Effects of pi-conjugation attenuation on the photophysics and exciton dynamics of poly(p-phenylenevinylene) polymers incorporating 2,2 '-bipyridines

The effect of pi-conjugation attenuation on the photophysics and exciton dy namics of two conjugated polymers 1 and 2 are examined in solution. The str uctures of polymers 1 and 2 have 2,2'-bipyridyl-5-vinylene units that alter nate with one and three 2,5-bis(n-decyloxy)-1,4-phenylenevinylene monomer u nits, respectively. The photophysics and exciton dynamics of polymers 1 and 2 were compared to those of the homopolymer, poly(2,5-bis(2'-ethylhexyloxy )-1,4-phenylenevinylene) (BEH-PPV). A series of changes in the photophysics of polymers 1 and 2 were found as a result of pi-conjugation attenuation. These changes include blue shifts in absorption and emission spectra, spect ral diffusion in stimulated emission, enhancement in photoluminescence quan tum yields and lifetimes, and increases in photoinduced absorption intensit ies and lifetimes. These changes are systematically more pronounced in poly mer 1 than in, polymer 2 and are correlated with pi-conjugation attenuation in the polymers due to twisting of the 2,2'-bipyridine groups about the 2, 2' single bond. An exciton dynamics model involving an ensemble of initial exciton states localized on oligomeric segments within the polymer with dif ferent conjugation lengths is proposed to describe the observed differences between polymers 1 and 2 and BEH-PPV. When the electronic coupling between these segments is strong, the polymer displays characteristics that are cl ose to those of a one-dimensional semiconductor. However, when these coupli ngs are weakened by groups, such as the 2,2'-bipyridine that attenuate pi-c onjugation, the polymer displays properties of an ensemble of oligomers.