Electronic Excitations in Polysilanes: Frenkel Excitons of a Disordered Chain
The soluble, disubstituted polysilanes, (SiXY)n, present a class of novel materials with interesting electronic, optical, conformational and photochemical properties,1 useful for a number of applications (photoresists, photoconductors, non-linear optical media). The lowest energy electronic states correspond to σσ excitations of the polymer backbone, delocalized over several tens of monomer units, and give rise to a strong absorption in the near UV. Strong electronic conformational coupling manifests itself in phase transitions, observed in solid films as well as liquid solutions and accompanied by large shifts of the near UV absorption band.2 Many issues that were, and still are, a matter of debate for Π-conjugated polymers also arise for the σ-conjugated polysilanes. Among these are: Of what type is the elementary excitation? What limits excitation delocalization? Is the phase transition an inter- or intramolecular process? The present paper focuses on the second question and discusses the spectra of poly (di-n-hexyl) silane (PDHS). At liquid helium temperatures the absorption spectrum in glassy solutions consists of a band at ca. 350 nm (as compared to 320 nm in room temperature solutions), which is only 2.5 nm broad and composed of still narrower lines (1–4 cm-1), as revealed by spectral hole-burning.3,10,11 This spectrum is successfully simulated by numerical modelling, describing the excitation as a Frenkel exciton of a disordered linear chain.
KeywordsOscillator Strength Acceptor State Site Energy Liquid Helium Temperature Participation Ratio
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