(Invited) Mechanistic Studies for Electrochemical Self-Assembly of Cuscn/Stilbazolium Dye Hybrid Thin Films

We have established electrochemical self-assembly (ESA) of inorganic / organic hybrid thin films in which the inorganic is CuSCN, known as 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) known to exhibit second-order nonlinear optical property for terahertz emitters. The CuSCN/DAS hybrid thin films by ESA show concerted photoluminescence based on energy transfer from inorganic CuSCN to organic DAS, making us anticipate its potential use in optoelectronic devices [1]. Understanding the mechanism of ESA to obtain tuning knob to maximize its functionality is therefore important.

In our previous study, switching of dye loading mechanism has been suggested, depending on DAS concentration (C_DAS) in the bath [2]. With low C_DAS, 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 with high C_DAS, resulting in formation of unique nanostructures as well as phase separation of inorganic and organic domains. The border for the switching of dye loading mechanism was found as the ratio between bulk concentrations of dye and the [Cu(SCN)]⁺ complex, C_DAS/C_comp = ca. 1/31 in case of DAS⁺ [3].

In this study, electrochemical analysis has been performed employing rotating disk electrode (RDE) under variation of bulk concentration of dye (DAST and CNST = 4-cyano-4'-(N'-methyl)stilbazolium tosylate for comparison) to verify the proposed mechanism. CNS⁺ has a strongly electron withdrawing -CN group and therefore bears a much larger dipole moment of 23.5016 Debye than that of DAS⁺ (10.1442 Debye) as determined by DFT calculation. Thus, a much stronger interaction of CNS⁺ with CuSCN (essentially with SCN^- ion) is expected than DAS⁺.

The overall behavior of CNS⁺ has been found exactly the same as DAS⁺ such as changing the crystal orientation of β-CuSCN to lay down its c-axis and phase transition from β- to α-CuSCN. However, such transitions were found for smaller concentrations of CNS⁺ than DAS⁺ because of its stronger affinity with CuSCN. The switching dye loading mechanism and associated change of the hybrid structure has also been observed for CNS. Fig. 1 shows plots of amount of dye loaded into the hybrid thin films against the dye concentrations in the electrolytic bath for DAS⁺ and CNS⁺. The steep slopes in low dye concentration range represent diffusion limited loading, whereas the moderate slopes are those limited by surface reaction. The bending points, C_DAS = 60 μmol dm^-3 and C_CNS = 200 μmol dm^-3, indicate the switching borders for C_comp = 2.5 mM. Because of the increased stability of adsorption of CNS⁺ on CuSCN surface, diffusion limited loading, thus all dyes reaching the electrode surface buried inside the film, continues to operate up to this high dye concentration. The increased stability of CuSCN/CNS surface complex also resulted in about 3 times more efficient dye uptake in the surface reaction limited regime. These results nicely confirm the validity of the proposed mechanism both for DAS⁺ and CNS⁺, and that the switching border can be varied by the stability of CuSCN/dye complex. Full analysis should be able to determine C_CNS/C_comp ratio as the switching border.

[1] K. Uda et al. ACS Omega, 4, 4056-4062 (2019).

[2] Y. Tsuda et al., Monatshefte für Chemie, 148, 845-854 (2017).

[3] Y. Tsuda et al., J. Electrochem. Soc. 166(9) B3096-B3102 (2019).

Figure 1