| Summary: | This study examines the synthesis of two geminal bisphosphonate ester-supported Ln<sup>3+</sup> complexes [Ln(<b>L3</b>)<sub>2</sub>(NO<sub>3</sub>)<sub>3</sub>] (Ln = Nd<sup>3+</sup> (<b>5</b>), La<sup>3+</sup> (<b>6</b>)) and optical properties of the neodymium(III) complex. These results are compared to known mono-phosphonate ester-based Nd<sup>3+</sup> complexes [Nd(<b>L1</b>/<b>L2</b>)<sub>3</sub>X<sub>3</sub>]<sub>n</sub> (X = NO<sub>3</sub><sup>−</sup>, n = 1; Cl<sup>−</sup>, n = 2) (<b>1</b>–<b>4</b>). The optical properties of Nd<sup>3+</sup> compounds are determined by micro-photoluminescence (µ-PL) spectroscopy which reveals three characteristic metal-centered emission bands in the NIR region related to transitions from <sup>4</sup>F<sub>3/2</sub> excited state. Additionally, two emission bands from <sup>4</sup>F<sub>5/2</sub>, <sup>2</sup>H<sub>9/2</sub> → <sup>4</sup>I<sub>J</sub> (J = 11/2, 13/2) transitions were observed. PL spectroscopy of equimolar complex solutions in dry dichloromethane (DCM) revealed remarkably higher emission intensity of the mono-phosphonate ester-based complexes in comparison to their bisphosphonate ester congener. The temperature-dependent PL measurements enable assignment of the emission lines of the <sup>4</sup>F<sub>3/2</sub> → <sup>4</sup>I<sub>9/2</sub> transition. Furthermore, low-temperature polarization-dependent measurements of the transitions from R<sub>1</sub> and R<sub>2</sub> Stark sublevel of <sup>4</sup>F<sub>3/2</sub> state to the <sup>4</sup>I<sub>9/2</sub> state for crystals of [Nd(<b>L3</b>)<sub>2</sub>(NO<sub>3</sub>)<sub>3</sub>] (<b>5</b>) are discussed.
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