1. Introduction

Recently, zero-dimensional (0D) metal hybrid halides have shown tremendous potential as luminescent materials due to their diverse structures and tunable photoluminescence (PL) (Han et al., 2021; Haque et al., 2025). Generally, 0D halides are com­posed of isolated metal halide clusters or units and bulky organic cations. In contrast to the high-dimensional halides, 0D hybrid halides always show strong quantum/dielectric confinement and possess much softer lattices, leading to an increased exciton binding energy, and allowing for the emission of self-trapped excitons (STEs) with wide broadband and large Stokes-shifted properties (Morad et al., 2019; Dastidar et al., 2024; Wang et al., 2024a). As a result, lower-energy green, yellow and red light emissions are easily realized in 0D halides, while higher-energy blue-light emission remains extremely challenging, especially in the pure-blue region (460–480 nm) (Ma et al., 2021; Zhang et al., 2021; Yang et al., 2025). Great effort has been made to explore the preparation of blue-emitting 0D halides. For instance, Lei and co-workers have reported a series of 0D hybrid zinc halides of the form AZnBr₄ [A = 1-ethylpiperazine (EP), 1-phenylpiperazine (BP) and N,N,2,2-tetra­methyl­propane-1,3-di­amine (TMPDA)] based on discrete [ZnBr₄]²⁻ tetra­hedra exhibiting blue emission (452–485 nm) and a maximum photolu­mi­nes­cence quan­tum yield (PLQY) of 35.47% (Ma et al., 2022a). Saparov and co-workers reported two halides, namely, (P-xd)ZnCl₄ and (P-xd)CdCl₄, where P-xd is p-xylylenedi­am­mo­nium, showing blue emission and a PLQY of 23% for (P-xd)ZnCl₄ (Popy et al., 2023). This group also utilized trimeth­yl(4-stilben­yl)methyl­ammonium (R⁺) as the organic cations to pre­pare indium(III) halides and the halide RInBr₄ displays high-efficiency blue emission with a PLQY value of 16.36% (Fattal et al., 2021). In addition, Zhang's group prepared a 0D hybrid zinc halide, i.e. (C₉H₁₄N)₂[ZnBr₄] (C₉H₁₄N⁺ is the N-butyl­pyridinium cation) based on the pyridine quaternary ammonium salt. This crystal shows efficient blue-light emission centred at 426 nm and a high quantum yield of 49.95% (Cheng et al., 2023). Despite the significant achievements that have been made in the development of 0D lead-free halides, achieving highly efficient blue-light emission is extremely challenging due to the limitation of an intrinsic large Stokes shift and an unavoidable self-absorption effect (Ma et al., 2022b; Peng et al., 2021; Cho et al., 2025). However, after summarizing the reported studies about blue-emitting 0D halides, we found that the construction of blue-emitting 0D halides is mainly concentrated on two strategies. The first common strategy is to adopt d¹⁰ metal ions, such as Zn²⁺ and Cd²⁺, which have a relatively stable outer electronic structure (Liu et al., 2024; Tan et al., 2025; Hao et al., 2022). The other is to focus on the modulation of organic cations. The organic com­ponents play a crucial role in generating blue emission and the introduction of an organic cation into a hybrid structure can improve its blue-emission efficiency (Deng et al., 2023; Belikova et al., 2024). These findings provide important inspiration for the design of hybrid halides as blue-light emitters.

In this study, a new 0D organic–inorganic hybrid halide, namely, bis­[4-(4-chloro­phen­yl)pyridinium] tetra­bromido­zinc­ate, [HCPP]₂[ZnBr₄], was prepared by adopting chlorine-de­cor­ated 4-phenyl­pyridine as the cation source. This halide exhibits an intense blue emission centred at 432 nm through STEs of the [ZnBr₄]²⁻ unit and a singlet excited state of the organic cations with a PLQY of 56.35%, surpassing almost all previously reported 0D zinc halide counterparts. This work provides an efficient way to achieve high-efficiency blue emission through the modulation of appropriate organics.

2. Experimental

2.3. Refinement

Crystal data, data collection and structure refinement details are summarized in Table 1. H atoms connected to C atoms were located at geometrically calculated positions.

Table 1 Experimental details Crystal data Chemical formula (C11H9ClN)2[ZnBr4] Mr 766.27 Crystal system, space group Monoclinic, C2/c Temperature (K) 285 a, b, c (Å) 12.7583 (6), 14.1507 (5), 15.2706 (5) β (°) 110.067 (1) V (Å3) 2589.57 (18) Z 4 Radiation type Mo Kα μ (mm−1) 7.34 Crystal size (mm) 0.16 × 0.16 × 0.15   Data collection Diffractometer Bruker APEXII CCD Absorption correction Multi-scan (SADABS; Bruker, 2016) Tmin, Tmax 0.321, 0.333 No. of measured, independent and observed [I > 2σ(I)] reflections 42246, 3223, 2597 Rint 0.074 (sin θ/λ)max (Å−1) 0.667   Refinement R[F2 > 2σ(F2)], wR(F2), S 0.027, 0.067, 1.02 No. of reflections 3223 No. of parameters 145 H-atom treatment H atoms treated by a mixture of independent and constrained refinement Δρmax, Δρmin (e Å−3) 0.26, −0.61 Computer programs: APEX2 (Bruker, 2016), SAINT (Bruker, 2016), SHELXT (Sheldrick, 2015a), SHELXL2014 (Sheldrick, 2015b) and OLEX2 (Dolomanov et al., 2009).

3. Results and discussion

Single-crystal X-ray diffraction analysis reveals that [HCPP]₂[ZnBr₄] crystallized in the monoclinic space group C2/c. The asymmetric unit consists of two [HCPP]⁺ cations and one [ZnBr₄]²⁻ anion. As shown in Fig. 1(a), each Zn²⁺ ion is coordinated to four Br⁻ anions to form a [ZnBr₄]²⁻ tetra­hedron, yielding a zero-dimensional (0D) structure, with an average Zn—Br bond length of 2.4055 (4) Å and an average Cl—Zn—Cl bond angle of 109.04 (3)°, which are close to those of [ZnBr₄]²⁻ tetra­hedra in previously reported Zn-based hybrid halides (He et al., 2023). Each [ZnBr₄]²⁻ tetra­hedron connects two [HCPP]⁺ cations via inter­molecular hy­dro­gen bonds to form a 0D dimer; the Br⋯H—N angle is 140.47 (3)°. One [HCPP]⁺ cation of this dimer is arranged parallel to one [HCPP]⁺ cation of another dimer, with an inter­planar distance of 3.81 (1) Å [Fig. 1(b)]. These dimers are further stacked into a 3D supra­molecular structure [Fig. 1(c)]. Notably, the pyridine and arene rings are nearly coplanar in the [HCPP]⁺ cation, with a small dihedral angle between their planes of 5.05 (3)°. The degree of structural distortion of the ZnBr₄ tetra­hedron was evaluated by calculating the distortion of the bond lengths (Δd) according to the equation of Robinson et al. (1971). The calculated Δd value is 1.14 × 10⁻⁴ and such a small degree of distortion (Liu et al., 2023; Wang et al., 2024b) suggests the weak STEs of the [ZnBr₄]²⁻ cluster, which is good for the generation of the narrow emission band of [HCPP]₂[ZnBr₄].

Figure 1 The mol­ecular structure of [HCPP]2[ZnBr4], showing (a) the tetra­hedral [ZnBr4]2− anion and [HCPP]+ cation, (b) the inter­actions between the cations and inorganic nodes, and (c) a packing diagram of [HCPP]2[ZnBr4].

The powder X-ray diffraction (PXRD) pattern of [HCPP]₂[ZnBr₄] matches well with that simulated from the obtained single-crystal data without any extra diffraction peaks, indicating their high purity [Fig. 2(a)]. The thermogravimetric (TG) curve shows that the [HCPP]₂[ZnBr₄] crystals have excellent thermal stability; they begin to decom­pose from ∼250 °C [Fig. 2(b)].

Figure 2 The basic characterization of [HCPP]2[ZnBr4], showing (a) experimental and simulated PXRD patterns of [HCPP]2[ZnBr4] and (b) thermal analysis of [HCPP]2[ZnBr4].

The photophysical properties of [HCPP]₂[ZnBr₄] were investigated. As shown in Fig. 3(a), the solid-state UV–Vis absorption spectrum of [HCPP]₂[ZnBr₄] shows that the band edge is at about 400 nm. Upon 360 nm excitation, [HCPP]₂[ZnBr₄] exhibits a narrow blue-emission band centred at 432 nm, with the full-width at half-maximum (FWHM) of 76 nm, a large Stokes shift of 46 nm and a high PLQY of 56.35%. The Commission Inter­nationale de l'Éclairage (CIE 1931) chromaticity coordinates were determined as (0.16, 0.07) [Fig. 3(c)]. The PL decay lifetime is 1.76 ns at 432 nm [Fig. 3(d)].

Figure 3 The luminescence properties and theoretical calculations for [HCPP]2[ZnBr4], showing (a) the solid-state UV–Vis absorption spectrum of [HCPP]2[ZnBr4], (b) the excitation and emission spectra of organic mol­ecule CPP and [HCPP]2[ZnBr4], (c) the CIE chromaticity coordinates, (d) the PL decay curves of CPP and [HCPP]2[ZnBr4], (e) DOS and (f) calculated band structure.

In addition, the luminescence behaviour of CPP powder was tested and it exhibited a weak blue emission with a peak at 375 nm under 320 nm excitation [Fig. 3(b)], a PLQY of 11.36% and the CIE chromaticity coordinates (0.15, 0.06). The time-resolved spectrum of CPP under ambient conditions revealed a delay time of 0.86 ns at 375 nm [Fig. 3(d)]. Compared to the PLQY of CPP, the PLQY of [HCPP]₂[ZnBr₄] shows a 4.96-fold improvement, which may be attributed largely to the ordered arrangement and rigid enhancement of CPP mol­ecules in the halide matrix, leading to nonradiative transition suppression and PLQY improvements.

To better understand the luminescence mechanism of [HCPP]₂[ZnBr₄], DFT calculations were performed to investigate the band structure and density of states (DOS). The valence band maximum (VBM) of [HCPP]₂[ZnBr₄] is mainly located in the [HCPP]⁺ cation and the Br 4p orbital, and the conduction band maximum (CBM) is mainly distributed in the [HCPP]⁺ cation [Fig. 3(e)]. The energy band gap between the valence and conduction band of [HCPP]₂[ZnBr₄] is 2.48 eV [Fig. 3(f)], with such a small value indicating an intense quantum confinement effect. These results further confirm that the enhancement of blue emission of [HCPP]₂[ZnBr₄] originates from the organic cations and the STE of the [ZnBr₄]²⁻ node (Qi et al., 2022).

4. Summary

By adopting chlorine-decorated 4-phenyl­pyridine, a zinc-based hybrid halide has been prepared. It exhibits efficient blue emission with a high PLQY of 56.35%, among the highest reported in the literature to date. Experimental and theoretical studies indicate that the emission originates from the synergistic function of the electron transitions in the organic cations and the STE states. This work lays the foundation for synthesizing more efficient blue-emission hybrid halides.