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

Abstract

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. Vshape (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.

Methoxycarbazolyl-disubstituted dibenzofuranes as holes- and electrons-transporting hosts for phosphorescent and TADF-based OLEDs

“…PL decay curves of the doped films recorded at room temperature contained components in a μs range which are attributed to TADF as it was proved elsewhere (Table S1, Fig. S2) [21,22]. These data show that the synthesized compounds can operate as hosts transferring electronic excitation energy to guest emitters without suppressing TADF.…”

“…CIE coordinates and wavelengths of the spectral peaks (Fig. 7a, S3-9) correlate to a great extent with the performances of OLEDs based on other hosts reported earlier [21,22]. Good electronic excitation energy transfer from host to guest molecules is also evident from the absence of collateral peaks in EL spectra of OLEDs.…”

“…Secondly, due to enhancement of hydrogen bonding between oxygen atoms of methoxy groups and hydrogen atoms of other molecules efficient packing is achieved in films resulting in high thermal stability, high glass transition and effective charge transport [8,9,17,18]. Thirdly, it is known that upon methoxy substitution, HOMO energies of the compounds are increased and ionization potential values are decreased resulting in more effective hole injection [19][20][21]. However, to our knowledge, no derivatives of methoxy-substituted carbazole capable of effectively transporting both holes and electrons were reported.…”

Conformational Dependence of Triplet Energies in Rotationally Hindered N‐ and S‐Heterocyclic Dimers: New Design and Measurement Rules for High Triplet Energy OLED Host Materials

“…This gives carbazole and its derivatives optical and electronic properties well suited for use in emissive devices based upon organic molecules. Carbazole has great synthetic versatility permitting systematic variations through substitution, interconversion and coupling reactions, allowing optimisation of the optoelectronic properties of carbazole derivatives for many applications [1–11] . Among the most important of these applications are their use as hole transport materials (HTMs), [12] or as host materials in the emissive layer (EML) of organic light emitting diodes (OLEDs) [13–17] .…”

An experimental and theoretical study of exciplex-forming compounds containing trifluorobiphenyl and 3,6-di-tert-butylcarbazole units and their performance in OLEDs

“…An increase in the dihedral angle of B20-301 when using a tuned o-value compared to the default (0.2 Bohr À1 ) is consistent with observations reported by Sallenave and co-workers studying carbazole-based materials. 30…”