Experimental determination of excitonic structure in polythiophene

The clearly resolved ${}^{1}{B}_{u}$ $(\ensuremath{\nu}=1)$ 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 ${}^{1}{A}_{g}$ $(\ensuremath{\nu}=2)$ exciton state lying 0.5 eV above the ${}^{1}{B}_{u}$ $(\ensuremath{\nu}=1)$ exciton state, while the electroluminescence spectrum shows the emission from the triplet exciton ${(}^{3}{B}_{u})$ lying 0.45 eV below the corresponding ${}^{1}{B}_{u}$ exciton. On the basis of the excitonic level diagram as well as the FeCl${}_{4}^{\ensuremath{-}}$-doping induced spectral change, the distinct photoinduced band observed at 1.0 eV is assigned to the internal transition between the triplet-exciton levels, ${}^{3}{B}_{u}$ $(\ensuremath{\nu}=1)$ $\ensuremath{\rightarrow}$ ${}^{3}{A}_{g}$ $(\ensuremath{\nu}=2)$.