Manifestations of strong electron correlation in polyacene: Fundamental gap, density of states, and photoconductivity

The quasi-particle (QP) bands of polyacene (PAC) – an infinite linear chain of fused benzene rings – are reconstructed from the QP spectra of oligoacenes (OACs) with N∈[5,30] (N is the number of fused rings) obtained using a computational methodology capable of incorporating the effect of strong non-dynamic electron correlation into the computation of the QP (electrons and holes) energy levels. As the result of mixing with the doubly excited electronic configuration, the fundamental gap in the ground electronic state of PAC widens to ca. 1.8 – 2.2 eV depending on the density functional employed. In the excited singlet electronic state, the band gap closes; thus, leading to photoconductivity of PAC, as well as longer (N≳9) OACs. The calculated optical gap of OACs converges to a finite value of ca. 1 eV in a reasonable agreement with the experiment. The experimentally observed sudden opening of the transmission gap in scanning tunneling spectroscopy of dodecacene (N = 12) is explained by the effect of strong correlation, which shifts the first maxima of the density of states of the ground and excited states towards larger electron binding energies.