Utilization of a Pt(ii) di-yne chromophore incorporating a 2,2′-bipyridine-5,5′-diyl spacer as a chelate to synthesize a green and red emitting d-f-d heterotrinuclear complex

Article abstract of DOI:10.1039/d0dt04198j

A new heterotrinuclear (d-f-d) complex[Eu(btfa)₃1c](btfa = 4,4,4-trifluoro-1-phenyl-1,3-butanedione and1c= [(Ph)(Et₃P)₂Pt-CC-R-CC-Pt(Et₃P)₂(Ph)] (R = 2,2′-bipyridine-5,5′-diyl) has been synthesized by utilizing the N,N-donor sites of the organometallic chromophore. The complex was characterized by analytical and spectroscopic methods. Photophysical properties of the complex were analysed in detail using both steady-state and time-resolved emission and excitation spectroscopy. The optical absorption spectrum of the complex is dominated by the spin allowed π-π* transitions of the btfa and1cunits in the UV-visible region (200-418 nm) and thus is excitable over a wide range of wavelengths across the UV into the visible region of the electromagnetic spectrum. The complex displays typical red Eu(iii) emission when excited at 345 nm. However, it also shows green emission when excited at 464 nm and, thus could be an interesting candidate for full colour display applications. The change in the colour could be a result of the high value of the energy back-transfer rate (6.73 × 10⁵s^?1) from the triplet state of the organometallic chromophore to the⁵D₁state of Eu(iii). Judd-Ofelt (J-O) intensity parameters (Ω₂andΩ₄), radiative (A_R), non-radiative (A_R) decay rates and intrinsic quantum yield (QEuEu) have been calculated.

Full text of DOI:10.1039/d0dt04198j

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PAPER
Utilization of a Pt(II) di-yne chromophore
incorporating a 2,2’-bipyridine-5,5’-diyl spacer as
a chelate to synthesize a green and red emitting
d–f–d heterotrinuclear complex†
Cite this: Dalton Trans., 2021, 50,
1465
a
a
b
b
Idris Juma Al-Busaidi,
Rashid Ilmi, * José D. L. Dutra,
Willyan F. Oliveira,
a,c
a
d
d
Ashanul Haque,
Nawal K. Al Rasbi, Frank Marken,
Paul R. Raithby * and
a
Muhammad S. Khan*
A new heterotrinuclear (d–f–d) complex [Eu(btfa)
3
1c] (btfa = 4,4,4-trifluoro-1-phenyl-1,3-butanedione
3 2 3 2
and 1c = [(Ph)(Et P) Pt–CuC–R–CuC–Pt(Et P) (Ph)] (R = 2,2’-bipyridine-5,5’-diyl) has been synthesized
by utilizing the N,N-donor sites of the organometallic chromophore. The complex was characterized by
analytical and spectroscopic methods. Photophysical properties of the complex were analysed in detail
using both steady-state and time-resolved emission and excitation spectroscopy. The optical absorption
spectrum of the complex is dominated by the spin allowed π–π* transitions of the btfa and 1c units in the
UV–visible region (200–418 nm) and thus is excitable over a wide range of wavelengths across the UV
into the visible region of the electromagnetic spectrum. The complex displays typical red Eu(III) emission
when excited at 345 nm. However, it also shows green emission when excited at 464 nm and, thus could
be an interesting candidate for full colour display applications. The change in the colour could be a result
Received 9th December 2020,
Accepted 23rd December 2020
of the high value of the energy back-transfer rate (6.73 × 10 s−1) from the triplet state of the organo-
5
DOI: 10.1039/d0dt04198j
5
metallic chromophore to the D
1
2 4
state of Eu(III). Judd–Ofelt (J–O) intensity parameters (Ω and Ω ), radia-
Eu
rsc.li/dalton
R R
Eu
tive (A ), non-radiative (A ) decay rates and intrinsic quantum yield (Q ) have been calculated.
exploit both the singlet and triplet excitons and produce
1
. Introduction
1
,3
light. Additionally, trivalent lanthanide [Ln(III)] ions provide
Organo-transition metal complexes with triplet (T) and singlet a fascinating platform because of their atom-like emission
S) harvesting features have emerged as attractive materials for spectra with very sharp emission bands (full width at half
(
1
efficient optoelectronic (O-E) applications. The introduction maxima (FWHM) less than 10 nm), long luminescence life-
4
of heavy transition metals such as rhenium (Re), ruthenium times and ion-specific emissions. Coordinating Ln(III)
(Ru), osmium (Os) and platinum (Pt) into these systems, with complex ions to organo-transition metal complexes could
their associated high spin–orbit coupling (SOC), facilitates endow the resultant complexes with enhanced PL properties
5
intersystem-crossing (ISC) of the excited singlet state S
1
to the for many potential applications. The unique optical pro-
2
lower-lying triplet state T . As a result, highly efficient photo- perties of the Ln(III) ions render them useful in a range of
1
5
h,6
luminescence quantum yields (PLQY) approaching 100% applications such as in lasers,
light-emitting sensors for
7
efficiency at ambient temperature are achievable. Hence, in hetero- and homogeneous fluoroimmunoassays, organic
the electroluminescence experiments, Pt(II) complexes can light-emitting diodes (OLEDs) used in display devices,
5
c,e,8
and
9
biological imaging. Hetero-multimetallic d–f complexes have
shown enhanced PL properties via energy transfer from the
transition metals acting as an antenna triplet state for the
Ln(III) ions. Compared to organic sensitizer chromophores,
the metal-based chromophores possess both a low energy
singlet state and a low triplet excited state and thus can be used
to extend the excitation window of the luminescent lanthanide
a
Department of Chemistry, Sultan Qaboos University, P.O. Box 36, Al Khoud 123,
Oman. E-mail: rashidilmi@gmail.com, msk@squ.edu.om
1
0
b
Pople Computational Chemistry Laboratory, Department of Chemistry, UFS,
4
9100-000 São Cristóvão, Sergipe, Brazil
c
Department of Chemistry, College of Science, University of Hail, Ha’il 81451,
Kingdom of Saudi Arabia
d
Department of Chemistry, University of Bath, Claverton Down, Bath BA2 7AY, UK.
11
complexes. Hence, optimized and tuned photophysical pro-
perties of luminescent lanthanide complexes such as narrow
E-mail: p.r.raithby@bath.ac.uk
†
Electronic supplementary information (ESI) available. See DOI: 10.1039/
d0dt04198j
emission bands, large Stokes’ shifts and long-lived excited
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state lifetimes can be easily enhanced. Moreover, 3d/5d coordi- fully conjugated while its 6,6′-bis(ethynyl) counterpart has
nation complexes can facilitate highly efficient Electronic broken conjugation and that 2,2′-bipyridine-5,5′-diyl spacer
Energy Transfer (EET) to the excited states of chelated 4f* imparts more desirable optical properties to the Pt(II) di-yne
1
2
20
cations. The latter process can be performed via two possible and poly-yne complexes. Based on this idea, in the present
1
3
pathways: (a) MLCT– MLCT intersystem crossing arising from work, we have utilized the ‘rigid-rod’ Pt(II) di-yne chromophore
the heavy-atom effect of the transition metals upon coordi- incorporating 2,2′-bipyridine-5,5′-diyl chelating spacer to syn-
nation to the lanthanide unit, (b) efficient MLCT–Ln* process thesize a new heterotrinuclear Pt–Eu–Pt complex (Chart 1).
3
3
that can compete with bimolecular quenching of the MLCT The photophysical properties of the heterotrinuclear complex
1
3
state by triplet oxygen. The coordination of 3d/5d metals con- have been investigated. The optical absorption spectrum of the
taining appropriate ligands allows the excitation in the visible present heterotrinuclear complex [Eu(btfa) 1c] displays a red
region. In addition, the inclusion of acetylide-functionalized shift compared to its kinked counterpart [Eu(btfa) 1d] [1d] =
3
1
4
3
3 2 3 2
chelating ligands allows light emission from the triplet excited [(Ph)(Et P) Pt–CuC–R–CuC–Pt(Et P) (Ph)] (R = 2,2′-bipyri-
1
5
state of the conjugated ligand. Moreover, introducing the dine-6,6′-diyl) because of the uninterrupted conjugation
acetylide unit may endow the complex with intriguing photo- present in the former complex. Moreover, the newly syn-
physical properties by additional intra-ligand and/or inter- thesized heterotrinuclear complex displays fascinating dual
ligand charge-transfer excited states (ILCT). Therefore, a strong emissions i.e., typical red emission due to the Eu(III) ion
sensitization of Eu(III) in the visible domain (up to 460 nm) by occurs when excited at 345 nm while a strong green emission
the use of a phosphorescent Pt(II) complex raises the overall is observed when the complex is excited at 464 nm. Finally,
1
2,13
4c
luminescence QY of the complexes.
We have recently
DFT calculations have been carried out to understand the
reported a heterotrinuclear ‘Pt–Eu–Pt’ complex of a sterically photophysical properties and the nature of the excited state.
hindered kinked/V-shaped Pt(II) di-yne antenna chromophore Furthermore, the results of the DFT in conjunction with the
of the type trans-[(Ph)(Et
3 2 3 2
P) Pt–CuC–R–CuC–Pt(Et P) (Ph)] observed photophysical data were utilized to gain an insight
21
(R = 2,2′-bipyridine-6,6′-diyl) (1d) as shown in Chart 1 and into the IET processes with the help of the LUMPAC software
described its photophysical properties. The complex displayed package.
typical red emission of the Eu(III) centre; however, the exci-
tation window was confined to the UV region. This can be
attributed to conjugation interruption in the system due to
2
. Experimental
ligand topology (presence of the ethynyl groups at 6,6′-posi-
2.1. General
1
6
tions of 2,2′-Bipy). A general strategy to shift the excitation
window of a complex is to design and engineer the molecule(s)
with extended/uninterrupted π-conjugation. We have pre-
viously reported that Pt(II) di-ynes and poly-ynes incorporating
All reactions were performed under a dry Argon (Ar) atmo-
sphere using standard Schlenk line techniques. Solvents were
pre-dried and distilled before use according to standard pro-
cedures. Europium chloride and gadolinium nitrate were pur-
chased from Strem, USA. All other chemicals were obtained
from Sigma Aldrich except 5,5′-dibromo-2,2′-bipyridine which
was obtained from Tokyo Chemical Industry (TCI) and all were
used as received. The compound trans-[Ph(Et P) PtCl] was pre-
pared according to reported procedures. Column chromato-
graphy was performed either with Kieselgel 60 (230–400 mesh)
silica gel or alumina (Brockman Grade I). Elemental analysis
was performed in the Department of Chemistry, Sultan
Qaboos University. Attenuated total-reflectance (ATR) infrared
1
5a,17
15a,18
a variety of carbocyclic,
heterocyclic
heterocyclic
and mixed
1
5a,19
spacers with extended conjugation exhibit
improved photophysical properties for various opto-electronic
applications. We noted that 5,5′-bis(ethnyl)-2,2′-bipyridine is
3
2
4
c
(
IR) spectra were recorded on pure samples on diamond using
a Cary 630 FT-IR spectrometer. Mass spectra were obtained
using the Agilent 6530 Accurate-Mass Quadruple Time-of-
Flight (Q-TOF) LC/MS.
2
.2. Synthesis of hetero-trinuclear complex [Eu(btfa) 1c]
3
5,5′-Bis(trimethylsilylethynyl)-2,2′-bipyridine
(1a),
5,5′-bis
(
ethynyl)-2,2′-bipyridine (1b) and trans-[(Ph)(Et P) Pt–CuC–R–
3 2
CuC–Pt(Et
3
P)
2
(Ph)] (R
=
2,2′-bipyridine-5,5′-diyl (1c)
1
6
(
Scheme 1) were synthesized as reported by us previously.
solution of 1c (0.073 g, 0.06 mmol) in MeOH/DCM (10 mL)
was added to a solution of [Eu(btfa) (H O) ] (0.050 g,
.06 mmol) in MeOH/DCM (10 mL). The reaction mixture was
A
4
c
3
2
2
Chart 1 Structure of the present and previously designed Pt(II) di-yne
organometallic chromophore and their heterotrinuclear (d–f–d)
complex.
0
left overnight with stirring at RT. The solution was left for slow
1466 | Dalton Trans., 2021, 50, 1465–1477
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Scheme 1 Synthetic protocol for the Pt(II) di-yne organometallic chromophore and the heterotrinuclear (d–f–d) complex.
ꢀ
ꢁ
5
7
5
7
solvent evaporation, after which solid product was obtained as
a yellow solid. The solid product was washed with cold ethanol
ꢀ
ꢁ
ν½ D0 ! F1ꢀ A D0 ! FJ
5
7
5
7
AR D0 ! FJ ¼ ꢀ
ꢁꢁ
AR½ D0 ! F1ꢀ
5
7
5
7
ν D ! F
0
J
A½ D ! F ꢀ
0
1
−
1
and dried in vacuo to obtain a light yellow solid. IR: ν/cm
ð3Þ
−
1
−1
–
CuC– 2087 cm ; C = O 1610 cm
94EuF Pt ; calc. C, 47.65; H, 4.70; N, 1.39%; found:
C, 47.63; H, 4.67; N, 1.36%. (Q-TOF) LC/MS = 2016.5935 m/z
(Fig. S1, ESI†).
C
80
H
9
N
2
O
6
P
4
2
1
τobs
Atot ¼
¼ AR þ ANR
ð4Þ
ð5Þ
ð6Þ
+
1
[
M ] (Fig. S2, ESI†). H NMR (400 MHz, CDCl ): δ 12.45 (b,
3
1
2
8
H); δ 9.34 (d, 2H, J = 7.2 Hz), 9.17 (d, 2H, J = 7.2 Hz), δ
.60–6.0 (m, 25 H); δ 3.34 (ss, 3H); 1.95 (b, 24H), 1.24 (b, 36H)
τrad ¼
AR
1
3
(Fig. S3, ESI†). C NMR (100.6 MHz, CDCl ): δ 187.48, 176.99,
τobs
AR
3
Eu
Eu
Q
¼
¼
τRAD
1
1
55.10, 138.10, 132.48, 126.44, 125.92 125.70, 124.32, 120.43
3
21.89 (CuC), 94.29, 57.82, 28.67, 14.08 (CH ), 7.39 (CH )
AR þ ANR
2
Photoluminescent quantum yield (PLQY) of the complex
was measured using previously reported methods. Both the
standard complex [Eu(btfa) (H O) ] and [Eu(btfa) 1c] were dis-
(Fig. S4, ESI†). NB. The NMR spectroscopic signals were broad-
2
2
ened by the presence of the highly paramagnetic Eu(III) ion.
3
2
2
3
solved in DCM and treated similarly in terms of absorption
and emission measurements. The excitation wavelength was
2
.3. Spectrofluorimetric measurements
UV–visible spectra were recorded on a Varian Cary 50 spectro-
photometer in the 200–600 nm range. Quartz cuvettes with
1
tions were applied. The steady state emission spectrum of the
Eu(III) complex and its phosphorescence lifetime in solution
were recorded on an Edinburgh FS5 fluorimeter at room-temp-
erature (RT). The measured time dependent emission signal
intensity was fitted to exponential functions convoluted with
the system response function. The goodness of the fit was
3
60 nm and measurements were carried out at room tempera-
ture, with a slit width of 1.0 nm for the excitation, and 1.0 nm
for the emission. The Φ of [Eu(btfa) 1c] was measured relative
cm path length were used, and solvent background correc-
3
23
to solution of [Eu(btfa)
3 2 2
(H O) ].
2.4. Cyclic voltammetry (CV)
Cyclic voltammetry (CV) was performed at room temperature at
a scan rate of 100 mV s− using a standard three-electrode con-
figuration employing (i) a Pt wire counter electrode, (ii) a
1
2
judged by the value of the reduced chi-squared (χ ). In all the
experiments, samples were contained in a 1 cm path length
quartz cell, and the measurements were conducted at 24 ±
3
.0 mm diameter glassy carbon working electrode (GCE)
immersed in deaerated dichloromethane solution with approx.
mM concentration of redox active material and (iii) an Ag/AgCl
1
1
°C. Important photophysical parameters such as the Judd–
non-aqueous reference electrode kit (all from BASi, USA). The
Ag/AgCl non-aqueous reference electrode was filled with 0.1 M
n-Bu NPF containing ca. 10 mM AgCl in order to provide a con-
4 6
Ofelt (Ω and Ω ), radiative (A ) and non-radiative (ANR) decay
rates, radiative lifetime (τrad), and intrinsic quantum yield
2
4
R
Eu
Eu
(
Q
) of the heterotrinuclear complex was estimated with the
stant reference potential; to allow comparison with theoretical
calculations, this was subsequently corrected to the ferrocene/
ferrocenium potential in the same electrolyte on the day of use.
help of the following equations and the details are discussed
in our recent report.
5
c
ꢀ
ꢁ
5
7
3
ℏAR D0 ! FJ
exp
Ωλ
¼
ꢀ
ꢁꢄꢂ
ꢃꢄ
ꢄ
ð1Þ
ð2Þ
3
2
2
3
5
7
ꢄ 5
D0kU k7FJ
ðλÞ
2.5. Theoretical analysis
3
2e π χν D0 ! FJ
2.5.1. Elucidation of ground-state geometry and determi-
4
X
ꢀ
ꢁ
5
7
nation of singlet and triplet state. The crystallographic struc-
ture of the organometallic chromophore 1c was obtained pre-
AR ¼
AR D0 ! FJ
J¼0
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5
c
viously by us. This was utilized to aid the drawing of the (CI) and exchange (Ex) mechanisms, respectively. As a conse-
5
7
5
starting geometry of the hetero-trinuclear complex quence, the electronic excitation of the states D ← F , G ←
4
0
6
7
5
7
[Eu(btfa)
3
1c] and was first optimized with the semiempirical
0 6 0
F , and L ← F for the Eu(III) ion are governed by the multi-
quantum mechanical Sparkle/PM6 model implemented in polar mechanism, while the exchange mechanism dominates
the MOPAC2016 software, without imposing any symmetry the D ← F and D ← F excitations. Reference presents
2
4
2
5
5
7
5
7
21
1
0
0
1
restrictions. The optimized Sparkle/PM6 geometry was used to the details of all terms contained in the eqn (7)–(9).
finally elucidate the structure of [Eu(btfa) 1c] by the DFT
The (1 − σ ) shielding factor, from eqn (9), deserves particu-
3
0
approach at the B3LYP level of theory with the help of the lar mention and is estimated as a function of the distance
2
6
27
ORCA 4.2.1 software package. The SVP basis set was used
to represent the electronic structure of the hydrogen (H) atom
as (4s1p)/[2s1p], while the carbon (C), nitrogen (N), oxygen (O)
and fluorine (F) atoms are treated as (7s4p1d)/[3s2p1d] by SVP.
The electronic structure of the Eu and Pt atoms were rep-
from the Ln(III) ion nucleus to the electronic barycentre of the
ligand donor (or acceptor) state, such distance is known as
RL (eqn (11)).
2
9
ꢆ
ꢇ
7
=2
Rmin
RL
2
8
ð1 ꢂ σ0Þ ¼ ρ
ð10Þ
ð11Þ
resented by the MWB(52) and LANL(68) effective core poten-
2
8
tials (ECP), respectively, which considered 52 and 68 elec-
trons in each of the respective cores. For Eu and Pt atoms the
remaining electrons were treated as (7s6p5d)/[5s4p3d] and
7s6p5d1f)/[6s3p2d1f], respectively. Finally, the phosphorus
P) atom was represented by the LANL(10) ECP with the
P
2
ci RL
i
i
RL ¼
P
ci2
(
(
i
2
8
remaining electrons being treated as (10s7p1d)/[4s3p1d].
where c is a given molecular orbital coefficient of the i atom
i
Using the B3LYP geometry the singlet and triplet states of that contributes to the ligand state, RL,i is the distance from i
the coordinated ligands were determined with the time-depen- atom to the Ln(III) ion, and Rmin is the smallest distance
dent density functional theory (TD-DFT) approach, applying between the atoms coordinated directly and the Ln(III) ion. The
2
8
the CAM-B3LYP functional. The elements were represented ρ overlap integral between the valence shells of the Ln(III) ion
by the same basis set and ECPs specified for the procedure of
and the ligating atom usually assumes a value equal to 0.05.
The F term, from eqn (7) and (9), corresponds to the energy
mismatch spectral overlap and is approximately calculated by:
5
a
geometry optimization. As in a previous study the effect of
the dichloromethane solvent was implicitly considered and
did not promote change in the excited states energies, there-
fore such effect was not considered in the current work. The
corresponding absorption spectrum was obtained from fitting
to a Lorentzian function with a half-width at half maximum
ꢊ
ꢋ
2
rffiffiffiffiffiffiffiffi
Δ
ℏγL
1
ln 2 ꢂ
π
ln 2
F ¼ ℏγ
e
ð12Þ
L
−
1
(HWHM) arbitrarily attributed as 10 nm.
where the ligand bandwidth at half-height, γ (in s ) is con-
L
2
.5.2. Intermolecular energy transfer (IET) rate. The most
sidered as being much larger than the widths of the 4f–4f tran-
sitions of Ln(III) ions (γLn). The Δ quantity (in erg) is the energy
difference between donor and acceptor energy states involved
in the ligand–metal ET. Back ET rate is calculated multiplying
2
9
21
recent Malta’s model implemented into LUMPAC 1.4.1 was
employed in the ET rates calculation involving the main ET
channels for [Eu(btfa) 1c]. Such a model considers the contri-
3
bution from two mechanisms, being the ligand–metal ET in
the corresponding direct ET rate by the Boltzmann factor
view only of the direct coulombic interaction calculated by:
ꢂjΔj=kBT
e
, where k stands for the Boltzmann constant and T is
B
e2SLF
X
D
E
assumed as being the environment temperature.
ꢅ
ꢅ
ðλÞꢅ
ꢅ
2
2
π
C
ðλÞꢅ
W
¼
Λλ ψ′J′ U
ψJ
ð7Þ
FED
ET
The Ωλ
corresponds to the forced electric dipole (FED)
ℏ Gð2J þ 1Þ
λ¼2;4;6
intensity parameters, which is derived from the Judd–Ofelt
theory (eqn (13)), and is taken from the eqn (14) neglecting the
contribution from the dynamic coupling (DC). The FED and
DC contributions depend on the so-called odd-rank ligand
ꢆ
ꢇ
D
E
ꢅ
ꢅ
ꢅ
2
1
FED
2
λ
2
2
Λλ ¼ 2Ωλ ð1 ꢂ σ1Þ
þ hr i 3 C
3
ð1 ꢂ σλÞ
6
R
L
!
λ þ 1
t
p
t
p
field parameter (γ ) and Γ , respectively.
ꢁ
ꢈ
ꢉ
ð8Þ
2
λþ2
R
L
ꢄ
ꢄ
ꢄ
λꢂ1; λþ1ðoddÞ tðallÞ
2
X
X ꢄ
Bλtp
calc
λ
The ligand–metal ET rates considering only the exchange
Ω
¼ ð2λ þ 1Þ
ð13Þ
ð14Þ
ð15Þ
ð2t þ 1Þ
mechanism is given by:
t
p¼ꢂt
2
2
ꢌ
ꢍ
8
3
π e ð1 ꢂ σ0Þ F
1=2
EX
ET
2
2
ΔE
ðλ þ 1Þð2λ þ 3Þ
2λ þ 1
W
¼
hψ′J′kSkψJi
hrtþ1iθðt; λÞγ ꢂ
t
4
Bλtp ¼
ℏ R Gð2J þ 1Þ
p
L
*
ꢄ
ꢄ
+
2
ð9Þ
D
E
ꢄ
ꢄ
ꢅ
ꢅ
X
X
X
ꢄ
ΨNꢂ1Πꢄ
ꢄ
ð1Þ
ꢄ
*
λ
ꢅ
ðλÞꢅ
t
hr ið1 ꢂ σλÞ f C
f Γ δt;λþ1
rjC ðjÞsꢂmðjÞꢄΨNꢂ1Π
p
0
ꢄ
m
j
ꢆ
ꢇ
1
=2
(λ)
2
2
gj
4
t þ 1
π
The 〈ψ′J′∥U ∥ψJ〉 and 〈ψ′J′∥S∥ψJ〉 reduced matrix elements
t
e2
tþ1
t*
γ ¼
ρ ð2β Þ
Y ðθj; ϕ Þ
p
j
j
tþ1
p
j
2
R
provide the selection rules for the direct coulombic interaction
j
j
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ꢆ
ꢇ
1
=2
X
From a system of equations in the steady-state condition
the normalized population of a given a level considered in the
ET modelling (η ) is calculated analytically by:
j
4
t þ 1
π
αj
t
t*
Γ ¼
Y ðθj; ϕ Þ
ð16Þ
p
tþ1
p
j
2
R
j
j
The j index runs over the atoms coordinated directly to the
X
X
dηa
dt
lanthanide ion, R corresponds to the distance between a given
¼ ꢂ
Wbaηb þ
Wabηa ¼ 0
ð21Þ
j
a=b
a=b
j atom and the lanthanide ion, θ and ϕ are the corresponding
j
j
angular coordinates. The charge factors (g
j
) and polarizabil-
where Wab is the transfer rate from level a to level b, and if a =
b, Wab = 0.
ities (α ), present in eqn (15) and (16), respectively, were
j
3
0
adjusted using the QDC model to reproduce the experi-
mental values of Ω . QDC model is implemented into
LUMPAC and postulates that the g are obtained from the
λ
2
1
j
3. Results and discussion
product between an adjustable parameter (Q) and each ZDO
3.1. Synthesis, characterization and elucidation of the
(Zero Differential Overlap) electronic density (q
j
) of each j atom
ground state geometry
(
eqn (17)). As shown in eqn (18), the α quantities are calcu-
j
In this work, we have synthesized a new hetero-trinuclear
complex (Scheme 1) bearing a rigid dinuclear Pt(II) di-yne
chromophore. The chemical structure of the Pt(II) di-yne
chromophore, heterotrinuclear (d–f–d) complex and their step-
ð17Þ wise synthesis is depicted in Scheme 1. Trans-[(Ph)(Et P) Pt–
lated by using the D and C adjustable parameters together
with the electrophilic superdelocalizabilities (SE). The refer-
2
1
ence details all terms present in eqn (13)–(18).
gj ¼ Q ꢁ qj
3
2
CuC–R–CuC–Pt(PEt ) (Ph)] (R = 2,2′-bipyridine-5,5′-diyl) was
3
2
1
6
αj ¼ SEj ꢁ D þ C
ð18Þ synthesized following our previously reported method.
Briefly, 5,5′-dibromo-2,2′-bipyridine was protected with tri-
methylsilylethyne yielding 1a (Scheme 1) by a Sonogashira
cross-coupling reaction. Smooth removal of trimethylsilyl
From the energy of the occupied molecular orbital and the
corresponding linear coefficients calculated with Sparkle/PM6,
31
3 j j
considering the B3LYP geometry of [Eu(btfa) 1c], the q and SE
groups with KOH/MeOH (Scheme 1) followed by dehydrohalo-
quantities were obtained using LUMPAC.
2
genation reaction of the diterminal alkyne 1b with trans-[Ph
.5.3. Theoretical quantum yield. The PLQY (q) is an
(
Et
solid. The heterotrinuclear complex [Eu(btfa)
was synthesized in 74.0% yield by reacting [Eu(btfa) (H O) ]
3
P)
2
PtCl] yielded the Pt(II) di-yne 1c (Scheme 1) as a yellow
important spectroscopic parameter and is defined as the ratio
of the number of emitted photons by the lanthanide ion to the
number of absorbed photons by the ligand. For the Eu(III) ion,
q is calculated using the following equation:
3
1c] (Scheme 1)
3
2
2
and Pt(II) di-yne (1c) in 1 : 1 ratio. The formulation of [Eu
btfa) 1c] was confirmed by elemental analysis, ESI-MS and IR
(
3
spectroscopy. The elemental analysis shows satisfactory results
Aradη
5D0
q ¼
ð19Þ and confirms the proposed formulation for the newly syn-
φηS0
thesized complex. The result of the elemental analysis was
where φ is the absorption rate (s− ) from the fundamental
1
further confirmed by the ESI-MS. The spectrum of [Eu
(
btfa)
complex displayed a molecular ion peak at 2016.59 m/z [M ]
Fig. S2, ESI†) and thus confirms the proposed stoichiometry
3
1c] in positive ion mode was recorded in MeOH. The
singlet to the excited singlet of the ligand. η 0
S
and η
5D0
are the
+
energetic population of the S fundamental state of the ligand
0
5
3+
(
and the D emitting state of the Eu , respectively.
0
shown in Chart 1. The spectrum also displayed various peaks
which could be due to fragmentation of the fluorinated
β-diketonate ligands. The results of the elemental analysis
and ESI† were further confirmed by IR spectroscopy. The IR
The radiative emission rate (Arad) depends on the theore-
tical intensity parameters (eqn (13)) and for the europium ion
is calculated by:
3
2
2
3
X
3
2e π χ
spectra of [Eu(btfa) 1c] together with those of [Eu
5
7
3
3
Arad ¼
ν½ D0 ! FJ¼λꢀ Ωλ
3
ℏð2J þ 1Þ
(btfa) (H O) ] and 1c are shown in Fig. S1, ESI.† The spectrum
3
2
2
λ¼2;4;6
ð20Þ
ꢄD
Eꢄ
3
3
5
7
3
of [Eu(btfa)
3
1c] displayed a single, sharp –CuC– stretch at
ꢅ
ꢅ
2
ꢄ
ꢄ
5
ðλÞꢅ7
ꢄ
32π n ν½ D0 ! F1ꢀ
Smd
ꢅ
2087 cm− , consistent with a trans configuration of the ethynyl
1
ꢁ
D0 U
FJ¼λ ꢄþ
3
ℏ
1
5b
units around the Pt(II) centres.
The presence of this stretch-
where e is the elementary charge; 2J + 1 is the degeneracy of ing absorption clearly supports the formation of the hetero-tri-
3
+
5
the initial state, for Eu the D
0
emitting is considered (J = 0). nuclear complex. Moreover, the IR spectra of the complex exhi-
The energies of the barycentres of the respective transitions bits CvO and CvC stretching vibrations at 1610 and
5
7
−1
0 J
are represented by the ν[ D → F ] quantities. χ is the Lorentz 1571 cm , respectively, characteristics of the lanthanide
2
2
4c
local-field correction term, and is given by χ = n(n + 2) /9, chelate complexes containing β-diketone ligand. The appear-
where the n refractive index was considered as equal to 1.424, ance of (PCH CH ) twisting and bending vibration bands in
2
3 3
regarding the DCM solvent. The magnetic dipole strength the spectrum also provide additional evidence of complex for-
5
7
(
1
S
md) of the D
0
→ F
1
transition is theoretically equals 9.6 × mation; however, the position of the peaks remain unaltered
0− esu cm .
42
2
2 30
appearing at 3041, 2961 and 2871 cm as observed in the Pt
−1
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Fig. 1 Ground state geometry of the heterotrinuclear complex optimized at the B3LYP level of theory.
(
II) di-yne chromophore. The results clearly provide unambigu- β-diketonate ligands. Spherical coordinates of coordination
ous support for the formation of the heterotrinuclear complex polyhedron are gathered in Table S1, ESI,† and, as can be
Eu(btfa) 1c]. seen, the Eu–N bonds predicted by the applied DFT approach
Repeated attempts to obtain single crystals of the complex are 2.684 and 2.647 Å, in line with literature values. On the
[
3
3
4
suitable for X-ray analysis failed. In fact, for predicting the other hand, the Eu–O bonds are in the ranges of 2.399–2.422 Å
photophysical properties of the given Eu complex, one needs and plainly the coordination polyhedron is of low symmetry.
to have the theoretical structure of the complex, even though
5
a
3.2. Optical absorption spectroscopy
the experimental structure is available. Given this fact, the
ground state geometry of the present hetero-trinuclear Fig. 2 shows the optical absorption spectra of the Pt(II) di-yne
complex was elucidated by the DFT method shown in Fig. 1, chromophore (1c) and of [Eu(btfa) 1c]. The spectrum of the Pt
3
and the coordination geometry of EuN O polyhedron is high- (II) chromophore displays a strong split peak at 382 nm due to
2
6
lighted. The crystallographic structure of the 1c [EWOQEM the π–π* transitions. It is important to emphasize that in the
1
6
Cambridge Structural Database (CSD) code] shows a linear spectrum of 1c with the ethynyl groups at the 5,5′-positions
orientation around the –CuC– triple bond. However, in the there is a 50 nm red shift compared to 6,6′-ethynyl based Pt(II)
present case, we noted distortion when 1c is coordinated to chromophore. This shift is a result of uninterrupted conju-
the Eu(III) ion. Such a structural modification is observed in gation along the chromophore and attests to our assumption
the literature when ligands comparable to 1c are coordinated as stated in the introduction. The absorption spectrum of
to a given metal centre (e.g., ASUVER and KIRSOU CSD [Eu(btfa) 1c] is dominated by spin allowed π–π* transitions of
3
3
3
refcodes), and could be due to the interactions involving the the β-diketone ligand and 1c in the UV–visible region
peripheral substituents in the different ligands of 1c with the (200–418 nm) with three major absorption peaks at 331 nm,
Fig. 2 (a) Optical absorption spectra of 1c and [Eu(btfa)
using the B3LYP geometry for [Eu(btfa) 1c].
3
1c] at RT. (b) Theoretical electronic spectrum calculated at the CAM-B3LYP level of theory
3
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3
89 nm and 416 nm. As noted in the organometallic chromo- nated by electronic transitions involving MOs centred on the
phore 1c, the spectrum of the present heterotrinuclear β-diketonate ligands.
complex displays a red shift and this extends into the visible
region of the spectrum compared to previously reported
3.3. PL properties
[
(
Eu(btfa) 1d] [1d] = [(Ph)(Et P) Pt–CuC–R–CuC–Pt(Et P) (Ph)] The PL and excitation spectrum of [Eu(btfa) 1c] recorded in
3 3 2 3 2
3
R = 2,2′-bipyridine-6,6′-diyl) (Fig S4, ESI†). It is thus excitable DCM at RT is shown in Fig. 4a. The excitation spectrum of the
4
c
across a wide range of wavelengths from UV to the visible. The complex was recorded by monitoring the most intense emis-
5
7
spectral differences of the two systems encouraged us to study sion transition D → F at 616 nm. The excitation spectrum
0
2
the photoluminescence (PL) of the new system further.
contains a broad band in the region between 270–400 nm.
To underpin the optical absorption spectroscopy result and Apart from this, the spectrum exhibits Eu(III) excitation tran-
7
5
−1
7
to clearly understand the origin of transition, we obtained the sitions assigned to F → D (21 551 cm ; 464 nm) and F →
0
2
0
5
D (18 657cm−1; 536 nm). The intensity of the excitation tran-
theoretical absorption spectrum of the present complex
Fig. 2b). As shown in Fig. 2b, the theoretical absorption spec- sition is higher than the broad band suggesting that the het-
1
(
trum is in qualitative agreement with the experimental spec- erotrinuclear complex could be excitable at the blue 464 nm
trum. The aforementioned assigned electronic transition is wavelength.
further supported by the molecular orbital (MO) analysis.
The emission spectrum of [Eu(btfa) 1c] at RT was obtained
3
Table 1 shows that the electronic transition highest occupied by exciting it at λmax 345 nm (Fig. 4a). The spectrum displays
molecule orbital (HOMO) → lowest unoccupied molecular typical well resolved sharp Eu(III) emission transitions at
orbital (LUMO) dominates the band located at the highest 579 nm, 594 nm, 616 nm, 653 nm and 701 nm, which corres-
7
5
7
5
7
5
7
5
wavelengths, and such MOs are centred in the molecular frag- ponds to D → F , D → F , D → F , D → F and D →
ment present in the 1c complex (Fig. 3). On the other hand,
0
0
0
1
0
2
0
3
0
7
4
F , respectively. The spectrum is dominated by the narrow
the band corresponding to the lowest wavelengths is domi- hypersensitive electric dipole D → F transitions (FWHM =
5
7
0
2
1
2.57 nm, Table 2). Moreover, the spectrum does show some
residual ligand fluorescence in the region between
25–531 nm, suggesting less efficient energy transfer (ET) from
the newly synthesized organometallic chromophore compared
to [Eu(btfa) 1d]. The 1931 CIE (Commission internationale de
Table 1 Electron transition configurations calculated at the TD-DFT
CAM-B3LYP level of theory for the most relevant singlet states of [Eu
4
3
(btfa) 1c]
3
Oscillator
Wavelength Ligand strength Major contribution
l’éclairage) colour coordinate of the heterotrinuclear complex
calculated from the PL spectrum shows a typical red emission
with colour coordinates (x = 0.6358; y = 0.3381, Table 2), which
Total
3
2
38.09 nm 1c
63.82 nm 1c/btfa 1.44
1.79
HOMO → LUMO
87.30%
HOMO−2 → LUMO+3 (25.15%) 60.91% is very close to National Television System Committee (NTSC)
HOMO−5 → LUMO+1 (22.94%)
and, thus, could be beneficial for the design of monochro-
HOMO−8 → LUMO+2 (12.81%)
HOMO → LUMO+1 (41.21%)
HOMO → LUMO+4 (30.33%)
5
a
matic red emitter for full colour displays. Moreover, as men-
tioned previously, we excited the complex at 464 nm.
2
77.85 nm 1c/btfa 0.34
71.54%
Fig. 3 Most relevant MOs calculated at the TD-DFT CAM-BLYP level of theory that explain the main electronic transitions.
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Fig. 4 (a) Excitation (broken line) and emission (solid line) spectra (λex = 345 nm) (b) CIE-1931 chromaticity diagram demonstrating the calculated
colour coordinate (x, y) values for [Eu(btfa) 1c] in DCM at RT.
3
Table 2 Experimental intensity parameters (Ω
2
and Ω
1c]
4 R
), radiative (A ) and non-radiative (ANR) rates, Intensity ratio, observed luminescence lifetime
(
τobs), and intrinsic quantum yield (ΦEu) of [Eu(btfa)
3
Ω
×10
2
Ω
4
FWHM
(nm)
τ
(μs)
obs
τ
rad
A
R
A
NR
−
20
2
(s−1)
L
Eu
Eu
Complex
Eu(btfa)
cm
Q
(%)
Q
(%)
R
21
CIE(x,y)
Eu
a
a
b
c
c
d
[
3
1c] 20.61
14.80
12.57
τ
1
= 39.52
1347 742.51
1184.28
15.10
38.54
11.65
[16.60]
0.6358; 0.3381
[0.668; 0.324]
[
29.45]*
[8.90]*
(6.91%) and
= 519.86
93.09%)
[912.81]*
[779.24]
[53.95]
τ
2
(
Theoretical 20.61
14.79
—
—
—
760.94
1162.66
19.8
39.56
—
—
4
c
a
*
Value in the square brackets are for [Eu(btfa)
3
1d] 1d = [(Ph)(Et
3
P)
2
Pt–CuC–R–CuC–Pt(Et
3
P)
2
(Ph)] (R = 2,2′-bipyridine-6,6′-diyl). Calculated
b
c
d
using eqn (1). Calculated using eqn (5). Calculated using eqn (2)–(4). Calculated using eqn (6).
Interestingly, the complex displays a totally different PL spec-
trum with a broad band that covers a region between decay lifetime (τobs) of the D excited state of the complex in
2 4
ANR, τrad, Ω and Ω , we first determined the experimental
5
0
max
4
75–750 nm (Fig. 5a). A close analysis of the spectrum shows solution by exciting it at λEx and fitting the decay curves
that the broadband also comprises very faint Eu(III) emission (Fig. S6, ESI† and Table 2). The decay curves fit with biexpo-
5
7
transitions D → F at 613 nm. This could be caused by the nential behaviour with τobs value τ = 39.52 (6.91%) and τ =
0
2
1
2
5
back-energy transfer from the
D
0
state to the ligand triplet 519.86 μs (93.09%) (Table 2). The A
R
and ANR decay rates of the
state leading to very severe quenching of the Eu(III) emission. complex were estimated utilizing the steady-state PL spectrum
Furthermore, the PL spectrum was utilized to calculate the CIE and τobs (the longer lifetime component was utilized for the
colour coordinates which is in green region with (CIE)x,y
.3011, 0.5726 (Fig. 5b). Thus, the dual emitting properties of Table 2. The complex displayed a larger ANR = 1184.28 s
the complex could be utilized in full colour display value compared to the reported 2,2′-bipyridine-6,6′-diyl based
=
calculation and refractive index 1.424) and are summarized in
−
1
0
3
c,35
devices.
complex as shown in Chart 1. Despite the same chemical com-
NR displayed by
A range of important photophysical parameters can be position, interestingly, the larger
A
determined from the PL spectrum of Eu(IIII) coupled with the [Eu(btfa) 1c] can be attributed to the linearity of the present
3
experimental decay lifetime. Firstly, the important parameter chromophore compared to the bent shape of 2,2′-bipyridine-
4
c
is intensity ratio ‘R21’ which is a measure of asymmetric 6,6′-diyl. Finally, Ω
2 4
and Ω were calculated (Table 2); the
−
20
2
environment around Eu(III) ion and can be estimated by the high values of Ω = 20.61 × 10
cm suggest that the Eu(III)
0 2
D → F tran- ion in the present complex is surrounded by a highly polariz-
2
5
7
ratio of intensity of the electric dipole (ED)
sition to the magnetic dipole (MD) D
5
7
5a
0 1 4
→ F transition. The able environment. The parameter Ω is related to the rigidity
high R21 = 11.65 value for the present complex indicates that it and long-range effects such as hydrogen bonding, π–π stacking
−
20
2
is not a centrosymmetric complex and the coordination sphere and large value (Ω
around Eu(III) is highly asymmetric. As mentioned above, and presence of these effects. A comparison of the Ω
to evaluate important photophysical parameters such as AR, of the present complex suggests that 2,2′-bipyridine-5,5′-diyl
4
≈ 14.80 × 10
cm ) is an indication of the
6
2
parameter
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Fig. 5 (a) Emission spectrum in DCM at RT at 464 nm, (b) CIE-1931 chromaticity diagram demonstrating the calculated color coordinate (x, y)
values for [Eu(btfa) 1c] complex.
3
Table 3 Energy of the singlet and triplet excited states most relevant, of [Eu(btfa)
distance from energy donor to acceptor centre (R
), and electronic tran- responsible for the lowest triplet state and as pictured in
sition configurations calculated at the TD-DFT CAM-B3LYP level of
theory for the corresponding triplet state of [Eu(btfa) 1c]
3
1c]. The HOMO → LUMO transition is the one
L
Fig. 4, such orbitals are centred on the ligands present in 1c.
In this manner, the lowest triplet state (T = 20 442.2 cm ) cal-
1
3
−1
culated at the CAM-B3LYP level of theory using the B3LPY geo-
metry is characterized as involving MOs centred on the ligands
State Energy
R
L
Major contribution
Total
−1
−1
−1
S
1
29 577.5 cm
20 442.2 cm
22 664.2 cm
5.49 Å HOMO → LUMO
4.98 Å HOMO → LUMO (79.55%)
4.35 Å HOMO−2 → LUMO+2 (26.69%) 62.03%
87.30%
79.55%
present in the 1c molecular fragment. The energy of the next
T
T
1
2
1
lowest triplet state (T
2
) is equal to 22 664 cm− and the most
HOMO−2 → LUMO+3 (19.00%)
HOMO−8 → LUMO+3 (16.34%)
relevant electronic transitions involve MOs centred mainly on
the btfa ligands. R values present in Table 3 show that the
donor centre is more distant from the Eu(IIII) centre for T in
comparison with T due to the contribution of the btfa ligands
L
2
1
unit is more symmetrical than the 2,2′-bipyridine-6,6′-diyl unit in T2.
−
20
2 4c
(
Ω = 29.45 × 10
cm ) and results in less hypersensitivity
In order to take account the effect of the distinct molecular
2
5
7
of the ED D
0
→ F
2
transition. This is further supported by groups (Table 3) in the modelling of the ligand – Eu(III) energy
the observed lower R21 = 11.65 value compared to that in the transfer, the diagram pictured in Fig. 6 was proposed, in
4
c
2
,2′-bipyridine-6,6′-diyl based complex (R21 = 16.60). The which the ET rates were calculated applying the LUMPAC
Eu
intrinsic quantum yield (Q ) is calculated by utilizing eqn (6) program. In the ET modelling, for Eu(III) only three states were
and the data is summarized in Table 3. The complex considered, namely, D4 (ca. 27 586 cm ), D1 (ca.
Eu(btfa) 1c] displayed high Q = 38.45%, however, it is lower 19 027 cm ), and D (ca. 17 293 cm ), with the energy values
Eu
5
−1
5
Eu
−1
5
−1
[
3
Eu
0
Eu
Eu
4c
36
than [Eu(btfa)
increased ANR value of the present complex.
3
1d] Q
= 53.95%, which is due to the obtained from Carnall et al. To calculate the energetic popu-
lation involving the states shown in Fig. 6, typical experimental
The fitted Ω and Ω values using the QDC model are in values for the non-radiative rates in coordination compounds
2
4
3
6
perfect agreement with the corresponding experimental were considered.
values. This procedure is important to obtain the forced elec-
For complexes containing β-diketonate ligands it is known
tric dipole contribution for the intensity parameters, which are that the ET channel is the one dominated by the exchange
presented in Table S2, ESI† and are used to evaluate the mechanism (Dexter mechanism), and the ET value the order
8
−1
energy transfer rates via the direct coulombic interaction of 10 s (Table 4) calculated with the exchange mechanism
5
5
mechanism.
0 1
confirms this. The triplet → ( D , D ) channels are related to
5
7
5
7
the D ← F and D ← F excitations, respectively, where
1
0
0
1
3
.4. Intermolecular energy transfer (IET) process
7
5c
1
the F state is considered as being thermally populated. By
The energy of the most relevant triplet excited states, as well as considering the schematic energy-level diagram postulated in
the respective distance between donor and acceptor centre Fig. 6 together the ET rates presented in Table 4, the popu-
5
(
R
L
), calculated at the TD-DFT CAM-B3LYP level of theory are lations of the S
0
and D
0
states were calculated (ηS0 = 0.28 and
shown in Table 3. In addition, the table presents the electronic
η
5D0
¼ 0:52), which provided a theoretical PLQY of 19.8% that
transition configurations for each of the relevant triplet states is close to experimentally determined value 15.10%.
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Fig. 6 Schematic energy-level diagram showing the most important ET channels for [Eu(btfa)
3
1c]. Please see Table 5 for the corresponding values
of ET rates.
4 6
Table 4 Energy transfer rates calculated by the Malta’s model using ammonium hexafluorophosphate (Bu NPF ) as the electrolyte
LUMPAC for the [Eu(btfa)
calculated at the TD-DFT CAM-B3LYP level of theory
3
1c] complexes with the ligand state energies
and approx. 1 mM metal complex. The CV experiment was
performed to investigate the electrochemical behaviour of
[
2
Eu(btfa)
3 3 2 2
1c] and the related [Eu(btfa) (H O) ] in the −2.0 to
Energy
.0 V range. Table 5 contains all the electrochemical parameters
Ligand state
4f state
ET rate
back-transfer
−
1
−1
−1
−1
(cm
)
(cm
)
(s
)
rate (s
)
and data obtained. The protonated btfa ligand showed two
reductions peaks with an initial broad shoulder in the region of
−2.0 to 0.0 V and there was no oxidation peak in the positive
potential region. Both [Eu(btfa) (H O) ] and [Eu(btfa) 1c] show
an oxidation signal and a complex sequence of reduction
signals. We noted a considerable difference in reduction peaks
between [Eu(btfa) (H O) ] and [Eu(btfa) 1c]. New chemically
5
4
9
8
8
8
1
S
1
(29 577.5)
(22 664.2)
(22 664.2)
(20 442.2)
(20 442.2)
→
→
→
→
→
D
4
D
1
D
0
D
1
D
1
(27 586)
(19 027)
(17 293)
(19 027)
(19 027)
1.60 × 10
1.27 × 10
8.72 × 10
5.96 × 10
6.81 × 10
1.14 × 10
3.36 × 10
5.66 × 10
6.73 × 10
1.88 × 10
5
5
5
5
1
T
T
T
T
2
2
1
1
−3
5
3
2
2
3
2
3
2
2
3
irreversible oxidation peaks appear in positive region at 1.74 V
and 1.57 V for [Eu(btfa) (H O) ] and for [Eu(btfa) 1c] respect-
3
.5. Electrochemical properties
3
2
2
3
Preliminary CV experiments were conducted with a glassy ively. The latter complex shows a reduction peak at −0.77 V
carbon disk, Pt wire, and Ag/AgCl electrode as the working elec- (Fig. 7), which we tentatively assign to the reduction peak of the
37
trode, counter electrode, and reference electrode, respectively, Eu(III). The reduction peak of [Eu(btfa) (H O) ] was ca. 0.49 V
3
2
2
in deaerated DCM solution containing 0.1 M tetrabutyl- less negative compared to [Eu(btfa) 1c]. This observation
3
Table 5 Cyclic voltammetry data of btfa, [Eu(btfa)
3
(H
2
O)
2 3
] and [Eu(btfa) 1c]
onset
Ox
onset
red
Ox
P
RE
ele
g
opt
Compound
btfa
E
(V)
E
(V)
E
(V)
E
(V)
HOMO (eV) LUMO (eV)
E
(eV) Eg (eV)
P
a
a
a
a
b
b
c
c
c
−0.82
−0.67
−0.06
−0.41
—
−0.53 , −1.13
1.74
1.57
—
−0.86 , −1.22
—
—
— f
1.6f
1.5
—
—
a
b
c
c
c
d
e
e
e
[
[
Eu(btfa)
Eu(btfa)
3
(H
2 2
O) ]
1.44
−0.28 , −1.20 , −1.63
−5.85d
−5.60
−5.40
−2.49 (−4.35)
3.36
2.57
3.19
a
b
c
c
c
e
3
1c]
1.19
−0.77 , −1.22 , −1.36 , −1.77
−3.03 (−4.00)
Theoretical (B3LYP)
—
—
−2.18
a
onset
Ox
onset
Re
b
c
d
E
= the onset oxidation potentials; E
= the onset reduction potentials. Anodic peak potential. Cathodic peak potential. HOMO
e
f
ele
onset
Ox
onset
eV. The energy
red
energy level. LUMO energy level. Energy gap measured from electrochemical data according to the Eonset, Eg = E
LUMO values were calculated by cyclic voltammogram and are displayed in parentheses.
− E
1474 | Dalton Trans., 2021, 50, 1465–1477
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Fig. 7 Cyclic voltammograms of btfa (red), [Eu(btfa)
electrode in 0.1 M Bu NPF solution in DCM at a scan rate of 100 mV s .
4 6
3
(H
2
O)
2
] (black) and [Eu(btfa)
3
1c] (blue) in the region between −2.0 to 2.0 V on glassy carbon
−1
suggests that primary ligands play a significant role in changing thesize a novel hetero-trinuclear (d–f–d) complex [Eu(btfa)
3
1c].
the electrochemical behaviour of Eu(III) complexes and the shift The structure of the complex was elucidated using DFT
38
of reduction peak of Eu(III). The other peaks could correspond methods. The optical absorption spectrum of the complex
to the reduction/oxidation of the electrochemically active exhibited transitions in the 200–418 nm range. Interestingly,
3
9
β-diketonate groups or further fragmentation products.
the complex displayed a red shift compared to the analogous
4c
For calibration, the redox potential of ferrocene/ferroce- kinked hetero-trinuclear complex reported earlier by us. This
nium (Fc/Fc ) was measured under the same experimental shift is due to uninterrupted conjugation in the present
+
conditions, and it is located at 0.39 V vs. the Ag/AgCl electrode complex (Chart 1). The PL studies revealed dual emissions, with
(Fig. S7. ESI†). HOMO energy values were calculated based on a typical red emission (CIE; x = 0.6358; y = 0.3381) when excited
the value of −4.8 eV for Fc with respect to a zero vacuum at 345 nm and a green emission (CIEx,y = 0.3011, 0.5726 at
4
0
level. The energy levels of the HOMO was estimated accord- 464 nm excitation. The dual emission could be due to two
4
1
onset
−1
ing to the following equation:
EHOMO = −(EOx + 4.41) eV different triplet states centred at: (a) btfa 22 664.2 cm with the
onset
9
−1
where EOx is the onset oxidation potential vs. Ag/AgCl. The ET rate 1.27 × 10 s with the negligible energy back-transfer
energy band gap (Eg ) was calculated from the onset wave- rate (Table 4) and (b) organometallic chromophore
length of the UV absorption (Eg = 1240/λ). The LUMO (20 442.2 cm ) with an ET rate 5.96 × 10 s with the energy
energy level was obtained from the HOMO energy and the back-transfer rate of 6.73 × 10 s for the T
energy bandgap (Eg = LUMO–HOMO) and ELUMO = −(ERe
.41) eV where ERe = onset reduction potential vs. Ag/AgCl. typical for europium β-diketonate complexes. The inter-
ele
ele
42
−1
8
−1
5
−1
5
1
→ D
1
. The excited
ele
onset
+
state lifetime of the present complex is in microsecond range
onset
4
An important disparity was ascertained between the measured molecular energy transfer in the complex on the basis of experi-
optical and electrochemical energy bandgap values. However, mental results and DFT calculations is discussed in detail.
there is no contradiction between these values, since they refer Moreover, the calculated photophysical parameters are in excel-
to different processes. The optical absorption data provides an lent agreement with the experimentally observed results.
estimation of the energy difference of the HOMO–LUMO orbi-
tals, while the electrochemical band gap is consistent with the
energy required to form a tightly bound exciton with
additional contributions from charging. The redox potentials
Conflicts of interest
4
3
measured by CV afford the absolute energetic positions of the The authors declare no conflicts of interest.
HOMO and LUMO levels (assuming insignificant effects from
charging), and the electrochemical or transport bandgap
4
3,44
depicts the energy required to form free charge carriers.
Acknowledgements
Moreover, the theoretically estimated HOMO and LUMO
values are almost similar to the experimentally determined
value (HOMO ≈ −5.60 eV, Table 5) especially at the B3LYP level
M. S. K. acknowledges the Research Council (TRC), Oman
(
Grant: RC/RG-SCI/CHEM/20/01) for funding. I. J. A. B.
acknowledges the Ministry of Education, Oman for a Ph.D.
scholarship. R. I. thanks HM’s Trust Fund (Grant No. SR/SQU/
SCI/CHEM/16/02) for a postdoctoral fellowship. W. F. O.
thanks PIBIC/CNPq/UFS (02/2019 COPES/POSGRAP/UFS) for a
scientific initiation fellowship. P. R. R. is grateful to the
(
HOMO ≈ −5.40 eV, Table S3, ESI†).
4
. Conclusion
In summary, we have utilized a Pt(II) di-yne chromophore incor- Engineering and Physical Sciences Research Council (EPSRC)
porating a bipyridine-5,5′,-diyl spacer (1c) as a chelate to syn- for continued funding (Grant EP/K004956/1).
This journal is © The Royal Society of Chemistry 2021
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S. Liu, Q. Zhao and W. Huang, Chem. Rev., 2018, 118, 1770–
References
1839.
1
A. Haque, R. A. Al-Balushi, I. J. Al-Busaidi, M. S. Khan and 10 (a) S. I. Klink, H. Keizer and F. C. van Veggel, Angew. Chem.,
P. R. Raithby, Chem. Rev., 2018, 118, 8474–8597.
2000, 112, 4489–4491; (b) T. J. Sørensen and S. Faulkner,
Acc. Chem. Res., 2018, 51, 2493–2501; (c) M. Pan,
W.-M. Liao, S.-Y. Yin, S.-S. Sun and C.-Y. Su, Chem. Rev.,
2018, 118, 8889–8935.
2
(a) A. F. Rausch, H. H. H. Homeier and H. Yersin, in
Photophysics of Organometallics, ed. A. J. Lees, Springer
Berlin Heidelberg, Berlin, Heidelberg, 2010, pp. 193–235;
(
b) I. J. Al-Busaidi, A. Haque, N. K. Al Rasbi and 11 (a) K.-J. Chen, H.-B. Xu, L.-Y. Zhang and Z.-N. Chen, Inorg.
M. S. Khan, Synth. Met., 2019, 257, 116189; (c) C.-L. Ho,
Z.-Q. Yu and W.-Y. Wong, Chem. Soc. Rev., 2016, 45, 5264–
Chem. Commun., 2009, 12, 744–746; (b) O. J. Stacey,
A. J. Amoroso, J. A. Platts, P. N. Horton, S. J. Coles,
D. Lloyd, C. F. Williams, A. J. Hayes, J. J. Dunsford and
S. J. A. Pope, Chem. Commun., 2015, 51, 12305–12308.
5
295; (d) W.-Y. Wong, Dalton Trans., 2007, 4495–4510;
(
e) J. Zhang, L. Xu, C.-L. Ho and W.-Y. Wong, in Polymer
Synthesis Based on Triple-Bond Building Blocks, ed. 12 I. M. Etchells, M. C. Pfrunder, J. A. G. Williams and
B. Z. Tang and R. Hu, Springer International Publishing, E. G. Moore, Dalton Trans., 2019, 48, 2142–2149.
Cham, 2018, pp. 157–189; (f) L. Xu, C.-L. Ho, L. Liu and 13 R. Ziessel, S. Diring, P. Kadjane, L. Charbonnière,
W.-Y. Wong, Coord. Chem. Rev., 2018, 373, 233–257;
g) C.-L. Ho and W.-Y. Wong, Coord. Chem. Rev., 2013, 257,
614–1649.
P. Retailleau and C. Philouze, Chem. – Asian J., 2007, 2,
975–982.
14 M. D. Ward, Coord. Chem. Rev., 2007, 251, 1663–1677.
(
1
3
(a) H. Yersin, Highly Efficient Oleds with Phosphorescent 15 (a) J. S. Wilson, N. Chawdhury, M. R. A. Al-Mandhary,
Materials, John Wiley & Sons, 2008; (b) S. Wang, H. Zhang,
B. Zhang, Z. Xie and W.-Y. Wong, Mater. Sci. Eng., R, 2020,
M. Younus, M. S. Khan, P. R. Raithby, A. Köhler and
R. H. Friend, J. Am. Chem. Soc., 2001, 123, 9412–9417;
(b) M. Jayapal, A. Haque, I. J. Al-Busaidi, R. Al-Balushi,
M. K. Al-Suti, S. M. Islam and M. S. Khan, J. Organomet.
Chem., 2017, 842, 32–38.
1
40, 100547; (c) Z. Chen, C.-L. Ho, L. Wang and
W.-Y. Wong, Adv. Mater., 2020, 32, 1903269; (d) G. Zhou,
W.-Y. Wong and X. Yang, Chem. – Asian J., 2011, 6, 1706–
1
727; (e) X. Yang, G. Zhou and W.-Y. Wong, Chem. Soc. Rev., 16 M. S. Khan, M. R. A. Al-Mandhary, M. K. Al-Suti,
2
015, 44, 8484–8575.
A. K. Hisahm, P. R. Raithby, B. Ahrens, M. F. Mahon,
L. Male, E. A. Marseglia, E. Tedesco, R. H. Friend,
A. Kohler, N. Feeder and S. J. Teat, J. Chem. Soc., Dalton
Trans., 2002, 1358–1368.
4
5
(a) A. N. Al-Khalili, I. J. Al-Busaidi, R. Ilmi, M. Al-Mandhary,
M. S. Khan and N. K. Al-Rasbi, Inorg. Chim. Acta, 2020, 501,
1
19226; (b) K. Binnemans, Coord. Chem. Rev., 2015, 295, 1–
4
5; (c) R. Ilmi, A. Haque, I. J. Al-Busaidi, N. K. Al Rasbi and 17 (a) M. S. Khan, M. R. A. Al-Mandhary, M. K. Al-Suti,
M. S. Khan, Dyes Pigm., 2019, 162, 59–66.
F. R. Al-Battashi, S. Al-Saadi, B. Ahrens, J. K. Bjernemose,
M. F. Mahon, P. R. Raithby, M. Younus, N. Chawdhury,
A. Köhler, E. A. Marseglia, E. Tedesco, N. Feeder and
S. J. Teat, Dalton Trans., 2004, 2377–2385; (b) R. A. Al-
Balushi, A. Haque, M. Jayapal, M. K. Al-Suti, J. Husband,
M. S. Khan, J. M. Skelton, K. C. Molloy and P. R. Raithby,
Inorg. Chem., 2016, 55, 10955–10967; (c) M. S. Khan,
M. R. A. Al-Mandhary, M. K. Al-Suti, T. C. Corcoran, Y. Al-
Mahrooqi, J. P. Attfield, N. Feeder, W. I. F. David,
K. Shankland, R. H. Friend, A. Köhler, E. A. Marseglia,
E. Tedesco, C. C. Tang, P. R. Raithby, J. C. Collings,
K. P. Roscoe, A. S. Batsanov, L. M. Stimson and
T. B. Marder, New J. Chem., 2003, 27, 140–149;
(d) M. S. Khan, A. K. Kakkar, N. J. Long, J. Lewis,
P. Raithby, P. Nguyen, T. B. Marder, F. Wittmann and
R. H. Friend, J. Mater. Chem., 1994, 4, 1227–1232;
(e) M. S. Khan, M. K. Al-Suti, M. R. A. Al-Mandhary,
B. Ahrens, J. K. Bjernemose, M. F. Mahon, L. Male,
P. R. Raithby, R. H. Friend, A. Köhler and J. S. Wilson,
Dalton Trans., 2003, 65–73.
(a) M. S. Khan, R. Ilmi, W. Sun, J. D. L. Dutra,
W. F. Oliveira, L. Zhou, W.-Y. Wong and P. R. Raithby,
J. Mater. Chem. C, 2020, 8, 5600–5612; (b) R. Ilmi, W. Sun,
J. D. L. Dutra, N. K. Al-Rasbi, L. Zhou, P.-C. Qian,
W.-Y. Wong, P. R. Raithby and M. S. Khan, J. Mater. Chem.
C, 2020, 8, 9816–9827; (c) R. Ilmi, M. S. Khan, Z. Li,
L. Zhou, W.-Y. Wong, F. Marken and P. R. Raithby, Inorg.
Chem., 2019, 58, 8316–8331; (d) R. Ilmi, S. Kansız, N. K. Al-
Rasbi, N. Dege, P. R. Raithby and M. S. Khan, New J. Chem.,
2
020, 44, 5673–5683; (e) R. Ilmi, M. S. Khan, W. Sun,
L. Zhou, W.-Y. Wong and P. R. Raithby, J. Mater. Chem. C,
019, 7, 13966–13975; (f) J. Li, J.-Y. Wang and Z.-N. Chen,
2
J. Mater. Chem. C, 2013, 1, 3661–3668; (g) M. D. Ward,
Coord. Chem. Rev., 2007, 251, 1663–1677; (h) H.-B. Xu,
L.-X. Shi, E. Ma, L.-Y. Zhang, Q.-H. Wei and Z.-N. Chen,
Chem. Commun., 2006, 1601–1603.
R. Ilmi, S. Anjum, A. Haque and M. S. Khan, J. Photochem.
Photobiol., A, 2019, 383, 111968.
E. G. Moore, A. P. S. Samuel and K. N. Raymond, Acc.
Chem. Res., 2009, 42, 542–552.
J. Kido and Y. Okamoto, Chem. Rev., 2002, 102, 2357–
6
7
8
9
18 (a) N. Chawdhury, A. Köhler, R. H. Friend, W. Y. Wong,
J. Lewis, M. Younus, P. R. Raithby, T. C. Corcoran,
M. R. A. Al-Mandhary and M. S. Khan, J. Chem. Phys., 1999,
110, 4963–4970; (b) L. Sudha Devi, M. K. Al-Suti, N. Zhang,
S. J. Teat, L. Male, H. A. Sparkes, P. R. Raithby, M. S. Khan
and A. Köhler, Macromolecules, 2009, 42, 1131–1141;
2
368.
(a) S. Zhu and Z. Gu, in Lanthanide-Based Multifunctional
Materials, ed. P. Martín-Ramos and M. Ramos Silva,
Elsevier, 2018, pp. 381–410; (b) K. Y. Zhang, Q. Yu, H. Wei,
1476 | Dalton Trans., 2021, 50, 1465–1477
This journal is © The Royal Society of Chemistry 2021

View Article Online
Dalton Transactions
Paper
(
c) R. A. Al-Balushi, A. Haque, M. Jayapal, M. K. Al-Suti, 30 J. D. L. Dutra, N. B. D. Lima, R. O. Freire and A. M. Simas,
J. Husband, M. S. Khan, O. F. Koentjoro, K. C. Molloy, Sci. Rep., 2015, 5, 13695.
J. M. Skelton and P. R. Raithby, Inorg. Chem., 2016, 55, 31 K. Sonogashira, Y. Tohda and N. Hagihara, Tetrahedron
465–6480; (d) J. S. Wilson, A. S. Dhoot, A. J. A. B. Seeley, Lett., 1975, 16, 4467–4470.
M. S. Khan, A. Köhler and R. H. Friend, Nature, 2001, 413, 32 S. V. Eliseeva, O. V. Kotova, F. Gumy, S. N. Semenov,
6
8
28–831; (e) M. Younus, A. Köhler, S. Cron, N. Chawdhury,
V. G. Kessler, L. S. Lepnev, J.-C. G. Bünzli and
M. R. A. Al-Mandhary, M. S. Khan, J. Lewis, N. J. Long,
N. P. Kuzmina, J. Phys. Chem. A, 2008, 112, 3614–3626.
R. H. Friend and P. R. Raithby, Angew. Chem., Int. Ed., 33 (a) J. E. Barnsley, S. Ø. Scottwell, A. B. S. Elliott,
1
998, 37, 3036–3039; (f) P. Li, B. Ahrens, K.-H. Choi,
K. C. Gordon and J. D. Crowley, Inorg. Chem., 2016, 55,
8184–8192; (b) F. Paul, S. Goeb, F. Justaud, G. Argouarch,
L. Toupet, R. F. Ziessel and C. Lapinte, Inorg. Chem., 2007,
46, 9036–9038.
M. S. Khan, P. R. Raithby, P. J. Wilson and W.-Y. Wong,
CrystEngComm, 2002, 4, 405–412.
9 M. S. Khan, M. R. A. Al-Mandhary, M. K. Al-Suti, N. Feeder,
1
2
S. Nahar, A. Köhler, R. H. Friend, P. J. Wilson and 34 S. A. Cotton and P. R. Raithby, Coord. Chem. Rev., 2017,
P. R. Raithby, J. Chem. Soc., Dalton Trans., 2002, 2441–2448. 340, 220–231.
0 M. S. Khan, M. R. A. Al-Mandhary, M. K. Al-Suti, 35 (a) F. Wu, H. Tong, Z. Li, W. Lei, L. Liu, W.-Y. Wong,
A. K. Hisahm, P. R. Raithby, B. Ahrens, M. F. Mahon,
L. Male, E. A. Marseglia, E. Tedesco, R. H. Friend,
A. Köhler, N. Feeder and S. J. Teat, J. Chem. Soc., Dalton
Trans., 2002, 1358–1368.
W.-K. Wong and X. Zhu, Dalton Trans., 2014, 43, 12463–
12466; (b) X. Yang, G. Zhou and W.-Y. Wong, J. Mater.
Chem. C, 2014, 2, 1760–1778.
36 W. T. Carnall, H. Crosswhite and H. M. Crosswhite, Energy
Level Structure and Transition Probabilities in the Spectra of
2
1 J. D. Dutra, T. D. Bispo and R. O. Freire, J. Comput. Chem.,
2
014, 35, 772–775.
3
the Trivalent Lanthanides in Laf , United States, 1978.
2
2
2 D. F. Eaton, J. Photochem. Photobiol., B, 1988, 2, 523–531.
3 N. B. Lima, S. M. Goncalves, S. A. Junior and A. M. Simas,
Sci. Rep., 2013, 3, 2395.
37 M. Gál, F. Kielar, R. Sokolová, Š. Ramešová and
V. Kolivoška, Eur. J. Inorg. Chem., 2013, 2013, 3217–3223.
38 (a) E. L. Yee, R. J. Cave, K. L. Guyer, P. D. Tyma and
M. J. Weaver, J. Am. Chem. Soc., 1979, 101, 1131–1137;
(b) F. Murauer, L. Kišová, J. Komenda and G. Gritzner,
J. Electroanal. Chem., 1999, 470, 1–8.
2
4 M. S. Khan, M. K. Al-Suti, B. Ahrens, P. R. Raithby and
S. J. Teat, Acta Crystallogr., Sect. E: Struct. Rep. Online, 2004,
6
0, m735–m737.
2
2
2
5 R. O. Freire and A. M. Simas, J. Chem. Theory Comput., 39 M. M. Richter and A. J. Bard, Anal. Chem., 1996, 68, 2641–
010, 6, 2019–2023. 2650.
6 F. Neese, Wiley Interdiscip. Rev.: Comput. Mol. Sci., 2018, 8, 40 (a) M. Thelakkat and H.-W. Schmidt, Adv. Mater., 1998, 10,
2
e1327.
219–223; (b) L. Dou, J. Gao, E. Richard, J. You, C.-C. Chen,
K. C. Cha, Y. He, G. Li and Y. Yang, J. Am. Chem. Soc., 2012,
134, 10071–10079.
7 F. Weigend and R. Ahlrichs, Phys. Chem. Chem. Phys., 2005,
7, 3297–3305.
2
2
8 P. J. Hay and W. R. Wadt, J. Chem. Phys., 1985, 82, 270–283.
9 A. N. Carneiro Neto, E. E. S. Teotonio, G. F. de Sá,
41 J. Yu, M. Xiao, H. Tan and W. Zhu, Thin Solid Films, 2016,
619, 1–9.
H. F. Brito, J. Legendziewicz, L. D. Carlos, 42 X. Zhang, Z. Chi, H. Li, B. Xu, X. Li, S. Liu, Y. Zhang and
M. C. F. C. Felinto, P. Gawryszewska, R. T. Moura, J. Xu, J. Mater. Chem., 2011, 21, 1788–1796.
R. L. Longo, W. M. Faustino and O. L. Malta, in Handbook 43 A. Bejan, S. Shova, M.-D. Damaceanu, B. C. Simionescu
on the Physics and Chemistry of Rare Earths, ed. and L. Marin, Cryst. Growth Des., 2016, 16, 3716–3730.
J.-C. G. Bünzli and V. K. Pecharsky, Elsevier, 2019, vol. 56, 44 Y. Takeda, T. L. Andrew, J. M. Lobez, A. J. Mork and
pp. 55–162. T. M. Swager, Angew. Chem., Int. Ed., 2012, 51, 9042–9046.
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