Senior Research Fellow
Prevention of azobenzene’s photoisomerization is expected to be fluorescent-Application for OLEDs
Azobenzene is one of the most studied photochromic molecule that shows reversible cis↔trans isomerism by thermal and photochemical pathways which enables them to be useful in phototriggered biological systems. Since the photochemical azobenzene isomerization is the dominant excited state deactivation pathway, an another, competitive, photophysical property, i.e. fluorescence becomes negligible and in fact, has not experimentally been observed many cases. The fluorescence properties of azobenzene are highly desirable, which would be promising skeleton in the fields of light-emitting devices, fluorescent probes, and fluorescent sensors etc. Hence, making fluorescent azobenzene derivatives are quite challenging in the current scenario. Though few examples of fluorescent azobenzene are available, the quantum yield is quite low. Here we have designed and synthesized a novel azobenzene appended rhodamine derivative (Azo-Rho) and their photochemical and photophysical properties were studied. Though, Azo-Rho undergoes photoisomerization similar to unsubstituted azobenzene, presence of Al3+ ion induces the rhodamine ring opening rather than the transcis photoisomerization. As a consequence, fluorescence enhancement of both azobenzene and rhodamine unit was observed. The enhanced fluorescence would have originated by restricting the excited state photochemical pathways and also the energy transfer from azobenzene to rhodamine via Chelation Enhancement Fluorescence (CHEF) assisted Fluoresence Resonance Energy Transfer (FRET) was observed. The mechanism involved in this process was monitored by both fluorescence and 1H NMR spectroscopy. The selectivity of Al3+ towards the Azo-Rho over various metal ions were examined after irradiation with λ=365 nm (Cu2+, Fe2+, Fe3+, Pb2+, Zn2+, Ni2+ as a perchlorate salts). Only Al3+ was found to show significant enhancement in fluorescence, however, a slight enhancement is also observed for Hg2+ and Fe3+ ions. The stoichiometric ratio of AZO-Rho to the Al3+ was found to be 1:1 by Job’s plot and the binding constant calculated using Benesi-Hildebrand plot corresponds to 5.2X103M-1. The competitive ring opening of Rho resulted in fluorescent azobenze and subsequent energy transfer by CHEF assisted FRET pathway.
Abstract: Recently, there has been a great deal of interest in using the photoisomerization of azobenzene compounds to control specific biological targets in vivo. These azo compounds can be used as research tools or, in principle, could act as optically controlled drugs. Such "photopharmaceuticals" offer the prospect of targeted drug action and an unprecedented degree of temporal control. A key feature of azo compounds designed to photoswitch in vivo is the wavelength of light required to cause the photoisomerization. To pass through tissue such as the human hand, wavelengths in the red, far-red, or ideally near infrared region are required. This Account describes our attempts to produce such azo compounds. Introducing electron-donating or push/pull substituents at the para positions delocalizes the azobenzene chromophore and leads to long wavelength absorption but usually also lowers the thermal barrier to interconversion of the isomers. Fast thermal relaxation means it is difficult to produce a large steady state fraction of the cis isomer. Thus, specifically activating or inhibiting a biological process with the cis isomer would require an impractically bright light source. We have found that introducing substituents at all four ortho positions leads to azo compounds with a number of unusual properties that are useful for in vivo photoswitching. When the para substituents are amide groups, these tetra-ortho substituted azo compounds show unusually slow thermal relaxation rates and enhanced separation of n-π* transitions of cis and trans isomers compared to analogues without ortho substituents. When para positions are substituted with amino groups, ortho methoxy groups greatly stabilize the azonium form of the compounds, in which the azo group is protonated. Azonium ions absorb strongly in the red region of the spectrum and can reach into the near-IR. These azonium ions can exhibit robust cis-trans isomerization in aqueous solutions at neutral pH. By varying the nature of ortho substituents, together with the number and nature of meta and para substituents, long wavelength switching, stability to photobleaching, stability to hydrolysis, and stability to reduction by thiols can all be crafted into a photoswitch. Some of these newly developed photoswitches can be used in whole blood and show promise for effective use in vivo. It is hoped they can be combined with appropriate bioactive targets to realize the potential of photopharmacology.
Pub.: 29 Sep '15, Pinned: 09 Jun '17
Abstract: The photoisomerization of azobenzenes provides a general means for the photocontrol of molecular structure and function. For applications in vivo, however, the wavelength of irradiation required for trans-to-cis isomerization of azobenzenes is critical since UV and most visible wavelengths are strongly scattered by cells and tissues. We report here that azobenzene compounds in which all four positions ortho to the azo group are substituted with bulky electron-rich substituents can be effectively isomerized with red light (630-660 nm), a wavelength range that is orders of magnitude more penetrating through tissue than other parts of the visible spectrum. When the ortho substituent is chloro, the compounds also exhibit stability to reduction by glutathione, enabling their use in intracellular environments in vivo.
Pub.: 12 Jun '13, Pinned: 09 Jun '17
Abstract: Most azobenzene-based photoswitches use UV light for photoisomerization. This can limit their application in biological systems, where UV light can trigger unwanted responses, including cellular apoptosis. We have found that substitution of all four ortho positions with methoxy groups in an amidoazobenzene derivative leads to a substantial (~35 nm) red shift of the n-π* band of the trans isomer, separating it from the cis n-π* transition. This red shift makes trans-to-cis photoswitching possible using green light (530-560 nm). The cis state is thermally stable with a half-life of ~2.4 days in the dark in aqueous solution. Reverse (cis-to-trans) photoswitching can be accomplished with blue light (460 nm), so bidirectional photoswitching between thermally stable isomers is possible without using UV light at all.
Pub.: 16 Nov '11, Pinned: 09 Jun '17
Abstract: The photoisomerization of azobenzene has been known for almost 75 years but only recently has this process been widely applied to biological systems. The central challenge of how to productively couple the isomerization process to a large functional change in a biomolecule has been met in a number of instances and it appears that effective photocontrol of a large variety of biomolecules may be possible. This critical review summarizes key properties of azobenzene that enable its use as a photoswitch in biological systems and describes strategies for using azobenzene photoswitches to drive functional changes in peptides, proteins, nucleic acids, lipids, and carbohydrates (192 references).
Pub.: 13 Apr '11, Pinned: 09 Jun '17
Abstract: A versatile synthetic method has been developed to incorporate photochromic azobenzene moieties into tetraphenylporphyrin frameworks in an orthogonal fashion, positioning the phenylazo substituents above and below the plane of the macrocycle. Surprisingly, photoisomerization is completely suppressed in the resulting azobenzene-confined porphyrins.
Pub.: 26 Sep '06, Pinned: 09 Jun '17
Abstract: Azobenzenes are constituents of the commonly and widely used azo dyes. Many dyes, except for the azo dyes, have been utilized for fluorescent materials. However, there are only a few fluorescent azobenzene derivatives and their fluorescence efficiencies are quite low. The current perspective provides an account of the fluorescent azobenzenes and aromatic aldimines featuring an N-B interaction. Incorporation of the intramolecular N-B interaction by using the bis(pentafluorophenyl)boryl group makes the azobenzenes and aromatic aldimines fluorescent with a range of colours. Some of them fluoresce with extraordinarily high fluorescence quantum yields. Their synthesis, structures, fluorescence properties, and applications are discussed.
Pub.: 10 Sep '13, Pinned: 09 Jun '17
Abstract: The azobenzene-containing photoswitchable piperidine general base catalyst is a prototype structure for light control of catalysis. Its azobenzene moiety moves sterically-shielding groups to either protect or expose the active site, thereby changing the compound's basicity and hydrogen-bonding affinity. The catalyst's reversible switching dynamics is probed in the infrared spectral range by monitoring hydrogen-bond formation between its active site and methanol (MeOH) as hydrogen-bond donor. Steady-state infrared (IR) and ultrafast IR and UV/Vis-spectroscopies are used to uncover ultrafast expulsion of MeOH from the active site within a few picoseconds. Thus, the force generated by azobenzene even in the final phase of its isomerisation is sufficient to break a strong hydrogen bond within 3 ps and to shut down access to the active site.
Pub.: 02 Jun '17, Pinned: 09 Jun '17