DOI:
10.1039/B005644H
(Paper)
New J. Chem., 2001,
25, 63-65
Strongly luminescent metal–organic compounds: spectroscopic properties and crystal structure of substituted 1,8-naphthyridine and its zinc(II) complex
Received (in Montpellier, France) 12th July 2000, Accepted 6th October 2000
First published on 8th December 2000
Abstract
N,N′-Bisbenzyl-2,7-diamino-1,8-naphthyridine (L) and its zinc(II) complex, [Zn(L)2(OAc)2], are strongly luminescent materials
with emission quantum yields in methanol being 0.38 and 0.59, respectively. The crystal structure of [Zn(L)2(OAc)2]
features a one-dimensional chain structure through intermolecular π–π stacking interactions. A broad emission
band ranging from 400
to 600 nm is observed
from the [Zn(L)2(OAc)2]
solid at room temperature.
Introduction
There is a current interest in strongly luminous materials due
to their potential applications in light-emitting diode devices.1–4
However, few metal–organic compounds showing intense
fluorescence or phosphorescence at wavelengths <450 nm are
reported in the literature. Notable examples are those containing
either 8-hydroxyquinoline or 7-azaindolate (aza) ligands.3,4
While polydentate nitrogen donor ligands are known
to have high binding affinity toward Zn(II), Al(III) and some heavy non-transition metal ions,5,6 previous studies suggest
that these metal–organic compounds are potentially new luminescent
materials exhibiting blue and/or white emissions.6 The strong binding affinity of the 1,8-naphthyridine moiety toward metal ions is well documented in the literature.7 Recent
studies by various workers have demonstrated that aromatic
amides can be effective chelating and/or bridging ligands
in a variety of metal complexes as exemplified by those linear
metal atoms arrays.5,8 In this work, we prepared N,N′-bisbenzyl-2,7-diamino-1,8-naphthyridine
(L) which combines the structural
features of naphthyridine and aromatic amide ligands.
Results and discussion
L was synthesized by the reaction of 2,7-dichloro-1,8-naphthyridine9
with benzylamine in toluene according to the reported
procedure i.e. refluxing a methanolic solution of Zn(OAc)2·2H2O and L for 30 min, and a pale yellow crystalline
solid was isolated by slow diffusion of diethyl ether into the
resultant solution. The solid was subsequently identified by X-ray
crystal analysis as [Zn(L)2(OAc)2].Fig.
1 depicts a perspective view of the [Zn(L)2(OAc)2] molecule.
The Zn atom adopts a distorted tetrahedral coordination
geometry; the measured Zn–N(3) and Zn–N(7) distances
of 2.043(2) and 2.050(2) Å, respectively, are comparable to
the related bond distance (2.055 Å) found for [Zn(dpa)(OAc)2]5b
(dpa = 2,2′-dipyridylamine). In principle, L can exhibit
a mondentate, bidentate or dinuclear bridging binding mode. As shown in the crystal packing diagram (Fig. 2), the molecules
are self-organized through extensive π–π stacking interactions
between the naphthyridyl rings. The interplanar separations
are 3.489 Å, and the molecules are linked to generate a supramolecular one-dimensional chain structure. We cannot identify
any significant π–π stacking interaction between the phenyl substituents
as reflected by the rather large interplanar separation
(>4.0 Å).
![Perspective
view of [Zn(L)2(OAc)2] (50% thermal ellipsoids) and atom-numbering scheme. Significant bond distances (Å) and angles (°): Zn(1)–N(3) 2.043(2), Zn(1)–N(7) 2.050(2), Zn(1)–O(1) 1.937(2), Zn(1)–O(3) 1.965(2); N(3)–Zn(1)–N(7) 129.00(9), N(3)–Zn(1)–O(1) 107.21(10), N(3)–Zn(1)–O(3) 99.07(9), N(7)–Zn(1)–O(1) 99.58(9), N(7)–Zn(1)–O(3) 109.22(10); O(1)–Zn(1)–O(3) 113.08(9).](/image/article/2001/NJ/b005644h/b005644h-f1.gif) |
| Fig. 1 Perspective
view of [Zn(L)2(OAc)2] (50% thermal ellipsoids) and atom-numbering scheme. Significant bond distances (Å) and angles (°): Zn(1)–N(3) 2.043(2), Zn(1)–N(7) 2.050(2), Zn(1)–O(1) 1.937(2), Zn(1)–O(3) 1.965(2); N(3)–Zn(1)–N(7) 129.00(9), N(3)–Zn(1)–O(1) 107.21(10), N(3)–Zn(1)–O(3) 99.07(9), N(7)–Zn(1)–O(1) 99.58(9), N(7)–Zn(1)–O(3) 109.22(10); O(1)–Zn(1)–O(3) 113.08(9). | |
![[Zn(L)2(OAc)2] molecules are self-assembled to form infinite one-dimensional chains by π–π stacking interactions.](/image/article/2001/NJ/b005644h/b005644h-f2.gif) |
| Fig. 2 [Zn(L)2(OAc)2] molecules are self-assembled to form infinite one-dimensional chains by π–π stacking interactions. | |
The photophysical properties of L and [Zn(L)2(OAc)2] are listed
in Table 1. The zinc complex shows two intense absorptions at λmax
= 236 (ε
= 1.48 × 105) and 367 nm (ε
= 8.3 × 104
dm3 cm−1 mol−1), which are
assigned to intraligand π–π* transitions of L. At room temperature, the zinc complex shows an intense emission at λmax
= 395 nm (τ
= 4.5 ns) in degassed MeOH, the excitation spectrum of which is the same as the absorption spectrum.
The emission is poorly vibronically-resolved with the vibra
tional spacing (ca. 1100 cm−1) comparable to the skeletal vibrational frequency of the free ligand (Fig. 3). We assign the emission to an intraligand 1(π–π*) fluorescence. It is noteworthy that the [Zn(L)2(OAc)2] complex has a higher emission quantum yield
than L (0.59
s. 0.38) in methanol solution at room temperature. Presumably, coordination of L to Zn(II) increases the ligand conformational rigidity, thereby reducing the non-radiative decay of the intraligand 1(π–π*) excited state. Similar enhancement of the intraligand fluorescence has also been reported for the [Zn(terpyridine)2]2+
system.6 At room temperature, the crystalline [Zn(L)2(OAc)2] solid shows a broad emission band (solid line in Fig. 3) ranging from 400 to 600 nm (peak maxima
at 434 and 470 nm), which is significantly red-shifted from the solution emission spectrum. As revealed by the crystal
packing diagram of the complex, the interplanar separation of 3.489 Å should allow excimeric interaction of the 1,8-naphthyridine moieties in the solid state, which may be responsible for the low energy solid state emission at wavelengths >450 nm.
Table 1
Photophysical data for L and the zinc(II) complex
Solid emission maxima | Methanol
solution at 298
K |
---|
| |
---|
UV-vis λmax/nm (ε/dm3 mol−1 cm−1)a | 298 K | 77 K | λmax/nmb | Φem | τ/nsc |
---|
|
---|
In MeOH at 298 K. Excitation wavelength = 367 nm. Excited at 266 nm picosecond pulses (300 K). |
---|
L | 236 (54000) | 418, 434 | 389, 410 | 396, 413(sh) | 0.38 | 3.6 |
367 (32000) | 500(sh) |
[Zn(L)2(OAc)2] | 236 (148000) | 434, 470 | 407 | 395, 414(sh) | 0.59 | 4.5 |
367 (83000) |
![Emission spectra of [Zn(L)2(OAc)2] in degassed MeOH (···) and in the solid state (——) at room temperature. Excitation: 367 nm.](/image/article/2001/NJ/b005644h/b005644h-f3.gif) |
| Fig. 3 Emission spectra of [Zn(L)2(OAc)2] in degassed MeOH (···) and in the solid state (——) at room temperature. Excitation: 367 nm. | |
Conclusion
In conclusion, the naphthyridyl ligand and its zinc(II) complex
exhibit a high energy blue emission in solution and a broad
white emission in the solid form. Compared with
[Zn4O(Aza)6]4 (Φ
= 0.17), the [Zn(L)2(OAc)2] molecules self-assemble through
intermolecular π–π interactions to form a supramolecular structure.
The high UV intraligand emission together with the visible
emission arising from intermolecular ligand–ligand interactions
suggest a future for supramolecular zinc(II) complexes as advanced materials in the design of white light emitters.6Experimental
All
the starting materials were used as received and solvents
were purified according to standard methods. 2,7-Dichloro-1,8-naphthyridine
was obtained commercially. The UV-vis spectra
were recorded on a Perkin-Elmer Lambda 19 spectrophotometer,
emission spectra on a SPEX Fluorolog-2 Model F11 fluorescence
spectrophotometer. Emission lifetimes of the compounds
were measured with a Quanta Ray DCR-3 Nd-YAG laser
as the excitation light source (pulse output 266 nm, 8 ns).
The 1H NMR spectra were recorded on a DPX-300 Bruker FT
spectrometer with chemical shifts (in ppm) relative to tetramethylsilane.
Elemental analyses were performed by the Institute
of Chemistry, Chinese Academy of Sciences, Beijing.Preparation
of
N,N′-bisbenzyl-2,7-diamino-1,8-naphthyridine (L)
A
mixture of 2,7-dichloro-1,8-naphthyridine7(0.5 g, 2.5 mmol) and
benzylamine (30 ml) was refluxed at 150°C for 8 h. After solvent evaporation, the residue was recrystallized from toluene
(20 ml) to afford a yellow crystalline solid. Overall yield:
0.48 g, 56%. Found: C, 77.59; H, 5.95; N, 16.50%. Calc. for C22H20N4: C, 77.62; H, 5.92; N, 16.46%. 1H NMR (270
MHz, CD3OD,
TMS):δ 7.59 (d, 2H, 3J
= 8.7), 7.38 (d, 4H, 3J
= 7.4), 7.29 (t, 4H, 3J
= 7.1), 7.27 (t, 2H, 3J
= 6.1), 6.46 (d, 2H, 3J
= 8.64 Hz), 4.68 (s, 4H). IR (KBr) ν/cm−1: 1600s, 1528s, 1342m, 1147m, 796w, 796w, 695w. EI-MS: m/z 340 [M]+.Preparation of [Zn(L)2(OAc)2]
A
methanolic solution (20 ml) of L (0.34 g, 1 mmol) was added
to a refluxing solution of Zn(OAc)2·2H2O (0.11 g, 0.5 mmol) in
MeOH (30 ml), and the mixture was refluxed for 3 h. After cooling
to room temperature, the solvent was removed by rotary
evaporation, and the white residue was recrystallized by
slow diffusion of diethyl ether into a methanolic solution to afford
colorless crystals. Overall yield: 0.3 g, 70%: Found: C, 66.45;
H, 5.51; N, 12.84%. Calc. for C48H46N8O4Zn: C, 66.70; H, 5.37; N, 12.96%. 1H NMR (270 MHz, CD3OD, TMS):δ
7.62 (d, 2H, 3J
= 8.5), 7.34 (m, 8H), 7.25 (m, 2H), 6.48 (d, 2H, 3J
= 8.6), 4.60 (d, 4H, 3J
= 5.5 Hz), 1.90 (s, 3H). FAB-MS: m/z 805 [M+
− OAc], 463 [M+
− L − OAc].X-Ray crystallographic chracterisation
of [Zn(L)2(OAc)2]
Crystals suitable for X-ray structure determinations were obtained
by slow diffusion of diethyl ether into a methanol solution
of the complex at room temperature. Data were collected
on a Bruker CCD SMART system, the crystal data and structure
refinement are: triclinic, space group P
, a
= 9.612(9),
b
= 15.884(15), c
= 15.884(15) Å, α
= 105.35(2), β
= 92.65(3),
γ
= 92.65(3)°, U
= 2332(4) Å3, Z
= 2, Dc
= 1.231 Mg m−3, μ(Mo-Kα) = 0.577
mm−1, F(000) = 904, T
= 294(2) K, 8311 independent reflections with I>2σ(I) were used in the refinement. The data were refined by full matrix least squares on F2, and R
= 0.085, Rw
= 0.21 with a goodness-of-fit of 0.82 were obtained. Note that the C(1)–C(6) and C(39)–C(41) atoms are disordered and were refined by isotropic refinement.CCDC
reference number 440/222. See http://www.rsc.org/suppdata/nj/b0/b005644h/
for crystallographic files in .cif format.
Acknowledgements
We
acknowledge support from The University of Hong Kong
and The Hong Kong Research Grants Council (HKU 7298/99P).References
- A. Kraft, A. C. Grimsdale and A.
B. Holmes, Angew. Chem., Int. Ed., 1998, 37, 402 CrossRef.
- M. Fukuda, K. Sawada and K. Yoshino, Jpn. J. Appl. Phys., 1989, 28, L1433 CAS; M. Fukuda, K. Sawada and K. Yoshino, J. Polym. Sci., Polym. Chem., 1993, 31, 2465 Search PubMed; G. Grem, G. Leditzky, B. Ullrich and G. Leising, Ad
. Mater., 1992, 4, 36 Search PubMed;
see also D. L. Gin and V. P. Conticello, Trends Polym. Sci., 1996, 4, 217 for a review. Search PubMed. - A. Hassan and S. Wang, Chem. Commun., 1998, 211
and references therein RSC.
- C.-F. Lee, K.-F. Chin, S.-M. Peng and C.-M. Che, J. Chem. Soc., Dalton Trans., 1993, 467 RSC; Y. Ma, H.-Y. Chao, Y. Wu, S.-T. Lee, W.-Y. Yu and C.-M. Che, Chem. Commun., 1998, 2491 RSC.
-
(a) K.-Y. Ho, W.-Y. Yu, K.-K. Cheung and C.-M. Che, Chem. Commun., 1998, 2101 RSC;
(b) K.-Y. Ho, W.-Y. Yu, K.-K. Cheung and C.-M. Che, J. Chem. Soc., Dalton Trans., 1999, 1581 RSC.
- N. W. Alcock, P. R. Barker, J. M. Haider, M. J. Hannon, C. L. Painting, Z. Pikramenou, E. A. Plummer, K. Rissanen and P. Saarenketo, J. Chem. Soc., Dalton Trans., 2000, 1447 RSC.
- R. L. Bodner and D. G. Hendricker, Inorg. Chem., 1970, 9, 1255 CrossRef CAS; A. Clearfield, R. Gopal and R. W. Olsen, Inorg. Chem., 1977, 16, 911 CrossRef CAS.
- R. Clerac, F. A. Cotton, K. R. Dunban, T. Lu, C. A. Murillo and X. Wang, Inorg. Chem., 2000, 39, 3065
and references therein CrossRef CAS; E. C. Yang, M. C. Cheng, M. S. Tsai and S. M. Peng, J. Chem. Soc., Chem. Commun., 1994, 2377 RSC.
- G. R. Newkome, S. J. Garbis, V. K. Majestic, F. R. Fronczek and G. Chiari, J. Org. Chem., 1981, 46, 833 CrossRef CAS.
|
This journal is © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2001 |
Click here to see how this site uses Cookies. View our privacy policy here.