DOI: 10.1002/cphc.200800658 A Femtosecond Study of Solvation Dynamics and Anisotropy Decay in a Catanionic Vesicle: Excitation- Wavelength Dependence Shantanu Dey, Dibyendu Kumar Sasmal, Dibyendu Kumar Das, and Kankan Bhattacharyya* [a] 1. Introduction The study of the dynamics of water in nanoconfined systems has attracted great attention during the last years. [1] Most re- cently, there have been attempts to understand the dynamics in different regions of an organized assembly. By using com- puter simulations, several groups have studied the dynamics of water in different regions of micelles, [2] surfaces, [3] many hy- drophobic cavities, [4] and most recently, in ionic liquids. [5] . The simulations suggest that the dynamics vary markedly from one region to another. Several recent experimental works also indi- cate a variation of the dynamics in different regions of micelles and other organized assemblies. [6–8] In a protein, the relaxation dynamics may vary widely from site to site. For instance, from computer simulations, Makarov and Petit proposed that there are many (294) hydration sites in myoglobin with varying resi- dence/relaxation times of the water molecules. [9] According to computer simulations, about 85 % of the solva- tion dynamics arises from the first solvation layer around the solvation probe. [10] Thus, the inherent spatial resolution of sol- vation dynamics is roughly equal to the size of the fluorescent probe (  1 nm). As a result, solvation dynamics may be utilized to probe regions of sizes around 1 nm in a heterogeneous as- sembly. By using solvatochromism, the fluorescent probes in different regions of such an assembly may be selectively excit- ed through variation of the excitation wavelength (l ex ). Excita- tion at a short wavelength (blue end) selects the solvatochro- mic (e.g. coumarin 480, C480) probe in a relatively nonpolar (hydrophobic) environment and gives rise to a blue-shifted emission spectrum. On the other hand, excitation at a longer wavelength (red end) selects the probe in a relatively polar (hydrophilic) environment and gives rise to a red-shifted emis- sion spectrum. This is known as red-edge excitation shift (REES). [11] Recently, the l ex dependence has been applied to study the dynamics in different regions of many organized as- semblies, such as neat ionic liquids, [12a] ionic-liquid microemul- sions, [12a] ionic-liquid mixed micelles, [12b] bile-salt aggregates, [12c] triblock copolymer micelles, [12d] gels, [12e] lipid vesicles, [12f] and polymer-surfactant aggregates. [12 g] The fluorescence resonance energy transfers (FRET) in different regions of a reverse micel- le [13a] and a polymer gel [13b] have also been studied by means of l ex variation. Herein, we apply this method to study the sol- vation dynamics in different regions of a catanionic vesicle. We use coumarin 480 (C480) as a fluorescent probe. Self-assembly of a cationic surfactant (e.g. dodecyl trimethyl ammonium bromide, DTAB) and an anionic surfactant (sodium dodecyl sulfate, SDS) leads to the formation of a catanionic vesicle [14] and a microemulsion. [15] Both DTAB and SDS contain an identical number of carbon atoms (i.e. 12) in the hydropho- bic tail. The strong electrostatic interaction between the oppo- The structure and dynamics of a catanionic vesicle are studied by means of femtosecond up-conversion and dynamic light scatter- ing (DLS). The catanionic vesicle is composed of dodecyl-trimeth- yl-ammonium bromide (DTAB) and sodium dodecyl sulphate (SDS). The DLS data suggest that 90 % of the vesicles have a di- ameter of about 400 nm, whereas the diameter of the other 10 % is about 50 nm. The dynamics in the catanionic vesicle are com- pared with those in pure SDS and DTAB micelles. We also study the dynamics in different regions of the micelle/vesicle by varying the excitation wavelength (l ex ) from 375 to 435 nm. The cata- nionic vesicle is found to be more heterogeneous than the SDS or DTAB micelles, and hence, the l ex -dependent variation of the sol- vation dynamics is more prominent in the first case. The solva- tion dynamics in the vesicle and the micelles display an ultraslow component (2 and 300 ps, respectively), which arises from the quasibound, confined water inside the micelle, and an ultrafast component (< 0.3 ps), which is due to quasifree water at the sur- face/exposed region. With an increase in l ex , the solvation dy- namics become faster. This is manifested in a decrease in the total dynamic solvent shift and an increase in the contribution of the ultrafast component (< 0.3 ps). At a long l ex ACHTUNGTRENNUNG(435 nm), the surface (exposed region) of a micelle/vesicle is probed, where the solvation dynamics of the water molecules are faster than those in a buried location of the vesicle and the micelles. The time con- stant of anisotropy decay becomes longer with increasing l ex , in both the catanionic vesicle and the ordinary micelles (SDS and DTAB). The slow rotational dynamics (anisotropy decay) in the polar region (at long l ex ) may be due to the presence of ionic head groups and counter ions. [a] S. Dey, D. K. Sasmal, D. K. Das, Prof. K. Bhattacharyya Physical Chemistry Department Indian Association for the Cultivation of Science Jadavpur, Kolkata 700 032 (India) Fax: (+ 91) 33-2473-2805 E-mail : pckb@mahendra.iacs.res.in 2848  2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim ChemPhysChem 2008, 9, 2848 – 2855