Synthesis, characterization, and luminescent property of amide functionalized ruthenium(II) complexes

Article abstract of DOI:10.1080/15421406.2014.939573

Amide functionalized bipyridine ligands and their ruthenium(II) complexes of the type [Ru(bipyridine)₂(L)](PF₆)₂ were synthesized and characterized by UV/Visible, emission, FTIR, ¹H NMR spectroscopies and elemental analysis. Thermal properties of the ruthenium(II) polypyridyl complexes have been investigated using thermogravimetric analysis (TGA) and differential thermal analysis (DTA) techniques. These complexes show remarkable thermal stability at high temperatures under nitrogen atmosphere. The ruthenium(II) complexes show increasing fluorescence intensity in the presence of the amide groups. The increase of the emission intensity and quantum yield of the molecules may be attributed to the change of dipole moment of the amide group on electronic excitation. The effects of substituent (-CH₃, -OCH₃, -COOC₂H₅, -COOH) on photophysical properties of molecules were correlated with the Hammett Substituent Constants. The molecules exhibit linear correlation for absorption and emission maxima.

Full text of DOI:10.1080/15421406.2014.939573

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Synthesis, Characterization, and
Luminescent Property of Amide
Functionalized Ruthenium(II) Complexes
Cigdem Sahina
a Department of Chemistry, Art & Science Faculty, Pamukkale
University, Denizli, Turkey
Published online: 26 Feb 2015.
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To cite this article: Cigdem Sahin (2015) Synthesis, Characterization, and Luminescent Property
of Amide Functionalized Ruthenium(II) Complexes, Molecular Crystals and Liquid Crystals, 607:1,
250-263, DOI: 10.1080/15421406.2014.939573
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Mol. Cryst. Liq. Cryst., Vol. 607: pp. 250–263, 2015
Copyright © Taylor & Francis Group, LLC
ISSN: 1542-1406 print/1563-5287 online
DOI: 10.1080/15421406.2014.939573
Synthesis, Characterization, and Luminescent
Property of Amide Functionalized
Ruthenium(II) Complexes
CIGDEM SAHIN∗
Department of Chemistry, Art & Science Faculty, Pamukkale University,
Denizli, Turkey
Amide functionalized bipyridine ligands and their ruthenium(II) complexes of the type
[Ru(bipyridine)2(L)](PF6)2 were synthesized and characterized by UV/Visible, emis-
sion, FTIR, 1H NMR spectroscopies and elemental analysis. Thermal properties of the
ruthenium(II) polypyridyl complexes have been investigated using thermogravimetric
analysis (TGA) and differential thermal analysis (DTA) techniques. These complexes
show remarkable thermal stability at high temperatures under nitrogen atmosphere.
The ruthenium(II) complexes show increasing fluorescence intensity in the presence
of the amide groups. The increase of the emission intensity and quantum yield of the
molecules may be attributed to the change of dipole moment of the amide group on elec-
tronic excitation. The effects of substituent ( CH3, OCH3, COOC2H5, COOH)
on photophysical properties of molecules were correlated with the Hammett Substituent
Constants. The molecules exhibit linear correlation for absorption and emission max-
ima.
Keywords Amide group; bipyridine; hydrogen bonding; ruthenium(II) complexes
Introduction
The 2,2ꢀ-bipyridine-based ligands have played an important role in coordination chem-
istry due to forming complexes with many transition metals [1]. Among the transition
metal complexes, ruthenium polypyridyl complexes are attracting interest because of their
applications [2] in nc- TiO2 based dye sensitized solar cells (nc-DSSCs) [3], organic light-
emitting diodes (OLEDs) [4], biosensors [5], molecular wires [6], and DNA structure probes
[7]. Ruthenium(II) polypyridyl complexes containing 4,4ꢀ-dicarboxy-2,2ꢀ-bipyridine as a
photosensitizer attached to nanocrystalline TiO2 have been widely studied in solar energy
conversion processes due to their stability, photophysical, photochemical, and electro-
chemical properties [8, 9]. The ruthenium(II) polypyridyl complexes are good candidates
for OLEDs because of their phosphorescence emission that results in the red-orange re-
gion [10]. This application of ruthenium complexes is related to the strong orange-red
triplet metal ligand charge transfer (3MLCT) emission, high luminescence quantum yield
∗Address correspondence to Cigdem Sahin, Department of Chemistry, Art & Science Faculty,
Pamukkale University, Denizli, Turkey. Tel.: +90 258 2963526; Fax: +90 258 2963535. E-mail:
cigdemsahin82@gmail.com
Color versions of one or more of the figures in the article can be found online at
www.tandfonline.com/gmcl.
250

Synthesis, Characterization, and Luminescent Property of Amide
251
and the reversible redox processes [11]. The efficiency of an organic light emitting diode
strongly depends on the choice of molecules which can improve performance and stability
of an OLED [4]. One of the most used ruthenium complexes in OLED applications is tris-
bipyridyl ruthenium(II) complex [Ru(bpy)3]2+ [12]. However, the compounds containing
amide groups are rare used as light emitting materials. The lone electron pairs on nitrogen
in amide groups can delocalize into the carbonyl group and lead the change of the dipole
moment of amide group on electronic excitation. This electronic polarization influences
electroluminescence properties of the compounds [12, 13]. Furthermore, the amide groups
affect nonradiative decay pathways of the electronic excited state of the ruthenium com-
plexes [14]. Our aim in synthesis of ruthenium(II) complex containing amide groups on
bipyridine ligands is to increase the emission intensity and quantum yield of the molecules
which are important for OLED application. The present study reports the synthesis and
characterization of amide functionalized bipyridine ligands and their ruthenium(II) com-
plexes. The correlation of the photophysical properties with Hammett substituent constants
has been investigated. The spectroscopic and thermal properties of synthesized molecules
suggest that these molecules can be utilized in OLED applications.
Experimental
Materials
Ruthenium(III) chloride hydrate, 4,4ꢀ-dimethyl-2,2ꢀ-bipyridine, p-toluidine, 3,5-
dimethoxyaniline, and chromiumtrioxide were obtained from Aldrich. 2,2ꢀ-bipyridine (bpy)
and tetrabutyl ammonium hexafluorophosphate (TBAPF6) were purchased from Fluka.
Thionyl chloride and 4-aminobenzoic acid were provided by Merck. [Ru(bpy)2Cl2]·2H2O
and [Ru(bpy)3](PF6)2 were prepared according to the literature methods [15, 16]. All re-
actions and manipulations were carried out under an argon atmosphere using standard
Schlenk techniques. Solvents were dried and distilled under a nitrogen atmosphere prior
to use. All other chemicals were used as received. The molecular structures of the ruthe-
nium(II) complexes are given in Scheme 1.
Measurements
UV-Vis and fluorescence spectra were recorded in a 1 cm path length quartz cell using
an Shimadzu UV-1601 UV-Vis spectrophotometer and Perkin Elmer LS55 fluorescence
spectrometers, respectively. The spectra were measured in acetonitrile at a concentration
of 4 × 10−5 M. Infrared spectra were recorded on a Perkin Elmer Spectrum Two FT-IR
Spectrometer with a diamond ATR. NMR spectra were recorded at probe temperature on
a Varian Mercury AS 400 NMR instrument. The reported chemical shifts are referenced
to tetramethylsilane (TMS). Elemental analyses were performed using a Carlo Erba 1106
elemental analyzer. The thermal properties of the complexes were obtained by a Shimadzu
DTG-60 equipped with DTA and TGA units under nitrogen atmosphere.
Synthesis of Ethyl-4-aminobenzoate
4-aminobenzoic acid (300 mg, 2.19 mmol) and sulfuric acid (0.25 mL) in ethanol (5 mL)
was refluxed for 5 h. After cooling of the mixture to room temperature, sodium carbonate
solution was added until the solution was neutralized. The resulting product was collected
on a sintered glass crucible by suction filtration and was obtained as white solid (60%

252
C. Sahin
CH3
O
CH3
O
O
CH3
NH
NH
N
NH
NH
O
N
N
N
N
N
N
N
Ru
Ru
N
N
O
N
N
CH3
O
O
O
CH3
O
CH3
CS109
CS110
O
O
O
OH
O
CH3
NH
NH
N
NH
NH
N
N
N
N
N
Ru
Ru
N
N
N
N
N
N
O
O
O
O
CH3
O
OH
CS111
CS112
Scheme 1. Molecular structures of the ruthenium(II) complexes.
yield). 1H NMR (CDCl3) δ ppm: 7.80 (d, J = 8.7 Hz, 2H), 6.59 (d, J = 8.7 Hz, 2H), 4.25
(q, J = 7.0 Hz, 2H), 4.01 (s, 2H), 1.31 (t, J = 6.8 Hz, 3H). 13C NMR (CDCl3) δ ppm:
166.8, 151.2, 131.6, 120.5, 113.9, 60.3, 14.5.
Synthesis of 4,4ꢀ-Dicarboxy-2,2ꢀ-bipyridine
Starting from 4,4ꢀ-methyl-2,2ꢀ-bipyridine was synthesized according to the literature [17].
The product was purified and characterized as 4,4ꢀ-dicarboxy-2,2ꢀ-bipyridine [9]. The yield
1
was 96%. FTIR (ATR, cm−1): 3449, 1719, 1604, 1563, 1290. H NMR (D2O+NaOH)
δ ppm: 7.73 (d, J = 5.2 Hz, 2H), 8.24 (s, 2H), 8.64 (d, J = 5.2 Hz, 2H). 13C NMR
(D2O+NaOH) δ ppm: 173.2, 155.9, 149.9, 146.6, 123.5, 121.5.
Synthesis of 2,2ꢀ-Bipyridine-4,4ꢀ-dicarbonyl dichloride
4,4ꢀ-dicarboxy-2,2ꢀ-bipyridine (400 mg, 1.64 mmol) and thionyl chloride (10 mL) were
reflux for 18 h. The excess of thionyl chloride was evaporated under reduced pressure. The
product was obtained a pale yellow solid with 98% yield. This product was used without
further purification.
Synthesis of 4,4ꢀ-Bis[(4-methylphenyl)carbamoyl]-2,2ꢀ-bipyridine (1)
4,4ꢀ-bis[(4-methylphenyl)carbamoyl]-2,2ꢀ-bipyridine was synthesized by modification of
literature procedure [18]. P-toluidine (41 mg, 0.38 mmol) was dissolved in dry
dichloromethane (10 mL) under argon. Then 2,2ꢀ-bipyridine-4,4ꢀ-dicarbonyl dichloride

Synthesis, Characterization, and Luminescent Property of Amide
253
(50 mg, 0.18 mmol) was added in five portions over a period of 1 h. The mixture was stirred
at room temperature for 8 h. The resulting product was collected on a sintered glass cru-
cible by suction filtration. The solid was washed with distilled water and diethyl ether. The
compound was obtained as a white powder. Yield: 50%. FTIR (ATR, cm−1): 3282, 3113,
3037, 2919, 1647, 1597, 1553, 1526, 1364, 1266, 1011. 1H NMR (DMSO-d6, 400MHz) δ
ppm: 10.59 (s, 2H), 8.93 (d, J = 4.0 Hz, 2H), 8.88 (s, 2H), 7.96 (d, J = 4.0 Hz, 2H), 7.67 (d,
J = 8.1 Hz, 4H), 7.18 (d, J = 8.1 Hz, 4H), 2.48 (s, 6H). Anal. Calcd. for C26H22N4O2(%):
C, 73.92; H, 5.25; N, 13.26. Found: C, 73.95; H, 5.23; N, 13.28.
Synthesis of 4,4ꢀ-Bis[(3,5-dimethoxyphenyl)carbamoyl]-2,2ꢀ-bipyridine (2)
4,4ꢀ-bis[(3,5-dimethoxyphenyl)carbamoyl]-2,2ꢀ-bipyridine was synthesized from 3,5-
dimethoxyaniline (58 mg, 0.38 mmol) and 2,2ꢀ-bipyridine-4,4ꢀ-dicarbonyl dichloride
(50 mg, 0.18 mmol) by using the same reaction procedure of compound 1. Yield: 54%.
FTIR (ATR, cm−1): 3270, 3114, 2933, 2836, 1659, 1598, 1536, 1365, 1295, 1156, 1012. 1H
NMR (DMSO-d6, 400 MHz) δ ppm: 10.61 (s, 2H), 8.96 (d, J = 5.2 Hz, 2H), 8.89 (s, 2H),
7.96 (d, J = 5.2 Hz, 2H), 7.11 (d, J = 2.4 Hz, 4H), 6.33 (s, 2H), 3.76 (s, 12H). Anal. Calcd.
for C28H26N4O6(%): C, 65.36; H, 5.09; N, 10.86. Found: C, 65.40; H, 5.14; N, 10.89.
Synthesis of 4,4ꢀ-Bis[(4-(ethoxycarbonyl)phenyl)carbamoyl]-2,2ꢀ-bipyridine (3)
4,4ꢀ-bis[(3,5-dimethoxyphenyl)carbamoyl]-2,2ꢀ-bipyridine was synthesized from ethyl-4-
aminobenzoate (63 mg, 0.38 mmol) and 2,2ꢀ-bipyridine-4,4ꢀ-dicarbonyl dichloride (50 mg,
0.18 mmol) by using the same reaction procedure of compound 1. Yield: 52%. FTIR (ATR,
cm−1): 3312, 3113, 2986, 2907, 1711, 1655, 1601, 1552, 1523, 1365, 1279, 1170, 1011.
1H NMR (DMSO-d6, 400MHz) δ ppm: 10.99 (s, 2H), 8.98 (d, J = 4.4 Hz, 2H), 8.92 (s,
2H), 7.99 (d, J = 4.8 Hz, 4H), 7.91 (d, J = 4.8 Hz, 6H), 4.31 (q, J = 4.4 Hz, 4H), 1.34 (t,
J = 7.6, 7.2 Hz, 6H). Anal. Calcd. for C30H26N4O6(%): C, 66.51; H, 4.87; N, 10.40. Found:
C, 66.56; H, 4.83; N, 10.45.
Synthesis of 4,4ꢀ-Bis[(4-(hydroxycarbonyl)phenyl)carbamoyl]-2,2ꢀ-bipyridine (4)
4,4ꢀ-bis[(3,5-dimethoxyphenyl)carbamoyl]-2,2ꢀ-bipyridine was synthesized from 4-
aminobenzoic acid (39 mg, 0.29 mmol) and 2,2ꢀ-bipyridine-4,4ꢀ-dicarbonyl dichloride
(40 mg, 0.14 mmol) by using the same reaction procedure of compound 1. Yield: 56%. FTIR
(ATR, cm−1): 3313, 3114, 2920, 2780, 1711, 1658, 1603, 1562, 1364, 1282, 1170, 1011.
1H NMR (DMSO-d6, 400MHz) δ ppm: 10.95 (s, 2H), 8.95 (d, J = 4.4 Hz, 2H), 8.91 (s,
2H), 7.95 (d, J = 4.8 Hz, 4H), 7.90 (d, J = 4.8 Hz, 6H). Anal. Calcd. for C30H26N4O6(%):
C, 64.73; H, 3.76; N, 11.61. Found: C, 64.77; H, 3.73; N, 11.64.
Synthesis of [Ru(bpy)2(1)](PF6)2 (CS109)
[Ru(bpy)2Cl2].2H2O (13.5 mg, 0.028 mmol) and compound 1 (11.4 mg, 0.028 mmol)
were heated under reflux in ethanol (15 mL) for 10 h under argon. The solution was
concentrated to 5 mL and then aqueous solution of KPF6 was added into the mixture. The
brown precipitate was collected by filtration. The product was washed with water followed
by ether and then dried in air. Yield: 62%. FTIR (ATR, cm−1): 3390, 3121, 3085, 2979,
1
2937, 2878, 1644, 1604, 1567, 1542, 1446, 1314, 1243, 1023, 832, 761, 556. H NMR
(Acetone-d6, 400 MHz) δ ppm: 9.46 (s, 2H), 8.86 (d, J = 7.6 Hz, 6H), 8.30 (d, J = 5.6 Hz,

254
C. Sahin
2H), 8.26 (m, 4H), 8.13 (d, J = 6.0 Hz, 2H), 8.09 (d, J = 4.8 Hz, 2H), 8.04 (d, J = 4 Hz,
2H), 7.68 (d, J = 8.8 Hz, 4H), 7.61 (m, 4H), 7.21 (d, J = 8 Hz, 4H), 2.32 (s, 6H). Anal.
Calcd. for C46H38N8O2RuP2F12(%): C, 49,07; H, 3.40; N, 9.95. Found: C, 49,02; H, 3.36;
N, 9.90.
CS110, CS111, and CS112 were synthesized starting from compounds 2, 3, and 4,
respectively, by using the same reaction conditions and purification steps of CS109.
Synthesis of [Ru(bpy)2(2)](PF6)2 (CS110)
The compound was obtained as a brown powder. Yield: 67%. FTIR (ATR, cm−1): 3393,
3113, 3101, 2939, 2937, 2843, 1682, 1603, 1548, 1460, 1365, 1283, 1154, 1067, 834, 764,
556. 1H NMR (Acetone-d6, 400 MHz) δ ppm: 9.58 (s, 2H), 8.85 (d, J = 7.6 Hz, 4H), 8.29
(d, J = 6.0 Hz, 4H), 8.25 (m, 4H), 8.12 (d, J = 5.6 Hz, 2H), 8.08 (d, J = 5.6 Hz, 2H), 8.02
(dd, J = 1.6 Hz, 2H), 7.59 (m, 4H), 7.14 (s, 4H), 7.14 (s, 2H), 6.33 (s, 2H), 3.77 (s, 12H).
Anal. Calcd. for C48H42N8O6RuP2F12 (%): C, 47,34; H, 3.48; N, 9.20. Found: C, 47.28; H,
3.49; N, 9.23.
Synthesis of [Ru(bpy)2(3)](PF6)2 (CS111)
The compound was obtained as a brown powder. Yield: 63%. FTIR (ATR, cm−1): 3383,
3114, 3102, 2984, 2937, 2907, 1717, 1682, 1600, 1529, 1365, 1278, 1104, 1013, 834, 764,
556. 1H NMR (Acetone-d6, 400 MHz) δ ppm: 9.56 (s, 2H), 8.86 (d, J = 8.4 Hz, 4H), 8.33
(d, J = 6 Hz, 2H), 8.25 (m, 4H), 8.12 (d, J = 6 Hz, 2H), 8.08 (t, J = 6.8, 6 Hz, 4H), 8.03
(d, J = 8.4 Hz, 5H), 7.97 (d, J = 9.2 Hz, 5H), 7.61 (m, 4H), 4.34 (q, J = 6.8 Hz, 4H), 1.36
(t, J = 6.8, 7.2 Hz, 6H). Anal. Calcd. for C50H42N8O6RuP2F12 (%): C, 48,36; H, 3.41; N,
9.02. Found: C, 48.33; H, 3.37; N, 9.06.
Synthesis of [Ru(bpy)2(2)](PF6)2 (CS112)
The compound was obtained as a brown powder. Yield: 69%. FTIR (ATR, cm−1): 3383,
3117, 386, 2970, 2931, 2872, 1728, 1644, 1605, 1545, 1467, 1370, 1264, 1167, 1108, 1024,
808, 553. 1H NMR (Acetone-d6, 400 MHz) δ ppm: 9.53 (s, 2H), 8.84 (d, J = 8.4 Hz, 4H),
8.31 (d, J = 6 Hz, 2H), 8.25 (m, 4H), 8.13-7.94 (m, 16H), 7.59 (m, 4H). Anal. Calcd. for
C46H34N8O6RuP2F12 (%): C, 46.59; H, 2.89; N, 9.45. Found: C, 46.61; H, 2.90; N, 9.40.
Results and Discussion
Synthesis and Structural Characterization
The summary of the synthetic route for the synthesized materials is shown in Scheme 2.
4,4ꢀ-dimethyl-2,2ꢀ-bipyridine was oxidized to 4,4ꢀ-dicarboxy-2,2ꢀ-bipyridine which was
converted into 2,2ꢀ-bipyridine-4,4ꢀ-dicarbonyl dichloride in the presence of the excess
thionyl chloride. Amide functionalized bipyridine ligands were obtained from the conden-
sation between 2,2ꢀ-bipyridine-4,4ꢀ-dicarbonyl dichloride and 2.1 equiv of the aromatic
amines. The reactions were conducted in anhydrous dichloromethane and at low tem-
perature to achieve a clean reaction without undesirable product [19]. The reaction of
[Ru(bpy)2Cl2]·2H2O with amide functionalized bipyridine derivatives followed by the ad-
dition of an aqueous solution of KPF6 to precipitate the products. The complexes were
obtained as a brown powder with 62–69% yield.

Synthesis, Characterization, and Luminescent Property of Amide
255
O
O
O
O
H3C
CH3
HO
OH
Cl
Cl
CrO3
SOCl2
H2SO4
N
N
N
N
N
N
R1
R2
R2
R1
O
O
R2
R2
R2
Cl
Cl
O
O
NH
NH
H2N
R1
+
N
N
R2
N
N
R2
R1
R2
O
R2
R2
R1
R1
NH
NH
N
R2
R2
N
N
N
O
O
Ru
NH
NH
N
Ru(bpy)2Cl2
KPF6
R2
N
N
N
O
R1
-
(PF6
)
2
R2
CS109: R1 = CH3; R2 = H
CS110: R1 = H; R2 = OCH3
R = COOCH CH ; R = H
1 2 3 2
CS111:
CS112: R1 = COOH; R2 = H
Scheme 2. The synthetic route for ligands and ruthenium(II) complexes.
1
The structures of synthesized ligands and complexes were confirmed by FT-IR, H
NMR spectroscopies. The FT-IR spectra of ligands (1–4) indicate characteristics the
N–H stretching bands and carbonyl group of amide bands between 3312–3282 and
1647–1659 cm−1, respectively [20]. The peaks at 1711 cm−1 are assigned to the carbonyl
ν(C O) group bands for ligands 3 and 4. The remaining peaks in the range 1010–1600 cm−1
are attributable to ν(C O) and ring stretching of the ligands. In the FT-IR spectra of the
ruthenium complexes (CS109, CS110, CS111, CS112), the bands for the stretching vi-
brations of amide group (C O) and (N H) are shifted to higher wavenumbers by about
25 cm−1 compared to free ligands. The absorption band at 556 cm−1 relative to Ru-N bond
is observed, when the bipyridine ligands coordinate to ruthenium metal [21].
1
The H NMR spectrum of ligand 1 (Fig. 1) shows signals attributable to the amide
N H and methyl proton signals as singlets at 10.60 ppm and 2.48 ppm, respectively.
Characteristic proton signals of methoxy substituents of ligand 2 are observed as singlets
at 3.76 ppm. In ligands 3 and 4 containing electron-withdrawing ethoxycarbonyl and
hydroxycarbonyl groups on phenyl ring lead to a downshift of amide protons. The 1H NMR
spectrum of complex CS109 (Fig. 1) shows 8 resonance peaks relative to bipyridine rings
in the aromatic region, although it was expected to give 11 proton signals. It is attributed to
different magnetic environments of the pyridine rings on the bipyridine ligands. The proton
signals of the pyridine rings can overlap and appear nearly equivalent [22]. In the complex
CS109, the amide proton signal in the aromatic region, and the methyl resonance peaks in
the aliphatic region are shifted upfield compared to free ligand. In the 1H NMR spectra of

256
C. Sahin
Figure 1. 1H NMR spectra of ligand 1 in DMSO-d6 and CS109 complex in acetone-d6.
other complexes (CS110, CS111, CS112), the ratios of aromatic resonance peaks prove the
presence of three bipyridine ligands and two phenyl rings.
Absorption and Emission Studies
The UV-Vis absorption of ruthenium complexes in acetonitrile solvent are shown in Fig. 2.
The maximum absorption wavelengths and the molar extinction coefficients are summarized

Synthesis, Characterization, and Luminescent Property of Amide
257
600
CS109
CS110
1,2
0,8
0,4
0,0
CS111
450
300
150
0
CS112
[Ru(bpy)3].(PF6)2
300
400
500
600
700
800
Wavelength (nm)
Figure 2. UV-Vis absorption and emission spectra of 4 × 10−5 M solution of the ruthenium(II)
complexes in acetonitrile.
in Table 1. The absorption spectra of the ruthenium complexes show the bands in the range
288–480 nm. The absorption bands at 288 nm correspond to the π-π∗ ligand-centered
charge transfer (LC) transitions of bipyridine ligands and the visible bands around 480 nm
are assigned to metal to ligand charge transfer (MLCT) transitions [23]. The lowest energy
MLCT absorption bands are attributed to the transition from a filled dπ metal orbital (t2g) to
a empty ligand-based orbital (π∗) [24]. The complexes show an additional band at 360 nm
due to the presence of the amide groups [25]. These spectra are similar to [Ru(bpy)3](PF6)2
complex, except red shift in the MLCT bands. The red shift is attributed to the electron
withdrawing characteristics of the amide groups lowering the energy MLCT transitions
[18].
Table 1. Absorption and emission data of the Ru(II) complexes
λambxs, (nm) (ε/104)
λemmx(nm)
ꢀem
(M−1.cm−1) (CH3CN) (CH2Cl2) (CH3CN) (CH3OH) (CH3CN)
Complex
CS109
σp
288
(3.06) (0.61) (0.58)
289 359 475
(2.85) (0.68) (0.63)
289 360 480
(2.99) (0.67) (0.59)
289 360 479
(2.38) (0.40) (0.40)
287 451
(2.69) (0.42)
359
472
650
654
660
663
608
658
661
665
667
624
673
672
676
686
618
0.15
0.15
−0.07
CS110
CS111
CS112
0.12
0.45
0.45
0.14
0.092
0.095
[Ru(bpy)3]
(PF6)2
The emission spectra of the Ru(II) complexes were obtained in different solvents by excitation at
480 nm. σp values obtained from [31].

258
C. Sahin
The emission spectra of the ruthenium complexes (Fig. 2) were obtained at room
temperature in acetonitrile. The ruthenium complexes exhibit broad emission band due
to the 3MLCT state [14]. The emission maximum of ruthenium(II) polypridyl complexes
containing amide groups were red-shifted by 33 nm compared to [Ru(bpy)3](PF6)2 complex.
The stronger acceptor property of the amide group on bipyridine ligand decreases the
energy of the excited state [18]. The emission quantum yield of ruthenium complexes
were calculated according to Eq. (1), where ꢀf is the fluorescence quantum yield, A is the
absorption intensity, S is the integrated emission band area, and n is the solvent reflective
index, u and s refer the unknown and standard, respectively [26]
Su As n2u
Ss Au n2s
φf = φfs
.
(1)
The fluorescence quantum yield measurements for solutions were prepared in acetoni-
trile and [Ru(bpy)3](PF6)2 was used as the reference materials (ꢀf = 0.095 in acetonitrile)
[27]. In comparison with [Ru(bpy)3](PF6)2 complex, there is an important difference in the
emission intensities and quantum yields of the complexes (CS109, CS110, CS111). The
emission intensities of these complexes are 1.7–2.5 times higher than the [Ru(bpy)3](PF6)2
complex and they exhibit high fluorescence quantum yield (ꢀf = 0.15–0.14), except for
CS112 complex. The increase of the emission intensity and quantum yield of the molecules
may be attributed to the change of dipole moment of the amide group on electronic excita-
tion [12, 13]. This electronic polarization allows the π extended electron system between
amide bond and aromatic rings [12, 13]. Although it was expected that complex CS112
may have similar emission properties like other complexes (CS109, CS110, CS111), there
is a decrease in the emission intensity and quantum yield of CS112 (ꢀf = 0.092). When the
carboxylic acid group binds to an aromatic ring, the carboxylic acid group can be close to
the planarity of the ring, and this results in an increase of intramolecular interaction [28].
Therefore, it leads to a decrease in fluorescence.
The emission spectra of the ruthenium complexes in the different polarities of solvents
were obtained at room temperature by excitation at 480 nm (Fig. 3). All of the complexes
show similar solvatochromic behavior in polar aprotic solvents (dichloromethane and ace-
tonitrile). The emission maxima of the ruthenium complexes are slightly blue-shifted from
acetonitrile to dichloromethane with the decreasing polarity of solvents. This shift can be
attributed to the less interactions between the excited molecules and solvents [29]. In polar
protic solvent (methanol), the compounds containing the aromatic amide groups induce
intermolecular hydrogen bonding and lead to spectral red shift. The intermolecular hydro-
gen bonding also decreases fluorescence intensity due to quenching effect in polar protic
solvent [13].
The photoluminescent properties of the complexes may be appropriate to be used as
red emitters in OLEDs [4].
Correlation of the Photophysical Properties with Hammett Substituent
Constants
Hammett correlations can be used as a useful model to investigate substituents’ effect
on photophysical properties of molecules [30]. The absorption and emission maxima of
the complexes as a function of Hammett Substituent Constants (σp) [31] were plotted in
Fig. 4. The linear correlations was observed for absorption and emission maxima with
regression coefficients R2 = 0.9867 and R2 = 0.9589, respectively. This correlation gives

Synthesis, Characterization, and Luminescent Property of Amide
259
600
500
400
300
200
100
0
CH Cl
CH Cl
2 2
CS110
2
2
CS109
500
400
300
200
100
0
CH CN
CH CN
3
3
CH OH
3
CH OH
3
600
650
700
750
550
600
650
700
750
Wavelength (nm)
Wavelength (nm)
500
400
300
200
100
0
200
150
100
50
CH2Cl2
CH3CN
CH3OH
CH Cl
CS112
2
2
CS111
CH CN
3
CH OH
3
0
550
550
600
650
700
750
600
650
700
750
Wavelength (nm)
Wavelength (nm)
Figure 3. The emission spectra of the ruthenium(II) complexes in different solvents: Acetonitrile
(CH3CN), dichloromethane (CH2Cl2), methanol (CH3OH).
b)
a)
CS112
CS111
668
666
664
662
660
658
CS112
CS111
y = 14,27x + 473,11
R² = 0,9867
y = 15,339x + 659,11
R² = 0,9589
480
478
476
474
472
CS110
CS110
CS109
CS109
-0,1
0,0
0,1
0,2
0,3
0,4
0,5
-0,1
0,0
0,1
0,2
0,3
0,4
0,5
σ
p
σ
p
c)
0,16
0,14
0,12
0,10
0,08
y = -0,0727x + 0,1503
R² = 0,4554
-0,1
0,0
0,1
0,2
0,3
0,4
0,5
σ
p
Figure 4. The correlation between σp and (a) absorption maxima, (b) emission maxima, (c) fluores-
cence quantum yield of ruthenium(II) complexes in acetonitrile.

260
C. Sahin
688
680
672
664
656
648
CS109
CS110
CS111
CS112
R2=0.9961
R2=0.9995
R2=0.9997
0,3
0,4
0,5
0,6
0,7
0,8
N
T
E
Figure 5. The emission maxima of ruthenium(II) complexes as a function of the normalized molar
electronic transition energy (ETN). ETN (CH2Cl2): 0.309, ENT (CH3CN): 0.460, ETN (CH3OH): 0.762 [34].
an opportunity to predict the spectroscopic data of many substituted molecules which σp
values are known from literature [31, 32]. The electron withdrawing groups ( COOC2H5,
COOH) linked to aromatic ring in ruthenium complexes (CS111, CS112) make more
positive σp values of substituents and cause red shift compared to the complexes containing
electron donor substituents (CS109, CS110) [33]. The red shift is related to decreasing the
band gap between the ground and excited states of molecules. When the substituent effects
of molecules ( CH3, OCH3, COOC2H5, COOH) on fluorescence quantum yields
were investigated, it was found that there is no correlation between σp and fluorescence
quantum yield (Fig. 4c).
The emission maxima of the complexes were seen to correlate with the normalized
molar electronic transition energy (ENT ) (Fig. 5) [34]. The regressions were linear for
the molecules (CS109, CS110, CS111) in polar aprotic solvents (dichloromethane and
acetonitrile) which the emission maxima are red-shifted with increasing the solvent polarity.
However, the molecule CS112 shows deviation in polar protic solvent (methanol) due to
the interaction of carboxylic acid groups [28].
The Thermal Behavior of the Ruthenium Complexes
The TGA and DTA curves of the ruthenium complex were obtained at heating rate of
20◦C/min in the temperature range of 30–900◦C under N2 atmosphere (Fig. 6). The ther-
moanalytical data of the ruthenium complex are summarized in Table 2. The TGA curve of
CS109 shows four decomposition steps in the range 149–782◦C (Fig. 6). The first decom-
position occurs between 149◦C and 160◦C with 16.57% mass loss which is attributed to
the elimination of 2 mol methyl phenyl units. In the second step, the mass loss of 21.41%
was observed between 160◦C and 380◦C due to the elimination of the one bipyridine ligand
containing two amide groups. The mass loss corresponding to the elimination of 2PF6
group was 25.11% between 380◦C and 595◦C in the third step. Finally, mass loss (27.37%)
in the temperature range 595–782◦C shows the removal of the two bipyridine groups of
the complex (C10H8N2). After this decomposition step, the black colored residue with the
mass 9.54% may be assigned to the ruthenium. The DTA curves of CS109 are given in

Synthesis, Characterization, and Luminescent Property of Amide
261
30
0
20
100
100
80
60
40
20
0
CS110
CS109
0
80
60
40
20
0
-20
-40
-60
-80
-30
-60
-90
100 200 300 400 500 600 700 800 900
100 200 300 400 500 600 700 800 900
o
Temperature (oC)
Temperature ( C)
20
20
0
100
80
60
40
20
0
100
80
60
40
20
0
CS112
CS111
0
-20
-40
-60
-80
-20
-40
-60
100 200 300 400 500 600 700 800 900
100 200 300 400 500 600 700 800 900
Temperature (oC)
Temperature (oC)
Figure 6. TGA and DTA curves of the ruthenium(II) complexes.
Table 2. Mass loss (%) of the ruthenium(II) complexes in the different temperature ranges
TGA temp.
range (◦C)
Decomposition
product loss
Complex
Stage
Mass loss (%)
Found
Calc.
16.37
21.34
25.75
27.75
9.54
CS109
I
II
149–160
160–380
380–595
595–782
>782
16.57
21.41
25.11
27.37
8.98
2C6H5-CH3
C10H8N2+2CONH
2PF6
III
IV
Residue
I
2 C10H8N2
Ru
CS110
CS111
CS112
313–350
29.72
29.59
2C6H5+2CONH
+4OCH3
II
350–467
467–868
>868
12.52
49.64
8.12
12.82
49.45
8.29
C10H8N2
III
Residue
I
2 C10H8N2+2PF6
Ru
305–362
31.85
30.97
2C6H5+2CONH
+2COOCH2CH3
C10H8N2
II
362–461
461–800
>800
12.60
48.31
7.24
12.58
48.49
7.96
III
Residue
I
2 C10H8N2+2PF6
Ru
390–640
39.82
40.68
C10H8N2+2CONH
+2C6H5+2COOH
C10H8N2
II
640–900
13.64
13.16
Residue
>900
The mass loss is
53.46% up to 900◦C

262
C. Sahin
Fig. 6. The exothermic peaks are observed at 395, 593, and 618◦C due the formation of
intermediate products [35].
The decomposition temperature range and percentage of mass losses of CS110 and
CS111 are listed in Table 2. Thermal behaviors of CS110 and CS111 result in similar
decomposition steps which give higher thermal stability than CS109 in the presence of
the –OCH3 and –COOCH2CH3 moieties on phenyl groups. The thermal decomposition of
these complexes appears at three steps. The mass loss under 365◦C for these complexes are
attributed to the 4OCH3 (2COOCH2CH3 for CS111), 2C6H5, and 2CONH groups in the first
step. The second decomposition occurs with the elimination of the one bipyridine ligand.
In the third step, two bipyridine and 2PF6 groups of the complexes undergo decomposition
in the temperature range 460–870◦C. The complexes exhibit a mass loss above 800◦C,
corresponding to the formation of Ru as a residue. The exothermic and endothermic peaks
are observed from DTA curves of CS110 and CS111 complexes. The peaks are related to
the decomposition of ruthenium complexes.
TGA curve of CS112 (Fig. 6) shows interesting thermal behavior in comparison to
other complexes (CS109, CS110, CS111). The mass loss of CS112 is 53.46% up to 900◦C,
although the decomposition of other complexes (CS109, CS110, CS111) occur with around
91% mass loss under 900◦C (Table 2). In addition, the CS112 leads to higher thermal
degradation temperature (458◦C) in the presence of the carboxylic acid group [36]. The
enhanced thermal stability is attributed to strong H-bond interaction of carboxylic acid.
Furthermore, theexothermicpeaks intheDTAcurveof CS112shows that thedecomposition
of complex shifts to higher temperature due to in the presence of the carboxylic acid group.
Conclusion
The bipyridine ligands containing aromatic amide groups and their ruthenium(II) com-
plexes were synthesized by using simple methods. It was found that the ruthenium(II)
complexes show a red shift in emission maximum and an increase in emission intensity in
the presence of the amide groups. The effects of substituent on photophysical properties
of molecules were investigated by Hammett correlation. The linear correlation was found
between spectroscopic data and σp. Thermoanalytical results show that these complexes
have high thermal degradation temperature. The new molecules with amide groups can be
a good alternative for OLED applications.
Funding
The authors acknowledge the project support fund of Pamukkale University.
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