Synthesis of Cs 2 TiX 6 . For the synthesis of Cs 2 TiX 6 (X = I, Br), 4 ml of cesium oleate (0.62 mmol) and 60 mg of DMOP were weighted in the glovebox and loaded in a 50 ml three-neck flask together with 2 ml of ODE, 2 ml of OA and 238 µl of titanium(IV) isopropoxide (0.78 mmol). The solution was degassed through 3 cycles of argon and vacuum and heated at 110 °C for 35 minutes under vacuum. At this point, for the case of Cs 2 TiI 6 synthesis, 1238 µL (8.436 mmol) of TMSI was injected and after 20 seconds the reaction was quenched by diluting the solution in 10 ml of toluene. For the synthesis of Cs 2 TiBr 6 , 574 µL (4.218 mmol) of TMSBr was injected and the reaction was quenched as for Cs 2 TiI 6 synthesis.