Synthesis, photophysical, and electrochemical properties of a new family of trinuclear Ru(II) polypyridine complexes

Article abstract of DOI:10.1080/00958972.2011.646998

A series of four tripodal ligands L^1-4 were prepared by the reaction of 9-(2-hydroxy)phenylimino-4,5-diazafluorene with 1,3,5- tris(bromomethyl)benzene, 1,3,5-tris(bromomethyl)-2,4,6-trimethylbenzene, tris(2-chloroethyl)amine hydrochloride, and

Full text of DOI:10.1080/00958972.2011.646998

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Synthesis, photophysical, and
electrochemical properties of a new
family of trinuclear Ru(II) polypyridine
complexes
Feixiang Cheng a , Jishu Chen a , Fan Wang a , Ning Tang b &
Longhai Chen b
a College of Chemistry and Chemical Engineering, Qujing Normal
University, Qujing 655011, P.R. China
b College of Chemistry and Chemical Engineering and State Key
Laboratory of Applied Organic Chemistry, Lanzhou University,
Lanzhou 730000, P.R. China
Version of record first published: 21 Dec 2011.
To cite this article: Feixiang Cheng , Jishu Chen , Fan Wang , Ning Tang & Longhai Chen (2012):
Synthesis, photophysical, and electrochemical properties of a new family of trinuclear Ru(II)
polypyridine complexes, Journal of Coordination Chemistry, 65:2, 205-217
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Journal of Coordination Chemistry
Vol. 65, No. 2, 20 January 2012, 205–217
Synthesis, photophysical, and electrochemical properties of a
new family of trinuclear Ru(II) polypyridine complexes
FEIXIANG CHENG*y, JISHU CHENy, FAN WANGy,
NING TANGz and LONGHAI CHENz
yCollege of Chemistry and Chemical Engineering, Qujing Normal University,
Qujing 655011, P.R. China
zCollege of Chemistry and Chemical Engineering and State Key Laboratory of Applied
Organic Chemistry, Lanzhou University, Lanzhou 730000, P.R. China
(Received 5 December 2010; in final form 16 November 2011)
A series of four tripodal ligands L1–4 were prepared by the reaction of 9-(2-hydroxy)pheny-
limino-4,5-diazafluorene with 1,3,5-tris(bromomethyl)benzene, 1,3,5-tris(bromomethyl)-2,4,6-
trimethylbenzene, tris(2-chloroethyl)amine hydrochloride, and pentaerythrityl tetratosylate,
respectively, in DMF solution under nitrogen. For each ligand, Ru(II) complexes were
prepared by refluxing Ru(bipy)2Cl2 ꢀ 2H2O and ligand in 2-methoxyethanol. Photophysical
behaviors of these Ru(II) complexes have been investigated by UV-Vis absorption and
luminescence spectrometry. They display metal-to-ligand charge transfer absorption at 442 nm
in MeCN solution at room temperature and emission at 574 nm in EtOH–MeOH (4 : 1, v/v)
glassy matrix at 77 K. Electrochemical studies of the Ru(II) complexes show one Ru(II)-
centered oxidation at 1.33 V and three ligand-centered reductions.
Keywords: Tripodal ligand; Ru(II) complex; UV-Vis absorption; Luminescence;
Electrochemistry
1. Introduction
Ru(II) polypyridine complexes have attracted much interest in molecular recognition,
artificial photosynthesis, DNA intercalation, pH switching, etc. due to their unique
combination of chemical stability, redox properties, reactivity, and emission [1].
Polynuclear complexes incorporating Ru(II) polypyridine units have received special
attention in connection with development of artificial multicomponent systems
for photoinduced electron or energy transfer and other related photonic devices [2].
2þ
2þ
For instance, RuðbipyÞ3 and OsðbipyÞ3 , covalently attached to the 30- and 50-
phosphates of two oligonucleotides, are juxtaposed when hybridized contiguously to a
fully complementary DNA target. Upon excitation into the metal-to-ligand charge
2þ
transfer (MLCT) band of RuðbipyÞ3 leads to resonance energy transfer to the MLCT
*Corresponding author. Email: chengfx2010@163.com
Journal of Coordination Chemistry
ISSN 0095-8972 print/ISSN 1029-0389 online ß 2012 Taylor & Francis
http://dx.doi.org/10.1080/00958972.2011.646998

206
F. Cheng et al.
state of OsðbipyÞ23þ; the system is capable of detecting mutations of DNA [3]. In the
design of such polynuclear systems, the bridging ligands used to connect two or more
metal polypyridine subunits are crucial because interactions between the bridged units,
and thereby the properties of polynuclear complexes, are strongly dependent on the
size, shape, and electronic nature of the bridging ligands [4]. A wide range of bridging
ligands have been used in recent years and many of them contain 2,20-bipyridine (bipy),
1,10-phenanthroline (phen), or 2,20 : 60,200-terpyridine [5]. Benniston and co-workers
prepared a linear 2,20 : 60,200-terpyridine-based trinuclear Ru(II)–Os(II) nanometer-sized
complex. This complex comprises two bis(2,20 : 60,200-terpyridine)ruthenium(II) termi-
nals connected via alkoxy-strapped 4,40-diethynylated biphenylene units to a central
bis(2,20 : 60,200-terpyridine) osmium(II) core. Energy transfer occurs with high efficiency
from Ru(II) units to the Os(II) center [6]. Cooke et al. have shown a new series
of supramolecular complexes consisting of Ru(II) polypyridine units bound to
dirhodium(II,II) tetracarboxylate cores. Efficient energy transfer from the MLCT
triplet state of the Ru-based components to the lowest energy excited state of the
dirhodium core takes place at 298 K in MeCN [7]. Much effort has been devoted to the
design and synthesis of polypyridine that lead to Ru(II) complexes with interesting
photophysical and electrochemical properties. 4,5-Diazafluoren-9-one (dafone) is
structurally similar to bipy and phen. However, the rigid structure imposed by the
central five-member ring means that the two nitrogen atoms are always held in the same
direction to avoid rotational conformation problems. Dafone has a much larger chelate
˚
˚
˚
bite compared with bipy and phen (N ꢀ ꢀ ꢀ N: dafone, 3.00 A; bipy, 2.62 A; phen, 2.64 A).
As a consequence, Ru(II) complexes including 4,5-diazafluorene have different
2þ
2þ
photophysical and electrochemical properties than RuðbipyÞ3 and RuðphenÞ3
complexes [8]. Recently, we described the synthesis, photophysical, and electrochemical
properties of trinuclear Ru(II) complexes derived from 4,5-diazafluoren-9-oxime [9]
with the goal of synthesizing new 4,5-diazafluoren-9-one based trinuclear Ru(II)
complexes with distinct properties. We have now extended this study to the synthesis
and characterization of four tripodal ligands and their Ru(II) complexes derived from
9-(2-hydroxy)phenylimino-4,5-diazafluorene. The photophysical and electrochemical
properties of these Ru(II) complexes are also presented.
2. Experimental
2.1. Materials
2,20-Bipyridine, 1,10-phenanthroline, 2-aminophenol, p-toluenesulfonyl chloride, 1,3,5-
tris(bromomethyl)benzene, pentaerythritol, tris(2-chloroethyl)amine hydrochloride,
mesitylene, ammonium hexafluorophosphate, hydrated ruthenium trichloride, MeCN,
CH2Cl2, EtOH, MeOH, and DMF were purchased from the Tianji Chemical Reagent
Factory. All solvents and raw materials were of analytical grade and used as received,
with the exception of MeCN, which was filtered over activated alumina and distilled
from P2O5 immediately prior to use. Tetrabutylammonium perchlorate (TBAP) [10],
4,5-diazafluoren-9-one [11], 9-(2-hydroxy)phenylimino-4,5-diazafluorene [12], 1,3,5-
tris(bromomethyl)-2,4,6trimethylbenzene [13], pentaerythrityl tetratosylate [14], and
Ru(bipy)2Cl2 ꢀ 2H2O [15] were synthesized according to literature procedures.

Trinuclear Ru(II) polypyridine complexes
2.2. Physical measurements
207
Liquid chromatography-mass spectrometry (LC-MS) spectra were recorded on a
Bruker Daltonics Esquire 6000 mass spectrometer. 1H NMR spectra were performed on
a Mercury Plus 300 spectrometer using TMS as internal standard. Elemental analyses
were obtained using a Perkin-Elmer 240C analytical instrument, absorption spectra
on a Varian Cary-100 UV-Visible spectrophotometer, and emission spectra with a
Hitachi F-4500 spectrophotometer. Emission quantum yields were calculated relative
2þ
to RuðbipyÞ3 (Èstd ¼ 0.376) in an EtOH–MeOH (4 : 1, v/v) glassy matrix and the
uncertainty in quantum yields was 15% [16]. Electrochemical measurements were
carried out at room temperature using a CHI 660B electrochemical workstation.
Cyclic voltammetry and differential pulse voltammetry were performed in MeCN and
DMF solutions by using a microcell equipped with a platinum disc working electrode,
a platinum auxiliary electrode, and a saturated potassium chloride calomel reference
electrode with 0.1 mol Lꢁ1 TBAP as supporting electrolyte. All samples were purged
with nitrogen prior to measurement.
2.3. Preparations
1,3,5-Tris[2-(4,5-diazafluoren-9-ylimino)phenoxymethyl]benzene (L1):
A
mixture
of 1,3,5-tris(bromomethyl)benzene (213 mg, 0.60 mmol), 9-(2-hydroxy)phenylimino-
4,5-diazafluorene (571 mg, 2.09 mmol), and K2CO3 (306 mg, 2.22 mmol) in DMF
(20 mL) was heated to 80ꢂC for 24 h under nitrogen. The solution was poured into
200 mL of water after cooling to room temperature. A red precipitate was formed and
collected by filtration. The precipitate was chromatographed on silica, eluted first with
CH2Cl2–ethyl acetate (1 : 1, v/v) to remove impurities, then with CH2Cl2–EtOH (25 : 1,
1
v/v) affording the desired product as a red solid. Yield: 189 mg (33.6%). H NMR
(300 MHz, CDCl3): ꢀ 4.50 (s, 6H), 6.58 (s, 3H), 6.86–6.99 (m, 12H), 7.08 (t, J ¼ 6.9 Hz,
3H), 7.16–7.23 (m, 6H), 8.30 (dd, J ¼ 7.2, 1.2 Hz, 3H), 8.54 (dd, J ¼ 3.3, 2.4 Hz, 3H),
8.70 (d, J ¼ 4.2 Hz, 3H). LC-MS: m/z 934.5 (M þ H)þ, 956.2 (M þ Na)þ.
1,3,5-Tris[2-(4,5-diazafluoren-9-ylimino)phenoxymethyl]-2,4,6-trimethylbenzene (L2):
L2 was prepared by the same process as described for L1, except that 1,3,5-
tris(bromomethyl)-2,4,6-trimethylbenzene (243 mg, 0.61 mmol) was used instead
of 1,3,5-tris(bromomethyl)benzene to react with 9-(2-hydroxy)phenylimino-4,5-
1
diazafluorene (592 mg, 2.17 mmol). Yield: 216 mg (36.1%) of a red solid. H NMR
(300 MHz, CDCl3): ꢀ 1.76 (s, 9H), 4.58 (s, 6H), 6.67 (d, J ¼ 7.5 Hz, 3H), 6.79
(d, J ¼ 7.2 Hz, 3H), 6.92 (dd, J ¼ 8.1, 5.1 Hz, 3H), 6.98-7.06 (m, 6H), 7.18
(t, J ¼ 7.8 Hz, 3H), 7.26–7.30 (m, 3H), 8.07 (d, J ¼ 7.5 Hz, 3H), 8.55 (d, J ¼ 5.4 Hz,
3H), 8.75 (d, J ¼ 4.5 Hz, 3H). LC-MS: m/z 976.3 (M þ H)þ, 998.2 (M þ Na)þ.
2,20,200-Tris[2-(4,5-diazafluoren-9-ylimino)phenoxyethyl]amine (L3): L3 was prepared
by the same process as described for L1, except that tris(2-chloroethyl)amine
hydrochloride (169 mg, 0.71 mmol) was used instead of 1,3,5-tris(bromomethyl)benzene
to react with 9-(2-hydroxy)phenylimino-4,5-diazafluorene (676 mg, 2.48 mmol). Yield:
1
268 mg (41.3%) of a red solid. H NMR (300 MHz, CDCl3): ꢀ 2.36 (t, J ¼ 5.0 Hz, 6H),
3.43 (t, J ¼ 5.0 Hz, 6H), 6.57 (d, J ¼ 8.4 Hz, 3 H), 6.87–6.95 (m, 9H), 7.00 (t, J ¼ 7.5 Hz,
3H), 7.11 (t, J ¼ 7.5 Hz, 3H), 7.28 (t, J ¼ 3.9 Hz, 3H), 8.19 (dd, J ¼ 7.8, 1.2 Hz, 3H),
8.59 (dd, J ¼ 4.5, 1.2 Hz, 3H), 8.75 (dd, J ¼ 6.3, 4.8 Hz, 3H). LC-MS: m/z 915.4
(M þ H)þ, 938.2 (M þ Na)þ.

208
F. Cheng et al.
1,10,100-Tris[2-(4,5-diazafluoren-9-ylimino)phenoxymethyl]-1000-(p-tosyloxymethyl)-
methane (L4): L4 was prepared by the same process as described for L1, except
that pentaerythrityl tetratosylate (716 mg, 0.95 mmol) was used instead of 1,3,5-
tris(bromomethyl)benzene to react with 9-(2-hydroxy)phenylimino-4,5-diazafluorene
(855 mg, 3.13 mmol). Yield: 153 mg (15.3%) of a red solid. 1H NMR (300 MHz,
CDCl3): ꢀ 2.29 (s, 3H), 3.39 (s, 6H), 3.58 (s, 2H), 6.18–6.21 (m, 3H), 6.80–6.87 (m, 9H),
6.96–7.03 (m, 8H), 7.25–7.28 (m, 2H), 7.36 (dd, J ¼ 7.8, 5.4 Hz, 3H), 8.08 (dd, J ¼ 7.5,
1.8 Hz, 3H), 8.50 (t, J ¼ 3.2 Hz, 3H), 8.79 (dd, J ¼ 4.5, 1.5 Hz, 3H). LC-MS: m/z 1056.2
(M þ H)þ.
[(bipy)6Ru3L1](PF6)6 (Ru-L1):
A
mixture of L1 (76 mg, 0.085 mmol) and
Ru(bipy)2Cl2 ꢀ 2H2O (147 mg, 0.28 mmol) in 2-methoxyethanol (20 mL) was heated to
120ꢂC for 12 h under nitrogen to give a clear deep red solution, then solvent was
evaporated under reduced pressure. The residue was purified twice by column
chromatography on neutral alumina, eluted first with MeCN–EtOH (10 : 1, v/v) to
remove impurities, then with MeCN–EtOH (2 : 1, v/v) affording [(bipy)6Ru3L1]Cl6,
which was then dissolved in a minimum of water followed by dropwise addition
of saturated aqueous NH4PF6 until no more precipitate formed. The precipitate was
recrystallized from MeCN–Et2O (vapor diffusion method) yielding a red solid. Yield:
1
91 mg (36.7%). H NMR (300 MHz, DMSO-d6): ꢀ 4.93 (s, 6H), 7.02–7.15 (m, 12H),
7.22 (t, J ¼ 8.4 Hz, 3H), 7.28 (s, 3H), 7.35 (t, J ¼ 8.4 Hz, 3H), 7.37–7.58 (m, 15H), 7.62
(d, J ¼ 5.4 Hz, 3H), 7.73 (d, J ¼ 5.1 Hz, 3H), 7.83 (dd, J ¼ 9.9, 5.4 Hz, 6H), 8.07
(d, J ¼ 5.1 Hz, 3H), 8.08–8.21 (m, 15H), 8.44 (d, J ¼ 7.5 Hz, 3H), 8.81 (t, J ¼ 8.4 Hz,
12H). LC-MS: m/z 869.4 (M–3PF6)3þ, 615.8 (M–4PF6)4þ. Elemental anal. Found:
C, 47.54; H, 2.77; N, 9.47. Calcd for C120H87F36N21O3P6Ru3: C, 47.35; H, 2.88; N, 9.66.
[(bipy)6Ru3L2](PF6)6 (Ru-L2): Ru-L2 was prepared by the same process as described
for Ru-L1, except that L2 (86 mg, 0.088 mmol) was used instead of L1 to react with
Ru(bipy)2Cl2 ꢀ 2H2O (182 mg, 0.35 mmol) affording a red solid. Yield: 89 mg (32.7%).
1H NMR (300 MHz, DMSO-d6): ꢀ 2.04 (s, 9H), 4.98 (s, 6H), 7.05–7.14 (m, 9H), 7.36–
7.40 (m, 9H), 7.51–7.58 (m, 15H), 7.66 (d, J ¼ 5.4 Hz, 3H), 7.72 (d, J ¼ 5.7 Hz, 3H), 7.83
(t, J ¼ 6.6 Hz, 6H), 8.06–8.21 (m, 18H), 8.33 (d, J ¼ 7.5 Hz, 3H), 8.79–8.84 (m, 12H).
LC-MS: m/z 883.7 (M–3PF6)3þ, 626.2 (M–4PF6)4þ, 472.3 (M–5PF6)5þ. Elemental anal.
Found: C, 48.06; H, 3.17; N, 9.67. Calcd for C123H93F36N21O3P6Ru3: C, 47.87; H, 3.04;
N, 9.53.
[(bipy)6Ru3L3](PF6)6 (Ru-L3): Ru-L3 was prepared by the same process as described
for Ru-L1, except that L3 (92 mg, 0.10 mmol) was used instead of L1 to react with
Ru(bipy)2Cl2 ꢀ 2H2O (186 mg, 0.36 mmol) affording a red solid. Yield: 117 mg (38.4%).
1H NMR (300 MHz, DMSO-d6): ꢀ 2.59 (s, 6H), 3.74 (s, 6H), 6.80–6.82 (m, 3H),
6.96–7.01 (m, 3H), 7.08–7.14 (m, 6H), 7.37 (t, J ¼ 6.3 Hz, 3H), 7.52–7.59 (m, 18H), 7.64
(d, J ¼ 5.7 Hz, 3H), 7.75 (d, J ¼ 5.4 Hz, 3H), 7.86 (dd, J ¼ 10.5, 5.4 Hz, 6H), 8.10–8.22
(m, 18H), 8.30–8.34 (m, 3H), 8.79 (d, J ¼ 8.1 Hz, 6H), 8.85 (d, J ¼ 8.1 Hz, 6H). LC-MS:
m/z 1367.0 (M–2PF6)2þ, 864.1 (M–3PF6)3þ, 611.0 (M–4PF6)4þ. Elemental anal. Found:
C, 46.59; H, 3.16; N, 10.03. Calcd for C117H90F36N22O3P6Ru3: C, 46.45; H, 3.00;
N, 10.19.
[(bipy)6Ru3L4](PF6)6 (Ru-L4): Ru-L4 was prepared by the same process as described
for Ru-L1, except that L4 (83 mg, 0.079 mmol) was used instead of L1 to react with
Ru(bipy)2Cl2 ꢀ 2H2O (148 mg, 0.28 mmol) affording a red solid. Yield: 87 mg (34.9%).
1H NMR (300 MHz, DMSO-d6): ꢀ 2.17 (s, 3H), 3.49 (s, 6H), 3.63 (s, 2H), 6.80–7.04 (m,
6H), 6.97–7.07 (m, 9H), 7.18–7.32 (m, 5H), 7.40–7.61 (m, 15H), 7.67 (t, J ¼ 6.3 Hz, 3H),

Trinuclear Ru(II) polypyridine complexes
209
7.79–7.84 (m, 9H), 7.99–8.06 (m, 3H), 8.09–8.22 (m, 17H), 8.28 (t, J ¼ 6.3 Hz, 3H), 8.79–
8.85 (m, 12H). LC-MS: m/z 911.0 (M – 3PF6)3þ, 646.1 (M–4PF6)4þ, 487.7 (M–5PF6)5þ
.
Elemental anal. Found: C, 46.86; H, 3.18; N, 9.50. Calcd for C123H93F36N21O6P6Ru3S:
C, 46.66; H, 2.96; N, 9.29.
3. Results and discussion
3.1. Synthesis
The outline of the synthesis of the four tripodal ligands L1–4 and their Ru(II) complexes
[(bipy)6Ru3L1–4](PF6)6, abbreviated as Ru-L1–4, is presented in scheme 1. The starting
compound 9-(2-hydroxy)phenylimino-4,5-diazafluorene was synthesized from 4,5-
diazafluoren-9-one according to literature procedure [12]. L1, L2, and L3 were prepared
in good yields by the reaction of 9-(2-hydroxy)phenylimino-4,5-diazafluorene with
1,3,5-tris(bromomethyl)benzene, 1,3,5-tris(bromomethyl)-2,4,6-trimethylbenzene, and
tris(2-chloroethyl)amine hydrochloride, respectively, in DMF. Initial attempt to
prepare the polypodal ligand of 9-(2-hydroxy)phenylimino-4,5-diazafluorene with
pentaerythrityl tetratosylate under the same experimental conditions did not afford
tetrapodal diimine ligand, but instead afforded the tripodal ligand L4. It may be noted
that the reaction conditions, namely temperature, duration of reaction, and Lewis base,
are very important. High temperature and NaOH resulted in the formation of a
tetrapodal diimine ligand [17]. The Ru(II) complexes were prepared by refluxing
Ru(bipy)2Cl2 ꢀ 2H2O and the ligands in 2-methoxyethanol solution, and isolated as their
PFꢁ6 salts. These compounds were characterized by 1H NMR, MS, and elemental
analyses.
Rillema and co-workers reported the electronic and 1H NMR properties of a series of
polypyridyl ligands derived from 4,5-diazafluoren-9-one [12]. Due to sp2 hybridization
of the nitrogen in the bridge, the structure of the phenylimino-4,5-diazafluorene group
is asymmetric, the protons in the two pyridine units of each 4,5-diazafluorene group are
non-equivalent. As shown for L2 (figure S1), the chemical shifts for the ꢁ, ꢂ, and ꢃ
protons are 8.75, 7.18, and 8.07 ppm, respectively; the chemical shifts for the ꢁ0, ꢂ0, and
ꢃ0 protons are 8.55, 7.06, and 7.30 ppm, respectively.
Elemental analyses are consistent with the formation of trinuclear systems.
Octahedral metal centers with bidentate ligands generally show stereoisomerism. The
number of stereoisomeric possibilities in polynuclear complexes increases exponentially
with the number of metal centers. Although 1H NMR spectra of some Ru(II)
1
polypyridine complexes have been clearly described [18], in most cases, the H NMR
spectra of polynuclear Ru(II) complexes are complicated. The protons in the two
pyridine units of each 4,5-diazafluorene group of Ru-L1–4 are unequal; therefore, the
1H NMR spectra of the complexes are complicated and the assignment of the proton
signals is difficult. The structures of trinuclear Ru(II) complexes are further established
by LC-MS spectra. This technique has proven to be very helpful for identifying
polynuclear transition metal complexes with high molecular masses [19]. The data with
the assignments of the peaks are given in section 2. Usually, the mass is calculated
from a series of multiply charged ions obtained by successive loss of counteranions.
LC-MS spectra for the complexes exhibit some expected peaks due to (M–nPF6)nþ
.

210
F. Cheng et al.
Scheme 1. Synthesis of tripodal ligands L1–4 and corresponding Ru(II) complexes Ru-L1–4
.

Trinuclear Ru(II) polypyridine complexes
211
Figure S2 shows the LC-MS spectrum of Ru-L2. Clearly, the main peak at m/z 626.2 is
assigned to (M–4PF6)4þ and the other two peaks at m/z 883.7 and 472.3 are assigned to
(M–3PF6)3þ and (M–5PF6)5þ, respectively. The measured molecular weights are
consistent with expected values.
3.2. Absorption spectra
Absorption spectra of the ligands are studied in CHCl3 and their Ru(II) polypyridine
complexes are studied in MeCN solution. The concentration of all samples is
10ꢁ5 mol Lꢁ1. The spectra are shown in figure 1 with the data summarized in table 1.
Absorptions of the ligands can be assigned to ligand-centered intraligand ꢄ!ꢄ* or
n!ꢄ* transitions. Assignments of the absorptions of Ru-L1–4 were made on the basis
of well-documented optical transitions of analogous Ru(II) polypyridine complexes [20].
Absorption spectra of the complexes show three well-resolved bands. Those at
ca 285 and 236 nm can be assigned to intraligand ꢄ!ꢄ* transitions centered on
2,20-bipyridine. The lowest energy band at 442 nm is attributed to MLCT transition,
which consists of overlapping dꢄ(Ru) ! ꢄ*(bipy) and dꢄ(Ru) ! ꢄ*(L) components.
The lowered symmetry removes the degeneracy of the ꢄ* levels, which results in the
appearance of a non-symmetrical MLCT band. The MLCT absorption maxima of the
2þ
complexes are blue-shifted by 8 nm compared with that of RuðbipyÞ3 [21], suggesting
that the donor properties of the ligands are weaker than that of 2,20-bipyridine. The
extinction coefficients of the MLCT bands of Ru-L1–4 are larger than those of trinuclear
Ru(II) complexes containing 2,20-bipyridine or 1,10-phenanthroline, but smaller than
those of trinuclear Ru(II) complexes containing 2,20 : 60,200-terpyridine [22].
3.3. Emission behavior
Ru-L1–4 are non-emissive in MeCN at room temperature upon excitation into the
MLCT band. The emission properties of Ru(II) polypyridine complexes generally
2.5
2.0
1.5
1.0
0.5
0.0
300
400
500
600
Wavelength (nm)
Figure 1. Absorption spectra of Ru-L1 (10ꢁ5 mol Lꢁ1, (—), typical for Ru-L1–4) in MeCN and L1 (10ꢁ5
mol Lꢁ1, (—-), typical for L1–4) in CHCl3 solution.

212
F. Cheng et al.
Table 1. Photophysical data of ligands and their Ru(II) polypyridine complexes.a
Compound
ꢅ
max, nm (104 ", (mol L
)
ꢁ1 ꢁ1 cmꢁ1
)
ꢅmax (nm)
È
L1
429 (0.47)
428 (0.48)
428 (0.37)
427 (0.36)
443 (5.84)
442 (5.29)
443 (5.63)
442 (5.44)
302 (2.96)
302 (2.78)
302 (2.61)
302 (2.66)
285 (21.95)
285 (20.25)
286 (21.21)
285 (20.55)
244 (8.48)
244 (8.07)
244 (7.96)
244 (10.04)
236 (17.53)
235 (16.67)
236 (16.53)
235 (16.25)
L2
L3
L4
Ru-L1
Ru-L2
Ru-L3
Ru-L4
573
574
574
575
0.085
0.088
0.090
0.081
2þ
aEmission quantum yields are calculated relative to RuðbipyÞ3 (Èstd ¼ 0.376) in an EtOH–MeOH
(4 : 1, v/v) glassy matrix at 77 K, the uncertainty in quantum yields is 15%.
1
MLCT
isc
LF
DE
3
MLCT
hv
k
o
k
k
nr
r
1
GS
Figure 2. Energy state diagram based on the Crosby–Meyer model.
follow the energy gap law [23]. The 3MLCT state is reasonably long-lived and is thought
to be deactivated by 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 (LF) excited
state. The energy of this presumed LF state should depend on the LF strength.
Emission intensities follow the model shown in figure 2 originally proposed by Crosby,
Meyer, and others [24]. The values of DE for the diimine complexes containing
2þ
diazafluorene are substantially lower than the corresponding value for RuðbipyÞ3
.
These results are consistent with ligand field theory. Diazafluorene derivatives
are known to be lower than 2,20-bipyridine in the spectrochemical series [25]. Hence,
substitution of diazafluorene derivatives for 2,20-bipyridine results in a decreased ligand
3
field and therefore, a lower LF excited state energy. Since the MLCT excited state is
not significantly affected, the variation in the LF state dictates the energy gap [26].
Consequently, population of the LF state is very efficient for these complexes at room
temperature and they are essentially non-emissive at room temperature. However,
energy transfer is inhibited at 77 K, so they show vibrational components similar to
2þ
that of RuðbipyÞ3 in EtOH–MeOH (4 : 1, v/v) glassy matrix at 77 K (figure 3) [27].

Trinuclear Ru(II) polypyridine complexes
213
2000
1500
1000
500
0
500
550
600
650
700
Wavelength (nm)
Figure 3. Luminescence spectra of Ru-L1 (green), Ru-L2 (black), Ru-L3 (red), and Ru-L4 (blue) in
EtOH–MeOH (4 : 1, v/v) glassy matrix at 77 K.
The complexes (10ꢁ5 mol Lꢁ1) exhibit characteristic emission at 574 nm in EtOH–
MeOH glassy matrix at 77 K with an excitation wavelength at 436 nm (table 1).
3.4. Electrochemistry
Electrochemical behaviors of the complexes have been studied in DMF and MeCN
solutions with 0.1 mol Lꢁ1 TBAP as supporting electrolyte. Reduction processes of the
complexes are not well-behaved in MeCN solution due to adsorption of the reduced
species onto the surface of the platinum electrode. In DMF solution, the complexes
display clear reduction processes, but do not exhibit oxidative waves due to the
insufficient anodic window of the solvent. So, the reduction processes were recorded
in DMF and oxidation processes in MeCN (table 2).
Ru-L1 exhibits a Ru(II)-centered reversible oxidation couple at 1.33 V (figure 4).
2þ
This potential is slightly more positive (by 50 mV) than that of RuðbipyÞ3 (þ1.28 V vs.
SCE) [28], but slightly more negative (by 60 mV) than that of the parent complex
[(bipy)2Ru(dafone)]2þ [25], which indicates that L1 is a stronger ꢄ-acceptor than
bipy but a weaker ꢄ-acceptor than dafone. In this study, the complexes show a single
wave in cyclic voltammetry and a single peak without broadening in differential

214
F. Cheng et al.
Table 2. Redox potentials of the Ru(II) polypyridine complexes.a
E1/2 (V) (DEp, mV)
Complex
Oxidation
Reduction
Ru-L1
Ru-L2
Ru-L3
Ru-L4
1.33 (61)
1.33 (64)
1.34 (67)
1.33 (62)
ꢁ0.84
ꢁ0.84
ꢁ0.86
ꢁ0.85
ꢁ1.41 (90)
ꢁ1.67 (82)
ꢁ1.67 (84)
ꢁ1.68 (92)
ꢁ1.67 (89)
ꢁ1.41 (120)
ꢁ1.42 (97)
ꢁ1.40 (102)
Oxidation potentials are recorded in 0.1 mol Lꢁ1 TBAP/CH3CN, reduction poten-
tials are recorded in 0.1 mol Lꢁ1 TBAP/DMF and potentials are given vs. SCE; scan
rate ¼ 200 mVsꢁ1; E1/2 refers to (Epa þ Epc)/2, where Epa and Epc are the anodic and
cathodic peak potentials, respectively; DEp ¼ Epa ꢁ Epc
.
0.00003
0.00002
0.00001
0.00000
1.0
1.2
1.4
1.6
–0.00001
–0.00002
–0.00003
–0.00004
–0.00005
Potential (V)
0.00005
0.00004
0.00003
0.00002
0.00001
0.00000
–0.00001
–0.6
–0.9
–1.2
–1.5
–1.8
Potential (V)
Figure 4. Cyclic voltammetry of Ru-L1; oxidation potential was recorded in 0.1 mol Lꢁ1 TBAP/CH3CN,
reduction potentials were recorded in 0.1 mol Lꢁ1 TBAP/DMF.
pulse voltammetry. A three-electron process for each oxidation wave of the complexes
was confirmed by coulometry. So, the oxidation wave can be ascribed to a three-
electron reversible process.
In the reduction processes of the complexes, the first reduction process is irreversible
due to adsorption of reduced species onto the surface of the platinum electrode.

Trinuclear Ru(II) polypyridine complexes
215
Table 3. Predicted UV-Vis absorption maxima from electrochemistry data.
E1/2 (V)
1st reduction
1st
oxidation
DE1/2(1)
ꢅ1max
DE1/2(2)
ꢅ2max
Exptl ꢅmax
(nm)
Complex
L
bpy
(V)a
(nm)b
(V)c
(nm)d
Ru-L1
Ru-L2
Ru-L3
Ru-L4
1.33
1.33
1.34
1.33
1.28
ꢁ0.84
ꢁ0.84
ꢁ0.86
ꢁ0.85
ꢁ1.41
ꢁ1.41
ꢁ1.42
ꢁ1.40
ꢁ1.32
2.17
2.17
2.20
2.18
573
573
565
570
2.74
2.74
2.76
2.73
2.60
453
453
450
455
478
443
443
443
442
450
2þ
RuðbpyÞ3
aDE1/2(1) ¼ E1/2 (1st oxidation) ꢁ E1/2 (1st L reduction).
bꢅ1max was calculated on the basis of DE1/2(1) data.
cE1/2(2) ¼ E1/2 (1st oxidation) ꢁ E1/2 (1st bipy reduction).
dꢅ2max was calculated on the basis of DE1/2(2) data.
Diazafluorene derivatives are known to be lower than 2,20-bipyridine in the spectro-
chemical series [25] and promoted electrons should be associated with the phenylimino-
4,5-diazafluorene of L1, so the first reduction process of Ru-L1 at ꢁ0.84 V is consistent
with the addition of electrons to the LUMO localized on L1, giving
[(bipy)2RuIIL3ꢁRuII(bipy)2RuII(bipy)2]3þ. The second quasi-reversible reduction pro-
cess at ꢁ1.41 V is located on one of the two 2,20-bipyridine ligands on each metal
terminal, adding electrons to the 2,20-bipyridine localized LUMO þ 1 orbitals yielding
[(bipy)(bipy. ꢁ)RuIIL3ꢁRuII(bipy. ꢁ)(bipy)RuII(bipy. ꢁ)(bipy)]. Similar to the oxidation
process, the reductions of the remote 2,20-bipyridine appear at the same potential,
indicating little interaction between the three sites. The third reduction at ꢁ1.67 V is
quasi-reversible and affords [(bipy. ꢁ)(bipy. ꢁ)RuIIL3ꢁRuII(bipy. ꢁ)(bipy. ꢁ)RuII(bipy. ꢁ)
(bipy. ꢁ)]3ꢁ. It is well-established that the lowest energy MLCT transitions (ꢅmax) are,
for a related class of ligands, linearly related to the difference between the potential
of the first reduction (LUMO) and the first oxidation (HOMO) (DE1/2) [29]. In all the
complexes reported here, the oxidation is localized on Ru(II) and the first reduction is
localized on the tripodal ligand. Electrochemical data are used to calculate the expected
energy transitions for the MLCT bands and are listed in table 3. As a result, the lowest
energy MLCT transition of Ru-L1 is expected to be dꢄ(Ru) ! ꢄ*(L) at 573 nm, but
experimentally no absorption in this region is observed. The MLCT band more
closely matches the dꢄ(Ru) ! ꢄ*(bipy) transition of RuðbipyÞ23þ, which suggests the
dꢄ(Ru) ! ꢄ*(L) transitions are forbidden. The electrochemical behaviors of Ru-L2–4
are similar to that of Ru-L1.
4. Conclusion
Four trinuclear Ru(II) complexes derived from 9-(2-hydroxy)phenylimino-4,5-
diazafluorene have been synthesized and characterized. These complexes show
2þ
rich photophysical and redox properties. Replacement of bipy in RuðbipyÞ3 based
complexes for a 4,5-diazafluorene based ligand leads to dramatic change of the
photophysical and redox behavior. Electrochemical properties show little interaction

216
F. Cheng et al.
between the four Ru(II) polypyridine complexes. It is well-documented that an
interaction of a few reciprocal centimeters (which cannot be noticed in spectroscopic
and electrochemical experiments) is sufficient to cause fast intercomponent electron
or energy transfer processes [30], so the complexes have potential applications in
photoinduced electron or energy transfer.
Supplementary material
1H NMR and MS of the ligands and Ru(II) complexes are included.
Acknowledgments
We are grateful to the Yunnan Provincial Science and Technology Department
(2010ZC148, 2006B0081M, 2009ZD008) for the financial support.
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