Applications

Historically, azobenzenes were employed as dyes owing to the bright, intensive colors of certain derivatives and their high fatigue resistance with little to no photodegradation. Since their spectral features, however, are dependent on the electronic characteristics of the attached functional groups, an application as colorimetric pH sensors by modification with substituents responding to acids and bases is equally obvious. Well-known examples are methyl yellow/orange/red, Congo red, and Alizarine yellow R providing sulfonate, amine, or carboxyl groups as pH responsive moieties.

More advanced applications rely on the configurational change of azobenzene caused by photoisomerization that directly converts light into mechanical work, or the difference in polarity of both forms that can change solvation and aggregation behavior. As basis for light-driven molecular machines or as simple on-off switches these photochromic molecules made their way into various fields of life and material sciences such as cytology,^[33] pharmacology,^[34] self-assembly processes,^[35] and polymer networks,^[36] among others. Chosen examples are presented in the upcoming paragraph to give an impression of the diversity of possible azobenzene applications.

Turning to the field of biochemistry, the activity of proteins and enzymes highly relies on their 3D structure and accurate folding. Taking advantage of the conformational change of azobenzene, the photochromic molecules can be connected to subunits of complex organic macromolecules, reversibly change motifs by isomerization, and, thus, enable or disable accessibility of substrates or receptors. An example is depicted in Figure 7a where the photoswitch controls the association of a protein with DNA by altering an α-helix structure essential for proper interaction of both units.^[33c]

a) b)

Figure 7. Reversible binding of azobenzene-substituted proteins to DNA controlled by light; a) isomerization to the Z configuration causes the formation of an α-helical structure that can bind in the major groove of DNA, the E state cannot adopt this structure and, therefore, does not associate;^[33c] b) isomerization changes the polarity of the peptidomimetic causing aggregation accompanied by compaction of the genetic material.^[37]

In this way, the activity of transcription factors (proteins that bind to DNA and initiate the RNA synthesis as the primal step of the expression of genetic information) can be influenced.

Furthermore, due to specific binding to DNA these peptides can be exploited as carriers for therapeutics or for blocking gene sequences still providing the option for later disassembly. In another work, a photoresponsive peptidomimetic was synthesized that similarly binds with DNA in one switching state but does not in the other (Figure 7b).^[37] In this case however, the process is not a result of geometrical alterations but a polarity change that triggers the binding event leading to a reversible compaction of the genetic material. Densely packed DNA is desirable for gene delivery into cells where the cargo could be spread again by irradiation with light once the target is reached.

In material sciences, azobenzenes are blended in polymer matrices or covalently integrated in their main or side chains aiming to control properties such as tensile strength, elasticity, or glass transition temperature. Due to the macromolecular environment, the geometrical or polarity change as a result of isomerization requires a response of the whole polymeric surrounding leading to an amplification of the single switching event and, thus, a possible macroscopic effect.

An example for the latter is depicted in Figure 8 where a self-oscillating soft actuator was prepared by incorporating fluorinated azobenzene in polymer films with liquid-crystalline properties.^[38] A continuous chaotic motion was observed under exposure to normal sunlight which could be reproduce by irradiation with green and blue light, the usual wavelengths to address the nπ*

bands of both isomers. While the effect has not been completely rationalized yet, one important contribution is the iterative change of the isomer ratio. In this case, however, the light source is not considered a stimulus but rather a constant energy source. Applications in self-cleaning surfaces and coatings are conceivable that are activated and driven automatically when the sun is

a) b)

Figure 8. a) Fluorinated azobenzene-based crosslinker providing methacrylates for covalent incorporation in the polymer network; b) liquid-crystalline film containing the azo crosslinker, produced by photopolymerization, a continuous chaotic motion is observed by exposure to normal sunlight, the pictures show snapshots within a short period of the oscillation.^[38]

Covering the medical field, the administration of most pharmaceuticals is not conducted in a directed fashion, but compounds are spread out all over the body, thus, causing possibly serious side effects at locations they are not meant to act. In this context, photoswitches are exploited to control the activity of drugs and therapeutics enabling activation only in the body parts of interest.

The challenge in that process is the modification of the molecular structure of existing pharmaceuticals by exchanging subunits for azobenzene or attaching them without loss of activity in one isomeric form but a significant decrease in activity in the other.

a) b)

Figure 9. a) Photoresponsive microtubule inhibitor Photostatin-1 obtained by modification of stilbene derivative Combretastatin A-4; b) pictures of cells exposed to different concentrations of Photostatin-1 in the dark (top) or after irradiation at 390 nm (bottom), in the higher concentrated, irradiated samples the destruction of the microtubules is observed while the non-irradiated samples remain intact.^[33b]

One elegant example is depicted in Figure 9 where the stilbene double bond of Combretastatin A-4, a microtubule inhibitor, is exchanged for a nitrogen double bond creating an azobenzene analogue.^[33b] Depending on the concentration, the destruction of the microtubules is observed under irradiation with 390 nm light. Since only the Z isomer is active the non-irradiated cells

remain intact. An application in cancer therapy is conceivable as only tumor cells are affected when precisely targeted by light, thus, preserving healthy tissue.

Numerous other possible applications of photoswitches and azobenzenes in particular are reported ranging from modified surfaces that enable directed motion on them,^[39] reversible ion channel blockers in cells,^[33a] and generation of anisotropy in polymeric material,^[40] to the controlled release of small molecules,[36d, 36e, 41] and self-healing materials.^[42]