Key words
mechanochemistry - fluorescence - polymers - cycloaddition - chromophores
The prerequisite for the comprehensive understanding of the mechanical properties of polymers is the correlation of their macroscopic mechanical properties with those of their molecular constituents. Optical force probes (OFPs) are force-responsive molecules (mechanophores)[1]
[2] that, when embedded into a polymer material, report mechanically induced transformations by the alteration of their optical properties.[3–5] Thereby, they are rendered promising candidates for this endeavor. Several OFPs have been reported to the literature including, but not limited to spiropyran,[6,7] dioxetane,[8]
[9] persistent delocalized radicals,[10]
[11]
[12] or aggregachromic dyes.[13]
[14]
[15] Diels–Alder adducts of anthracene and maleimide,[16]
[17]
[18] particularly those that bear extended π-electron systems (Figure [1]),[19]
[20]
[21] have been used to display forces with high sensitivity,[19]
[20]
[22]
[23] high spatial resolution,[21]
,
[24]
[25]
[26] and for the quantification[27] of bond-scission events.[21]
[25] Yet, the eventual success of OFPs will be determined not only by their properties and insights that can be obtained but also by their straightforward synthetic access and their ability to be incorporated within complex, nonuniform materials at desirable positions. Therefore, simplifications in both OFP synthesis and their conjugation to the polymer architecture are desirable.
Figure 1 Comparison of previously investigated π-extended anthracene OFP and the triazole-extended anthracene OFP of this work. Dashed bonds signify the connection to the polymer.
Here we report a new, synthetically facile approach to extend anthracene-maleimide Diels–Alder adduct OFPs with triazoles using the azide-alkyne Huisgen 1,3-dipolar cycloaddition (Figure [1]). By this, we equip these OFPs with an additional site for ‘click’ conjugation to the polymer architecture while maintaining a brightness comparable to the established π-extended anthracene fluorophores.
OFP synthesis was conducted with the reported[19]
[20]
[21] terminal alkyne Diels–Alder adduct 1 (Scheme [1]). This was subjected to a reductive azide–alkyne cycloaddition[28] using sodium l-ascorbate and CuSO4 together with 3-azido-1-propanol to form the OFP diol 2 with quantitative conversion and 70% yield[29] due to losses during workup (Figures S1–S3). Importantly, no purification besides collection of the product precipitate by filtration and washing was necessary. OFP diol 2 was then esterified using α-bromoisobutyryl bromide yielding the bifunctional initiator 3 for controlled radical polymerization (CRP, Figures S4–S6).[19]
[20]
[21] To assess the OFP performance, initiator 3 was incorporated into the center of linear poly(methyl acrylate) (PMA) chains via Cu0-catalyzed CRP[19]
[20]
[21] whereupon polymer 4 (Mn
= 60.9 kDa, ĐM
= 1.17, Figure [2a]) was obtained. It should be noted here that this reaction order was chosen to fully characterize the OFP in a defined polymer chain. In principle, both the Huisgen and Diels–Alder reactions can be performed using polymer-terminated azides or maleimides allowing access to complex conjugation sequences.
Scheme 1 Synthesis of triazole-extended anthracene OFP 3, its polymer 4, and the subsequent mechanochemical activation of 4
Figure 2 Monitoring of the sonochemical scission of 4. (a) GPC RI elugrams showing the molar mass distribution over sonication time. (b) UV/Vis spectroscopy showing the increasing absorption over sonication time. (c) Comparison of absorption and normalized fluorescence (λexc = 365 nm) before and after sonication.
To assess the mechanochemical performance of the OFP, a solution of polymer 4 was then subjected to pulsed (1 s ‘on’, 2 s ‘off’) ultrasonication using an immersion probe sonicator (20 kHz)[30] for low conversions to allow the initial slope approximation for rate constant determination. The proceeding mechanochemical reaction was followed by gel permeation chromatography (GPC) via refractive index (RI) detector and by UV/Vis absorption spectroscopy. Over the course of the sonication, a decrease of the peak molar mass of 4 (ca. 60 kDa) was observed and chain fragments at approximately half the initial peak molar mass emerged (Figure [2a]). This hinted towards the expected force-induced chain scission in the central region of the polymer chain.
The accompanying UV/Vis absorption measurements additionally revealed that a chromophore was generated during this process transitioning from a mostly transparent 4 only absorbing at λ < 300 nm to a species bearing the vibronic signature of anthracene between λ = 325–425 nm (Figure [2b]). Fluorescence spectroscopy was performed before and after the sonication and revealed a transition from an expectedly nonfluorescent 4 to a fluorescent species with an emission maximum at λem = 424 nm (Figure [2c]).
To verify the mechanochemical origin of the fluorogenic reaction, to determine the molar absorptivity ε, and to obtain the fluorescence quantum yield Φ of the emitter, control compound 6 resembling the activated mechanofluorophore was synthesized from 9-ethynylanthracene (5, Scheme [2], Figures S7–S9). The absorption spectra of 6 in MeCN were recorded with concentrations between 5–70 μmol L–1 and the measured absorbance at λ = 365 nm was plotted against the dye concentration (Figure [3a]). The linear fit with a fixed intercept at 0 resulted in ε
6
= 8148 L mol–1 cm–1 as indicated by the slope of the regression curve. A comparison with the spectral data derived from the sonication experiments (Figure [3b]) indicated a strong agreement between the two fluorophores both in absorption and emission. This corroborated a successful and reasonably selective force-induced cycloelimination reaction. Φ
6
was determined to 0.77 (MeCN, λexc = 365 nm) using an Ulbricht sphere. This value significantly exceeded the fluorescence quantum yield of regular anthracene with Φ = 0.27 (C6H6, λexc = 365 nm).[31] Φ
6
was comparable to 9-π-extended anthracene which we have reported before (cf. Figure [1]) where Φ = 0.72 (MeCN, λexc = 372 nm).[19] A comparable anthracene-triazole structure bearing a C8H17 chain reported in the literature also showed an increased Φ = 0.45 (MeCN)[32] indicating that extensions with triazoles could be a generally viable approach to achieve high fluorophore brightness in anthracene-based OFPs.
Scheme 2 Synthesis of control compound 6 by Huisgen cycloaddition
Figure 3 Optical properties of control dye 6. (a) Determination of the molar absorptivity ε
6
and (b) UV/Vis absorption and normalized fluorescence spectra of 4 after 80 min sonication and 6 (λexc = 365 nm, c
6
= 8.7 μmol L–1, MeCN).
The apparent sonochemical scission rate constants k were determined from GPC measurements (Figure [2a]) over the course of sonication using the Nalepa method[33]
[34]
[35] and via UV/Vis spectroscopy (Figure [2b]) using ε
6
.[19] Both approaches led to comparable values, hinting towards a preferred chain scission at the anthracene-maleimide OFP. Values obtained via GPC were slightly higher due to the contribution of nonspecific chain scission. The apparent scission rate constants were calculated to k
4,GPC = 2.13·10–3 min–1 and k
4,UV/Vis = 1.27·10–3 min–1 and were comparable to those measured for other 9-π-extended anthracene OFPs.[19]
[20]
[21] A notable contribution of the cycloelimination of the triazole moiety was not observed.[36]
[37]
In conclusion, we presented a new mechanofluorophore design based on a combination of the established 9-π-extended anthracene-maleimide Diels–Alder adducts and the 1,4-triazole moiety. The sonochemical bond scission in solution was observed by GPC, UV/Vis, and fluorescence spectroscopy. Both absorption and emission were only slightly bathochromically shifted compared to nonfunctionalized anthracene. Therefore, triazole extension in 9-positition of the anthracene led to little conjugation with the chromophore. However, the fluorescence quantum yield was significantly increased maintaining the brightness on a comparable level to previously reported 9-π-extended anthracenes.[19]
[20]
[21] The combination of two modular ‘click’ reactions potentially allows various reaction pathways to incorporate these OFPs at desirable positions in complex polymer materials and therefore expands the strategic toolbox for polymer mechanochemistry.
