Eu3+ ternary and tetrakis complexes with carbazole and methyl group substituted dibenzoylmethane derivatives: Induction of aggregation enhanced emission

Article abstract of DOI:10.1016/j.dyepig.2018.11.060

Two dibenzoylmethane (DBM) derivatives with methyl (Me-DBM) or carbazole (CBZ-DBM) substituents at the para-position of the phenyl ring and their four novel ternary and tetrakis Eu³⁺ complexes with 1,10-phenanthroline (PHEN) as a secondary liga

Full text of DOI:10.1016/j.dyepig.2018.11.060

Accepted Manuscript
3+
Eu ternary and tetrakis complexes with carbazole and methyl group substituted
dibenzoylmethane derivatives: Induction of aggregation enhanced emission
Ilze Malina, Kaspars Traskovskis, Natalija Lesina, Aivars Vembris
PII:
S0143-7208(18)32202-2
https://doi.org/10.1016/j.dyepig.2018.11.060
DYPI 7204
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 4 October 2018
Revised Date: 28 November 2018
Accepted Date: 30 November 2018
3+
Please cite this article as: Malina I, Traskovskis K, Lesina N, Vembris A, Eu ternary and tetrakis
complexes with carbazole and methyl group substituted dibenzoylmethane derivatives: Induction
of aggregation enhanced emission, Dyes and Pigments (2019), doi: https://doi.org/10.1016/
j.dyepig.2018.11.060.
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Eu3+ Ternary and Tetrakis Complexes with Carbazole
and Methyl Group Substituted Dibenzoylmethane
Derivatives: Induction of Aggregation Enhanced
Emission
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Ilze Malinaa,*, Kaspars Traskovskisa, Natalija Lesinab, Aivars Vembrisb
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Institute of Applied Chemistry, Riga Technical University, Paula Valdena Str. 3, Riga, LV-1048,
Latvia
bInstitute of Solid State Physics, University of Latvia, Kengaraga Str. 8, Riga, LV-1063, Latvia
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Two dibenzoylmethane (DBM) derivatives with methyl (Me-DBM) or carbazole
(CBZ-DBM) substituents at the para-position of the phenyl ring and their four novel
ternary and tetrakis Eu3+ complexes with 1,10-phenanthroline (PHEN) as a secondary
ligand or tetraethylamonium ion (N+(Et)4) as the counter-cation were synthesized and
characterized. The investigation of the optical properties of the complexes revealed
that Me-DBM based compounds exhibit aggregation enhanced emission (AEE), while
in the case of CBZ-DBM this effect is not observed. At the same time, the
introduction of a carbazole substituent reduces the emission quantum yield of the
complexes. The decrease in luminescence efficiency for CBZ-DBM based Eu3+
complexes is mainly attributed to the closely situated S1 and T1 energy levels of the
ligand that obstruct the intersystem crossing process.
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*corresponding author
E-mail adress: malina.ilze1@gmail.com or Ilze.Malina_1@rtu.lv (I. Malina)
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Keywords: Europium ternary complex, Europium tetrakis complex, partial energy
transfer process, aggregation enhanced emission, carbazole.
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1.
INTRODUCTION
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Over the previous years the research interest in Eu3+ complexed β-diketones is
not decreasing due to a wide variety of potential application directions found in
amplifiers for optical communication [1], as luminescent probes in biological samples
[2], in solar cells [3], as well as in the emissive layers of organic light emitting diodes
(OLEDs) [4]. Also Eu3+ complexes have been used to fabricate red or white light
emitting diodes (LEDs) [5, 6]. The utilization of Eu3+ organic complexes in OLEDs
and LEDs can be specifically outlined, because of their distinctive emission properties
such as high emission efficiencies and sharp emission bands in the red-light region,
which provide high color purity for the devices. Additionally, Eu3+ complexes with β-
diketone ligands usually have relatively simple and inexpensive steps of synthesis and
purification.
In order to obtain a brightly luminescent Eu3+ complex one must pay attention
to the design of organic ligands, so that their electronic configuration is compatible to
that of Eu3+ ion. To observe a full energy transfer process between the attached
ligands and the central metal ion, it is necessary that the triplet energy level of the
ligand is higher than the resonance level of the Eu3+ ion (17 250 cm-1) [7]. Usually it
means that absorption bands of ligands should lie in near-UV region, however there
are some reports on the Eu3+ organic complexes with blue light absorbing ligands [8,
9]. Furthermore, for the purpose of practical applications it is also necessary, that the
Eu3+ complex exhibits a good solubility in commonly used solvents, so that the
fabrication process of the thin films containing these compounds would be simpler
and less costly. Finally, Eu3+ organic complexes should exhibit high thermal and
chemical stability.
In the recent years many authors have designed ligands, which contain organic
substituents that provide good solubility, charge transporting properties and high
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thermal stability of the corresponding Eu3+ complexes. Carbazole, being an
inexpensive starting material and showing the previously discussed properties, is one
of the most commonly used amongst such chemical building blocks. Many
complexes, where carbazole is introduced to phenyltrifluorobutane-1,3-dione [10],
biphenyltrifluorobutane-1,3-dione [11] and phenylpropane-1,3-dione [12] para-
position through a nitrogen atom have been synthesized. Alternatively, Eu3+
complexes containing ligands, where carbazole is attached to β-diketone through 2-
[8, 9, 13] and 3- [14-16] position of carbazole ring, are reported. Some authors have
chosen to attach the carbazole group in secondary ligands, for example in 1,10-
phenanthroline unit [17, 18]. Independently from the position of the attachment, Eu3+
complexes with carbazole-functionalized β-diketone ligands usually exhibit bright red
emission and they have potential application possibilities in biological imaging [14]
and in fabrication of white organic light emitting diodes (WOLED) [11] and red-light
OLEDs [10].
Dibenzoylmethane (DBM), without the doubt, is one of the most used β-
diketone type chelating ligands for the Eu3+ organic complexes. However, the
understanding of the consequences of carbazole group attachment to para-position of
this molecule through N-atom (CBZ-DBM) and its effects on the corresponding Eu3+
complexes is poor. Only one report characterizing Eu3+ tris complex with this ligand
can be found [19]. It is known that ternary and tetrakis Eu3+ complexes usually
exhibit higher photoluminescence quantum yields (PLQY), longer lifetimes [7] as
well as higher thermal stability and solubility. This improvement is attributed to the
replacement of the chelating solvent molecules in the complex by a secondary or the
fourth β-diketone ligand. Furthermore, only two tetrakis Eu3+ complexes with β-
diketone ligands that contain carbazole group have been reported to date [10, 11], but
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the mentioned compounds also contain a trifluorated methyl group in DBM fragment.
This study consists of the synthesis of two DBM derivatives – 4-
methyldibenzoylmethane (Me-DBM) and 4-carbazol-9-yl-dibenzoylmethane (CBZ-
DBM) and the corresponding four novel Eu3+ complexes C1-C4 (Scheme 1), with an
initial aim to assess the effects of the carbazole introduction. Structural
characterization as well as investigation of optical properties in different media and
evaluation for OLED application use is reported. Ground state and excited state
energy level calculations are presented and discussed for ligands in order to explain
the experimental observations. The acquired results show that the emissive properties
of the compounds can be drastically changed depending on the structure of the used
dibenzoylmethane derivative.
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Scheme 1. Synthesis of ligands Me-DBM, CBZ-DBM and complexes C1-C4; (a) – 1) NaH, THF, 65
°C; 2) H2O, 10% HCl; (b) – 10% NaOH/H2O, 1,10-phenanthroline, EuCl3 6H2O; (c) – 10%
NaOH/H2O, N(Et)4Br, EuCl3 6H2O; (d) Carbazole, Cu, 18-Crown-6, K2CO3, DMF, 155 °C.
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2. EXPERIMENTAL SECTION
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2.1. Materials and instrumentation
EuCl3 6H2O (99.99%) was purchased from Acros Organics, 1,10-
phenanthroline (99%), tetraethylammonium bromide (98%), 4-methylacetophenone
(96%), 4-iodoacetophenone from Alfa Aesar, methylbenzoate (99%) and carbazole
(≥95%) from Sigma Aldrich, but poly(N-vinylcarbazole) (PVK) from ABCR
(GmbH&Co). All substances were used without further purification.
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1H NMR and 13C NMR spectra were recorded on a Brucker Avance 300 MHz
spectrometer in CDCl3, DMSO-D6 or CD3CN solutions. Chemical shifts were
expressed in parts per million (ppm) relative to solvent signal. CHN elemental
analysis was carried on Euro Vector EA 3000 analyzer. Infrared spectra were
recorded on a Perkin-Elmer Spectrum 100 FTIR spectrometer using KBr pellets (4000
to 650 cm-1). The thermogravimetric analysis was performed on a Perkin Elmer STA
6000 instrument. Each sample was heated from 30-900
°C with heating rate 10
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C/min in the nitrogen atmosphere. Low resolution mass spectra of ligands were
acquired on a Waters EMD 1000MS mass detector (ESI+ mode, voltage 30 V). The
UV/Vis absorption spectra were acquired with Perkin-Elmer 35 UV/Vis spectrometer
using 1 cm length quartz cuvettes. Emission and excitation spectra were measured on
QuantaMaster 40 steady state spectrofluorometer (Photon Technology International,
Inc.). For triplet level measurements low-temperature phosphorescence spectra were
acquired at 77 K in 2-methyltetrahydrofuran solution on the same device. The singlet
and triplet energy levels were determined at the emission band maxima. The lifetime
measurements were carried out in room temperature using the same
spectrofluorometer equipped with high power Xenon flash lamp as excitation source.
Lifetime data were processed by OriginPro 8 program. Absolute photoluminescence
quantum yields (PLQY) were determined using the same spectrofluorometer equipped
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with 6 inch integrating sphere by LabSphere and special holders for cuvette, film and
powder samples. The PLQY were calculated by software supplied by Photon
Technology International, Inc. Polymer films were obtained with Laurell WS-400B-
NPP/LITE spin coater (Parameters: speed 800 rpm, acceleration 800 rpm/s for 1 min).
Density functional theory (DFT) has been performed using ORCA program version
4.0.1 [20]. The geometry optimization of ligands was obtained using non-local
functional B3LYP with 6-311G** [21] basis set. For singlet and triplet energy level
calculations def2-TZVP basis [22] and def2/J [23] auxiliary basis were used.
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2.2. Synthesis
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The synthesis of dibenzoylmethane derivatives as well as complexes C1-C4 is
shown in Scheme 1.
2.2.1. Synthesis of compounds DBM derivatives
(Z)-3-hydroxy-3-phenyl-1-(p-tolyl)-prop-2-en-1-one Me-DBM . To a methyl benzoate
(1) (1.15 g, 8.45 mmol), placed in two-neck round-bottom flask, dry THF (15 mL)
and NaH 60% dispersion in mineral oil (0.34 g, 8.45 mmol) was added. Reaction
mixture was refluxed until all H2 is released from solution. Afterwards, 4-
methylacetophenone (2) (1.03 g, 7.68 mmol) dissolved in dry THF (5 mL) was added
dropwise with dropping funnel and resulting mixture was refluxed for four hours.
Then reaction mixture was cooled with ice bath to room temperature and neutralized
with 10% HCl solution until pH ~7. Water (10 mL) and CHCl3 (40 mL) was added
and organic fraction was separated, washed with water (2 x 50 mL) and dried with
anhydrous Na2SO4. The Solution was filtered and filtrate was evaporated. Obtained
precipitate was crystalized from i-PrOH (30 mL) and 1.01 g of compound Me-DBM
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was obtained as yellow powder. Yield 55%; m.p.= 80-81 °C. H NMR (300 MHz,
DMSO-D6, ppm): δ= 8.17 (2H, d, J=6.0 Hz, H-Ph); 8.12 (2H, d, J=6.0 Hz, H-Ph);
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7.69-7.62 (1H, m, H-Ph); 7.58 (2H, d, J=9.0 Hz, H-Ph); 7.38 (2H, d, J=9.0 Hz, H-Ph);
7.32 (1H, s, -CH=C(OH)-); 2.41 (3H, s, -CH3). 13C NMR (75 MHz, DMSO-D6, ppm):
δ= 186.1; 185.1; 144.0; 135.0; 133.4; 132.4; 129.9; 129.3; 128.0; 127.8; 93.4; 21.6.
FT-IR, (KBr, cm-1): 3129, 3066, 3038, 2924, 2855, 1599, 1520.
MS, C16H14O2 requires [M+H]+ 239.10; found [M+H]+ 239.2. Anal. Calcd. For
C16H14O2: C, 80.65; H, 5.92; found C, 80.44; H, 5.67.
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(Z)-3-hydroxy-3-(4-iodophenyl)-1-phenylprop-2-en-1-one (4). Two-neck round-
bottom flask containing dry THF (15 mL) and NaH 60% dispersion in mineral oil
(1.60 g, 40.00 mmol) was purged with Argon and cooled to 0 °C with ice bath. Then
methyl benzoate (1) (3.00 g, 22.00 mmol) was added. After 10 minutes 4-
iodoacetophenone (3) (5.00 g, 20.00 mmol) dissolved in dry THF (5 mL) was added
dropwise with dropping funnel and resulting mixture was refluxed for six hours.
Afterwards, ethylacetate (15 mL) and water (10 mL) was added and reaction mixture
was neutralized with 10% HCl solution until pH ~7. Organic fraction was separated,
washed with water (2 x 50 mL) and brine (2 x 25 mL), and dried with anhydrous
Na2SO4. Solution was filtered and filtrate was evaporated. Obtained precipitate was
crystalized from CH2Cl2: ethylacetate (2:1) system and 3.24 g of compound 4 was
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obtained as light brown powder. Yield 46%; m.p.= 105-107 °C. H NMR (300 MHz,
CDCl3, ppm): δ= 7.99 (2H, d, J=6.0 Hz, H-Ph), 7.85 (2H, d, J=9.0 Hz, H-Ph), 7.71
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(2H, d, J=9.0 Hz, H-Ph), 7.61-7.48 (3H, m, H-Ph), 6.83 (1H, s, -CH=C(OH)-). C
NMR (75 MHz, CDCl3, ppm): δ= 186.1; 184.7; 138.0; 135.3; 135.0; 132.7; 128.8;
128.6; 127.2; 99.9; 93.0. FT-IR, (KBr, cm-1): 3436, 3059, 2955, 2926, 1584, 1564,
1508. MS, C15H11IO2 requires [M+H]+ 350.98; found [M+H]+ 351.1. Anal. Calcd. For
C15H11IO2 : C, 51.45; H, 3.17; found C, 51.22; H, 2.97.
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(Z)-1-(4-(9H-carbazol-9-yl)phenyl)-3-hydroxy-3-phenylprop-2-en-1-one CBZ-DBM .
Two-neck round-bottom flask was purged with Argon and then diketone (4) (1.50 g,
4.29 mmol), carbazole (0.72 g, 4.29 mmol), K2CO3 (2.30 g, 16.80 mmol), activated
Cu (0.53 g, 8.58 mmol), 18-Crown-6 (0.38 g, 1.43 mmol) and dry DMF (7.5 mL)
were added. Reaction mixture was refluxed at 155 °C for four hours. Then it was
cooled to room temperature and water (10 mL) was added. Reaction mixture was
extracted with CH2Cl2 (3 x 25 mL) and organic fraction was separated, evaporated
and obtained precipitate was dissolved in CH2Cl2:MeOH (1:1) system. From this
solution green precipitate was obtained, filtered and then dried in room temperature.
Afterwards to green precipitate ethanol (30 mL) and 10% NaOH solution (5 mL) was
added and resulting mixture was refluxed for 20 minutes. The insoluble precipitate
was filtered off and filtrate was neutralized with 10% HCl solution. Yellow
precipitate (0.24 g) was formed, filtered and dried. Yield 14%; m.p.= 126-129 °C. 1H
NMR (300 MHz, CDCl3, ppm): δ= 8.27 (2H, d, J=9.0 Hz, H-Ph-CBZ), 8.19 (2H, d,
J=9.0 Hz, H-Ph), 8.07 (2H, d, J=6.0 Hz, H-Ph), 7.76 (2H, d, J=9.0 Hz, H-Ph-CBZ),
7.62-7.45 (7H, m, H-Ph-CBZ, H-Ph), 7.36 (2H, t, J=9.0 Hz, H-Ph-CBZ), 6.98 (1H, s,
-CH=C(OH)-). 13C NMR (75 MHz, CDCl3, ppm): δ= 186.0; 184.7; 141.6; 140.3;
135.4; 134.1; 132.7; 129.0; 128.8; 127.3; 126.7; 126.2; 123.9; 120.6; 120.4; 109.8;
93.2. FT-IR, (KBr, cm-1): 3436, 3055, 2926, 1602, 1516, 1224. MS, C27H19NO2
requires [M+H]+ 390.14; found [M+H]+ 390.3. Anal. Calcd. For C27H19NO2: C, 83.27;
H, 4.92; N, 3.60; found C, 83.01; H, 4.71; N, 3.72.
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2.2.2. Synthesis of complexes C1-C4
Eu(Me-DBM)3PHEN (C1). Solution of diketone Me-DBM (0.22 g, 0.92 mmol) in
ethanol (10 mL) was refluxed until complete dissolution. 1,10- Phenanthroline (0.06
g, 0.31 mmol) in ethanol (3 mL) was added, followed by addition of 10% NaOH
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solution until pH~7-8. Afterwards, distilled water solution (5 mL) containing
EuCl3 6H2O (0.08 g, 0.31 mmol) was added dropwise and resulting solution was
stirred for 2 hours in the room temperature. Formed precipitate was separated by
suction filtration, washed with ethanol (10 mL) and distilled water (10 mL) and dried
in vacuum at 50 °C for 24 hours. Complex C1 was obtained as light yellow powder
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(0.18 g). Yield 56%. H NMR (300 MHz, CDCl3, ppm): δ= 10.96 (2H, br s, H-
PHEN), 10.58 (2H, br s, H-PHEN), 10.00 (2H, br s, H-PHEN), 8.97 (2H, br s, H-
PHEN), 6.75 (9H, br s, H-Ph), 6.59 (6H, br s, H-Ph), 5.94 (12H, br s, H-Ph), 2.90
(3H, s, -CH=C(O)-), 2.40 (12H, s, -CH3). FT-IR, (KBr, cm-1): 3059, 3028, 2921,
1593, 1549, 1520, 1499, 1422. Anal. Calcd. For EuC60H47N2O6: C, 69.03; H, 4.54; N,
2.68; found C, 68.59; H, 4.32; N, 2.81.
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[Eu(Me-DBM)4]-N+(Et)4 (C2). Solution of diketone Me-DBM (0.25 g, 1.05 mmol) in
ethanol (10 mL) was refluxed until complete dissolution. Tetraethylammonium
bromide (0.06 g, 0.26 mmol) in ethanol (3 mL) was added, and then 10% NaOH
solution was added until pH~7-8. Afterwards, EuCl3 6H2O (0.07 g, 0.26 mmol) in
distilled water (5 mL) was added dropwise and resulting solution was stirred for 2
hours in the room temperature. Formed precipitate was filtrated, washed with ethanol
(10 mL), distilled water (10 mL) and dried in vacuum at 50 °C for 24 hours. 0.19 g of
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light yellow powder (C2) was obtained. Yield 59%. H NMR (300 MHz, CD3CN,
ppm): δ=7.70 (8H, br s, H-Ph), 7.65 (8H, d, J=9.0 Hz, H-Ph), 7.25 (12H, br s, H-Ph),
7.08 (8H, d, J=9.0 Hz, H-Ph), 5.93 (4H, s, -CH=C(O)-), 3.23 (8H, q, (CH3-CH2)4N+),
2.53 (12H, s, Ph-CH3), 1.25 (12H, m, (CH3-CH2)4N+). FT-IR, (KBr, cm-1): 3056,
3029, 2985, 2921, 2859, 1596, 1552, 1519, 1498, 1423, 1307, 1289. Anal. Calcd. For
EuC72H72NO8: C, 70.23; H, 5.89; N, 1.14; found C, 70.11; H, 5.75; N, 0.98.
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Eu(CBZ-DBM)3PHEN (C3). Complex was synthesized by the same procedure as for
C1, except diketone CBZ-DBM was used instead of compound Me-DBM. Light
brown powder, Yield 67%. 1H NMR (300 MHz, CDCl3, ppm): δ= 11.08 (2H, br d, H-
PHEN), 10.67 (2H, br s, H-PHEN), 10.24 (2H, br s, H-PHEN), 9.08 (2H, br d, H-
PHEN), 8.12-8.10 (6H, br d, H-Ph-CBZ), 7.34-7.21 (24H, m, H-Ph-CBZ, H-Ph),
6.95-6.90 (15H, m, H-Ph-CBZ, H-Ph), 6.44-6.08 (9H, m, H-Ph, -CH=C(O)-). FT-IR,
(KBr, cm-1): 3051, 2959, 2926, 2852, 1598, 1547, 1500, 1423. Anal. Calcd. For
EuC93H62N5O6: C, 74.59; H, 4.17; N, 4.68; found C, 74.12; H, 4.32; N, 4.29.
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[Eu(CBZ-DBM)4]-N+(Et)4 (C4). Complex was synthesized by the same procedure as
for C2, except diketone CBZ-DBM was used instead of compound Me-DBM. Light
brown powder, Yield 30%. 1H NMR (300 MHz, CDCl3, ppm): δ= 8.27 (8H, d, J=9.0
Hz, H-Ph-CBZ), 8.18 (8H, d, J=6.0 Hz, H-Ph-CBZ), 8.06 (8H, d, J=6.0 Hz, H-Ph-
CBZ), 7.78 (8H, d, J=9.0 Hz, H-Ph-CBZ), 7.62-7.32 (36H, m, H-Ph), 6.97 (4H, s, -
CH=C(O)-), 3.79-3.75 (8H, m, CH3-CH2)4N+), 1.32-1.24 (12H, m, (CH3-CH2)4N+).
FT-IR, (KBr, cm-1): 3057, 2926, 1596, 1548, 1500, 1306, 1226. Anal. Calcd. For
EuC116H92N5O8: 75.89; H, 5.05; N, 3.81; found C, 75.44; H, 4.93; N, 4.08.
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2.3. OLED preparation
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Sandwich type samples with the pixel size of 16 mm2 were prepared with the
following structure ITO/PEDOT:PSS(40nm)/C1(50nm)/TPBi(20nm)/LiF(1nm)/
Al(100nm) for the host free device and ITO/PEDOT:PSS(40nm)/PVK:C1(50nm)
(10wt%)/TPBi(20nm)/LiF(1nm)/Al(100nm) for hosted device, where Poly(3,4-
ethylenedioxythiophene)-poly(styrenesulfonate)
(PEDOT:PSS),
2,2′,2"-(1,3,5-
Benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) (TPBi) and LiF were used as hole-
injection, electron transport and electron-injection layers, respectively, and poly(N-
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vinylcarbazole) (PVK) as hole-transporting host for the emitter. Indium Tin Oxide
(ITO) glass (Präzisions Glas & Optik GmbH) with a sheet resistivity of 15 Ω/square
was used as a substrate. A 12 mm wide ITO strip line was made by wet etching at the
middle of the substrate. ITO substrates were cleaned by the following method:
sonicated in CHCl3; sonicated in acetone; 2x rinsed with deionized (DI) water;
sonicated in water with 3 vol% of Hellmanex II detergent; rinsed with DI water;
sonicated in DI water and isopropyl alcohol. Before deposition of PEDOT:PSS the
ITO glass was blown dry with nitrogen and treated by UV-ozone for 20 min. ITO
layer was covered with PEDOT:PSS (from H.C. Starck, Al4083) using spin coater
Laurell WS650. Rotation lasted for 1 minute with speed 2000 rpm. The sample was
moved in a glovebox and heated at 200 °C for 10 minutes. The solution of emitting
layer compounds with a concentration of 5 mg/mL was spin-coated on the
PEDOT:PSS layer with 2000 rpm for 40 sec and heated at 120 °C for 15 minutes
afterwards. Further the samples were moved from glovebox to vacuum chamber,
without the exposure to air, for thermal evaporation of electron transport TPBi (Sigma
Aldrich 806781) electron-injection LiF (Sigma Aldrich 449903) and electrodes at the
pressure 6 10-6 Torr. The deposition speed was 1Å/s, 0.1 Å/s and 5 Å/s for TPBi, LiF
and Al, respectively. In the end the samples were encapsulated using two component
epoxide. The current–voltage characteristics of the OLEDs were measured by the
multimeter Keithley 2700 and the voltage source was Keithley 230 unit. The
electroluminescence brightness was measured by Konica Minolta Luminance and
Color Meter CS-150.
264 3. RESULTS AND DISCUSSION
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3.1.
Synthesis and characterization of ligands and Eu3+ complexes
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The synthetic route to the β-diketone ligands Me-DBM and CBZ-DBM and their
Eu3+ ternary and tetrakis complexes is shown in Scheme 1. The ligand Me-DBM was
prepared by Claisen condensation reaction between methyl benzoate (1) and 4-methyl
acetophenone (2) with a moderate yield – 55%. The other β-diketone CBZ-DBM was
obtained by two-step synthesis. In the first stage 4-iododibenzoylmethane (4) was
obtained in the Claisen condensation reaction between methyl benzoate (1) and 4-
iodoacetophenone (3). Afterwards, CBZ-DBM was prepared in Cu catalyzed Ullman
condensation reaction between 4-iododibenzoylmethane (5) and carbazole, with a
relatively low yield (14%). Eu3+ ternary complexes C1 and C3 were prepared by
mixing ligands PHEN, Me-DBM or CBZ-DBM and EuCl3 6H2O in the ratio 1:3:1 in
the presence of 10% NaOH. However, tetrakis complexes C2 and C4 were obtained
by taking tetraethylammonium bromide, Me-DBM or CBZ-DBM and EuCl3 6H2O in
the molar ratio 1:4:1 in the presence of 10% NaOH. The obtained ligands were
1
analyzed by FT-IR, H NMR, 13C NMR spectroscopy and mass spectra data, but
1
newly synthesized Eu3+ complexes C1-C4 by FT-IR, H NMR, elemental analysis
1
data and thermogravimetric analysis (TGA). Comparison of H NMR spectra of the
ligand CBZ-DBM and its Eu3+ ternary complex C3 are given in Fig. 1. As it can be
seen from Fig. 1 (a) free ligand CBZ-DBM in CDCl3 solution exists in enol form and
methine proton (H4) shows a signal at 6.98 ppm. Enolic proton (-OH) was not
observed, due to a rapid exchange of this proton with H2O (traces in CDCl3), which
1
usually render the signal too broad to be observed in H NMR spectra. Protons from
carbazole cycle (H7-H10, H7’-H10’) exhibit three well resolved signals at 8.27, 7.76
and 7.36 ppm with spin coupling constant of 9.0 Hz. However, signals of protons H8,
H8’ overlap with protons from DBM fragment (H2, H2’, H3, H6, H6’) and form a
multiplet at 7.53 ppm. Remaining protons H1, H1’ and H5, H5’ show two well
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resolved doublets at 8.19 and 8.07 ppm. 1H NMR spectra of ligand CBZ-DBM show
changes upon complexation with Eu3+ ion (Fig.1 (b)). First of all, complexation with
Eu3+ ion leads to a proton signal broadening and shift to higher field. Four new broad
1
signals at 11.08, 10.67, 10.24 and 9.08 ppm appear in the H NMR spectra and
correspond to the protons from 1,10-phenanthroline molecule (H11-H14, H11’-H14’).
Methine proton (H4) from CBZ-DBM molecule shows a shift to higher field at 6.19
ppm and overlaps with proton signals from phenyl group of DBM unit (H6 and H6’).
Furthermore, proton signals from the carbazole cycle (H7-H10, H7’-H10’) are not
well resolved as it was for the free ligand and even overlaps with some proton signals
of DBM unit. Two signals – a broad doublet (8.11 ppm) and multiplets at 7.28 ppm
and 6.96 ppm were signed to protons H7-H10, H7’-H10’. Finally, remaining protons
from DBM unit exhibit two broad multiplets at 7.28 and 6.96 ppm. In conclusion, 1H
NMR spectra of complex C3 confirms that ligand ratio of PHEN to CBZ-DBM is 1:3
1
(8 protons:54 protons). Similar H-NMR spectra and the same ligand ratio was
obtained for other newly synthesized ternary complex C1.
The 1H NMR spectra of tetrakis complexes C2 and C4 contain two signals at
3.23 and 1.25 ppm (C2) (3.79 and 1.28 ppm for C4) in higher field region, which
correspond to the protons of ethylene and methyl group in N+(Et)4 unit. Moreover, six
signals of four Me-DBM ligands with an integral intensity of 52 protons or seven
signals of four CBZ-DBM ligands with an integral intensity of 68 protons were
observed at lower field and confirmed that these complexes contain one
tetraethylammonium cation and four DBM ligands.
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Fig. 1. (a) - 1H NMR spectra of compound CBZ-DBM in CDCl3 at 298 K; (b) – 1H NMR spectra of
Eu3+ ternary complex C3 in CDCl3 at 298 K.
FT-IR spectra of free ligands Me-DBM and CBZ-DBM exhibit a broad band
around 3200-3600 cm-1, which attributes to enolic –OH stretching vibrations.
Carbonyl group (C=O) stretching vibrations were found at 1599 (Me-DBM) and 1601
cm-1 (CBZ-DBM). The observed strong absorption bands at region 1573-1500 cm-1
probably corresponds to enolic and aromatic C=C bond vibrations. However, for
complexes C1-C4 no broad band in the region above 3000 cm-1 was observed,
indicating that DBM derivatives are coordinated to Eu3+ ion through diketone
fragment. For tetrakis complexes C2 and C4 sharp absorption bands in the region
3060-2850 cm-1 are observed and attributes to aliphatic C-H vibrations of N+(Et)4
unit. Furthermore, carbonyl group (C=O) stretching vibrations were found at lower
frequencies – at 1593 and 1594 cm-1 for complexes C1 and C2, and at 1597 and 1596
cm-1 for C3 and C4. Similar changes between FT-IR spectra of the ligands and their
Eu3+ complexes were reported for other β-diketone complexes [24, 25]. Elemental
analysis data of complexes C1-C4 were in a good agreement with the proposed
structures.
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Thermal stability of complexes C1-C4 was investigated by thermogravimetric
analysis (TGA) in inert atmosphere with a heating rate of 10 °C/min. Decomposition
temperatures of complexes C1 and C2 (275 and 234 °C) with Me-DBM ligands are
higher than the ones for complexes C3 and C4 with CBZ-DBM ligands (180 and 166
°C). It is apparent, that the introduction of carbazole moiety in DBM molecule
slightly decreases the thermal stability of Eu3+ complexes, independently from a
complex structure. Although Td for complexes C3 and C4 are below 200 °C, these
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values are close to the reported decomposition temperatures (190-200 °C) for other
Eu3+ complexes with β-diketones that contain carbazole moiety [10, 11].
Ternary and Tetrakis complexes C1-C4 exhibit excellent solubility in
commonly used solvents, such as CH2Cl2, THF, CHCl3, MeCN, EtOH.
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3.2. Absorption properties of ligands and Eu3+ complexes
The UV/Vis absorption spectra of compounds PHEN, Me-DBM, CBZ-DBM
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346
and complexes C1-C4 were measured in THF solutions (c~1.5 10-5M) and are shown
in Fig. 2.
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Fig.2. UV/Vis absorption spectra of compounds PHEN, Me-DBM, CBZ-DBM and their Eu3+
complexes C1-C4 in THF solutions (c 1.5 10-5M).
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Secondary ligand PHEN shows a broad absorption band with maximum at
289 nm and high molar absorption coefficient ε (Table 1). β-Diketone Me-DBM
exhibits a broad absorption band in near UV region (300-380 nm), centered at 347
nm, which attributes to the π-π* enolic transitions from β-diketone fragment.
Introduction of substituent (-Me) in DBM molecule leads to a small bathochromic
shift of the absorption spectra maxima by 5 nm, compared to the value reported for
DBM (λabs =343 nm in THF) [26]. Ligand CBZ-DBM exhibits a wide absorption
band with maxima at 291 and 375 nm and two well separated low energy shoulders at
320 and 335 nm. These two low energy shoulders together with absorption at 291 nm
correspond to π-π* transitions of carbazole moiety [27], whereas λabs at 375 nm marks
charge transfer (CT) transition due to a fact that carbazole moiety possesses electron
donating but carbonyl groups – accepting properties. To prove this statement DFT
calculation were carried for ligands Me-DBM and CBZ-DBM in order to obtain
optimized ground state structure geometries as well as HUMO and LUMO orbitals
(Fig. 3). HOMO orbital for Me-DBM are localized on 4-methylphenyl-β-diketone
unit, while LUMO resides on the whole molecule, illustrating a large HOMO-LUMO
overlap. However, for compound CBZ-DBM HOMO is localized on 1-(4-
carbazolephenyl)-1,3-diketone unit and LUMO is contained on 3-phenyl-1,3-diketone
part, what indicates that, upon the excitation, a CT transition is expected for CBZ-
DBM.
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Fig. 3. HOMO and LUMO orbitals of optimized ground state structures of compounds Me-DBM and
CBZ-DBM, obtained using B3LYP6-311G** set.
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The absorption spectra of Eu3+ complexes C1-C4 show a slightly different
behavior between Me-DBM and CBZ-DBM based compounds. While the band shape
in all cases almost completely mimics that of the free ligands, upon the complexation
the absorption peak of Me-DBM experiences a redshift. For CBZ-DBM based C4, on
the other hand, it remains almost unchanged, while for C3- slightly blueshifts. The
explanation of this difference between the two ligands can, once again, be explained
by CT nature of CBZ-DBM, as upon the complexation the increased electron density
in diketonate fragment reduces the push-pull character of the molecule, increasing the
bandgap value. The complexation with Eu3+ ion is accompanied with an increase in
molar absorption coefficients for the lowest energy transitions: for ternary complexes
C1 and C3 ε is approximately three times higher, but for tetrakis complexes C2 and
C4 ~ four times higher than the values measured for the free ligands, confirming yet
again, that in complexes C1, C3 Eu3+ is coordinated with three β-diketone ligands, but
in complexes C2 and C4 – with four β-diketone ligands.
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Table 1
UV/Vis absorption and luminescence spectral data of the ligands PHEN, Me-DBM, CBZ-DBM and
their complexes C1-C4 in THF (c~1.5 10-5M).
a
λabs, nm
λem, nm
τb, µs
Compound
PLQY
I2/I1
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(ε, 104 M-1 cm-1)
PHEN
Me-DBM
C1
C2
CBZ-DBM
289 (5.09)
347 (2.45)
356 (7.11)
-
-
-
-
-
-
-
-
611
611
478
0.03
0.02
0.26
0.10
0.26
29.8
23.4
-
12.1
-
143.60±0.24
141.49±0.35
354 (9.45)
291, 326, 341, 375 (2.11)
292, 327, 342, 369 (6.34) 475, 611
291, 326, 341, 374 (9.15)
-
38.88±0.22
-
C3
C4
476
394
395
a- intensity ratio between transitions 5D0 →7F2/5D0 →7F1 from emission spectra.
b - luminescence lifetime.
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3.3. Photoluminescence studies of Eu3+ complexes in solution and solid state
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Photoluminescence properties of the synthesized ligands and their Eu3+
complexes were investigated in THF solutions (c~1.5 10-5M). Relevant data are
summarized in Table 1 and emission spectra are shown in Fig. 4 (a).
Ligand Me-DBM is completely non-emissive in THF solution. CBZ-DBM,
on the other hand, exhibits bright cyan-blue emission (Fig. 4. (b)) with maxima at 478
nm and moderate absolute photoluminescence quantum yield (PLQY) of 0.26.
Eu3+ ternary and tetrakis complexes C1 and C2 with Me-DBM ligand show
only the narrow Eu3+ ion emission bands with multiple maximums at 580, 594, 611,
5
650 and 703 nm, corresponding to D0→7FJ (J=0-4) transitions. The emission
efficiencies are very low, with PLQY values in THF being 0.03 and 0.02 for C1 and
C2.
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Fig. 4. (a) - Emission spectra of Me-DBM, CBZ-DBM and their Eu3+ complexes C1-C4 in
THF solutions (c 1.5 10-5M); (b) – photograph of Me-DBM, CBZ-DBM and their Eu3+ complexes
C1-C4 in THF solutions under excitation with 365 nm UV-lamp.
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CBZ-DBM based Eu3+ ternary and tetrakis complexes C3 and C4 in THF
solutions exhibit quite different emission spectra. In the case of ternary complex C3
the emission spectra show the characteristic red emission from Eu3+ ion and also an
emission band ranging from 400 to 600 nm that is attributed to the luminescence of
CBZ-DBM. Intensity ratio between emission at 611 and 475 nm are 2:1. It is likely
that the incomplete energy transfer process from ligand to Eu3+ ion is a reason for
such behavior. Similar observations were made for other Eu3+ complexes with β-
diketone ligands containing carbazole groups [17, 19, 28]. PLQY for complex C3 in
THF solution is 0.10 and CIE color coordinates (x=0.26; y=0.28) lie in cyan blue
region, respectively. Complex C3 is also characterized with, relatively to C1 and C2,
5
short mono-exponential lifetime value (38.88 µm) for D0→7F2 transition of Eu3+. In
the emission spectra of tetrakis complex C4 no characteristic Eu3+ ion emission bands
are observed and only the emission of the ligand is present. This could be explained
with a known tendency of tetrakis complexes to dissociate into tris complexes and the
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corresponding tetraethylammonium salt [29], what could greatly affect the emission
intensity of the Eu3+ ion and intensify the emission of the ligand.
Solid-state photoluminescence properties of the Eu3+ complexes are
summarized in Table 2 and photoluminescence excitation and emission spectra are
shown in Fig. 5.
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Fig. 5. (a) – Excitation spectra (λem=611 nm) of powders of complexes C1-C4; (b) – Emission spectra
(λexc=400 nm) of C1-C4 powders. (c) Aggregation enhanced emission of the compound C2, illustrated
by UV irradiated solutions with different THF:hexane volume fractions.
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In solid powdered state all complexes exhibit wide excitation bands ranging
from 300 to even 470 nm with maxima around 400 nm. These wide bands correspond
very well to the absorption spectra of the complexes (Fig. 2), therefore confirming
assumption that the emission of Eu3+ ion arises from the energy that is absorbed by
the coordinated ligands. Furthermore, the excitation spectra in solid-state also
contains some Eu3+ ion excitation bands at 396 (7F0→5G4), 417 (7F0→5L6) and 467
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nm (7F0→5D3), which overlap with the much stronger excitation bands of the ligands.
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Excitation window for four newly synthesized complexes C1-C4 in solid-state have
been extended to the visible light region, what can be considered as an advantage,
because Eu3+ complexes with β-diketones usually show low stability under the UV-
light irradiation.
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Table 2
Excitation and emission spectra maximums (λexc, λem), absolute photoluminescence quantum yields
(PLQY), luminescence lifetimes (τ1, τ2) and intensity ratios I2/I1 between transitions 5D0 →7F2/5D0
→7F1 of complexes C1-C4 in solid-state and doped PVK films.
Compound
PLQY
0.53
0.57
0.09
0.10
0.22
0.18
0.07
0.09
I2/I1
29.3
29.9
28.1
29.7
31.6
27.3
22.9
23.4
λexc, nm
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399
412
398
351
361
346, 379
346, 379
λem, nm
611
611
611
611
611
611
611
611
τ1, µs
τ2, µs
431±2 (67%)
410±1 (68%)
170±2 (13%)
191±1 (15%)
364±1(71%)
354±1(67%)
362±17(23%)
295±2 (26%)
157±4 (33%)
124±2 (32%)
51±1 (87%)
48±0.5 (85%)
116±1(29%)
104±1(33%)
93±3 (77%)
84±1 (74%)
C1
C2
C3
C4
C1-PVK
C2-PVK
C3-PVK
C4-PVK
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457
Upon the excitation with 400 nm, the powders of complexes C1-C2 show only
the typical Eu3+ ion emission bands, with a significant enhancement in emission
efficiency (PLQY = 0.53 and 0.57) in comparison to the measurements in solution.
Such behavior strongly infers that these compounds exhibit the aggregation enhanced
emission (AEE). Indeed, this effect can be unambiguously demonstrated in the case of
C2 in the THF:hexane solvent system, where with an increasing hexane fraction a
suspension of the complex particles forms, “turning-on” the bright red emission (Fig.
5 (c)). To the best of our knowledge, only one report has attributed aggregation
induced emission (AIE) or AEE effects to Eu3+ complexes, where the metal was
sensitized with tetraphenylethene, a well-known AIE emitter [30]. At the same time,
DBM and its close structural analogues, in their complexed state with boron
trifluoride, are very well known frameworks for AIE-active compounds [31-34]. So it
is not surprising, that the same molecular system, but complexed with Eu3+ ion, would
exhibit AEE effect.
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Complexes C3 and C4 with CBZ-DBM ligands, which in solution spectra
showed the emission of ligand or the combination of emission of ligand and Eu3+ ion,
in solid-state exhibit only metal centered luminescence. This can be explained by a
combination of two effects: a stronger bonding between ligand and metal takes place
in the solid-state and the emission of the ligand is completely quenched due to the
aggregation. In comparison to Me-DBM based counterparts, no enhancement is
observed in emission efficiency of the C3 and C4. The relatively low PLQY values of
these compounds, once more, point to a poor excitation transfer between the ligand
and Eu3+ ion.
Emission spectra of complexes C1-C4 in solid-state contains five typical Eu3+
ion emission lines in the range from 575 to 720 nm. Emission band at ~580 nm
(5D0→7F0) shows a weak intensity due to the fact, that its corresponding transition is
forbidden by electric dipole and magnetic dipole moments. Unlike in the solution
spectra, this transition is not observed as a sharp peak, but rather than a poorly
5
resolved wide band. Transition D0→7F1 (594 nm), which is forbidden by electric
dipole moment but allowed by magnetic dipole moment, shows two to three Stark
components. Emission spectra is dominated by sharp, hypersensitive electric dipole
allowed transition at 611 nm (5D0→7F2) with full half width of ~ 6 nm, likewise it was
observed for solution samples. It is known, that electric dipole transitions of
lanthanide ions are quite sensitive to ligand field. Usually strongly asymmetric or
strongly interacting ligand fields lead to relatively intense electric dipole transitions
5
[35]. In our case the intensity ratio I2/I1 between transitions D0→7F2/5D0→7F1 are
very high (28.1-29.9), which shows, that not only Eu3+ ion is located in a site without
an inversion symmetry, but strong coordination interactions also take place.
Moreover, I2/I1 in solid-state is higher than in solutions, pointing to weaker
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interactions between ligands and central metal ion in the dissolved state. Similar
observations about I2/I1 in different states were reported for other Eu3+ complexes [36,
37]. Such intensity of 5D0→7F2 transition in comparison to the intensity of other Eu3+
emission lines is the reason why emission color of all four complexes is in red region,
with CIE x=0.67; y=0.33.
To further understand the photoluminescence behavior, luminescence lifetimes
5
of D0 excited state of Eu3+ ion were determined by monitoring the transition at 611
nm at room temperature. All luminescence decay curves were fitted with bi-
exponential functions and have two decay time values τ1 and τ2 (Table 2). Bi-
exponential lifetimes point to the presence of two sites of symmetry around central
5
metal ion. This hypothesis is confirmed by the relatively broad emission of D0→7F0
transition, observed at 580 nm. The splitting of the bands, marking this transition in
photoluminescence spectra, points to different non-equivalent sites in Eu3+ complexes
[38]. In solid-state the distance between two emitting Eu3+ ion sites are relatively
short and the interaction between Eu3+- Eu3+ centers can take place. Multiple
aggregated forms of the compound with alternative local chemical environments
would lead to species with different emission kinetics. Bi-exponential lifetime in
solid-state were also reported for other Eu3+ ternary and tetrakis complexes [25, 37,
39, 40]. As it is the case for PLQY, lifetime values are not greatly affected by the
structure of complexes (ternary or tetrakis) and τ1 and τ2 values vary in the range
from 3 to 33 µs, if the same ligands are present.
3.4. Eu3+ complex behavior in host materials and OLEDs
The incorporation of Eu3+ complexes in practically applicable devices often
involves a use of guest-host systems, where the emitting molecule is dispersed in
polymeric or molecular matrix. This has to be done in order to improve optical
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transparency and ensure plasticity to the initially crystalline materials. Additionally, a
host material with charge transporting properties is a necessity in order to acquire
efficient OLEDs with Eu3+ based emitters [10, 11]. To investigate the effects of
dispersion in polymer matrix, the synthesized compounds were dispersed in a
conventional hole-transporting material, poly(N-vinylcarbazole) (PVK). Four PVK
films doped with 8 weight percent (wt%) of complexes C1-C4 were prepared by spin
coating technique from THF solutions.
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529
Fig. 6. Emission spectra of PVK films doped with 8 wt% of complexes C1-C4.
530
531
532
533
534
535
536
537
Emission spectra of doped PVK films are shown in Fig. 6 and the relevant
emission data (PLQY, τ1 and τ2) are collected in Table 2. When excited at 380 nm, all
doped films exhibit only characteristic Eu3+ ion emission bands in the red-light region
and no emission from PVK are detected in shorter wavelengths. The films with
complexes C1 and C2 exhibit moderate PLQY of 0.22 and 0.18, but complexes C3
and C4 - 0.07 and 0.09. These values give an important insight about the differences
in the emission characteristics between the two subsets of compounds. The dispersion
in polymer matrix causes a drop in PLQY by more than 50% for Me-DBM based
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complexes, while CBZ-DBM shows no significant change in comparison to the
emission efficiency in the powdered form. This difference in the behavior can be
explained by the fact that C1 and C2 exhibits an AEE effect, while the emission of
C3 and C4, apparently, is not affected by a solid state packing arrangement. The
dispersion in polymer can be considered as a pseudo-solvation, where molecules are
encapsulated by the host material. However, for solution-processed guest-host
systems, unless the polar guest is completely encapsulated by isolating groups, a large
portion of the dispersed molecules still forms aggregates [41]. This is an explanation
for the PLQY drop for guest-host systems of C1 and C2, where only the aggregated
fraction of the molecules emit, while the emission in the isolated molecules is
quenched. In C3 and C4 the both fractions of the molecules are able to emit, hence no
apparent change in PLQY is observed.
Regarding the discussed difference between the emissive properties of Me-DBM
and CBZ-DBM based compounds, it can be attributed to the dissimilar electronic
nature of these ligands. It is known that AIE and AEE effects largely take place due to
the restriction of the emission-quenching intramolecular rotation, when the molecules
are packed in crystal lattice [42]. In Me-DBM this rotation, most likely, can be related
to the torsion changes between phenyl ring and diketone fragment. As discussed
previously, in CBZ-DBM the lowest energy electronic transition has CT character
that involves the push-pull system consisting of carbazole electron donating group
and diketone acceptor. As it is well known, the conjugation energy in push-pull
molecules causes their planarization and reduces conformational freedom. This can be
considered as the reason why the compounds based on CBZ-DBM do not exhibit
AEE effect and this ligand is strongly emissive in solution, while Me-DBM does not
emit.
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In order to evaluate a possible enhancement in solid-state emission quenching for
CBZ-DBM based compounds, PLQY for this ligand in a host-free film was
measured, yielding a value - 0.05. While a steep drop in PL efficiency can be seen for
CBZ-DBM in comparison to solution, indicating a strong concentration quenching,
solid phase emission efficiency notably increases for CBZ-DBM based Eu3+
complexes. In addition, PLQY of C3 and C4 is almost identical in host-free and PVK-
dispersed samples, showing no significant concentration dependence. These
observations let assume that, while CBZ-DBM is highly susceptible to photoinduced
electron transfer quenching, this effect is suppressed by the complexation to Eu3+.
This can be explained by a fast excited state migration to the metal center, which is
sterically encapsulated by the ligands.
Taking into the consideration the previously discussed results, two OLEDs
were prepared containing complex C1 as the emitter, where this compound was either
dispersed in PVK host [ITO/PEDOT:PSS(40nm)/PVK:C1(50nm)(10wt%)/
TPBi(20nm)/LiF(1nm)/Al(100nm)] or used in a host-free layer [ITO/PEDOT:PSS
(40nm)/C1(50nm)/TPBi(20nm)/LiF(1nm)/Al(100nm)]. In the presence of the host
material no characteristic Eu3+ emission band was observed in the spectra of the
device, as it emitted only blue light, associated with the electroluminescence (EL) of
the charge transporting compounds. On the other hand, the host-free OLED showed
only red Eu3+ related emission. The lack of the host and the poor charge transporting
characteristics of the compound determines that the device exhibits low maximal
brightness of 10 cd/m2, with current efficiency 0.004 cd/A and turn-on voltage 9 V.
These results reaffirm that due to AEE effect the ability to emit is greatly reduced for
Me-DBM based complexes, as they are dispersed in host material.
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3.5. Energy transfer between ligands and Eu3+ ion
To better understand the observed partial and complete energy transfer
between ligands CBZ-DBM or Me-DBM and Eu3+ ion, the singlet and triplet excited-
state energy levels were determined experimentally in the case of CBZ-DBM and
using the ORCA quantum chemical calculation program in the case of Me-DBM. A
simplified energy transfer process between ligands and Eu3+ ion can be expressed as a
following exited state transition sequence: S1–T1–Eu*. First, energy from the excited
singlet state (S1) of ligand is transferred to its excited triplet state (T1). Second, energy
from T1 state is transferred onto the excited states of the Eu3+ ion (usually 5D0). Some
authors have proposed interaction mechanisms involving these energy states to
evaluate the effectiveness of the excitation transfer process: (1) Reinhoudt empirical
rule [43] states, that for an efficient transfer between S1-T1 levels, the energy
difference between these two states should be around 5000 cm-1; (2) however, for
effective energy transfer between T1–Eu*, the difference needs to be in the range
2500-5500 cm-1 (Latva empirical rule) [44].
For ligand Me-DBM singlet and triplet energy levels are located at 28 218 and
21 928 cm-1, and for ligand CBZ-DBM at 21 929 and 18 416 cm-1, respectively (Fig.
7). All calculated energy levels are higher than the D0 energy level of Eu3+ ion (17
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250 cm-1). In the case of Me-DBM ligand, the difference between levels S1–T1 is
6290 cm-1 and T1–Eu* – 4678 cm-1. This means that the both empirical rules
discussed previously are fulfilled and the energy transfer process is efficient. High
emission PLQY values in the solid-state for complexes C1 and C2 confirms this
statement. For ligand CBZ-DBM the situation is different and the obtained energy
gap values for S1–T1 and T1–Eu* are smaller than those for Me-DBM – 3513 and
1166 cm-1, respectively. The difference between the excited singlet and triplet energy
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states of CBZ-DBM is less than 5000 cm-1, what could indicate a non-efficient
intersystem crossing in this molecule. This is probably the reason, why the emission
of the ligand is also observed in the luminescence spectra of complexes C3 and C4 in
THF solutions. Not all energy from S1 level is transferred to T1 level and some part of
it manifests as the ligand fluorescence. Furthermore, energy difference between triplet
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state of CBZ-DBM and D0 energy level of Eu3+ ion is notably lower than the
preferable value (2500-5500 cm-1) [43]. In the cases, where the gap is less than 3500
5
cm-1, the energy back-transfer from D0 energy level of Eu3+ ion to T1 level of the
ligand could occur. This is another likely cause for the low PLQY values of CBZ-
DBM based complexes.
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Fig. 7. Schematic energy level diagram of energy transfer processes: Me-DBM-to-Eu3+, CBZ-DBM-
to-Eu3+ and PHEN-to- Eu3+.
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Fig. 7 also shows the transfer process between the energy levels of secondary
ligand PHEN and resonance level of the Eu3+ ion. It is known, that singlet and triplet
energy levels of PHEN are located at 31 000 and 22 100 cm-1 [45]. After a
comparison of singlet and triplet energy levels of β-diketone ligands to those of
PHEN, we can conclude that the energy levels of the latter one are located at higher
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