RESULTADOS ESPECIFICOS CRISIS HIPERTENSIVA.

For the first generation dendrimer with biphenyl-based dendrons (Ir-G1) the LUMO and HOMO energy levels have been calculated to be 2.5 eV and 5.6 eV respectively. In all the carbazole dendrimers while the LUMO energy has also been found to be 2.5 eV, the HOMO energy was at 5.7 eV slightly greater than that of the biphenyl dendronised dendrimer. While knowledge of such numbers was important, of more interest was how these energy levels were distributed across the dendrimer structure.

Figure 5.1: Structure of the first generation car- bazole, Dendrimer5, Ir-CarbG1

Figure 5.2: Structure of the second generation carbazole, Dendrimer6, Ir-CarbG2

Figure 5.3: Structure of the third generation carbazole, Dendrimer7, Ir-CarbG3

Figure 5.4: Structure of the first generation double dendron carbazole, Dendrimer 8, Ir- CarbDDG1

Figure 5.5: Structure of the second generation double dendron carbazole, Dendrimer9, Ir-CarbDDG2

Density functional theory calculations are a useful tool in explaining the optical and electrical prop- erties of organic molecules particularly when complementary experimental results are available. Pre- viously such calculations have been performed on both the first generation fac-tris(2-phenylpyridyl) iridium [Ir(ppy)3] (Ir-G1) dendrimer with phenylene based dendrons, and a dendrimer with carbazole dendrons [157]. The calculations allowed the determination of the distribution and relative energies of the highest occupied (HOMO) and lowest unoccupied molecular orbitals (LUMO). These studies found that for Ir-G1, both the HOMO and LUMO energies of the dendrimer were strongly localised on the [Ir(ppy)3] core. This result reflected the fact that the second generation phenylene dendronised dendrimer was more photoluminescent in the solid state, and had lower hole mobility than the first generation ver- sion of this dendrimer. These two properties were consistent with the optically and electronically active cores being on average held further apart by the larger dendrons in the second generation dendrimer [93]. In contrast for the dendrimer with carbazole dendrons the calculations showed that while the LUMO also resided on the core, the HOMO density was not only found on the [Ir(ppy)3] core, but was also heavily located on the carbazole units at the dendrons.

In order to determine the effect on the molecular orbital distribution of the higher generation car- bazole dendrimers a further set of calculations were performed on these carbazole dendrimers by Dr Chris Shipley at the University of Oxford and Dr Seth Olsen at the University of Queensland. The re- sults of these calculations are summarised in Table 5.1, and those for the first generation biphenyl (Ir-G1)

Figure 5.6: Target diagrams showing the divisions of the Mulliken populations of the frontier molecular orbitals for the first generation biphenyl and carbazole dendrimers used in the generation of Table 5.1. The percentages shown in Table 5.1 arise from the orbital populations in the area between a given circle the next inward circle. The fluorenyl surface groups for the carbazole dendrimers have been omitted to enable a simpler calculation [160].

and carbazole (Ir-CarbG1) dendronised dendrimers are shown as target diagrams in Figure 5.6, further details can be found in Reference [160]. For the carbazole dendrimer the fluorenyl surface groups were omitted to enable a simpler calculation. The angles between different aromatic units in the dendrons and their attachment points to the core were, also for simplicity, fixed in the calculation to an angle of 30o to take into account the interactions between neighbouring aromatic units. It is highly probable that the different generation of dendrimer could cause deviations from this structural arrangement due to differ- ences in the steric interactions that occur. If for example, such steric interactions caused larger twisting in the dendrons than used in the calculations, then the calculated energy gap for the orbitals may be less than would be observed experimentally. Therefore while the technique is a powerful one it only allows general trends to be drawn from the results [160].

For the first generation dendrimer with biphenyl dendrons almost all (97.4 %) of the LUMO density was calculated to reside on the 2-phenylpyridyl ligand. Only around 2 % was found on the iridium and the remainder on the dendron. In each of the three generations of the carbazole dendrimers, as the table shows, the LUMO distribution was very similar, which indicated that the carbazole units did not strongly affect the LUMO energy density distributions.

Table 5.1: Mulliken populations of the frontier molecular orbitals of the first generation biphenyl based dendrimer and the three generations of carbazolyl dendronised dendrimers. Ir = iridium(III), ppy = 2- phenylpyridyl ligand, Layer 1 = first level of carbazole or phenyl branching units, Layer 2 = second level of carbazole branching units, and Layer 3 = final level of carbazole units [160]

HOMO distributions. For Ir-G1 the HOMO distribution was such that nearly all of the HOMO orbital density was on the central iridium metal core complex, 52.6 % of the density lay here, with 41.2 % on the ppy ligand, and the remaining 6.2 % on the biphenyl dendron [160].

In contrast moving to the Ir-CarbG1 dendrimer with carbazole dendrons, the amount of HOMO density on the iridium at 24.8 % was less than half of that found in the biphenyl dendron dendrimer. There was a similar percentage of the HOMO density on the ligand: 34.3 % in Ir-CarbG1. The remaining 40.4 % was found on the first layer of the carbazole units of the dendron, a large increase over the 6.2 % found on the biphenyl dendron for Ir-G1. In the second-generation carbazole dendrimer (Ir-CarbG2), the HOMO orbital density on the iridium atom fell further to 9.7 %, with 21.5 % on the ligand. Most of the density was found to reside on the first (41.5 %) and second (27.3 %) layers of carbazole units. A further dilution of the HOMO density on the iridium(III) complex continued with the third generation carbazole dendrimer where only 2.1 % was found on the iridium, and 6.7 % was found on the ppy ligand. The remaining HOMO orbital density was distributed between the first (22.3 %), second (37.8 %), and third (31.1 %) carbazole layers [160].

In the biphenyl dendronised dendrimers, in which the HOMOs on adjacent cores are effectively isolated from one another, the hole transport is known to be via core-to-core hopping, with hole transport found to increase with dendrimer generation [42, 93]. The fact that the HOMO was distributed over the whole of the carbazole dendronised dendrimers, as opposed to being localised on the core complex, indicates the hole transport in the two types of dendrimer could be very different, an issue that was considered further in subsequent sections of this this chapter.