Effect of Ethynyl Linkages on the Properties of the Derivatives of Triphenylamine and 1,8-Naphthalimide

Three new derivatives of triphenylamine containing different number of 1,8-naphthalimide moieties linked via ethynyl linkages were obtained by a Sonogashira cross-coupling reaction and their properties were studied. The glass-forming derivatives showed glass transition temperatures ranging from 73 to 96 °C. They showed high thermal stabilities with the temperatures of the onset of thermal degradation ranging from 421 to 462 °C. Fluorescence quantum yields of the dilute solutions in nonpolar solvents of the compounds ranged from 0.063 to 0.94 while those of the solid films were in the range of 0.011–0.25. Small singlet–triplet gap was predicted by DFT calculations, which was confirmed by the similarity between the prompt and the delayed fluorescence energies. Cyclic voltammetry measurements revealed close values of the solid state ionization potentials ranging from 5.48 to 5.61 eV and electron affinities ranging from −3.29 to −3.16 eV. The layer of 4,4′-((di­(<i>N</i>-(2-ethylhexyl)-1,8-naphthalimide-4-yl)­phenyl)­ethynyl)­benzenamine exhibited effective charge-transport with hole drift mobilities exceeding 10^–2 cm²/V·s. Good intrinsic hole transport parameters were predicted by theoretical estimations. Both efficient fluorescence and high hole mobilities are found to be positively influenced by the presence of ethynyl bridges, (i) allowing for molecular planarization and high hole mobility and (ii) inducing large space separation between the HOMO and LUMO localizations, with direct impact in singlet–triplet splitting