A Femtosecond Study of Excitation Wavelength Dependence of a Triblock Copolymer-Surfactant Supramolecular Assembly: (PEO) 20 -(PPO) 70 -(PEO) 20 and CTAC Shantanu Dey, Aniruddha Adhikari, Ujjwal Mandal, Subhadip Ghosh, and Kankan Bhattacharyya* Physical Chemistry Department, Indian Association for the CultiVation of Science, JadaVpur, Kolkata 700 032, India ReceiVed: January 10, 2008 Solvation dynamics and anisotropy decay of coumarin 480 (C480) in a supramolecular assembly containing a triblock copolymer, PEO 20 -PPO 70 -PEO 20 (Pluronic P123) and a surfactant, CTAC (cetyl trimethylammonium chloride) are studied by femtosecond up-conversion. In a P123-CTAC complex, C480 displays a significant (22 nm) red edge excitation shift (REES) in the emission maximum as λ ex increases from 335 to 445 nm. This suggests that the P123-CTAC aggregate is quite heterogeneous. The average rotational relaxation time (〈τ rot 〉) of C480 in a P123-CTAC complex decreases by a factor of 2 from 2500 ps at λ ex ) 375 nm to 1200 ps at λ ex ) 435 nm. For λ ex ) 375 nm, the probe molecules in the buried core region of P123-CTAC are excited and the solvation dynamics displays three components, 2, 60, and 4000 ps. It is argued that insertion of CTAC in P123 micelle affects the polymer chain dynamics, and this leads to reduction of the 130 ps component of P123 micelle to 60 ps in P123-CTAC. For λ ex ) 435 nm, which selects the peripheral highly polar corona region, solvation dynamics in P123-CTAC and P123 are extremely fast with a major component of <0.3 ps (∼80%) and a 2 ps (∼20%) component. 1. Introduction Water soluble triblock copolymers display interesting struc- tures and rich diversity in phases and have versatile industrial applications. 1-9 In (PEO) 20 -(PPO) 70 -(PEO) 20 (Pluronic P123), the PEO block is highly hydrophilic while the PPO block is extremely hydrophobic and insoluble in water above 288 K. 1 At a temperature above 288 K, dehydration of the PPO blocks leads to the formation of a P123 micelle with a hydrophobic core (PPO block) of radius ∼5.8 nm and a hydrophilic corona (PEO) of thickness 4.2 nm (Scheme 1A). 1-3,8a From Scheme 1A, it is evident that a tri-block copolymer micelle is highly heterogeneous on a molecular length scale and a fluorescent probe of length ∼1 nm should reveal the heterogeneity. However, some of the recent studies on solvation dynamics 4b-c and isomerization 4d did not attempt to delineate the dynamics in different regions of the triblock copolymer micelle. Castner and co-workers studied anisotropy decay in different regions of such a micelle using fluorescent probes with varying hydrophobicity. 5a-b In order to study picosecond solvation dynamics in different regions of an amphiphilic di-block copolymer, Hof and co-workers decomposed the emission spectra into two subspectra and attempted to monitor their time evolution separately. 4a Though this method is quite logical, it involves too many parameters and is often very difficult to separately monitor the contributions of the different subspectra. Recently, we studied dynamics in different regions of a P123 micelle, 6a P123 gel, 6b and P123-SDS aggregate 6c by varying the excitation wavelength (λ ex ) using femtosecond up-conver- sion. Excitation at a shorter wavelength (“blue edge”) selects a solvatochromic probe (e.g., coumarin 480, C480) in a relatively nonpolar environment (e.g., PPO block) and gives rise to a blue- shifted emission spectrum. On the contrary, excitation at a longer wavelength (“red edge”) selects the probe residing at a relatively polar environment (PEO block) and gives rise to a red-shifted emission spectrum. This is known as red edge excitation shift (REES). 10 Thus, one may spatially resolve dynamics in different regions of a heterogeneous system by variation of the excitation wavelength (λ ex ). We have demonstrated this by studying λ ex dependence of solvation dynamics in the micellar 6a and gel 6b phase of a P123 micelle in a P123-SDS aggregate 6c and in a reverse micelle containing an ionic liquid 6d and also in the case of FRET in a reverse micelle 7a and P123 micelle. 7b * Corresponding author. E-mail: pckb@mahendra.iacs.res.in. Fax: (91)- 33-2473-2805. SCHEME 1: Schematic Representation of (A) P123 Micelle and (B) Coumarin 480 (C480) 5020 J. Phys. Chem. B 2008, 112, 5020-5026 10.1021/jp8002257 CCC: $40.75 © 2008 American Chemical Society Published on Web 04/03/2008