Six- and four-coordinated zinc(II) complexes exhibit strong blue fluorescent properties

Article abstract of DOI:10.1016/S1387-7003(03)00224-7

A new ligand, 4-(p-chlorophenyl)-1,2,4-triazole (^pCltrz), and its complexes, [Zn(^pCltrz)₆]₂(ClO ₄)₂·H₂O (1) and [Zn( ^pCltrz)₂Cl₂] (2), were synthesized. The structure of 1 exhibits the first case that zinc(II) ion coordinated with six triazole ligands to form octahedral environment. Both 1 and 2 display very strong blue photoluminescence as the result of the fluorescence from the intraligand emission excited state.

Full text of DOI:10.1016/S1387-7003(03)00224-7

Inorganic Chemistry Communications 6 (2003) 1209–1212
www.elsevier.com/locate/inoche
Six- and four-coordinated zinc(II) complexes exhibit strong
blue fluorescent properties
*
Long Yi, Li-Na Zhu, Bin Ding, Peng Cheng , Dai-Zheng Liao,
Shi-Ping Yan, Zong-Hui Jiang
Department of Chemistry, Nankai University Tianjin 300071, PR China
Received 5 May 2003; accepted 23 June 2003
Published online: 26 July 2003
Abstract
A new ligand, 4-(p-chlorophenyl)-1,2,4-triazole (pCltrz), and its complexes, [Zn(pCltrz)6]2(ClO4)2 ꢀ H2O (1) and [Zn(pCltrz)2Cl2]
(2), were synthesized. The structure of 1 exhibits the first case that zinc(II) ion coordinated with six triazole ligands to form oc-
tahedral environment. Both 1 and 2 display very strong blue photoluminescence as the result of the fluorescence from the intraligand
emission excited state.
Ó 2003 Elsevier B.V. All rights reserved.
Keywords: Zinc complex; Triazole; Crystal structure; Fluorescence properties
1. Introduction
2. Experimental
In the past two decades, a variety of coordination
compounds containing N4 substituted 1,2,4-triazoles as
ligands coordinated to metal ions have been reported [1–
10]. By blocking the N4 donor position through substitu-
tion, only the N1 monodentate and the N1N2-didentate
coordination modes are possible. Structures and proper-
ties of mono- and polynuclear compounds have been de-
scribed [5–8,10]. On the other hand, blue luminescent
metal complexes have been of our particular interest be-
cause they are one of the key color components required
for full-color electroluminescent displays [11–13]. How-
ever, there are relatively few reports on the luminescent
properties of triazole coordination compounds. In this
present contribution, we synthesized a new N4 substituted
1,2,4-triazole ligand (pCltrz ¼ 4-(p-chlorophenyl)-1,2,4-
triazole), and obtained two zinc(II) complexes, namely
[Zn(pCltrz)6](ClO4)2 ꢀ H2O (1) and [Zn(pCltrz)2Cl2] (2).
Both X-ray crystal structures and their strong blue lumi-
nescent propertiesform thesubjectofthis communication.
2.1. Synthesis
The new ligand pCltrz was synthesized using a method
similar to that reported by Bayer et al. for the preparation
of other 4-substituted-1,2,4-triazoles [14]. In this case
4-chloroaniline (1 mol) was added slowly to the boiling
solution of monoformylhydrazine (1 mol) and triethyl-
orthoformate (1.5 mol) in 400 ml of anhydrous methanol.
The solution was refluxed for 4 h and the solvent was then
removed under reduced pressure. After cooling to room
temperature, the solid was washed with a little hexane,
and then washed three times with tetrahydrofuran. A
snow-white powder was yielded (48%). W ¼ 179:5; m.p.:
130–133 °C; IR (cmꢁ1): 3120(s), 1600 (s), 1525(vs),
1420(s), 1370(vs), 1240(vs), 1090(vs), 865 (s), 825(vs),
738(s), 630(s). Anal. Calc. for C8H6N3Cl: C, 53.48; H,
3.34; N, 23.40%; Found: C, 53.56; H, 3.20; N, 23.21%.
A solution of the Zn(ClO4)2 ꢀ 6H2O (0.1 mmol) or
ZnCl2 (0.1 mmol) was dropped into a vigourously stir-
red solution of the ligand (0.4 mmol). Stirring was
continued for a given amount of time. Then the solution
was filtered and reduced in volume in vacuo until the
complex started to precipitate. Then the reaction flask
* Corresponding author. Tel.: +86-22-23509957; fax: +86-22-
23502779.
E-mail address: pcheng@nankai.edu.cn (P. Cheng).
1387-7003/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved.
doi:10.1016/S1387-7003(03)00224-7

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L. Yi et al. / Inorganic Chemistry Communications 6 (2003) 1209–1212
was kept at 0 °C for 3 days. The colorless crystalline
products suitable for X-ray structure analysis were
gradually formed in the solution. Anal. Calc. for
C48H38Cl8N18O9Zn (1): C, 42.36; H, 2.79; N, 18.53%;
Found: C, 42.10; H, 2.82; N, 18.45%. m.p.: 235–237 °C.
Anal. Calc. for C16H12Cl4N6Zn (2): C, 38.75; H, 2.42;
N, 16.95%; Found: C, 38.62; H, 2.50; N, 16.80%. m.p.:
217–219 °C.
2.2. Physical measurements
All the solvents and chemicals were reagent grade and
used without further purification. Elemental analyses of
carbon, hydrogen and nitrogen were carried out with a
Perkin–Elmer analyzer model 240. Infrared spectra were
measured on a Bruker Vector 22 FT-IR instrument from
KBr pellets. Diffuse reflectance absorption spectrum was
performed with a Shimadzu UV-2101PC spectropho-
tometer in the range of 200–2000 nm at room temper-
ature. The photoluminescence spectrum was measured
by MPF-4 fluorescence spectrophotometer with a xenon
arc lamp as the light source.
Fig. 1. The ORTEP drawing of [Zn(pCltrz)6]2þ. Selected interatomic
ꢀ
bond distances (A) and angles (°): Zn(1)–N(13) 2.132(4), Zn(1)–N(4)
2.156(4), Zn(1)–N(10) 2.175(3), Zn(1)–N(16) 2.177(4), Zn(1)–N(7)
2.202(4), Zn(1)–N(1) 2.221(4), N(13)–Zn(1)–N(4) 176.51(14), N(13)–
Zn(1)–N(10) 90.32(14), N(13)–Zn(1)–N(16) 90.01(14), N(16)–Zn(1)–
N(7) 179.17(14), N(13)–Zn(1)–N(7) 90.72(14), N(10)–Zn(1)–N(16)
89.15(14), N(13)–Zn(1)–N(1) 88.06(14).
2.3. Crystal structure determination
Crystal data for 1: C48H38Cl8N18O9Zn, M ¼ 1359:93,
monoclinic, P21=c, a ¼ 20:369ð12Þ, b ¼ 14:773ð8Þ, c ¼
The N(13)–Zn(1)–N(4), N(13)–Zn(1)–N(16) and N(13)–
Zn(1)–N(10) angles are 176.51(14), 90.01(14) and 90.32
(14)°, respectively, indicating a slight distortion. It is
noticeable that there is a considerable variation in the
Zn–N distances around the metal ion and Zn(1)–N(13)
3
ꢀ
ꢀ
20:416ð12Þ A, b ¼ 113:675ð8Þ°, V ¼ 5627ð6Þ A , Z ¼ 4,
Dc ¼ 1:605 g/cm3, R1 ¼ 0:0447, wR2 ¼ 0:0988 (I > 2rðIÞ);
R1 ¼ 0:1010, wR2 ¼ 0:1345 (for all data), and S ¼ 0:898.
Crystal data for 2: C16H12Cl4N6Zn, M ¼ 495:49, mono-
ꢀ
clinic, P21=c, a ¼ 14:892ð5Þ, b ¼ 18:594ð7Þ, c ¼ 7:147ð2Þ
bond length is almost 0.09 A shorter than Zn(1)–N(7)
3
ꢀ
ꢀ
A, b ¼ 99:000ð6Þ°, V ¼ 1954:7ð11Þ A , Z ¼ 4, Dc ¼ 1:684
g/cm3, R1 ¼ 0:0423, wR2 ¼ 0:0864 (I > 2rðIÞ); R1 ¼
0:0959, wR2 ¼ 0:1297 (for all data), and S ¼ 0:900. X-ray
diffractions were collected on a BRUKER SMART 1000
CCD detector with graphite-monochromatized Mo-Ka
distance, this may be due to the need to ÔeaseÕ the space
repulsion of the triazole ligands. Through a few
mononuclear compounds are known in which the 4-
substituted triazole coordinates through N1 [17–20],
this is the first example that metal ion coordinated via
six triazole ligands. The ORTEP of 2 is shown in Fig. 2
together with the atomic labeling scheme. The coordi-
nation sphere of the Zn(II) ion may be described as a
slightly distorted tetrahedron. The metal ion in 2 is
coordinated by two nitrogen atoms of two triazole li-
gands and two chloride atoms. The Zn–N bond lengths
are very close to each other with the average value of
ꢀ
radiation with radiation wavelength 0.71073 A at 293(2)
K. The structures were solved by direct-methods using the
program SHELXS-97 [15] and Fourier difference tech-
niques, and refined by full-matrix least-squares method
on F 2 using SHELXL 97 [16].
ꢀ
3. Results and discussion
2.018 A. The Zn–Cl distances are 2.2039 (17) and
ꢀ
2.2352 (18) A, respectively. The bond angles around the
Two zinc salts, Zn(ClO4)2 ꢀ 6H2O as species with non-
coordinating anions and ZnCl2 as species with coordi-
nating anions, were chosen for the reactions with pCltrz.
The structures of 1 and 2 have shown that the coordi-
native behaviour of the anions was as expected [17].
The cationic unit of 1 is depicted in Fig. 1. The
zinc(II) ion in 1 is located on a slightly distorted oc-
tahedral center relating six N-donating pCltrz ligands.
zinc atom deviate much with respect to the value ex-
pected for an ideal tetrahedron, 109.4°. Compared with
1, the dihedral angle between phenyl ring and triazole
ring of the pCltrz ligand is almost 4° bigger in com-
pound 2, this may attribute to the space repulsion li-
gands in 1.
The excitation and emission spectra of pure 1, 2 and
pCltrz in the DMF solution with the same concentra-
tion at room temperature are shown in Fig. 3. The very
strong blue fluorescence for the two complexes can be
ꢀ
The Zn–N (2.132(4)–2.221(4) A) distances are within
the range observed for other octahedral complexes [17].

L. Yi et al. / Inorganic Chemistry Communications 6 (2003) 1209–1212
1211
p
the coordination of the Cltrz ligand to the zinc(II) ion
that effectively increases the rigidity of the ligand and
reduces the loss of energy via radiationless thermal vi-
brations. The condensed structures for the two com-
plexes are also an attribution. The red shift of the
emission from the UV light region to the blue color
region is complicated. Compared with two compounds,
the structures are different, while the emission behavior
is similar. Thus, it should be concluded that the dom-
inated factor be due to the coordination of ligand to
metal ion. From the experiment we can see that the
fluorescence efficiency for 2 is higher than that for 1.
The reason for this may be due to the energy difference
in zinc ion caused by the coordination environment.
The high luminescence efficiency in blue light region as
well as the high thermally stability indicates the two
complexes may be excellent candidate for highly ther-
mally stable blue fluorescent materials.
In summary, we have successfully synthesized a new
triazole ligand and two Zn(II) complexes. 1 is the first
example that metal ion coordinated by six triazole li-
gands in mononuclear compounds. Both 1 and 2 show
strong blue photoluminescence as the result of the flu-
orescence from the intraligand emission excited state.
Efforts to further investigate other luminescent metal
complexes and synthesized novel ligands are underway
in our laboratory.
Fig. 2. The ORTEP drawing of [Zn(pCltrz)2Cl2]. Selected interatomic
ꢀ
bond distances (A) and angles (°): Zn(1)–N(1) 1.998(5), Zn(1)–N(4)
2.039(5), Zn(1)–Cl(4) 2.2039(17), Zn(1)–Cl(3) 2.2352(18), N(1)–Zn(1)–
N(4) 99.21(19), N(1)–Zn(1)–Cl(4) 114.70(15), N(4)–Zn(1)–Cl(4)
114.66(14), N(1)–Zn(1)–Cl(3) 103.60(14), N(4)–Zn(1)–Cl(3) 103.33(15),
Cl(4)–Zn(1)–Cl(3) 118.83(7), C(2)–N(1)–Zn(1) 131.0(4), N(2)–N(1)–
Zn(1) 121.3(4), N(5)–N(4)–Zn(1) 125.2(4).
Acknowledgements
This work was supported by the National Science
Foundation of China (Nos. 90101028 and 50173011)
and the TRAPOYT of MOE China.
References
[1] G. Vos, A.J. de Kok, G.C. Verschoor, Z. Naturforsch. Teil B 36
(1981) 809.
Fig. 3. The excitation and emission spectra of pure 1, 2 and pCltrz in
the DMF solution at room temperature (2, solid line; 1, dot line;
pCltrz, dash line).
[2] L.R. Groeneveld, G. Vos, G.C. Verschoor, J. Reedijk, J. Am.
Chem. Soc., Chem. Commun. (1982) 620.
[3] A.L. Spek, G. Vos, Acta Crystallogr., Sect. C 39 (1983) 990.
[4] G. Vos, R.A.G. de Graaff, J.G. Haasnoot, A.M. van der Kraan,
P. de Vaal, J. Reedijk, Inorg. Chem. 23 (1984) 2905.
[5] G. Vos, J.G. Haasnoot, G.C. Verschoor, J. Reedijk, Inorg. Chim.
Acta 102 (1985) 187.
observed, where the maximum emission wavelength at
383 nm for 1 and 381 nm for 2. Additional shoulder
peaks at 373 nm for 1 and 375 nm for 2 also can be
detected. The emission of the two complexes is due to
the fluorescence from the intraligand emission excited
state. Correspondingly, the broad peak with the maxi-
mum excited light at ca. 330 nm can be observed both
in the exciting spectra for the two complexes. Con-
tractively, the free neutral ligand pCltrz displays very
weak emission at 359 nm in the UV region, in corre-
sponding to the excited light at 300 nm. The enhanced
fluorescence efficiency of the complexes is attributed to
[6] G. Vos, J.G. Haasnoot, G.C. Verschoor, J. Reedijk, P.E.L.
Schaminee, Inorg. Chim. Acta 105 (1985) 31.
[7] W. Vreugdenhil, J.G. Haasnoot, J. Reeduk, Inorg. Chim. Acta
167 (1990) 109.
[8] Y. Garcia, P.J. van Koningsbruggen, G. Bravic, P. Guionneau, D.
Chasseau, G.L. Cascarano, J. Moscovici, K. Lambert, A. Mich-
alowicz, O. Kahn, Inorg. Chem. 36 (1997) 6357.
[9] J.C. Liu, G.C. Guo, J.S. Huang, X.Z. You, Inorg. Chem. 42
(2003) 235.
[10] Q. Zhao, H. Li, Z. Chen, R. Fang, Inorg. Chim. Acta 336 (2002)
142.

1212
L. Yi et al. / Inorganic Chemistry Communications 6 (2003) 1209–1212
[11] (a) S.F. Liu, Q. Wu, H.L. Schmider, H. Aziz, N.X. Hu, Z.
Popovic, S. Wang, J. Am. Chem. Soc. 122 (2000) 3671;
(b) W. Yang, H. Schmider, G. Wu, Y.S. Zhang, S. Wang, Inorg.
Chem. 39 (2000) 2397;
[14] U.S. Pat. 3 821 376 (1974) to H.O. Bayer, R.S. Cook, W.C. voa
Mever.
[15] G.M. Sheldrick, SHELXL-97, Program for the Refinement of
Crystal Structure, University of Gottingen, 1997.
(c) Q.G. Wu, G. Wu, L. Brancaleon, S. Wang, Organometallics
18 (1999) 2553.
[16] G.M. Sheldrick, SHELXL-97, Program for the Refinement of
Crystal Structures, University of Gottingen, 1997.
[17] U. Hartmann, H. Vahrenkamp, Inorg. Chim. Acta 239 (1995) 13.
[12] J.R. Sheats, H. Antoniadis, M. Hueschen, W. Leonard, J. Miller,
R. Moon, D. Roitman, A. Stocking, Science 273 (1996) 884.
[13] (a) S. Wang, Coord. Chem. Rev. 215 (2001) 79;
(b) P.G. Lacroix, R. Clement, K. Nakatani, J. Zyss, I. Ledoux,
Science 263 (1994) 658;
ꢁ
[18] E. Szlyk, A. Wojtczak, M. JaskOlski, M. Gilski, J.G. Hassnoot, J.
Reedijk, Inorg. Chim. Acta 260 (1997) 145.
[19] J.M. Kelly, C. Long, J.G. Vos, J.G. Hassnoot, G. Vos, J.
Organomet. Chem. 221 (1981) 165.
(c) J. Zyss, I. Ledoux, J.F. Nicoud, in: J. Zyss (Ed.), Molecular
Nonlinear Optics, Academic Press, Boston, 1993.
[20] J.G. Vos, J.G. Hassnoot, G.C. Verschoor, C. Long, J.G. Vos, J.
Organomet. Chem. 231 (1982) 315.