Effects of micelles on excited state intramolecular proton transfer activities of 2-hydroxy 1-naphthaldehyde Papia Chowdhury, Sankar Chakravorti * Department of Spectroscopy, Indian Association for the Cultivation Science, 2A & 2B Raja S C Mullick Road, Jadavpur, Kolkata 700 032, India Received 17 March 2004; in final form 21 July 2004 Available online 5 August 2004 Abstract The effects of ionic and nonionic micelles on the excited state proton transfer processes of 2-hydroxy 1-naphthaldehyde (HNL) have been reported in this Letter. Deprotonation of HNL is considerably retarded in neutral and anionic micelles than that in cat- ionic type as evinced from the increased neutral emission. Increased anion emission of HNL in cationic micelle is more due to the abundance of hydroxyl ions in the environment as well as less nonradiative deactivation. Anion emission is found to decrease in anionic micelle due to less formation of the conformer along with increase in nonradiative decay. The nonradiative processes from neutral to ionic form and ionic to ground state are less affected in neutral micelle as compared to HNL in ionic micelles. Ó 2004 Elsevier B.V. All rights reserved. 1. Introduction Dual fluorescence in organic system arises mainly due to electron and proton transfer reactions. These two processes are twisted intramolecular charge transfer process (TICT) [1,2] and excited state intramolecular proton transfer (ESIPT) process [3–5]. The emissions in both the phenomena are sensitive to environment. In the case of TICT process an extra emission band arises due to intramolecular rotation about a single bond con- necting donor and acceptor moieties in the excited state and in ESIPT a proton is transferred from acid to basic moieties in excited state and an extra emission band is observed due to this intramolecular proton transfer. These fluorescent proton and electron donor–acceptor molecules play an important role as reporter molecules in probing the microheterogeneous environments such as cyclodextrins and micelles [6–8]. With introduction of proper probe, micellization occurs and the micelle– water interface electric field changes the charge transfer process of the probe [1]. These sensitive probes may be utilized to examine the interior of micellar aggregates. In our lab we earlier probed different micellar and cyclo- dextrin media [9,10] with the help of a reporter molecule 4-(N,N-dimethylamino) cinnamaldehyde. Micelles are known to form microheterogeneous structures in aqueous solution [11]. Microheterogeneous systems are characterized by a nonuniform distribution of their components at the microscopic level, due to for- mation of molecular aggregates i.e., micelles in water form a relatively hydrophobic core and a hydrophilic surface. The polarity of micelles decreases gradually from boundary to core. Three different types nonionic, anionic and cationic micelles are known to exist [2]. The ionic micelles have a charged micelle–water inter- face and the interface electric field may be modified through counterion binding or incorporation of a neu- tral species. All these three types of micelles consist of a dry hydrocarbon core surrounded by a ÔwetÕ spherical shell called the Stern layer. The thickness of this layer is 6–9 A ˚ for ionic micelles. Generally, for nonionic micelle the Stern layer is much thicker (26 A ˚ ). In ionic micelles the Stern layer contains the small counterions and a con- 0009-2614/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2004.07.069 * Corresponding author. Fax: +91 033 24732805. E-mail address: spsc@iacs.res.in (S. Chakravorti). www.elsevier.com/locate/cplett Chemical Physics Letters 395 (2004) 103–108