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Communication

Luminescent Properties and Thermal Stability of [PPh4][Cu3I4] with a Unique Helical Structure

by
Luyi Chen
1,†,
Andrey A. Petrov
1,2,*,†,
Mingming Li
1,*,
Sergey A. Fateev
1,2 and
Alexey B. Tarasov
2
1
Department of Materials Science, Shenzhen MSU-BIT University, Shenzhen 518172, China
2
Laboratory of New Materials for Solar Energetics, Department of Materials Science, Lomonosov Moscow State University, 1 Lenin Hills, 119991 Moscow, Russia
*
Authors to whom correspondence should be addressed.
†
These authors contributed equally to this work.
Molecules 2025, 30(3), 543; https://doi.org/10.3390/molecules30030543
Submission received: 30 December 2024 / Revised: 22 January 2025 / Accepted: 23 January 2025 / Published: 25 January 2025
(This article belongs to the Section Inorganic Chemistry)
Browse Figure
Versions Notes

Abstract

:
Hybrid halocuprates (I) with organic cations show great potential for optoelectronic applications due to their tunable luminescence and high thermal stability. In this study, the iodocuprate (I) [PPh4][Cu3I4], featuring unique helical chains of face-sharing tetrahedra, was synthesized and characterized. This compound exhibits a bandgap of 3.1 eV and orange luminescence at low temperature, attributed to self-trapped exciton emission. [PPh4][Cu3I4] demonstrates exceptional thermal stability among hybrid halocuprates with decomposition above 380 °C, forming a stable melt at ~255 °C without Cu+ oxidation in ambient atmosphere.

1. Introduction

Hybrid halocuprates (I) with organic cations represent a promising class of materials for advanced optoelectronic devices including photovoltaics, LEDs, and X-ray scintillating detectors [1]. These compounds offer tunable luminescent properties, high quantum yields, and non-toxicity, making them ideal for sustainable applications [1,2,3]. One of the advantages of hybrid halocuprates is their structural and compositional variety which governs a wide spectrum of optical properties, including bright luminescence, often coupled with intriguing properties like thermochromism [4,5,6,7,8,9] and mechanochromism [4,6].
The luminescent properties of halocuprates (I) with organic cations are known to be determined by their inorganic framework. In particular, room-temperature bright luminescence is usually observed for compounds with isolated units such as [CuX2]−, [CuX3]2−, [Cu2X4]2−, and [Cu4X6]2−, whereas for a chained framework, photoluminescence is usually weak or absent [10,11]. However, it was also found that some individual compounds with infinite [Cu2X3]− and [Cu3X4]− chains also demonstrate luminescence, and the actual influence of structural parameters on luminescence is yet to be revealed [12,13,14,15].
Although the crystal structures of alkylphosphonium halocuprates (I) were extensively explored in the 1980s by S. Jagner and H. Rartl with over a dozen compounds discovered [16,17,18,19,20,21,22,23], it was found only recently that similarly to alkylammonium halocuprates, such compounds demonstrate bright luminescence originating from self-trapped exciton emission (STE) with high quantum yields approaching 100% [24,25,26,27,28,29,30,31,32].
Several luminescent compounds with tetraphenylphosphonium (PPh4+) cations have been reported so far. While the structures [PPh4]2[Cu2I4], [PPh4]2[Cu4I6]·2(CH3)2CO, and [PPh4]2[Cu4I6]·2DMSO with isolated planar dimer [Cu2X4]2− and [Cu4X6]2− cluster units expectedly demonstrated bright luminescence with PLQY = 99.5% for the latter compound [6,24,33], structures with isolated [CuX2]− units (PPh4CuX2, X = Br, Cl) showed weak luminescence [34], whereas structures with infinite [Cu3I4]− chains [PPh3R][Cu3I4] (R = Me, Et, Bu) also turned out to be luminescent [12]. Furthermore, many hybrid halocuprates (I) with alkylphosphonium cations demonstrated strong coupling between polymorphism and optical properties (e.g., for [PPh4]2[Cu2I4] with four polymorph modifications [6,35] and α/β-[PPh3Me]2[Cu4I6] [36]), which complicates the prediction of the luminescent behavior of materials based on their structure even further.
Apart from structural influence acting as a structure-directing agent, phosphonium cations that include aromatic substituents can also participate in the formation of the upper valence band (HOMO) and the conduction band (LUMO). In particular, phenyl rings and the phosphorus atom itself can introduce new electronic states, shifting the energy levels of the system, which can directly impact the bandgap and luminescent properties of materials [25,37]. Moreover, such cations may also take part in charge transfer processes upon excitation, changing the nature and probability of radiative transitions [36,38].
Whereas two polymorphs [PPh4]2[Cu2I4] with isolated dimers [Cu2I4]2− were recently found to be luminescent [6], the properties of [PPh4][Cu3I4] remain unknown. Among all known hybrid halocuprates, this compound exhibits a unique structure composed of helical chains of face-sharing tetrahedra [39], while this distinct structural arrangement highlights the need for an in-depth investigation of [PPh4][Cu3I4]’s optical properties to understand its potential as a functional material.
In this article, we present for the first time the optical absorption, luminescent properties, and thermal stability of [PPh4][Cu3I4] and compare it with similar iodocuprate (I) phases.

2. Results and Discussion

Transparent crystals of [PPh4][Cu3I4] were synthesized by cooling a saturated solution of [PPh4]I and CuI (1:3 molar ratio) in acetonitrile. The diffraction pattern (Figure 1a) of the powdered crystals matches well with the structure reported by Hartl et al. in 1992 [39]. The profile analysis confirmed that [PPh4][Cu3I4] crystallizes in an orthorhombic Ccce space group with parameters a = 11.5821(5) Å, b = 21.1227(9) Å, and c = 23.3228(9) Å.
Structurally, [PPh4][Cu3I4] is similar to other hybrid halocuprates like ACu3X4 and ACu2X3 (A = organic cation; X = I, Br, Cl) which consist of infinite chains of edge- and face-sharing CuI4 tetrahedra. However, no other halocuprates with only the face-sharing connectivity of CuX4 tetrahedra have been reported. Many ACu2I3 iodocuprates with a 1D [Cu2I3]− chain have been reported [13], but fewer ACu3I4 structures were found, namely three structures [PPh3R][Cu3I4] (R = Me, Et, Bu) [12], [bp4][Cu3I4] (bp4 = N-methyl-4,4′-bipyridinium) [40], and [EtS-4-C5H4NEt][Cu3I4] [41], all with infinite [Cu3I4]− chains of edge-sharing tetrahedra, [(DodecylMe2S)[(DodecylMeS)3)][Cu3I4] with isolated [Cu3I4]− (also with edge-sharing structure) stabilized by sulfur atoms [42], and [N(CH3CH2CH2)4][Cu3I4] with 1D chains made of edge-sharing units of three face-sharing tetrahedra [23,43]. Notably, no bromide structures [Cu3Br4]− have been found so far, which agrees with Pauling’s third rule, predicting the lower connectivity of CuBr4 polyhedra due to their smaller radius and polarizability [44].
The Cu-Cu distance in [PPh4][Cu3I4] measured from the crystal structure data is as small as 2.30 Å due to the face-shared structure of the helical [Cu3I4]− chains, suggesting strong Cu–Cu bonding interactions [39]. In contrast, the chains themselves are loosely packed compared to the majority of structures demonstrating hexagonal packaging. PPh4+ cations are also not tightly packed in the structure and do not form the π-π stacking of phenyl groups as was observed for [PPh3Et][Cu3I4]. It can be concluded therefore that the cations are likely to only produce templating effects with a weak influence on optoelectronic properties.
We found that at room temperature, [PPh4][Cu3I4] does not exhibit luminescence unlike [PPh3R][Cu3I4] (R = Me, Et, Bu) and other compounds with PPh4+ cations ([PPh4]2[Cu2I4], [PPh4]2[Cu4I6]·2OC(CH3)2, [PPh4]2[Cu4I6]·2DMSO) [12,24,33]. However, at 77 K (−196 °C), it demonstrates orange emission attributed to STE with a PL maximum at 643 nm (1.93 eV) corresponding to CIE color coordinates of (0.33, 0.36). An excitation peak was observed at 356 nm (3.48 eV), with a Stokes shift of 287 nm (1.55 eV) and an FWHM of 177 nm. (Figure 1b). The absence of luminescence at room temperature may be attributed to the small Cu-Cu distance (2.3 Å) in the face-sharing tetrahedra structure and effective charge transfer between the excited and unexcited states resulting in a quenching effect. Such a shortening of the Cu-Cu distance was suggested to be the reason for luminescence quenching in Cu6I6·2HMTA (HMTA = hexamethylenetetramine) crystals upon the partial substitution of Cu to Ag [45]. However, although the Cu-Cu distance was shown to be one of the main factors governing luminescence and responsible for thermochromism [46], the example of [TMA][Cu2I3] (TMA = tetramethylammonium) with a small 2.34 Å Cu-Cu distance and luminescence at room temperature indicates that other structural features such as interactions with organic cations also contribute to luminescent properties [13].
The optical bandgap of [PPh4][Cu3I4] is determined to be Eg = 3.10 eV, based on the diffuse reflectance spectrum plotted in Tauc coordinates (Figure 1c), and it matches well with the photoluminescence excitation spectrum with an onset at ~3.02 eV. This bandgap is similar to the values of [PPh3R][Cu3I4] (~2.9 eV for R = Me, Et, n-Bu) [12], α/β-[TMA][Cu2I3] (3.1/3.3 eV, TMA = tetramethylammonium) [13], and [NMP][Cu2I3] (2.85 eV, NMP = N-methylpyridinium) [14] and slightly lower than that for ACu2I3 iodocuprates such as [MA][Cu2I3] (3.62 eV, MA = methylammonium) [47], [TMS][Cu2I3] (3.54 eV, TMS = trimethylsulfonium), and [TMSO][Cu2I3] (3.63 eV, TMS = trimethylsulfoxonium) [48]. This observation agrees with the general trend that a lower degree of [CuX4] tetrahedra condensation results in a higher bandgap [10,11,49].
The low-temperature luminescence spectrum of [PPh4][Cu3I4] with PLmax = 643 nm is similar to that for other iodocuprates with infinite [Cu3I4]− and [Cu2I3]− chains. In particular, also non-emissive at room temperature, at 77 K, [MA]Cu2I3 and [TMS]Cu2I3 have yellow-orange luminescence with PLmax = 602 nm and PLmax = 660 nm, respectively [47,50]; [NMP][Cu2I3] has PLmax = 700 nm [14]; and [PPh3R][Cu3I4] has an active emission band at low temperature centered at ~580–600 nm [12].
TG/DSC measurements showed the excellent thermal stability of [PPh4][Cu3I4] with decomposition above 380 °C (Figure 1d) which is one of the highest among hybrid halocuprates [12,27,38,40]. The first step of weight loss is attributed to the loss of PPh4I and the formation of CuI which perfectly matches with the calculated value (44.9%), whereas in the second stage, it decomposes with some remains of Cu. DSC measurement shows an endo-effect at ~255 °C which corresponds to melting. Notably, upon melting, a stable, transparent light yellow liquid is formed (Figure 1d, inset) without the oxidation of Cu+ to Cu2+ unlike many other halocuprates (I) [10]. Upon cooling below 190 °C, it crystallizes spontaneously without signs of the glass transition, as was observed for some 0D iodocuprates such as [PPh3Me]2[Cu4I6] [36].

3. Materials and Methods

3.1. Synthesis of Crystals

A total of 387.7 mg of tetraphenylphosphonium iodide (98%, anhydrous, Macklin, Shanghai, China) and 777.2 mg of copper (I) iodide (99%, Macklin) were dissolved in a 1:3 molar ratio in 5 mL of acetonitrile (99.8%, anhydrous, Sigma Aldrich, Saint Louis, MO, USA) in a glovebox with an Ar atmosphere. The solution was stirred at 70 °C until the precursors completely dissolved. After the addition of 20 µL of H3PO2 to prevent Cu+ oxidation, the solution became transparent. Then, a hot solution was filtered through a PTFE syringe filter (0.45 µm) and left to cool (~0.5°/min) to room temperature. After 24 h, transparent crystals of [PPh4][Cu3I4] were collected and dried on a filter paper.

3.2. Measurements

PL and PLE spectra were recorded at 77 K using a HORIBA QuantaMaster 8000 spectrofluorometer (Kyoto, Japan) from [PPh4][Cu3I4] crystals ground in an Ar atmosphere sealed in an NMR quartz tube. The maximum in the PLE spectrum (356 nm) was chosen as the excitation wavelength for measuring the PL spectrum. Excitation spectra were recorded at a wavelength corresponding to the position of the maximum of the PL spectrum (643 nm). A background was subtracted from the obtained spectra, and the resulting data were smoothed.
A diffuse reflectance spectrum (DRS) was recorded from [PPh4][Cu3I4] powder crystals at room temperature on a PerkinElmer Lambda 950 spectrophotometer (Springfield, IL, USA) at 25 °C in the wavelength range from 250 to 650 nm.
Powder X-ray diffraction patterns were obtained on a Rigaku Smartlab SE diffractometer (Kyoto, Japan) using radiation from a Cu anode and recording the CuKa component via a 2D detector in the Bragg–Brentano geometry in the 2θ range of 5–60° with a step of 0.02°. Crystal lattice parameters were refined from powder diffraction patterns using Jana 2006 software (version 20/02/2023) [51]. The visualization and analysis of the crystal structure were carried out using VESTA software (ver. 3.4.7) [52].
Thermogravimetric measurements were conducted using a Mettler Toledo TGA 2 system (Greifensee, Switzerland) in the temperature range of 25–900 °C. A small crystal of [PPh4][Cu3I4] was placed in an alumina crucible with a pierced lid and measured in a flow of Ar (30 mL/min) at a scanning speed of 5 °C/min.
A differential scanning calorimetry curve was obtained on a Mettler Toledo DSC 3 device (Mettler-Toledo, Greifensee, Switzerland) in the temperature range 25–500 °C. A small crystal of [PPh4][Cu3I4] was placed in an aluminum crucible with a pierced lid and measured in a flow of Ar (30 mL/min) at a scanning speed of 5 °C/min.

4. Conclusions

In this work, we synthesized and investigated the optical properties and thermal stability of [PPh4][Cu3I4], a hybrid iodocuprate (I) with a unique helical chain structure. This compound was synthesized as transparent crystals and confirmed to have an orthorhombic crystal structure. Optical studies revealed a bandgap of 3.10 eV and low-temperature luminescence with an emission peak at 643 nm, attributed to self-trapped excitons. Thermal analysis showed one of the highest stabilities among hybrid halocuprates, with decomposition temperatures exceeding 380 °C and a melting point at ~255 °C, forming a stable liquid. This study provides valuable insights into the relationship between structure and luminescent properties, contributing to a deeper understanding of hybrid halocuprates.

Author Contributions

The synthesis of crystals was conducted by L.C. and A.A.P.; measurements were conducted by A.A.P., L.C. and S.A.F.; data processing and visualization were conducted by A.A.P. and L.C.; the writing of the original draft was conducted by A.A.P.; writing—review and editing was conducted by all authors; supervision and funding acquisition were conducted by A.A.P., M.L. and A.B.T. The manuscript was written through the contributions of all the authors. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Russian Science Foundation (Project No. 22-73-10226).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data are contained within the article.

Conflicts of Interest

The authors declare no conflicts of interest.

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Molecules 30 00543 g001
Figure 1. (a) X-ray diffraction pattern of [PPh4][Cu3I4], (b) PLE and PL spectra measured at 77 K, (c) absorption spectrum in Tauc coordinates and inset with photo of crystal (scalebar is 0.3 mm), (d) TG/DSC curves with inset image of melting crystals.
Figure 1. (a) X-ray diffraction pattern of [PPh4][Cu3I4], (b) PLE and PL spectra measured at 77 K, (c) absorption spectrum in Tauc coordinates and inset with photo of crystal (scalebar is 0.3 mm), (d) TG/DSC curves with inset image of melting crystals.
Molecules 30 00543 g001
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MDPI and ACS Style

Chen, L.; Petrov, A.A.; Li, M.; Fateev, S.A.; Tarasov, A.B. Luminescent Properties and Thermal Stability of [PPh4][Cu3I4] with a Unique Helical Structure. Molecules 2025, 30, 543. https://doi.org/10.3390/molecules30030543

AMA Style

Chen L, Petrov AA, Li M, Fateev SA, Tarasov AB. Luminescent Properties and Thermal Stability of [PPh4][Cu3I4] with a Unique Helical Structure. Molecules. 2025; 30(3):543. https://doi.org/10.3390/molecules30030543

Chicago/Turabian Style

Chen, Luyi, Andrey A. Petrov, Mingming Li, Sergey A. Fateev, and Alexey B. Tarasov. 2025. "Luminescent Properties and Thermal Stability of [PPh4][Cu3I4] with a Unique Helical Structure" Molecules 30, no. 3: 543. https://doi.org/10.3390/molecules30030543

APA Style

Chen, L., Petrov, A. A., Li, M., Fateev, S. A., & Tarasov, A. B. (2025). Luminescent Properties and Thermal Stability of [PPh4][Cu3I4] with a Unique Helical Structure. Molecules, 30(3), 543. https://doi.org/10.3390/molecules30030543

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