A novel approach towards crown-ether modified polyimides with affinity for alkali metal ions recognition

In the attempt to enlarge the family of imide-based materials suitable for sensor applications, novel five- and six-membered thermostable polyimides functionalized with benzo-15-crown-5 ether side chains were synthesized and thoroughly investigated with regard to their affinity for alkali metal ions recognition. For this purpose, a new diamine monomer containing as receptor triphenylmethane modified with 15-crown-5 ether (called by us crowned TPM) has been synthesized and structurally characterized. For the first time, we brought evidence for the occurrence of a ground-state intermolecular charge transfer (CT) complex between the donor crowned TPM diamine and the acceptor diimide segment which is actually responsible for the polyimides strong light emission in the green to yellow spectral range. All polyimides were electrochemically active in both anodic and cathodic regions, their voltammetric response being consistent with the coplanarity and electron accepting strength of the diimide segment. In the presence of alkali metal ions some changes appeared in the electronic absorption signals, although quite diffuse. Instead, these cations significantly altered the fluorescence response of polyimides in dependence on the excitation wavelength, cation type and polymer structure. These mostly caused the occurrence of a new emission band, when the green or yellow fluorescence turned in blue, thus proving the capability of our polyimides to detect alkali metal ions.