Acid-Induced Multicolor Fluorescence of Pyridazine Derivative - ACS

Dec 12, 2017 - Density functional theory (DFT) calculations were performed in Gaussian 09 software at the B3LYP functional with the 6-31G* basis set l...
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Acid-induced Multicolor Fluorescence of Pyridazine Derivative Mengwei Li, Yuan Yuan, and Yulan Chen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b16050 • Publication Date (Web): 12 Dec 2017 Downloaded from http://pubs.acs.org on December 14, 2017

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Acid-induced Multicolor Fluorescence of Pyridazine Derivative Mengwei Li,a,c Yuan Yuan,a,c and Yulan Chen*,a,b a. Tianjin Key Laboratory of Molecular Optoelectronic Science, Department of Chemistry, Tianjin University, Tianjin, 300354, P. R. China b. School of Materials Science and Engineering, Tianjin University, Tianjin 300354, China c. Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin 300072, P. R. China *E-mail: [email protected] Phone/Fax: +86-22-27404118

KEYWORDS pyridazine, acid sensing, multicolor fluorescence, sensor, patterning

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ABSTRACT: Smart luminescent materials that are responsive to external stimuli have received considerable attention. Here we report a new D-A type 1,2-pyridiazine derivative (TPP) exhibiting turn-on fluorescence upon acid both in solution and in solid state. The protonation of the 1,2-pyridiazine ring caused a variation in the emission colors of the acidification species from blue (406 nm) to orange-red (630 nm) with a huge ∆λem (224 nm). As a result, a synthetic rainbow of emission in solution could be achieved from one single molecule, and white photoluminescence was readily tuned by controlled protonation. A TFA-sensor film made from TTP was demonstrated as a TFA-sensitive surface with high sensitivity and reversibility. On the basis of these findings, we constructed a solid state TTP film with a photoacid generator and demonstrate data encryption and decryption via cascade protonation reaction well controlled by UV light.

INTRODUCTION As one of the most important N-heterocycles, diazine derivatives have attracted much attention nowadays, due to their numerous advantages, such as easy of structural modification,1-3 excellent optical4,5 and electronic propertie,6 fruitful biological activities7-10 etc. Consisting of six-membered aromatics with two nitrogen atoms, diazines can be classified into three different types according to the relative position of the nitrogen atoms: pyridazine (1,2-diazine),11 pyrimidine (1,3-diazine),12 and pyrazine (1,4-diazine).13 All the three isomers exhibit highly π-deficient character and thus can be used as the electron-withdrawing part in π-conjugated push−pull structures. In this way,

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diazine based conjugated molecules with favorable luminescence induced by significant intramolecular charge transfer (ICT) estimate their potential applicability in OLED,14,15 in liquid crystal design,16-18 as well as in non-linear optical materials.19-21 Among the three isomeric forms, the pyridazine units constitute key components of biologically active structures,22-28 however, their utility in creating optoelectronic functional materials has only recently received attention of scientists in the organic chemistry and material communitie.29,30 In general, since the two nitrogen atoms in the conjugation skeleton allowing the pyridazine ring to act like an effective complexing agent, the emission and the charge transport of this chromophore are highly sensitive to external stimuli such as polarity,31-34 pH35,36, anion37 ect., to this end, novel materials with "intelligent" and tunable properties are available using pyridazine as the building block. Compared with pyrimidine and pyrazine, the pyridazine ring involves the strongest dipole moment (m = 4.22 D) and is the most basic of the diazines (with a relatively large pKa of 2.3).38 Based on these characteristics, we reasoned that by rational design, pyridazine derivatives could be very promising to perform as fluorescent sensors with high sensitivity. Although the research efforts regarding the luminescence behaviors of various pyridazines have increased within the last few years, new pyridazine based conjugated materials that possess sensitive turn-on luminescence features upon external stimuli both in solution and the solid states are still highly desired, for the purpose of advanced fluorescence sensors with high resolutions and broad spectrum of optical applications.39,40 Recently, we developed a facile synthesis toward stereospecific (Z)-aryl- 1,4-enediones,

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which exhibited acid-induced turn-on red emission both in solution and in the solid state.41 The reversible fluorescence acid sensing was attributed to the stepwise protonation of the 1,4-enedione core to rigidify the molecular conformation and strengthen the D–A interaction. These interesting results raise the question of how key optophysical/sensing

properties

might

respond

to

introducing

more

strongly

electron-accepting part into such structures. In the current work, we demonstrated the versatility of (Z)-1,4-enedione to build up stimuli-responsive luminescent materials with modulated electron affinity. Considering the sophisticated synthetic chemistry towards pyridazine ring from 1,4-diketone, a new D-A type pyridazine derivative TPP was thus designed and synthesized by one-step condensation from its 1,4-enedione precursor (Scheme 1). The photophysical properties were systematically characterized, through which the fluorescence of TTP was found highly sensitive to acid, giving a synthetic rainbow of emission in solution. To the best of our knowledge, this is the first example that multicolor fluorescence was achieved based on a single pyridazine derivative. We also fabricated a TFA-sensor film made from TTP and demonstrated its ability as a TFA-sensitive surface. Moreover, by incorporating a photoacid generator onto the surface, light directed luminescent patterning was realized through a cascade reaction way.

EXPERIMENTAL SECTION Materials and instruments. Unless noted otherwise, all chemicals were purchased from Aldrich, Acros or Adamas and used without further purification. Solvents were dried by

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usual methods before use. All reactions were performed under an atmosphere of nitrogen and monitored by TLC. 1H NMR (400 MHz) and

13

C NMR (100 MHz) spectra were

recorded on a 400 MHz Bruker AV 400 spectrometer. Matrix assisted laser desorption ionization time-of-flight mass spectroscopy (MALDI-TOF) was performed on a Bruker Autoflex speed TOF/TOF mass spectrometer using 1,8,9-anthratriol acid as the matrix. UV-vis absorption spectra were obtained on a PerkinElmer Lambda 750 UV/VIS/NIR spectrometer. Fluorescence spectra were recorded on a Hitachi F-7000 fluorescence spectrophotometer. Geometric structures of molecules were studied theoretically in the gas phase. Density functional theory (DFT) calculations were performed in Gaussian 09 software at the B3LYP functional with the 6-31G* basis set level. The fluorescence quantum yields (ФF) of TPP in different solvents were measured using quinine sulfate (0.1 M H2SO4, 0.54) as standard, and the ФF of protonated TPP were measured using Rhodamine B (ethanol, 0.65) as standard.

RESULTS AND DISCUSSION

Scheme 1. The structure and synthetic route of TPP.

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The synthetic route of 3,4,5,6-tetrakis(4-methoxyphenyl)pyridazine (TPP) is depicted in

Scheme

1.

TPBD-1

was

prepared

from

the

commercially

available

4,4’-dimethoxybenzoin according to the reported processure.41 TPP was obtained via Schiff-base condensation of TPBD-1 and hydrazine, in a yield of 55%. The compound was unambiguously characterized via 1H and 13C NMR spectroscopy and matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) mass spectroscopy.

Figure 1. (a) UV-vis absorption and PL spectra of TPP in chloroform; (b) PL spectra of TPP in different solvents (Concentration: 3×10-5 M, excitation wavelength: 300 nm).

The optophysical properties of TPP were investigated by UV-vis absorption and photoluminescence (PL) spectroscopy. In chloroform (concentration: 3×10-5 M), TPP showed absorption band in the UV region, in the range of 250−380 nm. The maximum absorption was located at 260 nm (π-π* transition), with a shoulder peak at ca. 300 nm (Figure 1a). The latter one could be ascribed to the ICT transition, as the emission peak of TPP was red-shifted by about 20 nm with the increase of solvent polarity (Figure 1b).

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The absorption profiles are almost identical in varying solvents (Figure S1), indicating its solvent polarity independent ground state electronic structure associated with the ICT transition.42 All the solutions exhibited blue fluorescence with low to moderate efficiencies (0.46%–4.73%). Although TPBD-1, the precursor of TPP, was emissive in aggregated state, the formation of dazine ring varied the photo-physical properties with ACQ effect instead of AIE character. In detail, in the mixed solution of THF and water (fw > 60%), the fluorescence intensity of TPP decreased dramatically (Figure S2).43,44 The faint emission of TPP aggregates might originate from the 1,2-diazine ring, which was expected to consume excited state energy via enhancing the intersystem crossing efficiency.35

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Figure 2. (a) UV-vis absorption spectra, (b) fluorescence spectra, and (c) PL intensity at the maximum emission wavelength of TPP in chloroform with different concentrations of TFA. (d) Images of TPP in chloroform with TFA (from left to right, [TFA] = 0, 5×10-4, 6×10-3, 4×10-2 M and the acidified solution upon further treatment with TEA in chloroform, taken under both natural light and the illumination with 365 nm UV light). Concentration of TPP: 3×10-5 M, excitation wavelength: 410 nm.

Pyridazine nitrogen groups are identified as efficient receptor for proton. To examine the sensing potential of TPP, we first performed the titration experiment of TPP with TFA in chloroform. As shown in Figure 2, the chloroform solution of TPP underwent a multiple color changes in the presence of TFA. With the addition of TFA from 0 to 0.04 M, the absorbance at 260 nm decreased with a slight bathochromic shift, accompanied with the appearance of two new absorbance bands at 414 nm and 470 nm. The color of solution changed from colorless to yellow. The new absorption bands implied the appearance of protonated species and could be attributed to the enhanced charge transfer transition.45 More distinct change was evidenced by PL spectra as shown in Figure 2b and Figure S4. Progressive addition of acid caused the slight increase and subsequent vanishing of the emission peak at 400 nm, as well as the emergence of a new fluorescence emission band located at ca. 630 nm whose intensity increased steadily. The corresponding fluorescence quantum yield (ФF) increased from 0.46% (without TFA, relative to quinine sulphate in 0.1 M H2SO4) to 11.12% (with TFA up to 0.04 M, relative

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to rhodamine B in ethanol). As for TPP, the methoxyl group and dazine ring acted as donor and acceptor, respectively, in neutral state, whereas the protonated pyridazine reinforced the electron-withdrawing nature and enhanced the ICT in the molecule. The formation of orange-red emissive acidified species was thereafter reasonable.35 Besides, the turn-on fluorescence sensing upon acidification could be due to an intermolecular Förster resonance energy transfer (FRET).14,36 Indeed, the protonated form acts as the energy acceptor whereas the pristine form acts as the energy donor (the absorption band of the protonated form overlaps the emission band of the neutral form, as shown in Figure 1a and 2a). Although protonation of pyrazine derivatives always led to quenching of the fluorescence, in our case, the turn-on fluorescence from TPP induced by acid is very attractive to give greater sensitivity of the probe. Resemble responsive phenomenon was observed when titration experiment was performed in dichloromethane (DCM). Formation of protonated species resulted in the new absorption peak at 425 nm with an ambiguous shoulder peak at ca. 470 nm (Figure 3a). And with TFA concentration increasing from 1×10-5 to 4×10-2 M, the intensity of emission peak at 400 nm (ФF = 4.73%) increased firstly followed by decreased, accompanied by the increase of the emission at 580 nm and red-shifting to 630 nm (ФF = 8.84%) gradually (Figure 3b, Figure S5). It is worth noting that, compared to those in chloroform, the color and fluorescence of the protonated species in DCM could be tuned more subtly, leading to rainbow colored solutions derived from a single molecule (Figure 3c). More interestingly, the bathochromic shift of emission peak after treatment of TFA

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was as large as 230 nm. The two factors would be very favorable to readily construct acid detecting device or other ratiometric evaluations, with higher sensitivity than many conventional acid-responsive materials. Similar fluorescence changes upon treatment with different acids were observed (Figure S6). Since TPP can take different forms with complementary emitting colors, we are inspired to investigate the controlled protonation of TPP to achieve white photoluminescence.46 The chromacity coordinates of TPP in its neutral and protonated forms are displayed in Figure 3d. Nonprotonated and fully protonated solutions appeared blue and orange, respectively. The addition of 3.3 equiv of TFA gave white light emission with CIE coordinates of (0.30, 0.32). The results highlighted the promising applications of TPP for the fabrication of WOLEDs.15 All the optical changes mentioned above could be reversibly switched off by neutralization with Et3N (Figure 3c), owing to the reversible nature of protonation and deprotonation process.

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Figure 3. (a) UV-vis absorption spectra and (b) fluorescence spectra of TPP in DCM with different concentrations of TFA. The corresponding (c) images and (d) CIE coordinates. (For images, from left to right: [TFA] = 0, 1×10-4, 5×10-4, 1×10-3, 3×10-3, 6×10-3, 4×10-2 M and the acidified solution upon further treatment with TEA, taken under both natural light and the illumination with 365 nm UV light). Concentration of TPP: 3×10-5 M, excitation wavelength: 410 nm for (b) and 365 nm for lower images of (c).

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Figure 4. The HOMOs and LUMOs of TPP in neutral and protonated states.

In order to investigate the changes in electronic properties of TPP upon protonation, theoretical calculations were performed according to density functional theory (DFT) calculations at the level of B3LYP/6-31G*. The electronic distribution of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy levels of TPP in neutral and protonated states were shown in Figure 4. The π electrons of the HOMO molecular orbital are delocalized on pyridazine core and two benzene rings at the 3-, 6- positions. The LUMO is mainly on pyridazine core and

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partially on benzene rings at the 4-, 5- positions. Upon protonation, the HOMO of TPP(H+) is located on benzene rings at the 4-, 5-, 6- positions, while the LUMO is primarily on pyridazine core. The calculated bandgap of the protonated specie is narrower than that of TPP, in accordance with the red-shifted absorbance band of TPP upon treatment with TFA. Moreover, concerning the configuration of the molecules, the protonated TPP had a larger torsion angle between pyridazine core and the 3- substituted benzene ring (Figure S7). The increased torsion could be attributed to the steric hindrance induced by protonation, which is in good agreement with the improved luminescence efficiency after protonation.

Figure 5. (a) Photographs of TPP sensor fabricated on a piece of filter paper (size: 10 cm × 5 cm) after fumed with TFA and TEA vapor under natural light (upper) and the illumination with 365 nm UV light (lower). (b) Fluorescence intensity of TPP sensor as a

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function of the TFA-TEA cycle numbers. Excitation wavelength: 410 nm. The fluorescence intensity was monitored at the wavelength of 600 nm. (c) Left: schematic of solid state sensor for patterning (film made of TPP and PAG) through cascade protonation reaction triggered by UV light. Right: luminescent “umbrella” patterning after UV-irradiation (254 nm) with a mask in front of the paper, which can be reversibly recovered upon treated with TEA vapor.

Considering the unique acid response feature of TPP in solution, its applications in solid-state fluorescent sensing device were explored. Although TPP was found as an ACQphore, the undesirable effect could become productive when the material was utilized as the security ink in solid-state sensory system, since the fluorescence background was very faint in its off state. In detail, a chloroform solution of TPP (1×10-3 M) was firstly exploited as the ink, by which a word “TJU” was written on a piece of filter paper (10 cm × 5 cm). It was difficult to detect the presence of “TJU” by the naked eyes neither under day light nor UV light (Figure 5a). After fumed with TFA vapor, “TJU” appeared on the paper, with intense yellow light emission under UV irradiation, as well as yellow color observed under natural light. In other words, the encrypted information could be decoded by treated the paper with TFA. Subsequently, the yellow emission of “TJU” could be dramatically decreased and fully converted to off state when the paper was fumed with TEA vapor, resulting in the encryption of the decoded information on the paper. Within fluorescent and chromatic switching cycles, the information could be read

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and hidden several times (Figure 5b). Since the ability to optically control the fluorescence of the thin film is of particular interest for solid state sensor arrays,47,48 we attempted a combination of a photoacid generator (PAG, see Supporting information) with TPP film. The sensor was prepared by depositing TPP and PAG on a piece of filter paper. Upon UV-irradiation by a low pressure mercury lamp (254 nm) for 5 min with a mask in front of the paper, a bright “umbrella” appeared on the paper (Figure 5c). The control experiment of TPP with p-toluene sulfonyl acid (TsOH) was performed in DCM solution. Orange emission peaked at 600 nm emerged, consistent with the finding in the solid state sensor. Meanwhile, pristine TPP film without PAG did not emit orange fluorescence after UV irradiation (Figure S8). The results from these control experiments further demonstrated the acid response nature of TPP and the turn-on fluorescence in the solid state sensor caused by UV-light induced acidification of TPP. Rather than using volatile acid vapor, the luminescence can be controlled by light in a cascade way, by which photo induced acid triggered the acidification of TPP. Thereafter, the patterned fluorescence occurred, which can be recovered after treatment with TEA vapor. We envisioned that, if well controlled, UV light could be used for micropatterning and also have potential for microdevice fabrication.49,50 Taking the high sensitivity of fluorescent sensing into account, the turn-on fluorescence of TTP under stimuli (i.e., non-emissive for itself and yellow for its complex with TFA) is more favorable for optical sensing. In contrast, the fluorescent background of the precursor TPBD-1, and other reported materials for acid detection could not be negligible,

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with unavoidable cross-talk between different emission light. It was also notice that, unlike TPBD-1 whose original emission could be fully recovered by standing the acidified sensor in the ambient air for 1 h, the yellow emission from the acidified TTP was more stable and could maintain for more than a week. Such improved stability for TTP toward acid was essential for fabricating practical devices for data recording and security protection.

CONCLUSION In conclusion, a D-A type diazine derivative (TPP) with methoxyl groups as donor units and 1,2-pyridiazine as acceptor unit was facilely synthesized by cyclization of its 1,4-enedione precursor. The luminescence and acid responsive behaviors of TPP has been studied, which was demonstrated as a smart luminescent material. Intriguingly, upon titration with TFA, the protonated TPP exhibited turn-on fluorescence with multicolor emission. As a result, a synthetic rainbow of emission in solution was achieved, and white photoluminescence could be readily tuned by controlled protonation. Solid-state TPP sensor for TFA was fabricated, which presented switchable fluorescence color with good reversibility and high sensitivity. By integrated with photoacid generator, the turn-on and patterned fluorescence can be readily triggered by light in a controlled way, implying its practical applications in acid detection, data encryption, etc. Our results suggest that rationally designed pyridiazine derivatives may be promising candidates as functionally stimulus-responsive materials for advanced data recording and security protection.

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ACKNOWLEDGMENTS Financial support by the National Natural Science Foundation of China (Grants 21522405 and 51503142), the National Key Research and Development Program of China (Grants 2017YFA0204503 and 2017YFA0207800), the Thousand Youth Talents Plan is gratefully acknowledged.

Supporting Information Available: Synthetic procedures, 1H NMR and

13

C NMR

spectra, MALDI-TOF spectrum, more UV-Vis absorption and PL spectra, DFT calculation results. This material is available free of charge via the Internet at http://pubs.acs.org.

NOTES AND REFERENCES (1) Mathew, T.; Papp, A. A.; Paknia, F.; Fustero, S.; Prakash, G. K. S. Benzodiazines: Recent Synthetic Advances. Chem. Soc. Rev. 2017, 46, 3060–3094. (2) Tang, R. Z.; Zhang, F.; Fu, Y. B.; Xu, Q.; Wang, X. Y.; Zhuang, X. D.; Wu, D. Q.; Giannakopoulos, A.; Beljonne, D.; Feng, X. L. Efficient Approach to Electron-Deficient 1,2,7,8-Tetraazaperylene Derivatives. Org. Lett. 2014, 16, 4726–4729. (3) Li, J. B.; Gao, J. K.; Xiong, W. W.; Zhang, Q. C.

A Concise Method to Prepare

Linear 2,3-Diazaoligoacene Derivatives. Tetrahedron Lett. 2014, 55, 4346–4349. (4) Itami, K.; Yamazaki, D.; Yoshida, J. Pyrimidine-Core Extended π-Systems: General Synthesis and Interesting Fluorescent Properties. J. Am. Chem. Soc. 2004, 126, 15396– 15397.

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(5) Malval, J. P.; Achelle, S.; Bodiou, L.; Spangenberg, A.; Gomez, L. C.; Sopperaa, O.; Guen, F. R. Two-photon Absorption in a Conformationally Twisted D–π–A Oligomer: a Synergic Photosensitizing Approach for Multiphoton Lithography. J. Mater. Chem. C 2014, 2, 7869–7880. (6) Yasuda, T.; Sakai, Y.; Aramaki, S.; Yamamoto, T. New Coplanar (ABA)n-Type Donor-Acceptor π-Conjugated Copolymers Constituted of Alkylthiophene (Unit A) and Pyridazine (Unit B): Synthesis Using Hexamethylditin, Self-Organized Solid Structure, and Optical and Electrochemical Properties of the Copolymers. Chem. Mater. 2005, 17, 6060–6068. (7) Barraja, P.; Diana, P.; Lauria, A.; Passannanti, A.; Almerico, A. M.; Minnei, C.; Longu, S.; Congiu, D.; Musiu C.; Colla, P. L. Indolo[3,2-c]cinnolines with Antiproliferative, Antifungal, and Antibacterial Activity. Bioorg. Med. Chem., 1999, 7, 1591–1596. (8) Laddha, S. S.; Bhatnagar, S. P. A New Therapeutic Approach in Parkinson’s Disease: Some Novel Quinazoline Derivatives as Dual Selective Phosphodiesterase 1 Inhibitors and Anti-Inflammatory Agents. Bioorg. Med. Chem., 2009, 17, 6796–6802. (9) Xu, L.; Russu, W. A. Molecular Docking and Synthesis of Novel Quinazoline Analogues as Inhibitors of Transcription Factors NF-kB Activation and Their Anti-Cancer Activities. Bioorg. Med. Chem., 2013, 21, 540–546. (10) Sivakumar, R. S.; Gnanasam, K.; Ramachandran, S.; Leonard, J. T. Pharmacological Evaluation of Some New 1-Substituted-4-Hydroxyphthalazines. Eur. J. Med. Chem. 2002, 37, 793–801. (11) Barlin, G. B. in Chemistry of Heterocyclic Compounds, John Wiley and Sons, New York, 1982, vol. 41.

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(12) Brown, J. in Chemistry of Heterocyclic Compounds, John Wiley and Sons, New York, 1962, vol. 16. (13) Castle, R. N. in Chemistry of Heterocyclic Compounds, John Wiley and Sons, New York, 1962, vol. 23. (14) Liu, D.; Zhang, Z. Y.; Zhang, H. Y.; Wang, Y. A Novel Approach Towards White Photoluminescence and Electroluminescence by Controlled Protonation of a Blue Fluorophore. Chem. Commun. 2013, 49, 10001–10003. (15) Achelle, S.; Rodríguez-López, J.; Katan, C.; Guen, F. R. Luminescence Behavior of Protonated Methoxy-Substituted Diazine Derivatives: Toward White Light Emission. J. Phys. Chem. C 2016, 120, 26986–26995. (16) Wild, J. H.; Bartle, K.; Kirkman, N. T.; Kelly, S. M.; O’Neill, M.; Stirner, T.; Tuffin, R. P. Synthesis and Investigation of Nematic Liquid Crystals with Flexoelectric Properties. Chem. Mater. 2005, 17, 6354–6360. (17) Lifka, T.; Zerban, G.; Seus, P.; Oehlhof, A.; Meier, H. Alkoxy Substituted (E,E)-3,6-Bis(styryl)pyridazineda

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The

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Metallosalphen

Dimers:

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