SMMs on Surface

in electronic structure to be investigated for the same (or very similar) molecular structures, and the impact of making such modifications can be substantial.³¹ For instance it's been reported that single-electron oxidations cause changes in the molecular structure of the sandwich unit, which impact on the ligand field experienced by the lanthanide trications, and which result in greater energetic separation of the ground mJ sublevel from the excited states.

Ishikawa reported³² that two-electron oxidation of [DyPc2]⁻ to give [DyPc2]⁺ caused significant contraction of the sandwich structure, consistent with the removal of antibonding electrons: the two N4 planes containing the nitrogen atoms directly bonded to Dy were calculated to be closer (Figure 5.10). This structural modification resulted in drastic changes in dynamical magnetism including a doubling of the energy barrier Ueff and a significant rise of the blocking temperature from the original anionic form.

Figure 5.10 Contraction of the square-antiprismatic coordination environment upon two-electron oxidation of [LnPc2]^- to give [LnPc2]⁺.

To date several LnPc2 with different lanthanide ions and bearing different peripheral substituents have been synthesized, looking for the highest values of Ueff. At the time of writing, the record anisotropy barrier for an SMM of any kind is held by Torres and co-workers, for a TbPc2 peripherally functionalized with OC6H4-p-^tBu substituents for which Ueff = 652 cm^-1 was determined.³³

Molecular Magnetism

systems able to act as a device. The challenge is the definition of experimental strategies to properly assemble and integrate these molecular materials into functional devices without compromising their properties.³⁷ Transferring the magnetic properties from the bulk material to the surface is far from trivial:

perturbations arising upon molecule–surface interactions can induce numerous changes, from chemical modifications (redox processes, chemical decomposition, ...) that completely alter the magnetic properties of the molecule to molecular deformations. Such deformations, though not so drastic as a chemical decomposition, can induce a modification of the magnetic exchange pathway or simply alter the easy-magnetization axes, and thus, the magnetization relaxation mechanisms and barriers. Surface effects as well as interaction with the substrate can completely change the characteristic SMM behavior observed in crystals, either increasing its magnetic anisotropy or completely removing it.

The high thermal and chemical stability of LnPc2 molecules make them suitable for deposition on surface via sublimation under Ultra-High Vacuum (UHV) conditions: sub-monolayer deposition of LnPc2 on different substrates has been largely investigated.³⁸ However it has been observed that peculiar magnetic behavior of the SMMs can be altered, and even suppressed, after their deposition on surface. The magnetic hysteresis is strongly affected when the environment is different from the crystalline phase: in general, the magnetization dynamics of LnPc2 is severely altered in amorphous materials, where the packing of the molecules characterizing the crystalline phase is lost.

In a work reported by Sessoli group,³⁰ the magnetic behavior of TbPc2 was investigated in different phases (Figure 5.11).

Figure 5.11 TbPc2 magnetic hysteresis loop in a) microcrystals, b) heated powder in the crucible before the deposition of the film, c) sublimated film on Kapton.

Microcrystals of TbPc2 showed the typical magnetic profile of terbium double deckers, with an opening of the hysteresis at temperature as high as 15 K (Fig.

5.11a). In the deposited thick film (Fig. 5.11c) the hysteresis was retained, with the typical butterfly shaped loops opened at 10 K. Although the blocking temperature appearead to be slightly lower, the qualitative behavior remained unaltered. In contrast, the fraction of the powder sample extracted from the crucible before evaporating film did not show any comparable magnetic hysteresis over the whole investigated temperature range (Fig. 5.11b). The intactness of the complex was however confirmed by the fact that the subsequently evaporated film presents the typical magnetic hysteresis. The disappearance of the hysteresis in the heated powder and its reappearance once the treatment was extended to sublimation and reconstruction of a film with partial molecular ordering was intriguing and apparently lacks of explanation.

Crystal packing could be responsible for the stabilization of a molecular structure characterized by a lower deviation of the orientation of the two phthalocyaninato rings from the exactly staggered situation (45°) (Figure 5.12).

This limit case corresponds to D4d symmetry of the coordination sphere of the terbium(III) ion leading to quenching of transverse anisotropy.

Molecular Magnetism

Figure 5.12 Distortions from D4d symmetry induced by intermolecular interactions.

An alternative way to assemble LnPc2 on surface is the grafting of molecules derivatized with appropriate functional groups: this approach is preferable to the unspecific deposition by thermal evaporation in the realization of durable devices due to the formation of stable bonds. Grafting offers also much more control on the orientation and the density of the components on the substrate.

The possibility to chemically functionalize LnPc2 molecules makes them ideal candidates for grafting on surface. Ruben and co-workers already reported in 2009 the synthesis of a pyrene-functionalized TbPc2 and its grafting through π-π interactions on carbon nanotubes^39,40 and on graphene.^41,42 In 2012 the SAM formation on gold substrate of TbPc2 functionalized with peripheral thioether substituents was reported.⁴³ However the lability of the Au–S bond⁴⁴ cannot guarantee the long-term stability required for the development of an SMM-based device.

Covalent grafting is preferable as it guarantees a higher stability with respect to π-stacking or S–Au interactions in the realization of robust and durable devices.

To date, stable covalent grafting between LnPc2 molecules and surface has not been reported yet. Thus new approaches in design and synthesis of LnDD for robust integration of bistable magnetic molecules with retained magnetic behavior in electronic devices are desirable.

In this Thesis we will present our work on design and synthesis of new TbPc2

able to preserve the SMM behavior once grafted covalently on different surfaces. In particular the robust covalent grafting of TbPc2 functionalized complexes to a silicon surface yielding a partially oriented monolayer silicon surface will be presented in Chapter 6.

In Chapter 7 we will exploit the selective functionalization of one of the two Pcs forming the TbPc2 complexes in order to promote their oriented grafting on silicon surface.

In Chapter 8 we will present the synthesis and the magnetic properties of new functionalized TbPc2.

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