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Mechanically-interlocked photosensitizer–quencher systems based on free-base tetraphenylporphyrin (H₂TPP)–gold nanoparticle (AuNP) composites has been designed and synthesized by utilizing a rotaxane architecture comprised of secondary ammonium and crown ether subunit. The H₂TPP-substituted 24-crown-8 was able to shuttle along the alkanethiolate axle, triggered by deprotonation/protonation at the ammonium station, altering the H₂TPP–AuNP distance and the photoexcitation energy transfer efficiency. Upon switching, quantum yields for photosensitized singlet oxygen (¹O${}_2)$ generation and fluorescence after deprotonation were quenched by 46% and 42%, respectively. External environment-responsive ¹O₂ generation based on such a protonation/deprotonation-driven molecular switch is potentially advantageous for a variety of applications including photodynamic therapies.

DNA, RNA and proteins are among the most important macromolecules in a living cell. These molecules are polymerized by molecular machines. These natural nano-machines polymerize such macromolecules, adding one monomer at a time, using another linear polymer as the corresponding template. The machine utilizes input chemical energy to move along the template which also serves as a track for the movements of the machine. In the Alan Turing year 2012, it is worth pointing out that these machines are "tape-copying Turing machines". We review the operational mechanisms of the polymerizer machines and their collective behavior from the perspective of statistical physics, emphasizing their common features in spite of the crucial differences in their biological functions. We also draw the attention of the physics community to another class of modular machines that carry out a different type of template-directed polymerization. We hope this review will inspire new kinetic models for these modular machines.