Thermoluminescent Tb(III) and Dy(III) complexes with redox-active ligands: experimental and theoretical study

Thermoluminescence and persistent luminescent materials with unique delayed emission have attracted much attention and exhibit great promise in optical information storage. In this manuscript, to reveal the thermoluminescence mechanism, a combined experimental and theoretical study of ternary Ln(NO₃ )₂ Acac(Phen)₂ complexes, where Ln is Tb(III), Dy(III), Eu(III), Acac is acetylacetonate anion, and Phen is 1,10-phenanthroline, was carried out. The terbium and dysprosium complexes had thermoluminescence properties, while the europium complex did not. A thermoluminescence mechanism is proposed: the powerful double π-conjugate phenanthroline system appearance upon photoexcitation, the peculiarities of frontier orbitals, the abnormally small highest occupied molecular orbital-lowest unoccupied molecular orbital gap, and the geometrical changes in the terbium and dysprosium complexes led us to suggest that phenanthroline molecules serve as 'chemical' electron traps. Therefore, we succeeded in 'freezing' and storing the excited state of complexes I and II indefinitely. The obtained thermoluminescent materials with 'chemical' traps of electrons are capable of storing the energy from incident photons and exhibit a great opportunity in optical information storage and anticounterfeiting applications.