Lanthanide-Dependent\nPhotochemical and Photophysical\nProperties of Lanthanide–Anthracene Complexes: Experimental\nand Theoretical Approaches

The structural, photophysical, and photochemical properties\nof\nLn(depma)(hmpa)<sub>2</sub>(NO<sub>3</sub>)<sub>3</sub> (Ln = La,\nCe, Nd, Sm, Eu, Tb, Ho, Er, and Yb) complexes <b>1-Ln</b> were\ninvestigated with a multidisciplinary approach involving synthesis,\nphotocycloaddition-based crystal engineering, spectroscopic analytical\ntechniques and quantum chemical ab initio calculations. Depending\non the Ln<sup>3+</sup> ion the isostructural <b>1-Ln</b> complexes\nexhibit quite different behavior upon excitation at 350–400\nnm. Some <b>1-Ln</b> complexes (Ln = La, Ce, Sm, Tb, Yb) emit\na broad and strong band near 533 nm arising from paired anthracene\nmoieties, whereas others (Ln = Nd, Eu, Ho, Er) do not. <b>1-Eu</b> is not emissive at all, whereas <b>1-Nd</b>, <b>1-Ho</b>, and <b>1-Er</b> exhibit a Ln<sup>3+</sup> based luminescence.\nUpon irradiation with 365 nm ultraviolet (UV) light <b>1-Ln</b> (Ln = La, Ce, Sm, Tb, Yb) dimerize by means of a photochemically\ninduced [4 + 4] cycloaddition of the anthracene moieties, whereas <b>1-Ln</b> (Ln = Nd, Eu, Ho, Er) remain monomers. We propose three\nmodels, based on the matching of the energy levels between the Ln<sup>3+</sup> ion and the paired or dimerized anthracene units in the\nenergy-resonance crossing region, as well as on internal conversion-driven\nand intersystem crossing-driven energy transfer, which explain the\nLn<sup>3+</sup> ion regulated photophysics and photochemistry of the <b>1-Ln</b> complexes.