Polysilane chains band structure

As the number of silicon atoms in the delocalized backbone cr-electron system increases, the number of HOMO and LUMO states increases, resulting in a band structure for high molecular weight polymers. Electronic absorptions from the HOMO (cr) to LUMO (essentially a ) are responsible for the characteristic UV absorption of polysilanes observed between 300 and 400 nm, the transition moment for which is in the direction of the Si chain.198 Polysilanes are... 

Properties such as photoconduc.tivityl l t l and photoluminescence of silicon polymers have been reported because of their wider optical band gap compared with crystalline silicon. Theoretical investigations of silicon polymers have been also reported ll2l-[21] Xakeda, Matsumoto and Fukuchi calculated the electronic structure of polysilane chains using the semi-empirical approach called the Complete Neglect of Differential Overlaps (CNDO) Molecular-Orbital (MO) method They discussed the dependence of the size and... 

The next question is how side-chain substitutions and skeleton conformations affect the band structures of polysilanes. Side chains provide two interesting effects (7) band gap reduction caused by substitution of larger alkyl side chains and skeleton-side chain interaction (i.e., (j-tt mixing) in aryl poly silanes. This interaction was confirmed by UV photo spectroscopy (UPS) (8-9) and photoabsorption and luminescence measurements (iO, 11). Skeleton conformations are related to thermochromism (12-17). The ab initio... 

Substitution of aryl side chains results in a different band structure. A perspective end view of poly(diphenylsilane) is shown in Figure 11a. Electrons conduct along the red region under the influence of a potential barrier of phenyl groups. This electrical analogue of an optical fiber consists of an electrical core and an electrical clad. A perspective end view of poly-(methylphenylsilane) is shown in Figure 11b. In these aryl polysilanes, two important points should be considered the existence of states localized at phenyl side chains and the a-7T interaction between delocalized skeleton ct bands and localized tt states. 

Figure 9. Calculated band structures of parent and alkyl-side-chain-substituted polysilanes. The abbreviations and symbols are defined as follows CB, conduction band VB, valence band F, k = 0 point and X, Brillouin zone edge. Bsu, Bu, B2g, Bg, and Ag denote orbital symmetries.

Band Structure and Optical Absorption Properties of Polysilane Chains... 

An LCAO (linear combination of atomic orbitals) local-density functional approach was used to calculate the band structures of a series of polymer chain conformations unsubstituted polysilane in the all-trans conformation and in a 411 helical conformation, and all-trans poly(dimethylsilane). Calculated absorption spectra predict a highly anisotropic absorption for the all-trans conformation of polysilane, with the threshold absorption peak arising strictly from polarizations parallel to the chain axis. The absorption spectrum for the helical conformation is much more isotropic. Results for the dimethyl-substituted polysilane chain suggest that the states immediately surrounding the Fermi level retain their silicon-backbone a character upon alkyl-group substitution, although the band gap decreases by I eV because of contributions from alkyl substituent states both below the valence band and above the conduction band to the frontier states. 

In this study, we investigated a set of model polysilane chain systems that illustrate the basic physics and chemistry of some optical properties of these materials. In particular, we looked at the band structure for unsubstituted polysilane in an all-trans conformation, as well as in a 4/1 helical conformation with four silicon atoms contained in one translational repeat unit. In addition, we compared results for the dimethyl-substituted polysilane in an dl -trans conformation with the results for the unsubstituted poly silane. 

Figure 1. Summery of calculated results on the band structures of polysilanes. a and a means valence and conduction band formed by a-conjugation along a chain, respectively.

Polysilanes Having Multi-phases. Figure 3 schematically shows the band structure in poly(dihexylsilane). The arystal phase has a trans conformation with a smaller band gap and the amorphous phase has a helix conformation with a larger band gap. When these different band gap phases exist along a chain, each conduction and valence band forms a one-dimensional hetero-junction and makes a potential barrier at the junction point... 

The conformational structures of polysilane main chains at the macro-and microscopic levels are controllable by suitable choice of the side chain structures. Similarly, it is also the side chain which controls the optoelectronic properties by effecting the optical band gap. In the case of phenyl-substituted polysilanes, electronic interaction between the delocalized Si chain cr-bonding orbitals and the it-orbitals of the aryl groups causes a dramatic modification of both the band gap and conformational properties [61,83]. These aryl-containing polysilanes may be potential candidates for applications in a molecular-based chiroptical switch and memory in the UV/visible region. On the other hand, the precise control of helical polymers is now a subject of great interest and importance, due to the tech-... 

Side-chain substitutions and conformation variations perturb the detailed electronic structures of polysilanes. Substitution by larger alkyl side chains decreases the band gap slightly because of the electron-donating... 

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