Dependence of optical properties of oligo-para-phenylenes on torsional modes and chain length

A systematic characterization of excited-state properties of para-phenylene oligomers constructed from two to eight aromatic rings is presented using density functional theory (DFT) and the coupled-cluster singles and doubles (CC2) method. Geometry optimizations have been performed for the ground state and for the electronically excited state. Vertical excitations and the fluorescence transitions have been calculated. Time-dependent DFT (TDDFT) method underestimates excitation and fluorescence energies systematically in comparison with experimental results. The computed TDDFT lifetime for the polymer limit (0.43 ns) is in agreement with the experimental value of 0.55 ns. The TDDFT torsional potential curves were investigated for biphenyl, terphenyl, and quarterphenyl oligomers in their electronic ground and excited states. Our calculations show an increase in the separation of the lowest excited state (S-1) to the next higher one with increasing molecular size. No indication is found for state crossings of the S-1 state with higher ones from planar structures up to torsional angles of 60 degrees to 70 degrees. Thus, an adiabatic description of the dynamics of the S-1 state might significantly simplify any dynamics simulations of torsional broadenings.