The Prospect of Salophen in Fluorescence Lifetime Sensing of Al³⁺

We have assessed the potential of salophen, a tetradentate Schiff base, in fluorescence sensing of Al³⁺ ions. While performing this investigation, we have noticed conflicting literature reports on the fluorescence spectral maximum and quantum yield of salophen. So, the compound has been purified by repeated crystallization. Fluorescence studies have been performed on samples in which the absorption and excitation spectra are completely superimposable. The purified compound exhibits a feeble fluorescence at 545 nm, associated with an ultrafast fluorescence decay. This is rationalized by excited state proton transfer and torsional motions within the molecule, which provide efficient nonradiative channels of deactivation of its excited state. The fluorescence quantum yield increases upon complexation of salophen with Zn²⁺ as well as Al³⁺. The increase is significantly more upon complexation with Al³⁺. However, fluorescence maxima are similar for the two complexes. This indicates that fluorescence intensity may not be a good parameter for Al³⁺ sensing by salophen, in the presence of a large excess of Zn²⁺. This problem can be circumvented if fluorescence lifetime is used as the sensing parameter, as the lifetime of the Al³⁺ complex is in the nanosecond time regime while that of the Zn²⁺ complex is in tens of picoseconds. The significant difference in the fluorescence quantum yield and lifetime between the two complexes is explained as follows: the Al³⁺ complex is monomeric, but the Zn²⁺ complex is dimeric. Quantum chemical calculations indicate a higher density of states near the locally excited state for the dimeric complex. This may lead to more efficient nonradiative pathways

Substituted versus Naked Thiourea Ligand Containing Pseudotetrahedral Cobalt(II) Complexes: A Comparative Study on Its Magnetization Relaxation Dynamics Phenomenon

A series of mononuclear tetrahedral cobalt­(II) complexes with the general molecular formula [Co­(L₁)₂X₂] [where L₁ = tetramethylthiourea ([(CH₃)₂N]₂CS) and X = Cl (<b>1</b>), Br (<b>2</b>), and I (<b>3</b>)] were isolated, and their structures were characterized by single-crystal X-ray diffraction. The experimental direct-current magnetic data are excellently reproduced by fitting both χ_M<i>T</i>(<i>T</i>) and <i>M</i>(<i>H</i>) simultaneously using the spin Hamiltonian (SH) parameters <i>D</i>_<b>1</b> = −18.1 cm^–1 and <i>g</i>_<b>1</b>,iso = 2.26, <i>D</i>_<b>2</b> = −16.4 cm^–1 and <i>g</i>_<b>2</b>,iso = 2.33, and <i>D</i>_<b>3</b> = −22 cm^–1 and <i>g</i>_<b>3</b>,iso = 2.4 for <b>1</b>–<b>3</b>, respectively, and the sign of <i>D</i> was unambiguously confirmed from X-band electron paramagnetic resonance measurements. The effective energy barrier extracted for the magnetically diluted complexes <b>1</b>–<b>3</b> (10%) is larger than the barrier observed for the pure samples and implies a nonzero contribution of dipolar interaction to the magnetization relaxation dynamics. The SH parameters extracted for the three complexes drastically differ from their respective parent complexes that possess the general molecular formula [Co­(L)₂X₂] [where L = thiourea [(NH₂)₂CS] and X = Cl (<b>1a</b>), Br (<b>2a</b>), and I (<b>3a</b>)], which is rationalized by detailed ab initio calculations. An exhaustive theoretical study reveals that both the ground and excited states are not pure but rather multideterminental in nature (<b>1</b>–<b>3</b>). Noticeably, the substitution of L by L₁ induces structural distortion in <b>1</b>–<b>3</b> on the level of the secondary coordination sphere compared to <b>1a</b>–<b>3a</b>. This distortion leads to an overall reduction in |<i>E</i>/<i>D</i>| of <b>1</b>–<b>3</b> compared to <b>1a</b>–<b>3a</b>. This may be one of the reasons for the origin of the slower relaxation times of <b>1</b>–<b>3</b> compared to <b>1a</b>–<b>3a</b>