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

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.

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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).

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).

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 λ_max = 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 λ_max = 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 maxima		Methanol solution at 298 K
UV-vis λmax/nm (ε/dm^3 mol^−1 cm^−1)^a	298 K	77 K	λmax/nm^b	Φem	τ/ns^c
a In MeOH at 298 K.b Excitation wavelength = 367 nm.c 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)

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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 P1̄, 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) ų, Z = 2, D_c = 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, R_w = 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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