Luminescence Color Tuning by Regulating Electrostatic Interaction in

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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.

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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 metalto-metal interaction, 7−9 and hydrogen-bonding interaction,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 biomarkers,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 counterions,21,22 probably because of the formation of more stable © XXXX American Chemical Society

coordination bonds for these complexes. By contrast, coordination bonds are usually more flexible for transitionmetal 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 Received: September 23, 2016

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DOI: 10.1021/acs.inorgchem.6b02319 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

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 obtained complexes were characterized by 1H and 13C NMR spectroscopy, elemental analysis, MALDI-TOF mass spectrometry, and X-ray crystal structure analysis. Single-crystals of ZnBF4, 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

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.

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 successfully 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 distinguishable 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

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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) B

DOI: 10.1021/acs.inorgchem.6b02319 Inorg. Chem. XXXX, XXX, XXX−XXX

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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 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).

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 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-

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 electrostatic 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 advantages 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 C

DOI: 10.1021/acs.inorgchem.6b02319 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry 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)-blockpoly(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 ZnPF6 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 electricfield 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+. Information Recording Device Based on Electrochromic 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-butyl3-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-ofconcept 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.

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 fluorescence color near the anode. From the 1H 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 D

DOI: 10.1021/acs.inorgchem.6b02319 Inorg. Chem. XXXX, XXX, XXX−XXX

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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 ZnPF6 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 technique.10

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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 ZnPF6. 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 E

DOI: 10.1021/acs.inorgchem.6b02319 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

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 (65% yield). 1H NMR (400 MHz, CDCl3) δ = 8.73−8.71 (m, J = 8.8 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). Synthesis of Zn(II) Complexes. General procedure for the synthesis of Zn(II)-bis(terpyridine) complexes [Zn(L)2]2+(X)−2: A 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. 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). 1a. 1H 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+. 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.

development of novel stimuli-responsive photofunctional materials for optoelectronic applications.

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EXPERIMENTAL SECTION

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. 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 matrixassisted 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 F 2. 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 timedependent 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. 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). Synthesis of 4-([2,2′:6′,2″-Terpyridin]-4′-yl)-N,N-diphenylaniline. 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

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Inorganic Chemistry

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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.inorgchem.6b02319. Chemical structures of model Zn(II) complexes; UV− visible spectrum, photoluminescence spectrum; cyclic voltammograms; 1H NMR spectrum; and two-photon absorption cross-sections of complex Zn-PF6 (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*(Q.Z.) E-mail: [email protected]. *(W.-Y.W.) E-mail: [email protected]; 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 Notes

The authors declare no competing financial interest.

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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 Professionals, 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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REFERENCES

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DOI: 10.1021/acs.inorgchem.6b02319 Inorg. Chem. XXXX, XXX, XXX−XXX