ABSENCE OF PHOTOINDUCED ELECTRON-TRANSFER FROM THE EXCITONIC ELECTRON-HOLE BOUND-STATE IN POLYDIACETYLENE CONJUGATED POLYMERS

We present the results of comparative studies of photoinduced electron transfer from semiconducting polymers onto C-60, focusing on the effe ct of strong Coulomb correlations, which lead to excitons (bound elect ron-hole pairs) as the primary photoexcitations. For this purpose, we investigated two different soluble polydiacetylene (PDA) derivatives i n which the lowest-energy photoexcitations are excitons with a binding energy of approximately 0.5 eV. The experimental studies (absorption and emission spectroscopy, subpicosecond and millisecond photoinduced absorption spectroscopy, and picosecond transient and near-steady-stat e photoconductivity) were carried out in pristine materials and in com posites of PDA with C-60. The PDA data are compared with the results o f similar studies on semiconducting conjugated polymers such as poly(p ara-phenylene-vinylene) and its derivatives, the PPV's, and polythioph ene and its derivatives, the P3AT's. The results show clearly that in contrast to the high quantum efficiency photoinduced electron transfer discovered earlier in the PPV's and the P3AT's, the excitonic bound s tate of the electron and hole inhibits photoinduced electron transfer from PDA onto C-60. We conclude that in the PPV's and the P3AT's, the Coulomb interaction is sufficiently well screened that upon photoexcit ation free carriers are created, thereby making possible facile electr on transfer to a near-by acceptor.