Ultrafast transient absorption spectroscopy investigations of excited state dynamics in SWNT/polymer composites and energy transfer between covalently appended components in [Ru(dmb)2(bpy-an)]2+.

摘要

This dissertation is primarily concerned with the description of excited state dynamics within nanoscale and molecular systems. Particularly the relaxation of carriers in single-walled carbon nanotube bundles following photoexcitation is studied. These structures are imbedded within a polymer film and the dependence on environment is also examined. The work is divided into four chapters. Chapter 1 provides an introduction to nanoscale materials as well as an overview of energy transfer studies conducted in our lab. Chapter 2 describes the specifics of the experimental methods used. The final two chapters comprise the results of the dissertation work.; In Chapter 3 wavelength-resolved femtosecond transient absorption spectroscopy is used to follow the electronic dynamics of single-walled carbon nanotubes/polymer films following visible (550 nm) and near infrared (900 nm) photoexcitation. Electron-hole (e-h) pairs give rise to sharp features in the transient spectra that decay in amplitude and exhibit rapid spectral shifts. The decay reflects (e-h) recombination on both short (1.3 ps) and long (35 ps) time scales. The origin of the two timescales is unclear, but is consistent with either geminate/nongeminate recombination or trap assisted relaxation. Blue shifts in the excited state absorption bands are also observed and attributed to cooling of the excited state carriers to a low lying state. Upon visible excitation, transient spectra also exhibit a broad photobleach at early times that corresponds to the relaxation of a pi-plasmon band. Changing the polymer environment of the nanotubes does not affect the relaxation dynamics observed. There is, however, a shift in the spectral positions of some excited state absorptions. This may indicate the presence of nanotube bundle states.; Chapter 4 describes additional work using TA spectroscopy to measure the timescale for triplet-triplet energy transfer (EnT) between a Ru(II) chromophore and a ligand bound anthracene acceptor in [RuII(dmb)2 (bpy-an)]2+ (dmb= 4,4'-dimethyl-2,2 '-bipyridine; bpy-an = 4-(9-anthrylethylene), 4' -methyl-2,2'-bipyridine). The appearance of anthracene excited state absorption is monitored following photoexcitation at 450 nm to a metal-to-ligand charge transfer (MLCT) state. The anthracene absorption grows in with biphasic kinetics and the two components are attributed to energy transfer (16 ps) and interligand electron transfer (27 ps).