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The application of polyfluorenes in polymeric light-emitting diodes has been hampered because of the charge injection difficulties and the troublesome formation of a tailed emission band at long wavelengths (>500 nm) during device fabrication and operation, leading to both a color instability and reduced efficiency. The incorporation of the phenothiazine units has been proven to significantly enhance the hole injection and charge carrier balance and at the same time efficiently suppress the keto defect emission. In this contribution, we apply quantum-chemical techniques to investigate poly[10-(N-(2′-methyl)phenothiazine-3,7-diyl) and its fluorene copolymer poly[10-(N-(2′-methyl)-phenothiasine-3,7-diyl)-co-alt-2,7-(9,9-dimethylfluorene)] (PFPTZ) and gain a detailed understanding the influence of phenothiazine units on the electronic and optical properties of fluorene derivatives. Density functional theory (DFT) and time-dependent DFT approaches are employed to study the neutral molecules, HOMO-LUMO gaps (ΔH-L), the lowest excitation energies (Eg's), positive and negative ions, as well as the IPs and EAs, focusing on the superiority of the electronic and optical properties attributed to the introduction of electron-donating moiety phenothiazine (PTZ) through comparing with pristine polyfluorene. The outcomes show that the highly nonplanar conformation of phenothiazine ring in the ground state preclude sufficiently close intermolecular interactions essential to forming aggregates or excimers. Furthermore, the HOMO energies lift about 0.4 eV, and thus, the IPs decrease about 0.3 eV in PFPTZ, suggesting the significant improved hole-accepting and transporting abilities, due to the electron-donating properties of phenothiazine ring by the presence of electron-rich sulfur and nitrogen heteroatoms and highly nonplanar characters, resulting in the enhanced performances in both efficiency and brightness compared with pristine polyfluorene. In addition, even though the introduction of electron-donating moiety PTZ onto fluorene leads to a slight bathochromic shift in absorption and emission spectra, the copolymer still exhibited strong blue emission. [ABSTRACT FROM AUTHOR]

Ambipolar diphenylamino end-capped oligofluorenylthiophenes and fluoroarene-thiophene show great potential for application in organic light-emitting diodes (OLEDs). Here, we provide an in-depth investigation on the optical and electronic properties of OF(2)TP-NPh ( 1a), OF(2)DTP-NPh ( 2a), OF(2)TTP-NPh ( 3a), OF(2)QTP-NPh ( 4a), and 2,5-bis-(2,3,5,6-tetrafluoro-4-trifluoromethyl-phenyl)-2,2′:5′,2′′:5′′,2′′′-quaterthiophene ( 5a). The geometric and electronic structures of the oligomers in the ground-state are studied with density functional theory (DFT) and ab initio Hartree-Fock, whereas the lowest singlet excited states are optimized by ab initio CIS. The energies of the lowest singlet excited states are calculated by employing time-dependent density functional theory (TDDFT). The results show that the highest occupied molecular orbitals, lowest unoccupied molecular orbitals, energy gaps, ionization potentials, and electron affinities for the oligomers are affected by the thiophene chain length and the different end-caps. The absorption and emission spectra exhibit red shifts to some extent due to the increasing thiophene chain length and the enhancing electron-donating property of the end-caps. Furthermore, the large Stokes shifts ranging from 58 to 80 nm are examined, resulting from a more planar conformation of the excited-state between the two adjacent units in the oligomers. All the calculated data show that the fluoroarene-thiophene has improved electron transport rate and charge transfer balance performance, and all the studied molecules can be used as ambipolar-transporting materials in OLEDs. [ABSTRACT FROM AUTHOR]