Paper Introduction By, Amrutha A.S. 13th June 2014

Photoswitching using visible light: A new class of organic photochromic molecules Authors: Sameh Helmy, Frank A.Leibfarth, Saemi Oh, Justin E. Poelma, Craig J.Hawker, and Javier Read de Alaniz Department of Chemistry and Biochemistry and Materials Research Laboratory, Materials Department, University of California Published in: JACS Published on: 21st May 2014

Introduction Organic photochromic compounds - Reversibly undergo changes in, spectral absorption Volume Solubility This property is important for applications in Energy storage Chemical sensing Controlling the conformation and activity of biomolecules Photoswitches are valuable because, Triggered by light which is most widely available Non-invasive

Introduction Well known classes of organic photochromic molecules are, Azobenzenes – (change in volume) Spiropyrans – ( change in solubility) require UV light Diarylethenes – ( change in spectral properties) Usage of UV light, Hinders the potential use of above mentioned Photochromic molecules in biomedical applications and material science – because, UV light can damage healthy cells and results in degradation for many macromolecular systems Fatigue resistance also main concern in UV based photochromic switches

Donor-acceptor stenhouse adducts (dasaS)

Synthetic Scheme

Photoswitching and thermal reversion

Study of the Photochemical conversion by NMR Conversion of 4a to 4b by proton NMR in CD3OD over 4h

Study of the Photochemical conversion by UV-Vis Spectroscopy B) Absorption spectra of 5a to 5b on irradiation with visible light in MeOH c) Expanded view off absorption spectra of 5a to 5b between 5a to 5b between 250 and 280nm

Reversible switching of DASAs – solvent dependent – aromatic solvents are ideal. Rates of inter conversion between triene and cyclopentenone form were evaluated in toluene for both the photoswitches bearing different acceptor groups Approximately same rate for triene to cyclopentenone conversion Rate of thermal recolouration upon cessation of irradiation – 1,3- dimethyl barbituric acid derivative reverted back with the rate approximately twice than the Meldrum’s acid derivative. This process is solvent and temperature dependent

Effect of solvent on photoswitching and thermal recolouration

Effect of solvent on photoswitching and thermal recolouration In polar protic solvent no thermal reversion. Choice of acceptor group has an effect on λmax and associated color with the triene (for 4a λmax =545nm and for 5a λmax =570nm) Fatigue experiments – rapid and complete switching of 5a was observed consistently with negligible material degradation (0.05% loss per cycle) Switching occurs via reversible Nazarov type 4π electro-cyclisation mechanism Conversion to the zwitterionic form results in the bleaching of all derivatives providing colorless water soluble materials – demonstrated by dynamic phase transfer

Photoswitching of micelle forming polymer amphiphile The amphiphilic nature of 7a induced micelle formation in aqueous environments was determined by Nile Red encapsulation experiments and dynamic light scattering – confirmed that Nile Red is successfully encapsulated and solubilized in water. CMC-49μM

Schematic representation

Fluorescence intensity (Em at 588nm) vs log concentration

Fluorescence emission spectra of Nile red Upon visible light irradiation, the absorption peak of 7a at 550nm decreased steadily, indicating the photoswitching of the DASA to its cyclic, hydrophilic state. The fluorescence emission spectra of encapsulated Nile Red showed a sharp decrease in intensity and red shift – indicating that the hydrophobic dye was released to the aqueous phase

conclusions Introduced a new class of photoswitchable molecules that are synthetically versatile and exhibit a unique combination of physical and chemical properties. These DASAs show reverse photochromism under visible light, which is complementary to previous classes of photoswitches and display excellent fatigue resistance under ambient conditions. The potential of these photoswitches in materials science was demonstrated through the synthesis of a functional amphiphile that displays on-demand light – mediated disassembly and cargo release.

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