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

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 (LC)-L-3 (Ligand-Centered) character, which is evidenced by their structured emission profiles and low k(r) values (of about 10(3) 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 mu s and 1.2 mu s) comparable to that of the reference complex [Ir(tpy)(2)](3+) (1.0 mu s) 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 (tau/phi) of the complexes increase significantly up to 5 mu s/4.2% (1) and 3 mu s/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 O-2, restrains the mobility of the complex and stimulates the radiative decay of the excited state.