Molecular Excited-State Relaxation Dynamics at the Colloidal Microparticle Interface Monitored with Pump-Probe Second Harmonic Generation Louis H. Haber † and Kenneth B. Eisenthal* Department of Chemistry, Columbia University, New York, New York 10027, United States ABSTRACT: Time-resolved second harmonic generation is used to monitor the excited-state relaxation dynamics of molecules adsorbed to the surface of colloidal microparticles suspended in solution. The cationic organic dye, malachite green (MG), is adsorbed to the negatively charged surface of polystyrene sulfate microparticles in water. MG is photoexcited to the S 1 excited state by a 615 nm pump pulse. The time- dependent change of the S 0 ground-state depletion is probed by second harmonic generation of an 800 nm pulse as a function of pump-probe delay to obtain a lifetime of 5.7 ± 0.4 ps. This excited-state lifetime is approximately three times longer than the corresponding lifetime at the air/water interface, showing the significant effect of the negatively charged surface on local friction, which is important in the energy relaxation of photoexcited MG. 1. INTRODUCTION Understanding the chemical and physical properties of molecules at the surface of microparticles or nanoparticles in solution is of fundamental scientific interest and is important for the development and optimization of applications in a broad range of fields such as catalysis, photovoltaics, and biology. 1-4 Second harmonic generation (SHG), where two incident photons of frequencies ω combine to generate a photon at frequency 2ω, 5,6 is a powerful technique for studying molecules at the interface of colloidal particles and the liquid in which they are suspended. 4 Coherent SHG is forbidden in bulk media that have inversion symmetry or are isotropic. However, SHG can be generated at the surface of centrosymmetric particles of size that is roughly the length scale of the incident light. Thus SHG is a surface-sensitive spectroscopy that can probe the surface of small, nano- to microsized centrosymmetric particles. 7,8 Previous SHG studies on colloids have investigated adsorption isotherms of molecules to the surface of colloidal particles made of polystyrene, 9-14 clay, 15 TiO 2 , 16 carbon black, 17 gold, 18 and silver. 19 Kinetic SHG measurements have investigated processes such as molecular transport across a liposome membrane 20,21 as well as enzyme binding, cutting, and rehybridization of DNA attached to microparticles 22 on the time scales of seconds to hours. SHG spectroscopy, obtained by scanning the fundamental laser frequency, has measured the electric quadrupole plasmon energy of colloidal silver nano- particles 23 and the spectrum of the charge-transfer complex of catechol on colloidal TiO 2 particles. 16 The interference between the dipole and octupole SHG response from spherical gold nanoparticles has been determined under varying input polarizations for comparison to theoretical predictions. 24 SHG measurements have also been used to obtain the electrostatic potential 25 and surface acidity pK a value 26 of charged colloidal particle surfaces. In addition, by measuring the angular scattering dependence of SHG from colloidal particles, 27-30 information on the molecular orientation distribution at the particle surface has been recently determined. 30 Time-resolved second harmonic generation (TR-SHG) and time-resolved sum frequency generation (TR-SFG) are established techniques for studying molecular dynamics at planar interfaces such as the air/water interface. 31-33 TR-SHG has been used to investigate molecular rotational dynamics, 34 relaxation dynamics of electronic molecular excited-states, 35-40 electron and energy transfer between molecules, 41 and solvation dynamics 42,43 at the air/water and liquid/liquid interfaces. Recent work has also used TR-SHG to determine the orientational distribution of molecules 44 and the formation dynamics of hydrated electrons 45 at the air/water interface. Ultrafast dynamics of electron and hole carriers at solid state interfaces in organic light-emitting diodes, 46 fullerene/poly- imide double-layers, 47 and quantum dot thin films on titanium dioxide 48 have been studied using time-resolved electric-field induced second harmonic generation measurements. On much longer time scales of minutes to hours, TR-SHG has been used to study DNA hybridization at fused quartz/aqueous interfaces. 49 Special Issue: B: Paul F. Barbara Memorial Issue Received: May 2, 2012 Revised: August 13, 2012 Published: August 28, 2012 Article pubs.acs.org/JPCB © 2012 American Chemical Society 4249 dx.doi.org/10.1021/jp304242c | J. Phys. Chem. B 2013, 117, 4249-4253