Bi-nuclear luminescent europium(III) molecular complexes for white light emitting diodes: Experimental and theoretical study

Article abstract of DOI:10.1016/j.ica.2019.05.014

Two newly designed and synthesized ancillary ligands with N1 functionalization (phenyl and fluorene) and its influence on photophysical properties of their corresponding binuclear europium complexes were investigated systematically. Photoluminescence (PL)

Full text of DOI:10.1016/j.ica.2019.05.014

Accepted Manuscript
Research paper
Bi-nuclear luminescent Europium(III) molecular complexes for white light
emitting diodes: Experimental and theoretical study
Rajamouli Boddula, Sivakumar Vaidyanathan
PII:
S0020-1693(19)30528-6
https://doi.org/10.1016/j.ica.2019.05.014
ICA 18915
DOI:
Reference:
Inorganica Chimica Acta
To appear in:
Received Date:
Revised Date:
Accepted Date:
16 April 2019
8 May 2019
8 May 2019
Please cite this article as: R. Boddula, S. Vaidyanathan, Bi-nuclear luminescent Europium(III) molecular complexes
for white light emitting diodes: Experimental and theoretical study, Inorganica Chimica Acta (2019), doi: https://
doi.org/10.1016/j.ica.2019.05.014
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers
we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and
review of the resulting proof before it is published in its final form. Please note that during the production process
errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Bi-nuclear luminescent Europium(III) molecular complexes for white light emitting
diodes: Experimental and theoretical study
Rajamouli Boddula, and Sivakumar Vaidyanathan *
Department of Chemistry, National Institute of Technology Rourkela, Rourkela-769 008,
Odisha, India.
* To whom correspondence should be addressed. Email: vsiva@nitrkl.ac.in (V. Sivakumar)
Tel: +91-661-2462654;

Abstract
Two newly designed and synthesized ancillary ligands with N1 functionalization (phenyl and
fluorene) and its influence on photophysical properties of their corresponding binuclear
europium complexes were investigated systematically. Photoluminescence (PL) emission
spectral study was shown dominant electric dipole transition of EuIII ion (5D0→7F2) and it is
clearly indicated that the EuIII ion occupies in the non-centrosymmetric site. The calculated
parameter via Judd-Ofelt shown ligands are the best sensitizer for EuIII metal ion. In addition,
there is no emission in the region of 400-550 nm (belongs to ligand), indicates that the
efficient energy transfer from ligand to central EuIII ion in the complex. The energy transfer
mechanism was proposed by the help of DFT and TD-DFT calculations.The obtained highest
QY (59.5 %, thin film) of the complex, Eu2(TTA)6(L2) indicating efficient energy
sensitization from fluorene-based ligand than that of phenyl based Eu-complex. The
calculated CIE color coordinates are well matched with NTSC standard values. The binuclear
Eu-complexes were combined with InGaN near UV LED, obtained pure red emission
(forward bias 20 mA) with CIE color coordinate value x = 0.65, y = 0.34 and x = 0.66, y =
0.33 for Eu2(TTA)6(L1) and Eu2(TTA)6(L2), respectively.
Key words: Europium luminescence, spacer molecule, bi-nuclear complex.

Introduction
Brilliant luminescent performances of molecular lanthanide complexes have been
widely studied due to their line like and long life (lifetime ranging from s–ms)
emission ranging from visible and Near-IR region, which are caused by an inter and/or
intramolecular energy transfer from the coordinated ligands to the central metal ions
[1-3]. In general, the sensitized lanthanide luminescence is highly essential (due to low
absorption strength/coefficients of lanthanide ions) and it arises from the singlet
excited state or intra-ligand charge transfer states, the antenna chromophore-centred
triplet state (the most frequently encountered mechanism), which consequently must
be higher in energy than the lanthanide excited emissive state (triplet energies of the
well-known ligands are hexafluoroacetylacetonate (hfa, 22,000 cm-1),
benzoyltrifluoroacetylacetonate (bfa, 21,400 cm-1), dibenzoylmethane (dbm, 20,600
cm-1), 1,10-phenanthroline (phen, 21,480 cm-1) and tetraazatriphenylene (aza, 23,800
cm-1) [4-8]. This constrains the range of chromophores that can be used to sensitize
lanthanide ions that emit in the visible spectral region. β-diketonates and aromatic N-
donor ligands have recognized to be efficient chelators and/ sensitizers of lanthanide
emission [9-10]. Design of white light emitting molecular lanthanide complexes are
major importance and have attracted recent years by many researchers, because of
their potential applications in smart lighting and display devices [11-15].
Careful ancillary or anionic ligand engineering (increase in absorption (oscillator
strength) at the wavelength of interest) enables the design of molecular lanthanide
complexes leads to red emission with a high quantum yield (efficient energy transfer
from ligand to Ln ion) and tunable emission (partial or incompetent energy transfer).
Shielding of lanthanide coordination environment by ligand is highly necessary, to
prevent the non-radiative transition of lanthanides excited state, induced by high-

frequency oscillators, like the O−H and C−H vibrations of the solvent (play a vital role
in reducing the overall quantum yield) [16-17]. Previously we have reported,
designing of bipolar ligand and their influence on PL properties of EuIII ions and
explored their applications in the field of solid-state lighting [18-22]. One of our recent
contributions, we have explored phenanthroimidazole-CBZ based bipolar ligand for
Eu(III) complex with ethoxy spacer molecule [21].
In the present investigation, we designed and synthesized two new phenanthroline
based ancillary ligands (two phenanthroimidazole rings separated by phenyl and
ethoxy chain, functionalized in the N1 position with phenyl and fluorene group) and
used the same for the synthesis of binuclear Eu-complexes. The luminescent europium
site isolation (can prevent concentration quenching effect) can be achieved by using
the long ethoxy chain (O-C4H8-O) along with phenyl (Ph) group. We have shown that
the new two binuclear Eu-complex, thereafter labeled Eu2(TTA)3(L1), Eu2(TTA)3(L2)
allows both PL and EL show characteristic red emission. The majority of the
mononuclear Eu-complexes are proved to be the best materials, however, some of the
bi and trinuclear Eu-complexes are shown the finest results. As compared with some
of the mononuclear complexes, the dinuclear complex offers better thermal stability
and stronger luminescence (enhanced QY), which makes it a promising light-
conversion molecular device [23-25]. Some of the literature work [26-30] shown that
the quantum efficiencies of the bi and trinuclear europium complexes are higher
(about 10%) than that of mononuclear ones. In addition, their lifetime of these
europium complexes was longer than the mononuclear ones. The chosen
Phenanthroline moiety is a rigid planar, hydrophobic, electron-accepting hetero
aromatic system, whose nitrogen atoms act cooperatively in cation binding for Eu-
metal ion. In addition, it can reduce the non-radiative decay of the excited states of the

europium ion and improve the stability of the europium complexes. It can also benefit
to improve the thermal stability of the complex [31]. The chosen flexible alkyl spacer
molecule (C-C bond) can make the flexible nature in the complex and may affect on
stability [32]. It can also be used to arrest the electronic communication between the
two phenanthroimidazole moieties to get efficient emission from the Eu-complexes
[21, 33, 34].
Both the ligands and their binuclear Eu-complexes were characterized by various
spectroscopic techniques (NMR, FT-IR, Mass and elemental analysis). The ligand
structures were optimized by using DFT calculations and their excited state
photophysical properties were evaluated by time-dependent density functional theory
(TD-DFT) calculation. The designed and synthesized ancillary ligands (L1 and L2)
and
the
corresponding
binuclear
Eu(III)
complex
(Eu2(TTA)3(L1),
Eu2(TTA)3(L2))molecular structure is depicted in Fig. 1.
Spacer
R
N
N
N
N
N
N
O
O
N
N
R

R = Ph, Phen(Ph)-Ph-O-C4H8-O-Ph-(Ph)Phen, (L1)

R = Fl,
Phen(Fl)-Ph-O-C4H8-O-Ph-(Fl)Phen, (L2)
Spacer
F F
S
F
F
R
N
O
O
N
N
O
O
N
Eu
O
Eu
O
N
N
N
N
R
S
F
F
3
3

R = Ph, Eu2(TTA)6(L1)
Antenna
Antenna

R = Fl,
Eu2(TTA)6(L2)
Fig. 1. Chemical structure of bipolar ligand and the corresponding binuclear Eu(TTA)3
complex.

Characterization of the complexes
The detailed synthetic condition and reactions of ligands and their binuclear Eu(III)
complexes are summarized in Scheme 1.
O
O
OH
Br
Br
K2CO3,
O
H
O
H
O
R
O
N
diketo
NH2
N
ACN
dial
O H
NH4OAc,
NH2
or
R =
Acetic acid
Fl
Ph
R
*
N
N
N
N
*
R =
O
O
or
N
N
N
N
R
Fl
Ph
R = Ph, Phen(Ph)-Ph-O-C4H8-O-Ph-(Ph)Phen, (L1)
R = Fl, Phen(Fl)-Ph-O-C4H8-O-Ph-(Fl)Phen, (L2)
S
S
Cl
Eu
Cl
NaOH, Ethanol
O
OH2
Eu
OH2
O
O
6H2O
Cl
L1 / L2
H2O, RT- 60oC,
74%
O
F
F F
F
F F
Eu (TTA) 3 .2H2O
3
TTA
CHCl3
60 OC
F F
S
F
R
N
O
O
N
O
O
N
N
Eu
O
Eu
O
N
N
N
N
F
R
S
F
F
3
3
R = Ph, Eu2(TTA)6(L1)
R = Fl, Eu2(TTA)6(L2)
Scheme 1. Synthetic route of the spacer(ether) attached and N1-functionalized
phenthroimadazole ligand and the corresponding -diketonateEu(III) ternary binuclear
complex.

Experimental
Materials: The compounds 4-hydroxybenzaldehyde (starting material), aniline and 1,4-
dibromom butane, 1, 10-phenanthroline, fluorene and thenoyltrifluoroacetone (TTA) and
europium chloride hexahydrate (99.99%) and tetrabutylammonium perchlorate (nBu4NClO4)
electrolyte were purchased from Sigma-Aldrich chemicals company. All the reactions were
monitored by thin-layer chromatography (TLC) with silica gel 60 F254 Aluminium plates
(Merk). Column chromatography was carried out using silica gel from Aldrich (70 – 230
mesh, 60 Å). The solvents (tetrahydrofuran, ethanol, dimethyl sulfoxide, hexane etc.) which
are used in the current study were carefully dried and distilled from appropriate drying agents
prior to use.
1
Measurements: H- and 13C-NMR spectra were recorded using an AV 400 Avance-III
400MHz Fourier Transform nuclear magnetic resonance (FT-NMR) Spectrometer Bruker
Biospin International, Switzerland, in deuterated chloroform/dimethyl sulfoxide solution.
Chemical shifts were quoted relative to tetramethylsilane (TMS, standard reference). Fourier
Transform infrared (FT-IR) spectra were recorded on a Perkin-Elmer, USA/ RX-I FTIR
spectrophotometer and elemental analyses were measures by Elementar Analysen Systeme,
Germany/Vario EL spectrometer. Mass spectra were recorded by using LC-Mass
spectrometry (Perkin Elmer, USA/ Flexer SQ 300 M). Powder X-ray diffraction of ligand and
the Eu(III) complex were measured by powder X-ray diffractometer (XRD) using Cu-
K1radiation (Rigaku, ULTIMA IV). DSC-TGA (thermal decomposition) was performed by
using Netzsch, Germany, STA449C/4/MFC/G. The absorption spectrum in solution, solid
(Diffuse reflectance spectroscopy, DRS) and thin film were measured by using UV-Visible
spectrometer (Shimadzu Corporation, Japan or UV-2450 Perkin Elmer, USA/Lamda 25 and
Lamda Perkin Elmer). The PL emission spectrum in solution phase and thin film were
recorded by HoribaJobinYvon, USA/Fluoromax 4P spectrophotometer. The CIE coordinates

were calculated by using PL emission data with the help of MATLAB software. The
electrochemical properties of the complex and ligand were measured by using cyclic
voltammetry (CV), AUTOLAB 302N Modular potentiostat at RT in dichloromethane. The
working (glass-carbon rod), auxiliary (counter, Pt wire) and reference (Ag/Agcl wire)
electrodes were used for CV analysis. The dichloromethane which contains 0.1 M Bu4NClO4
was used as the supporting electrolyte and scan rate was maintained as 100 mV s-1 The
.
ligand was optimized within density functional theory (DFT) framework using B3LYP/6-31G
(d, p) level of theory.After conforming the ground state geometry for the ligand we have
vertically excited the molecule to get the UV-Visible spectra and first excited state using TD-
DFT. To understand PL spectra of the Eu(III) complex, triplet excited state of the ligand is
also obtained by using the same methodology mentioned above. The optimization of the
ligand structures was carried out in a G09W suit of the program. The triplet energy of the
synthesized ligand was estimated by measuring the phosphorescence spectra of the ligand
(Gd-complex) at 77 K by using Edinburgh FLS 920 spectrofluorimeter, UK system consists
of a 450 w Xenon lamp (Xe 900) mounted in an air-cooled lamp housing with a built-in de-
ozonizer filter. The ozone free Xe arc lamp emits continuous radiation from 190 to 2600 nm.
A lifetime of the Eu(III) complex, as well as ligand, were measured at 298 K with Edinburgh
Instruments FLS 980 based on the time-correlated single photon counting technology upon
the excitation at 360 nm for complexes. A pulsed xenon lamp was used as the excitation
source and the signals were detected with a photomultiplier.The solution and solid QY of the
complexes were measured by using Edinburgh spectrofluorometer FS5 (integrating sphere).
Synthesis: The intermediate compounds are synthesized according to the previously reported
literature [16,17]. 9,9-diethyl-9H-fluoren-2-amine (Fl), 1,10-phenanthroline-5,6-dione and
the metal compound tris(thenoyltrifluoroacetone)europium (III) (Eu(TTA)3.2H2O) was
synthesized by using the well-known method.

Synthesis of di-aldehyde(dial) compound:
4-hydroxybenzaldehyde was taken in a round bottom flask and added acetonitrile (CH3CN)
solution. To this reaction mixture added potassium carbonate base and stirred for 10 minutes
then added 1, 4-dibromo butane. The resulting reaction mixture was stirred for 48h at 85 C
under nitrogen atmosphere. The resulting of the reaction mixture was checked by TLC (ethyl
acetate (EtOAc) in pet ether 2:8, Rf-0.3). The resulting of the compound was filtered to
remove the excess of the base from the reaction mixture. After that, the reaction mixture was
extracted with ethyl acetate and followed by dried with anhydrous sodium sulphate. The
solvent was evaporated to get 8 g crude compound. The obtained crude was purified by
column chromatography by using silica gel (100-200 mesh), eluent with 15 % ethyl acetate in
pet ether. The obtained pale yellow color solid was 6.1 g (50.0 %). 1H-NMR Data (400 MHz,
DMSO-d6): δ 9.89 (s, 2H), 7.86-7.84 (m, 4H), 7.02-7.00 (m, 4H), 4.16-4.13 (m, 4H), 2.07-
13
2.04 (m, 4H). C-NMR Data (100 MHz, DMSO-d6): δ 190.8, 163.9, 132.0, 129.9, 114.7,
67.7, 25.7.
Synthesis
of
2-(4-(4-(4-(1-phenyl-1H-imidazo[4,5-f][1,10]phenanthrolin-2-
yl)phenoxy)butoxy)phenyl)-1-phenyl-1H-imidazo[4,5-f][1,10]phenanthroline (L1):
The aniline (0.1.22mL, 13.42mmol) was taken in a round bottom flask and added dial
compound (2.0 g, 6.771mmol) in glacial acetic acid (30 mL) at room temperature. To this
reaction mixture subsequently, ammonium acetate (5.16 g, 67.11mmol) and diketo compound
(2.81 g, 13.42mmol) were added. The resulting mixture was stirred for 12 hrs at 110 C under
nitrogen atmosphere. The resulting of the reaction mixture of was monitored by TLC (MeOH
in Chloroform 1:9, Rf-0.4). The RM was poured into the minimum amount of water and then
added the ammonium hydroxide solution to neutralize the reaction mixture (pH ~7). The
solid was filtered and dissolved in dichloromethane, followed by dried with anhydrous
sodium sulphate. The solvent was evaporated to get 3.50 g crude compound. The obtained

crude was purified by column chromatography by using silica gel (100-200 mesh), eluent
with 4% methanol in chloroform. The purified product was dissolved in minimum amount of
THF solution added an excess of hexane solvent, the pale yellow color solid was formed is
400 mg (55.0%). 1H-NMR Data (400 MHz, DMSO-d6): δ 9.08 (dd, 2H, J = 2 Hz, J = 4.4 Hz),
9.01 (dd, 2H, J = 1.6 Hz, J = 8 Hz), 8.94 (dd, 2H, J = 1.6 Hz, J = 4 Hz), 7.87-7.84 (m, 2H),
7.75-7.69 (m, 10H), 7.51 (d, 4H, J = 8.8 Hz), 7.48-7.45 (m, 2H), 7.33 (dd, 2H, J = 1.2 Hz, J =
8.4 Hz), 6.91 (d, 4H, J = 8.8 Hz), 4.04 (br-s, 4H), 1.85 (br-s, 4H).13C-NMR Data (100 MHz,
DMSO-d6): δ 160.1, 153.0, 148.8, 144.3, 138.0, 130.9, 130.9, 129.3, 126.8, 124.1, 122.8,
122.2, 121.0, 114.6, 67.5, 25.4.Elemental analysis: Anal. Calc. for C54H38N8O2: C, 78.05; H,
4.61; N, 13.49; Found: C, 78.01; H, 4.58; N, 13.53 %. EI-mass: m/z, Cal. 830.93; found
831.52 (M+1).
Synthesis
of
2-(4-(4-(4-(1-(9,9-diethyl-9H-fluoren-7-yl)-1H-imidazo[4,5-
f][1,10]phenanthrolin-2-yl)phenoxy)butoxy)phenyl)-1-(9,9-diethyl-9H-fluoren-7-yl)-1H-
imidazo[4,5-f][1,10]phenanthroline (L2):
The same procedure which is followed for the L1 was carried out ro synthesis of the ligand
L2.NMR Data (400 MHz, DMSO-d6): δ 9.07 (m, 1H), 9.01 (d, 1H, J = 8 Hz), 8.91 (d, 1H, J =
4 Hz), 8.17-8.10 (m, 1H), 7.99-7.97 (m, 1H), 7.89-7.87 (m, 2H), 7.68-7.62 (m, 1H), 7.55 (d,
2H, J = 9.2 Hz), 7.49-7.47 (m, 2H), 7.42-7.38 (m, 3H), 6.81 (dd, 2H, J = 2.4 Hz, J = 8.8 Hz),
3.95-3.94 (m, 2H), 3.80-3.79 (m, 1H), 3.68-3.65 (m, 1H), 2.03-2.00 (m, 4H), 0.32-0.28 (m,
3H), 0.16-0.11 (m, 3H). 13C-NMR Data (100 MHz, DMSO-d6): δ 165.0, 159.7, 152.3, 152.2,
150.3, 148.8, 147.9, 144.1, 143.3, 140.1, 136.9, 135.5, 130.9, 130.1, 128.7, 128.0, 127.8,
127.7, 126.9, 124.3, 124.2, 123.8, 123.4, 122.6, 122.4, 122.1, 121.2, 119.9, 114.5, 97.6, 82.9,
68.7, 67.6, 66.1, 56.75, 33.6, 32.3, 25.5, 23.7, 8.74, 8.51. Elemental analysis: Anal. Calc. for
C76H62N8O2: C, 81.55; H, 5.58; N, 10.01; Found: C, 81.52; H, 5.52; N, 10.05 %. EI-mass:
m/z, Cal. 1118.5; found 1120.24 (M+1).

Synthesis of Eu2(TTA)6(L1): Taken two neck round bottom flask (50 mL) with nitrogen-
containing balloon adaptor and poured Eu(TTA)3.2H2O (205 mg, 0.240mmol, 1 eq) dissolved
in dry chloroform (CHCl3) (15 mL). To this solution, an added mixture of compound L1 (200
mg, 0.240mmol, 1eq) in chloroform (10 mL) and the reaction mixture (RM) stirred for 12h at
60 C temperature. The resulting product was dissolved in a minimum amount of THF and
excess of hexane added to get a solid product. The obtained final pale yellow color product
was 414 mg (70.0%). Elemental analysis: Anal. Calc. for C102H62Eu2F18N8O14S6:C, 49.76; H,
2.54; N, 4.55; S, 7.81; Found: C, 49.72; H, 2.51; N, 4.59; S, 7.84%.
Synthesis of Eu2(TTA)6(L2):
The same procedure which is followed for the Eu2(TTA)6(L1) was carried out to the synthesis
of the ligand L2. Elemental analysis: Anal. Calc. for C124H86Eu2F18N8O14S6: C, 54.15, H,
3.15, N, 4.07, S, 7.00; Found: C, 54.12, H, 3.11, N, 4.02, S, 6.98%.
FT-Infrared spectroscopy:
The infrared spectra of the ligands and their complexes were measured in the room
temperature in the range of 500 - 4000 cm-1. The observed major strong carbonyl (C=O)
stretching frequency fictional group of Eu(TTA)3. 2H2O complex found to be at 1614 cm-1and
which is shifted to 1601 cm-1 for the Eu2(TTA)6(L1) and 1599 cm-1 for the Eu2(TTA)6(L2)
(Fig. 2). These changes are the evidence that the -diketonate (Eu(TTA)3) compound
coordinated with ligand (L1 and L2), respectively. It leads to a form of the C=N bond is
converted into a structure of C=N–Eu-O bond. In addition, the vibration frequency of C=N
appears at 1605 cm-1(L1) and 1602 cm-1 (L2) in the ligands were shifted to 1538 and 1537
cm-1 in the complex, respectively. These major functional group changing is indicated that the
ligand coordinated with Eu(III) ion. Furthered, it was also supported by remained functional
groups varying from ligand to complex which are tabulated in the Table 1. These results are
concluding that the N atom in ligand coordinates with Eu(TTA)3.

Fig. 2.FT-IR spectra of the ligands and corresponding Eu(III)-complexes as well as
comparison with Eu(TTA)3.
Table 1. The Infrared frequencies (wavenumber in cm-1) at room temperature (RT) for free
ligands and its corresponding Eu2(TTA)3(L1),Eu2(TTA)3(L2) complex and Eu(TTA)3.
(Wavenumbe Eu2(TTA)3(L1) Eu2(TTA)3(L2)
r in cm-1)
L1
L2
Eu(TTA)3
1601
1538
1249
1307
1599
1537
1250
1306
--.--
1605
1247
--.--
--.--
1602
1245
--.--
1614
--.--
 (C=O)
 (C=N)
 (C-N)
--.--
1300
1137
--.--
 (C-F)
1142
832,
786
1140
835,
785
--.--
836,
742
--.--
839,
740
 (C-CF3)
 (C-H phen)
Results and Discussion
The synthesis of binuclear Eu-complexes (Eu2(TTA)6(L1) and Eu2(TTA)6(L1)) and
their photophysical/luminescent properties were discussed in detail in the results and

discussion part. The effect of N1 functionalization of the ligand by phenyl and
fluorene moiety (L1 and L2) and the binuclear Eu-complexation by the same has been
achieved. The synthesized ligands and their corresponding binuclear Eu-complexes
were characterized by using various spectroscopic techniques (nuclear magnetic
13
resonance spectroscopy (1H and C NMR, Fig. S1-S3), ESI-Mass spectrometry (Fig.
S4-S5), Elemental analysis, Fourier transforms infrared spectroscopy (FT-IR), etc).
All the measurements were carried out at room temperature (RT). Coordination of the
ligand and complex confirmation made by FT-IR and elemental analysis made to
confirm the presence of the molecular complex. The thermal decomposition and
morphological study of the ligands as well as the corresponding binuclear Eu
complexes also been discussed. The synthesized binuclear EuIII complexes were
imbedded in a PMMA matrix with different EuIII complex concentrations and studied
their photophysical properties. The detailed PL excitation and emission spectra
express the essential information regarding the absorption characteristics of the
attached chromophoric ligating group and site symmetry with respect to EuIII ion in
the complex. The theoretical calculations were performed by using density functional
theory (DFT) frame work using B3LYP/6-31G (d, p) level of theory, to estimate the
energy levels to support the mechanistic pathway of energy transfer from ligand to Eu
ion in the complex.
Thermal Properties and Powder X-ray diffraction (PXRD) studies
The thermal behaviour of the ligand (L1 and L2) and the corresponding binuclear Eu-
complex (Eu2(TTA)6(L1) and Eu2(TTA)6(L2)) under a nitrogen atmosphere were
examined by means of thermogravimetric analysis (TGA). The general profile of the
weight loss for the ligands and the binuclear Eu-complexes are displayed in Fig. 3.
The TGA of the complexes, Eu2(TTA)6(L1) and Eu2(TTA)6(L2) shown 10 %

decomposition at 283 and 279 C, respectively. The TGA of the complexes,
Eu2(TTA)6(L1) and Eu2(TTA)6(L2) are shown first minor mass loss of approximately
6.01 % in the range of 110 to 165 °C which corresponds to the elimination of the
coordinated water and solvent molecules. The major second decomposition of the
Eu2(TTA)6(L1) and Eu2(TTA)6(L2) observed at ~320 C(lost 12.1 % and reached to
87.8 %) and ~316 C (lost 11.8 % and reached to 88.1 %), respectively which
corresponds to the degradation of the TTA ligands (ranging from 250-320 C) [35-
37].The third decomposition undergoes at 438 (lost 54 % and reached to 64 %) and
404 C (lost 61 % and reached 38.9 %) and final decomposition undergoes at 651 and
545 C, respectively. These correspond to the decomposition of the ancillary ligands
(presence of fluorene, Phenyl, phenanthroline groups). The ligand (L1 and L2) thermal
decomposition temperature is high as compared with Eu-complexes. The first small
decomposition of ligands (L1 and L2) shown at 184 and 170 C belongs to the water
molecules and major decomposition at 403 (lost 8.26 % and reached to 91.7 %) and
417 C (lost 14.1 % and reached to 85.8 %), respectively. The temperature at 442 and
225 C observed 10 % weight loss, which is attributed to the decomposition of ligand
fragments (fluorene, Phenyl, phenanthroline groups) presence in the ligand.As
compared with spacer alkyl chain attached mononuclear substituted Eu-complex
consisted of lower thermal stability than the currently synthesized bi-nuclear alkyl
chain spacer Eu-complex [21].
The DSC studies of the ligands and the corresponding binuclear complexes show the
glass transition temperature (Tg) ranging from 137 to 333 C. The ligand moieties are
shown good Tg values as compared to that of corresponding Eu complexes (the same
is marked in the DSC curve in Fig. 3 right). The Tg of the ligands L1 and L2 are 333
and 235 C as well as respective complexes, Eu2(TTA)6(L1) and Eu2(TTA)6(L2)

shown 142 and 137 C, respectively. The PXRD analysis of the complexes was shown
broadband spectra and it is indicating that the amorphous nature of the complexes. The
pictorial view of the PXRD and their2-theta (deg), d (ang.) are tabulated in Fig. S6 and
Table S1, S2; respectively.
Fig.3.TG analysis curves for the ligands (L1 and L2) and its corresponding binuclear Eu(III)
complexes ((Eu2(TTA)6(L1) and Eu2(TTA)6(L1))(left) and in onset mentioned their DSC
curves (right).
Photophysical properties
UV-Visible absorption and Diffuse Reflectance Spectral studies:
The absorption spectrum of the ligands (L1 and L2) and the corresponding binuclear Eu-
complexes is shown in Fig. 4, left. Singlet–singlet n–π* enolic transition was assigned to the
β-diketonate moiety in the complex. Both the ligands and the Eu complex show similar
absorption spectral profile, however, the absorption edge of the Eu complex is slightly shifted
towards near UV region (extending to a longer wavelength from 350 to 400 nm). In addition,
the absorption peak consists shoulder in the near UV region, which is not present in the case
of ligands. The strong absorption in the UV-Vis region is due to strong  to * transition of
aromatic chromophores present in the complex as well as ligand moieties. The occurrence or
coordination of the bipolar ligand not only enhances the absorption intensity but also fulfills

the high coordination number of the central metal EuIIIion. Thus, it improves the coordination
saturation as well as thermal stabilities of binuclear Eu-complexes. The calculated
absorptions peaks are tabulated in Table 2. The band gap was calculated for all the
compounds by converting the UV-visible data (Fig. 4, right) into Kubelka-Munk function.
The acquired spectrum is converted to Kubelka-Munk function ( = F(R)).38
αh = A(h - Eg)n
(1)
Fig. 4. The UV-Visible absorption spectra of the bipolar ligands and corresponding
binuclear Eu-complex (left). Bandgap calculations from solid UV-Vis spectra of
ligand and complexes (right).
Table 2. The UV-absorption and PL emission data of synthesized Eu(III) complexes and
ligands.
S.
Compound
name
FWHM I2/I1
ratio
UV max (abs)[nm] ex[nm]
Solution (a) (b) Solution/solid
PL em(c)
No.
[nm]
1.
2.
Eu2(TTA)6(L1) 246, 284, 335
Eu2(TTA)6(L2) 245, 285, 335
321, 374 /272, 580, 592, 612, 4.31/4.4 14.2/
230, 426(d)
652, 702
3(d)
13.8(d)
320, 365/270,
230, 438(d)
580, 592, 612, 4.78/
652, 702 6.26(d)
16.0/
10.2(d)

3.
4.
5.
L1
249, 269, 290, 324
248, 271, 289, 326
273, 340
347/275, 320,
365(d)
425/448(d)
425,/521(d)
--.--
--.--
L2
352/276, 315,
365(d)
--.--
--.--
∼ 6
∼11
Eu(TTA)3
275, 340
580, 592, 612,
652, 703
[a] measured at 298 K in chloroform solution, [b] absorption peaks from the UV-absorption spectra, [c]
emission peaks from PL emission spectra, [d] values of compounds in solid.
Photoluminescence studies:
The synthesized ligand and binuclear EuIII complexes PL in solution state excitation
spectra in chloroform solvent, c = 1 × 10−5 M were recorded at room temperature. The
ligands, L1 and L2 in solution form are shown emission at a blue region which is 425
nm at excitation wavelength 347 and 352 nm, respectively (Fig. 4). However, the solid
of the ligands, L1 and L2 shown emission at 448 and 521 nm with excitation
wavelength 365 nm, respectively (Fig. 5). The spectral shifting (bathochromic shift)
from the solution to solid was observed is due to aggregation. In the Eu-complexes the
excitation spectral profile was taken by monitoring the 5D0 → 7F2 (612 nm) electric
dipole transition of the EuIII ion. The spectra consist of broad absorption in the UV
region (broadband between 275 and 450 nm), which can be assigned to the π–π*
electronic transition of the aromatic moieties present in the ligand. The absence of f–f
transitions of the EuIII ion in the absorption spectrum evidence that the luminescence
sensitization via the excitation of the ligand is effective in the presently studied
complexes. In other wards it can says that the f-f transition are absent because they are
too weak to be observed in a diluted solution.

Fig. 5. The PL excitation and emission spectra of the ligands (L1 and L2) in solution (left)
and solid state (right).
The PL emission of the complexes and ligands were studied in solution and solid
phase (Fig. 6). The emission bands of the EuIII complexes are observed at around 579,
592, 612, 650 and 695 nm, and are attributed to the 5D0–7FJ (f–f transitions) with J = 0,
1, 2, 3 and 4, respectively.The 5D0 → 7F0 transition is weak and situated at 579 nm
5
(single peak) due to D0-7F0 transition indicates that all EuIII ions in the structure
occupy a site of the same symmetry and experience the similar crystal field
perturbation in the complex. The moderately strong 5D0 → 7F1 transition is indicated
when EuIII ion occupies the center of the symmetric site and this magnetic dipole
transition
is
independent
of
the
coordinate
environment.
The
hypersensitive 5D0 → 7F2 transition (electric dipole) consists of a strong band at 612
nm,it leads to red luminescence of the complex and the local environment of the Eu
ion sense to be more non-symmetrical. The observed broad peaks at about 650 as well
as 695 nm correspond to the D0 → 7F3 and 5D0 → 7F4 transitions, respectively. It is
worth to note that the absence of a broad-band in the 380–530 (solution) and 380-700
nm region corresponding to the ligand emission in the emission spectra highlights that

the efficient ligand-to-metal energy transfer occurs in the presently studied Eu-
complexes.
Fig. 6. The PL excitation and emission spectra of the binuclear Eu(III) complex in solution
(left) and solid state (right).
The intensity of the electric dipole transition is more intense and it reaches the characteristic
red emission. The europium ion site symmetry or asymmetric ratio was calculated from the
5
5
integration emission intensity ratio of the D0-7F2 transition to D0-7F1 transition (I2/I1) from
the PL emission spectra of the complexes in solution as well as solid.The intensity ratio (I2/I1)
of the Eu(III) complex, Eu2(TTA)6(L1) in solution and solid are 14.2 and 13.8;
Eu2(TTA)6(L2) in solution and solid are 16.0 and 10.2; respectively. The intensity ratio of the
solution was found to be higher than solid. It is also indicating that the Eu(III) ion occupied
non-center of symmetry site. In addition, it was clear that the Eu(III) ion and ligand have
strong coordination interaction. As compared with the intensity ratio (I2/I1) of Eu(TTA)3 was
around 11.5 which is indicating that the current synthesized Eu-complexes are in high-
intensity ratio than Eu(TTA)3 in solution. The calculated full width at half maximum
(FWHM) values are 4.31 and 4.78 nm (solution); 4.43 and 6.26 nm (solid) for Eu2(TTA)6(L1)
and Eu2(TTA)6(L2), respectively. The less value of FWHM for the complexes is indicating

that the narrow emission intensive characteristic band is very much suitable to achieve the
efficient red emission.
It is generally believed that the energy transfer process proceeds through triplet pathway in
the europium luminescent complexes. The energy transfer from ligand to center metal ion is
very much necessary to understand the excited energy levels. Since the energy transfer from
the organic part towards metal ions occurs efficiently by two different pathways. In which the
first one, described by Forster, it takes into an account of multipolar interactions between
distinct oscillating centers [39]. The second one is Dexter’s mechanism, which can describe
as postulates energy transfer as electron exchange between the organic ligand and lanthanide
ion [40,41]. The similar nature of this process requires an overlap of the emission spectra of
the donor (ligand) and the absorption spectra of the acceptor. Based on these criteria it was
examined by the overlap between the emission spectrum of the ligand (L1 and L2) and
absorption spectra of the TTA (acceptor). The overlap observed in the region of 375-450 nm.
It is clearly indicating that the energy transfer process can appear through ligand triplet to
TTA triplet excited state through intramolecular energy transfer [42,43]. The process
implicates of photoexcitation of the ligand (antenna or organic chromophore) energy to EuIII
ion via their triplet (3T) states by means of internal conversion; finally, the EuIII ion emits
photons to ground state from first excited levels (5D0).It is also well documented that the
5
5
5
EuIIIexcited levels were located at D3 (24,800 cm−1), D2 (21,500 cm−1), and D1 (19,100
5
cm−1), to D0 (17,500 cm−1) feasible by internal conversion (thermal activation) [44].The
energy level matching is the key factor one need to consider for efficient energy transfer from
ligands to EuIII ion in the complex. Thus, the energy level’s match of the triplet state of the
ligands to 5D0 of EuIII is one of the vital factors, which directly influence the PLQY of the Eu
complex. It is also well known that anionic ligand (TTA) energy levels located at 25,164
(singlet) and 18,954 (triplet) [19]. The calculated singlet (1S) and triplet (3T) energy levels of

the EuIII complex by DFT and TD-DFT analysis of L1 and L2 were located at 29,013(3.59
eV), 27,172 (3.36 eV) cm-1 (singlet)and 22,173(2.74 eV), 22,152 cm-1 (2.74 eV) (triplet)
shown in Fig.7. The triplet levels of the ligands were also calculated by using Gd-complex at
77 K, which results are shown similar to that of the theoretical (Fig. S12). These obtained
results are showing capable of transferring the populated energy from the triplet state of the
ligand to 5D0 of EuIII metal ion efficiently. These outcomes were also supported according to
the Latvaet al., calculations [9] ((ΔE = E(T1) - E(5D0) is 2500 - 4000 cm-1; singlet above
25,000 (3.09 eV) and triplet above 20,000 cm-1 (2.47 eV)). Since the energy difference
between the 3T and 5D0 level is comparatively high and hence there is no reversible emission
possible (due to back transfer) from the complex and the same was reveals in the PL emission
spectra (no emission was observed in the rage of 400 to 550 nm for ligand moieties). It could
be expected that the fluorene in the complex sensitization boosted by decrease the vibronic
quenching effect. We have also reported recently the fluorene molecule at the N1 position in
the ligand is better sensitizer than consist of the alkyl chain (ethyl) [21]. The observed
complete energy transfer process was also been supported by Reinhoudt’s empirical rule [45].
In addition, it was also discussed that in this energy transfer mechanism TTA excited singlet
and triplet state can act as a transitional state between the singlet to triplet excited states of
5
the ligand as well as triplet excited state to the excited D0 level of Eu(III) ion, respectively.
The effective overlap between the ligating moieties and the absorption spectrum of TTA
showed that the characteristic luminescence of the corresponding EuIII occurs through
intramolecular energy transfer which is shown in pictorial view in Fig. S7.

Fig. 7. The energy transfer diagram for the complexes and energy transfer to EuIII ion from
ligand as well as TTA molecule (S = singlet, T = triplet, ET = energy transfer).
The emission under UV lamp (365 nm) of the ligands and complexes are in solution, thin
film and solid form are shown in Fig. 8. The blue color emission was observed from the
ligand under the UV lamp, whereas the corresponding Eu complex shown red emission at
365 nm. The respective thin films of the complexes are shown bright red color. The
Eu(TTA)3 also shown which is shown red color emission.
Fig.8. Ligands and corresponding Eu(III) complexes under UV (365 nm) light are (a)
Eu(TTA)3, (b) Eu2(TTA)6(L1), (c) Eu2(TTA)6(L2); (b1) (c1) are under normal light; (b2) (c2)

are under UV light; (b3( (c3) are thin films; (d) (e) are L1 and L2 under normal light (d1) (e1)
are under UV light.
These obtained outcomes (PLQY) were compared with the unimolecular Eu-complex [21]
shown higher QY in the solution state and similar in the case of solid, respectively. The
obtained results clearly indicating in solution state the radiative pathway increasing by
reducing non-radiative transition. However, in the solid state, there is no effect was found.
The detailed calculations of the Eu-complex absolute quantum efficiency (measured by the
integration sphere) are reported elsewhere [18, 21]. The absolute quantum efficiency
(measured by the integration sphere) of the complex was carried out in solution as well as
solid by monitoring the higher electric dipole transition (612 nm). The obtained absolute QY
for the binuclear Eu-complex in solution is found to be are 40.7 and 30.0 %, respectively.
Whereas, the complexes are in solid state show 31.8 and 32.4 %, respectively. In addition,
QY of the complexes doping with PMMA thin film was also measured and shown 52.7 and
59.5 %. The observed QY is higher than the solution and solid. It indicates that the PMMA
leads to efficient energy transfer via reducing the non-radiative transitions. It is also worth to
note that the presence of PMMA matrix leads to closer the acceptor and donor. In addition,
QY of the thin film of Eu2(TTA)6(L2) consists higher than that of Eu2(TTA)6(L1). It is
expecting due to the presence of fluorene molecule in the complex, Eu2(TTA)6(L2) leads to
increase the efficient energy transfer to center EuIII metal via ligand. The similar observations
also found in the case of solid. These results can also support by the theoretical calculations
of the singlet and triplet energy levels location and singlet energy level (L1) is near to the
triplet level as compared with L1. So it may efficiently transfer energy in L2 than L1 via
singlet to triplet energy level. As compared with the reported phenanthroline with spacer
consisted complex (solution and solid are 11.5 and 5%) [21], currently synthesized
complexes were shown improved QY (solution above 3 times, solid above 6 times). These

results are suggesting that the fluorene with spacer molecule in binuclear complexes (2 Eu
metal ions) extensively increase the energy transfer process leads to high QY.
Judd-Ofelt and Life time analysis
The Judd-Ofelt parameters were essential to understand the physical implication of the
symmetric/asymmetric and covalent/ionic bonding environment between Eu(III) ion
and the surrounding ligand moiety. Since these calculations were initials to calculate
the sensitization energy transfers process and other parameters. The J-O parameters
were calculated for Eu(III) complexes to form the PL emission spectra by well-
reported method from literature [18,42,46]. The calculated value has been tabulated in
Table 3 and the Ω2, Ω4 values are 1.79, 0.23 (10-20 cm2) (Eu2(TTA)6(L1)) and 2.44,
0.244 (10-20cm2) Eu2(TTA)6(L2), respectively. To achieve the best Eu(III) radiative
rates, necessary to design asymmetrical Eu(III) complexes with having larger Ω2.
7
Intensity parameter Ω4 (5D0→ F2) is less sensitive to the coordination sphere than Ω2
in the complex and reflects chemical environment rigidity surrounding the Eu(III) ion.
The most important isolated magnetic dipole is 5D0 to 7F1 (J=1) transition and it has no
electric dipole involvement, which is practically independent of the ion’s chemical
environment. Based on these criteria it can be used as a reference for calculating of
J–O intensity parameters.
The lifetime of the ligands, as well as complexes, were measured in different solvents and in
solid form which is interpreted in Fig. 9. The lifetime of luminescent decay profiles (5D0
lifetimes (τobs)) for the complexes and ligands was the best fitting with mono-exponential
curves and measured at room temperature. The observed luminescence decay profile
corresponds to a single exponential function, thus implying the presence of only one emissive
‒ 푡
EuIII centre. The single exponential function is given by the equation
,
퐼(푡) = 퐼 + 퐴 exp
( )
0
1
휏
where I0 = 0 is the offset value, A1 is the scalar quantity (from curve fitting), t is the time in

ms and τ is the decay time value for the exponential module. The values are suggesting the
presence of a single chemical environment around the emitting EuIII ion and compiled in
Table 3. The lifetime variations was observed in different solvents and the following order
was followed CHCl3 (0.67) >Acetone (0.62) >DCM (0.60) > THF (0.59) > DMF (0.50)
>solid (0.43ms) forEu2(TTA)6(L1) andCHCl3 (0.68) >Acetone (0.64) > DCM (0.62) > THF
(0.61) > DMF (0.49) > solid (0.42 ms) for Eu2(TTA)6(L2) at 360 nm.Both the complexes
were followed same tthe rend in the solution form towards decreasing of the life time. The
highest life time of the solution (CHCl3) as compared with solid indicates that the non-
radiative transition decreases via reducing the vibrational coupling and leads ratiative
5
transition enhancement. In addition, the primary process that quenches D0 excited state of
EuIII in solution form by non-radiative relaxation via a vibronic coupling. The excited state
5
life time of D0 (Eu3+) level (μs to ms scale) was enable energy transfer by high frequency
vibrational oscillators such as O−H, N−H, and C−H and leads to favours quenching of the
luminescence. According to the gained results, the C-H vibrational oscillators frequency is
less as compare to that of O-H and N-H and CHCl3 and DCM are shown enhanced results.
Fig. 9. The lifetime of the Eu(III) complexes, Eu2(TTA)6(L1) (left) and
Eu2(TTA)6(L2)(right) in different solvents and in the form of solid.

Table 3. The Judd-Ofelt and lifetime analysis of the complexes and ligands in solid form as
well as in different solvents.
S.
Compound
Intensity Parameters
(10-20 cm2)
τ / msa
No.
Ω2
Ω4
Solid CHCl3 DMF THF DCM Acetone
1
Eu2(TTA)6(L1)
Eu2(TTA)6(L2)
L1(10-3 ms)
0.23
0.24
--.--
0.43b
0.42b
2.11
0.67
0.68
1.62
0.50
0.49
1.71
0.59 0.60
0.61 0.62
2.10 2.55
0.62
0.64
1.76
1.78
2.44
--.--
2
3

exc 280 nm
L1 (10-3 ms)
exc 360 nm
4
5
6
--.--
---.--
--.--
--.--
--.--
--.--
4.33
1.70
1.92
11.5
1.85
21.8
12.0
2.52
24.5
21.9 23.0
1.71 2.51
20.6 23.0
26.8
1.72
20.8

L2(10-3 ms)
exc 280 nm

L2 (10-3ms)
exc 360 nm

aThe for the β-diketonate europium(III) complex and its ligand.b decay curves were best fitted with bi-
exponential curves.
The intrinsic lanthanide quantum yield (ΦLn) rationalization in coordination complexes
depends on the Judd-Ofelt theory, which considers an odd number of crystal-field
contributions. The ΦLn,experimental branching ratio (β1-3), stimulated emission cross section
5
(
1-3), radiative (ARAD), nonradiative (ANR) decay rates, D0 lifetime (tobs) and energy transfer
efficiency (sen) parameters were calculated by reported literature [47-50,42,21]. The
obtained intrinsic quantum yield (ΦLn) and energy transfer efficiency (sen) indicates that the

currently studied ligand is an efficient sensitizer for EuIII ion in the complexes. The sen is
higher for Eu2(TTA)6(L2) is 91.81 than the complex Eu2(TTA)6(L1) is 85.14. The obtained
higher sensitization is leads to efficient energy transfer and it reveals the higher QY of the
Eu2(TTA)6(L2). In addition, radiative and non-radiative transitions can be expressed by
radiative rate constants, which are available from emission lifetimes and the quantum yields.
As know the relation between f and , if f is high and  is also extended. In case the
compounds are non-fluorescent, the lifetime are estimated to be very short and it indicate the
very fast decay. The rate constants are calculated by R = f / and the values are 7.395x10-4 s-
1, 7.714x10-4 s-1 found for Eu2(TTA)6(L1), Eu2(TTA)6(L2), respectively [51]. The
comparative radiative constants of the complexes are not much varied and since their life
time also almost similar. Rapid decay due to large nuclear distortion leads to decrease the
quantum yield and reflects the R value also decreases. In addition, the Eu2(TTA)6(L2) shown
comparatively high R and it reveals less nuclear distortions between the ground and the first
excited singlet state. The detailed calculated parameters are tabulated in Table 4 and Table 5.
Table 4. Experimental branching ratios (β1-3) and stimulated emission cross section (1-3) of
the complex.
S.
Complex name
1
2
3
1
2
3
No.
1.
2.
Eu2(TTA)6(L1)
Eu2(TTA)6(L2)
0.520
0.505
6.935
1.117
0.336
0.329
7.92
5.96
86.65
89.82
5.42
4.24

Table 5. The Radiative (ARAD) and nonradiative (ANR) decay rates, 5D0 lifetime (tobs), intrinsic
quantum yield (Ln), energy transfer efficiency (sen) and overall quantum yield (overall) for
thecomplex.
S.
Complex name
ARAD
(S-1)
ANR
(S-1)

obs(ms)
RAD
Ln
Sen
Overall
No.
(ms)
(%)
(%)
(%)
1.
2.
Eu2(TTA)6(L1)
Eu2(TTA)6(L2)
630.681
838.937
170
154
0.43
0.42
1.59
1.19
27.04
35.29
85.14
91.81
31.8
32.4
Electrochemical Properties
The electrochemical properties were carried out for newly synthesized ligands and the
corresponding binuclear complexes and shown in Fig. 10. From the voltammogram,
the oxidation potentials of the ligands (L1 and L2) and complexes (Eu2(TTA)6(L1) and
Eu2(TTA)6(L2)) were found to be1.38, 1.27 and 1.45, 1.39 eV, respectively. The
obtained oxidation potentials from the CV analysis and other values are tabulated in
Table 6. The obtained oxidation potentials of the complexes were decreased as
compared to that of ligand. A similar observation was also been seen in case of
reduction potential. The followed equation which is reported by de Leeuw et al., [52]
used to calculate the HOMO and LUMO energy levels.
푂푥푖
EHOMO
표푛푠푒푡→푆퐶퐸 +4.4) eV
=‒ (퐸
=‒ (퐸
푅푒푑
ELUMO
표푛푠푒푡→푆퐶퐸 +4.4) eV

Fig. 10. Cyclic voltammogram of ligands and its corresponding Eu(III)-complexes.
Table 6.Electrochemical properties of the ligands and respective Eu(III)-complexes.
퐎퐱퐢
S.No. Compound
Name
Band gap/Energy gap
퐕퐨퐥퐭퐚퐠퐞
퐕퐨퐥퐭퐚퐠퐞 퐑퐞퐝
퐨퐧퐬퐞퐭 [V]
onset,
퐨퐧퐬퐞퐭 [V]
[nm](a)
푬푶풑풕
, [eV] (b)
( EHOMO [eV] )
1.38 (-5.78)
1.27 (-5.67 )
1.45 (-5.85)
( ELUMO [eV] )
-1.38 (-3.02)
-1.23 ( -3.17)
-1.40 (-3.00)
-1.26 (3.14)
품
1.
Eu2(TTA)6(L1)
452.4
459.2
411.9
454.1
2.74 (2.76)
2.70 (2.50)
3.01(2.85)
2.73 (2.65)
2.
3.
4.
Eu2(TTA)6(L2)
L1
L2
1.39 (-5.79)
퐸표푛푠푒푡
퐸표푛푠푒푡
= the onset reduction potentials, 푂푥푑 = the onset oxidation potentials, Eg = band gap, Eg = ELUMO
−
푟푒푑
EHOMO. [a] Calculated from the optical absorption (DRS spectra), [b] Calculated the onset wavelength of
optical absorption (onset) in solid-state film (DRS spectra). The energy differences (HOMO and LUMO)
values calculated by cyclic voltammogram are mentioned in bracket.
The calculated optical energy gap (DRS spectral analysis) of the ligands as well as
complexes was similar to that of the results obtained from the electrochemical