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. 2022 Mar 1;119(9):e2121746119.
doi: 10.1073/pnas.2121746119.

Visualizing molecular weights differences in supramolecular polymers

Qingyun Li  1 , Hanwei Zhang  1 , Kai Lou  1 , Yabi Yang  1 , Xiaofan Ji  2 , Jintao Zhu  2 , Jonathan L Sessler  3
Affiliations

Visualizing molecular weights differences in supramolecular polymers

Qingyun Li et al. Proc Natl Acad Sci U S A. .

Abstract

Issues of molecular weight determination have been central to the development of supramolecular polymer chemistry. Whereas relationships between concentration and optical features are established for well-behaved absorptive and emissive species, for most supramolecular polymeric systems no simple correlation exists between optical performance and number-average molecular weight (Mn). As such, the Mn of supramolecular polymers have to be inferred from various measurements. Herein, we report an anion-responsive supramolecular polymer [M1·Zn(OTf)2]n that exhibits monotonic changes in the fluorescence color as a function of Mn Based on theoretical estimates, the calculated average degree of polymerization (DPcal) increases from 16.9 to 84.5 as the monomer concentration increases from 0.08 mM to 2.00 mM. Meanwhile, the fluorescent colors of M1 + Zn(OTf)2 solutions were found to pass from green to yellow and to orange, corresponding to a red shift in the maximum emission band (λmax ). Therefore, a relationship between DPcal and λmax could be established. Additionally, the anion-responsive nature of the present system meant that the extent of supramolecular polymerization could be regulated by introducing anions, with the resulting change in Mn being readily monitored via changes in the fluorescent emission features.

Keywords: J-aggregate; fluorescence; molecular weight; self-assembly; supramolecular polymers.

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Conflict of interest statement

The authors declare no competing interest.

Figures

Fig. 1.
Fig. 1.
Schematic representation of the anion-responsive supramolecular polymer of this study. Chemical structure of supramolecular monomers and cartoon representations of the proposed supramolecular polymerization process and anion-responsive behavior and the various fluorescent colors expected as the degree of intermonomer interaction increases.
Fig. 2.
Fig. 2.
Complexation study and degree of polymerization calculation. (A) UV-vis spectra of mixtures of Zn(OTf)2 and 1a in DMF/H2O (1/4, vol/vol) at different molar ratios. The total concentration of 1a and Zn(OTf)2 was kept at 5.00 μM. (Inset) chemical structure of compound 1a. (B) Job’s plot of compound 1a and Zn(OTf)2. (C) UV-vis spectra of compound 1a (0.280 μM) in 1.00 mL DMF/H2O (1/4, vol/vol) recorded upon the stepwise addition of Zn(OTf)2 (100 μM) at 298 K. (D) Plot of (A0/(A − A0) as a function of 1/[Zn(OTf)2]. The apparent association constant Ka corresponding to the interaction between 1a (0.280 μM) and Zn(OTf)2 (100 μM) was determined using the Benesi–Hilderbrand equation A0/(A − A0) = (A0/(Amax − A0))((1/Ka) [Zn(OTf)2]−1 + 1). (E) The degree of polymerization calculated (DPcal) for equimolar mixtures of M1 and Zn(OTf)2 (0.08 to 2.00 mM).
Fig. 3.
Fig. 3.
Morphology, fluorescence behavior, and proposed determinants of the observed M1 + Zn(OTf)2 emission colors. (A) SEM images, (B) AFM images, and (C) LSCM images of the equimolar mixtures of M1 and Zn(OTf)2 at different concentrations (0.08, 0.16, 0.20, 0.40, 0.80, 1.20, 1.60, and 2.00 mM in DMF/H2O (1/4, vol/vol)). λex = 405 nm. (D) Photographs of equimolar mixtures of M1 and Zn(OTf)2 (0.08 to 2.00 mM) under UV light. (E) Cartoon representations of the self-assembly process thought to govern equimolar mixtures of M1 and Zn(OTf)2 and the change in the fluorescence color observed as the concentration is increased. (Scale bars: A, 200 μm; B, 20.0 μm; C, 100 μm.)
Fig. 4.
Fig. 4.
Fluorescence characterization of [M1·Zn(OTf)2]n (A) Normalized fluorescent spectra of equimolar mixtures of M1 and Zn(OTf)2 in the concentration range (0.08 to 2.00 mM in DMF/H2O [1/4, vol/vol]) at 298 K. λex = 410 nm. Slit: 10/15. (B) Table showing the relationship between λmax and monomer concentration.
Fig. 5.
Fig. 5.
Relationship between λmax and DPcal. (A) Table and (B) plot of the corresponding dependent correlation between λmax and DPcal.
Fig. 6.
Fig. 6.
Morphology, fluorescence behavior, and origin of the M1-derived fluorescence. (A) SEM images, (B) AFM images, and (C) LSCM images of M1 solutions at different concentrations (0.04, 0.08, 0.10, 0.20, 0.40, 0.60, 0.80, and 1.00 mM in DMF/H2O [1/4, vol/vol]). λex = 405 nm. (D) Photographs of M1 solutions (0.04 to 1.00 mM) recorded using a hand-held UV lamp. (E) Cartoon representations showing the various limiting self-assembled form of M1 and the fluorescence color changes expected as the concentration of the monomers is increased. (Scale bars: A, 100 μm; B, 20.0 μm; C, 100 μm.)
Fig. 7.
Fig. 7.
Fluorescence characterization of M1. Normalized fluorescent spectra of solutions of M1 recorded from 0.04 to 1.00 mM in DMF/H2O (1/4, vol/vol) at 298 K. λex = 410 nm. Slit: 10/15.
Fig. 8.
Fig. 8.
Reaction mechanism, morphology, and fluorescence behavior of M1 + Zn(OTf)2 and TBAOH. (A) Cartoon representations of the reaction mechanism between [M1·Zn(OTf)2]n (2.00 mM) and 20.0 equiv. of TBAOH (5.00 mM), both in DMF/H2O (1/4, vol/vol). (B) SEM image, (C) AFM image, and (D) LSCM image of M1 + Zn(OTf)2 in DMF/H2O (1/4, vol/vol) before and after the addition of TBAOH. (E) Normalized fluorescent spectra of M1 + Zn(OTf)2 (2.00 mM) and a mixture M1 + Zn(OTf)2 (2.00 mM) and TBAOH (5.00 mM) at 298 K. λex = 410 nm. (Inset) Photographs of M1 + Zn(OTf)2 in DMF/H2O (1/4, vol/vol) before and after the addition of TBAOH recorded under a hand-held UV lamp. Slit: 10/15. (Scale bars: B, 20.0 μm; C, 20.0 μm; D, 100 μm.)
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