Unexpected photo-switching behavior of 3,3ʹ -azothiophenes

Recent publications have brought to the limelight the endless opportunities the diversity of photo-switching structural elements opens up in the engineering of light driven reshaping of matter, in catalysis on-click including photodynamic cancer therapy, in light sensitive transport control and in data storage. A thorough review of literature published over the last 20 years shows that molecules- the structures of which can be reversibly altered by light- have garnered considerable attention in the industrial and scientific community. Among the reported photo-switches, azobenzenes, stilbenes, and hemithioindigo, all undergo photo-induced trans–cis interconversion; with a further group including spiropyrans and diarylethenes undergoing reversible restructuring. Such photo-switches have found use in many fields, including the photoactive reshaping of polymers, bio-inhibition “on click”, and the activation of sensors.

Among the molecules studied, azobenzene based photo-switchable molecular systems are the most popular. Unsubstituted trans or anti azobenzene shows three absorption bands. Molecular engineering has been used to introduce azobenzene chromophores into polypeptides: in both, side chains and in the backbone, which has opened up new opportunities using photo-switchable bio-functional molecules. Furthermore, the synthesis of polypeptides, in which azobenzene was introduced into the backbone as a photo-switchable structural element, has opened up a novel experimental approach, transient two-dimensional infrared spectroscopy to explore the hierarchy of protein folding processes. Success in photo-switching of bioactive functionalities using a range of molecular mechanisms is quite inspiring. Recently, the synthesis of 3,3’-azothiophene has been reported. Unfortunately, structural and photo-physical properties of 3,3’-azothiophene and 2,2ʹ -bismethoxycarbonyl- 3,3ʹ -azothiophene have not been reported.

Recently, Nottingham Trent University researchers from the Interdisciplinary Biomedical Research Centre: Dr. Patrick Huddleston, Dr. Victor Volkov and Dr. Carole Perry explored the photophysical properties of the novel 3,3ʹ -azothiophene molecular systems with the aid of quantum calculations. Specifically, they correlated the observed spectral properties with possible structural conformations as predicted by theory and address mechanisms of isomerization, (photo-induced and thermally driven), accounting for the nature of the side groups. Their work is currently published in the research journal, Physical Chemistry Chemical Physics.

For the synthesized and purified 3,3ʹ -azothiophene (3,3ʹ -AT) and 2,2ʹ -bismethoxycarbonyl-3,3ʹ -azothiophene, ultraviolet visible spectroscopy and temperature dependent kinetic measurements were undertaken. FTIR studies were performed on solutions of the samples 2 mm thick calcium fluoride windows held apart using a Teflon spacer of 100 microns.

The authors noted that upon exposure to ultraviolet and visible radiation, an effective anti(trans) to syn(cis) and syn(cis) to anti(trans) isomerization of the –N=N– moiety, respectively, was observed in the considered azothiophenes. In contrast to azobenzene based photo-switchable molecular systems, the syn(cis) to anti(trans) isomerization in the azothiophenes studied did not take place at 22 ^oC in the dark, reflecting the effect of the differences in structure on the electronic properties of the considered azothiophenes.

In summary, the Nottingham Trent research team successfully defined the photo-physical and photochemical properties of novel 3,3ʹ -AT and 3,3ʹ -ATM molecular systems using ultraviolet-visible and FTIR spectroscopy and with the assistance of density functional theory calculations. Temperature dependent experiments and theoretical studies suggest a slightly higher barrier for such processes than for azobenzene, which was attributed to the specific structural and electronic properties of the thiophene ring and the nature of the side groups.