Ultrafast Energy Migration in Chromophore Shell-Metal Nanoparticle Assemblies Oleg P. Varnavski, Mahinda Ranasinghe, Xingzhong Yan, Christina A. Bauer, Sung-Jae Chung, Joseph W. Perry, Seth R. Marder, and Theodore Goodson, III* Department of Chemistry, UniVersity of Michigan, Ann Arbor, Michigan 48109, Department of Chemistry, UniVersity of Arizona, Tucson, Arizona 85721, and School of Chemistry and Biochemistry and Center for Organic Photonics and Electronics, Georgia Institute of Technology, Atlanta, Georgia 30333 Received February 27, 2006; E-mail: tgoodson@umich.edu There is great interest in ligand-coated nanoparticles for a variety of sensing and imaging applications. 1 The ability to attach functional ligands, including chromophoric ligands, to nanoparticles offers opportunities to prepare multifunctional systems with a high local concentration of chromophores, and such systems could have inter- esting light harvesting properties. 2 Stellacci et al. 3 have reported on silver nanoparticles coated with a self-assembled shell of thiol- functionalized bis-1,4-(4-nitrostyryl)benzene chromophores, with an average of ∼2000 chromophores per particle. These nanopar- ticles exhibited a large two-photon absorption cross section, δ, of ∼2.7 × 10 -45 cm 4 s, which corresponds roughly to a linearly addi- tive contribution of individual molecules on the surface. 3 Interest- ingly, the chromophores on the particles exhibited little fluorescence quenching (quantum yield ) 0.33 vs 0.47 for free chromophore) despite the high local concentration of the chromophore, which frequently leads to self-quenching interactions. Recently, chro- mophore-coated silver nanoparticles with greatly increased solubility have been synthesized, 4 and these particles are highly fluorescent (Φ ) 0.5), again with little fluorescence quenching. Given the high local concentration of chromophores and efficient fluorescence, we performed studies utilizing ultrafast time-resolved spectroscopic methods to probe whether interactions between chromophores could lead to energy migration on the surface of the particle. The structure of the 11-(2,5-bis(4-(diethylamino)styryl)-4-meth- oxyphenoxy)undecane-1-thiol chromophore, 1, and a schematic of the silver particle-chromophore assembly are shown in Scheme 1. From estimates based on chemical analysis and transmission electron microscopy (TEM), the number of chromophores on the particle is on the order of ∼2000. Similar to the previously examined system, 3 this silver- chromophore assembly exhibits a very large δ (1.5 × 10 -44 cm 4 s) and relatively high fluorescence quantum yield of 0.42 (0.8 for the free dye) in toluene. 4 The linear absorption and fluorescence spectra of a solution of the chromophore-shell silver particles system, and the free dye 5 in dilute and concentrated solutions is shown in Figure 1. In this system, the silver core acts primarily as a center for assembly of the chromophores. We observe essentially no evidence of significant local field effects on the two-photon spectrum or peak cross section or on the fluorescence spectrum or radiative decay, 4 which may be due in part to the distance of the chromophore from the surface (about 1.5 nm,) its orientation relative to the surface, and the detuning from the plasmon resonance. The absorption and fluorescence spectra for the dilute and concentrated solutions of the free chromophore are very similar. There is not a significant shift in the spectra even when the concentration is changed by more than 5 orders of magnitude. The fluorescence intensity exhibits a linear increase as a function of the concentration of chromophores (over the range of 2 × 10 -6 M to 10 -1 M), which is consistent with weakly interacting chromo- phores. This behavior is in contrast to that of many other chromo- phore systems with highly polarizable π-electron systems that show significant spectral shifts and self-quenching at large molar concentration. 6 To probe the excited state dynamics of the chromophore, time- resolved fluorescence upconversion measurements 7 were carried out in dilute and concentrated chromophore solutions as well as the chromophore-silver particle assembly using the second har- monic of a Ti-sapphire femtosecond laser for excitation (λ ex ) 400 nm). For the dilute chromophore solutions, one observes a short rise-time feature (∼150 fs), as shown in Figure 2, that is similar to what has been reported previously with other distyrylbenzene chromophores. 8 For highly concentrated solutions (10 -1 M), an additional rise-time component of 1.3 ps that is thought to be associated with dye-dye energy transfer is observed (Figure 2). The chromophore/metal particle assembly showed different dynam- ics with no detectable rise-time component. Comparison of this result with that of the concentrated dye suggests a much faster energy transfer time for the metal particle system. These results are consistent with a relatively dense chromophore packing, in accord with the large number of chromophores estimated by elemental and TEM analysis. 4 The different dynamics for the dye- nanoparticle assembly as compared to the diluted dye may also reflect the different solvation environment for densely packed Scheme 1. Structure of Thiolated Chromophore 1 and Schematic of Chromophore-Silver Particle Assembly Figure 1. Normalized absorption and fluorescence spectra of silver particle assemblies (dilute) and free dye (dilute: 2 × 10 -6 M and concentrated: 10 -1 M) in toluene solution. Published on Web 08/09/2006 10988 9 J. AM. CHEM. SOC. 2006, 128, 10988-10989 10.1021/ja061378l CCC: $33.50 © 2006 American Chemical Society