Chromotropism of Polysilanes

The conformation of the polysilane chain, and hence the amount of electron delocalization and the absorption wavelength, may change with temperature, solvent, pressure and so on. The result is that many polysilanes are chromotropic.65 The effect of temperature changes, leading to thermochromism, have been most thoroughly investigated. 

For many polysilanes in solution, a reversible shift of the 7- 7 absorption band to longer wavelength takes place when the temperature is decreased. Four types of behavior are found 65 

When the substituent groups are large (aryl or branched alkyl) normally no long-wavelength thermochromic shift takes place. This behavior is found for arylalkyl-polysilanes, diarylpolysilanes, and (cyclo-HexSiMe) . 

In a revealing experiment, Toyoda and Fujiki showed that the UV absorption and emission spectra of (PhMeSi) are virtually identical to those of the copolymer, (PhMeSi)o.95 [PhSiCH2C (H)(CH3)CH2CH3]005 . The latter is known to be helical in solution from its CD spectrum. The implication is that (PhMeSi) , which was for many years described in terms of the anti-gauche model, is actually helical in solution also.69 The evidence which is accumulating now suggests that nearly all polysilanes adopt helical conformations in solution. 

With this new information in mind, an interpretation of the thermochromic behavior of polysilanes in solution can be outlined. We can begin with two calibration points (1) The polymers ( -butyl2Si) and ( -pentyl2Si) absorb at 315 nm both in the solid state and in solution both polymers as solids are known to have an all-D conformation, with co 154°. (2) The well-studied polymer ( -hexyl2 Si) , in its low-temperature phase, absorbs at 375 nm and has a (nearly) all-A conformation. Values of A,nax between 315 and 375 nm should correspond to intermediate values of the average torsional angle, ox 

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In this chapter, the first section will deal with the complex question of conformations of polysilanes. This will be followed by sections on the chromotropism of polysilanes, beginning with their UV thermochromism. Despite intensive study this phenomenon is still not well understood, and is the subject of important recent findings. Solvatochromism, ionochromism and other types of chromotropic behavior of polysilanes will then be considered. A final brief section will deal with the electrical conductivity of polysilanes, a topic which has not previously been reviewed. 

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