Excited state dynamics of Zn–salophen complexes

Abstract

Zn–salophen complexes are a promising class of fluorescent chemosensors for nucleotides and nucleic acids. We have investigated, by means of steady state UV–Vis, ultrafast transient absorption, fluorescence emission and time dependent density functional theory (TD-DFT) the behavior of the excited states of a salicylidene tetradentate Schiff base (Sal), its Zn(II) coordination compound (Zn–Sal) and the effect of the interaction between Zn–Sal and adenosine diphosphate (ADP). TD-DFT shows that the deactivation of the excited state of Sal occurs through torsional motion, due to its rotatable bonds and twistable angles. Complexation with Zn(II) causes rigidity so that the geometry changes in the excited states with respect to the ground state structure are minimal. By addition of ADP to a freshly prepared Zn–Sal ethanol solution, a longer relaxation constant, in comparison to Zn–Sal, was measured, indicative of the interaction between Zn–Sal and ADP. After a few days, the Zn–Sal–ADP solution displayed the same static and dynamic behavior of a solution containing only the Sal ligand, demonstrating that the coordination of the ADP anion to Zn(II)leads to the demetallation of the Sal ligand. Fluorescence measurements also revealed an enhanced fluorescence at 375 nm following the addition of ADP to the solution, caused by the presence of 2,3-diamino naphthalene that is formed by demetallation and partial decomposition of the Sal ligand. The efficient fluorescence of this species at 375 nm could be selectively detected and used as a probe for the detection of ADP in solution. Graphical abstract

Periodic Trends in the Stabilization of Actinyls in Their Higher Oxidation States Using Pyrrophen Ligands

“…Although expanded-porphyrin macrocycles are characterized with favorable cavities for adsorption, their synthetic feasibility and kinetics are challenging and thus unfavorable toward actinyl complexation. 29−34 Compared to the macrocyclic ligands, acyclic ligands have been widely used in molecular recognition and self-assembly of main group elements, 35,36 and they can also be utilized for chemical bonding and complexation with actinyl cations. 27,37 Specifically, pyrrol-2-ylphenyleneamine was earlier reported for its self-assembly and coordination with Zn(II) ions.…”

“…Ligands with mixed nitrogen and oxygen (as hard and soft) donor atoms such as oxygen-substituted expanded porphyrin, acyclic salophen and salen -type ligands were investigated to be favorable toward the relative softness of the hard actinyl acceptors. Although expanded-porphyrin macrocycles are characterized with favorable cavities for adsorption, their synthetic feasibility and kinetics are challenging and thus unfavorable toward actinyl complexation. − Compared to the macrocyclic ligands, acyclic ligands have been widely used in molecular recognition and self-assembly of main group elements, , and they can also be utilized for chemical bonding and complexation with actinyl cations. , Specifically, pyrrol-2-ylphenyleneamine was earlier reported for its self-assembly and coordination with Zn­(II) ions . Recently, the benzyl ester derivative of the ethyl ester bis­(pyrrole)­phenylenediamine, also called pyrrophen, was tailor-made for the purpose of molecular recognition of UO 2 2+ forming a 1:1 complex .…”