No abstract

Some rare indigo derivatives have been known for a long time to be photochromic upon irradiation with red light, which should be advantageous for many applications. However, the absence of strategies to tune their thermal half-lives by modular molecular design as well as the lack of proper synthetic methods to prepare a variety of such molecules from the parent indigo dye have so far precluded their use. In this work, several synthetic protocols for N-functionalization have been developed, and a variety of N-alkyl and N-aryl indigo derivatives have been prepared. By installation of electron-withdrawing substituents on the N-aryl moieties, the thermal stability of the Z-isomers could be enhanced while maintaining the advantageous photoswitching properties upon irradiation with red light (660 nm LED). Both experimental data and computational results suggest that the ability to tune thermal stability without affecting the dyes' absorption maxima originates from the twisted geometry of the N-aryl groups. The new indigo photoswitches reported are expected to have a large impact on the development of optical methods and applications in both life and material sciences.

The success of photopharmacology is inevitably tied to the availability of photoswitches, which can be operated within the biological window (λ=650-1450 nm) to maximize penetration in tissue. A general design strategy has been devised and a dihydropyrene derivative is described here that displays negative T-type photochromism, allowing for efficient and nearly quantitative (95 %) switching induced by NIR light λ>800 nm. The thermal half-life of the decolored ring-open meta-cyclophanediene isomer ranges from minutes to hours, depending on the solvent polarity and hence serves as a probe of the local environment. Due to the rather subtle geometrical differences between the two isomers, suitably modified NIR photoswitches are potential candidates for switching when bound in the pocket of the biological target, in principle allowing for reversible light-induced inhibitor deactivation as an alternative approach to externally regulate biological functions.

The use of low-intensity NIR light to operate molecular switches offers several potential advantages including enhanced penetration into bulk materials, in particular biological tissues, and reduced radiation damage due to the limited photon energies. The latter, however, pose a challenge for designing reasonably bistable systems. We have developed a general design strategy for direct one-photon NIR photoswitches based on negative photochromic dihydropyrenes carrying opposing strong donor−acceptor substituents either along the long axis of the molecule or across it. Thus, two series of 2,7-and 4,9-disubstituted dihydropyrenes were synthesized, and their photothermal properties investigated as a function of the type, strength, and position of the attached donor and acceptor substituents as well as the polarity of the environment. By shifting the excitation wavelength deep into the NIR, both NIR one-photon absorption cross-section and photoisomerization efficiency could be maximized while retaining a reasonable thermal stability of the metastable cyclophanediene isomer. Thus, the lowest optical transition was shifted beyond 900 nm, the NIR cross-section was enhanced by two orders of magnitude, and the thermal half-lives vary between milliseconds and hours. These unique features open up ample opportunities for noninvasive, optically addressable materials and material systems.

We report on the impact of partial fluorination of para-sexiphenyl (6P) on the growth mode when deposited on the non-polar ZnO(101̄0) surface. The evolution of the thin film structure and morphology is monitored by in situ atomic force microscopy and in situ real-time X-ray scattering. Both 6P and its symmetrical, terminally fluorinated derivative (6P-F4) grow in a highly crystalline mode, however, with a distinctly different morphology. While 6P films are characterised by the formation of two different phases with three-dimensional nanocrystallites and consequently a rather rough surface morphology, layer-by-layer growth and phase purity in case of 6P-F4 prevails leading to smooth terraced thin films. We relate the different growth behaviour to specifics of the thin film structure.