Photoinduced Proton and Charge Transfers in a Dihydroxynaphthalene Derivative: Chromotropic Acid

Chromotropic acid (4,5-dihydroxynaphthalene-2,7-disulfonic acid) is a chelating agent whose fluorogenic character has never been explained yet. The excited-state behavior of chromotropic acid itself has never been reported, despite its relationship with naphthols that are well-known photoacids. The present paper is accordingly devoted to a thorough investigation of chromotropic acid emission in aqueous solutions, which revealed its excited-state prototropic behavior in relation with the key role of the ground-state internal H-bond between the two −OH groups (displayed by the very different p K values, 5.4 and 15.6). Experiments were carried out in the whole acidity range (8 M HClO 4 to 10 M NaOH) using steady-state and time-resolved fluorimetry. In concentrated HClO 4, competitive quenching processes occurred, due to HClO 4 itself and to water, respectively, but no photoinduced deprotonation was observed. The former quenching was confirmed by studying the emission of 2,6-naphthalenedisulfonic acid in the same media. Photoinduced deprotonation from the first −OH group was only observed in diluted acidic solutions ([HClO 4 ] ≤ 1 M), within 500 ps, leading to an intermediate species IS*, which in turn leads to the monoprotonated form HCA 3- * within ≈230 ps. The structure of HCA 3- * is resonance stabilized by an internal charge transfer. The lifetime of HCA 3- * is 2.1 ns. The structure of IS* was tentatively proposed to be an ion pair between H + and the hydroxylate form of HCA 3- *. The p K * value of the first −OH group (≈1.8) was deduced from the rate constants. A moderate photoacidity (Δp K ≈ −3.6 p K units on excitation) is then displayed. Regarding the second −OH group, its photoinduced deprotonation was never observed, even in strongly basic solutions (1 M ≤ [NaOH] ≤ 10 M), which points out the outstanding stability of HCA 3- *. The lifetime of the fully deprotonated form (1.76 ns) could only be measured after direct excitation in 10 M NaOH.