pH responsive off-on-off luminescent switch of a novel ruthenium(II) complex [Ru(bpy)2(pipipH2)]2+

Article abstract of DOI:10.1016/j.inoche.2006.10.009

A novel ruthenium(II) complex, [Ru(bpy)₂(pipipH₂)](ClO₄)₂ · H₂O (1) (pipipH₂ = 2-(4-(1H-phenanthro[9,10-d]imidazol-2-yl)phenyl)-1H- imidazo[4,5-f][1,10] phenanthroline, bpy = 2,2^′-bipyridine) has been found to act as a luminescent pH switch with extraordinary sensitivity through protonation and deprotonation of the bis-imidazole-containing ligand pipipH₂ in aqueous solution at room temperature.

Full text of DOI:10.1016/j.inoche.2006.10.009

Inorganic Chemistry Communications 10 (2007) 170–173
www.elsevier.com/locate/inoche
pH responsive ‘‘off–on–off’’ luminescent switch of a novel
ruthenium(II) complex [Ru(bpy)2(pipipH2)]2+
a,c,
a
a
a,b,
*
Feng Gao a,b, Hui Chao *, Feng Zhou , Bin Peng , Liang-Nian Ji
a
Key Laboratory of Bioinorganic and Synthetic Chemistry of Ministry of Education, School of Chemistry and Chemical Engineering, Sun Yat-Sen
University, XiGang XiLu 135#, Guangzhou 510275, China
b
Department of Chemistry, Tongji University, Shanghai 200092, China
State Key Laboratory of Coordination Chemistry, Nanjing University, Nanjing 210093, China
c
Received 18 September 2006; accepted 5 October 2006
Available online 20 October 2006
Abstract
A novel ruthenium(II) complex, [Ru(bpy)2(pipipH2)](ClO4)2 Æ H2O (1) (pipipH2 = 2-(4-(1H-phenanthro[9,10-d]imidazol-2-yl)phenyl)-
1H- imidazo[4,5-f][1,10] phenanthroline, bpy = 2,20-bipyridine) has been found to act as a luminescent pH switch with extraordinary sen-
sitivity through protonation and deprotonation of the bis-imidazole-containing ligand pipipH2 in aqueous solution at room temperature.
Ó 2006 Elsevier B.V. All rights reserved.
Keywords: Ruthenium(II) complex; Protonation; Deprotonation; Luminescent pH switch
Molecular systems that respond to external stimuli have
received much attention in recent years in connection with
the design of molecular switches and molecular machines
[1,2]. Transfer of protons can be regarded as one of the
simplest chemical signals. Many life processes, such as
enzymes, operate within a very narrow pH window, where
their function or activity can be described as being ‘‘on/off
switching’’ as a function of pH [3]. Attempts to mimic such
‘‘off–on–off’’ or ‘‘on–off–on’’ behavior by constructing lumi-
nescence devices that are modulated by a single input, e.g.,
pH, is of current interest [4–6]. However, it is noticed that
this activity has primarily focused on organic molecules.
Investigations of transition metal complexes have attracted
much less attention, and their vast potential as pH sensors
remains largely untapped. Recently, ruthenium complexes
have received considerable attention as pH sensors due to
their long excited-state lifetime and high luminescence
quantum yield [7–11]. In our previous study, a dinuclear
ruthenium(II) complex was developed to act as the pH sen-
sor in which ‘‘off–on–off’’ luminescent switching can
achieve via the protonation/deprotonation of imidazole-
containing ligand [11]. As part of an ongoing systematic
study aimed at the construction of sensitive pH-induced
luminescent sensors and pH luminescent switches and the
exploration of the underlying mechanism, we presented
here the synthesis and pH luminescent switch properties
of a mononuclear ruthenium(II) complex, [Ru(bpy)2(pip-
ipH2)](ClO4)2 Æ H2O (1), in which organic fluorescent group
phenanthrene was introduced at one terminal with the aim
of extending the p-electron systems and increasing the
energy gap between the luminescent levels. As expected,
complex 1 was found to be fully reversible pH controlled
‘‘off–on–off’’ luminescent signaling.
*
Corresponding authors. Address: Key Laboratory of Bioinorganic and
Synthetic Chemistry of Ministry of Education, School of Chemistry and
Chemical Engineering, Sun Yat-Sen University, XiGang XiLu 135#,
Guangzhou 510275, China. Fax: +86 20 84035497.
E-mail addresses: ceschh@mail.sysu.edu.cn (H. Chao), cesjln@mail.-
sysu.edu.cn (L.-N. Ji).
The synthesis of complex 1 was achieved as shown in
Scheme 1. The ligands were synthesized on the basis of
the method for imidazole ring preparation established by
Steck and Day [12]. 2-(4-formylphenyl)imidazo-[4,5-f]
1387-7003/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2006.10.009

F. Gao et al. / Inorganic Chemistry Communications 10 (2007) 170–173
171
0.4
OHC
CHO
H
0.10
0.08
0.06
N
N
O
O
N
N
(b)
(a)
CHO
N
N
NH4Ac, AcOH
0.3
0.2
0.1
0.0
02
4
6
8
10 12
pH
Ru(bpy)2Cl2
2+
N
H
N
N
N
N
CHO
Ru
N
N
N
300
400
500
600
Wavelength / nm
Fig. 1. The absorption spectra changes of complex 1 upon raising the pH.
(a) Spectra changes in the range of 1.52–9.43. (b) The pH (0.08–12.96)
effect on absorbance at 458 nm.
O
O
NH4Ac, AcOH
and luminescent spectra, respectively.2 All spectral
changes observed were reversible. Fig. 1a shows the
changes of the UV–Vis spectra of complex 1 in the pH
range 1.52–9.43 in aqueous Briton–Robinson buffer. An
increase in pH causes a much more pronounced reduction
in absorbance of the band at 286 nm (40%), 371 nm (43%)
and the MLCT band at 464 nm (44%). The MLCT band
displays a slight red shift (4 nm) with increasing pH. In
the pH range of 0.08–1.52 and 9.43–12.96, the increase
in pH causes significant increase in absorbance of all
the three bands. The above spectral changes are attributed
to the acid-base equilibria of complex 1 shown in Fig. 2.
Three ground-state ionization constants obtained are
pKa1 = 0.6, pKa2 = 4.7 and pKa3 = 10.7. The ligand pip-
ipH2 has two protonated sites and two deprotonated sites.
The first protonation (pKa2 = 4.7), according to the spec-
tral changes, should take place at the imidazole group of
uncoordinated subunit. On the contrary, only the dissoci-
ation of the imidazole group in the coordinated subunit
(pKa3 = 10.7) was observed in the pH range 0.08–12.96.
This result is rationalized by the fact that the pKa of
the imino NH protons of free benzimidazole derivatives
is usually around 14. Therefore, it shows clearly that in
complex 1 the imidazole of the coordinated subunit is a
stronger acid or a weaker base in comparison with that
of the uncoordinated subunit. In the process of the lumi-
nescence titration, the emission spectra of complex 1 are
strongly dependent on the buffer pH (Fig. 3) and three
excꢀited-state ionizꢀation constants are pKꢀa1 ¼ 0:7,
pKa2 ¼ 5:4 and pKa3 ¼ 11:4. The emission intensity vs.
pH profile of complex 1 (Fig. 3, inset), composed of three
sigmoidal curves representing three separate processes,
clearly shows the ‘‘off–on–off’’ switching process modulat-
2+
N
H
N
H
N
N
N
N
N
Ru
N
N
N
1
Scheme 1. Synthetic routines of Ru(II) complexes.
[1,10] phenanthroline (fmp) was obtained through conden-
sation of 1,10-phenanthroline-5,6-dione with terephthalic
aldehyde in refluxing glacial acetic acid containing ammo-
nium acetate at a molar ratio of 1:1. The ligand pipipH2
can be synthesized by the phenanthrene-9,10-dione with
fmp. However, pipipH2 is sparingly soluble in common
organic solvents. Therefore, to prepare the complex 1, we
have carried out the condensation of phenanthrene-9,10-
dione with the pre-coordinated fmp in [Ru(bpy)2(fmp)]2+
[13] as shown in Scheme 1. The product was purified by
column chromagraphy on alumina with acetonitrile as elu-
1
ent and characterized by H NMR, ES-MS, IR, UV–Vis
spectroscopy and elemental analyses.1 (Caution! Perchlo-
rate salts of metal complexes are potentially explosive
and should be handled in small quantity with care.)
The pH dependence of the ground-state and excited-
state properties of 1 were investigated using UV–Vis
1
Selected data for complex 1: Anal. Calcd for C54H36N10Cl2O8R-
u Æ H2O: C, 56.75; H, 3.35; N, 12.26. Found: C, 56.54; H, 3.41; N, 12.33%.
1H NMR (ppm, DMSO-d6): 14.12 (s, 1H), 13.85 (s, 1H), 9.10 (d, 2H,
J = 8.0), 8.86 (d, 2H,J = 8.0), 8.83 (d, 2H,J = 8.0), 8.61 (t, 2H), 8.53 (d,
4H,J = 8.0), 8.21 (t, 2H), 8.10 (t, 2H), 8.00 (d, 2H,J = 8.0), 7.90 (d,
2H,J = 8.0), 7.86 (d, 2H, J = 8.0), 7.76 (q, 4H), 7.62 (m, 4H), 7.56(d, 2H),
7.36 (t,2H). ES-MS [CH3CN, m/z]: 1025 ([M–ClO4]+), 924 ([M–2ClO4–
H]+), 463 ([M–2ClO4]2+). mmax/cmꢁ1: 3368w (br), 3075w, 1603 m, 1504 m,
1449 m, 1084vs (ClO4), 851 s, 760 s, 724 m, 621 s. UV-Vis (CH3CN):
2
The UV–Vis and emission spectrophotometric pH titrations of the
complex were investigated in aqueous Briton-Robinson buffer with 0.2 M
NaCl to keep constant ionic strength in order to avoid any changes arising
from a change in the environment of the medium.
k
max = 459 nm (e = 18800 dm3 molꢁ1 cmꢁ1), 382 (59200), 286 (82900).

172
F. Gao et al. / Inorganic Chemistry Communications 10 (2007) 170–173
800
600
400
200
0
(b)
4+
(a)
800
600
400
200
0
N
H
N
H
N
N
N
N
N
Ru
N
N
H
N
H
0
2
4
6
8
10 12
pH
+H+
-H+
3+
2+
+
N
H
N
H
N
N
N
N
N
500
550
600
650
700
750
Ru
N
Wavelength / nm
N
N
H
Fig. 3. The emission spectra changes of complex 1 upon raising the pH.
(a) Spectra changes in the range of 1.86–12.96. (b) The pH (0.08–12.96)
effect on emission intensity at 596 nm.
+H+ -H+
‘‘on–off’’ light switch with a large luminescence on–off
ratio of 65.3. This behavior may involve rapid radiation-
less decay [15]. It has been known that the energy of the
metal-centered (MC) excited states depends on the ligand
field strength, which in turn depends on the r-donor and
p-acceptor properties of the ligands [15]. The negative
charge on the deprotonated imidazol substituent may be
delocalized over the whole p framework, which decreases
the r-donor and increases the p-acceptor capacity of the
pipipH2 ligand, resulting in weakening of the ligand-field
strength around the metal center and in turn lowering
the metal r* orbitals [16–18]. Now we can see that, it is
the introduction of pipipH2 ligand that enables the com-
plex 1 to act as not only a diprotic acid but also a diprotic
base and displays an ‘‘off–on–off’’ luminescent switch
depending on the solution pH.
In summary we have developed a novel mononuclear
ruthenium(II) luminescence device that displays a clear
and complete bell-shaped luminescent ‘‘off–on–off’’ switch-
ing as a function of pH in water through two different
mechanisms, one involving luminescence quenching arising
from the positive charge and the other originating rapid
radiationless decay. Studies on the details of the mecha-
nisms involved and design of new pH luminescent switches
are in progress.
N
H
H
N
N
N
N
N
N
Ru
N
N
N
+H+ -H+
N
H
N
N
N
N
N
N
N
Ru
N
N
+H+ -H+
N
N
N
N
N
N
N
N
N
Ru
N
Fig. 2. The acid-base equilibria of complex 1.
Acknowledgements
ing by pH value. In the first region, the emission intensity
increases very sharply within the narrow range from pH
0.08 to 1.86, acting as a sensitive ‘‘off–on’’ switch with
an emission enhancement factor of 2.6. The luminescence
of complex 1 is quenched at a lower pH when the imidaz-
ole rings are protonated because the protonated species is
a better electron acceptor than [Ru(bpy)3]2+ itself [14].
Contrastively, in the following two successive wider
regions (1.86–10.25 and 10.25–12.96), the emission inten-
sity decreases steadily to less than 2% of the original emis-
sion intensity, displaying the character of wide range pH
We are grateful to the supports of the National Natural
Science Foundation of China, the Natural Science Founda-
tion of Guangdong Province, the Research Fund for the
Doctoral Program of Higher Education, and the State
Key Laboratory of Coordination Chemistry in Nanjing
University.
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