Lanthanide photoluminescence lifetimes reflect vibrational signature of local environment: Lengthening duration of emission in inorganic nanoparticles

Photoluminescence of trivalent lanthanides (Ln(3+)) needs protection from environmental quenching induced by vibrational overtones of ligand and solvent molecules. Incorporating (doping) Ln(3+) in an appropriate inorganic nanoparticle (NP) provides a way to minimize this nonradiative decay rate. In order to better understand the components of emission decay kinetics in inorganic NPs - Ln(3+) assemblies, an analysis of Ln(3+) emission lifetime in the Ca(Ln)F-2 [Ln = Sm, Eu, Tb, Dy] NPs in absence of optical antenna effect is undertaken; and these values were compared with the same in various other systems compiled from literature. These systems include inorganic NPs with an optical antenna effect, molecular complexes, and freely floating moieties. This analysis reveals that inorganic NPs provide a remarkable lengthening of emission duration, irrespective of operation of NP sensitized Ln(3+) emission. A general trend of emission lifetime tau(L) [inorganic NP] > tau(S) [inorganic NP] approximate to tau [molecular complexes] > tau [freely floating ions] is revealed, with L and S being the longer and shorter lifetime components in inorganic NPs respectively. This provides important design criterion for developing lanthanide containing luminophores. A correlation of emission lifetimes with the vibrational frequencies of local environment is established, suggesting that the lower frequency oscillation of inorganic matrices are responsible for lengthening of emission lifetime in the inorganic NPs. Furthermore, this quantitative correlation (a) strengthens the intuitive interpretation of spatial location dependent Ln(3+) nonexponential emission decay behaviour in inorganic NPs and (b) suggests that Ln(3+) emission lifetime can be used as a probe for the local environment even in inorganic NPs.