Electronic structure properties of fluorene-phenylene monomer and its derivatives: TD-DFT study

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Fluorene-phenylene (FP) based oligomers and polymers and their derivatives are good candidates for organic blue-light emitting diodes. In this work, the intrinsic properties of the ground and excited states of FP monomer and its derivatives are studied. The ground state optimized structures and energies were obtained using the molecular orbital theory at a restricted (closed-shell) Hartree–Fock level (RHF) and the density functional theory (DFT) as approximated by the various hybrid functionals (RB3LYP, RB3P86, RB3PW91, RMPW1PW91). The ground state potential energy curves or surfaces of FP and its derivatives were obtained using RHF. All derivatives are nonplanar in their electronic ground states. The character and energy of the first 20 singlet–singlet electronic transitions have been investigated by applying the time-dependent DFT approximations, TD-B3LYP, TD-B3P86, TD-B3PW91 and TD-MPW1PW91, to the correspondingly optimized ground state geometries. The lowest singlet state was also studied with the configuration interaction (singles) approach (CIS). For each monomer, the S 1←S 0 electronic transition involves primarily the promotion of an electron from the HOMO to the LUMO, and is strongly polarized along the monomer backbone. Excitation energies are red shifted when side chains are attached to the basic FP unit or when the unit is extended longitudinally. Similar shifts are observed in the HOMO–LUMO gaps as obtained from the ground state DFT calculations. CIS results suggest geometry relaxation in the first singlet excited state. Comparisons between the different theoretical methodologies used to study the electronic ground and excited states are made and discussed.