Sensitivity of Redox and Optical Properties of Electroactive Carbazole Derivatives to the Molecular Architecture and Methoxy Substitutions

In the domain of organic light-emitting diode (OLED) applications, organic electroactive materials play a crucial role. In order to allow for more flexibility in their properties, methoxy or other substituents are frequently used. However, undesirable modifications in their polarity may be additionally obtained, which is particularly important in the case of TADF-based OLEDs. In order to dissociate as much as possible intramolecular and bulk effects, we synthesized two series of methoxy-substituted carbazole-bridge-carbazole (bridge = carbazolyl, phenyl) compounds and characterized them by means of experimental and theoretical methods. V-shape (3,6) substitutions on the carbazole bridge and linear (para-phenyl) bisubstitutions were used in the new compounds. By varying the number of methoxy groups from 0 to 4 per carbazole unit, we analyze the effect of the number and the linking topology of the methoxy substitutions on the thermal, electronic, and optical properties of the molecules. The results indicate that the variations of the redox and fluorescence properties upon methoxy substitutions depend importantly on the linear- versus V-shape D-A-D molecular architecture, due to the absence and presence, respectively, of the dipolar moments and of the bulk polarity. The choice of the molecular architecture and methoxy substitutions can be used consequently to switch on/off the molecular polarity and to tune the sensitivity of redox and optical properties of electroactive materials with respect to medium electrostatic effects. These compounds were additionally utilized in blue phosphorescence organic light-emitting devices and showed good hole-transporting, exciton-blocking, and electron-blocking properties.