A Fluorescence-Detected Coordination-Induced Spin State Switch

Abstract

The response of the spin state to in situ variation of the coordination number (CISSS) is a promising and viable approach to smart sensor materials, yet it suffers to date from insensitive detection. Herein, we present the synthetic access to a family of planar nickel(II) complexes, whose CISSS is sensitively followed by means of fluorescence detection. For this purpose, nickel(II) complexes with four phenazine-based Schiff base-like ligands were synthesized and characterized through solution-phase spectroscopy (NMR and UV−vis), solid-state structure analysis (single-crystal XRD), and extended theoretical modeling. All of them reveal CISSS in solution through axial ligating a range of Nand O-donors. CISSS correlates nicely with the basicity of the axial ligand and the substitution-dependent acidity of the nickel(II) coordination site. Remarkably, three out of the four nickel(II) complexes are fluorescent in noncoordinating solvents but are fluorescence-silent in the presence of axial ligands such as pyridine. As these complexes are rare examples of fluorescent nickel(II) complexes, the photophysical properties with a coordination number of 4 were studied in detail, including temperature-dependent lifetime and quantum yield determinations. Most importantly, fluorescence quenching upon adding axial ligands allows a "black or white", i.e. digital, sensoring of spin state alternation. Our studies of fluorescence-detected CISSS (FD-CISSS) revealed that absorption-based CISSS and FD-CISSS are super proportional with respect to the pyridine concentration: FD-CISSS features a higher sensitivity. Overall, our findings indicate a favored ligation of these nickel(II) complexes in the excited state in comparison to the ground state.

Ni, Pd, and Pt complexes of a tetradentate dianionic thiosemicarbazone-based O^N^N^S ligand

“…28 We found excellent agreement of predicted and experimental metrics across the entire reference set (error in bond lengths <2 pm). Taken together with the general close compliance between model and experiment of planar d 8 configured complexes, [36][37][38][39] As could be concluded from strictly diamagnetic behaviour of the complexes in the 1 H NMR spectra, all complexes exhibit close-to-ideal square planar coordination of the S^N^N^O donor set. The bond length of the coordinated C-S moiety cluster at 1.78 ± 0.01 Å, indicating dominant ene-thiolate character.…”

“…These settings gave excellent agreement with experiment in several previous studies on square-planar d 8 -based systems. [37][38][39] In keeping with this, the experimental transition energy and intensity could be satisfyingly matched for all complexes in this study (cf., Fig. 4 for Ni; for more spectra see Fig.…”

DFT Investigation of the Molecular Properties of the Dimethylglyoximato Complexes [M(Hdmg)2] (M = Ni, Pd, Pt)

“…Recently reported DFT and TD-DFT calculations on the molecular and electronic structures of planar d 8 configured Ni II , Pd II , and Pt II complexes have shown that the reliability of calculated data with experimental data [58][59][60][61][62][63][64][65][66] is very much dependent on functionals and basis sets. Furthermore, comparative DFT and experimental studies for these three metals were recently reported [62][63][64][65][66][67] allowing to gauge for the differences of these three metals.…”

Hidden Lewis acidity: Studies on the medium and structure dependent fluorescence of zinc(II) complexes

“…Similarly, enhanced affinity for Lewisbases has been recently reported for nickel(II) complexes of CF3-decorated ligands. [20] [Zn( 1…”