# Excitation-Wavelength-Dependent Emission of Congruently Melting Iodocuprate Hybrid Materials

Clémence Cazals, Nicolas Mercier, Magali Allain, Chiara Botta, Mikaël Kepenekian, Maxime Deutsch, Sébastien Pillet, Florian Massuyeau, Romain Gautier

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Clémence Cazals, Nicolas Mercier, Magali Allain, Chiara Botta, Mikaël Kepenekian, et al., Excitation-Wavelength-Dependent Emission of Congruently Melting Iodocuprate Hybrid Materials. Angewandte Chemie International Edition, 2025, pp.e202506827. 10.1002/anie.202506827. hal-05230345

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# Excitation-Wavelength-Dependent Emission of Congruently Melting Iodocuprate Hybrid Materials

Clémence Cazals,[a] Nicolas Mercier,[a] Magali Allain,[a] Chiara Botta,[b] Mikaël Kepenekian,[c] Maxime Deutsch,[d] Sebastien Pillet,[d] Florian Massuyeau,[e] Romain Gautier[e]

[a] Clémence Cazals, Dr Magali Allain, Prof. Nicolas Mercier*
MOLTECH-ANJOU, UMR-CNRS 6200, Université d'Angers
2 Bd Lavoisier, 49045 Angers, France
E-mail: nicolas.mercier@univ-angers.fr

[b] Dr. Chiara Botta
Istituto di Scienze e Tecnologie Chimiche “G. Natta” (SCITEC), CNR.
Via Corti 12, 20133 Milano, Italy

[c] Dr Mikaël Kepenekian
Univ Rennes, CNRS, ENSCR, INSA Rennes, ISCR (Institute des Sciences Chimiques de Rennes)—UMR 6226, F-35000 Rennes, France

[d] Dr Maxime Deutsch, Dr Sébastien Pillet
Université de Lorraine, CNRS, CRM2, UMR 7036, Nancy 54000, France

[e] Dr Florian Massuyeau, Dr Romain Gautier
Nantes Université, CNRS, Institut des Matériaux de Nantes Jean Rouxel, IMN, F-44000 Nantes, France

**Abstract:** In the context of the emergence of metal halide perovskites for optoelectronic materials, the lead-free and earth-abondant copper(I)-based halide hybrids are of high interest. Here, we report on two iodocuprate hybrids $(HO_2C(CH_2)_{n-1}NH_3)CuI_2$ (n= 3,4) exhibiting excitation-wavelength dependent emission. The thermodynamically stable n=4 compound (2D-C4) is based on a rare 2D iodocuprate network, while a more common 1D network of edge-sharing tetrahedra is found both in the n= 3 compound (1D-C3) and in the metastable n= 4 (1D-C4) hybrid. Depending on the excitation wavelength, 2D-C4 and 1D-C3 hybrids exhibit yellow emission, red emission, or white light emission -resulting from the emission of both components-, while 1D-C4 does not emit in the red region. The description of the electronic structure based on density functional theory (DFT) indicates two different origins for the emissions of 2D-C4 and 1D-C3, in particular from iodide vacancies (red emission) which involve mid-gap states. The emission of 2D-C4 can be also modulated from red to yellow depending on applied pressure. 2D-C4 has also an exceptional low congruent melting temperature of 110 °C, allowing the solvent-free preparation of thin films. Finally, Phosphor-converted-LED based on 2D-C4 have been prepared showing that either pink or nearly white emission is produced.

## Introduction

Materials with excitation-dependent (Ex-De) emission have attracted particular attention.[1-10] Their emission properties can be tuned depending on the excitation wavelength, without changing their composition or particle size. Such systems find potential applications in bioimaging, anticounterfeiting applications, sensors or various optoelectronic devices.[5-7] Besides the development of organic materials built from multiple emitting centers, the search of single component of Ex-De emission materials, which has been observed in some metal complexes,[1,2] small molecules,[3,4] polymers[10] or halide semiconductors,[8,9] is still in infancy.
Organic-inorganic halide materials have attracted much interest these recent years, in particular due to their ability to be prepared as thin films by techniques such as spin-coating. The most famous family is the one of halide perovskites (HP) which have become superstar semiconductor materials for photovoltaics[11–13] but also for lighting applications.[14–16] Thus, 2D (layered) perovskites were known since the 90’s to exhibit narrow emission[17,18] owing to the recombination of free excitons (FE) at room temperature, due to strong exciton binding energies (up to 300 meV).[19] More recently, in combination with 3D perovskites as 2D-3D composites, efficient perovskite LEDs were demonstrated.[20] In addition, layered bromide or chloride perovskites can exhibit broad band (BB) emission in the whole visible range which has led to the emerging domain of halide layered perovskites for white light emission applications.[21–25] However, all these materials are based on the toxic lead element and suffer from unstability under ambient air, irradiation, etc. These concerns motivate the search for alternative lead-free metal halide materials. In this context, halocuprate hybrids based on organic cations and halocuprate anions have emerged these recent years as some of the highest efficiency light emitters that combine non-toxic earth-abundant element, synthesis via solution routes and thin film preparation from solution.[26–44]

Most of organic-inorganic halide materials decompose before melting due to the high volatility of the organic component. However, it has been shown as soon as in 2002 that the halide perovskite (2-FPEA)SnI₄ (2-FPEA= 2-fluorophenethylammonium) can melt congruently (melting giving a liquid of same composition) at relatively low temperature of Tₘ(melting)= 174 °C.[45] Besides some other examples reported by D. Mitzi,[46,47] we discovered that HO₂C(CH₂)ₙ-₁NH₃⁺ (Cn⁺) cations led to a series of layered perovskites (Cn)₂PbX₄ exhibiting congruent melting with Tₘ as low as 126°C (X= I, n= 4) or 145 °C (X= Cl, n= 4).[48,49] Congruent melting of materials offer two main opportunities: the solvent-free preparation of thin films, and the synthesis in the molten state which can allow stabilizing phases not accessible from solution routes, as illustrated by the formation of the mixed cations and mixed halide solid solution (Cn₁-xCn'_x)₂PbBr₄ and (Cn)₂Pb(Br₁-xClx)₄.[49]

In this context, we report on iodocuprates hybrids based on HO₂C(CH₂)ₙ-₁NH₃⁺ (Cn⁺, n= 3,4) cations, 2D-(C4)CuI₂ (2D-C4) and (C3)CuI₂ (1D-C3). We show that these materials melt congruently at quite low temperature (Tₘ= 110 °C for n=4), while depending on the recrystallization process both 2D-(C4)CuI₂ and a new metastable phase 1D-(C4)CuI₂ are obtained. While the structure of 2D-(C4)CuI₂ is based on a rare 2D network, the other two structures are based on chains of edge-sharing CuI₄ tetrahedra. We show that 2D-(C4)CuI₂ and (C3)CuI₂ are Ex-De emission materials which can exhibit strong yellow-green emission (λexc= 320 nm), red emission (λexc= 365 nm), or white light emission resulting from the combined emission of both components (red and yellow) (λexc= 345 nm, n= 4), while 1D-(C4)CuI₂ gives white emission due to dual emission. The description of the electronic structure based on density functional theory (DFT) sheds new light on the origin of the emission properties of these compounds. In particular, we show that iodine vacancies lead to the formation of mid-gap states for 2D-C4 and 1D-C3 but not in the case of 1D-C4. We also show that the emission of 2D-C4 can be modulated from red to yellow depending on applied pressure. The stability of these materials in the molten state also allowed the solvent-free preparation of emissive thin films and the synthesis of the (C4)(Cu₁-xAgx)I₂ solid solution with tunable emission properties. Finally, 2D-(C4)CuI₂ has been incorporated in pc-LED showing that depending on the used phosphor, either pink or nearly white emission is produced.
<figure><img src="image_2.png" /><figcaption>**Figure 1.** a) Differential scanning calorimetry (heating and cooling) of 2D-(C4)CuI2 (left) and schematic recrystallization process affording the metastable phase 1D-(C4)CuI2 or 2D-(C4)CuI2, as showed by PXRD (right, PXRD of (C4)AgI2 also given for comparison); b) General view of the structure of 2D-(C4)CuI2 showing its layered nature with an herringbone arrangement of organic cation similar to that found in (C4)2PbI4; c) The 2D inorganic framework offering windows occupied by NH3+ groups on either side of the plane; d) The symmetrical dimeric building block Cu2I6 (bond distances in Å).</figcaption></figure>

**Results and Discussion**

**Synthesis, thermal properties and crystal structures:** Crystals of 2D-(C4)CuI2 and (C3)CuI2 have been obtained from a slow evaporation (from 80°C to room temperature) of a concentrated HI solution containing reagents (CuI and (Cn)I, n=3,4). The (C4)AgI2 compound has been synthesized by the same procedure (see SI for details of all the syntheses). As revealed by powder X-ray diffraction (PXRD, see Figure S3), all these hybrid materials are obtained as pure phases. Thermogravimetric analyses show that these compounds are stable up to 170 °C for the lowest temperature (2D-(C4)CuI2) where a first weight loss starts (Figure S7). The DSC curves reveal single endothermic peaks at 106°C and 130 °C for 2D-(C4)CuI2 (Figure 1a) and (C3)CuI2 (Figure S8), respectively. These peaks correspond to the melting of samples, and it is worth noting that no exothermic peak corresponding to the expected recrystallization is observed upon cooling, which is explained by a very slow crystallization process. Thus, as an example, (C4)CuI2 naturally cooling down to room temperature needs more than 30 minutes to fully crystallized. Moreover, by shaking a spatula in the molten liquid (C4/Cu/I system), crystallization is accelerated, but the compound obtained is not the expected one. Its powder XRD pattern is identical to the one of (C4)AgI2 whose structure contains AgI2⁻ chains, indicating that the metastable polymorph 1D-(C4)CuI2 is crystallized (Figure 1a). While stable at room temperature in inert atmosphere for (at least) a few months, this 1D phase is transformed into the 2D-(C4)CuI2 phase in air after a few hours. Interestingly, this phase has remarkable properties.
The structure of the 2D-(C4)CuI2 compound has a layered nature, with the 2D inorganic network separated by the organic sheets (Figure 1b). It is worth noting that this structure looks like the one of the layered perovskite (C4)2PbI4 (Figure S2),[50] in particular with a same space group (Pbca) and a similar herringbone arrangement of C4⁺ cations which interact together through O...H-O hydrogen bonds thus defining 1D supramolecular chains of organic cations along a (Figure S1a). In the iodocuprate hybrid which can be written 2D-(C4)2Cu2I4, two Cu⁺ ions replace one Pb²⁺, while in both hybrids, two iodides lie in the layer plane, and the two others are on either side of the layer plane. 2D anionic iodocuprate network is very scarce,[27,29,30] and the one stabilized in 2D-(C4)CuI2 has been very recently reported.[30] The 2D network results from interconnected corner-sharing dimeric units, each dimer consisting of two edge-sharing tetrahedra, leading to an open network showing windows defined by six adjacent CuI4 tetrahedra which accomodate the ammonium ends of organic cations (Figure 1c). Bond distances and bond angles of the dimeric unit are displayed on Figure 1d. The quite large angle of 121° is correlated with the short Cu...Cu distance of 2.607 Å, which appears inferior to the sum of van der waals radius (2.80 Å), and to reported values for such dimeric units (2.74 Å for isolated dimers, 3.44 Å for edge-sharing dimers -2D network-, or > 3 Å in CuI2⁻ chains - see below-).

The two iodocuprate compounds (C3)CuI2 and 1D-(C4)CuI2, as well as the iodoargentate one (C4)AgI2, are based on 1D inorganic network consisting of edge-sharing tetrahedra (Figure 2). The crystal structures of (C3)CuI2 and (C4)AgI2 have been solved from single crystal data, while the monoclinic unit cell of 1D-(C4)CuI2 have been determined from powder diffraction (a= 22.53 Å, b= 6.32 Å, c= 14.25 Å, β= 103.36°, see profile fitting, Figure S4), indicating a similar atomic arrangement than that found in (C4)AgI2 (a=22.72 Å, b=6.59 Å, c=14.19 Å, β= 102.87°). The structures of (C4)AgI2 and (C3)CuI2 can be described as layers of organic cations separated by inorganic chains lying in the (b,c) plane ((C4)AgI2), or (b,a) plane ((C3)CuI2) (Figure 2). It is worth noting that organic cations interact together through hydrogen bonding in (C3)CuI2 (dimers) and in 2D-(C4)CuI2 (1D H-bonded network), but not in 1D-(C4)MI2 (M= Ag, Cu). In 1D-(C4)MI2, both NH3⁺ and CO2H ends interact through hydrogen bonding with the iodides of the inorganic networks (Figure S1c). As regards to the geometrical features of the 1D chain, they are all based on tetrahedra displaying regular Cu/Ag-I bond distances, while different M...M distances are encountered (Figure 2). In (C3)CuI2, two very different M...M distances, 3.17 Å and 3.47 Å, are observed, greater than the one found in 2D-(C4)CuI2 (2.607 Å, Figure 1d), while in (C4)AgI2, there are two close distances -3.258 Å and 3.332 Å-. In 1D-(C4)CuI2 (isotype of (C4)AgI2), the b parameter of 6.32 Å corresponds to two Cu...Cu distances with an average Cu...Cu distance of 3.16 Å.

<figure><img src="image_3.png" /></figure>
**Figure 2.** Structures of iodometalates hybrids based on M(II)I₂⁻ (M= Cu, Ag) chains of edge-sharing tetrahedra: general view and details of the 1D network (Cu-I, Ag-I, Ag...Ag and Cu...Cu bond distances in Å): a) (C4)AgI2; b) (C3)CuI2.

**Luminescence properties and electronic structure:** In Figure 3, we report the photoluminescence (PL) properties of 2D-(C4)CuI₂ and (C3)CuI₂ crystallized powders, displaying excitation-dependent-wavelength emission. When excited at high energy (322 nm for 2D-(C4)CuI₂ and 310 nm for (C3)CuI₂) the PL spectrum consists of a broad band centered at 540 nm and 509 nm for the 2D-(C4)CuI₂ and (C3)CuI₂ materials, respectively, characterized by a great FWHM (full width at half maximum) of 125 nm. Such emission properties are commonly encountered for iodocuprate hybrids.[29–36] When excited at lower energy (380 nm), a broad band peaked at 720 nm (FHWM of 206 nm) for 2D-(C4)CuI₂ or 730 nm (FWHM of 156 nm) for (C3)CuI₂ is observed in the red region, which is uncommon for such iodocuprate hybrids. When the red emission has been previously reported, the corresponding materials exhibit a dual emission (yellow region and red region) with a same origin (same PLE).[37–39] In our materials, different origins of the yellow and red emissions are observed as distinct PL excitation (PLE) spectra are obtained. A narrow peak at high energy (300-320 nm) is observed for the yellow emission, indicating a large Stokes shift of more than 200 nm. For the red emission, the PLE consists of multiple bands at lower energies, in the 325-475 nm and 325-425 nm region for 2D-(C4)CuI₂ and (C3)CuI₂, respectively. For this emission, large Stokes shifts of around 200 nm are also observed. The PLE spectra of the red emissions are similar to the absorption spectra for both compounds (Figures S9-S10). The absorption spectra are based on a main band at 375 nm (2D-(C4)CuI₂) and 350 nm ((C3)CuI₂), also found in the spectra of iodide salts (C4)I and (C3)I and a shoulder at lower energy (450 nm for 2D-(C4)CuI₂ and 410 nm for (C3)CuI₂), which appears in PLE spectra (Figure 3). Let's notice that (C3)I and (C4)I exhibit very weak emission in the blue region (400-500 nm, Figure S11). The photoluminescence quantum efficiency is by far higher for the yellow emission (45 % for 2D-(C4)CuI₂ and 39 % for (C3)CuI₂) than for the red emission (2 % for 2D-(C4)CuI₂, 0.29 % for (C3)CuI₂). By selecting a suitable excitation wavelength (355 nm for 2D-(C4)CuI₂, Figure 3a), both emission bands are activated, thus nearly covering the whole visible region and giving a white light emission. The PL time decays for 2D-(C4)CuI₂ are in the nanosecond range for the red emission (19 ns), and in the microsecond range for the yellow emission (0.87 µs, Figure S12).
<figure><img src="image_4.png" /><figcaption>**Figure 3.** PL and PLE spectra of 2D-(C4)CuI2 (a) and (C3)CuI2 (b) showing the Ex-De emission behavior; (c) PL and PLE spectra of 2D-(C4)CuI2 in the 77K-333 K range (left) and FWHM=f(T) showing a linear relationship in the studied temperature range (right); (d) PL (right) and PLE (left) spectra of CuI, 2D-(C4)CuI2 and (C3)CuI2 (red emission).</figcaption></figure>

The PL temperature dependence of crystallized powder of 2D-(C4)CuI2 has been studied. In Figure 3c, the temperature dependence of the yellow emission is reported (see Figure S13 for the red emission). By increasing the temperature in the 77K-333K range, the overall PL intensity is progressively reduced while the peak position is unchanged for both emissions. As regards to PLE spectra, while the one of the red emission remains similar in the whole temperature range (Figure S13), the one of the yellow emission is continuously red-shifted from 325 nm to 360 nm (band edge, Figure 3c). We must also notice the linear evolution of the FWHM, from 80 nm (100K) to 125 nm (300 K) for the yellow emission (Figure 3c, right). Such emissions, typically in the 425 nm-550 nm range, characterized by a large Stokes shift, a large FWHM which decreases when T decreases and a lifetime in the microsecond range is usually associated to the recombination of self-trapped exciton (STE)[40,41] or to cluster centered 3CC states,[51,52] a recent review claiming that these different terms refer to the same phenomenon.[43] As already highlighted, Ex-De emission, here resulting in both yellow and red emissions, is very rare, and this raises questions about the origin of the red emission. However, we found that a similar emission can be observed for CuI. The main peak of CuI is a narrow free exciton type emission around 410 nm, and, depending on its preparation, two other emissions associated to Cu+ and I- vacancies can be also observed around 425 nm (narrow peak) and at 725 nm (broad band type).[53] Figure 3d reports the PL and PLE spectra of CuI, and red emissions of 2D-(C4)CuI2 and (C3)CuI2 showing that, even if the broad band in the red region are alike, the PLE spectra are very different, precluding the presence of CuI in 2D-(C4)CuI2 and (C3)CuI2 samples.
<figure><img src="image_5.png" /><figcaption>**Figure 4.** (a) PL and PLE spectra of 1D-(C4)CuI2 (left) and 2D-(C4)CuI2 (right) resulting from the 1D-to-2D solid state transformation; (b) PL intensity = f(excitation power density) ($\lambda_{exc}$= 405 nm) for the red emission of 2D-(C4)CuI2.</figcaption></figure>

In contrast with 2D-(C4)CuI2, when excited at around 380 nm, 1D-(C4)CuI2 does not exhibit any emission whereas a dual emission, peaked at 446 nm and 568 nm, covering the whole visible range is observed when excited at 300 nm. These two emission bands have the same PLE spectrum consisting of a narrow band at high energy showing the same origins (Figure 4a). Such dual emission has been already observed in some other iodocuprate hybrids, with different ratio of intensities. [37-39] For the 1D-(C4)CuI2, due to the same intensity of the two bands covering nearly the whole visible range, the CIE coordinates are (0.32, 0.34) corresponding to almost pure white emission (Figure S14). Once transformed into the 2D compound through a solid state process, the sample exhibits the same emission profile than the previously studied powder of 2D-(C4)CuI2 (Figure 4a and Figure 3a) demonstrating that the red emission of the 2D-(C4)CuI2 sample is not due to CuI impurities. The intensity of the red emission has been measured depending on the excitation power density in the 0-600 mW/cm² range (Figure 4b). Up to 300 mW/cm², a linear relationship is observed, while at higher density, the PL intensity tends to saturate (reversible and reproducible process), in contrast with the known linear dependence of the main (yellow-green) PL emission of iodocuprate hybrids.[44] This result is in favor of defects/vacancies[54] as the origin of the red emission, which is in good agreement with the hypothesis of I⁻ vacancies. The solid state transformation from the non emissive 1D phase (red emission) to the emissive 2D phase could indicate that iodide vacancies are associated to the specific 2D network of 2D-(C4)CuI2. However, the similar red emission observed for (C3)CuI2 whose structure is based on 1D inorganic network similar to the one found in 1D-(C4)CuI2, shows that this is not a valuable hypothesis. The activation of the red emission in 2D-(C4)CuI2 and (C3)CuI2 might, instead originate from common structural and/or electronic features of both structures.

A deeper understanding of the optical properties of the compounds is gained by considering their electronic structures as described by DFT-based calculations (see Supporting Information for a complete description of the computational setup). The three systems 2D-(C4)CuI2, 1D-(C4)CuI2, and (C3)CuI2 present similar features in their band structures (Figure 5a and S18) with top valence bands comprised of hybridized copper 3d orbitals and iodine 5p orbitals (Figure S19). Above the gap is a first set of bands originating primarily from organic cation states coupled with the I(5p) orbitals of the inorganic layer ($E_G^1$ = 1.89, 1.90, and 1.68 eV for
2D-(C4)CuI2, 1D-(C4)CuI2, and (C3)CuI2, respectively), the conduction states arising predominantly from the copper-iodide networks are found at higher energies (E<sub>G</sub><sup>2</sup>-E<sub>G</sub><sup>1</sup> = 0.59, 0.93, and 0.74 eV for 2D-(C4)CuI2, 1D-(C4)CuI2, and (C3)CuI2, respectively). The bottom of those bands result primarily from the hybridization of the Cu(4s) orbital and the I(5p) orbital (Figure S19), a composition typically found as iodocupprates conduction band minimum, leading to a type-I band alignment.[43] The intercalation of predominantly organic states within the inorganic gap, i.e. a type-II band alignment, has already been described in a similar 2D CuI system exhibiting efficient photoluminescence.[30] Interestingly, the coupling between the organic and inorganic moities is such that, in each case, the computed band-to-band absorption threshold corresponds to smaller bandgap between the copper-iodide states and the organic cation states E<sub>G</sub><sup>1</sup> (Figure 5d and S20). One should keep in mind that such calculation (i) suffers from the well-known bandgap underestimation,[55] (ii) cannot describe many-body effects such as excitonic features and, therefore, cannot account for the large Stokes shift exhibited by the compounds.

A contrast between materials appears when considering the creation of iodide vacancies V₁. For each materials, we envisioned two cases in which the removal of an iodide ion leaves two copper ions with dangling bonds, noted Cu* (Figure S21, S22, and S23). In the case of 2D-(C4)CuI₂ and (C3)CuI₂, one of these defects creates a localized state within the bandgap (Figure 5b, S24 and S25) at E<sub>G</sub>* = 1.10 and 1.23 eV for 2D-(C4)CuI₂, and (C3)CuI₂, respectively. It consists in a combination of the Cu(3d) orbitals of the copper ions with dangling bonds left by the missing iodide ion (Figure 5c). These states lead to signatures in the computed absorption spectra of 2D-(C4)CuI₂ and (C3)CuI₂ with absorption thresholds significantly lower with respect to the pristine systems (Figure 5d and S20). Interestingly, mid-gap states are found only when the distance between Cu* is smaller than 3 Å (2.56 Å for the Edge-vacancy in 2D-(C4)CuI₂, and 2.80 Å for Vacancy #1 in (C3)CuI₂), whereas larger Cu*-Cu* distances (5.33 Å for the Corner-vacancy in 2D-(C4)CuI₂, and 3.31 Å for Vacancy #2 in (C3)CuI₂) lead to defect states coupled with the organic states. On the other hand, iodide vacancies in 1D-(C4)CuI₂ lead to longer Cu*-Cu* distances (3.16 Å in both cases) and defect states of coupled with the organic cations as shown by the broad peaks appearing at the position of the band of organic states in the projected density of states (pDOS, Figure 5b and S26). As a result, the absorption spectra of the defective systems nearly match the one of the pristine material (Figure 5d and S20) and no optical signature is obtained for the iodide vacancies.

Hence, the red emissions of 2D-(C4)CuI₂ and (C3)CuI₂ are expected to arise from the defect states let by iodide vacancies, while the second emission would come from STE states after excitation of the Cu-I network, as already observed and described (Figure 5e). Thanks to the stabilization of the defect state by shorter Cu*-Cu* distances in the case of 2D-(C4)CuI₂ and (C3)CuI₂, it is possible to selectively expressed the low-energy emission, while 1D-(C4)CuI₂ only allows the simultaneous excitation of both emission centers.
<figure><img src="image_6.png" /><figcaption>**Figure 5.** (a) Computed band structure of the pristine 2D-(C4)CuI2, and corresponding projected density of states (pDOS) on Cu (gray), I (violet), and C4 cation (green). (b) pDOS for 2D-(C4)CuI2, 1D-(C4)CuI2, and (C3)CuI2 with an iodine vacancy (V1). Cu* stands for the copper atoms holding the iodine vacancy. (c) Partial charge density of the mid-bandgap defect state for V1@2D-(C4)CuI2. Cu, I, O, N, C, and H atoms are depicted in blue, violet, red, blue, gray, and pale pink, respectively. (d) Computed band-to-band absorption for the pristine (plain line) and defective (dashed lines) 2D-(C4)CuI2, 1D-(C4)CuI2, and (C3)CuI2. The iodine vacancy causes a large reduction of the absorption threshold of 2D-(C4)CuI2 and (C3)CuI2, but has little impact on the one of 1D-(C4)CuI2. (e) Scheme illustrating the excitation-dependent emission (left) and the emission properties upon pressure when λexc= 375 nm for 2D-(C4)CuI2 (right).</figcaption></figure>

The effect of pressure on the structure and emission properties of 2D-(C4)CuI2 has been
also explored. It is indeed known that hydrostatic pressure can change the bond distances and
angular distorsion of the inorganic framework in metal-halide perovskite materials, and as such
tune the optical and electronic band gap.[56–59] PL spectra are recorded using an excitation
wavelength of 375 nm under pressure varying from 0.01 to 5.15 GPa pressure range (Figure
S15). This excitation wavelength, while falling into the PLE band associated to the red
emission, is out of the PLE band associated to the yellow emission (Figures 3a and 4a). As a
consequence, only the expected red emission is observed at ambient pressure. When the
pressure increases, the broad red peak centered at 670 nm becomes more intense before
disappearing, while a broad yellow peak centered at ~555 nm appears when a pressure of 0.3
GPa is reached (this signal is 15 nm red shifted with respect to the yellow emission obtained
upon excitation at 322 nm at ambient pressure). The intensity of this yellow signal increases
very strongly with pressure up to its maximum at ca.~1.3 GPa before slowly decreasing up to
5.15 GPa (Figure 6a, Figure S16a). Releasing the pressure back to ambient pressure allows the recovering of the initial red emission, indicating that this luminescence switch is fully reversible and intrinsic (Figure S16b). The corresponding crystal structures have been determined at selected pressure values. The unit cell parameters as well as the volume (20% smaller, from V= 1961.3 Å³ to V= 1656.75 Å³) continuously decrease upon pressure increasing, no structural changes being observed in the studied pressure range (Table S2, Figure S6). A similar continuous decrease of the shorter Cu...Cu distance (intra-dimer) is observed from d= 2.60 Å at ambient pressure to d= 2.50 Å at P= 3.8 GPa (Figure S6c).

The computational investigation of 2D-(C4)CuI2 under pressure correctly describe the shortening of the Cu...Cu distance up to 4 GPa (Figure S27a) and the absence of structural changes. Interestingly, under applied pressure $E_G^1$ and $E_G^2$ band gaps, related to the yellow emission, increase by ca. 0.15 eV (Figure S27b). This behavior is in agreement with the blue-shift under decreasing temperature of the absorption band around 320 nm identified in the PLE spectra of the yellow signal emission (Figure 3c). Indeed, the decreasing of temperature emulates an increase of internal pressure. On the other hand, the expected increase of the band gap appears in contrast with the reduction of the excitation energy ($\lambda_{exc}= 375$ nm) observed for the yellow emission under pressure. Indeed, it has been shown that, upon compression, the distortion of the inorganic lattice increases the lattice deformation energy thus reducing the exciton self-trapping energy.[55] In this frame, under compression a new de-excitation path from the secondary band (mainly of organic type) woud be activated to produce yellow emission (Figure 5e).

**Thin films, solid solution and pc-LED:** One opportunity for organic-inorganic compounds exhibiting a melting without decomposition (congruent melting), is the preparation of solvent-free thin films. Thus melt-processed thin films of 2D-(C4)CuI2 have been prepared according to the process described by D. B. Mitzi: crystallized powders are placed on a FTO substrate and covered by a Kapton film.[47] Once the melting temperature is reached, the melt liquid is pressed from the top to form a thin liquid layer. A few seconds after the melting, the substrate is removed from the heating source, allowing the solidification of thin films. As shown by PXRD, the expected compound is obtained as a thin film (Figure S5). Moreover, Ex-De photoluminescence properties similar to the ones of powder samples are observed (Figure 6b). The low melting temperature of 2D-(C4)CuI2 as well its very slow recrystallization also allow the custom shaping of the material on a surface. As an exemple we wrote "MOLTECH" on a glass substrate (see Figure 6c) illustrating that this material is well suited for anticounterfeiting applications.
<figure><img src="image_7.png" /><figcaption>**Figure 6.** (a) Photoluminescence spectra of a single crystal of 2D-(C4)CuI2 upon increasing pressure (b) Photoluminescence spectra and photo (365 nm irradiation) of a melt-processed thin film of 2D-(C4)CuI2; (c) the word "MOLTECH" written from a liquid consisting of 2D-(C4)CuI2 in the molten state (photos under ambient light, 254 nm and 365 nm); (d) pc-LED based on the 2D-(C4)CuI2 material (365 nm chip (left) and 395 nm chip (right)).</figcaption></figure>

Taking advantage of the stability of 2D-(C4)CuI2 in the molten state, a solid solution 2D-(C4)Cu1-xAgxI2 (x=0.25/0.5) was prepared by adding (C4)AgI2 into the melted Cu-based compound. The liquid was then stirred during one minute before cooling to room temperature has been applied. For x up to 0.5, a single phase is obtained, corresponding to the 2D copper structure, as revealed by PXRD (Figure S5). As expected, due to the substitution of Cu+ by Ag+, an increase of the unit cell volume (shift of lines to low angle) is observed. When x>0.5, a mixture of phase, consisting of 2D-(C4)Cu0.5Ag0.5I2 and (C4)AgI2, is obtained. The Cu+ to Ag+ substitution involves a modulation of the photoluminescence properties (Figure S17). Indeed, for x=0.25, a yellow emission (λex = 322 nm) similar to that of copper phase was observed, while the red emission is slightly blue-shifted (λ= 692 nm) compared to 2D-(C4)CuI2 (λ= 724 nm). For x=0.5, the broad band emission in the yellow region is centered at 535 nm (λex = 322 nm), slightly blue-shifted compared to the copper phase (λ= 540 nm). The most notable change concerns the emission pattern during low-energy excitation (λ= 365 nm): three emission peaks cover approximately the entire visible range leading to a near white light emission (CIE coordinates of (0.39; 0.40), Figure S17), the broad band emission in the red (711 nm), the 535 nm emission and an excitonic peak at 440 nm.
For 2D-(C4)CuI2, phosphor-converted LED (pc-LED) have been fabricated by using two different LED chips (365 nm and 395 nm, see SI for preparation details) (Figure 6d). When the LED chip emits at 395 nm, a resulting pink colored pc-LED is obtained, in good agreement with the red emission of the compound for low energy excitation (380-400 nm). When the LED chip emits at 365 nm an equal contribution of the two types of emission (λ= 575 nm and λ= 725 nm) is observed resulting into an overall white light emission. It must be noted that a weak contribution of the emission of the phosphor is present in both cases (λ= 375 nm and λ= 425 nm, respectively).

**Conclusion**

In this article, we show that two iodocuprate hybrid materials (HO₂C(CH₂)ₙ-₁NH₃)CuI₂ (n= 3,4) exhibit unprecedented excitation-wavelength and pressure dependent emission properties. (HO₂C(CH₂)₃NH₃)CuI₂ exist as two polymorphs, the thermodynamically stable 2D-C4 compound whose structure is based on a rare 2D iodocuprate network, and the metastable 1D-C4 compound whose structure is based on 1D network of edge-sharing tetrahedra, as found in the n= 3 1D-C3 compound. Depending on the excitation wavelength, 2D-C4 and 1D-C3 hybrids exhibit yellow emission (2D-C4: λexc= 320 nm, PLQE 45 %, τ= 1 µs), red emission (2D-C4: λexc= 365 nm, PLQE 2 %, τ= 20 ns), or white light emission when the emission of both components is activated (2D-C4, λexc= 345 nm). In contrast, 1D-C4 does not emit in the red region. The description of the electronic structure based on density functional theory (DFT) indicate two different origins for the emission of 2D-C4 and 1D-C3. In particular, it is demonstrated that iodine vacancies, lead to the formation of mid-gap states for 2D-C4 and 1D-C3 -but not in the case of 1D-C4-, which is in good accordance with experimental behaviors, that is a red emission for 2D-C4 and 1D-C3 compounds only. We also show that the emission of 2D-C4 can be modulated from red to yellow depending on the applied pressure, while due to an exceptional low congruent melting temperature of 110 °C and a very good stability in the molten state, solvent-free preparation of thin films, and the synthesis of a Cu/Ag solid solution have been demonstrated. Finally, Phosphor-converted-LED based on 2D-C4 have been prepared showing that, depending on the used phosphor, either pink or nearly white emission is produced. These results open opportunities for the preparation of lead-free Ex-De and pressure dependent-emission materials which can be prepared as thin film by a solvent-free method for optoelectronic application.

**Acknowledgements**

This work is supported by the EUR LUMOMAT program (MeltHP project) and the French PIA project "Lorraine Université d'Excellence", reference ANR-15-IDEX-04-LUE. We thank the PMD2X X-ray diffraction facility of CRM2 laboratory (Université de Lorraine) for X-ray diffraction measurements (https://crm2.univ-lorraine.fr/plateformes/pmd2x/).

**Keywords:** • excitation-dependent emission • iodocuprate hybrid material • white emission • congruent melting • pressure-dependent emission

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