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Langmuir 2002, 18, 7872-7879
Intramolecular Charge Transfer and Solvation Dynamics of Coumarin 152 in Aerosol-OT, Water-Solubilizing Reverse Micelles, and Polar Organic Solvent Solubilizing Reverse Micelles Partha Hazra, Debdeep Chakrabarty, and Nilmoni Sarkar* Department of Chemistry, Indian Institute of Technology, Kharagpur -721 302, West Bengal, India Received April 30, 2002. In Final Form: July 17, 2002
The relative retardation of intramolecular charge transfer (ICT) and the solvation dynamics of coumarin 152 in pure AOT, water, methanol, acetonitrile, and formamide reverse micelles have been investigated using picosecond time-resolved emission spectroscopy. The rate of ICT has been retarded almost 7 times at w0 ) 4 and 4 times at w0 ) 32 compared to that in pure water. The rate of retardation of ICT is also observed in the methanol and acetonitrile reverse micelles in comparison to that in pure methanol and acetonitrile. In pure AOT, the solvation time is 12.22 ns, but at w0 ) 4 of water reverse micelles the solvation time is 7.27 ns. The slow dynamics in methanol, acetonitrile, and formamide reverse micelles is also observed. The relative retardation of the ICT rate is much smaller compared to the several thousand fold decrease in the solvation dynamics in the pool of the reverse micelles. The w dependency of solvation time is observed for water and methanol reverse micelles, but it is little for acetonitrile and none for formamide reverse micelles. The various w dependencies of solvation dynamics in water, methanol, and acetonitrile reverse micelles are explained on the basis of the presence and the absence of hydrogenbonding networks in water, methanol, and acetonitrile, respectively.
1. Introduction Recently, the study of solvation dynamics in various organized assemblies starting from protein surfaces,1 reverse micelles,2-5,10-12,15,20-21 and others10-12 is of great interest. The reverse micelle is an interesting model for biomembranes among the different organized assemblies. In general, surfactant molecules in a nonpolar solvent can do self-aggregation to form reverse micelles. Depending upon the use of cosolvents (water or polar organic solvents), the reverse micelles are termed as watersolubilizing or polar organic solvent solubilizing reverse micelles and the chemistry occurring in them is partly guided by cosolvents. The reverse micelles are interesting microreactors for heterogeneous chemistry,2-5 templates for nanoparticles,4,5 and models for biological membranes.2,3,5 The development of ultrafast laser spectroscopy made it possible to examine the role of solvent structure and dynamics that has a profound role in controlling the * To whom correspondence should be addressed. E-mail:
[email protected]. Fax: 91-3222-755303. (1) Pal, S. K.; Peon, J.; Zewail A. H. Proc. Natl. Acad. Sci. U.S.A. 2002, 99, 1763. (2) Cho, C. H.; Singh, S.; Robinson, G. W. Faraday Discuss. 1996, 103, 19. (3) de Gennes, P. G. Rev. Mod. Phys. 1992, 64, 645. (4) Moulik, S. P.; Pal, B. K. Adv. Colloid Interface Sci. 1998, 78, 99. (5) Structure and Reactivity in Reverse Micelles; Pileni, M. P., Ed.; Studies in Physical and Theoretical Chemistry Vol. 65; Elsevier: Amsterdam, 1989. (6) Heitele, H. Angew. Chem., Int. Ed. Engl. 1993, 32, 359. (7) Yoshihara, K.; Tominaga, K.; Nagasawa, Y. Bull. Chem. Soc. Jpn. 1995, 68, 696. (8) Jimenez, R.; Fleming, G. R.; Kumar, P. V.; Maroncelli, M. Nature 1994, 369, 471. (9) Lang, M. J.; Jordanides, X. J.; Song, X.; Fleming, G. R. J. Chem. Phys. 1999, 110, 5884. (10) Nandi, N.; Bhattacharyya, K.; Bagchi, B. Chem. Rev. 2000, 100, 2013. (11) Bhattacharyya, K.; Bagchi, B. J. Phys. Chem. A 2000, 104, 10603. (12) Levinger, N. E. Curr. Opin. Colloid Interface Sci. 2000, 5, 118.
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10.1021/la025881x CCC: $22.00 © 2002 American Chemical Society Published on Web 09/06/2002
Intramolecular Charge Transfer of Coumarin 152
organized media.10,11 Fleming et al.8,9 observed the solvation time of water (
