Synthesis and characterization of a series of transition metal polypyridyl complexes and the pH-induced luminescence switch of Zn(II) and Ru(II) complexes

Article abstract of DOI:10.1016/j.poly.2011.11.049

Five new transition metal complexes, formulated as [Zn(DMBPIP) ₃](PF₆)₂ (1), [Ni(DMBPIP)₃](PF ₆)₂ (2), [Co(DMBPIP)₃](PF₆) ₂ (3), [Fe(DMBPIP)₃](PF₆)₂ (4), [Ru(DMBPIP)₃](PF₆)₂ (5) {where DMBPIP = 2-(3′,5′-dimethoxybiphenyl-4-yl)-1H-imidazo[4,5-f] [1,10]phenanthroline} were synthesized and characterized by means of elemental analysis, IR, ¹H NMR, ¹³C{¹H} NMR, standard spectroscopy techniques and cyclic voltammetry. The pH effects on UV-Vis absorption and luminescence spectra of the Zn(II) and Ru(II) complexes are studied, and ground- and excited-state ionization constants of the two complexes are derived. The two complexes acting as pH-induced "off-on-off" luminescence switches through protonation and deprotonation in buffer solution at room temperature are studied and discussed as well.

Full text of DOI:10.1016/j.poly.2011.11.049

Polyhedron 33 (2012) 347–352
Contents lists available at SciVerse ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Synthesis and characterization of a series of transition metal polypyridyl
complexes and the pH-induced luminescence switch of Zn(II) and Ru(II) complexes
⇑
Hui-Hua Xu, Xian Tao, Yue-Qin Li, Ying-Zhong Shen , Yan-Hong Wei
Applied Chemistry Department, School of Material Science & Engineering, Nanjing University of Aeronautics & Astronautics, Nanjing 210016, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Five new transition metal complexes, formulated as [Zn(DMBPIP)3](PF6)2 (1), [Ni(DMBPIP)3](PF6)2 (2),
[Co(DMBPIP)3](PF6)2 (3), [Fe(DMBPIP)3](PF6)2 (4), [Ru(DMBPIP)3](PF6)2 (5) {where DMBPIP = 2-(30,50-dime-
thoxybiphenyl-4-yl)-1H-imidazo[4,5-f] [1,10]phenanthroline} were synthesized and characterized by
means of elemental analysis, IR, 1H NMR, 13C{1H} NMR, standard spectroscopy techniques and cyclic vol-
tammetry. The pH effects on UV–Vis absorption and luminescence spectra of the Zn(II) and Ru(II) com-
plexes are studied, and ground- and excited-state ionization constants of the two complexes are derived.
The two complexes acting as pH-induced ‘‘off–on–off’’ luminescence switches through protonation and
deprotonation in buffer solution at room temperature are studied and discussed as well.
Ó 2011 Elsevier Ltd. All rights reserved.
Received 15 September 2011
Accepted 25 November 2011
Available online 3 December 2011
Keywords:
Imidazo[4,5-f][1,10]phenanthroline
Transition metal complex
UV–Vis absorption
Luminescence
Fluorescence
pH switch
1. Introduction
control orbital energies by proton transfer [8]. Their metal com-
plexes with imidazole rings uncoordinated to the metal center
Small molecules that can be used as sensors, switches and logic
gates has received much attention in the last three decades [1,2],
which are of tremendous significance to the development of min-
iaturized device components [3]. Molecular switches, which can be
converted from one state to another through molecular level prop-
erty changes by external stimuli such as light, electric field or
chemical reaction, have attracted a great deal of attentions [4].
Transfer of protons as an external stimulation has attracted a lot
of interest because it could be simply controlled and can induce
the switching of properties such as luminescence and UV–Vis
absorption for pH sensors. The pH sensors are not only fundamen-
tal molecular switches but also can be used to measure pH and
pCO2 in the biological and environmental systems [5]. Besides,
the pH responsive luminescence switches could be used to mimic
many of life processes, such as the functions and activities of
‘‘switch on’’ or ‘‘switch off’’ enzymes within a narrow pH window
[6,7]. However, it is noticed that many pH sensor researches have
primarily focused on organic molecules. Investigations of transi-
tion metal complexes have attracted less attention and their great
potential as pH sensors has not fully explored. In order to synthe-
size this class of sensors, the most import factor is the introduction
of a pH sensitive functional group into the ligand such as polypyr-
are good emitters with proton induced on–off emission switching
characteristics through a reversible acid–base interconversion of
imidazole group. Several mononuclear and dinuclear metal poly-
pyridine complexes of this type have been reported [8–14].
In this paper, a new imidazo[4,5-f][1,10]phenanthroline deriva-
tive, namely 2-(30,50-dimethoxybiphenyl-4-yl)-1H-imidazo[4,5-
f][1,10]phenanthroline (DMBPIP) and its transition metal com-
plexes [M(DMBPIP)3](PF6)2 (M = Zn, Ni, Co, Fe, Ru) were synthesized
and characterized. And UV–Vis absorption and luminescence spec-
tra of the Zn(II) and Ru(II) complexes in response to pH changes
are studied as well.
2. Experimental
1,10-Phenanthroline-5,6-dione [15], 3,5-dimethoxyphenylbo-
ronic acid [16] were synthesized according to the literature meth-
ods. All other chemicals used were of analytical reagent grade
quality from commercial sources and were used without further
purification.
Melting points were measured on a WRS-1B digital melting point
apparatus and are uncorrected. The 1H NMR and 13C NMR spectra
were recorded on a Bruker Advance 300 or a Bruker DRX500 spec-
trometer in DMSO-d6 solution with TMS as internal standard. IR
spectra were recorded on a Bruker Vector 22 as KBr pellets in the
400–4000 cmꢀ1 region. Elemental analyses were carried out on a
Perkin-Elmer 240 C elemental analyzer. Absorption spectra were
idine. Imidazo[4,5-f][1,10]phenanthroline derivatives are poor
p-
acceptors and good
p-donors and have the appreciable ability to
⇑
Corresponding author. Tel.: +86 25 52112906/84765; fax: +86 25 52112626.
E-mail address: yz_shen@nuaa.edu.cn (Y.-Z. Shen).
recorded with
a
UV-2550 UV–Visible spectrophotometer.
0277-5387/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2011.11.049

348
H.-H. Xu et al. / Polyhedron 33 (2012) 347–352
Luminescence spectra were obtained with an Eclipse Fluorescence
spectrometer. Cyclic voltammetry was recorded on a CHI 660C elec-
trochemical work station using a platinum wire as working elec-
trode, a platinum plate as counter electrode, and an Ag/AgCl
electrode as the reference. 0.1 M tetra-n-butylammonium perchlo-
rate (n-Bu4NClO4) dissolved in anhydrous acetonitrile was em-
ployed as the supporting electrolyte. The concentration of the
complexes were 2 ꢁ 10ꢀ3 mol/L. Prior to electrochemical measure-
ment, the solution was deoxygenated with bubbling nitrogen for
30 min.
>350 °C. Anal. Calc. for C81H60N12O6P2F12Zn: C, 58.87; H, 3.66; N,
10.17. Found: C, 58.79; H, 3.62; N, 10.12%. 1H NMR (DMSO-d6,
300.13 MHz): d 9.28 (d, J = 8.0 Hz, Hc, 2 H), 8.47–7.80 (m, Hd, Ha,
Hb, He, 8 H), 6.90 (s, Hf, 2 H), 6.56 (s, Hg, 1 H), 3.84 (s, OCH3, 6
H). 13C NMR (DMSO-d6, 75.47 MHz): d 160.3, 151.3, 145.9, 145.8,
140.9, 140.6, 137.1, 133.5, 127.9, 126.8, 126.2, 125.6, 104.2, 99.0,
54.7 ppm. IR (KBr):
m = 3382 (w), 3085 (w), 2939 (w), 2839 (w),
1605 (m), 1456 (m), 1156 (m), 849 (s), 735 (m), 559 (m) cmꢀ1
.
2.4. Synthesis of [Ni(DMBPIP)3](PF6)2 (2)
The pH dependence of the ground-state and excited-state prop-
erties of the complexes was studied with UV–Vis absorption and
luminescence spectra, respectively. UV–Vis absorption and lumi-
nescence pH spectroscopic titrations of the two complexes were
carried out in acetonitrile–Briton–Robinson buffer (1:1) solution
with 0.2 mol/L NaCl to maintain a constant ionic strength. The
pH changes were also measured with a PHS-3C pH meter. All spec-
tral changes with pH were reversible.
The synthesis of 2 was similar to that of 1 except that NiCl2ꢂ2H2O
(0.1185 g, 0.5 mmol) was used instead of Zn(OAc)2ꢂ2H2O. Yield:
0.6448 g, 78.4% based on DMBPIP. M.p.: >350 °C. Anal. Calc. for
C
81H60N12O6P2F12Ni: C, 59.10; H, 3.67; N, 10.20. Found: C, 59.15;
H, 3.69; N, 10.14%. IR (KBr): = 3376 (w), 3082 (w), 2939 (w),
2838 (w), 1605 (m), 1456 (m), 1155 (m), 847 (s), 734 (m), 558 (m)
cmꢀ1
m
.
2.1. Synthesis of 30,50-dimethoxybiphenyl-4-carbaldehyde
2.5. Synthesis of [Co(DMBPIP)3](PF6)2 (3)
4-Bromobenzaldehyde (4.63 g, 25 mmol) and 3,5-dimethoxyph
enylboronic acid (5.46 g, 30 mmol) were dissolved in toluene
(100 mL). A solution of 1 M K2CO3 (50 mL) was added, and the mix-
ture was stirred with nitrogen bubbling for a while, then Ph(PPh3)4
(0.4634 g, 0.4 mmol) was added. The mixture was refluxed for
10 h in an atmosphere of nitrogen. After cooling to the room temper-
ature, the organic layer was separated and the aqueous layer was ex-
tracted twice with 20 mL portions of toluene. The combined organic
layer was washed twice with NaHCO3 solution and water respec-
tively, then dried over MgSO4, filtered and the solvent was evapo-
rated. Recrystallized from petroleum ether/dichloromethane (1:4,
v/v), the pure 30,50-dimethoxybiphenyl-4-carbaldehyde (4.31 g,
71.2% based on 4-bromobenzaldehyde) was obtained as a white so-
lid. Mp: 68.1–68.5 °C.
The synthesis of 3 was similar to that of 1 except that CoCl2ꢂ6H2O
(0.1186 g, 0.5 mmol) was used instead of Zn(OAc)2ꢂ2H2O. Yield:
0.6546 g, 79.5% based on DMBPIP. Mp: >350 °C. Anal. Calc. for
C
81H60N12O6P2F12Co: C, 59.10; H, 3.67; N, 10.20. Found: C, 59.16;
H, 3.64; N, 10.17%. IR (KBr): = 3377 (w), 3080 (w), 2937 (w),
2838 (w), 1605 (m), 1456 (m), 1156 (m), 846 (s), 734 (m), 558 (m)
cmꢀ1
m
.
2.6. Synthesis of [Fe(DMBPIP)3](PF6)2 (4)
The synthesis of 4 was similar to that of 1 except that FeCl2ꢂ4H2O
(0.0997 g, 0.5 mmol) was used instead of Zn(OAc)2ꢂ2H2O. Yield:
0.6628 g, 80.7% based on DMBPIP. Mp: >350 °C. Anal. Calc. for
C
81H60N12O6P2F12Fe: C, 59.20; H, 3.68; N, 10.22. Found: C, 59.17;
H, 3.67; N, 10.25%. 1H NMR (DMSO-d6, 500.13 MHz): d 9.06–8.96
(m, Hc, 2 H), 8.42–8.39 (m, Hd, 2 H), 7.96–7.64 (m, Ha, Hb, He, 6H),
6.91 (s, Hf, 2H), 6.57 (s, Hg, 1H), 3.85 (s, OCH3, 6H) ppm. 13C NMR
(DMSO-d6, 75.47 MHz): d 162.9, 161.6, 152.4, 147.5, 142.2, 131.2,
131.2, 128.0, 127.9, 127.5, 127.2, 126.2, 106.5, 100.2, 55.9 ppm. IR
2.2. Synthesis of 2-(30,50-dimethoxybiphenyl-4-yl)-1H-imidazo[4,5-
f][1,10]phenanthroline (DMBPIP)
A mixture of 30,50-dimethoxybiphenyl-4-carbaldehyde (2.10 g,
10 mmol), 1,10-phenanthroline-5,6-dione (3.39 g, 14 mmol), ammo-
nium acetate (15.42 g, 200 mmol) in glacial acetic acid (100 mL) was
refluxed for about 5 h. After cooling to room temperature, the reaction
mixture was poured into 50 mL of distilled water and neutralized to
pH = 7 with an aqueous ammonia solution. The precipitate was filtered
off, washed with distilled water, and finally recrystallized from petro-
leum ether/ethanol (1:10, v/v) to afford 2-(30,50-dimethoxybiphenyl-4-
yl)-1H-imidazo[4,5-f][1,10]phenanthroline as a yellow product. Yield:
3.25 g (75.2% based on 30,50-dimethoxybiphenyl-4-carbaldehyde). Mp:
>350 °C. Anal.Calc. for C27H20N4O2: C, 74.98; H, 4.66; N, 12.95. Found: C,
74.87; H, 4.61; N, 12.98%. 1H NMR (DMSO-d6, 300.13 MHz): d 13.82 (s,
N–H, 1 H), 9.09–9.04 (m, Ha, 2 H), 8.97–8.94 (m, Hc, 2 H), 8.38 (d,
J = 8.5 Hz, Hd, 2 H), 7.96 (d, J = 8.5 Hz, Hb, 2 H), 7.90–7.81 (m, He, 2 H),
6.94 (d, J = 2.2 Hz, Hf, 2 H), 6.57 (t, J = 2.2 Hz, Hg, 1 H), 3.86 (s, OCH3, 6
(KBr):
m = 3377 (w), 3079 (w), 2936 (w), 2836 (w), 1602 (m), 1458
(m), 1155 (m), 845 (s), 727 (m), 558 (m) cmꢀ1
.
2.7. Synthesis of [Ru(DMBPIP)3](PF6)2 (5)
The synthesis of 5 was similar to that of 1 except that RuCl3ꢂ3H2O
(0.1278 g, 0.5 mmol) was used instead of Zn(OAc)2ꢂ2H2O and
dimethylformamide was used instead of ethanol. Yield: 0.6046 g,
71.6% based on DMBPIP. Mp: >350 °C. Anal. Calc. for C81H60N12O6P2-
F12Ru: C, 57.62; H, 3.58; N, 9.95. Found: C, 57.59; H, 3.60; N, 9.99%.1H
NMR (DMSO-d6, 500.13 MHz): d 9.11–9.03 (m, Hc, 2 H), 8.43–8.37
(m, Hd, 2 H), 8.06–7.96 (m, Ha, Hb, 4 H), 7.82–7.80 (m, He, 2 H),
6.94 (s, Hf, 2 H), 6.59 (s, Hg, 1 H), 3.86 (s, OCH3, 6 H). 13C NMR
(DMSO-d6, 75.47 MHz): d 162.9, 161.6, 150.6, 148.4, 145.9, 141.9,
130.9, 130.2, 128.1, 127.6, 127.2, 126.7, 105.6, 100.4, 55.9 ppm. IR
H). IR (KBr):
m = 3086 (w), 2939 (w), 2835 (w), 1599 (m), 1453 (m),
1157 (m), 742 (m) cmꢀ1
.
(KBr):
m = 3393 (w), 3081 (w), 2938 (w), 2838 (w), 1602 (m), 1458
(m), 1156 (m), 848 (s), 725 (m), 558 (m) cmꢀ1
.
2.3. Synthesis of [Zn(DMBPIP)3](PF6)2 (1)
A DMF (10 mL) solution of DMBPIP (0.6485 g, 1.5 mmol) was
added dropwise to Zn(OAc)2ꢂ2H2O (0.1095 g, 0.5 mmol) dissolved
in DMF (10 mL) and refluxed for 12 h. The resulting clear solution
was cooled to room temperature and treated in situ anion ex-
change reaction by adding excess NH4PF6 aqueous solution. A yel-
low precipitate was filtered, washed with water and diethyl ether,
and dried under vacuum (0.6596 g, 79.8% based on DMBPIP). Mp:
3. Results and discussion
3.1. Synthesis and characterization
An outline of the synthesis of the ligand 2-(30,50-dimethoxybi-
phenyl-4-yl)-1H-imidazo[4,5-f][1,10]phenanthroline (DMBPIP) is
presented in Scheme 1, and the chemical structure of the related

H.-H. Xu et al. / Polyhedron 33 (2012) 347–352
349
complexes was shown in Fig. 1. 30,50-Dimethoxybiphenyl-4-carb-
H3CO
2+ , 2PF6
g
f
-
aldehyde was prepared according to a Suzuki cross-coupling reac-
tion starting from 4-bromobenzaldehyde and 3,5-dimetho xyphe
nylboronic acid. The ligand DMBPIP was prepared by a condensa-
tion reaction between one equivalent of 1,10-phenanthroline-5,6-
dione and one equivalent of 30,50-dimethoxybiphenyl-4-carbalde-
hyde in the presence of an excess of ammonium acetate in reflux-
ing acetic acid. All the related transitional metal complexes 1–5
were produced by refluxing three equivalents of DMBPIP with
one equivalent of each metal chloride or metal acetate in DMF,
respectively and then treating in situ anion exchange reaction
by adding excess NH4PF6 in order to precipitate the complexes
from their mother solution.
OCH3
e
d
N
NH
c
N
H3CO
H3CO
H
N
N
N
N
b
a
M
N
N
N
The 1H NMR, 13C NMR data and the important IR frequencies
are given in Section 2, and the 1H NMR spectra of [Fe(DMB-
PIP)3](PF6)2 (4) and DMBPIP are compared in Fig. S1, the charac-
teristic infrared absorption bands of the free ligand DMBPIP and
its complexes are given in Table 1. In general, the NMR and IR
spectra of the complexes are quite similar and can be used to de-
tect the coordination of phenanthroline nitrogens by comparison
of the free ligand. The 1H NMR spectrum of DMBPIP displays 7
signals in the aromatic region between 9.09 and 6.57 ppm with
the expected multiplicities, a singlet at 13.82 ppm for the N–H
peak and a singlet at 3.86 ppm for the methoxy peak. The 1H
NMR spectra of the complexes shows the expected peaks in the
aromatic and aliphatic regions as well. Upon M(II) coordination,
the Ha chemical shift for DMBPIP experienced a large upfield shift.
This is because the coordination of nitrogen atoms on the aro-
matic ring to M(II) leads to the electron deficiency of Ha, and
accordingly the deshielding effect on the protons of Ha [17,18].
In addition, the chemical shifts of N–H protons in the complexes
have not been examined, probably because this proton is very ac-
tive and exchanges quickly between the two nitrogens of the
imidazole ring in solution [19,20]. As can be seen from Table 1,
the IR spectra of the complexes show bands at 1602–1605 cmꢀ1
(C@N) and 1456–1458 cmꢀ1 (C@C), all shifted to lower frequen-
cies when compared to free ligand DMBPIP, suggesting complex-
ation. A new band at 558–559 cmꢀ1 can be assigned to M–N
stretching and a strong band at 845–849 cmꢀ1 can be ascribed
N
DMBPIP
HN
OCH3
H3CO
Fig. 1. The chemical structure of [M(DMBPIP)3](PF6)2 (M = Zn, Ni, Co, Fe, Ru)
(general atom numbering also given for NMR description).
Table 1
Characteristic IR bands (cmꢀ1) of the ligand and its complexes.
ꢀ
Sample
m
(C@N)
m
(C@C)
PF6
m (M–N)
DMBPIP
1599
1605
1605
1605
1602
1602
1453
1456
1456
1456
1458
1458
1
2
3
4
5
849
847
846
845
848
559
558
558
558
558
3.2. Absorption and emission spectra
The UV–Vis absorption spectra of complexes 1–5 in CH3CN
solution at room temperature are displayed in Figs. 2 and 3, respec-
ꢀ
to the counter anion PF6 [21].
OCH3
OCH3
OCH3
OCH3
OCH3
Br
CHO
Mg / B(OCH3)3
THF
Br
(HO)2B
OHC
Pd(PPh3)4 / Toluene
K2CO3 / H2O
OCH3
N
N
N
N
O
O
KBr
HNO3 / H2SO4
OCH3
N
N
NH4Ac
HAc
N
N
H
OCH3
Scheme 1. The synthetic route of ligand DMBPIP.

350
H.-H. Xu et al. / Polyhedron 33 (2012) 347–352
tively. The corresponding spectral data are listed in Table 1. All the
complexes show three intense bands around 240–250 nm, 290–
301 nm and 314–332 nm, which could be assigned to intraligand
charge transfer (ILCT). Low-energy absorptions were observed at
531 nm for the Fe(II) complex 4 and 466 nm for the Ru(II) complex
5, corresponding to the metal–ligand charge transfer (MLCT)
absorptions. These values and assignments are consistent with
those of a number of related complexes in the literature [22–24].
The luminescence emission spectra of the complexes 1 and 5 in
CH3CN solution are shown in Fig. 3. It can be seen from Fig. 3 and
Table 1, the complex 1 exhibits maximum emission band at
485 nm in CH3CN with excitation wavelength at 330 nm. As Zn(II)
is difficult to oxidize or to reduce owing to their d10 electronic con-
figuration, the emission of 1 should be neither ligand–metal charge
transfer (LMCT) nor metal–ligand charge transfer (MLCT) in nature.
It is tentatively attributed to the intraligand charge transfer (ILCT)
of ligands [25]. A CH3CN solution of complex 5 on excitation at
465 nm exhibited an emission maximum at 600 nm, which could
be assigned as a triplet metal–ligand charge-transfer transition
(3MLCT), on the basis of literature assignment of analogous Ru(II)
metal complexes [26,27]. These results indicate that the complexes
1 and 5 emit cyan/orange luminescence in CH3CN solution respec-
tively. For the complexes 2, 3 and 4, there are no emission lumines-
cence spectra can be observed. The reasons are that Ni(II), Co(II)
and Fe(II) are open-shell cations with available oxidation states
[28] and that the transition metal (M)-fluorophore (F) communica-
tion is too strong compared with the other interactions for com-
monly used receptor (R) [29].
1
1
2.0
1.5
1.0
0.5
0.0
(a.u.)
1
5
5
0.5
300
400
500
600
700
800
Wavelength/nm
Fig. 3. UV–Vis absorption and normalized luminescence spectra of complexes
[Zn(DMBPIP)3](PF6)2 (1) and [Ru(DMBPIP)3](PF6)2 (5) in CH3CN with the concen-
tration of 2 ꢁ 10ꢀ5 M at room temperature.
to ꢀ1.67 V, which can be assigned to the reduction of the ligand
DMBPIP.
3.4. PH effects on UV–Vis and emission spectra
The UV–Vis absorption spectra of [Zn(DMBPIP)3](PF6)2 (1) and
[Ru(DMBPIP)3](PF6)2 (5) in neutral CH3CN/H2O (1:1, v/v) are shown
in Figs. S2 and 4, respectively. The two complexes exhibit intrali-
gand
p–
p⁄ absorption band at 222–228, 283–291 and 317–
324 nm. The metal–ligand charge transfer (MLCT) transition band
of complex 5 is shown at 466 nm. The emission peak of complex
1 and 5 in CH3CN/H2O (1:1, v/v) solution at pH = 7 located at 450
and 600 nm, respectively (Figs. S2 and 4).
3.3. Electrochemistry
Cyclic voltammograms of the complexes 2–5 are compared Ta-
ble 2. The Ni(II), Co(II), Fe(II), Ru(II) complexes display metal-based
irreversible cyclic voltammetric response in DMF–0.1 M TBAP. The
Zn(II) complex 1 does not show any metal-based redox process,
suggesting the stability of the bivalent metal oxidation state in
the ZnII species [30]. The Ni(II) complex 2 undergoes oxidation to
the NiIII species giving an E1/2 value of 0.98 V for the NiII/NiIII couple
[31]. The CoII/CoIII oxidation occurs at a higher potential, giving an
E1/2 value of 1.02 V. The FeII/FeIII couple is observed at 1.15 V. Such
a high redox potential renders stability to the FeII state with a low-
spin electronic configuration. The RuII/RuIII oxidation occurs at
0.93 V versus Ag/AgCl electrode which is lower than that of [Ru(b-
The UV–Vis absorption spectra of complex 1 and 5 in CH3CN/
Britton–Robinson (BR) buffer (1:1, v/v) as a function of pH are
shown in Figs. S3 and 5, respectively. It is clear from Figs. S3 and
5 that the two complexes underwent two successive deprotonation
processes over a pH range of 1.8–11.9. For 1, upon raising pH from
1.8 to 6.6, the
p–
p⁄ transitions at 222, 283 and 324 nm were all
slightly increasing in the intensities (Fig. S3a). Spectral changes
were attributed to the concurrent dissociation of three protons
on protonated imidazole rings. Further increasing pH from 6.6 to
11.9, the deprotonation on neutral imidazole rings occurred. Band
at 222 nm was obviously decreasing in the intensities. Bands at
283 and 324 nm were red-shifted to 289 and 337 nm, respectively,
and were slightly decreasing in the intensities (Fig. S3b). Quantita-
tive analysis of the absorption changes at 280 nm does show two
deprotonation processes, as shown by nonlinear sigmoidal fits of
the data in the inset of Fig. S3. So, two ground-state ionization con-
stants of pKa1 = 4.32, pKa2 = 10.28 were obtained. Absorption spec-
tral changes of complex 5 are similar to that of complex 1. Upon
raising pH from 1.8 to 6.8, the absorbance for the MLCT band at
py)3]2+ [32] complex due to the presence of more delocalized
systems. At negative potentials, each complex shows well shaped
reduction (one) waves with E1/2 in the potential range of ꢀ1.11
p
3.5
2
3
4
3.0
2.5
2.0
1.5
1.0
0.5
0.0
466 nm and the
p–
p⁄ transitions at 228, 291 and 317 nm were
all slightly increasing in the intensities (Fig. 5a). Further increasing
pH from 6.8 to 11.9, the absorption intensity at 228 nm was
slightly decreasing. Bands at 291, 317 and 466 nm were red-shifted
to 301, 344 and 485 nm, respectively, and were decreasing in the
intensity (Fig. 5b). Two ground-state ionization constants were de-
rived to be pKa1 = 4.36 and pKa2 = 8.04 by nonlinear sigmoidal fit of
the data in the insets of Fig. 5.
Luminescence spectral changes of complexes 1 and 5 in CH3CN/
Britton–Robinson (BR) buffer (1:1, v/v) as a function of pH is shown
in Figs. S4 and 6, respectively. For 1, upon raising pH from 1.8 to
6.8, emission intensity increased by about 14.5-fold, and the emis-
sion maxima were blue-shifted from 600 to 442 nm (Fig. S4a).
Complex 1 acted as an off–on switch over this pH range. Upon
300
400
500
600
Wavelength/nm
Fig. 2. UV–Vis absorption spectra of complexes [Ni(DMBPIP)3](PF6)2 (2), [Co(DMB-
PIP)3](PF6)2 (3) and [Fe(DMBPIP)3](PF6)2 (4) in CH3CN with the concentration of
2 ꢁ 10ꢀ5 M.

H.-H. Xu et al. / Polyhedron 33 (2012) 347–352
351
Table 2
Optical and electrochemical data of the complexes.
Compound
Absorptiona
kabs (nm) (
Emissiona
kmax (nm)
E1/2Ox/Vb
E1/2Red/Vb
e
(1000 L molꢀ1 cmꢀ1))
1
2
3
4
5
249 (43), 290 (109), 332 (107)
246 (69), 292 (169), 330 (165)
250 (52), 291 (130), 331 (132)
247 (60), 301 (137), 321 (139), 531 (19)
246 (73), 293 (180), 314 (160), 466 (33)
485
–
–
–
600
–
–
0.98
1.02
1.15
0.93
ꢀ1.53
ꢀ1.16
ꢀ1.60
ꢀ1.57
a
The data were obtained in CH3CN at room temperature.
The electrochemical data were measured in DMF containing a 0.1 M tetra-n-butylammonium perchlorate using a platinum wire electrode as a working electrode and
b
were reported vs. Ag/AgCl electrode.
increasing pH from 8.7 to 11.9, the emission intensity was found to
decrease by about 83.0% (Fig. S4b). Over this pH region, complex 1
acted as an on–off emission switch. For 5, upon raising pH from 2.0
to 5.7, the intensity of the emission increased by about 14.4% with
the emission maxima blue-shifted from 601 to 598 nm (Fig. 6a).
Complex 5 acted as an off–on switch over this pH range, as shown
in the inset of Fig. 6a. On the contrary, the emission intensity was
found to sharply decrease by 57.3% as the pH further increased
from 5.7 to 10.9 with the emission maxima red-shifted from 598
to 604 nm (Fig. 6b). Over this pH region, complex 5 acted as an
on–off emission switch. This process may involve rapid radiation-
less decay [33]. It has been well known that the energy of the me-
tal-centered excited state depends on the ligand field strength. The
negative charge on the deprotonated imidazole rings can be delo-
1
3.0
2.5
2.0
1.5
1.0
0.5
0.0
(a.u.)
0.5
200
300
400
500
600
700
Wavelength/nm
calized over the whole
p framework, which lowers the ligand field
strength around the metal center and in turn lowering the metal r⁄
orbitals [34,35]. The emission intensities versus pH profiles of the
complexes (the insets of Figs. S4 and 6) shows that a luminescence
Fig. 4. UV–Vis absorption and normalized luminescence spectra of complex 5 in
neutral CH3CN/H2O (1:1, v/v) at room temperature.
240
(a)
(a)
3
200
1.6
220
2
2
4
6
pH
2
4
6
100
pH
1
0
500
550
600
650
700
750
800
850
200
300
400
500
600
Wavelength/nm
Wavelength/nm
(b)
(b)
1.6
1.4
1.2
1.0
200
3
2
1
0
200
100
100
6
8
10
pH
7
8
9
10
11
12
pH
500
550
600
650
700
750
800
850
200
300
400
Wavelength/nm
500
600
Wavelength/nm
Fig. 5. Changes of UV–Vis absorption spectra of [Ru(DMBPIP)3](PF6)2 (5) upon
Fig. 6. Changes of emission spectra of [Ru(DMBPIP)3](PF6)2 (5) upon raising the pH:
raising the pH: (a) from 1.8 to 6.8, (b) from 6.8 to 11.9.
(a) from 2.0 to 5.7, (b) from 5.7 to 10.9.

352
H.-H. Xu et al. / Polyhedron 33 (2012) 347–352
off–on–off switching action was achieved by the two deprotona-
tion processes of the protonated imidazole moieties on the
complexes.
References
[1] P. Goswami, D.K. Das, J. Lumin. 131 (2011) 760.
[2] A.P. de Silva, K.C. Loo, B. Amorelli, S.L. Pathirana, M. Nyakirang’ani, M.
Dharmasena, S. Demarais, B. Dorcley, P. Pullay, A. Salih, J. Mater. Chem. 15
(2005) 2791.
[3] Y. Shiraishi, R. Miyamot, T. Hirai, Tetrahedron Lett. 48 (2007) 6660.
[4] S. Xiao, T. Yi, Y. Zhou, Q. Zhao, F. Li, C. Huang, Tetrahedron 62 (2006) 10072.
[5] F. Cheng, N. Tang, J. Chen, F. Wang, L. Chen, Inorg. Chem. Commun. 14 (2011)
852.
[6] T. Gunnlaugsson, J.P. Leonard, K. Snchal, A.J. Harte, J. Am. Chem. Soc. 125
(2003) 12062.
[7] P. Pallavicini, V. Amendola, C. Massera, E. Mundum, A. Taglietti, Chem.
Commun. (2002) 2452.
[8] F. Liu, K. Wang, G. Bai, Y. Zhang, L. Gao, Inorg. Chem. 43 (2004) 1799.
[9] F. Cheng, N. Tang, J. Chen, F. Wang, L. Chen, Inorg. Chem. Commun. 13 (2010)
757.
[10] F. Cheng, N. Tang, Inorg. Chem. Commun. 11 (2008) 506.
[11] S.H. Fan, A.G. Zhang, C.C. Ju, L.H. Gao, K.Z. Wang, Inorg. Chem. 49 (2010) 3752.
[12] M.J. Han, L.H. Gao, Y.Y. Lü, K.Z. Wang, J. Phys. Chem. B 110 (2006) 2364.
[13] L. Xu, P.X. Liu, G.L. Liao, X. Chen, H. Chao, L.N. Ji, Aust. J. Chem. 63 (2010) 700.
[14] J. Gao, Z.P. Wang, C.L. Yuan, H.S. Jia, K.Z. Wang, Spectrochim. Acta, Part A 79
(2011) 1815.
[15] W. Paw, R. Eisenberg, Inorg. Chem. 36 (1997) 2287.
[16] R.M. Rosen, D.A. Wilson, C.J. Wilson, M. Peterca, B.C. Won, C. Huang, L.R. Lipski,
X. Zeng, G. Ungar, P.A. Heiney, V. Percec, J. Am. Chem. Soc. 131 (2009) 17500.
[17] H. Chao, R.H. Li, C.W. Jiang, H. Li, L.N. Ji, X.Y. Li, J. Chem. Soc., Dalton Trans. 1
(2001) 1920.
[18] Y.Z. Ma, H.J. Yin, K.Z. Wang, J. Phys. Chem. B 113 (2009) 11039.
[19] J.Z. Wu, B.H. Ye, L. Wang, L.N. Ji, J.Y. Zhou, R.H. Li, Z.Y. Zhou, J. Chem. Soc.,
Dalton Trans. (1997) 1395.
[20] L.F. Tan, X.H. Liu, H. Chao, L.N. Ji, J. Inorg. Biochem. 101 (2007) 56.
[21] S. Vidhisha, K.L. Reddy, K.A. Kumar, Y.P. Kumar, S. Satyanarayana, Transition
Met. Chem. 35 (2010) 1027.
[22] D. Pomeranc, V. Heitz, J.C. Chambron, J.P. Sauvage, J. Am. Chem. Soc. 123 (2001)
12215.
[23] C. Goze, J.C. Chambron, V. Heitz, D. Pomeranc, X.J. Salom-Roig, J.P. Sauvage, A.F.
Morales, F. Barigelletti, Eur. J. Inorg. Chem. 20 (2003) 3752.
[24] D. Martineau, M. Beley, P.C. Gros, S. Cazzanti, S. Caramori, C.A. Bignozzi, Inorg.
Chem. 46 (2007) 2272.
[25] X. Duan, Y. Li, S. Zang, C. Zhu, Q. Meng, CrystEngComm 9 (2007) 758.
[26] C. Girardot, G. Lemercier, J.C. Mulatier, J. Chauvin, P.L. Baldeck, C. Andraud,
Dalton Trans. 31 (2007) 3421.
[27] C. Goze, C. Sabatini, A. Barbieri, F. Barigelletti, R. Ziessel, Eur. J. Inorg .Chem.
(2008) 1293.
[28] Ö.A. Bozdemir, O. Büyükcakir, E.U. Akkaya, Chem. Eur. J. 15 (2009) 3830.
[29] Z. Xu, Y. Xiao, X. Qian, J. Cui, D. Cui, Org. Lett. 7 (2005) 889.
[30] R. Mithun, P. Biswarup, K.P. Ashis, D.J. Eluvathingal, N. Munirathinam, R.C.
Akhil, Inorg. Chem. 46 (2007) 11122.
Excited-state ionization constants, pKa⁄, could be correlated
with pKa thermodynamically on the basis of the Förster cycle
[36] by Eq. (1), in which T is temperature (298 K) and mB and mHB
are pure 0–0 transitions in cmꢀ1 for the deprotonated and the pro-
tonated forms, respectively. In practice, mB and mHB are often diffi-
cult or even impossible to obtain. A good approximation is to use
the emission maxima for mB and mHB since the protonation equilib-
rium is almost certainly established between the 3MLCT states
[37].
ꢃ
pKa ¼ pKa þ ð0:625=TÞðmB
ꢀ
mHB
Þ
ð1Þ
Therefore, for the Ru(II) complex 5, by using the emission band
maxima of both deprotonated and protonated species studied for
⁄
mB and mHB in Eq. (1), two excited state ionization constants pKa
values of pKa1⁄ = 4.54 and pKa2⁄ = 7.71 were obtained.
4. Conclusions
In summary, a series of transition metal complexes with 2-(30,50-
dimethoxybiphenyl-4-yl)-1H-imidazo[4,5-f][1,10]phenanthroline as
ligand have been synthesized and fully characterized. The lumines-
cence of complexes 1 and 5 emit cyan/orange luminescence at 485
and 600 nm in CH3CN solution respectively, while the other complexes
quench luminescence. The Zn(II) and Ru(II) complexes, in CH3CN/Brit-
ton–Robinson (BR) buffer (1:1, v/v) were found to exhibit two sepa-
rated protonation/deprotonation steps in ground and excited states,
respectively, and act as on–off–on type emission switches. They have
potential utility as a chemical sensor to detect pH variations of external
environment due to their interesting photon-dependent photophysical
properties.
Acknowledgement
[31] J.C. Philip, P. George, A.B. Christos, Bioorg. Med. Chem. 17 (2009) 6054.
[32] D.I. Simone, C.A.L. Francisco, B.F.S. Albérico, R.S. Andreza, C.T. Antônio, F.S.D.
Juliana, S.L. Benedito, M.C. Rose, Inorg. Chem. 46 (2007) 5744.
[33] A. Juris, V. Balzani, F. Barigelletti, S. Campagna, P. Belser, A. von Zelewsky,
Coord. Chem. Rev. 84 (1988) 85.
This work was supported by National Science and Technology of
Major Project (2009ZX02039101-02).
[34] R.H. Fabian, D.M. Klassen, R.W. Sonntag, Inorg. Chem. 19 (1980) 1977.
[35] J.M. Kelly, C. Long, C.M. O’Connell, J.M. Vos, H.A. Tinnemans, Inorg. Chem. 22
(1983) 2818.
Appendix A. Supplementary data
[36] E.V. Donckt, Progress in Reaction Kinetic, fifth ed., Pergamon Press, Oxford,
England, 1970 (Ed.: G. Poter).
[37] G.Y. Zheng, Y. Wang, D.P. Rillema, Inorg. Chem. 35 (1996) 7118.
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.poly.2011.11.049.