Wide range zero-thermal-quenching ultralong phosphorescence from zero-dimensional metal halide hybrids

Materials with ultralong phosphorescence have wide-ranging application prospects in biological imaging, light-emitting devices, and anti-counterfeiting. Usually, molecular phosphorescence is significantly quenched with increasing temperature, rendering it difficult to achieve high-efficiency and ultralong room temperature phosphorescence. Herein, we spearhead this challenging effort to design thermal-quenching resistant phosphorescent materials based on an effective intermediate energy buffer and energy transfer route. Co-crystallized assembly of zero-dimensional metal halide organic-inorganic hybrids enables ultralong room temperature phosphorescence of (Ph4P)2Cd2Br6 that maintains luminescent stability across a wide temperature range from 100 to 320 K (ΔT = 220 °C) with the room temperature phosphorescence quantum yield of 62.79% and lifetime of 37.85 ms, which exceeds those of other state-of-the-art systems. Therefore, this work not only describes a design for thermal-quenching-resistant luminescent materials with high efficiency, but also demonstrates an effective way to obtain intelligent systems with long-lasting room temperature phosphorescence for optical storage and logic compilation applications.

M olecular luminescent materials have attracted increasingly widespread attention due to their extensive applications in illumination resources, light-emitting diodes (LEDs), and biological imaging among many others [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20] . It is well known that the luminescence of molecular systems is usually sensitive to temperature, such that increasing temperature tends to provoke with intensification of molecular rotation and lattice vibration in chromophores, thereby facilitating the universal thermal-quenching (TQ) effect 21,22 . For example, such thermal effects are obstacles to high luminescent efficiency and therefore restrict the practical suitability of phosphors for commercial applications involving LEDs at high temperature. Quite recently, it was demonstrated that the introduction of intermediate energy levels or defect states for energy transfer in pure inorganic phosphors can diminish the TQ effect to some extent 23-27 ; however, examples of materials with zero-TQ emission are still rather rare, particularly for the case of molecular systems (Supplementary Table 1) [28][29][30] . Therefore, obtaining TQresistant materials remains an open challenge.
The origin of phosphorescence usually involves a photoluminescence (PL) transition between the excited triplet and singlet states. The forbidden transition renders molecular phosphorescence generally observable at very low temperatures, but difficult to achieve at ambient conditions 31 . Until recently, molecules with long-lived room-temperature phosphorescence (RTP) garnered much interest due to their potential applications in displays, anti-counterfeiting, and information storage 22,[32][33][34][35][36][37][38][39][40] . To date, despite continuous reports of RTP systems with ultralong lifetimes ranging from the microsecond (ms) to second (s) timescale, such materials with high RTP efficiency are still quite limited, since pronounced quenching of the triplet excitons occurs via nonradiative transitions at elevated temperature 21,22 . Considering the long-standing scientific and technological demands for simultaneous thermally resistant luminescence and ultralong RTP with high efficiency, the facile design of materials with zero TQ phosphorescence could greatly enrich the RTP family, and accordingly enhance the achievable quantum yield. However, such systems are still merely speculated.
Low-dimensional organic-inorganic hybrid metal halides have exhibited novel optoelectronic properties in applications, such as perovskite solar cells and photoemission devices 41,42 . For example, in zero-dimensional (0D) metal-halide systems, isolation of the inorganic polyhedral units from each other by large organic cations can promote exciton self-trapping and/or excited-state structural reorganization, resulting in a high photoluminescence quantum yield. As such, the co-crystallization of single/multinuclear metal-halide anions and cations is a promising approach toward building the versatile structures and interactions necessary to discover new phosphor systems. To date, however, ultralong RTP for 0D metal-halide hybrid materials remains unexploited. Facilely tunable metal and halide elements can be expected to provide a means of efficient modulation of spin-orbital coupling toward high-efficiency RTP output for practical applications. Moreover, anticipated severe restriction of the rotation and vibration of organic units due to the multiple noncovalent interactions with the surrounding metal-halide clusters could effectively facilitate resistance of the heat-quenching effect within the metal halide.
Herein, 0D metal-halide hybrids serve as a model system to spearhead the challenge of developing materials that achieve zero-TQ phosphorescence. The tetraphenylphosphine cation (Ph 4 P + ) was selected to supply organic moieties due to its potential to obtain long-lived triplet excitons. Its large C 3 symmetric skeleton is predicted to be relatively resistant to rotation upon heating. Furthermore, energy transfer between different energy levels can also compensate for energy loss associated with heat quenching.
The optimized cluster units and halide components of the (Ph 4 P) 2 Cd 2 Br 6 enable it to exhibit a definitive zero-TQ phosphorescence and stable RTP intensity across a wide temperature range (100-320 K), together with ultralong RTP lifetime (τ = 37.85 ms, λ em = 500 nm) and high RTP quantum yield (QY phos = 62.79%). This QY value marks a new record among state-of-theart molecular long-afterglow RTP materials. Moreover, the wide tunability of excitation-dependent RTP also endows the metal halides with great potential for applications in information encoding and optical logic gates. Therefore, this work not only provides an alternative strategy to achieve wide-ranging zero-TQ luminescence, but also introduces a new perspective on designing high-efficiency and ultralong RTP based on 0D metal-halide hybrids.

Results
Construction of 0D metal-halide hybrids. To promote spin-orbital coupling for ultralong-lived RTP, cadmium and different halogens (X = Cl and Br) with tunable heavy atomic effects were selected to serve as the metal-halide units. Representative systems consisting of four new metal-halide organic-inorganic hybrids with 0D single-nuclear (CdX 4 2− ) and dinuclear (Cd 2 X 6 2− ) clusters were successfully designed and co-crystallized via a facile hydrothermal reaction (Fig. 1). Single-crystal structure analysis reveals that 0D (Ph 4 P) 2 CdX 4 metal halides adopt a monoclinic space group C2/c wherein one CdX 4 2− tetrahedron anion is surrounded by four Ph 4 P + cations, such that CdX 4 2− units are separated from each other. The Cd … Cd distance is 10.28 Å for CdCl 4 2− (or 10.38 Å for CdBr 4 2− ), while the P … P distance is 6.24 Å (or 6.36 Å for CdBr 4 2− ). The Ph 4 P + units are highly isolated, which has a prohibitive effect on the formation of H-or J-type molecular aggregation. For (Ph 4 P) 2 Cd 2 X 6 , the Cd 2 X 6 2− units exhibit a co-edge metal-halide tetrahedral structure, giving rise to molecular stacking between Ph groups of two adjacent cations to produce a staggered pattern and further form an extended 1D zig-zag linear structure (Fig. 1b, c, e). The molecular rigidity imparted by this close arrangement and relatively high degree of condensation may restrict local structural relaxation of excited states, thereby inhibiting molecular rotationinduced nonradiative transition. Moreover, since the crystal structure appears relatively unimpacted by whether the halogen atom is Cl or Br, the single-nuclear and dinuclear hybrids in (Ph 4 P) 2 CdX 4 (Supplementary Table 2) and (Ph 4 P) 2 Cd 2 X 6 (Supplementary Table 3) can be regarded as isostructural systems, respectively. In both co-crystallized structures, the inorganic metal halides and organic cations exhibit a long-range ordered arrangement through cation-anion electrostatic interaction and C-H … Cl (or C-H … Br) hydrogen halide bonds (Fig. 1a, e, f). Under the similar structure, the differences in C-H···π and X···π for different halogen atoms are only 0.1-0.2 Å (Fig. 1b-d). The Cl … H bond lengths in (Ph 4 P) 2 Cd 2 Cl 6 are mainly 2.5-2.8 Å, while in (Ph 4 P) 2 Cd 2 Br 6 , the bond lengths are ca. 3.0 Å. It is also shown that there are water molecules in the crystal lattice of (Ph 4 P)Cd 2 Cl 6 , while not in the case of (Ph 4 P)Cd 2 Br 6 . The inclusion of solvents can be attributed to the following facts: (1) the atomic radius of the Br (0.114 nm) is larger than that of Cl (0.099 nm), i.e., when Cl replaces the position of Br, small water molecules enter the structure to fill into the cavity and form Cl … H bonds for structural stability; (2) the Pauling electronegativity of Cl (3.0) is stronger than that of Br (2.8), and thus it is easier for Cl binding hydrogen in water molecules to form hydrogen bonds.
( Fig. 2a, b) exhibit a typical two-band emission, with one in the ultraviolet region (λ em ≈ 350 nm) and the other corresponding to green wavelengths and possessing three major peaks around 470, 500, and 540 nm. The delayed PL spectra of (Ph 4 P) 2 Cd 2 X 6 overlap nearly perfectly with the long-wavelength regions of the prompt one, suggesting that the photoluminescence of (Ph 4 P) 2 Cd 2 X 6 feature both fluorescence and phosphorescence two-band characteristics. To identify the luminescent contribution of these materials, we measured the spectra of their corresponding organic ionic cocrystals (Ph 4 PCl and Ph 4 PBr) and four metal halides dissolved in N,N-dimethylformamide, which show only a sharp luminescence peak around 320 nm ( Supplementary  Fig. 2). In contrast, the crystalline forms of Ph 4 PCl and Ph 4 PBr show double emission around 350 and 500 nm ( Supplementary  Fig. 3a, c). The luminescence at 350 nm is consistent with the redshifted singlet emission commonly observed in organic solid-state materials compared to their solutions 43 . The emission at 500 nm can be attributed to typical crystallization-induced RTP behavior. The RTP QY phos values for the Ph 4 PCl and Ph 4 PBr are 8.02% and 21.77%, respectively (Supplementary Table 4). The relative high photoluminescence for Ph 4 PBr is related to the increased intermolecular distance as the change of anions from Cl to Br, and consequently weaker intermolecular electronic coupling results in the decrease of aggregation-induced quenching. Moreover, Br atom has a stronger spin-orbit coupling effect relative to that of Cl atom, which promotes the intersystem crossing rate, and thus is more inclined to RTP emission.
Together with the observations above and obvious different emissions from pristine CdX 2 ( Supplementary Fig. 4), the PL nature of (Ph 4 P) 2 Cd 2 X 6 is thus originally derived from its organic Ph 4 P + unit. It is also noted that the RTP emission of (Ph 4 P) 2 Cd 2 Br 6 is largely dominant over that of (Ph 4 P) 2 Cd 2 Cl 6 in the PL spectra, which can be ascribed to the stronger spin-orbital coupling of Cd 2 Br 6 2− units compared to those of Cd 2 Cl 6 2− , and results in the acceleration of intersystem crossing processes. In addition, the relationship between the prompt and delayed PL spectra clearly varies depending on the identity of (Ph 4 P) 2 CdX 4 :(Ph 4 P) 2 CdCl 4 exhibits nearly complete overlapping of its delayed mode and prompt spectra, while (Ph 4 P) 2 CdBr 4 exhibits strong fluorescence but very weak RTP output. This behavior may be related to the RTP emission quenching through the triplet-triplet annihilation process 31 . The luminescence performance of (Ph 4 P) 2 Cd 2 X 6 is generally better than (Ph 4 P) 2 CdX 4 , which can be attributed to that the distance between organic emissive molecules is closer in (Ph 4 P) 2 CdX 4 (Supplementary Table 5), and thus the energy is easier to be dissipated in the form of nonradiative transitions. Therefore, introducing different metal-halide clusters is an effective means of adjusting the singlet-triplet energy distribution and photoluminescence. To detect the role of different metals in the RTP emission, we have further synthesized Pb halide-based system, (Ph 4 P) 2 Pb 3 Cl 8 as a control example. It shows much shorter luminescence lifetime of 9.56 μs at 500 nm excited at 300 nm. The structure also exhibits that the distance between organic molecules becomes larger and the interaction becomes smaller ( Supplementary Fig. 5). This indicates that both the metal halide with moderate heavy-atom effect and suitable packing fashion could lead to different long-lived RTP emission.
To better understand the excitation-state properties, the timeresolved PL spectra of the metal-halide hybrids were measured at room temperature. The longest RTP lifetime of 0.82 s occurs at (Ph 4 P) 2 Cd 2 Cl 6 , with the corresponding QY phos of 23.49%, while the (Ph 4 P) 2 Cd 2 Br 6 exhibits the RTP lifetime of 37.85 ms and QY phos of 62.79% (Fig. 2c, d). These results are consistent with the emissive characteristics of metal-free Ph 4 PCl and Ph 4 PBr cocrystals ( Supplementary Fig. 3), namely the long-lived phosphorescence but weak intensity of the former versus the shorter phosphorescence lifetime but enhanced RTP of the latter. Such behaviors are rooted in the inherently competitive relationship between RTP QY phos and lifetime. Notably, the QY phos value reported in this study for (Ph 4 P) 2 Cd 2 Br 6 exceeds those of other asreported molecule-based RTP systems (Supplementary Table 6). The demonstrated combination of high RTP QY phos and ultralong lifetime is an important advancement in the development of highquality solid-state light-emitting applications.
To obtain insight into the influence of water molecules on RTP of (Ph 4 P) 2 Cd 2 Cl 6 , we further performed a control experiment after dehydration by heating. The thermogravimetric analysis (TGA, Supplementary Fig. 6) curve shows that the first step weight loss occurs at around 110°C (384 K), which corresponds to the loss of water content (1.59%) determined by single-crystal XRD analysis. Thus, we have heated the sample at 110°C (384 K) for an hour to remove the water and then cooled down to room temperature. It was observed that the RTP intensity of the dehydration system has slightly decreased in the range of 450-550 nm ( Supplementary Fig. 7a), and the RTP lifetime has also decreased from 0.82 s to 0.71 s compared with the pristine form ( Supplementary Fig. 7b), indicating that the inclusion of water molecules could stabilize the RTP to some extent based on the hydrogen-bonding interactions, and similar behavior also occurs for some other molecular cocrystal and carbon dot systems 44,45 . Moreover, to study the stability of the watercontaining phase at and above room temperature under illumination, we selected the typical temperatures (300 K and 320 K) to detect the photostability. It was observed that the RTP intensity can be readily repeated during the heating-cooling process at two typical temperatures ( Supplementary Fig. 7c), suggesting that RTP emission of the water-containing phase is stable at room temperature. Moreover, we performed the alternate illumination on the sample at typical excitation 254 and 365 nm ( Supplementary Fig. 7d), and the luminescent intensity is also repeatable under different illumination, confirming the photostability of the sample upon irradiation.
Temperature-resistant properties in metal halides. To probe the thermally luminescent properties of the 0D metal-halide hybrids, the temperature-dependent emission measurements were performed on the (Ph 4 P) 2 Cd 2 Br 6 , which exhibited both high RTP QY and ultralong lifetime. The PL emission spectra (Fig. 3a) reveal three prominent phosphorescence bands with relatively close triplet states (T 1 , T 2 , and T 3 ), which undergo a slight redshift as temperature increases from 100 to 400 K 46 . The phosphorescence intensity of (Ph 4 P) 2 Cd 2 Br 6 is particularly stable over a wider temperature range (100-320 K), with the T 1 and T 3 band at 320 K maintaining intensities 95% of those at 100 K (Fig. 3b, c). The emission intensity finally begins to decline at~340 K.
Moreover, increasing temperature evokes only a slight decrease of the lifetimes for the (Ph 4 P) 2 Cd 2 Br 6 from 49.58 ms (100 K) to 33.50 ms (300 K) ( Fig. 3d and Supplementary Table 7), suggesting that it can maintain its ultralong phosphorescent nature across a broad temperature range (>200 K). Therefore, the (Ph 4 P) 2 Cd 2 Br 6 can be regarded as a new type of zero-TQ phosphorescent materials with ultralong lifetime. To the best of our knowledge, although the zero TQ luminescent materials based on an inorganic compound and metal complex have been developed quite recently [23][24][25][26][27] , thermally resistant systems involving ultralong RTP have not yet been reported, particularly for metalhalide materials. As a counterpart to (Ph 4 P) 2 Cd 2 Br 6 , evaluation of the TQ effect in metal-free ionic cocrystal Ph 4 PBr ( Supplementary Fig. 8) indicates that the luminous intensity systematically decreases as the temperature increases from 140 to 300 K. This divergence between the temperature-dependent behaviors of the Ph 4 PBr and (Ph 4 P) 2 Cd 2 Br 6 verifies that the metal-halide units play a key role in inhibiting the TQ effect of phosphorescence over a wide temperature range. It is further observed that increasing temperature provokes a more rapid decrease to the phosphorescence peak intensity of (Ph 4 P) 2 Cd 2 Cl 6 than to that of the (Ph 4 P) 2 Cd 2 Br 6 ( Fig. 4), with the former exhibiting about 20% emission loss from 100 to 260 K. This difference, which is attributed to the stronger heavy-atom effect in the Cd 2 Br 6 2− clusters than in Cd 2 Cl 6 2− , is indicative of the suitability of Cd 2 Br 6 2− for achieving zero-TQ phosphorescence. Moreover, comparisons of the crystal parameters of (Ph 4 P) 2 Cd 2 Br 6 at 100 K and room temperature (Supplementary Tables 8-11) reveal that increasing temperature evokes a slight increase of the Cd-Br distance of the Cd 2 Br 6 2− units (Supplementary Table 11), while the C-P distance of the Ph 4 P + is nearly unaffected, resulting iñ 3% expansion of the crystal lattices and slight structural torsion of the Ph 4 P + units. It is reported that the temperature-induced structural torsion or phase change may facilitate TQ resistance in pure inorganic solids [23][24][25][26][27] . From a structural perspective, the temperature effect on the Cd 2 Br 6 2− clusters and expansion of the crystal lattice may even reduce the thermally induced molecular vibrations of Ph 4 P + to some extent, which is also confirmed by theoretical simulation below. It is observed that initial increases in intensities of the major phosphorescent emission (T 2 ) and the shoulder band (T 1 ) occur as the temperature increases from 100 to 240 K (Fig. 3b), accompanied by a slight decrease of the high-energy T 3 band. These observations can be attributed to the thermally active phosphorescent enhancement, wherein thermal energy assists dynamic energy transfer by promoting the capturing and releasing of triplet excitons among three closely positioned energy levels 46 . Moreover, it is noted that at low temperature, additional luminescence at 410 nm for (Ph 4 P) 2 Cd 2 Br 6 (Supplementary Fig. 9) and 421 nm for (Ph 4 P) 2 Cd 2 Cl 6 (Supplementary Figs. 10 and 11) appears, although these signals decrease continuously with increasing temperature. Once the emission Intensity (a.u.)

Temperature (K) Time (ms)
=49.58 ms Fig. 3 Emission spectra of (Ph 4 P) 2 Cd 2 Br 6 under different temperatures and the analysis. Solid-state emission spectra of a (Ph 4 P) 2 Cd 2 Br 6 at different temperatures between 100 and 400 K upon excitation at 300 nm. b Temperature-dependent intensity of emission (469 nm, 500 nm, 541 nm) from 100 to 400 K. c Two-dimensional map of the PL spectra of (Ph 4 P) 2 Cd 2 Br 6 under the prompt mode at different temperatures ranging from 100 to 400 K. d Timeresolved PL decay curve of (Ph 4 P) 2 Cd 2 Br 6 at different temperatures. e Energy levels of different states (calculated with phosphorescence peak at 100 K). intensity at 410 or 421 nm decreases to almost zero, the phosphorescence starts to decrease. Thus, it can be reasonably deduced that the two weak bands at 410 and 421 nm largely comprise the energy loss observed upon increasing temperature. To figure out the contribution of the peaks at 410 and 421 nm, the photophysical properties of the pristine Ph 4 PCl and Ph 4 PBr cocrystals were analyzed and found to possess major emission peaks at 410 and 466 nm, respectively, upon excitation at 365 nm ( Supplementary Fig. 3e, f). The delayed spectrum evidences that Ph 4 PCl exhibits very weak luminescence with two peaks at 410 and 500 nm, while that of Ph 4 PBr reveals loss of long-lived emission. Moreover, the emission of Ph 4 PBr at about the 400-420-nm position can be obtained in the monomer solution state, indicating that its position can be attributed to a singlet state.
The PL decay curves of (Ph 4 P) 2 Cd 2 Br 6 show that the lifetime at ca. 410 nm increases with temperature from 5.28 (100 K) to 42.49 μs (300 K), revealing that this position is characterized by a certain degree of thermally activated delayed fluorescence (TADF) (Supplementary Figs. 9b and 12). Therefore, the reduced TADF strength and the slow decay of phosphorescent intensity with increasing temperature indicates that the adjacent singlet state (S 1 ) serves as an intermediate energy level that captures energy and transmits it to the triplet-energy state, thereby reducing the nonradiative transition, maintaining phosphorescence intensity, and increasing quantum yield. The adjacent singlet TADF prevents thermally induced energy loss via phosphorescence to some extent. Increasing the temperature cannot fully compensate for the phosphorescence attenuation caused by TQ, so that as the TADF gradually decays to zero, the phosphorescence begins to undergo a distinct decrease. Based on the S 1 and T 3 excited-state levels of (Ph 4 P) 2 Cd 2 Br 6 (S 1 : 2.98 eV, 416 nm; T 3 : 2.69 eV, 461 nm) and (Ph 4 P) 2 Cd 2 Cl 6 (S 1 : 2.95 eV, 421 nm; T 3 : 2.67 eV, 464 nm), their energy gaps (ΔE ST ) between singlet and triplet states are very similar, namely 0.29 and 0.28 eV, respectively (Fig. 3e). Comparing the temperature-dependent luminescence behaviors of Ph 4 PBr, (Ph 4 P) 2 Cd 2 Cl 6 , and (Ph 4 P) 2 Cd 2 Br 6 , Fig. 4 illustrates that both the heavy-atom effect and thermally active triplet-energy transfer are important for achieving effective temperatureresistant phosphorescence.
Hirshfeld surface and fingerprint plot analyses. To evaluate the detailed intermolecular interactions and molecular rigidification structure of the zero-TQ (Ph 4 P) 2 Cd 2 Br 6 from a quantitative view, we conducted the Hirshfeld surface and fingerprint plot analyses. The Hirshfeld surface of (Ph 4 P) 2 Cd 2 Br 6 is mapped over d norm , shape index, and curvedness. The d norm surface is the normalized function of d i and d e (Supplementary Figs. 13 and 14), with white-, red-, and blue-colored surfaces. The white surface indicates those contacts with distances equal to the sum of the van der Waals (vdW) distance; red and blue indicate shorter contact (<vdW distance) and the longer contact (>vdW distance), respectively. The deep red color indicates the presence of Br···H hydrogen-bonding contact, while blue and white color spots indicate the presence of other close contacts, such as H···H, Cd···H, Cd···C, Br···C, and C···H. The two-dimensional fingerprint plot analysis for all contacts of (Ph 4 P) 2 Cd 2 Br 6 are shown in Supplementary Figs. 13g and 14g. The relative contributions to the Hirshfeld surface area for each type of intermolecular contact are illustrated in Supplementary Figs. 13 and 14. As for the large number of white regions that still exist in the d norm surfaces, it indicates that there is also weak and distant contact between molecules.
The void space within crystals is considered as the free volume. The low free volume usually corresponds to high molecular rigidification and confinement. The fractional free volume (FFV) can be defined as where V f is the free volume and V o is the volume occupied by the materials. It is observed that the FFV for (Ph 4 P) 2 Cd 2 Br 6 at different temperatures (Supplementary Table 12) is very low (<10%) 47 , suggesting high rigidification with less molecular vibrational and rotational motions. In detail, the FFV is 9.30% at 289 K, slightly larger than that at 100 K (7.82%). The system does get enhanced molecular thermal motion under heating conditions, with a slightly looser structure and a slightly less interaction. Comparison results based on single-crystal XRD test ( Supplementary Fig. 15f) show that although the vibration becomes larger at high temperature, the equilibrium position does not change obviously.
Molecular dynamics simulation and density functional theory (DFT) calculations. To better understand how structural factors contribute to the zero-TQ phosphorescence of (Ph 4 P) 2 Cd 2 Br 6 , molecular dynamics simulations were performed at typical thermodynamic temperatures of 100 and 290 K (which closely correspond to the experimental conditions above). It is observed that the Cd 2 Br 6 2− is characterized by a much more prominent fluctuation of its torsion angle than the Ph 4 P + cations are (Supplementary Fig. 15a-d), indicating that the Cd 2 Br 6 2− clusters serve as the structural relaxation units to reduce the thermal effect on Ph 4 P + . The average calculated Cd-Br and P-P distances are also consistent with the trend obtained from single-crystal structures at high/low temperatures ( Supplementary Fig. 15e). Furthermore, periodic DFT calculations were performed to gain insight into band structure, density of states (DOS), and electron-density distributions. Total and partial electronic densities of states evidence that the (Ph 4 P) 2 Cd 2 Br 6 system has a small bandgap of 3.236 eV (Fig. 5c), and this value is close to the experimental result calculated from the UV-vis absorption edge (3.28 eV) ( Supplementary Fig. 16). Band structure calculations indicate that both the conduction band (CB) minimum and the valence band (VB) maximum are localized at the Z point, confirming that it is a direct bandgap semiconductor, in agreement with the high solidstate QY phos emission measured experimentally (Fig. 5c). Partial electronic density of states (PDOS) reveals that the CB and VB are derived from the p orbitals of C and P atoms ( Supplementary  Fig. 17b). The highest occupied molecular orbital (HOMO) predominantly consists of p orbitals of Br in Cd 2 Br 6 2− clusters and adjacent π orbitals of benzene in Ph 4 P + units, corresponding to halogen-π interactions (Fig. 1d). The lowest unoccupied molecular orbital (LUMO) is solely localized on the p orbitals of C atoms in four benzene groups in whole organic Ph 4 P + . Similar results are calculated for (Ph 4 P) 2 Cd 2 Cl 6 ( Supplementary Figs. 17a  and 18). This suggests that potential energy/electronic transfer between Cd 2 Br 6 2− and Ph 4 P + , along with the strong heavy-atom effect, promotes triplet excitons (Fig. 5a, b). Combining experimental and theoretical results suggests that the zero-TQ phosphorescence of (Ph 4 P) 2 Cd 2 Br 6 can be attributed to the following factors: (1) from an energy perspective, the heavy-atom effect of Cd 2 Br 6 2− with strong spin-orbital coupling facilitates the ultralong phosphorescence dominating the overall PL process, while weak emission from high-energy singlet excitons serves as an energy buffer layer to reduce the TQ effect on triplet excitons. (2) The proximity of the triplet-energy levels leads to efficient energy transfer between different triplet states, resulting in enhanced thermally active and ultralong-lived phosphorescence across a wide temperature range. (3) From a structural perspective, the thermally induced fluctuation of Cd 2 Br 6 2− clusters could act as a structural buffer layer to decrease thermal effects associated with the structural relaxation of Ph 4 P + chromophores ( Fig. 3 and Supplementary Fig. 9).  19 and 20). Figure 6 shows that the four materials in daylight are distributed on the same black background, showing a white powder state, so the code character defined as "L" for the normal Morse password is obtained. When applying the excitation light source, we can decode different emission colors displayed by the samples. Furthermore, when the light source is turned off after excitation, we can decode it with different afterglow time of different materials. This becomes programmable: for example, if we choose excitation at 254 nm, after closing the excitation condition for 3 s, the resulting character is decoded as "R". Like this manner and process, precise application and removal of the excitation light source can produce different Morse code patterns that correspond to different characters (defined in this work as A, E, I, L, and R). Thus, the only state in daylight can be split into five different states under UV excitation with different wavelengths. Although this procedure allows for only five different states, it is conceivable that careful selection of the excitation light source, setting of the information that is encrypted by the responding emission color, and controlled duration in the range of 1-9 s of exposure to excitation could increase the variety of data that can be stored. This encryption method is strongly secure since decryption of the data would require precise control of experimental conditions (Fig. 6).

Discussion
In summary, new 0D metal-halide organic-inorganic hybrid materials exhibit rare zero-TQ ultralong phosphorescence over a wide temperature range (above 220 K). Unlike the well-known mechanisms of zero-TQ luminescence (such as structural torsion and phase change) and long-lived RTP (such as molecular  NATURE COMMUNICATIONS | https://doi.org/10.1038/s41467-020-18482-w ARTICLE NATURE COMMUNICATIONS | (2020) 11:4649 | https://doi.org/10.1038/s41467-020-18482-w | www.nature.com/naturecommunications rigidification), this work illustrates weak TADF bands that can serve as energy buffer layers to compensate the phosphorescence loss upon increasing temperature, and thus enables a new route toward achieving zero-TQ visible luminescence. Moreover, the (Ph 4 P) 2 Cd 2 Br 6 system exhibits high QY phos of 62.79%, which may stand for a new record in ultralong RTP materials, to the best of our knowledge. Furthermore, by exploiting the interesting excitation-dependent RTP of four metal halides, different combinations of the hybrids can be used for Morse code encryption and time-resolved information logic gates. Therefore, this work demonstrates the example of organic-inorganic hybrids with both zero-TQ emission and very high solid-sate RTP QY. Facile design of 0D metal-halide hybrids can be expected to provide new ways to develop materials with high-efficiency ultralong RTP for thermally resistant luminescence and information-encoding applications. Further work is still underway to explore the thermally stable phosphorescence of the materials above room temperature.

Methods
Materials and reagents. All the reagents (tetraphenylphosphonium chloride, tetraphenylphosphonium bromide, cadmium chloride, cadmium bromide, hydrochloric acid, and hydrobromic acid) were purchased from Sigma Chemistry Co. Ltd. and used without further purification. Distilled water is prepared in our lab.
Synthesis of metal-halide hybrids. The crystalline (Ph 4 P) 2 CdX 4 samples were prepared via the hydrothermal method. A mixture of CdCl 2 ·4H 2 O or CdBr 2 ·4H 2 O (0.6 mmol) with tetraphenylphosphonium chloride (0.6 mmol, 0.24 g) or tetraphenylphosphonium bromide (0.6 mmol, 0.25 g) and 8 mL of H 2 O was stirred for 10 min. The mixture was then transferred to a Teflon reactor (23 mL) and heated at 160°C for 24 h. After that, the mixture was cooled at a rate of 5°C h −1 to room temperature. The colorless transparent crystals of (Ph 4 P) 2 CdX 4 were washed by ethanol (3 × 10 mL). The crystalline (Ph 4 P) 2 Cd 2 X 6 samples were prepared via the hydrothermal method. A mixture of CdCl 2 ·4H 2 O or CdBr 2 ·4H 2 O (1.2 mmol) with tetraphenylphosphonium chloride (0.6 mmol, 0.24 g) or tetraphenylphosphonium bromide (0.6 mmol, 0.25 g) and 8 mL of H 2 O was stirred for 10 min. The mixture was then transferred to a Teflon reactor (23 mL) and heated at 160°C for 24 h. After that, the mixture was cooled at a rate of 5°C h −1 to room temperature. The colorless transparent crystals of (Ph 4 P) 2 Cd 2 X 6 were washed by ethanol (3 × 10 mL).
Structural and morphology characterization. Single-crystal X-ray diffraction data of these samples were collected on an Oxford Diffraction SuperNova areadetector diffractometer using mirror optics monochromated Cu Kα radiation at room temperature. UV-vis absorption spectra were performed on Shimadzu UV-3600 spectrophotometer at room temperature. PL microscope images of crystals were taken under OLYMPUS IX71 fluorescence microscope. All the relevant PL tests, including fluorescence and phosphorescence and time-resolved lifetime, were conducted on an Edinburgh FLS980 fluorescence spectrometer. The PLQY of the crystals was determined by using a Teflon-lined integrating sphere (F-M101, Edinburgh, diameter: 150 mm and weight: 2 kg) accessory in FLS980 fluorescence spectrometer.
Molecular dynamics (MD) simulations. The calculations were performed by classical molecular dynamic simulation method employing a universal field. Charge equilibration (QEq) method 48 was used to calculate atomic charges. In potential energy calculations, the long-range Coulomb interactions between partial charges were computed by the Ewald summation technique, and a "spline cutoff" method was used to calculate the van der Waals interaction. After energy minimization was applied on the initial model, MD simulations were performed in isothermal-isobaric (NPT) ensemble with the typical thermodynamic temperatures of 100 and 290 K (corresponding to the typical measurement in the experiment) and the pressure of 0.1 MPa (~1 atm). The Andersen method 49 and Berendsen method 50 were used to control temperature and pressure, respectively. The total simulation time was 250 ps with the simulation time step of 1 fs. The result shows that the system reached equilibrium with lattice parameters and total potential energy fluctuating around a constant value within the first 50 ps, so the dynamic trajectories were recorded every 20 fs in the remaining 200 ps in order to analyze the ensemble average values. The simulations were performed using Forcite module in Material Studio software package 51 .
Electronic structure calculations. The calculations were performed with the periodic DFT method by using Dmol3 module in Material Studio software package [51][52][53] . The initial configurations were fully optimized by the Perdew-Wang (PW91) 54 generalized gradient approximation (GGA) method with the double numerical basis sets plus polarization function (DNP). The core electrons of metals were treated by effective core potentials (ECP). The self-consistent field (SCF) converged criterion was within 1.0 × 10 −5 hartree per atom and the converged criterion of the structure optimization was 1.0 × 10 −3 hartree per Bohr. The Brillouin zone was sampled by 1 × 1 × 1 k points, and test calculations reveal that the increase in k points does not affect the results.
Hirshfeld surfaces and 2D fingerprint plot calculation. The Hirshfeld surfaces and 2D fingerprint plots were calculated by using Crystal Explorer 17.5 55 .