Abstract

Photophysical properties of lanthanides complexes are of great relevance for several academic and industrial areas because of their peculiar and unique features. Amongst these features, the long emission lifetimes ranging from microseconds to milliseconds are important for applications and characterization. Indeed, this is one of the paramount properties of lanthanide complexes and it has been employed in applications and for comparisons of almost all compounds and materials. However, despite being a fundamental property, the determination, use and interpretation of the lifetimes of lanthanide emitting states are still causing some confusion. For instance, it has shown that the lifetime of a given emitting state is dependent upon the excitation wavelength [1], when it can transfer non-radiative energy to another state. In this case, the emission lifetime can increase significantly when these states are near-resonance. Interestingly, the 5D0 Eu3+ emission lifetime () in the [Eu(ketoprofen)3H2O] complex decreases when the excitation wavelength is varied from 560 nm to 465 nm [2]. More specifically,  (in 103 s) was determined as 0.689, 0.644 and 0.594 when excitation occurred at 560, 525 and 465 nm, respectively. When the excitation decreases wavelengths shorter than 395 nm, the lifetime increases and becomes constant at ca. 0.68103 s. The main goal of this contribution is to understand and explain these observations. Noteworthy that it was proposed that there are strong vibronic coupling in this complex. Thus, we propose that the radiative emission (Atot) of the 5D0 would be expressed as Atot = Arad + Avib, namely, the usual Judd-Ofelt electric dipole contribution plus the vibronic coupling term to the radiative transition rate. Because of the strong vibronic coupling between states near the ligand triplet states, the contribution of Avib would be significant, increasing the radiative rate, and thus decreasing the emission lifetime ( = 1/Atot). Whereas, excitations at wavelengths removed from the ligand triplet states would generate a negligible contribution from the Avib and thus making the emission lifetime independent of the excitation wavelength. Currently, we are working in calculating the structures at triplet and singlet states as well as the appropriate equations for the radiative rate taking into accounts these contributions.