Orbital polysilanes

We realize that a more advanced method of calculation might place a larger weight on the role played by the orbitals of the two substituents carried by the Si atoms, and thus produce a larger difference between the parent polysilanes and their permethylated derivatives (cf. Ref. 18), but an inspection of the results suggests that this will not affect our conclusions. Another possible concern has to do with the absence of d orbitals in the INDO/S basis set In a large basis set calculation, they would undoubtedly contribute to some degree to the description of both the a and the lower excited states. They... 

The next-nearest-neighbor-orbital resonance integrals, /JI3, also remain unaffected by the pure twisting motion. We conclude that a pure twisting motion can therefore represent at best only a relatively small perturbation of the electronic structure of the polysilane chain, suitable for treatment by first-order perturbation theory. The perturbation is represented by changes in the resonance integrals between more distant hybrid orbitals, among which / 14 clearly is the most important. 

Models for cyclo-oligomerization, 175-176 Molecular mechanics (QM/MM), 178 Molecular orbitals, 7-9, 10 polysilanes, 143-145, 149 Monodentate coordination, 107-110,... 

Since for the diarylpolysilanes, as for all polysilanes, the main chain is chromophoric due to the electronic transition between the delocalized silicon a and a orbitals, the electronic spectroscopies of CD, UV-Vis, and FL are particularly powerful probes of the structures of these materials. The magnitude of dimensionless quantity, gabs, is perhaps the most useful in comparing the CD spectra of optically active poly(diarylsilane)s. 

Photoluminescence (PL) in the polysilanes is well documented,34b,34c and for the poly(diarylsilane)s occurs typically with a small Stokes shift and almost mirror image profile of the UV absorption.59 This is due to the similarity of the chromophore and fluorophore structures in the ground and excited states, respectively, which is a result of the fact that little structural change occurs on excitation of the electrons from the a to the a orbitals. As PL is the emissive counterpart to UV, the emissive counterpart to CD is circularly polarized pho-toluminescence (CPPL). Where the fluorophore is chiral, then the photoexcited state can return to the ground state with emission of circularly polarized light, the direction of polarization of which depends on the relative intensities of the right-handed and left-handed emissions (/R and /l, respectively), which in turn depends on the chirality of the material, or more accurately, the chirality... 

The optical and electronic functions of polysilanes owe to their delocalized highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) that are occupied by holes and conduction electrons, respectively. The polymer does not show high conductivity or optical nonlinearity if the electrons or holes are localized on a small part of the polymer chain. To elucidate the structure of HOMO and LUMO is therefore important for the molecular design of polysilanes as functional materials. 

Figure 24 Isosurfaces at +0.02 of molecular orbitals of the model compounds of polysilanes obtained by semiempirical PM3 calculation.

Figure 25 Energy diagrams for orbital interactions between polysilane skeleton and pendant group. HOMOsi, HOMOp and LUMOs , LUMOp denote the HOMO and LUMO delocalized on the skeleton and a pendant group, respectively, osi-c and a si-c are bonding and antibonding Si-C a orbitals, respectively.

The mechanism of orbital interactions mentioned above is not applicable if each monomer unit of polysilanes has sufficient electron affinity and two pendant groups of the monomer unit have approximately the same electron affinity. The LUMO of polysilanes is then regarded as a series of the LUMOs of monomer units that is perturbed by Si-Si bonds. The LUMO of the monomer unit has some electron density on the Si atom, since the unpaired electron in the LUMO is delocalized over the two pendant groups with approximately the same electron density. Connection of the monomer units with Si-Si bonds therefore causes the delocalization of the LUMO on adjacent monomer units and therefore on both the Si skeleton and pendant groups. 

One of the clearest indications of the presence of acceptor orbitals on silicon is the ability of polysilanes to be reduced to radical anions (73). Although early work assigned the structure (Me2Si)6T to the radical anion produced by reduction of dodecamethylcyclohexasilane (75), subsequent investigations showed that cyclic (Me2Si)5, (Me2Si)6, and (Me2Si)7 all... 

In the model compounds, this red shift has been ascribed to a combination of silicon backbone with the x-orbitals of the aromatic substituent coupled with a decrease in the LUMO energy due to x -(a, d) interactions (15,16). Further examination of the data in Table III shows that the absorption maximum of the cyclohexylmethyl derivative, 9, is also somewhat red-shifted relative to the other alkyl polymers suggesting that the steric bulk of the substituents and/or conformational effects may also influence the polysilane absorption spectrum. 

Bond-order changes upon photoexcitation obtained from Sandorfy SCF-CI calculations have been employed to predict the ease of photoextrusion of a silylene from polysilane chains or rings236. Smaller bond-order changes with longer polysilane chains suggested that they are less efficient silylene sources. Similar conclusions were reached for branched and cyclic polysilanes. This approach focuses on a to a excitation without participation by Si 4s orbitals in descriptions of excited states237. 

---