Phenylazopyridines as photoswitchable and metal chelating motifs in supramolecular chemistry

Abstract

Azobenzene photoswitching has exciting applications in many fields such as biology, surfaces and supramolecular chemistry. Phenylazopyridine (PAP) derivatives are a class of azo compounds that not only exhibit isomerisation but also have the ability to form a variety of metal complexes. In this work, ortho-, meta- and para-PAP derivatives have been synthesised with the aim of studying their isomerisation behaviour. PAP-phenol derivatives were synthesised using procedures previously reported in the literature. In order to add "handles" that could allow ligation to biomolecules, acid termini were appended to these PAP-phenol derivatives by coupling with tert-butyl 2-bromoacetate. Photoswitching studies followed by UV/visible spectroscopy showed that the PAP-phenol derivatives did not undergo trans-to-cis isomerisation under UV light irradiation, whereas the PAP-ester analogues photoisomerised under the same conditions over periods of time that varied from 12 to 23 minutes of irradiation. Cis-to-trans isomerisation was observed for these compounds after relaxation in the dark. First order rate constants were calculated for both isomerisation processes. Deprotection of o-PAP-ester with TFA produced the carboxylic acid in excellent yield. This molecule underwent trans-to-cis and cis-to-trans isomerism at faster rates than the PAP-ester analogues. The o-PAP-ester gave a 1:1 stoichiometry upon complexation to copper, although the stoichiometry for the o-PAP-phenol/Cu(II) complex was not clear. Interestingly, the o-PAP-ester did not exhibit trans-to-cis photoisomerisation in the presence of Cu(II) ions, whereas the reverse process was accelerated in the presence of Cu(II). The p-PAP-ester was coordinated to a platinum(II) centre through the 4-pyridyl moiety, but the presence of the Pt(II) centre appeared to inhibit photoisomerisation.

Date of Award	1 Aug 2014
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Simon Webb (Supervisor)