Novel bright blue emissions IIB group complexes constructed with various polyhedron-induced 2-[2′-(6-methoxy-pyridyl)]-benzimidazole derivatives

Article abstract of DOI:10.1039/c4ce00382a

Five IIB group complexes, [ZnL¹Cl₂] (1), {[CdL ¹Cl₂]}₂ (2), [Cd(L¹) ₂(NO₃)₂]·H₂O (3), [HgL ¹Cl₂] (4), and [ZnL²Cl₂] (5) [L ¹ = 1-[2-(6-methoxy-2-pyridylmethyl)]-2-[2-(6-methoxy-pyridyl)] benzimidazole; L² = 2-[2-(6-methoxypyridyl)]benzimidazole] have been synthesized and structurally characterized by elemental analysis, IR spectroscopy, ¹H NMR, ¹³C NMR and X-ray single-crystal analyses. The structural investigations testify that the ionic radius and counterions (Cl^- and NO₃^-) cooperatively affect the coordination mode of central metal. As small and medium radii, four-coordinated Zn²⁺ (1 and 5) and five-coordinated Cd²⁺ (2) possess tetrahedron and trigonal bipyramid geometries, respectively. Though Hg²⁺ (4) has a larger radius, its three-coordinated geometry is trigonal planar in order to eliminate repulsive forces. Further observations illustrate that Cd²⁺ (3) is bound to two ligands L¹ when NO₃^- is the counter anion, forming seven-coordinated monocapped trigonal prismatic geometry. Complexes 1-5 display bright blue luminescence with the emission maxima (λ_max) ranging from 399 to 499 nm at 298 K, depending on the N,N′-chelating ligand-centered π* → π transition. Upon cooling to 77 K, the complexes show rich structured emission profiles compared to those at 298 K. The emission lifetimes of L and 1-5 are on the microsecond scale. The emission efficiency of 1-5 is shown by quantum yields ranging from 0.23 to 0.40. Complexes 1-5 offer a good insight into the opportunities in the utilization of blue materials for application and function. the Partner Organisations 2014.

Full text of DOI:10.1039/c4ce00382a

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Novel bright blue emissions IIB group complexes
constructed with various polyhedron-induced
2-[2′-(6-methoxy-pyridyl)]-benzimidazole
derivatives†
Cite this: CrystEngComm, 2014, 16,
6114
*
*
Shuo Chen, Rui-Qing Fan, Xin-Ming Wang and Yu-Lin Yang
Five IIB group complexes, [ZnL1Cl2] (1), {[CdL1Cl2]}2 (2), [Cd(L1)2(NO3)2]·H2O (3), [HgL1Cl2] (4), and
[ZnL2Cl2] (5) [L1 1-[2-(6-methoxy-2-pyridylmethyl)]-2-[2-(6-methoxy-pyridyl)]benzimidazole; L2
=
=
2-[2-(6-methoxypyridyl)]benzimidazole] have been synthesized and structurally characterized by elemental
analysis, IR spectroscopy, 1H NMR, 13C NMR and X-ray single-crystal analyses. The structural investigations
testify that the ionic radius and counterions (Cl− and NO3−) cooperatively affect the coordination mode of
central metal. As small and medium radii, four-coordinated Zn2+ (1 and 5) and five-coordinated Cd2+ (2)
possess tetrahedron and trigonal bipyramid geometries, respectively. Though Hg2+ (4) has a larger radius, its
three-coordinated geometry is trigonal planar in order to eliminate repulsive forces. Further observations
illustrate that Cd2+ (3) is bound to two ligands L1 when NO3 is the counter anion, forming seven-
−
coordinated monocapped trigonal prismatic geometry. Complexes 1–5 display bright blue luminescence
with the emission maxima (λmax) ranging from 399 to 499 nm at 298 K, depending on the N,N′-chelating
ligand-centered π* → π transition. Upon cooling to 77 K, the complexes show rich structured emission pro-
files compared to those at 298 K. The emission lifetimes of L and 1–5 are on the microsecond scale. The
emission efficiency of 1–5 is shown by quantum yields ranging from 0.23 to 0.40. Complexes 1–5 offer a
good insight into the opportunities in the utilization of blue materials for application and function.
Received 20th February 2014,
Accepted 7th April 2014
DOI: 10.1039/c4ce00382a
www.rsc.org/crystengcomm
it is not an easy task to design blue-emitting ligands with
high quantum yield, relatively reasonable cost and a simple
Introduction
Over recent decades, rational design and synthesis of func-
tional complexes with desired structures and properties have
received increasing attention. In this regard, transition-metal
complexes based on benzimidazole derivatives have proven to
be promising owing to their potential functions as new classes
of materials.1 As luminescent materials, benzimidazole deriv-
atives have been extensively studied because of their ideal
characteristics for highly energy-efficient, flat-panel displays
and the next generation of solid-state lighting.2 In general,
the scale of the π-conjugated system for the ligand and the
electronic effect of substituents on the ligand are the domi-
nating elements for luminescence properties.3 Both the bulky
aromatic coupling system and appropriate electron-donating
or electron-withdrawing substituents on the ligand may facili-
tate the bathochromic shift of the emission color.4 Therefore,
reaction procedure. It is well-known that diarylanthracene,
di(styryl)arylene, fluorene, pyrene, benzimidazole and their
derivatives are excellent blue emitters, as reported in the
literature.5 Among the different ligand candidates, the
deprotonated benzimidazole ligand is an excellent blue emit-
ter when bonded to a central metal atom in a bridging
mode.6 Moreover, based on the inherent character of the
benzimidazole group, deprotonation for the NH group on
imidazole can be realized to attach a wide variety of substitu-
ents. Benzimidazole allows readily modifying and creating
potentially strong π–π stacking interactions to form supramo-
lecular structures.7 A larger π delocalization system is antici-
pated to enhance the luminescence properties. Besides, these
multifunctional benzimidazole derivatives are capable of
forming complexes with certain metal ions which can exhibit
uncommon antibacterial activity. Their applications in the
fields of pesticides and medicine possess high efficiency,
broad spectrum and high selectivity.8
Department of Chemistry, Harbin Institute of Technology, Harbin 150001, PR China.
E-mail: fanruiqing@hit.edu.cn, ylyang@hit.edu.cn; Fax: +86 451 86413710
† Electronic supplementary information (ESI) available. CCDC 938861, 938858,
938859, 933809, 938860 and 946739. For ESI and crystallographic data in CIF
or other electronic format see DOI: 10.1039/c4ce00382a
Motivated by this research trend and our project on the
synthesis of functional complexes, the luminescence properties
of some benzimidazole derivatives complexes are reported.
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Besides, primary exploration of the antibacterial activity is
carried out (Fig. S10, ESI†). Herein, we present the photophysical
properties of five IIB group complexes bearing the extended
π-conjugated benzimidazole derivatives 1-[2-(6-methoxy-2-
pyridylmethyl)]-2-[2-(6-methoxy-pyridyl)]-benzimidazole (L1) and
2-[2-(6-methoxy-pyridyl)]-benzimidazole (L2); both ligands feature
two neutral nitrogen donors that can chelate a metal center.
L1 has a larger conjugated π-system than L2; the functional group
(–CH2–Py–OCH3) on the NH position of the imidazole ring may
play a crucial role in the formation of the complexes and their
luminescence properties. To further enhance the rigidity of the
chelate, IIB group metal salts with different counterions are suc-
cessfully employed in the isolation of complexes 1–5. The coun-
and τ2 are defined as the luminescence lifetimes. The average
lifetime was calculated according to the following eqn (1):
12 A % 22 A2 %
1
(1)
1A % 2 A2 %
1
The luminescence quantum yields of the complexes were
measured in CH3OH at room temperature and cited relative
to a reference solution of quinine sulfate (Φ = 0.546 in
0.5 mol dm−3 H2SO4) as a standard, and they were calculated
according to the well-known eqn (2):10
terion (Cl− and NO3 ) and ionic radius are two key parameters



2

−
overall
ref
n
A
I
ref
(2)

for controlling their structural variations.9 The luminescence
behavior of L and complexes 1–5 is determined in the solid state
and solution, which display intense blue emission at 298 K and
77 K. The Commission Internationale de L'Eclairage (CIE) coor-
dinates of the photoluminescence spectra of complexes 1–5 are
calculated, which further demonstrates that the color gamut is
located in the blue region.
nref
A Iref

In eqn (2), n, A, and I denote the refractive index of solvent, the
area of the emission spectrum, and the absorbance at the excita-
tion wavelength, respectively, and φref represents the quantum
yield of the standard quinine sulfate solution. The subscript ‘ref’
denotes the reference, and the absence of a subscript implies an
unknown sample. For the determination of the quantum yield,
the excitation wavelength was chosen so that A < 0.05.
Experimental section
Materials and measurements
Synthesis
of
1-[2-(6-methoxy-2-pyridylmethyl)]-2-[2-
mixture of
(6-methoxypyridyl)]benzimidazole (L1):
a
All reagents were of analytical grade from commercial sources
and were used directly without any further purification.
1,2-Diaminobenzene and 6-methoxy-2-pyridinecarboxaldehyde
were purchased from Aldrich Co., Ltd and Rui Yi Sci. & Tec.
Co. Ltd. (Shanghai, China), respectively. Metal salts were pur-
chased from Ji Nan Henghua Sci. & Tec. Co. Ltd. (Shandong,
China). Solvents for reaction and photophysical studies were
dried and freshly distilled under dry nitrogen gas before use.
1H NMR (400 MHz) and 13C NMR (100 MHz) spectra were
recorded on a Bruker Avance-400 spectrometer using Si(CH3)4
as an internal standard at room temperature. Elemental analy-
ses for C, H, and N were performed on a Perkin-Elmer 2400
automatic analyzer. Fourier transform (FT)-IR spectra were
measured on a Perkin-Elmer Spectrum 100 FT-IR spectrometer
with samples prepared as KBr discs. Thermogravimetric analy-
ses were performed on a Perkin-Elmer STA 6000 Simultaneous
Thermal Analyzer with a heating rate of 10 °C min−1 in a
flowing air atmosphere. UV-vis spectra were recorded on a
Perkin-Elmer Lambda 35 spectrometer. The solid-state and
solution photoluminescence analyses were carried out on an
Edinburgh FLS920 fluorescence spectrometer in the range of
200–800 nm. The visible detector as well as the lifetime setup
was a red-sensitive photomultiplier (type r928). Low tempera-
ture analyses were carried out at 77 K with an Oxford Optistat
DN™ cryostat (with liquid nitrogen filling). Lifetime studies
were performed using a photon-counting system with a micro-
second pulse lamp as the excitation source. The emission
decays were analyzed by the sum of exponential functions. The
decay curve is well fitted into a double exponential function: I =
I0 + A1 exp (−t/τ1) + A2 exp (−t/τ2), where I and I0 are the lumines-
cence intensities at time t = t and t = 0, respectively, whereas τ1
1,2-diaminobenzene (1.1 g, 10 mmol) and 6-methoxy-2-
pyridinecarboxaldehyde (2.7 g, 20 mmol) was refluxed in
anhydrous toluene (20 mL) in the presence of a catalytic
amount of formic acid for 24 hours. After the reaction was
over, the resulting solution was concentrated under reduced
pressure (oil pump) to obtain the yellow solid. The crude
product was recrystallized from methanol to give the white
block solid. Yield: 1.6 g (45%); UV (CH3OH, 25 °C): λmax (nm) =
232, 279, 319; FT-IR (KBr, cm−1): ν = 3446 (m), 3050 (m, νPy,
C–H), 2949 (m), 2860 (w), 1588 (s), 1471 (s), 1442 (s), 1391 (m),
1316 (m), 1266 (m), 1073 (m), 1027 (m), 901 (s), 746 (s), 431
1
(s); H NMR (400 MHz, DMSO-d6, ppm): δ = 8.00 (d, 1H, J =
7.2 Hz, Py-H), 7.87 (d, 1H, J = 7.2 Hz, Py-H), 7.77 (dt, 1H, J =
3.6, 6.4 Hz, Py-H), 7.62 (dt, 1H, J = 4.0, 7.2 Hz, Py-H), 7.53
(t, 1H, J = 8.0 Hz, Py-H), 7.28 (dt, 2H, J = 3.6, 6.4 Hz, Py-H,
Ph-H), 6.89 (d, 1H, J = 8.0 Hz, Ph-H), 6.61 (d, 1H, J = 8.4 Hz,
Ph-H), 6.42 (d, 1H, J = 7.2 Hz, Ph-H), 6.22 (s, 2H, CH2), 3.68
(s, 3H, OCH3), 3.63 (s, 3H, OCH3); 13C NMR (100 MHz,
DMSO-d6, ppm): δ = 162.92, 162.56, 154.83, 149.48, 147.41,
142.00, 139.81, 139.76, 136.77, 123.30, 122.57, 119.49, 117.17,
112.88, 111.01, 110.77, 108.84, 53.12, 52.71, 49.51; elemental
analysis calcd for L1 [C20H18N4O2 (346.38)]: C, 69.35; H, 5.24;
N, 16.17%; found: C, 69.58; H, 5.12; N, 16.39%.
Synthesis of 2-[2-(6-methoxy-pyridyl)]-benzimidazole (L2): L2
was prepared in a similar procedure as that described for L1,
except using 6-methoxy-2-pyridinecarboxaldehyde (1.4 g,
10 mmol) instead of 6-methoxy-2-pyridinecarboxaldehyde
(2.7 g, 20 mmol). After the reaction was over, the resulting
solution was concentrated under reduced pressure (oil pump)
to obtain a yellow solid. The crude product was recrystallized
in ether to give the white block solid. Yield: 1.3 g (60%); UV
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(CH3OH, 25 °C): λmax = 228, 268, 307 nm; FT-IR (KBr): ν = 3055 (w),
2984 (w, νPy, C–H), 2946 (m), 1579 (s), 1470 (s), 1415 (s), 1325 (m),
150.02, 147.64, 142.64, 140.45, 140.13, 137.28, 123.66, 123.18,
119.95, 117.68, 113.47, 111.69, 111.46, 109.64, 53.52, 53.46,
50.24; elemental analysis calcd for 2 [C20H18Cl2N4O2Cd
1
1267 (m), 1149 (m), 1032 (m), 750 (s), 433 (m) cm−1; H NMR
(400 MHz, DMSO-d6, 25 °C): δ = 12.95 (s, 1H, imidazole-H), 8.10
(d, 1H, J = 7.2 Hz, Py-H), 7.90 (d, 1H, J = 8.0 Hz, Py-H), 7.74
(t, 1H, J = 7.6 Hz, Py-H), 7.30 (m, 1H, Ph-H), 6.79 (d, 1H, J =
8.4 Hz, Ph-H), 6.59 (d, 1H, J = 8.4 Hz, Ph-H), 6.37 (d, 1H, J =
7.2 Hz, Ph-H), 3.87 (s, 3H, OCH3); elemental anal. calcd for
L2 [C13H10N3O (224.24)]: C 69.63; H 4.50; N 18.74%; found: C
69.56; H 4.57; N 18.81%.
(529.68)]: C, 45.35; H, 3.43; N, 10.58%; found: C, 45.46; H,
3.28; N, 10.64%.
Preparation of [Cd(L1)2(NO3)2]·H2O (3)
Complex 3 was prepared using a similar procedure to that
described for complex 2, except using Cd(NO3)2·4H2O (30.8 mg,
0.1 mmol) instead of CdCl2·2.5H2O and L1 (69.6 mg, 0.2 mmol)
instead of L1 (34.8 mg, 0.1 mmol). After the sample was cooled
to room temperature, the mixed solution was filtered and kept
for evaporation at room temperature. The yellow crystals
appeared after 7 days. Yield: 28.4 mg (30%); UV (CH3OH,
25 °C): λmax (nm) = 222, 295, 337; FT-IR (KBr, cm−1): ν = 3478
(m), 3084 (w, νPy, C–H), 2989 (w), 2950 (w), 2856 (w), 1596 (s),
1579 (s), 1501 (s), 1471 (s), 1428(s), 1382 (s), 1304 (s), 1060
(m), 1025 (m), 807 (m), 750 (m), 563 (m), 433 (m); 1H NMR
(DMSO-d6, 400 MHz, ppm): δ = 8.02 (d, 1H, Py-H), 7.87
(d, 1H, Py-H), 7.76 (d, 1H, Py-H), 7.63 (m, 1H, Py-H), 7.52
(m, 1H, Ph-H), 7.29 (m, 2H, CH2), 6.89 (d, 1H, Ph-H), 6.63
(t, 1H, Py-Hp), 6.43 (d, 1H, Py-H), 6.22 (d, 2H, Ph-H), 3.68
(s, 3H, OCH3), 3.62 (s, 3H, OCH3); 13C NMR (DMSO-d6,
100 MHz, ppm): δ = 162.86, 162.19, 154.21, 149.54, 147.21,
139.82, 136.81, 131.18, 122.22, 119.20, 116.69, 114.53, 112.88,
110.84, 110.54, 108.51, 99.18, 52.86, 52.26, 49.25; elemental
analysis calcd for 3 [C40H38N10O11Cd (947.21)]: C, 50.72; H,
4.04; N, 14.79%; found: C, 50.65; H, 3.87; N, 14.85%.
Preparation of [ZnL1Cl2] (1)
A solution ZnCl2 (13.6 mg, 0.1 mmol) in CH3CN (10 mL) was
added to a solution of L1 (34.8 mg, 0.1 mmol) in CH3CN
(10 mL). The mixture was stirred under reflux for 8 hours.
After cooling to room temperature, the resulting solution was
removed by filtration. The filtrate was kept for evaporation at
room temperature for 4 days. Crystals suitable for X-ray struc-
ture determination were picked from the solution. Yield:
38.7 mg (80%); UV (CH3OH, 25 °C): λmax (nm) = 239, 279,
318; FT-IR (KBr, cm−1): ν = 3446 (m), 3086 (w, νPy, C–H), 2949
(w), 2858 (w), 1605 (s), 1580 (s), 1491 (s), 1465 (s), 1428 (s),
1322 (m), 1151 (m), 1052 (m), 1031 (m), 1024 (m), 795 (m),
1
755 (m); H NMR (400 MHz, DMSO-d6, ppm): δ = 7.99 (d, 1H,
J = 7.6 Hz, Py-H), 7.84 (t, 1H, J = 8.4 Hz, Py-H), 7.74 (dt, 1H,
J = 4.0, 7.2 Hz, Py-H), 7.59 (dt, 1H, J = 4.0, 7.2 Hz, Py-H), 7.50
(t, 1H, J = 8.0 Hz, Py-H), 7.25 (dt, 2H, J = 4.0, 7.2 Hz, Ph-H,
Py-H), 6.86 (d, 1H, J = 8.0 Hz, Ph-H), 6.58 (d, 1H, J = 8.4 Hz,
Ph-H), 6.43 (d, 1H, J = 7.2 Hz, Ph-H), 6.18 (s, 2H, CH2), 3.66
(s, 3H, OCH3), 3.60 (s, 3H, OCH3); 13C NMR (DMSO-d6,
100 MHz, ppm): δ = 163.49, 163.12, 155.28, 150.00, 147.84,
142.60, 140.47, 140.25, 137.20, 123.81, 123.08, 119.98, 117.78,
113.46, 111.77, 111.44, 109.34, 53.62, 53.21, 50.00; elemental
analysis calcd for 1 [C20H18Cl2N4O2Zn (482.67)]: C, 49.77; H,
3.76; N, 11.16%; found: C, 49.92; H, 3.63; N, 11.31%.
Preparation of [HgL1Cl2] (4)
A solution of HgCl2 (37.2 mg, 0.1 mmol) in CH3OH (5 mL)
was added to a solution of L1 (34.8 mg, 0.1 mmol) in CH2Cl2
(25 mL). The mixture was stirred under reflux for 8 hours.
After cooling to room temperature, the resulting solution was
removed by filtration. The filtrate was kept for evaporation at
room temperature for 3 days. Crystals suitable for X-ray struc-
ture determination were picked from the solution. Yield:
42.3 mg (60%); UV (CH3OH, 25 °C): λmax (nm) = 237, 279,
319; FT-IR (KBr, cm−1): ν = 3444 (m), 3082 (w, νPy, C–H), 2989
(w), 2950 (w), 1604 (s), 1575 (s), 1470 (s), 1414 (s), 1350 (s),
1312 (s), 1264 (m), 1151 (m), 1022 (m), 820 (m), 755 (m), 439
Preparation of {[CdL1Cl2]}2 (2)
A mixture of L1 (34.8 mg, 0.1 mmol) and CdCl2·2.5H2O (34.7 mg,
0.1 mmol) was dissolved in CH3CN (8.0 mL), which was
placed in a 10 mL glass flask and heated at 80 °C for 4 days.
After the sample was cooled to the room temperature, the
mixed solution was filtered and kept for evaporation at room
temperature. The yellow crystals appeared after 7 days. Yield:
42.3 mg (40%); UV (CH3OH, 25 °C): λmax (nm) = 238, 279,
318; FT-IR (KBr, cm−1): ν = 3455 (m), 3286 (m), 3252 (m),
3085 (w, νPy, C–H), 2924 (w), 2856 (w), 1663 (s), 1593 (s), 1473
(s), 1450 (s), 1386 (s), 1290 (s), 1217 (m), 1174 (m), 1087 (m),
1010 (m), 1027 (m), 795 (m), 733 (m); 1H NMR (400 MHz,
DMSO-d6, ppm): δ = 7.99 (d, 1H, J = 7.2 Hz, Py-H), 7.90
(d, 1H, J = 8.0 Hz, Py-H), 7.74 (dt, 1H, J = 3.6, 6.4 Hz, Py-H),
7.61 (dt, 1H, J = 4.0, 7.2 Hz, Py-H), 7.27 (m, 4H, J = 3.6,
6.8 Hz, Py-H, Ph-H), 6.93 (d, 1H, J = 8.0 Hz, Ph-H), 6.62
(d, 1H, J = 8.4 Hz, Ph-H), 6.43 (d, 1H, J = 7.2 Hz, Py-H), 6.21
(s, 2H, CH2), 3.67 (s, 3H, OCH3), 3.61 (s, 3H, OCH3); 13C NMR
(DMSO-d6, 100 MHz, ppm): δ = 163.48, 163.15, 155.23,
1
(w); H NMR (DMSO-d6, 400 MHz, ppm): δ = 7.98 (d, 1H, J =
7.2 Hz, Py-H), 7.86 (t, 1H, J = 8.4 Hz, Py-H), 7.75 (dt, 1H, J =
3.6, 6.8 Hz, Py-H), 7.59 (dt, 1H, J = 3.6, 6.8 Hz, Py-H), 7.52
(t, 1H, J = 8.0 Hz, Py-H), 7.27 (dt, 2H, J = 4.0, 7.2 Hz, Py-H,
Ph-H), 6.88 (d, 1H, J = 8.4 Hz, Ph-H), 6.59 (d, 1H, J = 8.0 Hz,
Ph-H), 6.46 (d, 1H, J = 7.2 Hz, Ph-H), 6.17 (s, 2H, CH2), 3.68
(s, 3H, OCH3), 3.59 (s, 3H, OCH3); 13C NMR (DMSO-d6,
100 MHz, ppm): δ = 163.51, 163.19, 155.14, 150.08, 147.59,
142.38, 140.53, 140.28, 137.10, 123.91, 123.19, 119.94, 117.85,
113.57, 111.92, 111.52, 109.41, 53.67, 53.23, 50.01; elemental
analysis calcd for 4 [C20H18Cl2N4O2Hg (617.87)]: C, 38.88; H,
2.94; N, 9.07%; found: C, 38.76; H, 2.78; N, 9.20%.
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Preparation of [ZnL2Cl2] (5)
hydrogen atoms in L1 and complexes 1–5 were found in Fou-
rier difference maps. All non-hydrogen atoms were refined
anisotropic thermal parameters. Crystallographic data and
structural refinements for L1 and 1–5 are summarized in
Table 1. CCDC reference numbers: 938861 for L1, 938858 for
1, 938859 for 2, 933809 for 3, 938860 for 4, and 946739 for 5.
A solution of ZnCl2 (13.6 mg, 0.1 mmol) in CH3CN (10 mL)
was added to a solution of L2 (22.4 mg, 0.1 mmol) in CH3CN
(10 mL). The mixture was stirred under reflux for 8 hours.
After cooling to room temperature, the resulting solution was
removed by filtration. The filtrate was kept for evaporation at
room temperature for 5 days. Crystals suitable for X-ray struc-
ture determination were picked from the solution. Yield:
28.8 mg (80%); UV (CH3OH, 25 °C): λmax (nm) = 231, 271,
313; FT-IR (KBr): ν = 3058 (w, νPy, C–H), 2948 (w), 2858 (w),
1586 (s), 1469 (s), 1451 (s), 1317 (m), 1267 (m), 1069 (m),
Results and discussion
Synthesis and characterization
The synthetic route to the ligand is shown in Scheme 1. Benz-
imidazole derivatives L1 and L2 were synthesized according to
modified methods.12 As shown in Scheme 2, the salen-type
ligand L3 was obtained by condensation of 1,2-diaminobenzene
and 6-methoxy-2-pyridinecarboxaldehyde in a 1 : 2 molar ratio
at room temperature. L3 could convert into the benzimidazole
derivative L1 in refluxing toluene in reasonable yield. This type
of transformation is well known and in the case of L1 produces
two 2-methoxy-pyridyl substituents. Analogously, treatment of
1,2-diaminobenzene with 6-methoxy-2-pyridinecarboxaldehyde
in 1 : 1 molar ratio in toluene at ambient temperature produces
the yellow Schiff-base. Furthermore, the mono-imine Schiff-
base precursor is converted into the off-white L2 in refluxing
toluene. In addition, if this reaction is carried out in low
boiling point solvents, such as CH3OH, C2H5OH, CH3CN,
and THF, the yields of L1 and L2 are very low. Therefore,
temperature plays a subtle role in synthesis. The prepara-
tion of 1–5 includes two methods: for 1, 4 and 5, the typi-
cal reaction of zinc(II)/mercury(II) chloride with ligand is
carried out under refluxing conditions; for 2 and 3, the
reaction of cadmium salts with L1 is carried out under
1021 (m), 903 (m), 752 (s), 434 (m) cm−1; H NMR (400 MHz,
1
DMSO-d6, 25 °C): δ = 13.11 (s, 1H, imidazole-H), 8.18 (d, 1H,
J = 7.2 Hz, Py-H), 7.96 (d, 1H, J = 8.0 Hz, Py-H), 7.90 (t, 1H, J =
7.6 Hz, Py-H), 7.65 (m, 1H, Ph-H), 7.01 (d, 1H, J = 8.4 Hz,
Ph-H), 6.71 (d, 1H, J = 8.4 Hz, Ph-H), 6.52 (d, 1H, J = 7.2 Hz,
Ph-H), 3.74 (s, 3H, OCH3); elemental anal. calcd for 5
[C13H11Cl2N3OZn (361.54)]: C 43.19; H 3.07; N 11.62%; found:
C 43.25; H 3.22; N 11.54%.
X-ray structure determination
Suitable cubic single-crystals were selected for single-crystal
X-ray diffraction analysis. The data were collected on a Bruker
Apex II CCD diffractometer using graphite-monochromated
Mo-Ka radiation (λ = 0.71073 Å) at 293 2 K. Data processing
was accomplished with the SAINT processing program. The
structures were solved by direct methods and refined by full-
matrix, least squares based on F2 using the SHELXTL 5.1
software package.11 The IIB group metal atoms were first
located, and then the oxygen, carbon, nitrogen, chlorine and
Table 1 Crystal data and structure parameters for ligand L1 and complexes 1–5
L1
1
2
3
4
5
Empirical formula
Formula weight
C20H18N4O2
346.38
ZnC20H18Cl2N4O2
482.67
CdC20H18Cl2N4O2
529.68
CdC40H38N10O11
947.21
HgC20H18Cl2N4O2
617.87
C13H11Cl2ON3Zn
361.54
Crystal system
Space group
a (Å)
b (Å)
c (Å)
α (°)
β (°)
γ (°)
V (Å)
Monoclinic
P21/c
11.532(2)
13.619(3)
11.371(2)
90
95.74(3)
90
1776.9(6)
4
Monoclinic
P21/n
15.010(3)
9.253(19)
16.741(3)
90
113.64(3)
90
2130.0(7)
4
Monoclinic
P21/n
10.023(2)
14.669(3)
14.429(3)
90
96.56(3)
90
2107.6(7)
4
Monoclinic
P21/c
11.048(2)
18.326(5)
19.723(5)
90
91.20(2)
90
3992.3(17)
4
Triclinic
P1
Triclinic
P1
¯
¯
7.995(16)
11.283(2)
11.996(2)
97.51(3)
103.89(3)
102.50(3)
1006.3(3)
2
7.346(15)
9.400(19)
10.700(2)
101.75(3)
90.16(3)
92.82(3)
722.4(3)
2
Z
T (K)
293(2)
293(2)
293(2)
293(2)
293(2)
293(2)
λ (Å)
0.71073
1.295
0.087
0.71073
1.505
1.427
0.71073
1.669
1.313
0.71073
1.549
0.620
0.71073
2.039
7.938
0.71073
1.653
2.066
ρcalcd (g cm−3
)
μ (mm−1
F(000)
)
728
984
1056
1904
592
360
θ range(°)
3.04 to 27.48
1.034
R1 = 0.0435
wR2 = 0.1106
R1 = 0.0739
wR2 = 0.1229
3.09 to 27.48
1.070
R1 = 0.0411
wR2 = 0.0895
R1 = 0.0649
wR2 = 0.0839
3.12 to 27.47
1.153
R1 = 0.0291
wR2 = 0.0834
R1 = 0.0350
wR2 = 0.0866
3.01 to 29.22
1.028
R1 = 0.0414
wR2 = 0.0939
R1 = 0.0650
wR2 = 0.1103
3.18 to 27.48
0.879
R1 = 0.0337
wR2 = 0.0894
R1 = 0.0387
wR2 = 0.0963
3.23 to 27.47
0.986
R1 = 0.0785
wR2 = 0.2118
R1 = 0.1369
wR2 = 0.3374
GOF on F2
R1, wR2(I > 2σ(I))a
R1, wR2(all data)a
P
P
P
P
a
2
R1 = ‖Fo| − |Fc‖/ |Fo|; wR2 = [ [w(Fo − Fc2)2]/ [w(Fo2)2]]1/2
.
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frequency and are split, from 1316, 1031, 989 and 747 cm−1
for L1 (1308, 1024, 989 and 750 cm−1 for L2) to 1322, 1052,
1024 and 755 cm−1 for 1 (1350, 1087, 1010 and 768 cm−1
for 2; 1382, 1060, 1025 and 750 cm−1 for 3; 1350, 1079, 1022
and 755 cm−1 for 4; 1317, 1029, 1023 and 764 cm−1 for 5).
This is indicative of the coordination of the ligand through
the imidazolic and pyridinic nitrogen atoms. For 3, a broad
band at 3472 cm−1 indicates the presence of crystallisation
water molecules. A strong band at ca. 1600 cm−1 is assignable
to the pyridine-ring vibrations. Extensive studies have shown
that four bands of 3 near 1480 (ν1), 1304 (ν4), 1025 (ν2) and
806 (ν3) cm−1 can be assigned to vibrations of the coordinated
nitrate group.13 The difference between the ν4 and the ν1
peak positions is approximately 190 cm−1, which is typical
for η2-chelation of the nitrate groups.14 The much smaller
splitting of in-plane bending NO3 vibrations (at 766 and
750 cm−1, 16 cm−1) indicates the presence of a monodentate
nitrate group.15 Coordination of the nitrogen atoms to the
IIB group metal center from the benzimidazole and pyridine
groups is substantiated by the far-infrared band (430 cm−1)
corresponding to ν(M–N) vibrational mode.
Scheme 1 The synthetic procedure for ligands L1–L3.
solvothermal conditions. Crystals of L1 and 1–5 suitable for
X-ray diffraction studies were obtained by evaporating the
recrystallized solution and the resulting solutions of corre-
sponding complexes, respectively. 1–5 are stable in solution
and in the solid state upon extended exposure to air. This
remarkable stability is attributable to the bidentate chelation
of the ligand to the metal center. Ligands have good solubil-
ity in common organic solvents; 1–5 are slightly soluble in
low polarity solvents such as n-hexane, toluene and benzene,
and soluble in medium and high polarity solvents.
Description of crystal structures
The novel bis-methoxy pyridyl substituted benzimidazole L1
belongs to the centric monoclinic space group P21/c. Both
pyridyl groups are twisted with respect to the planar benz-
imidazole unit in order to reduce the steric interactions
(Fig. 1). The two pyridine rings are virtually perpendicular to
each other (dihedral angle 87.81°), and the dihedral angles
In the FT-IR spectra (Fig. S1, ESI†), the characteristic
vibrations of 2-substituted pyridines are shifted to higher
Scheme 2 The mechanism of the formation of L1–L3.
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Fig. 1 Molecular structures of ligand L1 and complexes 1–4, the water molecule in 3 is omitted for clarity. The coordination sphere of the M2+
centre displays tetrahedron (1), trigonal bipyramid (2), distorted monocapped trigonal prism (3) and triangle geometries (4), respectively.
between the planar benzimidazole and each pyridine ring are
2.49° and 85.87°, respectively.
gives rise to a five-membered ring (Cd1–N1–C8–C7–N3) after
coordination. The dihedral angle between the five-membered
ring and the benzimidazole group is 6.77°. Simultaneously,
the appearance of five-membered ring system enhances the
stability of the structure. The Cd–N (imidazole) bond length
[2.286(2) Å] is significantly shorter than the Cd–N (pyridyl)
bond length [2.393(2) Å], which is consistent with the
structural data reported previously.17 The N–Cd–N bond
angle is 70.55(4)°, which is much smaller than the ideal bond
angle of an sp3 hybrid orbital (109.28°). The neighboring
molecules are connected through two bridging Cl2 and Cl2A
atoms to construct a dimer structure (Fig. S5, ESI†). There is
no intermolecular packing in 2. However, the structure of 2 is
stabilized by hydrogen bonds between the Cl2 atom and C4
atom of the adjacent units (Fig. S5, ESI†). The distance of
Cl⋯C is 3.541 Å, and the bond angle of Cl⋯H–C is 141.93°,
which are similar to results from the literature reported
previously.18
Complex 1 belongs to the centric monoclinic space group
P21/n. The zinc(II) atom is four coordinated, and bonded to
two nitrogen atoms derived from the N^N ligand and two
chlorine atoms, forming a tetrahedral geometry. The dihedral
angle between the benzimidazolyl ring and pyridyl ring
linked by a C–C single bond is 3.34°. Compared to L1, the
dihedral angle between the other pyridyl ring and the
benzimidazolyl ring (76.96°) decreases upon coordination.
The Zn–N (pyridine) bond length [2.097(3) Å] is longer than
the Zn–N (imidazole) bond length [2.031(3) Å]. The average
Zn–N distance of 2.064 Å is close to the values found in other
Zn(II) complexes.16 The two chlorine atoms (Cl1 and Cl2) are
located above and below the planar benzimidazole and zinc
atom. The distances between benzimidazole and the two
coordinated chlorine anions are 1.917(2) Å and 1.881(1) Å,
respectively.
Complex 2 is a dimer structure in which the Cd2+ cation
takes a five-coordinated geometry with a typical trigonal
bipyramid (Fig. 1). In the asymmetric structural units, each
Cd(II) center is bidentately chelated by N^N L1 via two
nitrogen atoms (N1 and N2), and the remaining sites are
occupied by three chlorine atoms (fully occupied Cl1, half-
occupied Cl2). Examination of the dihedral angle between
the two pyridine groups reveals the angle is approximately
perpendicular (89.37°). Closer inspection reveals the dihedral
angles to planar benzimidazole are 21.35° and 71.69°, respec-
tively. The changes of dihedral angles are compressed com-
pared to the free L1 in order to accommodate metal ions. It
The asymmetric unit of 3 contains an independent Cd2+
cation, two benzimidazole ligands, two nitrate anions and a
solvated water molecule (Fig. 1). The CdII center is coordi-
nated to four nitrogen atoms from two L1, and three oxygen
atoms from monodentate and bidentate nitrate anions.
The 7-coordinated geometry around Cd2+ cation is a distorted
monocapped trigonal prism, with Cd–N and Cd–O distances
in the range of 2.279(2)–2.532(3) Å and 2.374(3)–2.801(2) Å.
The Cd-bridging angles of N1–Cd–N3, N5–Cd–N7 and
O5–Cd–O7 are 68.64(8), 71.38(9) and 48.12(3), respectively.
The Cd–N distance to the benzimidazole ring is slightly shorter
than that to the pyridyl donor, as was observed recently for an
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analog.17 The adjacent pyridyl groups and benzimidazole plane
are twisted by dihedral angles of 16.45° and 28.30°, respec-
tively. Two benzimidazole ligands exhibit the cross style, whose
dihedral angle is 71.45°. Intriguingly, the adjacent pyridyl
group in one L1 is parallel-like with a methylene group bridg-
ing the pyridyl group on the other L1, the corresponding
angles are 21.08° and 26.09°, respectively. The structure of 3
is also stabilized by the hydrogen bonds between the O1W
atom and O5 atom of the adjacent units (Fig. S6, ESI†). The
distance of O⋯O is 3.015 Å, and the bond angle of O⋯H–O
is 146.58°, which are similar to results from the literature
reported previously.19
Akin to complex 1, complex 4 belongs to the centric triclinic
II
¯
space group P1. The Hg center is linked through only one
nitrogen atom of the benzimidazole group and two chlorine
atoms (Fig. 1). The nitrogen atom from the pyridyl group could
not participate in the coordination. Thus, the three-
coordinated Hg(II) atom forms a planar triangle. As aforemen-
tioned, although the complexes 1–4 use the same ligand, the
reason for the rare three-coordinated system has two contexts
as follows: (1) the ionic radius clearly plays a role in directing
the architecture of the resulting complex. The ionic radius of
Hg(II) is bigger compared with that of Zn/Cd(II), which
decreases the electrostatic attraction between ligand and
metal atom. The intensity of the bond also weakens along
with the increase of ionic radius. (2) The steric hindrance of
ligand can influence the forming of bonds around the metal
ion. In order to eliminate repulsive forces, an optimized con-
figuration may stabilize the structure of complex. Inspection of
the parameters of 4 reveals the Hg–N1 bond distance is
2.318(3) Å and 2.845(4) Å for the distance between Hg(II) and
un-coordinated N3 atom. Similar to complexes 2 and 3, a
three-dimensional supramolecular network formed through
C–H⋯Cl hydrogen bonded interactions (Fig. S7, ESI†).
Fig. 2 (a) Molecular structure of complex 5, all hydrogen atoms are
omitted for clarity. (b) The stick view of a 1D chain generated by the
hydrogen bonds Cl⋯H–N in 5 along the [100] direction. (c) View of the
π–π stacking interactions in 5.
site, it plays the role of a neutral bidenatate chelator. Two
oxygen atoms from different –OCH3 groups do not coordi-
nate to the metal center due to the high nitrophilicity of the
transition metals.
Photophysical properties
¯
Complex 5 belongs to the centric triclinic space group P1.
The electronic absorption spectra of L and 1–5 were studied
in methanol solution (Fig. 3) and show similar ligand-centered
bands in the region of 220–340 nm (ε = 6328–38 581 M−1 cm−1).
The numerical values of the maximum absorption wavelength
and molar extinction coefficients (ε) are listed in Table 2. The
absorption bands with λmax ≈ 235, 280 and 320 nm were
In analogy to 1, the centre metal Zn2+ ion is four coordinate
with a typical tetrahedron geometry (Fig. 2a). Each zinc center
is bidentately chelated by N^N L2 via two nitrogen atoms
(N1 and N2), and the remaining sites are occupied by two
chlorine atoms (Cl1 and Cl2). The dihedral angle between
the pyridyl group and benzimidazole is approximate parallel
(1.69°). The Zn–N (imidazolyl) bond [2.054(2) Å] is significantly
shorter than the Zn–N (pyridyl) bond [2.116(2) Å], which is
consistent with the structural data reported previously.16 The
N–Zn–N bond angle is 79.60(4)°, which is much smaller than
the ideal bond angle of an sp3 hybrid orbital (109.28°). However,
intermolecular Cl⋯H–N hydrogen bonding interactions are
present within each layer and further linked to construct a
1D chain structure along the [010] direction (Fig. 2b). The
distances of Cl1⋯N3 and Cl2⋯N3 are 3.234 and 3.431 Å,
respectively. The bond angles of Cl1⋯H3–N3 and Cl2⋯H3–N3
are 124.53° and 145.36°, respectively. There are weak π–π
stacking interactions between benzimidazole ring and pyridine
ring [the distance is 3.702(2) Å] in the crystal lattices of
5 (Fig. 2c) that help to create self-assembled suparmolecular
frameworks. For 1–5, although L1 has a richly coordinated
Fig. 3 UV-vis absorption spectra of L1, L2 and complexes 1–5 in
CH3OH.
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Table 2 Photoluminescent data for ligands L1, L2 and complexes 1–5
Absorptiona (ε)
Emission
(λmax, nm)
Decay lifetime Quantum
FWHM (nm) (τ, μs)
Medium
Complex (nm, dm3 mol−1 cm−1
)
yieldsa (Φ) (T/K)
CIE (x, y)
L1
232 (10 522), 279 (10 864), 399
31.8
61.6
58.8
59.7
71.2
59.6
60.5
35.1
74.8
55.2
54.9
63.0
48.0
45.1
42.5
61.6
48.0
53.0
40.2
66.0
36.3
60.0
29.4
63.5
26.3
8.87
5.12
10.14
7.99
5.24
6.01
8.56
6.93
7.57
19.77
9.52
5.80
9.76
9.77
8.77
6.28
8.79
10.74
10.27
5.64
10.74
7.83
5.97
8.33
9.84
8.33
5.73
6.86
9.43
7.68
10.07
21.29
0.23
0.20
CH3OH (298)
Solid (298)
CH3OH (77)
Solid (77)
CH3OH (298)
CH2Cl2 (298)
Solid (298)
CH3OH (77)
CH2Cl2 (77)
Solid (77)
CH3OH (298)
Solid (298)
CH3OH (77)
Solid (77)
CH3OH (298)
Solid (298)
CH3OH (77)
Solid (77)
CH3OH (298)
Solid (298)
CH3OH (77)
Solid (77)
CH3OH (298)
Solid (298)
CH3OH (77)
Solid (77)
CH3OH (298)
CH2Cl2 (298)
solid (298)
CH3OH (77)
CH2Cl2 (77)
solid (77)
0.16, 0.08
0.21, 0.21
0.16, 0.11
0.23, 0.24
0.13, 0.32
0.22, 0.22
0.14, 0.31
0.13, 0.25
0.16, 0.17
0.19, 0.35
0.16, 0.05
0.18, 0.13
0.18, 0.15
0.18, 0.13
0.16, 0.05
0.17, 0.11
0.17, 0.12
0.20, 0.16
0.16, 0.03
0.21, 0.19
0.19, 0.18
0.24, 0.24
0.16, 0.02
0.21, 0.19
0.20, 0.22
0.26, 0.30
0.16, 0.07
0.18, 0.10
0.19, 0.11
0.18, 0.08
0.20, 0.22
0.17, 0.12
317 (20 662)
405
419 471sh
412 440sh 469sh 497sh
L2
228 (6328), 268 (6658),
307 (13 523)
470
452
480
474
455
499
1
2
3
4
5
239 (12 822), 279 (14 403), 406
0.38
0.32
0.40
0.33
0.26
318 (26 940)
422
407 497sh
426 471sh 496sh 516sh
238 (11 549), 279 (11 549), 402
318 (24 541)
421
408 478sh
419 443sh 472sh 495sh
222 (35 537), 295 (33 673), 402
337 (38 581)
415
399 499sh
412 440sh 496sh 497sh
237 (11 720), 279 (12 805), 400
319 (24 047)
411
402 505sh
406 486sh 529sh 566sh 615sh 159.6
231 (9381), 271 (9890),
313 (19 399)
426
418
434
427
420
449
72.6
55.3
64.4
32.7
85.5
57.5
a
Recorded in methanol at 298 K, concentration = 10−5 mol L−1. sh = shoulder.
observed for all the ligands and complexes. The bands in
the wavelength range 232–239 nm could be assigned to the
π → π* transition of the benzimidazolyl system. The bands
at ca. 280 nm were attributed to the π → π* transition centered
on CN groups of the benzimidazolyl units. The bands
ca. at 320 nm belong to the n → π* transition of the
benzimidazolyl units. Moreover, the low energy absorption
bands of 3 (295 nm and 337 nm) are bathochromically shifted
as compared with those of the free L1 (279 nm and 317 nm).20
Clearly, the ε value is linearly dependent on the number of
2-(2′-pyridyl)-benzimidazolyl units (one ligand: 1, 2, 4 and 5;
two ligands: 3). It is reasonable to infer that the addition of a
carrier-transporting group (6-methoxy-2-pyridylmethyl) is use-
ful to enhance the electronic absorption of benzimidazolyl
complexes.
The emission spectra of L and 1–5 were recorded in
CH3OH solution and in the solid state at 298 K and 77 K
(Fig. 4). The photophysical data for emission are gathered in
Table 2. The process involves light emission that is triggered
by crystals that have been previously irradiated with long
wavelength UV light (365 nm). The emission is intense and
clearly visible. In polar protic solvent CH3OH, corresponding
emission peak of L1 appeared at 399 nm. In comparison with
L1, complexes 1–4 in CH3OH give narrow emission bands
(bandwidth at half-height = 29–55 nm) with λmax = 406, 402,
402 and 400 nm, respectively (Fig. 4). The emission maxima
of 1–4 are almost the same as that of the free L1, which
should be attributed to the π* → π transition of ligand. In
order to investigate the temperature-dependant emission,
cryogenic luminescent behavior measurements were also
performed in CH3OH. When the excitation and emission
spectra are recorded in solution at 77 K, complexes 1–4 dis-
play similar maximum emissions at ca. 402–419 nm (Fig. 4c).
The distinction between room temperature and cryogenic
temperature is the structured bands centered from 471 to
505 nm. In contrast to L1, it exhibits hypsochromic shift
upon coordination. When the complexes 1–4 and L1 are
examined in the solid state (Fig. 4b and 4d), bathochromic
shift from solution to solid is likely to be caused by inter-
molecular hydrogen bonding interactions existing in the
solid form. The hydrogen bonds can effectively decrease the
HOMO–LUMO energy gap and influence the ligand-centered
π* → π transitions.21 At 77 K, the emissions of solid state
at 440–615 nm are more distinctly structured compared to
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Fig. 4 Emission spectra of L1 and 1–4 monitored at the longest wavelength of absorption peak (a: 298 K, CH3OH; b: 298 K, solid state; c: 77 K,
CH3OH; d: 77 K, solid state).
those of CH3OH solution. CIE 1931, which assists the under-
standing of the color gamut, is also provided. The dominant
color gamut of L1 and complexes 1–4 focuses on blue region
according to the calculated values (Fig. 5).
Given the different ligand in 5, we investigated the lumi-
nescence of L2 and 5 both in the solid state and solution at
298 K and 77 K (Fig. 6–7). Solid L2 and 5 show blue emission.
At 298 K, the corresponding emission peak of L2 in polar
aprotic solvent (CH2Cl2) appeared at 452 nm, emitting sky-blue
luminescence. In contrast to L2, it is shown that complex 5
displays deep-blue fluorescence emission with the maximum
emission at 418 nm. Due to ZnCl2 not being luminescent, the
luminescence emissions of complex 5 and L2 both can be
attributed to the ligand-centered π* → π transitions. With the
increase in the polarity (order: CH2Cl2 < CH3OH), the
bathochromic shift for 5 occurs from CH2Cl2 (417 nm) to
CH3OH (425 nm) owing to more polarity in the excited state.
A similar emission energy shift is also exhibited in the spec-
tra of L2 (452 nm in CH2Cl2 and 470 nm in CH3OH). It is
ascribed to the donor ability of CH3OH as a protic solvent.
The hydrogen bond donor nature of CH3OH seems to stabi-
lize the excited state of the electronic transitions through
Fig. 5 CIE chromaticity diagram (1931 CIE standard) for ligand L1
and 1–4.
Fig. 6 Normalized emission spectra of L2 and 5 in CH3OH, CH2Cl2 and
in the solid state at 298 K.
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and nonradiative transition at low temperature. Meanwhile,
the lifetimes of emissions for L1 and 1–4 in the solid state is
shorter than those in solution, which is due to aggregation
quenching existing in the solid state.24 Conversely, the life-
times of emissions for L2 and 5 in the solid state are longer
than those in solutions, which can be ascribed to the less polar
nature of the environment in the solid state.25 For 5 and L2,
the lifetimes of emissions in CH3OH are substantially short-
ened in comparison with those in CH2Cl2 under the same test
conditions. This phenomenon can be attributed to the follow-
ing reason: in CH3OH, a non-radiative process from the intra-
molecular charge transfer (ICT) state results in a small energy
gap between the singlet states and triplet states.26 To the best
of our knowledge, only two other luminescence lifetime mea-
surements of 2-(2-pyridyl)benzimidazole complexes have
been previously reported. The first values are in the range
of 7.1–8.4 μs, reported by Wang et al. in 2005.27 The second
of value is in the range of 0.93 ns, reported by Liang et al.
in 2009.28 Obviously, the lifetime of emission for 5 in the
solid state (9.43 μs at 298 K, 21.29 μs at 77 K) is longer than
that reported above.
The luminescence quantum yield, defined as the ratio of
the number of photons emitted to the number of photons
absorbed, gives the efficiency of the luminescence process.
The quantum yields of L1, L2 and 1–5 have been determined
in CH3OH solution and were determined to be 0.23, 0.20,
0.38, 0.32, 0.40, 0.33, and 0.26, respectively. As we expected,
it was found that the quantum yields of 1–5 are higher than
those of the corresponding ligands. In brief, the enhance-
ment after the connection of the ligand to metal center
increases the conformational rigidity of the ligands, and
hence reduces the non-radioactive deactivation pathways.29
Meanwhile the quantum yield of the Hg(II) complex 4 is lower
than that of Zn(II) and Cd(II) complexes 1–3. These results
Fig. 7 Normalized emission spectra of L2 and 5 in CH3OH, CH2Cl2 and
in the solid state at 77 K.
solute–solvent interactions and be responsible for the shift
of the emission energy.22 In the solid state, complex 5 and L2
profiles peak at 432 and 480 nm. Such bathochromic shift
relative to that in solution indicates that intermolecular
hydrogen bond interactions exist in the solid.23 The emission
spectra in solution at 77 K possessed the similar order com-
pared to those at 298 K. However, the emission maxima in
solutions for L2 and 5 are slightly red-shifted by 3–4 nm as
compared to the room temperature values. In the solid state,
the emission maxima for L2 and 5 red-shifted even further
(15–19 nm) at 77 K, to λmax = 499 nm (L2) and 449 nm (5). We
can observe color variations through the CIE coordinates dia-
gram (Fig. 8). The variation from sky-blue of L2 to deep-blue of
5 undoubtedly implies the coordination of the zinc atom.
The lifetimes of emissions for L and 1–5 are in the region
of the microsecond scale. The average lifetimes are listed in
Table 2. A general trend is that the lifetime of emission at
low temperature (77 K) is longer than that at room tempera-
ture, which is attributed to the decrease of thermal vibration
Fig. 8 The corresponding CIE chromaticity diagram of L2 (a) and 5 (b).
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chelate ligand. The luminescence of the complexes is caused
by π*–π electronic transitions centered on the N^N chelate
ligands. Our investigation shows that two series of IIB group
metal complexes with emission colors covering the visible
blue region can be achieved.
Acknowledgements
This work was supported by the National Natural Science
Foundation of China (Grant No. 21371040 and 21171044), the
National key Basic Research Program of China (973 Program,
No. 2013CB632900), the Fundamental Research Funds for the
Central Universities (Grant No. HIT. IBRSEM. A. 201409), and
the Program for Innovation Research of Science in Harbin
Institute of Technology (PIRS of HIT No. A201416 and B201414).
Chen wants to thanks the support from Academic New Talents
Award of Doctoral Post Graduate of Ministry of Education.
Fig. 9 TG curves of complexes 1–5.
can be easily understood considering the following factor:
the Hg(II) cation and the chloride anions can quench the
fluorescence and result in luminescence decay.30 The bulkier
configuration of 3 reduces the vibrational motion and avoids
the solvents to quench the emission, which is the reason for
the higher quantum yield.
Notes and references
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