Synthesis of tetranitro-oxacalix[4]arene with oligoheteroacene groups and its nonvolatile ternary memory performance

Abstract

To achieve ultra-high density memory devices with a capacity of 3ⁿ or larger, a novel larger and stable oxacalix[4]arene, 4N4OPz, is reported. 4N4OPz exhibited excellent ternary memory behavior with high ON2/ON1/OFF current ratios of 10^8.7/10^4.2/1, low switching threshold voltage of −1.80 V/−2.87 V, and good stability for these three states.

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Many applications of organic electronic devices are associated with memory devices.¹ The communication between equipment and its surroundings needs the devices to send and receive stored information frequently. Thus, it is necessary to develop new materials for fast, non-volatile, inexpensive, reliable and high-density data storage.² Organic memory devices have received a lot of scientific interest not only because of their recent remarkable progress but also because of their unique advantages: light weight, printability and flexibility.³ Up to now, most of the research in this field has been conducted on materials with binary storage performance, which do not meet the future storage requirements (ultra-high density: 3ⁿ or larger).⁴ Thus, developing ultra-high density memory devices is urgent. Recently, a handful of organic materials with multilevel stable states have already been demonstrated to show an increasing capacity of 3ⁿ or larger.⁵ Although “0”, “1”, and “2” tristable states have been realized in these organic systems, such examples are still rare and it is still highly desirable to develop new organic compounds with a reliable storage capacity of 3ⁿ or larger.

Calix[n]arenes have received much attention over the last three decades in supramolecular chemistry due to their specific molecular structure, which allows the formation of numerous host–guest complexes.⁶ Replacing the bridging carbon atoms with heteroatoms can result in new candidates with intriguing physical and chemical properties. As a matter of fact, oxacalixarenes are an important class of heterocalixarenes, formed by replacing the bridging carbon atoms of calixarenes with oxygen atoms.⁷ However, these materials are still in the synthetic stage and their properties remain largely unexplored, especially for organic electronic devices, which might be due to their poor electronic properties. In our research, we believe that new calix[n]arenes with interesting electronic properties could be achieved if catechol groups were replaced by oligoacenes/oligoheteroacenes, because they have been widely used as active layers in organic semiconductor devices such as organic field-effect transistors, organic light emitting diodes, organic solar cells and even memory devices.^8–10

In this report, we are interested in the larger oxacalix[4]arene with two phenazine groups and four nitro groups because multilevel oxidation states could be stabilized by heteroatoms, and these states are very important to achieve 3ⁿ or larger data storage capacity. Herein, a novel larger oxacalix[4]arene (4,6,25,27-tetranitro-2,8,23,29-tetraoxacalix[4]-36,37-bis(decyloxy)phenazine, abbreviated as 4N4OPz), which has two different types of electron-withdrawing groups (nitro and pyrazine), has been successfully synthesized and characterized. We believe that 4N4OPz should have the following advantages: (1) the introduction of O atoms could make 4N4OPz more stable both in the ground state and oxidation state; (2) the nitro group has been introduced in electroactive molecules for applications in memory devices;¹¹ and (3) the memory device based on 4N4OPz might exhibit multilevel stable conductivity states in response to the applied voltage because the electron-withdrawing abilities of the nitro and pyrazine groups are different.

The synthetic procedure for 4N4OPz is depicted in Scheme 1. The synthesis of the target molecule 4N4OPz was achieved in two steps. Firstly, the starting material 1,2-bis(decyloxy)-4,5-diaminobenzene was prepared according to the previous reports.¹² Then, the commercially available 2,5-dihydroxy-1,4-benzoquinone was reacted smoothly with 1,2-bis(decyloxy)-4,5-diaminobenzene in alcohol to afford 7,8-bis(decyloxy)phenazine-2,3-diol in 62% yield. Secondly, the as-prepared intermediate was used as the nucleophilic reagent, which condensed with 1,5-difluoro-2,4-dinitrobenzene to produce 4N4OPz in 12% yield.

Scheme 1 Synthetic route of compound 4N4OPz: (i) 1.1 equiv. 2,5-dihydroxy-1,4-benzoquinone, CH₃CH₂OH, N₂, reflux, 62%; (ii) 1 equiv. 1,5-difluoro-2,4-dinitrobenzene, 10 equiv. K₂CO₃, DMF, 80 °C, 12%.

Fig. 1a shows the normalized optical absorption and emission spectra of 4N4OPz in chloroform (CHCl₃) and in a thin film on a quartz substrate. The absorption spectrum of 4N4OPz exhibits two prominent bands at 276 nm and 434 nm in CHCl₃, which can be ascribed to a localized aromatic π–π* transition and intramolecular charge transfer, respectively. 4N4OPz emits strong green fluorescence with the maxima peak at 495 nm (λ_ex = 426 nm) in CHCl₃, and the fluorescent quantum yield is as high as 13%. Compared to the optical properties of 4N4OPz in solution, the absorption peaks at both short wavelength and long wavelength are blue-shifted. Note that the absorption edge of 4N4OPz extends to ∼478 nm in the film state (Fig. S7†), from which the band gap is estimated to be 2.59 eV. Although the emission wavelength of the as-prepared film is largely red-shifted (105 nm), the fluorescence density of 4N4OPz in film is significantly decreased, which might be due to the increasing interactions among the molecules in the film. These phenomena suggest H-aggregate formation in the film.¹³4N4OPz exhibits a very good thermal stability with an onset decomposition temperature of ∼369 °C (considering the 5% weight loss temperature, Fig. S6†). The excellent thermal properties of 4N4OPz are expected to meet the requirements of heat resistance in the electronics industry.

Fig. 1 Characterization of 4N4OPz. (a) Normalized optical absorption and emission spectra of 4N4OPz in CHCl₃ and the thin film on a quartz plate; the inset pictures were taken at room temperature under 365 nm UV light. (b) Cyclic voltammogram curve of 4N4OPz thin film on ITO glass in a 0.1 mol L⁻¹ solution of TBAPF₆ in acetonitrile solution. Scan rate: 100 mV s⁻¹.

The electrochemical properties of the 4N4OPz film on an indium–tin oxide (ITO) glass substrate were studied through cyclic voltammetry (CV) in a 0.1 mol L⁻¹ solution of tetrabutylammonium hexafluorophosphate (TBAPF₆) in anhydrous acetonitrile solution with a scan rate of 100 mV s⁻¹. As shown in Fig. 1b, 4N4OPz exhibits one reduction potential (−1.84 V) and two oxidation potentials (0.67 and 1.38 V), which correspond to the LUMO, HOMO and HOMO-1 energy levels of ∼ −2.56, −5.07 and −5.78 eV using the equation E_LUMO/HOMO = −e(4.40 + E^onset_red/oxd) eV.¹⁴ The calculated band gap using the CV data is 2.51 eV for 4N4OPz, which matches very well with the optical onset-edge band gap result (2.59 eV).

Fig. 2a shows the scheme of our prototype memory device, which is similar to most reported memory devices, with a sandwich structure using indium tin oxide (ITO) as the bottom electrodes, aluminium (Al, 120 nm thickness) as the top electrodes, and an organic layer of 4N4OPz molecules as an active layer. The 4N4OPz film thickness was ∼100 nm, as measured by SEM through a cross-section of the film (Fig. 2a). The atomic force microscopy (AFM) image (Fig. 2b) shows that the film is smooth without aggregation.

Fig. 2 Memory-device characteristics of 4N4OPz. (a) Scheme of the sandwich device and SEM image of a cross-section of the device. (b) Tapping-mode (5 μm × 5 μm) AFM topography and typical cross-section profile of an AFM topographic image of the 4N4OPz film on ITO substrate. (c and d) I–V characteristics of the memory devices (ITO/4N4OPz/Al and ITO/4N4OPz/LiF/Al, respectively) fabricated with 4N4OPz. (e and f) Stability of the memory devices (ITO/4N4OPz/Al and ITO/4N4OPz/LiF/Al, respectively) in three states under a constant “read” voltage of −1 V at 25 °C.

The current–voltage (I–V) characteristics of the device were measured by a Hachioji B1500A (Agilent Technologies) semiconductor parameter analyzer. Fig. 2c shows the typical I–V performance of the as-fabricated device. Starting with the low-conductivity state (OFF, “0”), the current increased slowly with the applied negative voltage sweep. However, a sharp transition from the OFF state to the intermediate-conductivity state (ON1, “1”) was observed at a switching threshold voltage (STV) of −1.80 V with the increased negative bias. When the negative bias was increased further, the current density increased abruptly to 10⁻² A (ON2, “2”) at −2.87 V (sweep 1). These OFF-to-ON1 and ON1-to-ON2 transitions can be regarded as a “writing” process. It remained in the ON2 state when the negative sweep was repeated (sweep 2) and for the reverse voltage sweep (sweep 3). These results suggested that once the device was switched to the ON2 state, the memory device cannot return to both the ON1 state and the OFF state after turning off the power. Another cell of the device was measured over a voltage range of 0 to −2.5 V (sweep 4) and showed one STV at −1.65 V, indicating the transition from the OFF state to the ON1 state. The device remained at the ON1 state in the next sweep from 0 to −2.5 V (sweep 5) and 0 to 2.5 V (sweep 6). This result indicated that once the cell reached the ON1 state, this state could be maintained even when the power was turned off. In sweep 7 from 0 to −4 V, the storage cell underwent a transition from the ON1 state to the ON2 state at −2.69 V. Once it was switched to the ON2 state, the memory device cannot return to both the ON1 state and OFF state (sweep 8–9). The distinctive OFF, ON1, and ON2 states (i.e., different responses to the external electric field) can be programmed to correspond to “0”, “1”, and “2” signals, respectively, suggesting the potential application of the device for ternary data storage. It should be worthy of note that the two STVs of our memory device are as low as 3 V, suggesting that the ternary memory device has low-power consumption and is a potential candidate for low-cost and high-performance memory chips in portable nanoelectronic devices. The low STVs of our memory device may be mainly attributed to the formation of H-aggregation, which is favorable for carrier transport. These three states of the ternary memory cell are distinct and the current ratio of the “OFF”, “ON1” and “ON2” states is 1 : 10^4.2 : 10^8.7. It is worth noting that the ON2/ON1 and ON1/OFF current ratios in the above device are as high as 10⁴, which is enough to promise a low misreading rate through the precise control of the ON2, ON1 and OFF states. This device exhibits typical write-once read-many-times (WORM) behaviour, which is similar to most of the reported “WORM” devices.^1e,4b The switching mechanisms could be further confirmed by inserting a LiF thin film as a buffer layer because we believe that holes may dominate the conduction process in the ITO/4N4OPz/Al devices, and here LiF can be used as a block layer to confirm the memory mechanism and performance.¹⁵ As shown in Fig. 2d, the behavior of ITO/4N4OPz/LiF/Al is similar to that of ITO/4N4OPz/Al. In the first sweep from 0 to −4 V, two sharp transitions from the low-conductivity (OFF, “0”) state to an intermediate-conductivity (ON1, “1”) state and to a high-conductivity (ON2, “2”) state were observed at STVs of −1.69 V and −2.61 V, respectively. The three states of the ternary memory cell are also distinct and the current ratio of the “OFF”, “ON1” and “ON2” states is 1 : 10^3.9 : 10^8.8. To further confirm the memory performance, we used metal Pt instead of the Al top electrode. As shown in Fig. S11,† the behavior of ITO/4N4OPz/Pt is similar to that of ITO/4N4OPz/Al.

Fig. 2e and f show the retention times and stress tests of the memory device for the OFF, ON1 and ON2 states. Under a constant stress of −1 V, no significant degradation in the current for the three different states could be observed for at least 40 000 s during the readout test. We also measured the retention times under a constant stress of high voltage or high temperature (50 °C), and there was no significant degradation in the current for the three different states (Fig. S9†). Moreover, the stimulus effect of continuous read pulses of −1 V on the three different states based on two devices (ITO/4N4OPz/Al and ITO/4N4OPz/LiF/Al) was also investigated. The inset in Fig. S10† shows the pulses (the pulse period and pulse width are 2 μs and 1 μs) used for the measurements. No current decay was observed after at least 10⁷ continuous read cycles. Therefore, the switching behavior of the remnant stored data and the nonvolatile nature of the memory device can explain the functionality of WORM-type memory characteristics.

To gain further insight into the electronic structure, theoretical calculations were performed using the B3LYP density functional theory (DFT) method with the 6-31G (d) basis set.¹⁶Fig. 3a shows that the HOMO (−6.07 eV) electrons were mainly on the central phenazine units while the LUMO (−2.87 eV) electrons were mainly distributed on the tetranitro group, which indicated intramolecular charge transfer from the HOMO to LUMO orbitals. Thus, the calculated HOMO–LUMO gap was 3.20 eV. From Fig. 3a, it can be observed that an open channel is formed from the molecular surface throughout the molecule backbone with continuous molecular electrostatic potential (ESP, red), where charge carriers can migrate. However, there are some negative electrostatic potential regions (black) caused by electron-acceptor groups, such as nitro and pyrazine groups. These negative regions can serve as “traps” to block the movement of charge carriers. When 4N4OPz obtained one electron, the HOMO and LUMO orbitals were greatly changed. As shown in Fig. 3b, the electron density distributions of the HOMO (−1.04 eV) are mainly located on the tetranitro group, while the LUMO (−0.37 eV) orbital is mainly distributed on the phenazine group. Moreover, the HOMO–LUMO gap was reduced to 0.67 eV and the negative regions at the tetranitro group were decreased. When 4N4OPz obtained two electrons, there were almost no changes to the HOMO (1.07 eV) and LUMO (1.27 eV) orbitals. The HOMO–LUMO gap was reduced by only 0.20 eV and the negative regions at the tetranitro group almost disappeared. If 4N4OPz obtained more electrons, the HOMO–LUMO gap might be close to 0 eV and the negative regions at the pyrazine group may also disappear. As a result, an open channel is formed from the molecular surface throughout the molecular backbone with continuous molecular ESP, where charge carriers can migrate. To gain insights into the switching mechanisms for the memory devices, an energy level diagram for the ITO/4N4OPz/Al device is shown in Fig. 4. The energy barrier (0.27 eV) between the work function of ITO and the HOMO of the active layer is much lower than the energy barrier (1.74 eV) between the work function of Al and the LUMO of the active layer. Thus, holes may dominate the conduction process in ITO/4N4OPz/Al devices. The hole injection barrier is only 0.27 eV, which indicates a low STV. Under a low negative voltage, the 4N4OPz thin film displays a low-conductivity (OFF) state and the current increases slowly because the energy barrier between the Al electrode and the LUMO of the active layer is as large as 1.74 eV, which also blocks the electron migration. Under a high bias, hole injection is easier due to the low hole injection barrier (0.27 eV), and 4N4OPz has better conductivity at the intermediate-conductivity (ON1) state. At the same time, the HOMO–LUMO gap was reduced and these negative regions at the tetranitro group almost disappeared. The lower band gap means better conductivity. However, two traps are not filled at the same time due to the different electron accepting abilities of the nitro and pyrazine groups: the trap of the tetranitro group is filled and the trap of the pyrazine group is partly filled, which could be attributed to the stronger electron accepting ability of the tetranitro group over the pyrazine group. With the increasing bias, the trap of the pyrazine group is eventually filled, which leads to the current transition from the ON1 state to the ON2 state. At the same time, the HOMO–LUMO gap may be reduced to ∼0 eV. Thus, it is easy to understand the high conductance of the 4N4OPz thin film at the ON2 state. Consequently, the device shows multilevel memory characteristics due to the two charge traps with different electron-withdrawing abilities. The trapped charge carriers were stabilized by intra- and inter-molecular charge transfer, forming a charge-separated state, and could not be easily de-trapped under a reverse bias, resulting in a high-conductivity state which is retainable for a long time.

Fig. 3 DFT molecular simulation results. (a–c) HOMO and LUMO of 4N4OPz, 4N4OPz⁻ and 4N4OPz²⁻ respectively.

Fig. 4 Energy level diagram of the HOMO, HOMO-1 and LUMO for 4N4OPz along with the work function of the electrodes.

In summary, we have successfully synthesized a novel larger and stable polycyclic aromatic compound, 4N4OPz, which has two different types of heteroatoms (O and N) and two different types of electron-withdrawing groups (nitro and pyrazine). The sandwich-structure memory devices based on 4N4OPz exhibited excellent ternary memory behavior with high ON2/ON1/OFF current ratios of 10^8.7/10^4.2/1, low switching threshold voltage of −1.80 V/−2.87 V, and good stability for these three states. The memory performance was confirmed by ITO/4N4OPz/LiF/Al and ITO/4N4OPz/Pt devices. The conduction mechanism through the ITO/4N4OPz/Al device shows multilevel memory characteristics due to the two charge traps with different electron-withdrawing ability of the tetranitro and pyrazine groups. We believe that our results could provide guidance for the design and synthesis of new heteroacenes, which could be used as promising candidates in nonvolatile memory devices.

Acknowledgements

The authors graciously thank the Chinese Natural Science Foundation (21371128 and 21336005) and Chinese-Singapore Joint Project (2012DFG41900). The work is also supported by the Singapore National Research Foundation through the Competitive Research Programme under Project no. NRF-CRP5-2009-04 (J. G. Y. Z. and C. X.). Q. Z. acknowledges the AcRF Tier 1 (RG 18/09) and Tier 2 (ARC 20/12) financial support from MOE, the CREATE program (Nanomaterials for Energy and Water Management) from NRF, and the New Initiative Fund from NTU, Singapore.

Notes and references

1. (a) J. Chang, C. Chi, J. Zhang and J. Wu, Adv. Mater., 2013, 25, 6442; (b) Y. Wu, B. Su, L. Jiang and A. J. Heeger, Adv. Mater., 2013, 25, 6526; (c) J. C. Scott and L. D. Bozano, Adv. Mater., 2007, 19, 1452; (d) S. Moller, C. Perlov, W. Jackson, C. Taussig and S. R. Forrest, Nature, 2003, 426, 166; (e) Q.-D. Ling, D.-J. Liaw, C. Zhu, D. S.-H. Chan, E.-T. Kang and K.-G. Neoh, Prog. Polym. Sci., 2008, 33, 917; (f) T. Kushida, C. Camacho, A. Shuto, S. Irle, M. Muramatsu, T. Katayama, S. Ito, Y. Nagasawa, H. Miyasaka, E. Sakuda, N. Kitamura, Z. Zhou, A. Wakamiya and S. Yamaguchi, Chem. Sci., 2014, 5, 1296; (g) B. Koo, H. Baek and J. Cho, Chem. Mater., 2012, 24, 1091; (h) Y. Ko, Y. Kim, H. Baek and J. Cho, ACS Nano, 2011, 5, 9918.
2. (a) J. Liu, Z. Yin, X. Cao, F. Zhao, L. Wang, W. Huang and H. Zhang, Adv. Mater., 2013, 25, 233; (b) J. Fang, H. You, J. Chen, J. Lin and D. Ma, Inorg. Chem., 2006, 45, 3701; (c) F. Zhao, J. Liu, X. Huang, X. Zou, G. Lu, P. Sun, S. Wu, W. Ai, M. Yi, X. Qi, L. Xie, J. Wang, H. Zhang and W. Huang, ACS Nano, 2012, 6, 3027; (d) J. Liu, Z. Zeng, X. Cao, G. Lu, L.-H. Wang, Q.-L. Fan, W. Huang and H. Zhang, Small, 2012, 8, 3517.
3. (a) G. Liu, Q.-D. Ling, E. Y. H. Teo, C.-X. Zhu, D. S.-H. Chan, K.-G. Neoh and E.-T. Kang, ACS Nano, 2009, 3, 1929; (b) C. Simão, M. Mas-Torrent, J. Casado-Montenegro, F. Otón, J. Veciana and C. Rovira, J. Am. Chem. Soc., 2011, 133, 13256; (c) C. Wang, J. Wang, P. Li, J. Gao, S. Y. Tan, W. Xiong, B. Hu, P. S. Lee, Y. Zhao and Q. Zhang, Chem. - Asian J., 2014, 9, 779.
4. (a) E. Kapetanakis, A. M. Douvas, D. Velessiotis, E. Makarona, P. Argitis, N. Glezos and P. Normand, Adv. Mater., 2008, 20, 4568; (b) C. W. Chu, J. Ouyang, J.-H. Tseng and Y. Yang, Adv. Mater., 2005, 17, 1440; (c) R. J. Tseng, J. Huang, J. Ouyang, R. B. Kaner and Y. Yang, Nano Lett., 2005, 5, 1077; (d) J. Ouyang, C.-W. Chu, C. R. Szmanda, L. Ma and Y. Yang, Nat. Mater., 2004, 3, 918; (e) L. D. Bozano, B. W. Kean, M. Beinhoff, K. R. Carter, P. M. Rice and J. C. Scott, Adv. Funct. Mater., 2005, 15, 1933.
5. (a) J.-S. Lee, Y.-M. Kim, J.-H. Kwon, J. S. Sim, H. Shin, B.-H. Sohn and Q. Jia, Adv. Mater., 2011, 23, 2064; (b) S.-J. Liu, P. Wang, Q. Zhao, H.-Y. Yang, J. Wong, H.-B. Sun, X.-C. Dong, W.-P. Lin and W. Huang, Adv. Mater., 2012, 24, 2901; (c) C. Ye, Q. Peng, M. Li, J. Luo, Z. Tang, J. Pei, J. Chen, Z. Shuai, L. Jiang and Y. Song, J. Am. Chem. Soc., 2012, 134, 20053; (d) A. J. Kronemeijer, H. B. Akkerman, T. Kudernac, B. J. van Wees, B. L. Feringa, P. W. M. Blom and B. de Boer, Adv. Mater., 2008, 20, 1467; (e) J. Wang and G. D. Stucky, Adv. Funct. Mater., 2004, 14, 409; (f) T. Lee, S.-U. Kim, J. Min and J.-W. Choi, Adv. Mater., 2010, 22, 510; (g) P. Y. Gu, F. Zhou, J. Gao, G. Li, C. Wang, Q. F. Xu, Q. Zhang and J. M. Lu, J. Am. Chem. Soc., 2013, 135, 14086.
6. (a) G. W. Orr, L. J. Barbour and J. L. Atwood, Science, 1999, 285, 1049; (b) B. H. Hong, J. Y. Lee, C.-W. Lee, J. C. Kim, S. C. Bae and K. S. Kim, J. Am. Chem. Soc., 2001, 123, 10748; (c) M. Touil, M. Elhabiri, M. Lachkar and O. Siri, Eur. J. Org. Chem., 2011, 1914.
7. (a) Y. Yang, M. Xue and C. F. Chem, CrystEngComm, 2010, 12, 3502; (b) A. Kleyn, T. Jacobs and L. J. Barbour, CrystEngComm, 2011, 13, 3175; (c) Y. Visitaec, I. Goldberg and A. Vigalok, Inorg. Chem., 2013, 52, 6779; (d) J. L. Katz, M. B. Feldman and R. R. Conry, Org. Lett., 2005, 7, 91.
8. (a) P. T. Herwig and K. Müllen, Adv. Mater., 1999, 11, 480; (b) G. Giri, E. Verploegen, S. C. B. Mannsfeld, S. Atahan-Evrenk, D. H. Kim, S. Y. Lee, H. A. Becerril, A. Aspuru-Guzik, M. F. Toney and Z. Bao, Nature, 2011, 480, 504; (c) A. R. Murphy and J. M. J. Fréchet, Chem. Rev., 2007, 107, 1066; (d) M. M. Payne, S. R. Parkin and J. E. Anthony, J. Am. Chem. Soc., 2005, 127, 8028; (e) H. Qu and C. Chi, Org. Lett., 2010, 12, 3360; (f) J. Xiao, H. M. Duong, Y. Liu, W. Shi, L. Ji, G. Li, S. Li, X.-W. Liu, J. Ma, F. Wudl and Q. Zhang, Angew. Chem., Int. Ed., 2012, 51, 6094; (g) Q. Zhang, Y. Divayana, J. Xiao, Z. Wang, E. R. T. Tiekink, H. M. Doung, H. Zhang, F. Boey, X. W. Sun and F. Wudl, Chem. - Eur. J., 2010, 16, 7422; (h) J. Xiao, Y. Divayana, Q. Zhang, H. M. Doung, H. Zhang, F. Boey, X. W. Sun and F. Wudl, J. Mater. Chem., 2010, 20, 8167; (i) J. Xiao, S. Liu, Y. Liu, L. Ji, X. Liu, H. Zhang, X. Sun and Q. Zhang, Chem. - Asian J., 2012, 7, 561; (j) J. Xiao, C. D. Malliakas, Y. Liu, F. Zhou, G. Li, H. Su, M. G. Kanatzidis, F. Wudl and Q. Zhang, Chem. - Asian J., 2012, 7, 672.
9. (a) Q. Maio, Synlett, 2012, 326; (b) D. Q. Liu, X. M. Xu, Y. R. Su, Z. K. He, J. B. Xu and Q. Miao, Angew. Chem., Int. Ed., 2013, 52, 6222; (c) Q. Miao, T. Q. Nguyen, T. Someya, G. B. Blanchet and C. Nuckolls, J. Am. Chem. Soc., 2003, 125, 10284; (d) Z. X. Liang, Q. Tang, R. X. Mao, D. Q. Liu, J. B. Xu and Q. Miao, Adv. Mater., 2011, 23, 5514; (e) Z. X. Liang, Q. Tang, J. B. Xu and Q. Miao, Adv. Mater., 2011, 23, 1535; (f) Q. Tang, Z. X. Liang, J. Liu, J. B. Xu and Q. Miao, Chem. Commun., 2010, 46, 2977; (g) G. Li, H. M. Duong, Z. Zhang, J. Xiao, L. Liu, Y. Zhao, H. Zhang, F. Huo, S. Li, J. Ma, F. Wudl and Q. Zhang, Chem. Commun., 2012, 48, 5974; (h) Y. Wu, Z. Yin, J. Xiao, Y. Liu, F. Wei, K. J. Tan, C. Kloc, L. Huang, Q. Yan, F. Hu, H. Zhang and Q. Zhang, ACS Appl. Mater. Interfaces, 2012, 4, 1883; (i) B. Gao, M. Wang, Y. Cheng, L. Wang, X. Jing and F. Wang, J. Am. Chem. Soc., 2008, 130, 8297.
10. (a) U. H. F. Bunz, J. U. Engelhart, B. D. Linder and M. Schafforth, Angew. Chem., Int. Ed., 2013, 52, 3810; (b) B. D. Lindner, J. U. Engelhart, O. Tverskoy, A. L. Appleton, F. Rominger, A. Peters, H. J. Himmel and U. H. F. Bunz, Angew. Chem., Int. Ed., 2011, 50, 8588; (c) U. H. F. Bunz, Pure Appl. Chem., 2010, 82, 953; (d) U. H. F. Bunz, Chem.–Eur. J., 2009, 15, 6780; (e) G. Li, Y. Wu, J. Gao, C. Wang, J. Li, H. Zhang, Y. Zhao, Y. Zhao and Q. Zhang, J. Am. Chem. Soc., 2012, 134, 20298; (f) G. Li, Y. Wu, J. Gao, J. Li, Y. Zhao and Q. Zhang, Chem. - Asian J., 2013, 8, 1574; (g) J. Li and Q. Zhang, Synlett, 2013, 24, 686.
11. (a) G. Liu, B. Zhang, Y. Chen, C. X. Zhu, L. Zeng, D. S. H. Chan, K. G. Neoh, J. Chen and E. T. Kang, J. Mater. Chem., 2011, 21, 6027; (b) X. D. Zhuang, Y. Chen, G. Liu, B. Zhang, K. G. Neoh, E. T. Kang, C. X. Zhu, Y. X. Li and L. J. Niu, Adv. Funct. Mater., 2010, 20, 2916.
12. (a) C. W. Ong, S.-C. Liao, T. H. Chang and H.-F. Hsu, J. Org. Chem., 2004, 69, 3181; (b) S. V. Bhosale, C. Jani, C. H. Lalander and S. J. Langford, Chem. Commun., 2010, 46, 973–975.
13. (a) A. D. Hendsbee, C. M. Macaulay and G. C. Welch, Dyes Pigm., 2014, 102, 204; (b) T. K. An, S.-H. Hahn, S. Nam, H. Cha, Y. Rho, D. S. Chung, M. Ree, M. S. Kang, S.-K. Kwon, Y.-H. Kim and C. E. Park, Dyes Pigm., 2013, 96, 756; (c) Y. Wang, D. Liu, S. Ikeda, R. Kumashiro, R. Nouch, Y. Xu, H. Shang, Y. Ma and K. Tanigaki, Appl. Phys. Lett., 2010, 97, 033305.
14. W. Zhang, X. Sun, P. Xia, J. Huang, G. Yu, M. S. Wong, Y. Liu and D. Zhu, Org. Lett., 2012, 14, 4382.
15. (a) S. Wang, P. K. L. Chan, C. W. Leung and X. Zhao, RSC Adv., 2012, 2, 9100; (b) T.-L. Choi, K.-H. Lee, W.-J. Joo, S. Lee, T.-W. Lee and M. Y. Chae, J. Am. Chem. Soc., 2007, 129, 9842.
16. (a) P.-Y. Gu, C.-J. Lu, Z.-J. Hu, N.-J. Li, T.-T. Zhao, Q.-F. Xu, Q.-H. Xu, J.-D. Zhang and J.-M. Lu, J. Mater. Chem. C, 2013, 1, 2599; (b) S. Kim, Q. D. Zheng, G. S. He, D. J. Bharali, H. E. Pudavar, A. Baev and P. N. Prasad, Adv. Funct. Mater., 2006, 16, 2317.

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Footnote

† Electronic supplementary information (ESI) available: Details of the characterization of 4N4OP. See DOI: 10.1039/c4mh00022f

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