Qualitatively new paradigms for materials design and functionality will be realized when optical and photonic devices are pushed down to the nanoscale. Broadly defined, nanophotonics is the study of light and its interaction with features and systems that are tens to hundreds of nanometers in size. Using a combination of ultrafast techniques, we are studying the flow of charge and energy within a variety of nanoscale systems, including supramolecular assemblies, SWCNT composites, and semiconducting nanowires.
C. N. Fleming, K. A. Maxwell, J. M. DeSimone, T.J. Meyer, and J. M. Papanikolas "Ultrafast Excited-State Energy Migration Dynamics in an Efficient Light-Harvesting Antenna Polymer Based on Ru(II) and Os(II) Polypyridyl Complexes" J. Am. Chem. Soc., 2001, 123, 10336-10347.
G. B. Shaw and J. M. Papanikolas "Triplet-Triplet Annihilation of Excited States of Polypyridyl Ru(II) Complexes Bound to Polystyrene" J. Phys. Chem. B, 2002, 106, 6156-6162.
C. N. Fleming, L. M. Dupray, J. M. Papanikolas, and T. J. Meyer "Energy Transfer Between Ru(II) and Os(II) Polypyridyl Complexes Linked to Polystyrene" J. Phys. Chem. A, 2002, 106, 2328-2334.
C.N. Fleming, P.Jang, T. J. Meyer, and J.M. Papanikolas "Energy Migration Dynamics in a Ru(II) and Os(II) Based Antenna Polymer Embedded in a Disordered, Rigid Medium" J. Phys. Chem. B, 2004, 108, 2205-2209.
While the nature of the [Ru(bpy)3]2+ and [Os(bpy)3]2+ excited states at long times after photoexcitation are generally agreed upon, many questions regarding evolution on faster time scales still persist. Our group is using femtosecond absorption and emission spectroscopies to study the excited state dynamics in functionalized compounds. We characterize in detail the relaxation processes (ISC, vibrational relaxation, interligand electron-transfer, solvent reorganization) that follow photoexcitation, with the goal of understanding how one can control the flow of charge and energy within and amongst the molecular subunits.
M.K. Brennaman, J. H. Alstrum-Acevedo, P. Jang, C. N. Fleming, T. J. Meyer, and J. M. Papanikolas “Turning the [Ru(bpy)2(dppz)]2+ Light-Switch On and Off with Temperature” J. Am. Chem. Soc., 2002, 124, 15094-15098.
G. B. Shaw, C. L. Brown, and J.M. Papanikolas “Investigation of Interligand Electron Transfer in Polypyridyl Complexes of Os(II) Using Femtosecond Polarization Anisotropy Methods: Examination of Os(bpy)32+ and Os(bpy)2(mab)2+” J. Phys. Chem. A, 2002, 106, 1483-1495.
G. B. Shaw, D. J. Styers-Barnett, E. Z. Gannon, J. C. Granger, and J. M. Papanikolas “Interligand Electron Transfer Dynamics in Os(bpy)32+: Exploring the Excited State Potential Surfaces Using Femtosecond Spectroscopy” J. Phys. Chem. A, 2004, 108, 4998-5006.
A second focus of my research effort is on the characterization of low-frequency (<200 cm-1) motion in interfacial environments. Although the study of low-frequency modes (e.g. rotation and translation) in bulk liquids has been an active area of study for many years, little is known about the low-frequency motion that characterizes molecules located at the interface. χ(2)-based techniques, such as surface second harmonic generation, directly probe the interface. But, while these methods are surface specific, they are not suited for the investigation of low-frequency motion.
Our research in this area is aimed at developing spectroscopic methods for characterizing these motions in interfacial environments. We recently demonstrated a new interface specific transient grating technique that can be used to study interfacial motion. This spectroscopic method, which is carried out in a degenerate five-wave mixing geometry (D5WM), is analogous to one class of D4WM experiments (i.e. χ(3)-based) used to study the low frequency motion of bulk liquids. The χ(4)-based techniques are an extension of χ(3)-based nonlinear spectroscopies to surface and interface environments, much in the same way that χ(2)-based techniques represent an extension of linear spectroscopies to provide interface specificity.
T. Kikteva, D. Star, A. M. D. Lee, G. W. Leach, and J. M. Papanikolas “Five Wave Mixing: Surface-Specific Transient Grating Spectroscopy as a Probe of Low Frequency Intermolecular Adsorbate Motion” Phys. Rev. Lett., 2000, 85, 1906-1909.