Twisted Hemithioindigo Photoswitches: Solvent Polarity Determines the Type of Light-Induced Rotations

Abstract

Controlling the internal motions of molecules by outside stimuli is a decisive task for the generation of responsive and complex molecular behavior and functionality. Light-induced structural changes of photoswitches are of special high interest due to the ease of signal application and high repeatability. Typically photoswitches use one reaction coordinate in their switching process and change between two more or less-defined states. Here we report on new twisted hemithioindigo photoswitches enabling two different reaction coordinates to be used for the switching process. Depending on the polarity of the solvent, either complete single bond (in DMSO) or double bond (in cyclohexane) rotation can be induced by visible light. This mutually independent switching establishes an unprecedented two-dimensional control of intramolecular rotations in this class of photoswitches. The mechanistic explanation involves formation of highly polar twisted intramolecular charge-transfer species in the excited state and is based on a large body of experimental quantifications, most notably ultrafast spectroscopy and quantum yield measurements in solvents of different polarity. The concept of pre-twisting in the ground state to open new, independent reaction coordinates in the excited state should be transferable to other photoswitching systems.

The Effect of Substituent‐Dependent Photoinduced Intramolecular Charge Transfer on the Photochromism of Hemithioindigos

“…We assume that a complete separation of HOMO and LUMO leads to a corresponding weak electronic coupling between donor and acceptor and a larger dipole moment in the TICT state, which is much lower than for the S 1Min state. A similar behavior has also been described for N ‐aryl‐substituted trans ‐aminostilbenes and HTI derivatives . During the isomerization of OMe‐substituted HTI, the TICT state (a single bond torsion of about 90° in strong D–A systems) is much easier to populate after the S 1Min state than C=C torsion.…”

Picosecond Switchable Azo Dyes

“…The temporal evolution of the PPA spectrum, monitored at 575 nm, reveals rather complex dynamics (Figure , b and e ; Table ). Up to the ≈20 ps scale, we have fitted the decay with a global fitting routine, which some of us have used previously for other photoswitches . We have four time constants (Table , τ ) that are characterized by an amplitude that changes with the wavelength (see the Supporting Information, Figure S4 and S5, decay‐associated spectra (DAS)).…”

“…Up to the % 20 ps scale, we have fitted the decay with ag lobal fitting routine, which some of us have used previously for other photoswitches. [44] We have four time constants (Table 1, t)t hat are characterizedb ya na mplitude that changes with the wavelength (see the Supporting Information, Figure S4 and S5, decay-associated spectra (DAS)). The uncertainty in the time constants is mainly given by the pulse duration of 100 fs, and the t 1 and t 2 constants become difficult to distinguish in some cases because they only differ by 100-300 fs.…”

Metal Complexes of Indigo and of Some Related Ligands

“…It may be noted that Dube and co‐workers, applied a derivative of thioindigo, hemithioindigo, for the development of a novel light driven molecular motor …”