Strongly luminescent metal-organic compounds: Spectroscopic properties and crystal structure of substituted 1,8-naphthyridine and its zinc(II) complex

Article abstract of DOI:10.1039/b005644h

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)

Full text of DOI:10.1039/b005644h

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Strongly luminescent metal–organic compounds: spectroscopic
properties and crystal structure of substituted 1,8-naphthyridine and
its zinc(II) complex
Chi-Ming Che,*a Chun-Wai Wan,a Kin-Ying Hoa and Zhong-Yun Zhoub
a Department of Chemistry, T he University of Hong Kong, Pokfulum Road, Hong Kong
b Department of Applied Biology and Chemical T echnology and Materials Research Center,
Hong Kong Polytechnic University, Hunghom, Hong Kong
Received (in Montpellier, France) 12th July 2000, Accepted 6th October 2000
First published as an Advance Article on the web 8th December 2000
N,N@-Bisbenzyl-2,7-diamino-1,8-naphthyridine (L) and its zinc(II) complex, [Zn(L) (OAc) ], are strongly
2
2
luminescent materials with emission quantum yields in methanol being 0.38 and 0.59, respectively. The crystal
structure of [Zn(L) (OAc) ] features a one-dimensional chain structure through intermolecular pÈp stacking
2
2
interactions. A broad emission band ranging from 400 to 600 nm is observed from the [Zn(L) (OAc) ] solid at
room temperature.
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Fig. 1 depicts a perspective view of the [Zn(L) (OAc) ] mol-
Introduction
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2
ecule. The Zn atom adopts a distorted tetrahedral coordi-
nation geometry; the measured ZnÈN(3) and ZnÈN(7)
distances of 2.043(2) and 2.050(2) A, respectively, are compara-
ble to the related bond distance (2.055 A) found for
There is a current interest in strongly luminous materials due
to their potential applications in light-emitting diode
devices.1h4 However, few metalÈorganic compounds showing
intense Ñuorescence 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 metalÈorganic compounds are potentially
new luminescent materials exhibiting blue and/or white emis-
sions.6 The strong binding affinity of the 1,8-naphthyridine
moiety toward metal ions is well documented in the liter-
ature.7 Recent studies by various workers have demonstrated
that aromatic amides can be e†ective chelating and/or bridg-
ing ligands in a variety of metal complexes as exempliÐed by
those linear metal atoms arrays.5,8 In this work, we prepared
N,N@-bisbenzyl-2,7-diamino-1,8-naphthyridine (L) which com-
bines the structural features of naphthyridine and aromatic
amide ligands.
[Zn(dpa)(OAc) ]5b (dpa \ 2,2@-dipyridylamine). In principle, L
2
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 pÈp
stacking interactions between the naphthyridyl rings. The
interplanar separations are 3.489 A, and the molecules are
linked to generate a supramolecular one-dimensional chain
structure. We cannot identify any signiÐcant pÈp stacking
interaction between the phenyl substituents as reÑected by the
rather large interplanar separation ([4.0 A).
The photophysical properties of L and [Zn(L) (OAc) ] are
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2
listed in Table 1. The zinc complex shows two intense absorp-
tions at j \ 236 (e \ 1.48 ] 105) and 367 nm (e \ 8.3 ] 104
max
dm3 cm~1 mol~1), which are assigned to intraligand pÈp*
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. reÑuxing a methanolic solution of
Zn(OAc) É 2H O and L for 30 min, and a pale yellow crys-
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2
talline solid was isolated by slow di†usion of diethyl ether into
the resultant solution. The solid was subsequently identiÐed
by X-ray crystal analysis as [Zn(L) (OAc) ].
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2
Fig. 1 Perspective view of [Zn(L) (OAc) ] (50% thermal ellipsoids)
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2
and atom-numbering scheme. SigniÐcant bond distances (A) and
angles (¡): Zn(1)ÈN(3) 2.043(2), Zn(1)ÈN(7) 2.050(2), Zn(1)ÈO(1)
1.937(2), Zn(1)ÈO(3) 1.965(2); N(3)ÈZn(1)ÈN(7) 129.00(9), N(3)ÈZn(1)È
O(1) 107.21(10), N(3)ÈZn(1)ÈO(3) 99.07(9), N(7)ÈZn(1)ÈO(1) 99.58(9),
N(7)ÈZn(1)ÈO(3) 109.22(10); O(1)ÈZn(1)ÈO(3) 113.08(9).
DOI: 10.1039/b005644h
New J. Chem., 2001, 25, 63È65
63
This journal is ( The Royal Society of Chemistry and the Centre National de la Recherche ScientiÐque 2001

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cm~1) comparable to the skeletal vibrational frequency of the
free ligand (Fig. 3). We assign the emission to an intraligand
1(pÈp*) Ñuorescence. It is noteworthy that the [Zn(L) (OAc) ]
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complex has a higher emission quantum yield than L (0.59 vs.
0.38) in methanol solution at room temperature. Presumably,
coordination of L to Zn(II) increases the ligand conforma-
tional rigidity, thereby reducing the non-radiative decay of the
intraligand 1(pÈp*) excited state. Similar enhancement of the
intraligand Ñuorescence has also been reported for the
[Zn(terpyridine) ]2` system.6 At room temperature, the crys-
2
talline [Zn(L) (OAc) ] solid shows a broad emission band
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2
(solid line in Fig. 3) ranging from 400 to 600 nm (peak
maxima at 434 and 470 nm), which is signiÐcantly red-shifted
from the solution emission spectrum. As revealed by the
crystal packing diagram of the complex, the interplanar
separation of 3.489 A 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 wave-
lengths [450 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
[Zn O(Aza) ]4 (U \ 0.17), the [Zn(L) (OAc) ] molecules self-
4
6
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2
assemble through intermolecular pÈp interactions to form a
supramolecular structure. The high UV intraligand emission
together with the visible emission arising from intermolecular
ligandÈligand interactions suggest a future for supramolecular
zinc(II) complexes as advanced materials in the design of white
light emitters.6
Fig. 2 [Zn(L) (OAc) ] molecules are self-assembled to form inÐnite
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one-dimensional chains by pÈp stacking interactions.
transitions of L. At room temperature, the zinc complex
shows an intense emission at j \ 395 nm (q \ 4.5 ns) in
max
degassed MeOH, the excitation spectrum of which is the same
Experimental
as the absorption spectrum. The emission is poorly
All the starting materials were used as received and solvents
were puriÐed 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 spectro-
photometer, emission spectra on a SPEX Fluorolog-2 Model
F11 Ñuorescence 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.
vibronically-resolved with the vibrational spacing (ca. 1100
Preparation of N,Nº-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 reÑuxed at 150 ¡C for 8 h. After
solvent evaporation, the residue was recrystallized from
toluene (20 ml) to a†ord a yellow crystalline solid. Overall
yield: 0.48 g, 56%. Found: C, 77.59; H, 5.95; N, 16.50%. Calc.
Fig. 3 Emission spectra of [Zn(L) (OAc) ] in degassed MeOH (É É É)
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2
and in the solid state (ÈÈ) at room temperature. Excitation: 367 nm.
for C
H
N : C, 77.62; H, 5.92; N, 16.46%. 1H NMR (270
22 20
4
Table 1 Photophysical data for L and the zinc(II) complex
Solid emission maxima
Methanol solution at 298 K
UV-vis
/nm (e/dm3 mol~1 cm~1)a
j
298 K
77 K
j
/nmb
U
s/nsc
3.6
max
max
em
L
236 (54 000)
367 (32 000)
236 (148 000)
367 (83 000)
418, 434
500(sh)
434, 470
389, 410
407
396, 413(sh)
395, 414(sh)
0.38
0.59
[Zn(L) (OAc) ]
4.5
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2
a In MeOH at 298 K. b Excitation wavelength \ 367 nm. c Excited at 266 nm picosecond pulses (300 K).
64
New J. Chem., 2001, 25, 63È65

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MHz, CD OD, TMS):d 7.59 (d, 2H, 3J \ 8.7), 7.38 (d, 4H,
CCDC reference number 440/222. See http://www.rsc.org/
suppdata/nj/b0/b005644h/ for crystallographic Ðles in .cif
format.
3
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) l/cm~1: 1600s,
1528s, 1342m, 1147m, 796w, 796w, 695w. EI-MS: m/z 340
[M]`.
Acknowledgements
Preparation of [Zn(L) (OAc) ]
We acknowledge support from The University of Hong Kong
and The Hong Kong Research Grants Council (HKU 7298/
99P).
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2
A methanolic solution (20 ml) of L (0.34 g, 1 mmol) was added
to a reÑuxing solution of Zn(OAc) É 2H O (0.11 g, 0.5 mmol)
in MeOH (30 ml), and the mixture was reÑuxed for 3 h. After
cooling to room temperature, the solvent was removed by
rotary evaporation, and the white residue was recrystallized
by slow di†usion of diethyl ether into a methanolic solution to
a†ord colorless crystals. Overall yield: 0.3 g, 70%: Found: C,
2
2
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66.45; H, 5.51; N, 12.84%. Calc. for C
H
N O Zn: C,
48 46
8 4
66.70; H, 5.37; N, 12.96%. 1H NMR (270 MHz, CD OD,
3
TMS):d 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-
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3
4
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2
2
Crystals suitable for X-ray structure determinations were
obtained by slow di†usion of diethyl ether into a methanol
solution of the complex at room temperature. Data were col-
lected on a Bruker CCD SMART system, the crystal data and
structure reÐnement are: triclinic, space group P1,
a \ 9.612(9), b \ 15.884(15), c \ 15.884(15) A, a \ 105.35(2),
b \ 92.65(3), c \ 92.65(3)¡, U \ 2332(4) A
Mg m~3, k(Mo-Ka) \ 0.577 mm~1, F(000) \ 904, T \ 294(2)
K, 8311 independent reÑections with I [ 2p(I) were used in
the reÐnement. The data were reÐned by full matrix least
5
6
7
8
Ž
3, Z \ 2, D \ 1.231
c
squares on F2, and R \ 0.085, R \ 0.21with a goodness-of-
Ðt of 0.82 were obtained. Note that the C(1)ÈC(6) and C(39)È
C(41) atoms are disordered and were reÐned by isotropic
reÐnement.
w
9
New J. Chem., 2001, 25, 63È65
65