Lanthanide Luminescence in Visible-Light-Promoted Photochemical Reactions

Abstract

The excitation of lanthanides with visible light to promote photochemical reactions has garnered interest in recent years. Lanthanides serve as initiators for photochemical reactions because they exhibit visible-light-promoted 4f→5d transitions that lead to emissive states with electrochemical potentials that are more negative than the corresponding ground states. The lanthanides that have shown the most promising characteristics for visible-light promoted photoredox are Sm^II, Eu^II, and Ce^III. By understanding the effects that ligands have on the 5d orbitals of Sm^II, Eu^II, and Ce^III, luminescence and reactivity can be rationally modulated using coordination chemistry. This review briefly overviews the photochemical reactivity of Sm^II, Eu^II, and Ce^III with visible light; the properties that influence the reactivity of these ions; and the research that has been reported towards modulating their photochemical-relevant properties using visible light and coordination chemistry.

Keywords: catalysis; lanthanides; luminescence; photoluminescence; photoredox; visible light.

Figure 1

Energies of 4f→5d transitions for divalent and trivalent lanthanides in CaF₂ versus the number of 4f electrons. Circles represent empirical data, and the dashed curve represents calculated values. The red and blue lines represent the boundaries of visible light. Lanthanides within or near that range are of interest because of their ability to be tuned into the visible-light range using coordination chemistry. From Dorenbos, P. FD transition energies of divalent lanthanides in inorganic compounds, J. Phys: Condens. Matter 2003, 15, 575–594. © IOP Publishing Ltd. Adapted with permission. All rights reserved [15].

Figure 2

Ligands studied for their effects on the luminescence of Sm^II [41].

Scheme 1

Photochemical reactions of Sm^II (a) with halides, tosylates, and chalcogenides and (b) in ring-closings [16,38,42].

Scheme 2

Reduction of cyanobenzene to benzylamine by visible-light-promoted SmI₂ in the presence of methanol [43] and hexametylphosphoramide [40].

Figure 3

Complexes 3–8 studied for the effects of cryptands on the luminescent properties of Eu^II [47,55,59,60].

Figure 4

Orbital energy diagram of the 5d_z², 5d_xy, and 4f_z³ orbitals of europium-containing complexes 3, 4, and 5 [58,59].

Figure 5

Comparison of the maximum of absorbance bands of complexes containing Eu^II in solution in methanol [57,59,60]. * Complexes in acetonitrile. ** from [60].

Figure 6

Comparison of the maximum emission wavelengths of complexes containing Eu^II in solution in methanol [57,59,60]. * Complexes in acetonitrile ** from [60].

Figure 7

Guanidinate, amido, and aryloxide complexes 13–24 studied with respect to the luminescence of Ce^III [64,65,66].

Figure 8

Comparison of the maximum of absorbance bands of Ce^III-containing complexes 13–24 [64,65,66].

Figure 9

Comparison of the maximum emission wavelength of cerium complexes 13–24 [64,65,66].

Figure 10

Comparison of the ground-state electrochemical potentials (E_1/2) of cerium complexes 13–17 and 21–24 [64,65,66].

Figure 11

Comparison of the calculated excited-state electrochemical potentials (E_1/2*) of complexes 13–17 and 21–24 [64,65,66].

Scheme 3

Catalytic reductive coupling of benzyl chloride using Ce^III [64].

Scheme 4

Arylation of 4-bromofluorobenzene promoted by Ce^III. ¹ 20 mol % for complex 13 [64,65,66].

Figure 12

Cryptand 25 which was studied for its effect on the luminescence properties of Yb^II [64].