A highly luminescent and stable copper halide ionic hybrid structure with an anionic CuBr2(tpp)2module

Article abstract of DOI:10.1039/d1tc00205h

Here, we report a novel copper bromide ionic compound (bz-ted)CuBr₂(tpp)₂·H₂O (1, tpp = triphenylphosphine, and bz-ted = 1-benzyl-1,4-diazabicyclo[2.2.2]octan-1-ium) by a facile wet chemistry method. The inorganic module o

Full text of DOI:10.1039/d1tc00205h

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A highly luminescent and stable copper halide
ionic hybrid structure with an anionic CuBr2(tpp)2
Cite this: DOI: 10.1039/d1tc00205h
module†
Hua Tong,‡a Chanchan Xu‡b and Wei Liu
Received 15th January 2021,
Accepted 24th June 2021
a
*
DOI: 10.1039/d1tc00205h
rsc.li/materials-c
Here, we report a novel copper bromide ionic compound (bz-ted) been studied intensively recently due to their unique photo-
CuBr2(tpp)2ÁH2O (1, tpp = triphenylphosphine, and bz-ted = 1-benzyl- physical and photochemical properties originating from the d10
1,4-diazabicyclo[2.2.2]octan-1-ium) by a facile wet chemistry method. electronic configuration of the Cu atoms.18–20 A variety of
À
The inorganic module of the compound is CuBr2 coordinated to two inorganic modules have been reported, from discrete inorganic
tpp molecules, generating strong blue photoluminescence originating units to extended infinite chains or layers.21 These inorganic
from the metal-to-ligand-charge-transfer (MLCT) process. The combi- motifs are combined with different types of organic ligand,
nation of both ionic bonds and covalent bonds in the hybrid cluster mostly N-containing ligands (N-ligands), either aliphatic or
makes compound 1 strongly emissive, robust and solution-processable. aromatic, to form molecular clusters, chains, layers, and
Such a coordinated anionic inorganic module presents a new approach extended networks.18,21 The bondings between the two compo-
for the development of highly luminescent and stable copper halide nents are coordinate bonds, ionic bonds or both. Their lumines-
ionic hybrid structures.
cence mechanisms include metal-to-ligand-charge-transfer (MLCT),
halide-to-ligand-charge-transfer (XLCT), metal-centered change
Crystalline organic–inorganic hybrid materials are composed of transfer (CC), or a combination of them.22,23 The remarkable
inorganic and organic moieties that are combined at the structural diversity of the copper halide hybrid structure family is
molecular scale.1–7 This type of hybrid compound is both a result of a vast variety of available inorganic modules and organic
fundamentally and technologically important because of the ligands, as well as many possible coordination modes between
structural versatility and interesting properties that result from them.18 It has been found that the ionic bonding in the hybrid
the incorporation of both inorganic and organic components in structures could enhance their stability, while the coordinative
a single crystal lattice.8–13 Such inorganic motifs are further bonding is important for their luminescence.24,25 Thermochromic,
connected by different kinds of organic ligands, either aliphatic vapochromic, and mechanochromic luminescence has been
or aromatic, forming zero-dimensional (0D) molecular clusters observed and well studied for this family of materials.26–28
to one-dimensional (1D) chains, and from two-dimensional
The remarkable structural diversity of the copper halide
(2D) sheets to three-dimensional (3D) frameworks.6,14,15 These hybrid structure family is a result of a vast variety of available
materials are a very important class of functional materials with inorganic modules and organic ligands, and many possible
structural variety and interesting properties, showing potential coordination modes between them.18 They have been grouped
applications in a variety of areas, including light-emitting based on the nature of chemical bonding: the neutral struc-
diodes (LEDs), chemical sensors, functional coatings, etc.16,17
tures are those built on coordinative Cu–ligand bonds, and
Among them, hybrid structures based on copper halides are therefore both the inorganic module (CuI) and organic ligands
a representative luminescent hybrid structure family that has are charge-neutral; the ionic structures are those that are made
of cationic ligands and anionic inorganic modules without
direct coordinative bonds between the two; all-in-one (AIO)
type structures are those with both coordinate and ionic bonds
between the inorganic module and organic ligand.18 It has been
found out that the ionic bonding in the hybrid structure could
enhance their stability, while the coordinative bonding is
important for their luminescence.24,25 The copper halide ionic
organic–inorganic hybrid structures are much more stable
compared to the neutral structures with similar structure and
a School of Chemical Engineering and Technology, Sun Yat-sen University, Zhuhai,
519082, China. E-mail: liuwei96@mail.sysu.edu.cn
b Hoffmann Institute of Advanced Materials, Shenzhen Polytechnic,
7098 Liuxian Blvd, Nanshan District, Shenzhen, 518055, China
† Electronic supplementary information (ESI) available: Experimental details,
HMNR spectra of the organic ligands, PXRD patterns, photoluminescence spec-
tra, and crystallographic data. CCDC 2013295. For ESI and crystallographic data
in CIF or other electronic format see DOI: 10.1039/d1tc00205h
‡ These authors contributed equally.
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composition. However, they generally display weak or no lumines-
cence due to the absence of metal-to-ligand charge transfer (MLCT)
luminescence mechanism since there are no coordination bonds
between the inorganic species and the organic species. The studies
on AIO compounds reveal that the coordination bonds between the
ligand molecules and copper atoms are the key factors for the
strong luminescence of these hybrid compounds.24,25
Here, a new design strategy has been developed for the devel-
opment of hybrid structures with both ionic bonds and coordina-
À
tive bonds. A coordinated anionicÀinorganic cluster CuBr2(tpp)2
has been constructed with CuBr2 anion coordinated with twÀo
neutral triphenylphosphine (tpp) molecules. Such CuBr2(tpp)2
clusters are surrounded by cationic bz-ted (bz-ted = 1-benzyl-1,
4-diazabicyclo[2.2.2]octan-1-ium), forming an ionic organic–inor-
ganic hybrid structure (bz-ted)CuBr2(tpp)2ÁH2O (1). In this structure,
the cationic cluster CuBr2(tpp)2À contains the Cu–ligand coordina-
tion bonds, generating strong blue luminescence due to the charge
transfer among the ligand molecules and the metal. The ionic
bonds between cationic and anionic species improve the stability
and solubility of the hybrid structures.
The preparation of bz-ted bromide is conducted by a mod-
ified method and its 1H NMR spectrum is shown in Fig. S1
(ESI†).29 1,4-Diazabicyclo[2.2.2]octane (ted) was first dissolved
in acetone (50 mL) and benzyl bromide was added dropwise
into it under magnetic stirring. The reaction mixture was
stirred for 12 h and a white precipitate formed that was
collected by filtration. Colorless plate-shaped single crystals
of 1 were obtained by heating the mixture of CuBr, tpp, and bz-
ted bromide in a mixed CH2Cl2/toluene (1 : 1/v : v) solvent
system at 80 1C. Crystals formed overnight and were typically
Fig. 1 (a) Structural plot of the compound 1 (cyan balls: Cu; dark yellow
balls: Br; pink balls: P atoms; grey balls: C atoms; blue balls: N atoms; dark
grey balls: H atoms; red ball: O atom). (b) Structural plot of coordinated
copper halide modules and the surrounding ligands (blue balls). Some
atoms are hidden for clarity.
suitable for X-ray single-crystal diffraction analysis. The reac- function, as shown in Fig. 2a. The absorption edge for compound
tion yield is 75% based on Cu. The single crystal X-ray diffrac- 1 was found to be 3.0 eV. This value is also under the density-
tion analysis shows that 1 crystallizes in the triclinic space functional theory (DFT) calculation results of compound 1
%
group P1 with 2 formula units in the unit cell and all atoms are (Table S3, ESI†). Single crystals of 1 are colorless and transparent
in general positions. There are also water molecules in the under natural light, suggesting almost no absorption in the visible
structure. Detailed cryptographic data are summarized in Table region with a wide band gap, while under photoexcitation at the
S1 (ESI†). Each Cu atom is coordinated by two Br atoms and two excitation energy (360 nm), the single crystal of 1 emits intense
tpp molecules in a tÀetrahedral geometry, forming a negatively blue light (Fig. 2b inset). Upon excitation, the electrons are excited
charged CuBr2(tpp)2 species (Fig. 1a). The ligand isÀpositively from the valence bands to the conduction bands, and the
charged and is surrounded by the CuBr2(tpp)2 species radiative recombination of electrons and holes leads to the strong
(Fig. 1b). There is no coordination bond between the N atoms blue luminescence of the compound. Fig. 2b is the photolumi-
and the Cu atoms, thougÀh there is an uncoordinated N atom in nescence of compound 1 under 360 nm excitation at room
the ligand. CuBr2(tpp)2 and the ligand are connected by temperature. As shown in this figure, it exhibits strong
hydrogen bonds. The distance between O(1) and N(2) is blue emission with a full width at half-maximum (fwhm) of
3.052(1) Å and the angle of +O(H(1C)Á Á ÁN(2)) is 161.1(2)1, while B75 nm. The emission peaks at 430 nm, and such a high-
the distance between O(1) and Br(2) is 3.550(4) Å and the angle energy and deep blue emission is very rare among copper
of +O(H(1B)Á Á ÁBr2 ) (symmetry code: i, x, y À 1, z) is 169.5(5)1. halide-based hybrid structures.16,18,20 The excitation spectrum of
i
The phase purity of compound 1 was confirmed by powder 1 covers most of the UV light region with the strongest peak of
X-ray diffraction analysis (PXRD, Fig. S2, ESI†). The peak 360 nm. Photoluminescence under various excitation energies has
positions observed in the PXRD patterns are in good agreement been investigated and the spectra are plotted in Fig. S3 (ESI†). The
with those simulated from single-crystal X-ray data, indicating spectra show that though the emission intensities change, the
that a pure phase is obtained. The compositions of the crystals emission range remains the same, indicating that the emission is
were further verified by elemental analysis (Table S2, ESI†). Details from a single excitation state.
of synthesis and characterization can be found in the ESI.†
Temperature-dependent photoluminescence emission was
The optical absorption spectrum for compound 1 was collected studied under the excitation of 360 nm and the spectra are
at room temperature and converted to the Kubelka–Munk plotted in Fig. 3a. At various temperatures from 77 K to 277 K,
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Fig. 2 (a) UV-vis absorption spectrum of 1. (b) Excitation (black) and
emission spectra (blue) of 1. lex = 360 nm, and lem = 430 nm. Inset:
Crystalline sample of 1 under UV light.
this compound emits stable blue emission without any change
in the emission energy and range, indicating that there is no
thermochromic behaviour found for 1. As the temperature
decreases, the photoluminescence intensity increases for this
compound. An AIO type of copper iodide-based cluster with bz-
ted as the organic ligand, 0D-Cu3I5(bz-ted)2, has been reported
Fig. 3 (a) Emission spectra at various temperatures, lex = 360 nm. (b) CIE
coordinates of 1, inset: luminescent coating made by 1.
The Commission Internationale de l’Eclairage (CIE) chromati-
as a yellow emitter with an emission maximum at 560 nm.25 It city coordinates for the compound are calculated to be (0.15, 0.11),
has been studied that for 0D-Cu3I5(bz-ted)2, the luminescence as shown in Fig. 3b. The room temperature internal quantum
is majorly contributed from Cu–Cu interaction, and the ligands yields (IQYs) were measured on a C9920-03 absolute quantum
have little influence on its luminescence. Thermochromic yield measurement system (Hamamatsu Photonics). The IQY of
behaviour has also been found for this compound, which is compound 1 is 81% when excited at the energy of 360 nm. Such a
the major characteristic of metal-centered change transfer.30 high IQY is rare among the blue-emitting hybrid compounds and
Similar luminescence behaviour has been observed for Cu4I4 Cu-based all inorganic materials (Table S4, ESI†).12,13,32–34 The
cubane-based structures and other ionic structures without Cu– luminescence decay curve of the compound at room temperature
ligand bonds.27,31 There is only one Cu atom in the inorganic is shown in Fig. S5 (ESI†), giving a long lifetime of approximately
module of 1, eliminating the possibility of Cu–Cu interaction. 8.89 ms by monoexponential fitting, which indicates the phos-
Also, the organic ligands are not fluorescent, as shown in phorescence nature. Copper halide ionic organic–inorganic
Fig. S4 (ESI†). The strong high-energy emission of 1 suggests hybrid structures are relatively stable but typically display weak
the MLCT mechanism as observed for other neutral copper luminescence due to the absence of the metal-to-ligand charge
halide molecular complexes.19,20 The intense blue emission of transfer (MLCT) process, which limits their practical applications.
the compound is originated from the charge transfer between Most of the copper halide ionic structures have very low IQY, since
the tpp molecules and the Cu atoms. Compared to the lumi- the MLCT luminescence is absent in these structures as there are
nescence of 0D-Cu3I5(bz-ted)2, though the inorganic modules of no covalent bonds between the Cu atoms and the organic ligands.
the two clusters are different, the blue shift in emission energy Though compound 1 has an ionic structure, there are Cu–tpp
of 1 is highly related to the introduction of tpp, confirming that covalent bonds that lead to the strong electron transfer between
the organic ligands play a vital role in the luminescence and the metal and the ligands, which results in the high quantum
this type of compound is optically tunable.
efficiency of compound 1.
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Most of the previously reported copper halide hybrid struc- Such structure design strategy represents a new method for the
tures with strong blue emission are neutral molecular synthesis of highly luminescent and stable copper halide hybrid
clusters.18 Though their IQYs are high, poor stability is the compounds.
major obstacle for their practical applications. The thermal
stability of these clusters is typically below 60 1C. The stability
of copper halide hybrid structures could be enhanced by the
Author contributions
incorporation of the emissive core into multi-dimensional
H. T. and C. X. carried out the experiment. H. T. carried out the
frameworks.30 However, hybrid structures prepared by such
DFT calculations. W. L. wrote the manuscript and supervised
an approach are generally extended structures that show poor
the findings of this work. All the authors discussed the results
solubility in common organic solvents. As shown from the TGA
and contributed to the final manuscript.
plot (Fig. S6, ESI†), the decomposition temperature of com-
pound 1 is as high as 200 1C. The decomposition of this
compound starts from the loss of the organic molecules and
water molecules. The improved stability of compound 1 is
Conflicts of interest
achieved by combining both the inorganic and organic bonds There are no conflicts to declare.
in the hybrid structures. In addition, the Cu–P bonds show
higher bond energy compared to that of Cu–N bonds, which
Acknowledgements
would form hybrid clusters with higher stability.18,19
Compound 1 is soluble in common organic solvents, such as
methanol, DMSO, etc., at room temperature. About 100 mg of 1
can be completely dissolved in 1 mL of DMSO within 5 min at
room temperature under ultrasonication. No luminescence has
been observed for the dissolved sample. The good solubility of
compound 1 in DMSO can be attributed to the formation of
CuI-L-DMSO complexes and the 1@DMSO solution is non-
luminescent due to the dissociation of Cu–N bonds, which
eliminates the MLCT and XLCT charge transfer in 1.24 The
luminescent sample of 1 could be recovered by mild heating of
the solution until it dried out and the sample could be
recrystallized. As shown in the inset of Fig. 3b, a thin film of
compound 1 has been prepared by drop-casting 1@DMSO
solution on a glass substrate, showing a strong blue signal
under UV light. This one-pot synthetic approach by using both
neutral and ionic ligands to combine both ionic and coordina-
tion bonds is an effective and facile strategy for the design and
synthesis of organic–inorganic hybrid structures with strong
luminescence, high stability and solution processability.
Further expansion of high-performance copper halide hybrid
structures with the coordinated anionic inorganic cluster is
currently underway by using such a synthetic strategy with
different kinds of organic ligands.
The National Natural Science Foundation of China (21901167),
the Shenzhen Sci. and Tech. Research grant (JCYJ201803071
02051326), the Start-up Fund of Sun Yat-Sen University (76110-
18841231), and the Science and Technology Program of
Guangzhou (202102021177) are greatly acknowledged.
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