Bipolar Molecules with High Triplet Energies: Synthesis, Photophysical, and Structural Properties

This article sheds new light on the interplay of electronic and conformational effects in luminescent bipolar molecules. A series of carbazole/1,3,4-oxadiazole hybrid molecules is described in which the optoelectronic properties are systematically varied by substituent effects which tune the intramolecular torsion angles. The synthesis, photophysical properties, cyclic voltammetric data, X-ray crystal structures, and DFT calculations are presented. Excited state intramolecular charge transfer (ICT) is observed from the donor carbazole/2,7-dimethoxycarbazole to the acceptor phenyl/diphenyloxadiazole moieties. Introducing more bulky substituents onto the diphenyloxadiazole fragment systematically increases the singlet and triplet energy levels (E-S and E-T) and blue shifts the absorption and emission bands. The triplet excited state is located mostly on the oxadiazole unit. The introduction of 2,7-dimethoxy substituents onto the carbazole moiety lowers the value of E-S, although E-T is unaffected, which means that the singlet triplet gap is reduced (for 7b E-S - E-T = 0.61 eV). A strategy has been established for achieving unusually high triplet levels for bipolar molecules (E-T = 2.64-2.78 eV at 14 K) while at the same time limiting the increase in the singlet energy.