Synthesis and structure-property relationships of 2,2'-bis(benzo[b] phosphole) and 2,2'-benzo[b]phosphole-benzo[b]heterole hybrid π systems

Article abstract of DOI:10.1002/chem.201203047

The first comprehensive study of the synthesis and structure-property relationships of 2,2'-bis(benzo[b]phosphole)s and 2,2'-benzo[b]phosphole- benzo[b]heterole hybrid π systems is reported. 2-Bromobenzo[b]phosphole P-oxide underwent copper-assisted homocoupling (Ullmann coupling) and palladium-catalyzed cross-coupling (Stille coupling) to give new classes of benzo[b]phosphole derivatives. The benzo[b]phosphole-benzo[b]thiophene and -indole derivatives were further converted to P,X-bridged terphenylenes (X=S, N) by a palladium-catalyzed oxidative cycloaddition reaction with 4-octyne through the C_β-H activation. X-ray analyses of three compounds showed that the benzo[b]phosphole-benzo[b]heterole derivatives have coplanar π planes as a result of the effective conjugation through inter-ring C-C bonds. The π-π* transition energies and redox potentials of the cis and trans isomers of bis(benzo[b]phosphole) P-oxide are very close to each other, suggesting that their optical and electrochemical properties are little affected by the relative stereochemistry at the two phosphorus atoms. The optical properties of the benzo[b]phosphole-benzo[b]heterole hybrids are highly dependent on the benzo[b]heterole subunits. Steady-state UV/Vis absorption/fluorescence spectroscopy, fluorescence lifetime measurements, and theoretical calculations of the non-fused and acetylene-fused benzo[b]phosphole-benzo[b]heterole π systems revealed that their emissive excited states consist of two different conformers in rapid equilibrium.

Full text of DOI:10.1002/chem.201203047

DOI: 10.1002/chem.201203047
Synthesis and Structure–Property Relationships of 2,2’-
(benzo[b]phosphole) and 2,2’-Benzo[b]phosphole–Benzo[b]heterole
Hybrid p Systems
BisACHTUNGTRENNUNG
Yukiko Hayashi,[a] Yoshihiro Matano,*[a] Kayo Suda,[b] Yoshifumi Kimura,[c]
Yoshihide Nakao,[d] and Hiroshi Imahori[a, e]
Abstract: The first comprehensive
study of the synthesis and structure–
property relationships of 2,2’-bis-
ACHTUNGTRENNUNG(benzo[b]phosphole)s and 2,2’-ben-
zo[b]phosphole–benzo[b]heterole
hybrid p systems is reported. 2-Bromo-
benzo[b]phosphole P-oxide underwent
copper-assisted homocoupling (Ull-
mann coupling) and palladium-cata-
lyzed cross-coupling (Stille coupling) to
give new classes of benzo[b]phosphole
derivatives. The benzo[b]phosphole–
benzo[b]thiophene and -indole deriva-
tives were further converted to P,X-
bridged terphenylenes (X=S, N) by a
palladium-catalyzed oxidative cycload-
dition reaction with 4-octyne through
their optical and electrochemical prop-
erties are little affected by the relative
stereochemistry at the two phosphorus
atoms. The optical properties of the
benzo[b]phosphole–benzo[b]heterole
hybrids are highly dependent on the
benzo[b]heterole subunits. Steady-state
UV/Vis absorption/fluorescence spec-
troscopy, fluorescence lifetime meas-
urements, and theoretical calculations
of the non-fused and acetylene-fused
benzo[b]phosphole–benzo[b]heterole p
systems revealed that their emissive ex-
cited states consist of two different
conformers in rapid equilibrium.
ꢀ
the Cb H activation. X-ray analyses of
three compounds showed that the ben-
zo[b]phosphole-benzo[b]heterole deriv-
atives have coplanar p planes as a
result of the effective conjugation
ꢀ
through inter-ring C C bonds. The p–
p* transition energies and redox poten-
tials of the cis and trans isomers of bis-
AHCTUNGTRENN(GUN benzo[b]phosphole) P-oxide are very
close to each other, suggesting that
Keywords: cross-coupling · hetero-
cycles · homocoupling · phospholes ·
photophysics
Introduction
Phosphole, a phosphorus-bridged 1,3-butadiene, is a poorly
aromatic heterole with a low-lying LUMO and a narrow
LUMO–HOMO gap, which can be finely tuned by chemical
functionalizations at the phosphorus center and the ring
carbon atoms.[1] Recently, many efforts have been devoted
to the development of arene-fused phosphole p systems for
use in materials chemistry.[2] In particular, benzo[b]phos-
[a] Y. Hayashi, Prof. Dr. Y. Matano, 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
AHCTUNGTRENNUNG
pholes[3] and related compounds[4–7] have been receiving in-
creasing attention because they possess rigid and extended
p skeletons, which would be beneficial in the molecular
design of phosphole-based sensitizers, fluorophores, and
semiconductors. The benzo[b]phosphole skeleton contains
two functionalization sites, that is, a- and b-carbon atoms,
on one side of the phosphole unit. It is therefore important
to establish convenient methods for the introduction of p-
conjugative substituents onto these carbon atoms as well as
to systematically understand the optical and electrochemical
properties of the resulting benzo[b]phosphole p systems.
The literature to date contains several methodologies for
the construction of p-conjugated benzo[b]phospholes of
types P1 and P2 (Scheme 1).[8] The most frequently studied
is the intramolecular cycloaddition of 2-alkynylphenyl-
[b] K. Suda
Department of Chemistry
Graduate School of Science, Kyoto University
Sakyo-ku, Kyoto 606-8502 (Japan)
[c] Prof. Dr. Y. Kimura
Department of Chemical Science and Technology
Faculty of Bioscience and Applied Chemistry
Hosei University, Tokyo 184-8584 (Japan)
[d] Prof. Dr. Y. Nakao
Fukui Institute for Fundamental Chemistry
Kyoto University, Sakyo-ku
Kyoto 606-8103 (Japan)
[e] Prof. Dr. H. Imahori
Institute for Integrated Cell-Material Sciences (WPI-iCeMS)
Kyoto University, Nishikyo-ku
Kyoto 615-8510 (Japan)
AHCTUNGTRENNUNG
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201203047.
AHCTUNGTRENNUNG
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FULL PAPER
Scheme 2. Regioselective synthesis of 2-bromobenzo[b]phosphole P-
oxide (2).
Scheme 1. Examples of p-conjugated benzo[b]phospholes (E=lone pair,
O, S).
oxide (1) was prepared from 1-bromo-1-(trimethylsilyl)sty-
action was reported by Winter and Butters in the 1970s[9]
and has recently been improved by several research
groups.[3] In recent syntheses, Tsuji et al.,[3a] Sanji et al.,[3b]
and Yamaguchi et al.[3c] independently achieved the sequen-
tial introduction of p-conjugative substituents at the phosp-
hole a- and b-carbon atoms in one-pot or two-step proce-
dures. Yamaguchi et al. also reported the tandem cyclization
of 2,2’-bis(diphosphanyl)diphenylacetylene derivatives for
the synthesis of P3 and related compounds.[6] It is worth
noting that some benzo[b]phosphole derivatives have exhib-
ited extremely high emitting abilities and/or high electron
drift mobilities in the amorphous state.
In the above-mentioned intramolecular cycloaddition
methods, the p-conjugative groups are integrated with the
starting acetylene substrates in advance. By contrast, in
metal-promoted coupling protocols that use halogenated
benzo[b]phospholes, the p-conjugative groups can be intro-
duced after construction of the phosphole ring, which ena-
bles the divergent synthesis of a variety of 2-arylbenzo[b]-
phosphole derivatives starting from a common benzo[b]-
phosphole synthon. To our knowledge, however, metal-pro-
moted coupling reactions have not been fully addressed in
the synthesis of 2-arylbenzo[b]phospholes.
rene according to Kuritaꢁs method.[11] Carbon silicon bond
ꢀ
cleavage in 1 with N-bromosuccinimide (NBS) afforded
compound 2 in 54% yield. Mathey and coworkers prepared
compound 2 in two steps (dibromination–dehydrobromina-
tion) from a,b-unsubstituted benzo[b]phosphole P-oxide; 2
was obtained as a mixture with its regioisomer, the 3-bromo
derivative.[10] The present route from 1 to 2 is promising due
to the fact that compound 2 can be prepared as a single re-
gioisomer in a short reaction sequence from the commer-
cially available phenyltrimethylsilylacetylene.
With compound 2 in hand, we first conducted an Ullmann
coupling (Scheme 3). Heating a mixture of 2 and metallic
copper in DMF for 5 h at 1208C gave 2,2’-bis-
AHCTUNGTREG(NNUN benzo[b]phosphole) P-oxides (cis-3 and trans-3), together
In this paper, we report the first examples of 2,2’-bis-
ACHTUNGTRENNUNG(benzo[b]phosphole) and benzo[b]phosphole–benzo[b]heter-
ole hybrid p systems, which were successfully constructed
starting from 2-bromobenzo[b]phosphole P-oxide by an Ull-
mann coupling and a Stille coupling reaction, respectively.
In addition, a convenient method for the ring fusion of the
benzo[b]phosphole–benzo[b]heterole hybrids was establish-
ed. The structure–property relationships of the newly pre-
pared benzo[b]phosphole derivatives were investigated by
using X-ray crystallography, UV/Vis absorption/fluorescence
spectroscopy, cyclic voltammetry, fluorescence lifetime
measurements, and theoretical calculations. We investigated
1) the influences of the relative stereochemistry of the bis-
Scheme 3. Synthesis of the 2,2’-bis
4.
ACHTUNGTRENN(UNG benzo[b]phosphole) derivatives 3 and
with a few byproducts. After column chromatography on
silica gel, the major isomer, that is, cis-3 (Rf =0.09, CH2Cl2/
acetone=5:1), was successfully isolated in 39% yield, but
the minor isomer, that is, trans-3 (Rf =0.35), could not be
completely separated from the byproducts. Hence, cis-3 was
reduced by using HSiCl3 to the s3-P derivative 4, which was
isolated by column chromatography. The re-oxidation of 4
with H2O2 proceeded cleanly and quantitatively to give a
mixture of cis-3 and trans-3 in a 2:1 ratio, from which trans-
3 was successfully isolated.
ACHTUNGTRENNUNG(benzo[b]phosphole) P-oxides on their structures and funda-
mental properties and 2) the effects of the benzo[b]heterole
subunits on the optical properties of the non-fused and acet-
ACHTUNGTRENNUNGylene-fused benzo[b]phosphole–benzo[b]heterole p systems.
The 2,2’-bisACTHNUGTRNEUNG(benzo[b]phosphole)s, cis-3, trans-3, and the
s3-derivative 4 (cis/trans mixture), were characterized by
using conventional spectroscopic methods. The 31P NMR
peaks of cis-3 and trans-3 appeared at dP =34.9 and
35.2 ppm, respectively, whereas those of the s3-derivative 4
appeared at dP =ꢀ4.4 and ꢀ4.0 ppm. In the IR spectra, the
P=O stretching bands of cis-3 and trans-3 were observed at
Results and Discussion
Scheme 2 depicts the synthesis of 2-bromobenzo[b]phos-
phole P-oxide (2),[10] the starting material in the present
coupling reactions. 2-(Trimethylsilyl)benzo[b]phosphole P-
ACHTUNGTRENNUNG
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Y. Matano et al.
n˜max =1185 and 1198 cmꢀ1, respectively, indicating that the
multiplicity of the P=O bond in cis-3 is slightly smaller than
that in trans-3. The structure of cis-3 was further elucidated
by X-ray crystallography (Figure 1).[12] Single crystals of cis-
Figure 1. Top (left) and side view (right) of cis-3 (30% probability ellip-
soids). Hydrogen atoms are omitted for clarity. Selected bond lengths [ꢂ]
ꢀ
ꢀ
ꢀ
and torsion angle [8]: C1 C2 1.357(3), C2 C3 1.472(2), C1 C9 1.452(2),
ꢀ
ꢀ
ꢀ
ꢀ
C9 C10 1.352(2), C10 C11 1.4715(19), P1 O1 1.4881(12), P2 O2
1.4892(11), C2-C1-C9-C10 160.76(17).
3, grown from CH2Cl2/hexane, were found to be racemic,
and the unit cell consists of two kinds of enantiomers. The
length of the bond connecting the two phosphole rings is
1.452(2) ꢂ and the torsion angle at the inter-ring linkage is
160.76(17)8, implying that the two phosphole rings are effec-
tively conjugated. In the packing structure of cis-3, (R,R)
and (S,S) stereoisomers are stacked in parallel and in a
head-to-tail orientation with p–p distances of 3.4–3.5 ꢂ (Fig-
ure S1 in the Supporting Information).
To examine the optical and electrochemical properties of
cis-3 and trans-3, UV/Vis absorption/fluorescence spectra
and redox potentials were measured in CH2Cl2 (Table 1,
Figure 2). It is known that the optical and electrochemical
Figure 2. a) UV/Vis absorption (solid line) and fluorescence (dashed line)
spectra of cis-3 and trans-3 in CH2Cl2; lex =379 (cis-3) and 380 nm (trans-
3). The fluorescence spectra are normalized for comparison. b) Cyclic
voltammograms of cis-3 and trans-3 in CH2Cl2 with 0.1m [nBu4N]ACHTUNGTRENNUNG[PF6] as
a supporting electrolyte. Scan rate=60 mVsꢀ1. Asterisks indicate the Fc/
Fc+ couple.
dimers, cis-3 and trans-3, are very close to each other,
except for the extinction coefficients (absorption spectra)
and the fluorescence quantum yields (fluorescence spectra).
The emax and the Ff values of trans-3 (emax =26300mꢀ1 cmꢀ1,
Ff =0.40) are higher than the respective values of cis-3
(emax =17900mꢀ1 cmꢀ1, Ff =0.24). This discrepancy probably
stems from the difference in the planarity of cis-3 and trans-
3 (see below, torsion angles in Table 2). The small Stokes
shifts (ꢁ1000 cmꢀ1) and clear appearance of vibronic struc-
Table 1. Optical and electrochemical data for the benzo[b]phosphole de-
rivatives.
tures suggest that the 2,2’-bisACTHNUGRTNEUNG(benzo[b]phosphole) frame-
works in these dimers are rigid even in solution. In the
cyclic voltammetry measurements of cis-3 and trans-3 (in
CH2Cl2 with 0.1m [Bu4N]ACHTUNTRGNEUNG[PF6] as a supporting electrolyte),
labs [nm] (loge)[a]
lem [nm] (Ff)[a,b]
Eox [V][c]
Ered [V][c]
2
cis-3
trans-3
5
6
7
8
9
326 (3.59)
379 (4.25)
380 (4.42)
372 (4.35)
377 (4.31)
393 (4.07)
344 (3.96)
355 (4.08)
395 (N.d.)
441 (0.24)
440 (0.40)
451 (0.22)
450 (0.17)
542 (0.44)
396 (0.13)
431 (0.24)
N.d.
N.d.
N.d.
N.d.
ꢀ1.71 (r)
ꢀ1.67 (r)
ꢀ1.61 (ir)
ꢀ1.86 (ir)
ꢀ2.00 (ir)
N.d.
N.d.
reversible first reduction processes were observed at ꢀ1.71
and ꢀ1.67 V (vs. Fc/Fc+; Fc=ferrocene), respectively. The
first reduction potentials (Ered) of cis-3 and trans-3 are com-
parable to those of cis/trans-P2 (E=O, R =Ph; Ered =ꢀ1.72
+0.98 (ir)
+0.53 (ir)
N.d.
N.d.
ꢀ2.34 (r)
and ꢀ1.74 V vs. Fc/Fc+)[3c] and cis/trans-P3 (E=O; Ered
=
[a] Measured in CH2Cl2. [b] Excited at labs (excited at 400 nm for the Ff
measurements). [c] Redox potentials were determined by DPV in CH2Cl2
(for 3, 5, and 6) or THF (for 7 and 9) with 0.1m [Bu4N]ACTHNUTRGNENU[G PF6] (Ag/
ꢀ1.63 and ꢀ1.67 V vs. Fc/Fc+)[6a] reported by Yamaguchi
and coworkers (see Scheme 1).
AgNO3). First oxidation (Eox) and reduction (Ered) potentials versus the
To determine the nature of the frontier orbitals of the
2,2’-bisACTHNUTRGNE(UNG benzo[b]phosphole) p systems, we carried out densi-
Fc/Fc+ couple; r=reversible; ir=irreversible. N.d.=Not determined.
ty functional theory (DFT) calculations for cis-3, trans-3,
and a reference monomer, that is, benzo[b]phosphole P-
oxide, at the B3LYP/6-31G* level (Table 2 and Figure 3). In
the optimized structures, trans-3 (torsion angle (q)=179.98)
properties of chain-type 2,2’-biphospholes are sensitive to
the relative conformations of the two phosphole rings. For
example, Hissler and coworkers rationalized the effects of
ꢀ
is more planar than cis-3 (q=159.48). The Cb Cb bond
ꢀ
the torsion angles at the inter-ring C C bonds on the optical
lengths (a in Table 2; 1.467–1.468 ꢂ) of cis-3 and trans-3 are
slightly shorter than that of the reference monomer (a=
properties of a series of methylene-bridged 2,2’-biphosphole
derivatives in detail.[13] As shown in Figure 2, the spectral
shapes and the absorption/emission maxima of the present
ꢀ
1.479 ꢂ), whereas the Ca Cb bond lengths (b in Table 2;
1.359–1.361 ꢂ) are longer than that of the monomer (b=
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Synthesis and Structure–Property Relationships of Hybrid p Systems
FULL PAPER
Table 2. Bond lengths (a–c) and torsion angles (q) of cis-3, trans-3, and
monomer.[a]
monomer[a]
cis-3 (X-ray)
cis-3[a]
1.468
1.359
1.444
trans-3[a]
a [ꢂ]
b [ꢂ]
c [ꢂ]
q [8]
1.479
1.345
–
1.47[b]
1.36[b]
1.45
1.467
1.361
1.441
–
160.8
159.4
179.9
[a] Calculated at the B3LYP/6-31G* level. [b] Average values.
Scheme 4. Synthesis of the benzo[b]phosphole–benzo[b]heterole deriva-
tives 5–7 (dba=dibenzylideneacetone).
ring (dH =6.87 ppm), which probably reflects the different
electron densities of the heterole subunits; in 7, the phos-
AHCTUNGTREGpNNUN hole ring is electron deficient, whereas the pyrrole ring is
electron rich. In the IR spectra of compounds 5–7, the P=O
stretching bands were observed at n˜max =1196, 1195, and
1191 cmꢀ1, respectively. The structures of compounds 5 and
6 were further elucidated by X-ray crystallography. As
shown in Figure 4, both 5 and 6 are mostly coplanar with
torsion angles at the inter-ring linkages of 175.73(11)8 for 5
and 177.27(14)8 for 6. In addition, the relatively short inter-
Figure 3. HOMOs (bottom) and LUMOs (top) of the reference mono-
mer, cis-3, and trans-3 and their energies in [eV] calculated at the
B3LYP/6-31G* level.
ꢀ
ring C C bond lengths (1.4458(17) ꢂ for 5 and 1.442(2) ꢂ
for 6) are responsible for the effective conjugation between
the benzo[b]phosphole and the benzo[b]heterole p systems.
It is worth noting that both compounds, 5 and 6, are racemic
crystals, in which the (R) and (S) stereoisomers make dense-
ly p–p stacked pairs in a head-to-tail orientation (Figure S1
1.345 ꢂ). Obviously, the dimerization of the benzo[b]phos-
ACHTUNGTRENNUNGphole units at their a-positions induces effective p conjuga-
tion; this is also reflected in the relatively short inter-ring
ꢀ
C C bond length (c in Table 2; 1.441–1.444 ꢂ).
As shown in Figure 3, dimerization greatly stabilizes the
LUMO and destabilizes the HOMO. As a result, the
HOMO–LUMO gap is 1.20–1.26 eV narrower than that of
the reference monomer. Furthermore, the orbital diagrams
and energies of the HOMO and LUMO of cis-3 resemble
those of trans-3, suggesting that the relative stereochemistry
at the two phosphorus centers does not significantly affect
the orbital nature of the entire bis
network.
ACHTUNGERTN(NUNG benzo[b]phosphole) p
As expected, bromide 2 underwent a Stille coupling with
2-(tributylstannyl)benzo[b]thiazole, 2-(tributylstannyl)ben-
zo[b]thiophene, and 2-(tributylstannyl)-N-methylindole to
give the respective benzo[b]phosphole–benzo[b]heterole
hybrid derivatives 5–7 in 48–63% yields (Scheme 4).
The 31P NMR spectra of compounds 5–7 in CD2Cl2
showed singlet peaks at dP =33.5, 34.3, and 35.5 ppm, re-
Figure 4. Top (left) and side view (right) of a) 5 and b) 6 (30% probabili-
ty ellipsoids). Hydrogen atoms are omitted for clarity. Selected bond
ꢀ
ꢀ
ꢀ
lengths [ꢂ] and angles [8]: a) C1 C2 1.3524(17), C2 C3 1.4658(18), C1
1
spectively. In the H NMR spectrum of compound 7, the b-
ꢀ
ꢀ
C9 1.4458(17), P O 1.4838(8); C2-C1-C9-N 175.73(11). b) C1 C2
proton of the phosphole ring appeared downfield (dH =
ꢀ
ꢀ
ꢀ
ꢀ
1.357(2), C2 C3 1.468(2), C1 C9 1.442(2), C9 C10 1.366(2), C10 C11
1.430(2), P O 1.4875(10), C2-C1-C9-C10 177.27(14).
ꢀ
7.52 ppm, J
N
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Y. Matano et al.
in the Supporting Information) with p–p distances of ap-
proximately 3.2–3.3 ꢂ between the two facing thiazolyl rings
(for 5) and thiophene rings (for 6). This suggests that the a-
heteroaryl rings play an important role in arranging the
non-fused benzo[b]phosphole–benzo[b]heterole p systems in
the solid state.
The UV/Vis absorption and fluorescence spectra of com-
pounds 5–7 in CH2Cl2 are shown in Figure 5a. Among these
three chromophores, the benzo[b]phosphole–indole deriva-
with an emission maximum at 542 nm and a modest fluores-
cence quantum yield (Ff =0.44). In our previous study on
chain-type phosphole-heterole-phosphole and heterole-
phosphole-heterole p systems, we demonstrated that the
combination of a phosphole P-oxide (acceptor) and a pyr-
role (donor) produced a charge-transfer (CT) character in
the excited state.[14] The large Stokes shift and the broad
emission band observed for 7 suggest that the benzo[b]-
phosphole-indole chromophore possesses considerable CT
character in the excited state. Indeed, the fluorescence spec-
tra of 7 exhibited clear solvatochromism (Figure 5b); with
increasing solvent polarity, the fluorescence maxima (lem
)
are gradually red shifted and the Stokes shifts (Dn˜) become
larger, in the order cyclohexane (lem =495 nm, Dn˜ =
5020 cmꢀ1)<benzene
(lem =517 nm,
Dn˜ =5910 cmꢀ1)<
CH2Cl2 (lem =542 nm, Dn˜ =7000 cmꢀ1)<MeCN (lem
=
542 nm, Dn˜ =7520 cmꢀ1).
ꢀ
As compounds 6 and 7 have two Cb H bonds at the con-
jugated phosphole and heterole (thiophene or pyrrole) rings,
we anticipated that an acetylene unit would be connected at
the b-carbon atoms, based on an oxidative cycloaromatiza-
ꢀ
tion through dual C H activation. Under the palladium cat-
alysis conditions reported by Jiao and coworkers,[15] com-
pounds 6 and 7 underwent oxidative coupling with 4-octyne
to afford the P,X-bridged (X=S, N) terphenylenes 8 and 9
(Scheme 5). The reaction of 7 proceeded more rapidly than
Scheme 5. Palladium-catalyzed oxidative coupling reaction of compounds
6 and 7 with 4-octyne (Piv=pivalyl).
ꢀ
that of 6, suggesting that an electrophilic C H activation is
ꢀ
involved in the rate-determining step; the Cb H bond of the
more electron-rich pyrrole ring in 7 is easier to activate than
that of the thiophene ring in 6. The structures of the new
arene-fused phosphole derivatives 8 and 9 were character-
ized by using conventional spectroscopic techniques. The
31P NMR peaks of compounds 8 and 9 were observed up-
field (dP =27.2–27.7 ppm) compared to those of the precur-
sors 6 and 7 (dP =34.3–35.5 ppm). The UV/Vis absorption
and fluorescence spectra of the acetylene-fused derivatives 8
and 9 were significantly blue shifted relative to those of the
respective non-fused derivatives 6 and 7 (Figure 5c). It
should also be noted that the Stokes shifts of compounds 8
(3820 cmꢀ1) and 9 (4810 cmꢀ1) are appreciably smaller than
those of 6 (4250 cmꢀ1) and 7 (7000 cmꢀ1), respectively. This
demonstrated the rigidity of the fused p frameworks in 8
and 9. Compound 9 also showed a solvatochromic behavior
in solution.[16]
Figure 5. UV/Vis absorption spectra (solid line) and fluorescence spectra
(dashed line; excited at the absorption maxima) of a) compounds 5–7 in
CH2Cl2, b) compound 7 in cyclohexane, benzene, MeCN, and CH2Cl2,
and c) compounds 8 and 9 in CH2Cl2.
tive 7 showed the most red-shifted absorption and fluores-
cence maxima. Furthermore, the Stokes shift of
7
(7000 cmꢀ1) is considerably larger than those of compounds
5 (4950 cmꢀ1) and 6 (4250 cmꢀ1), implying that compound 7
changes its conformational and/or electronic structure in the
excited state to a greater extent than compounds 5 and 6. It
should also be noted that 7 displays a broad emission band
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Synthesis and Structure–Property Relationships of Hybrid p Systems
FULL PAPER
To compare the characters of the HOMOs and LUMOs
of the benzo[b]phosphole–benzo[b]heterole hybrid p sys-
tems, we first performed DFT calculations for compounds
5–7, 8m, and 9m (models for compounds 8 and 9, respec-
tively; two n-propyl groups were replaced by two methyl
groups) at the B3LYP/6-31G* level. As shown in Figure 6,
both the HOMO and the LUMO levels rise in the order 5<
6<7, which basically reflects the electron-donating and elec-
as a fluorescence detector. The fluorescence decay curves of
compounds 8 and 9 in CH2Cl2 were fitted as a single expo-
nential with lifetimes (tf) of 2.2 and 7.9 ns, respectively.
From the Ff and tf values, the radiative and non-radiative
decay rate constants (kr and knr) were determined to be kr =
5.9ꢃ107 and knr =4.0ꢃ108 sꢀ1 for 8 and kr =3.0ꢃ107 and
knr =9.7ꢃ107 sꢀ1 for 9. In contrast, the fluorescence decay
curves of the non-fused derivatives 5–7 under the same
measurement conditions were
analyzed by using two-exponen-
tial decay fittings. Figure 7a
shows the fluorescence spectra
of
7 in CH2Cl2 at different
delay times (lex =390 nm). The
fluorescence line shape is de-
pendent on the delay time, sug-
gesting that there are different
species in the electronic excited
state. Figure 7b shows the
changes in the fluorescence in-
tensities of compound
7 in
CH2Cl2 monitored at 460 and
630 nm. The fluorescence inten-
sity at 460 nm shows a fast
decay component following a
slow decay component, whereas
the fluorescence intensity at
630 nm shows fast rise and slow
decay components. The fast
decay-time constant of the
Figure 6. HOMOs and LUMOs of compounds 5–7, 8m, and 9m and their
energies in [eV] calculated at the B3LYP/6-31G* level.
tron-accepting ability of the thiazole, thiophene, and pyrrole
rings. The HOMO–LUMO gaps decrease in the order 5>
6>7. Among these three p systems, the inclination of the
molecular orbital coefficients for the HOMOs and LUMOs
is most distinct for compound 7; the HOMO is mainly local-
ized on the indole unit, whereas the LUMO is mainly local-
ized on the benzo[b]phosphole unit. The calculated orbital
diagrams explain the distinct CT character of 7 in its excited
state. The acetylene fusion from 6/7 to 8m/9m widens the
HOMO–LUMO gap of the benzo[b]phosphole–benzo[b]-
ACHTUNGTRENNUNGheterole p systems, which agrees well with the experimental-
ly observed results. In the entirely fused p systems 8m and
9m, the HOMOs essentially possess the character of the
dibenzoACHTUNGTRENNUNG[b,d]thiophene and carbazole, respectively.
Most of the p-conjugated benzo[b]phosphole derivatives
have been reported to be fluorescent. To our knowledge,
however, little information is available for the photodynam-
ics of this class of compounds. To obtain a deep insight into
ꢀ
the effects of the rotation of the inter-ring C C bond on the
photophysical behavior of the present benzo[b]phosphole–
benzo[b]heterole hybrid p systems, we measured the fluores-
cence lifetimes of the non-fused derivatives 5–7 and the
Figure 7. a) Time-resolved fluorescence spectra (from 440–640 nm) at 0.1,
5, 10, and 15 ns, and b) time profiles at 460 and 630 nm of the fluores-
cence decay for 7 in CH2Cl2. lex =390 nm.
acetACHTUNGTRENNUNGylene-fused derivatives 8 and 9 by using a streak camera
Chem. Eur. J. 2012, 18, 15972 – 15983
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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15977

Y. Matano et al.
460 nm intensity matches the rise-time constant of the
630 nm intensity well. Detailed analysis of the time-resolved
fluorescence spectra by using the singular value decomposi-
tion (SVD) indicated that there are two emitting species in
the electronic excited state and that there is an equilibrium
between them (see the Experimental Section). The fact that
the acetylene-fused derivatives 8 and 9 obeyed first-order ki-
netics may suggest that the two emitting species of 5–7 are
probably formed through rotation at the inter-ring bond be-
therefore also monitored in benzene and in MeCN. The kfast
value increases with an increasing solvent polarity (3.3ꢃ
108 sꢀ1 in benzene, 10.3ꢃ108 sꢀ1 in MeCN), whereas the kslow
values are largely independent of the solvent polarity (1.4ꢃ
108 sꢀ1 in benzene, 1.6ꢃ108 sꢀ1 in MeCN). It seems likely
that the two emitting species possess different CT charac-
ters; the more stable species seems to be more polarized.
Again, it should be noted that the two emitting species
ꢀ
presumably arise from rotation at the inter-ring C C bond,
tween the phos
G
that is, a change in the anti–syn conformations in the excited
state. To evaluate the validity of this interpretation, we next
performed DFT calculations on compounds 5–7 by using the
B3LYP/cc-pVDZ method. In the ground state of 7 (Fig-
ure 9a), two conformers, syn (q=49.58; q is the P-C-C-N
torsion angle) and anti (q=201.88), are both energy minima.
This trend is similar to the theoretical prediction for the ro-
tational potential energies of 2,5-bis(2-thienyl)phosphole re-
ported by Rꢄau, and coworkers.[17] The energy differences
between the two conformers of 7 (EantiꢀEsyn) are 1.5 kcal
molꢀ1 in a vacuum and ꢀ0.2 kcalmolꢀ1 in CH2Cl2 as deter-
mined by using a polarizable continuum model (PCM), indi-
cating that the solvent effect on the anti conformer is more
distinct than on the syn conformer. This is probably because
the dipole moment of the anti conformer (5.37 D) is larger
than that of the syn conformer (3.31 D). The small rotation-
al energy barriers between the two conformers (3–4 kcal
molꢀ1) suggest that rotation occurs freely in the ground state
of compound 7. In other words, both conformers are likely
to be present in solution. The HOMOs and LUMOs at the
different torsion angles and their orbital energies are sum-
marized in Figure S3 in the Supporting Information. It is
evident that the electron densities are localized at the twist-
ed structures. In the ground states of compounds 5 and 6,
the syn and anti conformers are energy minima with an
energy difference of 0.6 kcalmolꢀ1 for 5 and 2.0 kcalmolꢀ1
for 6 (Figure S4 in the Supporting Information). The activa-
ble scheme is shown in Figure 8.
Figure 8. Plausible scheme for the photodynamics of 7; r=radiative
decay, nr=nonradiative decay.
If we assume that the rate constants of the equilibria in
the excited states (kAB, kBA) are larger than those of the sum
of the radiative and non-radiative decays from the two excit-
ed states (k1, k2; k1 =kr1+knr1, k2 =kr2+knr2), the observed
rate constants (kfast, kslow) can be estimated by using Equa-
tions (1) and (2) (for details, see the Experimental Section).
kfast ¼ kAB þ kBA
ð1Þ
ð2Þ
ꢀ
tion energies of the rotation at the inter-ring C C bonds are
k1kAB þ k2kBA
kAB þ kBA
5–6 kcalmolꢀ1 for 5 and 4–6 kcalmolꢀ1 for 6 (determined in
kslow
¼
CH2Cl2 by using PCM), which implies that the two confor-
AHCTUNGTRENNUNG
The kfast and kslow values of compounds 5–7 in CH2Cl2 de-
termined by SVD analyses of the time-resolved spectra (Fig-
ure S2 in the Supporting Information) are summarized in
Table 3. The kfast values thus obtained (0.55–1.47ꢃ109 sꢀ1)
are four to five times larger than the kslow values (1.3–3.0ꢃ
108 sꢀ1). The kfast value is directly related to the equilibrium
rate constants, whereas the kslow value essentially reflects the
fluorescence decay rate constants. As mentioned above,
compound 7 exhibited clear solvatochromism in the steady-
state fluorescence spectra. The fluorescence decay of 7 was
the syn conformer (q=14.88) is a global minimum, and the
anti conformer (q=212.18) is a local minimum; the syn con-
former is more stable by 1.5 kcalmolꢀ1 than the anti con-
former. The rotational energy barriers of 3–6 kcalmolꢀ1
imply that an equilibrium between the two conformers is
possible in the S1 state of 7. The emission energies from the
S1 state of 7 were calculated by considering vertical transi-
tions to the S0 state at the same geometries. As shown in
Figure 9c, the oscillator strengths (f) are highly dependent
on the q values and nearly equal to zero at q=90 and 2708
(f=0.002–0.005). Apparently, the perpendicularly aligned
conformers of 7 are non-fluorescent in nature. At the two
energy minima at q=14.8 and 212.18, the S1-to-S0 transition
energies are DE=2.13 eV (f=0.544) and DE=2.16 eV (f=
0.357), respectively.
Table 3. Decay rate constants for 5–7 in various solvents.[a]
5
6
7
CH2Cl2
CH2Cl2
C6H6
CH2Cl2
MeCN
kfast [nsꢀ1
kslow [nsꢀ1
]
]
1.47
0.30
1.29
0.14
0.33
0.14
0.55
0.13
1.03
0.16
[a] Determined by the SVD analyses.
15978
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Chem. Eur. J. 2012, 18, 15972 – 15983

Synthesis and Structure–Property Relationships of Hybrid p Systems
FULL PAPER
Although the calculated transition energies do not com-
pletely match the observed ones, the theoretical results indi-
cate that the two emission processes are conceivable after
rapid equilibrium between the syn and anti conformers of 7
in the excited state. Presumably the trans conformer (CT*A
in Figure 8) is converted to the cis conformer (CT*B) in the
S1 state, followed by dual emissions with different rates at
the two different wavelengths.[18] The local and global
minima of the excited states of compounds 5 and 6 were
also determined at the same level of calculations. As listed
in Table 4, the energy differences between the syn and anti
Table 4. Calculated parameters at the S0 and S1 states of 5–7.[a]
5
6
7
conformation
syn
anti
syn
anti
syn
anti
q(S0) [8][b]
12.5
0.00
0.4
0.00
2.37
0.833
175.3
ꢀ0.65
178.0
ꢀ0.01
2.41
26.2
0.00
1.8
0.00
2.29
193.5
ꢀ2.01
185.6
+0.11
2.40
49.5
201.8
ꢀ0.18
212.1
+1.49
2.16
[c]
[e]
E(S0) [kcalmolꢀ1
q(S1) [8][d]
]
0.00
14.8
0.00
2.13
E(S1) [kcalmolꢀ1
]
DE
(S1–S0) [eV][f]
ACHTUNGTRENNUNG
f
A
0.851 0.757
0.892 0.544
0.357
[a] Calculated at the B3LYP/cc-pVDZ level by using the PCM model
(CH2Cl2). [b] P-C-C-X torsion angles (X=S for 5 and 6, X=N for 7) at
the local and global minima of the S0 state. [c] Relative energies of the
syn and anti conformers of the S0 state. [d] P-C-C-X torsion angles at the
local and global minima of the S1 state. [e] Relative energies of the syn
and anti conformers of the S1 state. [f] S1–S0 vertical transition energies
from the local and global minima of the S1 state. [g] Oscillator strengths
of the S1–S0 vertical transitions from the local and global minima of the
S1 state.
conformers are 0.01 kcalmolꢀ1 for 5 and 0.11 kcalmolꢀ1 for
6. The calculated S1-to-S0 transition energies from the two
energy minima are DE=2.37 eV (f=0.833 at q=0.48) and
DE=2.41 eV (f=0.851 at q=178.08) for 5 and DE=2.29 eV
(f=0.757 at q=1.88) and DE=2.40 eV (f=0.892 at q=
185.68) for 6. The emission energies of 5 and 6 are apprecia-
bly larger than those of 7, which is in good agreement with
the experimental observations. This may be a result of the
different CT characters in their excited states.
Conclusion
We have established new divergent methods for the synthe-
sis of 2,2’-bis
ACHTUNGTREN(NUNG benzo[b]phosphole) and 2,2’-benzo[b]phos-
AHCTUNGTREGpNNUN hole–benzo[b]heterole p systems based on Ullmann and
Stille coupling reactions of 2-bromobenzo[b]phosphole P-
oxide. The benzo[b]phosphole–benzo[b]heterole p conju-
ꢀ
gates bearing Cb H bonds were further converted to P,X-
bridged terphenylenes (X=S, N) by palladium-catalyzed ox-
Figure 9. Dependences of the P-C-C-N torsion angles (q [8]) of 7 on
a) the relative energies (E [kcalmolꢀ1]) in the S0 state in vacuum and in
CH2Cl2, b) the relative energies (E [kcalmolꢀ1]) in the S1 state in CH2Cl2,
and c) the S1-to-S0 vertical transition energies (DE [eV]) and the oscilla-
tor strengths (f) in CH2Cl2 calculated at the B3LYP/cc-pVDZ level.
ꢀ
idative cyclization with 4-octyne through dual C H activa-
tion. These results unambiguously corroborate that metal-
promoted coupling reactions are highly promising for intro-
ducing p-conjugative functionalities onto the phosphole ring
of the benzo[b]phosphole skeleton. The relative stereochem-
istry at the phosphorus centers was found to have small im-
pacts on the optical and electrochemical properties of the
Chem. Eur. J. 2012, 18, 15972 – 15983
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15979

Y. Matano et al.
ing mixture was stirred for 4.5 h at 1208C. The insoluble copper powder
in the mixture was filtered off through a Celite bed and water was then
added. The aqueous layer was extracted with toluene and the organic
combines were washed with brine, dried over Na2SO4, and evaporated.
The residue was subjected to silica gel column chromatography (CH2Cl2/
acetone=5:1). The blue fluorescent fraction (Rf =0.09) was collected and
bisACHTUNGTRENNUNG(benzo[b]phosphole) p systems. In contrast, the optical
properties of the benzo[b]phosphole–benzo[b]heterole p
conjugates are highly dependent on the combination of ben-
zo[b]heterole subunits. In particular, the benzo[b]phosphole-
indole derivative exhibits intrinsic CT character in the excit-
ed state. Time-resolved fluorescence lifetime measurements
and DFT calculations of both the ground state and the excit-
ed state of the benzo[b]phosphole–benzo[b]heterole hybrid
p systems revealed that two kinds of conformers are formed
in rapid equilibrium, with small rotational barriers. Further
studies on the application of this class of compounds to or-
ganic devices are now in progress.
evaporated to give cis-2,2’-bisACTHNUTRGNE(NUG benzo[b]phosphole) P-oxide (cis-3)
(0.584 g, 39%) as a yellow solid. In this coupling, trans-3 was also formed
but could not be purified by column chromatography. M.p. 277–2798C
(decomp); 1H NMR (400 MHz, CD2Cl2): d=7.29–7.52 (m, 17H), 7.57
(dd, J=8.3, 8.3 Hz, 2H), 7.74 ppm (d, J=34.2 Hz, 1H); 13C{1H} NMR
(100 MHz, CD2Cl2): d=125.6 (t, J=4.5 Hz), 128.9 (d, J=6.6 Hz), 129.0
(d, J=4.9 Hz), 129.3 (dd, J=10.7, 92.2 Hz), 129.7 (pseudo t, J=5.8 Hz),
130.5 (pseudo t, J=5.4 Hz), 131.9 (dd, J=4.9, 115.3 Hz), 132.0 (d, J=
100.8 Hz), 132.3, 133.4, 140.0 (pseudo t, J=11.9 Hz), 141.8 ppm (pseudo
t, J=13.6 Hz); 31P{1H} NMR (162 MHz, CDCl3): d=34.9 ppm; IR (neat):
n˜max =1185 cmꢀ1 (P=O); HRMS (MALDI): m/z calcd for C28H21O2P2:
451.1011; found: 451.1003 [M+H+].
Experimental Section
Compound 4: HSiCl3 (2.5 mL, 25 mmol) was added to a solution of cis-3
(0.277 g, 0.62 mmol) in toluene (23 mL). After stirring for 19 h at 1108C,
the mixture was added into water and the insoluble white substances
were filtered off. The organic layer of the filtrate was separated, and the
aqueous layer was extracted with toluene. The organic combines were
evaporated. The residue was recrystallized from MeOH/CH2Cl2 to give
compound 4 as a pale yellow solid (0.116 g, 45%). M.p. 273–2758C
(decomp); 1H NMR (400 MHz, CDCl3): d=7.10–7.52 ppm (m, 20H);
31P{1H} NMR (162 MHz, CDCl3): d=ꢀ4.41, ꢀ3.96 ppm; HRMS (EI): m/
z calcd for C28H20P2: 418.1040; found: 418.1048 [M+].
All melting points were recorded on a Yanagimoto micro melting point
apparatus and are uncorrected. 1H, 13C{1H}, and 31P{1H} NMR spectra
were measured on a JEOL EX400, AL300, or ECA-600P spectrometer.
Chemical shifts are reported as the relative values versus the signal of
tetramethylsilane (1H, 13C) or H3PO4 (31P). Selected 1H NMR spectra are
shown in Figures S5–S12 in the Supporting Information. 1H–1H COSY
spectra were measured on a JEOL ECA-600P spectrometer. Matrix-as-
sisted laser desorption/ionization (MALDI) time-of-flight mass spectra
(TOF) were measured on a Shimadzu Biotech AXIMA-CFR spectrome-
ter by using CHCA as a matrix. High-resolution mass spectra (HRMS)
were obtained on a Thermo Fisher Scientific orbitrapXL spectrometer
for MALDI, JEOL JMS-SX102A spectrometer for EI and Thermo
Fisher Scientific EXACTIVE spectrometer for ESI measurements. IR
spectra were obtained on a Thermo Fisher Scientific NICOLET 6700
FTIR spectrometer. UV/Vis absorption and fluorescence spectra were
obtained on a Perkin–Elmer Lambda 900 UV/Vis/NIR spectrometer and
on a Horiba FluoroMax-3 spectrometer, respectively. Quantum yield
measurements were performed on a Hamamatsu Photonics Quantaurus-
QY spectrometer. Electrochemical measurements were performed on a
CH Instruments model 660A electrochemical workstation by using a
glassy carbon working electrode, a platinum wire counter electrode, and
Compound trans-3: An aqueous solution of H2O2 (30%, 3 mL) was
added to
a solution of compound 4 (86 mg, 0.21 mmol) in CH2Cl2
(30 mL) and the resulting mixture was stirred for 1.5 h at room tempera-
ture. The mixture was quenched with an aqueous solution of Na2SO3
(20%, 120 mL) and the organic layer was separated. The aqueous layer
was extracted with CH2Cl2 and the organic combines were washed with
brine, dried over MgSO4, and evaporated. The residue was subjected to
silica gel column chromatography (CH2Cl2/acetone=5:1). The fraction of
Rf =0.35 was collected and evaporated to give trans-3 (6 mg, 12%).
M.p.>3008C; 1H NMR (400 MHz, CD2Cl2): d=7.31–7.36 (m, 4H), 7.45–
7.57 (m, 12H), 7.77 ppm (dd, J=8.3, 12.7 Hz, 4H); 31P{1H} NMR
(162 MHz, CDCl3): d=35.2 ppm; IR (neat): n˜max =1198 cmꢀ1 (P=O);
HRMS (EI): m/z calcd for C28H20O2P2: 450.0939; found: 450.0935 [M+].
an Ag/Ag+ [0.01m AgNO3, 0.1m [nBu4N]
ACTHNUTRGENUN[G PF6] (MeCN)] reference elec-
trode. The potentials were calibrated with ferrocene/ferrocenium. Tetra-
hydrofuran and diethyl ether (Et2O) were distilled from sodium benzo-
phenone ketyl before use. Compound 1 was prepared according to the re-
ported procedures.[11] Other chemicals and solvents were of reagent
grade quality and used without further purification unless otherwise
noted. Thin-layer chromatography was performed with silica gel 60 F254
(Merck), and preparative column chromatography was performed by
using ultra pure silica gel (230–400 mesh; SiliCycle Inc). All reactions
were performed under an argon atmosphere unless otherwise noted.
Compound 5: A mixture of compound 2 (601 mg, 1.97 mmol), 2-(tributyl-
stannyl)benzothiazole
(ꢁ3.0 mmol),
and
[Pd
G
(254 mg,
0.220 mmol) in toluene (90 mL) was stirred for 13 h at 1108C. The aque-
ous layer was extracted with EtOAc and the organic combines were
dried over Na2SO4, filtered off through a Celite bed, and evaporated.
The residue was subjected to silica gel column chromatography (CH2Cl2/
acetone=10:1). The blue fluorescent fraction (Rf =0.31) was collected,
evaporated, and recrystallized from CH2Cl2/hexane to give compound 5
(341 mg, 48%) as a yellow solid. M.p. 257–2598C (decomp); 1H NMR
(400 MHz, CD2Cl2): d=7.37–7.54 (m, 6H), 7.57 (d, J=6.8 Hz, 1H), 7.61
(d, J=7.3 Hz, 1H), 7.69 (dd, J=8.3, 8.5 Hz, 1H), 7.81 (dd, J=13.2,
10.0 Hz, 2H), 7.88 (d, J=7.8 Hz, 1H), 8.00 (d, J=7.8 Hz, 1H), 8.02 ppm
(d, J=32.7 Hz, 1H); 13C{1H} NMR (75 MHz, CD2Cl2): d=122.0, 123.9,
126.3, 126.6 (d, J=9.2 Hz), 126.9, 129.2 (d, J=12.3 Hz), 129.7 (d, J=
9.9 Hz), 129.9 (d, J=102.4 Hz), 131.1 (d, J=10.5 Hz), 131.5 (d, J=
11.1 Hz), 132.9 (d, J=3.1 Hz), 133.8 (d, J=1.8 Hz), 134.6 (d, J=95.0 Hz),
134.8 (d, J=72.8 Hz), 140.8, 141.2, 143.8 (d, J=17.9 Hz), 154.2,
160.9 ppm (d, J=11.7 Hz); 31P{1H} NMR (162 MHz, CD2Cl2): d=
33.5 ppm; IR (neat): n˜max =1196 cmꢀ1 (P=O); HRMS (ESI): m/z calcd for
C21H15NOPS: 360.0606; found: 360.0591 [M+H+]; elemental analysis
calcd (%) for C21H14NOPS: C 70.18, H 3.93, N 3.90, S 8.92, P 8.62;
found: C 70.04, H 3.91, N 3.79, S 8.86, P 8.43.
Synthesis and characterization of compounds 2–9
Compound 2: NBS (1.118 g, 6.28 mmol) was added to a solution of com-
pound 1 (0.894 g, 3.00 mmol) in MeCN (26 mL). The resulting mixture
was stirred for 5.5 h at room temperature and water was then added. The
aqueous layer was extracted with EtOAc and the organic combines were
washed with an aqueous saturated solution of Na2S2O3 and brine. The or-
ganic layer was dried over Na2SO4 and evaporated. The residue was sub-
jected to silica gel column chromatography (CHCl3/acetone=20:1). The
purple fluorescent fraction (Rf =0.33) was collected and evaporated to
give compound 2[10] (0.491 g, 54%) as a pale yellow solid. 1H NMR
(400 MHz, CDCl3): d=7.30–7.39 (m, 2H), 7.43–7.52 (m, 3H), 7.47 (d, J=
28.8 Hz, 1H), 7.58 (t, J=8.8 Hz, 1H), 7.63 (dd, J=7.3, 10.2 Hz, 1H),
7.72 ppm (dd, J=7.8, 12.7 Hz, 2H); 31P{1H} NMR (CDCl3, 162 MHz): d=
30.7 ppm.
Compound 6: A mixture of [Pd2ACHTNUTRGNE(UGN dba)3] (18 mg, 0.020 mmol), (2-furyl)3P
(32 mg, 0.14 mmol), and N-methylpyrrolidone (NMP) (1.6 mL) was stir-
red for 1 h at room temperature. Compound 2 (201 mg, 0.66 mmol), 2-
(tributylstannyl)benzo[b]thiophene (0.30 mL, 0.84 mmol), NMP (6.4 mL),
and CuI (148 mg, 0.78 mmol) were added to this mixture and the result-
Compound cis-3: Copper powder was washed with HCl in acetone, fol-
lowed by acetone alone in advance. The copper powder (0.467 g,
7.3 mmol) was added to a solution of 2-bromobenzo[b]phosphole (2)
(2.01 g, 6.6 mmol) in DMF (28 mL) at room temperature and the result-
15980
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Chem. Eur. J. 2012, 18, 15972 – 15983

Synthesis and Structure–Property Relationships of Hybrid p Systems
FULL PAPER
ing mixture was stirred for 14 h at room temperature. A saturated aque-
ous solution of NH4Cl was then added and insoluble substances were fil-
tered off through a Celite bed. The organic layer was separated and the
aqueous layer was extracted with EtOAc/hexane (1:1). The organic com-
bines were washed with water, dried over Na2SO4, and evaporated. The
residue was subjected to silica gel column chromatography (CH2Cl2/ace-
tone=15:1 to 10:1). The blue fluorescent fraction (Rf =0.63 at CH2Cl2/
acetone=10:1) was collected, evaporated, and recrystallized from
CH2Cl2/hexane to give compound 6 (127 mg, 54%) as a yellow solid.
M.p. 206–2078C (decomp); 1H NMR (400 MHz, CD2Cl2): d=7.28–7.80
(m, 14H), 7.51 ppm (d, J=41.5 Hz, 1H); 13C{1H} NMR (100 MHz,
CD2Cl2): d=122.5, 124.7, 125.1 (d, J=11.5 Hz), 125.4 (d, J=9.9 Hz),
126.0, 129.3 (d, J=10.7 Hz), 129.4 (d, J=12.3 Hz), 129.7 (d, J=10.7 Hz),
130.3 (d, J=99.6 Hz), 131.1 (d, J=10.7 Hz), 133.0 (d, J=7.8 Hz), 132.9,
133.0 (d, J=107.8 Hz), 133.8, 134.3 (d, J=93.9 Hz), 136.7 (d, J=14.8 Hz),
137.0 (d, J=2.5 Hz), 137.2 (d, J=2.5 Hz), 139.6, 140.5, 142.1 ppm (d, J=
27.2 Hz); 31P{1H} NMR (162 MHz, CD2Cl2): d=34.3 ppm; IR (neat):
n˜max =1195 cmꢀ1 (P=O); HRMS (ESI): m/z calcd for C22H16OPS:
359.0654; found: 359.0640 [M+H+]; elemental analysis calcd (%) for
C22H15OPS: C 73.73, H 4.22, S 8.95, P 8.64; Found: C 73.83, H 4.22, S
8.89, P 8.44.
aloyl alcohol (16 mL, 0.14 mmol) in DMF (1.6 mL) was stirred for 3 d at
100–1208C under an oxygen atmosphere. The mixture was then diluted
with EtOAc/hexane (1:1) and insoluble substances were filtered off
through a Celite bed. The aqueous layer was extracted with EtOAc/
hexane (1:1) and the organic combines were washed with water, dried
over MgSO4, and evaporated. The residue was subjected to silica gel
column chromatography (CH2Cl2/acetone=10:1). The purple fluorescent
fraction (Rf =0.33) was collected and evaporated to give compound 9
(29 mg, 51%) as an orange solid. M.p. 209–2118C (decomp); 1H NMR
(300 MHz, CD2Cl2): d=1.25 (t, J=7.0 Hz, 3H), 1.27 (t, J=7.1 Hz, 3H),
1.81–1.91 (m, 4H), 3.20–3.25 (m, 2H), 3.33–3.38 (m, 2H), 3.96 (s, 3H),
7.24–7.37 (m, 5H), 7.43–7.49 (m, 2H), 7.59–7.70 (m, 4H), 8.04 (dd, J=
3.3, 8.0 Hz, 1H), 8.11 ppm (d, J=8.4 Hz, 1H); 31P{1H} NMR (162 MHz,
CDCl3): d=27.7 ppm; IR (neat): n˜max =1196 cmꢀ1 (P=O); HRMS (ESI):
m/z calcd for C31H31NOP: 464.2138: found: 464.2130 [M+H+].
X-ray crystallographic analyses: Single crystals of cis-3, 5, and 6 were
grown from CH2Cl2/hexane at room temperature. X-ray crystallographic
measurements were made on a Rigaku Saturn CCD area detector with
graphite monochromated MoKa radiation (0.71070 ꢂ) at ꢀ1308C. The
data were corrected for Lorentz and polarization effects. The structures
were solved by using a direct method[19] and refined by full-matrix least-
squares techniques against F2 by using SHELXL-97.[20] The non-hydrogen
atoms were refined anisotropically, and hydrogen atoms were refined by
using the rigid model. All calculations were performed by using the Crys-
talStructure crystallographic software package[21] except for the refine-
ment. CCDC-894432 (cis-3), 894434 (5), and 894433 (6), contain the sup-
plementary 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.
Compound 7: A mixture of [Pd2ACHTNUTRGNE(NUG dba)3] (18 mg, 0.02 mmol), (2-furyl)3P
(36 mg, 0.16 mmol), and NMP (1.6 mL) was stirred for 1 h at room tem-
perature. Compound 2 (201 mg, 0.66 mmol), N-methyl-2-(tributylstanny-
l)indole (0.30 mL, 0.65 mmol), NMP (6.4 mL), and CuI (142 mg,
0.75 mmol) were added to this mixture and the resulting mixture was stir-
red for 3.5 h at room temperature. A saturated aqueous solution of
NH4Cl was then added and insoluble substances were filtered off through
a Celite bed. The organic layer was separated, and the aqueous layer was
extracted with EtOAc/hexane (1:1). The organic combines were washed
with water, dried over Na2SO4, and evaporated. The residue was subject-
ed to silica gel column chromatography (CH2Cl2/acetone=10:1). The
yellow fluorescent fraction (Rf =0.41) was collected, evaporated, and re-
crystallized from CH2Cl2/hexane to give compound 7 (147 mg, 63%) as a
yellow-green solid. M.p. 172–1738C (decomp); 1H NMR (400 MHz,
CD2Cl2): d=3.93 (s, 3H), 6.87 (s, 1H), 7.05 (dd, J=6.8, 7.6 Hz, 1H), 7.23
(dd, J=6.8, 7.6 Hz, 1H), 7.32 (d, J=8.3 Hz, 1H), 7.35–7.42 (m, 3H),
7.46–7.58 (m, 4H), 7.52 (d, J=35.6 Hz, 1H), 7.62 (dd, J=7.8, 8.3 Hz,
1H), 7.74 ppm (dd, J=8.3, 13.7 Hz, 2H); 13C{1H} NMR (100 MHz,
CD2Cl2): d=32.5, 105.9 (d, J=3.3 Hz), 109.8, 120.4, 121.5, 123.6, 125.3 (d,
J=9.9 Hz), 127.9, 129.3 (d, J=19.8 Hz), 129.3 (d, J=12.3 Hz), 129.4 (d,
J=19.8 Hz), 130.8 (d, J=112.0 Hz), 131.1 (d, J=10.7 Hz), 131.9 (d, J=
67.5 Hz), 132.7 (d, J=2.5 Hz), 133.0 (d, J=16.5 Hz), 133.7 (d, J=1.6 Hz),
136.0 (d, J=18.1 Hz), 139.8, 142.4, 142.7 ppm; 31P{1H} NMR (162 MHz,
CDCl3): d=35.5 ppm; IR (neat): n˜max =1191 cmꢀ1 (P=O); HRMS (ESI):
m/z calcd for C22H19NOP: 356.1199; found: 356.1184 [M+H+].
Computational details: The structures of compounds 3–7 and the model
compounds 8m and 9m were optimized by using DFT with the 6-31G*
basis set.[22] The functional of the DFT calculations was the Becke, three-
parameter, Lee–Yang–Parr (B3LYP) exchange-correlation functional.[23]
We confirmed that the optimized geometries were not in saddle but in
stable points. For compounds 5, 6, and 7, the potential energy surfaces of
the S0 and S1 states along the dihedral angles were performed by B3LYP
and TD-B3LYP methods with the cc-pVDZ basis set to elucidate the rel-
ative stability and rotational barriers between two stable geometries.[24]
The effect of CH2Cl2 was estimated by the polarizable continuum model
(PCM) method. The Cartesian coordinates are summarized in Table S1
in the Supporting Information. All calculations were carried out by using
the Gaussian 09 suite of programs.[25]
Fluorescence lifetime measurement and analysis: The time-resolved fluo-
rescence spectra were measured by a streak camera (Hamamatsu Pho-
tonics, C4331). The output (780 nm, ꢁ120 fs FWHM) of the optical para-
metric amplifier (Spectra Physics, TOPAS) excited by an amplified Ti:-
sapphire laser system (Spectra Physics, Spitfire XP, 800 nm, ꢁ120 fs
FWHM) was frequency-doubled to 390 nm by a BBO crystal, and used
as an excitation pulse (for the excitation of compound 8, the excitation
pulse of 360 nm was produced). The color sensitivity and the time-zero of
the streak images were corrected as previously reported.[26] Dichloro-
Compound 8: A mixture of compound 6 (60 mg, 0.17 mmol), 4-octyne
(0.12 mL, 0.81 mmol), PdACHTUNGTRNEUG(N OAc)2 (14 mg, 0.060 mmol), K2CO3 (8 mg,
0.06 mmol), tetrabutylammonium bromide (27 mg, 0.085 mmol), and piv-
aloyl alcohol (0.020 mL, 0.17 mmol) in DMF (3 mL) was stirred for 9 d at
100–1208C under an oxygen atmosphere. The mixture was then diluted
with EtOAc/hexane (1:1) and insoluble substances were filtered off
through a Celite bed. The aqueous layer was extracted with EtOAc/
hexane (1:1), and the organic combines were washed with water, dried
over MgSO4, and evaporated. The residue was subjected to silica gel
column chromatography (CH2Cl2/acetone=20:1). The purple fluorescent
fraction (Rf =0.24) was collected and evaporated to give compound 8
(11 mg, 14%) as an orange solid. 1H NMR (400 MHz, CD2Cl2): d=1.26
(t, J=7.3 Hz, 3H), 1.27 (t, J=7.3 Hz, 3H), 1.80–1.89 (m, 4H), 3.22–3.27
(m, 2H), 3.33–3.37 (m, 2H), 7.36–7.41 (m, 2H), 7.42–7.53 (m, 4H), 7.64–
7.71 (m, 3H), 7.76 (dd, J=7.3, 11.2 Hz, 1H), 7.83 (d, J=7.8 Hz, 1H),
8.09 (dd, J=3.9, 8.3 Hz, 1H), 8.26 ppm (d, J=7.8 Hz, 1H); 31P{1H} NMR
(162 Hz, CD2Cl2): d=27.20 ppm; IR (neat): n˜max =1201 cmꢀ1 (P=O);
ACHUTNGRENUNmG ethACHTUGNTRENaNUGN ne, acetonitrile, and benzene (spectroscopic grade) were used as re-
ceived. As shown in Figure 7a, the spectral shape of compound 7 was de-
pendent on the delay time after the excitation, suggesting that there are
several species that show emission. In order to extract the number of spe-
cies, which contributed to the time-resolved spectra, SVD analysis was
applied to the time-resolved spectra.[27] In the SVD analysis, the time-re-
solved spectra are decomposed into the spectral components and their
time profiles weighted by their singular values (SVs). The spectral com-
ponents that have large singular values are considered to be real compo-
nents, and other components are disregarded as noise. The spectra com-
ponents obtained by the SVD analysis for typical experimental condi-
tions are given in Figure S2 in the Supporting Information. For example,
the SV of the third component in Figure S2 in the Supporting Informa-
tion is much smaller relative to the largest one (0.02) and could be ne-
glected. All fluorescence spectra of compound 7 in different solvents
were decomposed into two components spectra. Time profiles of the de-
composed spectra are shown in Figure S2 in the Supporting Information.
HRMSACHTUNGTRENNUNG(MALDI): m/z calcd for C30H28OPS: 467.1593; found: 467.1614
[M+H+].
Compound 9: A mixture of compound 7 (44 mg, 0.12 mmol), 4-octyne
(50 mL, 0.35 mmol), Pd
ACHTUGNTREN(NUNG OAc)2 (8 mg, 0.04 mmol), K2CO3 (7 mg,
0.05 mmol), tetrabutylammonium bromide (24 mg, 0.074 mmol), and piv-
Chem. Eur. J. 2012, 18, 15972 – 15983
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
15981

Y. Matano et al.
The largest one was simulated by a single exponential decay, whereas the
smaller one was simulated by a double exponential. In order to obtain
the correspondence between the SVD analysis and the real kinetics, we
assumed the following rate equations [Eqs. (3) and (4)] for the excited
state of 7 according to Figure 8.
R. J. K. Taylor), Elsevier, Oxford, 2008, Chapter 3.15, pp. 1029–
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Matano, H. Imahori, Org. Biomol. Chem. 2009, 7, 1258; k) Y. Ren,
T. Baumgartner, Dalton Trans. 2012, 41, 7792.
*
d½CTAꢂ
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Eur. J. 2009, 15, 10000; f) H. Tsuji, K. Sato, Y. Sato, E. Nakamura,
Chem. Asian J. 2010, 5, 1294; g) S. Durben, T. Baumgartner, Angew.
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ð3Þ
*
*
¼ ꢀðkAB þ k1Þ½CTAꢂ þ kBA½CTB ꢂ
dt
*
d½CTB ꢂ
ð4Þ
*
*
¼ ꢀðkBA þ k2Þ½CTB ꢂ þ kAB½CTAꢂ
dt
Under the assumption that kAB and kBA are faster than k1 and k2, the con-
centration profiles of two excited states ([CTA*] and [CTB*]) are given as
shown in Equations (5) and (6),[28] with kfast and kslow given by Equa-
tions (7) and (8), respectively.
ꢀkfast
ꢀkfast
t
slow t
½CTA ꢂ ¼ Ae
þ Beꢀk
*
ð5Þ
ð6Þ
ð7Þ
t
slow t
½CTB ꢂ ¼ Ce
þ Deꢀk
*
˝
Escande, R. Szucs, D. Szieberth, C. Lescop, L. Nyulꢅszi, M. Hissler,
kfast ¼ kAB þ kBA
k2kAB þ k1kBA
R. Rꢄau, J. Am. Chem. Soc. 2012, 134, 6524; j) J.-H. Wan, W.-F.
Fang, Y.-B. Li, X.-Q. Xiao, L.-H. Zhang, Z. Xu, J.-J. Peng, G.-Q. Lai,
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maki, H. Imahori, Chem. Asian J. 2012, 7, 2305.
ð8Þ
kslow
¼
kAB þ kBA
The pre-exponential factors A, B, C, and D are given as shown in Equa-
tions (9)–(12) in which [CTA*]0 and [CTB*]0 are the initial populations of
the excited states of the A and B form, respectively.
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Kosaka, S. Yamaguchi, Chem. Asian J. 2009, 4, 1729.
*
*
½CTA ꢂ0kAB ꢀ ½CTB ꢂ0kBA
ð9Þ
ð10Þ
ð11Þ
ð12Þ
A ¼
B ¼
C ¼
D ¼
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2425, and references therein.
kAB þ kBA
*
*
ð½CTA ꢂ0 þ ½CTB ꢂ0ÞkBA
kAB þ kBA
*
*
ꢀ½CTA ꢂ0kAB þ ½CTB ꢂ0kBA
kAB þ kBA
*
*
ð½CTA ꢂ0 þ ½CTB ꢂ0ÞkAB
kAB þ kBA
The SVD spectra are expressed by the linear combination of [CTA*] and
[CTB*], and therefore it was concluded that the faster decay component
obtained by the SVD time profile is ascribed to kfast in Equation (1), and
the slower one to kslow in Equation (2). Because we could not determine
the ratio of kAB and kBA and the initial concentration of each excited
state, it was impossible to separate the apparent fluorescence spectra into
two different components ascribed to CTA* and CTB*.
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Acknowledgements
We thank Dr. Keiko Kuwata and Dr. Haruo Fujita (Kyoto University)
for the measurements of the high-resolution mass spectra and the 2D
NMR spectra, respectively. Y.H. and Y.M. also acknowledge Dr. Yuta
Takano and Dr. Tomokazu Umeyama (Kyoto University) for their valua-
ble comments on the photodynamics. This work was partially supported
by Grants-in-Aid (No. 22350016) from MEXT, Japan.
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15982
www.chemeurj.org
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2012, 18, 15972 – 15983

Synthesis and Structure–Property Relationships of Hybrid p Systems
FULL PAPER
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[12] cis-3: C28H20O2P2·H2O;
clinic; P21/c; a=7.0334(15), b=22.173(5), c=14.968(3) ꢂ; b=
m=
MW =468.40; 0.20ꢃ0.10ꢃ0.04 mm; mono-
98.288(3)8; V=2309.9(9) ꢂ3; Z=4; 1calcd =1.347 gcmꢀ3
;
2.17 cmꢀ1; collected reflns 18377; independent reflns 5288; parame-
ters 310; Rw =0.1137; R=0.0429 (I>2s(I)); GOF=1.053. 5:
C21H14NOPS; MW =359.36; 0.40ꢃ0.30ꢃ0.25 mm; monoclinic; P21/n;
a=10.958(2), b=8.5098(7), c=18.458(2) ꢂ; b=101.878(6)8; V=
1684.3(4) ꢂ3; Z=4, 1calcd =1.417 gcmꢀ3
;
m=2.96 cmꢀ1
;
collected
reflns 12854; independent reflns 3838; parameters 226; Rw =0.0905;
R=0.0316 (I>2s(I)); GOF=1.077. 6: C22H15OPS; W =358.37;
M
0.40ꢃ0.30ꢃ0.15 mm; monoclinic; P21/n; a=11.0892(19), b=
8.4923(14), c=18.655(3) ꢂ; b=100.518(2)8; V=1727.3(5) ꢂ3; Z=4,
1calcd =1.378 gcmꢀ3; m=2.86 cmꢀ1; collected reflns 13082; independ-
ent reflns 3923; parameters 226; Rw =0.1041; R=0.0379 (I>2s(I));
GOF=1.057.
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[16] The fluorescence maxima (lem) of compound 9 are gradually red
shifted and the Stokes shifts (Dn˜) become larger, in the order cyclo-
hexane (lem =410 nm, Dn˜ =3900 cmꢀ1)<CH2Cl2 (lem =431 nm, Dn˜ =
4810 cmꢀ1)<DMF (lem =439 nm, Dn˜ =5470 cmꢀ1).
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2010, 114, 11847.
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[18] The calculated dipole moments of the syn and anti conformers of
compound 7 in the excited state (in vacuum) are 9.50 and 8.54 D, re-
spectively, which are much larger than those at the ground state.
[19] a) SIR92 (for cis-3): A. Altomare, G. Cascarano, C. Giacovazzo, A.
Guagliardi, M. Burla, G. Polidori, M. Camalli, J. Appl. Crystallogr.
Received: August 28, 2012
Published online: November 9, 2012
Chem. Eur. J. 2012, 18, 15972 – 15983
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
15983