Unprecedented Ir(iii) cationic complexes based on tridentate tetrazolate ligands: synthesis, photophysics and encapsulation in SiO2 nanoparticles

Abstract

The first examples of luminescence Ir(III) complexes derived from a tridentate tetrazole ligand, 2-(tetrazole-5-yl)-1,10-phenanthroline (Hphenttz), are reported here. Two cationic complexes, heteroleptic [Ir(tpy)(phenttz)]²⁺ (1) (tpy = 2,2′:6′,2′′-terpyridine) and homoleptic [Ir(phenttz)₂]⁺(2), have been synthesized and fully characterized from a chemical, structural and photophysical point of view. Both complexes exhibit green luminescence with a prevalent ³LC (Ligand-Centered) character, which is evidenced by their structured emission profiles and low k_r values (of about 10³ s⁻¹), and supported by density functional theory (DFT) and time-dependent density functional theory (TD-DFT) calculations. In air-equilibrated solutions, 1 and 2 show emission lifetimes (respective values of 1.6 μs and 1.2 μs) comparable to that of the reference complex [Ir(tpy)₂]³⁺ (1.0 μs) and quantum yields slightly lower (1.5% (1), 1.3% (2)) than that obtained for [Ir(tpy)₂]³⁺ (2.5%). Under an oxygen-free atmosphere, the emission lifetimes and quantum yields (τ/Φ) of the complexes increase significantly up to 5 μs/4.2% (1) and 3 μs/3.3% (2). 1 has been embedded within amorphous silica nanoparticles leading to the hybrid material 1@SiO₂. This material shows enhanced photochemical stability and higher luminescence efficiency compared to the free complex, which demonstrates that silica hampers the diffusion of O₂, restrains the mobility of the complex and stimulates the radiative decay of the excited state.