Retro Diels-Alder reaction, self-assembly and polymerization. The target bis-diacetylene derivative 1-endo was readily prepared by employing the general strategy developed for the synthesis of endo isomers of DA adducts using furans and maleimides, and it was fully characterized by using spectroscopic methods. The exo isomer 1-exo, as well as the furan-containing diacetylene F and maleimide M were also prepared for comparison purposes (see Supplementary Information and Supplementary Figs. 1–6). Analysis of differential scanning calorimetry (DSC) thermograms revealed that rDA reaction of 1-endo takes place at ca. 100 oC while a higher temperature of ca. 120 oC is required for rDA reaction of 1-exo (Supplementary Figs. 7a and 7b). As a consequence of this observation and the expectation that steric hinderance to prevent molecular assembly would be greater in the endo form, we selected 1-endo as the substance probed in further studies.
In order to gain information about rDA-promoted self-assembly, 1-endo coated filter paper was prepared by using the drop-casting method (Figs. 3a, i). Upon irradiation with a common hand-held laboratory UV lamp (254 nm, 1 mw/cm2, 1 min), the 1-endo-immobilized filter paper does not undergo an observable color change (Fig. 3a, ii). This finding indicates that the diacetylene moieties in 1-endo are not properly aligned for topochemical polymerization as a result of steric effects caused by its rigid polycyclic nature. Interestingly, an intense blue-color appears when the 1-endo coated filter paper is subjected to heating at 110 oC for 5 min (Fig. 3a, iii) followed by UV irradiation (254 nm, 1 mw/cm2) for 1 min (Fig. 3a, iv). Observation of the blue color demonstrates that PDAs are generated by UV irradiation33. PDA formation was further confirmed by using visible absorption spectroscopy, which showed that irradiation brings about formation of the typical absorption spectrum of blue-colored PDA with an absorption maximum at 620 nm (Supplementary Fig. 8).
Although PDA formation is promoted by first subjecting 1-endo-immobilized filter paper to thermally stimulated rDA reaction followed by cooling to room temperature and UV irradiation, we found that photopolymerization is more efficient (greater intensity of the 620 nm absorption band, Supplementary Fig. 8) when the heat-treated paper is placed in a freezer for 10 min or kept at room temperature for 20 min prior to irradiation. Moreover, the results of analysis of UV-irradiation-dependent absorbance curves suggest that the intensity of the absorption band at 620 nm reaches a maximum after 1 min irradiation (Supplementary Fig. 9). Analysis of the residual material after photopolymerization shows that ca. 40% of the rDA products F and M are converted to the corresponding PDA (Supplementary Fig. 10). Incomplete conversion to polymer is mainly caused by the fact that UV-induced polymerization occurs on and near the surface of the filter paper. The furan and maleimide moieties located inside the filter paper where UV light is unreachable remain unpolymerized. It is evident from inspection of the Raman spectrum shown in Fig. 3c (blue line) that no monomeric diacetylene bands are present after UV irradiation, indicating that almost complete polymerization occurs in the UV-exposed area.
Examination of the powder XRD spectrum obtained after rDA reaction showed the existence of a relatively sharp peak at 2θ = 23.12, which corresponds to a molecular repeat distance of 3.8 Å (Supplementary Fig. 11). This calculated repeat distance is in good agreement with the optimum geometrical parameter expected for PDA formation34. It should be noted that no sign of polymerization was observed upon sequential thermal and photoirradiation treatment of 1-exo coated filter paper under identical conditions (Supplementary Fig. 12).
To gain further information about the efficiency of the rDA-promoted self-assembly approach, a folded-in-half piece of 1-endo coated filter paper was placed on a hot plate at 110 oC for 5 min (Fig. 3b, i) and then entirely irradiated using 254 nm UV light for 1 min (Fig. 3b, ii). The images show that only the heat-treated area displays a blue color, indicating generation of PDA only in the heat-exposed area. Formation of a PDA on the resulting blue colored part of the filter paper was confirmed by the observation that (Fig. 3c) Raman bands associated with a conjugated ene-yne moiety at 1458 cm− 1 and 2074 cm− 1 exist in the Raman spectrum of the paper subjected to both heat and UV treatment (Fig. 3c, blue line) while only a monomeric diacetylene band at 2264 cm− 1 is present in the spectrum of the white part of the paper (Fig. 3c, black line). The finding that the PDA is generated selectively in only the heat-treated area is intriguing in that it suggests that it will be possible to create patterned PDA images by utilizing localized and controlled heating and UV irradiation. The validity of this proposal was assessed in preliminary experiments in which PDA and a star shape image were created by subjecting 1-endo coated filter papers to patterned hot metal wires followed by UV irradiation (Supplementary Fig. 13).
1H NMR spectroscopic analysis was conducted to prove that heat treatment of 1-endo produces the diacetylene-containing furan F and maleimide M through rDA reaction (Fig. 4a). rDA reaction was carried out by heating a glass substrate coated with 1-endo at 110 oC for 1 min. The 1H NMR spectrum of 1-endo prior to heating contains a characteristic AB quartet at 4.89 and 4.65 ppm that corresponds to methylene protons labeled c in Fig. 4b (i), and two multiplets at 3.82 and 3.55 ppm associated with the ring fusion protons a and b. Heat treatment of 1-endo at 110 oC for 1 min results in the generation of a new peak in the 1H NMR spectrum at 5.06 ppm, which corresponds to the c1 protons of the furan F (Fig. 4b (ii)). In addition, the peak arising at 6.85 ppm (assigned as a1) is associated with the maleimide vinyl protons in M. Comparison of 1H NMR spectra of heat-treated material and independently synthesized furan F (Fig. 4b (iii)) and maleimide M (Fig. 4b (iv)) confirms that F and M are cleanly generated by rDA reaction of 1-endo.
To gain more information about the nature of the rDA reaction, product distributions were determined for reactions of solid samples of 1-endo on a glass substrate at various temperatures and fixed times (1 min). It is clear from the appearance of the characteristic furan and maleimide protons (eg., c1 and a1) in the 1H NMR spectra (Fig. 5) that rDA reaction occurs at 90 oC. A further increase of the temperature to 110 oC results in an increase in the amount of furan F and maleimide M produced. In addition, endo-to-exo thermal isomerization begins to take place when the temperature reaches to 120 oC, as is evidence by peaks for the fused ring protons g and h at 3.14 and 3.05 ppm. The data obtained with 1H NMR spectral analyses at various reaction temperatures allowed calculation of the product distribution and the results are summarized in Fig. 6a. As displayed in Fig. 6a, the amount of starting 1-endo decreases as the reaction temperature increases, the quantities of the rDA products F and M increase up to 130 oC and then slightly decrease at 140 oC, and the degree of endo-exo isomerization steadily increases from 120 to 140 oC. The time dependence of the product distribution was also evaluated for reactions at a fixed temperature (110 oC) (Fig. 6b) (see also Supplementary Fig. 14). The results demonstrate that 1-endo consumption reaches ca. 50% after 1 min heating and reaches a maximum of 85% after 5 min. Moreover, the amounts of rDA products F and M reach maximum values after 5 min heating and steadily decrease upon continued heating. Isomerization product 1-exo forms after 5 min heating and reaches a maximum after 10 min. The combined results of temperature and time dependent product distribution studies suggest that heat treatment of 1-endo at 110 oC for 1 min is optimum for efficient rDA reaction. It should be noted that more time (ca. 5 min) for optimum rDA reaction of 1-endo when it is coated on a filter paper. This difference is mainly a result of slower heat transfer of filter paper as compared to that of a glass substrate, and that 1-exo is not formed by heating at 110 oC for 5 min (Supplementary Fig. 15).
DSC scans of a sample formed by rDA reaction of 1-endo display two endothermic peaks at 46 and 50 oC (Supplementary Fig. 16a). The peak at 46 oC corresponds to unreacted 1-endo and the one at 50 oC corresponds to a mixture of F and M. Thus, the rDA reaction generates a homogeneous mixture of F and M without a phase separation. This suggestion is supported by observing a single peak at 54 oC in the DSC thermogram, obtained after heating of a 1: 1 mixture of F and M at 110 oC for 1 min (Supplementary Fig. 16b). In addition, two endothermic peaks at 39 and 51 oC are present in the DSC scan of a 1:1:1 mixture of 1-endo, F and M after heating at 110 oC for 1 min (Supplementary Fig. 16c). The slightly different peak position temperatures are likely due to the existence of slightly different molecular environment.
Cysteine-promoted color change of PDA. Among the interesting properties of PDA is the well established blue-to-red color transition they undergo in response to physical and chemical/ biochemical stimuli. This feature is used extensively in the design of PDA-based colorimetric sensors35–38. The PDA produced by submitting 1-endo to sequential rDA-UV irradiation contains maleimide moieties in the polymer framework (see Fig. 2d). Maleimide derivatives are well known to undergo facile formation of adducts via 1,4-Michael addition reactions with thiols, a process that serves as the basis for the design of biothiol-specific chemosensors39. As part of an effort aimed at exploring its use as a thiol sensor, we observed that the blue-colored filter paper containing the new PDA derived from 1-endo undergoes a blue-to-red color change upon exposure to 10 mM of cysteine (Fig. 7a). In Fig. 7c is displayed the structure of thiol-maleimide adduct formed within the PDA structure. Importantly, no color change is promoted by other amino acids, indicating that the sensor system is selective for the thiol-containing amino acid. The cysteine-induced blue-to-red color change of PDA is accompanied by absorption spectral changes in the form of a decrease in the band at 620 nm and a simultaneous increase in a band at 535 nm (Fig. 7b).
It is difficult to obtain direct evidence to prove that thiol-maleimide adduct formation has occurred when the PDA derived from 1-endo is treated with cysteine because of the very small amount product generated. As a result, indirect evidence was accumulated through FT-IR spectroscopic analysis of a PDA prepared using self-assembled M. Thiol-maleimide adduct formation was demonstrated by the observation of a shift of the maleimide C = O stretching band from 1685 cm− 1 to 1712 cm1 and loss of the maleimide C = C stretching vibration at 1601 cm− 1 after exposure of the PDA to cysteine (Fig. 7d). Also, a blue-to-red color transition and associated absorption spectral changes of the self-assembled M derived PDA occurs when it is treated with cysteine with similar absorption spectral changes (Supplementary Fig. 17). The results strongly suggest that the cysteine-promoted colorimetric response of the PDA, derived from rDA reaction of 1-endo, is a consequence of 1,4-Michael addition reaction of the maleimide moieties.
Thermochromic reversibility assessment. Another fascinating feature of the PDA generated using the rDA-based self-assembly method arose from an investigation of the thermochromic response. Thermochromism of PDA has been a subject of keen interest in the context of mechanisms and practical applications40–43. Most PDAs display an irreversible color change when subjected to a heating-cooling cycle. Thus, the initial blue color is not regenerated after removal of a heat source that is used to promote the blue-to red color conversion. We and others demonstrated that the presence of strong headgroup interactions in the PDA can lead to reversible thermochromism because they enable the headgroups to serve a “molecular clamp” that facilitate return of the polymers to its initial structural arrangement even after it has been partially distorted44. In the current study, we found that the blue colored PDA obtained by the sequential rDA-UV irradiation of 1-endo displays reversible thermochromism (blue-to-purple) between 25 and 40 oC (Fig. 8a). To our surprise, the temperature range of colorimetric reversibility increases up to 60 oC when the UV-induced polymerization to form the PDA is carried out after annealing the rDA product for 18 h at 35 oC (Fig. 8b). In addition, 18 h annealing at 45 oC before UV irradiation leads a further increase of the colorimetric reversibility temperature. Accordingly, the color change remains reversible even up to 80 oC (Fig. 8c).
The unusual annealing temperature-dependent nature of the reversibility of thermochromism of the new PDA is very intriguing because, as stated above, most of the PDA systems described thus far incorporate special head group interactions to facilitate return to the original polymer. The unique features are believed to be associated with the propensity for DA reaction between the maleimide and furan head groups in the PDA (Fig. 8d). Accordingly, the furan and maleimide moieties in the self-assembled products F and M formed by rDA (Fig. 8d, top), can undergo DA reaction during annealing (Fig. 8d, middle). UV-induced polymerization of the assembled adducts of this process should yield a PDA that has a different structure (Fig. 8d, bottom) than that produced directly from the individually assembled products F and M. In this way, the DA adduct functions as a molecular clamp to facilitate return of the PDA to its initial (blue) conformation after removal of the heat source. Indeed, evidence for reformation of DA adducts by heat treatment of rDA products was gained by observing that 1-endo and 1-exo are produced during the annealing process (Supplementary Fig. 18). As can be seen by inspecting Supplementary Fig. 18, the amounts of F and M in the product mixture formed by heat promoted rDA decrease, and a simultaneous increase occurs in the amounts of DA adducts 1-endo and 1-exo as the annealing temperature is increased, resulting in the increased thermochromic reversibility temperature.