Donor influence on the optoelectronic properties of N-substituted tetraphenylimidazole derivatives

Abstract

Three new 1,2,4,5-tetraphenylimidazole derivatives, 9,9-dimethyl-10-(4-(2,4,5-triphenyl-1H-imidazol-1-yl)phenyl)-9,10-dihydroacridine ( DMAC - TPI ), 10-(4-(2,4,5-triphenyl-1H-imidazol-1-yl)phenyl)-10H-phenoxazine ( PXZ-TPI ), and 10-(4-(2,4,5-triphenyl-1H-imidazol-1-yl)phenyl)-10H-phenothiazine ( PTZ - TPI ), bearing different electron donors at the N1 position of the imidazole were synthesised and characterised. DMAC - TPI and PXZ - TPI showed narrow emission at λPL of 388 and 418 nm in toluene, and in doped films in Zeonex polymer (1 wt.%) at λPL 381 and 407 nm, respectively, with a full width at half maximum (FWHM) ranging 0.42-0.44 eV. DMAC - TPI and PXZ - TPI are predicted to show very low oscillator strength for the low-energy transitions, which aligns to the observed low photoluminescence quantum yields. Both molecules showed a singlet-triplet energy gap (ΔEST of around 1.2 eV) that is much too large to enable reverse intersystem crossing and thermally activated delayed fluorescence. Connecting a donor group to TPI at the N1 position can lead to room temperature phosphorescence (RTP), as the example of PTZ - TPI showed.