Molecular machines represent the quintessential realization of Feynmans 1959 vision - to fill the “plenty of room at the bottom” with intricate functions. We have contributed to this rapidly evolving field by generating fast and visible light driven molecular motors that are highly promising for applications not tolerant of damaging UV light or high temperatures.1 After developing new synthetic methods for their manufacturing and gaining control over their maximum rotation speeds2 we recently provided a comprehensive mechanistic picture of their mode of action.3 From these studies crucial factors to influence their efficiencies and future design opportunities could be gleaned. Our current efforts are directed at applications in integrated molecular machines and implementation of our motors into more complex environments.4
A very recent breakthrough in our laboratory is the development of an entirely new type of molecular motor working without thermal ratcheting steps in the ground state. This photon-only driven motor is powered by three consecutive light-driven steps and exhibits a reverse temperature dependency of its efficiency, i.e. it becomes faster at lower temperatures instead of slower. This development will open up new avenues for molecular machines and their applications in the near future.5
In this research line we develop and study functional supramolecular systems that can be controlled in their properties by external stimuli. We strive to go beyond the sole establishment of molecular recognition processes and implement responsive elements for smart and emerging behavior.
To this end we have created different photoresponsive receptor1 and molecular tweezers motives, which we can reversibly switch between high and low affinity states using visible light signals. In a recent effort we were able to elicit a complex and dynamic guest relocation in solution by realizing a new concept: “simultaneous complementary photoswitching”.2 Two complementary substituted molecular tweezers respond to the same wavelength of irradiation in opposite manners. If the first tweezers gain binding affinity the second tweezers lose it at the same time, leading to relocalization of the guest from one host to the other. At a different wavelength of light irradiation the binding affinities and guest residing can be reversed. Only minimal signaling is needed to obtain a complex supramolecular behavior as the result.
One core activity in our research concerns chromophore design and mechanistic studies to develop new photoresponsive molecules with unique property profiles. Our long-term goal is gaining absolute control over light-induced molecular motions enabling full spatial and temporal resolution of nano-, micro-, and macroscopic properties.
Focusing on the underexplored class of indigoid photoswitches1 we have established specific molecular designs, which allow us to evoke a range of distinctive bond rotations by irradiation and directly prove them experimentally. Using simple means, like solvent polarity or temperature, different types of such rotations can be interchanged within the same molecule providing exquisite control over multiple molecular motions. Examples are polarity dependent single or double-bond rotation in donor-substituted hemithioindigo2 or the long elusive hula twist, which we evidenced unambiguously in an axially chiral molecular setup.3 Apart from providing unprecedented insights into fundamental photochemical mechanisms these molecular systems possess especially high potential for the construction of complex and unique future nanomachinery.
In a second research line we are developing highly efficient bistable photoswitches with red light responsiveness, which are interesting for a variety of applications ranging from material sciences to biology and photopharmacology. Particularly impressive performances are given by donor-substituted hemiindigo chromophores allowing photoswitching at the biooptical window with half-lives of the metastable forms beyond 1000 years.4,5