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Electrochemical self-assembly of CuSCN/4-(N,N-dimethylamino)-4'-(N'-methyl)stilbazolium (DAS) hybrid thin films has been carried out on systematic variation of bulk concentrations of [Cu(SCN)]+, DAS tosylate (DAST) and changing their flux density by angular speed of rotation of the rotating disk electrode. Switching of DAS loading mechanism dependent on the DAST concentration in the bath has been verified and quantified. The switching occurs at the concentration ratio of the precursors DAST/[Cu(SCN)]+ = ca. 1/31, molar ratio of the product DAS/CuSCN = ca. 1/48, below which the loading becomes diffusion limited for DAS to be occluded in the CuSCN grains, whereas surface reaction for formation of complex between CuSCN and DAS begins to limit the DAS loading when the relative concentration of DAST exceeds this border, resulting in a phase-separated precipitation of CuSCN and (DAS)(SCN) aggregate in unique nanostructures.

Electrochemical self-assembly (ESA) of metal-organic framework (MOF) thin films of Zn2+ ions and terephthalic acids (TPA) in a layered structure and with a composition of Zn3(OH)4(TPA)•6H2O was made possible by addition of TPA into the bath for electrodeposition of ZnO, namely, O2-saturated aqueous solution of ZnCl2. The deposition of Zn-TPA MOF, however, resulted in a strong passivation of the electrode towards O2 reduction to limit the electrochemical growth. Interestingly, chemical precipitation of Zn-TPA MOF was found to take over the film growth to achieve a thickness of about 2 µm in the extended electrolysis, in spite of the negligibly small current. Due to decrease of TPA concentration in the solution bulk, the electrode finally recovered the activity for the O2 reduction, which, however, resulted in the electrodeposition of ZnO to be overlaid on the Zn-TPA MOF. The electrodeposited Zn-TPA MOF was well adherent to the substrate.

Introduction: Combination of inorganic and organic materials offers unlimited new opportunities. While “composites” stands for their physical mixtures, “hybrids” and “inorganic/organic compounds” are especially interesting for their new functionalities, as they are achieved by self-organization due to “chemistry” among the constituents. We have established electrochemical self-assembly (ESA) of inorganic/organic hybrid thin films from one pot containing all the ingredients. 1,2) In this work, we have employed cathodic electrodeposition of CuSCN as the test bed for the ESA with various organic dyes to understand its principle and evaluated optical properties of the product thin films. Experiments: CuSCN / dye hybrid thin films were cathodically electrodeposited according to the reaction, [Cu(II)(SCN)]+ + e- → Cu(I)SCN, in the presence of various organic dyes shown in Fig. 1. Photoluminescence (PL) and PL excitation (PLE) spectra were measured between 77 and 298 K on a Horiba Fluorolog-3 equipped with a high power Xe lamp excitation source and lq. N2-cooled PMT detector. Results and discussion: Anionic xanthene dyes which afford ESA with ZnO were not successful with CuSCN except FLNCS, whereas zwitter ionic RB and all the cationic dyes yielded colorful thin films as they were loaded into CuSCN. Soft and hard, acid and base (HSAB) principle nicely explains the behavior of FLNCS, as its –NCS is soft Lewis basic, finding stable coordination to soft Lewis acidic Cu(I) site of CuSCN, just like the hard –COO- being good with hard Zn(II) of ZnO. On the other hand, partial replacement of Cu+ ions with cationic dyes (RB with its ammonium moiety) can account for their successful ESA. These two mechanisms were confirmed by the decrease of dye loading in baths containing excess SCN− and Cu2+, respectively. RB, R6G, NB and DAST show strong PL both as their solutions and in solid states with their PLEs nicely matched with their absorption spectra, while that of the hybrid thin films was totally quenched except for DAST (CuSCN/DAS hybrid thin film). The energy diagram drawn for the energy levels of individual components suggest hole transfer from HOMO of photoexcited organic chromophore to valence band of inorganic CuSCN, except for NB. Hybridization of NB with CuSCN might have altered its energetic structure to exhibit photo-induced carrier generation of the hybrid thin films, thus suggesting usefulness of the hybrid thin films with RB, R6G and NB as light absorbers in solar cells. On the other hand, the DAS+ chromophore exhibits PL even when it is loaded into CuSCN, with its energy in between solution (= monomer) and powder of DAST. While PL of solution and powder become sharp, blue-shifted and intensified at 77 K, as typically expected by exciton confinement, that of the hybrid film is red-shifted and not much intensified, suggesting a strong excitonic interaction with CuSCN to offer a new type of luminescent material. References: T. Iwamoto et al., J. Phys. Chem. C, 118, 16581-16590 (2014). Y. Tsuda et al., Microsys. Tech., DOI 10.1007/s00542-017-3394-9. Figure 1

We have found electrochemical self-assembly (ESA) of inorganic / organic hybrid thin films in which the inorganic is CuSCN, known to be a wide bandgap p-type semiconductor, whereas the organic is 4-N,N-dimethylamino-4’-N’-methylstilbazolium chromophore (abbreviated as DAS+) as its salt with tosylate (DAST) is known to exhibit second-order nonlinear optical property for terahertz emitters [1]. DAS+ is also known to yield a layered inorganic-organic hybrid crystal in a (DAS)(Cu5I6) composition [2]. Strong dipole-dipole interaction of the DAS+ chromophores aligned between the CuI layers results in a spontaneous photocarrier generation and ambipolar transport in a single absorber solar cell. Thus, CuSCN/DAS hybrid thin films can be attractive alternatives for such opto-electrical applications, when ordered arrangement of DAS+ chromophore is achieved during ESA. In our previous study, switching of dye loading mechanism has been suggested, depending on DAS+ concentration in the bath [1]. In low DAS+ concentration range, the loading is limited by diffusion so that DAS+ is entrapped within CuSCN crystal grains, while surface reaction of hybridization begins to limit the dye loading in high DAS+ concentration range, resulting in formation of unique nanostructures as well as phase separation of inorganic and organic domains. In this study, electrochemical analysis by employing rotating disk electrode (RDE) has been performed to verify the mechanism of ESA and also to explore the limit of DAS+ loading. Electrodeposition of CuSCN undergoes as limited by transport of 1 : 1 complex between Cu2+ and SCN- ions, namely {[Cu(SCN)]+} species, near 100% Faradic efficiency with marginal influence by the presence of DAS+. Thus, the rate of CuSCN growth is always proportional to the concentration of the active species, and also to ω1/2 (ω = angular speed of rotation of RDE). On the other hand, the rate of DAS+ precipitation, v(DAS)(SCN), should also be proportional to ω1/2 under the regime of diffusion limited loading with a given [DAS+], and is independent of CuSCN formation rate. When the rate surface complex formation begins to limit the rate of DAS+ precipitation, the amount of DAS+ loading should become proportional to the rate of CuSCN deposition, if a second order rate law as shown below holds, v(DAS)(SCN) (mol s-1 cm-2) = k[DAS+][site] (1) where, k (mol-1 cm3 s-1) is the second order reaction rate constant for formation of surface complex and [site] is the surface concentration of newly formed CuSCN sites, which should be expressed as, [site] (mol cm-2) = A × 0.62 × {[Cu(SCN)]+} × D{[Cu(SCN)]+}2/3 × ν(methanol, 298 K)-1/6 × ω1/2 (2) which is simply derived from Levich equation for diffusion limited electrodeposition of CuSCN, multiplied by a proportionality constant, A (s), to express the activity of the surface site. The amount of DAS+ loaded into the film electrodeposited for a given [DAS+] under variation of {[Cu(SCN)]+} in the bath was examined, and indeed such a trend as predicted from the above-mentioned model was found (Fig. 1). In the high {[Cu(SCN)]+} range, DAS+ loading is independent of {[Cu(SCN)]+} and appears proportional to [DAS+], because of the diffusion limited loading mechanism. When {[Cu(SCN)]+} goes below certain concentration, DAS+ loading changes proportionally to {[Cu(SCN)]+}, for a given [DAS+], but is also proportional to [DAS+] for a given {[Cu(SCN)]+}, as expected for the surface reaction limitation expressed by Eq. (1). The switching from surface reaction to diffusion limitation occurs at the lower {[Cu(SCN)]+} when [DAS+] goes lower, as recognized by the shift of the border between {[Cu(SCN)]+} dependent and independent parts. [1] Yuki Tsuda et al., Monatshefte für Chemie, 148, 845-854 (2017) [2] Elena Cariati et al., Adv. Mater., 13, 1665-1668 (2001). Figure 1

Metal organic flamework (MOF) is a class of inorganic / organic compound whose crystalline structure is made by bridging inorganic ions with organic ligands. Due to its open porous structure, special functionalities are anticipated in applications such as sensors, catalysis and batteries. Variation of MOFs is almost unlimited, not only by choice of building blocks but also by change of composition and crystalline structure. We have recently reported microwave assisted hydrothermal synthesis of MOFs with various layered structures, in which Zn2+ ions are bridged by terephthalic acid (TPA). These Zn-TPA MOFs exhibited proton-selective reversible redox reactions, suggesting their usefulness in membrane-free redox batteries. On the other hand, we have established a method to electrochemically self-assemble ZnO / organic dye hybrid thin films. When there is a favorable chemistry between inorganic and organic constituents, their hybrid nanostructures are spontaneously formed during electrolysis. Self-assembly of Zn-TPA MOFs can therefore be expected by minor addition of TPA into the bath for cathodic electrodeposition of ZnO. Potentiostatic electrolysis at -1.0 V (vs. Ag/AgCl) in O2-saturated aqueous electrolyte containing 5 mM ZnCl2 and TPA up to 500 μM resulted in gradual decrease of cathodic current of oxygen reduction current (Fig. 1). Strong passivation occurs when TPA > 75 μM to reduce the steady-state current down to ca. 0.1 mA cm-2, as compared to the O2 transport limit (ca. 1 mA cm-2 for Zn2+ + O2 + H2O + 2e- → ZnO + H2O2). Minor addition of TPA at 5 μM resulted in appearance of thin flake-like deposits together with tiny grains of ZnO, which then changes into entire coverage of the FTO-glass substrate with the flakes at 200 μM while ZnO grains disappear (Fig. 2). X-ray diffraction (XRD) patterns of the deposits revealed that the flakes are those of Zn-TPA layered MOF in a Zn3(OH)4(TPA)•6H2O composition. The passivation is obviously caused by high electrochemical resistance of the electrodeposited MOF towards O2 reduction, making it difficult to obtain thick MOF films for battery applications. Exploring the conditions to promote continuous electrochemical growth of MOF is under way. Figure 1