Polysilane dendrimer

The dynamics of polysilane dendrimer excited states were also studied by Watanabe as mentioned above in a comparison with polysilynes,360,364 and suggested that a configuration coordinate model is applicable to the photophysics of branched silicon chains. Calculations showed a distorted geometry of the excited state localized at a branching point. 

Figure 61 UV spectrum of 31-Si polysilane dendrimer 139 with 13-Si longest chain.378 Reprinted with permission from Lambert, J. B. Wu, H. Organometallics 1998, 17, 4904-4909, 1998 American Chemical Society.

Lithiotrisilanes, which are used as precursors to polysilane dendrimers, have been obtained by a mercury-lithium exchange reaction (equation 25)56,61. The terminally metalated trisilane Me3Si(Me2Si)2Li has been synthesized analogously56. 

Network Polysilanes, Polysilane Dendrimers, and Ladder Polysilanes... 

In recent years polysilane dendrimers have been synthesized in several laboratories. Two examples are shown in schemes 5.7128 and 5.8.129 The dendrimers have electronic absorption spectra similar to those of the longest silicon chains present, but with intensity about ten times as great. Like the network polysilanes, these dendrimers display weak visible emission spectra, perhaps resulting from localized excitations at the branching points. The field of polysilane dendrimers in the subject of a recent authoritative review.130... 

Independently of each other, Lambert et al. [69] and Suzuki et al. [70] both gained access to low-generation silane dendrimers (Gl, G2) in 1995. The latter prepared a first-generation polysilane dendrimer 3 by a stepped growth polymerisation technique. Coupling of methyl[tris(chlorodimethylsilyl)]silane (1) to tris-(trimethylsilyl)silyllithiurn (2) led to the first-generation branched dendrimer 3 (Fig. 4.38). 

Fig. 4.38 Preparation of a first-generation polysilane dendrimers (according to Suzuki et al.)...

Therefore, single-crystal X-ray structural analysis has hitherto been of limited use for structure elucidation of dendrimers. Most of the examples found in the literature [42 a] are actually single-crystal X-ray structural analyses of rigid, dimensionally stable (shape resistent) polyphenylene dendrimers [42 b] or of me-tallodendrimers [43] or polysilane dendrimers. 

In 1995, the first syntheses of polysilane dendrimers were reported independently by Lambert, Sekiguchi, Suzuki and their coworkers. These reports will be discussed separately. 

The study of dendritic polysilanes is a relatively new trend in silicon chemistry [1], However, first results suggest interesting properties for these compounds. Lambert and coworkers pointed out that their polysilane dendrimer, which contains 27 heptasilane chains possesses a very similar A ax value (272 nm) compared to the parent heptasilane (266 nm) but shows an extinction coefficient about ten times larger. Comparison of the number of silicon atoms in the molecules (13 V5 7) indicates an amplification of extinction per silicon atom of about 5 [la]. 

Reaction of compound 4 with dimethylphenylsilyllithium leads to the first generation polysilane dendrimer 6, which has been synthesized already by Sakurai et al [lb]. Again, treatment with HBr under the described conditions gives hexabromide 7, also in a clean reaction as a crystalline compound. Reaction of 7 with lithium aluminum hydride finally gives hydrosilane 8. 

A proposal made recently by Lambert and coworkers [Id] for the nomenclature and structural description of polysilane dendrimers treats the problem of the high redundancy of very complex names of these compounds which possess a high degree of symmetry and might be described better by different ways. 

Sekiguchi A, Lee VY, Nanjo M (2000) Lithiosilanes and their application to the synthesis of polysilane dendrimers. Coord Chem Rev 210 11... 

Watanabe A, Nanjo M, Sunaga T, Sekiguchi A (2001) Dynamics of the excited state of polysilane dendrimers origin of the broad visible emission of branched silicon chains. J Phys Chem A 105 6436... 

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