Optical Response of Terpyridine Ligands to Zinc Binding: A Close Look at the Substitution Effect by Spectroscopic Studies at Low Temperature

Abstract

Terpyridine (tpy) ligands are popular building blocks to bind metal ions. Several tpy ligands with different substituents were synthesized and examined for their binding with zinc cation. The study revealed a large substituent effect on the zinc binding-induced fluorescence quenching. With the aid of a liquid nitrogen Dewar, the tpy molecules were frozen to their ground-state conformation, preventing (or minimizing) molecular reorganization in the photoinduced excited state. This allowed us to detect the fluorescence spectra from the locally excited state (having a minimum of charge transfer interaction) and the temperature-dependent fluorescence. The fluorescence response to low temperature provided useful information about the intramolecular charge transfer (ICT) interaction between the donor and acceptor groups. Furthermore, a strong donor substituent (such as Me2N) played an essential role in observed fluorescence quenching. The study also provides a useful example to elucidate the ICT mechanism by using low-temperature fluorescence spectroscopy.

Irreversible Solvatochromic Zn-Nanopaper Based on Zn(II) Terpyridine Assembly and Oxidized Nanofibrillated Cellulose

“…Molecular modeling revealed more information about the absorption and emission from the molecular orbital interaction (Figure c) . The findings of this effort supported the assumption that the binding of zinc to the complex significantly increased the ability of the tpy to accept electrons, thereby producing stronger donor–acceptor interactions in the excited state.…”

Fluorescence quantum yield rationalized by the magnitude of the charge transfer in π-conjugated terpyridine derivatives

“…Other approximations for the exchange-correlation energy such as B3LYP, originally used for the study of terpyridyl compounds 1a-1g, 12,13 are not considered in the present work due to their known unreliable performance in describing the excited states of charge-transfer character. 23,24 The computational costs of CAM-B3LYP calculations are significantly lower than either ADC(2) or CC2.…”

The Role of Zinc(II) Ion in Fluorescence Tuning of Tridentate Pincers: A Review

“…Tpy-type ligands with different substituents were examined for their binding ability specifically toward zinc(II) cation in 2016 [20] and in 2017 [21] (Figure 2). In [21], Zhen Ma and coworkers examined the compound obtained by reacting 4 -phenyl-terpyridine and ZnSO 4 •7H 2 O by X-ray crystallographic analysis.…”

“…The former higher energy band of the complex was assigned to LLCT whereas the latter band to LMCT [49,50]. In 2016, the influence of the coordination stoichiometry was explored by Yi Pang and coworkers [20], by reacting substituted Tpy-type ligands (R in Figure 2 is a p-substituted phenyl ring bearing a donor group and R'=H or R'=CH 3 on the lateral pyridine groups) with ZnCl 2 . The authors pointed out that the zinc complex forms in a 2:1 (ligand: metal) ratio with a low zinc(II) concentration and in a 1:1 ratio with a high concentration of metal.…”

“…Nevertheless, ICT or ILCT transitions can be responsible of zinc binding-induced fluorescence quenching. Fluorescence quenching is observed in rigid pyridine-based ligands upon coordination, due to a decreased HOMO-LUMO energy gap, or in zinc-induced quenching of the protein intrinsic fluorescence due to conformational perturbations [18][19][20][21].…”