Quaternary Phosphorus-Induced Iodocuprate(I)-Based Hybrids: Water Stabilities, Tunable Luminescence and Photocurrent Responses

Article abstract of DOI:10.1002/ejic.201800813

Four (triphenyl)phosphonium-based quaternary phosphorus salts with different substituents (varying from methyl to n-butyl) were selected to be structural directed agents (SDAs) to construct four iodocuprate(I) hybrids via solution method, i.e., [(PPh₃Me)(Cu₃I₄)]_n (1), [(PPh₃Et)(Cu₃I₄)]_n (2), (PPh₃iPr)₂(Cu₂I₄) (3), [(PPh₃nBu)(Cu₃I₄)]_n (4). The inorganic iodocuprates in 1, 2 and 4 are 1-D (Cu₃I₄)_n^n– chains constructed from Cu₅I₁₁ units, but (Cu₂I₄)^2– in 3 is a di-nuclear cluster. Interestingly, the strength of Cu···Cu and π–π stacking interactions are weakened with the lengthening of alkyl groups on P-atom. The best water stability of 4 can be ascribed to the better hydrophobicity of n-butyl group, which deters the dispersing of organic and inorganic moieties and as a result, inhibit hydrolysis reaction. Furthermore, all compounds exhibit typical reversible luminescent thermochromic behaviors, among which 4 exhibits blue emission and the quenching of higher energy (HE) zone in 1 and 2 are led by strong π–π stacking interactions. Besides, effective and repeatable photocurrent responses can be detected in these compounds. In all, by systematically introducing alkyl groups into (triphenyl)phosphonium as SDAs to prepare hybrid iodocuprates, we can find that the longer alkyl groups can achieve stronger tunable PL materials with enhanced water stabilities.

Full text of DOI:10.1002/ejic.201800813

DOI: 10.1002/ejic.201800813
Full Paper
Iodocuprate Hybrids
Quaternary Phosphorus-Induced Iodocuprate(I)-Based Hybrids:
Water Stabilities, Tunable Luminescence and Photocurrent
Responses
Wen-Ting Zhang,[a] Jian-Zhi Liu,[b] Jing-Bo Liu,[b] Kai-Yue Song,[a] Yi Li,[a] Zhi-Rong Chen,[a]
Hao-Hong Li,*[a] and Rong Jiang*[a]
Abstract: Four (triphenyl)phosphonium-based quaternary organic and inorganic moieties and as a result, inhibit hydrolysis
phosphorus salts with different substituents (varying from reaction. Furthermore, all compounds exhibit typical reversible
methyl to n-butyl) were selected to be structural directed luminescent thermochromic behaviors, among which 4 exhibits
agents (SDAs) to construct four iodocuprate(I) hybrids via solu- blue emission and the quenching of higher energy (HE) zone
tion method, i.e., [(PPh3Me)(Cu3I4)]n (1), [(PPh3Et)(Cu3I4)]n (2), in 1 and 2 are led by strong π–π stacking interactions. Besides,
(PPh3iPr)2(Cu2I4) (3), [(PPh3nBu)(Cu3I4)]n (4). The inorganic iodo- effective and repeatable photocurrent responses can be de-
n–
cuprates in 1, 2 and 4 are 1-D (Cu3I4)n chains constructed tected in these compounds. In all, by systematically introducing
from Cu5I11 units, but (Cu2I4)2– in 3 is a di-nuclear cluster. Inter- alkyl groups into (triphenyl)phosphonium as SDAs to prepare
estingly, the strength of Cu···Cu and π–π stacking interactions hybrid iodocuprates, we can find that the longer alkyl groups
are weakened with the lengthening of alkyl groups on P-atom. can achieve stronger tunable PL materials with enhanced water
The best water stability of 4 can be ascribed to the better stabilities.
hydrophobicity of n-butyl group, which deters the dispersing of
Introduction
this kind of (cluster-based) compounds are greatly determined
by the compositions, geometries, nuclear numbers and
metal···metal interactions.[16–19] However, compared with the
extensively reported cluster-based iodocuprates, the polymeric
anionic iodocuprates are still underdeveloped due to lack of
efficient synthetic strategies, and the commonly used structure
directing agents (SDAs) are N/S-bearing organic cations with
different skeleton flexibility, hydrophobicity and electronic
structures,[12,13,20,21] for example, methylnicotinohydrazide
dication (MNH2+).[12] The quaternary phosphorus salts were
seldom utilized as SDAs in iodocuprate system, and only
limited iodocuprates/quaternary phosphorus hybrids were re-
ported.[22–31]
For cluster-based iodocuprate PL materials, previous studies
have shown that their LUMO energies are commonly localized
on the phosphine ligands, while the HOMOs are occupied by
metal centers.[32–34] Therefore, the cationization of phosphine
ligands can change their electronic structure to increase LUMO
energies, and the polymerization of iodocuprate can augment
the HOMO energies, consequently the emission maximum
(λmax) might shift to deep-blue region with the enchanced
HOMO–LUMO gaps. The strategy might help to improve the
luminescence properties of iodocuprate-based PL materials. Be-
sides, the alkyl modification on phosphine ligands can improve
their water stability, which will obviously decrease the fabricat-
ing costs and elongate the working life of related devices.[13,35]
In this work, four kinds of (triphenyl)phosphonium-based qua-
ternary phosphorus salts containing different substituents
(varying from methyl to n-butyl) on P atoms were used as struc-
Luminescent materials are one of the most important materials
due to their wide applications in display visualization devices,
trace detection, chemical sensing, biological labeling, et al.[1,2]
In this realm, cuprous iodide-based photoluminescence (PL)
materials have captured special interests owing to their extraor-
dinary structural diversity and large variety of photophysical
properties,[3,4] and more importantly, their low cost and non-
toxicity compared to the noble and rare-earth metals.[5–7] So
far, from the viewpoint of their structures, the cuprous iodide-
based PL materials can be divided into two types: one is neutral
iodocuprate components coordinated by neutral organic
ligands (e.g., cubane-like [Cu4I4R4]),[8–11] the other is negative
inorganic iodocuprate species accompanied by positively
charged structure directing agents (SDAs), in which the iodo-
cuprates range from low dimensional clusters, chains to ex-
tended 2-D and 3-D frameworks.[12–14] In the former case, most
of the neutral organic ligands are phosphine with different
substituents on phosphorus or phenyl rings, such as
[PPh2(CH2CH=CH2)].[9,15] And the luminescence behaviors of
[a] College of Chemistry, Fuzhou University,
Fuzhou, Fujian, 350116, China
E-mail: lihh@fzu.edu.cn
https://chem.fzu.edu.cn
[b] Department of Otorhinolaryngology,
Fujian Medical University Union Hospital,
Fuzhou, Fujian, 350001, China
Supporting information and ORCID(s) from the author(s) for this article are
available on the WWW under https://doi.org/10.1002/ejic.201800813.
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tural directed agents (SDAs) to synthesize four new iodo- C–H···π interactions can also be detected among adjacent
cuprates(I) hybrids, i.e., [(PPh3Me)(Cu3I4)]n (1), [(PPh3Et)(Cu3I4)]n phenyl groups (Table S4). Based on these weak interactions, a
n–
(2), (PPh3iPr)2(Cu2I4) (3), [(PPh3nBu)(Cu3I4)]n (4). They exhibit 1-D cationic chain is presented (Figure 1d). The 1-D (Cu3I4)n
blue-to-red emission with good water stability, in addition, ex- chains, packing diagrams and 1-D cationic chains based on
cellent photocurrent responses can be detected, and the mech- C–H···π interactions of 2 and 4 can be seen in Figure S3 and
anisms were discussed and verified by theoretical calculations.
Figure S4.
Results and Discussion
Crystal Structures
Under the template of four structural directed agents (SDAs)
with different substituents on P atoms, the iodocuprates(I) moi-
eties exhibit structural dimensions from 0-D (Cu2I4)2– cluster (for
3) to 1-D (Cu3I4)nn– chains (for 1, 2, 4). And the (Cu3I4)nn– chains
in 1, 2 and 4 are isostructures. No hydrogen bonds can be
found in all compounds, but C–H···π interactions can be ob-
served for all compounds, π–π stacking interactions were only
detected in 1 and 2.
Structures of [(PPh3Me)(Cu3I4)]n (1), [(PPh3Et)(Cu3I4)]n (2)
and [(PPh3nBu)(Cu3I4)]n (4): Single-crystal X-ray diffraction
analysis reveals that hybrid 1 and 2 crystallizes in the mono-
clinic system with space group P21/c, but space group of 4 is
Pbca. The structure of 1 has been documented.[27] All of them
n–
consist of 1-D (Cu3I4)n chains and template cations PPh3Me+/
n–
PPh3Et+/PPh3nBu+. The (Cu3I4)n chains in 1, 2 and 4 are iso-
structures, here we focus our attention on 1 to depict the crys-
tal structure. Their asymmetric units contain three crystallo-
graphically distinct Cu+ ions, four I– ions and one cation. The
Cu centers are all coordinated by four I– ions to give distorted
tetrahedral geometries. In 1, Cu(1) is coordinated by two μ3-I
[I(2), I(4)] and two μ4-I [I(1), I(1)#1, #1 x, –y + 1/2,z-1/2], and
Cu(2), Cu(3) atoms are surrounded by one μ2-I [I(2)], two μ3-I
[I(2), I(4)] and one μ4-I [I(1)] (Figure 1a). The bond lengths of
Cu–μ2-I and Cu–μ3-I range among 2.6382(11)–2.6691(11) Å and
2.6232(11)–2.7335(12) Å, respectively, and the Cu–μ4-I distances
are in the range of 2.6901(11)–2.7201(11) Å (Table S1 in the
ESI). The Cu–I distances follows the following approximate
order: d(Cu–μ4-I) > d(Cu–μ3-I) > d(Cu–μ2-I). This trend can also
be observed in other iodocuprate hybrids.[36,37] Cu(1)I4,
Cu(2)I4, Cu(3)I4, Cu(2)#1I4 and Cu(3)#2I4 (#1 x, –y + 1/2,z-1/2;
#2 x, –y + 1/2,z + 1/2) tetrahedra are self-condensed by edge-
sharing to generate a Cu5I11 unit (Figure 1a). Neighboring Cu5I11
units are connected into a 1-D (Cu3I4)nn– chain via edge-sharing
model along the c-axis (Figure 1b). Cu···Cu distances of
2.8517(15)–2.9899(15) Å are comparable with the sum of van
der Waals radii of 2.80 Å, indicating the presence of weak
metal–metal interactions (Figure 1a). Some other similar 1-D
(Cu3I4)– chains have been reported, such as [Ph4P][Cu3I4], but its
chain is based on face-sharing CuI4 tetrahedra.[25] The adjacent
Figure 1. (a) Structure of Cu5I11 unit showing Cu–Cu interactions; (b) 1-D
(Cu3I4)n chain based on edge-sharing CuI4 tetrahedra; (c) packing diagram
showing the position of SDAs; (d) 1-D cationic chain based on π···π stacking
and C–H···π interactions in 1.
n–
n–
1-D (Cu3I4)n chains feature parallel packing along the c-axes,
and PPh3Me+ cations locate among the space of 1-D anionic
chains (Figure 1c). No hydrogen bonds can be observed be-
tween organic and inorganic moieties, and only electrostatic
From methyl substitution (PPh3Me+ in 1) to n-butyl substitu-
force contributes to the structural stabilization. In addition, tion (PPh3nBu+ in 4), only structural perturbation happens on
π–π stacking interactions with centroidal distance of 3.588(5) Å (Cu3I4)nn– chains. The Cu···Cu distances elongate to some extent
between adjacent phenyl rings can be observed (Table S3). [2.8356(17)–3.0591(16) Å for 2, 2.8740(15)–3.0774(7) Å for 4],
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and π–π stacking interactions were also weakened with longer Water Stability Studies
centroidal distance of 3.667(5) Å in 2. When the substituent is
Up to now, one of the critical drawbacks of photoluminescence
(PL) materials is their generally low water stability, which will
deter their real applications.[35] In this work, the alkyl modifica-
tion on phosphine ligands has been conducted, which might
improve their water stability. In the water stability study, the as-
synthesized crystalline samples of 1, 3 and 4 were ground as
fine powders and soaked in deionized water at room tempera-
n-butyl in 4, no π–π stacking interactions can be found. The
weakening of Cu···Cu and π–π stacking interactions in 1, 2 and
4 might be led by larger counter cations. This trend again sup-
ports the conclusion that small counterions such as tetrameth-
ylammonium tend to produce infinite [CuxIy](y–x)– chains, but for
larger counteractions isolated clusters will generate.[21]
The structural features of 1, 2 and 4 are the presence of 1-D
n–
(Cu3I4)n inorganic anionic chains with different Cu···Cu and
π–π stacking interactions. Generally, in the iodocuprate system,
tetrahedral (CuI4) and triangular (CuI3) may generate iodocu-
prate anions with the same simplest formula but exhibiting dif-
ferent real structures. For example, iodocuprate anions with the
simplest formula of (Cu2I3)–, including (Cu2I3)–, (Cu4I6)2–,
(Cu10I15)5– can exhibit 0-D, 1-D or 2-D structures. For 1-D (Cu3I4)–
structures, several isomers including [(N(C3H7))4(Cu3I4)]n,[21]
[(etpy)(Cu3I4)]n,[37]
[(N-Bz-Py)2(Cu6I8)]n,[38]
[(EtS4C5H4NEt)-
(Cu3I4)]n,[39] [(C15H14N)2(CuI2)(Cu3I4)]n,[40] [(C3H8NO)(C3H7NO)-
(Cu3I4)]n,[41] [(C24H20P)(Cu3I4)]n
and [(C24H32KO8)(Cu3I4)]n
[42]
[43]
have been reported, which were constructed by different con-
nections of tetrahedral (CuI4) and triangular (CuI3) units. For in-
–
stance, Cu3I4 in [(N-Bz-Py)2(Cu6I8)]n are 3-D network, compara-
bly, [(EtS4C5H4NEt)(Cu3I4)]n shows an infinite 1D chain con-
structed by incomplete cubane-like units, and that in
[(C3H8NO)(C3H7NO)(Cu3I4)]n is ladder-like motif.
Structure of (PPh3iPr)2(Cu2I4) (3): 3 has been previously
structurally determined, whose planar (Cu2I4)2– dimer is con-
structed from two edge-sharing CuI3 triangles.[44] The Cu–I
lengths and I–Cu–I angles are consistent with those found in
other iodocuprates,[45] and the Cu···Cu distance of 2.9380(11) Å
is somewhat longer than reported iodocuprates.[46] Specially,
there are three kinds of C–H···π interactions between adjacent
PPh3iPr+ cations (Table S4). In detail, in each PPh3iPr+ cation,
two phenyl groups involve in C–H···π interactions, upon which
a 2-D organic layer is generated. (Cu2I4)2– dimers fill in the resid-
ual interlayers (Figure 2). But no obvious I···I interactions are
found between the dimmers.
Figure 2. Packing diagram of 3 showing C–H···π interactions.
Figure 3. The PXRD patterns of 1 (a), 3 (b) and 4 (c) under differentconditions.
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ture. Afterwards, the samples were centrifuged and dried at fer (MLCT) transitions and halogen-to-ligand charge transfer
40 °C at different time intervals. The PXRD experiments were (XLCT) can be ruled out. Therefore, the corresponding elec-
conducted on the collected samples to check their phases.
The phase purities of bulk compounds 1–4 have been veri-
fied by powder X-ray diffraction (PXRD). It's obvious that the
peaks the experimental patterns are consistent with the corre-
sponding simulated ones, suggesting their good phase purities
(Figure 3 and Figure S5). Furthermore, the water stability meas-
urements (Figure 3) imply that the crystal structures of 1, 3 and
4 can maintain after soaking in water for 1 day. The relatively
good water stabilities of these compounds can be assigned to
the alkylation of PPh3 of counteractions, in which both the PPh3
and the alkyl groups are typical hydrophobic species. Specially,
the absence hydrogen-bond donors on quaternary phosphorus
salts can rule out the formation of typical strong hydrogen
bonds with water in an aqueous system or in moist air, which
further improve their water stabilities. However, when extend-
ing the soaking time to 5 days, the PXRD patterns of 1 and 3
changed obviously. Compared with the standard pattern of the
inorganic structure of CuI (cubic phase, with the space group
F-43m, and lattice constants a = b = c = 6.04 Å, α = ꢀ = γ =
90°, JCPDS No. 77–2391), the samples soaked for 5 days should
be the mixtures of bulk crystals and CuI. Their structural col-
lapse might be illustrated as the hydrolysis reactions
(Scheme 1).[13] But 4 is still stable after soaking in 5 days judg-
ing from their PXRD. From the above observation, it can be
concluded that hybrids 1, 3 have relatively strong water stabili-
ties, but suffer from hydrolysis reactions after excessive soaking
in water. The higher water stability of 4 can be ascribed to
the better hydrophobicity of n-butyl group, which deters the
dispersing of organic and inorganic moieties and as a result,
inhibit hydrolysis reaction. In all, introduction of alkyl groups
on aromatic molecules is an effective strategy to construct
functional hybrid iodocuprates containing positively charged
SDAs without introducing strong hydrogen bonding groups.
And in the quaternary phosphorus/iodocuprate system, the
longer alkyl is, the better water stability can be obtained.
tronic transitions of 1–4 can definitely assigned to the ligand
centred n–π*/π–π* transitions of phosphine ligands.[47] At
longer wavelengths over 350 nm, relatively weaker absorption
bands are observed, which are not observed for the free li-
gands. These adsorptions are relevant to ligand-to-ligand
charge transfer (LLCT) and charge transfer between inorganic
iodocuprates and organic moieties.[48] The former transitions
can be validated by their obvious π–π and C–H···π interactions
in their lattices,[13] and the latter charge transfers have already
been reported.[48]
Scheme 1. Possible hydrolysis reactions after excessive soaking in water.
Optical Diffuse-Reflection Spectra
Solid-state optical diffuse-reflection spectra of 1–4 and their
corresponding counter-cations were recorded from powder
samples at room temperature (Figure 4a). Organic quaternary
phosphorus salts and their hybrids 1–4 exhibit intense adsorp-
Figure 4. (a) Solid state electronic spectra of 1–4 and their organic counterca-
tions; (b) optical diffuse reflectance spectra for 1–4.
Solid-state UV/Vis diffuse spectra of 1–4 calculated from the
tion in ultraviolet zone (260–450 nm). The adsorption peaks of diffuse reflectance data by using the Kubelka–Munk function is
organic quaternary phosphorus salts appear at about 330 nm, plotted in Figure 4b. The band gaps can be estimated as 2.88,
corresponding to the n–π*/π–π* transitions of phenyl groups 2.86, 3.22 and 2.92 eV for 1–4 respectively, suggesting that all
in organic cations. After hybridization with iodocuprates, for 3, compounds are semi-conductors with broad gaps. Compared
their adsorption peak didn't change, but for 1, 2 and 4, the with the measured value of 2.95 eV for bulk CuI,[33] slightly red
peaks shift to longer waves at about 370 nm. And for all com- shifts for 1, 2 and 4 can be observed, and a 0.27 eV blue-
pounds, the absorption bands extend to different extent com- shift occurs in 3. The shifts reveal obvious intermolecular charge
pared with those of free quaternary phosphorus salts. Because transfer (CT) effect in 1, 2 and 4, which are probably led by the
there are no metal-ligand bonds, metal-to-ligand charge trans- strong donating ability of iodocuprate frameworks, rich
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π–π interactions, and consequent packing modes.[49] Notice- room temperature luminescence of organic cation (Ph3PnBu·I
ably, 1, 2 and 4 have obviously smaller band gaps than that of was set as example) was measured (Figure S6), and the free
3, which is in accord with its 0-D cluster structure.
organic quaternary phosphorus salt exhibits a strong emission
band with a maximum peak around 485 nm under excitation
at 340 nm. Compound 1 generates green emission in solid state
at room temperature, emitting a single emission band with the
maximum peak at about 512 nm (λex = 302 nm). With the tem-
perature cooled from 297 to 77 K, the intensity of this band
increases gradually (Figure 6a). Specially, when temperature was
at 197 and 177 K, a new emission at 605 nm appears, but it
disappears upon further cooling. And when the sample is grad-
ually warmed up to room temperature, the green emission is
back to the initial intensity, illustrating typical reversible lumi-
nescent thermochromic behavior. 2 exhibits generally the same
reversible luminescent thermochromic behavior with that of 1,
Photophysical Properties
The colors of compound 1–4 under UV, liquid nitrogen are
shown in Figure 5. Temperature-dependent luminescences of
1–4 were measured to investigate their response to tempera-
ture (Figure 6, Figure 7, Figure 8 and Figure 9). In addition, the
except that the emission is yellow with peak at 592 nm (λex
=
373 nm, Figure 7). 3 and 4 illustrate different temperature-
dependent luminescences (Figure 8 and Figure 9). At room tem-
perature, 3 gives a fresh green emission with maximum peak
Figure 5. Pictures of 1–4(from left to right) under different conditions.
Figure 6. (a) Emission spectra at varied temperatures (λex = 302 nm) and
(b) CIE-1931 chromaticity diagram of 1.
Figure 7. (a) Emission spectra at varied temperatures (λex = 373 nm) and
(b) CIE-1931 chromaticity diagram of 2.
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at about 558 nm [low-energy emission (LE)] at room tempera-
ture (λex = 360 nm). When the temperature is cooled to 237 K,
a new blue emission band appears at a higher energy around
447 nm [high-energy emission (HE)]. As further cooling occurs
to 77 K, the intensities of these bands increase gradually, but
the blue emission is dominated. For 4, blue and yellow emission
(peak at 437, 586 nm, λex = 370 nm) at room temperature can
be observed. When cooled to 177 K, the high-energy blue emis-
sion (HE) is greatly intensified and shifts to around 477 nm. As
further cooled to 77 K, the intensity of this band increases grad-
ually with the concomitant decreasing of the LE band and a
slight blue shift to 460 nm. 3 and 4 also exhibit typical reversi-
ble luminescent thermochromic behaviors. Their PL details at
room temperature including quantum yields are summarized in
Table 1. We can conclude that their colors at low temperature
are generally consistent with those in Figure 5. The CIE-1931
emission profiles also show the luminescent color changes of
five compounds upon cooling.
Figure 9. (a) Emission spectra at varied temperatures (λex = 370 nm) and
(b) CIE-1931 chromaticity diagram of 4.
Table 1. PL data of compound 1–4 at room temperature.
Compound
λex [nm]
λem [nm]
Φ [%]
1
2
3
4
302
373
360
370
512
602
558
437/586
6.55
6.29
6.22
6.19
based coordinated polymers,[50,51] but much less attention was
paid to the iodocuprate/SDA system.[14] In the former case, two
emission bands (LE and HE) can be seen in the temperature-
dependent luminescences, in which the LE bands are attributed
to the iodocuprate cluster-centered CT (ICCT) excited state and
the HE bands to an iodide to phosphine ligand charge transfer
(XLCT).[50,52] In the iodocuprate cluster-centered (ICCT) process,
Cu–Cu distances were changed as the temperature cooling,
leading to geometrical variations of iodocuprate clusters.[53] In
the iodocuprate/SDA system, the strong and broad single-band
Figure 8. (a) Emission spectra at varied temperatures (λex = 3602 nm) and
(b) CIE-1931 chromaticity diagram of 3.
About the photophysical properties of iodocuprate(I)-based emissions can also be assigned to iodocuprate moieties without
hybrids, great efforts have been devoted to the iodocuprate- the presence of organic countercations.[14] Based on these liter-
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atures and the emission of free counteraction (Ph3PnBu·I, Figure electrons to quaternary phosphorus moieties. Secondly, the
S6), the LE emissions of 1–4 stem from the iodocuprate moie- Ph3PR+· radicals transfer their electrons to the ITO electrodes
(y–x)–
ties, and the HE emissions of 3 and 4 are the contribution of with re-production of Ph3PR+ acceptor. The CuxIy
anions
ligand-centered transitions (LLCT). Iodide to phosphine ligand lose one electron to generate the CuxIy(y–x)–· radicals to produce
charge transfer (XLCT) can be ruled out due to longer I···P dis- the electron conductive pair of CuxIy(y–x)·–Ph3PR·. This process
tances (longer than 5.1 Å). In addition, the temperature stimulus can also be verified by components of top of VBs (Cu-3d,
can lead to the configuration change of iodocuprate moieties, I-5p) and the bottom of CBs (p-π* anti-bonding orbital of the
and furthermore, affect the frontier molecular orbitals.[53] Ac- quaternary phosphorus) in PDOS (Figure 11 and Figure S8).
cording to structural analysis, the Cu–Cu distances are elon- According to this mechanism, the more electronic rich in
gated to some extent when substituent lengths increase. Gen- CuxIy(y–x)– donors can facilitate the electronic transfer. Therefore,
–
erally, the more relaxed iodocuprates will be more sensitive to (Cu2I4)2– of 3 is more electronic rich than (Cu3I4) chain in 1
thermal stimulus. Therefore, from methyl to n-butyl substitu- and 4, which gives rise to a relative higher current intensity
ents, red-shift can be observed for 1, 2 and 4. It's interesting in 3.
that no HE emissions can be found in 1 and 2, we can explain
that the stacking modes of organic counteractions with strong
π–π stacking interactions quench these emissions. Besides, for
1, the appearance of new emission band of 605 nm at 197 and
177 K might be attributed to promoted structural torsion at
these temperatures and increased localization of the excited
state.[54] For 3, no HE emissions can be detected at room tem-
perature, but such blue emission appears in 4. This might be
led by stronger C–H···π interaction in 4, which enhance the
ligand-to-ligand transition. Judging from this trend, we can
tune the luminescences of this kind of compounds by introduc-
ing different substituents on Ph3P, which has never been stud-
ied in iodocuprate(I)-based hybrids.
Photocurrent Response Properties
Figure 10. Photocurrent response behaviors in this work.
Photocurrent generation has been observed in metal halide
based hybrids.[55,56] Photoelectrochemical experiments have
also been performed on 1, 3 and 4 according to typical
method. The SEM diagrams of films coated with 1, 3 and 4 are
shown in Figure S7. In detail, three electrode photo/electro-
chemical cell system was adopted (sample modified ITO elec-
trode, a Pt wire as auxiliary electrode and a Ag/AgCl electrode
as reference electrode). The photocurrent response behaviors
in Na2SO4 aqueous solution under the illumination from a
150 W Xe arc lamp with on–off cycles are shown in Figure 10.
The results show that under the repetitive irradiation (the on-
off interval of 10 s), repeatable and steady photocurrents with
rapid responses can be detected. Specially, the photocurrent of
1 decreases to some extent firstly but stabilizes after five cycles.
On the contrary, 4 increases gradually and stabilizes after four
cycles. The photocurrents of 1, 3 and 4 all stabilize at about
4.40 × 10–7, 4.55 × 10–7 and 4.75 × 10–7 A, respectively The cur-
rent intensities of this work are weaker than the other viologen-
containing compounds, such as (MV)2[Li4(L)2(H2O)6] and
MV[Ni(4-pedt)2]2,[57] but stronger than metalloviologen-contain-
ing hybrids.[55] It has been proved that that the pure organic-
coated sole system shows no photocurrent response,[58] there-
fore, in this work, iodocuprate–quaternary phosphorus donor–
Figure 11. Thermogravimetric analysis (TGA) traces for 1–4.
Thermoanalytical Properties
acceptor systems are the reason for photocurrent generation. To examine the thermal stabilities of 1–4, TGA were carried out
Judging from this fact, the photocurrent generation mechanism in the temperature range of 30–1000 °C with a rate of
can be proposed: upon irradiation, the photosensitive quater- 10 °C min–1 under Ar2 gas, the results of which are presented
nary phosphorus cations are excited to generate the Ph3PR+· in Figure 11. 1–4 exhibit similar weight loss trends, in which
(y–x)–
radicals, at the same time, CuxIy
donors can also provide two steps weight loss can be observed. The first occurred at
4240
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280–500 °C (for 1: weight loss 41.81 %, theoretical value the charge transitions between organic SDAs accompanied by
41.43 %; for 2: weight loss 42.62 %, theoretical value 42.473 %; charge transfer between inorganic-organic species. In addition,
for 4: weight loss 44.09 %, theoretical value 43.56 %), which are the components of CBs and VBs suggest the multiple energy
due to the loss of organic PPh3R·I quaternary phosphorus salts. transfer processes in their PL behavior, i.e., iodocuprate cluster-
But 3 is different from others, which loses its (PPh3iPr)+ cation centered CT (ICCT) and ligand-to-ligand charge transfer (LLCT).
in this stage (experimental 49.04 vs. theoretical 53.22 %). In the These calculated results are in agreement with the PL assign-
second stage occurring among 640–930 °C, the iodocuprates ments of 1–4. Besides, these electronic structures support the
collapse with sublimed I2 dissociated from skeletons. In all, all photocurrent generation mechanism.
compounds exhibit good thermal stabilities with decomposi-
tion temperature ranging among 280–350 °C. Comparably, ther-
mal stabilities of 1, 2, 4 are higher than that of 3, which is
consistent with their structural features.
Conclusions
In summary, iodocuprates(I) hybrids directed by alkylated (tri-
phenyl)phosphonium have been prepared. With the lengthen-
ing of alkyl groups on P-atom, the weakened Cu···Cu and π–π
stacking interactions were observed, which are relative to their
water stabilities and luminescent thermochromic behaviors. Be-
Electronic Structure and Photophysical Property
Mechanisms
To look further insight into the optical properties of 1–4, theo-
retical studies including the band structure along with high
symmetry points of the first Brillouin zone, and density of states
(DOS) were performed using the CASTEP code in Materials Stu-
dio 8.0. The X-ray crystallographic data were used to construct
the calculated modes without optimization. The bands can be
assigned according to total and partial densities of states
(PDOS, Figure 12 and Figure S8). As shown by the total and
partial DOS diagrams, they possess similar electronic structures.
In detail, the top of the VBs between –5 and 0 eV are mainly
from the contributions of the π bonding orbitals of organic
cation, I-5p state and Cu-3d state, but I-5p/Cu-3d are dominated
at around the Fermi level. The bottom of the CBs between 0
and 5 eV are mainly from the contribution of the p-π* anti-
bonding orbital of the quaternary phosphorus SDAs, and
Cu-4p, I-5s/5p also gives small amount of contribution. In the
band gap engineering of organic/metal halide system, it is uni-
versally accepted that the band gap adjustments of inorganic
metal halides can be realized by the insertion of molecular or-
bitals of organic cations into the empty band of parent metal
halides.[59,60] This trend can also be seen in this work. Accord-
ingly, the intrinsic absorption of 1–4 can be mainly ascribed to
sides, effective and repeatable photocurrent responses can be
detected. In all, employing alkylated (triphenyl)phosphonium as
SDAs to prepare hybrid iodocuprates is an effective approach
to obtain strong PL materials with enhanced water stability. This
strategy will be beneficial for the rational design and synthesis
of iodocuprate-based PL materials with higher performance.
The work about introduction of other functional groups on sub-
stituted alkyls as SDAs is still ongoing.
Experimental Section
Materials and Instruments: All chemicals except quaternary phos-
phorus salts were commercial products and used without further
purification. IR spectra were recorded on a Perkin–Elmer Spectrum-
2000 FTIR spectrophotometer (4000–400 cm–1) on powdered sam-
ple spread on KBr plate. Elemental analyses for C, H and N were
performed on a Vario MICRO elemental analyzer. Optical diffuse
reflectance spectra were measured on a Perkin–Elmer lambda 900
UV/Vis spectrophotometer equipped with an integrating sphere at
293 K, and BaSO4 plates were used as reference. Powder XRD pat-
terns were obtained using a Philips X'Pert-MPD diffractometer with
Cu-Kα radiation (λ = 1.54056 Å). Fluorescence spectra were carried
out on an Edinburgh FL-FS 920 TCSPC spectrometer. The lumines-
cence quantum yields were recorded on a Hamamatsu Photonics
C11347–11 absolute photoluminescence quantum yield spectrome-
1
ter. H NMR spectra were recorded on a Bruker Avance III 400 MHz
NMR spectrometer. The morphologies of electrodes with films of 1,
3 and 4 were observed by using a HIROX SH-4000 energy spectrom-
eter. The photocurrent experiments were performed on a CHI650
electrochemistry workstation with three-electrode systems. TGA for
1–4 were carried out under argon atmosphere in the range of 30–
1000 °C with a scan rate of 10 °C min–1 using a NETZSCH STA449F5
Jupiter thermogravimetric analyzer.
Single Crystal X-ray Diffraction Analyses: The intensity data of
1–4 were collected on a Bruker APEX II diffractometer using graph-
ite-monochromated MoKα radiation (λ = 0.71073 Å) at room tem-
perature, during which the Lp factor corrections and multi-scan ab-
sorption corrections were applied. The structure was solved by di-
rect methods with SHELX-97 program and refined by full-matrix
least-squares techniques on F2 with SHELXL-97 program.[61,62] For
all non-hydrogen atoms, anisotropical refinements were carried out.
All hydrogen atoms attached to carbon atoms were geometrically
placed. Crystal parameters of four compounds were summarized in
Figure 12. Total and partial density of states of 1.
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Table 2. Summary of the crystal data and structure determination for 1–4.
Compound
1
2
3
4
Empirical formula
Formula mass
Crystal system
Space group
a [Å]
b [Å]
c [Å]
ꢀ [°]
V [Å]
C19H18Cu3I4P
975.55
Monoclinic
P21/c
12.6129(16)
25.241(3)
8.0055(10)
107.327(2)
2433.0(5)
4
C20H20Cu3I4P
989.58
Monoclinic
P21/c
12.8246(11)
25.251(2)
8.0659(7)
106.683(2)
2502.1(4)
4
C21H22CuI2P
622.71
Monoclinic
P21/n
11.5661(4)
12.2769(4)
15.2877(6)
94.788(4)
2163.21(13)
4
C22H24Cu3I4P
1017.63
Orthorhombic
Pbca
8.0261(5)
25.1831(16)
27.2628(17)
90.00
5510.4(6)
8
2.453
6.843
3760
Z
Dc [g/cm3]
2.663
7.743
1784
2.627
7.532
1816
1.912
3.940
1192
μ[mm–1
F(000)
]
Reflections, total
8055
8020
11314
35863
Reflections, unique
Reflections, observed
Goodness-of-fit on F2
No. of parameters refined
R[I > 2σ(I)]
4253(Rint = 0.0234)
3516
1.081
245
0.0363
4299(Rint = 0.0272)
3269
1.020
333
0.0391
3591(Rint = 0.0157)
3364
1.367
292
0.0196
4855(Rint = 0.0522)
3640
1.087
271
0.0351
Rw [I > 2σ(I)]
0.0785
0.883, –1.295
0.0765
0.900, –1.344
0.0749
0.653, –1.172
0.0913
1.328, –1.710
Residual extremes [e/Å3]
Table 2. Selected bond lengths and angles are listed in Table S1–
S2, π–π stacking interactions and C–H···π interactions are given in
Table S3 and S4.
Synthesis
Synthesis of Quaternary Phosphorus Salts: Quaternary phos-
phorus salts were prepared by one step alkylated reaction of tri-
phenylphosphine with iodomethane, iodoethane, isopropane iod-
ide, n-butyl iodide in the toluene solvent according to literature
CCDC 1818467 (for 1), 1590153 (for 2), 1815248 (for 3), and 1812547
(for 4) contain the supplementary crystallographic data for this pa-
per. These data can be obtained free of charge from The Cambridge
Crystallographic Data Centre.
method.[65,66] The detail synthesis process, H NMR spectra of four
1
quaternary phosphorus salts can be seen in Figure S2 (electronic
supplementary material).
Electrode Preparation and Photocurrent Measurement: Typical
solution coating method is utilized to prepare the photocurrent
measurement electrodes of compounds 1, 3 and 4.[63] 5 mg as-
synthesized powder was dissolved in 0.3 mL of DMF, and the sus-
pension was dispersed evenly to obtain a slurry. The slurry was
spread onto pre-cleaned ITO glass (0.6 × 0.6 cm, 14 Ω per cm2),
whose side part was previously protected using scotch tape. The
working electrode was dried overnight under ambient conditions.
A copper wire was connected to the side part of the working elec-
trode using conductive tape. Uncoated parts of the electrode were
isolated with epoxy resin. A 150 W high-pressure xenon lamp, lo-
cated 15 cm away from the surface of the ITO electrode, was em-
ployed as a full-wavelength light source. The photocurrent experi-
ments were performed on a CHI660 electrochemistry workstation
in a three-electrode system, with the sample-coated ITO glass as
the working electrode mounted on the window with an area of
0.25 cm2, a Pt wire as auxiliary electrode, and a Ag/AgCl electrode
as reference electrode. The supporting electrolyte solution was a
0.2 mol L–1 sodium sulfate aqueous solution. The applied potential
was 0.5 V for all measurements. The lamp was kept on continuously,
and a manual shutter was used to block exposure of the sample to
the light. The sample was typically irradiated at intervals of 10 s.
Synthesis of [(PPh3Me)(Cu3I4)]n (1): Compound 1 was prepared by
solution evaporation method. PPh3Me·I (0.0808 g, 0.2 mmol) and
CuI (0.0190 g, 0.1 mmol) were dissolved in 45 mL of ethanol, and
kept stirring at 45 °C for 2 h. The obtained solution was filtered,
and red filtrate liquor was covered with a cling film at room temper-
ature for slow evaporation. Colorless transparent lumpy crystals
were obtained after the growth period of 3 d. Yield: 76.3 %
(0.0248 g, based on Cu). C19H18Cu3I4P (975.55): calcd. C 23.39, H
1.86; found C 23.75, H 1.72. IR: ν = 3070 (w), 1579 (w), 1479 (s),
˜
1422 (s), 1316 (w), 1181 (m), 1160 (m), 1096 (s), 1075 (m), 1003 (w),
748 (s), 691 (s), 613 (w), 485 (s) cm–1
.
Synthesis of [(PPh3Et)(Cu3I4)]n (2): The synthesis process of 2 is
similar to that of 1, expect PPh3Et·I (0.0836 g, 0.2 mmol) was used as
starting material. Colorless transparent block crystals were obtained
after 3 d. Yield: 63.2 % (0.0209 g, based on Cu). C20H20Cu3I4P
(989.58): calcd. C 23.39, H 1.86; found C 23.75, H 1.72. IR: ν = 3090
˜
(w), 2920 (m), 1584 (w), 1492 (s), 1487 (s), 1403 (m), 1235 (w), 1185
(m), 1190 (s), 998 (s), 769 (s), 726 (s), 495 (s), 454 (s) cm–1
.
Synthesis of (PPh3iPr)2(Cu2I4) (3): The synthesis process of 3 is
also similar to that of 1, expect PPh3iPr·I (0.0864 g, 0.2 mmol) and
CuI (0.0380 g, 0.2 mmol) were used as starting materials. Colorless
transparent block crystals with yield of 54.6 % (0.0680 g based on
Cu) were obtained. C21H22CuI2P (622.71): calcd. C 40.50, H 3.56;
Computational Details: The cif files of 1–3 were used to construct
the calculated models. The electronic structure calculations were
carried out using density functional theory (DFT) with the three
non-local gradient-corrected exchange–correlation functions (GGA-
PBE) and using CASTEP code,[64] which uses a plane wave basis set
for the valence electrons and norm-conserving pseudopotential for
the core electrons. The number of plane waves included in the basis
was determined by a cutoff energy Ec of 550 eV. The parameters
used in the calculations and convergence criteria were according to
the default values of the CASTEP program in Materials Studio 8.0.
found C 41.05, H 3.32. IR: ν = 3080 (w), 2932 (m), 1584 (w), 1480 (s),
˜
1482 (s), 1378 (m), 1220 (w), 1146 (m), 1198 (s), 872 (s), 724 (s), 492
(s), 409 (m) cm–1
.
Synthesis of [(PPh3nBu)(Cu3I4)]n (4): Compound 4 was synthe-
sized using the same condition with that of 1, in which PPh3nBu·I
(0.0651 g, 0.1 mmol) and CuI (0.0190 g, 0.1 mmol) were used as
starting materials. Colorless transparent lumpy crystals with yield of
Eur. J. Inorg. Chem. 2018, 4234–4244
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Keywords: Iodocuprate · Quaternary Phosphorus ·
Luminescence · Photocurrent Response
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