Chemical tuning of the photon upconversion properties in Ti2+-doped chloride host lattices

The photophysical properties of Ti2+-doped NaCl and MgCl2 at 15 K are compared. At this temperature both materials emit luminescence from their respective lowest excited states and from the T-3(1g)(t(2g)e(g)) higher excited state. In Ti2+:MgCl2 the ligand field is significantly stronger than in Ti2+:NaCl, leading to T-1(2g)(t(2g)(2)) and T-3(2g)-(t(2g)e(g)) lowest excited states, respectively, in these materials. The near-infrared to visible upconversion mechanisms of these two materials are identified. The upconversion efficiency calculated for Ti2+:MgCl2 is approximately 2 orders of magnitude higher than for Ti2+:NaCl. This is mainly due to the more efficient energy storage in the intermediate upconversion level in Ti2+:MgCl2, and its higher T-3(1g)(t(2g)e(g)) --> T-3(1g)(t(2g)(2)) luminescence quantum yield relative to Ti2+:NaCl.