A Binary Supramolecular Assembly with Intense Fluorescence Emission, High pH Stability, and Cation Selectivity: Supramolecular Assembly-Induced Emission Materials

We construct a fluorescent supramolecular system (TPE-Q4⊂ DSP5) of excellent tolerance to a wide range of pH by the facile self-assembly of a new pillar[5]arene bearing disulfonated arms (DSP5) with an AIE-active tetraphenylethene-based tetratopic guest bearing four quaternary ammonium binding sites (TPE-Q4), which exhibits strong blue emission even in dilute aqueous solutions along with much higher quantum yield and longer fluorescence lifetime than TPE-Q4 itself. This appreciable property can be attributed to the supramolecular assembly-induced emission (SAIE) mechanism endowed by the host-guest inclusion complexation based on synthetic macrocycles. Remarkably, the enhanced fluorescence of the supramolecular assembly is quenched efficiently and exclusively by ferric ions in water with a high Stern–Volmer formula constant of 1.3 × 105  mol−1, demonstrating the excellent cation selectivity and visualized responsiveness in ion sensing and detection.


Synthesis of TPE-Q4
Scheme S2. Synthetic route of the targeted compound TPE-Q4.

Synthesis of compound 4
A solution of bis(4-hydroxyphenyl)methanone (5 g, 23.4 mmol) and NaOH (3.7 g, 56 mmol ) in CH3CH2OH (250 mL) was stirred in a 500 mL round bottom flask. Then 1, 4-dibromobutane (16.7 mL) was added into the above solution and the mixture reacted at reflux temperature for 12 h. The crude product was washed with CH3CH2OH and water and dried under high vacuum oven to get a white powder (6.7g, 59%

Synthesis of compound 5
Znic (1.7 g) and titanium tetrachloride (1.7 mL) were added into dry tetrahydrofuran (100 mL) at 0 °C and the mixture was stirred for 30 min. Then compound 4 (2.4 g) was added into above solution and reacted at reflux temperature for 3 h. The crude product was separated, extracted and concentrated. The obtained product was subjected to column chromatograph (petroleum ether/dichloromethane, 2:1 v/v) to get a white powder (550 mg, 30%

Synthesis of TPE-Q
Scheme S3. Synthetic route of the targeted compound TPE-Q.

Synthesis of DSNa
Scheme S4. Synthetic route of the targeted compound DSNa.
A solution of butanesultone (12.0 g, 100 mmol) in 1, 4-dioxane (100 mL) was added into a solution of hydroquinone (4.4 g, 40.0 mmol) in aqueous NaOH solution (10 wt%, 60 mL). The mixture was stirred at room temperature for 12 h and filtered to collect the crude solid.

Host-guest interaction investigation
To quantitatively assess the inclusion complexation behavior, fluorescence titrations of DSP5 and TPE-Q guest were performed at 298 K in deionized water. Using a nonlinear least-squares curve-fitting method, the association constant was obtained for each host-guest combination from the following equation.

Calculation of radiative and non-radiative decay rate constants
Fluorescence typically follows first-order kinetics: [S] is the concentration of exited state molecules at time t, [S]0 is the initial concentration and is the fluorescence lifetime.
Decay rate (k) is the inverse of lifetime, consisting of radiative and non-radiative decay rate constants: where krad is the radiative decay rate constant and knrad is the nonradiative decay rate constant. The quantum yield (QE) is defined as the fraction of emission process in which emission of light is involved: The values of radiative and non-radiative rate constants of bare TPE-Q4 and TPE-Q4⊂DSP5 were tabulated in Table S1. Table S1. Emission lifetimes (τ), radiative decay rate constant (krad), and non-radiative decay rate constant (knrad) of bare TPE-Q4 and TPE-Q4⊂DSP5 in water.

Limit of detection
The limit of detection (LOD) was determined from the equation