Excited state proton transfer from pyranine to acetate in a CTAB micelle Durba Roy, Rana Karmakar, Sudip Kumar Mondal, Kalyanasis Sahu, Kankan Bhattacharyya * Physical Chemistry Department, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700 032, India Received 17 August 2004; in final form 24 September 2004 Available online 22 October 2004 Abstract Proton transfer process of pyranine (8-hydroxypyrene-1,3,6-trisulfonate, HPTS) at the surface of a cationic micelle (cetyl tri- methyl ammonium bromide, CTAB) has been studied by emission spectroscopy. At the CTAB micellar surface, pH is 9.5 and the ground state pK a of HPTS is 10.2. These are higher compared to bulk water (pK a = 7.4 and pH 7). At the surface of the micelle, emission of HPTS is quenched by acetate ion with a bimolecular quenching constant, 3.5 · 10 7 M 1 s 1 . This is markedly slower than the excited state proton transfer in a solvent separated HPTS–water–acetate complex in bulk water. Ó 2004 Elsevier B.V. All rights reserved. 1. Introduction Ground and excited state proton transfer of pyra- nine (8-hydroxypyrene-1,3,6-trisulfonate, HPTS) [1–14] and other probes [15–18] have been widely studied to understand proton transfer in biological systems. A re- cent femtosecond mid-IR study has revealed that in bulk water excited state proton transfer (ESPT) from HPTS to acetate in an intermolecular hydrogen bonded complex is extremely fast and occurs in 150 fs while for solvent separated HPTS–water–acetate complex ESPT occurs in 100 ps time scale [1]. In a mi- celle even at a very high local concentration of acetate, formation of intermolecular hydrogen bonds between HPTS and acetate may be prevented because of the intervention of the surfactant chains. Also, in a micelle the probe will be shielded from bulk water and insuf- ficiently solvated [18]. This is expected to hinder the ESPT process from HPTS to acetate. In this Letter, we address this issue. pK a of HPTS decreases from 7.4 in the ground state to 0.4 in the excited state [6]. Fendler and co- workers [7] studied protolytic processes of HPTS inside the water pool of an AOT microemulsion. They found that in a large water pool the negatively charged probe HPTS remains at the core of the water pool away from the negatively charged surfactant AOT and exhibits a bulk water like dynamics. However, in a small water pool there is considerable difference from the behavior in bulk water. HPTS does not bind to anionic (sodium dodecyl sulfate, SDS) and neutral (Triton X-100, TX- 100) micelles and hence, exhibits fast ESPT as in bulk water. The anionic probe HPTS binds very strongly to a cationic micelle, cetyltrimethylammonium bromide (CTAB). In a CTAB micelle, the ESPT process of HPTS is very slow and as a result, it displays both the neutral and anion emission [14]. In this work, we study ESPT from HPTS to acetate ion in a CTAB micelle. 0009-2614/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2004.09.125 * Corresponding author. Fax: +91 33 2473 2805. E-mail address: pckb@mahendra.iacs.res.in (K. Bhattacharyya). www.elsevier.com/locate/cplett Chemical Physics Letters 399 (2004) 147–151