Seven-Coordinate Tb3+ Complexes with 90% Quantum Yields: High-Performance Examples of Combined Singlet- and Triplet-to-Tb3+ Energy-Transfer Pathways

abstract

Seven-coordinate, pentagonal-bipyramidal (PBP) complexes [Ln(bbpen)Cl] and [Ln(bbppn) Cl], in which Ln = Tb3+ (products I and II), Eu3+ (III and IV), and Gd3+ (V and VI), with bbpen(2-) = N,N'-bis(2-oxidobenzyl)-N,N'-bis(pyridin-2-ylmethyl)-ethylenediamine and bbppn(2-) = N,N'-bis(2-oxidobenzyl)-N,N'-bis(pyridin-2-ylmethyl)-1,2-propanediamine, were synthesized and characterized by single-crystal X-ray diffraction analysis, alternating-current magnetic susceptibility measurements, and photoluminescence (steady-state and time-resolved) spectroscopy. Under a static magnetic field of 0.1 T, the Tb3+ complexes I and II revealed single-ion-magnet behavior. Also, upon excitation at 320 nm at 300 K, I and II presented very high absolute emission quantum yields (0.90 +/- 0.09 and 0.92 +/- 0.09, respectively), while the corresponding Eu3+ complexes III and IV showed no photoluminescence. Detailed theoretical calculations on the intramolecular energy-transfer rates for the Tb3+ products indicated that both singlet and triplet ligand excited states contribute efficiently to the overall emission performance. The expressive quantum yields, Q(Ln)(L), measured for I and II in the solid state and a dichloromethane solution depend on the excitation wavelength, being higher at 320 nm. Such a dependence was rationalized by computing the intersystem crossing rates (W-ISC) and singlet fluorescence lifetimes (tau(S)) related to the population dynamics of the S-1 and T-1 levels. Thin films of product II showed high air stability and photostability upon continuous UV illumination, which allowed their use as downshifting layers in a green light-emitting device (LED). The prototypes presented a luminous efficacy comparable with those found in commercial LED coatings, without requiring encapsulation or dispersion of II in host matrixes. The results indicate that the PBP environment determined by the ethylenediamine (en)-based ligands investigated in this work favors the outstanding optical properties in Tb3+ complexes. This work presents a comprehensive structural, chemical, and spectroscopic characterization of two Tb3+ complexes of mixed-donor, en-based ligands, focusing on their outstanding optical properties. They constitute good molecular examples in which both triplet and singlet excited states provide energy to the Tb3+ ion and lead to high values of Q(Ln)(L).

keywords

LUMINESCENT LANTHANIDE COMPLEXES; MOLECULAR-ORBITAL METHODS; ION MAGNET; SPECTROSCOPIC PROPERTIES; COORDINATION-COMPOUNDS; CRYSTAL-STRUCTURES; METAL-IONS; BASIS-SET; EMISSION; LIGANDS

subject category

Chemistry, Inorganic & Nuclear

authors

Aquino, LED; Barbosa, GA; Ramos, JD; Giese, SOK; Santana, FS; Hughes, DL; Nunes, GG; Fu, LS; Fang, M; Poneti, G; Neto, ANC; Moura, RT; Ferreira, RAS; Carlos, LD; Macedo, AG; Soares, JF

Groups

G1 - Porous Materials and Nanosystems
G2 - Photonic, Electronic and Magnetic Materials

Projects

CICECO - Aveiro Institute of Materials (UIDB/50011/2020)

CICECO - Aveiro Institute of Materials (UIDP/50011/2020)

SusPhotoSolutions - Soluções Fotovoltaicas Sustentáveis (SUSPHOTOSOLUTIONS)

acknowledgements

This study was financially supported by Fundacao Araucaria (Grants 20171010 and 283/2014; protocol 37509), Conselho Nacional de Desenvolvimento Cientifico e Tecnologico (CNPq; Grant 308426/2016-9), Coordenacao de Aperfeicoamento de Pessoal de Nivel Superior (CAPES; Finance Code 001), Serrapilheira Institute (Grant Serra-1709-17054), and Fundacao de Amparo a 'Pesquisa do Estado do Rio de Janeiro (FAPERJ; Grants E-26/202.912/2019, SEI-260003/001167/2020, and E-26/010.000978/2019). The work was also developed within the scope of the projects of CICECO-Aveiro Institute of Materials, UIDB/50011/2020 and UIDP/50011/2020, financed by national funds through the FCT/MEC and, when appropriate, cofinanced by FEDER under the PT2020 Partnership Agreement. L.E.N.A., G.A. B., J.L.R., F.S.S., S.O.K.G., D.L.H., G.P., A.G.M., and J.F.S. thank the Brazilian agencies CNPq, CAPES, FAPERJ, and Fundacao Araucaria for fellowships. A.N.C.N.'s work was financially supported by the project SusPhotoSolutions, CENTRO-01-0145-FEDER000005, funded by FEDER, through POCI, and by national funds through FCT/MCTES. G.P. gratefully acknowledges Prof. Luis Ghivelder (Institute of Physics, Federal University of Rio de Janeiro) for providing access to the magnetic characterization equipment.