Spectral and electrochemical studies of bis(diimine)copper(II) complexes in anionic, cationic and nonionic micelles N. Anitha, R. Balamurugan, M. Palaniandavar ⇑ Centre for Bioinorganic Chemistry, School of Chemistry, Bharathidasan University, Tiruchirapalli 620 024, India article info Article history: Received 31 October 2010 Accepted 27 May 2011 Available online 31 July 2011 Keywords: Bis(diimine)copper(II) complexes Micelles Redox behavior abstract The spectral and redox behavior of bis(diimine)copper(II) complexes, where diimine is bipyridine, 1, 10-phenanthroline, 4-methyl-1,10-phenanthroline, 5-methyl-1,10-phenanthroline, 5-nitro-1,10-phenan- throline, 4,7-dimethyl-1,10-phenanthroline, 5,6-dimethyl-1,10-phenanthroline, 2,9-dimethyl-1,10-phe- nanthroline, 3,4,7,8-tetramethyl-1,10-phenanthroline and dipyrido-[3,2-d:2 0 ,3 0 -f]-quinoxaline, are significantly different in aqueous and in aqueous SDS, CTAB and Triton X-100 micellar solutions. The 1 H NMR spectral study in aqueous (D 2 O) and aqueous micelles reveals that the Cu(II) complexes interact more strongly with SDS than with CTAB and Triton X-100 micelles and at sites on SDS micelles different from those on the latter. Ligand Field spectral studies reveal that the complexes exist as the dicationic aquated species [Cu(diimine) 2 (H 2 O) 2 ] 2+ , which interacts strongly with the anionic SDS micelles through columbic forces. However, they exist as [Cu(diimine) 2 (H 2 O)Cl] + and/or [Cu(diimine) 2 H 2 ] located in the hydrophobic microenvironments in Triton X-100 and CTAB micelles. The attainment of reversibility of the redox systems in the micellar microenvironments is remarkable and this illustrates that the Cu(II) and Cu(I) species undergo stereochemical changes suitable for reversible electron-transfer. The remark- able differences in spectral and electrochemical properties of Cu(II) complexes in aqueous and aqueous micellar solutions illustrate that the complexes are nestled largely within the micellar environments and imply that the accessibilities of the complexes to electron-transfer are different and are dependent on the nature of micelles as well as the nature and hydrophobicity of the diimine ligands. Ó 2011 Elsevier Inc. All rights reserved. 1. Introduction Micelles, dynamic aggregates of microscopic order, are known for their significance in biological, synthetic and energy-transfer systems wherein the solubilized species can under appropriate conditions serve either as an electron acceptor or as an electron donor. There are a variety of interactions, which may be operative between solubilized substrates and host micelles. Recent research [1–11] has focused on electron transfer reactions between mole- cules bound to micelles. Such systems could provide a route to pro- ducing long-lived charge separation and ultimately, artificial photosynthesis. Although a range of spectroscopic and chromato- graphic techniques has been employed to explore micellar binding, electrochemical and photochemical methods have received rela- tively little attention. The reactions and redox behavior of organic substrates in micellar media have been well studied [12–16] and the electrochemistry of redox active substances in micellar media has been reviewed recently [17–22]. However, the partition of transition metal complexes of hydrophobic ligands between aqueous and aqueous micellar pseudo phases, the nature of their interaction with surfactant micelles and the extent of their micellar solubilization and stabilization, which affect the reaction and redox behavior of the complexes, have not been clearly under- stood. Such an understanding is essential to throw light on the cat- alytic behavior and rate enhancement of reactions involving metal complexes [23,24]. Spectral and electrochemical studies on the interaction of metal complexes with micelles provide valuable information about mode of interaction; however, they are scarce [25,26,19,27–30] because structurally well-defined metal com- plexes are required to be used as probes. The study of redox reactions of simple copper(II) complexes in aqueous media are relatively sparse partly because of limitations imposed by the poor water solubility of many copper(II) complexes and their tendency to disproportionate, and also because of the wide difference in coordination structures, bond length and stereo- chemistry of the two oxidation states. Also, active studies are still carried out to stabilize Cu(I) oxidation state in coordination com- plexes, because of its interesting applications in photophysics and photoelectrochemistry [31]. Also, the Cu(II)/Cu(I) redox cou- ples are important in many biological redox systems and in other contexts. So, we have initiated a systematic electrochemical inves- tigation on the structurally well defined Cu(II) complexes in 0021-9797/$ - see front matter Ó 2011 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2011.05.075 ⇑ Corresponding author. E-mail addresses: palanim51@yahoo.com, palaniandavarm@gmail.com (M. Palaniandavar). Journal of Colloid and Interface Science 362 (2011) 243–252 Contents lists available at ScienceDirect Journal of Colloid and Interface Science www.elsevier.com/locate/jcis