Dynamics and interactions in complexx nanostructured molecular systems observed through time-resolved fluorescence spectroscopy
Keywords: Chemistry.

Time-correlated single photon counting (TCPSC) is a fluorescence spectroscopy technique used to investigate the behavior of chromophores from the hundreds of picoseconds to the tens of nanoseconds time regime. This technique works by measuring the time difference between two photon events: the initial excitation and the time at which a photon is emitted by the chromophore. Each measurement increments the appropriate bin in a histogram by one. As more and more measurements are made, the time-dependent fluorescent behavior is uncovered. With a single fluorescent decay, one can extract the fluorescent lifetime of a species or measure excitation transport. However, by varying the polarization of light exciting the chromophore relative to a fixed resolving polarizer, the orientational dynamics of a probe can also be extracted. The first set of experiments described in this thesis are measurements of orientational dynamics of the fluorophore, perylene, solvated in solutions of room temperature ionic liquids (RTILs). The D2h symmetry of perylene made it possible to extract dynamical information on both in-plane and out-of-plane reorientation from time-resolved fluorescence anisotropy measurements. The RTILs were a series of 1-alkyl-3-methylimidazolium tetrafluoroborates, where alkyl chain varied from butyl to decyl in increments of two and varying the water content. From the anisotropy, the corresponding friction coefficients were determined to eliminate the influence of changes in viscosity caused by both the addition of water and the different alkyl chain lengths. As chain length increased, the addition of water had less of an effect on the local alkyl environment surrounding the perylene. The friction coefficients generally increased with higher water contents. At high water content, the shortest alkyl chain --BmimBF4, broke this general trend, with both in-plane and out-of-plane rotational friction decreasing above a water content of one water per ion pair. While a lot can be learned through the measurements of single wavelength emissions, doing so greatly limits the types of experiments one can study. With complex models, it becomes difficult to justify the difference between different models as many will fit a single trace perfectly. By the construction of time-dependent emission spectra, much more information can be extracted. This is done by collecting fluorescent decays at regular intervals across the emission band under identical experimental conditions. The next study shows proton transfer in the nanoscopic water channels of polyelectrolyte fuel cell membranes using a photoacid, 8-hydroxypyrene-1,3,6-trisulfonic acid sodium salt (HPTS) into the channels. Three fully hydrated membranes, Nafion (DuPont) and two 3M membranes, were studied to determine the impact of different pendant chains and equivalent weights on proton transfer. Measurements of the HPTS protonated and deprotonated fluorescent bands' population decays provided information on the proton transport dynamics. The decay of the protonated band from ~0.5 ns to tens of ns is in part determined by dissociation and recombination with the HPTS. The dissociation and recombination is manifested as a power law component in the protonated band fluorescence decay. Proton transfer dynamics of HPTS in the aprotic solvent 1-methylimidazole (MeIm) were also investigated using fast fluorescence measurements. Wavelength-dependent population dynamics of HPTS in MeIm, resulting from its deprotonation following optical excitation, were collected over the entire fluorescence emission window. Analysis of the time-dependent fluorescence spectra reveal four distinct fluorescence bands that appear and decay on different time scales. We assign these bands to be the conventionally considered protonated (P) and deporotonated (D) HPTS states with 2 additional associated states (A1 and A2). The protonated states decays within the instrument response, but the remaining states were all able to be measured dynamically. The simplest kinetic model, P → A1 →A2→ D, in which the protonated state feeds into a single associated state which in turn feeds into the second associated state which deprotonates provided a quite poor fit. Detailed simulations were performed and identified the most likely kinetic model as protonated feeds into A1 which in turn feeds into both A2 and D, where A¬2 can return to A1 but not D. Another type of experiment in which the time-dependent spectra are necessary for the interpretation is in solvation dynamics of the fluorescent probe coumarin 153 (C153) in polyether sulfone membranes (PES 200) with an average pore size of ~350 nm. For instance, the structural dynamics of a series of 1-alkyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (CnmimNTf2, n = 2, 4, 6, 10) room temperature ionic liquids confined in the PES 200 membranes. The solvation dynamics of C153 in the ionic liquids are multiexponential decay, and the slowest decay component of each bulk liquid matches the slowest component of the liquid dynamics measured by optical heterodyne-detected optical Kerr effect (OHD-OKE) experiments, which is single exponential. The fact that the slowest component of the Stokes shift matches the OHD-OKE data in all four liquids identifies this component of the solvation dynamics as arising from the complete structural randomization of the liquids. Although the pores in the PES membranes are large, confinement on the mesoscopic length scale results in substantial slowing of the dynamics for short chain ionic liquids, however, in the longest chain ionic liquid there was no noticeable confinement effect. The dynamic Stokes shift measurements report on structural relaxation, driven by a dipole created in a chromophore by its excitation from the ground electronic state to the S1 state. In another experiment, we were able to demonstrate that it is also possible to have an additional contribution from orientational relaxation of the Stokes shift chromophore. This effect, called reorientation-induced Stokes shift (RISS), can be observed when the reorientation of the chromophore and the solvent structural relaxation occur on similar time scales. Through a vector interaction, the electronic transition of the chromophore couples to its environment. The orientational diffusive motions of the chromophores will have a slight bias toward reducing the transition energy (red shift) as do the solvent structural diffusive motions. RISS is manifested in the polarization-dependence of C153 in poly(methyl methacrylate) (PMMA). Expressions are derived that permit determination of the structural dynamics by accounting for the RISS contributions. Using these equations, the structural dynamics of the medium can be measured for any system in which the directional interaction is well represented by a first order Stark effect and RISS is observed. The theoretical results are applied to the PMMA data, and the structural dynamics are obtained and discussed. Finally, the time-dependent photoluminescence of a broadband white light emitting perovskite -- 2,2'-(ethylenedioxy)bis(ethylammonium) tetrabromoplumbate, (EDBE)PbBr4 -- was measured using TCSPC. Time-dependent spectra were generated over a very broad time range from 100 ps to 100 ns. The main luminescent decay is a tri-exponential with time constants, 1.20 ns, 8.48 ns, and 21.75 ns.. Analysis of the time-dependent spectra showed a distinct spectral side peak which has a decay constant of 0.6 ns. After this peak decays, the emission line shape remains constant throughout the entire photoluminescent decay, demonstrating that the emission occurring from all wavelengths arises from a single ensemble. Therefore, the white light emission does not arise from a set of subensembles with different emission wavelengths and lifetimes. However, experiments on (EDBE)PbBr4 spin coated thin films luminescence decay is different, but it is still non-exponential and, like the crystal samples, occurs from a single ensemble after the relaxation of the rapidly decay side band. The spin coated samples have faster decays than the large single crystals