Effect of positional isomerism on the functional properties of carbazole-phenanthroimidazole-triphenylamine triads

Abstract

A series of bipolar 2, 6 or 2, 7 substituted carbazole-based isomeric hybrids featuring phenanthroimidazole as acceptor and N-phenylcarbazole or triphenylamine as donor are synthesized. Structure-property relationship of these blue-emitting materials is established by detailed investigation of physiochemical, thermal and electroluminescence characteristics. The materials showed tunable absorption and emission spectra depending upon nature and position of chromophores attached to carbazole core. The triphenylamine substituted isomers exhibited red-shifted absorption and emission spectra when compared to their respective N-phenylcarbazole-based analogs. It is attributed to the increased intramolecular charge transfer (ICT) in the electron-rich triphenylamine derivatives as further confirmed in solvatochromism studies. However, the N-phenylcarbazole derivatives showed less solvent dependence in spectra attesting less polar ground and excited state due to comparatively poor donor strength of N-phenylcarbazole. Similarly, dyes containing electron-rich chromophores showed facile removal of electron with low oxidation potentials. The thermal robustness of the compounds was attested by high thermal decomposition temperatures (Td) which varied from 438 to 481 οC. The EL performance of 3 wt% doped device fabricated with emitter derived from 2,7-disubstituted carbazole featuring N-phenylcarbazole and PI chromophores showed deep-blue CIE coordinates of (0.16, 0.06) and maximum external quantum efficiency of 5.3%.