Article pubs.acs.org/JPCB
Effect of Hydroxyl Group Substituted Spacer Group of Cationic Gemini Surfactants on Solvation Dynamics and Rotational Relaxation of Coumarin-480 in Aqueous Micelles Amit K. Tiwari, Sonu, and Subit K. Saha* Department of Chemistry, Birla Institute of Technology and Science, Pilani 333 031, Rajasthan, India S Supporting Information *
ABSTRACT: The solvation dynamics and rotational relaxation of Coumarin 480 (C-480) have been investigated in the micelles of a series of gemini surfactants, 12-4(OH)n-12 (n = 0, 1, and 2), with increasing hydroxyl group substitution within the spacer group. Steady-state and time-correlated single photon counting (TCSPC) fluorescence spectroscopic techniques have been used to carry out such study. Steady-state and TCSPC fluorescence data support the location of probe molecule at the Stern layer. The solvation dynamics is found to be slower on hydroxyl substitution of spacer group due to the formation of hydrogen bonds between water molecules and hydroxyl group(s) of spacer group. Such kind of hydrogen bonding protects the probe molecule from its contact with water molecules and also results in restricted mobility of water molecules. The average rotational relaxation time increases on increasing number of substituted hydroxyl group on a spacer group. It is because of formations of more and more close packed micelles and larger extent of intermolecular hydrogen bonding interactions between C-480 and hydroxyl group(s). For micelles of each of 12-4-12 and 12-4(OH)-12, the slow rotational relaxation is dominated by the lateral diffusion of the fluorophore along the spherical surface of the micelle. However, for 12-4(OH)2-12, the slow rotational relaxation is mainly due to the rotational motion of the micelle as a whole. Because of high microviscosity of micelles of 12-4(OH)2-12 and greater extent of hydrogen bonding interactions with C-480, the relaxation time corresponding to the lateral diffusion of the fluorophore is very high in this case. Micelles14−17 and mixed micelles18−20 have been very fascinating systems to study the solvation dynamics of water. The bimodal behavior of solvation dynamics of water molecules in the micelles of conventional surfactants are well studied.16,17 Bhattacharyya and co-workers first reported the solvation dynamics of C-480 and 4-aminopthalimide (4-AP) in the micelles of conventional surfactants, i.e., Triton X-100 (TX), cetyltrimethylammonium bromide (CTAB) and sodium dodecyl sulfate (SDS). They did not observe much difference between the average solvation time using two probes in these micelles and concluded that the solvation dynamics in the micelles weakly dependent on the probe.16,17 Their timedependent Stokes’ shift studies with C-480 first showed that the
1. INTRODUCTION The water molecule plays a central role in biological systems.1 There are several techniques such as NMR relaxation dispersion,2 dielectric relaxation3 and solvation dynamics available4−7 to study the properties of water molecules in various systems of molecular assemblies. Because of higher sensitivity toward time and length scale, solvation dynamics has been found to be one of the best techniques.4−7 Maroncelli and co-workers8−11 have studied the solvation dynamics using Coumarin-480 (C-480) and Coumarin-343 (C-343) as probes and reported very fast (
