Band gap modifications in functionalized poly(methylphenylsilanes)

For many applications of polysilanes in optoelectronic devices, it is desir able that polymer properties, such as their band gap energy levels, their ( redox) stability, and their propensity to interact favorably with (semi)con ducting inorganic substrates, can be tailored. It has been demonstrated tha t, by introduction of substituents in the aryl moiety of poly(methylphenyls ilane) (1), i.e., poly(methyl-4-methylphenylsilane) (2), poly(4-methoxyphen ylmethylsilane) (3), poly[4-(dimethylamino)phenylmethylsilane] (4), poly(3- methoxyphenylmethylsilane) (5), and poly[4-(2-methoxyethoxy)phenylmethylsil ane] (6), these objectives can be achieved. For comparative purposes, poly( 4,7,10,13-tetraoxatetradecylmethylsilane) (7) was also taken into considera tion. Electrochemical measurements (cyclic voltammetry) in THF/LiClO4 of 1- 7 show that the onset of oxidation V-i of each polysilane provides a reliab le 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 cond uction band edge due to the available potential window of THF/LiClO4, the p osition of the conduction band edge of the polysilanes is derived from thei r 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 r eaction 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 semi empirical PM3 calculations on polysilane oligomers n = 1-10 of I and 4.