INVESTIGATIONS ON THE CONFORMATIONAL VARI ETY OF MONOALKOXY AND 2,5-DIALKOXY SUBSTITUTED POLY(1,4-PHENYLENEETHENYLENE)S

The alkoxy substituted poly(1,4-phenyleneethenylene)s 1 and 2 can prin cipally exist in an enormous variety of conformers (table 1). However, force field calculations (MMX) reveal a far-reaching restriction to a relatively small number of non planar conformers with minimized steri c energy. The decisive criterion is due to the interaction between the oxygen atoms and the olefinic hydrogens. An increasing steric hindran ce should lead to higher twist angles THETA in the main chain and henc e to a shorter effective conjugation length. Such an effect could play a minor role between the unsubstituted parent compound and 1/2, but U V/Vis- and fluorescence spectra don't give any hints for a change of t he effective conjugation length n almost-equal-to 8 between 1 and 2.