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
Highlights
Salophens (N,N-bis(salicylidene)phenylenediamine) are tetradentate ligands, an example of which is shown in Fig. 1a, and are extensively used in coordination1 3 Vol.:(0123456789)Photochemical & Photobiological SciencesIn particular, Zn–salophen complexes often display interesting photochemical and photophysical properties [8,9,10]
In light of all of these partially contrasting observations, we decided to perform a more in-depth investigation of the dynamics of the tetradentate Schiff base shown in Fig. 1a (Sal), its Zn(II) coordination compound (Zn–Sal, Fig. 1b) and the spectroscopic changes following the interaction between Zn–Sal and adenosine diphosphate (ADP) in ethanol solutions, since the most relevant change in the luminescence properties were observed when Zn–Sal is associated with the ADP anion as a guest [17]
The spectrum of the freshly prepared Zn–Sal ethanol solution saturated with ADP, namely recorded 30 min after the preparation of the sample (Fig. 2a red line), showed negligible changes in shape of the absorption spectrum when compared to the spectrum of Zn–Sal (Fig. 2a blue line) in the 250–600 nm range
Summary
Salophens (N,N-bis(salicylidene)phenylenediamine) are tetradentate ligands, an example of which is shown in Fig. 1a, and are extensively used in coordination. The addition of nucleotides caused a 90–95% quenching of the green–yellow fluorescence of Zn–Sal, so that the latter is an on–off sensor for specific detection of these nucleotides We tentatively attributed this effect to the existence of stacking interactions between the aromatic rings of the Zn–salophen sensor and the aromatic nucleotide bases, and this was confirmed in the case of the gas phase structure of the Zn–Sal–AMP anionic complex [13]. In light of all of these partially contrasting observations, we decided to perform a more in-depth investigation of the dynamics of the tetradentate Schiff base shown in Fig. 1a (Sal), its Zn(II) coordination compound (Zn–Sal, Fig. 1b) and the spectroscopic changes following the interaction between Zn–Sal and ADP in ethanol solutions, since the most relevant change in the luminescence properties were observed when Zn–Sal is associated with the ADP anion as a guest [17]. To gain information on the possible spatial and electronic structure of the Sal ligand, Zn–Sal and Zn–Sal–ADP, the complex system has been investigated by means of Density Functional Theory (DFT) and Time Dependent DFT (TD-DFT)
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