Water-Soluble Phospholo[3,2-b]phosphole-P,P′-Dioxide-Based Fluorescent Dyes with High Photostability

Article abstract of DOI:10.1002/asia.201800533

We recently reported that fluorescent dye PB430, which consisted of a 2-phenyl-substituted benzophosphole P-oxide skeleton that was reinforced by a methylene bridge, showed pronounced photostability and, thus, high utility for applications in super-resolution stimulated emission depletion (STED) microscopy. Herein, we replaced the methylene bridge with another P=O group to 1) investigate the role of the bridging moieties; and 2) further modulate the fluorescence properties of this skeleton. We synthesized a series of phospholo[3,2-b]phosphole-based dyes—trans-PO-PB430, cis-PO-PB430, and trans-PO-PB460—all of which showed sufficient water solubility. Moreover, trans-PO-PB430 and trans-PO-PB460 exhibited intense green and orange fluorescence, respectively, and a high photostability that was comparable to that of PB430. In contrast, cis-PO-PB430 underwent rapid photobleaching upon continuous photoirradiation, which demonstrated the importance of steric shielding of the polycyclic skeleton by the substituents on the bridging moieties. The fluorescence properties of these dyes were insensitive to concentration, pH value, and polarity changes of the environment in solution. In addition, even in the solid state, these dyes showed strong green to orange emissions. These results demonstrate the potential utility of trans-PO-PB430 and trans-PO-PB460 as highly photostable fluorescent dyes.

Full text of DOI:10.1002/asia.201800533

CHEMISTRY
AN ASIAN JOURNAL
www.chemasianj.org
Accepted Article
Title: Water-Soluble Phospholo[3,2-b]phosphole P,P'-Dioxide-Based
Fluorescent Dyes with High Photostability
Authors: Chenguang Wang, Aiko Fukazawa, Yoshiyuki Tanabe, Naoto
Inai, Daisuke Yokogawa, and Shigehiro Yamaguchi
This manuscript has been accepted after peer review and appears as an
Accepted Article online prior to editing, proofing, and formal publication
of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
published online in Early View as soon as possible and may be different
to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: Chem. Asian J. 10.1002/asia.201800533
Link to VoR: http://dx.doi.org/10.1002/asia.201800533
A Journal of
A sister journal of Angewandte Chemie
and Chemistry – A European Journal

10.1002/asia.201800533
Chemistry - An Asian Journal
FULL PAPER
Water-Soluble Phospholo[3,2-b]phosphole P,P'-Dioxide-Based
Fluorescent Dyes with High Photostability
Chenguang Wang,[a] Aiko Fukazawa,*[b] Yoshiyuki Tanabe,[b] Naoto Inai,[b] Daisuke Yokogawa,*[a,b] and
Shigehiro Yamaguchi*[a,b]
Dedication ((optional))
Abstract: We have recently reported the fluorescent dye PB430,
which consists of a 2-phenyl-substituted benzophosphole P-oxide
skeleton that is reinforced with a methylene bridge. PB430 showed
pronounced photostability and thus high utility for applications in
super-resolution stimulated emission depletion (STED) microscopy.
In the present study, we replaced the methylene bridge with another
P=O group to (i) investigate the role of the bridging moieties, and (ii)
further modulate the fluorescence properties. We synthesized a
series of phospholo[3,2-b]phosphole-based dyes, i.e., trans-PO-
PB430, cis-PO-PB430, and trans-PO-PB460, all of which show
sufficient water solubility. Moreover, trans-PO-PB430 and trans-PO-
PB460 exhibit intense green and orange fluorescence, respectively,
and a high photostability that is comparable to that of PB430. In
contrast, cis-PO-PB430 underwent rapid photobleaching upon
continuous photoirradiation, which demonstrates the importance of
steric shielding of the polycyclic skeleton with the substituents on the
bridging moieties. The fluorescence properties of these dyes were
insensitive to concentration, pH value, and polarity changes of the
environment in solution. In addition, even in the solid state, these
dyes show strong green to orange emissions. These results
demonstrate the potential utility of trans-PO-PB430 and trans-PO-
PB460 as highly photostable fluorescent dyes.
various water-soluble dyes, such as fluoresceins, rhodamines,
and cyanines, are commercially available and have been widely
used for fluorescence imaging,[2] most of these suffer from low
resistance to photobleaching, which severely limits the long-term
visualization of in vivo cellular events as well as the imaging by
advanced microscopy techniques that require irradiation from
high-power lasers,[3] e.g. stimulated emission depletion (STED)
imaging,[4] single molecule imaging,[5] and saturated excitation
imaging.[6] To overcome this longstanding obstacle, considerable
efforts have been devoted to improving the photostability of such
dyes.[7‒11] One of the most common strategies to achieve this
goal is the introduction of electron-withdrawing groups, such as
fluorine,[8] fluoroalkyl,[9] sulfonates,[10] or cyano groups[11] to the
fluorophores.
In this context, phosphole, a heavier analogue of pyrrole,
represents an attractive building block for fluorescent dyes.[12,13]
Whereas pyrrole is an aromatic ring with an electron-donating
character, phosphole is nonaromatic and exhibits an electron-
accepting character. In addition, the electron-accepting
properties of phospholes can be further increased by
transformation into the corresponding phosphine oxides,
phosphine sulfides, phosphonium salts, and metal complexes.
Among these derivatives, phosphine oxides and sulfides are
particularly attractive due to their high thermal and chemical
stability. Taking advantage of these features, phosphole-based
p-electron systems have been widely used for organic
electronics,[13,14] which include organic light-emitting diodes,[15]
organic field effect transistors,[16] and organic photovoltaics.[17]
However, their potential utility for fluorescence imaging has not
yet been fully explored.[18]
So far, we have synthesized a series of phosphole-based p-
systems and studied their utility for bioimaging by capitalizing on
the electron-accepting character of the phosphole P-oxide.[19] By
combining the electron-accepting benzophosphole P-oxide with
an electron-donating triphenylamine moiety, we developed the
environment-polarity-sensitive fluorescent probe LipiDye (Figure
1a).[20] The donor-p-acceptor-type (D-p-A) probe exhibited high
photostability, which was further increased by reinforcing the 2-
arylbenzophosphole P-oxide skeleton with a methylene bridge
(C-Bphox, Figure 1a).[21] To circumvent the fluorescence
quenching of such D-p-A type fluorophores in aqueous solution,
we have recently developed a super-photostable phosphole-
based dye PhoxBright 430 (hereafter abbreviated as PB430;
Figure 1b), which exhibits a p‒p* transition character in the
excited state.[22] To improve the solubility in water, two
hydrophilic sulfonic groups were introduced on the substituents
of the methylene bridge. Moreover, PB430 showed an intense
green fluorescence in aqueous media and contains a free
carboxylic acid group that enables the conjugation with a
biomolecule. Taking advantage of these features, in particular
Introduction
Fluorescence microscopy is one of the most powerful and
versatile tools to investigate biological systems. For such
imaging applications, an ideal fluorophore should have the
following characteristics: 1) sufficient water solubility, 2) high
brightness, 3) high photostability, 4) biocompatibility, 5) a
reactive group for the conjugation to the biomolecule(s) of
interest, and 6) a long-wavelength emission to avoid the
interference from the auto-fluorescence background.[1] Although
[a]
[b]
Dr. C. Wang, Prof. Dr. D. Yokogawa, Prof. Dr. S. Yamaguchi
Institute of Transformative Bio-Molecules (WPI-ITbM)
Nagoya University
Furo, Chikusa, Nagoya 464-8602 (Japan)
E-mail: d.yokogawa@chem.nagoya-u.ac.jp;
yamaguchi@chem.naogya-u.ac.jp
Prof. Dr. A. Fukazawa, Y. Tanabe, N. Inai, Prof. Dr. D. Yokogawa,
Prof. Dr. S. Yamaguchi
Department of Chemistry, Graduate School of Science, and
Integrated Research Consortium on Chemical Sciences (IRCCS),
Nagoya University
Furo, Chikusa, Nagoya 464-8602 (Japan)
E-mail: aiko@chem.nagoya-u.ac.jp
Supporting information for this article is given via a link at the end of
the document.
This article is protected by copyright. All rights reserved.

10.1002/asia.201800533
Chemistry - An Asian Journal
FULL PAPER
the high photostability, PB430 has been successfully employed
for STED imaging, even under photoirradiation with high-power
lasers.
with respect to the orientation of the P=O groups. This
isomerism should induce differences in the physical properties of
these isomers. Herein, we report the synthesis of a series of
phospholo[3,2-b]phosphole-based fluorescent dyes that include
trans-PO-PB430, cis-PO-PB430, and trans-PO-PB460 (Figure
1d), i.e., the P=O-bridged analogues of PB430. Their
photophysical properties and the impact of the bridging P=O
moiety on the photostability were examined. We discovered that
trans-PO-BP430 and trans-PO-PB460 exhibit high photostability,
which is comparable to PB430, albeit that the photostability of
the cis isomer is lower than that of the trans isomer. The details
of the structure-properties relationship are discussed in this
article.
Results and Discussion
Synthesis
A
series of phospholo[3,2-b]phosphole-based dyes was
synthesized via the PCl3-promoted nucleophilic cascade
cyclization of bis(o-phosphanylphenyl)acetylenes (Scheme 1).[24]
To impart the products with water solubility, sulfonate-
substituted phenyl groups were introduced at the bridging P=O
moieties. The dilithiation of (2-bromophenyl)(2-bromo-4-
chlorophenyl)acetylene (2a) with tBuLi was followed by a
treatment with (p-anisyl)(diethylamino)phosphine chloride (1),
which afforded bis(aminophosphanyl)-substituted intermediate
3a. Without isolation,
Figure 1. Chemical structures of phosphole-based compounds: a)
environment-sensitive fluorescent probe LipiDye and its structurally reinforced
derivatives C-Bphox and Si-Bphox; b) super-photostable PB430; c)
dibenzophospholo[3,2-b]phosphole P,P’-dioxide derivative POPO-stilbene; d)
phospholo[3,2-b]phosphole-based dyes trans- and cis-PO-PB430, as well as
trans-PO-PB460.
In light of the outstanding photostability of the structurally
constrained benzo- and naphthophosphole oxides, we
envisioned that a modulation of the bridging moiety should
enable the modification of the photophysical properties under
retention of the high photostability. We have recently reported
that the replacement of the methylene bridge of C-Bphox with a
silicon bridge generates Si-Bphox (Figure 1a), which exhibited a
red-shifted absorption and fluorescence as well as high levels of
photostability.[23] These results prompted us to revisit the P=O-
bridged analogues, i.e., dibenzophospholo[3,2-b]phosphole
P,P¢-dioxides (POPO-stilbene; Figure 1c) in order to gain more
promising super-photostable fluorophores.
We have previously synthesized this compound via a
intramolecular
double
cyclization
of
bis(o-
the
phosphanylphenyl)acetylene.[24]
A
comparison
of
photophysical properties of POPO-stilbene with stilbenes that
contain CR2, SiR2, and PR bridges demonstrated that the
bridging with the (P=O)R moiety results in long-wavelength
absorption and emission due to the strong electron-withdrawing
effect of the phosphine-oxide moiety.[24,25] Moreover, POPO-
stilbene exhibits the largest Stokes shift and the longest
fluorescence lifetime in this compound series. These
characteristics should increase the utility of this skeleton as
fluorescent probes. In addition, POPO-stilbene contains two
stereogenic P centers, which give rise to cis- and trans-isomers
Scheme 1. Synthesis of the phospholo[3,2-b]phosphole-based dyes, cis-PO-
PB430, trans-PO-PB430, and trans-PO-PB460.
This article is protected by copyright. All rights reserved.

10.1002/asia.201800533
Chemistry - An Asian Journal
FULL PAPER
this compound was treated with PCl3, which resulted in the
halogenation of the phosphorus moiety, followed by
a
spontaneous intramolecular cascade cyclization. Finally, the
reaction mixture was treated with H2O2. This one-pot procedure
furnished chloro-substituted dibenzophospholo[3,2-b]phosphole
P,P¢-dioxide 4a as a mixture of cis and trans isomers. Their
different polarities allowed us to separate them by silica gel
column chromatography, which furnished the cis and trans
isomers in 32% and 35% yield, respectively.
The Suzuki-Miyaura cross-coupling of the cis or trans
isomers 4a with 3-(t-butoxycarbonyl)phenylboronic acid,
followed by deprotection of the t-butyl group with CF3COOH,
produced 6a. Further deprotection of the methoxy groups with
BBr3 furnished 7a in high yield. The alkylation of 7a with 1,3-
propanesultone under basic conditions proceeded not only on
the phenolic OH group at the phosphorus atom, but also on the
carboxyl group. The thus obtained 8a was hydrolyzed under
acidic conditions to afford cis- and trans-PO-PB430. The
naphthophosphole analogue trans-PO-PB460 was also
prepared in a similar manner (Scheme 1). The structures of
these compounds were fully characterized by NMR
spectroscopy and mass spectrometry. Owing to the high
hydrophilicity of the two sulfonate moieties, these dyes are
highly soluble in water (trans-PO-PB430: 150 mg/mL, cis-PO-
PB430: 30 mg/mL, and trans-PO-PB460: 150 mg/mL), whereby
the water solubility of the trans-isomers is higher than that of the
cis isomer.
Figure 2. Absorption (solid lines) and fluorescence (dashed lines) spectra of
trans-PO-PB430 in PBS buffer (pH = 7.4) at various concentrations.
Photophysical Properties
The photophysical properties of the thus prepared dyes trans-
PO-PB430, cis-PO-PB430, and trans-PO-PB460 were
investigated in aqueous PBS buffer (pH = 7.4). Initially, we
examined the concentration dependence of the photophysical
properties. The absorption and fluorescence spectra did not
change in a range of 1 µM to 100 µM, indicating that these dyes
do not form aggregates in aqueous solution (Figure 2, Figures
S1‒S2 in the SI). In PBS buffer, trans-PO-PB430 showed
absorption and emission bands with maximum wavelengths of
labs = 422 nm (molar absorption coefficient e = 10300 M−1 cm−1)
and lem = 545 nm (Figure 3 and Table 1). A large Stokes shift of
123 nm (5350 cm− 1) was observed for trans-PO-PB430. This
Figure 3. Absorption (solid lines) and fluorescence (dashed lines) spectra of
trans-PO-PB430, cis-PO-PB430, and trans-PO-PB460 in PBS buffer (pH =
7.4).
1
cm− ) are almost identical to those of trans-PO-PB430 in terms
of absorption and emission spectra as well as brightness (Figure
3 and Table 1). The configuration of the phosphoryl groups does
not affect the character of the ground and excited states.[24,29]
In comparison with trans-PO-PB430, trans-PO-PB460
showed a red-shifted absorption (labs = 459 nm) in PBS buffer
on account of its extended p-skeleton. trans-PO-PB460 also
showed a large Stokes shift of 133 nm (4900 cm–1), which
resulted in an orange emission (lem = 592 nm). As its
fluorescence quantum yield was moderate (FF = 0.25) (Figure 3
and Table 1), the brightness of the fluorescence (2300 M−1 cm−1)
is smaller than that of trans-PO-PB430. A comparison between
PB430 and trans-PO-PB460 demonstrates the impact of the
second P=O bridge, i.e., the replacement of the methylene
bridge with the second P=O group induces the red-shift of the
absorption (Dl = 33 nm) and the emission (Dl = 50 nm).[24,25]
Apart from the large Stokes shift, the relatively long
fluorescence lifetime (t) is another characteristic feature of these
phospholo[3,2-b]phosphole-based dyes. The t values of the
was
a common feature of bis-phosphoryl-bridged stilbene
derivatives, and would be further discussed in calculation part.
Importantly, trans-PO-PB430 exhibited a high fluorescence
quantum yield (FF = 0.74) despite the large Stokes shift. The
brightness of the fluorescence, which is defined as e × FF,[26]
amounted to 7600 M−1cm−1, and is thus comparable to those of
representative dyes with similar absorption and emission
wavelengths, such as Alexa Fluor 430 (8800 M−1 cm−1) (Table 1).
In addition, the absorption and emission spectra of trans-PO-
PB430 are almost comparable to those of PB430 (labs = 426 nm;
l
em = 542 nm; e × FF = 6900 M−1 cm−1), which demonstrates that
the introduction of the second PO bridge in trans-PO-PB430 can
furnish properties similar to those of PB430 without extending
the p-conjugated skeleton from benzo to naphto-fused structures.
It should also be noted that the photophysical properties of
1
cis-PO-PB430 (labs = 421 nm; lem = 548 nm; e × FF = 7100 M−
green-emissive
dyes
trans-
and
cis-PO-PB430
are
approximately 10 ns (Table 1), and thus significantly longer than
This article is protected by copyright. All rights reserved.

10.1002/asia.201800533
Chemistry - An Asian Journal
FULL PAPER
Table 1. Photophysical properties of the phosphole-based dyes trans-PO-PB430, cis-PO-PB430, and trans-PO-PB460, as well as of selected representative
dyes with similar absorption and emission wavelengths in PBS buffer (pH = 7.4)
labs
e
lem
e × FF
t
[ns]
kr
knr
[b]
Dye
FF
[nm][a]
[nm]
[108 s–1
]
[108 s–1
]
[M–1 cm–1
]
[M–1 cm–1
]
trans-PO-PB430
7600
7100
422
421
10300
10000
545
548
0.74
0.71
10.3
9.9
0.72
0.72
0.25
0.29
cis-PO-PB430
trans-PO-PB460
PB430[c]
459
426
431
428
9000
10400
16000
11300
592
542
541
540
0.25
0.66
0.55
0.21
2300
6900
8800
2400
3500
4.9
10.6
3.3
0.51
0.62
1.7
1.5
0.32
1.4
Alexa Fluor 430[d]
Lucifer Yellow CH[e]
5.3
0.40
1.5
Star 470SXP[f]
472
29000
624
0.12
0.8
1.8
11
[a] Longest-wavelength absorption maximum. [b] Absolute quantum yield determined by a calibrated integrating sphere system (error: ± 3%). [c] Reference 22.
[d] Reference 27. [e] Reference 28. [f] Reference 27.
those of Alexa Fluor 430 and Lucifer Yellow CH, both of which
exhibit similar absorption and emission wavelengths. The rate
constants for the radiative (kr) and nonradiative (knr) decay from
the excited singlet state were determined based on the FF and t
values (Table 1). The green-emissive dyes trans-PO-PB430, cis-
PO-PB430, and PB430 exhibit comparable kr (0.62–0.72 × 108
s–1) and knr (0.25–0.32 × 108 s–1) values. While the orange-
emissive dye trans-PO-PB460 also has a comparable kr value
(0.51 × 108 s–1), its knr value is substantially increased (1.5 × 108
s–1), which would explain the deceased FF value.
Next, we examined the dependence of the fluorescence
properties on the pH value and the polarity of the environment.
The fluorescence spectra of trans-PO-PB430, cis-PO-PB430,
Figure 5. Fluorescence spectra of trans-6a, i.e., a model compound for trans-
PO-PB430, in which the substituents on the phosphorus bridges are changed
to 4-methoxyphenyl groups, in various solvents.
and trans-PO-PB460 were insensitive to variations of the pH
value within the physiologically relevant window (pH = 5‒11)
(Figure 4, Figures S3‒S4 in the SI). As these compounds are
insoluble in organic solvents, the dependence on the polarity of
the environment was studied using their carboxylic acid
precursors cis-/trans-6a and trans-6b as model compounds.
These compounds did not exhibit any significant differences of
the fluorescence properties, thus demonstrating that the
sensitivity toward the polarity of the environment is low (Figure 5,
Figures S5‒S6 in the SI).
Figure 6. Fluorescence spectra of trans-PO-PB430 in PBS solution (pH = 7.4)
and in the solid state (amorphous powder).
The phospholo[3,2-b]phosphole-based dyes displayed
intense green to orange fluorescence, even in the solid state as
amorphous powders (Figure 6, Figures S7‒S8 in SI). The lem
and FF values were 538 nm and 0.42 for trans-PO-PB430, 556
nm and 0.62 for cis-PO-PB430, as well as 584 nm and 0.22 for
trans-PO-PB460. The emission spectra of the powder samples
of these dyes were similar to those in PBS solution. The intense
varying pH value.
Figure 4. Fluorescence spectra of trans-PO-PB430 in buffer solutions of
This article is protected by copyright. All rights reserved.

10.1002/asia.201800533
Chemistry - An Asian Journal
FULL PAPER
emissions in the solid state should be relevant to the large
Stokes shifts of the phospholo[3,2-b]phosphole-based dyes,
which diminish the self-quenching of the fluorescence.[30] This
feature should be advantageous in order to use these dyes for
various applications in the dispersed and the condensed states,
such as nanoparticles.
Quantum Chemical Calculations
To gain further insights into the origin of the characteristic
photophysical properties of these PO-bridged stilbenes,
concerning e.g. the large Stokes shift and the long fluorescence
lifetime, we conducted (TD) DFT calculations on cis- and trans-
6a as model compounds for cis- and trans-PO-PB430,
respectively. The geometry optimizations were conducted both
in the ground state (S0) and in the first excited singlet state (S1),
and the character of the electronic transitions between these
states were analyzed using the (TD-)B3LYP (VWN 5) density
functional in combination with the aug-cc-pVDZ (O), aug-cc-
pV(D+d)Z (P), and cc-pVDZ (C, H) basis sets.
TD DFT calculations were carried out on the vertical
excitations, employing the optimized geometries in S0 for cis-6a
and trans-6a. These calculations demonstrated that the S0–S1
transition should be attributed to the electronic transition from
the HOMO to the LUMO, which correspond to p and p* orbitals
that are delocalized over the p-skeleton, respectively (Figure 7a).
The LUMOs of these compounds exhibit
a
significant
contribution from the s* orbital of the exocyclic C–P–O moiety,
which has also been observed in other phosphole-based p-
systems.[25] These p–p* transitions do not accompany
a
significant change in dipole moment between S0 and S1. This is
consistent with the solvent-independent fluorescence properties
of these dyes, and plays a crucial role for retaining high FF
values even in aqueous solution.[22] The orbital distributions and
energy levels, excitation energies, as well as oscillator strengths
of trans-6a are almost identical to those of cis-6a. These results
confirm that the configuration of the phosphoryl group does not
significantly affect the character of the electronic transition.[24,29]
The geometry optimization of cis-6a and trans-6a in S1 was
also carried out. The results revealed significant structural
changes from the structures in S0. The optimized geometries in
S1 exhibit a significant contribution of a quinoidal character
(Figure 7b). Specifically, the C20–C21 bond elongates by 0.08 Å
from S0 to S1 (S0: 1.362 Å; S1: 1.442 Å), while the C3–C20 and
C15–C21 bonds contract by 0.050 and 0.045 Å (1.469 Å and
1.470 Å for cis-6a, 1.468 Å and 1.470 Å for trans-6a in S0, 1.418
Å and 1.425 Å for cis-6a, 1.419 Å and 1.425 Å for trans-6a in S1),
respectively. The degree of bond alternation in the fused
benzene rings also increases slightly. These structural changes
in S1 should be responsible for the large Stokes shift of cis- and
trans-PO-PB430.
Figure 7. a) Energy diagrams of the Kohn-Sham HOMOs and LUMOs, their
pictorial representation, and the results of the TDDFT calculations for the
vertical excitation of the S0–S1 transitions in cis-6a and trans-6a; b) optimized
geometries and selected bond lengths for cis-6a and trans-6a in S0 and S1. All
calculations were conducted using the (TD-)B3LYP (VWN 5) density functional
in combination with the aug-cc-pVDZ (O), aug-cc-pV(D+d)Z (P), and cc-pVDZ
(C, H) basis sets.
eV (trans-12) are larger than that of 9 (0.599 eV), which is
consistent with the experimental results (Table 2). To gain
deeper insight into the structural changes in S1 for 9–12, the
changes of the bond lengths in the 5-membered ring
substructure (r1–r5 in Figure 8) from S0 to S1 were carefully
inspected. The changes of the bond lengths r1, r2, and r3 are
almost comparable, indicating that the pronounced quinoidal
structure in S1 is
a common feature of the structurally
constrained stilbenes (Figure 8a and c). In contrast, the changes
of the endocyclic C–E bonds (r4 and r5) between S0 and S1
exhibit pronounced differences between carbon-bridged 9 and
10, trans-11, and trans-12, which contain third-row elements
(Figures 8b, c). In particular, r5 in 10 (0.019 Å), trans-11 (0.014
Å), and trans-12 (0.034 Å) become shorter in S1, whereas that of
9 remained unchanged. These structural changes should be
attributed to the contribution of the s* orbitals of the exocyclic E–
To assess the impact of the bridging elements on the
fluorescence properties, a series of ladder stilbenes with various
bridging moieties, i.e., CMe2 (9), SiMe2 (10), PPh (trans-11), and
P(=O)Ph (trans-12), were calculated at the same level of theory
(Table 2, Figure 8, and Figure S9 in SI). The Stokes shifts
estimated by the calculated transition energies for the absorption
and emission of 0.719 eV (10), 0.664 eV (trans-11), and 0.764
This article is protected by copyright. All rights reserved.

10.1002/asia.201800533
Chemistry - An Asian Journal
FULL PAPER
were conducted using the (TD-)B3LYP (VWN 5) density functional with the
aug-cc-pVDZ (O), aug-cc-pV(D+d)Z (Si, P), and cc-pVDZ basis sets (C, H).
C bonds (E = Si, P) to the LUMO (Figure S10 in SI), and, at least
in part, be responsible for the larger Stokes shift compared to
that of carbon-bridged 9. Notably, PO-bridged 12 exhibits the
largest contribution of the s* orbital of the exocyclic C–P–O
moiety to the Kohn-Sham LUMO, and also the largest extent of
bond shortening. This should be due to the effective match of
the energy levels between the s* orbital and the stilbene p*
orbital.
Photostability
The photostability of these phospholo[3,2-b]phosphole-based
dyes was compared to that of the previously reported PB430 in
PBS buffer (pH = 7.4). The decay of the absorbance in PBS
solution at a concentration of ~40 µM was monitored under
continuous photo-irradiation using a Xe lamp (lirr = 350–600 nm;
1500 W m–2). After 4 h of such intense photo-irradiation, trans-
PO-PB430 and trans-PO-PB460 retained 96% and 99% of their
initial intensity, respectively, while PB430 retained 98%. This
comparison demonstrates the extremely high photostability of
bis(phosphoryl)-bridged trans-PO-PB430 and trans-PO-PB460
(Figure 9, Figure S11 in SI). Under the same irradiation
conditions, cis-PO-PB430 retained only 81%. The photostability
of cis-PO-PB430 is thus significantly lower than that of trans-PO-
PB430, while no significant difference with respect to other
photophysical properties was observed between the cis and
trans isomers.
Table 2. Calculated and experimental values for the Stokes shifts for a series
of element-bridged stilbenes
Compound
9
Calc / eV[a]
Exp / eV
Exp / nm
0.599
0.472[b,c]
45[b,c]
10
0.719
0.664
0.764
0.534[b,c]
0.549[d,e]
0.556[d,e]
66[b,c]
65[d,e]
85[d,e]
trans-11
trans-12
[a] Difference between the calculated transition energies for the vertical
excitations using the TD-B3LYP (VWN 5) density functional with the aug-cc-
pVDZ (O), aug-cc-pV(D+d)Z (Si, P), and cc-pVDZ (C, H) basis sets. [b] In THF.
[c] Ref 31. [d] In CH2Cl2. [e] Ref 24.
S0
S1
9 (E = CMe2)
10 (E = SiMe2)
trans-11 (E = PPh)
trans-12 (E = P(=O)Ph)
r4
r
5
E
r1
r3
r2
E
a)
b)
1.50
1.45
1.40
1.35
1.90
1.85
Figure 9. Monitoring the variation in absorbance of PBS buffer (pH = 7.4)
solutions of PB430 (blue), trans-PO-PB430 (green), cis-PO-PB430 (orange),
and trans-PO-PB460 (deep red) under continuous irradiation from a Xe lamp
(lirr = 350–600 nm; 1500 W m–2). The absorption variation of each sample at
the absorption maximum wavelength (labs) was plotted as a function of the
irradiation time. A0 = absorbance at t = 0. The concentrations of the PBS
solutions of the dyes were ~40 µM.
1.80
1.55
1.50
r1
r2
r3
r4
r5
As previously mentioned, no significant difference was
observed between cis- and trans-PO-PB430 in terms of the
energy levels of the frontier molecular orbitals as well as the
character of the electronic transition. Their only differences
manifest in dipole moment and steric bulk. Therefore, we
suspected two potential reasons for their different photostability:
1) the cis-isomer, exhibiting a larger dipole moment, might also
exhibit a higher reactivity toward O2 or nucleophilic OH− groups
in the excited state; 2) the sterically less shielded side of the p-
skeleton in the cis-isomer might be susceptible to intermolecular
reactions in the excited state.
c)
0.10
0.05
0
elongation
shortening
–0.05
r1
r2
r3
r4
r5
Figure 8. a,b) Plots of selected bond lengths for the optimized geometries of 9
(black), 10 (blue), trans-11a (orange), and trans-12a (red) in S0 (solid lines)
and S1 (dashed lines); c) their difference between S0 to S1. All calculations
To examine these possibilities, we initially evaluated the
photostability of cis-PO-PB430 in air and in N2, which revealed
This article is protected by copyright. All rights reserved.

10.1002/asia.201800533
Chemistry - An Asian Journal
FULL PAPER
Conclusions
comparable photobleaching rates. This result excludes the
accelerated photobleaching in the presence of O2 under
irradiation (Figure S12 in SI). We also evaluated the
photostability of cis-PO-PB430 in PBS buffer (pH = 7.4) and in
We have synthesized three phospholo[3,2-b]phosphole-based
fluorescent dyes, trans-PO-PB430, cis-PO-PB430, and trans-
PO-PB460, which are highly soluble in water without forming
aggregates. trans-PO-PB430 and trans-PO-PB460 exhibit green
and orange emission, respectively, both of which were
insensitive to the concentration, pH value, and the polarity of the
environment in solution. The fluorescence brightness of trans-
PO-PB430 and trans-PO-PB460 is comparable to that of
representative fluorescence dyes with similar absorption and
emission wavelengths. More importantly, the two dyes display
exceptionally high photostability, which is comparable to that of
PB430. These features should render these dyes attractive
prospectives for the use in the long-term visualization of in vivo
cellular events and for advanced fluorescence imaging that
requires irradiation from high-power lasers. In addition, a
comparison between cis- and trans-PO-PB430 demonstrated
that the cis or trans configuration of the phosphoryl groups does
not affect their absorption and emission wavelengths, but
significantly influences the photostability. The sterically less
shielded cis-PO-PB430 exhibited decreased photostability
relative to the trans isomer. Thus, the steric protection by the
substituents on the bridging moieties most likely plays a crucial
role for the suppression of photodegradation through
intermolecular reactions. All these results provide important
insights into the impact of the nature of the bridging moieties on
the fluorescence properties of such dyes, and design principles
for the development of advanced super-photostable fluorescent
dyes based on phosphole building blocks.
Na2CO3/NaHCO3 buffer (pH
= 9.0). Despite the different
concentration of OH−, the photobleaching rates in both buffers
were comparable, which demonstrates that the OH− group does
not play a significant role in the photodegradation (Figure S13 in
SI).
Finally, we assessed the concentration dependence of the
photostability in PBS buffer. While the photostability of trans-PO-
PB430 was insensitive to variations of the concentration
between 4 µM and 40 µM, the cis-isomer showed a pronounced
dependence on the concentration (Figure 10a, Figure S14 in SI).
The photobleaching rate of a cis-PO-PB430 solution significantly
was increased with increasing the concentration from 4 µM to 40
µM (Figure 10b, Figure S15 in SI). This comparison implies that
the sterically less shielded side of the p-skeleton of the cis-
isomer is prone to undergo intermolecular reactions in the
excited state. In other words, the steric shielding of the p-
skeleton by the phenyl group on the phosphorus atom plays a
crucial role for the high photostability of trans-PO-PB430.
Acknowledgements
This work was partly supported by JSPS KAKENHI grant
16K13949 (S.Y.). The authors gratefully acknowledge financial
support from the Nagase Science and Technology Foundation
and the Naito Foundation (S.Y). ITbM is supported by the World
Premier International Research Center (WPI) Initiative, Japan.
Keywords: fluorescent dye • phosphole • photostability • isomer
[1]
a) H. Kobayashi, M. Ogawa, R. Alford, P. L. Choyke, Y. Urano, Chem.
Rev. 2010, 110, 2620–2640; b) M. S. T. Gonçalves, Chem. Rev. 2009,
109, 190–212; c) J. Qu, C. Kohl, M. Pottek, K. Müllen, Angew. Chem.
2004, 116, 1554–1557; Angew. Chem. Int. Ed. 2004, 43, 1528–1531; d)
C. Kohl, T. Weil, J. Qu, K. Müllen, Chem.-Eur. J. 2004, 10, 5297–5310;
e) K. Peneva, G. Mihov, F. Nolde, S. Rocha, J.-i. Hotta, K. Braeckmans,
J. Hofkens, H. Uji-i, A. Herrmann, K. Müllen, Angew. Chem. 2008, 120,
3420–3423; Angew. Chem. Int. Ed. 2008, 47, 3372–3375; f) K. Huth, T.
Heek, K. Achazi, C. Kühne, L. H. Urner, K. Pagel, J. Dernedde, R.
Haag, Chem.-Eur. J. 2017, 23, 4849–4862; g) S.-l. Niu, C. Massif, G.
Ulrich, P.-Y. Renard, A. Romieu, R. Ziessel, Chem.-Eur. J. 2012, 18,
7229–7242; h) E. Heyer, P. Lory, J. Leprince, M. Moreau, A. Romieu, M.
Guardigli, A. Roda, R. Ziessel, Angew. Chem. 2015, 127, 3038–3042;
Angew. Chem. Int. Ed. 2015, 54, 2995–2999.
Figure 10. Dependence of the photostability on the concentration of a) cis-PO-
PB430 and b) trans-PO-PB430 in PBS buffer (pH = 7.4). Absorption variations
of PBS solutions of cis- and trans-PO-PB430 with concentrations of 4 µM
(green squares), 12 µM (orange squares), and 40 µM (deep red squares) at
the absorption maximum wavelength (labs) were plotted as a function of the
irradiation time using a continuous light source (Xe lamp; 350–600 nm; 1500
W m–2); A0 = absorbance at t = 0.
[2]
R. P. Haugland, M. T. Z. Spence, I. D. Johnson, A. Basey, The
Handbook: A Guide to Fluorescent Probes and Labeling Technologies,
10th ed. Molecular Probes: Eugene, OR, 2005.
This article is protected by copyright. All rights reserved.

10.1002/asia.201800533
Chemistry - An Asian Journal
FULL PAPER
[3]
[4]
For recent reviews, see: a) X. Wu, W. Zhu, Chem. Soc. Rev. 2015, 44,
4179–4184; b) M. Minoshima, K. Kikuchi, J. Biol. Inorg. Chem. 2017, 22,
639–652; c) K. Fujita, Microscopy (Oxf) 2016, 65, 275–281.
a) T. Müller, C. Schumann, A. Kraegeloh, ChemPhysChem 2012, 13,
1986–2000; b) C. Eggeling, K. I. Willig, F. J. Barrantes, J. Neurochem.
2013, 126, 203–212; c) M. Dyba, S. W. Hell, Appl. Opt. 2003, 42,
5123–5129; d) Y. Wu, X. Wu, R. Lu, J. Zhang, L. Toro, E. Stefani, Sci.
Rep. 2015, 5, 14766.
[15] a) C. Fave, T.-Y. Cho, M. Hissler, C.-W. Chen, T.-Y. Luh, C.-C. Wu, R.
Réau, J. Am. Chem. Soc. 2003, 125, 9254–9255; b) H.-C. Su, O.
Fadhel, C.-J. Yang, T.-Y. Cho, C. Fave, M. Hissler, C.-C. Wu, R. Réau,
J. Am. Chem. Soc. 2006, 128, 983–995; c) H. Tsuji, K. Sato, Y. Sato, E.
Nakamura, J. Mater. Chem. 2009, 19, 3364–3366; d) O. Fadhel, M.
Gras, N. Lemaitre, V. Deborde, M. Hissler, B. Geffroy, R. Réau, Adv.
Mater. 2009, 21, 1261–1265; e) D. Joly, D. Tondelier, V. Deborde, B.
Geffroy, M. Hissler, R. Réau, New J. Chem. 2010, 34, 1603–1611; f) D.
Joly, D. Tondelier, V. Deborde, W. Delaunay, A. Thomas, K.
Bhanuprakash, B. Geffroy, M. Hissler, R. Réau, Adv. Funct. Mater.
2012, 22, 567–576; g) H. Chen, W. Delaunay, J. Li, Z. Wang, P.-A.
Bouit, D. Tondelier, B. Geffroy, F. Mathey, Z. Duan, R. Réau, M. Hissler,
Org. Lett. 2013, 15, 330–333; h) P. Gong, K. Ye, J. Sun, P. Chen, P.
Xue, H. Yang, R. Lu, RSC Adv. 2015, 5, 94990–94996; i) F. Riobé, R.
Szűcs, P.-A. Bouit, D. Tondelier, B. Geffroy, F. Aparicio, J. Buendía, L.
Sánchez, R. Réau, L. Nyulászi, M. Hissler, Chem.-Eur. J. 2015, 21,
6547–6556; j) Y. Zhou, S. Yang, J. Li, G. He, Z. Duan, F. Mathey,
Dalton Trans. 2016, 45, 18308–18312; k) Z. Zhuang, F. Bu, W. Luo, H.
Peng, S. Chen, R. Hu, A. Qin, Z. Zhao, B. Z. Tang, J. Mater. Chem. C
2017, 5, 1836–1842.
[5]
a) T. Ha, P. Tinnefeld, Annu. Rev. Phys. Chem. 2012, 63, 595–617; b)
E. M. S. Stennett, M. A. Ciuba, M. Levitus, Chem. Soc. Rev. 2014, 43,
1057–1075; c) B. Hinkeldey, A. Schmitt, G. Jung, ChemPhysChem
2008, 9, 2019–2027; d) N. Olivier, D. Keller, P. Gönczy, S. Manley,
PLoS One 2013, 8, e69004.
[6]
[7]
K. Fujita, M. Kobayashi, S. Kawano, M. Yamanaka, S. Kawata, Phys.
Rev. Lett. 2007, 99, 228105.
a) J. B. Grimm, B. P. English, J. Chen, J. P. Slaughter, Z. Zhang, A.
Revyakin, R. Patel, J. J. Macklin, D. Normanno, R. H. Singer, T.
Lionnet, L. D. Lavis, Nat. Methods 2015, 12, 244–250; b) X. Liu, Q.
Qiao, W. Tian, W. Liu, J. Chen, M. J. Lang, Z. Xu, J. Am. Chem. Soc.
2016, 138, 6960–6963; c) X. Song, A. Johnson, J. Foley, J. Am. Chem.
Soc. 2008, 130, 17652–17653.
[16] a) M. J. Sung, Y. Kim, S. B. Lee, G. B. Lee, T. K. An, H. Cha, S. H. Kim,
C. E. Park, Y.-H. Kim, Dye. Pigm. 2016, 125, 316–322; b) J. E. Lee, T.
K. An, Y.-H. Kim, Macromol. Res. 2016, 24, 629–633.
[8]
a) W.-C. Sun, K. R. Gee, D. H. Klaubert, R. P. Haugland, J. Org. Chem.
1997, 62, 6469–6475; b) B. R. Renikuntla, H. C. Rose, J. Eldo, A. S.
Waggoner, B. A. Armitage, Org. Lett. 2004, 6, 909–912; c) C. C.
Spagnuolo, R. J. Vermeij, E. A. Jares-Erijman, J. Am. Chem. Soc. 2006,
128, 12040–12041; d) G. Y. Mitronova, V. N. Belov, M. L. Bossi, C. A.
Wurm, L. Meyer, R. Medda, G. Moneron, S. Bretschneider, C. Eggeling,
S. Jakobs, S. W. Hell, Chem.-Eur. J. 2010, 16, 4477–4488.
a) A. N. Fletcher, D. E. Bliss, Appl. Phys. 1978, 16, 289–295; b) H.
Schill, S. Nizamov, F. Bottanelli, J. Bierwagen, V. N. Belov, S. W. Hell,
Chem.-Eur. J. 2013, 19, 16556–16565.
[17] a) H. Tsuji, K. Sato, Y. Sato, E. Nakamura, Chem.-Asian J. 2010, 5,
1294–1297; b) A. Kira, Y. Shibano, S. Kang, H. Hayashi, T. Umeyama,
Y. Matano, H. Imahori, Chem. Lett. 2010, 39, 448–450; c) Y. Matano, A.
Saito, Y. Suzuki, T. Miyajima, S. Akiyama, S. Otsubo, E. Nakamoto, S.
Aramaki, H. Imahori, Chem.-Asian J. 2012, 7, 2305–2312; d) Y. Matano,
H. Ohkubo, T. Miyata, Y. Watanabe, Y. Hayashi, T. Umeyama, H.
Imahori, Eur. J. Inorg. Chem. 2014, 1620–1624; e) Y. Matano, Y.
Hayashi, H. Nakano, H. Imahori, Heteroat. Chem. 2014, 25, 533–547; f)
Y. J. Kim, M.-J. Kim, T. K. An, Y.-H. Kim, C. E. Park, Chem. Commun.
2015, 51, 11572–11575; g) K. H. Park, Y. J. Kim, G. B. Lee, T. K. An, C.
E. Park, S.-K. Kwon, Y.-H. Kim, Adv. Funct. Mater. 2015, 25, 3991–
3997; h) T. T. T. Bui, S. Park, Q. V. Hoang, C. E. Song, S. K. Lee, J.-C.
Lee, S.-J. Moon, W. S. Shin, Synth. Met. 2016, 215, 235–242.
[18] E. Öberg, H. Appelqvist, K. P. R. Nilsson, Front Chem. 2017, 5, 28.
[19] S. Yamaguchi, A. Fukazawa, M. Taki, J. Synth. Org. Chem., Jpn. 2017,
75, 1179–1187.
[9]
[10] a) N. Panchuk-Voloshina, R. P. Haugland, J. Bishop-Stewart, M. K.
Bhalgat, P. J. Millard, F. Mao, W.-Y. Leung, R. P. Haugland, J.
Histochem. Cytochem. 1999, 47, 1179–1188; b) V. P. Boyarskiy, V. N.
Belov, R. Medda, B. Hein, M. Bossi, S. W. Hell, Chem.-Eur. J. 2008, 14,
1784–1792.
[11] a) A. Toutchkine, D.-V. Nguyen, K. M. Hahn, Org. Lett. 2007, 9, 2775–
2777; b) N. I. Shank, K. J. Zanotti, F. Lanni, P. B. Berget, B. A.
Armitage, J. Am. Chem. Soc. 2009, 131, 12960–12969; c) N. I. Shank,
H. H. Pham, A. S. Waggoner, B. A. Armitage, J. Am. Chem. Soc. 2013,
135, 242–251.
[20] E. Yamaguchi, C. Wang, A. Fukazawa, M. Taki, Y. Sato, T. Sasaki, M.
Ueda, N. Sasaki, T. Higashiyama, S. Yamaguchi, Angew. Chem. 2015,
127, 4622–4626; Angew. Chem. Int. Ed. 2015, 54, 4539–4543.
[21] C. Wang, A. Fukazawa, M. Taki, Y. Sato, T. Higashiyama, S.
Yamaguchi, Angew. Chem. 2015, 127, 15428–15432; Angew. Chem.
Int. Ed. 2015, 54, 15213–15217.
[12] a) M. Hissler, P. W. Dyer, R. Réau, Coord. Chem. Rev. 2003, 244, 1–
44; b) T. Baumgartner, R. Réau, Chem. Rev. 2006, 106, 4681–4727; c)
Y. Matano, H. Imahori, Org. Biomol. Chem. 2009, 7, 1258–1271; d) Y.
Ren, T. Baumgartner, Dalton Trans. 2012, 41, 7792–7800; e) Z. Wang,
T. Baumgartner, Chem. Rec. 2015, 15, 199–217; f) C. Reus, T.
Baumgartner, Dalton Trans. 2016, 45, 1850–1855.
[22] C. Wang, M. Taki, Y. Sato, A. Fukazawa, T. Higashiyama, S.
Yamaguchi, J. Am. Chem. Soc. 2017, 139, 10374–10381.
[13] a) J. Crassous, R. Réau, Dalton Trans. 2008, 6865–6876; b) T.
Baumgartner, Acc. Chem. Res. 2014, 47, 1613–1622; c) M. Stolar, T.
Baumgartner, Chem.-Asian J. 2014, 9, 1212–1225; d) Y. Matano,
Chem. Rec. 2015, 15, 636–650; e) M. P. Duffy, W. Delaunay, P.-A.
Bouit, M. Hissler, Chem. Soc. Rev. 2016, 45, 5296–5310; f) D. Joly, P.-
A. Bouit, M. Hissler, J. Mater. Chem. C 2016, 4, 3686–3698; g) M. A.
Shameem, A. Orthaber, Chem.-Eur. J. 2016, 22, 10718–10735; h) P.
Hibner-Kulicka, J. A. Joule, J. Skalik, P. Bałczewski, RSC Adv. 2017, 7,
9194–9236.
[23] R. A. Adler, C. Wang, A. Fukazawa, S. Yamaguchi, Inorg. Chem. 2017,
56, 8718–8725.
[24] A. Fukazawa, M. Hara, T. Okamoto. E.-C. Son, C. Xu, K. Tamao, S.
Yamaguchi, Org. Lett. 2008, 10, 913–916.
[25] A. Fukazawa, Y. Ichihashi, Y. Kosaka, S. Yamaguchi, Chem.-Asian J.
2009, 4, 1729–1740.
[26] L. D. Lavis, R. T. Raines, ACS Chem. Biol. 2008, 3, 142–155.
[27] a) S. Nizamov, K. I. Willig, M. V. Sednev, V. N. Belov, S. W. Hell,
Chem.-Eur. J. 2012, 18, 16339–16348; b) H. A. Al Attar, A. P.
Monkman, Biomacromolecules 2009, 10, 1077–1083; c) M. V. Sednev,
V. N. Belov, S. W. Hell, Methods Appl. Fluoresc. 2015, 3, 042004.
[28] a) http://omlc.org/spectra/PhotochemCAD/html/065.html; b) A. S.
Kristoffersen, S. R. Erga, B. Hamre, Ø. Frette, J. Fluoresc. 2014, 24,
1015–1024.
[14] a) Y. Matano, T. Miyajima, T. Fukushima, H. Kaji, Y. Kimura, H. Imahori,
Chem.-Eur. J. 2008, 14, 8102–8115; b) A. Saito, T. Miyajima, M.
Nakashima, T. Fukushima, H. Kaji, Y. Matano, H. Imahori, Chem.-Eur.
J. 2009, 15, 10000–10004; c) Y. Matano, A. Saito, T. Fukushima, Y.
Tokudome, F. Suzuki, D. Sakamaki, H. Kaji, A. Ito, K. Tanaka, H.
Imahori, Angew. Chem. 2011, 123, 8166–8170; Angew. Chem. Int. Ed.
2011, 50, 8016–8020; d) Y. Matano, H. Ohkubo, Y. Honsho, A. Saito, S.
Seki, H. Imahori, Org. Lett. 2013, 15, 932–935; e) E. Y.-H. Hong, C.-T.
Poon, V. W.-W. Yam, J. Am. Chem. Soc. 2016, 138, 6368–6371.
[29] a) A. Fukazawa, T. Murai, L. Li, Y. Chen, S. Yamaguchi, C. R. Chimie
2010, 13, 1082–1090; b) A. Bruch, A. Fukazawa, E. Yamaguchi, S.
Yamaguchi, A. Studer, Angew. Chem. 2011, 123, 12300–12304;
This article is protected by copyright. All rights reserved.

10.1002/asia.201800533
Chemistry - An Asian Journal
FULL PAPER
Angew. Chem. Int. Ed. 2011, 50, 12094–12098; c) S. Furukawa, S.
Haga, J. Kobayashi, T. Kawashima, Org. Lett. 2014, 16, 3228−3231.
[30] a) M. Shimizu, T. Hiyama, Chem.-Asian J. 2010, 5, 1516‒1531; b) M.
Shimizu, Y. Takeda, M. Higashi, T. Hiyama, Angew. Chem. Int. Ed.
2009, 48, 3653–3656; Angew. Chem. 2009, 121, 3707–3710; c) M.
Shimizu, R. Kaki, Y. Takeda, T. Hiyama, N. Nagai, H. Yamagishi, H.
Furutani, Angew. Chem. Int. Ed. 2012, 51, 4095‒4099; Angew. Chem.
2012, 124, 4171‒4175; d) C.-H. Zhao, A. Wakamiya, Y. Inukai, S.
Yamaguchi, J. Am. Chem. Soc. 2006, 128, 15934–15935; e) A.
Wakamiya, K. Mori, S. Yamaguchi, Angew. Chem. Int. Ed. 2007, 46,
4273‒4276; Angew. Chem. 2007, 119, 4351‒4354.
[31] S. Yamaguchi, C. Xu, K. Tamao, J. Am. Chem. Soc. 2003, 125, 13662–
13663.
This article is protected by copyright. All rights reserved.

10.1002/asia.201800533
Chemistry - An Asian Journal
FULL PAPER
Entry for the Table of Contents (Please choose one layout)
Layout 1:
Fluorescent Dyes
Phospholo[3,2-b]phosphole-
based fluorescent dyes, which
exhibit green to orange
fluorescence with high brightness
in aqueous solution, were
Chenguang Wang, Aiko
Fukazawa,* Yoshiyuki Tanabe,
Naoto Inai, Daisuke Yokogawa,*
and Shigehiro Yamaguchi*
Page No. – Page No.
synthesized. The trans isomer
with respect to the orientation of
the P=O groups displayed
exceptionally high photostability,
while the cis-isomer showed a
substantially decreased
Water-Soluble Phospholo[3,2-
b]phosphole P,P’-Dioxide-Based
Fluorescent Dyes with High
Photostability
photostability, which should be
attributed to diminished steric
protection of one side of the cis-
isomer by the aryl groups on the
phosphorus atoms.
This article is protected by copyright. All rights reserved.