TUNING OF THE LUMINESCENCE IN MULTIBLOCK ALTERNATING COPOLYMERS .1. SYNTHESIS AND SPECTROSCOPY OF POLY[(SILANYLENE)THIOPHENE]S

Synthetic routes to alternating copolymers consisting of oligosilylene blocks and oligothiophene blocks (T-x; x = 1, 2, 3, 4, or 6 rings) ar e presented. Solubility requirements for obtaining acceptable molecula r weights and, eventually, for film formation are met by the introduct ion of butyl groups replacing methyls on the silicon atoms and by empl oying T-6 blocks carrying two octyl substituents. Additionally, substi tuted oligothiophenes are synthesized as an aid in the interpretation of NMR, absorption, and fluorescence spectra. Regarding the electronic configuration of the oligothiophene blocks, NMR spectra show clear di fferences between plain oligothiophenes, end-substituted oligothiophen es, and polymers, indicative of pi-sigma interactions with the oligosi lylene blocks and possible through-conjugation to adjacent blocks in p olymers. Red shifts in optical spectra show a parallel trend across th e various compounds based on the same oligothiophene unit, related to the stabilization of photoexcited states on the oligothiophene by the oligosilylene substituents. These effects are strong in T-2-based comp ounds and reduced fdr longer T-n. The main feature of the spectra is t he decrease of the transition energies with the size of the oligothiop hene blocks in the polymers. Since this effect is also found in fluore scence, it enables one to adjust the luminescence wavelength by choosi ng the proper block length (''chemical tuning''). Fluorescence quantum efficiencies in solution are found to be remarkably high in polymers based on T-2 blocks. Spin-coated films of T-2-based (or T-3-based) pol ymers show evidence of T-4 (T-6) impurity blocks that act as an excito n trap.