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.