Effects of rotational conformation on electronic properties of 4,4′-bis(carbazol-9-yl)biphenyl (CBP): the single-molecule picture and beyond

ABSTRACT

4,4${}^{\mathrm{\prime }}$-bis(carbazol-9-yl)biphenyl (CBP) is a commonly used hole-transport host material in phosphorescent organic light-emitting devices (PhOLEDs). Despite its widespread use, device degradation is observed that most likely involves excited states and ionic species. The present work aims at providing a first step towards a detailed CBP characterisation by computing excited-state properties and ionised species of CBP using time-dependent density functional theory (TD-DFT) and the Bethe-Salpeter equation (BSE) based on the GW method. The investigation reveals a strong dependence of the absorption spectrum on the interactions of the phenyl and carbazole units, which is computed for 1.6 k ground-state rotamers using the efficient GW-BSE method. A similar approach using a set of 1.6 k excited-state rotamers shows a significantly smaller dependence on fragment orientation for emission. In order to model ensembles of gas-phase CBP molecules, spectra are simulated employing a Boltzmann distribution for different temperatures based on the relative ground-state energies of the rotamers. This approach reveals that certain absorption bands experience almost no change in the ensemble picture while others vanish, which can be understood in terms of the geometry-dependence of the excited states due to the phenyl-carbazole interactions.

GRAPHICAL ABSTRACT

Keywords:

• 4,4${}^{\mathrm{\prime }}$-bis(carbazol-9-yl)biphenyl (CBP)
• absorption spectra
• TD-DFT
• GW-BSE
• charge transfer excitations

Acknowledgments

The authors gratefully acknowledge support of project P2 of the Research Training Group GRK 2450 ‘Tailored Scale-Bridging Approaches to Computational Nanoscience’ funded by the Deutsche Forschungsgemeinschaft (DFG).

Disclosure statement

No potential conflict of interest was reported by the author(s).

Notes

Note: ${\mathrm{R}}_1$ is the global minimum and also denoted ‘0-0-0’ in the following.

${}^{\mathrm{a}}$Dihedral angle.

${}^{\mathrm{b}}$Bond distance.

${}^{\mathrm{a}}$Dihedral angle. ${}^{\mathrm{b}}$Bond distance.

^a ΔSCF, adiabatic ionisation.^bΔSCF, vertical ionisation.^cGW approximation, without geometry relaxation, PBE0/def2-TZVP level of theory.^dGW approximation, with geometry relaxation, PBE0/def2-TZVP level of theory, cf. Table 4.

${}^{\mathrm{a}}$Dihedral angle. ${}^{\mathrm{b}}$Bond distance

Note: For each temperature, the mean value μ and corresponding variance $\mathrm{Var}$ of excitation energies ν and oscillator strengths f are shown.${}^{\mathrm{a}}$Excitation energy in eV.${}^{\mathrm{b}}$Oscillator strength, length gauge. ${}^{\mathrm{c}}$Mean value. ${}^{\mathrm{d}}$Variance.

Additional information

Funding

Deutsche Forschungsgemeinschaft (DFG) grant No. GRK2450/P2.