Phosphorescence can be switched on in the absence of ligand-to-metal ion energy transfer, for instance, upon coordination of the organic molecule to a La 3+, Gd 3+, or Lu 3+ ion 9. In optical applications, the properties of the triplet state (T 1: Fig. Organic complexes of lanthanide ions comprise a burgeoning research area with a wide spectrum of applications: optical and magnetic resonance imaging 4, 5, contactless thermometry 6, drug delivery 7, and organic light-emitting diodes 8, for example. Light-harvesting techniques therefore employ coordination to organic molecules 2, or doping into strongly-absorbing inorganic chromophore systems 3. Lanthanide ions (Ln 3+) exhibit sharp and specific emission wavelengths but have weak absorption 1. The discovery of the behaviour of this compound provides insight into important photophysical processes of the triplet state of organo-lanthanide systems and may open new fields of application such as data encryption, anti-counterfeiting and temperature switching. In particular, on-off continuous irradiation cycles reveal a charging behaviour of the emission which is associated with triplet-triplet absorption because it shows a shorter rise lifetime than the corresponding decay lifetime and it varies with illumination intensity. Optical spectroscopy, density functional theory (DFT) and time-dependent DFT techniques have been applied to investigate the photophysical processes of this compound. The compound LaL1(TTA) 3 reported herein exhibits 77 K phosphorescence observable by the naked eye for up to 30 s. The triplet state lifetime under cryogenic conditions for organo-lanthanide compounds usually ranges up to tens of milliseconds.

Emission from the triplet state of an organo-lanthanide complex is observed only when the energy transfer to the lanthanide ion is absent.

Lanthanide absorption spectra