The synthesis of the branched oligosilanes 7-11, precursor compounds for the construction of core structures and wings, is shown in Figure 3. Reductive coupling of mixtures of SiCl₄ and Me₃SiCl with elemental lithium in THF (Tetrahydrofuran) gives access to 7 in yields of 60–80% [59]. Reductive coupling of mixtures of MeSiCl₃ and Me₃SiCl with elemental lithium in THF produces 10 as the major product in ca. 60–75% yields and 11 as the byproduct in ca. 20–25% yield [60]. The phenyl functionalized silanes 8 and 9 were synthesized in yields of ca. 70% by treatment of LiSiMe₂Ph with PhSiCl₃ and MeSiCl₃, respectively [32,61]. All five oligosilanes are air- and moisture-stable compounds that can be prepared on a ca. 50 g scale and purified easily by vacuum distillation or sublimation.

The introduction of multiple functional leaving groups can be achieved by selective chlorodemethylation of 7, 10 and 11 [62,63,64,65,66] and by chlorodephenylation of 8 [67]. Both types of reactions proceed with high selectivities to furnish the chlorosilanes 12, 13, 15 and 16 each in excellent yields as moisture-sensitive but air-stable compounds. Again, purification of these compounds can be carried out easily by vacuum distillation or sublimation. The triflato-functionalized core structure 14 can conveniently be generated via protodesilylation of 9 with three equivalents of trifluoromethanesulfonic acid [20]. The hexachloro-functionalized silane 17 is generated by base-induced redistribution of Cl₂MeSi-SiMeCl₂ (derived from chlorination of the Mueller-Rochow disilane fraction) [68,69].

Wing structures that contain silyl anions (herein referred to as metalated wings) can conveniently be generated from oligosilanes 7, 9 and 10 by a synthetic approach that was first developed by the groups of Gilman and Brook and involves selective cleavage of silicon-silicon bonds by strong metallo-nucleophiles. For example, reaction of LiMe with 7 and 10, respectively, generates after 2–3 days the highly air- and moisture-sensitive branched lithium silanides Li-18 [70,71] and Li-19 [72] (Figure 4). Similarly, branched polysilane 21 [73] can be converted into Li-22 [74] in good yields (Figure 5). Recently, Marschner developed an easy and cheap synthetic method that gives access to a wide variety of potassium silanides via the selective cleavage of a Si-Si bond with KOBu^t in THF [75]. Using this straightforward synthetic protocol, the potassium silanides K-18 [75], K-19 [76] and K-20 [41] (Figure 4) and also K-22 [77] (Figure 5) could be generated within a few hours in almost quantitative yields. Both, lithium and potassium silanide wings are used as strong nucleophiles in the selective formation of Si-Si bond required for the assembly of the final polysilane dendrimer. The use of the more reactive potassium silanides, however, allows for the isolation of the raw dendrimer in higher purities and yields and facilitates the purification of the dendrimer.

An extension of this synthetic approach to oligosilanes that contain two Si-M functionalities is shown in Figure 5. Reaction of two equivalents of KOBu^t with branched oligosilane 21 in THF at elevated temperatures gave access to the dipotassium disilanide K-23 in excellent yields [77]. Dilithiated disilanide Li-23 was obtained in modest overall yields in two steps [74]. Compound K-23 proved to be useful as a metalated spacer unit for the construction of large double-core polysilane dendrimers of first generation via silicon-silicon bond formation chemistry.

In 1995 Lambert et al. [18,24] and Suzuki et al. [19] independently reported the convergent synthesis and structure of the first polysilane dendrimers of first generation (Figure 6). Both groups used a similar synthetic approach involving reactions of the lithiated wing units Li-18, Li-19 and Li-22, respectively, with core structure 15 which generated the regular single-core dendrimers 24 (S-1312), 25 (S-1313) and 26 (S-1343). These reactions proceeded rapidly with formation of three new silicon-silicon bonds via elimination of lithium chloride. Notably, dendrimer 26, structurally characterized by X-ray crystallography, is composed of 31 silicon atoms, has a molecular weight of 1833 and has 13 silicon atoms in the longest silicon chain. It is the largest single-core dendrimer prepared to date [24].

Attempts to synthesize first generation polysilane dendrimers with four wings emanating from the central core failed, primarily for steric reasons [46]. For example, treatment of core structure 12 with four equivalents of Li-19 led to partial reduction and fragmentation processes resulting in the formation of 27 and 28 as low-yield byproducts (Figure 7) [21,27]. Both molecules, however, represent further examples of regular polysilane dendrimers of first generation with three wings emanating from the central core. Hydrido-functionalized dendrimer 27 (S-1312) is closely related to 24 (S-1312), in which the methyl group bound to the central core is replaced by H.

Recently, Krempner et al. have shown that a variety of core- and spacer-functionalized single-core dendrimers of first generation can be prepared easily and in high yields by modifying the number and position of the leaving chloro groups at the iso-tetrasilane single-core units (Figure 8). For example, reaction of potassium silanide K-19 with iso-tetrasilane 13 gave the remarkably stable chloro-functionalized dendrimer 29 [43]. Subsequent hydrolysis in the presence of sulfuric acid slowly but almost quantitatively generated the hydroxo-substituted dendrimer 30 [50]. Treatment of hexachloro substituted iso-tetrasilane 17 with potassium silanide K-19 in hexanes furnished the trichloro-substituted dendrimer 31 in excellent yields [42]. The reaction proceeds with high regio- and diastereoselectivity to form the l,l-form of 31 as the only detectable diastereomer. Hydrolysis of l,l-31 in the presence of NH₄(H₂NCOO) gave racemic mixtures of the two possible diastereomers l,l-32 and l,u-32 with the latter form being the major diastereomer [42]. In contrast, hydrolysis of l,l-31 without base gave l,l-32 as the major diastereomer [48].

In 1996, Lambert et al. reported the first convergent synthesis and solid-state structure of permethylated polysilane dendrimer 33 (S-1302), the smallest dendritic polysilane with a total of 10 silicon atoms and a longest silicon chain with 5 silicon atoms [21] (Figure 9). Compound 33 was obtained in fairly low yields (16%) from the reaction of Li-19 with MeSiCl₃ in THF, along with a cyclic by-product arising from silicon-silicon bond cleavage processes. Krempner et al. found that formation of cyclic by-products can be suppressed by replacing the solvent THF with n-pentane and by adding neat MeSiCl₃ to a pentane solution of Li-19 at −78 °C. This synthetic protocol allowed for the preparation of 33 in much higher yields (78%). Reacting HSiCl₃ with Li-19 under similar experimental conditions gave the hydrido-functionalized dendrimer 34 in yields of 67%. 34 proved to be a useful precursor for the synthesis of various other core-functionalized first-generation polysilane dendrimers (Figure 9) [31,32]. Note, however, that in these dendrimers the three emanating wings are directly connected to the central core with its functional group because of the absence of spacer groups. This structural arrangement enforces steric repulsion between wings and core and is therefore inappropriate for the construction of stable polysilane dendrimers of higher generation. On the other hand, the functionalized core is fully encumbered by the three wings, which results in enhanced hydrolytic stabilities of, for example, dendritic bromo silane 35. Notably, no hydrolysis of 35 in water occurred; only in the presence of sulfuric acid 35 slowly hydrolyzed to the OH-functionalized dendrimer 36. Lithiation of dendritic bromosilane 35 led to the formation of the first metalated dendrimer 37 as an air- and moisture sensitive but thermally stable red solid; no fragmentation of the dendritic silicon backbone occurred even at higher temperature [38,39].

The synthesis of irregular single-core dendritic polysilanes was motivated by efforts to compare structural and spectroscopic properties of dendrimers with the same silicon chain length, but with different substitution patterns [45]. The preparation of irregular dendrimers requires a stepwise assembly of strategic silicon-silicon bonds using differently structured metalated wings to be attached to the central core unit. As shown in Figure 10, reactions of two equivalents of either K-19 or K-18 with 15 lead to the formation of the chloro-functionalized polysilane dendrons 38 and 39, respectively. Subsequent treatment of 38 with K-18 and 39 with K-19 furnished the irregular single-core polysilane dendrimers 40 and 41, respectively. On the other hand, reaction of the dichloro silane 42 (derived from the stoichiometric reaction of 15 with K-18) with dipotassium disilanide 43 [78] gave the first carbocyclic polysilane dendrimer 44 in modest yields.

Sekiguchi’s group was the first to report the synthesis of a single-core polysilane dendrimer of second generation 47 by using a divergent synthetic methodology (Figure 11) [20]. In the first step, silyl triflate 14 (X = OTf) was generated in situ from the reaction of TfOH with 9 (see also Figure 4) and subsequently reacted with 3 equivalents of lithium silanide Li-20 to afford the phenyl-functionalized first-generation dendrimer 45 (S-1312) in 43% yield. Finally, dendrimer 47 was obtained in yields of 29% by dephenylation of 45 with TfOH, followed by addition of Li-19. Later, Sekiguchi et al. [28] and Marschner et al. [41] reported an improved procedure for the synthesis of phenylated dendrimer 45. Replacement of silyl trifilate 14 (X = OTf) with either bromosilane 14 (X = Br) or chlorosilane 15 avoided frequently encountered side-reactions of strong nucleophiles such as metal silanides with silyl triflates and produced 45 in much better yields (74% in both cases).

Marschner et al. also reported the synthesis of 49, a phenyl-substituted regular polysilane dendrimer of second generation, in good overall yields (Figure 11) [41]. Dendrimer 45, derived from the reaction of bromosilane 14 (X = Br) with potassium silanide K-20, was treated with neat HBr at −78 °C to furnish the bromo-substituted first generation dendrimer 48 in 83% yield. Subsequent reaction 48 with K-20 gave access to dendrimer 49 in yields of 41%. Notably, compounds 47 and 49 are the only examples of a regular polysilane dendrimers of second generation; both are composed of 31 silicon atoms and have longest chains of 13 silicon atoms. The structure of 47 was unambiguously identified by single-crystal X-ray crystallography (Figure 15) [20].

The synthesis and solid-state structure of dendritic silane 51 (Figure 12), the first double-core polysilane dendrimer was reported by Lambert et al. in 1998 [24]. Later, Krempner et al. published an improved synthesis via reductive coupling of bromo-functionalized dendron 50 with Na-K alloy, which gave 51 in yields of 32% [40]. Dendrimer 51 is of first generation with a longest chain of 6 silicon atoms and a total of 14 silicon atoms. Again, because of the absence of any silicon containing spacer groups the cores of the two dendrons are directly connected with each other, which introduces significant steric strain in the molecule. These repulsive steric interactions render the construction of double-core polysilane dendrimers of higher generation impossible.

Shortly after, Krempner et al. synthesized a variety of new double-core polysilane dendrimers of first generation and showed by means of X-crystallography that introducing spacer silicon groups in various positions indeed reduces the steric strain significantly [37]. For example, dendrimer 52 (Figure 12), derived from lithiation of 50 and subsequent addition of ClMe₂SiSiMe₂Cl, contains two silicon spacer groups between both central cores, which minimizes steric interactions between the two dendrons.

Spacer silicon groups were also incorporated between core and branch point such as in dendrimer 53, which was prepared by treatment of 4 equivalents of potassium silanide K-19 with the chloro-functionalized double-core unit 16 via salt metathesis (Figure 13). Reductive coupling of dendron 38 with Na-K alloy furnished the double-core polysilanes dendrimer 54 that contains spacer silicon groups between both cores and branch point and core. Compound 54 has a longest chain of 10 silicon atoms and total of 20 silicon atoms [44].

Recently, the Krempner lab reported a synthetic strategy to even larger double-core polysilane dendrimers involving a convergent methodology [47]. Key to this approach is the generation of dimetalated oligosilane K-23 published by Marschner and coworker (see also Figure 5) [77], which in reactions with the chloro-functionalized dendrons 38 and 39 afforded the double-core dendrimers 55 (D-1312-6) and 56 (D-1313-6), respectively, in good yields (Figure 14). In particular polysilane 56 is worth mentioning as it represents the largest dendritic polysilane prepared to date; it has a longest chain of 14 silicon atoms and a total of 32 silicon atoms. Both compounds have been characterized by X-ray crystallography and NMR spectroscopy.

One of the most characteristic features of polysilanes, regardless of whether they are linear, branched, hyperbranched or dendritic in structure, is the extensive delocalization of σ-bonded electrons (σ-conjugation) along silicon chains, resulting in strong electronic absorptions in the near UV due to a σ-σ* transition [1,2,3]. Both intensity and energy of the absorption are extremely sensitive to the overall conformation and the number of silicon atoms in the longest silicon chain [1,2,3] as well as the electronic and steric nature of the attached substituents [85,86,87,88,89]. For example, in linear permethylated polysilanes of general formula Me(SiMe₂)_nMe [90,91,92,93] delocalization of σ-electrons along the silicon chain leads to UV absorption that increases in wavelength and intensity as the number of silicon atoms in the polysilane chain increases with a wavelength reaching an asymptotic value of around 293 nm for [Me(SiMe₂)₂₄Me] (see also Figure 21 (left)).

Table 5 and Figure 21 (right) summarize the electronic absorption data for dendritic polysilanes. The data show that only the extinction coefficient for permethylated polysilane dendrimers tends to increase as the total number of silicon atoms and the number of silicon atoms in the longest chain increase from 3.4 × 10⁴ to 13.0 × 10⁴. The intensity of the UV absorption maximum is stronger in polysilane dendrimers than in linear polysilanes, primarily because of the higher “concentration” of silicon chains. Accordingly, the permethylated single-core polysilane dendrimers with longest chains of 7 silicon atoms 24, 25, 40 and 41 exhibit similar extinction coefficients and absorption maxima ranging from 0.5 to 0.6 × 10⁵ and from 265 to 269 nm, respectively. However, unlike in linear polysilanes there is no correlation between the energy of the absorption maximum and the number of silicon atoms in the longest chain for permethylated polysilane dendrimer. For example, comparison of UV data of the single-core dendrimers 44 with 40, both have a total number of 14 silicon atoms and longest chains of 7 silicon atoms shows that the absorption maximum of 44 is red shifted by ca. 8 nm relative to 40. Furthermore, double-core dendrimer 52 with a longest chain of only 8 silicon atoms displays an absorption maximum at 285 nm, whereas 56 absorbs at only 269 nm even though it is the largest polysilane dendrimer reported to date with longest chains of 14 silicon atoms and a total of 32 silicon atoms per molecule. Clearly, there are other factors such as conformational arrangements, steric and electronic effects that may influence the electronic properties of polysilane dendrimers.

Peralkylated linear polysilanes can adopt various conformational arrangements that deviate from the two preferred CCCC dihedral angles ω in hydrocarbons, 180° (A, anti) and 60° (gauche, G). Deviations from these angles can reasonably be explained by repulsive interaction between substituents attached to the silicon silicon backbone, which increase as the size of the substituents is increased relative to the Si-Si bond length [94,95]. Steric effects are also responsible for the observation of three conformational families of dihedral angles: gauche (G, ω ~ 60°), ortho (O, ω ~ 90°) and anti (A, ω ~ 180°) dominating in linear polysilanes [96,97,98]. Even more conformers are possible in branched and dendritic polysilanes due to the redundancy of silicon-silicon chains combined with strongly repulsive interactions between substituents. Possible conformers in polysilane dendrimers can be classified according to their SiSiSiSi dihedral angles as syn (S, ω ~ 0°), cis (C, ω ~ 30°), gauche (G, ω ~ 60°), ortho (O, ω ~ 90°), eclipsed (E, ω ~ 120°), deviant (D, ω ~ 150°) and anti (A, ω ~ 180°).

Recent studies on permethylated linear polysilanes with discrete conformations have revealed the great impact of the overall silicon chain conformation on the degree of σ-conjugation and the UV spectroscopic properties. It was shown theoretically and experimentally that the σ-σ* absorption peak shifts dramatically to the red as the SiSiSiSi dihedral angle, ω, is increased from 0° to 180° [99,100,101,102,103,104]. This effect was explained by 1,4-orbital interactions (β_vic), being most effective (maximum orbital overlap) in the HOMO (σ-type orbital) of a syn-conformer, while not present in an anti-conformer. In fact, the 1,4-orbital interaction in syn-conformers lowers the energy of the HOMO (σ-orbital) and consequently increases the energy of the σ-σ* transition relative to that of the anti-conformer ([99] and references cited therein).

Perhaps, the most striking example provides a series of conformationally constrained hexasilanes 59-62 (composed of three dihedral angles) that have been structurally characterized by X-ray studies [100,101] (Figure 22). Within this series of hexasilanes both absorption maximum and extinction coefficient monotonically increase as the SiSiSiSi dihedral angles become larger from SAS (λ = 239 nm, ε = 1.9 × 10⁴), AAS (λ = 255 nm, ε = 3.1 × 10⁴), AEA (λ = 259 nm, ε = 5.4 × 10⁴) to AAD (λ = 267 nm, ε = 6.7 × 10⁴) (Figure 22). These findings clearly revealed that σ-conjugation in polysilanes is effectively extended by anti or deviant conformations, while conformations with small dihedral angles such as syn, cis or gauche do not contribute to the extent of σ-conjugation [102].

Similar conclusions can be drawn for polysilane dendrimer, even though the redundancy of silicon chains and the steric strain causes a variety of more or less flexible conformers to be formed, each contributing differently to the extent of σ-conjugation along the silicon backbone. The presence of these different conformers complicates analysis and assignment of the often broad and intense UV absorption bands and hampers deducing the relationship between conformation and photophysical properties. Moreover, dihedral angles such as ortho are common in dendritic structures and it is not clear yet to what extent ortho conformers would contribute to the degree of σ-conjugation along silicon chains. Table 6 summarizes the overall conformation and average silicon-silicon distances of the longest silicon chains in selected polysilane dendrimers obtained from the X-ray data along with the measured UV absorption maxima. Note, that the average silicon-silicon distances of the longest silicon chains of all structurally characterized polysilane dendrimers (except for 51) lie within the relatively narrow range of 236–238 pm, suggesting only minimal influence of silicon-silicon bond stretching on the energy of the σ-σ* transition [105] and allowing for comparison of both conformational arrangements and UV absorption characteristics of individual dendrimers with each other.

The longest silicon chains in the single-core dendrimers 24, 40 and 25 contain seven silicon atoms (heptasilane chain) and are defined by four tetrasilane dihedral angles. The conformational analysis revealed that conformers with exclusively large dihedrals angles such as all-anti (A-A) or all deviant (D-D) or mixed A-D segments exist only in several pentasilane subunits and that in most of the heptasilane chains anti (A) or deviant (D) segments are interrupted by additional ortho (O) or gauche (G) conformers, which are predicted to interrupt σ-conjugation along the chain. The observed conformational arrangements (DDOD, ADOD etc.) can be explained in terms of the bulkiness of the dendrimer wings. They are oriented in the same direction (clockwise or anti-clockwise), which reduces their repulsive interactions (see also Figure 18). In all three dendrimers, however, the heptasilane chains with largest dihedral angles are fairly similar to each other, which correlate with the similar UV spectroscopic data of 24, 40 and 25.

The strong impact of the silicon chain conformation on the energy of the UV absorption maxima of this class of single-core dendrimers is best demonstrated by comparing dendrimer 40 with conformationally constrained dendrimer 44; both have longest heptasilane chains and the same total number of silicon atoms in the molecule [45]. Assuming that the absorption maximum originates from silicon chains with largest dihedral angles, 44 is red-shifted by 8 nm as a result of a conformational change from a ADOD conformer in 40 to a ADAD conformer in 44. Clearly this conformational arrangement can only be enforced by connecting two of the three dendrimer wings with an alkylene linker as is the case for 44. The formation of all-anti or all-deviant conformers in unconstrained single-core dendrimers is not preferred, since this would lead to considerable repulsions of the bulky wings as shown in Figure 23.

Notably, the broad and intense absorption maxima of the majority of double-core polysilane dendrimers are at lower energy than to those of single-core dendrimers with comparable silicon chain length. Most remarkable in this respect are the double-core dendrimers 51 (longest hexasilane chain; λ_max = 276 nm), 52 (longest octasilane chain; λ_max = 285 nm) both being characterized by X-ray crystallography and 54 (longest decasilane chain; λ_max = 292 nm); the latter with the longest wavelength of all known permethylated polysilane dendrimers. The conformational analysis (from the X-ray data) reveals that the SiSiSiSi dihedral angles in at least one of the longest hexasilane and octasilane chains of the double-core species 51 (EAE) and 52 (DEDED) (Figure 24), respectively, are larger than any of the longest heptasilane and undecasilane chains of the single-core species 24 (DDOD) (Figure 24) and 47 (DDODODOD), respectively.

On the other hand, the absorption maximum of double-core dendrimer 53 (λ_max = 271 nm) is only slightly shifted to the red relative to single-core dendrimer 24 (λ_max = 269 nm). Again, the X-ray data of 53 revealed that conformers with all-deviant segments exist only in several pentasilane subunits and that in the octasilane chains deviant (D) segments are interrupted by additional ortho (O) or even cis (C) conformers. Thus, it seems that in this type of dendrimer the σ-electrons are fully delocalized along the dendrimer wings (dendrons), which, similar to 24, have longest chains of 7 silicon atoms and DDOD conformations (Figure 25). The observation that the double-core dendrimers 52 and 54 (no X-ray data) absorb at significantly longer wavelengths than 53 implies that the incorporation of silicon spacer groups between the two cores as in 52 and 54 reduces steric repulsion between the dendrimer wings and, consequently, allows for conformations being optimal for the delocalization of σ-electrons along the silicon main chains.

Another striking example, which emphasizes the importance of conformational effects with regard to the degree of σ-conjugation, is provided by the double-core dendrimers 55 and 56 (Figure 26). According to the X-ray data, 55 has longest chains of 14 silicon atoms but conformers with exclusively large dihedrals angles such as all-anti or all-deviant segments exist only in several hexa- and pentasilane subunits, which are interrupted by additional ortho (O) or gauche (G) units. Note, however, that the heptasilane subunits of the dendrimer wings of 55 (DDOD) have dihedral angles fairly similar to those of the irregular single-core dendrimer 40 (DAGD and DDOD). In fact, 55 may be described as a dimer of 40: dendrimer 55 has a total of 28 silicon atoms, whereas 40 is composed of 14 silicon atoms. Accordingly, the wavelengths of UV absorption maxima 55 (λ = 274 nm) and 40 (λ = 269 nm) are similar to each other, but the extinction coefficients differ by a factor of two (40, ε = 0.6 × 10⁵; 55, ε = 0.6 × 10⁵). Clearly, these examples show that it is primarily the conformation of the silicon chain that determines the optical properties of polysilane dendrimers.

From the discussion above it seems to be clear that the silicon backbone conformation is one the major factors in determining the degree σ-conjugation not only in linear polysilanes but also in branched and dendritic structures. For comparison, the silicon backbone conformations and average silicon-silicon distances, derived from X-ray diffraction data and the UV absorption data of linear, branched and dendritic polysilanes of chain lengths 6, 8 and 10 are summarized in Table 7.

Linear permethylated polysilanes can adopt various silicon chain conformations in solution primarily due to the low rotational barriers of the silicon-silicon bonds. As a result, mixtures of twisted conformers are observed in solution causing relatively broad UV absorption bands at room temperature. Upon cooling to 77 K, however, the absorption maxima of these linear oligosilanes are significantly red-shifted and the absorption bands become sharper due to a conformational transition from twisted to predominantly helical all-anti conformers, meant to be optimal for σ-conjugation.

Evidence that a transition from twisted to all-trans conformers is responsible for the observed red-shift of the absorption maxima upon cooling to 77 K was provided by the synthesis and UV spectroscopic measurements of conformationally rigid linear oligosilanes with exclusively large dihedral angles (ideally all-anti). In fact, hexasilane 62 and all-anti octasilane 63, reported by Michl [100] and Tamao [103] et al., respectively, and decasilane/cyclodextrin inclusion complex 64, reported by Kira’s group [106], display absorption maxima at room temperature that have the longest wavelengths and the highest extinction coefficients within the series of linear hexa- octa- and decasilanes.

Comparing the structural and electronic properties of these linear polysilanes with their branched and dendritic counterparts appears to be difficult as significant steric strain is introduced upon adding silyl groups to the inner silicons of the silicon main chain as is the case in branched and dendritic polysilanes. In fact, the introduction of further branch points leads to significant deviations of the Si-Si-Si angles from the ideal tetrahedral angles (see also discussion in section 4) and an increase of the average silicon-silicon distances of the longest silicon chains from ca. 235 pm for linear polysilanes to ca. 239 pm for some of the dendritic polysilanes. As demonstrated for various substituted disilanes by Michl et al., changes of the silicon-silicon distances may affect the energy of the σ-σ* transition and consequently the electronic properties of the bulk material quite dramatically [105]. Note, for example, that sterically crowded hexasilane dendrimer 51 (conformation EDE) clearly outperforms the conformationally rigid carbocyclic hexasilane 62 (conformation AAD) by 10 nm with respect to the wavelength of the absorption maximum (see Table 7). On the other hand, the absorption maximum of sterically less crowded octasilane dendrimer 52 (conformation DEDED) is slightly blue-shifted relative to that of all-anti octasilane 63, but red-shifted relative to that of its branched counterpart 65. It is also not clear to what extent the conformations of the longest silicon chains found in the solid-state (X-ray diffraction) for branched and dendritic polysilanes will retain in solution. However, according to the UV absorption characteristics of the branched silanes 65 and 66, there seem to be some control over the conformation in solution as the absorption maxima of both compounds are significantly red-shifted relative to that of the linear silanes Me(SiMe₂)₈Me and Me(SiMe₂)₁₀Me at room temperature. Obviously, similar conclusions can be drawn for polysilane dendrimers as steric strain appears to be the dominating factor in terms of controlling certain silicon main-chain conformations of these well-defined molecules in solution.

Recently, the groups of Stueger and Krempner independently investigated the molecular structures and electronic properties of oxo-functionalized cyclohexasilanes and branched heptasilanes [110,111,112,113,114,115], respectively. Strong electronic coupling of oxygen-containing donor groups such as OR, OH and OSiR₃ with the silicon-silicon backbone (σ-n mixing) was noticed, resulting in a substantial decrease of the optical band gaps of these compounds. The results of DFT calculations revealed that the energy of the HOMO (primarily σ-orbital of silicon backbone) in these compounds is almost unaffected by the introduction of one or even more of oxo group. In the LUMO, however, strong σ-n mixing of the oxygen lone pairs with the σ-orbitals of the silicon-silicon backbone occurs, which significantly lowers the energy of the LUMO relative to that of permethylated cyclohexasilane and branched heptasilane. Thus, the rather unusual electronic spectra of these oxo-functionalized polysilanes can be explained by a symmetry forbidden HOMO-LUMO transition, which causes the red-shifted UV absorption maxima to be reduced in intensity [116,117].

Table 8 summarizes the absorption data for various functionalized and non-functionalized single-core polysilane dendrimers of first generation along with the conformational data. All these dendrimers have the same numbers of silicon atoms in the longest chains, same total numbers of silicon atoms per molecule and fairly similar conformational arrangements. Note, however, that the UV absorption characteristics of 29 and 30 are strikingly different from that of the hydrido-functionalized analogue 27 and permethylated 24. In fact, replacing the hydride group attached to the central silicon core with either a chlorine atom or an OH group tremendously shifts the absorption maximum to the red by 23 nm for 29 and 41 nm for 30, coupled with some reduction in intensity. Again, these findings can be understood as an optical band gap reduction resulting from effective interactions between chlorine or oxygen lone pair (n) and the σ-orbitals of the silicon main chains, resulting in HOMO-LUMO transitions that are symmetry forbidden.

Notably, the absorption maxima of the OH functionalized dendrimers l,l-32 and l,u-32, each of which has a total number of three OH groups and two OH groups per heptasilane chain, are blue-shifted by ca. 20 nm relative to 30, which contains only one OH group attached to the central core silicon. This suggests that the position of the silicon at which the OH group is attached within the silicon chain is more influential in determining the optical band-gaps of these compounds than the absolute number of OH groups attached to the silicon main chain. It seems that strongest electronic coupling of the OH group with the silicon backbone (σ-n mixing) is seen when the OH group is attached to a tertiary silicon atom, whereas secondary and primary Si-OH groups show weaker coupling in the LUMO. This observation is consistent with previous results on the absorption spectra of alkoxy substituted polysilanes of general formula Me-(SiMeOR)_n-Me and –[Si(OR)₂]_n-, in which the OR groups are primarily attached to secondary silicon atoms [87,88]. It is, perhaps, the greater inductive effect associated with the central tertiary silicon compared to secondary or primary silicon atoms, that will more effectively equalize the energies of the oxygen p orbital and silicon main chain σ levels. This would lead to a greater overlap of the involved orbitals resulting in a substantial decrease of the optical band gap [117].

The emission properties of linear polysilanes are characterized by sharp emissions in the UV region with a relatively small Stokes shift. Branched polysilanes show a dual emission, one sharp emission at around 360 nm that is assignable to the linear silicon chain and a very broad one in the visible region at around 450 nm, which is related to a branching point (tertiary silicon atom).

Sekiguchi and coworker studied the emissive properties of the first and second generation polysilane dendrimers 24 (S-1312) and 47 (S-2312) at 77 K, and also found for each dendrimer two bands in time-resolved emission spectra [33,36]. From the observed small Stokes shift of the sharp emissions, they concluded that the excitons are fully delocalized over the silicon chains. The broad emission, on the other hand, was thought to be due to excitons that are localized at branching points [Si(Si)₂]. It was shown that the broad emission arises from an intramolecular energy transfer from the excited state in the silicon chains to that in the branching points. Similar observations were made by Krempner et al., who studied the emissive properties of the permethylated dendrimers 24, 25, 40, 55 and 56 at room temperature, which all showed similarly weak dual emissions (Table 9). Sharp emissions appeared in the UV region at ca. 330 nm and broad emissions in the visible region at around 450-460 nm. In the OH-containing dendrimers 30 and l,l-32 the broad emissions at 480 nm (30) and 465 nm (l,l-32) predominated. The most striking feature of 30 and l,l-32, however, was a marked enhancement of the signal intensity by about a factor of 100 relative to the permethylated counterpart 24. Interestingly, this behavior closely resembles that of the strongly photoluminescent 2D-siloxene, [Si(OH)₃H₃]_n and its counterpart the 2D-polysiline, [Si₆H₆]_n, which showed a rather weak emission [116,118].