Novel Diaryliodonium Salts as One-Component Photopolymerisation Agents

It is planned to investigate the use of a one-pot method to efficiently prepare a range of diaryliodonium triflates based on known light absorbing motifs such as fluorenones, pyrenes, anthraquinones, squaraines and thioxanthone dioxides (Scheme 1). The effectiveness of this approach to iodonium salts based on electron-deficient arenes has not been reported.
With our new robust procedures in place for the formation of diaryliodonium salts from a range of aromatic and heteroaromatic compounds, the preparation of compounds that are expected to absorb at longer wavelength than those previously synthesised will be undertaken. By judicious tailoring of the substitution pattern of these new compounds absorption maxima in the near IR region is anticipated. Fortuitously a range of iodoarenes and iodoheteroarenes, e.g. iodoindoles and iodothiophenes, are commercially available and these substrates can all potentially be harnessed thus offering ready structural diversity. In most cases, the absorption spectra of these iodo-compounds are unreported. In addition, these light absorbing aromatic cores can be easily modified by both traditional and modern synthetic chemistry strategies.
There are many commercially available dyes that are known to absorb above 500 nm that can be converted into diaryliodonium salts by our proposed reaction with iodoarenes. For example, Oil Blue N is a commercially available anthraquinone derivative which absorbs above 600 nm. Other examples of commercially available dyes include thioindigo, indigo and m-cresol purple. Thioindigo will be the easiest of these compounds to utilise as it is neutral and does not contain any hydrogen bond donors.
As soon as these new iodonium salts are prepared, their UV-vis-NIR spectra will be assessed. We will quickly build up a picture of which arenes and iodoarenes lead to easily isolable and handleable iodonium salts and which moieties are optimal for effecting red-shifts in absorption maxima. In addition, we will assess the rates of photolysis to cationic and radical species by irradiation at suitable wavelengths (>400 nm) and follow the reactions by gas chromatography. The structures of the photodecomposition products will be characterised thus enabling the precise mode of photo-fragmentation to be established; this knowledge is essential for developing photopolymerisation applications.
With the series of long wavelength absorbing diaryliodonium salts in hand, their efficacies in the polymerisation of simple monomers such as cyclohexene oxide or acrylates will be assessed. The molecular weight distributions of the polymers will be determined by multi-angle laser light scattering.