Novel intramolecular charge transfer effect-based ligands and aggregation-induced emission-active europium complexes: synthesis, characterization, and fluorescence properties

Article abstract of DOI:10.1002/bio.3925

Two novel coumarin derivatives and the corresponding europium complexes were prepared using a simple procedure. The pH response of the ligand and the aggregation-induced emission (AIE) properties of the target europium complex were studied. The ligand had an intramolecular charge transfer (ICT) effect and was linearly and sharply responsive under acidic conditions. The goal europium complexes exhibited excellent AIE performance when subjected to increasing concentrations of target europium complex or proportion of poor solvent. The effect of substituents on fluorescence strength or thermogravimetric and electrochemical properties was further investigated. The target complexes displayed the typical fluorescence of europium. The fluorescence amplitude of the target europium complexes was enhanced by the addition of electron-donating groups to ligands. Thermogravimetric research findings indicated that the target complexes possessed extreme thermal stability. Electrochemistry discovery findings indicated that the highest occupied molecular orbit energy level of EuL¹ was greater than EuL², but the lowest unoccupied molecular orbit energy level was smaller than that of EuL². These complexes could be applied in medicinal chemistry, substance chemistry, and fluorescence labelling areas.

Full text of DOI:10.1002/bio.3925

Guo Dongcai (Orcid ID: 0000-0002-5147-4201)
Novel ICT Effect-based ligands and AIE-active europium complexes:
synthesis, characterization and fluorescent properties
Shuaishuai Yanga, Zuping Guoa, Zhongqian Hua, Dongcai Guoa*
a State Key Laboratory of Chemo/Biosensing and Chemometrics, College of Chemistry and Chemical Engineering, Hunan
University, Changsha 410082, China
* Corresponding Author: Dongcai Guo, E-mail address: dcguo2001@hnu.edu.cn (D. Guo), Tel./fax: +86073188821449
Abstract: Two novel coumarin derivatives and corresponding europium complexes were
facilely prepared. The pH response of the ligand and the AIE properties of the target
europium complex were studied. The results demonstrate that the ligand has an
intramolecular charge transfer (ICT) effect and is linearly and sharply responsive under acidic
conditions. The goal europium complexes exhibit excellent AIE performance with increasing
concentration of target europium complex or proportion of poor solvent. The effect of
substituents on fluorescence strength, thermogravimetric and electrochemical properties was
further investigated. The target complexes display typical fluorescence of europium, and the
fluorescence amplitude of the target europium complexes was enhanced with the addition of
electron-donating groups to ligands. Thermogravimetric research findings indicate that the
target complexes possess extremely thermal stability. The electrochemistry discovery
findings indicate that the HOMO energy level of EuL1 is greater than EuL2, but LUMO
energy level is smaller than that of EuL2. The aim complexes can be applied in medicinal
chemistry, substance chemistry and fluorescence labeling areas.
Keywords: Europium complex; ICT effect; AIE-active lanthanide behaviors;
Electrochemistry
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doi: 10.1002/bio.3925
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1. Introduction
In recent years, lanthanide complexes have been extensively used in catalysis, sensors,
magnetic recording materials fields based on their unique porous cavities and notable
electronic features1–3. Specially, a great number of lanthanide complexes were investigated
because of their pure light color, high luminescence efficiency, low excitation energy
requirement and unusual ability to effectively minimize background interference in
fluorescent imaging4,5. Lanthanide complexes can be studied in the field of life science6,7,
analysis8,9, biological detection10 and other aspects. However, owing to the quenching effect
(ACQ) caused by aggregation, lanthanide complexes typically attenuate or even quench
fluorescence in their aggregated state and these materials are very constrained in practical
applications11. Tang's work team uncovered a new aggregation-induced emission
phenomenon (AIE), which is entirely opposite to the ACQ effect12. When aggregated into a
high concentration solution or solid state, the AIE substance emits strong light rather than
fluorescence quenching. While AIE has been made for some years13, the effect of AIE on
lanthanide complexes through antenna effects is rarely recorded and being challenging14.
Current work findings indicate that organic ligands with stable coordination to lanthanide
ions include β-diketone derivatives, carboxylic acid derivatives and macrocyclic compounds,
all of which have the coordination atoms O, N and S15,16. Coumarin compounds not only have
important biological activity17,18, they also have exceptional optical properties owing to their
broader conjugate structure19,20. Since the absorption wavelength of coumarin is over 300 nm
but smaller than 400nm, the use of coumarin as a sensitizer means that the excited state is not
too small.
Therefore, in this work, the coumarin was used as a sensitizer and carboxylic acid
derivative as coordination site to synthesize the target ligands, and characterized by 1H NMR,
13C NMR and ESI-MS. Corresponding europium complexes with AIE properties were
successfully synthesized, and characterized by UV-vis, FT-IR, molar conductivity and
thermogravimetric analysis. The synthetic route is illustrated in Scheme 1. The pH response
of the ligand was explored, and the AIE properties of the target europium complex were
further investigated.
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Scheme 1. Synthetic route of ligands and the target europium complexes.
2. Synthesis method
2.1 Synthesis of intermediates
Synthesis of Compound A
5-methyl salicylic aldehyde (10 mmol, 1.36g) and Meldrum’s acid (12 mmol, 1.44g)
were suspended in 25 mL of water in a 100 mL round flask, and refluxed for 1 h. The
precipitated product was filtered and recrystallized in methanol to get product A121. General
synthesis procedures of A2 were similar to that of A1.
6-methyl-2-oxo-2H-chromene-3-carboxylic acid (A1) A white powder, yield:98%;
1H NMR (400 MHz, DMSO) δ 13.24 (s, 1H, COOH), 8.67(s, 1H,ArH), 7.68 (s,1H,ArH),
7.55 (d, J=7.54Hz, 1H,ArH), 7.34 (d, J=7.33Hz, 1H,ArH), 2.38(s, 3H,CH3);
1
6-nitro-2-oxo-2H-chromene-3-carboxylic acid (A2) A yellow solid, yield: 95%; H
NMR (400 MHz, DMSO) δ 13.55 (s, 1H, COOH), 8.91(d, J=8.89Hz, 2H,ArH), 8.50 (d,
J=8.48Hz, 1H,ArH), 7.65 (d, J=7.63Hz, 1H,ArH);
Synthesis of Compound B
The compound A1 (5 mmol, 1.45 g) was dissolved in thionyl chloride (20 ml) and
refluxed for 3 h. The solvent was removed by decompression to yield the desired product B1
as fine light white22. The general synthesis procedures of B2 were similar to that of B1.
6-methyl-2-oxo-2H-chromene-3-carbonyl chloride (B1) An white solid, yield: 80 %;
1H NMR (400 MHz, DMSO) δ 8.69(s, 1H,ArH), 7.72 (s, 1H,ArH), 7.58 (d,
J=7.56Hz,1H,ArH), 7.37 (d, J=7.35Hz,1H,ArH), 2.40(s, 3H, CH3);
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6-nitro-2-oxo-2H-chromene-3-carbonyl chloride (B2) An yellow solid, yield: 75 %;
1H NMR (400 MHz, DMSO) δ 8.93(d, J=8.90Hz, 2H,ArH), 8.51 (d, J=8.49Hz, 1H,ArH),
7.67 (d, J=7.65Hz, 1H,ArH);
2.2 Synthesis of Ligand
Compound B1 (5 mmol, 1.11 g) and diethyl iminodiacetic acid (5 mmol, 0.95 g) were
dissolved in 25 mL dichloromethane in a 150 mL three-neck flask, then ice bath for 6 h, the
solvent was removed was removed under reduced pressure to yield the crude product. Then
add the crude product to 2M sodium hydroxide solution, stir at room temperature for 18 h,
then acidify with dilute hydrochloric acid to give a white solid. The target product L1 was
obtained by simple filtration. The general synthesis procedures of L2 were similar to that of
L1.
2,2'-((6-methyl-2-oxo-2H-chromene-3-carbonyl)azanediyl)diacetic acid (L1) A white
solid, yield: 70.8 %. m.p. 198-200 ℃. 1H NMR (400 MHz, DMSO) δ 12.80 (s, 2H, COOH),
8.01 (s, 1H), 7.58 (s, 1H), 7.50 (d, J=7.48Hz,1H,ArH), 7.35(d, J=7.32Hz,1H,ArH), 4.16 (S,
13
4H, CH2), 2.37 (s, 3H, CH3); C NMR (126 MHz, DMSO) δ 170.84 (s), 170.30 (s), 165.97
(s), 152.28 (s), 143.23 (s), 134.53 (s), 134.24 (s), 130.09 (s), 129.18 (s), 124.20 (s), 118.36 (s),
116.47 (s), 50.93 (s), 48.09 (s), 20.67 (s). IR (KBr) v/cm-1: 1736cm-1 (C=O), 1697cm-1 (C=O),
1623cm-1 (-CONR2), 1237cm-1 (C-O-C). ESI-MS (M+) m/z: 342.00(M+23).
2,2'-((6-nitro-2-oxo-2H-chromene-3-carbonyl)azanediyl)diacetic acid (L2) A white
1
solid, yield: 65.2 %. m.p. 202-205 ℃. H NMR (400 MHz, DMSO) δ 12.84 (s, 2H, COOH),
8.82 (s, 1H), 8.47 (d, J=8.45Hz,1H,ArH), 8.28(s, 1H), 7.67 (d, J=7.63Hz,1H,ArH), 4.23 (S,
2H, CH2), 4.17 (S, 2H, CH2);13C NMR (126 MHz, DMSO) δ 170.77 (s), 170.20 (s), 165.26
(s), 157.82 (s), 156.83 (s), 144.10 (s), 142.22 (s), 127.65 (s), 126.13 (s), 125.43 (s), 119.23 (s),
118.23 (s), 50.84 (s), 48.01 (s). IR (KBr) v/cm-1: 1727 cm-1 (C=O), 1648 cm-1 (C=O),
1617cm-1 (-CONR2), 1234 cm-1 (C-O-C), 1527 cm-1 (-NO2). ESI-MS (M+) m/z:
351.19(M+1).
2.3 Preparation of target europium complexes
Coumarin derivative was dissolved in ethanol at 60 °C, and Eu(NO3)3 in ethanol (0.2
mol·L-1) was added. Then, the pH was adjusted to 6-7 using sodium ethoxide solution (1
mol·L-1). The reaction solution was refluxed for 5 h and cooled to ambient temperature. Then
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poured petroleum ether into the mixture and keep stirring, the precipitation was filtered and
washed with ethanol, then dried to collect target europium complexes.
3. Results and discussion
3.1 pH response of ligand
The pH response of the L1-2 (5×10-5mol/L) was investigated in various acidic
environments. The experiment was measured in an aqueous solution (1:1, PBS buffer/ethanol,
v/v). The entrance slit was 5.0 nm, the exit slit was 5.0 nm, and the voltage was 700V. Since
there is no fluorescence and no pH response in L2, only the fluorescence spectral data of L1
are in Fig. 1.
(a)
(b)
Fig. 1. (a) Emission spectrum of L1 under different pH (3.5-9.0); (b) The fluorescence intensity of L1
under different pH (3.5-9.0)
Scheme 2. The mechanism of pH response of L1.
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As seen in Fig. 1, when the pH value in the range of pH=6.0-9.0, the fluorescence
intensity remains substantially unchanged, and the solution exhibits a light blue
fluorescence. Interestingly, as the pH value declined, the fluorescence intensity increased
sharply and responded linearly. At the same time, the solution changed from light blue to
dark blue under an ultraviolet lamp excited at 365 nm. It is ascribed to the L1 has
intramolecular charge transfer (ICT) fluorescence characteristics. ICT refers to the
mechanism of intramolecular electron transfer as a molecule with a push-pull electronic
configuration is excited, resulting in a mechanism of splitting positive and negative charges.
Since the L1 is under acidic conditions, the carboxylic acid group is easily deprotonated to
form a good electron withdrawing group, and the coumarin aromatic ether has a lone pair
of electrons and is easily protonated to bind to a proton originating from a carboxyl group.
In this situation, the push-pull mechanism and the ICT effect are enhanced, leading to
improved fluorescence. The above ligand’s fluorescence reaction is mainly based on the
shift of the ligand through the reversible carboxylic acid, influencing the ICT effect23,
thereby increasing the fluorescence intensity. Since the substituent of L2 is an electron
withdrawal group (-NO2), a push-pull structure is not created, resulting in fluorescence
quenching and without pH response.
From the above, it can be seen that the L1 has potential applications in imaging pH
changes in acidic cells.
3.2 AIE property of the target europium complex
Since L1 has excellent fluorescence characteristics, it is used to explore the AIE
phenomenon. The target europium complex is soluble in DMF, DMSO and other organic
solvents but insoluble 1,4-dioxane. Therefore, the AIE property was studied in DMF and
1,4-dioxane. The entrance slit was 5.0 nm, the exit slit was 5.0 nm, and the voltage was 700V.
The effect of the concentration of the EuL1 on the fluorescence intensity was explored in
DMF. The entrance slit was 2.5 nm, the exit slit was 5.0 nm, and the voltage was 700V.
Correspondingly, the fluorescence spectral data were in Fig. 2 and Fig. 3.
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(a)
(b)
Fig. 2. (a) Emission spectrum of EuL1 at different concentrations in DMF; (b) Excitation
spectrum of EuL1 at different concentrations in DMF
(c)
(d)
(e)
Fig. 3. (c) Emission spectrum of EuL1 at different fD (0%-95%); (d) The fluorescence intensity of
different fD (0%-95%); (e) Fluorescence photograph of EuL1
As can be seen from the Fig. 2, when the concentration of EuL1 increases from 0.03125
mg/mL to 1 mg/mL, the excitation peak red-shift from 346 nm to 369 nm, and the
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fluorescence intensity increases from 1048 to 5137, an increase of about 5-fold. At 369 nm
excitation, EuL1 exhibited typical Eu(III) emission at higher concentrations (1 mg/mL). This
can be attributed to the fact that the higher the concentration, the stronger intermolecular
interaction is formed, the mobility of the ligand molecule is enhanced, and the molecular
conjugation is enhanced, the triplet energy level of L1 is better matched with the resonance
emission level of Eu(III), and has excellent energy transfer efficiency, resulting in aggregates
exhibiting strong AIE behavior. As shown in the Fig.3, the fluorescence strength of EuL1
continues to increase by increasing the 1,4-dioxane content of the weak solvent in the mixed
solvent, thus significantly enhancing the emission dependent on Eu(III). This may be due to
the addition of 1,4-dioxane resulting in the aggregation of EuL1, forming nanoparticles in a
mixed solvent24. The fluorescence intensity of the Eu(III) characteristic light in 95%
1,4-dioxane content, is around 5-fold in the 1,4-dioxane content of 0%. Thus,
aggregation-induced excitation energy in EuL1 is efficiently transported to Eu(III), resulting
in aggregates exhibiting strong AIE behavior. This is caused by the shielding reaction and the
f-f9 electronic transition within the lanthanide(III) ion25. This AIE-active Ln complex has
potential applications in high concentration probes and sensors, and can also be used in other
related applications such as solid state emitters.
3.3 Fluorescence properties analysis
Fluorescence spectra of the target europium complexes were determined in the solid
state. The entrance slit was 2.5nm, the exit slit was 2.5nm, and the voltage was 700V.
Correspondingly, the fluorescence spectral data were in Table 1 and Fig. 4.
Table 1 Fluorescence spectral data of EuL1-2
5D0—7F1
λem/nm
5D0—7F2
λem/nm
Compounds
λex/nm
I/a.u.
1970
I/a.u.
5372
EuL1
EuL2
371
594
594
617
617
357
1022
2964
λex: exciting wavelength; λem: emission wavelength; I(a.u.): relative fluorescence intensity
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(a)
(b)
Fig. 4. (a) Excitation spectrum of EuL1-2 in the solid state; (b) Emission spectrum of EuL1-2 in the
solid state
From Table 1 and Fig. 4, both target complexes exhibited characteristic fluorescence of
the central europium, and the two substituents had different fluorescence intensities. EL1 has
an optimum excitation peak at 371 nm and a Stokes shift of up to 246 nm. The emission
5
5
peaks at 580 nm, 594 nm, 617 nm, 651 nm, and 694 nm belong to D0→7F0, D0→7F1,
5
5
5
5D0→7F2, D0→7F3, and D0→7F4 transitions, respectively. The D0→7F1 transition and the
5D0→7F2 transition are magnetic dipole transitions and electric dipole transitions, respectively,
and their fluorescence intensities are 1970, 5372. This indicates that the triplet energy level of
the ligand matches well with the excited state energy level of Eu(III), and the energy transfer
efficiency between the ligand and Eu(III) is high. Importantly, the emission bands for the
5
5D0→7F1 transition and the D0→7F2 transition are all less than 10 nm, indicating that the
monochromatic properties of these target complexes are excellent26. The ratio of the
fluorescence intensity η (5D0→7F2/5D0→7F1) due to the electric dipole transition and the
magnetic dipole transition is 2.67 and 2.90, respectively, indicating that Eu(III) is not to the
symmetry center of the complex27. The ligand structure designed in this paper introduces an
electron-withdrawing group at the 3-position of the coumarin ring, and introduces
electron-donating (-CH3) and electron-withdrawing (-NO2) at 6-positions for comparison.
Obviously, the fluorescence intensity of EuL1 is greatly enhanced compared with EuL2,
because the introduction of electron-donating groups forms a push-pull electronic system and
expands its conjugate structure, increases the density of electron clouds on the conjugate
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system, more electrons enter the empty orbit of Eu(III), which increases the energy transfer
efficiency between the ligand and Eu(III), thereby enhancing the fluorescence intensity of the
complex. Conversely, the introduction of an electron-withdrawing group (-NO2) does not
form a push-pull electronic system, which reduces the density of the electron cloud on the
conjugated system, resulting in a decrease in the coordination ability of the ligand, and thus
the energy conversion rate of the ligand and Eu(III) is lowered. The EuL1-2 energy level is
explained in Fig.5. This can provide a suitable study for designing good ligands.
Fig. 5. Energy transfer process of EuL1-2
3.4 Elemental analysis
To confirm the composition of title Eu(III) complexes, elemental analysis were
investigated. The date of elemental analysis was listed in Table 2. It can be seen from Table 2
that the experimental data and the calculated data are in good agreement. The proportion of C,
H, N, O and Eu in Eu(III) complexes confirms the molecular structure of EuL1-2.
Table 2 Elemental analysis of title Eu(III) complexes.
Found (Calcd) (%)
Complexes
C
H
N
O
Eu
EuL1
EuL2
26.12(26.06) 2.21(2.19) 8.18(8.11) 41.71(41.66) 22.01(21.98)
23.33(23.28) 1.72(1.67) 9.74(9.70) 44.35(44.30) 21.10(21.04)
3.5 UV-vis spectral analysis
The UV absorption spectra of L1-2 (5×10-5 mol/L) and the EuL1-2 (5×10-5 mol/L) were
determined in DMF solution. Since the structures are similar, only L1 and EuL1 are selected
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to determine. The data were listed in Table 3, and the UV spectral curves were shown in Fig.
6.
Table 3 The UV data of L1-2 and corresponding EuL1-2
Compounds
λ1
ε1
λ2
ε2
L1
EuL1
L2
294
303
279
288
1.17
1.74
0.92
1.23
335
348
326
333
0.63
1.62
0.32
0.52
EuL2
Fig. 6. The UV spectra of EuL1and L1
From Fig. 6, it can be observed that the absorption peak at 294 nm is found in L1, which
belong to π→π* transition, corresponding europium complex also presents an absorption peak
at 303 nm with 9 nm red shift. The second absorption peak of the L1 is located at 335 nm,
which belong to n→π* transitions and the absorption peak of the corresponding europium
complex is located at 348 nm, with 13 nm red shift. Both π→π* and n→π* transitions
absorption band of the europium complex have a red shift, which indicates the conjugated
system has undergone obvious changes, and L1 is coordinated with the europium successfully.
Due to the coordination of europium, the lone pair electrons on the oxygen atom of the C=O
group of the target L1 are shifted, and the molecular structure changes to form a larger π bond
conjugated system, thereby reducing the electronic transition energy. The energy absorption
and transition of the target europium complex are improved, so the π→π* absorption band
and the n→π* absorption band of the target europium complex are red-shifted. As shown in
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Fig. 6, the peak shape of the UV spectrum of the europium complex is basically the same as
that of the corresponding L1, but the absorption peaks present red shift, indicating that the
π→π* transition and the n→π* transition in the coordination are not affected by the
introduced Eu(III). Due to the nature of the lanthanide, f-f transition of Eu(III) is symmetry
Laporte-forbidden28.
Obviously, the molar absorbance of the europium complex is greater than that of the
corresponding L1. The target europium complex has excellent UV absorption, which also
reflects their potential applications in the optical field.
3.6 FT-IR spectral analysis and Molar conductivity
The structure of L1-2 and the EuL1-2 can be verified by infrared spectra. Because the
structures are similar, only L1 and EuL1 are selected to determine. The data were listed in
Table 4, and corresponding FT-IR spectral curves were shown in Fig. 7.
Table 4 FT-IR data of L1-2 and EuL1-2
Compounds
νO-H
νC=O νC=O ν-CONR2
νC-O-C
1275
1274
1217
1217
Λm
L1
EuL1
L2
-
1778 1757
1487
1488
1621
1619
-
3400 1609 1580
1743 1713
3424 1713 1702
22
-
-
EuL2
17
Fig. 7. FT-IR spectra of EuL1 and L1
As shown in Table 4 and Fig. 7, for the L1, the two absorption peaks of the carboxylic
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acid (vC=O) are located at 1778 cm-1 and 1757 cm-1, the tertiary amide (ν-CONR2) is present at
1487 cm-1 and the aromatic ether (νC-O-C) is found at 1275 cm-1, which correspond to the
structure of the ligand, and confirmed the structure of the ligand. Meanwhile, the target
europium complex has a broad absorption peak around 3400 cm-1, which represents an
absorption peak of v(-OH). This is because the complex contains two molecules of water
after coordination29. In addition, the peaks of v(C=O) of the carboxylic acid group in the
EuL1 are red-shifted to 1609 cm-1 and 1580 cm-1, respectively, offset by 169 cm-1 and 177
cm-1. It can be explained that when the oxygen atoms of the C=O groups coordinate with the
Eu(III), the electrons of the C=O bonds move toward the Eu(III) and the bond force constant
changes. Therefore, vibration absorption peaks of carbonyl groups show significant changes30.
The lone pairs of v(C=O) in the ligand enter the 4f orbital of Eu(III), forming a large
conjugated system, and the rigid structure of the molecule is improved, resulting in vibration
of the affected area. The vibration frequency is lowered, so the absorption peak shifts to the
high frequency31. Meanwhile, the measured values of the molar conductivity are in the range
of 17 to 22 Scm2.mol-1, indicates that the above materials are non-electrolytes32. In other
words, there is no free nitrate in the complex molecule and successfully participated in
coordination.
Therefore, we can deduce the ligands are coordinated with Eu(III) successfully. The
functional group specifically involved in the coordination was determined, and the three
nitrates successfully participated in the complexes.
3.7 Thermogravimetric analyses
Thermal properties of the target complexes are measured by thermal gravimetric (TG)
under static air. Since the thermal behaviors of target complexes are extremely similar, only
the TG curves of EuL1 are selected to illustrate and shown in Fig. 8.
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Fig. 8.TG curves of EuL1
As showed in Fig. 8, the TG curves of target Eu(III) complexes are consistent with the
facts that there are three mainly decomposition for target complexes. The first stage with 4.96%
(the theoretical value is 5.02%) mass loss is in the range of 45-114℃, this is attributed to the
loss of the water in the complex. The second mass loss ranged from 114°C to 385°C is due to
the complete decomposition of three coordination nitrate groups, the experimental value
(26.05%) is consistent with the theoretical value (25.94%). The third mass loss between
385°C and 639°C is allocated to the decomposition of ligand, and the experimental value
(43.86%) is corresponding to the theoretical value (44.50%). The residue (25.13%) tends to
stabilize after 640°C, which matches well with the theoretical value (24.50%) of Eu2O3,
indicating the complex was completely decomposition into europium oxide. The results of
thermal analysis implied a good thermal stability of target complexes, and further confirmed
the structure of target europium complexes.
3.8 Electrochemical properties
To study the lowest unoccupied molecular orbit (LUMO), the highest occupied
molecular orbit (HOMO) and the onset potential (EOX) of target complexes, the
electrochemical properties of target Eu(III) complexes were investigated by the cyclic
voltammetry (CV) in DMSO. HOMO and LUMO were calculated according to equations (1)
and (2) and (3).
EHOMO = – (4.74+EOX) eV (1); ELUMO = EHOMO + Eg (2); Eg= 1240 / λonset (3)
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In equations (1) and (2) and (3), 4.47 is the energy level of the saturated calomel
electrode relative to vacuum. Eg is energy gap, λonset is the wavelength of the maximum UV
absorption peak33,34. Electrochemistry data of EuL1-2 are given in table 5, and the cyclic
voltammetry curves of target complexes are shown in Fig. 9.
Table 5 Electrochemical data of the EuL1-2
Compounds
λonset(nm)
Eox (V)
EHOMO (eV)
Eg (eV)
ELUMO (eV)
EuL1
EuL2
303
287
0.6967
0.7426
-5.4367
-5.4826
4.0924
4.3205
-1.3443
-1.1620
Fig. 9. The cyclic voltammetry curves (CV) of EuL1-2
It can be observed in Fig. 9, that the oxidation potential peaks of EuL1 and EuL2 are
0.6967V and 0.7426V, respectively. Due to the influence of the electron donating group
(-CH3), L1 has a larger π-conjugated system and more π bond electron cloud density than L2.
It can be seen from table 5 that the HOMO energy level of the complex is EuL1 ˃ EuL2, and
the LUMO energy level of the complex is EuL1 ˂ EuL2. The results indicate that the
introduction of electron donating groups increases the HOMO level of complexity and lowers
the LUMO level. Conversely, the electron withdrawing group reduces the HOMO level and
increases the LUMO level. Obviously, the energy gap (HOMO-LUMO) order is EuL1 ˃
EuL2, which indicates the introduction of the electron donating group (-CH3), which leads to
the formation of a push-pull system for the ligand, thus enhancing the stability of the target
complex. Therefore, this provides an idea for designing a highly stable complex in the future.
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Meanwhile, the HOMO levels of the two complexes are about -5.5 eV, which is the
same as many reported OLEDS materials35. Therefore, this also explains to some extent the
possibility that this material has been used in optical materials.
4. Conclusion
1
Two novel coumarin derivatives were synthesized and characterized by H NMR, 13C
NMR and ESI-MS. The corresponding Eu(III) complexes were also prepared and
characterized by UV-vis spectra, molar conductance, FT-IR spectra, elemental analysis and
thermogravimetric analysis. Due to the ICT effect, when pH value of the ligand solution is
pH ˂ 6.0, the fluorescence intensity of L1 increases sharply and reacts linearly. Owing to the
AIE effect of the complex, the fluorescence intensity increases linearly as the concentration
of the complex increases or the proportion of the poor solvent increases. The effects of the
substituents of fluorescence properties are ordered: -CH3 > -NO2, and the electron donating
group complex has stronger fluorescence intensity than the electron withdrawing group
complex. Further, thermal analysis results show that the target complexes own highly thermal
stability. The HOMO level of the electron donor group complex is higher than the energy
level of the electron withdrawing group, but the LUMO level is lower than the energy level
of the electron withdrawing group. In summary, the material can be used in a variety of
bioassays and optical materials.
Acknowledgments
The authors are grateful for the financial support of the National Natural Science
Foundation of China (No.21341010), Hunan provincial Science &Technology Department
(No.2016SK2064), Changsha Science and Technology Bureau (No.kq1701164). We also
thank Dr. William Hickey, the U.S. professor of HRM, for the English editing on this paper.
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Highlights
(1) Novel Eu(III) complexes on the basis of coumarin derivatives have been firstly
synthesized and characterized.
(2) All the target complexes emit characteristic fluorescence of Eu(III).
(3) The ligand has an ICT effect and is linearly responsive under acidic conditions.
(4) As the proportion of poor solvent increases, the complex exhibits strong AIE
performance.
(5) HOMO and LUMO energy levels of target complexes have been researched.
This article is protected by copyright. All rights reserved.