Electrochemical Self-Assembly of CuSCN/4-Cyano-4’-(N’-Methyl)Stilbazolium Hybrid Thin Films

Abstract

Electrochemically self-assembled (ESA) CuSCN/4-( N , N -dimethylamino)-4’-( N ’-methyl)stilbazolium (DAS) hybrid thin film was found to exhibit concerted photoluminescent (PL) property based on energy transfer from CuSCN to DAS [1]. The amount of co-precipitated DAS ( P DAS ) during ESA linearly increased on increasing DAS concentration ( C DAS ) in the bath but changes its slope due to switching of dye loading mechanism from diffusion limited to surface reaction limited [2]. The full analysis revealed the switching border to be dependent on the ratio between the concentration of CuSCN precursor complex, [Cu(SCN)] + ( C comp ), and C DAS as C comp / C DAS = ca. 31 [3]. When the dye loading operates as surface reaction limited, phase separation of CuSCN and DAS in nano-scale occurs to exhibit efficient PL. Thus, finding an alternative chromophore with higher or lower affinity with CuSCN than DAS should result in shifting of the switching border with smaller or larger C comp / C dye than the above indicated value for DAS. In this study, we employed 4-cyano-4’-( N ’-methyl)stilbazolium (CNS) for ESA with CuSCN. CNS has a strong dipole moment due to the presence of strongly electron withdrawing cyano group and thus is expected to exhibit a higher affinity to CuSCN than DAS. CuSCN/CNS hybrid thin films were obtained by cathodic electrolysis with various C CNS while fixing C comp = 2.5 mM. P CNS linearly increased upon increase of C CNS and changes its slope at around 200 μM, far larger than the switching point observed for DAS (around 60 μM). The steep slope for the low C CNS range should be determined by the diffusion of CNS to yield its diffusion coefficient D CNS = 1.49×10-6 cm 2 s -1 , slightly larger than that found for DAS ( D DAS = 1.25×10-6 cm 2 s -1 ). On the other hand, about 3 times larger P CNS for given dye concentrations and steeper slope of surface reaction limited regime for CNS than those found for DAS indicate higher adsorption stability of CNS than DAS to CuSCN surface. Soaking the CuSCN/CNS hybrid thin films electrodeposited at C CNS = 60 and 800 μM to represent the dye loading as diffusion and surface reaction limited, respetively, into dimethylacetamide resulted in partial and total extraction of CNS to reveal entrapment and phase-separation of CNS, just like the case of CuSCN/DAS. The maximum P CNS appears to be saturated when C CNS > 800 μM for CNS aggregate to occupy close to 30% of the total film volume. The present results already nicely support the validity of the switching of ESA mechanism, being common for both DAS and CNS. Smaller C comp / C CNS for the switching border and larger stability constant for surface complex formation are expected for CNS than DAS from full analysis. [1] K. Uda et al., ACS Omega, 4 , 4056-4062 (2019). [2] Y. Tsuda et al., Monatsh. Chem., 148 , 845 (2017). [3] Y. Tsuda et al., J. Electrochem. Soc., 166 , B3096-B3102 (2019). Figure 1