A benzophosphole P-oxide with an electron-donating group at 3-position: Enhanced fluorescence in polar solvents

Article abstract of DOI:10.1246/bcsj.20150238

Fluorophores with intramolecular charge-transfer (ICT) character in the excited state exhibit significant solvatochromism of their fluorescence. Here, we report an example for such compounds, a benzophosphole P-oxide bearing an electrondonating p-(diphenylamino)phenyl group at the 3-position. While this compound shows only subtle dependence of the absorption maximum on the solvent polarity (λ_max = 383-392 nm), its emission maximum is significantly red-shifted upon increasing the solvent polarity (cyclohexane: λ_em = 457 nm; DMF: λ_em = 598 nm). Most notably, the fluorescence quantum yields gradually increase with increased Stokes shifts, ultimately reaching Φ_F = 0.28 in DMF. This trend is fundamentally different from that observed for the corresponding 2-(diphenylamino)phenyl-substituted benzophosphole congener, for which applications as a fluorescent bioimaging probe were previously demonstrated. In this study, the origins of this striking difference are examined by a combined experimental and theoretical approach. Our results suggest that the observed difference arises from a significant contribution of quinoidal resonance forms in the ICT excited state, which suppresses nonradiative decay and hence increases the quantum yield in polar solvents.

Full text of DOI:10.1246/bcsj.20150238

Selected Paper
A Benzophosphole P-Oxide with an Electron-Donating Group at
3
-Position: Enhanced Fluorescence in Polar Solvents
1
1
1
1,2
Eriko Yamaguchi, Aiko Fukazawa,* Youhei Kosaka, Daisuke Yokogawa,
Stephan Irle,*1,2 and Shigehiro Yamaguchi*1,2
1Department of Chemistry, Graduate School of Science, Nagoya University, Furo, Chikusa-ku, Nagoya, Aichi 464-8602
2Institute of Transformative Bio-molecules (WPI-ITbM), Nagoya University, Furo, Chikusa-ku, Nagoya, Aichi 464-8602
E-mail: yamaguchi@chem.nagoya-u.ac.jp
Received: July 9, 2015; Accepted: August 10, 2015; Web Released: August 18, 2015
Shigehiro Yamaguchi
Shigehiro Yamaguchi received his Dr.Eng. from Kyoto University under the supervision of Prof. Kohei Tamao
in 1997. Before receiving his Dr. degree, in 1993, he was appointed as an assistant professor at Institute for
Chemical Research, Kyoto University. In 2003, he moved to Nagoya University as an associate professor,
and in 2005, he promoted to a full professor at the same place. Meanwhile, he spent a year (2000–2001) as
a visiting scientist at Massachusetts Institute of Technology. His current research is broadly focused on the
development of new functional π-conjugated materials directed toward organic electronics and bioimaging
applications.
Abstract
solvatochromism of their fluorescence. Such fluorescent mole-
cules have great potential as environment-sensitive fluorescent
bioimaging probes, which can visualize polarity changes
Fluorophores with intramolecular charge-transfer (ICT)
character in the excited state exhibit significant solvatochrom-
ism of their fluorescence. Here, we report an example for such
compounds, a benzophosphole P-oxide bearing an electron-
donating p-(diphenylamino)phenyl group at the 3-position.
While this compound shows only subtle dependence of the
absorption maximum on the solvent polarity (­max = 383­392
nm), its emission maximum is significantly red-shifted upon
increasing the solvent polarity (cyclohexane: ­em = 457 nm;
DMF: ­em = 598 nm). Most notably, the fluorescence quantum
yields gradually increase with increased Stokes shifts, ulti-
2
associated with cellular events. This class of molecules usually
consists of electron-donating and electron-accepting moieties,
3
and several fascinating fluorescent probes, such as prodan,
4
5
6
1,8-ANS, Dapoxyl, and Nile red, have been developed.
One crucial requirement for this class of fluorophores is the
ability to maintain intense emissions even in polar solvents. As
the ICT excited state is usually associated with a large structural
relaxation, and tends to undergo an accelerated nonradiative
decay, a special mechanism or suitable skeleton is necessary in
order to simultaneously generate a large Stokes shift and an
mately reaching Φ = 0.28 in DMF. This trend is fundamen-
tally different from that observed for the corresponding 2-
F
3
,7
intense emission. In this context, we have recently reported
the outstanding behavior of a benzophosphole P-oxide skele-
ton to act as an effective electron-accepting unit for donor­
(diphenylamino)phenyl-substituted benzophosphole congener,
for which applications as a fluorescent bioimaging probe were
previously demonstrated. In this study, the origins of this strik-
ing difference are examined by a combined experimental and
theoretical approach. Our results suggest that the observed
difference arises from a significant contribution of quinoidal
resonance forms in the ICT excited state, which suppresses
nonradiative decay and hence increases the quantum yield in
polar solvents.
8
,9
acceptor-type fluorophores (Figure 1). Compound 1, bear-
ing an electron-donating p-(N,N-diphenylamino)phenyl group
(hereafter denoted simply aminophenyl group) at the 2-
position, maintains high fluorescence quantum yields even in
polar (Φ = 0.64 in DMSO) or protic (Φ = 0.58 in ethanol)
F
F
solvents, while significant Stokes shifts were maintained in
these solvents. However, this compound still obeyed the gen-
eral trend of gradually decreasing fluorescence quantum yields
as a function of increasing solvent polarity.
Intramolecular charge-transfer (ICT) transitions generate
highly polarized excited states, which are sensitive to the polar-
ity of the reaction medium. Accordingly, fluorophores that
In sharp contrast to the behavior of 1, we have now dis-
covered that an analogous benzophosphole P-oxide 2a, bearing
the aminophenyl group at the 3-position, exhibited diametrally
opposed fluorescence properties, i.e. a gradual increase in
1
are characterized by ICT in the excited state show significant
Bull. Chem. Soc. Jpn. 2015, 88, 1545–1552 | doi:10.1246/bcsj.20150238
© 2015 The Chemical Society of Japan | 1545

D
D
O
vs
O
P
P
N(1)
Ph
1
Ph
2
C(19)
C(20)
C(18)
C(17)
D =
electron-
NPh
a:
b:
NPh
c:
NPh
2
2
2
C(4)
C(8)
C(5)
C(15)
C(3)
C(16)
donating group
OMe d:
NPh
2
C(2)
C(9)
C(1)
Figure 1. Chemical structures of benzophosphole P-oxides
C(6)
C(7)
C(14)
1
and 2.
C(13)
C(12)
C(11)
P(1)
Br
C(10)
O(1)
a
P
Ph
O
Br
3
4
Figure 2. ORTEP drawing of 2a. Thermal ellipsoids are
drawn at 50% probability. Hydrogen atoms are omitted for
clarity. Selected bond lengths (¡), bond angles (deg), and
torsion angles (deg): N(1)­C(18) = 1.415(2), P(1)­C(1) =
D
D
NPh
2
1.8067(18),
P(1)­C(8) = 1.8017(19),
C(1)­C(2) =
2
a 68%
b
1.354(2), C(1)­C(9) = 1.483(2), C(2)­C(3) = 1.494(2),
C(2)­C(15) = 1.479(2), C(3)­C(8) = 1.401(3), C(9)­
OMe
2b 70%
2c 94%
2d 91%
P
Ph
NPh
2
C(10) = 1.391(3),
C(12) = 1.374(3),
C(10)­C(11) = 1.395(3),
C(15)­C(16) = 1.399(2),
C(11)­
C(16)­
O
NPh
2
2
C(17) = 1.380(3), C(17)­C(18) = 1.393(3), P(1)­C(1)­
C(2) = 110.64(13), C(1)­P(1)­C(8) = 92.38(8), C(1)­
C(2)­C(3) = 114.25(15), C(2)­C(3)­C(8) = 113.27(15),
C(3)­C(8)­P(1) = 109.42(13), C(2)­C(1)­C(9)­C(14) =
Scheme 1. Reagents and conditions: a, 1) t-BuLi, THF,
78 °C; 2) PhP(NEt2)Cl, ¹78 °C; 3) PBr3, rt, 4) H2O2 aq.;
¹
b, for 2a and 2b: ArB(OH)2 (2a: Ar = 4-(N,N-diphenyl-
amino)phenyl; 2b: Ar = 4-methoxyphenyl), [Pd2(dba)3]¢
CHCl3 (2.5 mol %), S-Phos (5 mol %), K3PO4, toluene/
H2O, 80 °C; for 2c: N,N-diphenyl-4-ethynylaniline,
4
6.7(3), C(3)­C(2)­C(15)­C(20) = 48.4(2).
on phosphorus. Further oxidation with hydrogen peroxide
afforded the 3-bromo-substituted benzophosphole P-oxide 4.
An advantage of this method in comparison with other benzo-
[
Pd(PPh3)4] (1 mol %), CuI (2 mol %), Et3N, THF, 80 °C;
for 2d: 4-[4-(N,N-diphenylamino)phenylethynyl]phenyl-
boronic acid, [Pd(PPh3)4] (10 mol %), K3PO4, toluene/
H2O, 80 °C.
1
2
phosphole syntheses is that it allows the facile introduction
of various aryl groups at the 3-position of the benzophosphole
skeleton via palladium-catalyzed cross-coupling reactions.
Indeed, Suzuki­Miyaura and Sonogashira coupling protocols
furnished target compounds 2a­2d in good yields (68­94%).
The molecular structure of 2a was obtained from X-ray
crystallographic analysis, and is shown in Figure 2; it revealed
a propeller-like arrangement of the peripheral aryl groups,
whereby the dihedral angles between the phosphole ring and
the benzene rings at the 2- and 3-positions are 46.7 and 48.4°,
respectively. Due to the fused benzene ring, the phosphole ring
is slightly distorted, exhibiting C(1)­C(2) and C(3)­C(8) bond
lengths of 1.354(2) and 1.401(3) ¡, respectively. The phenyl
groups at the 2- and 3-position did not exhibit any significant
bond alternations.
fluorescence quantum yields with increasing solvent polarity.
In the present article, we would like to disclose the details of
this phenomenon together with an examination of its origins,
based on comparative studies with analogues 2b­2d and quan-
tum chemical calculations. A key factor in suppressing the
nonradiative decay in this type of molecule is the more pro-
nounced quinoidal contribution to the character of its excited
state.
Results and Discussion
A series of benzophosphole oxides with electron-donating
aryl groups at the 3-position was synthesized following a
previously reported method (Scheme 1).1
0,11
As a key precur-
sor, 3-bromo-2-phenylbenzophosphole oxide 4 was obtained
in a one-pot synthesis from o-bromo-substituted diphenyl-
acetylene 3. Compound 3 was treated with t-BuLi followed by
addition of an aminochlorophosphine to produce the corre-
sponding aminophosphanyl-substituted intermediate. Without
isolation, treatment with PBr3 promoted an intramolecular
trans-halophosphanylation by bromination of the amino group
Benzophosphole P-oxide 2a, bearing an electron-donating
aminophenyl group at the 3-position, exhibited intriguing
photophysical properties (Figure 3 and Table 1). Its absorption
spectrum in DMF showed an absorption maximum at 384
nm with a moderate molar extinction coefficient (¾ = 7530
¹
1
¹1
M
cm ). In the same solvent, the fluorescence spectrum
showed a reddish-orange emission (­max = 598 nm), and
1546 | Bull. Chem. Soc. Jpn. 2015, 88, 1545–1552 | doi:10.1246/bcsj.20150238
© 2015 The Chemical Society of Japan

¹
1
1
despite the considerable Stokes shift (9320 cm ), the fluores-
sponding Lippert­Mataga plots for both compounds in vari-
2
2
cence quantum yield was quite substantial (Φ = 0.28). Given
ous solvents were highly linear (1: R = 0.93, 2a: R = 0.87;
Figure S6), suggesting that specific solvent effects, such as
hydrogen bonding, do not affect the photophysical properties.
Similar to other donor­acceptor-type fluorophores, 1 showed
a significant decrease in ΦF with increasing bathochromic
shifts. In contrast to that, 2a exhibited the opposite behavior
F
the low fluorescence quantum yield of 1,2,3-triphenylbenzo[b]-
phosphole P-oxide in THF (Φ = 0.01), this contrast is quite
F
1
3
remarkable.
Even though the photophysical properties of 2a are com-
parable to those of the 2-aminophenyl-substituted congener
(­abs = 409 nm, ­em = 598 nm, Φ = 0.71 in DMF), the sol-
¹
1
1
(Figure 4): in cyclohexane, a small Stokes shift of 4200 cm
F
vent dependence of the fluorescence of these two compounds
is intrinsically different. Both compounds showed significant
bathochromic shifts with increasing solvent polarity. The corre-
and a Φ of merely 0.05 were observed, whereby the value of
the latter increased with increasing solvent polarity.
To investigate the effect of the nature of the aryl group at the
F
3
-position, we compared the properties of 2a with 2b, which
c-hexane
CH
DMF
acetonitrile
2 2
Cl
benzene
contains a weaker electron-donating anisyl group at the same
position (Table 1). Notably, 2b did not exhibit any solvent
dependence; neither for the fluorescence maximum wave-
CHCl
3
3
2
2
1
1
0
5
0
5
0
0.93
0.94
0.94
1
0
0
0
0
.0
.8
.6
.4
.2
0
0
.90
0.71
0.28
5
0.18
0
.07
0
.05
0
.03
0
3
00
400
500
Wavelength / nm
Figure 3. Absorption (solid line) and fluorescence spectra
broken line) of 2a in various solvents.
600
700
800
cHex
benzene
CHCl
3
CH
2
Cl
2
DMF
Figure 4. Fluorescence quantum yield of 1 (dashed line)
and 2a (solid line) in various solvents (cHex: cyclohexane,
DMF: N,N-dimethylformamide).
(
Table 1. Photophysical properties of benzophosphole P-oxides 2a­2d in various solvents
­
/
abs
nma)
¾
­em
/nm
Stokes shift
/cm
τ
/ns
kr
/10 s
knr
/10 s
Cmpd
Solvents
ΦFb)
/M¹1 cm¹1
a)
¹1
8
¹1
8
¹1
2
a
cyclohexane
benzene
CHCl3
CH2Cl2
DMF
acetonitrile
cyclohexane
benzene
CHCl3
CH2Cl2
DMF
cyclohexane
benzene
CHCl3
CH2Cl2
DMF
cyclohexane
benzene
CHCl3
CH2Cl2
DMF
383
392
390
390
384
384
348
352
353
351
350
419
428
437
433
435
356
358
353
353
351
®c)
457
509
533
552
598
606
444
449
450
446
447
461
503
548
584
648
455
480
547
608
685
0.050
0.025
0.072
0.18
4200
5860
6880
7530
9320
9540
6200
6200
6100
6100
6200
2200
3500
4640
5970
7560
6100
7100
10000
11900
13900
0.28
0.34
0.73
2.0
1.7
33
29
13
8190
8280
8010
7530
7490
0.73
0.98
0.90
0.56
0.50
4.1
1.5
1.5
®
®
®
®
®
0.28
4.9
0.27
4.8
®
®
c)
d)
d)
d)
d)
2
2
2
b
®
®
®
®
®
d)
d)
d)
d)
d)
d)
d)
d)
d)
d)
5800
7120
7310
d)
d)
d)
0.009
0.010
0.009
0.62
0.62
0.86
0.68
0.13
0.46
0.53
0.79
0.70
0.027
®
®
®
®
12400
23800
22300
24500
24300
24500
33100
30000
31300
32100
32400
®
®
c
2.4
2.6
4.4
5.7
1.7
1.7
1.9
4.3
6.3
2.6
2.3
1.9
1.2
0.81
2.6
2.8
1.9
1.1
1.0
1.6
1.5
0.33
0.57
5.3
3.1
2.5
0.48
0.48
36
d
0.27
a) Only the longest absorption or emission maximum wavelengths are shown. b) Absolute quantum yield determined by a calibrated
integrating sphere system. c) Not determined due to very low solubility. d) Not determined due to insufficient fluorescence.
Bull. Chem. Soc. Jpn. 2015, 88, 1545–1552 | doi:10.1246/bcsj.20150238
© 2015 The Chemical Society of Japan | 1547

(
a)
radiative decay rate constant (k
nonradiative decay rate constant (knr
r
)
2
.5
1
)
1
0
.5
0
(
b)
4
3
2
1
0
0
0
0
0
(
c)
6
4
2
0
(
d)
4
0
0
0
0
0
3
2
1
Figure 5. Kohn­Sham molecular orbitals, excitation ener-
gies, and oscillator strengths calculated at the TD-CAM-
B3LYP/6-311G(d) level for the optimized structures of 1,
cHex
benzene
CHCl
3
CH
2
Cl
2
DMF
2
a, and 2b in their electronic singlet ground state (S0).
Figure 6. Rate constants for radiative (blue) and non-
radiative (green) decay of (a) 1, (b) 2a, (c) 2c, and (d) 2d
in various solvents.
Only the electronic transitions with the highest contribu-
tion are shown.
length, nor for the fluorescence quantum yield. Irrespective
of the solvent polarity, 2b showed weak fluorescence with a
two compounds exhibit a contrasting solvent dependence of the
fluorescence as described above. To determine the origin of this
discrepancy, we further studied the dynamics of 1 and 2a in
the excited state, starting by fluorescence lifetime (¸) measure-
ments in various solvents. Based on these, and the previously
established ΦF values, radiative (k ) and nonradiative (k )
maximum around 450 nm and low Φ values. This result indi-
F
cates that the strongly electron-donating amino group in 2a
plays a pivotal role in the unusual solvent dependence of the
fluorescence quantum yield. The low Φ observed for 2b may
F
r
nr
be partly due to the intramolecular rotation or vibration of
decay rate constants were calculated. Figure 6 clearly shows
1
4
the 2,3-aryl groups in solution, similar to tetraarylsiloles and
that for 1, the decrease in Φ with increasing solvent polarity
F
1
5
-
phospholes.
Density functional theory (DFT) calculations at the CAM-
B3LYP/6-311G(d) level highlighted a clear difference between
a and 2b in terms of the nature of the electronic transitions
Figure 5). The optimized structure of 2a was consistent with
that of X-ray crystal structure. The S ¼S transition in 2b is
is due to a combination of decreased kr and increased knr
values, while the Φ values of 2a are mainly governed by
F
the knr values. Upon increasing the polarity from nonpolar
cyclohexane to polar DMF, the knr values for 2a decrease by
ca. 90%.
2
(
The significant decline of knr values for 2a in polar solvents
0
1
1
6,17
mainly attributable to the π­π* transition of the 2-phenyl-
substituted benzophosphole moiety. In sharp contrast, the
S0¼S1 transition in 2a consists predominantly of the intra-
molecular charge transfer (ICT) from the HOMO localized on
the electron-donating 3-(aminophenyl) group to the LUMO
localized on the benzophosphole P-oxide skeleton. Therefore,
should be related to the structure of its excited state.
Therefore, we calculated the optimized structures of 2a and 1
in the first excited singlet S1 state at the TD-CAM-B3LYP/
6-31G(d) level of theory (Figures 7 and S8). While the phenyl
groups at the 2- and 3-positions in the S state of 2a are
0
oriented in a twisted fashion relative to the benzophosphole
plane (dihedral angles: 41.3°/57.9°), their orientation becomes
the ICT character of S is the key to induce the observed
1
solvent dependence of the fluorescence in 2a.
more coplanar in S (22.3°/41.0°). Simultaneously, the bond
1
The DFT calculations also revealed an interesting similarity
in the electronic structure between 1 and 2a (Figure 5). The
HOMO of benzophosphole oxide 1, which contains the donor
substituent at the 2-position, is localized primarily around the
aminophenyl moiety, whereas the LUMO is localized mainly
on the benzophosphole P-oxide moiety. Even though both
energy levels are comparable to those of 3-substituted 2a, these
alternation mode also changes significantly from S to S . Not
0
1
only the aminophenyl group at the 3-position, but also the
phenyl group at the 2-position adopt a more quinoidal structure
in S . From S to S , the lengths of the bonds connecting the
1
0
1
phosphole ring and the phenyl groups at the 2- and 3-position
are shortened by 0.047 and 0.043 ¡, respectively. This change
should reflect the ICT character of the S ¼S transition.
0
1
1548 | Bull. Chem. Soc. Jpn. 2015, 88, 1545–1552 | doi:10.1246/bcsj.20150238
© 2015 The Chemical Society of Japan

(
a)
70
gas phase
6
6
6.7
2.0
S
1
polar solvent
6
.2
3
(PCM, CH CN)
N
60
N
60.5
5
9.7
NPh
2
7.0
18
1
0
9
55.0
55.0
17
6
2
1
O
50
15
1
O
4
7
14 13
P
P
3
2
5
20
9
12
6
8
P
10 11
O
Ph
1
2a
S
0
10
(
(
b)
c)
1
.9
1
.2 1.7
0.0
0
NPh2
0.1
0
90
Dihedral angle ( ) / deg
180
P
O
Ph
1
'
Figure 8. Energy diagrams for S0 (bottom) and S1 (top)
of 2a in the gas phase (dashed line) and in solution
NPh2
NPh2
(acetonitrile; solid line) as a function of the dihedral angle
ª (C3­C2­C15­C20). Geometry optimizations and ener-
gies of S0 and S1 were calculated at the CAM-B3LYP/BS
(BS N: 6-31+G(d); BS C, H, O, P: 6-31G(d)) and TD-
CAM-B3LYP/BS levels, respectively. The solvent effect
was calculated using the polarizable convent model
P
P
O
O
Ph
Ph
(PCM).
2a'
2a''
dihedral angle ª (C3­C2­C15­C20) in increments of 10°.
Subsequently, the molecular geometries for the S and S states
Figure 7. Structural changes in 1 and 2a between ground
S0) and first excited singlet state (S1) geometries: (a)
0
1
(
were optimized for each dihedral angle using the CAM-B3LYP
functional and the 6-31+G(d) or 6-31G(d) basis sets for the
N and the C, H, O, and P atoms, respectively. Even though
energy barriers were found close to 90° for both the ground
and excited states, the activation energy in the excited state is
higher than that in the ground state. To investigate the solvent
effect, energy barriers were calculated by using the polarizable
convent model (PCM), employing acetonitrile as a representa-
Superimposed drawing of the structures for S0 (blue) and
S1 (red) for 1 and 2a (top panel), as well as dominant
resonance forms in S1 for (b) 1 and (c) 2a (bottom panel).
Indeed, a natural population analysis (NPA)18 of the S state at
1
the (TD)-CAM-B3LYP/6-311G(d) level suggests that the elec-
tronic charge is transferred from both phenyl groups at the 2-
and 3-position to the phosphole skeleton (Figure S9). Upon
changing from S0 to S1, Wiberg bond indices (WBI) increase
from 1.05 to 1.23 and from 1.01 to 1.16 for the bonds con-
necting the phosphole with the phenyl groups at the 2- and 3-
positions, respectively (Figure S10). Taking these features into
1
9
tive polar solvent. Compared to the gas phase, the relative
energies of S in the polar solvent are stabilized irrespective
1
of the dihedral angle. However, while the energy barrier for S
1
¹
1
in the gas phase is about 6.2 kcal mol , it is increased to 7.0
¹1
account, the structure of 2a in S may be best described by
kcal mol in acetonitrile. The intramolecular bond rotation/
vibration of the phenyl group in 3-position is therefore sup-
pressed to a greater extent in polar solvents relative to nonpolar
solvents. Accordingly, this result is consistent with the experi-
mental observation of decreased knr values in polar solvents.
Furthermore, we found that a direct attachment of the
electron-donating group to the benzophosphole skeleton is
1
resonance structures 2a¤ and 2a¤¤ (Figure 7c). This structural
change contrasts to that in 2-substituted congener 1, which only
shows a quinoidal contribution from the aminophenyl group at
the 2-position, like the canonical structure of 1¤ (Figures S8­
S10). The greater contribution of the quinoidal character in 2a
might be responsible for its smaller knr value compared to that
of 1.
crucial to induce increased Φ with increased solvent polarity.
F
We assumed that for compound 2a, the quinoidal character
This is demonstrated by comparing the fluorescence properties
of 2c and 2d, which connect the aminophenyl group with either
an ethynyl or phenylethynyl spacer. Whereas both compounds
in S is enhanced in polar solvents, which should result in a
1
suppression of the intramolecular rotation/vibration of the
phenyl groups at the 2- and 3-position, thus facilitating the
decrease of the knr value. In order to test this hypothesis, we
calculated gas phase rotational barriers of the 3-phenyl group in
showed increased Φ values upon increasing the solvent polar-
F
ity from cyclohexane to chloroform, the ΦF values significantly
decreased in more polar solvents such as DMF (Table 1). The
kr and knr values for 2c and 2d were also determined, whereby
the knr values were found to decrease from cyclohexane to
chloroform. In more polar solvents, however, the knr values
2
a for the ground and excited state (Figure 8). For that purpose,
we used separate relaxed potential energy scans for the S and
0
S1 surfaces. Relative energies were calculated by changing the
Bull. Chem. Soc. Jpn. 2015, 88, 1545–1552 | doi:10.1246/bcsj.20150238
© 2015 The Chemical Society of Japan | 1549

were found to increase (Figures 6c and 6d), which is in stark
contrast to the trend observed for 2a. This increase of the knr
values for 2c and 2d in polar solvents is in good explained by
the energy gap law for nonradiative decay associated with the
significant red shift of fluorescence.20
tography (CHCl /AcOEt 8:1, R = 0.30) to afford 0.186 g
3 f
(0.340 mmol, 68% yield) of 2a as yellow solids. Mp: 206.7­
1
207.0 °C (decomp.). H NMR (400 MHz, CD Cl ): ¤ 7.74­7.63
2
2
(m, 3H), 7.54­7.46 (m, 2H), 7.40­7.31 (m, 4H), 7.30­7.26
13
1
(m, 6H), 7.20­7.11 (m, 9H), 7.08­7.04 (m, 4H). C{ H} NMR
100 MHz, CD Cl ): ¤ 150.4 (d, J = 21.4 Hz, C), 148.9 (s,
(
2
2
CP
Conclusion
C), 147.8 (s, C), 144.2 (d, JCP = 26.4 Hz, C), 134.2 (d, JCP =
91.4 Hz, C), 133.7 (d, JCP = 5.0 Hz, C), 133.3 (d, JCP = 2.0 Hz,
We discovered an unusual solvent dependence of the fluo-
rescence properties in benzophosphole P-oxide 2a, which bears
an electron-donating aminophenyl group at the 3-position. In
contrast to 2-(aminophenyl)-substituted congener 1, 2a exhib-
CH), 133.0 (d, JCP = 105 Hz, C), 132.5 (s, C), 131.4 (d, JCP
=
9.9 Hz, CH), 130.8 (d, JCP = 99.0 Hz, C), 130.6 (s, CH), 129.8
(s, CH), 129.5 (d, JCP = 5.7 Hz, CH), 129.5 (d, JCP = 9.9 Hz,
CH), 129.3 (d, JCP = 12.4 Hz, CH), 129.1 (d, JCP = 9.1 Hz,
CH), 128.6 (s, CH), 128.2 (s, CH), 127.7 (d, JCP = 14.8 Hz, C),
ited increased Φ with increasing solvent polarity, while simul-
F
taneously showing a large Stokes shift. In general, the con-
tribution of a 3-aryl group to the π-conjugation in heterole or
benzoheterole rings is not considered to be significant. How-
ever, the present results demonstrate that the substituent at the
125.4 (s, C), 124.6 (d, J = 10.7 Hz, C), 124.0 (s, C), 123.1 (s,
CP
3
1
1
C). P{ H} NMR (162 MHz, CD2Cl2): ¤ 38.1. HRMS (APCI):
m/z calcd. for C H NOP: 545.1903 ([M] ); found. 545.1921.
+
3
8
28
3
-position influences the excited state through an intramolec-
1,2-Diphenyl-3-(4-methoxyphenyl)benzo[b]phosphole P-
Oxide (2b). This compound was prepared essentially in
ular charge-transfer transition. The observed unusual fluo-
rescence properties of 2a result most likely from a sub-
stantial contribution of a quinoidal resonance structure to the
the same manner as described for 2a using 4 (0.199 g, 0.523
mmol), 4-methoxyphenylboronic acid (83.1 mg, 0.547 mmol),
[Pd (dba) ]¢CHCl (12.4 mg, 0.0120 mmol), S-Phos (10.5 mg,
first excited singlet electronic state (S ). These results thus
1
2
3
3
provide important new guidelines for the design of unprece-
dented fluorescent molecules.
0.0256 mmol), and K PO (0.162 g, 0.763 mmol). The mixture
3 4
was purified by silica gel column chromatography (CHCl3/
AcOEt 8:1, R = 0.35), and further purified by GPC to afford
f
Experimental
0
.149 g (0.366 mmol, 70% yield) of 2b as colorless solids. Mp:
1
General. Melting points (mp) or decomposition temper-
atures were determined with a Stanford Research Systems
189.7­189.9 °C (decomp.). H NMR (400 MHz, CD Cl ): ¤
2 2
7.72 (dd, J = 12.0, 8.0 Hz, 2H), 7.63 (t, J = 8.4 Hz, 1H), 7.48
(t, J = 7.2 Hz, 2H), 7.41­7.36 (m, 3H), 7.29­7.26 (m, 3H),
7.23­7.21 (m, 2H), 7.23­7.11 (m, 3H), 6.95 (d, J = 8.0 Hz,
1
13
1
31
1
OptiMelt MPA100 instrument. H, C{ H}, and P{ H} spec-
tra were recorded with a JEOL AL-400 spectrometer (400 MHz
1
13
31
13
1
for H, 100 MHz for C, 162 MHz for P) in CD Cl . The
2H), 3.83 (s, 3H). C{ H} NMR (100 MHz, CD Cl ): ¤ 160.5
2 2
2
2
1
13
chemical shifts in H and C NMR spectra are reported in
ppm using the residual proton of the solvents (5.30 ppm) or
(s, C), 150.3 (d, JCP = 21.4 Hz, C), 144.3 (d, JCP = 26.4 Hz, C),
134.3 (d, JCP = 95.5 Hz, C), 133.7 (d, JCP = 9.9 Hz, C), 133.2
(s, CH), 133.0 (d, JCP = 104 Hz, C), 132.5 (d, JCP = 2.4 Hz,
CH), 131.4 (d, JCP = 10.7 Hz, CH), 131.0 (s, CH), 131.0 (d,
JCP = 98.0 Hz, C), 129.5 (d, JCP = 9.9 Hz, CH), 129.5 (d,
JCP = 4.9 Hz, CH), 129.3 (d, JCP = 12.3 Hz, CH), 129.1 (d,
JCP = 9.0 Hz, CH), 128.7 (s, CH), 128.1 (s, CH), 126.6 (d,
¤
the solvent signals of CD Cl (53.84 ppm) as an internal
2
2
standard, respectively. The chemical shifts in 31P NMR spectra
are reported using H PO (0.00 ppm) as an external standard.
3
4
Mass spectra were measured with a Bruker micrOTOF Focus
spectrometer with the APCI ionization method. Thin layer
chromatography (TLC) was performed on plates coated with
JCP = 15.7 Hz, C), 124.5 (d, J = 11.5 Hz, CH), 114.8 (s, CH),
CP
3
1
1
0
.25 mm thick silica gel 60F254 (Merck). Column chromatog-
55.7 (s, CH3). P{ H} NMR (162 MHz, CD2Cl2): ¤ 38.0.
HRMS (APCI): m/z calcd. for C H O P: 409.1274 ([M] );
+
raphy was performed using PSQ100B (Fuji Silysia Chemicals).
Recycling preparative gel permeation chromatography (GPC)
was performed using an LC-918 (Japan Analytical Industry)
equipped with polystyrene gel columns (JAIGEL 1H and 2H,
2
7
21
2
found. 408.1289.
1,2-Diphenyl-3-{[4-(N,N-diphenylamino)phenyl]ethynyl}-
benzo[b]phosphole P-Oxide (2c). To a solution of 4 (0.376 g,
Japan Analytical Industry) and CHCl as the eluent. All reac-
0.987 mmol), [Pd(PPh ) ] (13.1 mg, 0.0113 mmol) and CuI
3
3 4
tions were carried out under an argon atmosphere. 3-Bromo-
(3.8 mg, 0.020 mmol) in triethylamine (5 mL) was added N,N-
diphenyl-4-ethynylaniline in THF (5 mL) dropwise over 5 min.
The mixture was heated at 80 °C with stirring for 18 h, and then
water was added. The mixture was extracted with chloroform.
The combined organic layer was washed with brine, dried over
anhydrous Na SO , and filtered. After removing solvents under
1
,2-diphenylbenzo[b]phosphole P-oxide 411 and 4-[4-(N,N-
21
diphenylamino)phenylethynyl]phenylboronic acid were pre-
pared according to the literature methods.
1
,2-Diphenyl-3-[4-(N,N-diphenylamino)phenyl]benzo[b]-
phosphole P-Oxide (2a). To a solution of 4 (0.192 g,
.503 mmol) in a mixed solvent of toluene (10 mL) and water
2
4
0
reduced pressure, the red solid was subjected to silica gel
column chromatography (CHCl /AcOEt 8:1, R = 0.26). The
(
(
2.5 mL) was added 4-(N,N-diphenylamino)phenylboronic acid
0.160 g, 0.553 mmol), [Pd (dba) ]¢CHCl (13.9 mg, 0.0134
3
f
solid obtained was recrystallized from AcOEt to afford 0.527 g
(0.924 mmol, 94% yield) of 2c as yellow crystals. Mp: 117.8­
2
3
3
mmol), S-Phos (10.1 mg, 0.0246 mmol) and K PO (0.161 g,
3
4
1
0
4
.758 mmol). The mixture was heated at 80 °C with stirring for
4 h and then extracted with chloroform. The combined organic
118.2 °C (decomp.). H NMR (400 MHz, CD2Cl2): ¤ 8.13 (d,
J = 8.4 Hz, 2H), 7.85 (dd, J = 7.6, 3.2 Hz, 1H), 7.71­7.57 (m,
4H), 7.45­7.29 (m, 13H), 7.15­7.10 (m, 6H), 7.00 (d, J = 8.8
layer was washed with brine, dried over anhydrous Na SO ,
2
4
1
3
1
and filtered. After removing solvents under reduced pressure,
the yellow solid was subjected to silica gel column chroma-
Hz, 2H). C{ H} NMR (100 MHz, CD Cl ): ¤ 149.9 (s, C),
2 2
147.2 (s, C), 142.3 (d, JCP = 24.7 Hz, C), 138.0 (d, JCP = 97.9
1550 | Bull. Chem. Soc. Jpn. 2015, 88, 1545–1552 | doi:10.1246/bcsj.20150238
© 2015 The Chemical Society of Japan

Hz, C), 133.8 (d, JCP = 9.0 Hz, C), 133.7 (s, CH), 133.6 (d,
JCP = 9.1 Hz, C), 133.6 (s, C), 132.6 (d, JCP = 2.5 Hz, CH),
with the maximum 2ª angle of 50.0°, of which 4771 were
independent reflections (Rint = 0.0214). The structure was
2
2,23
1
1
31.8 (d, JCP = 105 Hz, C), 131.3 (d, JCP = 10.7 Hz, CH),
31.2 (d, JCP = 100 Hz, C), 130.0 (s, CH), 129.9 (d, JCP = 10.7
solved by direct methods (SHELXS-97 or SHELXS-2013)
2
and refined by the full-matrix least-squares on F (SHELXL-
2013). All non-hydrogen atoms were refined anisotropically
and all hydrogen atoms were placed using AFIX instruc-
tions. The crystal data are as follows: C38H28NOP; FW =
Hz, CH), 129.3 (d, JCP = 17.3 Hz, CH), 129.3 (d, JCP = 12.4
Hz, CH), 128.9 (s, CH), 128.7 (s, CH), 128.6 (d, JCP = 6.5
Hz, CH), 126.1 (s, CH), 124.8 (s, CH), 124.1 (d, JCP = 10.7
3
ꢀ
Hz, CH), 121.6 (s, CH), 114.2 (s, C), 104.4 (s, C), 84.2
545.58, crystal size 0.16 © 0.15 © 0.12 mm , triclinic, P1
(#2), a = 10.179(4) ¡, b = 10.284(4) ¡, c = 14.338(6) ¡,
¡ = 109.581(6)°, ¢ = 91.482(4)°, £ = 94.596(4)°, V =
1
(
d, JCP = 23.1 Hz, C). 31P{ H} NMR (162 MHz, CD Cl ): ¤
2
2
3
8.4. HRMS (APCI): m/z calcd. for C H NOP: 570.1981
40 29
+
3
¹3
¹1
([M + H] ); found. 570.1968.
1407.3(10) ¡ , Z = 2, D = 1.287 g cm , ® = 0.130 mm ,
c
1
,2-Diphenyl-3-{4-[4-(N,N-diphenylamino)phenylethyn-
R = 0.0399 (I > 2σ(I)), wR = 0.1042 (all data), GOF =
1
2
yl]phenyl}benzo[b]phosphole P-Oxide (2d). This compound
was prepared essentially in the same manner as described
for 2a using 4 (0.153 g, 0.400 mmol) in toluene (4 mL) and
water (1 mL), 4-[4-(N,N-diphenylamino)phenylethynyl]phenyl-
boronic acid (0.185 g, 0.476 mmol), [Pd(PPh ) ] (48.3 mg, 41.8
1.039. CCDC 1050579.
Computational Method. Geometry optimizations for elec-
tronic singlet ground S0 and first excited S1 states of 1 and 2a
were performed using the CAM-B3LYP and TD-CAM-B3LYP
theory, respectively, with the 6-31G(d) basis set, as imple-
3
4
2
4
¯
mol), and K PO (0.128 g, 0.601 mmol). The mixture was
mented in the Gaussian 09 program. All stationary points
were optimized without any symmetry constraint and stationary
points on the electronic ground states were characterized by
normal coordinate analysis at the same level of theory (the
number of imaginary frequencies, NIMAG, was 0 for minima).
TD-DFT vertical excitation calculations were also performed
using the TD-CAM-B3LYP theory with the 6-31G(d) basis set.
The solvent effect was included using the polarizable contin-
uum model (PCM). Natural population analysis (NPA) for the
ground and excited states were carried out using the Gaussian
09 program based on the optimized structures of 1 and 2 at the
(TD-)CAM-B3LYP/6-311G(d) level of theory.
3
4
purified by silica gel column chromatography (CHCl /AcOEt
3
1
0:1, R = 0.28). The obtained solid was further purified by
f
washing with small amount of CH Cl and hexane to give
2
2
0.236 g of 2d (0.365 mmol, 91% yield) as yellow solids. Mp:
1
34.5­135.4 °C (decomp.). 1H NMR (400 MHz, CD2Cl2): ¤
7.72 (ddd, J = 12.0, 8.4, 1.2 Hz, 2H), 7.65 (d, J = 8.4 Hz,
1H), 7.56 (d, J = 8.0 Hz, 2H), 7.52­7.47 (m, 2H), 7.42­7.21
(
m, 14H), 7.13­7.05 (m, 9H), 6.97 (d, J = 8.8 Hz, 2H).
1
3
1
C{ H} NMR (100 MHz, CD Cl ): ¤ 149.8 (d, J = 21.4 Hz,
2
2
CP
C), 148.8 (s, C), 147.6 (s, C), 143.8 (d, JCP = 26.4 Hz, C),
35.4 (d, JCP = 93.8 Hz, C), 134.3 (d, JCP = 14.8 Hz, C), 133.4
s, CH), 133.3 (d, JCP = 13.1 Hz, C), 133.0 (s, CH), 132.8 (d,
JCP = 110 Hz, C), 132.6 (d, JCP = 2.5 Hz, CH), 132.3 (s, CH),
1
(
This work was partly supported by JST, CREST (S.Y. and
S.I.) and a Grant-in-Aid for Scientific Research on Innovative
Areas “Organic Synthesis based on Reaction Integration”
(No. 2105) from MEXT, Japan (A.F). E.Y. thanks the JSPS for
a Research Fellowship for Young Scientists. Parts of the com-
putational calculations were carried out using resources of the
Research Center for Computational Science, Okazaki, Japan.
1
1
1
1
1
1
31.4 (d, JCP = 10.7 Hz, CH), 130.7 (d, JCP = 99.6 Hz, C),
29.9 (s, CH), 129.8 (s, CH), 129.7 (d, JCP = 13.2 Hz, CH),
29.5 (d, JCP = 5.8 Hz, CH), 129.3 (d, JCP = 11.5 Hz, CH),
29.3 (d, JCP = 9.1 Hz, CH), 128.8 (s, CH), 128.4 (s, CH),
25.9 (s, CH), 124.5 (s, C), 124.5 (d, JCP = 10.7 Hz, CH),
24.2 (s, CH), 122.3 (s, CH), 115.9 (s, C), 91.4 (s, C), 88.5
3
1
1
(s, C). P{ H} NMR (162 MHz, CD Cl ): ¤ 38.0. HRMS
2 2
+
Supporting Information
(APCI): m/z calcd. for C46H33NOP: 646.2294 ([M + H] );
found. 646.2274.
Photophysical Measurements. UV­vis absorption spectra
were measured with a Shimadzu UV-3150 spectrometer with a
Spectral data for all new compounds and results of theo-
retical calculations. These materials are available free of charge
on J-STAGE.
¹5
resolution of 0.2 nm using ca. 10 M sample solutions in spec-
tral grade solvents in a 1 cm square quartz cuvette. Emission
spectra were measured with a Hitachi F-4500 spectrometer with
a resolution of 1 nm. Absolute fluorescence quantum yields
were determined with a Hamamatsu photonics PMA-11 cali-
brated integrating sphere system. Time-resolved fluorescence
spectra were measured with a Hamamatsu Picosecond Fluores-
cence Measurement System C4780 equipped with a picosecond
light pulser PLP-10 (excitation wavelength 377 nm with a
repetition rate of 10 Hz). All the decay profiles were fitted
reasonably well using a single exponential function.
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1552 | Bull. Chem. Soc. Jpn. 2015, 88, 1545–1552 | doi:10.1246/bcsj.20150238
© 2015 The Chemical Society of Japan