Experimental and density functional theory insights into the effect of withdrawing ligands on the fluorescence yield of Ru(II)-based complexes

Article abstract of DOI:10.1002/aoc.4677

The quality of emission spectra of metal complexes gives good insights into their performance in many optoelectronic applications. Herein, the effect of the number and position of various ligand structures on the emission spectra of Ru bipyridine complexes was studied. Specifically, the use of a different number of withdrawing groups (COOH) was investigated in detail. The complexes were first investigated using density functional theory (DFT) and time-dependent DFT calculations and then confirmed experimentally. The bandgap energy, reactivity, emission spectra and Stokes shift were found to depend on the number and position of the withdrawing groups attached to the Ru(bpy)₂²⁺ complexes. Upon increasing the number of withdrawing groups, the electrons were found to be withdrawn from the carbon orbitals and resonated to reach the metal, and accumulated around it, thus enhancing the metal-to-ligand charge transfer mechanism instead of the ligand-to-ligand charge transfer mechanism. The complexes with more withdrawing groups showed spectra with more intense emission peaks with shorter lifetime, indicating the enhancement in the photoactivity of the complexes. Ligands with ring nitrogens with two COOH groups showed the greatest effect on the enhancement of the emission spectra with a lifetime of 0.5359?ns. The resulting collective emission spectra covered a wide wavelength range, making the investigated complexes a good choice for many optoelectronic applications.

Full text of DOI:10.1002/aoc.4677

Received: 2 August 2018
DOI: 10.1002/aoc.4677
Revised: 3 October 2018
Accepted: 8 October 2018
F U L L P A P E R
Experimental and density functional theory insights into
the effect of withdrawing ligands on the fluorescence yield
of Ru(II)‐based complexes
Basant A. Ali1† | Walid Sharmoukh2† | Mohamed M. Elnagar2 | Zeinab M. Hassan3 |
Nageh K. Allam1
1 Energy Materials Laboratory (EML),
The quality of emission spectra of metal complexes gives good insights into
their performance in many optoelectronic applications. Herein, the effect of
the number and position of various ligand structures on the emission spectra
of Ru bipyridine complexes was studied. Specifically, the use of a different
number of withdrawing groups (COOH) was investigated in detail. The com-
plexes were first investigated using density functional theory (DFT) and time‐
dependent DFT calculations and then confirmed experimentally. The bandgap
energy, reactivity, emission spectra and Stokes shift were found to depend on
School of Sciences and Engineering, The
American University in Cairo, New Cairo
11835, Egypt
2 Inorganic Chemistry Department,
National Research Centre, Tahrir St,
Dokki, Giza 12622, Egypt
3 Chemistry Department, Fayoum
University, Faculty of Science, Fayoum,
Egypt
2+
Correspondence
the number and position of the withdrawing groups attached to the Ru(bpy)2
Nageh K. Allam, Energy Materials
Laboratory (EML), School of Sciences and
Engineering, The American University in
Cairo, New Cairo 11835, Egypt.
Email: nageh.allam@aucegypt.edu
complexes. Upon increasing the number of withdrawing groups, the electrons
were found to be withdrawn from the carbon orbitals and resonated to reach
the metal, and accumulated around it, thus enhancing the metal‐to‐ligand
charge transfer mechanism instead of the ligand‐to‐ligand charge transfer
mechanism. The complexes with more withdrawing groups showed spectra
with more intense emission peaks with shorter lifetime, indicating the
enhancement in the photoactivity of the complexes. Ligands with ring nitro-
gens with two COOH groups showed the greatest effect on the enhancement
of the emission spectra with a lifetime of 0.5359 ns. The resulting collective
emission spectra covered a wide wavelength range, making the investigated
complexes a good choice for many optoelectronic applications.
Funding information
Science and Technology Development
Fund, Grant/Award Number: 5415;
Egyptian Government, Grant/Award
Number: 5415
KEYWORDS
DFT, fluorescence, lifetime, Ru(II) complexes, withdrawing groups
1 | INTRODUCTION
luminescence by controlling the structure of the
ligands.[1,2] The luminescence of these transition
Transition metals combined with ringed structures such
as bipyridine rings are attracting attention in many appli-
cations due to their luminescence properties.[1] Many
approaches have been developed for enhancing their
metalorganic complexes makes them good candidates
for many applications such as organic light‐emitting
diodes (OLEDs),[3] photodynamic therapy,[4] dye‐
sensitized solar cells[5] and many other applications.
Researchers have studied computationally those unique
†These authors contributed equally
luminescence properties to get
a more in‐depth
Appl Organometal Chem. 2018;e4677.
wileyonlinelibrary.com/journal/aoc
© 2018 John Wiley & Sons, Ltd.
1 of 9
https://doi.org/10.1002/aoc.4677

2 of 9
ALI ET AL.
understanding of the origin of the emission and how to
enhance it.[5–7] OLEDs have become widely used light
sources in daily life. Developing OLEDs with low weight
and strong emission that covers more than one colour
in the spectrum is becoming a persistent need.[8] Organo-
metallic complexes of the d6 transition metals, such as
Ir(III) complexes, have attracted the attention of the
studied the effect of the ligand on the charge transfer
mechanism of the Ru(II) complexes and on the emission
spectra. Density functional theory (DFT) and time‐
dependent DFT (TD‐DFT) have proven to be reliable
tools in predicting the properties of complexes used in
OLEDs.[8b, 12]
9]
researchers due to their wide emission range.[8b,
Although the majority of the reports in the literature
focus on Ir(III) complexes, Ru(II) complexes have emis-
sion properties and have not been explored exten-
sively.[9,10] To this end, Ru2+ complexes are promising
targets for OLEDs, especially in that the charged moieties
of the ligands can act as mobile counter ions with no
extra electrolyte being needed for devices.[10b, d] Ru(II)
complexes can also undergo stable and reversible redox
reactions with their triplet and singlet excitations both
being allowed.[10a, d, 11]
The structure of the ligands attached to the metal cen-
tre in Ru(II) complexes greatly affects their emission
spectra.[10a, 11] In this paper, we present four novel Ru(II)
complexes each with four ligands, two of which each vary
in the number of electron‐withdrawing COOH groups
and inductive ring nitrogens (Figure 1). The first complex
contains the extra two ligands each with a pyridine ring
and one COOH group (Ru(bpy)2(pyphCOOH)22+, 1N–
1COOH), the second with a pyrimidine ring and one
COOH group (Ru(bpy)2(pmphCOOH)22+, 2N–1COOH),
2 | EXPERIMENTAL AND
COMPUTATIONAL METHODS
2.1 | DFT calculations
All complexes under study were optimized to their lowest
energy of the singlet state and of the triplet state with
DFT[13] using the Gaussian 16 software package.[14] The
level of calculation was the mixed basis set 6‐31G*[15,16]
for H, O, N and C atoms and effective core potential
LANL2/LANL2DZ[15] for Ru atoms, while the method
of calculation was the hydride three‐parameter density
functional method (B3LYP).[17] The B3LYP method
includes Becke's three‐parameter gradient exchange cor-
rection function (B3) and the Lee, Yang and Parr correla-
tion functional (LYP)[18] which typically provides
energetic and geometrical parameters better than other
methods.[19] In order to investigate the excited state relax-
ation of the complexes, we performed geometry optimiza-
tion for the first 15 singlet excited states and the first 15
triplet excited states in separate calculations using
TD‐DFT[20] at the same level of theory as that of the
optimization calculations. Single‐point energy[14,21] was
also calculated for the optimized structures. Visualization
the third with a pyridine ring and two COOH groups
2+
(Ru(bpy)2(pyisophCOOH)2
,
1N–2COOH), and the
fourth with a pyrimidine ring and two COOH groups
(Ru(bpy)2(pmisophCOOH)22+, 2N–2COOH). Also, we
FIGURE 1 Molecular structures of the
synthesized complexes

ALI ET AL.
3 of 9
of the data was performed using ChemCraft 1.6 soft-
ware[22,23] and GaussView 5.0 software.[24,25] Figure 2
shows a representative diagram of the calculations
performed.
electrochemical study of the complexes (0.2 mM) was
carried out in 0.1 M TBAPF6 as the supporting electrolyte
in acetonitrile solution. The emission spectra of the com-
plexes were obtained using an FP‐6500 spectrofluorome-
ter (Jasco Japan).
2.2 | Materials and characterization
2.2.1 | Chemicals and solvents
3 | RESULTS AND DISCUSSION
3.1 | Synthesis of Ru(II) complexes
All solvents were of HPLC grade and used without further
purification. All chemicals including 5‐aminoisophthalic
acid, 4‐aminobenzoic acid, 4‐pyrimidineboronic acid,
4‐pyridineboronic acid, palladium(II) acetate, tri‐o‐
tolylphosphine, 2,2′‐bipyridine, ruthenium chloride
hydrates, ammonium hexafluorophosphate (NH4PF6)
and tetrabutylammonium hexafluorophosphate (TBAPF6)
were purchased from Sigma‐Aldrich, Acros or Alfa‐Aesar
and used as received.
Scheme 1 illustrates the synthetic pathway of the synthe-
sized 1N–1COOH, 1N–2COOH, 2N–1COOH and 2N–
COOH complexes. Compounds 1 and 4 were synthesized
according to the literature.[25,26] Compounds 2, 3, 5 and 6
were synthesized through palladium‐catalysed Suzuki
coupling reaction of pyridin‐4‐ylboronic acid or
pyrimidin‐5‐ylboronic acid with methyl 4‐iodobenzoate
or dimethyl 5‐iodoisophthalate to obtain highly pure
compounds in good yields of 65, 75, 60 and 80%, respec-
tively. Corresponding Ru(bpy)2Cl2 (7) was prepared fol-
lowing a literature procedure,[27] where bipyridine
ligand was reacted with RuCl3 in dry methanol for 5 h
and then after subsequent washing with cold methanol
and filtration, the solid was dried and further reacted
with the ligands 2, 3, 5 or 6 in DMF for 6 h. The reaction
mixture was evaporated to dryness under reduced pres-
sure prior to the hydrolysis step in KOH and methanol
at 100°C for 12 h. After hydrolysis, the product was then
precipitated by adding excess of a saturated aqueous solu-
tion of NH4PF6 to the reduced volume of acidic methanol
solution of the reaction mixture. The red‐coloured solids
were filtered, washed with water and diethyl ether and
dried under vacuum to afford pure 1N–1COOH,
1N–2COOH, 2N–1COOH and 2N–2COOH complexes in
good yields of 60, 66, 72 and 68%, respectively. The full
NMR characterization of the complexes is given in the
supporting information.
2.2.2 | Structural, optical and electro-
chemical characterization
1H NMR and 13C NMR spectra were recorded with
Bruker 500 MHz instruments using the residual signals
for CDCl3 at 7.26 and 77.0 ppm and for DMSO‐d6 at
1
2.50 and 39.4 ppm as internal references for H and 13C,
respectively. Elemental analyses were conducted with
an Elmentar Vairo EL elemental analyser (Germany).
The UV–visible electronic spectra of the complexes were
obtained with a PerkinElmer Lambda 750 UV/Vis spec-
trophotometer. Electrochemical measurements were car-
ried out with a three‐electrode system using an Autolab
potentiostat/galvanostat (PGSTAT302N) at a scan rate of
100 mV s−1, employing a platinum electrode (diameter
of 2 mm) as working electrode, a platinum column as
counter electrode and an Ag/AgCl electrode as reference
electrode. Ferrocene/ferrocenium couple (Fc/Fc+, 0.63 V
versus NHE) was utilized as an external reference. The
FIGURE 2 Graphical representation of
the DFT and TD‐DFT calculations

4 of 9
ALI ET AL.
O
O
O
N
O
N
HO
+
OH HO
OH
N
I
B
N
B
Pd(dba)2
or
N
or
N
Tolylphosphine
K2CO3
DMF 4h, 90 o
C
75%
O
O
N
3
2
1
65%
O
O
HO
OH
OH
B
HO
B
N
O
O
I
Pd(dba)2
+
or
or
N
N
Tolylphosphine
K2CO3
O
O
O
O
C
6
DMF 4h, 90 o
4
N
N
O
O
5
O
O
60%
80%
O
O
N
O
O
60%
72%
1) drops of TEA
1N-1COOH
2N-1COOH
Cl
N
DMF, 8 h 160 o
C
+
2eq
N
or
2eq
N
N
Ru
N
2) hydrolysis
Cl
7
N
N
O
O
O
O
66%
68%
1N-2COOH
2N-2COOH
Cl
1) drops of TEA
DMF, 8 h 160 o
N
Ru
N
C
+
2eq
or
2eq
N
N
2) hydrolysis
N
N
Cl
O
O
O
O
SCHEME 1 Synthetic pathways of the complexes
TABLE 1 Structural parameters for the studied complexes
3.2 | Computed structural properties
Bond length
(Å)
1N–
1COOH
1N–
2COOH
2N–
1COOH
2N–
2COOH
All the calculated geometries are of C1 point group with
distorted octahedral structure. Table 1 presents the
structural parameters for the studied complexes. As
noted from the calculated structural parameters in the
ground state, the C═O and C─O bond lengths are ca
1.21 and 1.35 Å, respectively, with the bond lengths in
the complexes with two COOH groups being shorter
than those in the complexes with one COOH group,
which are slightly higher than reported values.[28] The
Ru─N bond length (for the ligands with COOH groups)
is found to be slightly stretched to ca 2.18 Å for all com-
plexes. The Ru─N bond lengths (for the bipyridine
ligands) are about 2.13 Å for all complexes under study,
which are very close to reported values.[28] No other sig-
nificant changes were observed for any other bond
lengths. The N─Ru─N bond angle (between the ligands
with COOH groups) is found to be ca 92° for all com-
plexes and the N─Ru─N angle (between the porphyrin
ring and the ligand with the COOH group) is found to
be ca 85°.
Chain Ru─N
Prophy Ru─N
Chain N─C
Prophy N─C
Chain C─C
Prophy C─C
Chain C═O
Chain C─O
2.177
2.125
1.353
1.347
1.476
1.477
1.213
1.350
2.177
2.125
1.353
1.347
1.477
1.477
1.214
1.346
2.180
2.128
1.351
1.347
1.478
1.477
1.212
1.348
2.177
2.130
1.348
1.347
1.479
1.477
1.219
1.342
3.3 | Computed electronic and
photoactivity of the complexes
As presented in Table 2, the calculated energy gap
between ELUMO and EHOMO increases in the order
1N–1COOH < 1N–2COOH ~ 2N–1COOH < 2N–2COOH.

ALI ET AL.
5 of 9
Since reactivity is inversely proportional to the energy
gap, the reactivity slightly decreases with an increasing
number of anchoring groups and greatly decreases
with an increasing number of nitrogen atoms in the
ligand. The values of ET decrease in the order
1N–1COOH > 2N–1COOH > 1N–2COOH > 2N–2COOH.
Dipole moment values are comparable for the complexes
1N–1COOH, 2N–2COOH and 1N–2COOH, while appar-
ently increase for the complex 2N–1COOH. The calculated
natural bond order charges showed that the metal (Ru)
was the highest positively charged atom with a value of
ca 0.696 for all complexes under study. The highest
negatively charged atom observed was the oxygen of
the hydroxyl group with values ranging from −0.561
to −0.583.
The TD‐DFT calculations were performed on the
optimized complexes to investigate both the absorption
and emission spectra. The electronic excitation properties
are presented in Table 3. The highest excitations in
the oscillator strength are the fifth and sixth excited
states, in the order 1N–1COOH > 1N–2COOH > 2N–
1COOH > 2N–2COOH. The frontier molecular orbital
distributions of the highest transition are shown in
Figure 3 indicating that the orbital distribution of the Ru
atom is around its d orbital, and for the branched ligands
the orbital distribution is around the π orbitals. For the
bipyridine rings, the orbital distribution is around the π*
orbitals. From Figure 3, it is clear that by increasing the
number of withdrawing groups, either the COOH groups
or the number of ring nitrogens, electrons are withdrawn
from the carbon orbitals and resonate to reach the metal
and accumulate around it. It is evident from Table 3 that
an increase of the electron‐withdrawing groups enhanced
the metal‐to‐ligand charge transfer (MLCT) mechanism at
the expense of the ligand‐to‐ligand charge transfer (LLCT)
mechanism. Notably, upon excitation, the electrons accu-
mulate around the bipyridine rings.
The calculated absorption spectra of the studied com-
plexes are depicted in Figure 4a, showing a clear trend
indicating that either adding an extra COOH group or
increasing the number of nitrogen atoms in the ligand
has two main effects: a blue shift in the wavelength and
an increase in the oscillator strength. However, increas-
ing the number of nitrogen atoms plays the dominant
role compared to adding an extra COOH group. All the
wavelength maxima of the complexes fall in the visible
light region, resulting in good absorption of light. The
TD‐DFT emission calculations showed all the complexes
not to have a significant phosphorescence emission, indi-
cating that the triplet state excitations are not favoured
for the studied complexes. The calculated fluorescence
emission spectra of the studied complexes are shown in
Figure 4b. It is clear that the emission spectra follow the
same trend as the absorption spectra. The complexes with
more ring nitrogens have the more intense peaks than
TABLE 2 Calculated energetic properties (ET, total energy; ΔE,
ELUMO − EHOMO; μ, dipole moment; f , oscillator strength)
1N–
1N–
2N–
2N–
Parameter 1COOH
2COOH
1COOH
2COOH
ET (a.u.)
ELUMO (eV)
EHOMO (eV)
ΔE (eV)
μ (D)
−2420.1617 −2797.3017 −2452.2192 −2829.3655
−7.2497
−10.6484
3.3987
−7.1844
−10.6939
3.5095
−7.4810
−11.0697
3.5887
−7.2930
−11.0144
3.7214
13.2368
0.0048
14.4238
0.0030
16.5644
0.0029
14.0362
0.0028
f
TABLE 3 Calculated absorption and emission spectral parameters
Parameter
State
1N–1COOH
1N–2COOH
2N–1COOH
2N–2COOH
Absorption
S1
Transition (CT)
Wavelength (nm)
f
MLCT
456.96
0.0048
MLCT
455.85
0.003
MLCT
439.61
0.0029
MLCT
435.97
0.0028
Main configuration
(coefficient)
Transition (CT)
Wavelength (nm)
f
Main configuration
(coefficient)
Transition (CT)
Wavelength (nm)
f
H − 4 → L + 1
(0.49024)
LLCT
435.62
0.0524
H → L
(0.48634)
LLCT
426.72
0.0603
H − 2 → L + 1
(0.56211)
LLCT
429.33
0.0653
H → L
(0.47739)
LLCT
419.23
0.0642
H − 4 → L
(0.52724)
MLCT
406.47
0.0745
H − 8 → L
(0.29955)
LLCT
398.26
0.0519
H → L
(0.62035)
MLCT
399.86
0.0811
H − 2 → L + 1
(0.58794)
MLCT
392.7
0.0567
S5
S6
Main configuration
(coefficient)
H → L + 1
(0.50199)
H → L + 1
(0.50052)
H − 2 → L + 1
(0.32907)
H − 2 → L
(0.48836)
Stokes shift (nm)
61.6
49.18
46.92
46.26

6 of 9
ALI ET AL.
FIGURE 3 Frontier molecular orbitals of the maximum transition for the studied complexes
those with one ring nitrogen, and the complexes with
more COOH groups give more intense peaks than those
with one COOH group. The Stokes shift values for the
complexes were calculated as the difference between the
maxima of the absorption and emission spectra presented
in Figure 4c–f. It was found that the more electron‐
withdrawing groups that are added, the lower the Stokes
shift becomes.
isophthalic acid, 5‐(pyrimidin‐5‐yl)isophthalic acid,
4‐(pyridin‐4‐yl)benzoic acid and 4‐(pyrimidin‐5‐yl)
benzoic acid). The other electronic transitions of the
complexes (1N–1COOH, 1N–2COOH, 2N–1COOH and
2N–2COOH) showed strong absorption peaks in the visi-
ble region at 482, 488, 492 and 483 nm, respectively.
These bands are characteristic of MLCT absorption bands
similar to other polypyridyl ruthenium(II) complexes.
The molar absorptivity (ε) values of the low‐energy
MLCT absorption bands for the ruthenium complexes
(1N–1COOH, 1N–2COOH, 2N–1COOH and 2N–2COOH)
are 17.095 × 103, 17.650 × 103, 11.650 × 103 and
11.500 × 103 M−1 cm−1, respectively, which are lower
than the corresponding values of pyrimidineisophthalate
and pyrimidinebenzoate ligands. The photoluminescence
(PL)[15] emission spectra of the complexes in acetonitrile
solution are shown in Figure 5b. The maximum PL
emission peaks (λmax) of 1N–1COOH, 1N–2COOH, 2N–
1COOH and 2N–2COOH were observed at 623, 640, 613
and 580 nm, respectively. Figure 5c shows the emission
spectrum of a mixture of the four Ru(II) complexes in
acetonitrile, covering the whole visible light region. The
3.4 | Experimental photophysical
properties of the complexes
The electronic absorption spectra of the new ruthenium
complexes measured in acetonitrile are shown in
Figure 5a and the related data are summarized in
Table 4. The absorption spectra of the complexes show
two bands. Intense and sharp bpy‐centred π–π* transi-
tions are located at high energy. The electronic bands at
371, 373, 360 and 355 nm in the UV region are ascribed
to the intraligand (π–π*) charge transitions of the bpy
and conjugated ancillary ligands (L1–L4: 5‐(pyridin‐4‐yl)

ALI ET AL.
7 of 9
FIGURE 4 (a) Comparative absorption spectra of the studied complexes. (b) Comparative emission spectra of the studied complexes. (c–f)
Comparative absorption and emission spectra of each complex
FIGURE 5 (a) Absorption spectra of Ru(II) complexes in acetonitrile. (b) Emission spectra of Ru(II) complexes in acetonitrile. (c) Emission
spectrum of a mixture of the four Ru(II) complexes in acetonitrile
obtained experimental absorption data are in good agree-
ment with those calculated via TD‐DFT. However, the
trend was clearer in the calculated data as calculations
only included the molecular structure with no solvent
effect and with no inclusion of other factors that appear
in the experimental measurements. The experimental
emission data showed a much longer emission wave-
length than that calculated by TD‐DFT but with the same
trend. While the DFT and TD‐DFT calculations were
meant to show the effect of the withdrawing groups on
the orbital structure and on the optical performance of
the complexes, the experimental data were meant to ver-
ify the theoretical calculations and express the real values.
The fluorescence quantum yield (Φf) of the synthe-
sized complexes was calculated using [Ru(bpy)3]Cl2 as a
reference via equation (1):
Qr × Fs As × n2s
Qf ¼
×
(1)
Fr × Ar
nr2
where Qr is the fluorescence quantum yield of the
reference, which equals 0.064 in acetonitrile solvent. F s

8 of 9
ALI ET AL.
TABLE 4 Photophysical and electrochemical properties of the complexes
λ
abs,max, nm
Quantum
yield, Φf
ERed
(VNHE
EHOMO
E0–0
(eV)c
Eox
(VNHE
ELUMO
(VNHE)
a
b
d
e
Complex
(ε, M−1 cm−1
)
Lifetime[1b]
)
(VNHE
)
)
1N–1COOH
1N–2COOH
2N–1COOH
2N–2COOH
371 (17 095), 482 (14488)
373 (17 650), 488 (12 470)
360 (11 790), 492 (9260)
355 (11 500), 483 (9320)
0.9034
1.153
0.036778
0.034466
0.035179
0.030158
0.92
0.94
0.99
0.97
0.99
1.02
1.07
1.13
1.87
1.92
2.02
2.07
1.03
1.06
1.11
1.17
−0.88
−0.90
−0.95
−0.94
0.7248
0.5359
aOxidation and reduction potentials (versus Ag/AgCl) were measured in acetonitrile with 0.1 M (nBu4N)PF6 and at a scan rate of 100 mV s−1, was calibrated
with Fc/Fc+ and converted to the NHE scale by addition of 0.40 V.
bEHOMO was calculated as −4.8e (Eo − E1/2 of Fc/Fc+), where E1/2 of Fc/Fc+ is −0.40 V.
ox
cE0–0 was determined from the intersection of the absorption and emission peak (E0–0 = 1240/λmax).
dEox was calculated as Eox − E0–0
.
eELUMO was calculated as EHOMO + E0–0 (see supporting information).
and F r are the area under the emission curve for sample
and reference, respectively. As is the maximum absor-
bance value of the sample and Ar is the absorbance value
of the reference at the wavelength corresponding to the
maximum absorbance value of the sample. Parameters
ns and nr are the refractive indices of the solvents used
for sample and reference measurements, respectively.
The refractive index of acetonitrile, used for sample and
reference measurements, is 1.344. The Φf values of
the complexes (1N–1COOH, 1N–2COOH, 2N–1COOH
and 2N–2COOH) in acetonitrile solution were found
to be 0.036778, 0.034466, 0.035179 and 0.030158,
respectively. The PL of the complexes shows a single
exponential decay in acetonitrile solution with excited
state lifetimes (τ) of 0.9034, 1.153, 0.7248 and
0.5359 ns for 1N–1COOH, 1N–2COOH, 2N–1COOH
and 2N–2COOH, respectively (Figure 6). The lifetime
becomes shorter with an increase in the number of
withdrawing ring nitrogens, indicating the enhance-
ment in the photoactivity of the complexes.
4 | CONCLUSIONS
Four new Ru‐based complexes with various combinations
of COOH groups and ring nitrogens were successfully
synthesized, as confirmed via 1H NMR and 13C NMR
spectra and elemental analysis. The effect of the with-
drawing groups on the optical and structural properties
was investigated both theoretically and experimentally.
DFT and TD‐DFT calculations were found to be reliable
tools for predicting the emission performance of the com-
plexes. The energy gap was found to increase in the order
1N–1COOH < 1N–2COOH ~ 2N–1COOH < 2N–2COOH.
Consequently, the reactivity slightly decreased with
increasing number of anchoring groups and greatly
decreased with increasing number of nitrogen atoms in
the ligand. The emission spectra were found to depend
on the type and number of the withdrawing groups
attached to the bpy rings. The highest excitations in
the oscillator strength were the fifth and sixth excited
states, which were in the order 1N–1COOH > 1N–
2COOH > 2N–1COOH > 2N–2COOH. Upon increasing
the number of withdrawing groups, either COOH groups
or the number of ring nitrogens, the electrons were found
to be withdrawn from the carbon orbitals and resonated
to reach the metal and accumulated around it. An
increase in the electron‐withdrawing groups enhanced
the MLCT mechanism instead of the LLCT mechanism.
Increasing the number of nitrogen atoms was found to
play the dominant role compared to adding an extra
COOH group in the observed blue shift. Also, the more
electron‐withdrawing groups added, the lower the Stokes
shift became. The complexes with more withdrawing
groups showed more intense emission peaks with shorter
lifetime, indicating the enhancement in the photoactivity
of the complexes. The ring nitrogen showed the highest
effect on the enhancement of the emission spectra with
a lifetime of 0.5359 ns.
FIGURE 6 Voltage transients of the synthesized complexes in
acetonitrile

ALI ET AL.
9 of 9
[14] M. Frisch, G. Trucks, H. Schlegel, G. Scuseria, M. Robb,
J. Cheeseman, G. Scalmani, V. Barone, G. Petersson,
H. Nakatsuji, Gaussian Inc., Wallingford, CT, 2016.
ACKNOWLEDGEMENTS
The authors acknowledge financial support from the
Science and Technology Development Fund (STDF)
provided by the Egyptian Government (grant no. 5415).
We also acknowledge Bibliotheca Alexandrina HPC for
allowing us to use the HPC resources.
[15] W. J. Hehre, R. Ditchfield, J. A. Pople, J. Chem. Phys. 1972, 56, 2257.
[16] a) T. H. Dunning, P. J. Hay, Gaussian Basis Sets for Molecular
Calculations, Springer 1977 1. b) P. J. Hay, W. R. Wadt,
J. Chem. Phys. 1985, 82, 270.
[17] A. D. Becke, J. Chem. Phys. 1993, 98, 5648.
[18] a) C. Lee, W. Yang, R. G. Parr, Phys. Rev. B 1988, 37, 785. b)
B. Miehlich, A. Savin, H. Stoll, H. Preuss, Chem. Phys. Lett.
1989, 157, 200.
CONFLICT OF INTEREST
The authors declare no conflicts of interest.
[19] T. Yanai, D. P. Tew, N. C. Handy, Chem. Phys. Lett. 2004, 393, 51.
[20] E. Runge, E. K. Gross, Phys. Rev. Lett. 1984, 52, 997.
ORCID
[21] F. Huang, Y. Dkhissi, W. Huang, M. Xiao, I. Benesperi, S.
Rubanov, Y. Zhu, X. Lin, L. Jiang, Y. Zhou, A. Gray‐Weale, J.
Etheridge, C. R. McNeill, R. A. Caruso, U. Bach, L. Spiccia,
Y.‐B. Cheng, Y. Dkhissi, W. Huang, M. Xiao, I. Benesperi, S.
Rubanov, Y. Zhu, X. Lin, L. Jiang, Y. Zhou, A. Gray‐Weale, J.
Etheridge, C. R. McNeill, R. A. Caruso, U. Bach, L. Spiccia,
Y.‐B. Cheng, Nano Energy 2014, 10, 10.
Nageh K. Allam https://orcid.org/0000-0001-9458-3507
REFERENCES
[1] Y. Chi, P.‐T. Chou, Chem. Soc. Rev. 2007, 36, 1421.
[2] P.‐T. Chou, Y. Chi, Eur. J. Inorg. Chem. 2006, 17, 3319.
[3] P.‐T. Chou, Y. Chi, Chem. Eur. J. 2007, 13, 380.
[22] G. A. Zhurko, D. Zhurko, Chemcraft: graphical software for
visualization of quantum chemistry computations. http://
www.chemcraftprog.com/ (accessed June 16, 2018).
[4] F. Heinemann, J. Karges, G. Gasser, Acc. Chem. Res. 2017,
50, 2727.
[23] R. Dennington, T. Keith, J. Millam, K. Eppinnett, W. L. Hovell,
[5] S. Fantacci, F. De Angelis, Coord. Chem. Rev. 2011, 255, 2704.
R. Gilliland in GaussView, Version, 2009.
[6] F. Alary, J.‐L. Heully, L. Bijeire, P. Vicendo, Inorg. Chem. 2007,
46, 3154.
[24] G. A. Zhurko, D. A. Z. Chemcraft: graphical software for visu-
alization of quantum chemistry computations. http://www.
chemcraftprog.com/ (accessed Jun 16, 2018).
[7] B. A. Ali, N. K. Allam, Spectrochim. Acta A 2018, 188, 237.
[8] a) C. Bizzarri, C. Fléchon, O. Fenwick, F. Cacialli, F. Polo, M. D.
Gálvez‐López, C.‐H. Yang, S. Scintilla, Y. Sun, R. Fröhlich,
ECS J. Solid State Sci. Technol. 2016, 5, R83. b) D. Di Censo,
S. Fantacci, F. De Angelis, C. Klein, N. Evans, K.
Kalyanasundaram, H. J. Bolink, M. Grätzel, M. K.
Nazeeruddin, Inorg. Chem. 2008, 47, 980. c) N.‐Y. Chau,
P.‐Y. Ho, C.‐L. Ho, D. Ma, W.‐Y. Wong, J. Organometal.
Chem. 2017, 829, 92.
[25] W. Sharmoukh, K. C. Ko, J. H. Ko, H. J. Nam, D.‐Y. Jung, C.
Noh, J. Y. Lee, S. U. Son, J. Mater. Chem. 2008, 18, 4408.
[26] L. E. R. Buckley, B.J. Coe, D. Rusanova, S. Sanchez, M. Jirasek,
V. D. Joshi, J. Vavra, D. Khobragade, L. Pospisil, S. Ramesova,
I. Cisarova, D. Saman, R. Pohl, K. Clays, N. V. Steerteghem, B.
S. Brunschwig, F. Teply, Dalton Trans. 2017, 46, 1052.
[27] W. Sharmoukh, W. M. Hassan, P. C. Gros, N. K. Allam, RSC
Adv. 2016, 6, 69647.
[9] J. J. Kim, Y. You, Y.‐S. Park, J.‐J. Kim, S. Y. Park, J. Mater.
Chem. 2009, 19, 8347.
[28] F. H. Allen, O. Kennard, D. G. Watson, L. Brammer, A. G.
Orpen, R. Taylor, J. Chem. Soc. Perkin Trans. 1987, (12), 1.
[10] a) R. Lakshmanan, N. Shivaprakash, S. Sindhu, J. Fluoresc.
2018, 28, 173. b) J. A. Barron, S. Bernhard, P. L. Houston, H.
D. Abruna, J. L. Ruglovsky, G. G. Malliaras, Chem. Eur. J.
2003, 107, 8130. c) C.‐Y. Liu, A. J. Bard, J. Am. Chem. Soc.
2002, 124, 4190. d) H. Rudmann, S. Shimada, M. F. Rubner,
J. Am. Chem. Soc. 2002, 124, 4918.
SUPPORTING INFORMATION
Additional supporting information may be found online in
the Supporting Information section at the end of the article.
[11] Y. L. Tung, S. W. Lee, Y. Chi, L. S. Chen, C. F. Shu, F. I. Wu,
A. J. Carty, P. T. Chou, S. M. Peng, G. H. Lee, Adv. Mater.
2005, 17, 1059.
How to cite this article: Ali BA, Sharmoukh W,
Elnagar MM, Hassan ZM, Allam NK. Experimental
and density functional theory insights into the
effect of withdrawing ligands on the fluorescence
yield of Ru(II)‐based complexes. Appl Organometal
Chem. 2018;e4677. https://doi.org/10.1002/aoc.4677
[12] a) C. Sahin, A. Goren, S. Demir, M. S. Cavus, New J. Chem.
2018, 42, 2979. b) F. Cervantes‐Navarro, D. Glossman‐Mitnik,
Chem. Cent. J. 2012, 6, 70. c) Q. Peng, A. Obolda, M. Zhang,
F. Li, Angew. Chem. 2015, 127, 7197. d) E. M. Zayed, G. G.
Mohamed, W. M. I. Hassan, A. K. Elkholy, H. Moustafa, Appl.
Organometal. Chem. 2018, 32, e4375.
[13] W. Kohn, Rev. Mod. Phys. 1999, 71, 1253.