Cationic IrIII Emitters with Near-Infrared Emission Beyond 800 nm and Their Use in Light-Emitting Electrochemical Cells

Article abstract of DOI:10.1002/chem.201805902

Solid-state near-infrared (NIR) light-emitting devices have recently received considerable attention as NIR light sources that can penetrate deep into human tissue and are suitable for bioimaging and labeling. In addition, solid-state NIR light-emitting e

Full text of DOI:10.1002/chem.201805902

A Journal of
Accepted Article
Title: Cationic Ir(III) emitters with near-infrared emission beyond 800
nm and their use in light-emitting electrochemical cells
Authors: Guan-Yu Chen, Bo-Ren Chang, Ting-An Shih, Chien-Hsiang
Lin, Chieh-Liang Lo, Yan-Zhi Chen, You-Xuan Liu, Yu-Ru Li,
Jin-Ting Guo, Chin-Wei Lu, Zu-Po Yang, and Hai-Ching Su
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To be cited as: Chem. Eur. J. 10.1002/chem.201805902
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10.1002/chem.201805902
Chemistry - A European Journal
Cationic Ir(III) emitters with near-infrared emission beyond 800 nm and their use in light-
emitting electrochemical cells
Guan-Yu Chen,[a] Bo-Ren Chang,[b] Ting-An Shih,[a] Chien-Hsiang Lin,[a]Chieh-Liang Lo,[b] Yan-Zhi
[b]
Chen,
You-Xuan Liu,[b] Yu-Ru Li,[b] Jin-Ting Guo,[c] Chin-Wei Lu,*[a] Zu-Po Yang,[c] and Hai-
Ching Su*[b]
a Department of Applied Chemistry, Providence University, Taichung 43301, Taiwan
b Institute of Lighting and Energy Photonics, National Chiao Tung University, Tainan 71150, Taiwan
c Institute of Photonic System, National Chiao Tung University, Tainan 71150, Taiwan
To whom to correspond:
E-mail: cwlu@pu.edu.tw; Fax: 886-4-26327554; Tel: 886-4-26328001-15213
E-mail: haichingsu@mail.nctu.edu.tw; Fax: 886-6-3032535; Tel: 886-6-3032121-57792
Abstract
Near-infrared (NIR) solid-state light-emitting devices have recently received much attention as NIR
light sources, which can penetrate deep into human tissue and are suitable for bioimaging and labeling.
On the other hand, solid-state NIR light-emitting electrochemical cells (LECs) show several promising
advantages over NIR organic light-emitting devices (OLEDs). However, in the reported ionic
transition metal complex (iTMC)-based NIR LECs, there is currently no iridium-based LEC that can
display NIR electroluminescence (EL) peaks near or above 800 nm. In this study, we demonstrate a
simple method for adjusting the energy gap between the highest occupied molecular orbital (HOMO)
and the lowest unoccupied molecular orbital (LUMO) with iridium-based iTMCs to generate NIR
emission. We show a series of novel ionic-iridium complexes with very small energy gaps, namely
NIR1–NIR6, in which 2,3-diphenylbenzo[g]quinoxaline moieties mainly take charge of the HOMO
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energy levels, and 2,2'-biquinoline, 2-(quinolin-2-yl)quinazoline, and 2,2'-bibenzo[d]thiazole moieties
mainly control the LUMO energy levels. All complexes exhibited NIR phosphorescence, with
emission maxima up to 850 nm, and are applied to the LEC components, showing a maximum external
quantum efficiency (EQE) of 0.05% in EL devices. By using the host (RED)-guest emissive system,
the highest EQE of the LEC can be further enhanced to above 0.1%.
1. Introduction
Near-infrared (NIR) organic light-emitting devices (OLEDs) have recently received intense
attention for use as NIR light sources due to the advantages of low power consumption and
compatibility with large-area and flexible substrates.[1] However, sophisticated multilayer structures
and low-work-function cathodes are generally necessary for NIR OLEDs to achieve optimal device
efficiency. More complicated fabrication processes limit their competitiveness with other solid-state
NIR emitting technologies. On the other hand, solid-state NIR light-emitting electrochemical cells
(LECs) present several promising advantages over NIR OLEDs. They have simpler device architecture,
which is solution processable, and consequently they are easier and cheaper to fabricate. With in-situ
electrochemical doping,[2] carrier injection is insensitive to the electrode work function. It permits the
use of air stable metals and, thus, reduces encapsulation costs. Additionally, electrochemical doping
also decreases operation voltage and improves device carrier balance.
Some NIR LECs based on ionic transition metal complexes (iTMCs),[3] laser dyes,[4] and
polymers[5] have been reported. Among these reports, few studies have been able to demonstrate the
NIR electroluminescence (EL) peak approaching or beyond 800 nm,[3d, 4a, 5a] which penetrates deeper
into the tissues and, thus, is suitable for use in bio-imaging.[1] For NIR LECs based on the laser dye
(3,3′-diethylthiatricarbocyanine iodide (DTTCI)), laser dye is doped in the host material and NIR EL
is accompanied by significant residual host emission in the visible spectral region.[4a] NIR LECs
composed of polymers containing pendant ruthenium complex groups show rather low device
efficiencies (external quantum efficiency (EQE) < 10-5%).[5a] For NIR LECs based on iTMCs, only
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ruthenium complexes have been demonstrated to provide an NIR EL peak > 720 nm.[3d] To the best of
our knowledge, no LECs based on iridium complexes offer an NIR EL peak in this spectral region. To
explore the potential application in NIR light sources, NIR LECs based on several novel iridium
iTMCs are proposed to exhibit an EL peak at ~800 nm.
Iridium complexes with NIR emission are well suited for bio-imaging and labeling, but are
relatively rare for use in LECs. In iridium-based iTMCs, the highest occupied molecular orbital
(HOMO) is typically present in C^N ligands and metals, while the lowest unoccupied molecular orbital
(LUMO) is located in N^N ligands.[3j, 6] To successfully obtain iridium complexes with NIR
luminescence, useful synthetic strategies include: (i) introducing electron-donating substituents on
C^N
ligands,
such
as
2-phenylpyridine,
2-phenylbenzo[g]quinolone,
2-methyl-3-
phenylbenzo[g]quinoxaline, and 2,3-diphenylbenzo[g]quinoxaline, to make the HOMO less stable,
and/or introducing electron-withdrawing groups on N^N ligands, such as 2,2’-bithiazole, 2,2'-
bibenzo[d]thiazole, 2-(pyridin-2-yl)benzo[d]thiazole, 2,2'-biquinoline, 2-(quinolin-2-yl)quinazoline,
and 4,7-diphenyl-1,10-phenanthroline, to stabilize the LUMO; (ii) extending the π-conjugation system
in C^N or N^N ligand, thereby reducing the energy gap between HOMO and LUMO.[3j, 6b, 6c, 7]
In this study, we demonstrate a facile way for iTMCs to tune the energy gap between HOMO
and LUMO, generating NIR emission via ILCT (intra-ligand charge transfer) and MLCT (metal to
ligand charge transfer). We report a novel series of ionic iridium complexes with extremely small
energy gaps, namely NIR1–NIR6, in which 2,3-diphenylbenzo[g]quinoxaline moieties mainly control
the HOMO energy levels and 2,2'-biquinoline, 2-(quinolin-2-yl)quinazoline and 2,2'-
bibenzo[d]thiazole moieties mainly control the LUMO energy levels. All complexes showed NIR
phosphorescence, with an emission maximum ranging from 770–850 nm, and were applied to LEC
components, exhibiting a maximum EQE of 0.05% in the NIR EL devices. The highest EQE of the
LEC can be further enhanced to higher than 0.1% by employing a host (RED)-guest emissive layer.
2. Results and Discussion
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2.1 Synthesis and Structural Characterization
When LECs are fabricated using NIR materials alone, efficiency tends to be poor.[7d, 8] Therefore,
an iTMCs-based host-guest system was applied in this study. As host materials, phenylpyridine
derivatives decorating electron-donating groups were used as C^N ligands, and bibenzothiazole with
a deep LUMO was used as a N^N ligand to synthesize an ionic iridium complex with red emission.
The host materials are designed to effectively cover the energy gap of NIR materials for efficient
energy transfer. So, 2-(4-butylbenzene)-4,4,5,5-tetramethyl-dioxaborolane was first reacted with 2-
bromopyridine to give butylphenylpyridine (bppd). Then, bppd and IrCl3∙3H2O were refluxed in a
mixed solvent of 2-ethoxyethanol and water (v/v = 3/1) to obtain a yellow precipitated cyclometalated
[Ir(bppd)2Cl]2, which can be directly used without further purification. Afterwards, [Ir(bppd)2Cl]2
and bibenzothiazole were refluxed under methanol, and a saturated aqueous KPF6 was added after
cooling to produce complex RED. The molecular designs of the series of guest-ionic iridium
complexes were based on the addition of electron-donating groups to C^N ligands and the introduction
of electron-withdrawing groups on an N^N bidentate ligand, thereby generating a smaller energy gap.
In addition, extending the π-conjugated system was also utilized to shrink the gap, leading to NIR
emission.[3c] As shown in Scheme 1, the synthesis of 2,3-diphenylbenzo[g]quinoxaline derivatives
(mpqx or mopqx), were conducted by the reaction of commercially available 2,3-naphthalenediamine
and functional benzil in the presence of iodine as a catalyst for the condensation reaction. Then,
IrCl3·3H2O was refluxed with mpqx or mopqx in a mixed solvent of 2-ethoxyethanol and water (v/v
= 3/1) to obtain a dark red precipitated solid, which was washed with water and n-hexane to obtain
cyclometalated dimer without further purification. Subsequently, cyclometalated was refluxed with the
corresponding diamine ligands in methanol. After cooling, a saturated aqueous KPF6 solution was
added to precipitate the products, and the complexes NIR1–NIR6 were obtained by filtration. All
targeted complexes were purified using silica gel column chromatography followed by
1
13
recrystallization in CH2Cl2/n-hexane solution, and their structures were identified by H NMR, C
NMR, and HRMS-ESI. All complexes are soluble in CH2Cl2, CH3CN, and DMSO, but solubility in
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nonpolar solvents, such as n-hexane and toluene, is quite limited.
2.2 Solution photophysics
The solution state UV-Vis absorption spectra and emission spectra of all target complexes are
shown in Figure 1. The selected photophysical properties are listed in Table 1, and all experiments
were conducted at room temperature using acetonitrile (MeCN) as a solvent at a concentration of 1 ×
10-5 M. It was confirmed by variable concentration experiments that no ground state aggregation was
observed in the concentration range of 1 × 10-6 to 1 × 10-4 M. In the absorption spectra, the relatively
short wavelength signals for all complexes showed a high molar extinction coefficient (~5.0 × 104 M-
1 cm-1) with very similar spectral features and a negligible solvent effect. Based on the molar extinction
coefficients and time-dependent density functional theory (TD-DFT) calculations (Table S1), these
signals can be attributed to the spin-allowed ligand-centered 1π-π* transitions in both ligand moieties
(250–300 nm), overlapping, in part, with the singlet metal-to-ligand charge transfer (1MLCT), singlet
ligand-to-ligand charge transfer (1LLCT), and singlet ligand-to-metal charge transfer (1LMCT)
transitions (300–450 nm), while the broadened, weak absorption in the region of relatively long
3
3
wavelength signals (450–600 nm) can be assigned to the mixed, spin-forbidden MLCT and LLCT
transitions.[3j, 6a, 6c, 7c, 7d, 9] The phosphorescent nature of NIR1–NIR6 was supported by the nearly 1 µs
lifetimes (156–277 ns) in deoxygenated solution (Table 1). In the photoluminescence (PL) spectra, all
complexes show similar emission profiles, with a distinct peak around 750 nm and a shoulder signal
at 850 nm, which can be contributed by the mixed triplet charge transfer (3CT).[3j, 6b, 7d] According to
TD-DFT calculation results (Figure S21), the HOMO is mostly distributed on C^N ligands and metals,
and the LUMO is primarily distributed on N^N ligands. It can be observed that complex RED with
phenylpyridine as C^N ligand has a higher energy of emission than NIR3 and NIR6, with mpqx or
mopqx as a C^N ligand, on account of the relatively short conjugated system. On the other hand, the
emission peaks of complexes NIR1–NIR3 are negligibly red-shifted relative to the corresponding
NIR4–NIR6. The energy levels of HOMOs of NIR4–NIR6 (-4.78, -4.66, and -4.71 eV) are slightly
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higher than those of NIR1–NIR3 (-4.83, -4.70, and -4.73 eV), which are caused by strong electron-
donating properties of the methoxy groups attached to the C^N ligands (mopqx), resulting in a
substantial increase in the electron density and energy.[7b, 7e] In addition, NIR1 is red-shifted compared
to NIR2 and NIR3, this effect is not significant. From the measurement of redox potentials, it can be
found that the N^N ligand (2,2'-biquinoline, bqu) of NIR1 provides a deeper LUMO (-3.14 eV)
compared to those of NIR2 and NIR3 (-3.00 and -3.04 eV), leading to a relatively small energy gap.[6b,
7d]
2.3 Cyclic voltammetry and thermal properties
Cyclic voltammetry studies on complexes RED and NIR1–NIR6 reveal redox waves with the
electrochemical data summarized in Table 1. The series of target complexes were measured for
reduction potential (LUMO, -1.30 ~ -1.49 V) in MeCN solvent, and the HOMO levels were derived
using the optical energy gap. Based on the TD-DFT results, the HOMO is mostly distributed on C^N
ligands and metals, and the LUMO is primarily distributed on N^N ligands. Therefore, the HOMO of
complex RED (-4.87 eV) is significantly deeper than complexes NIR1–NIR6 (-4.66 ~ -4.83 eV),
mainly because its C^N ligand (bppd) has a relatively short conjugation system compared to other
C^N ligands (mpqx or mopqx). On the other hand, complexes NIR4–NIR6 possess relatively shallow
HOMO energy levels at -4.66 ~ -4.78 eV compared to NIR1–NIR3, which are in accordance to the
nature that the methoxyl groups attached to the C^N ligands have stronger electron-donating properties
than the methyl substituents; hence, the HOMO energy levels were raised slightly. To further evaluate
the thermal properties, a thermogravimetric analysis (TGA) for complex NIR1 was performed (Figure
S19), for which the recorded decomposition temperature (Td) was depicted at 248 °C, demonstrating
the potential for being an ideal NIR LECs emitter. However, the glass transition temperature (Tg)
cannot be observed presumably due to the rigidly chelated arrangement around the iridium metal center.
2.4 Electroluminescent characteristics of LECs
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LEC device characteristics based on the structure of the glass substrate/indium-tin-oxide (ITO)
(120 nm)/PEDOT:PSS (40 nm)/emissive layer (160 nm)/Ag (100 nm) under 4.0 V are summarized in
Table 2. Only the EL spectra of LECs based on NIR3 and NIR6 can be measured due to the sensitivity
limit of the spectrometer. The EL spectra of LECs based on complexes NIR3 and NIR6 under 4.0 V
are depicted in Figure 2. Both complexes show predominant EL emission beyond 800 nm. To the best
of our knowledge, no reported LECs based on iridium iTMCs have shown primary EL emission in this
spectral range. Similar to the PL spectra shown in Figure 1, two emission bands can be observed in the
EL spectra. For complex NIR3, the EL spectrum shows a peak at 882 nm and a shoulder at ca. 810
nm. On the other hand, the EL spectrum of complex NIR6 exhibits a peak at 790 nm and a shoulder
at ca. 865 nm. Compared to PL in solutions Figure 1, both complexes show red-shifted EL in solid thin
films (Figure 2), mainly resulting from the stabilization of excited states in the condensed phase. It is
interesting to note that the intensity of the second EL emission band of complex NIR3 is higher than
that of the first EL emission band. The reversed intensity order in the first and second emission bands
of solid-state EL compared with that of solution PL reveals altered vibronic features of the molecular
orbital of complex NIR3 in the condensed phase.
Time-dependent current densities of the LECs based on complexes NIR3 and NIR6 under 4.0 V
are shown in Figure 3a. The device current first increased with time after the bias was applied, reaching
a maximum before undergoing gradual decrease over time. After a bias was applied on the LEC, the
mobile ions in the emissive layer were driven toward the electrodes, and the electrochemically doped
layers were established gradually. Carrier injection was promoted and, thus, the device current was
enhanced with time. When the doped layers were well formed, carrier injection approached a steady
state, and the device current reached a maximum value. Notably, the LECs based on complex NIR6
exhibit faster increasing rate and higher maximum value in device current than the LECs based on
complex NIR3. The methoxyl groups on the ligands of complex NIR6 facilitate ion transport and
accelerate ion accumulation near the electrodes, rendering faster device response and higher device
current. Ether-based structures have been used as ion transporters, which enhance ionic mobility by
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solvating the cations and providing ionic transport paths in the emissive layer of LECs.[10] The time-
dependent light output power of both devices is shown in Figure 3b. Initially, the light output power
generally followed the temporal evolution of the device current. However, it started to decrease before
the device current reached maximum value. Deteriorated light output power results from exciton
quenching near the ionic species in continuously extended doped layers[11] and/or material degradation.
Time-dependent EQEs of both devices are shown in Figure 3c. When a bias was applied on the LEC,
the EQE rapidly rose since improved carrier balance was achieved by the formation of the doped layers.
Because of the enhanced ion transport induced by methoxyl groups, the EQE of the LECs based on
complex NIR6 also shows a higher increasing rate than that of the LECs based on complex NIR3.
After reaching the peak EQE value (0.036 and 0.05% for LECs based on NIR3 and NIR6,
respectively), the EQE gradually decreased due to the same reasons responsible for the degradation of
light output power. These results demonstrate that provide predominant EL emission beyond 800 nm
by employing iridium iTMCs for the first time and reveal their potential applications in NIR light
sources.
To reduce self-quenching of NIR complexes in neat films and to enhance the device efficiency of
LECs, complexes NIR3 or NIR6 was doped in the host complex RED. The novel low-gap host
complex RED was sophisticatedly designed to reduce the host-guest energy offset and thus to ensure
low-voltage operation of the host-guest LECs. The EL characteristics of the host-guest LECs based on
RED doped with NIR3 or NIR6 (20 wt.%) under 2.3 V were measured and are listed in Table 2. The
EL spectra of the host-only and host-guest LECs are shown in Figure 4. It supports that the main EL
emission comes from the dopant complex and, thus, efficient energy transfer is achieved. In spite of
the slightly blue-shifted and narrowed EL emission due to reduced intermolecular interactions, the
predominant EL spectra of both host-guest LECs are still in the NIR spectral region (ELpeak > 770 nm).
Time-dependent current densities of the LECs based on RED doped with 20 wt.% NIR3 or NIR6
under 2.3 V are shown in Figure 5a. Both the host-guest LECs show very similar temporal evolution
in device current densities since the host complex RED dominates the electrical characteristics. The
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device current of the host-guest LECs doped with NIR6 is larger than that of the host-guest LECs
doped with NIR3. This may be because the methoxyl groups on the ligands of complex NIR6 enhance
ion transport and accumulation at the electrodes, improving carrier injection. The light output powers
of the host-guest LECs under 2.3 V are shown in Figure 5b and both LECs exhibit similar light output
powers. Compared to neat-film LECs, the host-guest LECs can provide comparable light output
powers under significantly lower current densities (cf. Figure 3a and Figure 5a), implying their
preferred device efficiencies. As shown in Figure 5c, the host-guest LECs indeed show improved EQE
> 0.1%. Enhanced device efficiency comes from reduced self-quenching of the guest complex
dispersed in the host film. These results demonstrate that improved device efficiency of NIR LECs
based on NIR3 or NIR6 can be achieved by employing the host-guest strategy.
3. Experimental Section
3.1 General Information
1H and 13C NMR spectra of compounds were collected on a Bruker Ascend 400 MHz
spectrometer at room temperature. Photophysical characteristics of complexes in solutions were
collected at room temperature using 1 × 10-5 M MeCN solutions of all complexes on
spectrofluorometer Edinburgh FS5, which were carefully purged with nitrogen prior to measurements.
UV-Vis absorption spectra were recorded on Perkin Elmer Lambda 14 spectrophotometer. To gain
more insight into the electronic structure of iridium-based iTMCs, complete geometrical optimisations
were performed using TD-DFT with B3LYP/LANL2DZ[Ir]6-31G(d,p)[S,O,N,C,H] basis set within
Gaussian 09. Oxidation and reduction potentials of all complexes were determined by cyclic
voltammetry (CV) at a scan rate of 100 mV/s in MeCN solutions (1.0 mM) on ZIVE SP1. A glassy
carbon electrode and a platinum wire were used as the working electrode and the counter electrode,
respectively. All potentials were recorded versus the Ag/AgCl (sat’d) reference electrode. For
reduction voltammograms, 0.1 M tetra-n-butylammonium hexafluorophosphate (TBAPF6) in MeCN
was used as the supporting electrolyte. 2,3-di-p-tolylbenzo[g]quinoxaline (mpqx), 2,3-bis(4-
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methoxyphenyl)benzo[g]quinoxaline (mopqx), and 2-(quinolin-2-yl)quinazoline (qqz) were
synthesized following the literature procedures.[12, 13] Cyclometalated iridium dichlorobridged dimers
with the general formula (C^N)2Ir(μ-Cl)2Ir(C^N)2 were prepared according to literature procedures.[14]
All experiments involving IrCl3·3H2O or any other iridium species were carried out in an inert
‐
atmosphere. The series of (C^N)2Ir(N^N)+PF6 complexes were produced following the literature
procedures.[15]
3.2 Materials and Synthesis
Synthesis of 2-(4-butylphenyl)pyridine (bppd). To a solution of 2-(4-butylphenyl)-4,4,5,5-
tetramethyl-dioxaborolane (0.73 g, 2.8 mmol) in toluene (9 mL), ethanol (2 mL), H2O (9 mL) was
added Na2CO3 (2.0 g, 19 mmol) followed by Pd(PPh3)4 (90. mg, 0.078 mmol) and 2-bromopyridine
(0.40 g, 2.6 mmol) under argon in a two-necked flask. The reaction mixture was refluxed for 24 h, and
then cooled to room temperature. To the reaction mixture was added H2O, extracted with ethyl acetate
for three times, dried over Na2SO4, and evaporated in vacuum to afford the crude product, which was
purified by flash chromatography on silica gel (n-hexane/ethyl acetate = 9/1) gave compound bppd
(0.48 g, 89%) as a yellow oil liquid. 1H NMR (400 MHz, CDCl3, 298 K) δ 8.67 (d, J = 4.7 Hz, 1H),
7.91 (d, J = 7.4 Hz, 2H), 7.77 – 7.68 (m, 2H), 7.29 (d, J = 7.5 Hz, 2H), 7.22 – 7.16 (m, 1H), 2.67 (t, J
= 7.7 Hz, 2H), 1.69 – 1.63 (m, 2H), 1.45 – 1.32 (m, 2H), 0.94 (t, J = 7.3 Hz, 3H).
Synthesis of [Ir2(bppd)4Cl2]. bppd (0.45 g, 2.1 mmol) was dissolved in 2-ethoxyethanol (36 mL) in
a round-bottom flask. Iridium(III) chloride hydrate (0.32 g, 1.1 mmol) and 11 mL of water were then
added to the flask. The mixture was refluxed under nitrogen for 24 h and then cooled to room
temperature. The precipitate was collected, washed with water and dried under vacuum to give
1
cyclometalated dimer gave [Ir2(bppd)4Cl2] (0.46 g, 67%) as a yellow solid. H NMR (400 MHz,
CDCl3, 298 K) δ 9.26 (d, J = 5.3 Hz, 4H), 7.81 (d, J = 8.0 Hz, 4H), 7.70 (t, J = 7.16 Hz, 4H), 7.39 (d,
J = 7.9 Hz, 4H), 6.72 (t, J = 6.1 Hz, 4H), 6.57 (d, J = 7.1 Hz, 4H), 5.74 (s, 4H), 2.23 – 2.13 (m, 8H),
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1.31 – 1.21 (m, 8H), 1.16 – 1.07 (m, 8H), 0.76 (t, J = 7.3 Hz, 12H). 13C NMR (100 MHz, CDCl3, 298K)
δ 168.8, 151.7, 145.5, 144.0, 141.3, 136.0, 130.71, 123.61, 121.8, 121.4, 118.1, 35.6, 33.1, 22.3, 14.0.
Synthesis of RED. In a round-bottomed flask, [Ir2(bppd)4Cl2] (0.30 g, 0.23 mmol) and bbtz (0.12 g,
0.46 mmol) were mixed together in 68 mL of methanol. The solution was then refluxed overnight
‐
under an inert atmosphere. After cooling to room temperature, counter ion exchange from Cl‐ to PF6
was accomplished via a metathesis reaction in which complexes were precipitated from the methanol
solution with an excess of KPF6, washed with water and methanol, and dried under vacuum. The crude
product was purified by column chromatography on silica gel (dichloromethane/acetone = 9/1) to give
complex RED (0.41 g, 87%) as a dark red solid. 1H NMR (400 MHz, CDCl3, 298 K) δ 8.11 (d, J = 8.1
Hz, 2H), 7.83 (d, J = 7.9 Hz, 2H), 7.76 – 7.67 (m, 2H), 7.60 (d, J = 8.0 Hz, 2H), 7.53 (dd, J = 11.2, 4.0
Hz, 4H), 7.22 – 7.15 (m, 2H), 7.03 – 6.91 (m, 4H), 6.80 (d, J = 8.5 Hz, 2H), 6.10 (d, J = 1.4 Hz, 2H),
2.47 – 2.31 (m, 4H), 1.47 – 1.40(m, 4H), 1.25 – 1.16 (m, 4H), 0.81 (t, J = 7.3 Hz, 6H). 13C NMR (100
MHz, DMSO-d6, 298 K) δ 166.6, 164.0, 150.3, 149.8, 145.2, 144.2, 142.1, 138.8, 135.7, 131.3, 128.6,
128.4, 125.0, 124.6, 123.5, 123.2, 121.7, 119.5, 34.6, 32.6, 21.0, 13.6. HRMS (ESI+) m/z: calcd for
+
C44H40N4IrS2 [M+H]+: 881.2318, found: 881.2303. Elemental analysis calculated (%)
C44H40F6IrN4PS2: C 51.50, H 3.93, N 5.46, S 6.25; found: C 51.21, H 4.28, N 5.42, S 5.88.
Synthesis of 2,3-di-p-tolylbenzo[g]quinoxaline (mpqx). 4,4′-dimethylbenzil (0.50 g, 2.1 mmol) and
2,3-diaminonaphthalene (0.40 g, 2.5 mmol) in MeCN (10.5 mL) were mixed with iodine (53 mg, 0.21.
mmol) and stirred at rt for 30 min. After completion, the reaction mixture was diluted with water, the
product was extracted with ethyl acetate, and the resultant organic layer was dried over anhydrous
Na2SO4. After removal of the solvent, the crude product was purified by column chromatography on
silica gel (n-hexane/ethyl acetate = 4/1) to give mpqx (0.67 g, 88%) as a yellow solid. 1H NMR (400
MHz, CDCl3, 298 K) δ 8.71 (s, 2H), 8.11 (dd, J = 6.5, 3.1 Hz, 2H), 7.56 (dd, J = 6.6, 3.0 Hz, 2H), 7.48
(d, J = 7.8 Hz, 4H), 7.17 (d, J = 7.8 Hz, 4H), 2.39 (s, 6H).
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Synthesis of 2,3-bis(4-methoxyphenyl)benzo[g]quinoxaline (mopqx). Following the procedure for
mpqx, 4,4′-dimethoxybenzil (0.50 g, 1.9 mmol) and 2,3-diaminonaphthalene (0.35 g, 2.2 mmol) in
MeCN (9.2 mL) were mixed with iodine (50. mg, 0.19 mmol) to give mopqx (0.72 g, 99%) as a brown
solid. 1H NMR (400 MHz, CDCl3, 298 K) δ 8.68 (s, 2H), 8.10 (dd, J = 6.5, 3.1 Hz, 2H), 7.55 (d, J =
8.7 Hz, 6H), 6.90 (d, J = 8.4 Hz, 4H), 3.85 (s, 6H).
Synthesis of [Ir2(mpqx)4Cl2]. mpqx (0.20 g, 0.55 mmol) was dissolved in 2-ethoxyethanol (9.3 mL)
in a round-bottom flask. Iridium(III) chloride hydrate (80. mg, 0.28 mmol) and 2.8 mL of water were
then added to the flask. The mixture was stirred under nitrogen at 100 °C for 24 h and then cooled to
room temperature. The precipitate was collected, washed with water and dried under vacuum to give
[Ir2(mpqx)4Cl2] (0.19 g, 70%) as a dark red solid. 1H NMR (400 MHz, CDCl3, 298 K) δ 8.80 (s, 4H),
8.12 (d, J = 7.7 Hz, 4H), 7.94 (d, J = 8.3 Hz, 5H), 7.85 – 7.76 (m, 8H), 7.52 (d, J = 7.7 Hz, 4H), 7.45
(d, J = 8.2 Hz, 4H), 7.36 – 7.28 (m, 5H), 7.23 (s, 4H), 6.96 (d, J = 8.2 Hz, 4H), 6.35 (t, J = 7.4 Hz,
4H), 6.19 (d, J = 8.3 Hz, 4H), 5.41 (s, 4H), 2.63 (s, 12H), 1.31 (s, 12H).
Synthesis of [Ir2(mopqx)4Cl2]. Following the procedure for compound [Ir2(mpqx)4Cl2], mopqx
(0.20 g, 0.51 mmol), 2-ethoxyethanol (8.7 mL), iridium(III) chloride hydrate (80. mg, 0.26 mmol), and
2.6 mL of water gave [Ir2(mopqx)4Cl2] (0.23 g, 92%) as a dark red solid. 1H NMR (400 MHz, CDCl3,
298 K) δ 8.93 (s, 4H), 8.17 (d, J = 8.4 Hz, 4H), 7.89 (d, J = 8.5 Hz, 8H), 7.72 (s, 4H), 7.31 (t, J = 7.6
Hz, 9H), 7.26 – 7.23 (m, 4H), 7.18 (d, J = 8.0 Hz, 4H), 7.06 (d, J = 8.9 Hz, 4H), 6.43 (t, J = 7.3 Hz,
4H), 5.99 (d, J = 9.8 Hz, 4H), 5.13 (s, 4H), 4.06 (s, 12H), 2.58 (s, 12H).
Synthesis of 2,2'-bibenzo[d]thiazole (bbtz).[3j] 2-Iodobenzothiazole (1.00 g, 3.83 mmol), N,N′-
diisopropylethylamine (DIPEA) (0.50 g, 3.8 mmol), n-Bu4NBr (0.62 g, 1.92 mmol) and Pd(OAc)2 (86
mg, 0.38 mmol) were dissolved in nitrogen-degassed toluene (14 mL). The mixture was heated to
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reflux for 20 h, after which time distilled water (50 mL) was added to the dark-brown solution before
cooling down the solution to room temperature. The mixture was stirred at room temperature and
dichloromethane was added. The organic layer was carefully separated and this procedure was
repeated one more time with additional dichloromethane. The organic layers were combined and
evaporated to dryness to give dark brown oil. To this oil was added diethyl ether (200 mL) and the
mixture was sonicated for 30 min. The ether solution was carefully decanted out and evaporated to
dryness to give bbtz (0.32 g, 63%) as a dark-orange microcrystalline solid.1H NMR (400 MHz, CDCl3,
298 K) δ 8.17 (d, J = 8.2 Hz, 2H), 7.99 (d, J = 8.1 Hz, 2H), 7.57 (t, J = 7.7 Hz, 2H), 7.50 (t, J = 7.6
Hz, 2H).
Synthesis of NIR1. In a round-bottomed flask, [Ir2(mpqx)4Cl2] (0.20 g, 0.11 mmol) and 2,2'-
biquinoline (bqu, 60. mg, 0.22 mmol) were mixed together in 32.3 mL of methanol. The solution was
then refluxed overnight under an inert atmosphere. After cooling to room temperature, counter ion
‐
exchange from Cl‐ to PF6 was accomplished via a metathesis reaction in which complexes were
precipitated from the methanol solution with an excess of KPF6, washed with water and methanol, and
dried under vacuum. The crude product was purified by column chromatography on silica gel
(dichloromethane/acetone = 95/5) to give NIR1 (0.19 g, 83%) as a dark red solid. 1H NMR (400 MHz,
DMSO-d6, 298 K) δ 8.81 (d, J = 8.1 Hz, 2H), 8.74 (s, 2H), 8.53 (d, J = 8.2 Hz, 2H), 8.22 (d, J = 7.4
Hz, 2H), 8.10 (d, J = 8.3 Hz, 2H), 7.89 – 7.72 (m, 4H), 7.62 (d, J = 8.3 Hz, 2H), 7.46 (t, J = 7.2 Hz,
2H), 7.33 (d, J = 6.7 Hz, 6H), 7.23 (t, J = 7.0 Hz, 2H), 6.97 (d, J = 7.9 Hz, 2H), 6.69 (s, 2H), 6.60 (d,
J = 7.9 Hz, 2H), 6.20 (d, J = 8.2 Hz, 2H), 2.44 (s, 6H), 1.96 (s, 6H). 13C NMR (100 MHz, DMSO-d6,
298 K) δ 165.1, 159.0, 153.4, 152.8, 146.5, 142.0, 141.9, 141.3, 140.1, 136.8, 136.2, 135.8, 132.7,
132.6, 132.3, 131.8, 129.9, 129.5, 129.4, 129.3, 128.9, 128.5, 128.2, 128.1, 127.5, 127.2, 126.7, 123.2,
121.8, 121.6, 21.5, 21.1. HRMS (ESI+) m/z: calcd for C70H50N6Ir+ [M+H]+: 1167.3721, found:
1167.3707. Elemental analysis calculated (%) C70H50F6IrN6P: C 64.06, H 3.84, N 6.40; found: C 63.98,
H 3.47, N 6.24.
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Synthesis of NIR2. Following the procedure for NIR1, [Ir2(mpqx)4Cl2] (0.20 g, 0.11 mmol), qqz (60.
mg, 0.22 mmol), and 32.3 mL of methanol gave NIR2 (0.19 g, 66%) as a dark red solid. 1H NMR (400
MHz, DMSO-d6, 298 K) δ 9.56 (s, 1H), 8.81 (s, 2H), 8.65 (s, 1H), 8.59 (s, 1H), 8.48 (dd, J = 8.28,
6.80 Hz, 2H), 8.38 – 8.23 (m, 2H), 8.08 (d, J = 8.6 Hz, 4H), 7.97 (d, J = 7.2 Hz, 3H), 7.63 – 7.39 (m,
5H), 7.35 (d, J = 8.3 Hz, 1H), 7.29 – 7.03 (m, 5H), 6.72 (s, 2H), 6.64 – 6.41 (m, 3H), 6.15 (d, J = 8.0
Hz, 1H), 6.02 (d, J = 6.8 Hz, 1H), 2.54 (s, 3H), 2.40 (s, 3H), 2.02 (s, 3H), 1.83 (s, 3H). 13C NMR (100
MHz, DMSO-d6, 298 K) δ 165.6, 164.9, 162.2, 160.0, 155.5, 155.3, 154.4, 153.4, 151.0, 148.7, 144.6,
142.5, 142.0, 141.8, 140.8, 139.9, 139.5, 138.4, 137.1, 137.0, 136.6, 136.0, 134.6, 134.5, 133.0, 132.9,
132.7, 132.6, 132.5, 132.2, 131.9, 131.7, 131.3, 130.8, 130.1, 129.7, 129.6, 129.2, 129.0, 128.7, 128.5,
128.4, 128.3, 128.3, 128.2, 128.0, 127.9, 127.3, 126.5, 1263, 124.4, 123.4, 123.1, 121.3, 121.1, 21.6,
21.3, 21.2, 21.0. HRMS (ESI+) m/z: calcd for C69H49N7Ir+ [M+H]+: 1168.3673, found: 1168.3665.
Elemental analysis calculated (%) C69H49F6IrN7P: C 63.10, H 3.76, N 7.47; found: C 63.16, H 4.17, N
7.67.
Synthesis of NIR3. Following the procedure for NIR1, [Ir2(mpqx)4Cl2] (0.30 g, 0.16 mmol), bbtz
(86 mg, 0.32 mmol), and 47.1 mL of methanol gave NIR3 (0.10 g, 23%) as a dark red solid. 1H NMR
(400 MHz, CDCl3, 298 K) δ 8.54 (s, 2H), 8.26 (d, J = 8.2 Hz, 2H), 7.98 (s, 2H), 7.91 (t, J = 6.8 Hz,
4H), 7.71 (t, J = 7.7 Hz, 2H), 7.39 (m, 12H), 7.20 (t, J = 7.4 Hz, 2H), 7.14 (d, J = 8.4 Hz, 2H), 6.70 (d,
J = 8.4 Hz, 2H), 6.45 (s, 2H), 5.90 (d, J = 8.5 Hz, 2H), 2.52 (s, 6H), 1.98 (s, 6H). Due to the solubility
+
issue, a decent 13C spectrum could not be obtained. HRMS (ESI+) m/z: calcd for C66H46N6IrS2 [M+H]+:
1179.2849, found: 1179.2840. Elemental analysis calculated (%) C66H46F6IrN6PS2: C 59.85, H 3.50,
N 6.35, S 4.84; found: C 59.54, H 3.73, N 6.14, S 5.04.
Synthesis of NIR4. Following the procedure for NIR1, [Ir2(mopqx)4Cl2] (0.20 g, 0.10 mmol), 2,2'-
biquinoline (bqu, 50. mg, 0.20 mmol), and 29.4 mL of methanol gave NIR4 (0.13 g, 46%) as a dark
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red solid. 1H NMR (400 MHz, DMSO-d6, 298 K) δ 8.84 (d, J = 8.6 Hz, 2H), 8.68 (s, 2H), 8.57 (d, J =
8.7 Hz, 2H), 8.25 (d, J = 7.8 Hz, 2H), 8.08 (d, J = 8.3 Hz, 2H), 7.89 – 7.77 (m, 4H), 7.71 (d, J = 8.9
Hz, 2H), 7.50 – 7.36 (m, 4H), 7.22 (t, J = 7.0 Hz, 2H), 7.09 (d, J = 9.6 Hz, 6H), 6.48 (d, J = 9.0 Hz,
2H), 6.33 – 6.08 (m, 4H), 3.89 (s, 6H), 3.39 (s, 6H). 13C NMR (100 MHz, DMSO-d6, 298 K) δ 164.5,
161.2, 160.8, 159.1, 155.0, 153.1, 146.6, 141.9, 136.6, 136.5, 135.7, 134.1, 132.7, 132.4, 132.3, 131.2,
130.4, 129.9, 129.6, 129.5, 128.2, 128.0, 127.3, 127.1, 126.6, 121.9, 121.2, 117.7, 114.2, 107.8, 55.5,
55.0. HRMS (ESI+) m/z: calcd for C70H50O4N6Ir+ [M+H]+: 1231.3517, found: 1231.3504. Elemental
analysis calculated (%) C70H50F6IrN6O4P: C 61.08, H 3.66, N 6.11; found: C 61.20, H 3.30, N 6.08.
Synthesis of NIR5. Following the procedure for NIR1, [Ir2(mopqx)4Cl2] (0.20 g, 0.10 mmol), qqz
(50 mg, 0.20 mmol), and 29.4 mL of methanol gave NIR5 (0.16 g, 59%) as a dark red solid. 1H NMR
(400 MHz, DMSO-d6, 298 K) δ 9.73 (s, 1H), 8.81 (d, J = 8.4 Hz, 1H), 8.75 (s, 1H), 8.65 (s, 1H), 8.57
(s, 1H), 8.53 (d, J = 9.0 Hz, 2H),, 8.42 – 8.25 (m, 2H), 8.07 (m, 8H), 7.58 (s, 1H), 7.45 (d, J = 8.0 Hz,
3H), 7.33 (d, J = 7.7 Hz, 2H), 7.27 (s, 1H), 7.22 – 7.04 (m, 2H), 6.96 (s, 2H), 6.90 (d, J = 8.9 Hz, 1H),
6.61 (d, J = 8.4 Hz, 1H), 6.45 (d, J = 8.7 Hz, 1H), 6.23 (d, J = 7.9 Hz, 1H), 6.11 – 5.91 (m, 3H), 3.97
(s, 3H), 3.86 (s, 3H), 3.46 (s, 3H), 3.29 (s, 3H). 13C NMR (100 MHz, DMSO-d6, 298 K) δ 165.1, 163.7,
162.2, 161.2, 160.6, 160.7, 160.4, 160.1, 157.8, 155.4, 154.2, 153.2, 152.7, 148.7, 144.8, 142.0, 138.4,
137.9, 137.0, 136.7, 136.4, 136.1, 134.7, 134.3, 134.1, 132.9, 132.7, 132.6, 132.2, 132.0, 131.8, 131.3,
131.1, 130.9, 130.1, 130.0, 129.9, 129.8, 128.6, 128.2, 128.0, 127.8, 127.7, 127.1, 127.1, 126.4, 126.3,
124.5, 122.9, 121.4, 120.7, 119.6, 117.9, 114.5, 113.9, 108.0, 107.5, 55.5, 55.4, 54.8, 54.7. HRMS
(ESI+) m/z: calcd for C69H49O4 N7Ir+ [M+H]+: 1232.3470, found: 1232.3457. Elemental analysis
calculated (%) C69H49F6IrN7O4P: C 60.17, H 3.59, N 7.12; found: C 60.52, H 3.22, N 6.66.
Synthesis of NIR6. Following the procedure for NIR1, [Ir2(mopqx)4Cl2] (0.30 g, 0.15 mmol), bbtz
(82 mg, 0.31 mmol), and 44.2 mL of methanol gave NIR6 (0.18 g, 42%) as a dark red solid. 1H NMR
(400 MHz, CDCl3, 298 K) δ 8.48 (s, 2H), 8.25 (d, J = 8.3 Hz, 2H), 8.02 – 7.92 (m, 4H), 7.88 (d, J =
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8.6 Hz, 2H), 7.79 (t, J = 7.9 Hz, 2H), 7.57 (d, J = 8.5 Hz, 2H), 7.50 (s, 3H), 7.39 (t, J = 7.6 Hz, 2H),
7.28 (s, 1H), 7.18 (t, J = 7.7 Hz, 2H), 7.09 (d, J = 8.3 Hz, 4H), 6.47 (d, J = 9.1 Hz, 2H), 6.08 (s, 2H),
5.87 (d, J = 8.7 Hz, 2H), 3.98 (s, 6H), 3.43 (s, 6H). Due to the solubility issue, a decent 13C spectrum
+
could not be obtained. HRMS (ESI+) m/z: calcd for C66H46O4N6IrS2 [M+H]+: 1243.2646, found:
1243.2634. Elemental analysis calculated (%) C66H46F6IrN6O4PS2: C 57.09, H 3.34, N 6.05, S 4.62;
found: C 56.72, H 3.19, N 5.95, S 4.68.
3.3 LEC Device Fabrication and Characterization
To fabricate LEC devices, standard clean and UV/ozone treatment were performed on the ITO
coated-glass substrate. The substrate was spin-coated with
a
40-nm poly(3,4-
ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) layer at 4000 rpm and then was baked
at 150 °C for 30 min in ambient air. The emissive layer of the neat-film LEC was composed of the
complex (NIR3 or NIR6) (80 wt.%) and 1-butyl-3-methylimidazolium hexafluorophosphate
-
[BMIM+(PF6 )] (20 wt.%). The ionic liquid [BMIM+(PF6)-] was added to provide additional mobile
ions and, hence, an accelerated device response. For the host-guest LEC, the emissive layer contained
-
the host complex (RED) (60 wt.%), guest complex (NIR3 or NIR6) (20 wt.%) and [BMIM+(PF6 )]
-
(20 wt.%). The mixture of complex and [BMIM+(PF6 )] in MeCN solution with a concentration of 60
mg mL-1 was spun on top of the PEDOT:DSS layer at 3000 rpm in ambient air. The thickness of the
emissive layer measured by ellipsometry was 160 nm. The sample was then baked at 70 °C for 10 h in
a nitrogen glove box. Finally, a silver top contact was deposited by thermal evaporation in a vacuum
chamber (~ 10-6 torr). The EL emission properties of LECs were measured using a source-measurement
unit (Agilent 4155C) and a calibrated Si photodiode. The EL spectra of these LECs were taken with a
calibrated spectrometer (Ocean Optics HR2000CG-UV-NIR). Neat-film and host-guest LECs were
measured at constant voltages of 4 and 2.3 V, respectively, in a nitrogen glove box to avoid rapid
degradation.
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4. Conclusions
In summary, we demonstrate a facile way for iTMCs to adjust the energy gap between HOMO
and LUMO, and generate NIR emission through ILCT and MLCT. We report a series of novel ionic
iridium complexes with very small energy gaps, namely NIR1–NIR6, where 2,3-
diphenylbenzo[g]quinoxaline moieties mainly take charge of the HOMO energy levels, and 2,2'-
biquinoline, 2-(quinolin-2-yl)quinazoline, and 2,2'-bibenzo[d]thiazole moieties mainly control the
LUMO energy levels. All complexes exhibited NIR phosphorescence, with emission maxima up to
850 nm, and were applied to the LEC components, exhibiting a maximum EQE of 0.05% in NIR EL
devices. By using the host (i.e., RED)-guest emission layer, the maximum EQE can be further
enhanced to greater than 0.1%, suggesting reduced self-quenching for the guest complexes dispersed
in the host film.
Conflicts of interest
There are no conflicts of interest to declare.
Acknowledgements
The authors would like to thank the financial support from Ministry of Science and Technology
of Taiwan (contract no. MOST 107-2113-M-126-004-MY2, MOST 107-2221-E-009-112-MY3, and
MOST 106-2221-E-009-122-MY3).
Appendix. Supplementary data
Supplementary data associated with this article, such as 1H and 13C NMR spectra, high resolution
mass spectra, TGA thermograms, and TD-DFT calculation can be found in the online version.
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Table 1. Summary of physical properties of complexes RED and NIR1–NIR6.
τobs
Absorption
(nm)[a]
Emission
(nm)[a]
Ered
HOMO
(eV)[c]
LUMO
(eV)
ΦPL
1/2
Complex
(ns)[d]
(V)[b]
(%)[e, f]
253, 362, 380
290, 338, 419
270, 330, 414
287, 331
749
-1.49
-1.30
-1.44
-1.40
-1.35
-1.49
-1.44
-4.87
-4.83
-4.70
-4.73
-4.78
-4.66
-4.71
-2.95
-3.14
-3.00
-3.04
-3.09
-2.95
-3.00
24
2.67
RED
NIR1
NIR2
NIR3
NIR4
NIR5
NIR6
789, 852
777, 840
778, 849
776, 846
767, 827
770, 837
177
156
208
203
203
277
-
-
-
-
-
-
270, 344, 446
271, 342, 442
282, 343, 437
[a] Measured in MeCN at room temperature (1 × 10-5 M, λex = 365 nm). [b] Used TBAPF6 (0.1 M) as
electrolyte in MeCN. Potential vs. ferrocene/ferrocenium redox couple. [c] The potential was derived
from the onset of emission wavelength. [d] Lifetime measurements were carried out in degassed MeCN
(1 × 10-5 M, λex = 365 nm). [e] Measured in solid state at room temperature (λex = 365 nm). [f] Only
complex RED can be measured due to the sensitivity limit of the spectrometer.
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10.1002/chem.201805902
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Table 2. Summary of electroluminescent characteristics of LECs.
λmax, EL
Lmax
ηext, max
ηP, max
Complex
Bias (V)
(nm)[a] (μW cm–2)[b] (×10-2 %)[c] (×10-2 mW W-1)[d]
NIR3
4.0
4.0
2.3
2.3
882
790
778
778
44.1
56.9
12.6
13.6
3.6
5.0
12.2
18.1
74.7
63.1
NIR6
RED:20 wt.% NIR3
RED:20 wt.% NIR6
11.6
9.3
[a] EL peak wavelength. [b] Maximal light output power. [c] Maximal external quantum efficiency. [d]
Maximal power efficiency.
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Scheme captions
Scheme 1. Synthetic routes of cationic iridium complexes RED and NIR1–NIR6.
Figure captions
Figure 1. Absorption (left axis) and PL spectra (right axis) of complexes RED and NIR1–NIR6 in
MeCN solutions (1 × 10-5 M, λex = 365 nm).
Figure 2. EL spectra of the LECs based on complexes NIR3 and NIR6 under 4.0 V.
Figure 3. Time-dependent (a) current density, (b) light output power, and (c) external quantum
efficiency of the LECs based on complexes NIR3 and NIR6 under 4.0 V.
Figure 4. EL spectra of the LECs based on RED and RED doped with 20 wt.% NIR3 or NIR6 under
2.3 V.
Figure 5. Time-dependent (a) current density, (b) light output power, and (c) external quantum
efficiency of the LECs based on RED doped with 20 wt.% NIR3 or NIR6 under 2.3 V.
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10.1002/chem.201805902
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Scheme
Scheme 1.
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10.1002/chem.201805902
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Figures
RED
1.0
0.8
0.6
0.4
0.2
0.0
1.0
0.8
0.6
0.4
0.2
0.0
NIR 1
NIR 2
NIR 3
NIR 4
NIR 5
NIR 6
300
400
500
600
700
800
900
1000
Wavelength (nm)
Figure 1.
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1.0
0.8
0.6
0.4
0.2
0.0
NIR3
NIR6
500 600 700 800 900 1000 1100
Wavelength (nm)
Figure 2.
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(a)
600
500
400
300
200
100
0
NIR3
NIR6
0.0
0.5
1.0
1.5
2.0
Time (min)
(b)
60
50
40
30
20
10
0
NIR3
NIR6
0.0
0.5
1.0
1.5
2.0
Time (min)
(c)
5
4
3
2
1
0
NIR3
NIR6
0.0
0.5
1.0
1.5
2.0
Time (min)
Figure 3.
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RED
1.0
0.8
0.6
0.4
0.2
0.0
RED: 20wt.% NIR3
RED: 20wt.% NIR6
500 600 700 800 900 1000 1100
Wavelength (nm)
Figure 4.
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(a)
10
8
6
4
2
RED: 20wt.% NIR3
RED: 20wt.% NIR6
0
0
20
40
60
80
Time (min)
(b)
15
10
5
RED: 20wt.% NIR3
RED: 20wt.% NIR6
0
0
20
40
60
80
Time (min)
(c)
12
10
8
6
4
RED: 20wt.% NIR3
RED: 20wt.% NIR6
2
0
0
20
40
60
80
Time (min)
Figure 5.
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