Controllable FRET Behaviors of Supramolecular Host–Guest Systems as Ratiometric Aluminum Ion Sensors Manipulated by Tetraphenylethylene-Functionalized Macrocyclic Host Donor and Multistimuli-Responsive Fluorescein-Based Guest Acceptor

The novel multistimuli-responsive monofluorophoric supramolecular polymer Poly(TPE-DBC)/FL-DBA and pseudo[3]rotaxane TPE-DBC/FL-DBA consisted of the closed form of nonemissive fluorescein guest FL-DBA along with TPE-based main-chain macrocyclic polymer Poly(TPE-DBC) and TPE-functionalized macrocycle TPE-DBC hosts, respectively. By the combination of various external stimuli, these fluorescent supramolecular host–guest systems could reveal interesting photoluminescence (PL) properties in DMF/H2O (1:1, v/v) solutions, including bifluorophoric host–guest systems after the complexation of Al3+ ion, i.e., TPE-DBC/FL-DBA-Al3+ and Poly(TPE-DBC)/FL-DBA-Al3+ with their corresponding open form of fluorescein guest FL-DBA-Al3+. Importantly, the Förster resonance energy transfer (FRET) processes occurred in both bifluorophoric host–guest systems between blue-emissive TPE donors (λem = 470 nm) and green-emissive fluorescein acceptors (λem = 527 nm) after aluminum detection, which were further verified by time-resolved photoluminescence (TRPL) measurements to acquire their FRET efficiencies of 40.4 and 31.1%, respectively. Both supramolecular host–guest systems exhibited stronger green fluorescein emissions as well as appealing ratiometric PL behaviors within the desirable donor–acceptor distances of FRET processes in comparison with their detached analogous mixtures. Regarding the pH effects, the optimum green fluorescein emissions with effective FRET processes of all compounds and host–guest systems were sustained in the range pH = 7–10. Interestingly, both host–guest systems TPE-DBC/FL-DBA and Poly(TPE-DBC)/FL-DBA possessed high sensitivities and selectivities toward aluminum ion to display their strong green emissions via FRET-ON behaviors due to the chelation-induced ring opening of spirolactam moieties to become green-emissive guest acceptor FL-DBA-Al3+, which offered excellent limit of detection (LOD) values of 50.61 and 38.59 nM, respectively, to be further applied for the fabrication of facile test strips toward aluminum detection. Accordingly, the inventive ratiometric PL and FRET sensor approaches of supramolecular host–guest systems toward aluminum ion with prominent sensitivities and selectivities were well-established in this study.