Chalcogenide-bound erbium complexes: Paradigm molecules for infrared fluorescence emission

The near-infrared luminescence properties of the nanoscale erbium ceramic cluster (THF)(14)Er10S6-Se12I6 (Er10, where THF = tetrahydrofuran) and the molecular erbiurn thiolate (DME)(2)Er(SC6F5)(3) (Erl, where DME = 1,2-dimethoxyethane) were studied by optical absorption, photoluminescence, and vibrational spectroscopy. The calculated radiative decay time of 4 ms for the I-4(13/2) -> I-4(15/2) transition is comparable to the reported values for previously reported organic complexes. The recorded emission spectrum of the I-4(13/2) -> I-4(15/2) transition was centered at 1544 nm with a bandwidth of 61 and 104 nm for Er10 and Erl, respectively, with stimulated emission cross sections of 1.3 x 10(-20) cm(2) (Er10) and 0.8 x 10(-20) cm(2) (Er1) that are comparable to those of solid-state inorganic systems. Lifetime measurements of the 1544 nm decay showed a fluorescence decay time of the order of 3 ms for ErIO that, together with the radiative decay time, yielded a quantum efficiency above 78%, which is considered to be the highest reported value for a molecular erbiurn compound. This efficiency is attributed to the absence of direct Er coordination with fluorescence quenching vibrational groups such as hydrocarbon and bonding of hydroxide groups. The direct coordination of S, Se, and I accounts for the improved fluorescence spectral properties. These properties result because these anions are heavy and form stable but weak bonds to facilitate a low phonon energy host environment for the erbium.