Thiophene based europium β-diketonate complexes: Effect of the ligand structure on the emission quantum yield

Article abstract of DOI:10.1021/ic1021164

The synthesis and the molecular and photophysical characterization, together with solid state and solution structure analysis, of a series of europium complexes based on β-diketonate ligands are reported. The Eu(III) complex emission, specifically its photoluminescence quantum yield (PL-QY), can be tuned by changing ligands which finely modifies the environment of the metal ion. Steady-state and time-resolved emission spectroscopy and overall PL-QY measurements are reported and related to geometrical features observed in crystal structures of some selected compounds. Moreover, paramagnetic NMR, based on the analogous complexes with other lanthanides, are use to demonstrate that there is a significant structural reorganization upon dissolution, which justifies the observed differences in the emission properties between solid and solution states. The energy of the triplet levels of the ligands and the occurrence of nonradiative deactivation processes clearly account for the luminescence efficiencies of the complexes in the series.

Full text of DOI:10.1021/ic1021164

ARTICLE
pubs.acs.org/IC
Thiophene Based Europium β-Diketonate Complexes: Effect
of the Ligand Structure on the Emission Quantum Yield
Christelle Freund, William Porzio, Umberto Giovanella, Francesco Vignali, Mariacecilia Pasini, and
Silvia Destri*
Istituto per lo Studio delle Macromolecole-CNR, Via E. Bassini 15, 20133 Milano, Italy
Agnieszka Mech
Dipartimento di Scienza dei Materiali, Universit ꢀa di Milano Bicocca, Via R. Cozzi 53, 20125 Milano, Italy
Sebastiano Di Pietro and Lorenzo Di Bari
Dipartimento di Chimica e Chimica Industriale, Universit ꢀa di Pisa, Via Risorgimento 35, 56126 Pisa, Italy
Placido Mineo
Dipartimento di Scienze Chimiche, Universit ꢀa di Catania, Viale A. Doria 6, 95125 Catania, Italy
S Supporting Information
b
ABSTRACT: The synthesis and the molecular and photophysical characteriza-
tion, together with solid state and solution structure analysis, of a series of
europium complexes based on β-diketonate ligands are reported. The Eu(III)
complex emission, specifically its photoluminescence quantum yield (PL-QY),
can be tuned by changing ligands which finely modifies the environment of the
metal ion. Steady-state and time-resolved emission spectroscopy and overall PL-
QY measurements are reported and related to geometrical features observed in
crystal structures of some selected compounds. Moreover, paramagnetic NMR,
based on the analogous complexes with other lanthanides, are use to demonstrate
that there is a significant structural reorganization upon dissolution, which justifies
the observed differences in the emission properties between solid and solution
states. The energy of the triplet levels of the ligands and the occurrence of
nonradiative deactivation processes clearly account for the luminescence efficien-
cies of the complexes in the series.
’
INTRODUCTION
stability, ease of preparation, and noticeable emission properties
due to the effectiveness of the energy transfer (ET) from this
ligand to the Ln(III) ion.
Particularly, europium β-DK complexes have attracted more
The photophysical properties of lanthanide (Ln) complexes,
9
based on π-conjugated ligands, acting as light antennae, have
1
been fully assessed. Interestingly the emission of ions spans from
1
ꢀ3
interest in optoelectronic applications because of their strong and
visible to IR according to the Ln(III) chosen,
applications in disparate fields such as optoelectronic devices,
allowing for
8,10
1,2
narrow red emission. The intensity of this emission depends
on the type of β-diketone and on the type of complex. Con-
sidering ternary complexes of β-DK ligands and Lewis bases to
complete the coordination sphere only, the majority of the highly
luminescent complexes contain fluorinated groups which in-
crease volatility and improve thermal and oxidative stability of
4
5
6
sensors, bioassays, and telecommunication. Recent years have
witnessed a growing interest in their use both alone and as a
component in blends with polymers imparting mechanical
processing properties to the material, to fabricate organic light-
emitting diodes (OLEDs) of different colors and even white
11
7
the compounds. However, a combination of aromatic and
OLEDs, luminescent solar concentrators, lasers, and plastic
8
optical fibers for data transmission.
One of the most studied class of ligands is constituted by
β-diketonate derivatives (β-DK) in view of their chemical
Received: October 20, 2010
Published: May 26, 2011
r 2011 American Chemical Society
5417
dx.doi.org/10.1021/ic1021164
|Inorg. Chem. 2011, 50, 5417–5429

Inorganic Chemistry
ARTICLE
Scheme 1
fluoro-aliphatic substituents on the diketone gives europium(III)
induced by Eu(III). Fortunately, taking advantage also of nuclear
relaxation rates, we can demonstrate isostructurality in solution
upon substitution of Eu(III) with late lanthanides, notably Tb-
(III) and Yb(III), which induce much larger paramagnetic shifts
and derive a set of reliable pseudocontact terms, which allows us
to put forward an alternative structure and dynamic picture for
the complexes in solution, accounting for the PL-QY differences
observed between solid and solution states.
complexes with a more intense luminescence because of the
9b
more efficient ET from the ligand to the lanthanide ion. As a
matter of fact, europium tris(2-thienoyltrifluoroacetonate) phe-
3
nantroline [Eu(TTA) Phen] is one of the most efficient euro-
3
pium complexes in the β-DK series. To date, the largest solid
state photoluminescence quantum yield (PL-QY) measured for a
Eu(III) complex, [Eu(TTA) DBSO] where DBSO = dibenzyl
3
1
2
sulfoxide, is 85%. It is well-known that complex PL-QY upon
As a consequence, the relevance of emission quenching
provoked by high energy CꢀH oscillators in this series, although
its effectiveness is lesser than that observed in Nd(III), Er(III)
ligand excitation results from a balance between the ligands-to-
5
Eu(III) ET rates and the D radiative and nonradiative decay
0
14
rates. The nonradiative decay rates may have contributions from
several processes: multiphonon relaxation, thermally assisted
back-ET (BT) from lanthanide ion to ligand excited levels, relaxa-
tion to the ground state via crossover to another excited state, for
example, the ligand-to-metal charge-transfer state of the Eu(III)
complexes, has been recognized.
’
EXPERIMENTAL SECTION
13
Materials. Sodium amide, sodium methoxide, methanol, tetrahy-
ion, or ET between the lanthanide ions themselves. In this con-
tribution a series constituted by thienoyl ligands only is considered
to maximize the intersystem crossing process (ISC) between S1
drofuran (THF), ethanol, hexane, dichloromethane (CH
2 2
Cl ), chloro-
form (CHCl ), ethyl ether (Et O), ethyl acetate, sodium hydroxide,
3
2
anhydrous sodium sulfate, hydrochloric acid, and phosphoric acid were
purchased from Aldrich and Merck, and deuterochloroform (CDCl
and dideuteromethylenchloride (CD Cl ) from Cambridge Isotope
and T levels of the ligand (Figure 1 below). A complete character-
1
3
)
ization of these β-DK complexes, particularly the study of the PL
properties in the series related to photophysical considerations,
crystal structures, where available, and to complexes geometry
determined by NMR studies is presented. The ligands consid-
ered in the present study are reported in Scheme 1 where Ln(III)
is Eu(III) for the complexes actually discussed or Gd(III) for energy
triplet determination, and Pr, Tb, Tm, Yb, Lu for paramagnetic
NMR (Lu provides the necessary diamagnetic reference).
Chemical and magnetic properties of lanthanides offer a
unique tool to investigate in detail the structure of these
complexes in solution and to get further insight into the relation
2
2
Laboratories-Inc. Methanol, ethanol, and THF were purified by stan-
dard distillation methods.
4,4,4-Trifluoro-1-(2-thienyl)-1,3-butanedione (HTTA), 2-acetyl-3-
bromothiophene, 2-acetylthiophene, 2-bromothiophene, ethyltrifluor-
oacetate, 2-thiophenecarboxylic acid ethyl ester, 5-(2-methylthiophene)-
4
,4,5,5-tetrametyl-1,3,2-dioxoborolane, 1,8-dibromooctane, acetic anhy-
dride, tetrakis(triphenylphosphine)palladium [Pd(PPh ], 1,10-phe-
nantroline (Phen), and europium(III) chloride hexahydrate (EuCl3
3 4
)
3
6H O) were purchased from Aldrich and used without any further purifica-
2
tion. The synthesis of ligands is reported in the Supporting Information.
1
13
between geometry and luminescence properties. H and
C
Apparatus and Procedure. Crystals suitable for X-ray diffrac-
tion (XRD) analysis were obtained by slow crystallization from THF
for Eu(DTDK) Phen and from CHCl for Eu(Br-TTA) Phen and
spectra of the Eu(III) complexes do not lend themselves to an
accurate analysis because of the limited pseudocontact shifts
3
3
3
5
418
dx.doi.org/10.1021/ic1021164 |Inorg. Chem. 2011, 50, 5417–5429

Inorganic Chemistry
ARTICLE
Table 1. Crystallographic Details of Eu(Br-TTA) Phen and Eu(DTDK) Phen Complexes
3
3
chemical formula
formula weight
space group
a/nm
C
36
H
17Br
3
EuF
9
2
N O
S
6 3
C
53
H
45EuN
1182.23
P2 /c (No.14)
2 8 6
O S
1232.39
P1 (No.2)
0.9804(1)
1.3673(2)
1.6067(2)
95.118(3)
95.479(3)
98.792(3)
2106.9(5)
2
1
1.2039(8)
3.0280(2)
1.4916(11)
90.0
b/nm
c/nm
R (deg)
β (deg)
93.89(4)
90.0
γ (deg)
3
V/nm
5425.0(15)
4
Z
ꢀ3
D
calc/g cm
1.943
1.447
radiation
Mo KR (0.71073 Åꢁ )
Mo KR (0.71073 Åꢁ )
ꢀ
1
μ/cm
F(000)
4.568
1.44
1184
2400
temperature/ K
293
293
anomalous dispersion
all non-H atoms
all non-H atoms
parameters refined in full-matrix least-squares
unweighted agreement factor
weighted agreement factor
541
632
0.057
0.058
0.156
0.122
a
goodness of fit
1.016
0.965
least-squares weights
largest shifts
b
c
0.00
0.05 σ
high and low peaks in final diff. map/e Å
crystal color
2.77 and ꢀ1.09
yellow
0.75 and ꢀ0.53
yellow
crystal shape
prism
prism
3
crystal dimensions/mm
0.44 ꢁ 0.25 ꢁ 0.19
Bruker-AXS SMART-APEX
0.50 ꢁ 0.23 ꢁ 0.20
Bruker-AXS SMART-APEX
0.53ꢀ0.76
instrument
empirical absorption
maximum θ/deg
0.24ꢀ0.48
25
25
reflections included F
o
> 4σ(F
o
)
5711 over 7406
4495 over 9524
monochromator graphite crystal, attenuator Zr foil, factor 17.0
scan type ω
scan rate
ω
ω
d
d
10 s/image
10 s/image
scan width/deg
0.2
) þ (0.1125P) þ 2.6021P] where P = (F
0.2
a
2
2
b
2
2
2
2
2
c
2
2
2
Minimization function ∑w(F
.0000P] where P = (Fo þ 2F )/3. That is, 0.02ꢀ/s.
o
ꢀ F ). w = 1/[\σ (F
c
o
o
þ 2F
c
)/3. w = 1/[\σ (F
o
) þ (0.0723P) þ
2
2
d
0
c
Eu(MeT-TTA)
3
Phen, respectively. XRD experiments were carried out
oxyphenyl)-10,20-bis(p-hydroxyphenyl)porphyrin (C68
H
78
N
N O
4 4
4
O
4
,
,
using an Enraf Nonius CAD4 instrument for single crystal analysis, while
films and powder were examined using a computer controlled Siemens
D-500 diffractometer equipped with Soller slits and an Anton-Paar camera
for variable temperature experiments under nitrogen atmosphere.
Crystal structure resolution was performed according to the condi-
tions shown in Table 1 and Supporting Information, Table S2, using
1014 Da) and tetrakis(p-dodecanoxyphenyl) porphyrin (C92
H
126
1350 Da).
UVꢀvis absorption spectra for both solutions and films were
measured with a Lambda 900 Perkin-Elmer spectrometer. Steady-state
photoluminescence (PL) spectra were recorded using a SPEX 270 M
monochromator equipped with a N -cooled CCD detector, by exciting
2
15
WINGX package.
with a monochromated Xe lamp. The overall PL-QY were measured in
ꢀ5
ꢀ1
NMR spectra were recorded on Bruker Advance 400 and 270
spectrometers and on a Varian Inova 600 (14.1 T) instrument equipped
with triple resonance gradient probe.
CH
2 2
Cl solution [10 Mol L ] at room temperature with respect to a
reference solution of quinine sulfate in 1 N H SO and exciting at
2
4
350 nm (PL-QY = 54.6%). The overall PL-QY for the films obtained by
spin-coating from CH Cl solutions, was determined under ligand
excitation (360 nm) and is based on the absolute method using a
FTIR spectra were taken on a Perkin-Elmer 2000 FTIR spectrometer.
Positive MALDI-TOF mass spectra were acquired by a Voyager DE-
STR (PerSeptive Biosystem) using a delay extraction procedure (25 kV
applied after 2600 ns with a potential gradient of 454 V/mm and a wire
voltage of 25 V) and detection in linear mode. The instrument was equipped
with a nitrogen laser (emission at 337 nm for 3 ns) and a flash AD con-
verter (time base 2 ns). trans-2[3-(4-tert-Butylphenyl)-2-methyl-2-pro-
penylidene]-malononitrile (DCTB) or dithranol were used as a matrix.
Mass spectrometer calibration was performed using 5,15-bis(p-dodecan-
2
2
1
6
calibrated integrating sphere. The estimated error for the solid-state
PL-QY is 10%.
The emission lifetimes were collected while monitoring 615 nm
5
7
Eu(III) D - F hypersensitive transition. All the samples were excited
0
2
with a beam of 355 nm Nd:YAG laser, with the power of 100 μW and the
repetition rate of 200 Hz (for the sample with the shorter lifetime the
repetition rate was 3000 Hz), a PCI plug-in multichannel scaler ORTEC
5
419
dx.doi.org/10.1021/ic1021164 |Inorg. Chem. 2011, 50, 5417–5429

Inorganic Chemistry
353 100-ps Time Digitizer/MCS has been used in a photon counting
acquisition mode, with a time resolution better than 100 ns. The
estimated error for the solution emission lifetimes is 2%.
ARTICLE
9
J
3,4 = 7.0, 2H, Phen), 6.50 (d, J4,3 = 4.1, 3H, Th), 5.64 (d, J3,4 = 4.1, 3H,
Th), 3.29 (s, 3H, CH β-diketonate). C NMR (CD Cl , 100 MHz,
1
3
2 2
δ
ppm, JHz): 179.5 (CO-Th), 168.3 (COꢀCF
126.7, 124.8, 111.8, 106.2, 97.5 CH β-diketonate. Elemental analysis was
performed for C, H, S, atoms: Calc. for EuBr 17: C,
35.09; H, 1.39; S, 7.81. Found: C, 34.87; H, 1.43; S, 7.58.
Eu(TTA) (Br-TTA)Phen. Starting products: Phen, HTTA, and Br-
HTTA as ligands and EuCl 6H O as the metal precursor, in the
3
), 162.8, 150.4, 127.1,
3
Synthesis of Complexes: General Procedure. Eu(DTDK) -
Phen. 1,3-Di(thien-2-yl)propane-1,3-dione (HDTDK) (708.9 mg, 3 mmol)
and Phen (180.2 mg, 1 mmol) were dissolved in hot EtOH (7 mL).
To this solution cooled to room temperature was added an aqueous
S F O N C H
3 3 9 6 2 36
2
ꢀ
1
NaOH solution (3 mL, 1 mol L , 3 mmol). After stirring for 20 min,
EuCl 6H O (366.2 mg, 1 mmol) in water (7 mL) was added to the
3
2
3
respective stoichiometry of 1, 2, 1, and 1. Yield 85%. The complex was
obtained as a light pink beige powder, which emits strongly under
excitation at 360 nm, in the solid state and in solution as well. No
attempts for purification were performed, to avoid any decomposition.
MALDI-TOF (matrix dithranol, m/z): [2L1 Phen Eu] 773.6; [L1 L2
Phen Eu] 851.5; [2L2 Phen Eu] 931.3; [2L1 2Phen Eu] 953.6; [L1
3
2
3
solution. The addition was accompanied by a large precipitation. The
mixture was then heated for 3 h at 60 ꢀC. After cooling to room
temperature, the yellow precipitate was collected by filtration and dried
under vacuum to afford 91% of the complex (0.9437 g).
þ
þ
þ
þ
ꢀ
1
FT-IR [film from CH Cl ν (cm )]: 1585, 1560, 1545, 1530, 1515,
2
2,
þ
þ
þ
L2 2Phen Eu] 1031.6; [2L2 2Phen Eu] 1111.4; the ions were
1
1
480, 1430ꢀ1410 (br), 1345, 1295 (br), 1235, 1195, 1150, 1110, 1075,
030, 855, 840, 875, 750, 730ꢀ710 (br), 660, 620, 590. UVꢀvis
ꢀ
ꢀ
detected as M species because of loss of a TTA or Br-TTA anion
1
fragment from the corresponding molecular species. H NMR (CD Cl ,
absorption: solution (CH Cl ) λ
= 272 and 373 nm; film λ =
max
2
2
2
2
max
4
00 MHz, δppm, JHz): 10.88ꢀ10.82 (2H, Phen), 10.12ꢀ10.02 (2H,
2
80 and 400 nm. MALDI-TOF (matrix DCTB, m/z): main peak, [2 L
þ
þ
Phen), 9.37ꢀ9.25 (2H, Phen), 8.49ꢀ8.45 (2H, Phen), 6.96 (d, 2H,
Phen Eu] 802.47; [2 L 2 Phen Eu] 982.54; the ions were detected
as M species because of loss of a DTDK anion fragment from the
corresponding molecular species. Taking into account all the peaks
þ
ꢀ
J
(
1
= 4.6, Th TTA), 6.47ꢀ6.40 [m, 3H, Th TTA (2H) and Th Br-TTA
1H)], 5.94 and 5.86 (2d, 2H, J3,4 = 3.6, Th TTA), 5.71 and 5.63 (2d,
5,4
H, J3,4 = 3.8, Th Br-TTA), 3.50 and 3.28 (2s, 1H, CH β-diketonate of
present in the spectrum and their intensity, a DTDK:Phen:Eu ratio of
13
1
Br-TTA), 3.05 and 2.88 (2s, 2H, CH β-diketonate of TTA). C NMR
3
:1:1 could be calculated. H NMR (CD
2
Cl
2
, 400 MHz, δppm, JHz):
(
1
CD Cl , 100 MHz, δppm, JHz): 179.5, 168.6, 162.8, 150.2, 135.6, 135.5,
27.2, 127.1, 127.0, 126.9, 126.5, 124.5, 123.7, 123.6, 112.0, 106.2, 95.6,
and 95.1 (CH β-diketonate). Elemental analysis was performed for C, H,
S, atoms: Calc. for EuC H BrF N O S : C, 40.24; H, 1.78; S, 8.95.
Found: C, 40.04; H, 1.88; S, 8.78.
Eu(BrC8-TTA) Phen. Starting products: Phen and BrC8-HTTA as
ligands, and EuCl3 6H O as the metal precursor Yield 48%. The
complex was obtained as a yellowish solid after precipitation from hexane
solution. MALDI-TOF (matrix dithranol, m/z): main peak [2 L Phen
major form (∼ 85%): 12.95 (s br, 2H, Phen), 10.71 (d, 2H, J3,4 = 7.6,
2 2
Phen), 10.06 (s, 2H, Phen), 9.29 (d, 2H, J4 = 7.6, Phen), 6.68 (d, 6H,
,3
J
5,4 = 4.8, Th), 6.20 (dd br, 6H, J4,3 = 3.6, Th), 5.59 (d, 6H, J3,4 = 2.8, Th),
2
.96 (s, 3H, CH β-diketonate); minor form (∼15%): 12.05 (s br, 2H,
36 19
9 2 6 3
Phen), 10.63 (d, 2H, J3 = 7.6, Phen), 9.99 (s, 2H, Phen), 9.07 (d, 2H,
,4
J4,3 = 7.6, Phen), 6.55 (d, 6H, J5,4 = 4.4, Th), 6.07 (dd br, 6H, J = 3.6,
3
4,3
1
3
2
Th), 5.23 (s br, 6H, Th), 2.59 (s, 3H, CH β-diketonate). C NMR
CD Cl , 100 MHz, δppm, JHz): 181.9 (CdO), 164.4, 162.7, 149.9,
31.3, 128.1, 123.1, 122.7, 115.1, 109.3, 94.0 (CH β-diketonate), 57.9.
Elemental analysis was performed for the C, H, S atoms: calc. for
EuC53 using crystal for XRD: C, 53.84; H, 3.84; S, 16.27.
3
(
2
2
1
þ
þ
þ
Eu] 1154.9; [2 L 2Phen Eu] 1333.7; the ions were detected as M
species, because of loss of a BrC8-TTA anion fragment from the
corresponding molecular species. Taking into account all the peaks
ꢀ
45 8 2 6
H O N S
Found C, 53.72; H, 3.92; S, 16.13.
Eu(TTA) Phen. Starting products: Phen and HTTA as ligands, and
present in the spectrum and their intensity a BrC8TTA:Phen:Eu ratio of
3
1
EuCl
3
6H
2
O as the metal precursor. The complex was obtained as a
3:1:1 could be calculated. H NMR (CD
2
Cl
2
, 270 MHz, δppm, JHz) 10.35
(s br, 2H, Phen), 10.15 (d, 2H, J4,3 = 8.1, Phen), 9.47 (s, 2H, Phen), 8.48
(d br, 2H, J3,4 = 8.1, Phen), 6.14 (d, J3,4 = 3,4, 3H, Th), 5.83 (d J4,3, 3H,
3
pale rose powder with a yield of 65% after crystallization from CHCl
MALDI-TOF (matrix dithranol, m/z): [2 L Phen Eu] 773.4, main
peak; [2 L 2Phen Eu] 953.7; the ions were detected as M species
because of loss of a TTA fragment from the corresponding molecular
species. Taking into account all the peaks present in the spectrum and
3
.
þ
þ
þ
Th), 3.50 (t, 6H, CH
ꢀTh), 3.42 (t, 6H, CH ꢀBr), 2.47 (s, 3H, CH β-
2
2
diketonate), 1.89ꢀ1.26 (36H, alkyl chain). Elemental analysis was
performed for C, H, S, atoms: Calc. for EuC H F N O S Br : C,
6
0
65
9
2
6
3
3
1
their intensity a TTA: Phen: Eu ratio of 3:1:1 could be calculated. H
45.93 ; H, 4.18; S, 6.13. Found: C, 46.03 ; H, 4.01; S, 6.00.
NMR (CD
2
7
2
Cl
H, J4,3 = 8.0, Phen), 9.44 (s, 2H, Phen), 8.56 (d, 2H, J3,4 = 7.6, Phen),
.04 (d, 3H, J5,4 = 4.8, Th), 6.50 (dd, 3H, J4,5 = 4.2, Th), 6.02 (s large, 3H,
2
, 400 MHz, δppm, JHz): 10.89 (s br, 2H, Phen), 10.17 (d,
Eu(MeT-TTA) Phen. Starting products: Phen and MeT-HTTA as
3
ligands, and EuCl3 6H O as the metal precursor. Solvent EtOH:THF
2
3
(1:5). The complex was obtained as a yellow powder with a yield of 72%
13
Th), 3.13 (s, 3H, CH β-diketonate). C NMR (CD Cl , 100 MHz,
after crystallization from CHCl . MALDI-TOF (matrix dithranol, m/z):
2
2
3
þ
þ
δ
ppm, JHz): 179.5 (CO-Th), 168.8 (COꢀCF
27.1, 127.0, 123.7, 112.2, 106.3, 95.6 (CH β-diketonate). Elemental
analysis was performed for C, H, S, atoms. Calc. for EuC H F O N S :C,
3
), 162.9, 150.2, 135.5,
[2 L Phen Eu] 965.57; [3 L Eu]H 1102.26; the ions were detected as
þ
ꢀ
1
M
species, because of loss of a MeT-TTA anion fragment from the
corresponding molecular species, and as MH species, respectively.
Taking into account all the peaks present in the spectrum and their
þ
36 20 9 6 2 3
4
3.43 ; H, 2.02; S, 9.66. Found: C, 43.70; H, 1.92; S, 9.48.
Eu(Br-TTA) Phen. Starting products: Phen and Br-HTTA as ligands,
and EuCl 6H O as the metal precursor. The complex was obtained as a
1
3
intensity a MeT-TTA:Phen:Eu ratio of 3:1:1 could be calculated. H
3
2
NMR (CD Cl , 270 MHz, δppm, JHz): 11.09 (s br, 2H, Phen), 10.06
2
2
3
light orange powder which was crystallized from CHCl (yield 70%).
(d,2H,J4,3 = 7.8, Phen), 9.31 (s, 2H, Phen), 8.53 (d, 2H, J3,4 =7.6,Phen),7.26
(d,3H,J= 3.2, Th), 6.90 (d, 3H, J=2.4,Th),6.45(d,3H,J= 3.8, Th), 5.76 (d,
3H, J = 3.8, Th), 2.97 (s, 3H, CH β-diketonate), 2.73 (s, 9H, Me). Elemental
analysis was performed for C, H, S, atoms: Calc. for EuC H F N O S : C,
3
þ
MALDI-TOF (matrix dithranol, m/z): [2 L Phen Eu] 931.3, main
peak; [2 L 2Phen Eu] 1111.4; the ions were detected as M species,
because of loss of a BrTTA anion fragment from the corresponding
þ
þ
ꢀ
51 32 9 2 6 6
molecular species. Taking into account all the peaks present in the
47.70; H, 2.51; S, 14.98. Found C, 47.82; H, 2.55; S, 14.80.
spectrum and their intensity a Br-TTA:Phen:Eu ratio of 3:1:1 could be
The same synthetic procedures were used in the preparation of the
1
calculated. NMR H (CD
2
Cl
2
, 400 MHz, δppm, JHz): 10.94 (s large, 2H,
corresponding Ln(TTA) Phen, Ln = Gd, Pr, Tb, Tm, Yb, Lu, as well as
3
Phen), 10.12 (s large, 2H, Phen), 9.35 (s, 2H, Phen), 8.53 (s large, 2H,
for the systems of empirical formula Gd(Br-TTA) Phen, Gd(TTA) -
3
2
Phen), 6.50 (s br, 3H, Th), 5.66 (s br, 3H, Th), 3.24 (s, 3H, CH
3 3 2
(Br-TTA)Phen, Gd(BrC8-TTA) Phen, Gd(DTDK) Phen, Yb (TTA) -
1
β-diketonate). H NMR (acetone d
6
, 270 MHz, δppm, JHz): 11.82 (s large,
(Br-TTA)Phen, and Yb(BrC8-TTA) Phen. Elemental analyses were
3
2H, Phen), 10.21 (d, J4,3 = 7.8, 2H, Phen), 9.28 (s, 2H, Phen), 8.69 (d,
performed for C, H, S, atoms:
5
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Inorganic Chemistry
ARTICLE
Calc. for GdC36
3.47; H, 2.12; S, 9.44.
Calc. for GdBr
C, 34.80; H, 1.43; S, 7.55.
Calc. for GdC H BrF N O S : C, 40.06; H, 1.77; S, 8.91. Found:
H
20
F
9
N
2
O
6
S
3
: C, 43.20; H, 2.01; S, 9.61. Found C,
a more polar solvent as deuterated acetone); while for MeT-
HTTA as well as for HTTA only the signal of enolic form is
apparent. The stronger pushꢀpull character of the latter group of
4
3 3 9 6 2 36
S F O N C H17: C, 34.97; H, 1.39; S, 7.78. Found:
2
1
molecules can be related to the enolic form stabilization.
36
19
9
2
6
3
The complexes Eu(Ligand) Phen were synthesized from
3
C, 39.93; H, 1.85; S, 8.72.
Calc. for GdC60
C, 45.93; H, 4.21; S, 6.00.
EuCl 6H O and the corresponding diketonate in EtOH/water
3
3
2
65 9 2 6 3 3
H F N O S Br : C, 45.81; H, 4.16; S, 6.11. Found:
with phenanthroline as the ancillary ligand as described by Melby
22
et al. and reported in Scheme 2.
Calc. for GdC H O N S : C, 53.60; H, 3.82; S, 16.20. Found C,
53
45
8
2
6
In the case of 5-methylthien-2-yl, higher yields of complex
were attained only by using 1:5 EtOH:THF as the solvent, to
maintain the ligand enolic anion in solution. The complexes are
poorly soluble in most organic solvents except for acetone. They
are partially soluble in CH Cl , CHCl , and EtOH. The com-
5
3.49; H, 3.94; S, 16.01.
Calc. for PrC36
4.12; H, 2.15; S, 9.58.
Calc. for TbC H F N O S : C, 43.12; H, 2.01; S, 9.59. Found C,
H
20
F
9
N
2
O
6
S
3
: C, 43.91; H, 2.05; S, 9.77. Found C,
4
3
6 20 9 2 6 3
2
2
3
4
3.30; H, 2.13; S, 9.40.
Calc. for TmC36
2.85; H, 2.07; S, 9.37.
1
13
plexes were characterized by H, C NMR spectroscopy, (see
also below results and discussion on paramagnetic NMR).
H
20
F
9
N
2
O
6
S
3
: C, 42.70; H, 1.99; S, 9.50. Found C,
13
C
4
NMR spectrum was carried out only in the case of adequate
solubility of the complex. FTIR and UVꢀvis spectroscopy,
cyclovoltammetry (presented in Supporting Information, Table
S1 and Figure S1), elemental analysis, MALDI-TOF mass
spectrometry determination together with XRD analysis, when
single crystals could be obtained, were performed to complete
the characterization. The two last analyses and NMR integrals are
in agreement with a Ligand:Phen:Ln ratio of 3:1:1. In the
MALDI-TOF experiments, in some cases, exchanges among
the ligands was observed, and also the matrix taking part in this
process. Deeper investigations are in progress with different
matrixes to clarify the matrix effect and will be the object of a
forthcoming paper.
Calc. for YbC H F N O S : C, 42.52; H, 1.98; S, 9.46. Found C,
36
20
9
2
6
3
42.72 ; H, 2.09; S, 9.26.
Calc. for LuC36
H
20
F
9
N
2
O
6
S
3
: C, 42.44; H, 1.98; S, 9.44. Found C,
: C, 39.46; H, 1.75; S, 8.78. Found: C,
Br : C, 45.32; H, 4.12; S, 6.05. Found:
42.63; H, 2.08; S, 9.27.
9 2 6 3
Calc. for YbC36H19BrF N O S
39.60; H, 1.88; S, 8.59.
Calc. for YbC60
H
65
F
9
N
2
O
6
S
3
3
C, 45.54; H, 4.01; S, 5.91.
’
RESULTS AND DISCUSSIONS
Synthesis of Ligands and Complexes. 5-Bromo-2-thienoyl-
trifluoroacetone Br-HTTA was synthesized by the Claisen con-
densation of 2-acetyl-5-bromothiophene and ethyltrifluoroacetate
Photophysical Characterization. The optical properties of
Eu(III) complexes have been studied for both CH Cl diluted
2 2
with sodium methoxide as the base and MeOH/Et O mixture as
solutions and solid-state (thin film) samples by absorption,
steady-state and time-resolved emission spectroscopy, and over-
all PL-QY measurements.
In Figure 1, the absorption spectra of the ligands in solution
(a) and of all the complexes in solution and as thin film (b) are
shown. The broad bands of the ligand absorption show their
maximum (λmax) ranging from 250 to 375 nm and absorption
edges up to 430 nm. The exception being MeT-HTTA whose
2
1
7
the solvent. The pure product was obtained in a 65% yield after
sublimation. In the synthesis of ligand HDTDK, sodium amide
replaced sodium methoxide as the base, and THF was used as the
1
8
solvent. To obtain MeT-HTTA the strategy of modifying the
acetylthiophene reagent via Suzuki coupling was adopted. Spe-
cifically 5-bromo-2-acetylthiophene reacted with the 5-(2-methyl-
thiophene)-boronic acid pinacol ester to afford the intermediate
ketone, which was later transformed by a Claisen type condensa-
tion with ethyltrifluoroacetate, into the corresponding diketone
MeT-HTTA. 8-Bromooctyl-2-thienoyltrifluoroacetone (BrC8-
HTTA) was prepared by acylation with acetic anhydride and
λ
max is equal to 415 nm. The absorption profiles of the complexes
dominated by the ligand absorption are almost identical for the
samples measured in solution as well as in thin films. As expected
the edge of the absorption band for the thin films is slightly red-
shifted.
1
9
phosphoric acid of 2-(8-bromooctyl)thiophene, obtained fol-
20
lowing the procedure reported in the literature, and subsequent
The sensitization pathway in Ln(III) complexes generally
consists of excitation of the ligand into its singlet excited state,
subsequent ISC to its triplet state, and ET from the triplet state of
Claisen condensation of ethyltrifluoroacetate on the terminal
17
ketonic group. NMR spectroscopy of the β-DK ligands in CDCl
3
1,5a,9b
as the solvent shows the presence of both ketonic and enolic
tautomers for HDTDK, BrC8-HTTA, and Br-HTTA, although
in a different ratio (15% for the first, 10% for the second, and less
than 1% for the third one, this last percentage slightly increases in
the ligand to the Ln(III) ion.
To determine the energy of the triplet state of the ligands
versus energy states of Eu(III) ions, the phosphorescence spectra
3
þ
of the frozen solution of the corresponding Gd complexes,
Scheme 2
5
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Inorganic Chemistry
ARTICLE
ꢀ
5
ꢀ1
ꢀ5
ꢀ1
2 2 2 2
Figure 1. Normalized Absorbance of ligands in CH Cl solution [10 mol L ] (a), Eu(III) complexes (b) in CH Cl solution [10 mol L ]
(
3
3
solid-line) and in film (dotted-line). In the inset, diagram of the most probable states involved in the photophysics of the Eu(III) complexes.
7b
considering the 0ꢀ0 transition, were measured (Supporting
Information, Figure S2). The energy values of the triplet levels of
the ligands in complexes were as follows: TTA (502 nm,
ꢀ
1
ꢀ1
1
9920 cm ), BrC8-TTA (511 nm, 19569 cm ), and Br-
ꢀ
1
TTA (521 nm, 19194 cm ), which places them between the
D (465 nm, 21500 cm ) and D (524 nm, 19070 cm
excited states of the metal ion. As already shown by Chen
5
ꢀ1
5
ꢀ1
)
2
1
23
2
4
et al., the introduction of an electron donor substituent on the
TTA ligand provokes a bathochromic shift of the triplet state
energy level. The energy of the triplet state of DTKT (531 nm,
ꢀ
1
5
ꢀ1
1
8832 cm ) is close to the D (578 nm, 17300 cm ) level of
0
2
Eu(III). The energy of the triplet state of the ligand MeT-TTA
was hard to determine because the ligand fluorescence in the
corresponding Gd(MeT-TTA) Phen frozen solution overlaps
3
the phosphorescence, indicating that the triplet state was scarcely
ꢀ
1
5
populated. A value around 526 nm, 19011 cm , too close to D1
to prevent metalꢀligand BT, was estimated. A diagram of the
most probable states involved in the photophysics of the Eu(III)
complexes is reported as inset in Figure 1.
ꢀ
5
Figure 2. PL spectra of the complexes in CH Cl solution [10 Mol
L
complexes in solution.
2
2
ꢀ
1
] by exciting at 350 nm. Inset, time-resolved emission spectra of the
Figure 2 presents the emission spectra recorded for all the
complexes in solution (Supporting Information, Figure S3
for PL spectra of the films). These emission spectra show typical
3
þ
5
7
Eu PL features corresponding to the transitions D f F
0
0
5
7
Eu(TTA) Phen, Eu(Br-TTA) Phen, Eu(TTA) (Br-TTA)Phen,
3
3
2
(
around 580 nm), D f F (592 nm), the hypersensitive
0 1
7 5 7 5
5
and Eu(BrC8-TTA) Phen complexes, while for Eu(DTDK) -
3
3
D f F (615 nm), D f F (around 635 nm), and D f
0
2
0
3
0
7
5
7
Phen and Eu(MeT-TTA) Phen the feeding state becomes too
3
close to the energy of the D (the emitting state) and the D
F (around 685 nm). The intensity of the D f F transition
4
0
2
5
5
5
7
(
electric dipole) is greater than that of the D f F transition
0
1
0
1
respectively, and BT operates.
(
magnetic dipole) suggesting that the coordination environment
3
þ
To verify this behavior, the steady-state PL-QY of solutions
and films (presented in Table 2) as well as the emission
decay measurements of solutions at room temperature were
carried out. The recorded PL-QY values for thin film are in
of the Eu ion is free from an inversion center. The emission
spectra of Eu(III) complexes do not exhibit the ligand centered
transition, signifying that there is an efficient intramolecular ET
3
þ
from the β-DK ligand to the Eu ions.
It is well established in the literature that for Eu(III) β-DK
the range from 0.72 for Eu(TTA) Phen (which is in agree-
ment with the value reported in literature) to 0.41 for both
3
2
6
complexes, the largest PL-QY is observed when the energy of the
5
triplet state of the ligand is close to the energy of D level of
Eu(TTA) (Br-TTA)Phen and Eu(BrC8-TTA) Phen, 0.34 for
1
2
3
Eu(III) by keeping to the following phenomenological rule ΔE
Eu(Br-TTA) Phen, down to less than 0.01 for Eu(DTDK) Phen
3 3
ꢀ
1 5a,25
(
T * ꢀ Ln* emissive level) in the range 2500ꢀ3500 cm
.
and Eu(MeT-TTA) Phen, indicating an almost complete quen-
1
3
For these reasons, high PL-QY should be expected from
ching of the emission in these latter ones.
5
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Inorganic Chemistry
Table 2. Overall PL-QY of All the Complexes in Film and Solution [CH Cl 10 M ], the Relative Integrated Intensity of the
ARTICLE
ꢀ
5
2
2,
5
7
5
7
5
D f F Transition with Respect to That of the D f F Transition Band (A ), D Lifetime (τobs), Radiative (ARAD) and Non-
0
2
0
1
21
0
Radiative (ANR) Decay Rates, Intrinsic PL-QY of Eu(III) (PL-QYintr), and the ET Efficiencies (ηsens) in Solution
film
solution
ꢀ1
ꢀ1
complex
PL-QY
A
21
PL-QY
A
21
τ
obs (μs)
ARAD (s
)
ANR (s
)
PL-QYintr
ηsens (%)
a
Eu(TTA)
3
Phen
0.72
18.4
0.48
0.43
0.43
0.37
0.01
<0.01
15.6
14.7
14.9
13.3
14.7
710
610
550
500
5
713
736
748
729
843
695
903
0.51
0.45
0.41
0.36
0
95
b
Eu(TTA)
2
(Br-TTA)Phen
0.41
96
b
Eu(BrC8-TTA)
Eu(Br-TTA) Phen
Eu(DTDK) Phen
Eu(MeT-TTA) Phen
3
Phen
0.41
1070
1271
≈100
≈100
a
3
0.34
17.3
14.8
a
3
<0.01
a
3
<0.01
a
b
Semicrystalline film. Amorphous film. Estimated relative errors: τobs, ( 2%; PL-QY, ( 10%; τrad, ( 10%; PL-QYintr, ( 12%; ηsens, ( 22%.
In CH Cl solution, the PL-QY of Eu(TTA) Phen decreases
simple explanation, and nonradiative decay mechanisms must be
taken into consideration. Indeed the nonradiative deactivation
rates are two times higher for all the complexes than the one
2
2
3
26
to 0.48 as reported in the literature, while the PL-QY values of
the other complexes are substantially unmodified with respect to
the corresponding film (Table 2).
calculated for Eu(TTA) Phen.
3
To evaluate the efficiency of the Eu(III) sensitized emission,
its 615 nm luminescence decay profiles (Figure 2 inset) were
measured in solution upon excitation of the ligand absorption
band at 355 nm. The PL decay profiles measured for all the
samples are well fitted by a single-exponential function, which
indicates that the Eu(III) ions are placed in a unique symmetry
site with a lifetimes (τobs) in the range 0.50ꢀ0.71 ms (see
In Table 2 the PL-QY measured on a films of all the samples
are reported too. As already mentioned an increase of the value
related to Eu(TTA) Phen complex with respect to that one of
3
solution is observed. Paramagnetic NMR analysis clearly indi-
cates an augmented molecular symmetry in solution as compared
with solid state (cf. XRD and NMR data below).
5
7
As the magnetic-dipole transitions D f F are nearly in-
0
1
27
Table 2), typical for Eu(III) organic complexes with β-DKs.
dependent of the ligand field, they can be used as an internal
29
Such a long τobs is, however, not observed for Eu(DTDK) Phen
standard to account for ligand differences. The electric-dipole
5 7
3
that exhibits 0.005 ms lifetime (i.e., strong emission quenching).
It is worth to underline that the trend observed for transient
decays in the solutions is the same as for PL-QY.
transitions D f F , are sensitive to the symmetry of the
0 2
coordination sphere. The A21 ratio in the lanthanide complex
30
measures the symmetry of the coordination sphere.
The ligands must protect Eu(III) from the external sources of
nonradiative deactivation (like water and solvents molecules),
and provide efficient light harvesting and ligand-to-metal ET
In the solid state the distortion of the symmetry around the
Eu(III) ion enhances the probability of the electric-dipole
transition implying increased PL-QY values. Moreover, PL-QY
depends on the ηsens values which are in their turn related to the
(
ηsens) to achieve high PL-QY values. To get a better under-
1
standing of the radiative and nonradiative pathways, the PL
distance of the Eu ligands. Considering that the intramolecular
efficiencies of Eu(III) complexes in solution are analyzed in
terms of the equation
ligand-to-Eu ET are almost complete in solution and the
8b,28
ligandꢀmetal distances vary less than 0.01 nm in the solid state
for all the TTAs complexes (see below), the variation of η
ꢀ
ꢁ
sens
τobs
should not influence PL-QY in our case. Indeed the PL-QY
PL-QY ¼ ηsens ꢁ PL-QYintr ¼ ηsens
ꢁ
τrad
increment is verified for Eu(TTA) Phen only. The deactivation
3
processes already evidenced in solution must take place in the
solid state too and are probably even more important.
where PL-QYintr is the intrinsic PL-QY of Eu(III), ηsens is the
efficiency of the ligand-to-metal ET, and τobs and τrad are the
observed and the radiative lifetimes. The details of the calcula-
tions were reported in ref 28.
Although OH oscillators, for example, as in bound water
molecules, are the most effective quenchers both in the solid
state and in solution, clear evidence for the quenching effect of
higher harmonics of NꢀH, CꢀH and CdO, which are less
The overall PL-QY, the relative integrated intensity of the
5
7
5
7
31
D f F transition with respect to that of the D f F transi-
efficient oscillators, is provided; thus, a deeper analysis of the
0
2
0
1
tion band (A ), the radiative (ARAD) and nonradiative (ANR
)
possible deactivation processes has to be performed.
2
1
decay rates, and ηsens for all complexes in solution are presented
in Table 2.
Because of the extremely fast emission transient decay and
To give further insight into the quenching processes, specifi-
cally to justify the trend observed for PL-QY and the lifetime of
the complexes, structural determinations both in solid state and
in solution have been carried out. The geometry of the complexes
has been compared with that of other complexes whose crystal-
lographic data are reported in the literature.
very low PL-QY of the Eu(DTDK) Phen, the calculated values of
3
ηsens and ANR are senseless. The very low PL-QY of this complex
is probably related to inefficient intramolecular ET between
ligand and metal ion caused by the very low energy of the triplet
state of DTDK.
Crystal Structures of Eu(Br-TTA) Phen and Eu(DTDK) -
3
3
Phen. The crystal and molecular structures of three Eu(III)
complexes of the series were solved. We report here data related
to Eu(Br-TTA) Phen and Eu(DTDK) Phen, while the data for
On the contrary, all the remaining complexes exhibit the same
high ηsens as the Eu(TTA) Phen suggesting that the matching
3
3
3
differences between the energy of the triplet level of the ligand
Eu(MeT-TTA) Phen are provided in the Supporting Informa-
tion in view of the mosaic spread of the crystals, preventing
3
5
and the D energy level of the Eu(III) ion cannot be applied as a
1
5
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Inorganic Chemistry
ARTICLE
3
Figure 3. (a) a axis view of the crystal packing of Eu(Br-TTA) Phen complex; (b) ORTEP plot with thermal ellipsoid probability of 50%.
accurate crystal structure refinement. All of them are character-
ized by quite expected geometries on the basis on the crystal-
allowing for a EuꢀEu separation over 0.9 nm (Table 3 below).
Conversely when solvent molecules are included into the crystal,
Phen molecules cannot face, adjacent β-DK residues partially overlap,
and a remarkable reduction of the packing factor occurs, namely,
lographic findings of the archetype complex, that is, Eu(TTA) -
3
3
2
Phen crystal. The main features of the crystal data and the
structural resolution are reported in Table 1, while in Figure 3
and 4 the corresponding crystal packings are shown.
in crystals of Eu(DTDK) Phen the packing factor value drops to
3
0.62 to be compared with the close packed crystal value (g0.7).
Comparison of Structures of Eu(III) Complexes. To shed
light onto photophysical properties observed in the series of
Eu(III) complexes containing thiophene β-DK and Phen ligands,
shown in Scheme 1, some structural features were taken into
account and compared, for example, the shortest metalꢀmetal
distances, the europium-ligand distances, and the angle formed
by EuOO and β-DK residues planes in the crystals considered.
All these values are reported in Table 3. A more complete analysis
including crystal structures of complexes containing bisphenyl-
(1,3-β-diketonate) and bipyridyl ligand is reported in the Sup-
porting Information.
The octa-coordination at Eu-atom is quite distorted, namely,
instead of the expected value of 180ꢀ, the average plane of two
opposite β-DK forms an angle ranging from ∼164ꢀ for Eu-
(
DTDK) Phen to 151ꢀ for Eu(Br-TTA) Phen and to 146ꢀ for
3
3
Eu(MeT-TTA) Phen, while the corresponding angle between
3
Phen and the remaining β-DK residue is contracted from ∼146ꢀ
in Eu(DTDK) Phen to ∼140ꢀ in Eu(Br-TTA) Phen or constant
3
3
(
151ꢀ) in Eu(MeT-TTA) Phen. The different packing arrange-
3
ment motif in these three crystals is clearly attributed to the
presence of a solvent molecule, completely confirmed in the
3
2
crystal structure of the parent complexes Eu(TTA) (Phen)
3
33
and Eu(TTA) BipyCO, namely, in the latter case a toluene
The closest EuꢀEu distances range from 0.893 to 1.054 nm
(see line 4 of Table 3); this fact implies that the metalꢀmetal
interaction, possibly contributing to quenching the emission,
cannot be accounted for as the main parameter affecting the
3
molecule is present.
Specifically, in the absence of any clathrated molecule the
crystal packing consists in the facing of adjacent Phen ligands
5
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Inorganic Chemistry
ARTICLE
3
Figure 4. (a) Crystal packing of Eu(DTDK) Phen complex, viewed along b axis; (b) ORTEP plot with thermal ellipsoid probability of 50%.
Table 3. Comparison among EuꢀO, EuꢀN, and EuꢀEu Distances (nm) in Eu(III) Complexes with Phenanthroline and
Thiophene β-DK Ligands, Together with the Angles (deg) Formed by EuOO and β-DK Residue Planes in the Crystals of the
Considered Complexes
complex
a
b
geometric parameter
Eu(TTA)
3
Phen
Eu(Br-TTA)
3
Phen
Eu (DTDK)
3
Phen
Eu(MeT-TTA)
0.2628
3
Phen
average EuꢀO distance
crystal esd e0.0016 nm
average EuꢀN distance
crystal esd e0.0010 nm
shortest EuꢀEu distance
crystal esd e0.0010 nm
EuOOꢀβ-DK1
0.2362
0.2594
0.976
0.2359
0.2357
0.2628
0.893
0.2594
0.980
0.2572
1.054
12
1
21
g1
3
20
6
20
EuOOꢀβ-DK2
EuOOꢀβ-DK3
g2
17
e2
14
a
b
Two tetrahydrofuran molecules are clathrated in the crystal. Because of the low quality of the crystals, the values are affected by a large esd of 0.005.
emission properties. As a matter of fact, an energy minimization
calculation, using the COMPASS program of the MATSTUDIO
ligands allow for larger metal separation, gives values ranging
from 1350 to 1400 kcal/mol by varying the EuꢀEu distances
from 0.95 to 1.7 nm.
3
4
package, on Eu(BrC8-TTA) Phen molecule, whose bulky
3
5
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Inorganic Chemistry
ARTICLE
a
Table 4. Comparison among Shortest EuꢀH Distances (nm) in Selected Complexes
b
c
d
d,e
complex/type
Phen
CH-β-DK
closest-residue
next closest-residue
Eu(TTA)
Eu(Br-TTA)
Eu(DTDK) Phen
Eu(MeT-TTA) Phen
3
Phen
0.350(2)/0.552(2)
0.344(2)/0.550(2)
0.346(2)/0.552(2)
0.333(2)/0.551(2)
0.476(3)
0.471(3)
0.470(3)
0.467(3)
0.646(3)
0.621(3)
0.628(6)
0.609(3)
0.662(4)
0.651(4)
0.651(4)
0.639(4)
3
Phen
3
3
a
H-atoms are conventionally placed at 0.093 nm to the carbon to which they are bonded. Only the average values less than 0.7 nm are considered. The
average contact number is indicated in parentheses. For the H-type see Scheme 3. H-atoms belonging to Phen residues (H1, H2). H-atom of three
β-DK moieties (H3). H-atoms of thienyl rings (H4). H-atom of Phen residues (H5).
b
c
d
e
The range of EuꢀO distances is comprised within 3σ (line 2 of
Table 3), hence fairly significant, while the range of EuꢀN
distances exceeds 6σ (line 3 of Table 3), clearly meaningful.
Specifically the EuꢀN distances enlarge in the case of short
Scheme 3
EuꢀEu contacts, as detected in Eu(MeT-TTA) Phen character-
3
ized by very loose crystal-packing.
However, a clear correlation with photophysical behavior of
TTA type Eu(III) complexes cannot be evinced; hence, we
conceived the angles between mean molecular planes involving
EuOO and β-DK residues, which map the juxtaposition of
molecular orbitals (MO) raised from ligand field theory of f
electrons of the Eu atom with the lone pair n-orbitals of the ligand
3
5
oxygen atoms. This determines the effectiveness of the ET
between metal to ligand or vice versa, even in the case of distorted
octahedral coordination. Specifically the a2u MO antibonding of
Eu(III) will overlap with the n-orbitals of oxygen of the β-DK
groups linked to the metal, and such a superimposition is as
efficient as small are the angles between the planes. In lines 5ꢀ7
of Table 3 such values are reported for selected complexes.
section). However, for the third complex and for Eu(MeT-
TTA) Phen the main cause of PL-QY decrease should be
3
attributed to the reduced ET related to the corresponding ligand
triplet state energy. In fact, moving from solid state to solution no
variation of PL-QY value can be observed. In summary, both the
angles between planes (Table 3) and HꢀEu distances (Table 4)
can tune the observed variation of PL-QY in the selected
complexes.
It is evident that in the case of the Eu(TTA) Phen complex the
3
sum of the angles is clearly reduced (<15ꢀ) while in all the other
compounds, the value exceeds 25ꢀ, clearly indicating a different
electronic environment to the metal, that is, less efficient over-
lapping of f (Eu) and n (O) orbitals.
1
Solution NMR Analysis. The H NMR spectrum of Eu-
(TTA) Phen (see Scheme 4 for the numbering) in CDCl
3
3
solution consists of eight reasonably narrow lines displaying a
moderate paramagnetic shift (on the average about 1.5 ppm),
allowing for the immediate assignment based on the coupling
patterns of thiophene and Phen.
Finally a crucial effect onto emission properties has been
checked, namely, the distance of the aromatic H atoms from the
31b
metal and the number of such contacts.
In Table 4 such
relevant distances, lower than 0.7 nm, are reported for selected
crystal structures of Eu(III) complexes. The H atoms were
considered conventionally at 0.093 nm from the carbon atom
to which they are bonded. Different types are distinguished in
Scheme 3, with the label corresponding to the column of Table 4.
The number of aromatic-H contacts, playing a relevant role in
quenching the complex emission, is indicated in parentheses for
each H-type. In all crystal structures examined the closest EuꢀH
contacts involve H atoms near to Phen N-atoms (column 2 of
Table 4); however other significant contacts should be considered,
that is, the H atom of the β-DK residue (column 3 of Table 4)
and the H atom of the thienyl ring closer to the β-DK group
The presence of one set of signals is at odds with the XRD
geometry, where the complex displays C symmetry and conse-
1
quently the three TTA ligands should give rise to different sets of
signals. Lowering the temperature to ꢀ40 ꢀC did not lead to
significant line-broadening, which suggests a static structural
rearrangement leading to a real C structure, rather than a
3
dynamic process. To gain insight into the geometry of this
system, we decided to switch to lanthanides inducing larger
paramagnetic shifts. The rationale for doing so is that a dynamic
process which appears fast on the time scale dictated by the small
paramagnetic shifts induced by Eu(III) may become slow or at
least intermediate for a lanthanide spreading the signals over a
much wider range. The first choice in this respect was Tb(III),
which is reasonably close to Eu(III), but is associated to a much
(
column 4 of Table 4). The other closer contacts are indicated in
column 5 of Table 4. According to different chemical constitu-
36ꢀ38
1
tion of the ligands, the number of such contacts changes, namely,
larger magnetic anisotropy.
As expected, the H spectrum
it doubles when the CF group is substituted by a thienyl residue.
spans a much wider range, from ꢀ30 to þ23 ppm with some line
broadening, which is anyway not too severe, given the relaxation
properties of this ion (the broadest line has a width less than 150
Hz, but there are two lines less than 15 Hz wide), which once
more points against the dynamic rearrangement. We may recall
that at 14.1 T, the expected line-width of a proton separated by
0.5 nm from Tb(III), in a complex of size comparable to the
3
Indeed only an accurate analysis permits to extract significant
differences to evince any trend, that is a reduction of distances of
type 3, 4, and 5 is observed comparing the first, second, and
fourth complexes, while the type 4 number of contacts is doubled
in the third compound with respect to Eu(TTA) Phen. Hence
3
the effectiveness of aromatic-H quenching should be augmen-
ted, consistent with the trend of measured PL-QY (see optical
38
present ones, is around 210 Hz.
5
426
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Inorganic Chemistry
ARTICLE
Scheme 4
1
13
a
Because this matter appeared of great relevance for under-
standing the geometry of the red-emitting species in solution, we
Table 5. Observed H and C Shifts and Longitudinal
ꢀ
1
Relaxation Rates (s ) for Yb(TTA) Phen and
3
prepared a series of Ln(TTA) Phen, with Ln = La, Pr, Eu, Tb,
Tm, Yb, and Lu, and measured H spectra and longitudinal
Lu(TTA) Phen
3
3
1
1
13
H
C
relaxation times T on all of them (Supporting Information,
1
Table S6). In no case did we observe more than the expected 8
signals, but sometimes we could not identify some. While the
assignment of the diamagnetic species together with Eu was
straightforward, because of the coupling and chemical shift
patterns, for the other paramagnetic systems we took advantage
of the longitudinal relaxation rates, as described below, and
verified this assignments with two-dimensional (2D) correlation
Yb
Lu
Yb
Lu
Yb
Lu
position
δ
δ
F
1
F
1
δ
δ
T3
T4
T5
CH
P2
P3
P4
P5
3.53
5.89
6.93
ꢀ10.2
3.8
7.57
7.02
7.49
6.12
9.67
7.81
8.37
7.82
21.3
2.5
0.57
0.44
0.31
0.55
0.61
0.60
0.61
0.61
123.1
126.0
130.5
56.3
129.8
127.9
132.4
92.3
2.5
40.0
>100
18.2
10.6
8.4
ND
151.8
ND
(
COSY) for Yb and Pr.
In paramagnetic systems, when one can exclude chemical
12.8
18.2
19.3
136.0
155.3
144.8
137.6
125.8
exchange and contact relaxation, nuclear relaxation is affected
and often dominated by two interactions, dipolar and Curie
a
3
9
Parts per million (ppm) referred to the residual solvent signal at 7.26
ppm and 77.0 ppm, respectively.
terms. These two mechanisms provide terms to the observed
obs
relaxation rate F , that add up to what can be regarded as an
dia
intrinsic diamagnetic term F , which is what one would have in
dia
As a side observation we can mention that indeed F plays a
minor role.
This procedure allowed us to assign the proton resonances.
Only proton 2 on Phen here indicated as P2 is elusive, but in fact
it appears broadened not only in Eu, but also in La and Lu
spectra.
the absence of paramagnetism and can be very appropriately
estimated through the diamagnetic La or Lu analogues. Both
dipolar and Curie terms contain an explicit dependence to the
3
þ
6
nucleus-Ln distance, r, and are proportional to 1/r . Thus the
observed relaxation rate can be written as
const
r6
obs
dia
dipolar
Curie
dia
Once this assignment was made, we verified it for Pr and Yb, by
homonuclear correlation (COSY) (Table 5 and Supporting
Information, Figure S9ꢀS10).
F
¼ F þ F
þ F
¼ F
þ
ð1Þ
where the constant in the last term depends on the electronic and
reorientational correlation times but can ultimately be treated as
a heuristic parameter to be fitted. The above equation holds for
both longitudinal and transverse rates.
The observed shifts allowed us to extract the paramagnetic
term, which could be used for a further check of the isostructur-
para
ality along the series, by plotting δ (Ln)/ÆS (Ln)æ. These plots
z
were linear for Ln = EuꢀYb, but not for Pr, which indicates that
To assign the spectrum of a paramagnetic Ln(TTA) Phen
3
Pr(TTA) Phen may have a different structure. This, put together
3
(
Ln = Pr, Tb, Tm, Yb), in a very first approximation, we neglected
with the observation made above of proportionality in the
relaxation rates, may be interpreted with a different coordination
number because of different axial ligation for Pr. Since anyway we
were interested in the Eu complex, we did not further investigate
the lighter lanthanide.
dia
the diamagnetic term F and tentatively postulated isostructurality.
In this case, the i-th proton in the various complexes with
different Ln would have the same distance r from Ln, and the
paramagnetic longitudinal relaxation rate for proton i, in the
para
i
complex with Ln, F (Ln), would be proportional to the one of
The separation of contact and pseudocontact terms was
the Eu complex. With all these approximations, we may expect
attained by means of Reilley’s method on the data for EuꢀYb,
PC
obs
obs
and the pseudocontact shifts (δ or PCS) were used for
F ðLnÞ ꢁ F ðEuÞ
ð2Þ
i
i
structural optimization, together with the relaxation rates by
means of the program PERSEUS. As the diamagnetic refer-
ence, we used the data of Lu(TTA) Phen.
Since we know that there is a structural rearrangement leading
to C symmetry, it is clear that the X-ray crystal structure cannot
be used as the input. Therefore, we optimized separately the TTA
part and Phen.
For TTA we could find 4 H PCS and 4 longitudinal relaxation
rates. The latter ones are fully compatible with the X-ray structure,
which means that the location of Ln with respect to the individual
3
8
For each complex with a certain Ln, we sorted the observed
rates in ascending order, and we could immediately verify the
proportionality with the data obtained for Eu. This procedure
3
obs
i
obs
i
leads to linear plots of F (Ln) vs F (Eu) (Supporting In-
3
formation, Figure S5ꢀS8). This simple fact demonstrates two
important things: (1) chemical exchange (as well as other
mechanisms) can be ruled out as a source of longitudinal
relaxation; (2) from the point of view of LnꢀH distances, the
complexes are isostructural.
1
5
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Inorganic Chemistry
ARTICLE
Table 6. Experimental Total Paramagnetic Shifts and Cal-
culated PCS for Yb(TTA) Phen, Relative to the Yb(TTA)
3
3
a
Moiety Only
para
pc
position
δ
(exp)
δ (calc)
1
H
T3
T4
T5
CH
T3
T4
T5
CH
ꢀ4.04
ꢀ1.13
ꢀ0.56
ꢀ16.32
ꢀ6.7
ꢀ1.9
ꢀ1.9
ꢀ36
ꢀ4.18
ꢀ1.51
ꢀ1.95
ꢀ11.8
ꢀ5.18
ꢀ2.63
ꢀ2.86
ꢀ37.75
1
3
C
3
Figure 5. Optimized structure for the Ln(TTA) portion in solution
based on the analysis of paramagnetic NMR data. Left: view from the top
along the C axis); right: view from the side. Phen moiety is omitted for
(
3
clarity.
a
The agreement factor R = 1.8%; the magnetic anisotropy constant D =
200 (200) ppm Å .
3
2
β-DK is correct to the best of our paramagnetic NMR data.
Because the complex is axially symmetrical, the magnetic anisot-
ropy is fully described by one parameter D and two Euler angles
defining the orientation of this rod-shaped anisotropy with
respect to the ligand. These two parameters were optimized
through PERSEUS, affording the structure of the Ln(TTA)3
moiety represented in Figure 5.
Table 7. Experimental Total Paramagnetic Shifts and Cal-
culated PCS for Yb(TTA) Phen, Relative to the Yb(Phen)
Moiety Only
3
3
a
para
pc
position
δ
(exp)
δ (calc)
For the Phen ligand we identified only 3 PCS because protons
P2 and P3 were not detected with certainty in the Tb spectrum;
on the other hand, because P3 was clearly seen in Eu, Tm, and Yb,
we could use its relaxation rate, as well. For symmetry reasons,
1
H
P3
P4
P5
P4
P5
4.99
9.83
1.23
8.41
11.48
17.70
19.00
10.80
14.30
17.29
Phen must lie exactly on the C axis; thus, in principle the
13C
3
parameters D and angle θ determined above should completely
fit the experimental data, which appeared unsatisfactory, leading
to an agreement factor R > 9%. Accordingly, the geometry was
allowed to vary, by changing the distance of the ligand from Ln,
a
The Yb-Phen distance was allowed to change and increased by 0.4 Å,
with respect to the x-ray structure, while the magnetic anisotropy
constant D was kept fixed at the value found above (D = 2200
ppm Å ). Agreement Factor R = 3.4%.
3
while maintaining it along C . By so doing a good solution was
3
found, where the ligand is moved 0.04 nm away from Ln.
All of the above findings may suffer of the very limited number
of experimental data because of the few protons in the ligands.
Observing that the contact shifts for Yb are usually small and that
they anyway provide a contribution to the total paramagnetic
shift which is scattered in magnitude and sign, we decided to
neglect it altogether, but at the same time to take full advantage of
the C shifts, as well, as determined by heteronuclear correla-
tion. The result is not significantly different from what was found
above and is summarized in Tables 6 and 7.
We moved on to study the complex including the brominated
ligand Br-TTA shown in Scheme 4. In this case we focused only
on the Yb system. By mixing 1 equiv of Br-TTA with 2 equiv of
TTA during the synthesis of the complex, we obtained a perfectly
statistical distribution of the species incorporating 0,1,2,3 bro-
minated units. These are most easily identified by monitoring the
CꢀH resonances, which are the most spread. No exchange
between these forms in the time scale of our EXSY spectra
TTA portion and with the ligand Phen capping the distorted
trigonal antiprism of the 6 oxygen atoms of TTA ligands. The
LnꢀPhen distance is somewhat elongated (e.g., YbꢀN attains a
value of 0.286 nm). It is interesting to observe that by means of
EXSY experiments we observed that there is exchange between
free and bound Phen. This process is slow on the time scale of the
paramagnetic spectrum provided by Yb, that is, neither shift nor
relaxation rates of the bound form are affected by this exchange.
On the contrary, TTA is firmly bound to the lanthanide.
PL-QY Correlation. Optical and structural characterizations
strongly address the ranking of PL-QY in the series of complexes
both in solid state and in solution. XRD solid state structural
determination and solution paramagnetic NMR experiments
contribute to deepen the knowledge of TTA Eu complexes,
particularly they evidence along the series a change of sym-
metry and a variation of the Eu-ligand distances below 0.05 nm
moving from solution to solid state. Moreover, fine structure
details in crystal structures reveals that the number of H-atom
near to Eu(III) ion together with the angle between EuOO
and diketonate planes strongly affect the Eu emission, namely,
the increase of both factors largely contributes to lumines-
cence quenching.
1
3
(
100 ms) takes place. Although a detailed analysis is rather
involved, the observed shifts are compatible with the structure
described above for all of the complexes of this mixture. The
spread in the observed shifts has to be attributed only to a modest
difference in the constant D, but we are presently unable to
speculate on its origin.
In solution, paramagnetic NMR shows that the symmetry
The situation is not much different for the complex incorpor-
level increases, producing a decrease of Eu(TTA) Phen PL-
3
ating three alkylated TTA units in Ln (BrC8-TTA) Phen.
QY although the accuracy of their determination prevents an
appreciation of how the Euꢀligand distances vary along with
the TTA series; hence, we cannot fully correlate these with the
different nonradiative (ANR) decay rates calculated for TTA
complexes.
3
As a conclusion, Ln(TTA) Phen in CHCl solution, as well as
3
3
the derivatives containing brominated or alkylated TTA, must be
depicted as undergoing a major structural rearrangement with
respect to the crystal state, leading to a real C symmetry of the
3
5
428
dx.doi.org/10.1021/ic1021164 |Inorg. Chem. 2011, 50, 5417–5429

Inorganic Chemistry
ARTICLE
’
CONCLUSIONS
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’
ASSOCIATED CONTENT
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Supporting Information. Synthesis of the ligands, cyclo-
b
voltammetric data, phosphorescence spectra of frozen solution of
Gd(III) complexes, photoluminescence spectra of complexes in
solid state, analysis of crystals of Eu (III)-Complexes based on β-
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(
’
AUTHOR INFORMATION
(
Corresponding Author
*E-mail: s.destri@ismac.cnr.it.
(
(
’
ACKNOWLEDGMENT
(
The work has been partially supported by PRIN 2007 project
2
007PBWN44 of Italian MIUR
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