Fluorene-decorated Ir(iii) complexes: synthesis, photophysics and tunable triplet excited state properties in aggregation

Abstract

Two new heteroleptic cationic Ir(III) complexes bearing benzothiazole and pyridine motifs on fluorene groups were synthesized and characterized. Complexes Ir1 and Ir2 exhibit ¹π,π* transitions below 430 nm, with broad but weak metal-to-ligand and ligand-to-ligand charge transfer absorption bands above 430 nm. Both complexes possess long-lived emissions (τ_em = 0.37 μs for Ir1, τ_em = 5.41 μs for Ir2) and triplet excited states (τ_TA = 0.14 μs for Ir1, τ_TA = 6.06 μs for Ir2). Their optical properties in solution and aggregated states were also investigated. Both Ir(III) complexes exhibit aggregation-induced phosphorescence emission behavior in an acetonitrile–water mixture. As the water content increased to 90%, the emission intensities of complexes Ir1 and Ir2 increased nearly 3 times and 2.8 times, respectively. Moreover, nonlinear transmittance experiments were performed in CH₃CN and a CH₃CN–H₂O mixture (f_w = 90%), and the strength of the reverse saturable absorption (RSA) at 532 nm followed the trend: Ir2 (CH₃CN) ≥ Ir1 (CH₃CN) > Ir2 (CH₃CN–H₂O) > Ir1 (CH₃CN–H₂O). Abundant triplet state excitons participated in the radiative transition progress, which inhibited the RSA process. These results indicate that the fluorene-decorated Ir(III) complexes are suitable for aggregation emission and optical power limiting applications.