Diaryltriazolium Photoswitch: Reaching a Millisecond Cycloreversion with High Stability and NIR Absorption.

The exceptional thermal stability of diarylethene closed isomers enabled many applications but also prevented utilization in photochromic systems that require rapid thermal reversibility. Herein, we report the diaryltriazolium (DAT+ ) photoswitch undergoing thermal cycloreversion within a few milliseconds and absorption of the closed form in the near-infrared region above 900 nm. Click chemistry followed by alkylation offers modular and fast access to the electron-deficient DAT+ scaffold. In addition to excellent fatigue resistance, the introduced charge increases water solubility, rendering this photoswitch an ideal candidate for exploring biological applications.

DiI-CT-A bimodal neural tracer for X-ray and fluorescence imaging.

Here, we present an X-ray-visible neural tracer, referred to as DiI-CT, which is based on the well-established lipophilic indocarbocyanine dye DiI, to which we conjugated two iodine atoms. The tracer is visible with microfocus computed tomography (microCT) imaging and shares the excellent fluorescent tracing properties of DiI. We document the discovery potential of DiI-CT by analyzing the vibrissa follicle-sinus complex, a structure where visual access is poor and 3D tissue structure matters and reveal innervation patterns of the intact follicle in unprecedented detail. In the brain, DiI-CT tracing holds promise for verification evaluation of indirect connectivity measures, such as diffusion tensor imaging. We conclude that the bimodal dye DiI-CT opens new avenues for neuroanatomy.

Donor-Acceptor Dihydropyrenes Switchable with Near-Infrared Light.

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.

Taking Photochromism beyond Visible: Direct One-Photon NIR Photoswitches Operating in the Biological Window.

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.

Dihydropyrene as an Aromaticity Probe for Partially Quinoid Push-Pull Systems.

Although aromaticity is a key concept in chemistry that is frequently used to explain the structure and reactivity of organic compounds, it remains challenging to actually measure this property. Proper probes allow for an experimental quantification of aromaticity using nuclear magnetic resonance (NMR) spectroscopy and in this study 15,16-dimethyl-15,16-dihydropyrene (DHP) is demonstrated to be particularly well suited for this purpose. DHP has two internal methyl groups, which are positioned within the π cloud of this bridged, planar, and rigid [14]annulene, and are therefore shifted to higher field in proton NMR spectra owing to ring-current effects. A specific DHP derivative bearing strongly electron-donating dimethylamino and strongly electron-accepting nitro groups in a relative pseudo-para orientation has been synthesized and characterized with respect to its quinoid character. Solvatochromism as well as a reduced ring current show that the partial quinoid character in this push-pull DHP derivative is about 12 %. This study extends the scope of DHP as an aromaticity probe and aids in the better understanding of the phenomenon of aromaticity.