Phenanthroline chromophore as efficient antenna for Tb3+ green luminescence: A theoretical study

Abstract

The photophysical processes, from excitation in the UV range, energy migration pathways to luminescence of Tb(phen)2(NO3)3 complex in the Vis region have been studied by theoretical approaches and spectroscopic measurements. The UV–Vis absorption, diffuse reflectance, excitation and luminescence spectra indicated that the ligand 1,10-phenanthroline (phen) was a suitable antenna chromophore for Tb3+ luminescence at excitation of wavelength range 350–352 nm, corresponding to the S1 state of phen. The energy level diagram and character of the singlet and triplet excited states of the ligand-chromophore and Tb(phen)2(NO3)3 in gas phase and polar solution were obtained by the DFT/TDDFT/ωB97xD/B1 method and multireference ab-initio calculations with perturbative spin-orbit coupling corrections. It was also established that the vibro-electronic contribution to the excitation energy is important for correct reproduction and interpretation of the experimental absorption and emission spectra of Tb(phen)2(NO3)3. The rate constants for competitive singlet-triplet intersystem crossing relaxation transitions, as well as radiative and non-radiative deactivation of the ligand-centered excited states were qualitatively estimated and the fastest electronic relaxation mechanism was revealed. The excitation channel S0→S1→T2→T1→5D4 was predicted as the most probable one for effective phen→Tb3+ energy transfer. The sensitization results in a high luminescent quantum yield of 13%, which suggests energy transfer rates, sufficiently higher than the rates of radiative and non-radiative depopulation of the T1 state.