Experimental determination of excitonic structure in polythiophene

The clearly resolved B-1(u) (nu=1) exciton and its vibronic structures show up in ordinary absorption and photoluminescence spectra of films of highly ordered poly(3-octylthiophene). The electroabsorption and the two-photon absorption spectra have unraveled the charge-transfer type (1)A(g) (nu=2) exciton state lying 0.5 eV above the B-1(u) (nu=1) exciton state, while the electroluminescence spectrum shows the emission from the triplet exciton (B-3(u)) lying 0.45 eV below the corresponding B-1(u) exciton. On the basis of the excitonic level diagram as well as the FeCl4--doping induced spectral change, the distinct photoinduced band observed at 1.0 eV is assigned to the internal transition between the triplet-exciton levels, B-3(u) (nu=1) --> (3)A(g) (nu=2).