Mechanochemical generation of singlet oxygen

Abstract

Controlled generation of singlet oxygen is very important due to its involvement in scheduled cellular maintenance processes and therapeutic potential. As a consequence, precise manipulation of singlet oxygen release rates under mild conditions, is crucial. In this work, a cross-linked polyacrylate, and a polydimethylsiloxane elastomer incorporating anthracene-endoperoxide modules with chain extensions at the 9,10-positions were synthesized. We now report that on mechanical agitation in cryogenic ball mill, fluorescence emission due to anthracene units in the PMA (polymethacrylate) polymer is enhanced, with a concomitant generation of singlet oxygen as proved by detection with a selective probe. The PDMS (polydimethylsiloxane) elastomer with the anthracene endoperoxide mechanophore, is also similarly sensitive to mechanical force.

Fig. 1. Synthesis scheme for the monomer 5.

Fig. 2. The structure and the preparation of PMA-EPO (anthracene-endoperoxide polymethacrylate copolymer) and PDMS-EPO elastomer.

Fig. 3. Application of mechanical stress leads to the emergence of fluorescent anthracene cores presumably via cycloreversion with a concomitant singlet oxygen release. Distortion of the endoperoxide bridgehead positions is expected initiate cycloreversion. Inset pictures: appearance of the polymer under irradiation with a hand held-UV lamp at 360 nm, left before (left), and after (right) cryomilling.

Fig. 4. Evolution of fluorescence emission spectra of the singlet oxygen probe SOSG in a cryomill: (a) singlet oxygen sensor milled alone for 10 min, and (b) 20 min; PMA-EPO (240 mg) together with singlet oxygen sensor (c) after 10 min, (d) and 20 min of milling. Singlet oxygen sensor concentration was 5 μM, in 3 mL DMSO. Excitation wavelength is 480 nm.

Fig. 5. Application of mechanical stress on PDMS-EPO strips leads to the enhancement of fluorescence as mechanically triggered cycloreversion generates singlet oxygen and diphenylanthracene cores. (a) Strips before (left) and after (right) being subjected to oscillating force in DMA at 25 °C, (b) strips before (left) and after (right) being subjected to oscillating force in DMA at 45 °C, (c) mechanical force introduced in rheometer also triggers an enhancement of the fluorescence emission at 25 °C (before, left; after, right). Pictures were taken while the strips were illuminated at 360 nm with a hand-held UV lamp.