Viologen capped by nucleobase—building blocks for functional materials: synthesis and structure–properties relationship

The current study presents a new class of functional derivatives (1–3) consisting of a dicationic viologen (4,4’-bipyridinium unit) (V2+) capped by nucleobases thymine (NB1), adenine (NB2), thymine/adenine (NB1, NB2), and ion-paired with amphiphilic anion 3,4,5-tris(dodecyloxy)benzene sulfonate (DOBS−). The target of our work focuses on the design and synthesis of molecular building blocks in which three different functionalities are combined: chromophore (V2+ unit), molecular recognition (NB unit), and thermotropic liquid crystal (DOBS unit). The resulted materials exhibit liquid crystalline properties at ambient temperature with significant particularities-induced by nucleobases in the mesogen structure. Structure–properties relationship study focuses on providing knowledge about (1) how the thermotropic, redox properties, thermochromism, or ionic conductive properties are influenced by the presence of purinic or pyrimidinic nucleobases, and (2) how effective is their ability to self-assembly by hydrogen bonding in nonpolar solvents. The presence of nucleobases has been proved to have a substantial impact on electron transfer rate during the reduction of viologen moieties by intermolecular aggregation. Ionic conductivity and thermochromic properties of derivatives 1–3 were investigated and compared to a non-containing nucleobase analog methyl viologen with 3,4,5-tris(dodecyloxy)benzene sulfonate anion (MV) as reference.

The bulk solid-state behavior of such complexes, obtained by ion-pairing dicationic nucleobase-appended viologens with amphiphilic anion 3,4,5-tris(dodecyloxy)benzene sulfonate (1-3) was investigated. The systematic structure-properties relationship study focuses on (1) how the thermotropic, redox properties, thermochromism, and ionic conductive properties are influenced by the presence of nucleobases, and (2) how effective is their ability to self-assembly by hydrogen bonding in a nonpolar solvent.
Such multifunctional building-block molecules can form self-assembling structures by ionic interactions, hydrogen bonding, charge transfer, p-p, and van der Waals interactions that have potential applications in nanotechnology, crystal engineering, and medicine.

Experimental section
Synthesis procedure A solution in water (0.01 mol L -1 ) of 1, 2, or 3, as iodine salt, was vigorously stirred for 12 h at 21°C with an equal volume of cesium 3,4,5-tris(dodecyloxy)benzene sulfonate (0.01 mol L -1 ) solution in chloroform. Further on, the organic phase was separated, washed two times with 20 mL distilled water, dried over MgSO 4 , and subsequently evaporated under reduced pressure. The resulted yellow solid was freeze-dried from 40 mL benzene to obtain the ionic complexes 1, 2, and 3, respectively.

Thermotropic characterization
DSC measurements were taken on samples of approx. 4-12 mg. Phase transitions were measured on a Netzsch DSC 204 Phoenix differential scanning calorimeter. The heating and cooling rates were 10°C min -1 . Indium, tin, and cyclohexane were used as calibration standards. Thermo-optical polarizing microscopy (TOPM) was realized with a Zeiss AXIOPLAN 2 polarizing microscope, equipped with a Mettler FP 80 hot stage. Pictures were taken using a digital Zeiss AxioCam MRC4 camera with a resolution of 4 megapixels combined with Zeiss AxioVision software.

Diffusion-ordered spectroscopy
DOSY experiments were performed on a Bruker AMX-500 spectrometer using the Bruker pulse program ledbpgp2s. Diffusion time (D20) was 60 ms, the length of the diffusion gradient variable between 600 and 2500 ls (P30) and the gradient recovery delays 200 ls (D16) in deuterated chloroform or THF. Temperature-dependence DOSY measurements were applied to a sample of respective ionic complex 1-3 (50 mM) in the temperature range from 20 to 50°C. Concentration-dependence DOSY experiments were performed at 30°C on samples of different concentrations ranging from 3 to 50 mM of ionic complex 1-3. The deuterated solvent used for temperature and concentration-dependence experiments, respectively, was THF-d 8 .

Electrochemical characterization
Cyclic voltammetry experiments were performed under argon in a three-electrode electrochemical cell with the potentiostat PGSTAT 302 N from AUTO-LAB controlled by a PC running under GPES from Windows, version 4.9 (ECO Chemie B.V.). A glassy carbon electrode (GCE) from Metrohm (Germany) with an electrochemically active surface area of A = 0.031 cm 2 was used as a working electrode. The surface of the working electrode was polished before the measurement with Al 2 O 3 . The reference electrode was Ag/AgCl (3 M, KCl in water), and the counter electrode was a Pt wire. The analyte concentration was 1 mmol/L dissolved in THF containing lithium bis(trifluoromethane)sulfonimide (0.1 mol L -1 ) as a supporting electrolyte.
Ionic conductivity 5-7 mg solid material 1, 2, and 3, respectively, was placed between two ITO conductive glass slides, heated at 51°C, and compressed to prepare a uniform film with a rectangular surface area of 0.25 cm 2 and approx. 50 lm thickness. The conductivity experiments were performed in a two-electrode system by applying a constant potential (-2 V) between the two ITO electrodes. The flowing current in the external electrical circuit was monitored upon the time while the temperature was ramped with 4°C min -1 in the range from 30 to 90°C.
During the ionic exchange reaction, the color of the organic phase changed from colorless to yellow, which is attributed to the transfer of viologen dication from the aqueous to chloroform phase. The ionic complexes, formed between dicationic viologens 1-3 and 3,4,5-tris(dodecyloxy)benzene sulfonate, were isolated from the organic phase in excellent yield ([ 93%) and characterized using 1 H-, 13 C-NMR and IR spectroscopy. (cf. SI for detailed spectroscopic data). Their purity exceeded 98% accordingly with elemental analysis results. Viologen 2? /DOBSratio was deduced from the relative integral peaks of the respective ionic components and was found to be 1/2 (Fig. 1). The experimentally determined value of Viologen 2? /DOBSratio in the corresponding ionic complex demonstrates that: (i) the interaction between Viologen 2? and DOBS -, respectively, is electrostatic as per one unit of Viologen 2? it is required theoretically two DOBSunits to ensure electrostatic neutrality, and (ii) the phase transfer reaction occurred quantitatively until the dicationic viologen from chloroform solution completely replaced the cesium.

Thermotropic behavior
Thermal properties of ionic complexes 1-3 were investigated using differential scanning calorimetry (DSC) and thermo-optical polarizing microscopy (TOPM). A comparison with the previously reported analog MV 12 was performed in order to understand the influence of nucleobases capping groups on the thermal behavior of ionic complexes 1-3. Figure 2a-c illustrates the corresponding 2nd heating-cooling cycles of ionic complexes 1-3. At 8.2°C for 1, -17.7°C for 2, and -24°C for 3, respectively, the endothermic peak corresponds to the melting transition from crystalline to mesophase state. Figure 2d-f shows the optical micrographs of derivatives 1-3 at 110°C. The colored domains seen between the cross-polarizers above melting transition indicate that all three nucleobase-appended viologens exhibited optical anisotropy. Shearing experiments performed by mutual sliding of the microscope slides proved the liquid nature of the observed material; hence, the compounds are in a mesomorphic state.
The transition from crystal to LC state of all three complexes that occurred at low temperature was compared to analog, derivative MV 21 (DOBS)2 (T m-= 64.7°C) [12]. This is attributed to the presence of thymine and adenine in the mesogenic unit's chemical structure, which influence the supramolecular arrangement, i.e., destabilize the crystalline state.
The undisturbed texture of the mesophase, as obtained by cooling from the isotropic state, has been obtained only for compound 1 (Fig. 2d), which is the only ionic complex stable up to its isotropization temperature. The fan-like texture of 1 suggests a columnar type mesophase, an expectable supramolecular arrangement for a discotic mesogen such as 3,4,5-tris(dodecyloxy)benzene sulfonate anion [22]. Notably, the mesophase texture is different from the analog MV [16] revealing the influence of thymine capping units on supramolecular arrangement. The isotropization of 1 (192°C) occurred at a lower temperature than MV (T iso = 270°C).
The compounds containing adenine as capping groups (2 and 3) were not thermally stable up to the isotropization point. They changed the color irreversibly into dark brown at elevated temperatures ([ 160°C) concomitantly with the loss of optical anisotropy. For this reason, it was impossible to obtain an undisturbed texture with characteristic features of the mesophase by cooling from the isotropic state. Ionic complexes 2 and 3 are thermally unstable due to irreversible oxidation of adenine in a media containing a suitable electron acceptor such as viologen. Irreversible electro/oxidation of adenine is well known and follows a complex mechanism [23,24]. TGA analysis showed the ionic complexes losing mass at 296°C for compound 1, 306°C for 2, and 300°C for compound 3 (cf. supporting information).
The phase transition schemes of derivatives 1-3 are summarized in Table 1.

Self-association by hydrogen bonding in low-polar solvents
Ionic complexes of 4,4'-bipyridinium capped by nucleobases with amphiphilic anion DOBSare expected to self-associate in solvents with low polarity due to the ability of nucleobase moieties to undergo in-plane hydrogen bonding. The self-association of ionic complexes 1-3 were investigated in solution by 1 H-NMR spectroscopy (i.e., one-dimensional 1 H-NMR and DOSY) and compared with nucleobase-free MV. The influence of solvent nature, temperature, and concentration were systematically checked.
The 1 H-NMR spectra at 30°C in deuterated chloroform of the individually ionic complexes 1-3 and MV, respectively, are represented in Fig. 3. Except for the compound MV, all the other ionic complexes showed a line broadening of the spectra, increasing in order 1 \ 3 \ 2.
The 1 H-NMR line broadening effect arises from the self-association of the molecules into higher molecular weight aggregates [25]. Previous research showed that thymine (T) and adenine (A) nucleobases could undergo many hydrogen bonding motifs in low-polar solvents. These motifs include self-associated cyclic dimers T-T, A-A, trimer A-A-A (k ass-\ 4 M -1 ), or complementary T-A base-pairing with higher association constant (k ass [ 100 M -1 ) [26][27][28][29][30]. Our results show that the self-association of nucleobase-appended viologens in solvents with low polarity is induced by adenine and thymine, which are capable of hydrogen bonding.
The magnitude of signal attenuation seen in 1 H-NMR spectra is interpreted as a qualitative appreciation of the self-association degree. Comparing the relative signal broadening of the ionic complexes 1-3 (Fig. 3), the compound with the highest degree of self-association is 2, containing two adenine units, while the ionic complex with the lowest degree of self-association is 1 containing two thymine units. The ionic complex 3, comprising an adenine and a thymine group, has an intermediary self-association degree. Remarkably, the ionic complexes capped by adenine units exhibited the highest degree of selfassociation. This is explained by the nature of purinic adenine moiety, which contains more hydrogen bonding sites over the thymine [28], which enhances the supramolecular self-assembly process of the corresponding nucleobase-appended viologens.
Imino proton of thymine in compound 3 (N 3 -H, 11.46 ppm, ma inset in Fig. 3) appeared broadened, and downfield shifted compared to imino proton (N 3 -H, 10.35 ppm) in compound 1. This observation suggests that the thymine unit in compound 3 is associated with hydrogen bonding with the complementary adenine. Isotropization temperature determined by thermo-optical analysis; c Thermally unstable beyond 160°C; Cr = solid crystal, U = mesophase, I = isotropic phase An overlap of the three individual DOSY spectra of ionic complexes 1, 2, and 3, respectively, in CDCl 3 at 30°C, is shown in Fig. 4. The diffusion coefficients of the ionic complexes increase in the order 1 \ 3 \ 2. The observed difference in the diffusion of ionic complexes 1-3 is explained by the different self-association degrees. This trend is in complete agreement with the assumption derived from previously presented one-dimensional 1 H-NMR experiments.
The self-association by hydrogen bonding of ionic complexes 1-3 is an equilibrium process. The molar ratio of associated and non-associated molecular species depends on the thermodynamic parameters (temperature, concentration) and association constant (K ass ). In respect to this, a series of DOSY experiments were performed to investigate the influence of thermodynamic parameters, i.e., temperature and concentration. Figure 5 shows the measured diffusion coefficients of compound 2 as a function of temperature and concentration. From the trend of diffusion coefficients, it can be concluded: (1) by temperature increase, the self-association degree decreases because of thermal-induced hydrogen bond breaking, and (2) by concentration increase, the self-association degree of the molecular species increases.
Further on, it was investigated the self-association behavior of ionic complexes 1-3 in three different solvent media: chloroform, THF, and a mixture of THF: water (4:1 volume ratio) (see Fig. 6). The highest degree of self-association was observed in chloroform, where peak signals of respective ionic complexes appeared significantly diminished and broadened compared with the other two solvent systems. In THF, a solvent with higher polarity than   chloroform, the ionic complexes exhibited a lower broadening of the peaks suggesting a lower degree of self-association. When the solvent was a mixture of THF: water, practically no self-association occurred since no line broadening was observed. The self-association of nucleobase-appended viologens was inhibited in THF: water, obviously due to the competitive hydrogen bonding with water molecules [31][32][33].
Combined experiments, DOSY, and one-dimensional 1 H-NMR proved that the self-association of nucleobase-appended viologens 1-3 is a process strongly influenced by the type of nucleobase (thymine or adenine), solvent media as well as by the thermodynamic parameters, temperature, and concentration. The crucial role of nucleobases in the selfassociation behavior of complexes 1-3 was highlighted by direct comparison with the analogs derivative MV, which did not self-associate in the same environmental conditions. It is noteworthy that viologens capped dihexadecylviologen bistriflimide can self-associate by electrostatic forces in low-polar solvents [34]. In our system, the self-association by electrostatics was insignificant, presumably due to a much bulky counter anion DOBS.

Electrochemical properties
Electrochemical characterization of solution of 1-3 was performed to regard the influence of the self-association process on the redox properties of the 4,4'-bipyridinium unit. The compounds 1-3 and analog, derivative MV, respectively, were investigated by means of cyclic voltammetry.
The cyclic voltammograms in Fig. 7 represent the typical reversible reduction process of the 4,4'bipyridinium unit from dicationic state (V 2? ) to radical cation (V ?• ) and further to the neutral species (V 0 ). The cathodic and anodic current peaks (Ipc/Ipa) ratio was determined after baseline correction of the corresponding cyclic voltammograms and satisfies the condition for reversibility (I pc /I pa = 1) for all ionic complexes. Separation peak potential (DE = E pa--E pc ) for both reduction steps overcome the value required for the one-electron transfer process (\ 59 mV). We believe this effect is attributed to the iR drop rather than to the reversibility of the electrochemical process. The iR drop is expected in a solvent with low dielectric permittivity, such as THF [35].
Derivatives 1-3 were reduced at much positive potential than the analogs derivatives MV. The value of formal potentials E 0 1 and E 0 2, respectively, of ionic complexes 1-3 followed the same trend as for the analogs derivatives as PF 6 salt [21] and were not affected by the self-assembly process in THF. The linear trend of the plot of current peak I pc1 against the square root of the scan rate indicates a diffusion-controlled redox process in the range from 0.05 to 1 VÁs -1 . The limiting process is related to the electron transfer kinetics between the electrode and the redox species. Remarkably, the rate of electron transfer was diminished as a result of the self- Figure 7 Cyclic voltammograms of (0.001 mol L -1 ) ionic complexes 1-3 and MV, respectively, at 0.6 VÁs -1 scan rate in LiTFSI/THF (0.1 mol L -1 ).

Figure 8
Plot of the first peak current (I pc1 ) versus the square root of the scan rate (v) for 1-3 and MV, respectively. association process. That is reflected in the small value of the slope for derivatives 1-3 compared to analog MV (Fig. 8).
The diffusion coefficients calculated from cyclic voltammetry data (D CV ) of ionic complexes 1-3 and MV, respectively, increase in the order 2 \ 3 \ 1 \ MV and follow the same trend as DOSY experiments. Detailed electrochemical parameters derived from CV measurements can be found in supporting information.
Ionic conductivity and thermochromism in solid state Figure 9a illustrates two different temperature ranges with distinctive ionic conductive properties. In the first low temperature range below 60°C, no passing current between the electrodes was observed. The ionic complexes 1-3 are highly viscous. The ionic mobility is moderate; they act as insulator materials; In the second temperature range above 60°C, complexes 1-3 are less viscous, and the conductivity increases exponentially with the temperature. In this temperature regime, the ionic complexes 1-3 behaved as 2nd order conductors. The 2nd order conductors are defined as materials of which electrical conductivity increases with the increase in temperature. Figure 9b presents the Arrhenius plot of conductivity in the high-temperature regime (above 60°C) for the ionic complexes 1-3, respectively. The activation energy (E a ) was calculated with Formula 1 [36]: where m is the slope of the Arrhenius plot (1000 K), and R is the gas constant (8.314 J K -1 mol -1 ). The activation energy values were 65.85 kJ/mol for 1, 71.67 kJ mol -1 for 2, and 74.41 kJ mol -1 for 3. When a compact film of compounds 1, 2, and 3, respectively, was heated between two glass plates without any applied voltage, a color change was observed. The corresponding materials 1-3 changed the color instantly (small DT) from pale-yellow to blue-green above 110°C (Fig. 10). Similar behavior was previously reported for the analog, derivative MV [16]. Figure 11 illustrates the Vis spectra of the thin films of respective compounds 1-3 after exposure at 110°C between two glass plates without any applied voltage. The typical absorption band with a maximum wavelength of 600 nm demonstrates the formation of monomeric radical and cationic species (V ?• ). The oxidation of the film in the presence of atmospheric oxygen (color change from blue-green to yellow) occurred when the film was cooled down at room temperature (approx. 21°C) and stand for more than 12 h. It is noteworthy that the color change did not occur at the outer rim of the thin films where contact with the air is imminent, and the formed radical species are spontaneously oxidized back in the presence of the atmospheric oxygen.
Above 60°C, the liquid crystalline compounds 1-3 started to behave as 2nd order conductors because of the exponential increase in ionic mobility. Under these conditions, the electron transfer from the anion Figure 9 a Plot of current versus temperature at a constant applied voltage (-2 V) for ionic complexes 1-3; b Arrhenius plots of ionic conductivity versus reciprocal temperature for ionic complexes 1-3.
3,4,5-tris(dodecyloxy)benzene sulfonate, which acts as an electron donor source, to the viologen moiety (electron acceptor) is facilitated. Extensive work of Haramoto et al. [37] and Nanasawa et al. [3] also demonstrated that the long alkylbenzene sulfonate counter anion might act as an electron source for photoreduction N,N'-diphenyl bipyridinium derivatives in LC state. On the other hand, the group of Kato [38,39] concluded that the mobility of the ionic species plays a crucial role in the redox activity of the liquid crystal materials combining ionic and electronic functions. Beneduci et al. [40] emphasized the importance of ionic conductivity on the electrochromic properties in the bulk state of the p-conjugated thienoviologens liquid crystals, compounds with similar redox activity such as the one reported in our study. Remarkably, the electrochromic reaction of derivatives 1-3 did not require any external applied potential like the nanostructured redox liquid crystals reported by Kato or Beneduci [27][28][29]. The reduction of viologen units occurred spontaneously at a temperature at which the material existed in LC state with sufficiently low viscosity to allow the charge transport in the bulk liquid crystal.

Conclusions
A novel class of redox-active liquid crystalline materials (1-3) capable of self-associating in solution by hydrogen bonding was synthesized and characterized concerning structure, self-assembly, and physicochemical properties. Such materials are based on ion pairing of amphiphilic anion 3,4,5-tris(dodecyloxy)benzene sulfonate (DOBS -) with dicationic nucleobase-appended viologens. The compounds 1-3 exhibited liquid crystalline properties at room temperature, while the analogs paraquat derivative MV changed from solid crystalline to LC state above 60°C. The fan-shaped texture of compound 1, observed on a polarized light microscope, suggested a columnar type mesophase different from that reported for MV analog [12]. Compound 1, containing thymine units in its structure exclusively, exhibited a stable mesophase until isotropization. By contrast, compounds 2 and 3 were decomposed by the temperature before transition into the isotropic state, revealing the influence of adenine on the thermal instability. Derivatives 1-3 self-associate in solvents with low polarity by hydrogen bonding. 1 H-NMR and DOSY experiments demonstrated that the degree of self-association depends strongly on the nature of the capping groups (i.e., thymine or adenine), solvent polarity, temperature, and concentration. Due to self-association by hydrogen bonding in solution, the electron transfer rate during electrochemical reduction of 1-3 was significantly diminished. The ionic complexes 1-3 behaved as 2nd order conductors in the mesophase state, at a temperature at which viscosity increased significantly as well as theirs analog MV derivative. Ionic mobility, induced by thermal energy, facilitated the electron transfer from the anion DOBSto the viologen unit, which was readily transformed in the radical, cationic state Figure 10 Ionic complexes 1-3 compressed between two glass slides, before and after thermal treatment. Figure 11 VIS spectra of a thin film of ionic complexes 1-3 after exposure to 110°C.
under anaerobic conditions. Compounds 1-3 show thermochromic and ionic conductive properties similarly to MV but in mild temperature conditions. The new class of compounds, combining the hydrogen bonding abilities of nucleobases, redox function of viologen, and thermotropic properties of dodecyl sulfonate, has a great potential for the construction of optoelectronic components for optoelectronic devices.

Funding
Open Access funding enabled and organized by Projekt DEAL. The authors did not receive support from any organization for the submitted work. No funds, grants, or other support were received for conducting this study.

Declarations
Conflict of interests The authors declare that they have no known competing financial interests or personal relationships that could have influenced the work reported in this paper. All authors certify that they have no affiliations with or involvement in any organization or entity with any financial or non-financial interest in the subject matter or materials discussed in this manuscript. The authors have no financial or proprietary interests in any material discussed in this article.
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