61st Annual Report on Research 2016

This report summarizes progress towards the synthesis and study of new organic dyes based on the peri-naphthingo (PNI) structure. Our proposal focused on development of new synthetic routes to the “PNI” core exploiting some well-known routes to make the famous indigo pigment. As such, we initially focused on applying the Bayer-Drewson synthesis. This method proved to be successful – condensation of 8-nitro-1-naphthaldehyde with acetone in aqueous base led to the parent PNI chromophore. However, this synthesis exposed two challenges. Firstly, the PNI molecule was fairly insoluble in all common solvents (perhaps not unsurprising given that it is a large, flat hydrocarbon. We tackled this issue by preparing PNI derivatives in which the outer benzene rings had substituents such as t-Butyl, bromo- or 2-thienyl. Indeed these compounds (particularly the t-Bu and thienyl derivatives) were more freely soluble in a range of organic solvents.

The second, and larger, challenge, concerned the yield of the oxidative dimerization step. Yields for all derivatives were routinely in the 5-15% range – unacceptably poor particularly given our plan to perform subsequent chemistry on the PNI units. We have made progress in adapting a modified Bayer-Drewson synthesis in which nitromethane replaces acetone as the substrate in the aldol condensation reaction (the nitromethane reaction is a pseudo-Henry reaction). For selected substrates the yields of these reactions improved markedly (close to 50%) though more work is needed here to further increase yields.

Finally we have made inroads in the functionalization of PNI to further explore and modulate electro-optical properties.  Suzki-Miyaura cross-coupling of dibromo-PNI compounds with arylboronic acids (aryl = phenyl, thiophene) yielded the corresponding diaryl PNIs. These derivatives can also be made by performing the cross-coupling reactions on the nitro/aldehyde precursor compounds. In the case of the 2-thienyl derivative, large red shifts in the electronic absorption spectra (660 nm for the bis(thienyl) PNI compared to 580 for the corresponding diboromo precursor). These large spectral shifts are an important development because they demonstrate that the PNI chromophore – in contrast to indigo, for which substituent effects on spectral properties are minimal – is sensitive to substituent effects.