A new spirobifluorene-bridged bipolar system for a nitric oxide turn-on fluorescent probe

Article abstract of DOI:10.1021/ol200463m

Chemical equations presented. A new spirobifluorene-bridged bipolar molecule (EDADO) as a nitric oxide (NO) turn-on fluorescent probe was designed and synthesized. The fluorescence of EDADO is strongly quenched by photoinduced electron transfer (PET) from the electron-donating o-phenylenediamine-containing biphenyl branch to the orthogonally arranged electron-accepting 1,3,4-oxadiazole-containing conjugated oligoaryl system. Upon reacting with NO, EDADO is converted to EDADO-T, which exhibits strong fluorescence due to the suppression of PET.

Full text of DOI:10.1021/ol200463m

ORGANIC
LETTERS
2011
Vol. 13, No. 9
2216–2219
A New Spirobifluorene-Bridged Bipolar
System for a Nitric Oxide Turn-On
Fluorescent Probe
Li-Yen Lin, Xiauo-Yun Lin, Francis Lin, and Ken-Tsung Wong*
Department of Chemistry, National Taiwan University, Taipei 10617, Taiwan
kenwong@ntu.edu.tw
Received February 21, 2011
ABSTRACT
A new spirobifluorene-bridged bipolar molecule (EDADO) as a nitric oxide (NO) turn-on fluorescent probe was designed and synthesized. The
fluorescence of EDADO is strongly quenched by photoinduced electron transfer (PET) from the electron-donating o-phenylenediamine-containing
biphenyl branch to the orthogonally arranged electron-accepting 1,3,4-oxadiazole-containing conjugated oligoaryl system. Upon reacting with
NO, EDADO is converted to EDADO-T, which exhibits strong fluorescence due to the suppression of PET.
Nitric oxide (NO) is an important biomolecule in a
variety of physiological and biological processes, and has
a critical role as a signal transmitter in the cardiovascular
system, the central and peripheral nervous systems, and the
immune system.1 The pivotal biological role of NO has
triggered studies of molecular probes responsive to it. A
number of methodologies2 for detecting NO have been
developed, including electrochemical,3 electron paramag-
netic resonance,4 chemiluminescence,5 and fluorescence-
based6,7 techniques. Among these methods, fluorimetry
holds promise because of its high sensitivity, high selectiv-
ity, real-time detection, and simplicity. Several fluorescent
probes for NO have been reported to date. They can be
divided into two principal categories: (1) organic molecule-
based sensors, most notably, o-diaminofluoresceins and
related molecules, which can detect intracellular NO
through the modulation of photoinduced electron transfer
(PET) of the probes;6 and (2) transition metal-based
probes, which have also been used for NO sensing via
fluorophore displacement.7
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We previously developed a series of spirobifluorene-
bridged bipolar systems that exhibit ultrafast and highly
efficient PET behavior.8 With effective use of such spirobi-
fluorene-bridged bipolar systems, a number of applications,
such as electrogenerated chemiluminescence devices,9
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r
10.1021/ol200463m
Published on Web 03/29/2011
2011 American Chemical Society

Scheme 1. Synthesis of Spiro-Bridged Bipolar Molecule EDADO and Related Intermediates
ultraviolet photodetectors,10 and phosphorescent organic
light-emitting diodes,11 have been successfully demon-
strated. Herein, we report the design and synthesis of a
new spirobifluorene-bridged bipolar molecule (EDADO,
Scheme 1) as a NO-sensitive fluorescent probe. Spirobi-
fluorene appears to be a suitable fluorophore core for
highly sensitive fluorescent probes because spirobifluo-
rene-based molecules generally have high extinction coeffi-
cients and photoluminescence (PL) quantum yields.12 The
nature of each individual chromophore orthogonally se-
parated by a rigid σ-spacer in spirobifluorene-based mo-
lecules can be independently tailored. In our design of
EDADO, the modification involves introduction of o-
phenylenediamine, which reacts with high selectivity to-
ward NO, to serve as the electron donor and the NO
receptor.6 The 1,3,4-oxadiazole-containing conjugated
branch acts as the electron acceptor counterpart. With
proper positioning (spiro-configuration) of these two
branches, the nonfluorescent EDADO ascribed to the
strong PET from the o-phenylenediamine moiety to the
1,3,4-oxadiazole-containing conjugated oligoaryl moiety
can be converted into a highly fluorescent EDADO-T by
reacting with NOþ or N2O3 under aerobic conditions.
Such a change involves transformation of o-phenylenedi-
amine to benzotriazole, rendering the resulting chromo-
phore with different electronic character for suppressing
the PET. The incorporation of four triethyleneglycol
monomethyletherside chains increasesthe water solubility
of EDADO, which may enhance its compatibility with
potential biological applications.
The synthetic route for EDADO is depicted in Scheme 1.
2,7-Dibromo-20,70-dicyano-9,90-spirobifluorene8 was hy-
drolyzed under basic conditions to yield the dicarboxylic
acid derivative 1 in 88% yield.13 The acid groups of 1 were
converted to acid chlorides and subsequently reacted with
a tetrazole derivative 3 in toluene to give the 1,3,4-oxadia-
zole-containing derivative 4 in 70% yield.8,14 Compound 3
was prepared by 1,3-dipolar cycloaddition of sodium azide
and the nitrile compound 2, which was obtained by the
benzylation of 3,4-dihydroxybenzonitrile with benzyl
chloride. Removal of the benzyl groups in molecule 4 with
BBr3 in CH2Cl2 resulted in compound 5, which was then
subjected to nucleophilic substitution with triethylene
glycol monomethyl ether tosylate15 in dimethylformamide
(DMF) to afford molecule 6 in 49% yield. Suzukiꢀ
Miyaura cross-coupling of the dibromo compound 6 with
(10) Lin, H.-W.; Ku, S.-Y.; Su, H.-C.; Huang, C.-W.; Lin, Y.-T.;
Wong, K.-T.; Wu, C.-C. Adv. Mater. 2005, 17, 2489.
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Phys. Chem. Chem. Phys. 2008, 10, 5822.
(12) (a) Wong, K.-T.; Chien, Y.-Y.; Chen, R.-T.; Wang, C.-F.; Lin,
Y.-T.; Chiang, H.-H.; Hsieh, P.-Y.; Wu, C.-C.; Chou, C. H.; Su, Y. O.;
Lee, G.-H.; Peng, S.-M. J. Am. Chem. Soc. 2002, 124, 11576. (b) Chiang,
C.-L.; Tseng, S.-M.; Chen, C.-T.; Hsu, C.-P.; Shu, C.-F. Adv. Funct.
Mater. 2008, 18, 248. (c) Saragi, T. P. I.; Spehr, T.; Siebert, A.;
Fuhrmann-Lieker, T.; Salbeck, J. Chem. Rev. 2007, 107, 1011. (d) Poriel,
(13) Holy, P.; Havlik, M.; Tichy, M.; Zavada, J.; Cisarova, I. Collect.
Czech. Chem. Commun. 2006, 71, 139.
ꢀ
C.; Liang, J.-J.; Rault-Berthelot, J.; Barriere, F.; Cocherel, N.; Slawin,
(14) Pudzich, R.; Salbeck, J. Synth. Met. 2003, 138, 21.
(15) Heathcote, R.; Howell, J. A. S.; Jennings, N.; Cartlidge, D.;
Cobden, L.; Coles, S.; Hursthouse, M. Dalton Trans. 2007, 1309.
A. M. Z.; Horhant, D.; Virboul, M.; Alcaraz, G.; Audebrand, N.;
Vignau, L.; Huby, N.; Wantz, G.; Hirsch, L. Chem.;Eur. J. 2007, 13,
10055.
Org. Lett., Vol. 13, No. 9, 2011
2217

the boronic ester derivative 7, which was synthesized by
Pd-catalyzed borylation of 4-bromo-2-nitroaniline with
bis(pinacolato)diboron, produced 8 in 94% yield. Finally,
the nitro groups of 8 were reduced with tin powder to
generate EDADO in 67% yield.16 For comparison, the
benzotriazole-containing EDADO-T was synthesized by
treating EDADO with NaNO2 under acidic conditions
(50% yield).17
exhibit remarkable photoluminescence quantum yields of
0.72 and 0.83, respectively. In contrast, the bipolar probe
EDADO comprising both donor and acceptor chromo-
phores into a spiro-configuration exhibits weakly detect-
able fluorescence (ΦF < 0.003). This result suggests that
efficient intramolecular PET occurs in EDADO, leading to
a charge-separated excited state, which is typically
nonfluorescent.
In addtion, model compounds DAS, containing the o-
phenylenediamine chromophore, DAS-T, bearing the ben-
zotriazole chromophore, and EDO, containing the 1,3,4-
oxadiazole chromophore, were also synthesized (Scheme 2).
The Pd-catalyzed Suzuki coupling reaction of 4-bromo-2-
nitroaniline with 2,7-bis(4,4,5,5-tetramethyl-1,3,2-dioxa-
borolan-2-yl)-9,90-spirobifluorene produced intermediate
9 in 50% yield. The reduction of the nitro groups of 9 gave
rise to DAS, which was subsequently reacted with NaNO2
under acidic conditions to give DAS-T in 47% yield. The
bromo and benzyl groups of compound 4 could be simul-
taneously removed by Pd/C-catalyzed hydrogenation to
produce compound 10 in 89% yield, which underwent
nucleophilic substitution with triethyleneglycol mono-
methyl ether tosylate in DMF to afford EDO in 58% yield.
Figure 1. Absorption (solid lines) and emission (broken lines)
spectra of EDADO (blue), DAS (green), and EDO (red) in
MeCN solution.
Scheme 2. Synthesis of the Model Compounds DAS, DAS-T,
and EDO
Table 1. Photophysical Parameters of DAS, DAS-T, EDO,
EDADO, and EDADO-T in MeCN Solution
ε/Mꢀ1 cmꢀ1
λ
em/nma
ΦF
b
compd
λabs /nm
DAS
359
336
354
49 589
40 691
62 803
500
0.72
0.69
0.83
DAS-T
EDO
388
445
EDADO
EDADO-T 339 (339)c 75 360c
355 (354)c 74 784c
ꢀ (ꢀ)c
0.003 (0.002)c
443 (461)c 0.60 (0.12)c
a Excited at the absorption maxima. b Determined by using a cali-
brated integrating sphere system. c Measured in MeCN/H2O (1:1, v/v)
solution.
To shed light on the emission origin of EDADO-T,
which is the expected product upon treating EDADO with
nitric oxide. A comparison study on the photophyscial
behavior of compounds DAS-T, EDO, and EDADO-T in
MeCN was conducted (Figure 2). In sharp contrast to
EDADO, a strong emission band peaked at 443 nm is
observed for EDADO-T with a high ΦF of 0.60, indicating
that the PET is efficiently blocked due to the weak
electron-donating ability of the benzotriazole branch
in EDADO-T.6 Apparently, the emission spectrum of
EDADO-T superimposes to that of the 1,3,4-oxadiazole-
containing model compound EDO. Therefore, the emis-
sion of EDADO-T can be unambiguously assigned from
the 1,3,4-oxadiazole-containing branch. It is also note-
worthy that the emission spectrum of DAS-T partially
overlaps with the absorption spectrum of EDO. This result
indicates that the benzotriazole-containing chromophore
(present in DAS-T) of EDADO-T could potentially serve
Figure 1 depicts the photophysical behavior of model
compounds DAS and EDO, which are composed of
individual donor and acceptor chromophore respectively,
as compared to those of EDADO in MeCN solution. The
photophysical data are summarized in Table 1. DAS
containing o-phenylenediamine donor and EDO bearing
1,3,4-oxadiazole acceptor display fluorescence emission
maxima centered at 500 and 445 nm, respectively. Due to
the rigid nature of the spirobifluorene core, DAS and EDO
(16) Piersanti, G.; Giorgi, L.; Bartoccini, F.; Tarzia, G.; Minetti, P.;
Gallo, G.; Giorgi, F.; Castorina, M.; Ghirardi, O.; Garminati, P. Org.
Biomol. Chem. 2007, 5, 2567.
(17) Hirano, T.; Hiromoto, K.; Kagechika, H. Org. Lett. 2007, 9,
1315.
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Org. Lett., Vol. 13, No. 9, 2011

as an energy donor for fluorescence resonance energy
transfer (FRET) to the energy acceptor, the 1,3,4-oxadia-
zole-containing chromophore (present in EDO). The reac-
tion of NO with o-phenylenediamine apparently not only
suppresses PET in EDADO but also produces a new
chromophore that may additionally contribute to the final
fluorescence of EDADO-T (Figure 3).
spectrum of presynthesized EDADO-T (Figure S1, Sup-
porting Informaton (SI)). In addition, the formation of
EDADO-T by mixing EDADO and NOC-9 was further
confirmed by electrospray ionization mass spectroscopy
(ESI-MS) of the resulting mixture, in which a molecular
ion (m/z 1487.62) corresponding to [EDADO-TþH]þ was
detected (Figure S2, in SI) .
Figure 2. Absorption (solid lines) and emission (broken lines)
spectra of EDADO-T (black), DAS-T (purple), and EDO (red)
in MeCN solution.
Figure 4. Fluorescence spectra (excitation at 339 nm) of EDA-
DO (6 ꢁ 10ꢀ5 M) in the presence of NOC-9 in MeCN/H2O (1:1,
v/v) at pH 7.0.
In summary, we have synthesized a spiro-configured
bipolar molecule (EDADO) as a new turn-on fluorescent
probe for NO. The success of the designed molecule was
primarily due to the judicious selection of the o-phenyle-
nediamine moiety as the electron-donating group aswell as
the NO receptor, which is orthogonally bridged to the
highly fluorescent and electron-accepting 1,3,4-oxadi-
azole-containing oligoaryl via a saturated σ-spacer. Upon
treatment of EDADO with NO (generated either from
NaNO2 in acidic conditions or from NOC-9), the non-
fluorescent EDADO, due to efficient PET, was converted
to the highly fluorescent EDADO-T. Comparisons of the
photophysical characteristics of DAS-T and EDO with
those of EDADO-T indicate that the PL chromophore
of EDADO-T originated from the 1,3,4-oxadiazole-con-
taining biphenyl branch. The results also indicate the
participation of the newly formed benzotriazole-bearing
chromophore in intramolecular FRET. These results can
provide new ideas for the molecular design of potential
molecular probes for NO.
Figure 3. Proposed emission mechanism of EDADO-T.
Fluorescence titration experiments were conducted by
treating EDADO solution [6 ꢁ 10ꢀ5 M in H2O/CH3CN
(1:1, v/v) at pH 7.0] with various concentrations of the NO
precursor, NOC-9 (2.7 min half-life for releasing NO at
25 °C),18 and then measuring the fluorescence after 30 min.
These were done to evaluate the performance of EDADO
as a practical probe for NO in the aqueous solution.
EDADO showed almost no fluorescence in the absence
of NOC-9, but the EDADO solution was converted to a
highly fluorescent mixture upon its addition of NOC-9
(Figure 4).19 The emission intensity increased incremen-
tally with the loading amount of NOC-9. These results
indicate that the emission of this resulting mixture is from
EDADO-T, as verified by comparing with the emission
Acknowledgment. This work was financially supported
by the National Science Council of Taiwan.
Supporting Information Available. Synthesis, charac-
1
terization, copies of H and 13C NMR spectra of new
(18) Hrabie, J. A.; Klose, J. R.; Wink, D. A.; Keefer, L. K. J. Org.
Chem. 1993, 58, 1472.
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compounds, and ESI-MS spectrum of the fluorescence
titration experiments. This material is available free of
charge via the Internet at http://pubs.acs.org.
Org. Lett., Vol. 13, No. 9, 2011
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