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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 transcis 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.


Red-Shifting Azobenzene Photoswitches for in Vivo Use.

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