Lithium sensors based on photophysical changes of 1-aza-12-crown-4 naphthalene derivatives synthesized Buchwald-Hartwig amination

Lithium detection is of great significance in many applications. Lithium-sensing compounds with high selectivity are scarce and, if any, complicated to synthesize. We herein report a novel yet simple compound that can detect lithium ions in an organic solvent through changes in absorbance and fluorescence. Naphthalene functionalized with 1-aza-12-crown-4 ( 1 ) was synthesized via one step from commercially available 1-bromonaphthalene through Buchwald-Hartwig amination. In order to obtain a structure-property relationship, we also synthesized two other compounds that are structurally similar to 1 , wherein the compounds 2 and 3 include an imide moiety (an electron acceptor) and do not include a 1-aza-12-crown-4 unit, respectively. Upon the addition of lithium ions, compound 1 displayed a clear isosbestic point in the absorption spectra and a new peak in the fluorescence spectra, whereas the compounds 2 and 3 indicated miniscule and no spectroscopic changes, respectively. 1 H NMR titration studies and the calculated optimized geometry from density functional theory (DFT) indicated the lithium binding on the aza-crown. The calculated limit of detection (LOD) was 21 μM. The lithium detection with 1 is selective among other alkali metals (Na + , K + , and Cs + ). DFT calculation indicated that the lone pair electrons in the nitrogen atom of 1 is delocalized yet available to bind lithium, whereas the nitrogen lone pair electrons of 2 showed significant intramolecular charge transfer to the imide acceptor, resulting in a high dipole moment, and thus were unavailable to bind lithium. This work elucidates the key design parameters for future lithium sensors. Lithium sensor based on 1-aza-12-crown-4 naphthalene that can detect lithium ions through absorption and emission changes with the detection limit of 21 μM in an organic solvent.