On the design of new europium heteroaromatic carboxylates for OLED application

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*

a

b

Daniil S. Koshelev , Tatyana Yu. Chikineva , Vladislava Yu. Khudoleeva , Alexey V. Medvedko , Andrew A.

c

d

b

e

f,g

Vashchenko , Alexander S. Goloveshkin , Dmitriy M. Tsymbarenko , Alexey A. Averin , Anna Meschkov , Ute

f,g

b

a,b

Schepers , Sergey Z. Vatsadze , and Valentina V Utochnikova .

a. Faculty of Materials Science, M.V. Lomonosov Moscow State University, 1/73 Leninskye Gory, Moscow

1

19991, Russia.

b. Department of Chemistry, M.V. Lomonosov Moscow State University, 1/3 Leninskye Gory, Moscow 119991,

Russia.

c. P.N. Lebedev Physical Institute, Leninsky prosp. 53, Moscow 119992, Russia.

d. A. N. Nesmeyanov Institute of Organoelement Compounds, Vavilova str., 28, Moscow 117312, Russia.

e. A. N. Frumkin Institute of Physical Chemistry and Electrochemistry, Leninsky prosp. 31/5 p.2, Moscow 119071,

Russia.

f. Institute of functional interfaces, Karlsruhe Institute of Technology, Karlsruhe, Germany.

g. Institute of organic chemistry, Karlsruhe Institute of Technology, Karlsruhe, Germany.

†

Electronic Supplementary Information (ESI) available

*

Corresponding author: Daniil S. Koshelev dan_kosh@mail.ru)

Abstract:

The approach to the directed synthesis of lanthanide aromatic carboxylates – precursors to the electroluminescent

materials, - was proposed, namely the conjugation length increase and heteroatom introduction in the appropriate

position in combination with the neutral ligand introduction. This resulted in the isolation of a series of new

lanthanide complexes, among which the highest electroluminescence efficiency was obtained for mixed-ligand

europium benzothiazole-2-carboxylate with bathophenanthroline in a solution-processed OLED. The peculiarities of

energy transfer processes allowed obtaining luminescence thermometer materials based on this system, which

demontrated the sensitivity of 2.8%/K in the physiological range.

Keywords: Luminescence; Lanthanides; Carboxylates; OLED; Benzoxazole; Benzothiazole;

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1

. Introduction

Luminescent lanthanide (III) coordination compounds (CC) are unique candidates for

electroluminescent materials as they offer ultimate colour saturation due to their narrow luminescence

bands. They also have potential for a high internal quantum efficiency of electroluminescence because they

are capable of harvesting triplet excitons for light emission [1], so they have attracted considerable interest

as OLED emission layer (EML) materials [2–8]. The mostly utilized technique of OLED processing is

vacuum deposition, as it ensures high efficiency. However, volatility is required of luminescent materials in

that case; besides, thermal evaporation is cost-ineffective and provides lower resolution than solution

process [3,9,10]. In contrast, solution deposition of the emitting layers requires only solubility. In addition,

this technique offers a whole number of variations, i.e. slot-die coating, blade coating, ink-jet printing, roll-

to-roll process [11–15] and enables the obtaining of OLEDs of any size. But the most critical problem of

solution-processed devices is orders of magnitude lower efficiency. So, even europium mixed-ligand (ML)

aromatic carboxylates, among which is the one with the record photoluminescence quantum yield (PLQY)

2

value [16], demonstrated electroluminescence with the efficiency of only up to 25 Cd/m that is too low

2

even in comparison to europium volatile complexes, such as β-diketones (i.e. 400 Cd/m for europium

2

dibenzoylmethanate with phenanthroline [17] or 72 Cd/m for recently reported europium pyrazole-

substituted β-diketone with batophenanthroline [18]. Still solution processing is promising in manufacturing

and material cost decreasing, which requires simultaneous solution of two tasks: the search of the new

materials and the development of the solution procession of OLEDs. The present work is aimed to give

impact to the first one.

Thus, design of the luminescent material precursors based on lanthanide CCs must be started from the

ligand design instead of the undirected synthesis. We selected aromatic carboxylates for the investigation,

as they ensure high chemical stability and the high PLQY values of lanthanide CCs [19–28]. Their crucial

issues usually are low solubility, particularly in water, due to the tendency to polymerize [22,29–33] and

low charge carrier mobility [20,34–36]. Thus, the ligand design was aimed on the increase of the complex

solubility and mobility, as well as absorption. The goal of the work was to verify the prospectiveness of

such a material design for obtaining of the materials for potential use in bioimaging and OLEDs. In the

present work their solution-processed OLED performance was demonstrated in particular.

There are several methods to increase the solubility, including in water for bioimaging and in organic

solvents for solution-processable OLED. However, even though there is a large amount of experimental

data that allows scientists to selectively modify the properties of the complexes (for example, solubility and

luminescence intensity) unpredictable results still occur. So, the ML complex formation may result in both

solubility increase [37–40] or decrease [16,41,42]. The introduction of the aliphatic chain into the ligand

usually increases solubility [43–46] but decreases the luminescence quantum yield due to the quenching,

originating from the CH-vibrations [44,47–50]. Ligand fluorination helps avoiding the latter problem, but

leads to the decrease of the ligand molar absorption coefficient and complex stability in solution [24,51–

5

4]. The introduction of a heteroatom (for instance N) into the α-position relative to a carboxylic group of

the ligand was recently shown as another efficient approach, still lacking the listed drawbacks [55]. It will

be used in the present work.

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Important is that the heteroatom introduction (not only N but also S,O) was also shown to increase

electron mobility, particularly of the highly conjugated compounds [56]. It also helps increasing the triplet

-

1

-1

-

state energy (i.e. benzoate (24800 cm ) [57] vs picolinate (25773 cm ) [58] or even 2-furanoate (29200 cm

1

)

[58]). In order to achieve high sensitization efficiency of lanthanide luminescence, ligands triplet state

energy should be above lanthanides excited state energy, and the optimal difference is usually believed to

-

1

5

3+

-1

make up about 2500-4000 cm [59]. So, for instance, europium excited state ( D (Eu ) = 17200 cm )

0

-1

5

3+

-1

requires triplet state energy over 20000 cm , while for terbium ( D (Tb ) = 20500 cm ) the triplet state

4

-

1

energy over 23000 cm is preferential, though there are some exceptions [60].

Increasing of conjugation length, that is the number of conjugated double bonds, can also increase ligand

molar absorption coefficient [61–65]. At the same time, it results in decrease of the triplet excited state

energy [61], which can lead to the decrease of sensitization efficiency; but as mentioned above, the

heteroatom introduction can help avoiding this problem. There are many studies concerning coordination

compounds based on monocyclic benzoic[24,66–68] and picolinic [43,58,69] acids derivates as well as on

bicyclic acids derivates (naphthoic [70], benzofuran carboxylic[71,72], quinoline carboxylic acid[73–76]).

But basing on the mentioned above dependence it can be suggested new forms of perspective ligands.

a)

b)

Figure 1 a) Benzoxazole-2-carboxylic acid (H(boz)) b) benzothiazole-2-carboxylic acid (H(btz)). Positions of H

1

atoms are enumerated for signals in H NMR spectra assignment.

So, in order to combine high solubility, high absorption and charge carrier mobility, conjugated

heteroaromatic carboxylate ligands were used in the present work, namely benzoxazole-2-carboxylate

-

(

boz) and benzothiazole-2-carboxylate (btz) were obtained, and the role of the second heteroatom (O or S)

was studied. Lanthanide homo- and mixed-ligand carboxylates with these ligands and neutral

o

-

phenanthroline (Phen) and bathophenanthroline (BPhen) ligands were obtained and studied as precursors to

OLED emitting layer materials [27,77–79].

2

. Experimental section

Materials and methods.

6

All solvents, including deuterated solvents (D O, DMSO-d ), and chemicals were purchased from

2

commercial sources.

Aniline and 2-aminophenol were purchased from SigmaAldrich.

Lanthanide salts were purchased from Huizhou GL Technology Co.

All other chemicals were purchased from Component-Reactive. Deuterated solvents were purchased

from Carl Roth Gmbh.

1

13

H, C NMR spectra were recorded at 25 °C using Agilent 400 MR spectrometer with the operating

frequency of 400 and 100 MHz respectively. Chemical shifts are reported in ppm relative to residual

solvent signals. Elemental analysis was performed on the microanalytical device Heraeus Vario Elementar.

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Thermal analyses were carried out on a thermoanalyzer STA 409 PC Luxx (NETZSCH, Germany) in the

temperature range of 20–1000 ºC in argon atmosphere, heating rate 10 º/min. The composition of formed

gases was determined by quadrupole mass spectrometer QMS 403C Aёolos (NETZSCH, Germany),

combined by thermoanalizer NETZSCH STA 409 PC Luxx. Mass spectra were recorded for mass numbers

1

8 (Н О), 44 (CO ) и 58 (CH C(O)CH ).

2 2 3 3

IR spectra in the ATR mode were recorded on a spectrometer SpectrumOne (Perkin-Elmer) in the

region of 400-4000 cm-1.

X-ray powder diffraction (XRD) measurements were performed on a Bruker D8 Advance Vario

diffractometer with LynxEye detector and Ge (111) monochromator,

λ(CuKα ) = 1.54060 Å, θ/2θ scan

1

from 6.0° to 60°, step size 0.02°, in transmission mode, with the sample deposited between Mular films.

Single crystal X-ray diffraction (SC-XRD) studies were carried out on a Bruker APEX-II CCD

diffractometer. The crystal was kept at 120 K during data collection. Using Olex2 [80], the structure was

solved with the olex2.solve [81] structure solution program using Charge Flipping and refined with the XL

[

82] refinement package using Least Squares minimisation.

The solubility of the CС lanthanides was measured at room temperature according to the following

procedure: a suspension of 150 mg of each compound in 10 mL of solvent was refluxed for 1 hour and

thereafter cooled to room temperature. Then 5 mL of the solution were filtered into a cup with known mass,

the solvent was evaporated. and the change of the cup’s mass corresponded to the amount of the dissolved

product.

Absorption spectra were recorded in the region 250–800 nm with a Perkin–Elmer Lambda 650

spectrometer. Emission spectra was measured with a Fluorolog 3 spectrofluorometer over excitation with a

xenon lamp. For the determination of the photoluminescence quantum yields in solid state, an integration

sphere Quanta Phi was used. Luminescence lifetime measurements were recorded and detected on the

same system. Lifetimes are averages of at least three independent measurements. All luminescence decays

proved to be perfect single-exponential functions. Photoluminescence spectra at two temperatures (77,

2

98 K) were obtained on a S2000 (Ocean Optics) multichannel spectrometer with excitation by LGI-21

nitrogen laser (λ = 337 nm). Thermal dependence of luminescence properties was measured on the same

ex

device using thermal control device and heating element. Electroluminescence spectra were measured on

a PicoQuant time-correlated single photon counting system used as a conventional spectrofluorimeter.

Spectral resolution was 4 nm. Electroluminescence intensity was normalized by integration time, which

allows comparing different spectra.

OLED manufacturing took place in a clean room class 10000 (Lebedev Physical Institute, Moscow,

Russia) in a glove-box under an argon atmosphere. The substrates were cleaned by ultrasonication in the

following media: NaOH aqueous solution, distilled water, acetone and 2-propanol for 16 min each. A 40

nm-thick poly(3,4-ethylenedioxythiophene):poly(styre-nesulfonate) (PEDOT:PSS) hole-injective layer was

first deposited. An aqueous solution of PEDOT:PSS (5 mL) was poured onto the preheated (70 °C)

patterned ITO glass substrate, after which the substrate was rotated for 60 seconds at 2000 rpm. Finally, the

deposited film was annealed in air at 80 °C for 60 min. As a hole-transport layer (20 nm) poly-TPD solution

−

1

was spin-coated from chlorobenzene (c = 5 gL ) and was annealed in argon at 220 °C for 30 min. The

−

1

emission layer was spin-coated from the solution (c = 5 gL ) on the heterostucture or on the glass substrate.

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The

∼

15 nm-thick electron-transport/hole-blocking layer (TPBi) was thermally evaporated (Univex-300,

LeybordHeraeus) followed by a

∼

1 nm-thick LiF layer and a >100 nm-thick aluminium layer as the cathode

−

6

under a pressure below 10 mm Hg. The thickness of the layers was controlled by using a quartz indicator.

The contacts were attached to the electrodes, and the device was sealed with epoxy resin (Norland Optical

Adhesive).

Quantum chemical calculations. The geometry of the ground state of the K(carb) and Eu(carb)₃

molecules was optimized using Firefly 8.2 package [83] in a framework of the DFT theory with PBE0

functional, full electron basis set 6-31G(d, p) for C, N, O, S and H atoms, and quasi-relativistic Stuttgart-

Koeln ECP52MWB pseudopotential (f-electrons in core) with appropriate basis set for Eu atom [84].

For each molecule all relevant structural isomers were considered, optimized and the lowest-energy

isomer was chosen for further calculations. The optimized molecular geometries were checked for the

absence of imaginary frequencies in the vibrational spectra.

TD-DFT calculations were performed at the same PBE0/6-31G(d, p)+ECP52MWB level for the

optimized geometry of molecules, 20 singlet single excited states were considered. The energies of the

frontier orbitals were calculated using the DFT and TDDFT results. The energy of the HOMO was obtained

as the eigenvalue of DFT calculation for the ground state, while the energy of the LUMO was computed as

E(HOMO) + Eg, where Eg is the lowest vertical transition energy in TD-DFT specturm. The details of this

approach were reported in [39].

Synthesis

Synthesis of K(boz)

Methyl benzoxazole-2-carboxylate. A mixture of 2-aminophenol (2.11 g, 19.4 mmol) and methyl

2

,2,2-trimethoxyacetate (10.17 g, 62.0 mmol) were flushed with argon and stirred at 100°C for 42 h. Excess

of methyl 2,2,2-trimethoxyacetate was distilled off under reduced pressure. The residue was suspended in

diethyl ether (100.0 mL), filtered, and obtained powder was washed with diethyl ether until discoloration of

filtrate. The beige powder was purified with column chromatography (silica gel, particle size 40-63 µm)

eluting with petroleum ether – ethyl acetate 3:1. R (PE:EA 3:1) 0.28. Yield 1.04 g (31%). Light yellow

f

powder.

1

H-NMR (400 MHz, CDCl ) δ (ppm): 7.90 (d, J = 8 Hz, 1H), 7.68 (d, J = 8 Hz, 1H), 7.51 (t, J = 8 Hz,

3

1

3

2

H), 4.10 (s, 3H). C-NMR (100 MHz, CDCl ) δ (ppm): 156.89, 152.49, 150.87, 140.45, 128.24, 125.83.

3

NMR spectra coincide with the literature data [85].

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Scheme 1. Synthesis of K(boz) and K(btz). a) (MeO) CCO Me, 100°C, 42 h. b) KOH, THF, H O, rt. c) 2-

3

2

chloroacetamide, S, DMF, Et N, rt. d) K Fe(CN) , NaOH, H O, reflux.

3

6

2

Potassium benzoxazole-2-carboxylate was obtained according to the Scheme 1. 0.87 M aq. KOH

(

6.06 mL) were added to the solution methyl-benzoxazole-2-carboxylate (0.93 g, 5.3 mmol) in THF

10.5 mL). Orange solution was stirred at rt for 1 h and solvent removed completely. Yield 1.04 g (98%).

-

Beige solid. Mp. 307-310°C (dec.). C H NO : HRMS, m/z: calcd. for [M-H] , 162.0197; found, 162.0189.

8

5

3

-

1

IR-ATR (KBr, cm ): 3279, 1652, 1542, 1389, 1330, 1255, 1161, 843, 738. H-NMR (400 MHz, D O) δ

2

1

3

(

ppm): 7.69 (d, J = 8 Hz, 1H), 7.65 (d, J = 8 Hz, 1H), 7.37 (t, J = 8 Hz, 1H), 7.32 (t, J = 8 Hz, 1H). C-

NMR (100 MHz, D O) δ (ppm): 161.66, 157.92, 149.93, 139.32, 127.01, 125.11, 120.26, 111.40. Calcd: C,

2

5

8.90; H, 3.09; N, 8.59%; found: C, 59.02; H, 2.98; N, 8.60%.

Synthesis of K(btz)

-(Phenylamino)-2-thiooxoacetamide. The freshly distilled aniline (3.82 g, 40.5 mmol) were dissolved

2

in dry DMF (37 mL). To prepared solution of trimethylamine (7.4 mL) and sulphur (5.14 g, 161.1 mmol)

were sequentially added under vigorous stirring. The light brown solution was stirred for 30 min and then

chloroacetamide (3.43 g, 36.7 mmol) was added. The resulted red solution was stirred for 14 h at room

temperature and diluted with water (440 mL). The sulfur was removed by filtration and washed with

acetone until discoloration of filtrate. Solvents from filtrate were removed and the residue was recrystallized

1

from ethanol. Yield 3.06 g (46%). Orange needles. H-NMR (400 MHz, DMSO-d ) δ (ppm): 12.10 (s, 1H),

6

8

.14 (d, J = 16 Hz, 2H), 7.92 (d, J = 8 Hz, 2H), 7.42 (t, J = 8 Hz, 2H), 7.28 (t, J = 8 Hz, 1H).

1

3

C-NMR (100 MHz, DMSO-d ) δ (ppm): 186.43, 162.31, 138.47, 128.58, 126.82, 123.41. All NMR

6

spectra coincide with the literature data [86].

Benzothiazole-2-carboxamide. To the suspension of K Fe(CN)₆

(

215.00 g, 653.00 mmol

)

in water

3

(

400 mL) the solution of 2-(phenylamino)-2-thiooxoacetamide (9.20 g, 51.1 mmol) in 10% aq NaOH (810

mL) was added under stirring. Suspension was stirred for 1 h at room temperature and filtered. Brown

product was filtered, washed with water and dried on air. Yield 6.37 g (70%). Light brown powder.

1

R (DCM:MeOH 3:1) 0.66. H-NMR (400 MHz, DMSO-d ) δ (ppm): 8.48 (s, 1H), 8.20 (d, J = 8 Hz, 1H),

f

6

8

.11 (d, J = 8 Hz, 1H), 8.07 (s, 1H), 8.20 (d, J = 8 Hz, 1H), 7.61 (t, J = 8 Hz, 1H), 7.55 (t, J = 8 Hz, 1H).

1

3

C-NMR (DMSO-d ) δ (ppm): 165.01, 161.35, 152.85, 136.40, 127.04, 126.66, 124.02, 123.01. All NMR

6

spectra coincide with the literature data [86].

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Potassium benzothiazole-2-carboxylate was obtained according to the Scheme 1. The benzothiazole-2-

carboxamide (6.40 g, 35.8 mmol) were mixed with 10% aq KOH (194 mL) and refluxed for 45 min.

Reaction mixture was filtered while hot and cooled to room temperature. Precipitate was filtered, washed

with water and dried. Yield 4.15 g (53%). Beige flakes. Mp. >360°C (dec.). C H NO S: HRMS, m/z: calcd.

8

5

2

-

-1

for [M-H] , 162.0197; found, 162.0189. IR (KBr, cm ): 3233, 1613, 1491, 1362, 1316, 1125, 845, 757, 731.

1

H-NMR (400 MHz, D O) δ (ppm): 7.80 (d, J = 8 Hz, 1H), 7.73 (d, J = 8 Hz, 1H), 7.36 (t, J = 8 Hz, 1H),

2

1

3

7

1

.29 (t, J = 8 Hz, 1H). C-NMR (100 MHz, D O) δ (ppm): 167.23, 165.93, 152.05, 136.02, 126.66, 126.58,

2

23.36, 122.32. Calcd: C, 53.62; H, 2.81; N, 7.82%; found: C, 53.57; H, 2.76; N, 7.92%.

3

+

3+

Synthesis of lanthanide complexes (Ln = Eu , Gd , Tb )

Synthesis of Ln(boz) ·3H O. To the solution potassium benzoxazole-2-carboxylate (0.06 g, 0.30 mmol)

3

2

in acetone (10 mL) the solution lanthanide chloride hydrate (0.05 g, 0.12 mmol) in acetone (10 mL) was

added. After prolonged stirring, the precipitate was filtered off. The filtrate was evaporated to dryness,

redissolved in water (30 mL) and evaporated to dryness. Yield 0.05 g (65%). Pale brown powder.

Eu(boz) ·3H O: Calcd. for EuC H N O : C 41.63, H 2.62, N 6.07%; found: C 41.78, H 2.56, N 6.13%.

3

2

24 16

3

11

-

1

IR (KBr, cm ): 3279, 1650, 1545, 1393, 1332, 1256, 1164, 845, 741.

Gd(boz) ·3H O: Calcd. for Gd C H N O : C 41.32, H 2.60, N 6.02%; found, %: C 41.48, H 2.66, N

3

2

24 16

3

11

-1

6

.18. IR (KBr, cm ): 3280, 1649, 1539, 1385, 1334, 1252, 1158, 843, 737.

Tb(boz) ·3H O: Calcd. for Tb C H N O : C 41.22, H 2.59, N 6.01%; found %: C 41.65, H 2.61, N

3

2

24 16

3

11

-1

6

.07. IR (KBr, cm ): 3279, 1649, 1543, 1387, 1329, 1253, 1162, 842, 735.

Synthesis of Ln(btz) ·3H O. To the solution lanthanide chloride hexahydrate (0.05 g, 0.12 mmol) in

3

2

water (5 mL) the solution of the potassium benzothiazole-2-carboxylate (0.1 g, 0.46 mmol) in water (10

mL) was added. The precipitate was filtered and dried on air. Yield 0.07 g (70%). White powder.

Eu(btz)₃·3H O: Calcd. for EuC H N O S : C 38.92, H 2.45, N 5.67%; found: C 39.04, H 2.38, N 5.51%.

2

24 18

3

9 3

-

1

IR (KBr, cm ): 3232, 1611, 1490, 1364, 1315, 1131, 844, 758, 728.

Gd(btz)₃·3H O: Calcd. for GdC H N O S : C 38.65, H 2.43, N 5.63%; found: C 38.93, H 2.44, N 5.72%.

2

24 18

3

9 3

-

1

IR (KBr, cm ): 3234, 1615, 1487, 1362, 1317, 1129, 847, 757, 732.

Tb(btz)₃·3H O: Calcd. for TbC H N O S : C 38.56, H 2.43, N 5.62%; found: C 38.62, H 2.45, N

2

24 18

3

9 3

-1

5

.59%. IR (KBr, cm ): 3237, 1617, 1497, 1359, 1312, 1125, 855, 764, 725.

Synthesis of Eu(boz) ·Q, Q=Phen, BPhen (Phen – 1,10- phenanthroline, BPhen – 4,7-diphenyl-

3

1

,10-phenanthroline). To the solution lanthanide chloride hexahydrate (0.05 g, 0.12 mmol) in acetone (10

mL) the solution of Phen/BPhen (0.02/0.05, 0.12 mmol) in acetone (10 mL) was added. After prolonged

stirring, the solution of 0.06 g of potassium benzoxazole-2-carboxylate in 10 mL of acetone was added to a

mixture. Then mixture was stirred for 30 min and the precipitate was filtered off. The filtrate was

evaporated to dryness, redissolved in 30 mL of water and evaporated on a rotary evaporator. Yield

0

.05/0.05 g (48/41%). Pale brown powder.

Eu(boz) ·Phen: Calcd. for EuC H N O : C 52.81, H 2.44, N 8.56, found: C 51.96, H 2.38, N 8.60%.

3

24 16

3

11

-

1

IR (KBr, cm ): 3186, 1653, 1540, 1381, 1328, 1249, 1004, 840, 730.

Eu(boz) ·BPhen: Calcd. for EuC H N O : C 59.38, H 2.89, N 7.22, found: C 58.98, H 2.93, N 7.27%.

3

24 16

3

11

-

1

IR (KBr, cm ): 3203, 1642, 1542, 1382, 1330, 1248, 1160, 841, 741, 702.

ACCEPTED MANUSCRIPT

Synthesis of Eu(btz) ·Q, Q=Phen, BPhen. To the solution lanthanide chloride hexahydrate (0.045 g,

3

0

.12 mmol) in water (10 mL) the solution of Phen/BPhen (0.022/0.045 g, 0.12 mmol) in ethanol (10 mL)

was added. After prolonged stirring, solution of the potassium benzothiazole-2-carboxylate (0.1 g, 0.46

mmol) in water (10 mL) was added to the mixture. The precipitate was filtered and dried on air. Yield

0

.08/0.09 g (75%). White powder.

Eu(btz) ·Phen: Calcd. for EuC H N O S : C 49.88, H 2.31, N 8.08, found: C 50.01, H 2.35, N 8.02%.

3

24 16

3

8 3

-

1

IR (KBr, cm ): 3255, 1624, 1493, 1364, 1314, 1127, 845, 760, 730.

Eu(btz) ·BPhen: Calcd. for EuC H N O S : C 56.58 H 2.77, N 6.87, found: C 56.61, H 2.76, N 6.91%.

3

24 16

3

8 3

-

1

IR (KBr, cm ): 3135, 1620, 1494, 1358, 1315, 1113, 835, 762, 701.

Cellular uptake. Confocal microscopy (using a Leica SPE Scanning confocal inverted microscope) was

performed in order to investigate the cellular uptake and the distribution of the compounds. Therefore,

HeLa (human cervical carcinoma) cells were seeded in 8-well µ-chamber slides (ibidi) the at the density of

4

2

x10 cells/well in DMEM (Gibco) supplemented with 10% FCS (Gibco) and 1% penicillin/streptomycin

(

Gibco). After 24 h of incubation at 37°C, 5% CO , the medium was removed and the cells were treated

2

with the compounds (10 µM) in DMEM and incubated for 72 h at 37°C, 5% CO . For the negative control,

2

the cell culture medium was exchanged in some wells without addition of the compounds. After

incubation, cells were washed three times with DPBS (Gibco). The compounds were excited at 405 nm,

emission was measured at 550-750 nm. Images were taken as sequential scans at 400 Hz with a resolution

of 1024×1024 pixels.

Cytotoxicity studies. HeLa (human cervical carcinoma) cells were seeded in the 96 well plates at the

4

density of 1x10 cells/well in DMEM (Gibco) supplemented with 10% FCS (Gibco) and 1%

penicillin/streptomycin (Gibco). After 24 h of incubation at 37°C, 5% CO , the medium was removed and

2

the cells were treated with various concentrations of the complexes in DMEM (solvent concentration ≤

0

,5% v/v) and incubated for 72 h at 37°C, 5% CO . For the negative control, the cell culture medium was

2

exchanged in some wells without addition of the compounds, for the solvent control the corresponding

amount of the solvent was added. Thereafter, 15 µl of the MTT reagent (Promega) were given in each well.

For the positive control, Triton X-100 (1%) (Serva) was added in some wells before treating them with the

MTT reagent. After 3 h of incubation the cells were lysed using the Stop Solution (Promega) to release the

blue-purple formazan. The cell viability was determined by measuring the absorbance of the resulting

formazan at 595 nm using a multiwell plate reader (SpectraMax ID3, Molecular Devices, USA) and

calculated in relation to the negative/solvent control.

3

. Results and discussion

Synthesis of heteroaromatic acids. Both acids were prepared in the form of potassium salts since the

corresponding acids are unstable against decarboxylation in acidic environment [87]. Benzoxazole-2-

carboxylate was synthesized by ring closure reaction of ortho-ester with 2-aminophenol [88] with

subsequent hydrolysis of obtained methyl ester. Benzothiazole-2-carboxylate was synthesized by the novel

approach consisting of the sequential alkylation-oxidation of aniline [89] and oxidative ring closure with

potassium ferricyanide [90] followed by hydrolysis. This approach was proved effective and will allow

ACCEPTED MANUSCRIPT

obtaining other substituted benzothiazoles from commercially available anilines. The composition of the

1

13

obtained compounds was established based on H and C NMR data; in addition, single crystals of 2-

phenylamino)-2-thiooxoacetamide and methyl benzoxazole-2-carboxylate were obtained (CCDC 1887593

(

and 1887587; see Electronic Supporting Information (ESI); Figure S 1, Figure S 2).

Lanthanide complexes Ln(btz) ·3H O (Ln = Eu, Gd, Tb) were obtained by the interaction between

3

2

aqueous solutions of potassium benzothiazole-2-carboxylate and lanthanide chloride hexahydrate solution.

While Ln(boz) ·3H O (Ln = Eu, Gd, Tb) were obtained by the same exchange reaction in acetone due to

3

2

K(boz) was insoluble in water. The obtained complexes were recrystallized from water to avoid toxic

acetone in their composition. It turned out that Ln(boz) ·3H O rapidly degraded in water solution which

3

2

was obvious from fast decrease of product yield. In contrast, Ln(btz) ·3H O was stable in the solution.

3

2

Mixed-ligand (ML) lanthanide complexes Eu(carb) ·Q (carb =boz, btz, Q=Phen, BPhen) were

3

obtained by the reaction between EuCl Q (in situ from EuCl 6H O and Q in EtOH) and corresponding

3

3·

2

K(carb) in ethanol. The ML complexes precipitated from ethanol, were filtered off, washed with ethanol

and dried in air.

According to the XRD data all the complexes Ln(carb) ·3H O and Eu(carb) ·Q (Ln = Eu, Gd, Tb;

3

2

3

carb=boz, btz, Q = Phen, BPhen) were amorphous (Figure S 6;Figure S 7), which is important for OLED, as

crystallization of the emission layer would immediately result in the device shorting. Thus their

1

composition was determined by combination of TGA and elemental analyses, H NMR spectroscopy and IR

spectroscopy. The TGA data of Ln(carb) ·Solv (Ln = Eu, Gd, Tb; carb=boz, btz) was carried out in the

3

temperature range 200-800°C; mass-spectrometry of released gases was used (mass numbers 18 (H O), 44

2

(

CO ), 58 (MeC(O)Me)). It was shown that Ln(boz) ·Solv (Ln = Eu, Gd, Tb) obtained from acetone

2 3

contained both coordinated water and acetone molecules, while in the TGA pattern of Ln(boz) ·3H O (Ln =

3

2

Eu, Gd, Tb), recrystallized from water, acetone ionic current (M=58) demonstrated no maxima, witnessing

the success of recrystallization in avoiding acetone presence (Figure 2). Hydrate composition of

Ln(carb) ·3H O (Ln = Eu, Gd, Tb; carb=boz, btz) was determined from the weight loss in the temperature

3

2

range of ca. 50-200 °, determined from water ionic current, and confirmed by elemental analysis. Water

coordination was also evident from the IR spectroscopy data (Figure S 3).

TGA data of ML complexes proved the absence of coordinated water molecules and the presence only

one neutral ligand according to weight loss (clcd 22% vs found 25% for Phen loss, clcd 33% vs found 30%

for BPhen loss) in the range of 200 to 400 °С (Figure 2; Figure S 4;Figure S 5).

1

00

1

00

5

8 (MeC(O)Me)

8

6

4

2

0

8

6

4

2

0

58 (MeC(O)Me)

1

8 (H O)

2

1

8 (H O)

2

4

⁴⁽^C^O2⁾

44 (CO₂)

800 1000

0

200

400

600

0

200

400

600

800

1000

Temperature (°С)

a)

b)

Figure 2 TGA data with normalized ionic currents of a) Eu(boz) ·Solv, b) Eu(boz) ·3H O

3

2

1

ACCEPTED MANUSCRIPT

H NMR spectroscopy of homoligand complexes in bioinert solvents (water and DMSO) was used to

prove the presence of the metal-to-ligand bond in solution, which would ensure the sensitization of

europium ionic luminescence and ensure bright luminescence of the complex in solution, which is

important i.e. for bioimaging application. This is possible due to the shift and broadening of the proton

1

signals occur in the H NMR spectra, if the ligand, to which the protons belong, is coordinated by the

europium ion, which is due to the europium magnetic moment, of the europium CC. While in the case of

complete dissociation there is no influence of the ligand proton signals on the europium ion luminescence

[

91–93]. As a result of partial dissociation, there would be both coordinated and non-coordinated

x+

-

carboxylate anions present in the solution; however fast exchange [Eu(carb)₃

↔ Eu(carb)₃_-_x+ x(carb) ]

results in the presence of only one set of signals, their position intermediate between that of free and

-

coordinated (carb) . For example, fluorobenzoates demonstrated complete dissociation in water, metal to

ligand bonds were observed only in organic solvents [24,94].

(

1,4) (2,3)

K(boz)

8

,4 8,2 8,0 7,8 7,6 7,4 7,2 7,0

(

4)

Eu(boz) ⋅3H O

3

2

(

3)

(1)

(2)

8

,4 8,2 8,0 7,8 7,6 7,4 7,2 7,0

2,3)

(

(1,4)

K(btz)

8

,4 8,2 8,0 7,8 7,6 7,4 7,2 7,0

Eu(btz) ⋅3H O (4)

3

2

(2,3)

(1)

8

,4 8,2 8,0 7,8 7,6 7,4 7,2 7,0

Chemical Shift (ppm)

a)

b)

1

Figure 3. a) Proton numbering and suggested ligand coordination mode and b) H-NMR spectra of K(carb) (5 g/L)

and Eu(carb) ·3H O (2 g/L) in D O (carb= boz, btz)

3

2

1

Indeed, H NMR spectra of Eu(carb) ·3H O (carb = boz, btz) in D O revealed the proton signals shifts;

3

2

their values depended on the proton position (Figure 3b). So, the largest shift (ca. 0.4 ppm) was observed

for proton (1) (Figure 3a) for both Eu(carb) ·3H O (carb = boz, btz) witnessing not only the presence of the

3

2

Eu–carb bond, but also the nitrogen atom participation in the coordination. Indeed, in the absence of the

heteroatom coordination, such as for europium benzofuranoate, both protons of the benzene ring (1) and (4)

underwent the same shift (Figure S 9) [72]. The reason for the presence of N,O-coordination of lanthanide

atom and the absence of E,O-coordination (E= O, S) of lanthanide atom is apparently the possibility of

decoupling an unbound electron pair by the nitrogen atom, while oxygen or sulfur have a saturated electron

shell, which is not capable of forming a bond with lanthanide atom. Indeed, according to CSD, O and S

heteroatom coordination is untypical, unlike N heteroatom coordination.

1

H NMR spectra of ML complexes also approved the suggested composition Eu(carb) ·Q (carb = boz,

3

btz; Q=Phen, BPhen) according to the integral intensity ratio of anionic and neutral ligand proton signals

(

Figure S 10;Figure S 11).

Solubility of the obtained complexes was measured in H O, C H OH, MeC(O)Me, MeOH, CHCl

3

2

5

which is important for OLED thin film deposition (C H OH, MeC(O)Me, MeOH, CHCl ) and for

2

5

3

ACCEPTED MANUSCRIPT

bioimaging (H O) (Table 1). It turned out that the heteroatom introduction into the aromatic ligand keeps

2

high solubility of complexes even when ligand conjugated length increases for both ligands. Thus, the

-

solubility of Eu(carb) ·3H O (carb = boz , btz ) reaches 2.2-2.4 mmol/L which is lower than of

3

2

fluorobenzoates [23,24,94], but higher than of the whole number of europium aromatic carboxylates, i.e.

,4-dimethoxybenzoate Eu(dmb) ·4H O (0.97 mmol/L) [66] and naphthoate Eu(nph) ·2H O (0.0353

3

2

3

2

mmol/L) [95]. Solubility in organic solvents is even higher and riches 20.2 mmol/L for Eu(btz) ·3H O in

3

2

chloroform. This is an indirect proof of the presence of the N–Eu bond.

Neutral phenanthroline-based ligand introduction decreases the solubility: so, water solubility of ML

complexes is too low to be measured. Nevertheless, the solubility in ethanol and chloroform still reaches 5

mmol/L, which is enough for thin film deposition.

So, ligand design allows us to obtain highly soluble complexes in organic solvents and water, which did

not completely dissociate in solution due to suggested N,O-coordination.

6

5

4

3

2

1

0000

0

60000

Eu(boz) ⋅3H O

3

2

5

4

3

2

1

0000

0

Eu(btz) ⋅3H O

3

2

K(boz)

K(btz)

2

50

300

350

400

450

500

250

300

350

400

Wavelength, nm

a)

b)

TD-DFT calculations

K(boz)

TD-DFT calculations

K(btz)

Eu(boz)₃

Eu(btz)₃

2

50

300

350

400

250

300

350

400

Wavelength, nm

c)

d)

ACCEPTED MANUSCRIPT

Eu(boz) ⋅3H O

3

2

100000

Eu(btz) ⋅3H O

3

2

Eu(boz) ⋅Phen

60000

40000

20000

0

3

8

6

0000

Eu(btz) ⋅Phen

3

Eu(boz) ⋅BPhen

3

Eu(btz) ⋅BPhen

3

40000

2

0000

0

2

250

300

350

400

450

500

50

300

350

400

450

500

Wavelength, nm

e)

f)

-

5

-5

Figure 4. Absorption spectra of a) K(carb) (acetonitrile, 10 M), b) Eu(carb) ·3H O (EtOH, 10 M), c) K(carb) (TD-

3

2

-5

DFT calculated) d) Eu(boz) (TD-DFT calculated) e) Eu(boz) ·Q (EtOH, 10 M), (Q=Phen, BPhen) f) Eu(btz) ·Q

3

-5

(

EtOH, 10 M), (Q=Phen, BPhen)

The ligand optical absorption determination revealed that molar extinction coefficient was, indeed,

-

1

very high and reached ε(λ = 275 nm) = 13500 (M·cm) for K(boz), while for K(btz) was slightly lower

-1

(

Figure 4a). For comparison, ε (λ = 265 nm) = 500 (M·cm) and (λ = 270 nm) 4000 (M·cm) were

measured for pentafluorobenzoate [24] and benzoate anions [51], correspondingly. Both homoligand

-

1

europium complexes demonstrated ε up to 60000 (M·cm) , which ensures their visually bright

luminescence. Even the introduction of highly absorbing Phen ligand did not further increase absorption,

unlike Phen-containing fluorobenzoates [16,23]. The use of BPhen instead of Phen resulted in

5

-1

Eu(btz) ·BPhen absorption up to 10 (M·cm) , which is a very high value.

3

+

3+

Luminescent properties. Prior to examination of the luminescence properties of Tb and Eu

complexes, triplet states of the obtained ligands were determined from the phosphorescence spectra of

-

corresponding Gd(carb) ·3H O (carb =boz , btz ) from the maximum of the 0-0 phonon band [34,38,58,96]

3

2

-

1

(

Figure S 12). The triplet states of both ligands were about 20400 cm , demonstrating no dependence on

3

+

second heteroatom, and should be optimal for sensitization of Eu luminescence [97] and coincided with

3

+

the excited state energy of Tb .

5

7

^D4

^FJ

J=2

5

7

J=5

D₀

F_J

Eu(boz) ⋅3H O

Tb(boz) ·3H O

3 2

3

2

J=1

J=6

J=4

J=3

J=4

J=3

J=5

Eu(btz) ⋅3H O

Tb(btz) ·3H O

3

2

3

2

J=4

700

J=3

650

4

00

450

500

550

600

750

400

450

500

550

600

650

700

Wavelength, nm

a)

b)

Wavelength, nm

Figure 5. Luminescence spectra (λ = 337 nm) at 298 K a) of Eu(carb) ·3H O (carb = boz, btz), b) of

ex

3

2

Tb(carb) ·3H O (carb = boz, btz).

3

2

In the case of Tb(carb) ·3H O for both carb = boz, btz pure terbium ionic luminescence was observed,

3

2

-

carb luminescence was not present in the spectra (Figure 5b). However, the quantum yield of terbium

ACCEPTED MANUSCRIPT

luminescence reached only 1-2% (Table S 1). At the same time, Tb(boz) ·Solv obtained from acetone

3

(

Table S 1), demonstrated quantum yield of 20%, which seem to witness the quenching role of water

molecules. However, water quenches terbium luminescence rather ineffectively; so, benzoate hydrate [98],

phenoxybenzoate hydrate [40], pyrazolecarboxylate hydrate [20] of terbium demonstrate quantum yields

up to 100%. We suggest that in present case we observe the cumulative quenching effect by the ligand,

which provides the back energy transfer, and the water molecules. The same effect was observed in the case

of Tb(carb) ·Phen [99], containing hydroxocarboxylate anions (carb): they demonstrated low intensity or no

3

terbium luminescence at room temperature due to the cumulative effect of OH-groups in the composition of

anionic ligand and Phen molecules. This suggestion is supported by the absence of ligand luminescence in

the spectra (Figure 5b) as well as by much higher quantum yields of europium complex luminescence, since

europium is known to be quenched by water molecules much more effectively [100–102]. In case of

benzofuranoates [103] terbium complex demonstrated low luminescence intensity at room temperature due

to back energy transfer (Tb → carb) was dominating, which was indicated by presence of ligand

luminescence in the spectra.

The cumulative influence of the back energy transfer and vibrational quenching is solidly demonstrated

by the study of luminescence thermal dependence in solution. While the intensity of europium

luminescence is almost independent on temperature in ethanol and almost independent even in water,

terbium luminescence decreases significantly in water, ethanol, chloroform and acetone upon heating (),

witnessing that back energy transfer is responsible for this behavior. The value of the luminescence

intensity decrease depends on the solvent, witnessing the role of OH-vibrational quenching.

2

1

,4

,2

,0

,8

,6

,4

7

6

5

,0

,5

,0

,5

,0

x = 0,025

2

,8

,6

,4

2

,6

x = 0,01

2,4

2,2

,0

1,8

2,2

2,0

1,8

2

1

,6

,4

1,2

3

00

305

310

315

320

305

310

315

T(K)

320

T(K)

a)

b)

Figure 6. LIR and S values of Eu Tb (btz) ·3H O, dissolved in ethanol (2.5 g/l):a) x = 0.01 and b) x = 0.025

x

1-x

3

2

Such a behavior allowed suggesting these complexes for luminescent thermometry applications.

The solutions in ethanol, containing the mixture of xEu(btz) ·3H O and (1-x)Tb(btz) ·3H O with x =

3

2

3

2

0

.01 and 0.025 were tested as luminescence thermometry materials (Figure 6). As the luminescence

intensity ratio (LIR) the ratio between the intensities of terbium and europium luminescence bands at 545

and 612 nm, correspondingly, was taken. While the sensitivity of the obtained systems was characterized by

the commonly used parameter S = 1/LIR(dLIR/dT), where T is temperature [104,105]. Both systems

r

demonstrated S values up to 2.4-2.8, which are rather high: indeed, the sensitivity of 2.8 in the

r

physiological range (35-50℃) is the highest obtained for the Tb-Eu system. This is not surprising: as some

ACCEPTED MANUSCRIPT

of us have theoretically shown [106] and experimentally verified [107], the sensitivity of the thermometer

based on the energy transfer between Tb and Eu only depends on temperature and is limited by ca. 0.5%/K

in the physiological range. In the present system the ligand triplet state is involved in the temperature-

dependent luminescence processes, which allows the significant increase in the sensitivity up to the record

value. Important is that the luminescence is reversible when the samples were cooled down to its original

temperature (Figure S 28).

It is also important that the sensitivities of both systems are very similar despite very different LIR

values. This allows suggesting that in 4-level systems (ground state + 3 excited states, i.e. of Tb, Eu and btz

ligand) the sensitivity is also dependent only on the level energies and temperature.

Both europium carboxylate hydrates demonstrated solely europium luminescence with the quantum

yields of 7%, which is higher than of terbium complexes, additionally speaking in favour of the suggested

mechanism of terbium luminescence quenching. These quantum yields, though rather moderate, were not

limited by quenching of the water molecules. Indeed, introduction of the neutral ligands – both

o-

phenanthroline (Phen) and batophenanthroline (BPhen) – into the europium coordination sphere resulted in

a very moderate increase of the quantum yield (in case carb = btz) or even decrease of the quantum yield

(

carb = boz). At the same time sensitization efficiency reaches 50%, which means that quantum yields of all

complexes Eu(carb) Q (carb = boz, btz; Q = 3H O, Phen, BPhen) was caused by a quenching mechanism,

3

2

which is, however, not associated with water OH vibrations.

ACCEPTED M-1 AN-1USCRIPT

Table 1. Solubility of europium complexes (mmol·L /g·L )

Eu(btz)₃Eu(dmb)₃·

Eu(bfc)₃·

H O*

Eu(boz)₃·

Phen

Eu(boz)₃·

BPhen

Eu(btz)₃·

Phen

Eu(pfb)₃·

H O [24]

2

Eu(nph)₃·

Sample

·BPhen

4H O

2

3

3H O

2H O [95]

2

[108]

H O

1.4/1.0

6.2/4.3

0.58/0.4

4.6/3.2

5.5/3.8

2.2/1.5

11.5/8.0

2.6/1.8

15.9/11.0

17.3/12.0

0.5/0.4

6.1/5.0

n/a

4.9/4.0

0.1/0.1

5.2/5.0

n/a

4.1/4.0

2.4/1.8

9.5/7.0

1.4/1.0

3.5/2.6

20.2/15

0.2/0.2

5.8/5.0

n/a

4.6/4.0

0.1/0.1

4.9/5.0

n/a

4.9/5.0

0.97/0.75

n/a

68.5/55.21 0.035/0.02

2

C H OH

n/a

2

5

Me C(O)

2

MeOH

CHCl₃

n/a – not available

*

Eu(bfc) ·3H O was obtain in previous work [72] and tested in the present work for comparison.

3

2

The latter conclusion was supported by the values of luminescence lifetimes which were almost

independent on the neutral ligand Q in the europium coordination sphere (Q = H O, Phen, BPhen) and

2

rather slightly depended on the anionic ligand (boz, btz) (Table S 1).

Despite rather low quantum yield values, luminescence intensity of europium and terbium complexes

was visually bright due to the high absorption of the ligands. Indeed, luminosity ꢀꢁ = ꢁꢂ ∗ ꢃꢀꢄꢅ of the

-1

complexes reached 9800 (M·cm) ; this is lower than luminosity of europium 2-fluorobenzoate with

-

1

phenanthroline (26550 (M·cm) ) with the record quantum yield among europium complexes, however, is

higher than that of some widely known luminescent materials as europium complexes with dipicolinate [43]

and salicylic derivates [109].

Thus, ligand design allowed obtaining brightly luminescent europium complexes due to the ligand high

absorption, though the PLQY values turned out to be rather moderate. This allowed successfully using them

as bioimaging materials (see Figure S 18).

Quantum chemical calculations. Since the X-ray structures of considered metal carboxylates were not

available, we have performed a series of DFT calculations to determine the geometry. First of all, the most

–

energetically favourable carb coordination mode, namely O,O’; O,Ch or O,N, was determined (see Figure

6

a for atom assignment).

The localization and the energy of frontier orbitals, HOMO and LUMO, are of crucial importance for

luminescent molecules use in OLEDs. It is well known that the DFT method, suitable for ground state

calculations, significantly overestimates the energies of virtual orbitals containing no electrons, to which the

LUMO belongs. Therefore, in our work we used a dual-stage approach to determine the LUMO energy,

based on the calculation of vertical excitations and then the energy gap (E ) from TD-DFT, followed by the

g

calculation of the LUMO energy as E(LUMO) = E(HOMO) + E as reported in previous work of some of

g

us [39]. To verify the reasonableness of vertical excitation calculations, the TD-DFT absorption spectra

were compared with those experimentally observed and were found to be in a good agreement as described

above (Figure 4).

ACCEPTED MANUSCRIPT

a)

b)

Figure 7. DFT optimized geometry of K(carb) (a) and Eu(carb) (b) molecules (carb= boz. btz).

3

Therefore, accurate calculations of HOMO and especially LUMO energy levels for voluminous Eu tris-

carboxylates in the framework of DFT and TD-DFT require remarkable computation resources. One can

expect that ionic bond nature in both rare earth and alkaline carboxylates, as well as the localization of

frontier orbitals on organic ligand species, make it possible to perform calculation of K(carb) molecules

instead of Eu(carb) to simplify the system. In the present work, we have performed calculation of HOMO

3

and LUMO levels for both K and Eu carboxylates to check the correctness of this simplification to reveal

the impact of central ion on coordination mode and frontier molecular orbitals.

DFT calculations of K(carb) molecules (carb= boz, btz) revealed that the geometry with bidentate O,O’

-

coordination mode of carb has the lowest energy (Figure 7), while the structure containing O,N or O,Ch

coordinated ligand is less favourable (Table S 1). In contrast to K(carb), for Eu(carb) the N,O ligand

3

coordination is energetically favourable (Figure 7b, Table S 1). This difference the most probably originates

3

+

from the smaller ionic radius of Eu (1.17Å for CN = 6) rather than of K (1.38Å) which is comparable

with the bite size of carb ligand (2.70Å). Therefore, based on the combination of theoretical DFT results

1

and experimental data of H NMR and solubility measurements we can conclude the participation of

nitrogen heteroatom in the coordination of europium ion.

a)

b)

Figure 8. Localization of HOMO (a) and LUMO (b) of K(boz) based on DFT converged density.

-

Difference in coordination mode of carb and especially in number of coordinated ligands in molecules

of K(carb) and Eu(carb) resulted in different localization of frontier orbitals. So, in K(carb) HOMO is

3

localized on the organic ligand (Figure 8a), while LUMO is mostly localized on the potassium ion (Figure

8

b).

a)

b)

Figure 9. Localization of HOMO (a) and LUMO (b) of Eu(boz) based on DFT converged density.

3

ACCEPTED MANUSCRIPT

For both europium complexes, HOMO and LUMO are localized on different organic ligands in

europium coordination sphere (Figure 9). This significantly reduces the energy levels of HOMO and

LUMO by ca. 1.5eV (Table 2) and makes it impossible to use the energies calculated for K(carb) as an

estimation of those for Eu(carb) (See ESI for more details).

3

According to DFT and TD-DFT results, Eu(boz) and Eu(btz) molecules demonstrate quite similar

3

values of frontier orbital energies. However, it is worth noting that HOMO level of Eu(btz) is by ca. 0.2 eV

3

higher, what is not important for photoluminescence properties but may play an important role in OLED.

Table 2. Calculated DFT and TD-DFT calculated energy levels of frontier molecular orbitals for K(carb) and

Eu(carb) , (carb= boz. btz).

3

HOMO.

eV

Gap.

eV

LUMO*.

eV

K(boz)

K(btz)

-6.128

-6.027

-7.554

-7.322

4.443

4.331

4.751

4.294

-1.685

-1.696

-2.803

-3.028

Eu(boz)₃

Eu(btz)₃

*

- estimation of LUMO energy value is done on the basis of HOMO level and Gap values.

Thus, the obtained europium CCs possessed photoluminescence and sufficient solubility, and they can

be expected to demonstrate the charge carrier mobility as a consequence of the electron-depleted

heteroaromatic ring system and the dipole-dipole interaction of conjugated systems presence, and their

HOMO and LUMO energies were calculated. This made it possible to test the europium CC as a material

for the emission layer in OLED devices

OLED preparation and testing. Calculated HOMO energies are very low, which made it impossible to

properly select the hole-transport layer (HTL) for OLED fabrication based on these values. Thus, poly-TPD

was selected as hole transport layer due to its insolubility in most solvents after post-treatment (annealing at

2

20 ºC for 30 minutes in argon-glovebox atmosphere) [110], which allowed us to investigate solvent

influence on emission layer (EML) performance in OLED. As electron transport layer (ETL) well known

and often used TPBi material was selected [5,23,39,111].

a)

b)

ACCEPTED MANUSCRIPT

Figure 10. The scheme of energy levels of obtained OLED devices with EML = Eu(btz) ·Q. Q=Phen. BPhen.

3

So, OLED devices with heterostructure ITO/PEDOT-PSS/poly-TPD/EML/TPBi/ LiF/Al, where EML =

Eu(carb) Q (carb= boz, btz Q = 3H O, as well as Phen and BPhen), were obtained. The lifetimes of the

3

,

2

emission layers coincided with those of powders, obtained from corresponding solvents (see ESI). To be

able to properly compare the performance of the obtained OLED devices, the electroluminescence (EL)

spectra were measured at the same conditions (see Experimental section), only varying the integration time;

-

1

thus, intensity is given in sec , which corresponds to the counts divided by the integration time.

I-V curves of the devices with both EML = Eu(carb) ·3H O (carb = boz, btz) demonstrated rather high

3

2

current densities, witnessing that indeed the obtained complexes possessed enough charge carrier mobility

proving our complex design hypothesis (Figure S 19). At the same time only the device with EML =

Eu(btz) ·3H O possessed europium ionic luminescence, in contrast to the one with EML = Eu(boz) ·3H O.

3

2

3

2

Indeed, as mentioned above, Eu(boz) ·3H O is unstable in solution, so it may undergo rapid degradation in

3

2

the thin film. In addition, it was shown, that Eu(btz) ·3H O thin film obtained from ethanol demonstrated

3

2

better EL performance than the one obtained from chloroform (Figure S 20).

Table 3. The list of obtained OLED devices:

Device

designation

OLED 1

OLED 2

OLED 3

OLED 4

OLED 5

OLED 6

EML

Solvent

Eu(btz) ·3H O

EtOH

CHCl₃

EtOH

CHCl₃

3

2

Eu(btz) ·3H O

3

2

Eu(boz) 3H O

3

2

Eu(btz) ·Phen

3

Eu(btz) ·BPhen

3

Eu(btz) ·BPhen

3

The presence of the poly-TPD emission band in the EL spectra of OLED1 and OLED2 reveals that

electron-hole recombination occurs on the interface between EML and the hole transport layer. Indeed, it

witnesses the electron, rather than hole, transport properties of EML; besides, due to very low HOMO level

of EML, this layer plays rather hole-blocking role. Therefore, to improve the situation Phen and BPhen

neutral ligands with relatively high HOMO energies were introduced to decrease HOMO-HOMO gap

between poly-TPD and EML. It is worth noting that BPhen, known as ETL material, possesses own charge

carrier mobility.

ACCEPTED MANUSCRIPT

(

OLED 4) (OLED 6)

1

400

200

000

(

OLED 5)

(

OLED 5)

7

6

5

4

0

(OLED 6)

(OLED 1)

8

6

4

2

00

0

(

OLED 1)

OLED 4)

poly-TPD

TPBi

30

20

(

10

0

-

10

3

00

400

500

600

700

800

0

1

2

3

4

5

6

7

8

9 10 11 12 13 14

Wavelength, nm

U, V

a)

b)

2

Figure 11. a) The OLEDs electroluminescence spectra at 0.2 mA/mm : OLED 5, OLED 6, OLED 1, OLED 4 and b)

their I-V curves.

Indeed, OLED devices with EML = Eu(btz) ·Q (Q = Phen, BPhen) demontrated no poly-TPD

3

luminescence (Figure 11a (1), (2)). However, it turned out that Phen performed as an electron blocking

layer due to the high LUMO energy. It was witnessed by the presence of TPBi emission band in the

electroluminescence spectra, and resulted in the absence of the overall EL intensity increase. While BPhen

introduction resulted in the disappearance of the parasitic HTL emission, and OLED5 and OLED6

demonstrated pure europium ionic electroluminescence, twice more intense than the one of OLED1 at the

same current density; the solvent comparison again proved that ethanol is preferential over chloroform.

Important is that PLQYs of Eu(btz) ·Q (Q = 3H O, Phen, BPhen) are similar (Table S 1), thus their

3

2

different performance in OLEDs is due to different HOMO and LUMO energies.

So, the ligand design allowed us to obtain soluble europium CC with high enough electron mobility.

Though their application in OLEDs was hampered by too low HOMO energy, introduction of neutral ligand

allowed solving this problem. It is well-known that BPhen is usually used in order to increase efficiency of

OLED device [7,25,112,113] because of its high electron mobility. However, in present work the I-V

curves only differ in the V_o_nvalue due to the different HOMO-LUMO gaps of Eu(btz) ·3H O, Phen, and

3

2

BPhen (Figure 11) that indicates enough mobility of the designed carboxylate ligands, and BPhen helps

solving the problem of too low HOMO energy.

4

. Conclusions

The approach toward the directed synthesis of new lanthanide CC as a precursor to OLED emission

layer material was proposed, and its effectiveness was shown. As such an approach, the combination of

ligand conjugation extension together with heteroatom introduction was proposed. The appropriate

selection of the heteroatom nature and position, namely the introduction of nitrogen heteroatom, able to be

coordinated to lanthanide ions, in the α-position to the carboxy-group, allowed obtaining europium

benzoxazole-2-carboxylates and benzothiazole-2-carboxylates with enough solubility and stability in

solution, while the extended conjugation length together with the presence of two heteroatoms ensured both

bright europium ionic luminescence and electron mobility. The properties of the obtained compounds were

ACCEPTED MANUSCRIPT

mainly independent on the second non-nitrogen heteroatom nature; the biggest difference was observed in

-

the complex stability in solution, which made btz derivative prospective candidates to luminescent

-

materials, unlike boz derivatives. At the same time, quantum yield was rather moderate, while HOMO

energy was too high for effective electroluminescence; these obstacles were overcome by the neutral ligand

introduction in the complex composition. As a result, mixed ligand Eu(btz) ·BPhen with effective

3

electroluminescence was obtained.

5

. Conflicts of interest

There are no conflicts to declare.

6

. Acknowledgements

This work was supported by RFBR (project №№ 18-33-00250, 16-53-76018).

The research is carried out using the equipment of the shared research facilities of HPC computing

resources at Lomonosov Moscow State University.

AM and US thank the Research Training Group GRK2039.

7

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ACCEPTED MANUSCRIPT

New benzoxazolates and benzothiazolates of some lanthanides were obtained.

Complex properties are mainly independent on the second (O or S) heteroatom nature.

Mixed-ligand europium benzothiazolate with batophenanthroline was successfully used in

solution-based OLED.

Bimetallic terbium-europium benzothiazolate demonstrates temperature-dependent luminescence

with high sensitivity (2.8%/K).

Article Doi

On the design of new europium heteroaromatic carboxylates for OLED application

DOI: 10.1016/j.dyepig.2019.107604

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