Optical Absorptions of New Blue-Light Emitting Oligoquinolines Bearing Pyrenyl and Triphenyl Endgroups Investigated with Time-Dependent Density Functional Theory

Abstract

Abstract:The optical absorption spectra of a family of four n-type conjugated oligomers, oligoquinolines, which can be commercially used to develop high-performance light-emitting diodes for their many desirable properties, have been recently calculated from time-depedent density functional theory (TDDFT) within the adiabatic approximation for the dynamical exchangecorrelation potential. In this work, we investigate the optical absorption of two new family members of the blue-light emitting oligoquinolines bearing pyrenyl and triphenyl endgroups in gas phase and chloroform (CHCl 3 ) solution employing the adiabatic TDDFT. The ionization potentials and electron affinities of these two oligoquinoline molecules are also calculated with the groundstate DFT, from which the adiabatic dynamical exchange-correlation potential is constructed. We show that the calculated optical absorptions are in good agreement with experiments. The ionization potentials obtained with the DFT methods agree well with the experimental estimates, while the electron affinities are significantly underestimated in comparison with experiments. A natural transition orbital analysis for selected excited states with the largest oscillator strengths shows that the electronic charge is slightly redistributed in the process of electronic excitations.

Prediction of Excitation Energies for Conjugated Oligomers and Polymers from Time-Dependent Density Functional Theory

“…In order to make a comprehensive assesment of the density functionals that were originally developed for ground-state properties, we applied [46,47] these functionals to complex systems [30,71,76,107]. In our assesment of the performance of several popular density functionals for conjugated oligomers and polymers that will be discussed in section V, the PBE GGA functional has been excluded, because it gives the least accurate excitation energies of atoms and small molecules in comparison with other density functionals we tested.…”

“…To provide a deep physical insight into these phenomena, we calculated [46,47] the optical absorption of oligoquinolines in gas phase and chloroform (CHCl 3 ) solution, respectively, with the adiabatic TDDFT methods. The excitation energies of oligoquinolines in solution were calculated with PCM (polarizable continuum model) [112].…”

“…(See Refs. [46,47] for B2PPQ, BPYPQ, and B3PPQ.) We plotted the orbitals derived from PBE0 computational results, since this method provides the most accurate results in comparison with experiment across the entire molecular family considered.…”

“…The segment of this chain length contains at least 16 molecular rings, which mimics quite welll the optical properties of polymers with infinite chain [110,111]. The groups of -(CH 2 ) n CH 3 has little effect on the optical properties of the polymers [46,47]. These side chains only affect some physical and chemical properties, such as phase transition temperature, solubilities, etc.…”

“…Recently we have tested [45] a sophiscated nonempirical density functional and its one-parameter hybrid version on small molecules and found that the calculated excitation energies are in fairly good agreement with experiment. Motivated by their practical success on small molecules, we then applied [46,47] them and several other commonly-used density functionals to the study of the optical absorption spectra of light-emitting conjugated oligomers. A striking difference between small molecules and conjugated oligomers is that electronic excitations of this kind of oligomers are often accompanied with some amount of charge transfer, and even singlet-triplet excitation can occur [48] (see discussion in section V).…”