Deuterium Isotope Effect on Femtosecond Solvation Dynamics in an Ionic Liquid Microemulsion: An Excitation Wavelength Dependence Study Dibyendu Kumar Sasmal, Supratik Sen Mojumdar, Aniruddha Adhikari, and Kankan Bhattacharyya* Physical Chemistry Department, Indian Association for the CultiVation of Science, JadaVpur, Kolkata 700 032, India ReceiVed: NoVember 18, 2009; ReVised Manuscript ReceiVed: January 18, 2010 The deuterium isotope effect on the solvation dynamics and the anisotropy decay of coumarin 480 (C480) in a room temperature ionic liquid (RTIL) microemulsion is studied by femtosecond up-conversion. The microemulsion consists of the RTIL 1-pentyl-3-methyl-imidazolium tetra-fluoroborate ([pmim][BF 4 ]) in triton X-100 (TX-100)/benzene. Replacement of H 2 O by D 2 O in the microemulsion causes retardation of solvation dynamics. The average solvation time of C480 (〈τ s 〉) in RTIL microemulsion with 5 wt % D 2 O is ∼1.5-1.7 times slower compared to that in the H 2 O containing RTIL microemulsion. This suggests that the main species in the microemulsion responsible for solvation is the water molecules. In both D 2 O and H 2 O containing RTIL microemulsion, the solvation dynamics exhibits marked dependence on the excitation wavelength (λ ex ) and becomes about 15 times faster as λ ex increases from 375 to 435 nm. This is ascribed to the structural heterogeneity in the RTIL microemulsion. For λ ex ) 375 nm, the region near the TX-100 surfactant is probed where bound water molecules cause slow solvation dynamics. At 435 nm, the RTIL pool is selected where the water molecules are more mobile and hence gives rise to faster solvation. The average time constant of anisotropy decay shows opposite dependence on λ ex and increases about 2.5-fold from 180 ps at λ ex ) 375 nm to 500 ps at λ ex ) 435 nm for D 2 O containing RTIL microemulsion. The slower anisotropy decay at λ ex ) 435 nm is ascribed to the higher viscosity of RTIL which causes greater friction at the core. 1. Introduction Room temperature ionic liquids (RTILs) have received a lot of recent attention as an environmentally benign (“green”) solvent and for their diverse applications. 1–5 The heterogeneous (bicontinuous) nature of RTIL renders it a phase-transfer catalyst-like behavior and may be responsible for the unique effects of a RTIL on a chemical reaction. The polar solvation dynamics in neat RTIL have been addressed in many recent works. 6–8 The unique properties of RTIL have inspired many theoretical studies and computer simulations. 9,10 The simulations predict nanostructural organization with clear segregation of polar (ionic) and nonpolar (organic) domains in a neat RTIL. 9,10 The predicted microheterogeneity is confirmed by recent small- angle X-ray scattering, 11a optical Kerr effect, 11b and Raman scattering 11c studies. Recently, our group 12c and Maroncelli and co-workers 12d demonstrated dynamic heterogeneity in neat RTIL by excitation wavelength (λ ex ) dependence of solvation dynam- ics. We have showed that, in neat RTIL, the average solvation time decreases 6 times from 860 to 135 ps as λ ex increases from 375 to 435 nm. 12c It is proposed that, at a short λ ex (375 nm), the fluorescent probes at the nonpolar domain (near the alkyl chains) are excited while a long λ ex (435 nm) selectively excites the polar regions near the counterions. Recent computer simulations have carefully delineated the role of collective motion of the cation and the anion and polarizability in a RTIL. 10 Many amphiphilic molecules self-assemble in a RTIL to form micelles. 13–18 Several groups studied the solvation dynamics of coumarin 153 (C153) in amphiphilic ionic liquids, and their micelles in water. 12,13,18 The magnitude of retardation (∼2 times) of solvation dynamics for a micelle in a RTIL compared to that in a neat RTIL is much smaller than that (100-1000 times) observed in the case of a micelle in water. 19–23 Recently, many groups reported the formation of reverse micelles and microemulsions involving RTILs. 16,17 Gao et al. 16 and Eastoe et al. 16d reported the formation of microemulsion with a pool of the RTIL ([bmim][BF 4 ]) inside a reverse micelle containing the surfactant triton-X-100 (TX-100) in a hydrocar- bon. From SANS studies, Eastoe et al 16d concluded that the ionic liquid pool is ellipsoidal in shape with a semiminor radius of 2.4 nm and a length of 11 nm for an equimolar ratio of [bmim][BF 4 ] and the surfactant TX-100. Gao et al. reported that about 6 wt % water may be encapsulated in the polar region of such a microemulsion. 16a We have previously studied a micro- emulsion consisting of the ionic liquid 1-pentyl-3- methylimidazolium tetraflouroborate ([pmim][BF 4 ]), in TX-100/benzene. 12a,c In a RTIL microemulsion, many species may affect solvation dynamics and anisotropy decay (molecular rotation). These include the polar head groups of the surfactant, counterions of RTIL, and water. In the present work, we attempt to examine the role of water by studying the deuterium isotope effect. We will show that replacement of H 2 O by D 2 O markedly affects solvation dynamics in a RTIL microemulsion while anisotropy decay remains unaffected. This shows that solvation dynamics and anisotropy decay in RTIL microemulsion originate from different sources. We also examine the red edge excitation shift (REES) 24,25 like λ ex dependence in the H 2 O and D 2 O containing microemulsions. Many groups have previously studied the deuterium isotope effect on solvation dynamics in bulk water 26 and other * To whom correspondence should be addressed. E-mail: pckb@ iacs.res.in. Fax: (91)-33-2473-2805. J. Phys. Chem. B 2010, 114, 4565–4571 4565 10.1021/jp910948w  2010 American Chemical Society Published on Web 03/17/2010