Cocrystal 55DMBPH 2 ba was synthesized and characterized. The crystal structure is isostructural to cocrystal 55DMBP H 2 ia with iodanilic acid. The heat capacity, dielectric permittivity, and polarizationelectric field hysteresis showed that the 55DMBPH 2 ba undergoes a paraelectricferroelectic phase transition at 253 K on cooling. The transition mechanism is discussed through the analysis of isotropic temperature factors of atoms.Recently, organic ferroelectrics having a hydrogen bond (H bond), e.g., phenazinechrolanilic acid 1 (PhzH 2 ca), have been paid increasing attention.2,3 In those compounds, dielectric properties depend on a combination of H-bond donor and acceptor.2 A representative series is cocrystals between 5,5¤-dimethyl-2,2¤-bipyridine (55DMBP) and H 2 xa: 55DMBPH 2 ca is antiferroelectric 4 whereas 55DMBPH 2 ia (iodanilic acid) ferroelectic.5 Since both ferroelectricity and antiferroelectricity are found, the series is very important for understanding the mechanism that determines the dielectric nature of such organic complexes. However, there has been no report on 55DMBP H 2 ba (bromanilic acid) so far. In this letter, we report its synthesis, structure, and properties.Single crystals of 55DMBPH 2 ba were grown by diffusion using dehydrated acetone (Wako, >99.5%) as a solvent. Commercial 55DMBP (Aldrich, 98%) and H 2 ba (TCI, >98%) were purified by fractional sublimation in vacuum before dissolving them in dehydrated acetone. The growth took about 2 months. The yield was very low (15%) even in successful cases. Due to the difficulty in crystal growth, only a limited number of single crystals were available for measurements of physical properties. This prevented the authors from performing precise characterization at the highest level.Structure analyses were performed using a SMART APEX diffractometer (Bruker AXS KK). Data collections were performed at room temperature (295 K) and 200 K. Unit-cell refinement and data reduction were carried out using SAINT.

Viologens with linear alkyl chains from propyl to hexyl were synthesized to complete a series from methyl to octyl viologens, together with commercial samples, to study formation of their co-crystals with 1,3-bis(dicyanomethylidene)indan anion (TCNIH-). Co-crystals in a (viologen)(TCNIH)2 composition were obtained for all the viologens without inclusion of any other ions or solvent molecule. A border was clearly found between pentyl and hexyl for the evolution of charge transfer (CT) band in the absorption spectrum for electron transfer from TCNIH- donor to viologen acceptor. Strong coulombic interaction for close packing of co-crystals obviously contributes to the evolution of CT, although it is suppressed when the side chain is bulkier than hexyl to weaken the donor/acceptor interaction, as supported by drastic decrease of melting point and drastic increase of the largest crystal lattice spacing, found by DSC and PXRD, respectively. An exception was the salt with propyl viologen which did not show the CT band, in spite of the relatively strong donor/acceptor interaction.

Electrochromic displays (ECD) are non-emissive, reflective, non-volatile and energy-saving, as they are based on reversible color change by electrochemical redox reactions. Unlike the electrophoretic e-ink displays already used in book readers, which can only switch between two tones (black and white), ECD can produce gray tones. While the established inorganic WO3 EC device used as automatic antiglare mirror is monotonous, full color ECD can be envisioned by employing reversible color change of organic chromophore. However, most of solid thin films of organic dye molecules cannot achieve electrical conduction and associated ion exchange needed for near-complete redox. A promising strategy is to use mesoporous inorganic electrode with adsorbed monolayer of organic dyes, just like that used in dye-sensitized solar cells. While sufficient color density can be assured by the virtue of high surface area, electrical conduction and ionic transport are also facilitated. The challenge is to find combination of the organic chromophore which undergo reversible color change, preferably in W/Y, W/M, W/C to achieve full color, and suitable electrolyte to prevent dye desorption from the electrode surface. In this study, we have synthesized carboxylated viologen derivatives (N,N’-bis(2-carboxyethyl)-4,4’-bipyridinium dichloride (CE2V) and 3-[4-(N-methyl-4-pyridino)-1-pyridinio]carboxyethyl diiodide (CEMV)) (Fig. 1a), which are adsorbed onto mesoporous ZnO nanoparticulate electrode to test their electrochromism. Viologens are known to exhibit a sharp color change from colorless to deep blue upon its reduction to its cation radical that is stable and reversibly oxidized. In situ spectroelctrochemical measurements were performed to evaluate the capabilities of the newly developed systems. CE2V was synthesized by carboxylation of 4,4’-bipyridine with acrylic acid and subsequent conversion to chloride with hydrochloric acid (yield 73%). CEMV was prepared by monomethylation of 4,4’-bipyridine with methyl iodide and subsequent carboxylation with acrylic acid (yield 46%). Mesoporous ZnO film (10 μm thick) prepared by doctor blading was soaked in ethanolic solution 500 µM of CE2V or CEMV for 3 hours. Transmission absorption spectra were measured in situ by Otsuka Electronics MDPC-7000 diode array spectrophotometer during cyclic voltammetry on the viologen modified ZnO electrodes in a 0.1 M tetrabuthylammonium hexafluorophosphate (TPAPF6) in acetonitrile under N2. The amounts of CE2V and CEMV adsorbed on ZnO were 1.31×10-7, 7.28×10-8 mol cm-2, respectively, indicating superior adsorption stability of double anchor molecule. Both of them exhibited stable reversible redox in two-steps (Fig. 1b). Associated color changes were observed from colorless to purplish blue and yellow to purple for CE2V and CEMV, respectively, which were then bleached in the second reduction. Singly reduced cation radicals intensely absorb in the visible, but doubly reduced states are not colored. The oxidation back to the singly reduced state regenerated the color, which was again bleached on its oxidation back to the original dication state. The in situ spectra monitored for the reversible color change are shown for ZnO/CE2V electrode (Figs. 1c,d). The charges consumed in the primary reduction were calculated from the integral of the peak areas to determine the redox active fractions as 6.83×10-8 and 4.48×10-8 mol cm-2 for CE2V and CEMV, respectively, which correspond to 52 and 62% of the totally adsorbed amounts. Although the spectra of cation radicals of viologens developed in the present study are bit too broad to serve as white to “cyan” chromic device, the relatively fast and stable reversible electrochromism has been achieved by combination of porous inorganic electrode and adsorbed monolayer of organic chromophore. Figure 1

Charge transfer (CT) salt by combination of ionic organic donor and acceptor can be an interesting alternative to eliminate large voltage loss in bulk-heterojunction organic solar cells. We have succeeded in synthesis of novel CT co-crystals of 1,3 bis(dicyanomethylidene)indane anion (TCNIH-) donor and N-alkyl substituted viologen cations acceptor to exhibit broad and intense light absorption from visible to NIR. However, it is not straightforward to employ heavily crystallizing organic salts in thin film devices. As an alternative approach to prove the concept, here we propose attachment of the CT pair as a dye to porous oxide semiconductor to dissociate its CT exciton and to harvest it as current in a dye sensitized solar cell (DSSC) fashion. In this work, carboxylated viologen derivatives, N,N’-bis(carboxymethyl)-4,4’-bipyridinium chloride (CE2V) and N-(2-carboxymethyl)N’-methyl-4,4’-bipyridinium iodide (CEMV) (Fig. 1) were synthesized for their attachment to porous ZnO. Their formation of CT salts with TCNIH- has also been studied. CE2V was synthesized by carboxylation of 4,4’-bipyridinium with acrylic acid and subsequently converted to chloride with hydrochloric acid. CEMV iodide was prepared by monomethylating 4,4’-bipyridinium with methyl iodide and carboxylation with acrylic acid. The synthesized viologen was mixed with TCNIH- in 1 : 2 ratio in water, filtrated and recrystallized in ethanol to identify the structure of the co-crystals. Mesoporous ZnO film prepared by doctor blading was sequentially soaked in ethanolic solution of CE2V or CEMV, and acetone/toluene solution of TCNIH-, to obtain ZnO/viologen/TCNIH hybrid structure in the end. The attachment of respective molecules was checked by UV-vis absorption both for the film samples and the ammonia solution dissolving the entire film. Formation of CT salts was confirmed by significant broadening of the absorption towards NIR as compared to that of the Na+ salt of TCNIH- (Fig. 1), so that the presence of the carboxylic acid anchor group in viologens do not disturb their intimate ionic interaction with TCNIH-. Triclinic single crystalline structure of the co-crystals was identified by XRD. CE2V and CEMV were adsorbed at high concentrations (500 μM and 500 μM, respectively) onto ZnO, so that the carboxylic acid group indeed acted as an anchor for their attachment to ZnO. The anionic TCNIH- in fact could directly be adsorbed onto ZnO to yield bluish films, whose spectrum, however, differed from those with pre-attached viologens (Fig. 1). Those of ZnO/viologen/TCNIH indicate clear broadening towards red, which can be attributed to the formation of CT pairs. Photoelectrochemical analysis in the presence of suitable redox system is expected to result in photocurrent from the CT state. Figure 1

Combination of ionic organic donor and acceptor to form crystalline salt can be an interesting strategy to obtain new types of organic semiconductors, since charge transfer (CT) excitonic properties are anticipated. We have previously achieved a novel CT salt from deprotonated 1,3-(bisdicyanomethylidene)indan anion (TCNIH-) and methyl viologen cation (MV2+) in a 2:1 composition. In this study, the n-alkyl chain length in viologen was systematically changed to find out the border for the evolution of CT character of the mixed salts. [1] Propyl to hexyl viologens (PrV to HxV) were synthesized by quaternization of 4,4’-bipyridine using the corresponding 1-haloalkanes in the yields of 60-62%, whereas methyl, ethyl, heptyl and octyl viologens (MV, EV, HpV, OV) were commercially available. Co-crystals with TCNIH- were precipitated by mixing with viologens at 2 : 1 ratio in water, filtrated and recrystallized from ethanolic solutions. While their crystal structures were examined on powder samples and single crystals, their optical properties were studied by measuring UV-vis and photoluminescence (PL) spectra for CT excitation. Co-crystals with TCNIH- were obtained for all viologens in a 2:1 ratio in the yields of 39-44%. While Na+ salt with TCNIH- preserves the character of the deprotonated TCNIH- in solution, showing a peak at around 590 nm and its absorption extending up to around 850 nm, those with RV2+ having short alkyl chains exhibit absorption towards NIR range and concomitant decrease of visible absorption, due to CT from TCNIH- to RV2+ (Fig. 1). A clear border can be found between PnV and HxV, as the CT absorption is significantly decreased when R is longer than Hx. Crystal structures of the mixed salts were identified as monoclinic for MV, BV, HxV salts, while triclinic for the others. The powder XRD data as well as melting points determined by differential scanning calorimetry indicated significant enlargement of donor-acceptor distances, decrease of melting point, and thereby weakened interaction to account for the observed CT characters. Coulombic interaction between donor and acceptor result in close packing of these co-crystals for their CT characters, whereas it is hindered when alkyl chains longer than Hx is introduced. However, PL from CT states was observed for all the compounds, as the salts with HxV, HpV and OV also showed clearly longer emission than that of Na salt, indicating primary excitation confined in TCNIH-, followed by its relaxation down to the CT states in such salts. Reference [1] E. Saito, et al., ECS Trans., 2018, 88, 301-311 Figure 1