Thermotropic Liquid-Crystalline and Light-Emitting Properties of Bis(4-Aalkoxyphenyl) Viologen Bis(Triﬂimide) Salts

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Article Thermotropic Liquid-Crystalline and Light-Emitting Properties of Bis(4-aalkoxyphenyl) Viologen Bis(triﬂimide) Salts

Pradip K. Bhowmik 1,* , Muhammed Kareem M. Al-Karawi 1, Shane T. Killarney 1, Erenz J. Dizon 1, Anthony Chang 1 , Jongin Kim 1, Si L. Chen 1, Ronald Carlo G. Principe 1, Andy Ho 1, Haesook Han 1, Hari D. Mandal 2, Raymond G. Cortez 2, Bryan Gutierrez 2, Klarissa Mendez 2, Lewis Sharpnack 3, Deña M. Agra-Kooijman 4, Michael R. Fisch 5 and Satyendra Kumar 6 1 Department of Chemistry and Biochemistry, University of Nevada Las Vegas, 4505 S. Maryland Parkway Box 454003, Las Vegas, NV 89154-4003, USA; [email protected] (M.K.M.A.-K.); [email protected] (S.T.K.); [email protected] (E.J.D.); [email protected] (A.C.); [email protected] (J.K.); [email protected] (S.L.C.); [email protected] (R.C.G.P.); [email protected] (A.H.); [email protected] (H.H.) 2 Department of Biology and Chemistry, Texas A & M International University, 5201 University Blvd., Laredo, TX 78041, USA; [email protected] (H.D.M.); [email protected] (R.G.C.); [email protected] (B.G.); [email protected] (K.M.) 3 Department of Earth Science, 1006 Webb Hall, University of California, Santa Barbara, CA 93106, USA; [email protected] 4 Advanced Materials and Liquid Crystal Institute, Kent State University, Kent, OH 44242, USA; [email protected] 5 College of Aeronautics and Engineering, Kent State University, Kent, OH 44242, USA; mﬁ[email protected] 6 Division of Research, University at Albany, Albany, NY 12222, USA; [email protected] * Correspondence: [email protected]; Tel.: +1-(702)-895-0885; +1-(702)-895-4072

Academic Editor: Roberta Cassano Received: 28 April 2020; Accepted: 21 May 2020; Published: 23 May 2020 Abstract: A series of bis(4-alkoxyphenyl) viologen bis(triflimide) salts with alkoxy chains of different lengths were synthesized by the metathesis reaction of respective bis(4-alkoxyphenyl) viologen dichloride salts, which were in turn prepared from the reaction of Zincke salt with the corresponding 4-n-alkoxyanilines, with lithium triflimide in methanol. Their chemical structures were characterized by 1H and 13C nuclear magnetic resonance spectra and elemental analysis. Their thermotropic liquid-crystalline (LC) properties were examined by differential scanning calorimetry, polarizing optical microscopy, and variable temperature X-ray diffraction. Salts with short length alkoxy chains had crystal-to-liquid transitions. Salts of intermediate length alkoxy chains showed both crystal-to-smectic A (SmA) transitions, Tms, and SmA-to-isotropic transitions, Tis. Those with longer length of alkoxy chains had relatively low Tms at which they formed the SmA phases that persisted up to the decomposition at high temperatures. As expected, all of them had excellent thermal stabilities in the temperature range of 330–370 ◦C. Their light-emitting properties in methanol were also included.

Keywords: extended viologens; Zincke salt; metathesis reaction; ionic liquid crystals; thermotropic; smectic phase A; diﬀerential scanning calorimetry; polarizing optical microscopy; X-ray diﬀraction; thermogravimetric analysis

1. Introduction Thermotropic ionic liquid crystals (ILCs) are an important class of materials that combine the properties of both ionic liquids and liquid crystals. The liquid-crystalline (LC) phases oﬀer many

Molecules 2020, 25, 2435; doi:10.3390/molecules25102435 www.mdpi.com/journal/molecules Molecules 2020, 25, 2435 2 of 20 advantages over the liquid phases. For example, ion conduction is significantly enhanced in smectic A (SmA) and columnar phases as compared with the isotropic liquid phases. The unique properties of ILCs make them suitable for many technological applications including display technology, solar cells, ion conductors in batteries, and templates for the synthesis of nanomaterials [1–14]. They are even considered as organized reaction media in which many organic reactions including Diels–Alder reactions; intramolecular Diels–Alder reactions can be performed with highly regio- and chemoselective reactions, with the additional advantage of being recycled [15–17]. They are usually composed of varied suitably modified organic cations and organic and inorganic anions. Among the common cations are quaternary ammonium, quaternary phosphonium, pyrrolidinium, piperidinium, guanidinium, imidazolium, pyridinium pyrazolium, triazolium, among other cations and the common anions are Br−, NO3−, BF4−, CF3SO3−,PF6−, ClO4−, N(SO2CF3)2−, OTs−, ReO4−, among other anions. These are examples of monocations and monoanions employed for the synthesis of ILCs [1–14]. Additionally, they can be versatile in their chemical architectures. They can be dicationic, tricationic, and multicationic in their makeups [18–30]. For example, viologens are examples of dicationic salts that are symmetric (alkyl groups similar) and exhibit LC phases (n = 6, 7, 8, 14–20) even at room temperature [31–33]. Asymmetric (alkyl groups dissimilar) types with triflimides as counterions exhibit smectic LC phases with a wide range of stability that range from as low as 0 ◦C to as high as 146 ◦C. Consequently, they have increased ranges of stability of LC phases as compared with those for symmetric viologen salts [34]. In the further development of ILCs, there are several reports in the literature on extended viologen salts (I–VI), in which viologen moiety is elongated with the phenyl residue, as shown in Figure1, and also display smectic LC phases [35–37]. As a continuation of our research efforts on ILCs, herein, we describe the synthesis of a series of extended viologen salts with bis(triflimide) (n = 1, 6, 8, 10, 12, 14, 16, 18, and 20), where n denotes the carbon atoms in the alkoxy chain. We determine their chemical structures by 1H and 13C NMR spectra, as well as elemental analysis, and the characterization of their thermotropic LC properties by several experimental techniques including differential scanning calorimetry (DSC), polarizing optical microscopy (POM), and variable temperature X-ray diffraction (VT-XRD) studies. Their thermal stabilities by thermogravimetric analysis (TGA) are also included. The general structures and designations for these synthesized extended viologen salts, and their synthetic routes are shown in Scheme1. The LC properties of these salts with these triflimide anions enable one to establish the structure–property relationship of this important class of ILCs. Additionally, in contrast to other anions, the reports of ILCs on triflimide anions are relatively less studied [24,31–34,38–40] which give the impetus for this study. Molecules 2020, 25, 2435 3 of 20 Molecules 2020, 25, x FOR PEER REVIEW 3 of 20

FigureFigure 1. 1. ChemicalChemical structuresstructures ofof extended extended viologen viologen salts salts that that exhibit exhibit smectic smectic A A (SmA) (SmA) phases. phases.

As a continuation of our research efforts on ILCs, herein, we describe the synthesis of a series of extended viologen salts with bis(triflimide) (n = 1, 6, 8, 10, 12, 14, 16, 18, and 20), where n denotes the carbon atoms in the alkoxy chain. We determine their chemical structures by 1H and 13C NMR spectra, as well as elemental analysis, and the characterization of their thermotropic LC properties by several experimental techniques including differential scanning calorimetry (DSC), polarizing optical microscopy (POM), and variable temperature X‐ray diffraction (VT‐XRD) studies. Their thermal stabilities by thermogravimetric analysis (TGA) are also included. The general structures and designations for these synthesized extended viologen salts, and their synthetic routes are shown in Scheme 1. The LC properties of these salts with these triflimide anions enable one to establish the structure–property relationship of this important class of ILCs. Additionally, in contrast to other anions, the reports of ILCs on triflimide anions are relatively less studied [24,31–34,38–40] which give the impetus for this study.

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Scheme 1. Synthetic routes for the preparation of extended viologen bis(triflimide) salts (EVn). Note Scheme 1. Synthetic routes for the preparation of extended viologen bis(triﬂimide) salts (EVn). that the compounds n = 10, 12, and 14 corresponding to structure IV were reported in [36]. Note that the compounds n = 10, 12, and 14 corresponding to structure IV were reported in [36].

2.2. Results and Discussion InIn thisthis study, aa seriesseries ofof extendedextended viologenviologen bis(triﬂimide)bis(triflimide) saltssalts (EVn)(EVn) withwith variablevariable alkoxy chainschains werewere synthesized,synthesized, characterizedcharacterized forfor theirtheir chemicalchemical structures,structures, andand furtherfurther characterizedcharacterized forfor theirtheir thermotropicthermotropic LCLC propertiesproperties usingusing DSC,DSC, POM,POM, andand VT-XRDVT‐XRD techniques.techniques. TheThe thermalthermal stabilitiesstabilities ofof thethe saltssalts werewere determineddetermined usingusing TGATGA technique.technique. Their light-emittinglight‐emitting properties includingincluding theirtheir chloridechloride precursorsprecursors inin methanolmethanol werewere studiedstudied using using UV-Vis UV‐Vis and and photoluminescence photoluminescence spectrometer. spectrometer.

2.1.2.1. Synthesis of Bis(4-n-alkoxyphenyl)-4,4Bis(4‐n‐alkoxyphenyl)‐4,40-bipyridinium‐bipyridinium Bis(triflimide)Bis(triflimide) SaltsSalts (EV1, (EV1, EV6,EV6, EV8,EV8, EV10,EV10, EV12, EV14,EV12, EV16,EV14, EV18,EV16, andEV18, EV20) and EV20) TheThe 4-n-alkoxyanilines4‐n‐alkoxyanilines were prepared in accordance with the modified modified literature procedure [41]. [41]. TheThe bis(4-alkoxyphenyl)-4,4bis(4‐alkoxyphenyl)‐4,40-bipyridinium‐bipyridinium dichloride dichloride (EV1Cl, (EV1Cl, EV6Cl, EV6Cl, EV8Cl, EV8Cl, EV10Cl, EV10Cl, EV12Cl, EV12Cl, EV14Cl, EV16Cl,EV14Cl, EV18Cl,EV16Cl, andEV18Cl, EV20Cl) and withEV20Cl) alkyl with chains alkyl of chains different of lengthsdifferent were lengths synthesized were synthesized as shown inas Schemeshown in1[ Scheme42–44]. 1 The [42–44]. synthetic The routesynthetic was route based was on abased two-step on a proceduretwo‐step procedure involving involving the aromatic the nucleophilicaromatic nucleophilic substitution substitution between thebetween 1-chloro-2,4-dinitrobenzene the 1‐chloro‐2,4‐dinitrobenzene and 4,40-bipyridine and 4,4‐bipyridine to give the to Zinckegive the salt Zincke and itssalt subsequent and its subsequent ANRORC ANRORC (anionic ring(anionic opening ring andopening ring closing)and ring reaction closing) withreaction the correspondingwith the corresponding 4-n-alkoxyanilines. 4‐n‐alkoxyanilines. For the preparation For the ofpreparation Zincke salt, of the Zincke usual solventsalt, the acetonitrile usual solvent was used.acetonitrile For the was ANRORC used. For reaction, the ANRORC the usual solventreaction, ethanol the usual was used solvent except ethanol for EV18Cl was used and EV20Cl.except for In theseEV18Cl cases, and DMAc EV20Cl. was In used these for cases, the better DMAc solubility was used of these for the long better carbon solubility chains 4-n-alkoxyanilines of these long carbon [36]. 1 Thechains structures 4‐n‐alkoxyanilines of chlorides [36]. salts The of shorter structures carbon of chlorides chains, up salts to n of= shorter12, were carbon established chains, from up to their n = 12,H 13 andwere establishedC NMR spectra from obtained their 1H inand CD 133COD NMR (Figures spectra S1–S5). obtained For the in CD longer3OD carbon (Figures chains, S1–S5).n = For14–20, the 1 welonger were carbonable to chains, verify n their = 14–20, chemical we were structures able to from verifyH their NMR chemical spectra structures in CD3OD from only 1H (Figures NMR spectra S6–S9). Then.in CD3 theseOD only chlorides (Figures salts S6–S9). were Then. converted these chlorides to bis(triflimides) salts were salts converted by the to metathesis bis(triflimides) reaction salts with by lithiumthe metathesis triflimides reaction in a common with lithium solvent triflimides methanol in [31 a– common33]. For EV1, solvent the methanol metathesis [31–33]. reaction For was EV1, carried the outmetathesis in water. reaction Then, the was bis(triflimide) carried out in salts water. with Then, shorter the carbonbis(triflimide) chains, salts up to withn = 12,shorter were carbon characterized chains, 1 13 analyzingup to n = 12, the wereH and characterizedC-NMR spectra analyzing obtained the 1H in and CD 313ODC‐NMR (Figures spectra S10–S14), obtained while in those CD3OD with (Figures longer 1 13 carbonS10–S14), chains, whilen those= 14, 16,with 18, longer 20, were carbon characterized chains, n = analyzing 14, 16, 18, the 20, Hwere and characterizedC NMR spectra analyzing obtained the in1Hd and6-DMSO 13C NMR (Figures spectra S15–S18). obtained Their in purityd6‐DMSO was (Figures also determined S15–S18). from Their their purity elemental was also analysis. determined from their elemental analysis.

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2.2. Thermotropic LC Properties of EVn by DSC, POM, and VT‐XRD Figure 2 displays the DSC thermograms of EV1 in the heating and cooling cycles. It clearly shows two endotherms in each of the two heating cycles. The peak maxima of the endotherms, as well as heat of enthalpies in the second heating cycle, were slightly lower than those in the first heating cycle [45–49]. In the first cooling cycle, there were cooling exotherms with large supercooling and decreased heat of enthalpies. In the second cooling cycle, there were no exotherms which suggested that no Molecules 2020, 25, 2435 5 of 20 crystallization occurred under the experimental conditions used. Figure 3 shows diffraction patterns of EV1 that indicated that it has crystalline phase at room temperature, an isotropic phase of diffuse 2.2.scattering Thermotropic rings LCat 200 Properties °C, and of incipient EVn by DSC, crystalline POM, phase and VT-XRD at 160 °C on slow cooling from the isotropic phase. These results suggested that the high‐temperature endotherm corresponded to crystal‐to‐ Figure2 displays the DSC thermograms of EV1 in the heating and cooling cycles. It clearly shows liquid transition, Tm, at 164 °C. The low‐temperature endotherm corresponded to crystal‐to‐crystal two endotherms in each of the two heating cycles. The peak maxima of the endotherms, as well transition. The POM studies also corroborated these statements. Thus, EV1 did not form any LC as heat of enthalpies in the second heating cycle, were slightly lower than those in the ﬁrst heating phase. cycle [45–49].

Figure 2. Diﬀerential scanning calorimetry (DSC) thermograms of EV1 obtained at heating and cooling

ratesFigure of 10 2.◦ CDifferential/min. scanning calorimetry (DSC) thermograms of EV1 obtained at heating and cooling rates of 10 °C /min. In the first cooling cycle, there were cooling exotherms with large supercooling and decreased heat ofFigure enthalpies. S19 shows In the the second DSC thermograms cooling cycle, of there EV6 were in its no heating exotherms and whichcooling suggested cycles. In thatthe first no crystallizationheating cycle, occurred it showed under three the endotherms experimental located conditions at 81, 93, used. and Figure 114 °C.3 showsIn the second di ffraction heating patterns cycle, ofthe EV1 absence that indicated of low‐temperature that it has crystalline endotherms phase and at only room the temperature, presence of an a large isotropic endotherm phase of at di 105ffuse °C scatteringwith the ringsincreased at 200 heat◦C, andof enthalpy incipient suggested crystalline that phase low at‐ 160temperature◦C on slow endotherms cooling from were the isotropicrelated to phase.crystal These‐to‐crystal results transition suggested and that high the high-temperature‐temperature was endotherm related to corresponded crystal‐to‐liquid to crystal-to-liquid transition. The transition,presence Tofm cooling, at 164 ◦exothermC. The low-temperature in each of the cooling endotherm cycles corresponded suggested that to crystal-to-crystal the crystallization transition. from the Theisotropic POM studies liquid alsooccurred corroborated under the these experimental statements. conditions Thus, EV1 didused. not The form first any cooling LC phase. and second coolingFigure exotherms S19 shows underwent the DSC thermograms a relatively low of EV6 degree in its of heating supercooling and cooling of 14 cycles. and 6 In°C, the respectively. first heating cycle, it showed three endotherms located at 81, 93, and 114 ◦C. In the second heating cycle, the absence of low-temperature endotherms and only the presence of a large endotherm at 105 ◦C with the increased heat of enthalpy suggested that low-temperature endotherms were related to crystal-to-crystal transition and high-temperature was related to crystal-to-liquid transition. The presence of cooling exotherm in each of the cooling cycles suggested that the crystallization from the isotropic liquid occurred under the experimental conditions used. The first cooling and second cooling exotherms underwent a relatively low degree of supercooling of 14 and 6 ◦C, respectively. MoleculesMolecules 20202020,, 2525,, x 2435 FOR PEER REVIEW 6 6of of 20 20 Molecules 2020, 25, x FOR PEER REVIEW 6 of 20 (a) (b) (c) (a) (b) (c)

Figure 3. The X‐ray diffraction patterns of EV1. (a) Crystalline phase at room temperature; (b) Figure 3. The X-ray diffraction patterns of EV1. (a) Crystalline phase at room temperature; (b) Isotropic IsotropicFigure phase3. The takenX‐ray at diffraction 200 ℃; (c )patterns Incipient of crystalline EV1. (a) Crystallinephase at 160 phase ℃ on at cooling room temperature;from isotropic (b ) phase taken at 200 ◦C; (c) Incipient crystalline phase at 160 ◦C; on cooling from isotropic phase. It can phase.Isotropic It can phase be seen taken that at this 200 compound ℃; (c) Incipient scatters crystalline X‐ray poorly. phase at 160 ℃ on cooling from isotropic be seen that this compound scatters X-ray poorly. phase. It can be seen that this compound scatters X‐ray poorly. TheThe POM POM studies studies also also corroborated corroborated by by the the fact fact that that it it formed formed the the typical typical crystal crystal texture. texture. Thus, Thus, The POM studies also corroborated by the fact that it formed the typical crystal texture. Thus, similarsimilar to to EV1, EV1, EV6 EV6 did did not not form form any any LC LC phase. phase. similar to EV1, EV6 did not form any LC phase. FigureFigure 44 shows thethe DSC thermogramsthermograms ofof EV8EV8 inin itsits heating and coolingcooling cycles.cycles. In the first first heating cycle itFigure showed 4 shows three theendotherms DSC thermograms located at , of108 EV8 122, in itsand heating 190 °C. and In coolingthe second cycles. heating In the cycle, first heatingthere cycle it showed three endotherms located at 108, 122, and 190 ◦C. In the second heating cycle, there werecycle four it showed endotherms three locatedendotherms at 104, located 112, 122, at 108,and 122,190 °C.and In 190 each °C. ofIn the the cooling second cycles,heating there cycle, were there were four endotherms located at 104, 112, 122, and 190 ◦C. In each of the cooling cycles, there were threewere exotherms. four endotherms In conjunction located atwith 104, VT 112,‐XRD 122, studies, and 190 it °C.was In found each ofthat the the cooling endotherm cycles, atthere 190 were°C three exotherms. In conjunction with VT-XRD studies, it was found that the endotherm at 190 ◦C correspondedthree exotherms. to SmA In ‐conjunctionto ‐isotropic transition,with VT‐XRD Ti, and studies, that at it 122 was °C found it corresponded that the endotherm to crystal‐ toat‐ SmA190 °C corresponded to SmA-to-isotropic transition, Ti, and that at 122 ◦C it corresponded to crystal-to-SmA transition,corresponded Tm, since to SmA it showed‐to‐isotropic diffraction transition, patterns Ti, andof diffuse that at rings 122 °C at it200 corresponded °C, inner arcs to of crystal SmA‐ toat ‐190SmA transition, Tm, since it showed diffraction patterns of diffuse rings at 200 ◦C, inner arcs of SmA at °C,transition, and sharp T minner, since ring it showed and outer diffraction diffuse ring patterns at 180 of °C diffuse (Figure rings 5). Theat 200 endotherm(s) °C, inner arcs prior of SmA to the at T 190m 190 ◦C, and sharp inner ring and outer diffuse ring at 180 ◦C (Figure5). The endotherm(s) prior to corresponded°C, and sharp to inner crystal ring‐to ‐andcrystal outer transition(s). diffuse ring The at 180exotherm °C (Figure prior 5). to The the endotherm(s)recrystallization prior exotherm to the T m the Tm corresponded to crystal-to-crystal transition(s). The exotherm prior to the recrystallization corresponded to crystal‐to‐crystal transition(s). The exotherm prior to the recrystallization exotherm inexotherm each of inthe each cooling of the cycles cooling presumably cycles presumably corresponded corresponded to the transition to the transition from the fromSmA theto another SmA to (monotropic)in each of the mesophase. cooling cycles Interestingly, presumably both corresponded Tm and Ti underwent to the transition very little from supercooling the SmA to inanother the another (monotropic) mesophase. Interestingly, both Tm and Ti underwent very little supercooling in (monotropic) mesophase. Interestingly, both Tm and Ti underwent very little supercooling in the coolingthe cooling cycles cycles which which was was in stark in stark contrast contrast to tomany many ILCs ILCs [50–54]. [50–54 ].Additionally, Additionally, the the POM POM studies studies also also cooling cycles which was in stark contrast to many ILCs [50–54]. Additionally, the POM studies also indicatedindicated that that EV8 EV8 formed formed an an SmA SmA phase, phase, since since its its optical optical texture texture exhibited exhibited the the so so-called‐called focal focal-conic‐conic textureindicated taken that at EV8160 formed°C on cooling an SmA from phase, the since isotropic its optical phase texture (Figure exhibited 6) [45–49]. the Thus, so‐called these focal results‐conic texture taken at 160 ◦C on cooling from the isotropic phase (Figure6)[ 45–49]. Thus, these results suggestedtexture taken that EV8at 160, unlike °C on EV1 cooling and EV6, from showed the isotropic an SmA phase phase (Figure after T6)m and[45–49]. a Ti leadingThus, these to the results LC suggested that EV8, unlike EV1 and EV6, showed an SmA phase after Tm and a Ti leading to the LC phasesuggested range thatof 68 EV8 °C. ,The unlike analogous EV1 and salt EV6, containing showed nan‐octyl SmA group phase has after a T Tmm at and 103 a °C Ti andleading Ti at to 180 the °C LC phase range of 68 ◦C. The analogous salt containing n-octyl group has a Tm at 103 ◦C and Ti at 180 ◦C phase range of 68 °C. The analogous salt containing n‐octyl group has a Tm at 103 °C and Ti at 180 °C resultingresulting in in the the LC LC phase phase range range of of 77 °CC[ [37].37]. resulting in the LC phase range of 77◦ °C [37].

107 °C 107 癈

118 °C 188 °C 2C 107 118°C 癈 188 癈 2C 107 癈

119 °C 188 °C 1C

119 癈 188 癈 Heat Flow (W/g) 2H1C

Heat Flow (W/g) 104 °C 190 °C 2H 10411 2癈 °C 122 190 癈 °C 1H 112 癈 122 癈 108 °C 190 °C 1H 108 癈122 °C 190 癈 122 癈 50 100 150 200 250 50Temperature 100 150(°C) 200 250 Temperature (癈 ) FigureFigure 4. 4. DSCDSC thermograms thermograms of of EV8 EV8 obtained obtained at at heating heating and and cooling cooling rates rates of of 10 10 °C◦C /min./min. Figure 4. DSC thermograms of EV8 obtained at heating and cooling rates of 10 °C /min.

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Molecules 2020, 25, x FOR PEER REVIEW 7 of 20 (a) (b) (c) (a) (b) (c)

FigureFigure 5. The 5. X-rayThe X‐ dirayﬀ ractiondiffraction patterns patterns of EV8.of EV8. (a )(a Isotropic) Isotropic phase phase taken taken at at 200 200◦ ℃C;; ((b) Sharp inner inner arcs Figure 5. The X‐ray diffraction patterns of EV8. (a) Isotropic phase taken at 200 ℃; (b) Sharp inner of SmAarcs taken of SmA at taken 190 ◦ C;at 190 (c) SmA℃; (c) of SmA inner of inner ring sharpring sharp and and outer outer ring ring di ﬀdiffuseuse at at 180 180◦ C.°C. arcs of SmA taken at 190 ℃; (c) SmA of inner ring sharp and outer ring diffuse at 180 °C.

Figure 6. The focal‐conic texture of EV8 obtained at 160 °C on cooling from the isotropic phase Figure 6. The focal-conic texture of EV8 obtained at 160 ◦C on cooling from the isotropic phase (magnification(magnification 200 200×).). × Figure 7 shows the DSC thermograms of EV10 in its heating and cooling cycles. In the first FigureFigure 6.7 showsThe focal the‐conic DSC thermogramstexture of EV8 ofobtained EV10 in at its 160 heating °C on and cooling cooling from cycles. the isotropic In the first phase heating heating cycle, it showed three endotherms located at 106, 127, and 253 °C. In the second heating cycle, cycle,(magnification it showed three 200×). endotherms located at 106, 127, and 253 ◦C. In the second heating cycle, there there were also three endotherms located at 104, 123, and 251 °C. In each of the cooling cycles, there were also three endotherms located at 104, 123, and 251 ◦C. In each of the cooling cycles, there were were four exotherms. In the POM studies, we determined that its Tm at 104 °C and Ti at 251 °C resulted Figure 7 shows the DSC thermograms of EV10 in its heating and cooling cycles. In the first four exotherms.in an LC phase In the range POM of 147 studies, °C and we confirmed determined the thatresults, its Tasm reportedat 104 ◦C in and the Tliteraturei at 251 ◦[36].C resulted The in heatingan LCexotherm, phase cycle, range it showedprior of 147to threethe◦C andrecrystallization endotherms confirmed located theexotherm results, at 106, in as 127, reportedeach and of 253the in the°C.cooling literatureIn the cycles, second [36 ].presumably heating The exotherm, cycle, thereprior werecorresponded to the also recrystallization three to theendotherms transition exotherm from located the in SmAat each 104, to of another123, the coolingand (monotropic) 251 cycles, °C. In mesophase.each presumably of the The cooling corresponded corresponding cycles, there to the weretransition foursalt having exotherms. from n the‐decyl SmA In group the to POM another has astudies, Tm (monotropic) at 75 we °C anddetermined Ti mesophase. at 175 °Cthat resulting its The Tm correspondingat in 104 the °CLC andphase T i saltrange at 251 having of °C 100 resulted n-decyl°C ingroup an [37].LC has phaseHowever, a Tm rangeat we 75 ◦wereofC and147 at variance°C Ti atand 175 withconfirmed◦C the resulting fact thethat in results,there the was LC as phasean reported additional range in endotherm of the 100 literature◦C[ in37 between]. [36]. However, The Tm and Ti in each of the heating cycles, although the nature of this endotherm remains unknown. This exotherm,we were at prior variance to withthe recrystallization the fact that there exotherm was an additional in each endotherm of the cooling in between cycles, Tm andpresumably Ti in each correspondedof theendotherm heating to cycles, wasthe transitionprobably although related from the naturetheto the SmA LC of‐ to this‐ anotherLC endotherm transition. (monotropic) Similar remains to mesophase.EV8, unknown. both Tm This Theand endothermcorrespondingTi exhibited was saltprobably having related n‐decyl to group the LC-to-LC has a T transition.m at 75 °C and Similar Ti at to 175 EV8, °C bothresulting Tm and in the Ti exhibited LC phase little range supercooling of 100 °C [37].in contrast However, to many we were ILCs at that variance undergo with large the fact supercooling, that there was also an known additional as hysteresis endotherm [50–57 in]. between Tm and Ti in each of the heating cycles, although the nature of this endotherm remains unknown. This endotherm was probably related to the LC‐to‐LC transition. Similar to EV8, both Tm and Ti exhibited

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little supercooling in contrast to many ILCs that undergo large supercooling, also known as hysteresis [50–57]. Molecules 2020, 25, x FOR PEER REVIEW 8 of 20

little supercooling in contrast to many ILCs that undergo large supercooling, also known as hysteresis Molecules[50–57].2020, 25 , 2435 8 of 20

Figure 7. DSC thermograms of EV10 obtained at heating and cooling rates of 10 °C /min. Figure 7. DSC thermograms of EV10 obtained at heating and cooling rates of 10 ◦C /min. Figure 7. DSC thermograms of EV10 obtained at heating and cooling rates of 10 °C /min. FigureFigure8 8 shows shows the the DSC DSC thermograms thermograms of of EV12 EV12 in in its its heating heating and and cooling cooling cycles. cycles. In In the the first first heating cycle,Figure it showed8 shows thea large DSC endotherm thermograms located of EV12 at 105in its °C heating and a smalland cooling endotherm cycles. at In 131 the °C. first In the heating cycle, it showed a large endotherm located at 105 ◦C and a small endotherm at 131 ◦C. In the secondheating heating cycle, cycle, it showed there was a large also endotherm a large endotherm located at located 105 °C atand 104 a small°C and endotherm a small end at 131otherm °C. In at the 130 °C. second heating cycle, there was also a large endotherm located at 104 ◦C and a small endotherm at In eachsecond of the heating cooling cycle, cycles, there there was alsowere a largethree endotherm exotherms. located In the at POM 104 °C studies, and a small we determined endotherm at that 130 its°C. large 130 ◦C.In Ineach each of the of cooling the cooling cycles, cycles, there were there three were exotherms. three exotherms. In the POM studies, In the POM we determined studies, wethat determinedits large endotherm corresponded to Tm at 104 °C which was in excellent agreement as reported in the literature that itsendotherm large endotherm corresponded corresponded to Tm at 104 °C to Twhichm at 104was ◦inC excellent which wasagreement in excellent as reported agreement in the literature as reported [36]. The exotherm prior to the recrystallization exotherm in each of the cooling cycles presumably in the[36]. literature The exotherm [36]. The prior exotherm to the recrystallization prior to the exotherm recrystallization in each of exotherm the cooling in cycles each presumably of the cooling cyclescorrespondedcorresponded presumably to the to corresponded transitionthe transition from from to the the SmA transition SmA to to another another from the(monotropic)(monotropic) SmA to another mesophase. mesophase. (monotropic) We We tried tried to confirm mesophase. to confirm its reportedits reported Ti at T 305i at 305°C and°C and found found that that this this transition transition appeared in in the the first first heating heating cycle cycle followed followed by by We tried to confirm its reported Ti at 305 ◦C and found that this transition appeared in the first heating decompositiondecomposition (Figure (Figure S20), S20), since since the the T mT mendotherm endotherm in thethe first first heating heating cycle cycle was was not not reproducible reproducible in in cycle followed by decomposition (Figure S20), since the Tm endotherm in the first heating cycle the second heating cycle. However, we were at variance with the fact that there was an additional wasthe second not reproducible heating cycle. in the However, second heating we were cycle. at variance However, with we the were fact at that variance there was with an the additional fact that endotherm after Tm in each of the heating cycles, although the nature of this endotherm remains thereendotherm was an after additional Tm in each endotherm of the afterheating T incycles, each although of the heating the nature cycles, of although this endotherm the nature remains of this unknown. Presumably, they were relatedm to the LC‐to‐LC transition. Unlike EV8 and EV10, EV12 endothermunknown. Presumably, remains unknown. they were Presumably, related to they the were LC‐to related‐LC transition. to the LC-to-LC Unlike transition. EV8 and EV10, Unlike EV12 EV8 showed Tm after which it formed an SmA phase that persisted up to the decomposition temperature showed Tm after which it formed an SmA phase that persisted up to the decomposition temperature and EV10,at high EV12 temperature showed (vide Tm after infra) which [45–49]. it formed an SmA phase that persisted up to the decomposition temperatureat high temperature at high temperature(vide infra) [45–49]. (vide infra) [45–49].

95 癈 95 °C 127 癈 97 癈 127 °C 2C 97 °C 95 癈 2C 95 °C 128 癈 98 癈 1C Heat Flow(W/g) 128 °C 2H 98 °C 1C Heat Flow(W/g) 104 癈癈 130 1H2H 105 癈 104 °C 131 癈 130 °C 1H 50 100 150 200 250 105 °C Temperature131 °C (癈)

50 100 150 200 250 Figure 8. DSC thermograms of EV12 obtained at heating and cooling rates of 10 °C /min. Temperature (°C)

FigureFigure 8.8. DSC thermograms of EV12 obtained at heating andand coolingcooling ratesrates ofof 1010 ◦°CC //min.min. Figure9 shows the DSC thermograms of EV14 in its heating and cooling cycles. In the ﬁrst heating cycle, it showed four endotherms located at 11, 44, 76, and 106 ◦C. In the second heating cycle, there were three endotherms located at 12, 44, and 104 ◦C. The POM studies suggested that the highest-temperature endotherm at 104 ◦C is its Tm after which it transformed into SmA which persisted up to the decomposition at high temperature. This Tm was in excellent agreement with that Molecules 2020, 25, x FOR PEER REVIEW 9 of 20

Figure 9 shows the DSC thermograms of EV14 in its heating and cooling cycles. In the first heating cycle, it showed four endotherms located at 11, 44, 76, and 106 °C. In the second heating cycle, there were three endotherms located at 12, 44, and 104 °C. The POM studies suggested that the Moleculeshighest‐2020temperature, 25, 2435 endotherm at 104 °C is its Tm after which it transformed into SmA which9 of 20 persisted up to the decomposition at high temperature. This Tm was in excellent agreement with that in the literature [36]. However, it did not show Ti. In contrast, the corresponding salt with n‐tetradecyl in the literature [36]. However, it did not show Ti. In contrast, the corresponding salt with n-tetradecyl group has a Tm at 80 °C and Ti at 320 °C resulting in the LC phase range of 240 °C [37]. Again, we group has a Tm at 80 ◦C and Ti at 320 ◦C resulting in the LC phase range of 240 ◦C[37]. Again, were at variance with the Ti at 328 °C which was also determined by POM and not detected by DSC we were at variance with the Ti at 328 ◦C which was also determined by POM and not detected by [36]. Additionally, the additional endotherms prior to the Tm corresponded to crystal‐to‐crystal DSCtransition [36]. Additionally,which is known the additionalas polymorphism endotherms [58–62]. prior It to was the Tverifiedm corresponded by observations to crystal-to-crystal of a highly transitionbirefringence which texture. is known There as were polymorphism sharp textural [58 defects–62]. Itand was absence verified of homogeneity by observations in the of adomains highly birefringencesuggesting its texture. crystal phases. There were The features sharp textural for the defects DSC thermograms and absence of of EV16, homogeneity EV18, and in EV20 the domains in their suggestingheating and its cooling crystal cycles phases. were The essentially features identical for the DSC (Figures thermograms S21–S23) with of EV16, a minor EV18, exception. and EV20 They in their heating and cooling cycles were essentially identical (Figures S21–S23) with a minor exception. showed essentially similar Tms (101, 106, and 107 °C, respectively,) similar to EV14 (104 °C), after Theywhich showed they transformedessentially similar into SmA Tms phases (101, 106, (Figure and 107 10)◦ whichC, respectively,) persisted similarup to their to EV14 decomposition (104 ◦C), after at whichhigh temperatures. they transformed They into also SmA showed phases the (Figurepolymorphism 10) which phenomenon persisted up since to their there decomposition were additional at high temperatures. They also showed the polymorphism phenomenon since there were additional endotherms prior to the Tms. In the cases of EV16 and EV20, each of them exhibited an exotherm at endothermsca. 260 °C in prior the tofirst the heating Tms. In cycle the cases in the of SmA EV16 phase. and EV20 The, each reason of them for this exhibited exotherm an exotherm remains to at ca.be 260explored,◦C in the although first heating there cycle exists in a the precedence SmA phase. in the The literature reason for [22], this such exotherm as exotherms remains toin be the explored, smectic althoughphases of there other exists ILCs a without precedence any in definitive the literature answers. [22], suchHowever, as exotherms we speculate in the the smectic reason phases for the of otherexotherm ILCs was without related any to definitive conversion answers. of metastable However, state we of speculate smectic the phase reason to stable for the state exotherm of smectic was relatedphase, since to conversion it appeared of only metastable in the first state heating of smectic cycle. phase to stable state of smectic phase, since it appeared only in the first heating cycle.

90 °C

23 °C 2C

90 °C

22 °C 1C

Heat Flow (W/g) 12 °C 40 °C 104 °C 1H 11 °C 44 °C 76 °C 106 °C

0 50 100 150 200 250 Temperature (°C)

Figure 9. DSC thermograms of EV14 obtained at heating and cooling rates of 10 ◦C /min. Figure 9. DSC thermograms of EV14 obtained at heating and cooling rates of 10 °C /min. The thermodynamic properties of phase transition temperatures of EVn determined from DSC measurements and decomposition temperatures from TGA measurements are compiled in Table1. It shows that EV1 had the highest melting point of 164 ◦C in the series but no LC property. EV6 had the lower melting point of 105 ◦C as compared with EV1 and had no LC property, similar to EV1. Interestingly, all other members in the series formed SmA phases at their respective Tms. EV8 had the highest Tm and the rest of the members had essentially similar low Tms. EV8 and EV10 also exhibited Tis, thus resulting in the LC phase ranges of 68 and 147 ◦C, respectively. EV12–EV20 formed SmA phases at low Tms that persisted up to decomposition temperature at high temperatures. On the one hand, these results can be rationalized by the fact that the short alkyl (alkoxy) chain does not permit the nanosegregation between the ionic groups and hydrophobic groups, and thus prevents the formation layer structures. On the other hand, the long alkyl (alkoxy) chain permits the nanosegregation between the ionic moieties and hydrophobic moieties leading to layered smectic phases, in which ionic layers are alternated with hydrophobic alkoxy/alkyl layers, which is the essential criterion for the formation of ILCs [63–65]. The results also suggested that the length of the alkoxy had little effect on Tms but a significant effect on the Tis, which was in stark contrast to molecular LCs [66]. The longer the alkoxy chain, the higher the temperature at which an LC phase transformed into an isotropic phase Molecules 2020, 25, 2435 10 of 20 and the larger the LC temperature range [67]. These results are in excellent agreement with those of other ILCs including the Cnmim family (1-alkyl-3-methylimidazolium salts) [54,68,69]. Although triflimide counterion is a large size ion with diffuse electron cloud, it can provide low electrostatic attraction interactions with many cations, thus, leading to many ionic liquids, but it is also capable of forming ILCs with suitably designed cations such as extended viologen cation moieties which are similar to the viologen moieties reported earlier in the literature [31–34]. Specifically, the thermotropic liquid-crystallinity of the short alkoxy chain, as low as octyloxy in the extended viologen moiety, with this triflimide ion is quite an interesting result in the field of ILCs. Molecules 2020, 25, x FOR PEER REVIEW 10 of 20

Figure 10. Optical textures of EV16, EV18, and EV20 taken at (a) 240, (b) 150, and (c) 290 °C, Figure 10. Optical textures of EV16, EV18, and EV20 taken at (a) 240, (b) 150, and (c) 290 ◦C, respectively, respectively, displaying SmA phases (magnification 400×). displaying SmA phases (magniﬁcation 400 ). × Table1.TheThermodynamic thermodynamic properties properties of of phase phase transition transition temperatures temperatures of of extended EVn determined viologen bis(triﬂimide)from DSC measurements and decomposition temperatures from TGA measurements are compiled in Table 1. salts (EVn) obtained from DSC measurements and decomposition temperatures obtained from It shows that EV1 had the highest melting point of 164 °C in the series but no LC property. EV6 had thermogravimetric analysis (TGA) measurements. the lower melting point of 105 °C as compared with EV1 and had no LC property, similar to EV1. Interestingly, alla other members in the seriesb formed SmA phases at theirc respectived Tms. EV8e had the f Sample Tm ◦C Tm ◦C TLC-LC ◦C Ti ◦C ∆T ◦C Td ◦C highest Tm and the rest of the members had essentially similar low Tms. EV8 and EV10 also exhibited EV1 104 g (17.4), 164 (25.8) - - - - 370 EV6Tis, thus 75 g resulting(0.84), 105 in (38.2) the LC phase ranges -of 68 and 147 °C, respectively. - EV12–EV20 - formed - SmA 328 EV8phases at low T -ms that persisted 104 g up(4.6), to 112decompositiong (0.36), 122 (43.5) temperature -at high temperatures. 190 (1.8) On 68 the one 330 EV10hand, these results - can be rationalized 104by the (46.1) fact that the short 123 alkyl (0.6) (alkoxy) 251 chain (2.3) does not 147 permit 332 h EV12the nanosegregation - between the ionic 104 groups (56.8) and hydrophobic 130 (0.3) groups, 305and (3.4)thus prevents- the 331 g g h EV14formation layer - structures. 12On (2.1),the 40other(5.5), hand, 104 (55.9) the long alkyl - (alkoxy)328 chain permits- the 337 EV16 - 54 g (1.0), 82 g (3.3), 101 (49.2) - - - 337 EV18nanosegregation - between the 63 ionicg (2.4), moieties 70 g (6.2), and 106 hydrophobic (69.6) moieties - leading - to layered -smectic 331 EV20phases, in which - ionic layers 71 areg (2.9), alternated 83 g (4.5), with 107 (71.8) hydrophobic alkoxy/alkyl - layers, - which - is the 330 a essential criterion for the formation of ILCs [63–65]. The results also suggested that the length of the Tm = crystal-to-isotropic phase transition. Datum was taken from the second heating cycle of the DSC thermogram m i b at aalkoxy heating had rate little of 10 effect◦C/min. on TheT s value but a in significant the parentheses effect was on thethe enthalpy T s, which in kJwas/mol in for stark this transition.contrast to Tm = crystal-to-LCmolecular LCs phase [66]. transition. The longer Datum the alkoxy was taken chain, from the the higher second the heating temperature cycle of at the which DSC an thermogram LC phase at a c heatingtransformed rate of 10 into◦C/ min.an isotropic The value phase in the and parentheses the larger was the theLC enthalpytemperature in kJ /rangemol for [67]. this These transition. resultsT areLC-LC = d LC-to-LC phase transition. Ti = LC-to isotropic transition. Datum was taken from the second heating cycle of the in excellent agreement with those of other ILCs including the Cnmim family (1‐alkyl‐3‐ DSC thermogram at a heating rate of 10 ◦C/min. The value in the parentheses was the enthalpy in kJ/mol for this methylimidazoliume salts) [54,68,69]. Although triflimidef counterion is a large size ion with diffuse transition. ∆T = (Ti-Tm), that is, the LC phase range. Td = the temperature at which a 5% weight loss of the g saltelectron occurred cloud, at a heating it can provide rate of 10 low◦C /electrostaticmin in nitrogen. attractionCrystal-to-crystal interactions with transition. many Datum cations, was thus, taken leading from the secondto many heating ionic cycle liquids, of the but DSC it thermogram is also capable at a of heating forming rate ILCs of 10 ◦withC/min. suitably The value designed in the parenthesescations such was as the h enthalpyextended in kJ viologen/mol for this cation transition. moieties fromwhich reference are similar [36]. to the viologen moieties reported earlier in the literature [31–34]. Specifically, the thermotropic liquid‐crystallinity of the short alkoxy chain, as low as octyloxy in the extended viologen moiety, with this triflimide ion is quite an interesting result in the field of ILCs.

Molecules 2020, 25, x FOR PEER REVIEW 11 of 20

Table 1. Thermodynamic properties of phase transition temperatures of extended viologen bis(triflimide) salts (EVn) obtained from DSC measurements and decomposition temperatures obtained from thermogravimetric analysis (TGA) measurements.

Sample Tm a °C Tm b °C TLC‐LC c °C Ti d °C ΔT e °C Td f °C EV1 104 g (17.4), 164 (25.8) ‐‐ ‐ ‐370 EV6 75 g (0.84), 105 (38.2) ‐‐ ‐ ‐328 EV8 ‐104 g (4.6), 112 g (0.36), 122 (43.5) ‐ 190 (1.8) 68 330 EV10 ‐ 104 (46.1) 123 (0.6) 251 (2.3) 147 332 EV12 ‐ 104 (56.8) 130 (0.3) 305 h (3.4) ‐ 331 EV14 ‐ 12 g (2.1), 40 g (5.5), 104 (55.9) ‐ 328 h ‐ 337 EV16 ‐ 54 g (1.0), 82 g (3.3), 101 (49.2) ‐ ‐ ‐ 337 EV18 ‐ 63 g (2.4), 70 g (6.2), 106 (69.6) ‐ ‐ ‐ 331 EV20 ‐ 71 g (2.9), 83 g (4.5), 107 (71.8) ‐ ‐ ‐ 330

a Tm = crystal‐to‐isotropic phase transition. Datum was taken from the second heating cycle of the DSC thermogram at a heating rate of 10 ℃/min. The value in the parentheses was the enthalpy in kJ/mol for this transition. b Tm = crystal‐to‐LC phase transition. Datum was taken from the second heating cycle of the DSC thermogram at a heating rate of 10 ℃/min. The value in the parentheses was the enthalpy in kJ/mol for this transition. c TLC‐LC = LC‐to‐LC phase transition. d Ti = LC‐to isotropic transition. Datum was taken from the second heating cycle of the DSC thermogram at a heating rate

of 10 ℃/min. The value in the parentheses was the enthalpy in kJ/mol for this transition. e ΔT = (Ti‐ Tm), that is, the LC phase range. f Td = the temperature at which a 5% weight loss of the salt occurred at a heating rate of 10 ℃/min in nitrogen. g Crystal‐to‐crystal transition. Datum was taken from the Molecules 2020, 25, 2435 11 of 20 second heating cycle of the DSC thermogram at a heating rate of 10 ℃/min. The value in the parentheses was the enthalpy in kJ/mol for this transition. h from reference [36]. 2.3. Thermal Stabilities of Extended Viologen Bis(triflimide) Salts (EVn) 2.3. Thermal Stabilities of Extended Viologen Bis(triflimide) Salts (EVn) The stabilities for all extended viologen salts were studied by TGA analyses. They are defined as the The stabilities for all extended viologen salts were studied by TGA analyses. They are defined temperature (◦C) at which a 5% weight loss for each of the salts occurred at a heating rate of 10 ◦C/min inas nitrogen.the temperature Despite (°C) the at presence which a of 5% flexible weight alkyl loss chains, for each the of representative the salts occurred TGA at thermograms a heating rate of of five 10  saltsC/min were in plotted, nitrogen. as shownDespite in Figurethe presence 11, and displayof flexible relatively alkyl highchains, thermal the representative stabilities that areTGA in thermograms of five salts were plotted, as shown in Figure 11, and display relatively high thermal the temperature range of 330–370 ◦C (Table1, Figure S24). There were no apparent specific trends  alongstabilities the series.that are However, in the temperature EV1 had the range highest of thermal330–370 stability C (Table and 1, EV20 Figure had S24). the lowestThere were thermal no stability.apparent Triflimide specific trends is one along of best the counterion series. However, that imparts EV1 the had high the thermal highest stabilitythermal of stability any ionic and liquids, EV20 ILCs,had the and lowest ionic polymersthermal stability. reported Triflimide in the literature is one [of70 –best72], counterion because it hasthat non-nucleophilic imparts the high character thermal beingstability the of conjugate any ionic base liquids, of a super ILCs, acid.and ionic Therefore, polymers it causes reported decomposition in the literature of the associated[70–72], because cationic it moietieshas non‐ nucleophilicallynucleophilic character at relatively being high the temperatures. conjugate base of a super acid. Therefore, it causes decomposition of the associated cationic moieties nucleophilically at relatively high temperatures.

EV1 100 EV6 EV8 EV10 80 EV12

40 Weight (%) Weight

0 50 100 150 200 250 300 350 400 450 500 550 Temperature (°C)

Figure 11. TGA thermograms of EV1 EV1–EV12–EV12 obtained a heating rate of 10 ◦°C/minC/min in nitrogen. 2.4. Light Emission Properties of EVCl6 and EV6 All the extended viologens with dichloride and bis(triﬂimide) showed essentially identical, absorption, excitation, and emission spectra in methanol as measured by UV-Vis and luminescence spectrometer. Figure 12 shows the emission spectra of EV6Cl and EV6 in methanol that are representative of the light emission properties of this class of salts (Figures S25–S27). Each of them showed a major peak at 572 nm when excited at various excitation wavelengths of 205–260 nm. This major emission peak was independent of the counterions (chloride and triﬂimide) and the length of alkoxy groups attached to the extended viologen moiety. Molecules 2020, 25, x FOR PEER REVIEW 12 of 20

2.4. Light Emission Properties of EVCl6 and EV6 All the extended viologens with dichloride and bis(triflimide) showed essentially identical, absorption, excitation, and emission spectra in methanol as measured by UV‐Vis and luminescence spectrometer. Figure 12 shows the emission spectra of EV6Cl and EV6 in methanol that are representative of the light emission properties of this class of salts (Figures S25–S27). Each of them showed a major peak at 572 nm when excited at various excitation wavelengths of 205–260 nm. This Moleculesmajor2020 emission, 25, 2435 peak was independent of the counterions (chloride and triflimide) and the 12length of 20 of alkoxy groups attached to the extended viologen moiety. (a)

1000 205 nm 240 nm 900 252 nm 260 nm Monitored at 572 nm 800 572 nm 205 nm 532 nm 700 512 nm 280 nm 600

500 252 nm 400

Intensity(a.u) 300 424 nm

200 391 nm

100

0 200 300 400 500 600 700 800 Wavelength (nm)

(b)

1000 206 nm 252 nm 900 572 nm 260 nm 205 nm 280 nm 800 Monitored at 572 nm 280 nm 532 nm 700 512 nm 600

500 252 nm 400

Intensity (a.u) 300 430 nm 200

100

0 200 300 400 500 600 700 800 Wavelength (nm)

Figure 12. Emission spectra of (a) EV6Cl and (b) EV6 in methanol at various excitation wavelengths.

3. MaterialsFigure and 12. MethodsEmission spectra of (a) EV6Cl and (b) EV6 in methanol at various excitation wavelengths.

3.1.3. Instrumentation Materials and Methods

1 13 3.1.The InstrumentationH and C nuclear magnetic resonance (NMR) spectra of all the extended viologen bis(triﬂimide) salts in CD3OD or d6-DMSO were recorded by using VNMR 400 spectrometer operating 1 13 (Varian Inc.,The PaloH and Alto, C CA, nuclear USA.) atmagnetic 400 and resonance 100 MHz, at(NMR) room temperature.spectra of all Elemental the extended analysis viologen was performedbis(triflimide) by Atlanta salts Microlabin CD3OD Inc., or (Norcross, d6‐DMSO GA, were USA). recorded Diﬀerential by using scanning VNMR calorimetry 400 spectrometer (DSC) measurementsoperating (Varian of all theInc., salts Palo were Alto, conducted CA, USA.) on at a400 TA and module 100 MHz, DSC Q200at room series temperature. (TA Instruments, Elemental

New Castle, DE, USA) in nitrogen at heating and cooling rates of 10 ◦C/min. The temperature axis of the DSC thermograms was calibrated before use with reference standards of high purity indium and tin. Their thermogravimetric analyses (TGA) were performed using a TGA Q50 instrument (TA Instruments, New Castle, DE, USA) at a heating rate of 10 ◦C/min, in nitrogen. Optical studies were performed on these viologen salts sandwiched between a standard microscope glass slide and coverslip. The samples were heated and cooled on a Mettler hot-stage (FP82HT) and (FP90) Molecules 2020, 25, 2435 13 of 20 controller (Mettler Toledo, Columbus, OH, USA) and observations of the phases made between crossed polarizers of an Olympus BX51 microscope (Olympus America Inc., New York, NY, USA). In short, salts were heated above their clearing transitions, whenever possible, and cooled at 5 ◦C/min to room temperature, with brief pauses to collect images and observe specific transitions. In some cases, the salts were heated well above the Tms to make thin films, and photomicrographs were taken at specific temperatures. X-ray diffraction studies of salts contained in flame sealed 1 mm quartz capillaries were performed using a Rigaku Screen Machine (Rigacu, Inc., The Woodlands, TX, USA). The salt under study was placed inside the Linkam HFS350X-Cap capillary (Linkam Scientific Instruments, Tadworth, UK) hot-stage 78 mm away from the two-dimensional (2D) detector, with temperature controlled to the accuracy of 0.1 C. A magnetic field of ~2.5 kG was applied to the samples using a pair of ± ◦ samarium cobalt permanent magnets with B nearly parallel to the beam stop. Scattering patterns were collected using a Mercury 3 CCD detector (Rigacu, Inc., The Woodlands, TX, USA) with resolution 1024 1024 pixels (size, 73.2 µm 73.2 µm) and copper Kα radiation generated by a microfocus sealed × × X-ray tube with copper anode (λ = 1.542 Å). The UV-Vis absorption spectra in methanol were recorded with a Varian Cary 3 Bio UV-Vis spectrophotometer (Agilent Technologies, Santa Clara, CA, USA) at rt. Photoluminescence spectra in methanol solutions were recorded with a Perkin–Elmer LS 55 luminescence spectrometer (Perkin Elmer, Akron, OH, USA) with a xenon lamp light source.

3.2. Materials 4-Hydoxyacetanilide, n-hexyl bromide, n -octyl bromide, n -decyl bromide, n -dodecyl bromide, n -tetradecyl bromide, n -hexadecyl bromide, n -octadecyl bromide, eicosyl bromide, 4,40-bipyridine, 1-chloro-2,4-dintrobenzene, 4-methoxyaniline, potassium carbonate, sodium hydroxide, and lithium triﬂimide were purchased from TCI America (Portland, OR, USA) and used as received. For synthesis and puriﬁcation purposes, reagent grade solvents including acetonitrile, ethanol, methanol, ethyl acetate, N,N-dimethylacetamide (DMAc), spectral grade methanol were used as obtained from Sigma-Aldrich (Milwaukee, WI, USA).

3.3. Synthesis of 4-n-Alkoxyanilines (n = 6, 8, 10, 12, 14, 16, 18, 20) All the 4-n-alkoxyanilines were prepared according to the slightly modiﬁed literature procedure [41]. The modiﬁcation was the use of acetone instead of N,N-dimethylformamide in the alkylation of 4-hydroxyacetanilide with the respective n-alkyl bromide followed by hydrolysis of 4-n-alkoxyacetanilide. The overall yields for the synthesis of 4-n-alkoxyamines in two-step reaction were 74%, 60%, 67%, 62%, 845, 805, 85%, and 86%.

3.4. Synthesis of Zincke Salt Zincke salt was prepared from the reaction of 1-chloro-2,4-dinitrobenzene (2.5 equiv) with 4,40-bipyridine (1 equiv) on heating in acetonitrile according to the procedure described in the literature [42,43].

3.5. General Procedure for the Synthesis of Bis(4-alkoxyphenyl)-4,40-Bipyridinium Dichloride (EV1Cl, EV6Cl, EV8Cl, EV10Cl, EV12Cl, EV14Cl, EV16Cl, EV18Cl, and EV20Cl) The procedure that was adopted for the synthesis of EV1Cl from the reaction of Zincke salt with 4-methoxyaniline [44], as an example, is as follows: Ethanol was added to 1,10-bis(2,4-dinitrophenyl)-(4,40-bipyridine)-1,10-diium chloride (2.00 g, 3.56 mmol) and 4-methoxyaniline (1.10 g, 8.90 mmol) to form a yellow solution. The reaction mixture was heated on stirring to reflux under nitrogen, for 24 h. At the end of the reaction, it was brought to room temperature. Ethanol was removed from the reaction mixture using a rotary evaporator to yield the reaction products that were, then, dried overnight in vacuum to remove any residual solvent. Then, the desired crude product was obtained by removing 2,4-dintroaniline with plenty of hot acetone and collected by vacuum filtration. Finally, it was purified from recrystallization from methanol Molecules 2020, 25, 2435 14 of 20 yielding (1.44 g, 3.26 mmol) a yellow to dark yellow powder. Similarly, EV6Cl, EV8Cl, EV10Cl, EV12Cl, EV14Cl, and EV16Cl were prepared from the corresponding 4-n-alkoxyanilines and recrystallized from an appropriate solvent or solvent mixtures (vide infra). In the cases of EV18Cl and EV20Cl, the solvent DMAc was used because of better solubility of these amines instead of ethanol according to the procedure reported in the literature [36].

Data for EV1Cl: Yield 92%. δH (CD3OD, 400 MHz, ppm): 9.50 (4H, d, J = 7.2 Hz), 8.90 (4H, d, J = 7.2 Hz), 7.90 (4H, br), 7.32 (4H, br), 3.96 (6H, s). δC (CD3OD, 400 MHz, ppm): 162.48, 149.69, 145.35, 135.47, 126.87, 125.49, 115.41, 55.17.

Data for EV6Cl: Recrystallized from methanol/ethyl acetate, yield 81%. δH (CD3OD, 400 MHz, ppm): 9.49 (4H, d, J = 7.2 Hz), 8.79 (4H, d, J = 7.2 Hz), 7.89 (4H, br), 7.30 (4H, br), 4.15 (4H, t, J = 6.4 Hz), 1.81–1.87 (4H, m), 1.38–1.55 (12H, m), 0.96 (6H, t, J = 6.8 Hz). δC (CD3OD, 400 MHz, ppm): 161.94, 149.64, 145.30, 135.32, 126.88, 125.47, 115.85, 68.54, 31.30, 28.74, 25.36, 22.26, 12.98.

Data for EV8Cl: Recrystallized from methanol/ethyl acetate, yield 78%. δH (CD3OD, 400 MHz, ppm): 9.50 (4H, d, J = 7.2 Hz), 8.90 (4H, d, J = 7.2 Hz), 7.90 (4H, br), 7.31 (4H, br), 4.16 (4H, t, J = 6.4 Hz), 1.82–1.88 (4H, m), 1.33–1.53 (20H, m), 0.92 (6H, t, J = 6.8 Hz). δC (CD3OD, 400 MHz, ppm): 161.93, 149.64, 145.30, 135.32, 126.89, 125.48, 115.85, 68.54, 31.58, 29.04, 28.98, 28.78, 25.69, 22.31, 13.05.

Data for EV10Cl: Recrystallized from ethanol/ethyl acetate, yield 80%. δH (CD3OD, 400 MHz, ppm): 9.49 (4H, d, J = 7.2 Hz), 8.90 (4H, d, J = 7.2 Hz), 7.89 (4H, br), 7.30 (4H, br), 4.16 (4H, t, J = 6.4 Hz), 1.83–1.87 (4H, m), 1.32–1.53 (28H, m), 0.92 (6H, t, J = 6.8 Hz). δC (CD3OD, 400 MHz, ppm): 161.94, 149.64, 145.30, 135.31, 126.87, 125.46, 115.85, 68.54, 31.65, 29.31, 29.28, 29.07, 29.04, 28.78, 25.69, 22.32, 13.05.

Data for EV12Cl: Recrystallized from ethanol, yield 60%. δH (CD3OD, 400 MHz, ppm): 9.49 (4H, d, J = 7.2 Hz), 8.90 (4H, d, J = 7.2 Hz), 7.89 (4H, br), 7.30 (4H, br), 4.16 (4H, t, J = 6.4 Hz), 1.83–1.87 (4H, m), 1.32–1.53 (36H, m), 0.92 (6H, t, J = 6.8 Hz). δC (CD3OD, 400 MHz, ppm): 161.94, 149.64, 145.30, 135.31, 126.87, 125.46, 115.85, 68.54, 31.65, 29.31, 29.28, 29.07, 29.04, 28.78, 25.69, 22.32, 13.05.

Data for EV14Cl: Recrystallized from ethanol, yield 80%. δH (CD3OD, 400 MHz, ppm): 9.49 (4H, d, J = 5.6 Hz), 8.88 (4H, d, J = 6.0 Hz), 7.87 (4H, br), 7.30 (4H, br), 4.16 (4H, t, J = 6.4 Hz), 1.82–1.87 (4H, m), 1.30–1.54 (44H, m), 0.92 (6H, t, J = 6.8 Hz).

Data for EV16Cl: Recrystallized from ethanol, yield 82%. δH (CD3OD, 400 MHz, ppm): 9.50 (4H, d, J = 6.8 Hz), 8.88 (4H, d, J = 7.2 Hz), 7.87 (4H, br), 7.30 (4H, br), 4.14 (4H, t, J = 6.4 Hz), 1.87–1.78 (4H, m), 1.22–1.42 (52H, m), 0.91 (6H, t, J = 6.8 Hz).

Data for EV18Cl: Recrystallized from ethanol, yield 45%. δH (CD3OD, 400 MHz, ppm): 9.49 (4H, d, J = 7.2 Hz), 8.80 (4H, d, J = 7.2 Hz), 7.87 (4H, br), 7.30 (4H, br), 4.16 (4H, t, J = 6.4 Hz), 1.83–1.87 (4H, m), 1.29–1.54 (60H, m), 0.91 (6H, t, J = 6.8 Hz).

Data for EV20Cl: Recrystallized from ethanol, yield 45%. δH (CD3OD, 400 MHz, ppm): 9.50 (4H, d, J = 6.8 Hz), 8.87 (4H, d, J = 7.2 Hz), 7.87 (4H, br), 7.30 (4H, br), 4.16 (4H, t, J = 6.4 Hz), 1.82–1.87 (4H, m), 1.29–1.54 (68H, m), 0.91 (6H, t, J = 6.8 Hz).

3.6. Synthesis of bis(4-Methoxyphenyl)-4,40-Bipyridinium Bis(triﬂimide) Salt (EV1) A clear solution of 0.95 g (3.31 mmol) lithium triflimide in 5 mL water was added dropwise to the clear solution of 0.66 g (1.50 mmol) of EV1Cl in 25 mL water on stirring at room temperature. The precipitation occurred immediately in the reaction mixture and it was stirred overnight to complete the reaction. Then, the product was filtered, washed with plenty of water, and dried in a vacuum to yield 1.19 g (1.28 mmol) of the pure product EV1. Yield 85%. δH (CD3OD, 400 MHz, ppm): 9.44 (4H, d, J = 6.8 Hz), 8.80 (4H, d, J = 6.8 Hz), 7.84 (4H, br), 7.30 (4H, br), 3.95 (6H, s). δC (CD3OD, 400 MHz, ppm): 162.49, 149.87, 145.34, 135.26, 126.83, 125.44, 121.28, 118.09, 115.38, 55.13. Anal. calcd Molecules 2020, 25, 2435 15 of 20

for C28H22N4O8F12S4 (930.74): C, 36.13; H, 2.38; N, 6.02; S, 13.78%. Found C, 36.42; H, 2.26; N, 6.09; S, 13.66%.

3.7. General Procedure for the Synthesis of Bis(4-n-alkoxyphenyl)-4,40-bipyridinium Bis(triflimide) Salts (EV6, EV8, EV10, EV12, EV14, EV16, EV18, EV20) by Metathesis Reaction The following procedure [31–33], as an example, was adopted for the synthesis of EV20 from the metathesis reaction of EV20Cl with lithium triflimide. An amount of 0.33 g (0.34 mmol) EV20Cl was dissolved in methanol on boiling to get a clear solution to which clear methanol solution of lithium triflimide 0.22 g (0.75 mmol) dissolved in 5 mL was added on stirring. The reaction mixture was heated to reflux on stirring overnight for the completion of the metathesis reaction. The product precipitated out on cooling the reaction mixture to the ambient temperature. It was collected by filtration, washed with plenty of water, and recrystallized from methanol yielding 0.24 g (0.16 mmol). In the cases of EV6, EV8, EV10, and EV12, after the completion of the metathesis reaction, methanol was removed by rotatory evaporator to give the crude product that was repeatedly washed with water to give the pure product.

Data for EV6: Yield 67%. δH (CD3OD, 400 MHz, ppm): 9.43 (4H, d, J = 6.8 Hz), 8.79 (4H, d, J = 6.8 Hz), 7.83 (4H, br), 7.29 (4H, br), 4.15 (4H, t, J = 6.4 Hz), 1.81–1.87 (4H, m), 1.36–1.55 (12H, m), 0.96 (6H, t, J = 7.2 Hz). δC (CD3OD, 400 MHz, ppm): 161.97, 149.82, 145.29, 135.26, 126.82, 125.41, 121.28, 118.09, 115.82, 68.53, 31.28, 28.71, 25.33, 22.23, 12.93. Anal. calcd for C38H42N4O10F12S4 (1071.00): C, 42.61; H, 3.95; N, 5.23; S, 11.98%. Found C, 42.38; H, 3.38; N, 5.20; S, 12.27%.

Data for EV8: Yield 79%. δH (CD3OD, 400 MHz, ppm): 9.43 (4H, d, J = 7.2 Hz), 8.79 (4H, d, J = 7.2 Hz), 7.84 (4H, br), 7.29 (4H, br), 4.15 (4H, t, J = 6.4 Hz), 1.81–1.87 (4H, m), 1.33–1.54 (20H, m), 0.93 (6H, t, J = 7.2 Hz). δC (CD3OD, 400 MHz, ppm): 161.96, 149.82, 145.29, 135.26, 126.83, 125.41, 121.28, 118.09, 115.83, 68.52, 31.55, 29.01, 28.96, 28.75, 25.66, 22.28, 13.00. Anal. calcd for C42H50N4O10F12S4 (1127.11): C, 44.76; H, 4.47; N, 4.97; S, 11.38%. Found C, 44.54; H, 4.48; N, 4.92; S, 11.40%.

Data for EV10: Yield 80%. δH (CD3OD, 400 MHz, ppm): 9.43 (4H, d, J = 6.8 Hz), 8.79 (4H, d, J = 6.8 Hz), 7.82 (4H, br), 7.28 (4H, br), 4.15 (4H, t, J = 6.4 Hz), 1.81–1.88 (4H, m), 1.32–1.54 (28H, m), 0.92 (6H, t, J = 7.2 Hz). δC (CD3OD, 400 MHz, ppm): 161.96, 149.82, 145.28, 135.25, 126.83, 125.41, 121.28, 118.09, 115.83, 68.52, 31.64, 29.35, 29.26, 29.05, 29.02, 28.75, 25.66, 22.30, 13.02. Anal. calcd for C46H58N4O10F12S4 (1183.21): C, 46.69; H, 4.94; N, 4.71; S, 10.84%. Found C, 46.44; H, 4.84; N, 4.73; S, 10.73%.

Data for EV12: Yield 79%. δH (CD3OD, 400 MHz, ppm): 9.44 (4H, br), 8.79 (4H, br), 7.83 (4H, d, J = 8.8 Hz), 7.28 (4H, d, J = 8.8 Hz), 4.15 (4H, t, J = 6.4 Hz), 1.83–1.87 (4H, m), 1.31–1.54 (36H, m), 0.92 (6H, t, J = 7.2 Hz). δC (CD3OD, 400 MHz, ppm): 161.98, 149.82, 145.29, 135.25, 126.81, 125.40, 121.29, 118.09, 115.83, 115.75, 68.53, 31.64, 29.35, 29.32, 29.29, 29.04, 28.76, 25.66, 22.30, 13.01. Anal. calcd for C50H66N4O10F12S4 (1239.32): C, 48.46; H, 5.37; N, 4.52; S, 10.35%. Found C, 48.48; H, 5.32; N, 4.51; S, 10.20%.

Data for EV14: Recrystallized from ethanol, yield 72%. δH (d6-DMSO, 400 MHz, ppm): 9.60 (4H, d, J = 6.8 Hz), 8.98 (4H, d, J = 6.8 Hz), 7.88 (4H, br), 7.30 (4H, br), 4.11 (4H, t, J = 6.8 Hz), 1.73–1.76 (4H, m), 1.22–1.44 (44H, m), 0.85 (6H, t, J = 6.8 Hz). δC (d6-DMSO, 400 MHz, ppm): 161.39, 148.75, 146.04, 145.99, 135.52, 126.90, 126.59, 124.70, 121.50, 118.30, 116.11, 68.79, 31.73, 29.49, 29.48, 29.45, 29.20, 29.15, 28.93, 25.87, 22.53, 14.36. Anal. calcd for C54H74N4O10F12S4 (1295.43): C, 50.07; H, 5.76; N, 4.32; S, 9.90%. Found C, 50.61; H, 5.77; N, 4.31; S, 9.79%.

Data for EV16: Recrystallized from ethanol, yield 74%. δH (d6-DMSO, 400 MHz, ppm): 9.60 (4H, br), 8.98 (4H, br), 7.87 (4H, br), 7.29 (4H, br), 4.11 (4H, t, J = 5.6 Hz), 1.71–1.78 (4H, m), 1.22–1.42 (52H, m), 0.84 (6H, t, J = 6.4 Hz). δC (d6-DMSO, 400 MHz, ppm): 161.39, 148.75, 146.08, 146.03, 145.99, 135.52, 126.90, 126.59, 124.70, 121.50, 118.30, 116.11, 68.80, 31.73, 29.49, 29.48, 29.45, 29.20, 29.15, 28.93, 25.88, 22.52, 14.37. Anal. calcd for C58H82N4O10F12S4 (1351.53): C, 51.54; H, 6.12; N, 4.15; S, 9.49%. Found C, 51.63; H, 6.11; N, 4.06; S, 9.31%. Molecules 2020, 25, 2435 16 of 20

Data for EV18: Recrystallized from ethanol, yield 61%. δH (d6-DMSO, 400 MHz, ppm): 9.59 (4H, br), 8.98 (4H, br), 7.87 (4H, br) 7.29 (4H, br) 4.11 (4H, t, J = 6.4 Hz), 1.71–1.78 (4H, m), 1.22–1.42 (60H, m), 0.84 (6H, J = 6.4 Hz). δC (d6-DMSO, 400 MHz, ppm): 161.39, 148.76, 146.04, 145.99, 135.52, 126.90, 126.59, 124.70, 121.50, 118.30, 116.11, 68.79, 31.74, 29.48, 29.45, 29.40, 29.39, 29.38, 29.23, 29.15, 28.95, 25.89, 22.53, 14.34. Anal. calcd for C62H90N4O10F12S4 (1407.64): C, 52.90; H, 6.44; N, 3.98; S, 9.11%. Found C, 53.10; H, 6.53; N, 4.01; S, 8.99%.

Data for EV20: Recrystallized from methanol, yield 47%. δH (d6-DMSO, 400 MHz, ppm): 9.63 (4H, d, J = 6.4 Hz), 9.02 (4H, d, J = 6.4 Hz), 7.90 (4H, br) 7.33 (4H, br) 4. 14 (4H, t, J = 6.4 Hz), 1.75–1.79 (4H, m), 1.24–1.46 (68H, m), 0.87 (6H, J = 6.8 Hz). δC (d6-DMSO, 400 MHz, ppm): 161.38, 148.69, 146.01, 135.52, 126.87, 126.61, 124.70, 121.50, 118.30, 116.12, 68.80, 31.72, 29.47, 29.46, 29.44, 29.21, 29.13, 28.94, 25.88, 22.52, 14.38. Anal. calcd for C66H98N4O10F12S4 (1463.75): C, 54.16; H, 6.75; N, 3.83; S, 8.76%. Found C, 54.27; H, 6.60; N, 3.86; S, 8.87%.

4. Conclusions A series of bis(4-alkoxyphenyl) viologen bis(triflimide) salts with alkoxy chains of different lengths were synthesized by the metathesis reaction of respective bis(4-alkoxyphenyl) viologen dichloride salts, which in turn prepared the reaction of Zincke salt with the corresponding 4-n-alkoxyanilines, with lithium triflimide in methanol. Their chemical structures were established using spectroscopic techniques and elemental analysis. Their thermotropic LC properties as determined by DSC, POM, and VT-XRD suggested that the salts with short alkoxy chains (n = 1 and 6 carbon atoms) did not form LC phase on melting, but the salts with long alkoxy chains (n = 8 and 10 carbon atoms) exhibited crystal–LC transitions, Tms, and LC–isotropic transitions, Tis, and showed Schlieren or focal conic textures indicative of their SmA phases. Those of higher alkoxy chains (n = 12, 14, 16, 18, and 20) exhibited SmA phases at relatively low Tms and their SmA phase persisted up to their decomposition at high temperatures. The majority of them also exhibited crystal-to-crystal transitions prior to the Tms. They had good thermal stability in the temperature range of 330–370 ◦C. These results suggest that they exhibited LC phases at relatively low Tms which have practical implications. Importantly, triflimide counterion is not only interesting for the preparation of ILs but also interesting for the preparation of ILCs, an important class of LC.

Supplementary Materials: The following are available online at http://www.mdpi.com/1420-3049/25/10/2435/s1, Figures S1–S5: 1H and 13C spectra of EV1Cl, EV6Cl, EV8Cl, EV10Cl, and EV12Cl, Figures S6–S9: 1H spectra of EV14Cl, EV16Cl, EV18Cl and EV20Cl, Figures S10–S18: 1H and 13C NMR spectra of EV1, EV6, EV8, EV10, EV12, EV14, EV16, EV18, and EV20, Figures S19–S23: DSC thermograms of EV6, EV12, EV16, EV18, and EV20, Figure S24: TGA thermograms of EV14, EV16, EV18, and EV20, Figures S25–S27: Emission spectra of EV1Cl and EV1, EV8Cl and EV8, and EV10Cl and EV10. Author Contributions: Conceptualization, P.K.B., H.H., and H.D.M.; methodology, M.K.M.A.-K, S.T.K., E.J.D., J.K., S.L.C., A.H., R.G.C., B.G., and K.M.; software, E.J.D., A.C., R.C.G.P, D.M.A.-K., and L.S.; validation, P.K.B., H.H., and R.G.C.P.; formal analysis, R.C.G.P. and H.H.; investigation, P.K.B., H.H., and M.R.F.; resources, P.K.B.; data curation, H.H., E.J.D., and R.C.G.P.; writing—original draft preparation, P.K.B., E.J.D., and S.L.C.; writing—review and editing, P.K.B., H.H., and S.L.C.; visualization, P.K.B., R.C.G.P., and M.R.F.; supervision, P.K.B. and H.H.; project administration, P.K.B. and S.K.; funding acquisition, P.K.B., S.K., and H.D.M. All authors have read and agreed to the published version of the manuscript. Funding: This work is in part supported by the NSF under grant no. 0447416 (NSF EPSCoR RING-TRUE III), grant no. DMR-1410649 (US Ireland Partnership), NSF-Small Business Innovation Research (SBIR) Award (grant OII-0610753), NSF-STTR Phase I IIP-0740289, NASA GRC contract no. NNX10CD25P, and Welch Foundation grant no. BS0040. Acknowledgments: We at the University of Nevada Las Vegas thank Jung Jae Koh for his help in the measurement of light-emitting properties of extended viologen bis(triflimide) salts. We (H.D.M., R.G.C., B.G., and K.M.) at Texas A & M International University, thank the Department of Biology and Chemistry for financial support of this research project. Conflicts of Interest: The authors declare no conflict of interest. Molecules 2020, 25, 2435 17 of 20

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Sample Availability: Samples of the compounds are not available from the authors.

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