Fluorescence-based nitric oxide detection by ruthenium porphyrin fluorophore complexes

Article abstract of DOI:10.1021/ic035418n

The ruthenium(II) porphyrin fluorophore complexes [Ru(TPP)(CO)(Ds-R)] (TPP = tetraphenylporphinato dianion; Ds = dansyl; R = imidazole (im), 1, or thiomorpholine (tm), 2) were synthesized and investigated for their ability to detect nitric oxide (NO) based on fluorescence. The X-ray crystal structures of 1 and 2 were determined. The Ds-im or Ds-tm ligand coordinates to an axial site of the ruthenium(II) center through a nitrogen or sulfur atom, respectively. Both exhibit quenched fluorescence when excited at 368 or 345 nm. Displacement of the metal-coordinated fluorophore by NO restores fluorescence within minutes. These observations demonstrate fluorescence-based NO detection using ruthenium porphyrin fluorophore conjugates.

Full text of DOI:10.1021/ic035418n

Inorg. Chem. 2004, 43, 6366−6370
Fluorescence-Based Nitric Oxide Detection by Ruthenium Porphyrin
Fluorophore Complexes
Mi Hee Lim and Stephen J. Lippard*
Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139
Received December 10, 2003
The ruthenium(II) porphyrin fluorophore complexes [Ru(TPP)(CO)(Ds-R)] (TPP
) tetraphenylporphinato dianion;
Ds dansyl; R imidazole (im), 1, or thiomorpholine (tm), 2) were synthesized and investigated for their ability
)
)
to detect nitric oxide (NO) based on fluorescence. The X-ray crystal structures of 1 and 2 were determined. The
Ds-im or Ds-tm ligand coordinates to an axial site of the ruthenium(II) center through a nitrogen or sulfur atom,
respectively. Both exhibit quenched fluorescence when excited at 368 or 345 nm. Displacement of the metal-
coordinated fluorophore by NO restores fluorescence within minutes. These observations demonstrate fluorescence-
based NO detection using ruthenium porphyrin fluorophore conjugates.
Introduction
Research in our laboratory focuses on the synthesis of
fluorescence-based sensors in which NO-induced dis-
placement of a fluorophore, quenched when bound to a
metal center, is accompanied by light emission upon excita-
tion at a proper wavelength. Previous applications of this
strategy revealed that [Co(RDATI)2], [Co(DATI-4)], and
[Rh2(µ-OAc)4(Ds-R)] complexes, where DATI is dansyl
aminotroponiminate and Ds-R is an imidazole or piperazine
derivatized dansyl group, display dramatic increases in
fluorescence upon exposure to NO.16,17 These complexes are
stable in the presence of O2, an important requirement for
biological applications, but additional tactics are required,
including faster response rates and water compatibility. We
have therefore been exploring other synthetic platforms to
approach these goals.
Nitric oxide, well-known as an atmospheric pollutant, also
serves as a messenger in the cardiovascular, immune, and
nervous systems.1-5 To understand these diverse biological
functions, directly sensing of NO in a manner that maps its
spatial and temporal distribution would be most valuable.
Currently, NO can be monitored6 by chemiluminescence,5
amperometry,7 EPR spectroscopy,8-10 or fluorescence.11-15
* To whom correspondence should be addressed. E-mail: lippard@
mit.edu.
(1) Furchgott, R. F. Angew. Chem., Int. Ed. 1999, 38, 1870-1880.
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2453.
Nitric oxide binds to the heme iron of soluble guanylyl
cyclase (sGC) selectively over O2.18,19 The coordination
environment of this NO-binding iron center has inspired the
fabrication of fiber optic probes that are embedded with a
fluorescent dye-labeled heme domain of sGC or cytochrome
c′ to detect NO.20,21 These probes, however, report only local
(15) Meineke, P.; Rauen, U.; de Groot, H.; Korth, H.-G.; Sustmann, R.
Chem. Eur. J. 1999, 5, 1738-1747.
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2004, 126, 4972-4978.
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6366 Inorganic Chemistry, Vol. 43, No. 20, 2004
10.1021/ic035418n CCC: $27.50
© 2004 American Chemical Society
Published on Web 09/08/2004

Fluorescence-Based NO Detection
) 7.25, 1.5 Hz), 7.57-7.51 (2H, m), 7.19 (1H, dd, J ) 7.5, 0.5
Hz), 3.55-3.53 (4H, m), 2.89 (6H, s), 2.67-2.64 (4H, m). 13C NMR
(125 MHz, CDCl3): δ 152.0, 133.6, 130.9, 130.5, 130.3, 130.2,
128.3, 123.3, 119.5, 115.5, 47.5, 45.6, 27.5. IR (KBr; cm-1): 2985
(w), 2953 (w), 2917 (w), 2862 (w), 2825 (w), 2783 (w), 1612 (w),
1590 (m), 1579 (m), 1572 (m), 1501 (w), 1478 (w), 1463 (w), 1443
(w), 1411 (m), 1401 (m), 1379 (w), 1355 (m), 1319 (m), 1288 (m),
1228 (w), 1199 (w), 1183 (w), 1171 (w), 1140 (m), 1102 (w), 1080
(m), 1043 (w), 1018 (w), 965 (m), 942 (w), 914 (s), 835 (w), 814
(w), 801 (w), 789 (m), 775 (w), 689 (s), 659 (m), 623 (m), 569 (s),
530 (w), 502 (w), 452 (m). ESI(+)MS (m/z) [M + H]+ Calcd for
C16H21N2O2S2: 337.1. Found: 337.4.
NO concentrations at their tips and are unsuitable for
intracellular work. An iron complex of a methoxycoumarin-
pendant cyclam and 2,2,5,5-tetramethylpyrrolidine-N-oxide
covalently linked to fluorescamine was designed as a
fluorescent model of sGC. Unfortunately, this sensor is
unstable in air and displays only a weak fluorescent response
to NO.22
The reactivity of nitric oxide with metalloporphyrins has
been extensively investigated.23,24 Ruthenium porphyrins
form stable nitrosyl adducts upon exposure to NO.24-29 In
the present Article, we describe the synthesis and charac-
terization of ruthenium porphyrin complexes with axially
bound fluorophores. We demonstrate that a fluorophore
coordinated to ruthenium in this manner can be released by
NO, resulting in turn-on fluorescence upon excitation. This
ruthenium porphyrin fluorophore scaffold is, to the best of
our knowledge, the first example of a fluorescent NO sensor
incorporating a metalloporphyrin.
[Ru(TPP)(CO)(Ds-im)] (1). A portion of Ds-im (55 mg, 0.18
mmol) was added to a solution of [Ru(TPP)(CO)] (45 mg, 0.060
mmol) in 2 mL of CH2Cl2, after which Et2O was slowly diffused
into the solution at 0 °C. Purple crystals of X-ray quality were
grown over 1 day and isolated in 93% yield (0.058 g, 0.056 mmol).
IR (KBr; cm-1): 3164 (w), 3144 (w), 3126 (w), 3103 (w), 3074
(w), 3045 (w), 3022 (w), 2972 (w), 2945 (w), 2971 (w), 2945 (w),
2864 (w), 2830 (w), 2788 (w), 2771 (w), 1938 (s), 1593 (m), 1568
(w), 1437 (m), 1387 (m), 1350 (m), 1304 (m), 1261 (w), 1202 (w),
1176 (m), 1163 (m), 1062 (m), 1008 (s), 934 (w), 834 (w), 796
(m), 757 (m), 751 (m), 736 (w), 717 (m), 700 (m), 677 (w), 664
(w), 636 (m), 591 (m), 559 (w), 538 (w), 525 (w), 492 (w), 461
(w). UV-vis in CH2Cl2 [λmax/nm (ꢀ, M-1 cm-1)]: 313 (2.0 × 104),
413 (2.3 × 105), 534 (2.0 × 104), 567 (4.6 × 103), 601 (1.3 ×
103). 1H NMR (300 MHz, CD2Cl2): δ 8.60 (8H, s), 8.43 (1H, d, J
) 8.5 Hz), 8.26-8.22 (4H, m), 7.90 (4H, dm, J ) 7.3 Hz), 7.78-
7.64 (14H, m), 7.52 (1H, dd, J ) 7.6, 1.1 Hz), 7.32-7.19 (3H, m),
7.13 (1H, d, J ) 7.7 Hz), 6.46 (1H, d, J ) 8.8 Hz), 2.83 (6H, s).
13C NMR (100 MHz, CDCl3): δ 180.1, 152.3, 143.3, 142.4, 134.0,
133.8, 133.1, 131.4, 131.2, 130.0, 129.9, 129.1, 128.1, 127.0, 126.3,
126.0, 123.4, 122.4, 121.2, 115.5, 115.3, 114.3, 45.1. Anal. Calcd
for C60H43N7O3RuS: C, 69.08; H, 4.15; N, 9.40. Found: C, 68.78;
H, 4.21; N, 9.09.
Experimental Section
General Considerations. All reagents for syntheses were
purchased from Aldrich and used without further purification.
Dichloromethane (CH2Cl2) and tetrahydrofuran (THF) were purified
by passage through alumina columns under an Ar atmosphere.
Diethyl ether (Et2O), hexanes, and ethyl acetate (EtOAc) were used
as received. Ds-im was synthesized as previously reported.17 Nitric
oxide (Matheson 99%) was purified as described.17,30 NO was
transferred by syringe in a glovebox. NO reactions were performed
under anaerobic conditions to avoid adventitious reactions of the
gas with O2. Fluorescence emission spectra were recorded at 25.0
( 0.2 °C on a Hitachi F-3010 spectrophotometer. NMR spectra
were measured on a Varian 300 spectrometer or an Inova 500 MHz
spectrometer at ambient temperature and referenced to internal 1H
and 13C solvent peaks. FT-IR spectra were obtained on an Avatar
360 spectrophotometer and UV-vis spectra on a Hewlett-Packard
8453 diode array spectrophotometer. ESI-MS analysis was per-
formed on an Agilent 1100 series instrument.
Dansyl-thiomorpholine (Ds-tm). To a solution of dansyl
chloride (2.90 g, 10.7 mmol) in 200 mL of THF were added
thiomorpholine (1.11 g, 10.7 mmol) and Cs2CO3 (4.18 g, 12.8
mmol). The reaction was allowed to stir overnight and filtered, and
the solvent was removed by rotary evaporation. The crude solids
were purified by column chromatography (silica, 6:1 hexanes/
EtOAc; Rf ) 0.29 by TLC), yielding a yellow product (2.82 g,
8.38 mmol, 77%): mp 144-146 °C. 1H NMR (500 MHz, CDCl3):
δ 8.56 (1H, d, J ) 5 Hz), 8.3 (1H, d, J ) 10 Hz), 8.19 (1H, dd, J
[Ru(TPP)(CO)(Ds-tm)] (2). A portion of Ds-tm (9.1 mg, 0.027
mmol) was added to a solution of [Ru(TPP)(CO)] (10 mg, 0.013
mmol) in 2 mL of CH2Cl2. The resulting solution was layered with
hexanes and cooled to 0 °C. Purple crystals of X-ray quality were
grown over 4 days and collected (0.013 g, 0.012 mmol, 91%). UV-
vis in CH2Cl2 [λmax/nm (ꢀ, M-1 cm-1)]: 312 (2.5 × 104), 412 (2.1
× 105), 531 (2.1 × 104), 569 (5.1 × 103), 602 (2.0 × 103). IR
(KBr; cm-1): 3104 (w), 3075 (w), 3052 (w), 3022 (w), 2985 (w),
2943 (w), 2937 (w), 2865 (w), 2832 (w), 2790 (w), 1951 (s), 1595
(m), 1574 (w), 1568 (w), 1527 (m), 1503 (w), 1486 (w), 1477 (w),
1453 (w), 1440 (m), 1405 (w), 1394 (w), 1373 (w), 1350 (m), 1320
(w), 1305 (m), 1282 (w), 1264 (w), 1230 (w), 1216 (w), 1201 (w),
1175 (m), 1157 (w), 1141 (m), 1094 (w), 1071 (s), 1008 (s), 962
(w), 945 (w), 909 (m), 885 (m), 846 (w), 834 (w), 793 (s), 754
(m), 737 (m), 716 (m), 700 (s), 672 (w), 664 (w), 637 (w), 619
(w), 595 (w), 577 (w), 567 (m), 540 (w), 527 (w), 499 (w), 462
(21) Barker, S. L. R.; Clark, H. A.; Swallen, S. F.; Kopelman, R.; Tsang,
A. W.; Swanson, J. A. Anal. Chem. 1999, 71, 1767-1772.
(22) Soh, N.; Imato, T.; Kawamura, K.; Maeda, M.; Katayama, Y. Chem.
Commun. 2002, 2650-2651.
(23) Hoshino, M.; Laverman, L.; Ford, P. C. Coord. Chem. ReV. 1999,
187, 75-102 and references therein.
(24) Ford, P. C.; Lorkovic´, I. M. Chem. ReV. 2002, 102, 993-1017.
(25) Miranda, K. M.; Bu, X.; Lorkovic´, I.; Ford, P. C. Inorg. Chem. 1997,
36, 4838-4848.
1
(w),452 (w), 415 (w). H NMR (500 MHz, CD2Cl2): δ 8.64 (8H,
s), 8.51 (1H, s), 8.22 (4H, br, s), 8.00 (4H, br, s), 7.76-7.64 (14H,
m), 7.36 (2H, s), 7.16 (1H, s), 2.86 (6H, s), 1.11 (4H, br, s), -2.23
(4H, br, s). Anal. Calcd for C61H48N6O3RuS2‚CH2Cl2: C, 64.02;
H, 4.33; N, 7.22. Found: C, 64.47; H, 4.29; N, 7.21.
(26) Lorkovic´, I. M.; Miranda, K. M.; Lee, B.; Bernhard, S.; Schoonover,
J. R.; Ford, P. C. J. Am. Chem. Soc. 1998, 120, 11674-11683.
(27) Lorkovic´, I. M.; Ford, P. C. Inorg. Chem. 1999, 38, 1467-1473.
(28) Lorkovic´, I. M.; Ford, P. C. Chem. Commun. 1999, 1225-1226.
(29) Kadish, K. M.; Adamian, V. A.; Van Caemelbecke, E.; Tan, Z.;
Tagliatesta, P.; Bianco, P.; Boschi, T.; Yi, G.-B.; Khan, M. A.; Richter-
Addo, G. B. Inorg. Chem. 1996, 35, 1343-1348.
X-ray Crystallographic Studies. A suitable crystal was mounted
in Paratone N oil on the tip of a glass capillary and frozen under
a -100 °C nitrogen cold stream. Data were collected on a Bruker
APEX CCD X-ray diffractometer with Mo KR radiation (λ )
0.71073 Å) controlled by the SMART software package and refined
and solved with the SAINTPLUS and SHELXTL software
(30) Lorkovic´, I. M.; Ford, P. C. Inorg. Chem. 2000, 39, 632-633.
Inorganic Chemistry, Vol. 43, No. 20, 2004 6367

Lim and Lippard
packages.31-33 The general procedures used for data collection are
reported elsewhere.34 Empirical absorption corrections were cal-
culated with the SADABS program.35 The structures of 1 and 2
were solved by standard Patterson and difference Fourier methods.
All non-hydrogen atoms were refined anisotropically, and the
structure solution was checked for higher symmetry with PLA-
TON.36 In the crystal structure of 1, a disordered Et2O molecule in
the lattice was refined over two positions, each with a 0.5 occupancy
factor. One and one-half CH2Cl2 molecules in the structure of 2
were disordered. In the first disordered CH2Cl2, one of the chlorine
atoms resides in two positions with occupancy factors of 0.6 and
0.4. In the remaining 0.5 CH2Cl2, the carbon atom was disordered
over two sites modeled with occupancy factors of 0.6 and 0.4. The
highest electron density in the final difference Fourier maps for 1
and 2 was 1.57 and 1.40 e/Å3, respectively, in the vicinity of the
ruthenium atom.
Table 1. Summary of X-ray Crystallographic Data
[Ru(TPP)(CO)-
(Ds-im)]‚0.5Et2O (1)
[Ru(TPP)(CO)(Ds-tm)]‚
1.5CH2Cl2 (2)
formula
fw
space group
a, Å
b, Å
c, Å
C62H48N7O3.5RuS
1080.20
P21/c
12.773(3)
20.264(4)
20.711(4)
C62.5H51Cl3N6O3RuS2
1205.63
P1h
11.241(2)
15.233(3)
16.615(3)
97.00(3)
97.90(3)
103.00(3)
2711.2(9)
2
1.477
-100
0.568
19865
9094
770
0.0744
R, deg
â, deg
γ, deg
V, Å3
Z
107.79(3)
5104.3(18)
4
1.406
-100
0.405
37390
9023
694
0.0514
0.1224
1.570, -0.557
F
calc, g/cm3
T, °C
µ(Mo KR), mm-1
total no. of data
no. of unique data
no. of params
Ra
Results and Discussion
wR2 b
0.1489
1.403, -0.716
max, min peaks, e/Å3
Syntheses of Fluorophore-Derived Ruthenium Por-
phyrin Complexes. Ruthenium carbonyl tetraphenylpor-
phyrin complexes [Ru(TPP)(CO)(L)], with L ) Ds-im (1)
or Ds-tm (2), were readily prepared from solutions of
[Ru(TPP)(CO)] and the dansyl-derivatized axial base, imid-
azole or thiomorpholine, in CH2Cl2. Crystals of 1 were grown
by vapor diffusion of Et2O into the resulting solution over 1
day at 0 °C and isolated in 93% yield. When a CH2Cl2
solution of [Ru(TPP)(CO)] and Ds-tm was layered with
hexanes, X-ray quality crystals of 2 were grown over 4 days
at 0 °C in 91% yield.
a R ) ∑||Fo| - |Fc||/∑|Fo|. b wR2 ) {w(Fo - Fc )2/∑[w(Fo )2]}1/2
.
2
2
2
Table 2. Selected Bond Distances (Å) and Angles (deg)a
[Ru(TPP)(CO)(Ds-im)] (1)
Ru1-N1
Ru1-N2
Ru1-N3
Ru1-N4
Ru1-N5
2.056(3)
2.049(3)
2.045(3)
2.055(3)
2.166(3)
Ru1-C45
1.834(4)
1.139(5)
179.58(15)
179.3(3)
C45-O1
C45-Ru1-N5
O1-C45-Ru1
[Ru(TPP)(CO)(Ds-tm)] (2)
Ru1-N1
Ru1-N2
Ru1-N3
Ru1-N4
Ru1-S1
2.045(5)
Ru1-C45
1.817(8)
1.140(8)
173.2(2)
177.4(7)
2.051(5)
2.040(6)
2.054(5)
2.500(2)
C45-O1
C45-Ru1-S1
O1-C45-Ru1
X-ray Crystal Structure Determinations of [Ru(TPP)-
(CO)(Ds-im)] (1) and [Ru(TPP)(CO)(Ds-tm)] (2). Crystal-
lographic data for 1 and 2 are summarized in Table 1, and
selected bond distances and angles are contained in Table
2. The crystal structures of 1 and 2 indicate that the
fluorophores are coordinated to the axial site of the ruthenium
center trans to the carbonyl group via the nitrogen atom of
imidazole and the sulfur atom of thiomorpholine, respectively
(Figure 1). In the crystal structure of 1, the Ru-CCO and
Ru-Nim distances, 1.834(4) Å and 2.166(3) Å, and the Ru-
C-O and Nim-Ru-CCO angles, 179.3(3)° and 179.58(15)°,
are consistent with those in the [Ru(TPP)(CO)(1-MeIm)]
analogue reported previously.37,38 Compound 2 is the first
crystallographically characterized ruthenium porphyrin com-
plex that contains sulfur-donor axial ligand trans to a
carbonyl group. The Ru-S bond length, 2.500(2) Å, is the
longest reported for ruthenium porphyrin complexes contain-
a Numbers in parentheses are estimated standard deviations of the last
significant figures. Atoms are labeled as indicated in Figure 1.
ing S-donor axial ligands,39,40 reflecting the strong trans
influence of the carbonyl ligand.
Fluorescence Properties. Fluorescence studies revealed
39-fold and 2.0-fold quenching of the dansyl group fluores-
cence in 1 and 2, respectively, when compared to that of
the free Ds-im or Ds-tm ligands (Figures 2 and S1). In the
solid state, 1 and 2 are not fluorescent. Upon addition of
NO to solutions of these compounds, an increase in
fluorescence was observed. Reaction of a 10 µM dichloro-
methane solution of 1 with 100 equiv of NO afforded a 19-
fold increase in the integrated fluorescence emission (Figure
3). The fluorescence response was complete in less than 20
min. Restoration of fluorescence to the value of free Ds-im
in the reaction of 1 with NO does not occur, most likely
due to an inner filter effect. Ruthenium porphyrin complexes
have strong absorbance bands at the wavelengths where
excitation and emission of the fluorophore occurs, thus
absorbing some of the light excitation and emission, resulting
in a diminished fluorescence response. A similar effect was
observed in the reaction of 2 with NO. When 100 equiv of
(31) SMART: Software for the CCD Detector System, version 5.626; Bruker
AXS: Madison, WI, 2000.
(32) SAINTPLUS: Software for the CCD Detector System, version 5.01;
Bruker AXS: Madison, WI, 1998.
(33) SHELXTL: Program Library for Structure Solution and Molecular
Graphics, version 6.2; Bruker AXS: Madison, WI, 2000.
(34) Kuzelka, J.; Mukhopadhyay, S.; Spingler, B.; Lippard, S. J. Inorg.
Chem. 2004, 43, 1751-1761.
(35) Sheldrick, G. M. SADABS: Area-Detector Absorption Correction;
University of Go¨ttingen: Go¨ttingen, Germany, 2001.
(36) Spek, A. L. PLATON, A Multipurpose Crystallographic Tool; Utrecht
University: Utrecht, The Netherlands, 2000.
(37) Slebodnick, C.; Seok, W. K.; Kim, K.; Ibers, J. A. Inorg. Chim. Acta
1996, 243, 57-65.
(38) Salzmann, R.; Ziegler, C. J.; Godbout, N.; McMahon, M. T.; Suslick,
K. S.; Oldfield, E. J. Am. Chem. Soc. 1998, 120, 11323-11334.
(39) James, B. R.; Pacheco, A.; Rettig, S. J.; Ibers, J. A. Inorg. Chem.
1988, 27, 2414-2421.
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2045-2046.
6368 Inorganic Chemistry, Vol. 43, No. 20, 2004

Fluorescence-Based NO Detection
Figure 3. Fluorescence response of 10 µM [Ru(TPP)(CO)(Ds-im)] in
CH2Cl2 upon addition of 100 equiv of NO (s). Individual spectra were
recorded at 1, 3, 5, 10, 15, and 20 min. Dashed line is at 0 min.
Scheme 1
Nature of the Reaction of [Ru(TPP)(CO)(Ds-im)] (1)
with Nitric Oxide. When [Ru(TPP)(CO)] is treated with
excess NO, the product is [Ru(TPP)(NO)(ONO)].25,29 In order
to determine whether similar chemistry might apply in the
present case, the Ru-containing product from the reaction
of 1 with NO was isolated from a CH2Cl2/pentane solution
under anaerobic conditions and characterized by IR and UV-
vis spectroscopy. The data were consistent with those
previously reported for [Ru(TPP)(NO)(ONO)].25,29 An X-ray
analysis of crystals grown by the slow evaporation of a
CH2Cl2 solution of the complex isolated from the NO
reaction revealed the presence of [Ru(TPP)(NO)(ONO)]
(57% yield), with an 8-fold disorder in the axial ligands.
The X-ray crystal structure is consistent with those previously
reported in the literature.25 In addition, the 1H NMR spectrum
of the reaction of 1 with NO indicated the presence of free
Ds-im. Taken together, these results demonstrate that nitric
oxide treatment causes both Ds-im and CO to dissociate from
the axial sites of the Ru(II) center (Scheme 1). We therefore
conclude that the fluorescence enhancement of 1 and 2
observed upon reaction with NO arises from displacement
of Ds-im or Ds-tm from their axial positions, liberating the
fluorophores from the quenching environment of the Ru(II)
center and restoring fluorescence.
Figure 1. ORTEP diagrams of [Ru(TPP)(CO)(Ds-im)] (top) and [Ru(TPP)-
(CO)(Ds-tm)] (bottom) showing 50% thermal ellipsoids.
Conclusions
Figure 2. Fluorescence emission spectra of [Ru(TPP)(CO)(Ds-im)] (s)
and Ds-im (- - -) in CH2Cl2.
New fluorophore-derived ruthenium porphyrin complexes
have been prepared, which can be used for direct fluorescence-
based detection of NO. The fluorescence increase observed
during the reaction of these nonfluorescent complexes with
NO is the result of the dissociation of fluorophore from the
axial site of the ruthenium center. This study further
demonstrates the value of fluorophore displacement as a valid
NO was introduced into a 10 µM dichloromethane solution
of 2, a 1.3-fold increase in fluorescence was exhibited (Figure
S2). The response is much more rapid, however, being
complete in 3 min. Compounds 1 and 2 display turn-on
fluorescent detection of NO that is 1-2 orders of magnitude
more rapid than our previously reported Co(II) sensors.16
Inorganic Chemistry, Vol. 43, No. 20, 2004 6369

Lim and Lippard
strategy for the development of NO sensors and paves the
way for the development of water-soluble, even more rapidly
responding metalloporphyrins toward the ultimate goal of
sensing nitric oxide in living cells.
discussions. The MIT DCIF NMR spectrometer was funded
through NSF Grants CHE-9808061 and DBI-9729592.
Supporting Information Available: Figure S1 and S2 for the
fluorescence studies of 2 with and without NO, and X-ray
crystallographic files (CIF). This material is available free of charge
via the Internet at http://pubs.acs.org.
Acknowledgment. This work was supported by NSF
Grant CHE-0234951. We thank Dr. Sumitra Mukhopadhyay
and Dr. Peter Mueller for assistance with the X-ray structure
determinations and Dr. Scott A. Hilderbrand for helpful
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