Synthesis and luminescent properties of 4′-phenyl-2,2′:6′,2″-terpyridyl compounds bearing different aryl substituents

A series of new 4′-phenyl-2,2′:6′,2″-terpyridine compounds bearing different aryl substituents (Ar=phenyl (1a), 2,4-difluorophenyl (1b), 4-(trifluoromethyl) phenyl (1c), 4-methoxyphenyl (1d), 9-hexyl-9H-carbazolyl (1e), 9,9-dihexyl-9H-fluorenyl (1f), 4-(diphenylamino)phenyl (1g)) were synthesized and characterized. The influence of the aryl substituents on the luminescence of these compounds is systematically investigated by spectroscopic methods and simulated by density functional theory (DFT) calculations. All compounds exhibit structured absorption bands in the UV region; and broad, structureless charge-transfer bands/shoulders for 1d-1g, which systematically red-shifts when electron-donating substituents are introduced to the phenyl rings, but blue-shifts when electron-withdrawing substituents are attached on the phenyl rings. All compounds are emissive in solution at room temperature (λemmax=358-489 nm, Φ_F=0.16-0.92, τ_F=0.77-2.24 ns), which can be attributed to ¹π, π∗/¹ICT (intramolecular charge transfer) state. Their fluorescent quantum yields increases when the electron-donating aryl substituents attached on the phenyl rings. DFT calculations on 1a-1g were also performed to gain insight into the nature of the ground electronic state. As the representative of compounds with electron-donating substituents, 1f exhibited high fluorescent quantum yields (Φ_f≥ 0.90 in solvents) while the compounds with electron-withdrawing substituents showed relative low fluorescent quantum yields. Their photophysical properties have been investigated with the aim to provide a basis for elucidating the structure-property correlations and developing new blue light-emitting materials.