Mechanistic Studies for Electrochemical Self-Assembly of CuSCN/Stilbazolium Dye Hybrid Thin Films

We have found 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. DAS is also known to yield a layered inorganic-organic hybrid crystal in a (DAS)(Cu5I6) composition [1]. 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 (C DAS) in the bath [2]. In 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 in high C DAS, 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 precipitation, P CuSCN, is always proportional to the concentration of the complex (C comp), and also to ω 1/2 (ω = angular speed of rotation of RDE) therefore P CuSCN can be expressed Eq. (1) from Levich equation. P CuSCN = J/F = 0.62×C comp×D comp 2/3×ν -1/6×ω 1/2 ··· (1) On the other hand, the rate of DAS precipitation, P DAS, should also be proportional to ω 1/2 under the regime of diffusion limited loading with a given C DAS, and is independent of CuSCN formation rate. P DAS = 0.62×C DAS×D DAS 2/3×ν -1/6×ω 1/2 ··· (2) Plots of P DAS against C comp for variation of C DAS were examined, and indeed such a trend as predicted from the above-mentioned model was found (Fig. 1). In the high C comp range, P DAS is independent of C comp and appears proportional to C DAS, because of the diffusion limited loading mechanism. When C comp goes below certain concentration, P DAS changes proportionally to C comp, while its slope also increase as dependent on C DAS. The relationship between the switching C comp and C DAS yields C DAS / C comp = 1/31 as the border for the switching of the mechanism [3], whereas the dependence of the slope on C DAS in the low C comp range should reveal the stoichiometry of surface complex between CuSCN and DAS. When the rate surface complex formation, P DAS become proportional to the P CuSCN, assuming n of DAS complexes with single CuSCN site yields the equilibrium as the right and the surface is expected to be always equilibrated such equation, P CuSCN + nC DAS ⇄ P DAS ··· (3) Stability constant K should be expressed like Eq. (4), K = P DAS/P CuSCN × C DAS n ··· (4) Eq. (4) can be re-written by inserting Eq. (1), P DAS/C comp= 0.62×D comp 2/3×ν -1/6×ω 1/2 ×K×CDAS n ··· (5) Plotted slope of Fig. 1 in the low C comp range (P DAS/C comp) against C DAS and did fitting with n as variable and then resulted n = 0.6. It means approximately one DAS coordinating two CuSCNs and stability constant K can be calculated as 6.78×102 mol−0.5 cm2.5. [1] Elena Cariati et al., Adv. Mater., 13, 1665-1668 (2001). [2] Yuki Tsuda et al., Monatshefte für Chemie, 148, 845-854 (2017). [3] Yuki Tsuda et al., J. Electrochem. Soc. 166(9) B3096-B3102 (2019). Figure 1