Functionalized Lanthanide(III) Complexes Constructed from Azobenzene Derivative and β-Diketone Ligands: Luminescent, Magnetic, and Reversible Trans-to-Cis Photoisomerization Properties

Article abstract of DOI:10.1021/acs.inorgchem.6b02819

The coordination ability of ligands functionalized by azobenzene was manipulated, and two novel chelating ligands, (E)-4-(phenyldiazenyl)-N,N-bis(pyridin-2-ylmethyl) benzohydrazide (C₂₅H₂₂N₆O, PBPM) and (E)-4-((4-(dimethyl

Full text of DOI:10.1021/acs.inorgchem.6b02819

Article
pubs.acs.org/IC
Functionalized Lanthanide(III) Complexes Constructed from
Azobenzene Derivative and β‑Diketone Ligands: Luminescent,
Magnetic, and Reversible Trans-to-Cis Photoisomerization Properties
Li-Rong Lin,*,† Hui-Hui Tang,† Yun-Guang Wang,†,‡ Xuan Wang,†,‡ Xue-Ming Fang,†
and Li-Hua Ma*,§
†Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, People’s Republic
of China
§Department of Chemistry, College of Science and Computer Engineering, University of HoustonClear Lake, 2700 Bay Area
Boulevard, Houston, Texas 77058, United States
S
* Supporting Information
ABSTRACT: The coordination ability of ligands functionalized by azobenzene
was manipulated, and two novel chelating ligands, (E)-4-(phenyldiazenyl)-N,N-
bis(pyridin-2-ylmethyl) benzohydrazide (C25H22N6O, PBPM) and (E)-4-((4-
(dimethylamino)phenyl)diazenyl)-N,N-bis(pyridin-2-ylmethyl) benzohydrazide
(C27H27N7O, dmPBPM), were synthesized. The ligands can offer four
coordinating atoms (one oxygen and three nitrogens) to act as tetradendate
ligands, together with the two β-diketonates (4,4,4-trifluoro-1-phenylbutane-1,3-
dionate, tfd), and the trifluoroacetate anion presented as a ligand and a
counterion to form the quaternary units with lanthanide(III) ions (La, Eu, and
Gd), [Ln(tfd)2(PBPM)(CF3CO2)] (LnC47H34F9N6O7) and [Ln-
(tfd)2(dmPBPM)(CF3CO2)] (LnC49H39F9N7O7), where the lanthanide(III)
ions are nine coordinated with N3O6 donor sets. All six complexes were
structurally characterized, and four crystals were obtained and further analyzed
by means of single-crystal X-ray diffraction. They all crystallized in the
monoclinic P21/c space group with very similar lattice parameters, forming a monocapped twisted square antiprism. We
successfully observed the photoluminescent properties of Eu(III) complexes at a wavelength of 614 nm in both solution and the
solid state, as well as the trans-to-cis photoisomerization with the quantum yield (Φt→c = 10−2) of [Eu(tfd)2(PBPM)(CF3CO2)]
complex that was comparable to that of PBPM. Moreover, the trans-to-cis photoisomerization rates of complexes
[Ln(tfd)2(PBPM)(CF3CO2)] (La, Eu, Gd) (10−3−10−2
s
−1) were also at the same level as that of PBPM and much higher
than azobenzene itself (10−5−10−4 s−1). With the aid of TD-DFT calculations, the luminescence of Eu(III) complexes was found
to originate from the attenuation effect of β-diketonates. These features provide the foundation for the development of
azobenzene-derived β-diketonates lanthanide(III) complexes with photoisomerization and photoluminescence dual functions.
azobenzene,10,13 bis- or tris(bipyridine),14 terpyridine metal
INTRODUCTION
■
complexes with azobenzene conjugation,15 metalladithiolenes
with azobenzene groups, and azobenzene-containing tris-
cyclometalated complexes.16,17 Most of the studied complexes
displayed trans-to-cis photoisomerization upon UV light and
the reverse cis-to-trans isomerization by excitation of the n−π*
band of azobenzene.18,19 However, the efficiency of the
photoisomerization was quite low, and the quantum yields of
the trans-to-cis photoisomerization were much lower than
those of the organic azobenzenes.20 Some of the metal
complexes such as Ir(III) and Ru(II) complexes even showed
decreased quantum yields of photoisomerization.19,21
Azobenzene is well known for its trans-to-cis isomerization
upon irradiation with UV light, and the reverse isomerization
can take place by irradiation with visible light or by heating.1 A
large number of azobenzene derivatives have been studied in
solution as well as in polymer matrices.2−4 These compounds
are promising for application in a wide variety of areas, from
biological research to information technology.5−9 Azobenzene-
functionalized transition metal complexes are an alternative to
pure organic azobenzene derivatives and can provide more
versatile molecular materials.10−12 Combining an azo moiety
within coordination compounds can lead to systems capable of
exhibiting novel and unusual properties, such as redox, optical,
and magnetic properties, of the metal complexes with the
photoisomerization reaction. Representative examples are the
families of azobenzene-functionalized transition metal com-
plexes that have been studied in detail, such as ferrocene−
To overcome this difficulty, instead of the d-block group VIII
transition metal ions, we focused on the f-block lanthanide ions
Received: November 22, 2016
© XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.6b02819
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Scheme 1. Synthetic Procedures Used in the Preparation of PBPM, dmPBPM, and Complexes [Ln(tfd)2(PBPM)(CF3CO2)]
and [Ln(tfd)2(dmPBPM)(CF3CO2)]
(Ln(III)), a particularly interesting class that possesses unusual
electronic properties resulting from shielding of the 4f orbitals
by the filled 5s25p6 subshells. Recently, we investigated the
photoisomerization properties of a series of azobenzene-derived
tris-β-diketonate lanthanide complexes.22 It was found that the
presence of Ln(III) enhanced the reversible trans-to-cis
isomerization properties of the azobenzene group and increased
the fatigue resistance. However, a luminescent property of such
azobenzene-derived tris-β-diketonates lanthanide complexes
has not been observed at room temperature owing to the
forbidden nature of their electronic transitions. The energy
could not be transferred from the chromophore to the metal
center, which was an obvious drawback for performing useful
functions in the fields of fundamental research and possible
applications in biological systems and optical data transport.
Combining the information above with the idea to explore
the possibility of a luminescent property of azobenzene-derived
diketonate lanthanide complexes, we designed and synthesized
two new ligands, (E)-4-(phenyldiazenyl)-N,N-bis(pyridin-2-
ylmethyl) benzohydrazide (C25H22N6O, PBPM) and (E)-4-
((4-(dimethylamino)phenyl)diazenyl)-N,N-bis(pyridin-2-yl-
methyl) benzohydrazide (C27H27N7O, dmPBPM), which are
functionalized with an azobenzene group and able to play the
role of a chelating ligand for Ln(III) ions. PBPM or dmPBPM
(Scheme 1) consist of an azobenzene moiety, a bis(pyridin-2-
ylmethyl)benzohydrazide group, to offer one oxygen and three
nitrogen atoms to strongly coordinate with Ln(III) ions. The
tetradendate PBPM or dmPBPM ligands, together with two β-
diketonates coordinated to the lanthanide center, can form
higher coordination number complexes and favor shielding
from the solvent molecules.23 It is well known that the electron-
donating substituent groups on the aromatic ring significantly
affect the photoisomerization properties of the azo group or
photoluminescence of lanthanide(III) complexes.1,22,24,25 How-
ever, the effect of the electron-donating group on the
complexes with photoisomerization and photoluminescence
dual functions has barely been reported. We therefore prepared
dmPBPM containing the dimethylamino electron-donating
group to address the effects of amino N substituents on the
photoluminescence and photoisomerization quantum efficien-
cies of both the free ligand and its Ln(III) complexes and
compared their characteristics with those of PBPM. Herein, we
present the structural characterization of PBPM and dmPBPM
in complexation with the Ln(III) β-diketonates, where 4,4,4-
trifluoro-1-phenylbutane-1,3-dionate (tfd) is introduced to the
molecular system to construct luminescent Ln(III) β-
B
DOI: 10.1021/acs.inorgchem.6b02819
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diketonates.23,26,27 The results reveal the photoluminescent
properties of the Eu(III) complexes and the magnetic
properties of the Gd(III) complexes as well as an efficient,
rapid, reversible, and high quantum yield photoisomerization.
We report here the first promising case of an Eu(III) complex
with PBPM attached ([Eu(tfd)2(PBPM)(CF3CO2)]) capable
of generating both efficient photoisomerization and impressive
photoluminescence. Additionally, easy modification of certain
positions of the ligand scaffold would enable the spread of
azobenzene-derived ligands and β-diketonate Ln(III) multinary
complexes, which will contribute even more to the dual-
function materials exploration for applications in the fields of
lasers, displays, sensors, solar cells, electroluminescent devices,
and biomedical research.
J = 6.5 Hz, 2H), 7.97 (d, J = 8.3 Hz, 4H), 7.56−7.51 (m, 3H), 4.44 (q,
J = 7.1 Hz, 2H), 1.45 (t, J = 7.1 Hz, 3H).
4-Phenylazobenzohydrazide (L2). A 0.51 g amount of L1
compound (2.0 mmol) and 10 mL of hydrazine hydrate (85%) were
added into 15 mL of ethanol solution. The solution was refluxed at
100 °C for 12 h under vigorous stirring. A pale yellow solid appeared
with time after the mixture was stored in a refrigerator for several
hours. The precipitate was filtered and washed with 15 mL of ethanol,
30 mL of cool water, and 15 mL of ethanol, affording a pale yellow
solid L2 0.38 g (yield 79%): mp 146−147 °C; ESI(+)-MS (m/z,
methanol) 240.1 [M+]; 1H NMR (400 MHz, DMSO-d6) δ ppm = 9.99
(s, 1H), 8.06−8.01 (m, 2H), 7.96−7.90 (m, 4H), 7.65−7.58 (m, 3H),
4.70 (s, 2H).
(E)-4-(Phenyldiazenyl)-N,N-bis(pyridin-2-ylmethyl) Benzohydra-
zide (PBPM). 2-Picolyl chloride hydrochloride (0.49 g, 3 mmol) and
compound L2 (0.24 g, 1 mmol) were dissolved in 30 mL of 1:1
ethanol/water and heated to 80 °C. To this solution, 6 mL of aqueous
NaOH (1.0 mmol/L) was added dropwise over a period of 30 min,
and the resulting solution was additionally stirred for 6 h. The solution
was cooled to room temperature, and an orange yellow solid product
appeared, which was filtered and washed with mixed water/ethanol
(1:1) solvents and then recrystallized from dichloromethane/
petroleum ether (1:1) to yield orange PBPM, 0.49 g (yield 58%);
mp 151 °C; ESI(+)-MS (m/z, methanol) 445.1 [M + Na+]; 1H NMR
(400 MHz, DMSO-d6) δ ppm = 9.82 (s, 1H), 8.48 (s, 2H), 7.93−7.86
(m, 4H), 7.81 (d, J = 22.2 Hz, 6H), 7.61 (s, 3H), 7.25 (s, 2H), 4.31 (s,
4H). FT-IR (KBr, cm−1): 3433 (s, O−H), 1654 (s, CO), 1591 (m,
CC), 1569 (m, N−H), 1477 (s, NN), 1291 (s, C−N), 855, 820,
768 (m, Ph−H). Anal. Calcd for C25H22N6O: C, 71.07; H, 5.25; N,
19.89. Found: C, 70.86; H, 5.21; N, 19.82.
Ethyl 4-((4-Dimethylamino)phenyl)azobenzoate (L3). Ethyl 4-
aminobenzoate (0.83 g, 5.0 mmol) was dissolved in HCl solution
(12.0 mL, 3.0 mol·L−1). A solution of sodium nitrite (0.38 g, 5.5
mmol) in distilled water (12.0 mL) was prepared in a test tube and
added dropwise to the acidic solution of ethyl 4-aminobenzoate over 5
min at 0 °C. The mixture was stirred at 0 °C for 30 min. N,N-
Dimethylaniline (0.61 g, 5.0 mmol) was dissolved in 5 mL of ethanol
and cooled to 0 °C, which was added slowly to the above
aryldiazonium salt solution at 0 °C. The pH of the resultant colored
mixture was adjusted by saturated sodium carbonate solution. After the
pH reaching 7−8, the solution immediately turned red, and in a few
seconds a red crystalline precipitate of the azo compound was formed.
The solution was kept stirring for another 2 h at 0 °C. The precipitate
was collected by filtration and washed with water/ethanol (1:1), which
was recrystallized from dichloromethane/petroleum ether (1:1) to
yield the red compound L3, 1.262 g (yield 85%); mp 153−154 °C;
ESI(+)-MS (m/z, methanol) 297.2 [M+]; 1H NMR (400 MHz,
CDCl3) δ ppm = 8.16 (d, J = 8.4 Hz, 2H), 7.91 (dd, J = 19.0, 8.6 Hz,
4H), 6.78 (d, J = 9.0 Hz, 2H), 4.42 (q, J = 7.1 Hz, 2H), 3.13 (s, 6H),
1.44 (t, J = 7.1 Hz, 3H).
4-((4-Dimethylamino)phenyl)azobenzohydrazide (L4). Hydrazine
hydrate (85%, 15 mL) and compound L3 (0.60 g, 2 mmol) were mixed
and refluxed for 12 h under vigorous stirring. After the mixture was
kept in a refrigerator for several hours, the yellow solid product
appeared, which was filtered and washed with 50 mL of water/ethanol
(1:1) mixed solvent precooled on ice, generating compound L4, 0.369
g (yield 65%); mp 146−147 °C; ESI(+)-MS (m/z, methanol) 283.1
[M+]; 1H NMR (400 MHz, DMSO-d6) δ ppm = 9.87 (s, 1H), 7.96 (d,
J = 8.5 Hz, 2H), 7.81 (dd, J = 8.8, 5.7 Hz, 4H), 6.84 (d, J = 9.2 Hz,
2H), 4.55 (s, 2H), 3.07 (s, 6H).
EXPERIMENTAL SECTION
■
Materials and Methods. LnCl3·3H2O (Ln = La, Eu and Gd; Jinan
Camolai Trading Co.), 4,4,4-trifluoro-1-phenylbutane-1,3-dione
(Htfd), ethyl p-aminobenzoate and 2-(chloromethyl)pyridine hydro-
chloride (Leon Technology(Beijing) Co., Ltd.), tetrabutylammonium
perchlorate (Adamas Reagent Co., Ltd.), and poly(methyl meth-
acrylate) (PMMA) (Alfa Aesar) were used, and all other chemicals
were obtained from Sinopharm Chemicals Group Co. Analytical-grade
solvents were redistilled before use. ESI-MS data were acquired on a
Bruker ESQUIRE-3000 Plus LC-MS/MS spectrometer. The melting
point was determined with an X-4 micromelting point apparatus
without correction. Elemental analyses were performed on a Vario EL
III elemental analyzer. Solution 1H NMR spectra were recorded at 400
MHz using a Bruker Advance 400 spectrometer at 25 °C. Chemical
shifts (ppm) were determined with TMS as the internal reference. IR
spectra (KBr pellets) were recorded on a Nicolet AVATAR FT-IR 330
spectrometer. Thermogravimetric analyses (TGA) were carried out on
a SDTQ 600 thermogravimetry under nitrogen atmosphere with a
heating rate of 10 °C/min. Absorption spectra were scanned using a
Shimadzu UV224012PC absorption spectrometer. The luminescent
excitation and emission spectra were recorded using a Hitachi F7000
fluorescence spectrophotometer equipped with a 150 W xenon lamp
as excitation source. The maximal wavelength of the Eu(III)
complexes is at 614 nm, which is beyond the spectra correction
region (less than 600 nm) of this instrument. Therefore, the spectra
were uncorrected for detector response, and the slit widths with
excitation and emission slits were set at 2.5/5.0 nm. The overall
quantum yields of complexes II and V were measured in organic
solvent at room temperature, and the emission quantum yield standard
was Ru(bpy)2Cl2 in air-saturated aqueous solution (Φ = 0.028).28
Magnetic measurements were carried out at a sweeping rate of 1.5 K
min−1 on a Quantum Design SQUID XL7 magnetometer in the 2−
300 K temperature range. DFT calculations were performed at the
three-parametrized Becke−Lee−Yang−Parr (B3LYP) hybrid ex-
change-correlation functional and 6-31+G(d) basis set using the
Gaussian 09 program.29 Low-lying singlet and triplet excitation
energies were calculated at the optimized geometries by TD-DFT at
the same basis set. The geometries of these compounds were
optimized with the DFT(B3LYP) method without considering the
solvent effect.
Ligands Syntheses. Ethyl 4-phenylazobenzoate (L1). Nitro-
sobenzene (0.16 g, 1.5 mmol) was dissolved in glacial acetic acid (6
mL), followed by adding ethyl p-aminobenzoate (0.17 g, 1 mmol) and
heating to reflux at 90 °C overnight. The resulting solution was cooled
to room temperature and poured into water (7 mL). The aqueous
solution was extracted three times, each with 20 mL of dichloro-
methane. The combined organic layers were treated with anhydrous
sodium sulfate and filtered. The collected solution was concentrated
under reduced pressure and purified by silica gel chromatography with
eluting solvent petroleum ether/ethyl acetate (20/1), yielding 0.54 g
L1 (yield 70%) as orange solid: mp 126−127 °C; ESI(+)-MS (m/z,
methanol) 254.1 [M+]; 1H NMR (400 MHz, CDCl3) δ ppm = 8.21 (t,
(E)-4-((4-(Dimethylamino)phenyl)diazenyl)-N,N-bis(pyridin-2-yl-
methyl) Benzohydrazide (dmPBPM). 2-(Chloromethyl)pyridine
hydrochloride (0.10 g, 6 mmol) and compound L3 (0.57 g, 2
mmol) were dissolved in 15 mL of absolute ethanol and heated to 78
°C. To this solution, 6 mL of 1.0 mmol/L NaOH aqueous solution
was added dropwise over a period of 30 min, and the resulting mixture
was stirred for an additional 6 h. The forming orange red precipitate
was filtered while still hot. The product was washed with water/
ethanol (1:1) and then recrystallized from 1:1 dichloromethane/
petroleum ether to yield compound dmPBPM as a red solid, 0.065 g
C
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Table 1. Crystallographic Data for Complexes [La(tfd)2(PBPM)(CF3CO2)] (I), [Eu(tfd)2(PBPM)(CF3CO2)] (II),
[La(tfd)2(dmPBPM)(CF3CO2)] (IV), and [Gd(tfd)2(dmPBPM)(CF3CO2)] (VI)
crystal data
CCDC number
I
II
1437960
IV
1437963
VI
1437961
1437962
empirical formula
fw
C47H34F9LaN6O7
1104.71
C47H34F9EuN6O7
1117.76
C49H39F9LaN7O7
1147.78
C49H39F9GdN7O7
1167.13
temp (K)
wavelength (Å)
cryst syst
space group
a (Å)
203(2)
203(2)
203(2)
203(2)
0.71073
0.71073
0.71073
0.71073
monoclinic
P21/c
monoclinic
P21/c
monoclinic
P21/c
monoclinic
P21/c
24.515(5)
9.862 (2)
19.371(4)
90
24.109(7)
9.717(3)
19.229(5)
90
25.981(1)
9.741(4)
19.625(7)
90
25.785(5)
9.695(2)
19.500(3)
90
b (Å)
c (Å)
α (deg)
β (deg)
101.521(3)
90
102.483(5)
90
95.271(1)
90
96.132(4)
90
γ (deg)
V (Å)
4589.0(15)
4
4398.0(2)
4
4945.8(3)
4
4846.8(15)
4
Z
calcd density (Mg m−3
)
1.599
1.688
1.541
1.599
abs coeff (mm−1
)
1.026
1.525
0.956
1.462
θ range
3.39−52.00
24 537
3.46−51.00
22 957
6.01−51.00
38 361
3.17−52.00
26 112
no. of reflns collected
data/restraints/parameters
goodness of fit on F2
8985/0/631
1.144
8188/0/631
0.906
9156/0/661
1.072
9470/0/660
1.149
final R indexes [I ≥ 2σ(I)]
R1 = 0.0619
wR2 = 0.1299
R1 = 0.0725
wR2 = 0.1357
R1 = 0.0414
wR2 = 0.0964
R1 = 0.0632
wR2 = 0.1023
R1 = 0.0495
wR2 = 0.1104
R1 = 0.0950
wR2 = 0.1404
R1 = 0.0827
wR2 = 0.1484
R1 = 0.1257
wR2 = 0.1648
R indexes [all data]
(yield 70%); mp 196−197 °C; ESI(+)-MS (m/z, methanol) 465.1
(s, CC), 1577 (m, N−H), 1538 (s, NN), 1384, 1132 (s, C−N),
1287, 1185 (s, C−F), 858, 789, 761 (m, Ph−H). Anal. Calcd for
EuC47H34F9N6O7: C, 50.50; H, 3.07; N, 7.52. Found: C, 50.55; H,
3.07; N, 7.87.
1
[M+]; H NMR (400 MHz, CDCl3) δ ppm = 9.48 (s, 1H), 8.60 (s,
2H), 7.98−7.85 (m, 6H), 7.77 (t, J = 7.4 Hz, 2H), 7.67 (d, J = 7.5 Hz,
2H), 7.26 (s, 2H), 6.81 (d, J = 8.4 Hz, 2H), 4.40 (s, 4H), 3.16 (s, 6H).
FT-IR (KBr, cm−1): 3433 (s, O−H), 2923 (m, C−H), 1687 (w, C
O), 1603 (m, CC), 1547 (m, N−H), 1518 (s, NN), 1367, 1140
(s, C−N), 858, 820, 767 (m, Ph−H). Anal. Calcd for C27H27N7O: C,
69.66; H, 5.85; N, 21.06. Found: C, 69.10; H, 5.84; N, 21.29.
Synthesis of Lanthanide(III) Complexes. General Procedures.
All six complexes were synthesized by the same method described as
follows. PBPM (or dmPBPM) (0.25 mmol) and 4,4,4-trifluoro-1-
phenylbutane-1, 3-dione (Htfd, 0.162 g, 0.75 mmol) were added to 15
mL of absolute ethanol. The reaction mixture was stirred and heated
to 78 °C. To this solution 0.75 mL of 1.0 mmol/L CF3COONa
aqueous solution was added dropwise in 10 min. Then 0.25 mmol of
LnCl3·3H2O in 5 mL of deionized water was added dropwise, and the
resulting mixture was stirred for an additional 10 h at 78 °C. The
precipitate appeared after cooling. The crude product was filtered and
washed with 30 mL of 1:1 water/ethanol several times and then dried
under reduced pressure to give pure solid powder. Crystals of
complexes suitable for X-ray analysis were obtained by slow
evaporation from their DMF and ethanol mixed solution for about 1
month.
[La(tfd)2(PBPM)(CF3CO2)] (I). Orange solid (0.27 g, yield 83%); mp
180−182 °C; ESI(+)-MS (m/z, methanol) 991.4 [M+]; 1H NMR
(400 MHz, DMSO) δ ppm = 9.86 (s, 1H), 8.49 (s, 2H), 7.91 (dd, J =
18.1, 8.6 Hz, 7H), 7.83−7.72 (m, 6H), 7.64−7.37 (m, 10H), 7.26 (s,
2H), 6.34 (s, 2H), 4.32 (s, 4H). FT-IR (KBr, cm−1): 3433 (s, O−H),
1682 (m, CO), 1627 (m, CC), 1577 (m, N−H), 1536 (s, N
N), 1384, 1131 (s, C−N), 1286, 1185 (s, C−F), 819, 795, 761, 629
(m, Ph−H). Anal. Calcd for LaC47H34F9N6O7: C, 51.10; H, 3.10; N,
7.61. Found: C, 50.76; H, 3.08; N, 7.62.
Gd(tfd)2(PBPM)(CF3CO2) (III). Orange solid (0.26 g, yield 78%); mp
260−262 °C; ESI(+)-MS (m/z, methanol) 1010.2 [M+]. FT-IR (KBr,
cm−1): 3441 (s, O−H), 1687 (s, CO), 1630 (s, CC), 1578 (m,
N−H), 1537 (s, NN), 1384, 1132 (s, C−N), 1286, 1185 (s, C−F),
876, 828, 798, 761 (m, Ph−H). Anal. Calcd for GdC47H34F9N6O7: C,
50.27; H, 3.05; N, 7.48. Found: C, 50.20; H, 3.01; N, 7.34.
La(tfd)2(dmPBPM)(CF3CO2) (IV). Dark red solid (0.30 g, yield 87%);
mp 243−244 °C; ESI(+)-MS (m/z, methanol) 1034.6 [M+]; 1H NMR
(400 MHz, DMSO-d6) δ ppm = 9.81 (s, 1H), 8.50 (s, 2H), 7.93 (d, J =
7.4 Hz, 4H), 7.82−7.65 (m, 10H), 7.48 (d, J = 40.9 Hz, 6H), 7.26 (s,
2H), 6.83 (d, J = 9.2 Hz, 2H), 6.34 (s, 2H), 4.32 (s, 4H), 3.07 (s, 6H).
FT-IR (KBr, cm−1): 3433 (s, O−H), 2970 (m, C−H), 1682, 1628 (m,
CO), 1602 (m, CC), 1578 (m, N−H), 1523 (s, NN), 1369,
1132 (s, C−N), 1283, 1182 (s, C−F), 828, 792, 757 (m, Ph−H). Anal.
Calcd for LaC49H39F9N7O7: C, 51.27; H, 3.42; N, 8.54. Found: C,
51.23; H, 3.45; N, 8.52.
[Eu(tfd)2(dmPBPM)(CF3CO2)] (V). Dark red solid (0.30 g, yield
85%); mp 248−249 °C; ESI(+)-MS (m/z, methanol) 1048.1 [M+];
1H NMR (400 MHz, DMSO-d6) δ ppm = 9.75 (s, 1H), 8.47 (d, J = 4.2
Hz, 2H), 7.80−7.74 (m, 10H), 7.25 (s, 9H), 6.82 (d, J = 9.0 Hz, 2H),
6.59(s, 3H), 4.40 (s, 2H), 4.30 (s, 4H), 3.07 (s, 6H). FT-IR (KBr,
cm−1): 3423 (s, O−H), 2973 (m, C−H), 1686, 1629 (m, CO),
1602 (m, CC), 1576 (m, N−H), 1538 (s, NN), 1369, 1132 (s,
C−N), 1286, 1185 (s, C−F), 825, 792, 762 (m, Ph−H). Anal. Calcd
for EuC49H39F9N7O7: C, 50.70; H, 3.39; N, 8.45. Found: C, 50.61; H,
3.33; N, 8.39.
[Gd(tfd)2(dmPBPM)(CF3CO2)] (VI). Red solid (0.26 g, yield 74%);
mp 268−269 °C; ESI(+)-MS (m/z, methanol) 1053.1 [M+]. FT-IR
(KBr, cm−1): 3442 (s, O−H), 2917 (m, C−H), 1687, 1629 (m, C
O), 1602 (m, CC), 1576 (m, N−H), 1539 (s, NN), 1369, 1133
(s, C−N), 1286, 1185 (s, C−F), 858, 825, 792, 762 (m, Ph−-H). Anal.
Calcd for GdC49H39F9N7O7: C, 50.46; H, 3.37; N, 8.41. Found: C,
50.33; H, 3.35; N, 8.49.
Eu(tfd)2(PBPM)(CF3CO2) (II). Orange solid (0.26 g, yield 87%); mp
1
198−200 °C; ESI(+)-MS (m/z, methanol) 1005.2 [M+]; H NMR
(400 MHz, DMSO-d6) δ ppm = 9.83 (s, 1H), 8.48 (s, 2H), 7.94−7.72
(m, 10H), 7.60 (s, 3H), 7.30−6.97 (m, 9H), 6.58 (s, 3H), 4.46−4.15
(m, 6H). FT-IR (KBr, cm−1): 3444 (s, O−H), 1687 (m, CO), 1629
D
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PMMA Thin Film Preparation. PMMA (100.0 mg) was dissolved
in dichloromethane (10.0 mL) with mild sonication for 10 min. The
compound (10.0 mg) was added to this solution, which was sonicated
mildly for another 15 min. The polymer thin film with similar
thickness was coated on a 12 mm × 45 mm quartz plate substrate.
Measurements of Photoisomerization Quantum Yields and
Isomerization Rate Constants. UV light (365 nm) was acquired
from a ZF-2 UV analyzer (Shanghai Yihui). Visible light (450 nm) was
obtained from a 300 W Xe lamp (PLS-SXE300CUV) and isolated
using a sharp cutoff filter. A 1 cm path length quartz cell was used for
the photoisomerization measurements. The sample concentration was
approximately 2.0 × 10−5 M for all complexes. The light intensity was
measured by a UV-A radiation meter (Optical electrical apparatus
factory of Beijing Normal University). The photoisomerization
quantum yield was evaluated based on eq a18,30
Gd(tfd)2(dmPBPM)(CF3CO2) (VI) were obtained by slow
evaporation from their DMF and ethanol mixed solutions after
approximately 1 month. Unit cell determinations and further
analyses revealed that all four complexes crystallized in the
monoclinic P21/c space group with very similar lattice
parameters. The molecular geometry of the crystal structures
was determined by single-crystal X-ray crystallography. The
crystallographic data are provided in Table 1, and selected bond
lengths and angles of the coordination environment of the
metal centers are listed in Tables S1 and S2 in the Supporting
Information. These four complexes were isomorphic, forming a
monocapped highly twisted square antiprism. The PBPM (or
dmPBPM) ligand chelates with the lanthanide in a tetradentate
fashion through one oxygen and three nitrogens from the
bis(pyridin-2-ylmethyl)benzohydrazide group. Two pairs of
oxygens from the 4,4,4-trifluoro-1-phenylbutane-1,3-dionate
(tfd) ligands chelate with the lanthanide ion in a normal way.
The coordination sphere is completed by a trifluoroacetate
(CF3COO−) to give a nine-coordinate lanthanide center with a
{LnN3O6} coordination environment, corresponding to a
formula of [Ln(L)(tfd)2(CF3CO2)] (L = PBPM or dmPBPM).
In the structures of complex I, II, and VI, the distorted
monocapped square antiprism has the N(6), O(1), O(6), and
O(3) four atoms forming one face, the N(4), O(5), O(2), and
N(5) four atoms forming the opposite face, and the atom O(4)
occupies the capped position, while in the structure of complex
IV, because the trifluoroacetate anion was close to the pyridine
group with the N(6) atom, the distorted monocapped square
antiprism has the O(6), N(6), O(5), and O(3) four atoms
forming one face, the O(1), N(4), N(5), and O(2) four atoms
forming the opposite face, and the atom O(4) occupies the
capped position. The Ln−O bond lengths are in the range of
2.460−2.508 Å for complex I, 2.331−2.394 Å for complex II,
2.467−2.523 Å for complex IV, and 2.361−2.420 Å for complex
VI. The Ln−N bond lengths are in the range of 2.734−2.861 Å
for complex I, 2.614−2.748 Å for complex II, 2.732−2.867 Å
for complex IV, and 2.633−2.767 Å for complex VI, which are
in agreement with the typical values for some lanthanide
complexes previously reported.22,33 Complexes I and IV have
similar ranges of La−O and La−N bonds distances, presumably
because the individual La−O and La−N bond distances are
very similar. The ligands in both La(III) complexes (I and IV)
are virtually “identical” except for a long-range p-dimethylamino
substituent in the dmPBPM ligand; however, the p-
dimethylamino substituent results in a shortened bond length
of the azo group in complex IV. The lengths of NN bonds in
complexes I, II, IV, and VI are 1.201(8), 1.163(8), 1.179(7),
and 1.186(13) Å, respectively, which are all slightly shorter than
the corresponding distances observed for pure azobenzene and
its derivatives.34 Taking complex I as an example, the
perspective view of the molecular structure, unit cell packing
diagram, and its coordination polyhedron geometry of central
La(III) are shown in Figure 1; the others are displayed in
Figures S1−S3.
1
1
Φ = k
0 I
0 1 − 10−εcl
(a)
where k0 is a zero-order rate constant for the decrease of the initial
isomer concentration in mol L−1 s−1, I0 is the intensity of incident
irradiation light in einstein L−1 s−1, ε is the extinction coefficient in L
mol−1 cm−1 at the irradiation wavelength of the solution, c is the
concentration of the solution in mol L−1, and l is the path length of
light through the sample in cm. The trans-to-cis photoisomerization or
cis-to-trans thermal isomerization obeyed first-order kinetics, so the
isomerization rate constant, kiso, was calculated from the change in
absorbance at a certain wavelength with time by eq b19,31
A∞ − A0
A∞ − At
ln
= kisot
(b)
where At, A0, and A∞ denote the absorbance at time t, time zero, and at
the end of the reaction, respectively. The data of the quantum yields
and the rate constants for all compounds were the average of the three
independent replicates performed for each concentration.
The thermal recoverable experiment was performed by a solution of
the ligand or the complex in a 1 cm quartz cuvette irradiated with a
365 nm UV lamp for 30 min, and then the cuvette was placed in the
thermostated cell of a spectrophotometer, and the spectrum was
measured as a function of time or as a function of temperature. The
temperature error was controlled as 1 °C.
The uncertainty of the quantum yields and the rate constants for all
compounds were calculated according to three independent replicates
performed for each concentration.
Structure Determination of Complexes I, II, IV, and VI by X-
ray Diffraction. Diffraction intensity data for single crystals of the
complexes were collected on a Bruker SMART Apex CCD
diffractometer equipped with graphite-monochromatic Mo Kα (λ =
0.71073 Å) radiation using the ω scan mode at 173(2) K. The crystal
data integration and reduction steps were performed using the SAINT
software. Lorentz-polarization and empirical absorption corrections
were applied to the data. Structures were solved by direct methods and
refined by full-matrix least-squares calculations based on F2 using the
SHELXTL-2014 software package.32All non-hydrogen atoms were
refined anisotropically. Treatment of hydrogen atoms in the least-
squares refinement resulted in different data, where some of the
hydrogen atoms were constrained by locating them at the calculated
positions and the others were independent. Crystallographic details for
complexes I, II, IV, and VI are summarized in Table 1. Data for the
crystal structures of complexes I, II, IV, and VI have been deposited at
the Cambridge Crystallographic Data Centre (CCDC 1437962,
1437960, 1437963, and 1437961 for complexes I, II, IV, and VI,
respectively); these data are obtainable free of charge via the Internet
at www.ccdc.cam.ac.uk/conts/retrieving.
Thermal Analysis of the Complexes. To provide more
insight into the structure of the complexes, the thermal stability
of complexes I−VI was investigated under a nitrogen
atmosphere by thermogravimetric analysis (TGA) in the
temperature range of 30−800 °C on the powder samples. As
shown in Figure 2, the TG plots provide evidence that
complexes I−III exhibit a very similar thermal decomposition
process and complexes IV−VI also have the same trend. All
complexes underwent no decomposition below 240 °C. The
RESULTS AND DISCUSSION
■
X-ray Crystallographic Analysis. Suitable crystals of
complexes La(tfd)2(PBPM)(CF3CO2) (I), Eu(tfd)2(PBPM)-
(CF3CO2) (II), La(tfd)2(dmPBPM)(CF3CO2) (IV), and
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dmPBPM ligands are strongly coordinating tetradentate
ligands.
UV−Vis Absorption Spectra. The UV−vis absorption
spectra of free ligands Htfd, PBPM, and dmPBPM and their
complexes I−VI in ethanol and acetonitrile at room temper-
ature are displayed in Figure 3, and the spectral data are listed
Figure 1. Crystal structure of complex [La(tfd)2(PBPM)(CF3CO2)]
(I) (all hydrogen atoms are omitted for clarity) (a), coordination
polyhedron geometry of the central La(III) (b), and the packing
diagram in the unit cell (c).
Figure 3. UV−vis absorption spectra of PBPM, complexes [Ln-
(tfd)2(PBPM)(CF3CO2)] (I for La, II for Eu, III for Gd) (a, b) and
dmPBPM, complexes [Ln(tfd)2(dmPBPM)(CF3CO2)] (IV for La, V
for Eu, VI for Gd) (c, d) (2.0 × 10−5 mol/L) in different solvents.
in Table S3. As shown in Figure 3, the UV−vis absorption
spectrum of the ligand PBPM exhibits two typical absorption
bands of the azo group in the region of 250−500 nm, a strong
π−π* transition band (325 nm/3.07 × 104 L mol−1 cm−1), and
a weak absorption band (440 nm/900 L mol−1 cm−1) from the
forbidden symmetry of the n−π* transition. However, owing to
the contribution of the electronic-donating substitutent, the
N,N-dimethylamino group, dmPBPM exhibits only strong
intensity π−π* (268 nm/1.80 × 104, 428 nm/3.31 × 104 L
mol−1 cm−1) absorption bands. The broad band around 350−
550 nm of dmPBPM may be attributed to the red shift of the
π−π* band arising from the strong electron-donating group,
leading to the weak n−π* transition absorption band buried in
its π−π* band.1,22
Figure 2. Thermogravimetric analysis (TGA) curves of complexes
[Ln(tfd)2(PBPM)(CF3CO2)] (I for La, II for Eu, III for Gd) and
[Ln(tfd)2(dmPBPM)(CF3CO2)] (IV for La, V for Eu, VI for Gd).
The spectra of complexes I−VI show the iteration of the free
ligands tfd and PBPM (or dmPBPM) absorption spectra,
accompanied by a small red shift at the maximum wavelength.
Likewise, the spectra of complexes I−VI show a strong azo
group π−π* transition absorption close to 328 nm and the
symmetry forbidden n−π* transition within the range of 400−
500 nm. Furthermore, the molar absorption coefficients of
complexes I−VI around 328 nm are almost doubled compared
with those of the free ligands PBPM or dmPBPM (shown in
Table S3), perhaps owing to the simple iteration of the two
localized chromophores from the free ligands tfd and PBPM
(or dmPBPM), and also the ligands absorption being perturbed
by lanthanide ion coordination. Finally, no significant effect of
the solvent polarity on the absorption spectra of complexes I−
VI was observed.
first decomposition stage occurred in the temperature range of
242−368 °C with a mass loss of 50.35%, 49.21%, and 48.93%
for complexes I, II, and III, respectively. The percentages are
consistent with material decomposition and elimination of one
trifluoroacetate anion and two tfd dionates from the complexes.
The theoretically calculated losses are 49.36%, 48.74%, and
48.56% for complexes I, II, and III, respectively. Upon further
heating, the complexes started to decompose up to 800 °C.
Similarly, the TGA curves showed the first stage mass losses
48.68%, 47.98%, and 47.19% for complexes IV, V, and VI from
244 to 363 °C, respectively, indicating the relative mass loss of
one trifluoroacetate anion and two tfd dionates from the
complexes. These data are consistent with the theoretically
calculated losses of 47.51%, 46.94%, and 46.76% for complexes
IV, V, and VI, respectively. From the thermal analysis it could
be inferred that the trifluoroacetate anion and tfd dionate are
more easily removed from the complexes than the PBPM or
dmPBPM ligands, indicating that the designed PBPM and
Photoisomerization Behavior of PBPM and dmPBPM
Ligands. Figure 4 shows the absorption spectral changes of
PBPM and dmPBPM upon UV irradiation at 365 nm and
visible-light irradiation at 450 nm as a function of time in an
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acetonitrile are 24.5 and 37.5, respectively. Therefore,
acetonitrile can stabilize the cis form to some extent. It should
be mentioned that even though observed in acetonitrile, the
photoisomerization of dmPBPM was still much less than that of
PBPM. The electron-donating group reduces the π−π* energy
level of the azo group; therefore, the energy difference between
the π−π* level and the n−π* level decreased, resulting in the
fact that the spectrum showed different changes in different
solvents as far as the absorption intensity was concerned. It
appears that the para electron-donating substituent N,N-
dimethylamino group produced substantial changes on the
absorption and was unfavorable to the photoisomerization
properties of azobenzene in the molecules.
Photoisomerization Behavior of Complexes I−VI.
Complexes I−III ([Ln(tfd)2(PBPM)(CF3CO2)]) constructed
from a PBPM ligand showed similar spectral changes upon
irradiation in different solvents. Taking complex II ([Eu-
(tfd)2(PBPM)(CF3CO2)]) as an example, Figure 5 shows the
Figure 4. UV−vis spectral changes of PBPM and dmPBPM in ethanol
solution (2.0 × 10−5 mol/L) upon irradiation at 365 nm and
recoverable irradiation at 450 nm as a function of time.
ethanol solution. The similar absorption spectral changes in
acetonitrile solution are shown in the Supporting Information
(Figure S4). For PBPM, the intensity of the π−π* absorption
band at 325 nm gradually decreased with increasing irradiation
time, while the absorbance at 440 nm (n−π* transitions band)
increased. A photostationary state was reached after 10 min,
which indicates completion of isomerization from trans to cis.
Subsequently, irradiation of the solution with visible light (450
nm) led to 80% recovery of the absorption spectrum. This
reversible spectral change was observed repeatedly (Figure S5).
Ten cycles of maximum absorption intensity at 325 and 440 nm
were modulated by the irradiation of UV light at 365 nm and
visible light at 450 nm, alternatively, which exhibited no
variation. The first-order rate constants of the photoisomeriza-
tion reaction of PBPM in solutions were 3.0 × 10−3 s−1 in
ethanol and 2.0 × 10−3 s−1 in acetonitrile, which are much
higher than those of azobenzene and its derivatives (10−5−10−4
s−1).18,19 An estimate of the photoreaction quantum yields
(Φt→c) in different solvents was evaluated based on eq a (see
Experimental Section). The photoisomerization quantum yields
(Φt→c) of PBPM were 0.045 in ethanol and 0.037 in
acetonitrile, which are smaller than those of a substituted
azobenzene, such as 4-methoxy-4′-nitroazobenzene or 2,4,6-
trimethylazobenzene.35,36 Data are listed in Table 2. Evidently
the spectra of dmPBPM in an ethanol solution underwent no
significant change upon irradiation with UV or visible light with
increasing time. However, in an acetonitrile solution, the
spectra of dmPBPM showed some change under the same
conditions (Figure S4). The dielectric constants of ethanol and
Figure 5. UV−vis spectral change of complex II ([Eu(tfd)2(PBPM)-
(CF3CO2)]) in ethanol solution (2.0 × 10−5 mol/L) upon irradiation
at 365 nm and recoverable irradiation at 450 nm as a function of time.
typical spectral change of complex II in an ethanol solution
upon irradiation with 365 nm and its recoverable spectra after
irradiation with 450 nm. The absorption maximum of complex
II in ethanol solution appeared at 328 nm, characteristic of the
π−π * absorption band, with a very weak n−π* absorption
band at 448 nm. Upon UV 365 nm light irradiation, a band at
448 nm was developed at the expense of the corresponding
original band at 328 nm with two isosbestic points located at
287 and 386 nm. This spectral change indicates the occurrence
of the photoisomerization in solution. The complex reached a
photostationary state within 10 min with irradiation at 365 nm.
Table 2. Quantum Yields (Φt−c) and Photoisomerization Rate Constants (s−1) of PBPM, dmPBPM, and Complexes I−VI in
Different Solvents
ethanol
acetonitrile
ethanol
acetonitrile
104Φt→c
compounds
102Φt→c
104kiso
102Φt→c
104kiso
compounds
104Φt→c
104kiso
104kiso
PBPM
4.5 0.4
3.9 0.4
3.6 0.3
3.8 0.3
30.0 1.5
40.1 1.9
50.0 2.1
30.2 1.3
3.7 0.3
1.1 0.1
2.5 0.2
2.8 0.2
20.1 1.1
13.0 0.6
26.3 1.2
20.5 1.2
dmPBPM
a
a
a
a
0.74 0.1
a
a
I
IV
V
a
II
III
0.67 0.1
1.80 0.3
0.49 0.02
1.11 0.05
1.90 0.4
0.83 0.2
0.99 0.4
VI
a
a
No significant photoisomerization behavior was observed.
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When the UV irradiation was ceased, 75% of the compound
returned to the trans form with visible 450 nm irradiation for 2
s. The recovery percentage of the trans form was a little lower
than that of the pure PBPM ligand, because the photochemical
pathway was not simple in such a complex, where the visible
absorption would be a mixture of other bands, not just the azo
n−π* band. Similar spectral changes for complex II in an
acetonitrile solution are shown in Figure S6.
The photoisomerization reaction of complex II in solutions
followed first-order kinetics with rate constants of 5.0 × 10−3
s−1 in ethanol and 2.6 × 10−3 s−1 in acetonitrile and quantum
yields of 0.039 in ethanol and 0.011 in acetonitrile (Table 2),
which are similar to that of pure PBPM. Compared with other
azobenzene-containing metal complexes, such lanthanide-
coordinated complexes show active photoisomerization. The
trans-to-cis photoisomerization quantum yields are comparable
with their pure ligands. It is obvious that the trans-to-cis
photoisomerization rates of complexes I−III (10−2 s−1) are
comparable to or even higher than those of the reported Ru(II)
or Rh(III) bis(terpyridine) metal complexes (10−5−10−3 s−1),19
as well as azobenene itself (10−5−10−4 s−1).18,36
electron-donating dimethylamino group. Although the absorp-
tion maximum at 426 nm could decrease upon UV 365 nm
irradiation and be recovered by 450 nm irradiation (Figure
S11), the absorption intensity at 326 nm, which can be assigned
to the main absorption of the tfd ligand, decreased upon
irradiation with UV 365 nm light or visible 450 nm light. This
spectral change indicated the occurrence of the photobleaching
of complex IV in solution, where the tfd ligand was removed
from the molecule. There are two reasons to interpret the
instability of complex IV in the solution state under UV
irradiation. First, La(III) is the most kinetically labile among the
lanthanide ions.37 Its complexes are not as stable as the
complexes of other lanthanide ions. Second, the dmPBPM is
larger in size than PBPM due to the electron-donating
dimethylamino group.
Compared with the easy-to-observe photoisomerization
properties of PBPM, the quantum yield and photoisomerization
rate constants of dmPBPM were too low to be determined. It
was interesting that complexes V and VI containing dmPBPM
showed a more sensitive photoisomerization than dmPBPM
itself. This indicates that the mechanism of the isomerization is
mediated through the ligand excited states. The coordination of
a lanthanide ion presumably increased the spin−orbit coupling
which allowed isomerization to occur. As shown in Figures S12
and S13, upon UV 365 nm light irradiation in different solvents,
complexes V and VI exhibited a similar trend to that of
dmPBPM. With the photoisomerization, the intensities of the
bands at 325 and 426 nm decreased, accompanied by a small
red shift of the band around 426 nm. When irradiating the
sample using 450 nm light, the absorption intensities around
325 and 426 nm increased. The quantum yields of isomer-
ization of complexes V and VI are much smaller than those of
PBPM and complexes I−III owing to the para electron-
donating substituent, the dimethylamino group in dmPBPM
ligand (Table 2).
To test the stability of the trans−cis−trans photoisomeriza-
tion, 10 cycles of complex II switching in different solvent were
conducted by irradiating the sample with 365 nm light for 10
min, followed by 450 nm light for 2 min. Complex II was stable
under these conditions (Figures 6 and S7), and no signs of
Photoisomerization Behavior of PBPM and Com-
plexes I−III in PMMA Film. PBPM and its complexes I−III
presented good reversible and repeatable photoisomerization
reactions in solutions. For the purpose of implementing such
photoresponsive substances to practical applications, their
reversible photoisomerization in the solid state was also
studied. The measurements of the UV−vis absorption spectra
of PBPM and complexes I−III doped in PMMA film were
performed. The PBPM isomerization behavior in PMMA film
showed a similar trend to that in solution (Figure S14). Upon
irradiation with 365 nm UV light, the intensity of its π−π*
transition absorption band at 328 nm decreased while its n−π*
transition absorption band at 440 nm gradually increased. A
photostationary state was reached after 10 min. Subsequently,
irradiating the solution with visible light at 450 nm for only 5 s
led to 80% recovery of the absorption spectra, indicating that its
cis form was unstable in PMMA film. The cis form could easily
and quickly transfer to its trans form. This reversible spectral
changes were also observed repeatedly (Figure S16a). The first-
order rate constant of the photoisomerization reaction of
PBPM in PMMA was 3.1 × 10−4 s−1, very close to that
obtained in solution.
Figure 6. UV−vis spectral changes of complex II ([Eu(tfd)2(PBPM)-
(CF3CO2)]) in ethanol solution (2.0 × 10−5 mol/L) by irradiation of
UV light at 365 nm and visible light at 450 nm. (Inset) Ten cycles of
the maximum absorption intensity at 328 and 448 nm alternatively
modulated by irradiation of UV light at 365 nm and visible light at 450
nm.
photodegradation were detected. The UV−vis absorption
spectra changes of complexes I and III in ethanol and
acetonitrile solutions were similar to those of complex II
(Figures S8−S10 in the Supporting Information). Complexes
I−III showed an overall reversible isomerization reaction, and
more than 10 cycles could be repeated, demonstrating their
excellent fatigue resistance and reversibility. The good
reversibility of trans−cis−trans photoisomerization of these
complexes in solutions denied the possibility of photo-
degradation.
Different from complex I ([La(tfd)2(PBPM)(CF3CO2)]),
complex IV ([La(tfd)2(dmPBPM)(CF3CO2)]) was unstable
under UV light irradiation both in ethanol and in acetonitrile
solutions, simply owing to the dmPBPM ligand that bears the
The UV−vis spectral changes of complex I in a PMMA film
upon irradiation at 365 nm and its recoverable spectra upon
irradiation at 450 nm are shown in Figure S15. The π−π*
absorption band of complex I in a PMMA film also appeared at
329 nm, and a weak n−π* absorption band was observed at 450
nm, with two isosbestic points located at 287 and 386 nm,
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respectively. Upon 365 nm UV light irradiation, the maximum
absorption intensity at 450 nm increased accompanied by a
decrease of the intensity at 329 nm. A photostationary state was
reached in just 10 min with irradiation at 365 nm. The first-
order reaction rate constant of trans to cis for complex I in
PMMA film was 7.4 × 10−3 s−1, which is higher than those in
solution and for PBPM itself. When the UV irradiation was
ceased, 75% of the system returned to the trans form with 450
nm irradiation for 30 s. The recovery of the trans form by
visible light demonstrates that the photochemical pathway of
complex I is similar to those in solution and for PBPM itself.
Moreover, complex I was stable in PMMA without signs of
photodegradation when undergoing 10 cycles of alternating
irradiation of UV light at 365 nm and visible light at 450 nm
(Figure S16b). Complex III showed similar isomerization
properties as those of complex I (Figure S17). The first-order
reaction rate constant for trans-to-cis isomerization of complex
III in PMMA film was 5.0 × 10−3 s−1, which was also higher
than those in solution and for PBPM itself. In the same
condition, complex II showed a poor photoisomerization
reaction owing to its insolubility in PMMA.
Thermal Cis-to-Trans Isomerization of PBPM and
Complexes I−III. PBPM and its complexes I−III were
sensitive to light-triggered azobenzene trans-to-cis isomer-
ization with 365 nm light. The isomerization could be reversed
by irradiation with visible light at 450 nm, which also occurred
spontaneously in the dark, because of the thermodynamic
stability of the trans isomer.1 Cis-to-trans thermal isomerization
can range from an order of hours to days for azobenzenes and
from seconds to milliseconds for pseudostilbenes.38,39 To
examine the cis-to-trans thermal isomerization of the ligands
and complexes, first, the solutions of PBPM and complexes I−
III in ethanol or acetonitrile were irradiated for 30 min at 365
nm light and then kept in the dark at room temperature; they
began to reverse to the trans isomer after standing in the dark
for 16 h and longer, which was too slow to find the first-order
rate constants and half-life. The reverse process of cis-to-trans
isomerization was actively monitored by UV−vis absorption
spectroscopy at 50 °C. The calculated first-order rate constants
(k) and half-life time (τ1/2) for this process are presented in
Table 3. A representative example is shown in Figure 7, and the
Figure 7. UV−vis spectral changes before and after 30 min irradiation
with 365 nm, and thermal cis-to-trans isomerization of complex I
([La(tfd)2(PBPM)(CF3CO2)]) in ethanol solution (2.0 × 10−5 mol/
L) at 50 °C.
isomerization of the Ln(III) complex was higher. For instance,
at 70 °C for 15 min, PBPM was able to achieve 70% cis-to-trans
reversal while the La(III), Gd(III), and Eu(III) complexes
accomplished 85%, 95%, and 100% reversal, respectively. At 78
°C for 15 min, the refluxing temperature of ethanol, all ligands
and complexes can achieve 100% recovery. In the solid state,
the PMMA film of PBPM ligand and its Ln(III) complexes
were able to recover 100% at 80 °C.
As far as dmPBPM and complexes IV−VI are concerned,
since the cis form was difficult to be obtained under the same
conditions, the thermal stability of the cis forms of dmPBPM
and complexes IV−VI was not investigated.
Photoluminescent Properties of Complexes II and V.
It is well established that some Eu(III) complexes involving β-
diketonates would yield bright red emission under near-
ultraviolet or visible-light irradiation owing to the efficient
energy transfer from the β-diketonate ligand to the central
Eu(III) ions (antenna effect).42−44 Their unique optical
properties, such as narrow emission bands, high luminescence
efficiency, and long excited-state lifetimes, have resulted in
them being of interest for a wide range of photonic applications
including light-emitting diodes, a laser system, and luminescent
probes for bioanalyses.27,33,45−52 We therefore investigated the
luminescent properties of europium complexes II and V in
ethanol and acetonitrile solutions at room temperature. The
photoluminescence spectra of complexes II and V in different
solvents are shown in Figure 8. Upon excitation at 330 nm,
complexes II and V showed the characteristic emission bands of
Table 3. Thermal Isomerization Rate Constants (kiso, s−1)
and Half-Life (τ1/2, min) of PBPM and Complexes I−III in
Different Solvents at 50 °C
ethanol
acetonitrile
104kiso
τ1/2
104kiso
τ1/2
5
7
compounds
an Eu(III) ion originating from the D0 → FJ (J = 0−4)
PBPM
2.2 0.11
2.4 0.12
3.0 0.15
1.8 0.08
52.5 2.6
47.8 2.3
37.8 1.9
64.5 3.4
0.65 0.03
1.9 0.08
2.7 0.14
1.6 0.07
105.6 4.8
62.4 2.5
41.9 2.1
72.9 3.1
transitions. Among them, the induced electric dipole transition
5
7
I
of D0 → F2 at 614 nm was the strongest. The intensity of
II
III
emission bands at 578, 590, and 650 nm corresponding to the
7
5D0 → F0,1,3 transitions were relatively weak owing to the
magnetic and electric dipole-forbidden schemes. The overall
luminescence quantum yields (Φo) were determined exper-
imentally under excitation of the ligand in acetonitrile and
ethanol solutions at room temperature. The Φo values of
complexes II and V in ethanol were 0.049 and 0.028,
respectively, which were slightly lower than those obtained in
acetonitrile solvent (0.074 for II and 0.034 for V). By
comparing the absorption spectra of complexes II and V with
their excitation spectra, it was observed that the excitation and
absorbance spectra of complexes II and V overlapped well with
the tfd dionate ligand, indicating that Eu(III) emission was
others are organized in Figure S18. At 50 °C, the first-order rate
constants for the cis-to-trans thermal isomerization of PBPM
and complexes I−III in ethanol and acetonitrile solutions are
larger than those of other azobenzene-containing metal
complexes and smaller than those of organic pull−push
azobenzene.24,40,41
The behavior of the thermal cis-to-trans isomerization of
both ligand PBPM and complexes I−III as a function of
temperature was also examined in ethanol (Figure S19). In
contrast to the PBPM, it was found that the rate of the thermal
I
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Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Figure 8. Emission spectra of complexes II ([Eu(tfd)2(PBPM)-
(CF3CO2)]) (λex = 355 nm) and V ([Eu(tfd)2(dmPBPM)-
(CF3CO2)]) (λex = 370 nm) in different solvents (2.0 × 10−5 mol/L).
○
Figure 9. χMT ( ) and 1/χM (Δ) versus T plots for complexes III
([Gd(tfd)2(PBPM)(CF3CO2)]) and VI ([Gd(tfd)2(dmPBPM)-
(CF3CO2)]) measured at 1000 Oe.
mainly sensitized from the tfd dionate ligand. The ancillary
ligands of PBPM and dmPBPM did not participate in the
energy transfer to the lanthanide ion, which might be attributed
to their lower triplet state energy levels. To gain insight into the
nature of the electronic transitions of the systems, TD-DFT
was carried out for geometry optimization and the excitation
energies of the lowest excited triplet states of tfd, PBPM, and
dmPBM were calculated to be 2.4817, 1.7846, and 1.8328 eV,
respectively. Obviously, the excitation energies of the lowest
excited triplet states of both PBPM and dmPBPM are lower
than the resonance level of the Eu3+ ion (5D0, 2.1434 eV),
which agrees well with the experimental results. Hence, the
experimental results prove the theoretical prediction, where
only the excitation energy of tfd could be transferred to the rare
earth ion according to the triplet state of the ligand being
higher than the resonance level of the rare earth ion.53
Furthermore, the luminescence intensity did not change upon
UV−vis light irradiation (Figure S20). As a result, the present
findings demonstrate that complex II will be a good dual-
function smart material for biological system applications and
light conversion molecular devices.
Curie−Weiss equation, and the values of C (cm3 K mol−1) and
θ (K), obtained from the best fitting, were, respectively, 8.84
and −0.76 for complex III and 9.66 and −1.24 for complex VI,
indicating that both complexes were antiferromagnetic.63
SUMMARY
■
A series of novel multinary β-diketonate lanthanide complexes
(La(III), Eu(III), Gd(III)) of PBPM and dmPBPM function-
alized with azobenzene were synthesized and structurally
1
characterized by FT-IR, HNMR, TGA, ESI-MS, and single-
crystal X-ray diffraction crystallography. The X-ray crystal
structures indicated that the Ln(III) ion was nine coordinate by
one oxygen atom and three nitrogen atoms from the PBPM
ligand (or dmPBPM), four oxygen atoms from the two β-
diketonate ligands, and the one oxygen atom from trifluor-
oacetate, forming a monocapped twisted square antiprism. The
thorough trans-to-cis photoisomerization study of the com-
plexes revealed that all complexes were able to undergo
reversible trans-to-cis photoisomerization in different organic
solvents as well as in solid PMMA film. The [Ln(tfd)2(PBPM)-
(CF3CO2)] complexes bearing azobenzene groups without a
donor substituent showed a more rapid reversible trans-to-cis
photoisomerization compared with that of the [Ln-
(tfd)2(dmPBPM)(CF3CO2)] complexes. Remarkably, a lumi-
nescence investigation demonstrated that the ligands exhibited
a good antennae effect with respect to the Eu(III) ion owing to
the efficient intersystem crossing and ligand-to-metal energy
transfer mechanism, which was confirmed by the TD-DFT
calculation. Interestingly, the magnetic properties of the
Gd(III) complexes were antiferromagnetic. The present results
signified the photoisomerization and photoluminescence dual
functions of Eu(III) multinary complexes, which would be
anticipated for potential applications as smart materials for
biological systems or as a light conversion molecular device.
Magnetic Properties Studies of Complexes III and VI.
Since the Gd(III) ion has seven unpaired f electrons with a high
magnetic moment and long electronic relaxation time, it has
been intensively investigated for magnetic refrigeration and
magnetic resonance imaging materials.54−59 The magnetic
properties of the synthesized [Gd(tfd)2(PBPM)(CF3CO2)]
(III) and [Gd(tfd)2(dmPBPM)(CF3CO2)] (VI) complexes
have also been studied. The variable-temperature molar
magnetic susceptibilities χM of complexes III and VI were
measured under an applied field of 1000 Oe and in the range of
2−300 K. The χMT versus T plots are shown in Figure 9. The
room-temperature χMT values of complexes III and VI were
found to be 8.85 and 9.65 cm3 K mol−1, respectively, which are
a little higher than the theoretical values (7.94) owing to the
orbital magnetic moment contribution to the central ion.60
Since the shortest distance of the nearest Gd(III) ions between
the adjacent molecules was 9.695 Å, it can be assumed that no
direct magnetic interaction existed with a high possibility of a
weak dipole−dipole role among Gd(III) complex molecules.61
The χMT values for the two complexes hold above 100 K and
then slowly decreased down to 12K because of the single-ion
crystal field effects.62 The χMT values decreased quickly in the
range of 12K−2K owing to the zero-field splitting of Gd(III)
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.6b02819.
■
S
NMR spectra for ligands and complexes and IR spectra
for all complexes (PDF)
X-ray crystallographic files for complexes I, II, IV, and VI
(CIF)
−1
ions. The χMT data were fitted (χM versus T) using the
J
DOI: 10.1021/acs.inorgchem.6b02819
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
(15) Yamamura, M.; Yamakawa, K.; Okazaki, Y.; Nabeshima, T.
Coordination-driven Macrocyclization for Locking of Photo- and
Thermal Cis-Yrans Isomerization of Azobenzene. Chem. - Eur. J. 2014,
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AUTHOR INFORMATION
■
Corresponding Authors
*E-mail: linlr@xmu.edu.cn.
*E-mail: mal@uhcl.edu.
ORCID
Li-Rong Lin: 0000-0002-2521-3809
Author Contributions
‡H.-H.T. and Y.-G.W. contributed equally to this work.
Notes
The authors declare no competing financial interest.
(18) Sierocki, P.; Maas, H.; Dragut, P.; Richardt, G.; Vogtle, F.; De
Cola, L.; Brouwer, F. A.; Zink, J. I. Photoisomerization of Azobenzene
Derivatives in Nanostructured Silica. J. Phys. Chem. B 2006, 110,
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(19) Yutaka, T.; Mori, I.; Kurihara, M.; Mizutani, J.; Kubo, K.;
Furusho, S.; Matsumura, K.; Tamai, N.; Nishihara, H. Synthesis,
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ACKNOWLEDGMENTS
■
This work is supported by the National Natural Science
Foundation of China (Grant No. 21271150) and the Natural
Science Foundation of Fujian province (Grant No.
2015J01060).
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DOI: 10.1021/acs.inorgchem.6b02819
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