Thermochromism polysilanes

The recent interest in substituted silane polymers has resulted in a number of theoretical (15-19) and spectroscopic (19-21) studies. Most of the theoretical studies have assumed an all-trans planar zig-zag backbone conformation for computational simplicity. However, early PES studies of a number of short chain silicon catenates strongly suggested that the electronic properties may also depend on the conformation of the silicon backbone (22). This was recently confirmed by spectroscopic studies of poly(di-n-hexylsilane) in the solid state (23-26). Complementary studies in solution have suggested that conformational changes in the polysilane backbone may also be responsible for the unusual thermochromic behavior of many derivatives (27,28). In order to avoid the additional complexities associated with this thermochromism and possible aggregation effects at low temperatures, we have limited this report to polymer solutions at room temperature. 

With the success of CgK in the syntheses of 41 and related co-polymers, the same methodology was applied to the synthesis of polysilanes bearing pendant crown ether substituents, viz. 42-44,150,151 which were synthesized in yields from 6% to 28%, with molecular weights of the order of 104. The polymers were found to be water soluble and thermochromic in accordance with the Schweizer thermochromism theory (see Section 3.11.5.2.4).152... 

Figure 9 Thermochromic UV properties of (a) end-graft polysilane 79, (b) dilute solution, and (c) film analogs 80.65 Reprinted with permission from Ebata, K. Furukawa, K. Matsumoto, N. Fujiki, M. Polym. Prepr. (Am. Chem. Soc. Div. Polym. Chem.) 1999, 40, 157-158, 1999 American Chemical Society.

When COMC II (1995) was published, two general types of thermochromism (reversible) had been recognized for polysilanes on lowering the temperature a gradual bathochromic transition, and an abrupt transition to longer wavelength absorption. From the vast amount of experimental data now accumulated, five different types of thermochromic behavior on cooling can be characterized ... 

Figure 20 Temperature-dependent UV spectra of polysilanes illustrating five different types of thermochromism (in hexane for (a)-(c)246 and in toluene for (d) and (e)247). Figures 20(a)-20(c) reprinted with permission from Sanji, T. Sakamoto, K. Sakurai, H. Ono, K. Macromolecules 1999, 32, 3788-3794. 1999 American Chemical Society.

The origin and mechanism of thermochromism in the polysilanes is an area of ongoing discussion, and it is likely that under different conditions, different mechanisms are operative, which could be usefully clarified by further study. 

Finally, concerning dialkylpolysilanes, it is interesting to note that the solid-state (film) UV spectral profile of the thermochromism exhibited by 49,157 shown in Figure 28, almost exactly matches that of the solvatochromism (see Figure 16 above). This indicates that the before and after conformations are essentially the same and that the reduction of temperature or addition of HFIP are responsible for similar conformational changes in the polymer an abrupt straightening of the polysilane backbone. 

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. 

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) . 

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... 

There has been a great deal of interest in the UV-visible spectroscopy of the polygermanes, particularly in comparison with the analogous chains that have silicon or tin backbones.41,42 Both conventional and Raman spectroscopy have been employed. One interesting observation is that the symmetrically disubstituted polyfdi- n - a I k y I g e n nanes) exhibit thermochromic transitions at temperatures below those of their polysilane analogues. Another is the conclusion that in poly(di-n-hexylgermane) the side chains adopt trans-planar conformations as in the polysilane counterpart. The two chains are also similar in that both backbones can, under certain circumstances, also adopt planar zig-zag conformations. 

Specific properties of polysilanes have been linked to the method of synthesis.35 For example, in the case of anionic polymerization of poly[l-(6-methoxy-hexyl)-l,2,3-trimethyldisilanylene] a new type of chromism was induced in the polysilane film by the difference in the surface properties of substrates and was termed a surface-mediated chromism. The polysilane exhibited thermochromism with an absorption maximum at 306 nm at 23°C, but <15°C a band at 328 nm began to appear. A monolayer of the polysilane was transferred onto both a clean hydrophilic quartz plate and a hydrophobic one treated with hexamethyldisilazane by the vertical dipping method. With the hydrophobic plate, a broad UV absorption at 306 nm is obtained, whereas the absorption on a hydrophilic plate shifts to 322 nm. The conformation of the polysilane is preserved by hydrogen bonding between the silica surface and the ether section of the substituent on the hydrophilic plate. The polysilane is attached to the hydrophobic surface only by van der Waals forces, and this weaker interaction would not sustain the thermodynamically unstable conformational state that is attained on the water surface. 

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. 

FIGURE 2. Thermochromic behavior of some polysilanes in hexane solution (a) ( -HexSiMe)n (b) ( -PrSiHexn)n (c) [(n-Hex)2Si]n. Reprinted with permission from Reference 23. Copyright 1999 American Chemical Society... 

In the light of these new findings, an interpretation of the thermochromic behavior of polysilanes in solution can be offered. In Type 1 behavior, gradual ordering of the polysilane chain into a strongly-twisted helical form takes place as the temperature is lowered. The absorption wavelength of ca 315 nm is close to that of the known all-D helices in... 

---