Dye Chemistry

Single Isomer Rhodamine Synthesis

CTB has developed a method to directly synthesise single isomer rhodamine dyes. This is achieved by replacing the commonly used functionalised anhydride with a phthalaldehydic acid, which provides only one point of reactivity for the aminophenol in the form of an aldehyde. The phthalaldehydic acids can be accessed from phthalides in two high yielding steps. We have carried out the first 2 steps up to a 5 gram scale. The final rhodamine forming step has been carried out up to a 1 gram scale. The route has been utilised to generate various rhodamine types including tetramethylrhodamine, rhodamine B, X-rhodamine and rhodamine G. With regards to cost, all of the rhodamine compounds produced so far can be made from commercially available phthalides apart from the methyl ester variant, which is synthesised from 5-cyanophthalide in two high yielding steps. Bromination is then achieved using N-bromosuccinimide with a radical initiator. Purification can be achieved using a standard silica gel column using a mixture of acetonitrile and water as eluents. However, due to the highly charged nature of these compounds we recommend using HPLC purification if possible. This synthetic route does not allow for two isomers to form. We have confirmed that this is the case using NMR, an extract of the proton NMR of a batch of 5-carboxytetramethylrhodamine showing the aromatic signals is shown in the accompanying image. This clearly shows the presence of only one species.

• Mudd, Gemma, et al. "A general synthetic route to isomerically pure functionalized rhodamine dyes." Methods and Applications in Fluorescence 3.4 (2015): 045002.