Substituent effects on the apparent pK of the reversible open-to-closed transition of sultams derived from sulforhodamine dyes

Sultams derived from sulforhodamine dyes by reaction with primary amines undergo reversible ring-chain interconversion that is pH-dependent. The apparent pK for the transition has been modulated in the range 4.40–8.04 by variation of the amino substituents at the 3′,6′-positions of the sulforhodamine. Alterations of the substituent on the sulfonamide nitrogen atom had much smaller effects. The pH-dependent kinetics of ring-closure after photolytic opening of the closed form of one of the compounds studied here have been measured over the pH range 6.02–8.51.