ChemInform Abstract: Excited States of Titanium(2+): Sharp‐Band and Broad‐Band Near‐Infrared Luminescence from Ti2+ in MgCl2 and MgBr2.

Abstract

Absorption and luminescence spectral investigations of Ti2+(3d2) doped into the MgCl2 host show a sharp-band near-IR emission at low temp. with a radiative decay time of 109 ms. From the sharp emission an accurate determination of the energy splitting within the 3T1g(Oh) ground state of Ti2+ in this crystal environment is obtainable. Besides the near-IR emission Ti2+:MgCl2 exhibits a broad-band emission, which dominates above 70 K. In the bromide host a broad-band near-IR luminescence at low temp. with a decay time of 750 µs is observed. A combination of the spectroscopic results with lifetime measurements as a function of temp. leads to radiative and nonradiative mechanisms for both systems.

Design of Luminescent Inorganic Materials: New Photophysical Processes Studied by Optical Spectroscopy

“…The first TM upconversion observed in our laboratories was found in Ti 2+ doped into MgCl 2 . 13 As shown in the d 2 energylevel diagram of Figure 2a, octahedral Ti 2+ has an orbitally degenerate 3 T 1 (t 2 2 ) ground state. This is split in MgCl 2 due to a weak trigonal (D 3d ) distortion, yielding a 3 A 2 ground state.…”

“…This is split in MgCl 2 due to a weak trigonal (D 3d ) distortion, yielding a 3 A 2 ground state. 13 The 10 K axial absorption spectrum of 5% Ti 2+ : MgCl 2 is shown in Figure 3a. Two features are observed in the visible and near-IR (NIR) energy regions, corresponding to the first two spin-allowed electronic transitions from the ground state ( 3 T 1 (t 2 2 ) f 3 T 2 (t 2 e) and 3 T 1 f 3 T 1 (t 2 e)).…”

“…Detailed spectroscopic studies have been used to quantify the radiative and nonradiative decay processes involving these two excited states in Ti 2+ :MgCl 2 . 13 The upper-excited-state ( 3 T 1 ) luminescence is observed not only with higher-energy excitation, but also with broadband excitation in the NIR using a lamp. 13 At low excitation powers, this upconversion luminescence shows a quadratic dependence on excitation power, as would be expected from a two-photon process.…”

“…13 The upper-excited-state ( 3 T 1 ) luminescence is observed not only with higher-energy excitation, but also with broadband excitation in the NIR using a lamp. 13 At low excitation powers, this upconversion luminescence shows a quadratic dependence on excitation power, as would be expected from a two-photon process. Shown in Figure 3c is the excited-state excitation (ESE) spectrum of this sample, obtained by pumping the 3 T 2 (t 2 e) absorption at 9300 cm -1 and monitoring the visible upconversion luminescence intensity as a function of the wavelength of a second NIR beam.…”

“…Magnetic exchange interactions are more easily induced in TM systems than in RE systems due to the smaller orbital shielding in TMs, and such effects may therefore be far more pronounced in this relatively unexplored area of TM or mixed TM/RE upconversion. Future studies will address the influence of magnetic perturbations on the excited-state properties of a variety of TM and TM/RE systems, including Ti 2+ , 13 Ni 2+ , 23 and Mo 3+ , 15 all of which have shown the ability to mediate upconversion and also have relatively small spinorbit coupling parameters (Table 1), leading to stringent selection rules restricting some potentially interesting upconversion processes.…”