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.
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
Spintronic devices employing both semiconducting and ferromagnetic properties of materials are to be realized by doping of high spin transition metal ions to semiconductors such as ZnO. Transition metal oxides are also interesting as electrocatalysts for electrochemical conversion of renewable electricity to chemical energy for its storage. In this study, we have attempted microwave (MW)-assisted hydrothermal synthesis of Co, Mn-doped ZnO nanoparticles. Aqueous precursor solutions containing ZnCl2 and CoCl2 or MnCl2 at various ratios in a total concentration of 0.2 M were basified with NaOH to ca. pH 13, and subjected to a 2.45 GHz MW[1,2] to promote reaction at 160°C for 30 min. The products were characterized by FE-SEM, XRD and UV-Vis. Mesoporous electrodes of ZnO and Zn0.92Co0.08O were fabricated by doctor blading method. The green-colored Co-doped samples exhibit characteristic absorption peaks at around 560, 610 and 650 nm due to d-d transition of Co(II) and a red shift of the bandgap absorption onset. Phase separation of Co(OH)2 has also been suggested by XRD and FE-SEM images for x > 0.1, so that doping up to about 10% was apparently possible. Systematic shift of the three major XRD peaks of Wurtzite ZnO towards low angles was recognized upon increasing x, suggesting substitution of the lattice Zn2+ ions with Co2+. Lattice constants were calculated as a = b = 3.252 Å, c = 5.204 Å for ZnO, whereas they were enlarged to 3.277, 5.244 Å, respectively, for x = 0.20. CVs of ZnO and Zn0.92Co0.08O electrodes in a neutral electrolyte are compared in Fig. 1. Besides the clear decrease of cathodic charging and discharging currents, Co-doping results in an anodic wave at around + 1 V due to Co2+ → Co3+, followed by a large increase of irreversible anodic current, as compared to the non-doped sample. Chronoamperograms for a few minutes confirmed this current enhancement and gas bubble evolution from the anode due to catalytic oxidation of water (2H2O → O2 + 4H+ + 4e-). [1] Y. Hirai, K. Furukawa, H. Sun, Y. Matsushima, K. Shito, A. Masuhara, R. Ono, Y. Shimbori, H. Shiroishi, M. S. White & T. Yoshida, “Microwave-assisted hydrothermal synthesis of ZnO and Zn-terephthalate hybrid nanoparticles employing benzene dicarboxylic acids” Microsyst Technol (2017). doi: 10.1007/s00542-017-3392-y [2] H. Sun, L. Sun, T. Sugiura, M. S. White, P. Stadler, N. S. Sariciftci, A. Masuhara & T. Yoshida, “Microwave-assisted hydrothermal synthesis of struture-controlled ZnO nanocrystals and their properties in dye-sensitized solar cells” Electrochemistry, in press. Figure 1
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