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 Wana, Kin-Ying Hoa and Zhong-Yun Zhoub
aDepartment of Chemistry, The Uni[italic v]ersity of Hong Kong, Pokfulum Road, Hong Kong
bDepartment of Applied Biology and Chemical Technology and Materials Research Center, Hong Kong Polytechnic Uni[italic v]ersity, Hunghom, Hong Kong

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.


ugraphic, filename = b005644h-u1.gif

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).
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.
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 [italic v]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 maximaMethanol solution at 298 K
  
UV-vis λmax/nm (ε/dm3 mol−1 cm−1)a298 K77 Kλmax/nmbΦemτ/nsc
 
a In MeOH at 298 K.b Excitation wavelength = 367 nm.c Excited at 266 nm picosecond pulses (300 K).
L236 (54000)418, 434389, 410396, 413(sh)0.383.6
367 (32000)500(sh)
[Zn(L)2(OAc)2]236 (148000)434, 470407395, 414(sh)0.594.5
367 (83000)



Emission spectra of [Zn(L)2(OAc)2] in degassed MeOH (···) and in the solid state (——) at room temperature. Excitation: 367 nm.
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.6

Experimental

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[1 with combining macron], 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).

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