Comparative study on the structural, optical, and electrochemical properties of bithiophene-fused benzo[c]phospholes

Article abstract of DOI:10.1002/chem.200801017

Three types of bithiophenefused benzo[c]phospholes were successfully prepared by Ti¹¹-mediated cyclization of the corresponding dialkynylated bithiophene derivatives as a key step. Each σ³- phosphorus center of the benzo[c]phosphole subunits was readily transformed into σ⁴-phosphorus center by Au coordination or oxygenation. In addition, the bithiophene subunit was functionalized at the αα ′-carbon atoms by Pd-catalyzed cross-coupling reactions with heteroarylmetals and by an S_NAr reaction with hexafluorobenzene. The experimentally observed results (NMR spectroscopy, X-ray analysis, UV/Vis absorption/fluorescence spectroscopy, and cyclic/differential-pulse voltammetry) have revealed that the structural, optical, and electrochemical properties of the bithiophene-fused benzo[c]phospholes vary considerably depending on the jr-conjugation modes at the bithiophene subunits and the substituents of the heterocyclopentadiene components. The appropriately ring-annulated σ³-P derivatives and σ⁴P-AuCl complexes were found to emit fluorescence in the orange-red region, and the σ⁴-P-oxo derivatives proved to undergo reversible one-electron reduction at -1.4 to -1.8V (vs ferrocene/ ferrocenium). These results indicate that the bithiophene-fused benzo[c]phospholes possess narrow HOMOLUMO gaps and low-lying LUMOs, which was confirmed by density functional theory calculations of their model compounds. The time-of-flight measurement of an ITO/benzo[c]phosphole/Al device showed that the electron mobility in the P-oxo derivative is one-order higher than that in AIq₃ at low electric fields. The present study demonstrates that the arenefused benzo[c]phosphole skeleton could be a highly promising platform for the construction of a new class of phosphole-based optoelectrochemical materials.

Full text of DOI:10.1002/chem.200801017

DOI: 10.1002/chem.200801017
Comparative Study on the Structural, Optical, and Electrochemical
Properties of Bithiophene-Fused Benzo[c]phospholes
Yoshihiro Matano,*[a] Tooru Miyajima,[a] Tatsuya Fukushima,[b] Hironori Kaji,[b]
Yoshifumi Kimura,[c] and Hiroshi Imahori[a, d, e]
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FULL PAPER
Abstract: Three types of bithiophene-
fused benzo[c]phospholes were success-
fully prepared by TiII-mediated cycliza-
tion of the corresponding dialkynylated
bithiophene derivatives as a key step.
Each s3-phosphorus center of the ben-
zo[c]phosphole subunits was readily
transformed into s4-phosphorus center
by Au coordination or oxygenation. In
addition, the bithiophene subunit was
functionalized at the a,a’-carbon atoms
by Pd-catalyzed cross-coupling reac-
tions with heteroarylmetals and by an
SNAr reaction with hexafluorobenzene.
The experimentally observed results
(NMR spectroscopy, X-ray analysis,
UV/Vis absorption/fluorescence spec-
troscopy, and cyclic/differential-pulse
voltammetry) have revealed that the
structural, optical, and electrochemical
properties of the bithiophene-fused
benzo[c]phospholes vary considerably
depending on the p-conjugation modes
at the bithiophene subunits and the
substituents of the heterocyclopenta-
diene components. The appropriately
ring-annulated s3-P derivatives and s4-
P-AuCl complexes were found to emit
fluorescence in the orange–red region,
and the s4-P-oxo derivatives proved to
undergo reversible one-electron reduc-
tion at À1.4 to À1.8 V (vs ferrocene/
ferrocenium). These results indicate
that the bithiophene-fused benzo[c]-
phospholes possess narrow HOMO–
LUMO gaps and low-lying LUMOs,
which was confirmed by density func-
tional theory calculations of their
model compounds. The time-of-flight
measurement of an ITO/benzo[c]-
phosphole/Al device showed that the
electron mobility in the P-oxo deriva-
tive is one-order higher than that in
Alq3 at low electric fields. The present
study demonstrates that the arene-
fused
benzo[c]phosphole
skeleton
could be a highly promising platform
for the construction of a new class of
phosphole-based optoelectrochemical
materials.
Keywords: conjugation · fused-ring
systems · phosphole · pi interac-
tions · thiophene
Introduction
is also capable of changing its optical and electrochemical
properties by chemical functionalizations (metal coordina-
tion, oxygenation, alkylation, etc.) at the phosphorus center.
These prominent features of phosphole are beneficial for
developing new classes of heterole-based p-conjugated ma-
terials for use in optoelectrochemical applications, such as
organiclight-emitting diodes (OLEDs), field-effetc transis-
tors (FETs), and nonlinear optical (NLO) devices.[2] In
search of potential candidates for this purpose, much effort
has been devoted to reveal the relationship between the
structure and the fundamental properties of phospholes
bearing p-conjugative substituents at the 2,5-positions (a,a’-
positions). For example, RØau and co-workers systematically
investigated the optical and electrochemical properties of
2,5-diarylphospholes,[3] and demonstrated the potential utili-
ty of this class of compounds as components of the emissive
layer for OLEDs.[4] Several other groups have also focused
their attention on the light-emitting ability and/or electro-
chemical properties of oligomers, polymers, and dendrimers
incorporating 2,5-diarylphosphole units.[5] Furthermore,
phospholes bearing push–pull-type (captodative) p-conjuga-
tive substituents at the 2,5-positions were found to behave
as potential NLO-phores in solution.[6] Theoretical studies
on the phosphole-containing p-conjugated systems have pro-
moted a better understanding of their structure–property re-
lationships.[7]
Heterocyclopentadienes (heteroles) containing the third-row
elements such as silicon, phosphorus, and sulfur exhibit char-
acteristic optical and electrochemical properties that origi-
nate from the intrinsicnature of their 3s and 3p orbitals.
Phosphole, the phosphorus analogue of pyrrole, is a poorly
aromaticheterole with a low-lying LUMO due to the effec-
tive s*
(P-R)–p*(1,3-diene) orbital interaction.[1] Like other
heteroles, phosphole changes its electronic structure by the
introduction of p-conjugative substituents at the P-connect-
ed cis-1,3-diene function. It is worth noting that phosphole
G
[a] Prof. Dr. Y. Matano, T. Miyajima, Prof. Dr. H. Imahori
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
[b] T. Fukushima, Prof. Dr. H. Kaji
Institute for Chemical Research
Kyoto University, Uji, Kyoto 611–0011 (Japan)
[c] Prof. Dr. Y. Kimura
Department of Chemistry, Graduate School of Science
Kyoto University, Sakyo-ku, Kyoto 606–8502 (Japan)
[d] Prof. Dr. H. Imahori
Institute for Integrated Cell-Material Sciences (iCeMS)
Kyoto University, Nishikyo-ku, Kyoto 615–8510 (Japan)
In addition to the above 2,5-disubstituted systems, ring-
fused p-systems, such as dibenzo
A
[e] Prof. Dr. H. Imahori
Fukui Institute for Fundamental Chemistry
Kyoto University, Takano-Nishihiraki-cho
Sakyo-ku, Kyoto 606–8103 (Japan)
dithieno[b,d]phospholes, have been receiving growing inter-
AHCTREUNG
est, as they possess rigid and elongated p-networks that are
prerequisite for exhibiting high light-emitting ability.[8,9] In
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200801017. It contains X-ray
packing structures, absorption/fluorescence spectra, CV/DPV volta-
particular, Baumgartnerꢁs dithieno[b,d]phospholes have
G
proven to emit intense fluorescence covering the blue–
mograms, theoretical results, and H NMR spectra.
yellow region.[2,9] Quite recently, Yamaguchi and co-workers
1
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8103

Y. Matano et al.
also provided highly emissive bisphosphoryl-bridged stil-
benes, the fluorescence quantum yields of which were re-
ported to have reached 99%.[10] Except for these examples,
however, the number of ring-fused phosphole-based p-sys-
tems is quite limited, and their potential in optoelectro-
chemical applications has not been fully addressed.
In 2002, Dinadayalane and Sastry reported a theoretical
study on the electronic structures and reactivities of a series
of unsubstituted benzo[c]heteroles based on density func-
tional theory (DFT) calculations.[11] Taking the calculated
HOMO and LUMO energies into consideration, the authors
predicted that benzo[c]phosphole would be an attractive
platform for the construction of novel phosphole-based p-
conjugated systems with a low-lying LUMO and a relatively
small HOMO–LUMO gap. Noticeably, the LUMO energy
and the HOMO–LUMO gap of benzo[c]phosphole
(ELUMO =À1.97 eV and DEH-L =3.29 eV calculated at the
B3LYP/6–31G* level) are much lower and narrower, respec-
Figure 1. Bithiophene-fused benzo[c]phospholes studied here (E=lone
pair, AuCl, O).
ponents. Additionally, we evaluated the electron mobility of
a P-oxo derivative by the time-of-flight (TOF) method.
Results and Discussion
Synthesis and characterization of bithiophene-fused ben-
zo[c]phospholes: Bithiophene-fused benzo[c]phospholes 4–
12 were successfully prepared starting from dialkynylated bi-
thiophene derivatives 1–3 as depicted in Scheme 1. The syn-
thesis of the s3-P derivatives 4–6 includes the TiII-mediated
cyclization[16] of diynes 1–3 as a key step. Reaction of 3,3’-bi-
tively, than those of dibenzo
A
to À0.97 eV and DEH-L =4.94–4.95 eV calculated at the same
level),[12] implying that the position of fused ring(s) dramati-
cally affects the nature of frontier orbitals. Despite this sug-
gestive result, however, the chemistry of benzo[c]phospholes
remains unveiled, due to the lack of thermally and kinetical-
ly stable derivatives. In other words, the highly reactive o-
quinonoid character of benzo[c]phosphole has precluded its
isolation under ambient conditions.[13] For instance, ben-
zo[c]phospholes with no substituent at the benzo backbone
readily undergo Diels–Alder reactions and have not been
characterized well.[14] With this in mind, we designed bithio-
phene-fused benzo[c]phospholes as a new family of ben-
zo[c]phosphole. It was anticipated that the fusion of bithio-
phene subunit at the benzo backbone would reduce the o-
quinonoid character and would then bring kinetic and ther-
mal stabilization to the ben-
s(phenylethynyl)-2,2’-bithiophene (1a)[17] with [Ti
propene)], generated in situ from [Ti(OiPr)4] and two equiv-
alents of iPrMgCl,[18] followed by addition of dichloro-
(phenyl)phosphane, gave a mixture of target compound 4a
A
AHCTREUNG
AHCTREUNG
and unreacted 1a, which were not separable by column
chromatography. Subsequent treatment of the mixture of 4a
and 1a with AuCl
(SMe2) selectively converted 4a to AuI–
N
phosphole complex 7a, which could be easily separated
from 1a by column chromatography and was isolated as a
red solid in 57% yield (based on 1a). When treated with an
excess amount of P
C
zo[c]phosphole skeleton. It was
also expected that further ex-
tension of p-network would be
accomplished by chemical
functionalizations at the fused
bithiophene ring.
Here, we report the first
comparative study on the
structural, optical, and electro-
chemical properties of three
types of bithiophene-fused
benzo[c]phospholes
(Figure 1).[15] Both experimen-
tal and theoretical results have
revealed that the intrinsic
nature of the frontier orbitals
of the bithiophene-fused ben-
zo[c]phospholes varies signifi-
cantly depending on the p-con-
jugation mode of the bithio-
phene subunits and/or the sub-
stituents at the heterole com-
Scheme 1. Synthesis of bithiophene-fused benzo[c]phospholes.
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Bithiophene-Fused Benzo[c]phospholes
FULL PAPER
1
derivative 4a in 91% yield after reprecipitation from
MeOH. This two-step protocol enabled us to isolate 4a in
pure form. According to the similar procedures, 2-thienyl-
substituted derivatives (4b and 7b) and other two types of
bithiophene-fused benzo[c]phospholes (5, 6, 8, and 9) were
prepared from the corresponding dialkynylated bithiophene
derivatives (1b, 2a,b, and 3a,b). The isolated s3-P deriva-
tives 4a,b, 5a,b, and 6a reacted with m-chloroperbenzoic
acid (m-CPBA) at room temperature to give the respective
P-oxides 10a,b, 11a,b, and 12a in 92–96% yields (Method
A; see the Supporting Information). Alternatively, 10–12
were obtained from 1–3 without forming the Au complexes
7–9 (Method B; see the Supporting Information). As illus-
trated in Figure 1, 4, 7, 10 are classified to the type I, 5, 8, 11
to the type II, and 6, 9, 12 to the type III derivatives.
spectroscopy) and elemental analysis. The H NMR spectra
of 4–17 in CD2Cl2 clearly show CS symmetry for all the com-
pounds. The a-phenyl ortho-protons of the s3-P derivatives
4a–6a were observed equivalently as sharp peaks at room
temperature, suggesting that the inversion at the phosphorus
center occurs rapidly on the NMR timescale. By contrast,
the a-phenyl ortho-protons of the s4-P derivatives 7a–12a
were observed as broad and split peaks at room tempera-
ture. The above spectral behavior reflects the difference in
hybridization of the phosphorus atom between 7a and 10a.
The 31P peaks of the s3-P derivatives 4–6 appeared at d=
24.2–29.6 ppm, whereas those of Au complexes 7–9 and P-
oxides 10–12 appeared at d=46.5–51.4 ppm and d=35.3–
39.5 ppm, respectively. These results support the fact that
the phosphorus center is functionalized by Au coordination
and oxygenation. The degree of downfield shifts observed
for bithiophene-fused benzo[c]phospholes (Dd=19.0–
23.9 ppm for 7, 8, 9 versus 4, 5, 6; Dd=6.1–13.8 ppm for 10,
11, 12 versus 4, 5, 6) differs from that reported for 3,4-C4-
methylene-bridged 2,5-diphenylphospholes[3c,4b,5e] (Dd=
28.1 ppm for 19 versus 18; Dd=30.1 ppm for 20 versus 18)
To investigate substituent effects on the optical and elec-
trochemical properties, we also prepared a,a’-diarylbithio-
phene-fused benzo[c]phospholes 14–17 starting from the
type-I s3-P compound 4a (Scheme 2). Treatment of 4a with
an excess amount of n-butyllithium in THF at À788C, fol-
lowed by sequential addition of iodine and m-CPBA afford-
ed a,a’-diiodo P-oxo derivative 13 in 40% yield. The
Suzuki–Miyaura coupling of 13 with 5-methylthiophen-2-yl-
boronicacid in DMF at 110 8C produced a,a’-bis(5-methyl-
thiophen-2-yl) derivative 14 in 31% yield, whereas the Ne-
gishi coupling of 13 with thiazol-2-ylzincbromide in THF
produced a,a’-bis(thiazol-2-yl) derivative 15 in 20% yield.
Dilithiated 4a underwent aromaticnucleophilicsubstitution
(SNAr) reaction with hexafluorobenzene to afford a,a’-bis-
and a-phenyl-substituted dithieno
[b,d]phospholes[9c] (Dd=
28.7 ppm for 22 versus 21; Dd=39.6 ppm for 23 versus 21).
A
(pentafluorophenyl) s3-P derivative 16 in 40% yield. The
oxygenation of 16 with m-CPBA produced P-oxide 17 in
91% yield.
Compounds 4–17 are air- and thermally stable solids and
were fully characterized by conventional spectroscopic tech-
niques (1H, 13C, and 31P NMR spectroscopy, MS, and IR
Apparently, the P-substituent effect on the chemical shift is
related to the p-conjugated framework at the phosphole
Scheme 2. Functionalization of bithiophene-fused benzo[c]phospholes.
Chem. Eur. J. 2008, 14, 8102 – 8115
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Y. Matano et al.
ring. The 31P peaks of the s3-P derivative 16 (d=26.3 ppm)
and the P-oxo derivatives 13–15 and 17 (d=37.3–37.9 ppm)
are close to the respective 31P chemical shifts of 4a (d=
24.6 ppm) and 10a (d=37.8 ppm). It seems that the a,a’-
substituents of the fused bithiophene subunit do not affect
the electronic character at the phosphorus center significant-
ly. The IR spectra of the P-oxides 10–15 and 17 showed
Table 1. Selected bond lengths, bond angles, and dihedral angles of 4a,
5a, and 7a–10a.
À
characteristic P O stretching bands at nmax =1187–
1200 cmÀ1.
4a
–
5a
–
7a
AuCl
8a[a]
AuCl
9a[a]
AuCl
10a
O
E
bond lengths [Š]
Crystal structures of bithiophene-fused benzo[c]phospholes:
The crystal structures of 4a, 5a, and 7a–10a were unambig-
uously elucidated by X-ray diffraction analyses. Single crys-
tals were grown from CH2Cl2–MeOH (for 4a, 5a, 7a–9a) or
CH2Cl2–hexane (for 10a) at room temperature. The ORTEP
diagrams are depicted in Figure 2, packing structures are
shown in Figure S1 in the Supporting Information, selected
bond lengths, bond angles, and dihedral angles are listed in
Table 1, and crystallographic parameters are summarized in
Table 3 in the Experimental Section.[19] Each phosphorus
center in the s3-P derivatives 4a and 5a is pyramidalized
with the sum of C-P-C bond angles (ꢀC-P-C) of 299.5–303.28,
whereas that in the s4-P derivatives 7a–10a adopts a distort-
ed tetrahedral geometry with ꢀC-P-C of 306.1–310.98. The p-
conjugated phosphole and bithiophene rings are nearly on
the same plane with dihedral angles (A–B and A–B’;
Table 1) of 1.5–10.68. For all the compounds, the phosphorus
atom is slightly deviated by 0.05–0.28 Š from a cyclic 1,3-
a
1.796(2)
1.793(2)
1.374(2)
1.374(2)
1.486(2)
1.461(2)
1.461(2)
1.386(2)
1.385(2)
1.424(2)
1.801(2)
1.805(2)
1.365(3)
1.366(3)
1.491(3)
1.446(3)
1.453(3)
1.383(3)
1.378(3)
1.454(3)
1.792(4) 1.802(6) 1.793(8)
1.787(4) 1.787(6) 1.795(8)
1.800(5)
1.806(5)
a’
b
b’
c
d
d’
e
1.353(6) 1.360(8) 1.359(11) 1.359(6)
1.373(6) 1.367(8) 1.351(11) 1.358(6)
1.502(6) 1.495(8) 1.499(11) 1.505(6)
1.453(6) 1.459(8) 1.465(11) 1.469(6)
1.457(6) 1.445(8) 1.470(11) 1.485(6)
1.368(6) 1.379(8) 1.437(11) 1.392(6)
1.389(6) 1.383(8) 1.445(11) 1.385(6)
1.436(7) 1.452(8) 1.442(11) 1.416(6)
e’
f
bond angles [8]
a
92.22(7)
91.85(1)
94.1(2)
94.3(3)
93.6(4)
93.6(4)
107.7(4)
104.8(4)
b
106.00(7) 103.14(1) 106.9(2) 106.5(2) 104.8(4)
104.96(7) 104.46(1) 109.9(2) 108.6(3) 107.7(4)
b’
dihedral angles [8][b]
A–B 6.0
1.6
1.5
82.9
81.9
3.3
7.7
9.5
2.6
10.6
2.8
3.0
5.5
83.7
62.3
A–B’ 8.8
A–C 53.1
A–C’ 54.7
88.0
51.8
61.5
80.4
64.7
76.7
[a] Data of one of the pairing molecules. [b] Dihedral angles between the
mean planes A and B/B’/C/C’.
À
diene plane of the phosphole ring. The C C/C=C bond
length alternation observed for the benzo[c]phosphole skel-
eton (b–f in Table 1) is little affected by the P functionaliza-
tion, suggesting that the P lone pair in the s3-P derivatives
hardly mixes into the 1,3-dienic p-system of the phosphole
ring. The bonds at e/e’ of 9a are longer by 0.06–0.07 Š than
those of 7a and 8a, and the carbon–carbon bonds making
the central six-membered ring (c, d/d’, e/e’, and f) of 9a have
considerable single-bond character. These data clearly ex-
hibit the difference in the p-conjugation mode of bithio-
phene subunits; the benzo[c]phosphole character of the
type-III derivatives should be very weak as compared to
that of the type-I and type-II derivatives. Among the type-I
À
À
derivatives, the Ca Cb and Cb Cb bond lengths in the phosp-
hole ring of 4a (b/b’=1.374(2) Š; c=1.486(2) Š) differ only
slightly from those of 7a (b/b’=1.353(6)-1.373(6) Š; c=
1.502(6) Š) and 10a (b/b’=1.358(6)-1.359(6) Š; c=
1.505(6) Š). The packing structures of the present P,S-
hybrid p-systems definitely depend on hybridization at the
phosphorus center: the s3-P derivative 4a adopts head-to-
tail orientation, whereas the
s4-P-Au complex 7a adopts
head-to-head orientation (Fig-
ure S1 in the Supporting Infor-
À
mation). The Au P bond
lengths
(2.2198(15)–
2.2283(10) Š) and the P-Au-Cl
bond angles (175.17(6)–
177.57(9)8) of 7a–9a are within
the range of typical values of
known Au–phosphole com-
plexes, including 19 and 22.[20]
In contrast to the previously
reported Au complexes, how-
ever, there is no Au–Au inter-
action in the packing structures
of 7a–9a, which adopt head-
to-head orientation with a par-
Figure 2. The ORTEP diagrams (50% probability ellipsoids) of a) 4a, b) 5a, c )7a, d) 8a, e) 9a, and f) 10a.
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Bithiophene-Fused Benzo[c]phospholes
FULL PAPER
allel p–p stacking motif (7a) or a twisted p–p stacking motif
(8a and 9a) as shown in Figure S1 in the Supporting Infor-
mation.[21] It is likely that stericcongestion around the phos-
phorus center directs the head-to-head orientation of the
present Au complexes.[22] In all the compounds character-
ized, two benzene rings attached to the a-positions are twist-
ed significantly out of the phosphole ring plane with torsion
angles (A–C and A–C’) of 53.1–88.08. This may be partly
due to packing effects derived from the close proximity of
the two hybrid p-systems.
Optical properties of bithiophene-fused benzo[c]phosp-
holes: Compounds 4–17 are yellow, orange, or purple solids
soluble in CHCl3, CH2Cl2, THF, and toluene. To disclose the
optical properties of a series of bithiophene-fused benzo[c]-
phospholes, we measured UV/Vis absorption and fluores-
cence spectra of 4–17 in CH2Cl2 (Table 2, Figure 3, and Fig-
ure S2 in the Supporting Information). The absorption spec-
tra of the phenyl-substituted derivatives 4a and 5a display
broad p–p* transitions at around 400–550 nm, and the fluo-
rescence spectra of 4a and 5a show broad emissions in the
orange region (500–800 nm) with fluorescence quantum
yields (FF) of 17.6% and 9.7%, respectively. The absorption
and fluorescence maxima of 4a (labs =462 nm; lem =600 nm)
and 5a (labs =472 nm; lem =609 nm) are largely red-shifted
relative to those of 18 (labs =354 nm; lem =466 nm)[3c] and 21
(labs =431 nm; lem =470 nm),[9c] which represents the effec-
tive p-extension in the present P,S-hybrid p-systems of
types I and II. In marked contrast to 4a and 5a, the type-III
compound 6a displays weak absorption and emission bands
at labs =395 nm (shoulder) and lem =548 nm (FF =0.6%). A
Figure 3. a) UV/Vis absorption and fluorescence spectra of 4a, 5a, and
6a in CH2Cl2. b) UV/Vis absorption spectra (normalized) of 4a, 7a, 10a,
14, 15, and 17 in CH2Cl2.
similar tendency was observed for a series of the 2-thienyl-
substituted derivatives 4b–6b, which show absorption and
Table 2. Optical and electrochemical data for bithiophene-fused benzo[c]phospholes.
Absorption[a]
Fluorescence[a]
Stokes shift
[cmÀ1
Redox potentials[b]
labs [nm][c]
e [mÀ1 cmÀ1
]
lem [nm][d]
FF [%][e]
G
]
Eox [V][f]
Ered [V][f]
DE [V][g]
4a
4b
5a
5b
6a
6b
7a
7b
8a
8b
9a
9b
10a
10b
11a
11b
12a
13
462
483
472
491
395(sh)
409(sh)
502
535
503
7700
8700
4600
4900
2300
3200
5300
6200
2900
3700
1400
2800
4500
5400
2400
3000
1400
6800
7600
9200
15900
8000
600
646
609
656
548
582
661
710
675
720
596
606
–
–
–
–
–
–
–
–
603
–
17.6
2.9
9.7
2.6
0.64
1.1
0.30
0.05
0.10
0.02
0.47
0.67
–
–
–
–
–
–
–
–
6.7
–
4980
5220
4770
5120
7070
7270
4790
4610
5070
4660
7030
6490
–
–
–
–
–
–
–
–
0.45
0.36
0.47
0.39
0.66
0.56
0.83
0.66
0.86
0.70
1.12
0.88
0.72[h]
0.54
0.78[h]
0.59
1.00
0.78
0.44
0.73
0.55
0.83
À2.14
À2.00
À2.09
À1.97
À2.35
À2.18
À1.63
À1.55
À1.62
À1.50
À1.86
À1.76
À1.58
À1.45
À1.54
À1.41
À1.80
À1.45
À1.50
À1.43
À2.01
À1.47
2.59
2.36
2.56
2.36
3.01
2.74
2.46
2.21
2.48
2.20
2.98
2.64
2.30
1.99
2.32
2.00
2.80
2.23
1.94
2.16
2.56
2.30
539
420(sh)
435(sh)
530
566
533
573
427(sh)
544
603
561
14
15
16
17
473
535
4560
–
[a] Measured in CH2Cl2. [b] Determined by DPV in CH2Cl2 with 0.1m nBu4N+PF6À. [c] The longest absorption maxima. [d] Excited at 440 nm for 4, 5, 7,
8 and 16 and at 420 nm for 6 and 9. [e] Fluorescence quantum yields determined by comparison with the reported value of RØauꢁs 3,4-C4-bridged 1-
phenyl-2,5-bis(2-thienyl)phosphole–Au Complex (FF =12.9%; reference [4b]). [f] Redox potentials versus Fc/Fc+ couple. [g] EoxÀEred. [h] The second ox-
idation potentials of 10a and 11a are 0.83 V and 0.91 V, respectively.
Chem. Eur. J. 2008, 14, 8102 – 8115
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8107

Y. Matano et al.
emission maxima at longer wavelengths than do the phenyl-
substituted derivatives 4a–6a (Dlabs =14–21 nm; Dlem =34–
47 nm: 2-thienyl versus phenyl). The Stokes shifts of 6a,b
(7070–7270 cmÀ1) are considerably larger relative to those of
4a,b (4980–5220 cmÀ1) and 5a,b (4770–5120 cmÀ1). Evident-
ly, the p-electrons in 6 are less efficiently delocalized over
the fused rings than those in 4 and 5, as all the three hetero-
le rings are linked at the respective b-positions in 6.
to LUMO+1 (vide infra). The above results demonstrate
that the optical properties of bithiophene-fused benzo[c]-
phospholes are also tunable by chemical modifications at
the fused bithiophene backbone.
Electrochemical properties of bithiophene-fused benzo[c]-
phospholes: Redox potentials of the bithiophene-fused ben-
zo[c]phospholes were determined by means of cyclic voltam-
metry (CV) and differential pulse voltammetry (DPV). The
measurement conditions are summarized in the caption of
Table 2, and selected voltammograms are shown in Figure 4
To further our understanding of the photodynamics of the
three types of bithiophene-fused benzo[c]phospholes, we
measured fluorescence lifetimes of 4a–6a in CH2Cl2 at
room temperature. The fluorescence decays were well-fitted
by the single exponential component, and fluorescence life-
times (ts) of 4a–6a were determined to be 3.2, 4.1, and
0.18 ns, respectively. Note that ts value of the type-III com-
pound (6a) is considerably smaller than ts values of type-I
and type-II compounds (4a and 5a). To shed light on nonra-
diative pathway from the S1 state of these chromophores, we
next evaluated the possibility of triplet formation of 4a–6a
in CH2Cl2 by the transient grating (TG) method.[23] The TG
signals of the samples 4a and 5a showed only the thermal
grating ones, which were built within a system response time
(ca. 50 ns) and decayed by the thermal diffusion. On the
other hand, the TG signal of the sample 6a showed another
slower buildup of the thermal grating signal (ca. 200 ns),[24]
which was due to the nonradiative process from the triplet
state produced by the intersystem crossing from the S1 state.
This slower rise component was amount to 80% of the total
signal intensity. With these results in hand, we can safely
conclude that the main pathway from the S1 state of 6a is
not the direct relaxation to the S0 state, but the intersystem
crossing to the T1 state. These results clearly show that the
p-conjugation mode plays a crucial role in photodynamics of
the bithiophene-fused benzo[c]phospholes.
It is known that chemical functionalizations at the phos-
phorus atom of phospholes change their optical properties
dramatically.[1,2] In the present P,S-hybrid chromophores, the
P-coordination to AuCl salt induces a bathochromic shift of
the absorption and emission maxima (Dlabs =31–52 nm and
Dlem =61–66 nm; for 7/8 versus 4/5) and a considerable de-
crease in emission efficiency (FF =0.02–1.1% for 7–9 versus
FF =0.6–17.6% for 4–6). The oxygenation at the phosphorus
atom causes more pronounced effects on the p–p* transi-
tions, that is, the larger bathochromic shifts of absorption
maxima (Dlabs =61–83 nm for 10/11 versus 4/5) and com-
plete suppression of emission.
In the absorption spectra of 14, 15, and 17, labs of the
lowest excitation bands were observed at 603, 561, and
535 nm, respectively. The bathochromic shift of labs relative
to that of the parent type-I P-oxide 10a (labs =530 nm) is in
the order: 14 (Dlabs =73 nm)>15 (Dlabs =31 nm)>17
(Dlabs =5 nm). This indicates that the electron-donating aryl
substituents at the bithiophene a,a’-positions narrow the
HOMO–LUMO gap more significantly than the electron-
withdrawing ones. It should be noted that 14, 15, and 17 ex-
hibit other intense bands at labs =398, 400, and 373 nm, re-
spectively, assignable to the p–p* transitions from HOMO
Figure 4. Cyclic voltammograms (upper) and differential pulse voltammo-
grams (lower) of a) 10a and b) 14. Asterisks indicate Fc/Fc+ couple.
and Figure S4 in the Supporting Information. The electro-
chemical oxidation and reduction processes of the s3-P de-
rivatives 4–6 were found to be irreversible, and the redox
potentials determined by DPV are listed in Table 2. The first
oxidation potentials (Eox) and the first reduction potentials
(Ered) of the type-I compound 4a (Eox =0.45 V; Ered
=
À2.14 V versus ferrocene/ferrocenium) and the type-II com-
pound 5a (Eox =0.47 V; Ered =À2.09 V) are comparable,
which implies a subtle difference in their HOMO and
LUMO energies. By contrast, the type-III compound 6a
showed more positive Eox (0.66 V) and more negative Ered
(À2.35 V), thus indicating that the HOMO–LUMO gap of
6a (DE=3.01 V) is larger than that of 4a (DE=2.59 V) and
5a (DE=2.56 V). Replacing the 2,5-substituents at the
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Bithiophene-Fused Benzo[c]phospholes
FULL PAPER
phosphole ring from phenyl to 2-thienyl shifted Eox and Ered
to the negative and positive directions, respectively, thereby
narrowing the HOMO–LUMO gap by 0.20–0.34 V.
The chemical functionalizations at the phosphorus center
explicitly affect the redox potentials of bithiophene-fused
benzo[c]phospholes. The Au complexes 7–9 show irreversi-
ble voltammograms for oxidation and reduction processes at
more positive side relative to those observed for the corre-
sponding s3-P compounds 4–6. As was observed for RØauꢁs
phospholes 18 and 19,[3c,4b] the coordination to AuCl salt
lowers both HOMO and LUMO of the present P,S-hybrid
p-systems. The reduction potentials of 10–12 are further
shifted to the positive side (DEred =0.55–0.58 V versus 4–6)
as compared to those of 7–9 (DEred =0.42–0.51 V versus 4–
6), suggesting that the P-oxides are better electron acceptors
than the P–Au complexes. It is likely that the s*–p* orbital
interaction between the phosphorus center and the P-linked
diene unit becomes significant by the introduction of the
oxo group (vide infra). Noticeably, 10–12 show reversible
cyclic voltammograms for the reduction process as shown in
Figure 4a and Figure S3, implying that the P-oxides are elec-
trochemically stable in the one-electron reduction process.
The electronic effect of the p-conjugative heteroaryl sub-
stituents at the bithiophene moiety is also noteworthy. Intro-
duction of 5-methyl-2-thienyl groups raises the HOMO level
(Eox =0.44 V for 14 versus Eox =0.72 V for 10a), whereas
that of 2-thiazolyl groups lowers the LUMO level (Eox =
À1.43 V for 15 versus Eox =À1.58 V for 10a). As shown in
Figure 4b, both oxidation and reduction processes of 14 oc-
cured reversibly. These results primarily reflect the electron-
donating nature of the thienyl group and the electron-with-
drawing nature of the thiazolyl group. The HOMO–LUMO
gaps of 14 (DE=1.94 V) and 15 (DE=2.16 V) are smaller
than the gap of 10a (DE=2.30 V), implying that the p-net-
works at the bithiophene moiety in 14 and 15 are extended
efficiently to the attached heteroaryl groups. The substitu-
tion with pentafluorophenyl groups does not change the
HOMO–LUMO gap (DE=2.30 V for 17), although both
HOMO and LUMO levels are lowered in the same degree
by the inductive effect. It is now evident that the character
of frontier orbitals of bithiophene-fused benzo[c]phospholes
is tunable by suitably modifying the a,a’-positions of the bi-
thiophene subunit as well as the phosphorus center. The
HOMO–LUMO gaps determined by electrochemical meas-
urements are in good accordance with those estimated by
UV/Vis absorption/emission spectra.
respective values of 4a, 5a, and 10a determined by X-ray
crystallography (Figure S4 in the Supporting Information).
As visualized in Figure 5, the HOMO and the LUMO of
each bithiophene-fused benzo[c]heterole basically consist of
those derived from the heterole subunits. The HOMO of
4m and 5m holds antibonding character between the adja-
cent heterole rings, whereas the LUMO represents inter-
ring bonding interactions. The calculated HOMO and
LUMO energies (EHOMO and ELUMO) of the type-II model
5m are close to those of the type-I model 4m within 0.1 eV.
In contrast, the HOMO and LUMO energies of 6m are dis-
tinct from those of 4m and 5m. In particular, the LUMO of
6m represents almost no inter-ring bonding interaction and
is located at a much more positive level (ELUMO =À1.61 eV)
than that of 4m (ELUMO =À1.93 eV) and 5m (ELUMO
=
À2.00 eV). Consequently, the calculated HOMO–LUMO
gap of 6m (DE=4.08 eV) is considerably larger than that of
4m (DE=3.28 eV) and 5m (DE=3.28 eV).
As seen in Figure 5, the HOMO and LUMO energies of
the P-oxo models 10m–12m are lower by 0.40–0.42 eV and
0.78–0.81 eV, respectively, than those of the s3-P models
4m–6m. Hence, the HOMO–LUMO gaps of 10m–12m
become narrower by 0.36–0.41 eV relative to those of 4m–
6m. Evidently, the oxygenation at the phosphorus center
stabilizes the LUMO more than the HOMO, due to the ef-
fective s*–p* orbital interaction. The introduction of 2-
thienyl and 2-thiazolyl groups at the a,a’-positions of the bi-
thiophene subunit also perturbs the HOMO and LUMO en-
ergies. For example, replacement of the a-hydrogen atoms
by the 2-thienyl groups (10m–14m) causes more pro-
nounced effect on the HOMO (EHOMO =À5.63 eV to
À5.22 eV) than the LUMO (ELUMO =À2.71 eV to
À2.78 eV). The theoretically calculated substituent effects
are in good accordance with the experimentally observed re-
sults (vide supra). Note that the LUMO+1 of 14m and
15m, the wavefunctions of which are extended to the het-
Theoretical studies on bithiophene-fused benzo[c]heteroles:
To gain a deep insight into the electronic structures of bi-
thiophene-fused benzo[c]phospholes and related benzo[c]-
A
ed model compounds 4m–6m, 10m–12m, 14m, 15m, and
24m–27m. The structures of these compounds were opti-
mized at the B3LYP/6–31G* level, which is the same as that
used for the theoretical calculation of unsubstituted ben-
zo[c]phosphole.[11,25] The bond lengths and bond angles at
the p-conjugated units of 4m, 5m, and 10m are close to the
eroaryl substituents (Figure S5 in the Supporting Informa-
tion), lie at À1.80 eV and À2.15 eV, respectively. The calcu-
lated HOMO–LUMO+1 gaps of 14m and 15m (3.42–
Chem. Eur. J. 2008, 14, 8102 – 8115
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8109

Y. Matano et al.
corresponding heteroles (thio-
phene>s3-phosphole>silo-
À
le),[1e] implying that the s*
(E
N
H)–p* orbital interaction (E=
Si, P) intrinsically determines
the LUMO levels of bithio-
phene-fused benzo[c]heteroles.
It should be emphasized again
that the making of the P-oxo
bond stabilizes the LUMO
more significantly than the
HOMO due to the enhanced
s*–p*
orbital
interaction.
Namely, EHOMO and ELUMO of
10m are lower by 0.42 and
0.78 eV than EHOMO and ELUMO
of 4m, respectively. The calcu-
lated LUMO level of 10m is
lower than that of 25m, sug-
gesting the high electron-ac-
cepting ability of the P-oxo-
benzo[c]phosphole p-network
as compared to that of the
benzo[c]silole p-network.
A
series of the type-II models
5m, 11m, 26m, and 27m ex-
hibit almost the same tenden-
cy; the LUMO level lowers in
the order: benzo[c]thiophene
(26m)>s3-P-benzo[c]phos-
A
(5m)>benzo[c]silole
AHCTREUNG
Figure 5. HOMO (lower) and LUMO (upper) of a) type-I bithiophene-fused benzo[c]heterole models 4m,
10m, 14m, 15m, 24m, and 25m, b) type-II models 5m, 11m, 26m, and 27m, and c) type-III models 6m and
12m. The orbital energies are given in eV.
Electron mobility of a bithio-
phene-fused benzo[c]phos-
phole: As mentioned above,
AHCTREUNG
the P-oxo-type bithiophene-
fused benzo[c]phospholes have
3.33 eV) reasonably explain the appearance of the excita-
tions at labs =398–400 nm in the UV/Vis spectra of 14 and
15 (Figure 3b).
been found to possess high electron-accepting ability. In ad-
dition, these P-oxides are reasonably stable in electrochemi-
cal reduction processes. Such properties are beneficial for
designing efficient electron-transporting organic materials.[26]
With this in mind, we conducted a time-of-flight (TOF)
measurement to determine the electron mobility of 10a as a
representative example. The well-dried 10a was deposited
onto the surface of indium–tin–oxide (ITO) by sublimation
in vacuo (0.5–0.7 nmsÀ1; 10À3–10À4 Pa). The device was once
exposed to the atmosphere, and the aluminum electrode
To compare the electronic effects of heteroatoms on the
frontier orbitals of bithiophene-fused benzo[c]heteroles, we
also calculated benzo[c]thiophene models 24m (type I) and
26m (type II) and benzo[c]silole models 25m (type I) and
27m (type II) by using the same computational method. As
shown in Figure 5, the HOMOs of 4m, 24m, and 25m re-
semble one another, wherein the heteroatoms are on the
node of orbitals. In fact, EHOMO of 24m and 25m differ only
slightly from EHOMO of 4m (DEHOMO =À0.08 to +0.03 eV).
On the other hand, the LUMOs of 24m and 25m differ
markedly from the LUMO of 4m in shape and energy. That
is, ELUMO of benzo[c]thiophene 24m is higher by 0.45 eV
than that of 4m, whereas ELUMO of benzo[c]silole 25m is
lower by 0.26 eV than that of 4m. The order of ELUMO
(24m>4m>25m) is in good agreement with that of the
was made by vapor deposition (0.04 nmsÀ1
; 2.5–3.4”
10À3 Pa). The thickness of the organic layer was 5.0 mm,
which was determined by a stylus surface profiler (ULVAC
Dektak 6M). The thickness of aluminum electrode was esti-
mated to be ꢀ20 nm. Figure 6 depicts the field dependency
of the logarithmicelectron mobility of 10a. The results of
Alq3 (film thickness=5.3 mm), prepared and measured by
the same vacuum deposition and TOF apparatuses in our
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Chem. Eur. J. 2008, 14, 8102 – 8115

Bithiophene-Fused Benzo[c]phospholes
FULL PAPER
Experimental Section
General: All melting points are uncorrected. 1H, 13C{1H} and 31P{1H}
NMR spectra were recorded using CDCl3 as the solvent unless otherwise
noted. Chemical shifts are reported as the relative value versus tetrame-
thylsilane (1H and 13C) and phosphoricaicd ( 31P). MALDI-TOF mass
spectra were measured by using CHCA as a matrix. All solvents were
distilled from sodium benzophenone ketyl (diethyl ether), or calcium hy-
dride (CH2Cl2, toluene, Et3N, iPr2NH) before use. All the reactions were
performed under an argon or nitrogen atmosphere. Column chromatog-
raphy on silica gel was performed by using UltraPure Silicagel (230–400
mesh; SiliCycle), and thin-layer chromatography (TLC) was performed
on silica gel 60 F254 (Merck). The reaction procedures for the synthesis
of 1–9 and the 1H NMR spectra of 4–9 were reported in the Supporting
Information of reference [15]. In the present paper, the reaction proce-
dures and spectral data for 10–17 are described.
Synthesis of 10a
Figure 6. Electron mobilities (m) of 10a and Alq3 versus square root of
electric field (E). Circles: 10a; squares: Alq3 (measured in our lab). The
inset shows a schematic image of the device used for the TOF measure-
ment.
Method A: m-CPBA (max 77%, 240 mg) was added to a solution of 4a
(470 mg, 1.0 mmol) in CH2Cl2 (30 mL). The color of the solution turned
to purple in a few seconds, and the mixture was concentrated under re-
duced pressure. The solid residue was subjected to silica-gel column chro-
matography (CH2Cl2/acetone). The purple fraction was collected, evapo-
rated, and reprecipitated from hexane/CH2Cl2 to give 10a (460 mg, 94%)
as a purple solid.
group, respectively, were also shown for reference. In sharp
contrast to Alq3, 10a showed the negative field dependency
of the electron mobility (m), which increases with decreasing
the electric field (E) and reaches the highest value (m=3.9”
10À5 cm2 VÀ1 sÀ1) at E=5.0”105 VcmÀ1.[27] At the low elec-
tricfields, the mobilities of 10a (m=2.9–3.9”10À5 cm2 VÀ1 sÀ1
at E=5.0–6.0”105 VcmÀ1) are about one-order higher than
the mobility of Alq3 (m=2.3”10À6 cm2 VÀ1 sÀ1 at E=5.7”
105 VcmÀ1).[28] This preliminary result suggests that the ring-
fused benzo[c]phosphole oxides are promising candidates as
the electron transporting materials.[29]
Method B: Alternatively, the crude mixture containing the corresponding
s3-compound (prepared from 1a, Ti
(OiPr)4, iPrMgCl, and PhPCl2) was
G
treated with m-CPBA. After column chromatography and reprecipita-
tion, 10a was obtained (Yield 63% from 1a). M.p. 2708C (decomp);
1H NMR (400 MHz, CD2Cl2): d=6.54 (d, J=5.4 Hz, 2H), 6.91 (d, J=
5.4 Hz, 2H), 6.9–7.5 (m, 10H), 7.43 (ddd, J=7.4, 7.4 Hz, 4JP, H =2.8 Hz,
2H), 7.47 (t, J=7.4 Hz, 1H), 7.90 ppm (dd, J=7.4, 3JP,H =12.2 Hz, 2H);
13C{1H} NMR (100 MHz, CD2Cl2): d=123.8, 126.9 (d, JP,C =94.6 Hz),
127.3, 128.7 (d, JP, C =2.5 Hz), 129.1 (brs), 129.4, 129.4 (d, JP, C =13.2 Hz),
131.3, 131.6 (d, JP, C =93.0 Hz), 131.6 (d, JP, C =9.9 Hz), 132.7 (d, JP,C
=
2.5 Hz), 134.1 (d,
JP, C =8.2 Hz), 137.6, 140.5 ppm (d, JP,C =29.6 Hz);
31P{1H} NMR (162 MHz, CD2Cl2): d=+37.8 ppm; IR (KBr): n˜ =
1190 cmÀ1 (P=O); MS (MALDI-TOF): m/z: 490 [M+]; elemental analysis
calcd (%) for C30H19OPS2: C 73.45, H 3.90; found: C 73.29, H 3.63.
Synthesis of 10b: This compound was prepared from the corresponding
s3-compound 4b and m-CPBA according to a similar procedure (Meth-
od A) described for the synthesis of 10a. Yield: 92%, purple solid; m.p.
2758C (decomp); 1H NMR (400 MHz, CD2Cl2): d=6.94 (d, J=5.4 Hz,
Conclusion
The TiII-mediated cyclization protocol has proven to be ef-
fective for the preparation of three types of bithiophene-
fused benzo[c]phospholes, which are the first examples of
thermally and kinetically stable benzo[c]phosphole. Both
the experimental results (NMR spectroscopy, X-ray crystal-
lography, absorption/emission spectroscopy, and CV/DPV)
and the DFT computational results have revealed that the
ring-fused pattern and the substituents at the heterole rings
have a significant impact on the optical and electrochemical
properties of bithiophene-fused benzo[c]phospholes. The ap-
propriately fused s3-P derivatives and P–AuCl complexes
are potential emitters with small HOMO–LUMO gaps cov-
ering the orange–red region, and the P-oxo derivatives are
good electron acceptors with reasonable electrochemical sta-
bility. In addition, we have conducted the time-of-flight
measurement to evaluate the electron mobility of the type-I
P-oxo derivative, which exhibits high electron-transporting
ability relative to Alq3 at low electric fields. The present
study demonstrates that the arene-fused benzo[c]phosphole
skeleton could be a promising platform for the construction
of a new class of phosphole-based p-conjugated materials in
optoelectronic applications.
2H), 7.00 (d, J=4.9 Hz, 2H), 7.02 (d, J=5.4 Hz, 2H), 7.06 (dd, J=4.9,
4
4.9 Hz, 2H), 7.40 (d, J=4.9 Hz, 2H), 7.45 (ddd, J=7.3, 7.3 Hz, JP, H
=
3
2.8 Hz, 2H), 7.57 (t, J=7.3 Hz, 1H), 7.90 ppm (dd, J=7.3, JP, H =12.2 Hz,
2H); 13C{1H} NMR (75 MHz, CDCl3): d=123.5, 124.9 (d, JP, C =97.4 Hz),
125.5 (d, JP, C =97.4 Hz), 126.9, 127.7 (d, JP, C =1.9 Hz), 127.8, 128.3 (d,
J
J
P, C =5.0 Hz), 129.0 (d, JP,C =12.4 Hz), 130.5 (d, JP, C =18.0 Hz), 131.6 (d,
P, C =10.5 Hz), 132.6 (d, P, C =3.1 Hz), 133.6 (d, P, C =8.7 Hz), 137.9,
J
J
141.5 ppm (d,
J
P, C =29.2 Hz); 31P{1H} NMR (162 MHz, CDCl3): d=
+35.8 ppm; IR (KBr): n˜ =1196 cmÀ1 (P=O); MS (MALDI-TOF): m/z:
502 [M+]; elemental analysis calcd (%) for C26H15OPS4: C 62.13, H 3.01;
found: C 62.25, H 3.13.
Synthesis of 11a: This compound was prepared from the corresponding
s3-compound 5a and m-CPBA according to a similar procedure (Meth-
od A) described for the synthesis of 10a. Yield: 96%; wine red solid;
m.p. >3008C; 1H NMR (400 MHz, CD2Cl2): d=7.17 (d, J=5.4 Hz, 2H),
7.2–7.3 (m, 4H), 7.25 (d, J=5.4 Hz, 2H), 7.35–7.4 (m, 6H), 7.44 (ddd, J=
7.4, 7.4 Hz, 4JP, H =2.9 Hz, 2H), 7.56 (t, J=7.4 Hz, 1H), 7.91 ppm (dd, J=
7.4, 3JP,H =12.2 Hz, 2H); 13C{1H} NMR (100 MHz, CD2Cl2): d=123.4,
126.7 (d, JP, C =96.3 Hz), 129.1 (d, JP, C =1.6 Hz), 129.3 (d, JP, C =4.1 Hz),
129.5 (d, JP, C =11.5 Hz), 129.6 (d, JP, C =1.6 Hz), 130.4 (d, JP, C =95.5 Hz),
130.6, 130.7, 131.7 (d, JP, C =9.9 Hz), 132.7 (d, JP, C =2.5 Hz), 133.0 (d, JP, C
=
8.2 Hz), 137.9, 140.5 ppm (d, JP, C =30.5 Hz); 31P{1H} NMR (162 MHz,
CDCl3): d=+38.0 ppm; IR (KBr): n˜ =1200 cmÀ1 (P=O); MS (MALDI-
TOF): m/z: 490 [M+]; elemental analysis calcd (%) for C30H19OPS2: C
73.45, H 3.90; found: C 73.43, H 3.71.
Chem. Eur. J. 2008, 14, 8102 – 8115
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www.chemeurj.org
8111

Y. Matano et al.
Synthesis of 11b: This compound was prepared from the corresponding
s3-compound 5b and m-CPBA according to a similar procedure (Meth-
od A) described for the synthesis of 10a. Yield: 95%; purple solid; m.p.
2708C (decomp); 1H NMR (400 MHz, CD2Cl2): d=7.05–7.13 (m, 4H),
7.22 (d, J=5.4 Hz, 2H), 7.35 (d, J=5.4 Hz, 2H), 7.4–7.5 (m, 4H), 7.59 (t,
J=7.3 Hz, 1H), 7.90 ppm (dd, J=7.3, 3JP, H =12.2 Hz, 2H); 13C{1H} NMR
times. The combined organic extracts were washed with brine, and dried
over Na2SO4. After removal of the volatile components under reduced
pressure, the residue was subjected to silica-gel column chromatography
(CH2Cl2/acetone). The purple fraction was collected, evaporated, and
reprecipitated from MeOH to give 14 (13 mg, 20%) as a deep blue solid.
M.p. 2908C (decomp); 1H NMR (400 MHz, CD2Cl2): d=6.87 (s, 2H),
6.9–7.5 (m, 10H), 7.27 (d, J=3.4 Hz, 2H), 7.46 (ddd, J=7.5, 7.5 Hz,
4JP, H =2.9 Hz, 2H), 7.58 (t, J=7.5 Hz, 1H), 7.72 (d, J=3.4 Hz, 2H),
7.94 ppm (dd, J=7.5, 3JP, H =12.2 Hz, 2H); 31P{1H} NMR (162 MHz,
CD2Cl2): d=+37.5 ppm; IR (KBr): n˜ =1195 cmÀ1 (P=O); MS (MALDI-
TOF): m/z: 657 [M+]; HRMS (FAB): m/z calcd for C36H21ON2PS4:
656.0274; found: 656.0269 [M+]. 13C NMR data could not be obtained
due to low solubility.
(100 MHz, CDCl3): d=123.0, 124.1 (d, JP, C =100.0 Hz), 125.2 (d, JP, C
99.2 Hz), 128.0 (d, JP,C =1.7 Hz), 128.5 (d, JP,C =2.5 Hz), 128.7 (d, JP, C
=
=
5.0 Hz), 129.0 (d, JP, C =12.4 Hz), 129.8 (d, JP, C =18.2 Hz), 130.9, 131.6 (d,
P, C =10.7 Hz), 132.0 (d, P, C =9.1 Hz), 132.7 (d, P, C =3.3 Hz), 138.0,
141.9 ppm (d,
J
J
J
J
P, C =30.6 Hz); 31P{1H} NMR (162 MHz, CDCl3): d=
+35.3 ppm; IR (KBr): n˜ =1200 cmÀ1 (P=O); MS (MALDI-TOF): m/z:
502 [M+]; elemental analysis calcd (%) for C26H15OPS4: C 62.13, H 3.01;
Found: C 62.12, H 3.08.
Synthesis of 16: nBuLi (1.61m”0.27 mL, 0.44 mmol) was slowly added at
À788C to a solution of 4a (95 mg, 0.20 mmol) in THF (10 mL). The mix-
ture was kept at this temperature for 20 min, and hexafluorobenzene
(0.40 mL) was added. The resulting mixture was slowly warmed to room
temperature and stirred for 8 h. Then water was added and the mixture
was extracted with CH2Cl2 several times. The combined organic extracts
were washed with brine and dried over Na2SO4. After removal of the vol-
atile components under reduced pressure, the residue was subjected to
silica-gel column chromatography (CH2Cl2). The orange fraction was col-
lected, evaporated, and reprecipitated from MeOH to give 16 (65 mg,
Synthesis of 12a: This compound was prepared from the corresponding
s3-compound 6a and m-CPBA according to a similar procedure (Method
A) described for the synthesis of 10a. Yield: 94%; pale yellow solid;
m.p. >3008C; 1H NMR (400 MHz, CD2Cl2): d=7.05 (d, J=2.9 Hz, 2H),
7.1–7.45 (m, 10H), 7.44 (ddd, J=7.5, 7.5 Hz, 4JP, H =2.8 Hz, 2H), 7.50 (d,
3
J=2.9 Hz, 2H), 7.53 (t, J=7.5 Hz, 1H), 7.80 ppm (dd, J=7.5, JP, H
=
12.2 Hz, 2H); 13C{1H} NMR (100 MHz, CD2Cl2): d=118.0, 127.8 (d,
P, C =98.0 Hz), 127.9, 128.7 (d, P, C =1.7 Hz), 129.3 (d, JP,C =12.4 Hz),
129.9, 130.6, 130.9 (d, JP, C =96.7 Hz), 131.6 (d, JP, C =9.9 Hz), 132.3, 132.5
J
J
(d,
J
P, C =2.5 Hz), 133.8, 134.5 (d,
J
P, C =8.3 Hz), 138.8 ppm (d, JP, C
=
40%) as a
reddish orange solid. M.p. 2058C (decomp); 1H NMR
28.9 Hz); 31P{1H} NMR (162 MHz, CD2Cl2): d=+39.5 ppm; IR (KBr):
n˜ =1195 cmÀ1 (P=O); MS (MALDI-TOF): m/z: 490 [M+]; elemental
analysis calcd (%) for C30H19OPS2: C 73.45, H 3.90; found: C 73.05, H
3.72.
(400 MHz, CD2Cl2): d=7.1–7.3 (m, 5H), 7.3–7.45 ppm (m, 12H); 13C{1H}
NMR (100 MHz, CD2Cl2): d=124.6 (m), 127.8, 127.9 (d, JP,C =8.3 Hz),
128.9 (d, JP, C =12.4 Hz), 129.0, 129.1 (d, JP, C =9.1 Hz), 129.7 (d, JP,C
7.4 Hz), 130.6 (d, JP,C =2.5 Hz), 133.5 (d, JP, C =3.5 Hz), 135.0 (d, JP, C
21.7 Hz), 135.0 (d, JP,C =17.4 Hz), 135.2 (d, JP,C =3.0 Hz), 137.4 (d, JP,C
=
=
=
Synthesis of 13: nBuLi (1.61m”1.37 mL, 2.2 mmol) was slowly added at
À788C to a solution of 4a (470 mg, 1.0 mmol) in THF (50 mL). The mix-
ture was kept at this temperature for 20 min, and a solution of I2
(640 mg, 2.5 mmol) in THF (5 mL) was added. The resulting mixture was
slowly warmed to room temperature. After stirring for 2 h, the mixture
was treated with saturated aq. Na2S2O3 and extracted with CH2Cl2 twice.
To the combined organic extracts was added m-CPBA (max 77%,
240 mg). The mixture was dried over Na2SO4, and concentrated under re-
duced pressure. The solid residue was subjected to silica-gel column chro-
matography (CH2Cl2/acetone). The purple fraction was collected, evapo-
rated, and reprecipitated from diethyl ether/CH2Cl2 to give 13 (300 mg,
40%) as a purple solid. M.p. >3008C (decomp); 1H NMR (400 MHz,
15.7 Hz), 138.1 (dm, JF,C =250 Hz), 140.5 (dm, JF,C =250 Hz), 144.3 (dm,
J
F,C =250 Hz), 147.5 ppm (d, J
P, C =1.7 Hz); 31P{1H} NMR (162 MHz,
CD2Cl2): d=+26.2 ppm; MS (MALDI-TOF): m/z: 807 [M+]; HRMS
(FAB): m/z calcd for C42H17F10PS2: 806.0350; found: 806.0377 [M+]. One
of the carbon atoms could not be detected clearly in the 13C NMR spec-
trum.
Synthesis of 17: This compound was prepared from the corresponding s3-
compound 16 and m-CPBA according to a similar procedure (Method A)
described above for the synthesis of 10a. Yield: 92%, purple solid; m.p.
3008C (decomp); 1H NMR (400 MHz, CD2Cl2): d=7.08 (s, 2H), 7.0–7.5
4
(m, 10H), 7.46 (ddd, J=7.4, 7.4 Hz, JP,H =2.9 Hz, 2H), 7.57 (t, J=7.4 Hz,
CD2Cl2): d=6.66 (s, 2H), 6.9–7.5 (m, 10H), 7.43 (ddd, J=7.4, 7.4 Hz,
1H), 7.92 ppm (dd, J=7.4, 3JP, H =12.2 Hz, 2H); 13C{1H} NMR (100 MHz,
CD2Cl2): d=109.1 (m), 125.0, 125.6 (brs), 126.4 (d, JP, C =95.1 Hz), 128.9
(brs), 129.1 (d, JP, C =1.7 Hz), 129.6, 129.6 (d, JP, C =12.4 Hz), 131.6 (d,
3
4JP, H =2.9 Hz, 2H), 7.55 (t, J=7.4 Hz, 1H), 7.90 ppm (dd, J=7.4, JP,H
=
12.2 Hz, 2H); 31P{1H} NMR (162 MHz, CD2Cl2): d=+37.3 ppm; IR
(KBr): n˜ =1187 cmÀ1 (P=O); MS (MALDI-TOF): m/z: 742 [M+]; ele-
mental analysis calcd (%) for C30H17I2OPS2: C 48.54, H 2.31; found: C
48.70, H 2.31. 13C NMR data could not be obtained due to low solubility.
J
P, C =10.7 Hz), 132.1, 132.2 (d,
JP, C =73.6 Hz), 132.9 (d, JP,C =2.5 Hz),
133.5 (d, JP, C =8.3 Hz), 138.0, 138.3 (dm, JF,C =250 Hz), 139.3 (d, JP, C
=
29.8 Hz), 140.9 (dm, JF,C =250 Hz), 144.4 ppm (dm, JF,C =250 Hz); 31P{1H}
NMR (162 MHz, CD2Cl2): d=+37.9 ppm; IR (KBr): n˜ =1192 cmÀ1 (P=
O); MS (MALDI-TOF): m/z: 823 [M+]; HRMS (FAB): m/z calcd for
C42H17OF10PS2: 822.0299; found: 822.0284 [M+].
Synthesis of 14: A mixture of 13 (74 mg, 0.10 mmol), 5-methyl-2-thio-
phene-boronicaidc (31 mg, 0.22 mmol), [Pd
A
(200 mg), and DMF (10 mL) was stirred for 3 h at 1108C. Then water
was added and the mixture was extracted with toluene several times. The
combined organic extracts were washed with brine and dried over
Na2SO4. After removal of the volatile components under reduced pres-
sure, the residue was subjected to silica-gel column chromatography
(CH2Cl2/acetone). The deep green fraction was collected, evaporated,
and reprecipitated from MeOH to give 14 (21 mg, 31%) as a deep green
solid. M.p. 2908C (decomp); 1H NMR (400 MHz, CD2Cl2): d=2.42 (s,
6H), 6.46 (s, 2H), 6.62 (d, J=3.4 Hz, 2H), 6.80 (d, J=3.4 Hz, 2H), 6.9–
7.5 (m, 10H), 7.45 (ddd, J=7.5, 7.5 Hz, 4JP, H =2.9 Hz, 2H), 7.54 (t, J=
7.5 Hz, 1H), 7.93 ppm (dd, J=7.5, 3JP, H =12.2 Hz, 2H); 31P{1H} NMR
(162 MHz, CD2Cl2): d=+37.4 ppm; IR (KBr): n˜ =1190 cmÀ1 (P=O); MS
(MALDI-TOF): m/z: 683 [M+]; HRMS (FAB): m/z calcd for
C40H27OPS4: 682.0682; found: 682.0682 [M+]. 13C NMR data could not
be obtained due to low solubility.
X-ray crystal crystallographic analysis: Single crystals were grown from
CH2Cl2–MeOH (for 4a, 5a, 7a, 8a, and 9a) or CH2Cl2–hexane (for 10a)
at room temperature. All measurements were made on a Rigaku Saturn
CCD area detector with graphite monochromated MoKa radiation (l=
0.71069 Š). The structures were solved by direct methods,[31] and expand-
ed using Fourier techniques.[32] Non-hydrogen atoms were refined aniso-
tropically, and hydrogen atoms were refined by using the rigid model. All
calculations were performed using CrystalStructure crystallographic soft-
ware package[33] except for refinement, which was performed using
SHELXL-97.[34] The structural parameters are summarized in Table 3.
CCDC-660029 (4a), CCDC-687636 (5a), CCDC-660028 (7a), CCDC-
687637 (8a), CCDC-687635 (9a), and CCDC-687638 (10a) contain the
supplementary crystallographic data for this paper. These data can be ob-
tained free of charge from The Cambridge Crystallographic Data Centre
via www.ccdc.cam.ac.uk/data_request/cif.
Synthesis of 15: A solution of 2-thiazolylzincbromide in THF (ca. 0.5 m”
0.5 mL, prepared from 2-bromothiazole and activated zinc[30]) was added
Fluorescence lifetime and transient-grating measurements: The fluores-
cence lifetime was measured in CH2Cl2 by using a streak camera as a
fluorescence detector. Samples 4a and 5a were excited by the 480 nm
laser pulse produced by an optical parametric amplification system oper-
to a mixture of 13 (74 mg, 0.10 mmol), [Pd(PPh3)4] (10 mg), and THF
A
(8 mL); the resulting mixture was heated under reflux for 7 h. Then
water was added and the mixture was extracted with CH2Cl2 several
8112
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ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2008, 14, 8102 – 8115

Bithiophene-Fused Benzo[c]phospholes
FULL PAPER
Table 3. X-ray structural parameters of 4a, 5a, 7a, 8a, 9a, and 10a.
4a
5a
7a
8a
9a
C30H19AuClPS2
10a
formula
formula weight
crystal size (mm)
crystal system
a [Š]
b [Š]
c [Š]
a [8]
b [8]
C30H19PS2
474.54
0.45”0.20”0.15
monoclininc
6.2062(2)
21.4229(9)
17.4039(8)
90
97.372(3)
90
2294.80(16)
P21/n
C30H19PS2
474.54
0.25”0.20”0.10
triclinic
C31H21AuCl3PS2
791.88
0.20”0.15”0.05
triclinic
C30H19AuClPS2
706.96
C30H19OPS2
490.54
0.30”0.15”0.05
triclinic
706.96
0.20”0.15”0.04
monoclinic
19.141(3)
13.200(2)
21.155(3)
90
102.716(2)
90
5214.1(14)
P21/a
0.30”0.20”0.03
monoclininc
19.210(3)
13.1441(18)
21.304(3)
90
102.890(2)
90
5243.6(13)
P21/a
9.930(2)
9.888(3)
10.044(8)
10.278(8)
12.845(9)
93.010(9)
98.884(4)
116.591(9)
1160.4(15)
10.098(2)
12.819(3)
85.808(9)
87.573(10)
63.481(5)
1147.0(4)
12.243(4)
12.481(4)
100.709(4)
102.781(4)
95.077(4)
1434.5(8)
g [8]
V [Š3]
¯
¯
¯
space group
Z
P1
P1
P1
4
2
2
8
8
2
1calcd [gcmÀ3
]
1.374
3.19
1.374
3.19
1.833
56.29
À150
55.0
1.791
59.51
1.801
59.85
1.404
3.21
m [cmÀ1
T [8C]
]
À150
À150
À150
À150
À150
2qmax [8]
55.0
55.0
55.0
55.0
55.0
independent reflns
variables
5203
299
4980
299
6302
344
11949
632
11595
632
5089
308
R [I>2.00s(I)]
wR (all)
goodness of fit
0.0383
0.0752
1.015
0.0440
0.0896
1.034
0.0329
0.0678
1.012
0.0427
0.0959
1.009
0.0699
0.1218
1.066
0.0829
0.1575
1.052
ated by the output from an amplified femto-second Ti:Sapphire laser
system. Sample 6a was excited by the second harmonic output (400 nm)
from the same Ti:Sapphire laser system.
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The production of the triplet state was confirmed by the transient grating
measurement. The experimental setup was almost the same as that in ref-
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was monitored by the 633 nm CW laser. Before the TG measurement,
the sample solution (CH2Cl2) was bubbled by argon gas for more than
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Electrochemical measurements: These were performed in CH2Cl2 with a
glassy carbon working electrode, a platinum wire counter electrode, and
an Ag/Ag+ [0.01m AgNO3, 0.1m nBu4NPF6 (MeCN)] reference elec-
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Acknowledgement
This work was partially supported by Japan Science and Technology
Agency in Research for Promoting Technological Seeds (Grant No. 10-
034).
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[22] The electronic factor, namely dipole-dipole interaction of the adja-
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7a, 8a, and 9a.
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[24] The slower rise component was quenched by the addition of oxygen
into the solution.
[25] In our preliminary communication, we reported the optimized struc-
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energies are as follows: 4m–Ph (À5.10 eV and À1.89 eV), 5m–Ph
(À5.16 and À1.96 eV), 6m–Ph (À5.55 and À1.60 eV). See refer-
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[26] There are some reports on OLEDs using triorganylphosphine
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8114
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Chem. Eur. J. 2008, 14, 8102 – 8115

Bithiophene-Fused Benzo[c]phospholes
FULL PAPER
M. S. Gordon, D. J. DeFrees, J. A. Pople, J. Chem. Phys. 1982, 77,
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Received: May 27, 2008
Published online: August 19, 2008
Chem. Eur. J. 2008, 14, 8102 – 8115
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
8115