Self-Assembly Properties of Amphiphilic Iron(III) Spin Crossover Complexes in Water and at the Air–Water Interface

Paulo N Martinho,Tibebe Lemma,Brendan Gildea,Tia E Keyes,Grace G Morgan,George Mckerr,Irina A Kühne,Barry O’Hagan,Claudio Gandolfi,Martin Albrecht

doi:10.3390/magnetochemistry4040049

Abstract

The assembly properties of three known spin crossover iron(III) complexes 1–3, at the air–water interface, are reported. All three complexes are amphiphiles, each bearing a pair of Cn alkyl chains on the polyamino Schiff base sal2trien ligand (n = 6, 12, or 18). Complex 1 is water-soluble but complexes 2 and 3 form Langmuir films, and attempts were made to transfer the film of the C18 complex 3 to a glass surface. The nature of the assembly of more concentrated solutions of 3 in water was investigated by light scattering, cryo-SEM (scanning electron microscopy), and TEM (transmission electron microscopy), all of which indicated nanoparticle formation. Lyophilization of the assembly of complex 3 in water yielded a powder with a markedly different magnetic profile from the powder recovered from the initial synthesis, notably, the spin crossover was almost completely quenched, and the thermal behavior was predominantly low spin, suggesting that nanoparticle formation traps the system in one spin state.

Highlights

The continuing pressure to intensify hard drive bit densities has prompted research into molecular alternatives to the current methodology of using nanoislands of magnetic metal oxides, which are starting to reach the superparamagnetic size limit [1,2]
We have investigated the behavior of three known spin crossover (SCO) iron(III) complexes which host hydrophobic alkyl chains varying from C6 to C18
The ability of these compounds to form monolayers at the air–water interface was investigated, and we have found that the C6 compound 1 does not form monolayers at the air–water interface, due to partial solubility of the compound in water

Summary

Introduction

The continuing pressure to intensify hard drive bit densities has prompted research into molecular alternatives to the current methodology of using nanoislands of magnetic metal oxides, which are starting to reach the superparamagnetic size limit [1,2]. There exist many external perturbation methods to effect a spin state change including thermal [16,17,18], irradiation [19,20,21], magnetic field [22,23], pressure [24,25,26], or electric [27,28] inputs, and an wide range of detection methods, including measurement of magnetic properties [29], vibrational [21,30,31] and electronic [32,33,34] spectroscopic signatures, conductance [35], or surface plasmonic response [36] This activity has intensified research into assembly protocols for spin crossover (SCO) complexes, and methods to prepare thin films [37,38], nanocrystals [39,40], and nanoparticles [41,42,43,44,45,46] have all been reported in recent years. Such amphiphilic complexes show markedly different magnetic behavior to the parent complex in both the solid state and in solution, and these and related Mn(III) SCO systems [53], may be candidates for Langmuir–Blodgett (LB) [54,55] monolayer formation and transfer to a surface

Methods

Results

Conclusion