Electronic, Structural, and Optical Properties of Conjugated Polymers Based on Carbazole, Fluorene, and Borafluorene

• Published in: The journal of physical chemistry. B Vol. 108; no. 10; pp. 3123 - 3129
• Main Authors: Brière, Jean-François, Côté, Michel
• Format: Journal Article
• Language: English
• Published: American Chemical Society 11.03.2004
• ISSN: 1520-6106; 1520-5207
• DOI: 10.1021/jp035363a

We present a first-principles study of the structural, electronic, and optical properties of four conjugated polymers, poly(p-phenylene) (PPP), poly(2,7-fluorene) (PF), poly(2,7-carbazole) (PC), and poly(2,7-borafluorene) (PBF), and of their isolated constituent molecules. All the calculations were done using density-functional theory (DFT) with a plane-wave basis set, pseudopotentials, and local exchange-correlation energy. Resemblances in the atomic structures of the systems studied allow us to make correspondences between their wave functions near the Fermi energy. The dihedral angles of the polymers under consideration are all similar, varying between 26° and 27°. In agreement with experimental data, we find smaller energy gaps for carbazole and borafluorene compared to biphenyl and fluorene, which is due to differences in the highest occupied molecular orbital (HOMO) or lowest unoccupied molecular orbital (LUMO) wave functions. However, for the polymers, the experimental gap for PC was found to be almost the same as for PPP and PF. Our calculations explain this experimental observation which is attributed to a change in the ordering of the last two valence bands between carbazole and its polymer. We also find that the energy gap of PBF, which has not been synthesized yet, should be smaller than the minimum energy gaps of the other studied polymers by ≃0.5 eV. The polarizations for the lowest electronic transitions are presented from the analysis of the wave functions' symmetries. We find excellent agreement between the calculated and available experimental data, validating the predictions made.