Preparation, photophysical, and electrochemical properties of two tetranuclear ruthenium(II) polypyridyl complexes containing 4,5-diazafluorene

Article abstract of DOI:10.1007/s11243-012-9686-0

Two tetrapodal ligands L¹ and L² containing 4,5-diazafluorene units have been synthesized and characterized. Both ligands are composed of two kinds of nonequivalent coordinating sites: one involves the 4-(4,5-diazafluoren-9-ylimino)p

Full text of DOI:10.1007/s11243-012-9686-0

Transition Met Chem (2013) 38:259–265
DOI 10.1007/s11243-012-9686-0
Preparation, photophysical, and electrochemical properties
of two tetranuclear ruthenium(II) polypyridyl complexes
containing 4,5-diazafluorene
•
•
•
Feixiang Cheng Chixian He Hongju Yin
Ning Tang
Received: 20 November 2012 / Accepted: 18 December 2012 / Published online: 9 January 2013
Ó Springer Science+Business Media Dordrecht 2013
Abstract Two tetrapodal ligands L1 and L2 containing
4,5-diazafluorene units have been synthesized and char-
acterized. Both ligands are composed of two kinds of
nonequivalent coordinating sites: one involves the 4-(4,
5-diazafluoren-9-ylimino)phenoxy moiety, and the other
one involves the 2-(4,5-diazafluoren-9-ylimino)phenoxy
moiety. The Ru(II) complexes [(bpy)8Ru4(L1)](PF6)8 and
[(bpy)8Ru4(L2)](PF6)8 (bpy = 2,20-bipyridine) have been
obtained by refluxing Ru(bpy)2Cl2ꢀ2H2O and each ligand
in 2-methoxyethanol. Both complexes exhibit metal-to-
ligand charge transfer (MLCT) absorptions at around
443 nm and emission at around 574 nm. Electrochemical
studies of both complexes display one Ru(II)-centered
oxidation at around 1.33 V and three ligand-centered
reductions.
photoinduced electron or energy transfer and other related
photonic devices [4–7]. In the design of such Ru(II) sys-
tems, the bridging ligands that are used to link two or more
metal polypyridine subunits are crucial because the inter-
actions between the bridged units, and thereby the ground
and excited state properties of the polynuclear complex, are
strongly dependent on the size, shape, and electronic nature
of the bridging ligands [8, 9]. Thus, the synthesis of
appropriate bridging ligands is the most important factor in
realizing molecular devices based on polynuclear Ru(II)
complexes. A wide range of bridging ligands have been
prepared in order to assemble Ru(II) polypyridine building
blocks over the past decade. However, the vast majority of
such studies have focused on systems containing sym-
metric bridging ligands. The study of polynuclear Ru(II)
complexes, bridged with ligands containing two kinds of
nonequivalent coordinating sites, has attracted less atten-
tion [10–13]. Toward the aim of synthesizing new poly-
nuclear Ru(II) complexes with interesting photophysical
and electrochemical properties, herein, we describe the
synthesis and characterization of two tetrapodal ligands
incorporating two kinds of nonequivalent chelating sites:
one involving the 4-(4,5-diazafluoren-9-ylimino)phenoxy
moiety, and the other involving the 2-(4,5-diazafluoren-9-
ylimino)phenoxymethyl moiety. The absorption and
emission spectra, and electrochemical properties of both
complexes are also presented and discussed.
Introduction
There is at present a great interest in the chemistry of
polypyridyl complexes of ruthenium because of their out-
standing photophysical and electrochemical properties and
their extensive use in luminescence sensing, solar energy
conversion, DNA intercalation, pH switching, etc. [1–3].
Polynuclear Ru(II) polypyridyl complexes have received
special attention in recent years in connection with the
development of artificial multicomponent systems for
F. Cheng (&) ꢀ C. He ꢀ H. Yin
Experimental
College of Chemistry and Chemical Engineering, Qujing Normal
University, Qujing 655011, People’s Republic of China
e-mail: chengfx2010@163.com
2,20-Bipyridine, 1,10-phenanthroline, 4-aminophenol, 2-ami-
nophenol, p-toluenesulfonyl chloride, pentaerythritol, tetrabu-
tylammonium perchlorate (TBAP), ethyl acetate, RuCl3ꢀ3H2O,
NH4PF6, K2CO3, CH3CN, CH2Cl2, EtOH, and DMF were
N. Tang
College of Chemistry and Chemical Engineering, Lanzhou
University, Lanzhou 730000, People’s Republic of China
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Transition Met Chem (2013) 38:259–265
purchased from the Tianjin Chemical Reagent Factory.
Solvents and raw materials were of analytical grade and used as
received, apart from CH3CN, which was filtered over acti-
vated alumina and distilled from P2O5 immediately prior
to use. 4,5-Diazafluoren-9-one [14], 9-(4-hydroxy)phenyl-
imino-4,5-diazafluorene [15], 9-(2-hydroxy)phenylimino-4,5-
diazafluorene [15], pentaerythrityl tetratosylate [16], and
Ru(bpy)2Cl2ꢀ2H2O [17] were prepared according to literature
procedures.
1H NMR spectra were obtained on a Mercury Plus 300
spectrometer and a Mercury Plus 400 spectrometer using
TMS as internal standard. ESI-HRMS spectra were obtained
on a Bruker Daltonics APEXII47e mass spectrometer and
ESI–MS spectra with a Bruker Daltonics Esquire 6000 mass
spectrometer. Elemental analyses were obtained using a
Perkin-Elmer 240C analytical instrument. Absorption spectra
were obtained on a Varian Cary-100 UV–Visible spectro-
photometer and emission spectra with a Hitachi F-4600
spectrophotometer. Emission quantum yields were calculated
relative to Ru(bpy)32? (Ustd = 0.376) in EtOH:MeOH (4:1)
glassy matrix [18]. Electrochemical measurements were car-
ried out at room temperature using a CHI 660B electro-
chemical workstation. Cyclic voltammetry and differential
pulse voltammetry were performed in CH3CN and DMF
solutions using a micro cell equipped with a platinum disk
working electrode, a platinum auxiliary electrode, and a sat-
urated potassium chloride calomel reference electrode with
0.1 mol/L TBAP as supporting electrolyte. All samples were
purged with nitrogen prior to measurement.
Found: C, 66.5; H, 4.3; N, 8.6. Calcd for C53H42N6O8S2: C,
66.7; H, 4.4; N, 8.8.
Synthesis of 1,10,100-tris[4-(4,5-diazafluoren-9-ylimino)-
phenoxymethyl]-1000-(p-tosyloxymethyl)-methane
(compound 2)
A
mixture of pentaerythrityl tetratosylate (815 mg,
1.08 mmol), 9-(4-hydroxy)phenylimino-4,5-diazafluorene
(1,162 mg, 4.26 mmol), and K2CO3 (623 mg, 4.51 mmol)
in DMF (20 mL) was heated to 80 °C for 24 h under
nitrogen atmosphere. The solution was poured into 200 mL
of water after cooling down to room temperature, and a red
precipitate which formed was collected by filtration. The
crude product was chromatographed on silica, being eluted
first with CH2Cl2-ethyl acetate (2:1, v/v) to remove
impurities, then with CH2Cl2-EtOH (20:1, v/v) to afford
the desired product as a red solid. Yield: 193 mg (16.9 %).
1H NMR (300 MHz, CDCl3): d = 2.43 (s, 3H), 4.38 (s,
6H), 4.55 (s, 2H), 6.94–7.12 (m, 18H), 7.29 (d, J = 7.5 Hz,
2H), 7.39 (dd, J = 8.1, 5.4 Hz, 3H), 7.80 (d, J = 8.1 Hz,
2H), 8.24 (dd, J = 7.8, 1.2 Hz, 3H), 8.66 (dd, J = 4.8,
1.2 Hz, 3H), 8.80 (dd, J = 6.3, 1.5 Hz, 3H). ESI-HRMS:
m/z 1,056.3291 (M ? H)?, 1,078.3136 (M ? Na)?.
Found: C, 71.3; H, 4.1; N, 11.7. Calcd for C63H45N9O6S:
C, 71.6; H, 4.3; N, 11.9.
Synthesis of 1,10-di[4-(4,5-diazafluoren-9-
ylimino)phenoxymethyl]-100,1000-di[2-(4,5-diazafluoren-
9-ylimino)phenoxymethyl]-methane (L1)
Synthesis of 1,10-di[4-(4,5-diazafluoren-9-ylimino)-
phenoxymethyl]-100,1000-di(p-tosyloxymethyl)-methane
(compound 1)
A mixture of compound 1 (641 mg, 0.67 mmol), 9-(2-
hydroxy)phenylimino-4,5-diazafluorene (613 mg, 2.25 mmol),
and K2CO3 (330 mg, 2.39 mmol) in DMF (50 mL) was
heated to 90 °C for 72 h under nitrogen atmosphere. The
solution was poured into 500 mL of water after cooling
down to room temperature, and a red precipitate which
formed was collected by filtration. The crude product was
purified twice by column chromatography on silica, being
eluted with CH2Cl2-EtOH (20:1, v/v) to afford the desired
A
mixture of pentaerythrityl tetratosylate (652 mg,
0.87 mmol), 9-(4-hydroxy)phenylimino-4,5-diazafluorene
(486 mg, 1.78 mmol), and K2CO3 (273 mg, 1.98 mmol) in
DMF (20 mL) was heated to 80 °C for 24 h under nitrogen
atmosphere. The solution was poured into 200 mL of water
after cooling down to room temperature, and a red pre-
cipitate which formed was collected by filtration. The
crude product was chromatographed on silica, being eluted
first with CH2Cl2-ethyl acetate (2:1, v/v) to remove
impurities, then with CH2Cl2-EtOH (25:1, v/v) to afford
the desired product as a red solid. Yield: 233 mg (28.2 %).
1H NMR (300 MHz, CDCl3): d = 2.44 (s, 6H), 4.09 (s,
4H), 4.31 (s, 4H), 6.84 (d, J = 8.7 Hz, 4H), 6.97 (d, J =
9.0 Hz, 4H), 7.06 (d, J = 3.0 Hz, 4H), 7.31 (d, J = 8.1 Hz,
4H), 7.41 (dd, J = 7.5, 4.8 Hz, 2H), 7.76 (d, J = 8.4 Hz,
4H), 8.25 (d, J = 7.5 Hz, 2H), 8.67 (d, J = 3.6 Hz, 2H),
8.81 (d, J = 4.8 Hz, 2H). ESI–MS: m/z 955.4 (M ? H)?.
1
product as a red solid. Yield: 202 mg (26.0 %). H NMR
(400 MHz, CDCl3): d = 3.66 (s, 4H), 3.99 (s, 4H), 6.52 (d,
J = 8.4 Hz, 4H), 6.75 (d, J = 7.6 Hz, 2H), 6.81–6.85 (m,
6H), 6.98–7.02 (m, 10H), 7.04–7.08 (m, 2H), 7.33 (dd,
J = 7.6, 5.2 Hz, 2H), 7.41 (dd, J = 7.6, 5.2 Hz, 2H), 8.16
(dd, J = 7.6, 1.2 Hz, 2H), 8.26 (dd, J = 7.6, 1.2 Hz, 2H),
8.51 (dd, J = 7.6, 1.2 Hz, 2H), 8.67 (dd, J = 4.0, 2.4 Hz,
2H), 8.75 (dd, J = 4.8, 1.6 Hz, 2H), 8.81 (dd, J = 4.8,
1.6 Hz, 2H). ESI–MS: m/z 1,157.4 (M ? H)?. Found: C,
75.6; H, 4.1; N, 14.3. Calcd for C73H48N12O4: C, 75.8; H,
4.2; N, 14.5.
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Transition Met Chem (2013) 38:259–265
261
Synthesis of 1,10,100-tris[4-(4,5-diazafluoren-9-
ylimino)phenoxymethyl]-1000-[2-(4,5-diazafluoren-9-
ylimino)phenoxymethyl]-methane (L2)
J = 5.6 Hz, 6H), 7.88 (d, J = 5.6 Hz, 6H), 8.17 (d,
J = 4.8 Hz, 3H), 8.19–8.35 (m, 18H), 8.29 (d, J = 7.6 Hz,
3H), 8.38–8.45 (m, 6H), 8.82–8.86 (m, 16H). ESI–MS: m/z
1,179.0 (M—3PF6)3?, 847.8 (M—4PF6)4?. Found: C,
46.1; H, 2.6; N, 9.7. Calcd for C153H112F48N28O4P8Ru4: C,
46.3; H, 2.8; N, 9.9.
L2 was prepared by the same procedure as that described
for L1, except compound 2 (672 mg, 0.58 mmol) was used
instead of compound 1 to react with 9-(2-hydroxy)phe-
nylimino-4,5-diazafluorene (293 mg, 1.08 mmol). Yield:
281 mg (38.2 %) of a red solid. 1H NMR (400 MHz,
CDCl3): d = 4.12 (s, 6H), 4.47 (s, 2H), 6.81–6.84 (m, 6H),
6.88–6.93 (m, 7H), 7.02–7.05 (m, 4H), 7.06–7.11 (m, 5H),
7.22–7.24 (m, 1H), 7.29–7.32 (m, 2H), 7.40 (dd, J = 7.6,
4.8 Hz, 3H), 8.22 (dd, J = 7.6, 1.6 Hz, 1H), 8.25 (dd,
J = 7.6, 1.6 Hz, 3H), 8.55 (dd, J = 5.2, 1.6 Hz, 1H), 8.66
(dd, J = 4.8, 1.3 Hz, 3H), 8.72 (dd, J = 5.2, 1.6 Hz, 1H),
8.81 (dd, J = 5.2, 1.6 Hz, 3H). ESI–MS: m/z 1,157.4
(M ? H)?. Found: C, 75.5; H, 4.0; N, 14.3. Calcd for
C73H48N12O4: C, 75.8; H, 4.2; N, 14.5.
Results and discussion
Synthesis
The procedure of the synthesis of the two tetrapodal
ligands and their Ru(II) complexes is presented in
Scheme 1. Starting compounds 9-(4-hydroxy)phenylimino-
4,5-diazafluorene and 9-(2-hydroxy)phenylimino-4,5-di-
azafluorene were prepared from 4,5-diazafluoren-9-one
according to the literature procedure [15]. Tetrapodal
ligands L1 and L2 have been prepared by two steps, 9-(4-
hydroxy)phenylimino-4,5-diazafluorene reacted with pen-
taerythrityl tetratosylate for 24 h at different molar ratio
affording compounds 1 and 2, respectively. Compounds 1
and 2 reacted with 9-(2-hydroxy)phenylimino-4,5-diaza-
fluorene in DMF for 72 h yielding ligands L1 and L2,
respectively. Both Ru(II) complexes were obtained by re-
fluxing Ru(bpy)2Cl2ꢀ2H2O and the ligand in 2-methoxy-
Synthesis of [(bpy)8Ru4(L1)](PF6)8
A mixture of ligand L1 (79 mg, 0.07 mmol) and Ru-
(bpy)2Cl2ꢀ2H2O (187 mg, 0.36 mmol) in 2-methoxyethanol
(50 mL) was heated to 120 °C for 12 h under nitrogen to
give a clear deep red solution, and then the solvent was
evaporated under reduced pressure. The residue was puri-
fied twice by column chromatography on alumina, being
eluted first with CH3CN-EtOH (5:1, v/v) to remove
impurities, then with EtOH to afford the complex
[(bpy)8Ru4(L1)]Cl8. This complex was dissolved in the
minimum amount of water followed by dropwise addition
of saturated aqueous NH4PF6 until no more precipitate
formed. The precipitate was recrystallized from CH3CN-
Et2O mixture (vapor diffusion method) to afford a red
solid. Yield: 96 mg (35.4 %). 1H NMR (400 MHz, DMSO-
d6): d = 4.09 (s, 8H), 6.99–7.04 (m, 16H), 7.07–7.12 (m,
4H), 7.54–7.65 (m, 24H), 7.77 (d, J = 4.8 Hz, 4H),
7.83–7.86 (m, 8H), 8.03–8.08 (m, 4H), 8.16–8.20 (m,
24H), 8.34–8.41 (m, 4H), 8.83 (s, 8H), 8.85 (s, 8H). ESI–
MS: m/z 848.2 (M—4PF6)4?, 649.3 (M—5PF6)5?. Found:
C, 46.2; H, 2.7; N, 9.7. Calcd for C153H112F48N28O4P8Ru4:
C, 46.3; H, 2.8; N, 9.9.
-
ethanol solution and isolated as their PF6 salts in good
yields. Both complexes were characterized by elemental
1
analyses, ESI–MS, and H NMR spectroscopy.
Absorption spectra
The UV–Vis absorption spectra of both complexes were
recorded in CH3CN solution, at a working concentration of
5 9 10-6 mol/L. The energy maxima and absorption
coefficients are summarized in Table 1, and the spectra are
shown in Fig. 1. Assignments of the absorption bands of
the complexes have been made on the basis of the well-
documented optical transitions of analogous Ru(II) poly-
pyridyl complexes [19–21]. The absorption spectra of both
complexes show three well-resolved bands. Those at ca.
286 and 238 nm can be assigned to intraligand p ? p*
transitions centered on the 2,20-bipyridine. The lowest
energy band at around 443 nm is attributed to an MLCT,
dp ? p* transition, which consists of overlapping
dp(Ru) ? p*(bpy) and dp(Ru) ? p*(L) components. The
lowered symmetry removes the degeneracy of the p* lev-
els, which results in the appearance of a nonsymmetric
MLCT band. The MLCT absorption maxima of the com-
plexes are blue-shifted by about 7 nm compared with that
of Ru(bpy)23? [22], suggesting that the donor properties of
ligands L1 and L2 are weaker than those of 2,20-bipyridine.
Synthesis of [(bpy)8Ru4(L2)](PF6)8
[(bpy)8Ru4(L2)](PF6)8 was prepared by the same procedure
as that described for [(bpy)8Ru4(L1)](PF6)8, except L2
(85 mg, 0.07 mmol) was used instead of L1 to react with
Ru(bpy)2Cl2ꢀ2H2O (191 mg, 0.37 mmol). Yield: 127 mg
1
(43.5 %) of a red solid. H NMR (400 MHz, DMSO-d6):
d = 4.08 (s, 8H), 7.04–7.22 (m, 12H), 7.32–7.44 (m, 8H),
7.55–7.62 (m, 20H), 7.76 (d, J = 4.8 Hz, 6H), 7.85 (d,
123

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Transition Met Chem (2013) 38:259–265
O
S
O
O
S
O
O
N
N
O
S
O
O
O
S
N
O
O
O
N
O
O
N
O
S
N
S
O
O
O
O
HO
O
O
O
N
O
O
S
O
N
N
K2CO3
DMF
N
O
N
N
N
N
N
N
HO
N
N
N
HO
compound 1
K2CO3
DMF
compound 2
N
N
N
K2CO3
DMF
N
N
N
N
N
N
N
N
N
N
N
O
N
L1
N
O
N
O
O
O
N
O
N
N
L2
N
O
N
O
N
N
N
N
N
Ru(bpy)2Cl2
N
N
N
N
Ru(bpy)2Cl2
N
N
N
N
Ru2+
N
[(bpy)8Ru4(L1)]8+
[(bpy)8Ru4(L2)]8+
N
Ru2+
N
N
N
N
N
N
N
N
N
N
Ru2+
N
N
N
N
N
N
N
O
Ru2+
N
O
N
O
N
O
O
O
N
N
O
O
N
N
N
N
Ru2+
N
N
N
N
N
N
N
N
Ru2+
N
N
N
N
N
N
N
N
N
Ru2+
N
N
N
Ru2+
N
N
Scheme 1 Synthesis of tetrapodal ligands L1-2 and their Ru(II) complexes
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Transition Met Chem (2013) 38:259–265
263
Table 1 Photophysical and electrochemical data of Ru(II) polypyridyl complexes
Complex
Absorption
Emissiona
E1/2, V (DEp, mV)b
kmax (nm) (104e, M-1 cm-1
)
kmax (nm)
U
Oxidation
Reduction
[(bpy)8Ru4(L1)]8?
444 (7.04)
286 (26.19)
238 (21.46)
443 (6.70)
286 (25.21)
238 (21.93)
574
574
0.158
1.33 (59)
1.34 (65)
-0.86irr
-1.42 (96)
-1.68 (92)
-0.85irr
[(bpy)8Ru4(L2)]8?
0.152
-1.41 (98)
-1.69 (95)
a
The emission quantum yields are calculated relative to Ru(bpy)23? (Ustd = 0.376) in EtOH:MeOH (4:1, v/v) glassy matrix at 77 K, the
uncertainty in quantum yields is 15 %
b
Oxidation potentials are recorded in 0.1 mol/L TBAP/CH3CN, reduction potentials are recorded in 0.1 mol/L TBAP/DMF and potentials are
given versus SCE, scan rate = 200 mV/s and DEp is the difference between the anodic and cathodic waves
1.2
1.0
0.8
0.6
0.4
0.2
0.0
300
400
500
600
Wavelength (nm)
Fig. 2 Energy state diagram based on the Crosby–Meyer model
Fig. 1 Absorption spectra of complexes [(bpy)8Ru4(L1)](PF6)8
(black) and [(bpy)8Ru4(L2)](PF6)8 (red) in CH3CN solution at room
temperature. (Color figure online)
consistent with ligand field theory, since diazafluorenone
derivatives are known to be lower than 2,20-bipyridine in
the spectrochemical series [31], hence the ligand field
excited state energy will be lowered if 2,20-bipyridine
ligands are replaced by diazafluorenone derivatives. Con-
sequently, population of the ligand field state is very effi-
cient for these complexes, and they are essentially
nonemissive at room temperature. However, the energy
transfer is inhibited at 77 K, so both complexes show
vibrational components similar to that of Ru(bpy)23? in
EtOH:MeOH (4:1, v/v) glassy matrix at 77 K (Fig. 3) [32, 33].
The complexes (10-5 mol/L) show characteristic emission
at around 574 nm and a shoulder at around 620 nm in
EtOH/MeOH (4:1, v/v) glassy matrix at 77 K when excited
at 436 nm (Table 1).
Emission spectra
Upon excitation into the MLCT band, both Ru(II) com-
plexes are nonemissive in CH3CN solution at room tem-
perature. The emission properties of Ru(II) polypyridyl
complexes generally follow the energy gap law [23–25].
The 3MLCT state is reasonably long-lived and is thought to
be deactivated by three processes: radiative decay, kr,
radiationless decay, knr, and thermal population of a higher
lying excited state, koexp(-DE/RT). For the last process,
the thermally accessible excited state has been designated
as a ligand field excited state. The energy of the ligand field
state should depend on the ligand field strength. The
emission intensities follow the model shown in Fig. 2
originally proposed by Crosby, Meyer, and others [26–30].
The values of DE for the Ru(II) diimine complexes con-
taining diazafluorene are substantially lower than the cor-
Electrochemistry
The electrochemical behaviors of the complexes have been
studied in DMF and CH3CN solutions with 0.1 mol/L
TBAP as supporting electrolyte. The reduction waves of
responding value for Ru(bpy)32?
. These results are
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Transition Met Chem (2013) 38:259–265
0.00002
0.00001
0.00000
-0.00001
-0.00002
-0.00003
-0.00004
a
2000
1500
1000
500
0
1.6
1.4
1.2
1.0
Potential / V
0.000007
0.000006
0.000005
0.000004
0.000003
0.000002
0.000001
0.000000
1.1
1.2
1.3
1.4
1.5
1.6
1.7
Potential / V
500
550
600
650
700
b
0.00006
0.00005
0.00004
0.00003
0.00002
0.00001
0.00000
-0.00001
Wavelength (nm)
Fig. 3 Emission spectra of complexes [(bpy)8Ru4(L1)](PF6)8 (black)
and [(bpy)8Ru4(L2)](PF6)8 (red) in EtOH:MeOH (4:1, v/v) glassy
matrix at 77 K. (Color figure online)
the complexes are not well behaved in CH3CN solution due
to adsorption of the reduced species onto the surface of the
platinum electrode. In DMF solution, the complexes dis-
play clear reduction processes, but do not exhibit the oxi-
dative waves due to the insufficient anodic window of this
solvent. Therefore, the oxidation potentials were recorded
in CH3CN solution, and the reduction potentials were
recorded in DMF solution (Table 1).
-0.4 -0.6 -0.8 -1.0 -1.2 -1.4 -1.6 -1.8 -2.0
Potential / V
Fig. 4 Cyclic voltammetry of complex [(bpy)8Ru4(L1)](PF6)8
(5 9 10-4 mol/L), a oxidation potential is recorded in 0.1 mol/L
TBAP CH3CN solution, b reduction potentials are recorded in
0.1 mol/L TBAP DMF solution
The complex [(bpy)8Ru4(L1)]8? displays a Ru(II)-cen-
tered reversible oxidation couple at 1.33 V (Fig. 4a). This
potential is slightly more positive (by about 50 mV) than
that of Ru(bpy)23? (?1.28 V vs. SCE) [34], but slightly
more negative (by about 60 mV) than that of the parent
complex [(bpy)2Ru(dafone)]2? (dafo = 4,5-diazafluoren-
9-one) [31], which indicates that the ligand L1 is a stronger
p-acceptor than 2,20-bipyridine but a weaker p-acceptor
than dafone. Complex [(bpy)8Ru4(L1)]8? has four Ru(II)
centers; two of one type of coordination environment,
whilst the other two are different. The complex shows a
single wave in cyclic voltammetry and a single peak
without broadening in differential pulse voltammetry
(Fig. 4a), which indicates that the small redox potential
difference caused by these different coordination environ-
ments is not resolved by the electrochemical technique.
Electrochemical studies of both complexes reveal three
ligand-centered reductions. A four-electron process for each
couple of both complexes has been confirmed by coulome-
try. The first irreversible reduction wave of complex
[(bpy)8Ru4(L1)]8? at -0.86 V shows that this complex is a
better electron acceptor than [Ru(bpy)3]2? by about 0.7 V
(Fig. 4b), which is consistent with the addition of four elec-
trons to the LUMO localized on the ligand L1 to give the
species [(bpy)2Ru(bpy)2RuL4-Ru(bpy)2Ru(bpy)2]4?. The second
reduction at -1.42 V is located on one of the two 2,20-
bipyridine ligands of each Ru(II) center, thus adding four
electrons to the LUMO ? 1 orbital localized on the 2,20-
bipyridine ligand to give the species [(bpyꢀ-)(bpy)Ru(bpy)
(bpyꢀ-)RuL4-Ru(bpyꢀ-)(bpy)Ru(bpyꢀ-)(bpy)]. The third reduc-
tion at -1.68 V affords the species [(bpyꢀ-)(bpyꢀ-)
Ru(bpyꢀ-)(bpyꢀ-)RuL4-Ru(bpyꢀ-)(bpyꢀ-)Ru(bpyꢀ-)(bpyꢀ-)]4-.
The electrochemical behavior of complex[(bpy)8Ru4(L2)]8?
is similar to that of [(bpy)8Ru4(L1)]8?
.
Conclusion
In summary, two 4,5-diazafluorene-based tetranuclear
Ru(II) polypyridyl complexes incorporating two kinds of
nonequivalent chelating sites have been synthesized. Both
complexes exhibit intense emission at around 574 nm
originating from the lowest energy MLCT excited state in
EtOH:MeOH (4:1, v/v) glassy matrix at 77 K. Electro-
chemical studies of both complexes show one single
Ru(II)-centered oxidation wave without broadening. The
photophysical and electrochemical properties of both
complexes are somewhat different to those of Ru(bpy)32?
due to the different electronic nature of the tetrapodal
ligands L1 and L2. Taking into account the tetranuclear
structure of both Ru(II) complexes, they have potential
123

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