title: Fluorescent Sensor Materials based on 3d Transition Metal Complexes
Creators: name: Kurz, Hannah
Publisher: name: University of Bayreuth
Year: 2022
Subjects: subject: Schiff Base-Like
subject: Complexes, Ni(II)
subject: Zn(II)
subject: Sensor
subject: Fluorescence
subject: Self-Assembly
subject: 540

description: The aim of this work was the design of photoluminescent sensor materials based on 3d transition metal complexes composed of Schiff base-like ligands. In this work, the focus was on the influence of axial coordination on the emission properties. In the first project the effect of coordination-induced spin state switching (CISSS) on the optical properties of a series of nickel(II) complexes, bearing a fluorescent phenazine backbone was investigated. UV-vis measurements of titration series of the non-coordinating solvent chloroform and the strong Lewis base pyridine showed continuous and drastic changes of the absorption properties with growing pyridine amount. Thereby, the pyridine equivalents required to induce a CISSS strongly depend on the Lewis acidity of the nickel(II) metal center. While the complexes [Ni(1–3)] all require similar amounts of pyridine, the introduction of electron withdrawing CF3 substituents in [Ni(4)] and through this, the strongly enhanced Lewis acidity of the Ni(II) metal center, result in a drastically increased sensitivity towards pyridine. Due to the acid-base character of the coordination process, the sensitivity of the nickel(II) complexes towards axial donors correlates as well with the basicity of the donor. Remarkably, [Ni(1–3)] feature fluorescence in the non-coordinating solvent chloroform with quantum yields of 5 ± 1% and excited state lifetimes in the order of 1 ns at room temperature. In contrast, [Ni(4)] is non-emissive in all solvents. DFT calculations point to a substitution-dependent nature of the lowest excited states. Even though, the HOMOs and LUMOs of all complexes are predominantly phenazine centered, the transition dipoles of the leading optical transitions of [Ni(1–3)] and [Ni(4)] point in opposite directions. While for [Ni(1–3)] a charge shift from the [NiN2O2] chelate to the phenazine was observed, the opposite is the case for [Ni(4)]. This leads to a MLCT-like transition for [Ni(1–3)] and a LMCT-like transition for [Ni(4)], causing non-radiative deactivation. The emission spectra of chloroform/pyridine series of [Ni(1–3)] showed successive fluorescence quenching upon pyridine addition. Contrary to the implication of solely static quenching upon coordination in the ground state, the upward-curved Stern-Volmer plot hints towards additional dynamic quenching in the excited state. In agreement, comparing the pyridine concentration dependency of the absorbance and the emission reveals a higher sensitivity of the fluorescence-detected CISSS (FD-CISSS). This is also reflected in a solvent series of [Ni(1)], where most solutions are CISSS-silent in the absorbance spectra, but CISSS-active in the emission spectra. The degree of quenching can be clearly correlated to the basicity of the solvent. The enhanced sensitivity of the FD-CISSS can be attributed to a drastically increased Lewis acidity of the formally nickel(III) metal center of [Ni(1–3)] in the excited state due to the MLCT-like character of the lowest excited state. Coordination-induced emission changes are not limited to nickel(II) complexes. As zinc(II) complexes tend to overcome the unfavored square-planar coordination sphere through stacking into dimers/oligomers, coordination-induced destacking can affect the emission properties as well. The effect of targeted destacking on the emission was studied for a series of zinc(II) complexes bearing nitrile substituents in the ligand backbone. While the absorbance behavior of [Zn(5–7)] only slightly depends on the Lewis basicity of the solvent, drastic changes were observed in the emission behavior. Thereby, a strong emission was observed in coordinating solvents, whereas a close-to-zero emission was observed in non/weakly-coordinating solvents. As a result, chloroform/pyridine series of [Zn(5–7)] showed a remarkable turn-on emission behavior based on coordination-induced destacking. Even though the optical behavior is qualitatively similar to that of the closely related [Zn(sal)], an enormous sensitivity reduction was observed for [Zn(5–7)]. As DFT calculations revealed an even higher Lewis acidity for [Zn(5–7)] relative to [Zn(sal)], the sensitivity decrease must be based on strongly stabilized stacked dimers/oligomers. Indeed, DFT calculations revealed the formation of a completely new stacking motif based on (ZnO)2 macrocycles. In the next step, the complex [Zn(5)] was encapsulated into polystyrene-block-poly(4-vinylpyridine) (PS-b-P4VP) block copolymer micelles (BCP), that form through self-assembly in toluene. The micelles are composed of a PS shell and a P4VP core providing pyridine coordination sites. The [Zn(5)]@BCP hybrids show a strong green emission due to coordination of pyridine moieties to the zinc(II) complexes within the micelle core. All [Zn(5)]@BCP hybrids exhibit higher quantum yields and longer lifetimes than pure [Zn(5)] in pyridine, suggesting the prevention of dynamic quenching processes upon embedment in the P4VP core. Chloroform/toluene series were studied to investigate the effects of micelle formation on the emission properties. DLS measurements confirmed the absence of micelles in chloroform-rich solutions, whereas micelles were observed above a toluene content of 20 vol%. Since no turn-on behavior could be obtained due to a non-negligible amount of coordinated [Zn(5)] in chloroform, the chloroform was acidified to receive the nearly non-emissive ligand. This acidification indeed leads to a remarkable turn-on emission sensor behavior that correlates with the micelle formation.