Charging and ultralong phosphorescence of lanthanide facilitated organic complex

Compounds featuring long-lived luminescence have potential applications in a variety of fields, including anti-counterfeiting and switches. Here the authors report a lanthanide-based compound that exhibits phosphorescence observable by the naked eye for up to 30 s at 77 K; On-off continuous irradiation cycles reveal a charging behaviour associated with triplet-triplet absorption, showing a shorter rise lifetime than the decay lifetime. Emission from the triplet state of an organo-lanthanide complex is observed only when the energy transfer to the lanthanide ion is absent. The triplet state lifetime under cryogenic conditions for organo-lanthanide compounds usually ranges up to tens of milliseconds. The compound LaL1(TTA)(3) reported herein exhibits 77 K phosphorescence observable by the naked eye for up to 30 s. Optical spectroscopy, density functional theory (DFT) and time-dependent DFT techniques have been applied to investigate the photophysical processes of this compound. 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. 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.