From ligand exchange to reaction intermediates: what does really happen during the synthesis of emissive complexes?

Abstract

In situ monitoring of the formation of emissive complexes is essential to enable the development of rational synthesis protocols, to provide accurate control over the generation of structure-related properties (such as luminescence) and to facilitate the development of new compounds. In situ luminescence analysis of coordination sensors (ILACS) utilizes the sensitivity of the spectroscopic properties of lanthanide ions to their coordination environment to detect structural changes during crystallization processes. Here, ILACS was utilized to monitor the formation of [Eu(bipy)(NO)] (bipy = 2,2'-bipyridine) during co-precipitation synthesis. Validity of the ILACS results was ensured by concomitant utilization of in situ monitoring of other reaction parameters, including in situ measurements of pH value, ionic conductivity, and infrared spectra, as well as ex situ and synchrotron-based in situ X-ray diffraction analyses. Gradual desolvation of the Eu ions and attachment of ligands were detected by an exponential increase of the intensity of the D → F (J = 0-4) transitions in the emission spectrum. Additionally, the in situ emission spectra show a decrease in the crystallization rate and an increase in the induction time in response to a reduction in the concentration of the starting solutions from 12 mM until crystallization ceased at starting reactant concentrations <6 mM. An increase to a three-fold higher concentration leads to the formation of a reaction intermediate, and its stability was determined to be highly concentration-dependent. The in situ luminescence measurements also demonstrated the existence of a ligand exchange process within the [Eu(bipy)(NO)] complex upon addition of a phen (phen = 1,10'-phenanthroline) solution and the generation of a new phen-containing emissive complex. In attempting to solve the structure of this new phen-containing complex, a different, but nevertheless previously unsynthesized complex, [Eu(phen)(NO)]bipy, was obtained, which shows characteristic Eu luminescence in the red spectral range.

Real‐time Probing the Formation of [M(2,2‐bipyridine)2(NO3)3] (M = Ce, La, Tb) Complexes and Influence of Synthesis Parameters

“…Previous experiments showed how in situ luminescence analysis can be used to monitor the formation of [Ce(bipy) 2 (NO 3 ) 3 ] and [Eu(bipy) 2 (NO 3 ) 3 ] . The next experiments aimed at testing if this principle can also be used to study the formation of [La(bipy) 2 (NO 3 ) 3 ].…”

“…The experiments were executed with three different setups, as described in detail in our previous works , , . In brief, setup A (Figure S32, Supporting Information) combines a workstation EasyMax® 102 (Mettler Toledo, Gießen, Germany) with a Fluorolog‐3 spectrometer (HORIBA, Jovin Yvon GmbH, Unterhaching, Germany), containing a R928P Photomultiplier, iHR‐320‐FA triple grating imaging spectrograph, a Syncerity charge‐coupled device (CCD) detector and a 450 W xenon lamp.…”

“…In addition, in situ XRD cannot be used to characterize the events occurring in solution leading up to the crystallization, like the desolvation process, which is essential for synthesis processes in liquid media such as co‐precipitation, solvothermal or ionic liquids‐, ultrasound‐ and microwave‐based approaches . These difficulties can be overcome by complementing in situ XRD measurements with the in situ luminescence analysis of coordination sensors (ILACS) method. Using this approach, changes in the coordination environment of the lanthanide ions (coordination sensors) can be tracked based on their influence on the respective spectroscopic properties.…”

Monitoring the mechanism of formation of [Ce(1,10-phenanthroline)2(NO3)3] by in situ luminescence analysis of 5d–4f electronic transitions

“…improving reaction conditions, increasing product yield or avoiding the formation of side products) but also allows targeted manipulation of synthesis parameters to alter the product structure and, consequently, their structure-related properties. 1,[4][5][6][7] The in situ luminescence analysis of coordination sensors (ILACS) approach [8][9][10][11] enables monitoring of synthesis processes in real time, under real reaction conditions. Using the spectroscopic sensitivity of lanthanide ions (Ln) [12][13][14][15][16] as coordination sensors, it is possible to obtain structural information about changes in the coordination environment of the Ln ions, throughout the entire reaction, from the early stages of the product formation by means of in situ luminescence measurements.…”

“…8 Traditionally, Eu 3+ and Tb 3+ have been used as coordination centers for ILACS measurements, following the behavior of the emission peaks generated by 4f-4f electronic transitions of these ions for monitoring the formation of solid materials. [8][9][10] These 4f-4f transitions are parity-forbidden, causing them to have a low absorption coefficient and sharp absorption bands, which, in turn, generally lead to low emission intensities. Due to shielding effects of the 5s 2 and 5p 6 subshells on the 4f-orbitals, the position of the emission peaks of the 4f-4f transitions are also almost invariable, being relatively insensitive to changes in coordination environment in comparison to the emission bands assigned to the 5d-4f electronic transitions of lanthanide ions.…”