Luminescence Color Tuning by Regulating Electrostatic Interaction in Light-Emitting Devices and Two-Photon Excited Information Decryption

Article abstract of DOI:10.1021/acs.inorgchem.6b02319

It is well-known that the variation of noncovalent interactions of luminophores, such as π- π interaction, metal-to-metal interaction, and hydrogen-bonding interaction, can regulate their emission colors. Electrostatic interaction is also an important noncovalent interaction. However, very few examples of luminescence color tuning induced by electrostatic interaction were reported. Herein, a series of Zn(II)-bis(terpyridine) complexes (Zn-AcO, Zn-BF₄, Zn-ClO₄, and Zn-PF₆) containing different anionic counterions were reported, which exhibit counterion-dependent emission colors from green-yellow to orange-red (549 to 622 nm) in CH₂Cl₂ solution. More importantly, it was found that the excited states of these Zn(II) complexes can be regulated by changing the electrostatic interaction between Zn²⁺ and counterions. On the basis of this controllable excited state, white light emission has been achieved by a single molecule, and a white light-emitting device has been fabricated. Moreover, a novel type of data decryption system with Zn-PF₆ as the optical recording medium has been developed by the two-photon excitation technique. Our results suggest that rationally controlled excited states of these Zn(II) complexes by regulating electrostatic interaction have promising applications in various optoelectronic fields, such as light-emitting devices, information recording, security protection, and so on.

Full text of DOI:10.1021/acs.inorgchem.6b02319

Article
pubs.acs.org/IC
Luminescence Color Tuning by Regulating Electrostatic Interaction
in Light-Emitting Devices and Two-Photon Excited Information
Decryption
Yun Ma,† Shujuan Liu,‡ Huiran Yang,‡ Yi Zeng,† Pengfei She,‡ Nianyong Zhu,† Cheuk-Lam Ho,†
Qiang Zhao,*,‡ Wei Huang,‡ and Wai-Yeung Wong*,†,§
†Institute of Molecular Functional Materials, Department of Chemistry and Institute of Advanced Materials, Hong Kong Baptist
University, Waterloo Road, Kowloon Tong, Hong Kong, P. R. China
‡Key Laboratory for Organic Electronics & Information Displays (KLOEID) and Institute of Advanced Materials (IAM), Nanjing
University of Posts & Telecommunications (NUPT), 9 Wenyuan Road, Nanjing 210023, Jiangsu, P. R. China
§Department of Applied Biology and Chemical Technology, The Hong Kong Polytechnic University, Hung Hom, Hong Kong, P. R.
China
S
* Supporting Information
ABSTRACT: It is well-known that the variation of noncovalent interactions of
luminophores, such as π−π interaction, metal-to-metal interaction, and hydrogen-bonding
interaction, can regulate their emission colors. Electrostatic interaction is also an important
noncovalent interaction. However, very few examples of luminescence color tuning
induced by electrostatic interaction were reported. Herein, a series of Zn(II)-
bis(terpyridine) complexes (Zn-AcO, Zn-BF4, Zn-ClO4, and Zn-PF6) containing different
anionic counterions were reported, which exhibit counterion-dependent emission colors
from green-yellow to orange-red (549 to 622 nm) in CH2Cl2 solution. More importantly,
it was found that the excited states of these Zn(II) complexes can be regulated by changing
the electrostatic interaction between Zn2+ and counterions. On the basis of this
controllable excited state, white light emission has been achieved by a single molecule, and
a white light-emitting device has been fabricated. Moreover, a novel type of data
decryption system with Zn-PF6 as the optical recording medium has been developed by
the two-photon excitation technique. Our results suggest that rationally controlled excited
states of these Zn(II) complexes by regulating electrostatic interaction have promising applications in various optoelectronic
fields, such as light-emitting devices, information recording, security protection, and so on.
coordination bonds for these complexes. By contrast,
coordination bonds are usually more flexible for transition-
metal complexes with d10 electronic configuration. Therefore,
the variation of electrostatic interaction between metal ions and
counterions would change the bond strength of the complex
and then influence the electron-withdrawing ability of the
coordination sites. These features offer the complex the
capability to achieve luminescence color tuning by controlling
the electrostatic interaction.23 Among all transition-metal
complexes with d10 electronic configuration, Zn(II) complexes
represent an emerging class of molecules for developing novel
photoelectronic materials because of their low cost, excellent
stability, and high photoluminescence quantum efficiency.24
For the zinc(II)-bis(terpyridine) complexes, their emission
often originated from the charge transfer (CT) state.24 While
the Zn2+ coordinated terpyridine unit can serve as an electron
acceptor, a strong electron donor has to be attached on the
terpyridine to facilitate the CT state, and a widely used
INTRODUCTION
■
Materials showing tunable luminescence properties are ideal for
various photoelectronic applications. Traditional strategies for
emission color tuning of materials are based on structural
modification of materials.1−3 In addition to covalent
modification, numerous examples showed that the variation
of noncovalent interactions, such as π−π interaction,4−6 metal-
to-metal interaction,7−9 and hydrogen-bonding interac-
tion,10−12 can also induce changes in the optical property of
the luminophores. However, examples of luminescence color
tuning induced by electrostatic interaction, which is also an
important noncovalent interaction, are quite rare.13,14
Ionic transition-metal complexes are an important class of
luminescent materials for various optical applications, including
photoelectronic devices,15,16 chemical sensors,17,18 bio-
markers,19,20 etc. These complexes often contain counterions
to maintain electroneutrality. The emission colors of Pt(II),
Ir(III), and Ru(II) complexes with d6 and d8 electronic
configuration normally remain unchanged by regulating the
electrostatic interaction between metal ions and counter-
ions,21,22 probably because of the formation of more stable
Received: September 23, 2016
© XXXX American Chemical Society
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−
triphenylamine donor group has been chosen in this work.
Herein, to demonstrate that the emission colors of
luminophores can be tuned by electrostatic interaction, a series
of Zn(II) complexes (Zn-AcO, Zn-BF4, Zn-ClO4, and Zn-PF6,
(Figure 1a) with different anionic counterions were prepared.
complexes [Zn(tpypa)2]2+2X− (X− = CH3COO−, BF4 ,
−
−
ClO4 or PF6 ) were obtained by refluxing ligand Tpypa
with zinc(II) acetate (0.5 equiv) in methanol, followed by anion
exchange with NaBF4, NaClO4, or NaPF6, respectively. The
1
obtained complexes were characterized by H and 13C NMR
spectroscopy, elemental analysis, MALDI-TOF mass spectrom-
etry, and X-ray crystal structure analysis. Single-crystals of Zn-
BF4, Zn-ClO4, and Zn-PF6 were obtained by slow diffusion of
hexane to their respective dichloromethane solutions. Basic
crystallographic data are summarized in Table S1, and selected
bond lengths for the complexes are given in Table S2. As
confirmed by X-ray crystallography, these Zn(II) complexes
exhibit a distorted octahedral six coordination geometry (Figure
1).
Luminescence Color Tuning by Counterions. The
photophysical data of Zn(II) complexes are listed in Table
S3. Their emission peaks in CH2Cl2 are gradually red-shifted
from 549 to 622 nm with the change of emission color from
green-yellow to orange-red in the order CH3COO− → BF4
→
−
−
−
ClO4 → PF6 (Figure 2b and c). By contrast, the emission
Figure 1. (a) Chemical structures of Zn(II)-bis(terpyridine)
complexes. (b) Single-crystal structure of Zn-PF6.
In addition, the model Zn(II) complexes 1a−1d ([Zn-
(ttpy)2]2+2X− (ttpy = 4′-(p-tolyl)-2,2′:6′,2′′-terpyridine; X− =
CH3COO−, BF4 , ClO4 , or PF6 )) were also prepared to
demonstrate the important role of a strong donor group on the
photoluminescence properties. It is found that these Zn(II)
complexes exhibit different emission colors from green-yellow
to orange-red (549 to 622 nm) in CH2Cl2 solution. The
negative solvatochromism of Zn-PF6 indicates that there are
two competitive intraligand charge transfer (ILCT) excited
states in these Zn(II) complexes and that the electrostatic
interaction between Zn2+ and counterions plays a crucial role in
the regulation of these two competitive ILCT excited states. To
control the electrostatic interaction between counterions and
Zn2+, thin solid films with tunable emission colors of blue, gray,
white, and yellow have been achieved by doping different
concentrations (0.2 wt %, 0.4, 0.6, and 3.0 wt %, respectively)
of Zn-PF6 into poly(ethylene glycol)-block-poly(propylene
glycol)-block-poly(ethylene glycol) (PEG−PPG-PEG). The
quantum efficiencies of these films were measured to be 67%,
56%, 41%, and 9% at the excitation wavelength of 365 nm.
Furthermore, multicolor light-emitting devices were success-
fully fabricated by coating these polymer films on commercially
available ultraviolet light-emitting diodes (LEDs). More
interestingly, it was found that the electric field can also induce
the variation of excited states by changing the electrostatic
interaction between counterions and Zn2+, resulting in the
tuning of luminescence colors. On the basis of this interesting
electrochromic luminescence behavior, a quasi-solid data
recording device has been fabricated by using the distinguish-
able luminescence color change of Zn-PF6 from orange to sky
blue under an electric field. Furthermore, by utilizing the
favorably large two-photon absorption (TPA) cross-section of
Zn-PF6, for the first time, data encryption and decryption were
achieved by the two-photon excitation technique.
−
−
−
Figure 2. (a) Absorption spectra and (b) room-temperature
normalized photoluminescence spectra of all Zn(II) complexes in
CH2Cl2 (10 μM). (c) Schematic illustration of the influence of
counterions on the HOMO and LUMO energy levels of Zn(II)
complexes. Insert: photographs of the emission colors of these
complexes in CH2Cl2 (10 μM).
peaks of 1a−1d are located at 386, 395, 398, and 398 nm,
respectively (Figure S2). For these model complexes, the
lowest unoccupied molecular orbital (LUMO) level is related
to the charge density of the zinc center because the ligands are
coordinated with Zn2+. It is known that the net charge quantity
of the zinc center of the complex increases with decreasing the
basicity of counterions.25 For Zn-PF6, the zinc center may have
−
more net charge due to the weak basicity of PF6 . The ionic
charge introduced by Zn2+ upon coordination to the ligand
makes the terpyridine unit a stronger electron-withdrawing
group, which is beneficial to stabilize the LUMO level. On the
contrary, when CH3COO− with strong basicity was used as a
counterion, the interaction between Zn2+ and CH3COO− is
more covalent in nature, leading to less net charge of the zinc
center and decreased electron-withdrawing ability of the
terpyridine unit. On the other hand, the highest occupied
molecular orbital (HOMO) level is barely affected because it is
located on the triphenylamine group. Next, the HOMO levels
RESULTS AND DISCUSSION
■
Synthesis and Characterization. The synthesis of ligand
4-([2,2′:6′,2″-terpyridin]-4′-yl)-N,N-diphenylaniline (Tpypa)
was carried out as published previously.24 The Zn(II)
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of Zn(II) complexes were determined by the electrochemical
measurements, and the energy gaps between HOMO and
LUMO were calculated from UV−visible absorption spectra.
Thus, the LUMO levels can be obtained. When the counterion
the solvent, the electrostatic interaction between counterions
and Zn2+ decreases because the polar solvent molecules
separate counterions out from Zn2+. With increasing the
polarity of the solvent, the electrostatic interaction between
Zn2+ and counterions decreases because the polar solvent
molecules separated counterions out from Zn2+. Besides, the PL
spectra of Zn-PF6 in CH3CN at different concentrations were
investigated (Figure S9). As the concentration decreases, the
shoulder peak at the long emission wavelength gradually
disappears, and the emission peak experiences a blue shift in
wavelength. Furthermore, the PL spectral change of Zn-PF6 in
DMF solution by the addition of Bu4N+PF6− was recorded. It is
expected that the long wavelength emission will increase in
intensity. The emission intensity at the long wavelength was
only slightly enhanced after the addition of Bu4N+PF6− (0−200
equiv) (Figure S11), which might be because the polarity of
DMF is too high. Then, the less polar CH2Cl2/DMF mixture
(v/v, 9/1) was used. The emission intensity at the long
wavelength was increased more obviously by the addition of
was changed from PF6 through ClO4 , BF4 to CH3COO−,
the LUMO levels were destabilized. Thus, the emission
wavelength exhibits a blue shift due to the destabilization of
the LUMO level induced by the increase of the basicity of
counterions (Figure 2c). In addition, the photoluminescence
(PL) spectra of these Zn(II) complexes in the solid state were
studied, and their emission peaks were located at 560 nm, 604
nm, 601 and 606 nm, respectively (Figure S5).
−
−
−
To further study how the counterions influence the
photophysical properties of Zn(II) complexes, the PL spectra
of Zn-PF6 were recorded in different solvents. Interestingly, the
emission band shows a hypochromic shift from 622 to 489 nm
with increasing solvent polarity from CH2Cl2 to DMF (Figure
3b and Table S4). To trace the detailed change of solvent-
Bu4N+PF6 (0−200 equiv) (Figure S12). The fact that the
−
increase in intensity of the long wavelength emission is not very
significant might be attributed to the electrostatic interaction
−
present between Bu4N+ and PF6 , which limits the influence of
the added PF6− on Zn2+. These observations confirmed that the
electrostatic interaction between Zn2+ and counterions indeed
plays a key role in regulating the excited state of these Zn(II)
complexes (Figure 4).
Figure 3. (a) Absorption spectra and (b) room-temperature
normalized photoluminescence spectra of complex Zn-PF6 (10 μM)
in CH2Cl2, CHCl3, CH3CN, DMSO, and DMF. (c) PL spectra of
complex Zn-PF6 in CH2Cl2−DMF mixtures (10 μM) with different
DMF fractions. (d) Photos of the emission colors of Zn-PF6 (10 μM)
in CH2Cl2, CHCl3, CH3CN, DMSO, and DMF.
Figure 4. Schematic illustration of the change of the lowest-energy
excited state of Zn(II) complex by pulling the counterion out from the
Zn(II) complex. d represents the distance between Zn2+ and
counterions.
dependent emission, the PL spectrum of Zn-PF6 was recorded
in CH2Cl2 with increasing the DMF portion (Figure 3c). It was
observed obviously that the long-wavelength emission (622
nm) was gradually quenched but a new emission (518 nm)
emerged. The excitation spectra monitored at 500 nm increase
in intensity significantly with the increase of DMF portion,
while the excitation spectra at 622 nm gradually decrease in
intensity and the excitation peak wavelength experiences a blue
shift (Figure S8). These observations suggest that there are two
competitive intraligand charge transfer excited states (ILCT1
and ILCT2) in these Zn(II) complexes, where ILCT1 and
ILCT2 states originated from the terpyridine unit with strong or
weak electron-withdrawing ability of the ligand. The electro-
static interaction between Zn2+ and counterions may provide a
stabilizing force for the ILCT2 excited state of Zn(II)
complexes. As we know, the greater the polarity of a solvent,
the smaller the electrostatic interaction between ions of
opposite charges in the solvent. In a less polar solvent, the
electrostatic interaction is believed to be stronger due to the
poorer polarity of the solvent. With increasing the polarity of
Fabrication of a White Light-Emitting Device.
Compared to traditional mercury-containing fluorescent
lamps, white light-emitting devices possess numerous advan-
tages since they are environment-friendly, safe, energy saving,
etc.26−28 Thus, tremendous efforts have been made to search
for white light-emitting materials. However, most of them rely
on the combination of different emitters with complementary
colors, which may lead to problems such as color aging or
specific requirements for device fabrication.28 Therefore, the
development of a single-molecule that can directly emit white
light is of great interest and importance. The insight we have
gained into the way of regulating two competitive ILCT excited
states of these Zn(II) complexes offers us an opportunity to
achieve white light emission by a single emitter. By blending the
Zn(II) complexes with a suitable concentration into the
polymer film with an appropriate polarity to regulate the
electrostatic interaction between Zn2+ and counterions, white
light emission is anticipated to be observed. Zn-PF6 was
selected as a representative compound to perform the following
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−
experiments because the basicity of PF6 is weaker than those
−
−
of ClO4 , BF4 , and CH3COO−, indicating that it can be
separated out from Zn2+ more easily by an external change.
When the Zn-PF6 content in poly(ethylene glycol)-block-
poly(propylene glycol)-block-poly(ethylene glycol) (PEG−
PPG-PEG) was increased from 0.2 to 3.0 wt %, the emission
colors of blue, gray, white, and yellow were obtained (Figure 5a
Figure 6. (a) Photographs of electrochromic luminescence of Zn-PF6
(20 μM) in CH2Cl2. (b) Emission spectra of Zn-PF6 before and after
applying a voltage in CH2Cl2. (c) Constructed rewritable information
recording device based on electrochromic luminescence.
Figure 5. (a) Normalized PL spectra of Zn-PF6 with increasing
concentrations of Zn-PF6 in PEG−PPG-PEG films. (b) CIE-1931
chromaticity diagram for Zn-PF6 in PEG−PPG-PEG films at different
concentrations. (c) Emission photograph of Zn-PF6 in polyether films
at different concentrations under UV lamp (excitation at 365 nm). (d)
Multicolor LEDs fabricated by coating a PEG−PPG-PEG film of Zn-
PF6 on the surface of a 5 mm reference ultraviolet LED (365−370 nm
emission, commercially available.): (1) illuminating reference UV
LED, the LED coated with a polymer film of (2) 0.2 wt %, (3) 0.6 wt
%, and (4) 3.0 wt % Zn-PF6.
−
show that PF6 near the cathode is migrated to the anode
through the electric field (Figure S15). When PF6− is separated
from Zn-PF6 near the cathode, the lowest-energy excited state
is altered to the ILCT1 state due to the destabilization of the
ILCT2 state (Figure 4). Thus, a fluorescence color change from
orange to sky blue was observed near the cathode. The same
phenomenon was observed for Zn-BF4 and Zn-ClO4. However,
for Zn-AcO, because the electrostatic interaction between the
complex and CH3COO− is more covalent in nature, the applied
voltage under our experimental conditions has a difficult time
extracting CH3COO− from Zn-AcO. Therefore, the electric-
field induced luminescence color change is not obvious (Figure
6a). These results further demonstrate that two competitive
ILCT excited states of Zn(II) complexes can be regulated by
controlling the electrostatic interaction of counterions with
Zn2+.
and c). Especially, a distinct white light emission was observed
by doping 0.6 wt % Zn-PF6 into PEG−PPG-PEG, and the
corresponding CIE coordinates are (0.29, 0.34), which is close
to the pure white light (Figure 5b). Thus, the CIE coordinates
can be easily tuned by adjusting the concentration of the
dopant Zn-PF6 in a polymer host. The PL quantum efficiencies
of these films were measured at room temperature. The values
of 67%, 56%, 41%, and 9% were achieved at the excitation
wavelength of 365 nm, which show a remarkable decrease with
increasing the Zn-PF6 content. Such high quantum efficiencies
in the solid state inspired us to explore its practical application
in lighting. Therefore, multicolor light-emitting devices utilizing
Zn-PF6 were successfully fabricated. By coating a polymer film
of 0.6 wt % Zn-PF6 on a commercially available ultraviolet
LEDs, a bright white light was observed (Figure 5d), and the
bright blue and yellow light-emitting devices were also available
by simply changing the Zn-PF6 content in the polymer film.
Electrochromic Luminescence. Next, we investigated
whether the electric field can manipulate the electrostatic
interaction between Zn2+ and counterions to realize the
electrochromic luminescence (ECL). Two Pt electrodes were
immersed in a CH2Cl2 solution of Zn-PF6. From Figure 6a, we
can see that without applying a voltage, the solution emits a
bright orange-red fluorescence. Upon applying a voltage of 10
V for a few seconds, the luminescence color near the cathode
immediately shows a drastic blue shift, and the fluorescence
color of the solution is changed to sky blue (Figure 6b). By
contrast, the applied electric field cannot change the
Information Recording Device Based on Electro-
chromic Luminescence. On the basis of the interesting
electrochromic luminescence behavior, we have successfully
demonstrated the use of these materials as an optical recording
medium. The quasi-solid film was fabricated by coating Zn-PF6
with electrochemically and thermally stable ionic liquid 1-butyl-
3-methylimidazolium hexafluorophosphate (BMIM+PF6 ) and
−
SiO2 nanoparticles on an indium−tin oxide (ITO) electrode
plate. When the movable needle Pt electrode pen with a voltage
of 4 V was in contact with the surface of the quasi-solid film, the
electric-field induced migration of PF6− can cause the switch of
excited state from ILCT2 to ILCT1. Thus, a sky blue
luminescence spot immediately appeared at the contact point
of the film. With the movement of the needle electrode pen, the
trace can be recorded on the surface of device as a
distinguishable sky blue fluorescence pattern. As a proof-of-
concept demonstration, the characters “HK” were written on
the surface of a quasi-solid film with a needle electrode pen
(Figure 6c). Interestingly, the recorded information can be
erased by reversing the voltage direction on the needle
electrode pen. As described in Figure 6c, the characters “HK”
were completely erased, and new letters “NJ” were recorded.
This “write-erase” process can also be repeated. Therefore, the
electrically manipulated information recording and erasing were
achieved by utilizing the electric field to regulate the excited
state of Zn-PF6.
1
fluorescence color near the anode. From the H NMR and
mass spectral data near the two electrodes, we can see that the
structure of Zn-PF6 remains intact under our experimental
conditions (Figures S13 and S14). 19F NMR spectra clearly
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Figure 7. (a) TPA cross-sections of complex Zn-PF6 and BODIPY. (b) Two-photon fluorescence spectra of Zn-PF6 in DMF and CH2Cl2, and
BODPIY in CH2Cl2 (10 μM) at 700 nm. (c) Schematic diagram of the information encryption and decryption processes. (d) Confocal microscopy
images collected at different excitation wavelengths for information decryption.
Information Decryption via Two-Photon Excitation
Imaging. Information security protection plays a significant
role in our daily life and has attracted widespread research
interest.29,30 Here, a new information protection strategy has
been developed to achieve this goal. The TPA cross-section of
Zn-PF6 was measured to be 298 GM at 950 nm in CH2Cl2
solution and 43.5 GM at 700 nm in DMF solution (Figure 7a),
indicating that the peak TPA cross-sections for the two
emissive excited states of Zn-PF6 are located at different
excitation wavelengths. In addition, their TPA cross-sections
are larger than some commonly used organic dyes, such as
BODIPY (Table S8). All of these features offer us the
opportunity to develop a new strategy for information security
protection. Figure 7b depicts the schematic illustration of the
process of information encryption and decryption. A character
was recorded with Zn-PF6 and encrypted by the strongly
emissive BODIPY, and thus this information was unreadable
under a normal UV lamp. A confocal microscope was employed
to acquire the concealed information. With the laser excitation
at 405 nm, the luminescence image shows a noise signal due to
severe interference fluorescence (Figure 7d).
By contrast, a blank luminescence image was acquired under
excitation at 635 nm because all luminophores cannot be
excited at this wavelength. Next, near-infrared light was applied
as the excitation source. With excitation at the wavelength
range from 700 to 740 nm, a clear character “W” was revealed
in the Figure as a result of the different emissive states of Zn-
PF6 and BODIPY having a distinct TPA cross-section of 43.5,
4.3, and 4.9 GM, respectively. On the other hand, only a noise
signal was detected when excitation was done with a longer
wavelength (such as 950 nm). In this way, only authorized
individuals who know the correct decryption strategy and the
exact excitation wavelength range can access the protected
information. This new approach shows numerous advantages,
including simple construction and quick readout as compared
to the conventional techniques, such as mass spectra and
Raman spectra.31,32 In addition, since only an excitation within
a narrow wavelength range (700−740 nm) can access the
protected information, it exhibits a higher security than the
recently developed time-resolved photoluminescence techni-
que.10
CONCLUSIONS
■
In this study, we demonstrated that the emission colors of
luminophores can be tuned by regulating the electrostatic
interactions. Specifically, luminescence color tuning of Zn(II)-
bis(terpyridine) complexes has been realized by the variation of
counterions (CH3COO−, BF4 , ClO4 , and PF6 ). In addition,
the excited state of the Zn(II) complex can be controlled by the
regulation of electrostatic interaction between Zn2+ and
counterions. On the basis of this controllable excited state, by
simply doping different concentrations (0.2 wt %, 0.4, 0.6, and
3.0 wt %, respectively) of Zn-PF6 into PEG−PPG-PEG, the
emission colors of blue, gray, white, and yellow were obtained.
Furthermore, a white light-emitting device was successfully
fabricated by coating the polymer films on commercially
available ultraviolet LEDs. Moreover, a rewritable information
recording device has been designed and constructed based on
the interesting electrochromic luminescence behavior of Zn-
PF6. More importantly, a new information decryption strategy
that utilized the favorably large TPA cross-section of Zn-PF6
was achieved by the TPA excitation technique.
−
−
−
In summary, the controllable excited-state property of Zn(II)
complexes by regulating electrostatic interaction makes them a
perfect candidate for the promising applications in various
optoelectronic fields, including light-emitting device, data
recording, security protection, information display, etc. It is
expected that this study will stimulate further work on the
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were added. After the dark pink solution had been stirred at 25 °C for
12 h, the precipitate was isolated by filtration and washed with EtOH.
Purification was accomplished readily by recrystallization from ethanol
development of novel stimuli-responsive photofunctional
materials for optoelectronic applications.
1
(65% yield). H NMR (400 MHz, CDCl3) δ = 8.73−8.71 (m, J = 8.8
EXPERIMENTAL SECTION
Hz, 4H), 8.68 (d, J = 8.0 Hz, 2H), 7.90 (t, J = 15.6 Hz, 2H), 7.81 (d, J
= 8.4, 2H), 7.37 (t, J = 12 Hz, 2H), 7.32 (t, J = 16 Hz, 4H), 7.19 (t, J =
15.2 Hz, 6H), 7.09−7.05 (m, 2H).
■
Materials. Unless otherwise stated, all starting materials and
reagents were purchased from commercial suppliers and used without
further purification. All solvents were purified before use. The solvents
were carefully dried and distilled from appropriate drying agents prior
to use.
Synthesis of Zn(II) Complexes. General procedure for the
synthesis of Zn(II)-bis(terpyridine) complexes [Zn(L)2]2+(X)− : A
2
solution of Zn(CH3COO)2·2H2O (0.5 mmol) in absolute methanol
(30.0 mL) was added to a solution of the terpyridine ligand (1.0
mmol) in absolute methanol (100.0 mL). After refluxing for 12 h, the
mixture was evaporated and the residue washed successively with
methanol (3 × 20 mL), water (2 × 20 mL), and diethyl ether (2 × 20
mL). Further purification of the crude product was achieved by
recrystallization from acetonitrile/diethyl ether. Complexes Zn-BF4,
Zn-ClO4, and Zn-PF6 were synthesized by anion exchange with
NaBF4, NaClO4, or KPF6 in CH3CN solution.
Measurements. NMR spectra were recorded on a Bruker Ultra
Shield Plus 400 MHz NMR instrument (1H, 400 MHz; 19F, 377
MHz). Mass spectra were obtained on a Bruker autoflex matrix-
assisted laser desorption ionization time-of-flight (MALDI-TOF/
TOF) mass spectrometer. UV−visible absorption spectra were
recorded with a HP UV-8453 spectrophotometer. Photoluminescent
spectra were measured with an Edinburgh Instrument FLS920
combined fluorescence lifetime and steady state spectrophotometer
that was equipped with a red-sensitive single-photon counting
photomultiplier in Peltier Cooled Housing. The fluorescence quantum
efficiency (Φf) of the samples was estimated using Rhodamine B (Φf =
65% in ethanol) as the standard, and Φf of the solid state was
measured on a FLS920 steady state and calibrated integrating sphere
system from Edinburgh Instrument. X-ray diffraction data were
collected at 293 K using graphite-monochromated Mo−Kα radiation
(λ = 0.71073 Å) on a Bruker APEX DUO diffractometer. The
collected frames were processed with the software SAINT, and an
absorption correction (SADABS) was applied to the collected
reflections. The structure was solved by the direct methods
(SHELXTL) in conjunction with standard difference Fourier
techniques and subsequently refined by full-matrix least-squares
analyses on F2. Two-photon absorption measurements were
performed using a mode-locked Ti:Sapphire laser (Chameleon,
Coherent Co.) with ∼130 fs pulse duration. The emission was
collected into a monochromator (Acton SP2500, PI Co.) coupled with
a Princeton Instrument SPEC-10:400B LN/CCD. The two-photon
imaging setup was integrated with an Olympus IX81 laser scanning
confocal microscope. The device with the written “W” was placed on
the platform, and a 2 mm × 2 mm sample area consisting of 200 × 200
pixels was scanned with an acquisition rate of 2 ms/pixel.
Theoretical Calculations. The calculations were carried out with
the Gaussian 09 software package.33 The optimizations of complex
structures were performed using B3LYP density functional theory. On
the basis of the ground- and excited-state optimization, the time-
dependent density functional theory (TDDFT) approach was applied
to investigate the excited-state electronic properties. The LANL2DZ
basis set was used to treat the zinc atom, whereas the 6-31G* basis set
was used to treat all other atoms. The contours of the HOMOs and
LUMOs were plotted.
TDDFT calculations were performed on these Zn(II) complexes to
explore the nature of their excited states. Figure S3 illustrates that the
HOMO is located primarily on the triphenylamine group, while the
LUMO and LUMO+4 mainly reside on the terpyridine moiety. For
them, the S1 is originated from the HOMO → LUMO (68%) and
HOMO → LUMO+4 (13%) transitions. These calculation results
demonstrate that intraligand charge transfer transitions participate in
the excited states of these Zn(II) complexes.
Zn-AcO. 1H NMR (400 MHz, CDCl3) δ = 9.04 (d, J = 8.8 Hz, 4H),
8.28 (s, 4H), 8.23−8.21 (d, J = 8.0 Hz, 4H), 8.03 (t, J = 15.6 Hz, 4H),
7.61−7.57 (m, 8H), 7.36 (t, J = 12 Hz, 8H), 7.20−7.18 (m, 12H), 7.16
(t, J = 15.2 Hz, 4H), 1.89 (s, 6H). 13C NMR (100 MHz, CDCl3) δ =
178.04, 139.21, 129.64, 128.06, 126.36, 125.17, 124.34, 122.91, 122.14,
121.83, 120.42, 118.19. MS (MALDI-TOF) [m/z]: 508.16 [M−
2CH3COO]2+. Elemental analysis (calcd, found for C70H54N8O4Zn):
C (73.90, 74.14), H (4.75, 4.93), N (9.85, 9.54).
Zn-BF4. 1H NMR (400 MHz, CD3CN) δ = 8.89 (s, 4H), 8.70 (d, J
= 8.4 Hz, 4H), 8.16 (t, J = 14.4 Hz, 4H), 8.10 (d, J = 8.8 Hz, 4H), 7.81
(d, J = 4.8 Hz, 4H), 7.45 (t, J = 15.6 Hz, 8H), 7.39 (t, J = 12.4 Hz,
4H), 7.25−7.21 (m, 16H). 13C NMR (100 MHz, CD3CN) δ = 155.03,
150.71, 149.30, 147.69, 147.58, 146.33, 140.78, 129.51, 128.71, 127.15,
129.99, 125.60, 124.56, 122.72, 120.59, 119.76. MS (MALDI-TOF)
[m/z]: 508.14 [M−2BF4]2+. Elemental analysis (calcd, found for
C66H48B2F8N8Zn): C (66.43, 66.71), H (4.03, 4.35), N (9.39, 9.24).
Zn-ClO4. 1H NMR (400 MHz, CD3CN) δ = 8.89 (s, 4H), 8.70 (d, J
= 8.0 Hz, 4H), 8.15−8.08 (m, 8H), 7.81 (d, J = 4.8 Hz, 4H), 7.44 (t, J
= 15.6 Hz, 8H), 7.38 (t, J = 12.4 Hz, 4H), 7.24−7.19 (m, 16H). 13C
NMR (100 MHz, CD3CN) δ = 155.08, 150.73, 149.33, 147.77, 147.59,
146.36, 140.81, 129.54, 128.72, 127.17, 127.02, 125.61, 124.56, 122.73,
120.60, 119.76. MS (MALDI-TOF) [m/z]: 508.16 [M−2ClO4]2+.
Elemental analysis (calcd, found for C66H48Cl2N8O8Zn): C (65.05,
65.24), H (3.94, 3.93), N (9.20, 9.24).
Zn-PF6. 1H NMR (400 MHz, CD3CN) δ = 8.90 (s, 4H), 8.70 (d, J
= 8.0 Hz, 4H), 8.17 (t, J = 16.8 Hz, 4H), 8.11 (d, J = 8.8 Hz, 4H), 7.83
(d, J = 4.4 Hz, 4H), 7.46 (t, J = 16 Hz, 8H), 7.40 (t, J = 12.4 Hz, 4H),
7.26−7.22 (m, 16H). 13C NMR (100 MHz, CD3CN) δ = 155.15,
150.81, 149.38, 147.79, 147.62, 146.39, 140.85, 129.56, 128.74, 127.22,
127.21, 125.66, 124.58, 122.74, 120.61, 119.77. MS (MALDI-TOF)
[m/z]: 508.33 [M−2PF6]2+. Elemental analysis (calcd, found for
C66H48F12N8P2Zn): C (60.53, 60.74), H (3.67, 3.93), N (8.56, 8.24).
1
1a. H NMR (400 MHz, DMSO-d6): δ = 8.92 (s, 4H), 8.82−8.78
(m, 8H), 8.20 (t, J = 15.6 Hz, 4H), 8.12 (d, J = 7.8 Hz, 4H), 7.75 (t, J
= 12.4 Hz, 4H), 7.39 (d, J = 8.0 Hz, 4H), 2.41 (s, 6H), 1.65 (s, 6H).
MS (MALDI-TOF) [m/z]: 355.13 [M−2CH3COO]2+.
1b. 1H NMR (400 MHz, DMSO-d6): δ = 9.37 (s, 4H), 9.16 (d, J =
8.0, 4H), 8.38 (d, J = 8.0, 4H), 8.28 (t, J = 15.2 Hz, 4H), 7.95 (d, J =
4.8 Hz, 4H), 7.59 (d, J = 8.0 Hz, 4H), 7.49 (t, J = 12.8 Hz, 4H), 2.49
(s, 6H). MS (MALDI-TOF) [m/z]: 355.14 [M−2BF4]2+.
Synthesis of 4-(Diphenylamino)benzaldehyde. POCl3 (2.0
mL) was dropped slowly into DMF (6.0 mL) at 0 °C and stirred for
30 min at room temperature. To the above solution was added a
CH2Cl2 solution of triphenylamine (3.8 g, 15.4 mmol). After the
mixture was refluxed overnight, the solution was poured into ice water
and then extracted by CH2Cl2. The product was obtained by
recrystallization in ethanol (95% yield). 1H NMR (400 MHz,
CDCl3) δ = 9.79 (s, 1H), 7.67 (d, J = 8.9, 2H), 7.18−7.15 (m,
6H), 7.36 (t, J = 7.8, 4H), 7.00 (d, J = 8.9, 2H).
1c. 1H NMR (400 MHz, DMSO-d6): δ = 9.36 (s, 4H), 9.16 (d, J =
8.0, 4H), 8.38 (d, J = 8.0, 4H), 8.27 (t, J = 15.2 Hz, 4H), 7.95 (d, J =
4.8 Hz, 4H), 7.59 (d, J = 8.0 Hz, 4H), 7.48 (t, J = 12.8 Hz, 4H), 2.49
(s, 6H). MS (MALDI-TOF) [m/z]: 355.14 [M−2ClO4]2+.
1d. 1H NMR (400 MHz, DMSO-d6): δ = 9.37 (s, 4H), 9.16 (d, J =
8.0, 4H), 8.38 (d, J = 8.0, 4H), 8.27 (t, J = 15.2 Hz, 4H), 7.95 (d, J =
4.8 Hz, 4H), 7.59 (d, J = 8.0 Hz, 4H), 7.48 (t, J = 12.8 Hz, 4H), 2.49
(s, 6H). MS (MALDI-TOF) [m/z]: 355.16 [M−2PF6]2+.
CCDC 1437630, CCDC 1437631, and CCDC 1437632 contain the
crystallographic data for this article. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via www.
ccdc.cam.ac.uk/data_request/cif.
Synthesis of 4-([2,2′:6′,2″-Terpyridin]-4′-yl)-N,N-diphenyla-
niline. To a stirred mixture of 4-(diphenylamino)benzaldehyde (1.0 g,
3.41 mmol) and 2-acetylpyridine (1.73 g, 14.31 mmol) in 15 mL of
EtOH, KOH powder (0.57 g, 14.1 mmol), and ammonia (5.0 mL)
F
DOI: 10.1021/acs.inorgchem.6b02319
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
From Phosphorescent Cationic OrganoPlatinum(II) Complexes in
Water. Angew. Chem., Int. Ed. 2009, 48, 7621−7625.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.6b02319.
■
S
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Chemical structures of model Zn(II) complexes; UV−
visible spectrum, photoluminescence spectrum; cyclic
1
voltammograms; H NMR spectrum; and two-photon
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AUTHOR INFORMATION
Corresponding Authors
*(Q.Z.) E-mail: iamqzhao@njupt.edu.cn.
*(W.-Y.W.) E-mail: rwywong@hkbu.edu.hk; wai-yeung.wong@
polyu.edu.hk
■
ORCID
Yi Zeng: 0000-0003-0694-1795
Wei Huang: 0000-0001-7004-6408
Wai-Yeung Wong: 0000-0002-9949-7525
(14) Ma, D. X.; Qiu, Y.; Duan, L. New Insights into Tunable
Volatility of Ionic Materials through Counter-Ion Control. Adv. Funct.
Mater. 2016, 26, 3438−3445.
Notes
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The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank Hong Kong Research Grants Council
(HKBU12304715), Areas of Excellence Scheme of HKSAR
(AoE/P-03/08), the Hong Kong Polytechnic University,
National Program for Support of Top-Notch Young Profes-
sionals, National Natural Science Foundation of China
(61274018), Natural Science Foundation of Jiangsu Province
of China (BK20160885), and Hong Kong Baptist University
(FRG1/14-15/084) for financial support.
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