A novel photochromic hybrid containing trinuclear [Cd3Cl12]6− clusters and protonated tripyridyl-triazines

Pengfei Hao*, Chunyu Guo, Junju Shen and Yunlong Fu*
Key Laboratory of Magnetic Molecules, Magnetic Information Materials Ministry of Education, School of Chemical and Material Science, Shanxi Normal University, Linfen 041004, P. R. China. E-mail: haopengfei_2015@126.com; yunlongfu@sxnu.edu.cn

Received 28th August 2019 , Accepted 24th September 2019

First published on 24th September 2019


Incorporation of electron-deficient N-protonated 2,4,6-tri(4-pyridyl)-1,3,5-triazine (H3TPT) into electron-rich chlorocadmate leads to a novel organic–inorganic hybrid [H3TPT]2[Cd3Cl12] (1), which features as a trinuclear anionic cluster [Cd3Cl12]6− with one CdCl6 octahedron and two CdCl4 tetrahedra via edge-sharing mode and exhibits excellent photochromic performance with fast photoresponsive rate, obvious coloration contrast and high thermal stability.


Electron donating-accepting hybrid materials have attracted intense interest due to the excellent designability and controllablity of compositions, structures and bandgaps, and their potential and/or real applications in data storage, decorations, display, memory, switches, photography, photometry, non-linear optics and so on.1 In particular, considerable efforts have been devoted to the development of organic–inorganic photochromic hybrids, which are largely focused on the elaborate selection and rational integration of electron donors and acceptors to modulate their intermolecular electronic behaviors including charge transfer (CT) and electron transfer (ET).2 Although intrinsic properties of anion/π-acceptor interactions in the solution between π-acids such as the naphthenediimides (NDIs) and 1,4,5,8,9,12-hexaazatriphenylene (HAT(CN)6) and Lewis basic anions (e.g., F, Cl, Br and I) have been systematically investigated by Saha3 and Ballester,4 the intermolecular electronic behaviors (occurrence of CT/ET) in the crystalline materials are largely dependent on the composition, aggregating modes and the interfacial variations of electron donors/acceptors.5 Generally, to ensure efficient photoinduced ET and subsequent photochromism, stronger electron acceptors such as viologens and NDIs6 are typically selected to match with the stronger electron donors such as chloride/bromide/oxide-based donors, in which simultaneous occurrence of CT and consequent coloration are usually viewed as an unfavorable factor to hinder effective photo-induced ET and chromism.7 Therefore, how to rationally optimize balance of CT/ET and photochromic performance, such as photoresponsive rate and range, coloration contrast, reversibility and thermal stability, are still remains a long-term challenge. In principle, elaborate selection of electron donors and acceptors constitutes the most crucial role.

Chlorocadmates are inclined to two-dimensional perovskite layers8 and one-dimensional chains9 similar to those of Pb(II) and Sn(II) halides,10 while the 0-D chlorocadmate clusters are relatively scarce.11 These chlorocadmates possess structural diversity, unique optical properties (i.e., confinement effects) and electron behaviors,12 which might be a promising candidate as novel electron donors for the design of photochromic hybrids. Up to date, photochromic chlorometallate hybrids are mainly viologens-based chlorobismuthates,5,13 while photochromic chlorocadmate hybrids have never been involved and high photochromic performance may be expected.

Tripyridyl-triazines (TPT) are a type of rigid polypyridine compounds14 possessing diverse coordination modes, high redox activity and strong π-acidity, which have been widely utilized to construct polynuclear complexes and coordination networks with fascinating properties, such as crystal structural rearrangements,15 photoluminescence,16 and photocatalysis.17 Noteworthy, in addition to typical electron-deficient NDI and viologens, TPT have also been selected as electron acceptors to construct the organic–inorganic photochromic systems with oxygen-containing groups such as phosphate and carboxylate.18 Unique spatial motif, tunable electron accepting ability and high thermal stability endow TPT a potential advantage for the construction of photochromic hybrids. More importantly, the protonated TPT could not only act as a template to direct the synthesis of the novel chlorocadmate framework, but also interplay with chlorocadmate anion by electrostatic interactions and hydrogen bonding interactions, which are beneficial for the stability of the hybrid. Furthermore, the protonation of TPT is anticipated to lower its LUMO energy level and thus increase the electron-deficient ability, which will be favorable to the photoinduced ET process and consequent improvement of photochromic property.

In this communication, we reported a novel inorganic–organic hybrid [H3TPT]2[Cd3Cl12] with excellent photochromic performance. Colorless crystals of 1 were obtained by a simple and effective reaction of TPT, CdCl2·2.5H2O, concentrated HCl and benzyl alcohol at room temperature for three days. 1 features a novel trinuclear anionic cluster [Cd3Cl12]6− with unique linkage mode and charge compensating agents of [H3TPT]3+ cations. The title compound displays unique photochromic behaviors possessing fast photoresponsive rate, obvious coloration contrast and high thermal stability. To the best of our knowledge, it is the first chlorocadmate-based photochromic hybrid with electron-deficient H3TPT as electron acceptor.

Single-crystal X-ray diffraction experiments reveal that compound 1 crystallizes in the triclinic space group P[1 with combining macron], and the asymmetric unit comprises of one and a half crystallographically independent Cd2+ ions, six Cl ions, and one [H3TPT]3+ cation (Fig. S2, ESI). The Cd2+ ions exhibit two kinds of coordination mode: octahedral geometry and tetrahedral geometry. The Cd1 ion is surrounded by six Cl ions in nearly ideal octahedral coordination environment, while the Cd2 ion is connected with four Cl ions in seriously distorted tetrahedral coordination environment. The Cd–Cl bond distances are in the range of 2.4367(6)–2.6721(5) Å and the Cl–Cd–Cl bond angles vary from 86.25(2) to 180.000(1)° (Table S2, ESI), which are quite consistent with those found in other hybrid chlorocadmates.8,9,11 As shown in Fig. 1a, one CdCl6 octahedron and two CdCl4 tetrahedra aggregate into a trimeric [Cd3Cl12]6− anionic cluster via edge-sharing mode, which is charge balanced by [H3TPT]3+ cations (Fig. S3, ESI). As far as we known, 0-D chlorocadmate clusters are usually condensed by edge-, or face-sharing of single [CdCln] primary units (typically n = 4, 6), the occurrence of [Cd3Cl12]6− anionic cluster with unique linkage mode adds a fundamentally new binary anion to cadmium halide chemistry. Notably, a pair of [H3TPT]3+ cations are assembled into a compact supramolecular dimer (Fig. 1b) via offset π–π interactions (interplanar distance is 3.319(1) Å, Cg⋯Cg = 3.505(5) Å). Meanwhile, there are a plethora of N–H⋯Cl interactions between inorganic clusters and organic counter cations (Table S3, ESI), in which each [H3TPT]3+ cations is hydrogen-bonded to a total of four [Cd3Cl12]6− anions (Fig. 1c) and each [Cd3Cl12]6− anions is hydrogen-bonded to a total of six [H3TPT]3+ cations (Fig. 1d). These multiple weak interactions in 1 play critical roles in stabilizing the crystal structure, photoinduced ET pathways and photochromic behaviors discussed below.


image file: c9dt03494c-f1.tif
Fig. 1 (a) View of [Cd3Cl12]6− anionic cluster. (b) View of the [H3TPT]3+ cationic dimer. (c) The pattern of N–H⋯Cl contacts around each [H3TPT]3+. (d) The pattern of N–H⋯Cl contacts around each [Cd3Cl12]6−. Partial hydrogen atoms are omitted for clarity.

The stability of 1 is investigated through thermogravimetric and differential scanning calorimetry (TG-DSC) studies. As shown in Fig. S6 (ESI), TGA curve of 1 exhibits multi-step weight losses. There is no obvious weight loss between atmospheric temperature and 298 °C. The first weight loss in the temperature regions of 298–375 °C (found: 29.85%) with a broad endothermic peak, may be ascribed to the elimination of one half [H3TPT]·3Cl (expected: 30.27%). Further heating to 550 °C causes a continuous weight losses due to the removal of the other half [H3TPT]·3Cl (found: 31.19%) and complete decomposition of inorganic frameworks. Compared with other TPT-based photochromic hybrids,18 the relatively higher thermal stability of 1 is attributable to the number and strength of the weak interactions, which is beneficial for its practical application in photochromic devices.

By comparison of the obvious coloration of TPT-based photochromic hybrid integrating with phosphate/carboxylate,18 the coloration of 1 is colorless (Fig. 2a), which corresponds to the CT absorption bands in ultraviolet region (Fig. 2b). The colorlessness of 1 is favorable for the photoinduced ET accompanying photochromic performance with high coloration contrast, which exhibits the delicate modulating effect of chlorocadmate as electron donor on balance of intermolecular CT/ET.


image file: c9dt03494c-f2.tif
Fig. 2 (a) Photographs showing the photochromic behavior of 1. (b) Time-dependent UV-vis absorption spectra of 1. (c) EPR spectra of 1 and 1P in solid state at room temperature.

Irradiated by the mercury (Hg) lamp (300 W, ∼365 nm) at room temperature in air, 1 displays a naked-eye detectable color transformation and changes from colorless to green (denoted as 1P) within only 3s and tends to be saturated in 10 min (Fig. 2a), which is more sensitive to UV light than those of reported ET photochromic materials (the photoresponsive rate usually more than 1 min and to be saturated for about 40 min).2,5,7–9,18,19 Elemental analysis, FT-IR spectra and powder X-ray diffraction (PXRD) measurements exhibit the phase purity of the bulky materials of 1. The single crystal structure of 1P shows that there is no obvious change in crystallographic data, spatial arrangement, bond lengths and bond angles (Tables S1 and S2, ESI), which are further verified by the consistent FT-IR spectra (Fig. S4, ESI) and PXRD patterns (Fig. S5, ESI) before and after UV light irradiation, ruling out the photoinduced isomerization or photolysis.20 As depicted in Fig. 2b, time-dependent UV-vis spectra of 1 shows a new broad absorption band centered at 718 nm in the whole visible region (400–900 nm) and drastically increases with the duration of light irradiation, which is good agreement with the characteristic absorption of colored organic radicals [H3TPT]2+˙ via photoinduced ET. Furthermore, a sharp and strong signal with g = 2.0011 after irradiation was observed in electron paramagnetic resonance (EPR) curve (Fig. 2c), which further confirms that the color change of 1 is attributable to photoinduced ET between the anionic clusters [Cd3Cl12]6− and the organic cations [H3TPT]3+. Note that a relatively weak EPR signal at 2.0015 before UV light illumination (Fig. 2c), may be ascribed to the generation of organic radicals induced by sunlight irradiation, which is further verified by a very weak characteristic absorption band centered at 718 nm (Fig. 2b, black line), suggesting the wide-range photoresponse of 1.

The ET mechanism could also be elucidated by X-ray photoelectron spectroscopy (XPS) experiments. As shown in Fig. 3, the core-level spectra of Cd 3d and C 1s before and after irradiation were almost the same, while the obvious changes take place in the core-level spectra of Cl 2p and N 1s. The Cl 2p peaks shift to higher binding energies (197.56 eV for 1 vs. 197.84 eV for 1P), while the N 1s peaks move to lower binding energies (399.26 eV for 1 vs. 399.11 eV for 1P). The result implies that the electrons transfer from the Cl atom of [Cd3Cl12]6− to the N atom of [H3TPT]3+ and consequent generation of [H3TPT]2+˙ radicals, which is similar to that of photochromic hybrids of TPT integrating with phosphate/carboxylate.18


image file: c9dt03494c-f3.tif
Fig. 3 Cd 3d (a), C 1s (b), Cl 2p (c) and N 1s (d) XPS core-level spectra of 1 and 1P.

Interestingly, the dark blue crystal of 1P is hyperstatic in the dark at room temperature for about eight months, implying formation of an ultra-long-lived charge-separated state,21 which is promising in constructing molecular electrodes.22 Noteworthy, 1P can completely revert to 1 on heating at 120 °C for 1 h under aerobic condition, which is proved by the basically disappearance of new absorption bands (Fig. S6, ESI). This reversible coloration–decoloration process can be repeated at least eight times by alternating light irradiation and thermal treatment, and no remarkable loss of photochromic properties is observed (Fig. S7, ESI).

In comparison of the reported TPT-based photochromic materials,18 the title compound displays excellent photochromic performances with fast photoresponsive rate, obvious coloration contrast, which can be illustrated from the following three aspects: (a) The combination of novel electron-rich chlorocadmate anionic clusters and electron-deficient [H3TPT]3+ cations realizes the subtle balance between intermolecular CT and ET, which is responsible for the obvious coloration contrast (initial color vs. photochromism). (b) The protonated TPT endow the number and strength of N–H⋯Cl interactions and associated Cl⋯N contacts (3.2026(17), 3.2237(18), 3.2468(18) and 3.2616(18) Å) in interface of chlorocadmate donors and [H3TPT]3+ acceptors, which is not only beneficial for the high thermal stability of the hybrid, but also provide a type of efficient ET pathway to produce the colored radicals of [H3TPT]2+˙. (c) The formation of compact supramolecular dimers of [H3TPT]3+ cations can effectively enhance the stability of the radicals, which is advantageous for the net photoinduced ET.

In summary, the first chlorocadmate-based photochromic hybrid was synthesized by marriage of electron-deficient N-protonated 2,4,6-tri(4-pyridyl)-1,3,5-triazine (H3TPT) and electron-rich chlorocadmate via a simple and effective procedure at ambient temperature. The observation of a unique trinuclear anionic cluster [Cd3Cl12]6− possessing an arrangement of one CdCl6 octahedron and two CdCl4 tetrahedra via edge-sharing mode, represents an important new development in cadmium halide chemistry. The title compound exhibits fast photoresponsive rate, obvious coloration contrast and high thermal stability due to the suitable matching role of electron-rich chlorocadmate donors and electron-deficient [H3TPT]3+ acceptors. Our research offers a new pathway for the design and exploitation of new photochromic hybrids with excellent properties.

Conflicts of interest

The authors declare no competing financial interest.

Acknowledgements

This work was supported by National Natural Science Foundation of China (21171110), Research Fund for the Doctoral Program of Higher Education of China (20131404110001), Natural Science Foundation of Science and Technology Agency of Shanxi Province (201601D202025, 201701D121022), and 1331 Project of Shanxi Province.

Notes and references

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Footnotes

Electronic supplementary information (ESI) available: Experimental section; asymmetric unit; packing diagram; TG-DSC curve; IR spectra; PXRD patterns; UV–vis absorption spectra; cyclic number of coloration–decoloration; crystallographic data; bond lengths and angles; and hydrogen bonds. CCDC 1935646 and 1947057. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c9dt03494c
These authors contributed equally to this work.

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