Band gap modifications in functionalized poly(methylphenylsilanes)

For many applications of polysilanes in optoelectronic devices, it is desirable that polymer properties, such as their band gap energy levels, their (redox) stability, and their propensity to interact favorably with (semi)conducting inorganic substrates, can be tailored. It has been demonstrated that, by introduction of substituents in the aryl moiety of poly(methylphenylsilane) (1), i.e., poly(methyl-4-methylphenylsilane) (2), poly(4-methoxyphenylmethylsilane) (3), poly[4-(dimethylamino)phenylmethylsilane] (4), poly(3-methoxyphenylmethylsilane) (5), and poly[4-(2-methoxyethoxy)phenylmethylsilane] (6), these objectives can be achieved. For comparative purposes, poly(4,7,10,13-tetraoxatetradecylmethylsilane) (7) was also taken into consideration. Electrochemical measurements (cyclic voltammetry) in THF/LiClO4 of 1-7 show that the onset of oxidation V-i of each polysilane provides a reliable estimate of its valence band edge; within series 1-7 V-i shifts over ca. 0.7 V. Although it is impossible to obtain a reliable estimate of the conduction band edge due to the available potential window of THF/LiClO4, the position of the conduction band edge of the polysilanes is derived from their optical band gaps using fluorescence excitation and emission spectroscopy. The electrochemical and optical properties of the related polysilanes 1-5 correlate with the substituent Hammett constants (sigma(R)). The Hammett reaction constants (rho) indicate that the optical band gap (rho = 0.29) is less sensitive to electronic pertubations induced by the substituents than the valence band edge (rho = 0.85). From these results the response of the conduction band edge toward substituent induced electronic pertubations was estimated to be rho = 0.60. The experimental results are supported by semiempirical PM3 calculations on polysilane oligomers n = 1-10 of I and 4.