44 (ir)
À1.00 (r)
À2.02 (r)
À1.82 (r)
1
31P and H (methyl) peaks at dP 19.6 ppm and dH 2.46 ppm
(JPH = 13.7 Hz), respectively. The structures of 3b and 7b
were further elucidated by X-ray crystallography.[12] As shown
in Figure 1, the phosphorus-linked p-systems are almost flat,
and two biacenaphthene planes are stacked in parallel and in
head-to-tail orientation with p–p distances of 3.32–3.47 ꢀ. In
both 3b and 7b, the phosphorus center adopts a distorted
tetrahedral geometry with the cyclohexyl group in chair
conformation. On the other hand, the whole packing arrange-
ment of 7b differs from that of 3b (Figure S1 in the
Supporting Information).
2[d]
[a] Measured in CH2Cl2; N.d.=Not determined. [b] lex =440 nm (except
for 6b). lex =580 nm for 6b. Fluorescence quantum yields (FF): <0.01
for 3–7 and 0.01 for 8. [c] Determined by differential pulse voltammetry
(0.1m Bu4N+PF6À; Ag/Ag+); First oxidation (Eox) and reduction (Ered
)
potentials vs. ferrocene/ferrocenium. r: reversible; qr: quasi-reversible;
ir: irreversible. [d] Data from Ref. [9b].
reversible voltammograms observed for 3a,b and 7b exhibit
their high stability in the electrochemical reduction processes.
In each pair of 3a,b and 4a,b, there are very small differences
in Eox values between the P-phenyl and P-cyclohexyl deriv-
atives. On the other hand, Ered of the P-phenyl derivatives are
anodically shifted by 0.04–0.12 V relative to Ered of the P-
cyclohexyl derivatives, which probably reflects the slight
difference in the inductive effects of these two organyl (R)
substituents on the LUMO energies.
To know the nature of frontier orbitals of the present p-
systems, we carried out density functional theory (DFT)
calculations of 3b, 4b, and 8 at the B3LYP/6-31G* level. The
P-cyclohexyl group in 3b and 4b was found to adopt a chair
conformation at their optimized structures. The calculated
bond parameters of 3b are almost identical to the observed
ones (by X-ray). As depicted in Figure 2, HOMOs possess the
heteroacene character to a large extent, whereas LUMOs
consist of the heterole- and biacenaphthene-derived orbitals
and are spread over the whole p-planes. The replacement of
the sulfur bridge with the P-R bridge (from 8 to 4b) stabilizes
Figure 1. Top and side views of a) 3b and b) 7b. Hydrogen atoms and
dichloromethane molecules are omitted for clarity. Selected bond
lengths [ꢀ]: 3b: P–C1 1.801(2), P–C4 1.801(2), P–C25 1.833(2), P–S
1.9322(9), C2–C3 1.451(3), C3–C4 1.381(3). 7b: P1–C1 1.787(7), P1–
C4 1.783(6), P1–C25 1.790(6), P1–C26 1.801(6), C1–C2 1.383(8), C2–
C3 1.470(9), C3–C4 1.380(8).
To reveal the optical and electrochemical properties of the
present p-systems, UV/Vis absorption/fluorescence spectra
and redox potentials of 3–7 and thiophene analogue 8[13] were
measured in CH2Cl2 (Table 1, Figure S2 and S3 in the
Supporting Information). The lowest p–p* transitions of
4a,b appeared at longer wavelengths than the corresponding
transition of 8. The P-functionalizations from 4a,b to 3a,b/
5a,b/6b/7b caused noticeable red shifts of absorption/emis-
sion maxima (Dlab/Dlem = 18–48/27–58 nm), which is a typical
trend observed for the p-conjugated phospholes.[1] All the
diacenaphtho[b,d]phospholes were found to be weakly fluo-
rescent (FF < 0.01).
As expected, the P-functionalizations induced anodic
shifts for both oxidation and reduction potentials (Eox and
Ered). In cyclic voltammetry (CV) measurements, 3a, 3b, and
7b showed reversible reduction processes at Ered = À1.36,
À1.40, and À1.00 V (vs. Fc/Fc+; Fc = [(C5H5)2Fe]), respec-
tively. These values are less negative than Ered values of Tsuji–
Nakamuraꢁs benzo[b]phosphole 1 (À2.08 V),[6] our acenaph-
tho[c]phosphole 2 (À1.82 V),[9b] and the thiophene analog 8
(À2.02 V), indicating that the electron-accepting ability of
3a,b and 7b is considerably higher than that of 1, 2, and 8. The
Figure 2. HOMOs (bottom) and LUMOs (top) of 3b, 4b, and 8 with
their orbital energies and HOMO–LUMO gaps (in eV) calculated at
the B3LYP/6-31G* level. Orange P, yellow S.
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LUMO level due to the s*(P-C)-p* orbital interaction. The P-
thioxidation (from 4b to 3b) enhances this interaction,
resulting in a further stabilization of LUMO level. Judging
from the orbital diagrams and energies, the electronic effect
by Marcus theory.[15] The lin value was quantified using the
DFT method[16] by considering energy differences between
neutral and charged geometries for two (neutral and radical
anion) states. As shown in Table S1 in the Supporting
=
À
of the P S linker on LUMO is more significant than that on
Information, the electron injection into 3b reduces the C C
HOMO, and as a consequence, the HOMO–LUMO gap
decreases in the order: 8 > 4b > 3b. These theoretical results
are in good qualitative agreement with the above-mentioned
experimental results.
To reveal a role of phosphorus(V) linkages in electron-
spin delocalization of two kinds of p-radical species derived
from 3b and 7b, we performed electrochemical ESR meas-
urements. The electrochemical single-electron reductions of
3b and 7b were conducted in an electrode-equipped ESR cell
with THF (for 3b) or CH2Cl2 (for 7b) as a solvent and
nBu4NPF6 as an electrolyte. The ESR spectra recorded at
183 K showed well-resolved hyperfine structure, centered at
g = 2.0031–2.0032 (Figure 3). The generated species were
bond alternation of the fused phosphole ring (Dl = 0.069 ꢀ for
À
À
3b and Dl = 0.008 ꢀ for 3b C; Dl is a difference of Ca Cb and
Cb Cb bond lengths), reflecting the enhanced p-conjugation
À
between the two acenaphthylene subunits. The spin density
distributions of 3bÀC obtained by the DFT calculations
(Figure S5 in the Supporting Information) support the hyper-
fine coupling constants of 3bÀC obtained by the ESR measure-
ments (see above). The calculated lin value of 3b/3bÀC
(0.198 eV) was significantly smaller than that of meridional
À
Alq3/Alq3 C (0.270 eV at the B3LYP/6-31G* level), which
suggested that 3b would be a potential candidate as an
electron-transporting material in terms of the reorganization
energy.[17] With this in mind, we finally measured electron
drift mobility of a vacuum-deposited film of 3b (Tm = 2838C,
Tg = 1358C; determined by differential scanning calorimetry
measurements) by a time-of-flight (TOF) method.[18] Fig-
ure S6 in the Supporting Information shows the field depend-
ency of the logarithmic electron mobility (mE) of 3b and 2.[9b]
The mE value of 3b slightly increased with decreasing the
electric field (E) and reached 2.4 ꢂ 10À3 cm2 VÀ1 sÀ1 at E =
4.3 ꢂ 105 VcmÀ1. It should be emphasized here that the
observed electron drift mobility of 3b is 1–2 orders of
magnitude larger than that of 2[9b] and is comparable to the
highest value (mE = 2 ꢂ 10À3 cm2 VÀ1 sÀ1 at E = 2.5 ꢂ 105 VcmÀ1
for 1; determined by the TOF method)[6a] ever reported for
the p-conjugated phosphole derivatives.
In summary, we successfully prepared diacenaphtho[1,2-
b:1’,2’-d]phospholes as a new class of arene-fused phosphole
p-systems with high electron affinity. The electrochemically
reduced states of the phosphorus(V)-linked derivatives were
stabilized by the effective delocalization of the unpaired
electron over the whole p-systems. Most importantly, the
cyclohexylphosphine–sulfide derivative exhibited a high elec-
tron-transporting ability in the vacuum-deposited film. The
present results unambiguously corroborate that the fusion of
phosphole and planar arene rings based on the rational design
concept is a promising strategy for the development of
phosphorus(V)-containing organic n-type semiconductors.
Figure 3. ESR spectra of a) 3bÀC in THF and b) 7bC in CH2Cl2 at 183 K
with g, aP, and selected aH values (>0.10 mT).
assigned to anion radical 3bÀC and neutral radical 7bC by
comparison with their simulated spectra (Figure S4 in the
Supporting Information). In each radical species, the major
contributions to the hyperfine splitting stem from the bridging
31P nucleus (aP = 1.45 or 1.70 mT) and eight of the peripheral
1H nuclei (aH = 0.25–0.28 mT) in the biacenaphthene
moiety.[14] The observed difference in aP values between
3bÀC and 7bC represents that the interaction of the unpaired
electron with the 31P nucleus varies depending on the charge
and the substituents of phosphorus. More importantly, it has
become evident that an unpaired electron delocalizes into the
phosphorus(V)-linked whole p-systems efficiently.
Received: April 21, 2011
Published online: June 22, 2011
Keywords: biacenaphthene · electron mobility ·
.
EPR spectroscopy · phosphole · radical ions
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in the Supporting Information), implying that the diacenaphtho-
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