Surprising variations in the rate of ring opening for a series of rhodamine lactams with similar equilibrium endpoints

• A series of structurally similar rhodamine lactam derivatives was easily prepared from rhodamine.
• The rate trend is different from the trend for the absorbance measured at equilibrium.
• The rate of conversion from the rhodamine lactam form to the rhodamine amide form can vary greatly (<1 s to >12 h).
• Similar rate trends were observed with both trifluoroacetic acid (TFA) and iron (III).
• The rate of conversion changed depending on the structure of the substituent attached to the lactam nitrogen.

Rhodamine amides are known to function as an important platform for molecular probe formation. The use of rhodamine-based probes has focused on the equilibrium between the off to on opening of the lactam form to the amide form. Herein, a series of four structurally similar rhodamine lactam derivatives is studied to show that the rate of conversion from the lactam form to the amide form can vary greatly (<1 s to >12 h) and that the rate trend is different from the trend for the absorbance measured at equilibrium. Slow or varying rates of ring opening for structurally similar rhodamine derivatives could produce inaccurate results. Rates were studied with both trifluoroacetic acid (TFA) and iron III. The rate of conversion changed depending on the structure of the substituent attached to the lactam nitrogen. Interestingly the rate of conversion varied even for compounds with similar absorbance at equilibrium. This work will help with the development of more effective rhodamine-based molecular probes.