Seven new derivatives of phenanthro[9,10-d]imidazole having differenet substituents at the 1st and the 2nd positions of the phenanthroimidazole moiety were synthesized and characterized. The comparative study of their properties was performed employing thermal, optical, electrochemical and photoelectrical measurements. The properties of the newly synthesized compounds were compared with those of earlier reported derivatives of phenanthroimidazole and several interesting new findings were disclosed. Density functional theory calculations accompanied by optical spectroscopy measurements have shown the possibility of tuning the emission properties (excited-stated decay rate, fluorescence quantum yield, etc.) of phenanthro[9,10-d]imidazole derivatives via attachment of different substituents to the 1st and the 2nd positions. The most polar and bulky substituents linked to the 2nd position were found to have the greatest impact on the emissive properties of compounds causing (i) fluorescence quantum yield enhancement of dilute liquid and solid solutions (up to 97%), (ii) suppression of intramolecular torsion-induced nonradiative excited-state relaxation in rigid polymer films as well as (iii) inhibition of aggregation-promoted emission quenching in the neat films. Most of the studied compunds exhibited ambipolar charge transport character with comparable drift mobilities of holes and electrons. The highest hole and electron mobilities approaching 10⁻⁴ cm² V⁻¹ s⁻¹ were observed for the derivative having a triphenylamino group at the 1st position of the imidazole ring and the phenyl group at the 2nd position. The estimated triplet energies of phenanthro[9,10-d]imidazole compounds were found to be in the range of 2.4–2.6 eV, which is sufficiently high to ensure effective energy transfer to yellow/red emitters.

Efficient TADF relies on effective reverse intersystem crossing (rISC) from triplet to singlet manifold, and therefore the transition rate associated with this process must outcompete non-radiative decay rate of the triplets. Herein the issue of non-radiative triplet quenching in benzophenone-derived compounds frequently employed as blue TADF emitters in OLEDs is addressed and an effective way to suppress this quenching is proposed. TADF emitters designed for this study were based on phenone-derived electron-accepting units linked with multiple carbazolyl donors to deliver similar and large rISC rates (k_rISC ∼ 4 × 10⁶ s⁻¹), yet significantly different non-radiative triplet quenching rates (k^T _nr). We found that substitution of a loose phenyl moiety by a methoxy group in benzophenone suppresses k^T _nr by one order of magnitude (from 3 × 10⁶ s⁻¹ down to 0.3 × 10⁶ s⁻¹), while leaving rISC rate k_rISC almost unaffected. A 10 times enlarged ratio k_rISC/k^T _nr in 5tCzMeB as compared to that in 5tCzBP emitter suffices to retrieve the vast majority of triplet population via the rISC resulting in PL quantum yields of 0.99 and 0.58 in doped DPEPO and neat films, respectively. Sky-blue TADF OLEDs with doped and non-doped emissive layers based on 5tCzMeB demonstrated impressive maximum external quantum efficiency (EQE) values of 24.6% and 13.4%, respectively, whereas at 100 cd m⁻² the devices exhibited 16.5% and 7.7% EQEs, respectively.