PERFIL DEMOGRÁFICO

This chapter has reported the results of DFT calculations of a Cu complex on a po- larising thin film, using a newly developed perfect conductor model to show that 2ML of NaCl are sufficient to stabilise two different charge states of the complex on a Cu substrate.

It has been shown that the inclusion of an orbital dependent screened Coulomb interaction changes the predicted geometry of the charged state in qualitative way in gas-phase, making the Td− _{conformation the most stable upon the charging.}

Table 5.8: Experimental and simulated STM images of Td Cu(dbm)2. Experimental

images taken on a NaCl 2ML Cu(111) substrate, simulated images calculated on a NaCl 2ML Cu(100) substrate. For the experimental images the value of the bias a < 2 V and b_≈2 V. For the simulated images a= 0.5 V, b= 1.5 V.

System Bias (V) −b a b Exp. Td Td− Td−_v Td−_rot −20 −10 0 10 20 PDO S (a) SP 0_{, PDOS} Molecule Carbon Carbonpz −3.0 −2.5 −2.0 −1.5 −1.0 −0.5 0.0 0.5 1.0 1.5 E - EFermi(eV) 0.000000 0.000001 0.000002 0.000003 0.000004 0.000005 0.000006 0.000007 LDO S (b) SP0_{, STS} −20 −10 0 10 20 PDO S (c) PDOS, SP − Molecule Carbon Carbonpz −3.0 −2.5 −2.0 −1.5 −1.0 −0.5 0.0 0.5 1.0 1.5 E - EFermi(eV) 0.000000 0.000001 0.000002 0.000003 0.000004 0.000005 0.000006 0.000007 LDO S (d) SP−_{, STS} SP0 _{(SUMO) SP}− _(LUMO) SP− _{(LUMO + 1)} SP− _(HOMO) SP0 _(LUMO) (e) SP0 _{(LUMO + 1)}

Figure 5.16: (a) PDOS and (b) STS of Cu(dbm)2 in the SP0 state. (c) PDOS and (d)

STS spectrum of Cu(dbm)2 in the SP− state. In (b) and (d) STS spectrum is taken

from the point indicated by the cyan star in (e). PDOS is of the states of the molecule and its carbon pz states.

of the complex is not sufficient to cause the switching from the SP to the Td confor- mation. From the calculations of the adsorbed complex, it has been shown that the switching of the complex from SP to Td occurs not because of a charging of the complex, with the charging of the complex occurring at a lower bias voltage and not necessitat- ing a substantial conformational change. This charged square planar conformation was predicted theoretically before being found in experimental STM studies. The switch to Td− _{conformation is instead shown to be the result of tunnelling through the LUMO} of the SP− _{state and the geometry of the adsorbed Td}− _{conformation consists of just} one raised phenyl ring with the remaining three rings adsorbed flat on the surface.

In summary, Cu(dbm)2 is a combination of a redox and a conformational switch. A

NaCl bilayer is shown to be sufficient to stabilise charge on the molecule with a small amount of conformational change and the charging of the complex is a necessary pre- requisite to the conformational switch to a Td conformation. Despite the complex not being a pure redox switch the experimental observations that it is stable and reversible means that it is still of interest for applications in molecular electronics.

Chapter 6

Calculation of Hydrogen Transfer

Barrier in Porphycene

This chapter looks at porphycene (Pc) adsorbed on Cu(110). Pc consists of a nitrogen cavity containing two hydrogen atoms which can exist in either a cis conformation (both hydrogens bonded on the same side of the cavity) or a trans conformation (both hydrogens bonded on opposite sides of the cavity).

Here, the activation energy for hydrogen transfer in porphycene (Pc) is calculated using the nudged elastic band (NEB) method. It is shown that there is an overestima- tion of the barrier by this method and that the quantum nature of the hydrogens must be taken into account in order to obtain a good agreement with experiment. This is done through the calculation of the vibrational frequencies of the normal modes of the hydrogens at the saddle points of the potential energy surface, from which an estimation of the zero point energy (ZPE) can easily be obtained.

The inclusion of these zero point effects lowers the barrier to hydrogen transfer, bringing the activation energy into line with experimental results determined from an Arrhenius plot. The hydrogen transfer from a cis-to-cis tautomerisation of Pc is, in such a way, seen to proceed via a two step mechanism whereby the Pc travels through a short-lived trans state.

In this chapter, the determination of the geometries of the cis and trans conform- ers and their energies was performed by Felix Hanke at the University of Liverpool.

N N H H N N

Figure 6.1: Molecular structure of porphycene.

Experimental work, classifying the hydrogen transfer and establishing its temperature dependence, were done by Takashi Kumagai in the group of Leonard Grill at the Fritz- Haber-Institute, Berlin [30, 31].

6.1

Introduction

Porphycene (Pc) (see Figure 6.1) is a planar and aromatic isomer of porphine (free-base porphyrin). Whereas the four N atoms of porphine form a square cavity, the cavity of Pc is rectangular. The N-N distance along the shortest side of the rectangle is 2.63 ˚

A and is shorter than the N-N distance in porphine (2.89 ˚A). The rectangular cavity means that rather than there being two isomers as in porphine, cis (with the two H atoms on adjacent vertices of the cavity) and trans (with the H atoms on diametrically opposed verticies), porphycene has three; trans and two inequivalent cis conformers, named cis and cis2.

The reduced distance between the H atoms and the N atoms opposite in the cis conformation of porphycene leads to a significantly different behaviour of the H atoms inside inside the cavity to porphine. This is predominantly due to a stronger hydrogen- bond existing between the H atom and the N atom opposite than in porphine. That this effect has a significant impact on the tautomerisation of Pc in comparison to porphine is illustrated by the different rates of tautomerisation in Pc and porphine. Braun et al. [173] used NMR techniques to determine the rate constant for trans- trans tautomerisation of porphine in liquid and polycrystalline phase. They found that porphine tautomerised through an intermediate cis tautomer with a rate constant that

could only be observed at elevated temperatures, taking a value of 2_×104_s−1 _{at 298 K.}

In contrast, the Pc molecule exhibits a much higher rate constant for tautomerisation, pump-probe spectroscopy showed that the tautomerisation proceeded via a concerted trans-trans motion of the H atoms, taking a value of 5.8×1011_s−1_{at room temperature}

[174].

That Pc can be reversibly switched between two states makes it a molecule of in- terest for applications in molecular electronics. There have been studies on similar phenomena in molecules structurally related to porphine, for example, on naphthalo- cyanine, which was adsorbed on a NaCl bilayer and switched between its two tautomers via inelastic electron tunnelling induced by an STM [29]. Also, Auw¨arter and coworkers [175] conducted a study of free-base tetraphenyl-porphyrin adsorbed on a Ag surface that was also switched between its two tautomers via electron tunnelling from an STM. Molecular switches of this type are of particular interest because they do not involve large conformational changes of the molecule and so would be more easily connected to any nanoscale circuit in a practical application in molecular electronics. In addition, they might be expected to be more robust than a switch involving large conformational changes.

The first part of this chapter discusses experimental work by Takashi Kumagai and theoretical work by Felix Hanke to characterise the adsorption geometry and energetics of Pc on Cu(110). Then the geometries resulting from this work are used as a basis for understanding theoretically the process of tautomerisation, first via calculation of the activation energy and reaction path of tautomerisation and then by calculating the vibrational modes that induce tautomerisation.