Experimental determination of excitonic structure in polythiophene

The clearly resolved ${}^1{B}_u$ $(\nu =1)$ 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 ${}^1{B}_u$ $(\nu =1)$ 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${}_4^{-}$-doping induced spectral change, the distinct photoinduced band observed at 1.0 eV is assigned to the internal transition between the triplet-exciton levels, ${}^3{B}_u$ $(\nu =1)$ $\to$ ${}^3{A}_g$ $(\nu =2)$.