Biologically compatible, phosphorescent dimetallic rhenium complexes linked through functionalized alkyl chains: Syntheses, spectroscopic properties, and applications in imaging microscopy

Article abstract of DOI:10.1021/ic201654d

A range of luminescent, dimetallic complexes based upon the rhenium fac-tricarbonyl diimine core, linked by aliphatic chains of varying lengths and functionality, have been synthesized and their photophysical properties examined. Each complex displays characteristic ³M_ReL _diimineCT emission in aerated acetonitrile solution, with long lifetimes in the range of 129-248 ns and corresponding quantum yields in the range 3.2-8.0%. In aqueous solution, as opposed to acetonitrile, the complexes generally show a small hypsochromic shift in λ_em and an extension of the ³MLCT lifetime, attributed to a hydrophobically driven association of the alkyl chains with the rhenium-bound diimine units. In live cell imaging experiments using MCF7 cells the complexes all show good uptake by non-energy dependent mechanisms without endosomal entrainment, and with varying propensity to localize in organelles. The degrees of uptake and localization properties are discussed in terms of the length and chemical nature of the linkers, and in terms of the likely interactions between these and the various cellular components encountered.

Full text of DOI:10.1021/ic201654d

Article
pubs.acs.org/IC
Biologically Compatible, Phosphorescent Dimetallic Rhenium
Complexes Linked through Functionalized Alkyl Chains: Syntheses,
Spectroscopic Properties, and Applications in Imaging Microscopy
Rebeca G. Balasingham,† Flora L. Thorp-Greenwood,† Catrin F. Williams,‡ Michael P. Coogan,*,†
and Simon J. A. Pope*,†
‡
†School of Chemistry and School of Biosciences, Main Building, Cardiff University, Cardiff CF10 3AT, Cymru/Wales, U.K.
ABSTRACT: A range of luminescent, dimetallic complexes based upon the rhenium
fac-tricarbonyl diimine core, linked by aliphatic chains of varying lengths and
functionality, have been synthesized and their photophysical properties examined.
3
Each complex displays characteristic MReLdiimineCT emission in aerated acetonitrile
solution, with long lifetimes in the range of 129−248 ns and corresponding quantum
yields in the range 3.2−8.0%. In aqueous solution, as opposed to acetonitrile, the
complexes generally show a small hypsochromic shift in λem and an extension of the
3MLCT lifetime, attributed to a hydrophobically driven association of the alkyl chains
with the rhenium-bound diimine units. In live cell imaging experiments using MCF7
cells the complexes all show good uptake by non-energy dependent mechanisms
without endosomal entrainment, and with varying propensity to localize in organelles.
The degrees of uptake and localization properties are discussed in terms of the length
and chemical nature of the linkers, and in terms of the likely interactions between these
and the various cellular components encountered.
functionality, and examples of rhenium-based agents now exist
which can target most of the important cellular organelles.21,22
The vast majority of the previous studies have, however,
focused exclusively upon functionalized monometallic com-
plexes and/or relatively small molecular weight complexes. In
this study, we present the possibilities afforded through larger,
lipophilic dimetallic rhenium species, which, because of their
dicationic nature, may show enhanced accumulation in healthy
cells over the monocationic analogues. The internal membranes
of certain organelles (including mitochondria) also have mem-
brane potentials, which would encourage the accumulation of
dicationic species over monocationic variants. As the rate of
membrane diffusion could also be expected to suffer as a
function of size (Fick’s law), the complexes are designed with
lipophilicity in mind to enhance permeancy. As it has previously
been noted that related lipophilic complexes show unusual
modulation of luminescence in aqueous media,23 because of
hydrophobically driven intramolecular interactions between
alkyl chains and diimine ligands, an investigation of the com-
parative photophysical behavior of the complexes in organic
solvent and water has also been undertaken.
INTRODUCTION
■
Luminescent diimine derivatives of the fac-{ReI(CO)3} core
have been known as efficient emitters for many years, and their
photophysical properties widely studied.1−5 In recent years they
have become recognized as valuable luminophores for
application in live cell imaging studies through confocal
fluorescence microscopy.6−12 Such complexes can be function-
alized in a stepwise manner allowing a number of physical
features to be tuned separately, with the possibility of com-
bining variations in the diimine fragment and the axial ligands
to give a large range of related systems, and offer many
attractive features in the design of cell imaging agents.6 These
attractive features include: aqueous stability; low intrinsic
cytotoxicity (due to the kinetically inert d6 electronic structure
impeding ligand exchange and associated heavy-metal toxicity);
cationic charge and lipophilicity, which assists in crossing bilipid
membranes, and accumulation inside cells with healthy
membrane potential; and their attractive photophysical proper-
ties.6 Visible wavelength excitation and detection are possible
3
through the formation of a long-lived MLCT excited state
which makes complexes of this nature attractive for imaging
applications. By comparison with the typical organic
fluorophores they have large Stokes’ shifts and long lumi-
nescence lifetimes, which can assist in differentiating their
emission from autofluorescence emanating from endogenous
material by wavelength filtering or time gating (recently
demonstrated in cell work with a related d6 IrIII complex),13−19
leading to an improved signal-to-background ratio.20 Such is
the utility of these complexes that organelle specificity has
been demonstrated through targeting vectors and tuned
EXPERIMENTAL SECTION
■
General Measurements and Analysis. All photophysical data
were obtained on a JobinYvon-Horiba Fluorolog spectrometer fitted
with a JY TBX picosecond photodetection module. Luminescence
lifetimes were obtained using either 295 or 372 nm nanoLEDs
Received: August 1, 2011
Published: January 20, 2012
© 2012 American Chemical Society
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Inorganic Chemistry
Article
operating at 1 MHz. All lifetime data were collected using the
JY-Horiba FluoroHub single photon counting module in multichannel
scaler mode. Lifetimes were obtained using the provided software,
DAS6. IR spectra were recorded on a Varian 7000 FT-IR. LR Mass
spectra were obtained using a Bruker MicroTOF LC. UV−vis data
were recorded as solutions on a Jasco 570 spectrophotometer. 1H and
13C{1H} NMR spectra were run either on a NMR-FT Bruker 250 or
400 MHz spectrometers and obtained using CDCl3, CD3OD, or
CD3CN solutions; 1H NMR chemical shifts (δ) were determined rela-
tive to internal Si(CH3)4 and are given in parts per million (ppm).
Elemental analyses were conducted by Medac Ltd. (U.K.).
Human Cell Incubation Studies. Human adenocarcinoma cells
(MCF-7), obtained from the European Collection of Cell Cultures,
Porton Down, Wiltshire, U.K., were maintained in Hepes modified
minimum essential medium (HMEM) supplemented with 10% fetal
bovine serum, penicillin, and streptomycin. Cells were detached from
the plastic flask using trypsin−EDTA solution, and suspended in an
excess volume of growth medium. The homogeneous cell suspension
was then distributed into 1 mL aliquots with each aliquot being subject
to incubation with the complexes final concentration 50 μg mL−1, at
4 °C for 30 min. Cells were finally washed three times in phosphate
buffer saline (PBS, pH 7.2), harvested by centrifugation (5 min, 800 g),
and mounted on a slide for imaging. Preparations were viewed using a
Leica TCS SP2 AOBS confocal laser microscope using X63 objective,
with excitation at 405 nm and detection at 510−580 nm. Z-plane slices
were used to record multiple single-plane views cell populations to
estimate percentage uptake of lumophores.
Co-Localization/Nucleolar Staining Experiments. MCF-7
adenocarcinoma cells were harvested as indicated above, resuspended
in HMEM, and incubated with both complex (final concentration
50 μg mL−1) and the nucleolar-specific stain SYTORNASelect (final
concentration 250 μM) for 1.5 h at 4 °C. Cells were then washed three
times in PBS, mounted, and viewed as above by confocal microscopy.
The excitation and detection wavelengths used for imaging were ex.
405 nm, em. 510−580 nm for complex, and ex. 488 nm, det. 520−
540 nm for SYTORNASelect.
aminomethyl pyridine (0.76 mL, 7.43 mmol) and the mixture heated
to 59 °C for 3 h. Solvents were removed under vacuum and the
product was washed using sat. aq. ammonium chloride (3 × 10 mL).
The organic layer was dried and solution evaporated to give L1 (Yield:
1.48 g, 95%). 1H NMR (CDCl3, 400 MHz, 298 K) δH: 8.51−8.44 (2H,
m), 7.80−7.72 (2H, m), 7.69−7.61 (2H, m), 7.55−7.52 (1H, m),
3
7.20−7.17 (1H, m), 6.47 (1H, s, NH), 4.32 (2H, d, JHH = 4.3 Hz),
3
3
3.58 (2H, t, JHH = 7.5 Hz), 2.19 (2H, t, JHH = 7.3 Hz), 1.66−1.39
(4H, m), 1.29−1.01 (14H, m) ppm. 13C{1H} NMR (CDCl3, 101
MHz, 298 K) δC: 173.3, 168.5, 149, 148.7, 135.6, 134.2, 133.9, 132.2,
123.5, 123.1, 40.9, 38.1, 36.7, 29.4, 29.4, 29.2, 29.1, 28.6, 26.8, 25.7
ppm. MS (ES+) m/z: 435.33 [M+H]+. HRMS (ES+) found m/z
436.2588 [M+H]+; [C26H33O3N3]+ requires 436.2595. UV−vis. (ε/
M
−1 cm−1) (MeCN) λmax: 259 (14700), 294 (6400) nm. IR (nujol) υ:
1772 (CO), 1715 (CO) cm−1.
Synthesis of L2. L1 (1.2 g, 2.84 mmol) was dissolved in
methoxyethanol (20 mL), hydrazine (0.41 mL, 0.854 mmol) added,
and the solution stirred at 115 °C for 12 h. The solution was then
allowed to cool, the white precipitate removed by filtration, and the
filtrate evaporated to dryness. The filtrate was then stirred in hot
chloroform and refiltered. The solvent was removed by evaporation,
producing a creamy white solid as precursor pro-L2 (Yield: 0.5 g,
1
58%). H NMR (CDCl3, 400 MHz, 298 K) δH: 8.46−8.38 (2H, m),
3
3
7.53 (1H, d, JHH = 7.5 Hz), 7.12−7.05 (1H, m), 4.44 (2H, d, JNH
=
5 Hz), 2.55 (2H, t, 3JHH = 12.5 Hz), 1.63 (2H, t, 3JHH = 7.5 Hz), 1.59
(2H, t, JHH = 6.2 Hz), 1.27−1.10 (16H, m) ppm. 13C{1H} NMR
3
(CDCl3, 101 MHz, 298 K) δC: 148.1, 147.7, 134.6, 122,6, 41.1, 39.9,
35.6, 33.5, 32.5, 28.5, 28.5, 28.4, 28.3, 28.2, 25.8, 24.7, 24.5 ppm. MS
(ES+) m/z: 306.3 [M+H]+. HRMS (ES+) found m/z 306.2537 [M
+H]+; [C18H32ON3]+ requires 306.2540.
This product (0.1 g, 0.38 mmol) was added to nicotinoyl chloride
(0.055 g, 0.39 mmol), chloroform (20 mL), triethylamine (0.14 mL,
0.93 mmol), and the mixture heated to 59 °C for 3 h. Solvents were
removed under vacuum, and the product was washed using sat. aq.
ammonium chloride (3 × 10 mL). The organic layer was dried
1
(MgSO4), and solvent removed to give L2 (Yield: 0.125 g, 93%). H
NMR (CDCl3, 400 MHz, 298 K) δH: 8.91−8.88 (1H, m), 8.56−8.51
Toxicity Analyses of the Rhenium Complexes on MCF-7
Cells. Complexes were incubated with MCF-7s for 1.5 h incubation at
a concentration of 50 μg/mL. After incubation, cells were washed and
incubated with 0.5 μg/mL propidium iodide for 30 min and the
number of nonviable cells (PI-stained) counted. Control (DMSO
only), 3 ( 2.92 SD)% dead cells; Re2-phen-L3, 7 ( 4.66 SD) %
dead cells; Re2-phen-L2: 1 ( 1.79) % dead cells; Re2-bipy-L5: 4
3
(1H, m), 8.48−8.39 (2H, m), 8.00 (1H, d), 7.50 (1H, d, JHH = 7.8
Hz), 7.34−7.25 (1H, m), 7.20−7.13 (1H, m), 6.48 (1H, s, NH), 6.1
3
(1H, s, NH), 4.45 (2H, d, JHH = 5.9 Hz), 3.38 (2H, m), 2.15 (2H, t,
3JHH = 7.6 Hz), 1.71−1.48 (4H, m), 1.35−1.02 (14H, m) ppm.
13C{1H} NMR (CDCl3, 101 MHz, 298 K) δC: 156.8, 149.1, 148.8,
147.8, 135.9, 135.3, 123.7, 123.6, 65.2, 49, 40.1, 36.7, 29.5, 29.3, 29.2,
26.9, 25.7 ppm. MS (ES+) m/z: 411.3 [M+H]+, 433.3 [M+Na]+.
HRMS (ES+) found m/z 411.2749 [M+H]+; [C24H35O2N4]+ requires
411.2755. UV−vis (ε/M−1 cm−1) (MeCN) λmax: 260 (9900) nm. IR
(nujol) υ:1633 (CO) cm−1.
(
3.51) % dead cells (SD = standard deviation of 5 different counts).
Materials and Procedures. Fac-[Re(CO)3(phen)Br],24 fac-[Re-
25
(CO)3(bipy)(MeCN)]BF4, and fac-[Re(CO)3(phen)(MeCN)]BF4
were prepared according to the literature procedures. Nicotinoyl
chloride was prepared by simply stirring and heating nicotinic acid in
SOCl2 until complete dissolution was evident; the solvent was then
removed in vacuo, and the crude product (nicotinoyl chloride) used
immediately, without further purification. L5 and L626 were prepared
according to the literature methods. All other starting materials were
used as purchased.
Synthesis of L3. Nicotinoyl chloride (1.77 g, 12.48 mmol) in
chloroform was added slowly to a solution of 1,12-dodecanediol (1.0 g,
4.99 mmol) and triethylamine (2.1 mL, 14.97 mmol) in chloroform
(40 mL). The solution was left to stir at 60 °C for 5 h. The solution
was washed with water (3 × 10 mL) and sat. aq. ammonium chloride
(10 mL). The organic layer was dried and the solvent evaporated to
dryness, yielding a white solid (Yield: 1.90 g, 93%). 1H NMR (CDCl3,
400 MHz, 298 K) δH: 9.12 (2H, s), 8.72−8.68 (2H, m), 8.27−8.21
Synthesis of L1. Laurinlactam (13 g, 0.065 mols) was dissolved in
6 M HCl (150 mL) and heated at 150 °C for 24 h. The green solution
was allowed to cool and the resultant precipitate was filtered giving
12-aminolauric acid as the hydrogen chloride salt (Yield: 10.2 g, 73%).
1H NMR (CD3OD, 400 MHz, 298 K) δH: 2.83 (2H, t, 3JHH = 7.6 Hz),
2.30 (2H, t, 3JHH = 7.4 Hz), 1.66−1.53 (4H, m), 1.38−1.15 (14H, s) ppm.
MS (ES+) m/z: 216.2 [M+H]+. 12-Aminolauric acid hydrogen
chloride (1.00 g, 3.98 mmol) and phthalic anhydride (0.589 mg,
3.98 mmol) were subsequently added to a Schlenk tube and heated
until evolution of water vapor ceased giving a brown solid. 1H NMR
(CD3OD, 400 MHz, 298 K) δH: 7.70−7.82 (4H, m), 3.63 (2H, t,
3
(2H, m), 7.36−7.30 (2H, m), 4.75 (4H, t, JHH = 6.7 Hz), 1.79−1.69
(4H, m), 1.48−1.43 (4H, m), 1.32−1.13 (12H, m) ppm. 13C{1H}
NMR (CDCl3, 101 MHz, 298 K) δC: 164.2, 152.1, 149.7, 136.2, 125.4,
122.3, 64.6, 28.9, 28.5, 28.2, 27.6, 24.9 ppm. MS (ES+) m/z: 413.2
[M+H]+.
Synthesis of L4. Pro-L2 (0.200 g, 0.655 mmol) and benzol-1,-
2,4,5-benzenetetracarboxylicdianhydride (0.071 g, 0.323 mmol) were
heated together in a Schlenk tube until the evolution of water vapor
ceased, producing a brown solid (Yield: 0.251 g, 98%). 1H NMR
(CDCl3, 250 MHz, 298 K) δH: 8.49 (4H, m), 8.22 (2H, d), 7.57 (2H,
3
3JHH = 6.3 Hz), 3.28−3.23 (2H, m), 2.23 (2H, t, JHH = 6.8 Hz),
3
3
1.62−1.50 (4H, m), 1.37−1.13 (12H, m) ppm.
d, JHH = 7.5 Hz), 7.21−7.18 (2H, m), 4.41 (4H, d, JHH = 6.3 Hz),
3
3
This crude product (1.3 g, 3.72 mmol) was then dissolved in thionyl
chloride (2 mL) and left at 71 °C for 1 h. The excess thionyl chloride
was removed under reduced pressure and to this was added
chloroform (20 mL), triethylamine (1.14 mL, 11.14 mmol), and 3-
3.17 (4H, t, JHH = 1.5 Hz), 2.18 (4H, t, JHH = 15 Hz), 1.64−1.60
(8H, m), 1.41−1.32 (28H, m) ppm. 13C{1H} NMR (CDCl3, 101
MHz, 298 K) δC: 172.3, 165.3, 165.1, 164.7, 149, 148.5, 147.8, 147.5,
136.4, 136.2, 135.9, 135.8, 134.9, 133.4, 122.8, 122.7, 117.4, 117.1,
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39.9, 38.7, 37.7, 35.6, 28.4, 28.3, 28.2, 28, 27.4, 25.7, 24.7 ppm. MS (ES+)
m/z: 793.5 [M+H]+, 815.5 [M+Na]+. HRMS (ES+) found m/z 793.4634
[M+H]+; [C46H61O6N6]+ requires 793.4647. UV−vis (ε/M−1 cm−1)
(MeCN) λmax: 274 (16100), 372 sh (7000) nm. IR (nujol) υ: 1740 (CO),
1696 (CO) cm−1.
Synthesis of f ac-{[Re(CO)3(1,10-phenanthroline)]2(L4)}(BF4)2
(Re2-phen-L4). To a round-bottom flask was added, [Re(CO)3
(phen)(MeCN)]BF4 (50 mg, 0.1 mmol), L4 (40 mg, 0.05 mmol)
and chloroform (10 mL) and heated to 59 °C for 12 h. The removal of
the solvent under high vacuum afforded an orange colored solid. The
crude product was purified using column chromatography, eluting
with dichloromethane:methanol (96:4) (Yield: 16 mg, 14%). 1H NMR
(CDCl3, 400 MHz, 298 K) δH: 9.49 (4H, m), 8.72 (4H, d), 8.1
Synthesis of f ac-[Re(CO)3(1,10-phenanthroline)(L1)]BF4 (Re-
phen-L1). To a round-bottom flask encased in aluminum foil, AgBF4
(31 mg, 0.14 mmol), [ReBr(CO)3(phen)] (57 mg, 0.1 mmol), L1
(50 mg, 0.12 mmol) and toluene (10 mL) were added and heated to
100 °C for 12 h. The solution was filtered through Celite and washed
repeatedly with acetonitrile until the solution ran clear. The removal of
the solvent under high vacuum afforded a yellow solid. The crude
product was purified using column chromatography eluting with
dichloromethane:methanol (98:2) and reprecipated from chloroform
3
(2H, s), 8.0 (12H, m), 7.49 (2H, d, JHH = 7.9 Hz), 7.05−7.00
3
(2H, m), 6.62 (2H, s, NH), 3.93 (4H, d, JHH = 6 Hz), 3.57 (4H, t,
3
3
3JHH = 7.2 Hz), 1.65 (4H, t, JHH = 6.7 Hz), 1.37 (4H, t, JHH = 7.0
Hz), 0.98−1.13 (32H, m) ppm. MS (ES+) m/z: 846.2 [M+H]2+,
1243.5 [M-(Re(CO)3Phen)]+, 1779.5 [M-BF4]+. HRMS (ES+) found
m/z 845.2373; [185Re2C76H76O12N10]2+ requires 845.2346. IR (nujol)
υ: 2032 (CO), 1917 (CO), 1717 (CO amide) cm−1. UV−vis (ε/M−1
cm−1) (MeCN) λmax: 274 (17500) nm, 375 (5100) nm. Anal. calcd
(%) for the corresponding hexafluorophosphate salt, C76F12-
H76N10O12P2Re2·1.5CH2Cl2: C, 44.08; H, 3.78; N, 6.64. Found (%):
C, 44.06; H, 3.80; N, 6.38.
1
and excess diethyl ether. (Yield: 27 mg, 28%). H NMR (CDCl3, 400
MHz, 298 K) δH: 9.52−9.49 (2H, m), 8.69−8.63 (2H, m), 8.17−8.10
(2H, m), 8.09−8.04 (1H, m), 8.0 (2H, s), 7.73−7.70 (3H, m), 7.62−
3
7.58 (4H, m), 7.69−7.03 (1H, m), 4.19 (2H, t, JHH = 6 Hz), 3.59
3
3
(2H, t, JHH = 7.3 Hz), 1.96 (2H, t, JHH = 7.7 Hz), 1.64−1.60 (2H,
m), 1.41−1.32 (2H, m), 1.20−1.11 (14H, m) ppm. 13C{1H} NMR
(CDCl3, 101 MHz, 298 K) δC: 174.6, 168.9, 154.6, 152.0, 150.0, 146.8,
140.5, 140.5, 139.9, 135.1, 134.3, 132.5, 131.8, 131.5, 128.8, 128.1,
126.5, 124.1, 123.6, 53.9, 40.6, 38.5, 36.4, 34.5, 29.9, 29.8, 29.7, 29.6,
27.3, 26.1, 25.9, 22.8 ppm. MS (ES+) m/z: 886.3 [M+H-BF4]+. HRMS
(ES+) found m/z 884.2570; [185ReC41H41O6N5]+ requires 884.2581.
IR (nujol) υ: 2032 (CO), 1921 (CO), 1711 (CO amide) cm−1. UV−
vis (ε/M−1 cm−1) (MeCN) λmax: 275 (29600), 365 (3400) nm. Anal.
calcd (%) for BC41F4H41N5O6Re·CHCl3: C, 48.93; H, 4.79; N, 5.94.
Found (%): C, 48.93; H, 4.65; N, 6.08.
Synthesis of f ac-{[Re(CO)3(2,2′-bipyridine)]2(L5)}(BF4)2 (Re2-
bipy-L5). The complex was prepared by the addition of L5 (hexyl
dinicotinamide; 0.050 g, 15 mmol) to [Re(CO)3(bipy)(MeCN)]BF4
(0.159 g, 31 mmol) in ethanol (10 mL). The solution was heated
to just below reflux with stirring overnight. The product precipitated
as the solution was cooled and the yellow solid was filtered under
vacuum and washed. Subsequent reprecipation from dichlorome-
thane with diethyl ether gave the pure yellow solid (Yield: 0.136 g,
1
3
69%). H NMR (CD3CN, 400 MHz, 298 K) δH: 9.15 (4H, d, JHH
=
3
3
5.2 Hz), 8.46 (2H, s), 8.32 (4H, d, JHH = 8.0 Hz), 8.25 (2H, d, JHH
=5.6 Hz), 8.21−8.18 (4H, m), 8.07 (2H, d, 3JHH = 6.8 Hz), 7.73−7.69
(4H, m), 7.17 (2H, bs), 3.21−3.16 (4H, m), 1.46−1.38 (4H, m),
1.23−1.18 (4H, bm) ppm. 13C{1H} (CD3CN, 62.5 MHz, 298 K) δC:
195.2 (CO), 162.3, 155.5, 153.5, 150.8, 140.9, 137.4, 132.8, 128.5,
125.7, 124.5, 39.9, 28.5, 26.2 ppm. MS (ES+) m/z: 1265.2 [M-BF4]+.
HRMS (ES+) found m/z 1262.1902; [185Re2C44H38O8N810BF4]+
requires 1262.1938. IR (CH3CN) ν: 2036 (CO), 1933 (CO), 1672
(CO amide) cm−1. UV−vis (ε/M−1 cm−1) (MeCN) λmax: 305 (15000),
319 (14900) 333 (5400) nm. Anal. calcd (%) for B2C44F8-
H38N8O8Re2·0.75CH2Cl2: C, 37.93; H, 2.82; N, 7.91. Found (%): C,
38.12; H, 2.45; N, 8.23.
Synthesis of f ac-{[Re(CO)3(1,10-phenanthroline)]2(L2)}(BF4)2
(Re2-phen-L2). To a round-bottom flask encased in aluminum foil,
AgBF4 (40 mg, 0.2 mmol), [ReBr(CO)3(phen)] (70 mg, 0.13 mmol),
L2 (27 mg, 0.06 mmol), and toluene (10 mL) were added and heated
to 100 °C for 12 h. The solution was filtered over Celite and washed
repeatedly with acetonitrile until the solution ran clear. The removal of
the solvent under high vacuum afforded a yellow solid. The crude
product was purified using column chromatography and eluting with
dichloromethane:methanol (96:4). (Yield: 21 mg, 24%). 1H NMR
(CDCl3, 400 MHz, 298 K) δH: 9.53−9.49 (5H, m), 8.78−8.71 (5H,
m), 8.49−8.44 (1H, m), 8.25 (1H, m), 8.03−7.61 (8H, m), 7.62−7.58
(1H, m), 7.52 (1H, d), 7.20−7.15 (1H, m), 7.06−7.04 (1H, m), 6.92
(1H, s, NH), 6.61 (1H, s, NH), 4.31 (2H, d), 3.95−3.90 (2H, m),
1.55−1.52 (2H, m), 1.40−1.36 (2H, m), 1.29−1.11 (16H, m) ppm.
MS (ES+) m/z: 861.29 [M-Re(CO)3(phen)], 655.66 [M+H]2+.
HRMS (ES+) found m/z 859.2737; [185ReC39H42O5N6]+ requires
859.2741. IR (nujol) υ: 2031 (CO), 1910 (CO), 1675 (CO amide)
cm−1. UV−vis (ε/M−1 cm−1) (MeCN) λmax: 252 (15760), 272 (15520),
342 (5100) nm. Anal. calcd (%) for B2C54F8H50N8O8Re2·3CH2Cl2 C,
39.35; H, 3.24; N, 6.44. Found (%): C, 39.24; H, 3.60; N, 6.69.
Synthesis of f ac-{[Re(CO)3(1,10-phenanthroline)]2(L3)}(BF4)2
(Re2-phen-L3). To a round-bottom flask encased in aluminum foil,
AgBF4 (8.5 mg, 4.4 × 10−5 mol), [ReBr(CO)3(phen)] (30 mg, 7.39 ×
10−5 mol), L3 (14 mg, 3.69 × 10−5 mol) and toluene (10 mL) were
added and heated to 100 °C for 12 h. The solution was filtered
through Celite and washed repeatedly with acetonitrile until the
solution ran clear. The filtrate was evaporated, and a yellow solid
recrystallized from dichloromethane and diethyl ether (Yield: 19 mg,
Synthesis of f ac-{[Re(CO)3(2,2′-bipyridine)]2(L6)}(BF4)2 (Re2-
bipy-L6). The complex was prepared by the addition of L6 (octyl
dinicotinamide; 0.050 g, 14 mmol) to [Re(CO)3(bipy)(MeCN)]BF4
(0.146 g, 28 mmol) in ethanol (10 mL). The solution was heated to
subreflux with stirring overnight. The product was reprecipated from
dichloromethane and diethyl ether and then filtered under vacuum to
1
give a yellow solid (Yield: 0.094 g, 51%). H NMR (CD3CN, 250
MHz, 298 K) δH: 9.17 (4H, d, 3JHH = 0.8 Hz), 8.45 (2H, d, 3JHH =1.6
3
3
Hz), 8.31 (4H, d, JHH = 8.4 Hz), 8.25 (2H, d, JHH = 5.2 Hz), 8.19
(4H, dd, J = 8.4 and 1.6 Hz), 8.05 (2H, dt, J = 1.6, 8.0 Hz), 7.73−7.68
(4H, m), 7.29 (2H, m), 7.09 (2H, bs), 3.21−3.16 (4H, m), 1.46−1.39
(4H, m), 1.24−1.17 (8H, m) ppm. 13C{1H} (CD3CN, 62.5 MHz,
298 K) δC: 195.3 (CO), 162.7, 155.5, 153.4, 150.8, 140.9, 137.4, 132.9,
128.5, 126.0, 124.5, 40.9, 39.0, 28.5, 25.5 ppm. HRMS (ES+) found
m/z 1290.2291; [185Re2C46H42O8N810BF4]+ requires 1290.2251. IR
(CH3CN) ν: 2036 (CO), 1933 (CO), 1635 (CO amide) cm−1. UV−
vis (ε/M−1 cm−1) (MeCN) λmax: 308 (26200), 319 (26100), 336
(8900) nm. Anal. calcd (%) for B2C46F8H42N8O8Re2·H2O·CH2Cl2: C,
38.05; H, 2.99; N, 7.55. Found (%): C, 38.29; H, 2.52; N, 7.78.
1
26%). H NMR (CD3CN, 400 MHz, 298 K) δH: 9.54−9.51 (4H, m),
8.72 (4H, d), 8.48 (2H, s), 8.31 (2H, d), 8.06−8.02 (6H, m), 7.99−
7.97 (4H, m), 7.42−7.23 (2H, m), 4.04 (4H, t, 3JHH = 6.6 Hz), 2.05−
1.92 (4H, m), 1.58−1.50 (4H, m), 1.31−1.13 (12H, m) ppm. 13C{1H}
NMR (CDCl3, 101 MHz, 298 K) δC: 162.9, 155.6, 154.6, 152.0, 146.7,
140.6, 140.4, 131.5, 129.2, 128.3, 127.2, 126.7, 66.0, 29.9, 29.4,
28.1 ppm. MS (ES+) m/z: 863.3 [M-(Re(CO)3(Phen)+H]+. HRMS
(ES+) found m/z 861.2404; [185ReC39H40O7N4]+ requires 861.2421.
IR (nujol cm−1) υ: 2034 (CO), 1917 (CO), 1722 (CO ester) cm−1.
UV−vis (ε/M−1 cm−1) (MeCN) λmax: 252 (24400), 275 (26600), 377
(2900) nm. Anal. calcd (%) for B2C54F8H48N6O10Re2·0.75CH2Cl2: C,
42.40; H, 3.22; N, 5.42. Found (%): C, 42.13; H, 3.87; N, 5.80.
RESULTS AND DISCUSSION
■
Synthesis of the Ligands and Complexes. Each of
the ligands was synthesized from commercially available pyri-
dine-type starting materials (nicotinic acid or 3-aminomethyl
pyridine). L1 was isolated by preparing the phthalimido pre-
cursor, through acid-promoted ring-opening hydrolysis of laurin-
lactam to give 12-aminolauric acid; subsequent reaction with
phthalic anhydride gave the corresponding phthalimide pre-
cursor. This synthon was converted to the corresponding acid
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chloride and then reacted with 3-aminomethyl pyridine to give
L1. Deprotection of L1 produced the corresponding free amine,
pro-L2 (Scheme 1), where L2 then required further addition of
Scheme 2. Rhenium Complexes Synthesized in This Study
a
Scheme 1. Synthesis of Selected Ligands
a
Reagents: (i) 6 M HCl; (ii) phthalic anhydride, Δ; (iii) SOCl2; (iv)
3-aminomethylpyridine; (v) hydrazine hydrate, Δ; (vi) nicotinoyl
chloride; (vii) pyromellitic dianhydride, Δ.
nicotinoyl chloride. Reaction of pro-L2 with 0.5 equiv of benzol-
1,2,4,5-benzenetetracarboxylicdianhydride (pyromellitic dianhy-
dride) gave L4, wherein a pyromellitic-diimide (PMDI) chro-
mophore is incorporated into the alkyl chain framework. L3 was
simply isolated following reaction of 1,12-dodecanediol with
excess nicotinoyl chloride. Similarly, L5 (via 1,6-diaminohexane)
and L6 (via 1,8-diaminooctane) were prepared through coupling
of the appropriate diamines with excess nicotinoyl chloride.10
Each of the new ligands were fully characterized using compre-
hensive spectroscopic analyses and mass spectrometry. The
syntheses of the cationic rhenium complexes (Scheme 2) were
achieved following literature precedent, using either fac-[Re-
(CO)3(bipy)(MeCN)](BF4) or fac-[Re(CO)3(phen)(Br)] as
required;2,27 the pure mono- and dimetallic complexes were iso-
lated either as their tetrafluoroborate or hexafluorophosphate
salts. Spectroscopic characterization of the complexes was achi-
1
short-lived fluorescence, and attributed to the π−π* state of
the chromophores in each case. The emission from L4 is
therefore characteristic of a monomeric PMDI, rather than
excimer-type, fluorescent state. For L1 these observations are
consistent with our previous work on phthalimide-appended
pyridyl-type ligands.
For the complexes, sensitization of the MLCT at around
380 nm resulted in characteristic long-lived MLCT emission
1
3
at around 540 nm (phenanthroline derivatives) or 560 nm
(bipyridine derivatives), with corresponding quantum yields in
the 3.2−8.0% range.24,25 In aerated MeCN the emission
profiles were typically broad and featureless in appearance, and
the lifetimes were satisfactorily fitted to single exponential
decays in each case, suggesting that for the dimeric species both
rhenium diimine chelates are in the same excited state environ-
ment. As reported for closely related systems,28,29 the lifetime
of the phenanthroline complexes are typically much longer than
related bipyridine analogues, reflecting both the enhanced
rigidity and the higher energy emission wavelength of the former.
For Re2-phen-L4, the overlapping absorption bands of the
1MLCT and bridging PMDI-type chromophore infer that both
chromophoric units can be sensitized (λex = 345 nm), revealing
a complex that is dual-emissive: a higher energy (i.e., monomer-
type) band at about 405 nm30 attributed to the PMDI fluo-
rophore that is relatively unperturbed compared to the free
ligand (cf. 420 nm) and second, a broad, lower energy band
1
eved using H and 13C{1H} NMR, IR and UV−vis. spectros-
copies. Additional analytical data was provided through elemental
analyses and mass spectrometry, which was obtained on the
complexes and frequently revealed peaks at m/z observed for the
dicationic species (z = 2), as well as fragmentation of the axial
ligands resulting in the loss of one “Re(CO)3(phen)” unit.
Luminescence Properties of the Ligands and Com-
plexes. The absorption properties of the complexes are
characterized by two common features: ligand-centered
(diimine, pyridine, or phthalimido) and metal-to-ligand charge
transfer transitions (1MLCT). The former typically predom-
inate <320 nm, whereas the latter contribute to the pale yellow
colored appearance of the complexes (>320 nm).24,25 In the
case of L4 there are additional contributions from the PMDI
chromophore between 300 and 350 nm.
3
(541 nm) assigned to the MLCT emitting state. The fluo-
rescence lifetime of the PMDI-based emission is shorter than
for the free ligand (2.6 vs 4.9 ns) suggesting some degree of
quenching in the presence of the rhenium units. In contrast to
In aerated MeCN solution at room temperature, the
emission properties of L1 and L4 show a longest wavelength
emission (ca. 400−410 nm), which was characterized as a
3
the other complexes, the decay profile for the MLCT band
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Table 1. UV-visible Absorption and Emission Data for the Complexes
a
a
ab
,
a
c
c
compound
λabs (1MLCT)/nm
λem/nm
Φ
τ3MLCT/ns
λem/nm
τ3MLCT/ns
[Re(CO)3phen(L1)]BF4
365
342
377
365
333
336
546
547
541
0.080
0.048
0.064
0.050
0.050
0.032
248
543
540
546
338
{[Re(CO)3phen]2L2}(BF4)2
{[Re(CO)3phen]2L3}(BF4)2
{[Re(CO)3phen]2L4}(BF4)2
{[Re(CO)3bipy]2L5}(BF4)2
215
510
248
444
405,541
551
46, 202
136
421, 537
555
27, 332
132
{[Re(CO)3bipy]2L6}(BF4)2
553
129
550
149
a
b
c
In aerated acetonitrile. Quantum yields obtained using a reference value of 0.016 for [Ru(bpy)3](PF6)2 in aerated acetontrile. In aerated water.
Steady state spectra obtained using λex = 345 nm. Lifetimes obtained using λex = 372 nm (errors 10%).
could not be fitted to a single-exponential component, but
rather, a biexponential decay provided a better fit for the data
(as judged by the summed residual squared errors of the
associated fit), giving two distinct lifetimes of 202 (∼75% of the
total decay intensity) and 46 ns (Table 1). The former lifetime
is clearly comparable in magnitude to τ(MLCT) of the complexes
of L1-L3, whereas the latter component is shorter than
intensity increased with decreasing temperature, together with a
concomitant hypsochromic shift in λem; such dependence was
not observed in MeCN over this temperature range. Although
these trends are less pronounced than for the related
monometallic complexes,23 they are still consistent with an
entropically driven chain-wrapping process which shields the
3MLCT excited state from water solvent. For Re2-phen-L4 the
corresponding VT spectra (Figure 1) revealed the ratiometric
3
expected for unperturbed MLCT emission originating from
the cationic complex core (cf. Re-phen-L1); the quantum yield
of Re2-phen-L4 is also lower than the structurally related
complex Re-phen-L1. With comparison to the other emissive
complexes in this series, prospective quenching pathways for
the 3MLCT state may involve the bridging PMDI-type
chromophore. A commonly observed quenching pathway for
3 MReLCT excited states involves forward electron transfer from
an available donor, generating a LLCT transient excited state
(back electron transfer regenerates the ground MLCT state).31
However, in this case the oxidation potential of PMDI (actually
a very strong electron acceptor) should render such a process
unfavorable from a thermodynamic perspective.32 Alternatively,
triplet−triplet energy transfer has been previously reported for
related Re(I)-spacer-anthracene (i.e., chromophore-quencher)
3
complexes where the observed MLCT lifetimes were also
shortened (mechanistically reported to proceed via Dexter
Figure 1. Steady state emission spectra for Re2-phen-L4 recorded in
water at temperatures between 278 and 318 K (excitation wavelength
345 nm).
electron exchange).33,34 The energy transfer between MLCT
3
and 3IL(anthracene) are much more favorable than those
described here since, energetically, the triplet excited state of
PMDI (19700 cm−1)32 is above that of anthracene. In
responses of the dual-emission between 278 and 318 K, nicely
demonstrating the relative sensitivities of the two different
3
comparison the PMDI-centered π−π* state of L4 (recorded
3
excited states (1π−π* of PMDI at about 420 nm, and MLCT
at 77 K; 1:1, EtOH:MeOH glass) possessed vibronically struc-
tured peaks at 467 (21400 cm−1) and 490 nm (20400 cm−1),
and these values suggest that there is some spectral overlap of
at about 535 nm) to the polar solvent environment.
3
Accordingly, the dipolar MLCT excited state showed marked
3
the PMDI triplet state with the MLCT emission profile,
temperature dependence, consistent with solvation changes in
the local environment.
although triplet−triplet energy transfer does not appear to be
particularly efficient (deaerated measurements did not result
The lifetimes were also extended in water, most noticeably
for the nicotinamide derivative Re2-phen-L2, again suggesting
that the excited state localized on the coordinated phenanthro-
line ligand was shielded from the quenching phenomena
associated with water solvent. In contrast, for the shorter chain
dimetallic species based on ligands L5 and L6, the measure-
ments in water did not show any significant deviations from
those obtained in MeCN, suggesting that the alkyl chain length
must exceed at least C8 to allow sufficient conformational
freedom.23
Confocal Fluorescence Microscopy: Cellular Imaging.
Having studied the photophysical properties of the dirhenium
complexes, and established that they had suitable emission and
excitation profiles for cell imaging applications, a series of live
cell imaging experiments were undertaken. To assess the
influence upon cell uptake and, in particular, intracellular
localization of changing the chelating ligand and the nature and
3
in an extension in the MLCT lifetimes), as evidenced by
the predominance of the longer lifetime value to the decay
profile.
Luminescence measurements in aqueous media (<10−5 M)
demonstrated that the majority of the complexes are sensitive
to the nature of the solvent environment. Our previous studies
have proposed the influence (and chain length dependence) of
hydrophobically driven intramolecular interactions between
long alkyl chains and the coordinated diimine unit, effectively
3
resulting in shielding of the MLCT excited state from the
(quenching) solvent environment.23 For the complexes of L1-
L4 the 3MLCT emission was retained and moderately
hypsochromically shifted, again consistent with shielding from
the polar aqueous solvent. For these complexes the aqueous
emission spectra also showed reversible temperature depend-
ence within a narrow temperature range, wherein the emission
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length of the linker, the complexes were each incubated under
the same conditions, and for the same period with MCF-7 cells.
The incubations were carried out at 4 °C at which temperature
energy-dependent uptake processes such as endocytosis are inhi-
bited,35 to avoid entrainment in endosomes, and to allow a
comparison of the intrinsic membrane permeancy of the species.
The cells were washed free of excess agent, then allowed to warm
to ambient temperature to allow energy-dependent intracellular
distribution pathways to re-emerge, while blocking energy-
dependent uptake before examination by confocal laser micros-
copy. In all cases the excitation wavelength was 405 nm (at the
the cellular localization with some general background emission
from membrane and cytoplasm, but with distinct bright spots,
characteristic of organelle staining. It is likely that the organelles
may include mitochondria, ER, and Golgi apparatus. The
mitochondrial uptake is encouraged by the dicationic nature of
the complex, enabling uptake across the mitochondrial
membrane because of the significant membrane potential,
maintained by the negative interior of the organelle with
respect to the cytoplasm. However, the pattern of distribution
is far too wide for specific mitochondrial uptake, in line with the
requirement for rhenium complexes to be thiol-reactive to
show specific mitochondrial staining,23 while several lipophilic
cationic examples have shown some, nonspecific mitochondrial
uptake.7
1
low-energy edge of the MLCT absorption band), and the
images were collected between 530 and 580 nm, correlating to
3MLCT emission, and well separated from autofluorescence.6
The reference monomeric species Re-phen-L1 with a C11-
linked phthalimide showed good uptake (Figure 2) with >80%
Similarly, the nonsymmetrical dirhenium diamide Re2-phen-
L2 showed good uptake (Figure 4), with >80% of cells showing
Figure 4. Uptake (left, scale bar = 48 μm) and localization (right, scale
bar = 10 μm) of Re2-phen-L2 in MCF7s.
Figure 2. Uptake (left, scale bar = 48 μm) and localization (right, scale
bar = 5 μm) of Re-phen-L1 in MCF7s.
rhenium based emission, but again there were wide variations in
the intensity between cells. Again, localization in a large number
of cytoplasmic structures giving a distinctive pattern of staining
of the cytoplasmic organelles, but without evidence of speci-
ficity. This is consistent with the cationic and lipophilic nature
of the complex, with no targeting vectors to direct specific
localization to any given organelle.
The highly lipophilic diester species Re2-phen-L3 showed
excellent uptake with uniformly intense emission from >90% of
the cell sample, with diffuse cytoplasmic and membrane
staining, with brighter staining characteristic of better uptake
than the amide analogues (Figure 5). This higher uptake is
of the cell sample showing rhenium-based emission, consistent
with the highly lipophilic structure, which will assist passive
diffusion across membranes. Closer examination of individual
cells (Figure 2) showed general staining, with plasma
membrane, internal membranes, and the perinuclear region
of the cytoplasm all showing strong emission with no evidence
of nuclear uptake. No localized intense spots were observed in
the cytoplasm, indicating nonspecific distribution throughout
lipophilic structures, rather than specific uptake in particular
organelles.
The dimeric, pyromellitate-linked species Re2-phen-L4 also
showed excellent uptake (Figure 3), with >80% of the cell
Figure 5. Uptake (left, scale bar = 14 μm) and nucleolar localization
(right, scale bar = 10 μm) of Re2-phen-L3 in MCF7s.
Figure 3. Uptake (left, scale bar = 48 μm) and localization (right, scale
bar = 7 μm) of Re2-phen-L4 in MCF7s.
assigned to the lipophilicity associated with the aliphatic chain,
combined with an absence of amide units, which have pre-
viously been noted7 as showing lower uptake than esters in
sample showing rhenium-based emission, although the intensity
was variable from cell to cell. There was a distinctive pattern to
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pairs of analogous rhenium complexes. In addition to the
excellent uptake observed with this complex, on a small number
of occasions distinct bright spots of emission from the dark
region of the cells correlating to the nucleus were observed,
which is usually considered to be characteristic of nucleolar
localization. As with the previous reports of highly lipophilic
rhenium complexes which show nucleolar localization,7 the
small percentage of cells showing nuclear uptake (on the
occasions that it was observed) made confirmation of nucleolar
localization by colocalization experiments impossible. In the
absence of colocalization, experiments were undertaken with
the same cell line, under the same incubation conditions, with a
known nucleolar stain, SYTORNASelect, (SRS) which is
known to selectively stain the nucleoli of healthy mammalian
cells (Figure 6). The patterns of staining of the nucleoli of the
Figure 7. Localization of Re2-bipy-L5 (left, scale bar = 6 μm) and
Re2-bipy-L6 (right, scale bar = 6 μm) in MCF7s.
that observed with Re2-bipy-L6, which showed clear ac-
cumulation in distinct organelles, as well as general back-
ground emission from the cytoplasm. This suggests that the
greater lipophilicity of Re2-bipy-L6 assists in permeating the
internal membranes protecting organelles and allowsit to
penetrate them and accumulate in the interior, rather than
be associated with the exterior of the membranes.
Overall, it is clear that dimeric rhenium complexes, even
those as large as Re2-phen-L4 are taken up efficiently by
MCF7s, by non-energy dependent mechanisms, without
endosomal entrainment. The degree of uptake and the patterns
of localization depend not only upon the length of the linker
chain but also upon the nature of the linkages. Thus, the diester
Re2-phen-L3 is taken up best of all the complexes examined,
even though on simple charge versus chain-length arguments
monocationic Re-phen-L1 might be expected to show best
uptake. It has previously been noted7 that the uptake of
rhenium complexes is highly dependent upon small changes in
structure which cannot be explained by simple lipophilicity
arguments, and that in most cases esters are taken up better
than the analogous amides. This phenomenon implies that even
in the absence of endocytosis, specific interactions between
the complex and the exterior of the cell membrane are impor-
tant to uptake, making simple assumptions about permeancy−
lipophilicity relationships unwise. Small changes in chain length
(e.g., from Re2-bipy-L5 to Re2-bipy-L6) can lead to noticeable
changes in uptake and localization, reinforcing similar previous
observations in related monomeric complexes. Finally, although
no exactly analogous complexes were examined, the trends of
uptake imply that 1,10-phenanthroline complexes are more
effectively taken up than 2,2′-bipyridine derivatives, in line
with the expected improvement in lipophilicity and the
literature precedent for related ruthenium complexes.
The cytotoxicity of a small range of complexes was examined
by the propidium iodide (PI) assay, in which cells are incubated
first with the species of interest, then with PI, which only enters
cells that have damaged membranes, itself an indicator of poor
health. The complexes tested showed low cytotoxicity levels,
with 1−7% of cells taking up PI, compared to 3% for a DMSO
control, indicating that these are viable fluorophores for use in
live cell imaging. The highest toxicity (7% PI stained cells) is
associated with the highly lipophilic diester, Re2-phen-L3, in
line with previous reports of highly lipophilic rhenium cations
causing membrane disruption,9 while the other complexes
tested showed cytotoxicity below, or barely above, the blanks.
In summary, a series of luminescent dimetallic rhenium com-
plexes linked through various chain lengths and functionalities
have been synthesized and fully characterized. For chain lengths
Figure 6. Localization of SRS (scale bar = 9 μm) in MCF7s.
cell samples with SRS were essentially identical to those
observed with the diester, although the diester also showed
cytoplasmic staining, providing strong support for the assign-
ment of the staining as nucleolar. In addition, a series of Z-
stacks36 were collected confirming that the emission assigned as
nucleolar came from within the central depth of the otherwise
dark nuclear region of the cell, confirming that an intranuclear
compartment had been stained, rather than a site on the cell
surface. Even in the absence of colocalization data therefore, the
confirmation that these bright spots represent staining of an
intranuclear compartment, similar in size and shape to the
nucleoli stained by SRS strongly indicates an assignment as
nucleolar staining. This apparent nucleolar localization is
assigned to the additional lipophilicity of ester species over
amide analogues, allowing permeation of the nuclear
membrane, and it is likely that the localization in the nucleolus
is a result of Coulombic attraction between the dicationic
complex and polyanionic poly- or oligo-nucleotides.
The pair of analogous diamide complexes, Re2-bipy-L5 (L5 =
1,6-hexanedinicotinamide) and Re2-bipy-L6 (L6 = 1,8-
octanedinicotinamide) were also studied to assess the change
in uptake upon replacing phenanthroline in the related
complex Re2-phen-L2 with bipyridine, and to compare the
C8 and C6 analogues. Both Re2-bipy-L5 and Re2-bipy-L6
showed cellular uptake (Figure 7), but the level was much
higher for the more lipophilic Re2-bipy-L6 complex. The
patterns of localization were similar with both agents
showing accumulation in the cytoplasm, but the emission
from cells stained with Re2-bipy-L5 was more diffuse than
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(20) See: Lakowicz, J.R. In Principles of Fluorescence Spectroscopy,
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(21) Amoroso, A. J.; Arthur, R. J.; Coogan, M. P.; Court, J. B.;
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Commun. 2011, 47, 3096−3098.
(23) Coogan, M. P.; Fernandez-Moreira, V.; Hess, J. B.; Pope, S. J.
A.; Williams, C. New J. Chem. 2009, 33, 1094−1099.
(24) Wrighton, M. S.; Morse, D. L. J. Am. Chem. Soc. 1974, 96, 998−
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(29) Thorp-Greenwood, F. L.; Coogan, M. P.; Hallett, A. J.; Laye, R.
H.; Pope, S. J. A. J. Organomet. Chem. 2009, 694, 1400−1406.
(30) Kato, S. I.; Nonaka, Y.; Shimasaki, T.; Goto, K.; Shinmyozu.
J. Org. Chem. 2008, 73, 4063−4075.
(31) Shanze, K. S.; MacQueen, D. B.; Perkins, T. A.; Cabana, L. A.
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greater than C8, the complexes show solvent-dependent
photophysical properties consistent with hydrophobically
driven intramolecular interactions in aqueous environments.
The incorporation of a PMDI-type chromophore into the
alkyl chain provides a complex that is dual emissive, with
ratiometric sensitivity to solvent environment, wherein the
PMDI unit appears to provide a potential quenching path-
3
way for the MLCT excited state. Each complex demon-
strated great utility as an intracellular imaging probe for
MCF7 cells, as evidenced through confocal fluorescence
microscopy studies. The ester-linked dimetallic complex
showed improved uptake over the related amide systems
and specific organelle localization was promoted through
the dicationic nature of the complexes.
AUTHOR INFORMATION
Corresponding Author
■
*E-mail: cooganmp@cardiff.ac.uk (M.P.C.), popesj@cardiff.ac.
uk (S.J.A.P.).
ACKNOWLEDGMENTS
■
We thank Cardiff University and EPSRC for financial support
and the staff of the EPSRC Mass Spectrometry National Service
(University of Swansea) for providing MS data.
(34) Thornton, N. B.; Schanze, K. S. New J. Chem. 1996, 20, 791−
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■
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(35) Pawlet, J. B., Ed.; Handbook of Biological Confocal Microscopy;
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(36) A series of Z-stack scans showing the nucleolar localization of
Re2-phen-L3 can be viewed as the original Z-stack series with LCS
Lite available free at ftp://ftp.llt.de/softlib/LCSLite/LCSLite2611537.
exe and can also be viewed as single images with a variety of image
handling software.
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