Origin of the blue emission from poly(1-phenyl-2-alkynes) and poly(phenylacetylenes)

We have studied the electronic structure, absorption, and photoluminescence of poly(1-phenyl-2-alkynes) - [C<SUB>6</SUB>H<SUB>5</SUB>)C- C(C<SUB>m</SUB>H<SUB>2m+1</SUB>)]<SUB>n</SUB>-(m = 1, 2), poly(phenylacetylene)-[HC=C(C<SUB>6</SUB>H<SUB>5</SUB>]<SUB>n</SUB>- and its derivatives -[HC=C(C<SUB>6</SUB>H<SUB>4</SUB>-p-R]<SUB>n</SUB> with various non- liquid crystal ring substitutes. For poly(1-phenyl-2- alkynes), the PL efficiency is very sensitive to the molecular structure of the alkyl pendant and can be enhanced up to 50 times as the alkyl side-chain increases in length. But for poly(phenylacetylenes), their luminescent efficiency can be improved several times only as the tail becomes bulky. Regardless of the types of the pendants, the emission color of the polymers is pineed at ~450 nm (2.7eV). The band structure of the polymers, which has been calculated using extended Huckel tight-binding method, is essentially an ensemble of the backbone (extended states) and the pendants (localized states), and the processes of optical absorption and blue emission are confined in the directly attached aromatic ring. The interaction between the pheny chromophore and its nearest neighbors is of vital importance in improving the emission efficiency. Although the band gap of the backbone can be enlarged by the pendant, its (pi) - (pi) <SUP>*</SUP> interband transistion is insignificant for the blue emission.