Multiresponsive UV-One-Photon Absorption, Near-Infrared-Two-Photon Absorption, and X/γ-Photoelectric Absorption Luminescence in One [Cu4I4] Compound

Article abstract of DOI:10.1021/acs.inorgchem.9b00876

A new Cu₄I₄-cluster-based compound is constructed to show multifaceted photoluminescent attributes: (1) ultraviolet (UV)-excited thermo-, mechano-, and rigido-chromic phosphorescence by the OPA (one-photon absorption) pathway, due to the interchanging emissions from cluster-centered (³CC) and halide-to-ligand charge-transfer (³XLCT) excited triplet states, (2) the ability to convert X/γ-ray and near-infrared (NIR) radiation to visible-light emission, in which the heavy Cu₄I₄ cores serve as the efficient X/γ-PEA (photoelectric absorption) or NIR-TPA (two-photon absorption) trapper and convertor to photons in the visible spectrum from the same emissive triplet states as those produced by UV excitation. This all-in-one compound affords a highly integrated nanolab for understanding and exploiting a wide range of photophysical phenomena simultaneously and is further fabricated into fiber-coupled long-range, in situ cryogenic thermometer and poly(methyl methacrylate) (PMMA)-embedded monolith gel, providing access to advanced applications in multifunctional optical materials and devices.

Full text of DOI:10.1021/acs.inorgchem.9b00876

Article
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Multiresponsive UV-One-Photon Absorption, Near-Infrared-Two-
Photon Absorption, and X/γ-Photoelectric Absorption Luminescence
in One [Cu4I4] Compound
Shao-Yun Yin,† Zheng Wang,† Zhi-Min Liu,‡ Hui-Juan Yu,† Jian-Hua Zhang,† Yong Wang,⊥
Rihua Mao,⊥ Mei Pan,*,† and Cheng-Yong Su†
†MOE Laboratory of Bioinorganic and Synthetic Chemistry, Lehn Institute of Functional Materials, School of Chemistry, Sun
Yat-Sen University, Guangzhou, Guangdong 510275, China
‡School of Chemistry and Chemical Engineering, Shanxi University, Taiyuan, Shanxi 030006, China
⊥Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 201800, PR China
S
* Supporting Information
ABSTRACT: A new Cu4I4-cluster-based compound is con-
structed to show multifaceted photoluminescent attributes: (1)
ultraviolet (UV)-excited thermo-, mechano-, and rigido-chromic
phosphorescence by the OPA (one-photon absorption) pathway,
due to the interchanging emissions from cluster-centered (3CC)
and halide-to-ligand charge-transfer (3XLCT) excited triplet states,
(2) the ability to convert X/γ-ray and near-infrared (NIR)
radiation to visible-light emission, in which the heavy Cu4I4 cores
serve as the efficient X/γ-PEA (photoelectric absorption) or NIR-
TPA (two-photon absorption) trapper and convertor to photons
in the visible spectrum from the same emissive triplet states as
those produced by UV excitation. This all-in-one compound
affords a highly integrated nanolab for understanding and
exploiting a wide range of photophysical phenomena simultaneously and is further fabricated into fiber-coupled long-range,
in situ cryogenic thermometer and poly(methyl methacrylate) (PMMA)-embedded monolith gel, providing access to advanced
applications in multifunctional optical materials and devices.
photodynamic therapy. This is highly preferable for industrial
flaw detection and biomedical treatment in living tissues to take
advantage of the deeper penetration ability of X- and γ-rays. For
this purpose, new modalities of X/γ-ray inducible materials are
demanding, for which transition metal cluster-based coordina-
tion compounds appear to be competent due to the combination
of high atomic numbers (Z) and good luminescence efficiency
within an integrated structure.30−33 To approach from another
end, TPA (two-photon absorption)-induced luminescence
undergoes a conversion process in which two photons from
low energy radiation are absorbed simultaneously to reach the
excited states and emit luminescence when transiting back to the
ground state. Near-infrared (NIR) photons can thus be
converted into emitting photons in the visible spectrum via
the TPA process and are therefore endowed with specific
advantages including tight focusing, deep penetration, low cell
damage, etc., which are also especially beneficial for biological
applications. However, although abundant TPA metal−organic
materials have been developed,34,35 the origination is basically
dependent on the TPA process involved with the excited states
INTRODUCTION
■
Photoluminescent metal−organic materials have been actively
pursued in the past decades due to their numerous applications
such as organic light-emitting diodes (OLEDs),1−3 sensors and
switches,4−6 anti-counterfeiting and barcodes,7−9 bioimages and
therapies,10−12 optical transductors and modulators,13−16 and so
on. Among which, tetranuclear Cu(I) compounds with
characteristic cubane-like {Cu4I4} core structures show
fascinating photoluminescent properties. Generally, emissions
from cluster-centered (3CC) and halide-to-ligand charge-
transfer (3XLCT) excited triplet states might be aroused by
ultraviolet (UV) excitation via one-photon absorption (OPA)
process. Meanwhile, the breadth of environmental sensitivity
can be found for these dual emissions by delicate alteration in
the coordination structure and related energy states, resulting in
solvo-, thermo-, mechanochromism, etc.17−29 All these afford
{Cu4I4} compounds potential applications in intelligent
materials and devices with multistimuli emission responsiveness.
Alternatively, radioluminescent materials, also called scintilla-
tors, can absorb higher energy radiation like X-ray or γ-ray by a
photoelectric absorption (PEA) process, can be converted into
visible light emissions, and are widely applicable in X/γ-ray
dosimetry, X/γ-ray-induced CT (computed tomography), and
Received: March 26, 2019
© XXXX American Chemical Society
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Figure 1. (a) Crystal structure of Cu4I4. (b) Proposed energy level diagrams of Cu4I4 (CC, cluster-centered; XLCT, halide-to-ligand charge transfer;
GS, ground state; OPA, one-photon absorption; TPA, two-photon absorption; PEA, photoelectric absorption; LE, low-energy emission; HE, high-
energy emission; Ea, activation energy). (c) UV-OPA induced thermochromism of Cu4I4 in the solid state (λex = 330 nm): temperature-dependent
emission spectra (upper left); corresponding CIE coordinates (upper right); PL photos taken at different temperatures (lower). (d) Illustration of a
fiber-coupled low temperature (77−200 K) PL thermometer from Cu4I4 (left, SMF, single mode fiber; OSA, optical spectrum analyzer); fitting curve
for the temperature-dependent intensity ratio of HE (445 nm) vs LE (535 nm) emissions measured by the thermometer.
Vario EL element analyzer. IR spectra were measured on a Nicolet FT-
IR-170SX instrument using KBr disks in the 4000−400 cm−1 regions.
Thermogravimetric analyses (TGA) were performed on a NETZSCH
TG209 system in nitrogen and under 1 atm of pressure at a heating rate
of 10 °C min−1. Crystal data were collected on a Super Nova X-ray
diffractometer system (Agilent Technologies, USA). PXRD data were
collected on an X-ray powder diffractometer (Rigaku, Japan). The
photoluminescence spectra and lifetime experiments were measured on
a FLS980 spectrophotometer with a xenon lamp and pulsed xenon lamp
as the light source, respectively. Absolute PL quantum yields (Φ) were
measured by the Quantaurus-QY (Hamamatsu, Japan) in the
integrating sphere. The X-ray-induced radioluminescence under
room temperature (RT) and 77 K were recorded by using the XEL
accessory in an Edinburg FLS 980 fluorescence spectrometer with a
tungsten X-ray tube as the X-ray source at Shanghai Institute of
Ceramics, Chinese Academy of Sciences. The absolute photoelectron
of organic ligands, while metal-cluster-based TPA has remained
rather unexplored so far. Considering the above aspects
together, novel photoluminescent materials to integrate UV-
OPA, NIR-TPA, and X/γ-PEA induced emission together and
simultaneously possess multistimuli PL responsiveness might be
designed from a Cu-cluster-based component, which will bring
new opportunities for both understanding and application of
advanced optical functional materials.
EXPERIMENTAL SECTION
■
Materials and Methods. All the experimental materials were
purchased commercially and used without further purification. The 1H
NMR spectra were recorded on a Bruker AVANCE III (400 MHz)
spectrometer, applying TMS as the internal standard and analyzed by
the MestReNova software. Elementary analysis was performed on a
B
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yield was obtained from the pulse height spectra measurements using
Am-241 as the radiation source of γ-rays. The scintillation light was first
detected by a PMT (Hamamatsu, R1306) and then recorded by a
multichannel analyzer (ORTEC-927). The test samples were wrapped
with a Tyvek high reflection film and then coupled to PMT using optical
grease (DC-200). NIR-TPA emission spectra were obtained using an
Astrella/OperA Solo femtosecond laser with the excitation from 680 to
960 nm, and the emissions were collected using an Ocean Optics
spectrophotometer (QE65 Pro).
Synthesis and Fabrication. 3-pmbtd Ligand. The ligand 3-
pmbtd (2,6-bis(pyridin-3-ylmethyl)-3a,4,4a,7a,8,8a-hexahydro-4,8-
ethenopyrrolo[3,4-f]isoindole-1,3,5,7(2H,6H)-tetraone) was synthe-
sized according to our earlier report (Scheme S1).36 1H NMR (400
MHz, CDCl3) δ = 8.52 (dd, J = 6.6, 1.8, 2H), 7.60 (dd, J = 6.0, 1.9, 1H),
7.21 (dd, J = 7.8, 4.8, 1H), 5.87 (dd, J = 4.3, 3.1, 1H), 4.55 (s, 2H), 3.75
(s, 1H), 2.99 (s, 2H).
Cu4I4 Crystal ([Cu4I4(3-pmbtd)2]·Solvents). A solution of CuI (9.5
mg, 0.05 mmol) in CH3CN (5 mL) was layered onto a solution of 3-
pmbtd (22 mg, 0.05 mmol) in CH2Cl2 (6 mL) in a test tube. The
solution was left to stand for 3 days at room temperature, and colorless
transparent big block crystals were obtained. Yield: 56%. Anal. Calcd for
C51H45Cl2Cu4I4N9O8: C, 35.11; H, 2.60; N, 7.23. Found: C, 35.48; H,
2.62; N, 7.06. 1H NMR (400 MHz, DMF) δ 8.84 (s, 1H), 8.41 (s, 1H),
7.62 (s, 1H), 6.97 (s, 1H), 5.83 (s, 0H), 4.73 (s, 2H), 3.50 (s, 3H), 3.44
(s, 1H). IR (KBr, cm−1): 3586 (m), 1771 (s), 1715 (s), 1694 (s), 1683
(m), 1434 (s), 1394 (m), 1344 (s), 1320 (m), 1175 (vs), 707 (s).
Cu4I4@PMMA Monolith Gel. 2.0 g of PMMA (poly(methyl
methacrylate)) was dissolved in chloroform (40 mL) in a 100 mL
beaker, and then, 8 mg of Cu4I4 was added. The suspension was stirred
for 2 days. This mixture was then loaded into a glass culture dish and
allowed to dry for 3 days; a hybrid monolith with a thickness of
approximately 2 mm was obtained.
Cu4I4-Based Optical Fiber Thermometer. A tiny amount (∼1 mg) of
Cu4I4 powder was inserted into a thin capillary (Φ = 300 μm, length = 2
cm), which was connected with single mode fibers (SMFs, 125 μm in
diameter, 1 m long each) from two sides. The capillary containing Cu4I4
powder was then placed into cryogenic equipment for low temperature
detection. A UV laser (355 nm) was applied from the input side after a
condenser lens, passing through the fiber to excite the Cu4I4 powder,
and the generated emission was collected from the output side of the
fiber linked with an OSA (optical spectrum analyzer, TeraSys 4000).
from 350 °C, showing the stability of the compound can be up to
350 °C (Figure S1).
UV-OPA Luminescence and Thermochromism. The 3-
pmbtd ligand shows absorption in 200−300 nm due to π−π*
transitions, and Cu4I4 shows absorption in 200−400 nm
attributed to π−π* and n−π* transitions of organic ligands
3
and the transition from ground state to the CC cluster-based
excited state (Figure S2). In the solid state, Cu4I4 manifests the
broad band excitation spectrum in the ultraviolet (UV) region
from 250 to 400 nm, due to the one-photon absorption (OPA)
process from the ground state to the Cu4 cluster-centered (CC)
excited states (Figure 1b,c).17 Upon irradiation of 330 nm UV
light at room temperature, Cu4I4 presents a broad band emission
with the maximum at 535 nm, showing bright green
luminescence. This low energy (LE) green emission has a
high absolute quantum yield (Φ = 58%) and long decay lifetime
(τ = 13.1 μs), which should be related with phosphorescence
from the cluster-centered (3CC) triplet excited states of the Cu4
center. When lowering the temperature from 300 to 175 K, the
intensity of the LE green emission is steadily increased, due to
the reduction of the thermal deactivation process. While further
lowering the temperature from 175 to 77 K, the LE green
emission is gradually weakened. This is simultaneously
accompanied by the appearance and further enhancement of
another high energy (HE) blue emission centered at 445 nm,
assignable to the halide to ligand charge-transfer (3XLCT)
triplet states. The alternative switching of the LE and HE
emission is characteristic of the equilibrium between two
thermally induced processes (Figure 1b).37 As a result, a naked-
eye detectable PL color change is clearly observed from room
temperature to liquid nitrogen temperature, shifting from green
to cyan, and then brilliant blue, which is also confirmed by the
calculated CIE coordinates (Figure 1c). Variant-temperature
crystal data were collected at 100, 150, 200, and 300 K,
respectively (Tables S1 and S2), which show normal reduction
of crystal lattice and Cu−Cu/Cu−I distances with a decrease in
temperature due to the thermal expansion and contraction
effect, but no tipping point was observed to suggest a direct
relationship with the switches in LE/HE emission states.
RESULTS AND DISCUSSION
■
To determine the energy barrier between these two emitting
states, an activation energy (Ea) of ∼1.5 kJ mol−1 was calculated
using the Arrhenius plot method (Figure S3).37 Compared with
the reported data, this rather low energy barrier indicates a
strong coupling between the two energy states in the Cu4I4
sample and affords a controlled thermochromism in a large
temperature range. Decay lifetime evolutions of the two
emissions also show temperature dependence (Table S3), albeit
with different degrees. It is interesting to find that the decay
lifetime of HE emission is increased almost 3-fold from 17.4 to
48.0 μs as the temperature is lowered from 200 to 77 K, while
only a slight increase from 14.5 to 21.3 μs is detected for the LE
emission. This shows that the HE blue emission assigned to the
3XLCT state is more sensitive to thermal deactivation, and in
such a case, the deactivation is also related to the energy transfer
Crystal Structure. The {Cu4I4}-cluster-based compound,
[Cu4I4(3-pmbtd)2]·solvents (abbreviated as Cu4I4), was
assembled from CuI salt and organic ligand 3-pmbtd ([2,6-
bis(pyridin-3-ylmethyl)-3a,4,4a,7a,8,8a-hexahydro-4,8-etheno
pyrrolo[3,4-f ]isoindole-1,3,5,7(2H,6H)-tetraone]) by a facile
layer method at room temperature. Cu4I4 has the typical
tetranuclear “cubane”-like core, which is chelated and embedded
by two almost orthogonal 3-pmbtd ligands with a dihedral angle
of ∼86° (each ligand curved to coordinate with two coppers by
the two terminal nitrogen atoms), forming a “distorted glasses”
configuration (Figure 1a). In the {Cu4I4} core, the four copper
atoms are arranged as a tetrahedron, which is encapsulated by a
larger I4 tetrahedron, with each I− on a triangular face of the Cu4
tetrahedron. The average Cu−Cu distance is 2.70 Å, which is
less than the sum of the van der Waals radii (2.80 Å), showing a
weak Cu···Cu interaction, and the average distance of Cu−I is
2.69 Å. Uniquely, the chelating encapsulation of two ligands
around the {Cu4I4} cluster might bring more intensified and
tunable luminescence in Cu4I4 compared with the commonly
monodentate ligand coordinated counterparts and prompts us
to explore it further. The TG curve of Cu4I4 shows a slight
weight loss in the range from 30 to 345 °C, which is attributed to
the evacuation of solvent molecules. Then, abrupt weight loss
related with the collapse of the coordination framework happens
3
to the LE CC state with green emission, whose population
becomes dominant at room temperature. Furthermore, on the
basis of the above temperature dependence of the dual emissions
of solid state Cu4I4, we fabricated a low temperature PL
thermometer using Cu4I4 as the thermal sensitive material to be
coupled with an optical fiber device (Figure 1d). Remote and in
situ detection of the cryogenic temperature from 77 to 200 K
was conducted, resulting in a good linear correlation between
the log10(I445/I535)−T plot. This ratio-type thermometer
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possesses long-term stability, as confirmed by PXRD tests of the
sample (Figure S4), as well as easy manipulation and self-
calibration accessibility, which affords practical applications with
performance comparable to some reported ratiometric ther-
mometers (Table S4).
Rigido- and Mechanochromism. When Cu4I4 is dissolved
in DMF solution at room temperature, the emission maximum is
red-shifted greatly to 660 nm (λex = 320 nm, Figure 2a), which is
125 nm longer than that in the solid state. This results in a red
luminescence color compared with the solid state green
emission. The red-shift tendency of Cu4I4 in the DMF solution
is different from most of the other metal−organic materials,
which usually have a longer-wavelength emission in the solid
state due to the aggregation effect. Such a phenomenon should
be correlated with the relaxed energy states in Cu4 clusters due
to the weakened rigidity effect when the molecule is scattered in
solution.19 Therefore, after the DMF solution is cooled down
and frozen under 150 K into a glass matrix state, the relaxed
energy states in Cu4 clusters will be rigidified again, switching
into green emission with a maximum at around 580 nm, falling
3
between the solid state and fluid state CC-originated LE
emissions. Such behavior has been named as a “rigidochromic
effect”, which can be attributed to the medium rigidity, leading
to distortion of the 3CC state relative to the ground state, and
observed before for the emission bands in some Cu(I),
rhenium(I), and Pt(II) complexes, etc.38−40 Here, the rigidified
energy states in Cu4I4 show a more complicated fluorescence
switch concerning the addition of HE emission under 150 K in
the DMF frozen glass matrix. As shown in Figure S5, in the DMF
solution state above 200 K, basically only the LE 3CC emission
appears in the red fluorescence region. While after being frozen
3
at 150 K, accompanying the blue-shift of the CC emission to
3
580 nm, the HE XLCT emission appears at around 490 nm,
longer than that in the solid state (445 nm). Furthermore,
lowering the temperature and increasing the excitation wave-
3
length will both result in steady transition from the LE CC
3
emission dominance to the HE XLCT emission dominance
(Figure S5d−f), showing a population transfer between the two
different excited states. In general, due to the rigido-effect
observed in the DMF solution and glass matrix state, the
emission color of Cu4I4 can be tuned in a much wider color range
(red−yellow−green−blue, Figure 2b) than in the solid
crystalline state (green−blue). The decay lifetime study
(Table S5) shows a fast decay rate in the DMF solution for
the red emission at room temperature (1.3 μs), but a longer
lifetime is observed for the rigidified 3CC and 3XLCT in the glass
matrix (31.3/53.1 μs for 570/490 emissions at 77 K, for
example) than in the solid crystalline state (21.3/48.0 μs for
535/445 emissions at 77 K, correspondingly), showing that the
energy state populations for the dual emissions in Cu4I4 can be
finely tuned by outer environments.
Mechanochromism was also observed in Cu4I4 when grinding
the sample at room temperature. As shown in Figure 2c, the LE
green emission of Cu4I4 will shift to a longer wavelength with
much broadened contour (400 to 750 nm) after grinding with a
laboratory mortar. As a result, the pristine green emission will be
switched into yellow emission. Figure S6 manifests the basic
coordination structure of the complex is preserved during the
process, and the mechanochromism phenomenon should be due
to the slight Cu−Cu distance changes in the {Cu4I4} cluster. As
revealed in the single crystal structure of Cu4I4, there exists Cu−
Cu interactions of direct bonding character. If the Cu−Cu
distances become shorter, the bonding character will increase
and the 3CC energy level will be lowered to result in emission
with a longer wavelength. Such is the case for the grinding
sample to show red-shifted and much broadened emissions,
suggesting shorter Cu−Cu distances and more structured
3
energy levels are generated for the CC excited states in the
crushed sample.41 Furthermore, after soaking in CH3CN
solvent, the crushed sample can recover the original green
emission, showing restoration of the Cu−Cu distances and 3CC
energy states. We also tried other solvents like EtOH, CH2Cl2,
and MeOH, and the restoration effect was not as good as MeCN
(Figure S7), which may be due to solubility, polarity, and so on.
Figure 2. (a) Rigidochromism of Cu4I4 in the DMF solution (200−300
K) and glass matrix state (77−150 K) by excitation of 320 nm UV light,
and (b) emission photos taken at different temperatures and excitation
wavelengths. (c) Mechanochromism of Cu4I4 before and after grinding
and recovered by soaking in CH3CN at room temperature (λex = 330
nm).
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A more detailed mechanism for this might deserve further
exploration.
X/γ-PEA Luminescence. Attributed to the heavy {Cu4I4}
cubane-like core and the high photoluminescence quantum
yield, Cu4I4 might exhibit radioluminescence (scintillating
luminescence) via a photoelectric absorption (PEA) process.
We first tested the single crystal sample of Cu4I4 with the size of
∼6 mm (Figure 3a) at room temperature. When excited by X-
ray (15 kV, 15 μA, 300 K), the block crystal of Cu4I4 emits bright
green luminescence (λmax = 524 nm), similar to the UV-excited
LE emission in the bulk powdered sample as described before. In
order to further detect and compare low temperature radio-
luminescence, we prepared a Cu4I4@PMMA (wt = 0.4%)
monolith gel sample. It shows green emission (λmax = 524 nm)
under X-ray (15 kV, 15 μA) at room temperature, identical with
the LE radioluminescence detected for the single crystal. When
excited by X-ray (50 kV, 50 μA) at 77 K, the radioluminescence
switches to a blue (λmax = 439 nm) color, as a counterpart of the
UV-excited HE emission (Figure 3b). The similar thermochro-
mic behavior confirms that the X-ray induced radiolumines-
cence in Cu4I4 should arise from the same emissive triplet states
as those generated by the UV excitation, and we postulate the
mechanism like this, upon X-ray radiation, photoelectric
absorption process happened for {Cu4I4} clusters to expel the
inner shell electrons. Relaxation of the ionized molecules is
accompanied by further ejection of the outer-shell electrons as
fast electrons to interact with the triplet 3CC and 3XLCT excited
states, leading to the observed X-ray fluorescence, production of
Auger electrons, and thermal dissipation simultaneously.30−33
The incorporation of Cu4I4 into a PMMA-based monolith gel
facilitates the material molding and device fabrication, and
preliminary tests proved that it is also responsive to γ-irradiation.
The detected γ-ray induced room temperature radiolumines-
cence has an absolute photoelectron yield of ∼7%. Considering
the moderate loading amount (0.4%) of the active Cu4I4
complex within the PMMA matrix, we anticipate the present
monolith gel can serve as an efficient model of both X- and γ-ray
scintillating material to combine the advantages of temperature
responsiveness and easy fabrication.
NIR-TPA Luminescence. We were motivated by the
observation of the strong luminescence behavior of Cu4I4;
thus, the crystalline powder samples were subjected to a 100 fs
laser at NIR wavelength from 740 to 960 nm at room
temperature. Typically, the power-dependent TPA emissions
excited by 750 nm NIR are shown in Figure 3c. As we can see,
the emission contour and position is coincident with the UV-
OPA excited LE green emission. The slope of the log(I)−log(P)
dependence of the emission intensity upon irradiation was
found to be ≈2 (Figure S8), which confirms the two-photon
absorption process in the material. The TPA threshold for the
sample was also obtained by determining the turning point on
the PL intensity−excitation power curves. It is found that the
sample has a low threshold energy of ∼74.3 μJ. TPA cross
sections of the sample at different NIR wavelengths were then
quantified by comparing the TPA emission intensities measured
in the solid state with that of Rhodamin B as a reference and
computed from the following equation,
Figure 3. X-ray-PEA emission of (a) the Cu4I4 single crystal (the inset
shows the crystal photo) and (b) PMMA-embedded monolith gel
(insets, left, photos for the monolith gel; right, CIE coordinates). (c, d)
NIR-TPA emission of the Cu4I4 solid powder. (c) Power-dependent
emission spectra at the excitation of 750 nm (inset, TPA emission
photo). (d) TPA cross sections from 740 to 960 nm.
IsampleNreference
ησ
= ησ
×
2,sample
2,reference
IreferenceN
emission intensities of the sample (Isample) are compared with
that of Rhodamin B (Ireference), and the concentration Nreference or
sample
where ησ2,sample and ησ2,reference represent the TPA action cross
section for the sample and reference, respectively. The 2PA
N
sample is calculated according to the molecular weight per unit
cell volume.
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ORCID
Within the laser excitation wavelength from 740 to 960 nm,
the measured ησ2 for the Cu4I4 sample is in the range of 1−30
GM with the maximum position at 780 nm (Figure 3d). Overall,
the measured ησ2 is small compared with the usual solution
samples, which might be due to reabsorption and scattering
effects of solid samples in the present test conditions.
Comparatively, we also tested the 3-pmbtd ligand, which
shows no TPA attributes within 680 to 960 nm at all. Further
considering the similarity between the UV-OPA and NIR-TPA
emissions of Cu4I4, we deduce that the present TPA
luminescence should occur via a two-photon absorption
pathway directly related with the {Cu4I4} cluster-centered
excited states. As far as we know, only one report of TPA
luminescence has been observed in the {Cu4I4} metal−organic
material before, in which the mechanism is based on the two-
photon absorption process of π-conjugated organic ligand, while
Mei Pan: 0000-0002-8979-7305
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by NSFC (21771197, 21720102007,
21821003, 11575276), Local Innovative and Research Teams
Project of Guangdong Pearl River Talents Program
(2017BT01C161), and FRF for the Central Universities.
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CONCLUSION
■
In summary, a {Cu4I4}-core-based metal−organic compound
has been successfully designed to possess multifaceted photo-
physical functions. First, it can be excited by a UV-light via one-
photon absorption (OPA) process, showing thermo-, rigido-,
and mechanochromism, due to the modifications and transitions
between the 3CC and 3XLCT dual energy states. Second, X/γ-
ray radioluminescence via photoelectric absorption (PEA) and
NIR-excited luminescence via two-photon absorption (TPA)
pathways related to identical energy states were also conducted.
Third, the metal−organic compound was further fabricated into
an optical fiber-coupled thermometer and PMMA-embedded
monolith gel, which prompts the facile applications in a remote,
in situ cryogenic temperature detector and novel type of X- and
γ-ray scintillator materials. Therefore, this study brings forward a
novel strategy to combine OPA, TPA, and PEA phenomena
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ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.9b00876.
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drug/cargo delivery and cancer therapy. Adv. Mater. 2017, 29, 1606134.
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2658−2669.
More spectroscopic graphs and data and single-crystal
data (PDF)
Accession Codes
CCDC 1889064 and 1907715−1907717 contain the supple-
mentary crystallographic data for this paper. These data can be
obtained free of charge via www.ccdc.cam.ac.uk/data_request/
cif, or by emailing data_request@ccdc.cam.ac.uk, or by
contacting The Cambridge Crystallographic Data Centre, 12
Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
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AUTHOR INFORMATION
■
Corresponding Author
*E-mail: panm@mail.sysu.edu.cn.
F
DOI: 10.1021/acs.inorgchem.9b00876
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