Conclusion

4.8. Experimental

4.9. References

Chapter 4

Ruthenium Acetylide Dendrimers and Related

Complexes

4.1. Introduction

The traditional design of molecules with high molecular nonlinearities has focused on dipolar complexes. These molecules tend to have high second-order nonlinearities because of the increased stability of the first excited state. This concept has been used in organic, inorganic and organometallic complexes. A different design motif, however, is to examine octupolar compounds. Many examples of octupolar compounds exhibit high second-order hyperpolarizabilities. Ul2 Octupolar molecules are less likely to pack in a centrosymmetric space group, or to have dipoles aligned in an antiparallel fashion, both of which would eliminate a bulk response. Additionally, octupolar molecules can possess substantial off- diagonal tensor components, which can lead to high nonlinearities.

Dendrimera are regular hyperbranched polymers which, because of their regular repeating units, possess well defined molecular weights. The dendrimers synthesized in this Chapter have three arms at each branching point. A dendrimer generation is defined by the number of branching points (Figure 4.1.) such that a zero-generation dendrimer possesses one branching point and a first-generation dendrimer possesses a second branching point at the terminus of each zero-generation arm.

Zero-Generation First-Generation Second-Generation

Figure 4.1. Idealized dendrimer geometry

Two different synthetic methodologies are used to construct dendrimers: divergent and convergent (Figure 4.2.). The divergent strategy involves reaction of a zero-generation core with a branching point, to give the first generation dendrimer. This, in turn, is reacted with another branching point to give the second-generation dendrimer. The convergent strategy involves construction of a dendritic wedge, which is then coupled to a core to give the desired dendrimer. Both methodologies involve trade-offs. Because the divergent route uses smaller branching points, there is less steric strain; however, separation of defective dendrimers tends to be more difficult, as the two dendrimers tend to be quite similar. The convergent route increases the problem of steric strain, but makes the purification of the final product simpler.

Zero-Generation First-Generation Second-Generation

Figure 4.2. Divergent (above) and convergent (below) dendrimer construction

methodologies

Dendrimers have been used for a variety of different purposes including drug delivery,1316 biochips,17 nanoscopic containers,15 and catalysts.16 Dendrimers have a well-established role in photonics; there have been a number of recent studies into their effectiveness as light harvesting molecules,18'24 as nonlinear optical materials,25'27 as m aterials for high performance waveguides28 and as electro-optical materials.29

While most dendrimers are organic,30'33 organometallic dendrimers have been reported in the literature.34'67 A review on organometallic dendrimers and other related complexes was published in 1998.68 There are a small number of transition-metal acetylide dendrimers that have been reported.26,27,62'67,69

The existing transition-metal acetylide dendrimers have utilized two different transition metals: platinum and ruthenium. Sections 4.2. and 4.3. include details of literature-extant ruthenium-acetylide dendrimers. Figure 4.3. displays an example of a platinum-acetylide dendrimer developed by Takahashi and co-workers.64 Dendrimers with the same design motif, and up to six generations in size, have been synthesized.67 Novel variants have also been synthesized, such as a dendrimer incorporating a porphyrin core. Figure 4.4. displays an example of a platinum-acetylide dendrimer with a porphyrin core, but it should be noted that examples of up to two generations larger have been reported.66

The synthetic work in this Chapter is divided into three broad themes. The first theme involves the incorporation of an electron donating group (diethylamino) onto the outside of a ruthenium acetylide dendrimer. The second theme is an investigation into dendrimers with an organic core and metals on the periphery. Because of the large numbers of chemical reactions required to synthesize these dendrimers, the last synthetic motif involves an investigation into new synthetic routes to reduce the number of steps required for the synthesis of new dendrimers.

Figure 4.3. Platinum-acetylide dendrimer64

Figure 4.4. Porphyrin-containing platinum-acetylide dendrimer66

4 . 2 .

Synthesis of Dendrimers with Electron-Donating Peripheral

Substituents

4.2.1.

Introduction

Two motifs are common in the design of materials with high nonlinearities: dipolar and octupolar geometry. Dipolar molecules rely on large charge asymmetry and extensive jt Ü delocalization to generate high nonlinearities. Dipolar complexes tend to use a donor-jt- conjugated bridge-acceptor system (Figure 4.5.), but while producing high nonlinearities, it also leads to a loss of optical transparency. Octupolar complexes provide a means of obtaining high nonlinearities without a corresponding loss of optical transparency. For example, the zero generation dendrimer l,3,5-{tran^-[Ru(C=CPh)(dppe)2](C=CC6H4-4- O C ) } 3C6H3 has a I y| of 3000 ± 600 x lO 36 esu, whereas the first generation analogue

1,3,5-C6H3-(C=CC6Fl4-4-C=C-?rafts-[Ru(dppe)2]C=C-3,5-C6H3-{C=CC6H4-4-CsC-?ranj'-

[Ru(C=CPh)(dppe)2]}2)3 has a | y | of 20 700 ± 2000 x 1CF36 esu. The two complexes display a MLCT band in almost identical locations (X.max = 411 nm and 402 nm, respectively).27 The incorporation of an electron-withdrawing nitro- group on the periphery of the first generation dendrimer mentioned above leads to a substantial increase in the nonlinearities.70 The purpose of this section is to investigate the effect of incorporation of an electron-donating substituent on the periphery of a dendrimer.

donor ji-conjugated bridge acceptor

Figure 4.5. Donor-jr-conjugated bridge-acceptor system

4.2.2.

Synthesis o f Et2NC()H4-4-C=CH

The standard literature preparation of a functionalized p-benzaldehyde involves reaction of the aldehyde (Et2NC6H4-4-CHO, in this case) with triphenylphosphine, Zn dust and carbon

tetrabromide to give the compound Et2NC6H4-4-CH=CBr2. This compound is then converted to Et2NC6H4-4-OCLi by reaction with n-butyllithium, and then protonated in situ by reaction with water (Scheme 4.1.) to give the NEt2 functionalized phenylacetylene.

An alternative synthetic route was devised. The compound Et2NC6H4-4-I was formed by reaction of NEt2Ph and iodine. The product, Et2NCöH4-4-I (1), was in turn reacted with

trimethylsilylacetylene via a Sonogashira coupling to give Et2NCöH4-4-OCSiMe3 (2) which was desilylated with tetrabutylammonium fluoride to give the desired acetylene (Scheme 4.2.). The identity of the acetylene 3 was confirmed by comparison of the NMR and low resolution El mass spectrometry data to the literature.72 The trimethylsilyl- protected precursor 2 was characterized by NMR and high resolution El mass spectrometry.

Zn pph3 CBr4