Two star-shaped tetranuclear Ru(II) complexes containing uncoordinated imidazole groups: Synthesis, characterization, photophysical and pH sensing properties

Article abstract of DOI:10.1002/bio.3015

Tetrapodal ligands H₄L¹ and H₄L² containing imidazole groups have been synthesized by the reaction of 1,10-phenanthroline-5,6-dione with 1,2,4,5-tetrakis[(4-formylphenoxy)methyl]benzene and 1,2,4,5-tetrakis[(3-formylphenoxy)methyl]benzene, respectively, in presence of NH₄OAc. Two star-shaped complexes [{Ru(bpy)₂}₄(μ₄-H₄L¹)](PF₆)₈ and [{Ru(bpy)₂}₄(μ₄-H₄L²)](PF₆)₈ (bpy = 2,2′-bipyridine) have been prepared by refluxing Ru(bpy)₂Cl₂·2H₂O and each ligand in ethylene glycol. The deprotonated complexes [{Ru(bpy)₂}₄(μ₄-L¹)](PF₆)₄ and [{Ru(bpy)₂}₄(μ₄-L²)](PF₆)₄ have been obtained by the reaction of sodium methoxide with [{Ru(bpy)₂}₄(μ₄-H₄L¹)](PF₆)₈ and [{Ru(bpy)₂}₄(μ₄-H₄L²)](PF₆)₈, respectively, in methanol. The pH effects on the UV-vis light absorption and emission spectra of both complexes have been studied, and ground- and excited-state ionization constants of both complexes have been derived. The photophysical properties of both complexes are strongly dependent on the solution pH. They act as proton-induced off-on-off luminescent sensors through two successive deprotonation processes of imidazole groups, with a maximum on-off ratio of 8 in buffer solution at room temperature. Theoretical calculations for the highest occupied molecular orbital (HOMO) and lowest occupied molecular orbital (LOMO) orbitals of bridging ligand are also presented for plausible explanations of the fluorescence changes.

Full text of DOI:10.1002/bio.3015

Research article
Received: 31 May 2015,
Revised: 22 July 2015,
Accepted: 24 July 2015
Published online in Wiley Online Library: 7 September 2015
(wileyonlinelibrary.com) DOI 10.1002/bio.3015
Two star-shaped tetranuclear Ru(II) complexes
containing uncoordinated imidazole groups:
synthesis, characterization, photophysical and
pH sensing properties
Feixiang Cheng,* Shiwen Yu, Chixian He, Mingli Ren and Hongju Yin
ABSTRACT: Tetrapodal ligands H4L1 and H4L2 containing imidazole groups have been synthesized by the reaction of 1,10-
phenanthroline-5,6-dione with 1,2,4,5-tetrakis[(4-formylphenoxy)methyl]benzene and 1,2,4,5-tetrakis[(3-formylphenoxy)methyl]
benzene, respectively, in presence of NH4OAc. Two star-shaped complexes [{Ru(bpy)2}4(μ4-H4L1)](PF6)8 and [{Ru(bpy)2}4
(μ4-H4L2)](PF6)8 (bpy = 2,2′-bipyridine) have been prepared by refluxing Ru(bpy)2Cl2·2H2O and each ligand in ethylene glycol.
The deprotonated complexes [{Ru(bpy)2}4(μ4-L1)](PF6)4 and [{Ru(bpy)2}4(μ4-L2)](PF6)4 have been obtained by the reaction of so-
dium methoxide with [{Ru(bpy)2}4(μ4-H4L1)](PF6)8 and [{Ru(bpy)2}4(μ4-H4L2)](PF6)8, respectively, in methanol. The pH effects on
the UV–vis light absorption and emission spectra of both complexes have been studied, and ground- and excited-state ioniza-
tion constants of both complexes have been derived. The photophysical properties of both complexes are strongly dependent
on the solution pH. They act as proton-induced off–on–off luminescent sensors through two successive deprotonation pro-
cesses of imidazole groups, with a maximum on–off ratio of 8 in buffer solution at room temperature. Theoretical calculations
for the highest occupied molecular orbital (HOMO) and lowest occupied molecular orbital (LOMO) orbitals of bridging ligand
are also presented for plausible explanations of the fluorescence changes. Copyright © 2015 John Wiley & Sons, Ltd.
Additional supporting information may be found in the online version of this article at the publisher’s web site.
Keywords: star-shaped Ru(II) complex; imidazole; pH sensor; theoretical calculation; photophysics
have small Stokes’ shifts, overlapping pKa values and limited
Introduction
photostability. Rather fewer systems employing luminescent tran-
Molecules responding to external stimulation have received much
sition metal complexes have been reported and their great poten-
attention in recent years, and are potentially useful in the design
of molecular switches and molecular machines (1–5). Transfer of
tial as pH sensors has not fully explored (15–19).
For direct pH sensing by the complex, the indicator must in-
protons can be regarded as one of the simplest but very impor-
clude a pH-sensitive group which is capable of reversible
tant chemical signals. Optical pH sensors are based on pH-
protonation–deprotonation and altering the emissive properties
dependent changes of the absorbance or luminescence of the
of the indicator with pH changes. This concept has been success-
molecules, which are of great importance in biological, chemical
fully applied to several kinds of Ru(II) polypyridyl complexes, and
and industrial areas such as biological and chemical analyses, acid
Ru(II) polypyridyl complexes with carboxylic acid, pyridine, amine
rain pollution, environmental monitoring and process control. In
the analytical application, the luminescent pH sensors have the
or phenol groups attached to the core ligands have been reported
(20–23). The choice of ligands is an crucial factor for a successful
advantages of higher sensitivity and selectivity over the
Ru(II) polypyridyl complex pH sensor. Imidazole-containing ligands
absorbance-based sensors. Currently, much pH sensor research
has focused on organic fluorophores, including fluorescein, naph-
thalene, corrole, and coumarin derivatives (6–11). Recently,
Teknikel and Unaleroglu reported a difluoroboradiaza-S-indacene
are poor π acceptors and good π donors compared with pyridine-,
pyrazine-, and pyrimidine-containing ligands. Furthermore,
imidazole-containing ligands possess dissociative imino N-H pro-
tons, which can perturb the electronic properties of their metal
dye substituted with bis(methoxycarbonyl)ethenyl at positions 3
complexes through protonation and deprotonation. Although
and 5 as a near-IR colorimetric and fluorometric on–off pH sensor
(12); Qui et al. constructed a series of novel fluorescent sensors for
determination of broad-range pH. These sensors use photo-
induced electron transfer as the signal transduction and employ
* Correspondence to: F. Cheng, College of Chemistry and Chemical Engineer-
ing, Qujing Normal University, Qujing 655011, People’s Republic of China.
4-amino-1,8-naphthalimide as the single fluorophore, ethyl moi-
ety as the spacer and a series of phenols and anilines as the recep-
tors (13); Aggarwal and Khurana reported a series of indeno-furan
based colorimetric and on–off fluorescent pH sensors, these sen-
sors were based on the change in structure of phenazine/
quinoxaline unit (14). However, these organic molecules usually
E-mail: chengfx2013@163.com
College of Chemistry and Chemical Engineering, Qujing Normal University,
Qujing 655011, People’s Republic of China
Abbreviation: HOMO, highest occupied molecular orbital; LOMO, lowest oc-
cupied molecular orbital; MLCT, metal-to-ligand charge transfer.
Luminescence 2016; 31: 712–721
Copyright © 2015 John Wiley & Sons, Ltd.

Tetranuclear Ru(II) complexes act as pH sensors
some Ru(II) polypyridyl complexes containing imidazole groups
have been prepared, in most cases, these complexes are non-
emissive or weakly emissive associated with deprotonation pro-
cesses (24–27); only those with imidazole units uncoordinated to
the Ru(II) centers are good emitters. To the best of our knowledge,
only several Ru(II) polypyridyl complexes of this type have been re-
ported (28–31). Therefore, in this work, two star-shaped
tetranuclear Ru(II) polypyridyl complexes with uncoordinated im-
idazole groups have been prepared. The photophysical properties
of the two complexes in response to the pH changes are presented
and discussed. Theoretical calculations for the highest occupied
molecular orbital (HOMO) and lowest occupied molecular orbital
(LUMO) orbitals for the protonated, neutral, and deprotonated
bridging ligand H4L1 are also presented for plausible explanations
of the fluorescence changes.
Synthesis of 1,2,4,5-tetrakis[(3-formylphenoxy)methyl]
benzenes
1,2,4,5-Tetrakis[(3-formylphenoxy)methyl]benzene was prepared
by the same procedure as that described for 1,2,4,5-tetrakis[(4-
formylphenoxy)methyl]benzene, except 3-hydroxybenzaldehyde
(792mg, 6.49 mmol) was used instead of 4-hydroxybenzaldehyde
to react with 1,2,4,5-tetrakis(bromomethyl)benzene (459 mg,
1.03 mmol). Yield: 181 mg (28.6%) of a white solid. 1H NMR
(400MHz, DMSO-d6): δ = 5.37 (s, 8H), 7.35 (dd, J = 7.6, 4.0Hz, 4H),
7.51 (s, 8H), 7.52 (s, 4H), 7.75 (s, 2H), 9.94 (s, 4H). ESI-MS: m/z =
615.4 (M + H)+. IR νmax (KBr, cmÀ1): 3437 m (br), 1693s, 1595s,
1480 m, 1451 m, 1395s, 1357w, 1321 m, 1255s, 1191w, 1149 m,
1078w, 1033 m, 885 m, 788 s, 764 m, 735w, 679 m, 647 m, 577w.
Synthesis of 1,2,4,5-tetrakis{4-((1,10-phenanthroline-[5,6-d]
imidazol-2-yl)phenoxy)methyl}benzene (H4L1)
A mixture of 1,2,4,5-tetrakis[(4-formylphenoxy)methyl]benzene
(155mg, 0.25mmol), 1,10-phenanthroline-5,6-dione (327 mg,
1.56 mmol), and NH4OAc (2.43 g, 31.15mmol) in glacial acetic acid
(32 mL) was heated to 130°C for 5 h, giving a suspension. The reac-
tion mixture was filtered hot, and the solid was washed succes-
sively with CH2Cl2, hot ethanol, hot DMF, and ethyl ether,
affording the desired product as a yellow solid. Yield: 149mg
(42.9%). 1H NMR (400 MHz, DMSO-d6): δ = 5.49 (s, 8H), 7.28 (t, J =
8.4 Hz, 8H), 7.74–7.87 (m, 10H), 8.20 (dd, J = 8.4, 6.0Hz, 8H), 8.82
(d, J = 8.0 Hz, 8H), 9.00–9.04 (m, 8H), 13.48 (s, 4H). ESI-MS: m/z =
1376.4 (M + H)+. Elemental analysis found: C 75.32, H 4.12, N
16.10; calculated for C86H54N16O4: C 75.10, H 3.96, N 16.29. IR νmax
(KBr, cmÀ1): 3411 s (br), 1659w, 1610s, 1520w, 1481s, 1451 m,
1394w, 1356w, 1295w, 1242s, 1179s, 1072 m, 1031 m, 835w,
809w, 741m, 693w, 648w.
Experimental
Materials
2,2′-Bipyridine, 1,10-phenanthroline, 4-hydroxybenzaldehyde, 3-
hydroxybenzaldehyde, 1,2,4,5-tetrakis(bromomethyl)benzene, eth-
ylene glycol, sodium, tetrabutylammonium perchlorate (TBAP),
RuCl3·3H2O, NH4PF6, K2CO3, NH4OAc, ethyl ether, AcOH, CH3CN,
CH2Cl2, EtOH, MeOH, and N,N-dimethylformamide (DMF) were
purchased from the Tianjin Chemical Reagent Factory. Solvents
and raw materials were of analytical grade and used as re-
ceived, apart from CH3CN, which was filtered over activated
alumina and distilled from P2O5 immediately prior to use.
1,10-phenanthroline-5,6-dione (32), and Ru(bpy)2Cl2·2H2O (33)
were synthesized according to literature procedures.
Synthesis of 1,2,4,5-tetrakis{3-((1,10-phenanthroline-[5,6-d]
imidazol-2-yl)phenoxy)methyl}benzene (H4L2)
Physical measurements
1H NMR spectra were recorded on a Mercury Plus 400 spectrome-
ter using tetramethylsilane (TMS) as internal standard. Electrospray
ionization mass spectrometry (ESI-MS) spectra were obtained on a
Bruker Daltonics Esquire 6000 mass spectrometer. Elemental
analyses were taken using a Perkin-Elmer 240C analytical instru-
ment. Absorption spectra were performed on a Varian Cary-100
UV–Visible spectrophotometer and emission spectra with a Hitachi
F-4600 spectrophotometer.
H4L2 was prepared by the same procedure as that described for H4L1,
except 1,2,4,5-tetrakis[(3-formylphenoxy)methyl]benzene (143 mg,
0.23 mmol) was used instead of 1,2,4,5-tetrakis[(4-formylphenoxy)
methyl]benzene to react with 1,10-phenanthroline-5,6-dione
1
(314 mg, 1.50 mmol). Yield: 119 mg (37.2%) of a yellow solid. H
NMR (400 MHz, DMSO-d6): δ = 5.55 (s, 8H), 7.23 (d, J = 8.0 Hz,
4H), 7.49 (t, J = 8.0 Hz, 4H), 7.68 (dd, J = 7.6, 4.4 Hz, 8H), 7.83 (d,
J = 7.6 Hz, 4H), 7.98 (s, 6H), 8.71 (d, J = 8.0 Hz, 4H), 8.75 (d, J =
8.0 Hz, 4H), 8.92 (s, 8H), 13.53 (s, 4H). ESI-MS: m/z = 1376.3 (M +
H)+. Elemental analysis found: C 75.28, H 4.09, N 16.13; calculated
for C86H54N16O4: C 75.10, H 3.96, N 16.29. IR νmax (KBr, cmÀ1):
3406 s (br), 1598s, 1562s, 1476s, 1391 m, 1320w, 1291w, 1224s,
1078 m, 1039 m, 875w, 805 m, 738 m, 692w, 620w.
Synthesis of 1,2,4,5-tetrakis[(4-formylphenoxy)methyl]
benzene
A mixture of 4-hydroxybenzaldehyde (811 mg, 6.65 mmol), 1,2,4,5-
tetrakis(bromomethyl)benzene (486 mg, 1.09 mmol), and K2CO3
(1.10 g, 7.97 mmol) in DMF (50 mL) was heated to 85°C for 48 h un-
der nitrogen. After cooling down to room temperature, the solu-
tion was diluted with 500mL of water. The white precipitate was
collected by filtration, then chromatographed on silica, eluting
with CH2Cl2-EtOH (20:1, v/v) to afford the desired product as a
Synthesis of [{Ru(bpy)2}4(μ4-H4L1)](PF6)8
A mixture of H4L1 (107 mg, 0.08mmol) and Ru(bpy)2Cl2·2H2O
(265mg, 0.51 mmol) in ethylene glycol (30 mL) was heated to
160 °C for 22 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 alumina, eluted
first with CH3CN-EtOH (6:1, v/v) to remove impurities, then with
MeOH to afford the complex [{Ru(bpy)2}4(μ4-H4L1)]Cl8. This com-
plex was dissolved in a minimum amount of water followed by
dropwise addition of saturated aqueous NH4PF6 until no more
1
white solid. Yield: 213 mg (31.8%). H NMR (400 MHz, CDCl3): δ =
5.28 (s, 8H), 7.05 (d, J = 8.8Hz, 8H), 7.70 (s, 2H), 7.83 (d, J = 8.8Hz,
8H), 9.89 (s, 4H). ESI-MS: m/z = 615.3 (M + H)+. IR νmax (KBr,
cmÀ1): 3454 m (br), 1689s, 1595s, 1510 m, 1447 m, 1391 m,
1314 m, 1258s, 1159s, 1111 m, 1030 m, 895w, 856 m, 830 m,
712w, 643w, 615 m, 510w.
Luminescence 2016; 31: 712–721
Copyright © 2015 John Wiley & Sons, Ltd.
wileyonlinelibrary.com/journal/luminescence

F. Cheng et al.
precipitate formed. The precipitate was recrystallized from CH3CN-
Et2O mixture (vapor diffusion method) to afford a red solid. Yield:
148 mg (45.4%). H NMR (400 MHz, DMSO-d6): δ = 5.49 (s, 8H),
1382 m, 1311w, 1241 m, 1180w, 1103w, 1044w, 843 s, 767 s,
731w, 557 m.
1
7.34 (t, J = 6.8 Hz, 8H), 7.41 (d, J = 8.4Hz, 8H), 7.58–7.62 (m, 16H),
7.86 (d, J = 5.6Hz, 8H), 7.91 (t, J = 6.8Hz, 8H), 7.96 (s, 2H), 8.04 (d,
J = 4.8Hz, 8H), 8.11 (t, J = 8.0 Hz, 8H), 8.23 (t, J = 8.0Hz, 8H), 8.36
(d, J = 8.4 Hz, 8H), 8.85 (d, J = 8.4Hz, 8H), 8.89 (d, J = 8.0Hz, 8H),
Synthesis of [{Ru(bpy)2}4(μ4-L2)](PF6)4
[{Ru(bpy)2}4(μ4-L2)](PF6)4 was prepared by the same process as de-
scribed for [{Ru(bpy)2}4(μ4-L1)](PF6)4, except that [{Ru(bpy)2}4(μ4-
H4L2)](PF6)8 (102 mg, 0.03mmol) was used instead of [{Ru(bpy)2}4
(μ4-H4L1)](PF6)8 to react with sodium metal (26 mg, 1.13 mmol)
affording a red solid. Yield: 51 mg (58.0%). 1H NMR (400 MHz,
DMSO-d6): δ = 5.63 (s, 8H), 7.28 (s, 4H), 7.36 (s, 12H), 7.54–7.61 (m,
10H), 7.64 (s, 8H), 7.84 (s, 12H), 8.01–8.03 (m, 12H), 8.10 (s, 12H),
8.22 (t, J = 7.6 Hz, 8H), 8.35–8.40 (m, 4H), 8.89 (d, J = 7.6Hz, 8H),
8.93 (d, J = 8.0 Hz, 8H), 9.13 (s, 8H). ESI-MS: m/z = 432.6 (M – 4PF6
+ 3H)7+, 505.0 (M – 4PF6 + 2H)6+, 605.6 (M – 4PF6 + H)5+, 634.4
(M – 3PF6 + 2H)5+, 664.2 (M – 2PF6 + 3H)5+, 692.6 (M – PF6 +
4H)5+, 756.2 (M – 4PF6)4+, 792.7 (M – 3PF6 + H)4+, 829.6 (M – 2PF6
+ 2H)4+, 865.9 (M – PF6 + 3H)4+, 1106.5 (M – 2PF6 + H)3+, 1154.7 (M
– PF6 + 2H)3+. Elemental analysis found: C 55.57, H 3.32, N 12.22;
calculated for C166H114F24N32O4P4Ru4: C 55.31, H 3.19, N 12.43. IR νmax
(KBr, cmÀ1): 3396 s (br), 1661m, 1602m, 1512w, 1463s, 1381m,
1313w, 1219 m, 1101w, 1041w, 844 s, 766 s, 728 m, 558m.
9.11 (d, J = 7.6Hz, 8H). ESI-MS: m/z = 505.2 (M – 8PF6 – 2H)6+
,
605.3 (M – 8PF6 – 3H)5+, 634.6 (M – 7PF6 – 2H)5+, 663.5 (M – 6PF6
– H)5+, 792.2 (M – 7PF6 – 3H)4+, 830.0 (M – 6PF6 – 2H)4+, 866.3 (M
– 5PF6 – H)4+, 902.3 (M – 4PF6)4+, 1154.5 (M – 5PF6 – 2H)3+
,
,
1202.4 (M – 4PF6 – H)3+, 1250.4 (M – 3PF6)3+, 1804.3 (M – 2PF6)2 +
1876.8 (M – 3PF6 – H)2+. Elemental analysis found: C 47.82, H 2.98,
N 10.51; calculated for C166H118F48N32O4P8Ru4: C 47.60, H 2.84, N
10.70. IR νmax (KBr, cmÀ1): 3389 m (br), 1607 m, 1451s, 1367w,
1310w, 1241 m, 1177w, 1041w, 842 s, 763 s, 557 s.
Synthesis of [{Ru(bpy)2}4(μ4-H4L2)](PF6)8
[{Ru(bpy)2}4(μ4-H4L2)](PF6)8 was prepared by the same process as
described for [{Ru(bpy)2}4(μ4-H4L1)](PF6)8, except that H4L2
(98 mg, 0.07 mmol) was used instead of H4L1 to react with Ru(-
bpy)2Cl2·2H2O (237 mg, 0.46 mmol) affording a red solid. Yield:
1
124 mg (41.5%). H NMR (400 MHz, DMSO-d6): δ = 5.56 (s, 8H),
Results and discussion
7.18 (s, 4H), 7.35 (s, 12H), 7.57 (s, 2H), 7.59 (s, 8H), 7.60 (s, 8H),
7.72 (s, 4H), 7.78 (s, 4H), 7.85 (d, J = 5.2 Hz, 8H), 7.88 (s, 8H), 8.03
(d, J = 6.4 Hz, 4H), 8.09 (t, J = 8.0Hz, 8H), 8.21 (t, J = 8.0 Hz, 8H),
8.27 (s, 4H), 8.86 (d, J = 8.0Hz, 8H), 8.90 (d, J = 8.4 Hz, 8H), 9.10 (s,
Synthesis and characterization
An outline of the synthesis of the two tetrapodal ligands and their
corresponding star-shaped Ru(II) complexes is presented in
Scheme 1. Both ligands were prepared on the basis of the method
for imidazole ring preparation established by Steck et al. (34). H4L1
and H4L2 were obtained in over 40% yields through condensation of
1,10-phenanthroline-5,6-dione with 1,2,4,5-tetrakis[(4-formylphenoxy)
methyl]benzene and 1,2,4,5-tetrakis[(3-formylphenoxy)methyl]ben-
zene, respectively, in refluxing glacial acetic acid at a molar ratio of
1:4. The Ru(II) complexes [{Ru(bpy)2}4(μ4-H4L1)]8+ and [{Ru(bpy)2}4(μ4-
H4L2)]8+ were prepared by refluxing Ru(bpy)2Cl2·2H2O and each ligand
in ethylene glycol solution, and isolated as their PF6À salts in about
42% yields. The deprotonated complexes [{Ru(bpy)2}4(μ4-L1)]4+
and [{Ru(bpy)2}4(μ4-L2)]4+ were achieved in over 58% yields by re-
action of sodium methoxide with [{Ru(bpy)2}4(μ4-H4L1)]8+ and [{Ru
(bpy)2}4(μ4-H4L2)]8+, respectively, in methanol. Both deprotonated
complexes can completely revert to the corresponding proton-
ated complexes by the addition of acid. The four complexes were
characterized by elemental analyses, ESI-MS, 1H NMR and IR.
Elemental analyses were very consistent with the formation of
tetranuclear systems. The structures of these Ru(II) complexes were
further established by ESI-MS spectra. This technique has proven
to be very helpful for identifying polynuclear transition metal com-
plexes with high molecular masses (35). The data with the peak as-
signments are given in the Experimental section. Usually, the mass
of Ru(II) polypyridyl complexes with imidazole fragments is calcu-
lated from a series of multiply charged ions obtained by the suc-
cessive loss of counter anions or hydrogen ions. Figure 1 shows
the ESI-MS spectrum of complex [{Ru(bpy)2}4(μ4-H4L1)](PF6)8; it ex-
hibits some expected peaks due to (M – nPF6)n+ or (M – nPF6 –
mH)(n–m)+ cations. The main peak at m/z = 605.3 is assigned to
(M – 8PF6 – 3H)5+, and the other nine main peaks at m/z = 505.2,
634.6, 792.2, 830.0, 866.3, 902.3, 1154.5, 1202.4, and 1250.4 are
assigned to (M – 8PF6 – 2H)6+, (M – 7PF6 – 2H)5+, (M – 7PF6 –
3H)4+, (M – 6PF6 – 2H)4+, (M – 5PF6 – H)4+, (M – 4PF6)4+, (M –
5PF6 – 2H)3+, (M – 4PF6 – H)3+, and (M – 3PF6)3+, respectively. All
8H). ESI-MS: m/z = 431.8 (M – 8PF6 – H)7+, 505.1 (M – 8PF6
–
2H)6+, 605.7 (M – 8PF6 – 3H)5+, 635.2 (M – 7PF6 – 2H)5+, 664.4 (M
– 6PF6 – H)5+, 756.3 (M – 8PF6 – 4H)4+, 793.4 (M – 7PF6 – 3H)4+
829.8 (M – 6PF6 – 2H)4+, 865.4 (M – 5PF6 – H)4+, 902.2 (M –
4PF6)4+, 1104.8 (M – 6PF6 – 3H)3+, 1154.1 (M – 5PF6 – 2H)3+
1250.7 (M – 3PF6)3+, 1730.3 (M – 5PF6 – 3H)2+. Elemental analysis
found: 47.84, 3.01, 10.48; calculated for
,
,
C
H
N
C
166H118F48N32O4P8Ru4: C 47.60, H 2.84, N 10.70. IR νmax (KBr,
cmÀ1): 3414 s (br), 1599s, 1453s, 1358 m, 1309w, 1216 m, 1088w,
1027w, 843 s, 807w, 766 s, 727 m, 556 m.
Synthesis of [{Ru(bpy)2}4(μ4-L1)](PF6)4
A solution of sodium methoxide solution, which was made by dis-
solving sodium metal (28 mg, 1.22 mmol) in MeOH (6mL), was
added to the complex [{Ru(bpy)2}4(μ4-H4L1)](PF6)8 (115 mg,
0.03mmol) in MeOH (6mL). The solution was heated while stirring
for 3 h under nitrogen and then cooled to 0°C in a refrigerator. A
red microcrystalline solid was collected by filtration. The solid
was purified by column chromatography on alumina, eluted with
MeOH to afford the desired product. The product was recrystal-
lized from MeCN-Et2O mixture (vapor diffusion method) to afford
1
a red solid. Yield: 65mg (65.7%). H NMR (400MHz, DMSO-d6): δ
= 5.48 (s, 8H), 7.28–7.36 (m, 16H), 7.58–7.62 (m, 16H), 7.85 (d, J =
5.6 Hz, 8H), 7.89 (s, 8H), 7.92 (s, 2H), 8.02 (d, J = 10.8 Hz, 8H), 8.10
(t, J = 8.0 Hz, 8H), 8.22 (t, J = 8.4Hz, 8H), 8.61 (d, J = 8.4 Hz, 8H),
8.89 (d, J = 8.4Hz, 8H), 8.93 (d, J = 8.4 Hz, 8H), 9.08 (d, J = 6.8Hz,
8H). ESI-MS: m/z = 504.6 (M – 4PF6 + 2H)6+, 605.7 (M – 4PF6 +
H)5+, 634.3 (M – 3PF6 + 2H)5+, 664.4 (M – 2PF6 + 3H)5+, 693.0 (M
– PF6 + 4H)5+, 756.8 (M – 4PF6)4+, 793.4 (M – 3PF6 + H)4+, 829.7
(M – 2PF6 + 2H)4+, 866.3 (M – PF6 + 3H)4+, 1105.9 (M – 2PF6 +
H)3+, 1154.8 (M – PF6 + 2H)3+. Elemental analysis found: C 55.60,
H 3.34, N 12.26; calculated for C166H114F24N32O4P4Ru4: C 55.31, H
3.19, N 12.43. IR νmax (KBr, cmÀ1): 3397 s (br), 1607 m, 1451s,
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Tetranuclear Ru(II) complexes act as pH sensors
Scheme 1. Synthesis of tetrapodal ligands H4L1–2 and corresponding Ru(II) complexes.
Luminescence 2016; 31: 712–721
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F. Cheng et al.
Molecular orbital calculations of bridging ligand
The compositions and energies of bridging ligand H4L1 orbitals
have been obtained to rationalize the spectroscopic results
(Table 1). Theoretical energy levels of ligand H4L1 have been cal-
culated using Gaussian 09D program at B3LYP/3–21G level.
Figure 3 shows the graphical illustrations for the HOMO and
the three lowest lying unoccupied orbitals (LUMO) of protonated,
neutral, and deprotonated bridging ligand H4L1. The HOMO orbit
of protonated ligand H4L1 is centered on the 1,10-phenanthroline
portion, whereas the HOMO orbits of neutral and deprotonated
species are extended on the 1,10-phenanthroline-[5,6-d]imidazole
portion. The MO LUMO of the protonated and neutral ligand H4L1
are mainly centered on the 1,10-phenanthroline-[5,6-d]imidazole
portion with the electronic distribution. The MO LUMO of
the deprotonated ligand H4L1 is constricted on the 1,10-
phenanthroline part and no more electronic density on the
imidazole-type nitrogens. This simple electronic approach
reflects the different σ-donor and π-acceptor properties of the
three species.
Photophysical properties
The UV–vis absorption spectra of the investigated complexes in
CH3CN solution (5 x 10–6 mol/L) are shown in Fig. 4. The energy
maxima and absorption coefficients are summarized in Table 2. As-
signments of the absorption bands are made on the basis of the
well documented optical transitions of analogous Ru(II) polypyridyl
complexes (28–31). The absorption spectrum of complex [{Ru(bpy)
2}4(μ4-H4L1)]8+ comprises four regions. The bands at around 287
and 241 nm are attributed to the intraligand π→π* transitions
based on 2,2′-bipyridine. The spectrum displays one characteristic
π(H4L1)→π*(H4L1) band of ligand H4L1 at around 320 nm. The low-
est energy band at around 460 nm is assigned to the singlet ex-
cited state of metal-to-ligand charge transfer (MLCT) transition,
which consists of overlapping dπ(Ru) → π*(bpy) and dπ(Ru) → π*
(H4L1) components. The lowered symmetry removes the degener-
acy of the π* levels, which results in the appearance of a non-
symmetrical MLCT band. The MLCT absorption maximum of
complex [{Ru(bpy)2}4(μ4-H4L1)]8+ is red-shifted by about 10 nm
compared with that of Ru(bpy)23+ (36) suggesting that ligand
H4L1 has a larger π framework. Deprotonation of ligand H4L1 in
the tetranuclear complex induces an obvious change in the ab-
sorption spectrum. Compared with the absorption spectrum of
complex [{Ru(bpy)2}4(μ4-H4L1)]8+, the deprotonated complex [{Ru
(bpy)2}4(μ4-L1)]4+ shows the MLCT absorption maximum at around
463 nm, which is much broader with a tail extending out to nearly
535 nm. The band at around 328 nm is attributed to the π(L1)→π*
(L1) transition. The red shifts of the MLCT band and π→π* band
suggest the deprotonated bridging ligand has a larger extent of
π delocalization. Upon excitation into the MLCT band of complex
[{Ru(bpy)2}4(μ4-H4L1)]8+, it shows intense emission at around
610 nm in CH3CN solution at room temperature, which is charac-
Figure 1. The ESI-MS spectrum of complex [{Ru(bpy)2}4(μ4-H4L1)](PF6)8.
the measured molecular weights are consistent with the ex-
pected values.
Octahedral metal centers with bidentate ligands generally show
stereoisomerism. A mononuclear octahedral complex can exist in
two enantiomeric forms, named Δ and Λ. The number of stereo-
isomeric possibilities in polynuclear complexes increases exponen-
tially with the number of metal centers. Complexes [{Ru(bpy)2}4
(μ4-H4L1)]8+ and [{Ru(bpy)2}4(μ4-H4L2)]4+ should be formed as a
mixture of 16 stereoisomers, because each Ru(II) complex sub-
unit is chiral, and can adopt either the Δ or Λ configuration.
1
The downfield region of the H NMR spectra of the complexes
[{Ru(bpy)2}4(μ4-H4L1)]8+ and [{Ru(bpy)2}4(μ4-L1)]4+ in (CD3)2SO
1
solvent is shown in Fig. 2. Two sets of H NMR signals were ob-
served for the four complexes; one set corresponds to the ancillary
ligand 2,2′-bipyridine, and the other set corresponds to the bridg-
ing ligand. Deprotonation of complex [{Ru(bpy)2}4(μ4-H4L1)]8+ has
influence on the resonance frequency of the protons in the
bridging ligand. In particular, the resonance frequency for Hd shifts
from 8.36 to 8.61 ppm. Shifts in the 2,2′-bipyridine resonance
frequency are small but nevertheless significant. The shifts in the
resonance positions of H4L1 protons can be explained by the
increased electron density in the imidazole groups, which delocal-
izes the electron density over the whole π framework of the bridg-
ing ligand and affect the electron density in the phenanthroline
ring and phenyl ring of bridging ligand. The chemical shifts of
the protons on the nitrogen atom of the imidazole groups have
not been observed for the complex [{Ru(bpy)2}4(μ4-H4L1)]8+, since
metal coordination causes electron deficiency in the ligand and, as
a result, these NH protons are more acidic and easily exchanged
between the two nitrogen atoms of the imidazole ring.
3
teristic of the MLCT dπ(Ru)→dπ* emission state. Photophysical
properties of complexes [{Ru(bpy)2}4(μ4-H4L2)]8+ and [{Ru(bpy)2}4
(μ4-L2)]4+ are similar to that of complexes [{Ru(bpy)2}4(μ4-H4L1)]8+
and [{Ru(bpy)2}4(μ4-L1)]4+.
pH-dependent photophysical properties
The pH dependence of the ground- and excited-state properties of
the complexes [{Ru(bpy)2}4(μ4-H4L1)]8+ and [{Ru(bpy)2}4(μ4-H4L2)]8+
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Luminescence 2016; 31: 712–721

Tetranuclear Ru(II) complexes act as pH sensors
Figure 2. Downfield region of the 1H NMR (400 MHz, DMSO-d6) spectra of complexes (a) [{Ru(bpy)2}4(μ4-H4L1)]8+; and (b) [{Ru(bpy)2}4(μ4-L1)]4+
.
the 2,2′-bipyridine centered intraligand π→π* transition band at
285 nm are slightly decreasing in their intensities with one
isosbestic point at 345 nm appearing. Several methods can be
used to determine the pKa values (37–40), for this work, the
spectrophotometric method already reported in the literature
for the analogous Ru(II) polypyridyl complexes was chosen
(41). A plot of absorbance at 236 nm versus pH for complex
[{Ru(bpy)2}4(μ4-H4L1)]8+ is given in the inset of Fig. 5(a). The plot
has the expected sigmoidal shape, with the pH at the point of
inflection giving the first ground-state ionization constant
pΚa1 value of 4.59. The observed spectral changes are attributed
to the concurrent dissociation of four protons on the proton-
ated imidazole rings. An increase in pH from 6.48 to 11.79 can
induce the deprotonation on neutral imidazole groups (Fig. 5b).
The acid–base equilibria of complex [{Ru(bpy)2}4(μ4-H4L1)]8+ is
shown in Fig. S1. The bands at 460 and 285 nm are red-shifted
to 463 and 288 nm, respectively, accompanied by decreases in
their intensities. The band at 318 nm is red-shifted to 327 nm
with significant increase in intensity. The minimum at 490 nm
is increased in intensity with a broader tail extending out to
nearly 540 nm. The occurrence of four isosbestic points at 246,
306, 400, and 468 nm indicates the presence of two species in
equilibrium. The value of the second ground-state ionization
constant pΚa2 is 9.46. pΚa1 and pΚa2 are compared with previ-
ously reported data of the analogous Ru(II) complexes. The first
Table 1. The calculated HOMO and LUMO energies of proton-
ated, neutral, and deprotonated ligand H4L1
Compound
EHOMO / ELUMO / ELUMO +1 / ELUMO +2
eV eV eV eV
/
H4L1 (protonation +4H) À0.435 À0.312 À0.311 À0.311
H4L1
À0.215 À0.066 À0.066 À0.065
H4L1 (deprotonation –4H) 0.029 0.139
0.142
0.143
has been investigated by UV–vis absorption and luminescence
spectra, respectively. UV–vis absorption and luminescence pH
spectroscopic titrations of both complexes were carried out in
acetonitrile—Britton–Robinson buffer (1:1, v/v) solution with
NaCl (0.2 mol/L) to keep a constant ionic strength. The pH titra-
tions were carried out over the pH range 1.82–11.79 and the
spectral changes with pH were found to be reversible.
Figure 5 shows the UV–vis absorption spectra of complex [{Ru
(bpy)2}4(μ4-H4L1)]8+ as a function of pH. The spectral changes indi-
cate that the complex undergoes two successive deprotonation
processes over the pH range 1.82–11.79. Upon increasing pH
from 1.82 to 6.48 (Fig. 5a), the MLCT band at 460 nm, the ligand
H4L1 centered intraligand π→π* transition band at 318 nm, and
Luminescence 2016; 31: 712–721
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F. Cheng et al.
Figure 3. The graphical illustrations for the HOMO and LUMOs of protonated, neutral, and deprotonated bridging ligand H4L1.
ionization constant of the protonated imidazole groups on the
complex is close to the corresponding pΚa value of 4.73 for
the complex Ru(Hipdpa) (29). The second ionization constant
of the neutral imidazole groups on the complex is comparable
to the corresponding value of 9.48 for the complex [(bpy)2Ru
(mbpibH2)]2+ (42).
stronger σ-donor ability compared with protonated ligand,
which increases the negative charge of central metal atom
and destabilises the HOMO orbital of corresponding complex.
The energy gap between HOMO and LUMO orbitals decreases,
emission of the complex is partly increased at higher pH (43).
But compared with the free complex [{Ru(bpy)2}4(μ4-H4L1)]8+
,
The emission spectra changes of complex [{Ru(bpy)2}4(μ4-H4L1)]8+
as a function of pH are shown in Fig. 6. Upon increasing pH from
1.82 to 6.48, the emission intensity increases by about 15%, but
the emission maxima at 609 nm is unchanged (Fig. 6a). Hence,
the complex acts as an off-on luminescent pH sensor during
the process. The HOMO of the complex [{Ru(bpy)2}4(μ4-H4L1)]8
the increase of emission intensity over the pH range is not
very significant.
On further raising the pH from 6.48 to 11.79 (Fig. 6b), the emis-
sion intensity decreases steadily to less than 12% of the original
emission intensity, and the emission maximum is obviously red-
shifted from 609 to 622nm. Especially over the pH range from
7.74 to 9.15, the complex [{Ru(bpy)2}4(μ4-H4L1)]8+ shows obvious
variations in the emission spectra. The maximum on–off ratio of
+
is basically localized on the metal center, and the LUMO is
located on the most reducible ligand. Neutral ligand H4L1 has
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Luminescence 2016; 31: 712–721

Tetranuclear Ru(II) complexes act as pH sensors
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
a
0.67
0.66
0.65
0.64
0.63
0.62
0.61
0.60
0.59
1.2
0.9
0.6
0.3
0.0
3
4
5
6
pH
300
400
500
600
Wavelength (nm)
300
400
500
600
Wavelength (nm)
Figure 4. Absorption spectra of complexes (5 x 10À6 mol/L) [{Ru(bpy)2}4(μ4-H4L1)]8+
(black), [{Ru(bpy)2}4(μ4-H4L2)]8+ (red), [{Ru(bpy)2}4(μ4-L1)]4+ ( green), and [{Ru(bpy)2}4
(μ4-L2)]4+ (blue) in CH3CN solution at room temperature.
0.50
b
1.2
0.9
0.6
0.3
0.0
0.45
0.40
0.35
0.30
8.3 of the complex [{Ru(bpy)2}4(μ4-H4L1)]8+ compares favourable
with the reported data for analogous imidazole-containing Ru(II)
polypyridyl complexes (28–31). Hence, [{Ru(bpy)2}4(μ4-H4L1)]8+
acts as an effective on–off emission pH sensor over the stated
pH range. This behavior may involve rapid radiationless decay
(44). It has been well documented that the energy of the metal
centered excited state depends on the ligand field strength, which
in turn depends on the σ-donor and π-acceptor properties of the
ligand. The negative charge on the deprotonated imidazole rings
can be delocalized over the whole π framework, decreasing the
σ-donor and increasing the π-acceptor capacity of the bridging li-
gand, resulting in weakening of the ligand field strength around
the metal center and in turn lowering the metal σ* orbitals
(28–31). The value of ΔE between the metal σ* orbital and metal
π orbital of the deprotonated complex [{Ru(bpy)2}4(μ4-L1)]4+ is
lower than that of the complex [{Ru(bpy)2}4(μ4-H4L1)]8+. Conse-
quently, population of the metal charge excited state is very effi-
cient for the deprotonated complex, and this complex is
essentially weakly emissive at room temperature. The excited-
state ionization constant pKa* could be estimated on the basis of
the Förster cycle (40), which correlates pKa* with pKa thermody-
namically by the equation pΚa* = pΚa + (0.625/T)(vB – vHB), in which
vB and vHB are pure 0–0 transitions in cmÀ1 for the basic and acidic
species, respectively. In practice, vB and vHB are often difficult or
even impossible to obtain. A good approximation is to use the
emission maxima for vB and vHB since a protonation equilibrium
is almost certainly established between the 3MLCT states (45).
Therefore, the energies of the emission maxima in wavenumbers
were used in equation, giving two excited-state ionization con-
stants of pΚa1* = 4.59 and pΚa2* = 9.79. The value of pΚa1* is equal
to the value of pΚa1, while the value of pΚa2* is 0.33 pΚa units
7
8
9
10
11
12
pH
300
400
500
600
Wavelength (nm)
Figure 5. Absorption spectra changes of complex [{Ru(bpy)2}4(μ4-H4L1)]8+ upon in-
creasing the pH: (a) from 1.82 to 6.48; (b) from 6.48 to 11.79.
greater than the value of pΚa2, indicating that the electron density
of the excited state is higher than that of the ground state and the
excited electron is directed to the ligand H4L1 rather than 2,2′-
bipyridine. The increase in electron density on the ligand H4L1 in-
creases its basicity and, therefore, increases the excited state
pKa* value.
The pH-dependent photophysical properties of the complex
[{Ru(bpy)2}4(μ4-H4L2)]8+ are similar to those of [{Ru(bpy)2}4(μ4-
H4L1)]8+. [{Ru(bpy)2}4(μ4-H4L2)]8+ also undergoes two successive
deprotonation processes over the pH range 1.82 to 11.79, with a
maximum on–off ratio of 8.5. The ground-state ionization con-
stants are pΚa1 = 4.65 and pΚa2 = 9.53, and the values of two
excited-state ionization constants pΚa1* and pΚa2* are 4.65 and
9.88, respectively.
Table 2. UV–vis absorption data of Ru(II) polypyridyl complexes
Complex
λ
max, nm (104ε, MÀ1 cmÀ1
)
[{Ru(bpy)2}4(μ4-H4L1)]8+
[{Ru(bpy)2}4(μ4-H4L2)]8+
[{Ru(bpy)2}4(μ4-L1)]4+
[{Ru(bpy)2}4(μ4-L2)]4+
460 (4.57)
460 (4.53)
463 (4.11)
463 (4.16)
320 (8.43)
321 (7.88)
328 (10.46)
328 (9.75)
287 (27.32)
287 (24.46)
289 (25.61)
289 (24.07)
241 (8.52)
240 (10.64)
241 (10.11)
242 (10.38)
Luminescence 2016; 31: 712–721
Copyright © 2015 John Wiley & Sons, Ltd.
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F. Cheng et al.
7000
6000
5000
4000
3000
2000
1000
0
a
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We are grateful to the National Natural Science Foundation of
China (21261019) and Shanghai key laboratory of rare earth func-
tional materials for financial support.
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