Photochemistry Controls Molecular Motion and Switching

Abstract

We chemists have always strived to control molecules.[1] An aspect of that control has been the emulation of larger-scale engineering phenomena, for example machinery of various kinds, on a molecular level. After all, the miniscule size of molecules allows the obvious advantage of operation in nanometric spaces where even modern engineering objects struggle to go. Then, engineering concepts can be applied in some of the smallest spaces relevant to biology. Light offers a direct way of communicating our commands to the molecules in this small space, providing a power supply, and also a way of indicating their responses back to us. So photochemistry becomes a powerful platform for developing molecular machinery.[2] A classical chemical education told us that photochemistry concerned the ability of light to break (and make) covalent bonds within molecules. We now realize this was its brutal face. It also has a gentle face, where the only damage done (at least initially) is the translocation of a tiny electron. However, this stores substantial energy within the molecule, besides bringing an electric field into being. Such a photoinduced electron transfer (PET),[3] so dominant in green plant photosynthesis, now stands ready to lend its power to our machine designs. It is particularly apt that Credi[4] summarizes the literature pertinent to this Research Front by distilling several decades of Bolognese photochemical wisdom for us. Photochemical bond-breaking and -making is involved in the elegant switching of photoactivity of porphyrins (Straight et al.[5]) and quantum dots (Tomasulo et al.[6]). Interestingly, the coloured form of the photochrome[7] is responsible for switching ‘on’ in the former case and ‘off’ in the latter. Electronic energy transfer plays a dominant role in both these cases. PET takes