Spacer Length Effect on the Photoinduced Electron Transfer Fluorescent Probe for Alkali Metal Ions

Abstract— We have synthesized four derivatives of aikyi pyrene covalently bonded to aza-lS-crown-6 at the nitrogen position, Py(CH₂)_n, (n = 1–4), to study the effect of spacer length on the emission properties of pyrene fluorophore upon complication of alkali metal ions by the crown moiety. In the absence of alkali metal ions, the parent molecule is weakly fluorescent because its emission is partially quenched by photoinduced electron transfer (PET) from nitrogen lone pairs to the excited singlet state of pyrene. Complication of alkali metal ions (e.g. K⁺) by the crown moiety prevents the nitrogen lone pair from participating in PET and results in an enhancement in the observed emission from pyrene (fluorescent turn on). Because the PET effect could be exerted through bonds as well as space, its magnitude may show a dependence on chain length. We have examined the fluorescence behavior of these pyrene aza-crown ether derivatives in the presence of alkali metal ions to determine the magnitude of such an effect and its impact on the sensitivity of the fluorescent probe for detection purposes. Our results indicate that maximum efficiency for PET between the pyrene moiety and aza-crown ether is achieved when n ≤ 3.