Electrochemical Self-Assembly of Cu­SCN / Dye Hybrid Thin Films and Their Optical Properties

Abstract

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.