Chemical tuning of the photon upconversion properties in Ti(2+)-doped chloride host lattices

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