Exploring the excited state character of nitroarylcarbazole derivatives using wavefunction analysis

The solid state fluorescence behaviour of a family of 9-(4-Nitroaryl)carbazoles is investigated via calculations of their electronically excited states. Ground state optimisations show that for many of the molecules, the most stable conformation has the nitroaryl moiety twisted relative to the carbazole. Vertical excitation calculations and subsequent wavefunction analysis [1] show that for molecules which are solid state emissive, the lowest lying singlet state has a major contribution from a charge transfer state from the carbazole donor to the nitroaryl acceptor. For molecules which are non-emissive, the lowest lying singlet state is found to be an n – π* state. We find that the energy of the charge transfer state decreases as the electron withdrawing power of the nitroaryl acceptor is increased. Calculations of excitations for molecules with orthogonal conformations suggest that the charge transfer state is dark, as indicated by a very small oscillator strength. The question, then, is: how can these molecules be solid state emissive if the lowest lying excited state is dark? A scan of the torsion angle shows that the oscillator strength increases significantly as the molecule becomes more planar, and the energy profile for this rotation is relatively flat. We therefore propose that provided that stacking in the solid state leads to small perturbations in the molecular geometry and the lowest lying singlet is a charge transfer state, the molecule will be solid state emissive. [1] F. Plasser “Theodore 2.0.2: a package for theoretical density, orbital relaxation and exciton analysis”; available from http://theodore-qc.sourceforge.net