Synthesis and photophysics of reverse saturable absorbing heteroleptic iridium(iii) complexes bearing 2-(7-R-fluoren-2′-yl)pyridine ligands

Abstract

Three new heteroleptic cationic iridium(III) complexes (1–3) bearing 5,5′-bis(7-benzothiazolylfluoren-2′-yl)-2,2′-bipyridine (N^N) and 2-(7-R-fluoren-2′-yl)pyridine (R = H, naphthalimide, NPh₂) (C^N) ligands were synthesized and characterized. Their photophysical properties were investigated systematically by UV-vis absorption, emission, and transient absorption spectroscopy, and the UV-vis absorption spectra were simulated by time-dependent density functional theory (TDDFT). All complexes exhibit ligand-centered ¹π,π* transitions with minor contributions from ¹ILCT (intraligand charge transfer, π(benzothiazolylfluorene) → π* (bpy)) and ¹MLCT (metal-to-ligand charge transfer, dπ(Ir) → π* (bpy)) transitions below 475 nm, and very weak ^1,3MLCT and ^1,3LLCT (ligand-to-ligand charge transfer, π(C^N) → π* (bpy)) transitions above 475 nm. Complexes 1–3 are weakly emissive in fluid solution at room temperature and in glassy matrix at 77 K. The emission of 2 in toluene solution is possibly dominated by the substituted bipyridine ligand-localized ³π,π* emission, while 1 and 3 likely exhibit ³MLCT/³LLCT dominated emission in toluene solution. All complexes possess relatively strong triplet transient absorption (TA) from visible to NIR region, where reverse saturable absorption (RSA) could occur. Nonlinear transmission experiments at 532 nm using ns laser pulses demonstrate that all three complexes exhibit strong RSA, with the RSA strength following this trend: 1 > 3 > 2. The different substituents on the fluorenyl groups of the C^N ligands alter the energy and lifetime of the lowest singlet and triplet excited states. They also affect the ground-state and excited-state absorption characteristics. The differences in the ratio of the excited-state absorption to that of the ground state and the triplet excited-state quantum yield result in the different degrees of RSA. Complexes 1–3 are strong reverse saturable absorbers at 532 nm and could potentially be used as broadband nonlinear absorbing materials.