ALBERT

Notes: It is becoming increasingly clear that the electronic properties of conjugated polymer films are strongly dependent on factors such as the conformation and the degree of aggregation of the polymer strands in the solution from which the film was cast. In this paper, we show how we can take advantage of conjugated ionomers (conjugated polymers that have been functionalized with side groups that can be electrically charged) to control the polymer morphology and degree of interchain interactions in both solutions and the films cast from them. The particular ionomer we study in this work, poly(2,5-bis[N-methyl-N-hexyl amino] phenylene vinylene) (BAMH-PPV), has dialkyl amino side groups that can be controllably charged by protonation with organic acids. In dilute BAMH-PPV solutions, protonation of just a few percent of the amino side groups leads to tight coiling of the polymer backbone, resulting in an enormous blueshift of the polymer's absorption and photoluminescence (PL) spectra. At higher BAMH-PPV solution concentrations, however, protonation of the side groups leads to redshifted emission, indicative of increased interactions between polymer chromophores that presumably result from counterion-mediated attractive interactions. The results suggest that conjugated polymer chromophores in solution interact by interpenetration of neighboring chains rather than by self-aggregation of the chromophores on a single chain. Scanning force microscopy experiments indicate that the surface topography of BAMH-PPV films varies directly with the degree of side-group protonation in the solution from which the film was cast. In addition, BAMH-PPV films cast from protonated solutions have a redder PL spectrum and a higher degree of exciton–exciton annihilation than films cast from neutral solutions, verifying that memory of the chain conformation and degree of chromophore interaction in solution carries through the spin-coating process. The charge-induced changes in the morphology of BAMH-PPV films also lead to dramatic differences in the performance characteristics of BAMH-PPV-based light-emitting diodes. Overall, we believe that the degree of control over the electronic properties of conjugated ionomers makes them attractive candidates for use in a wide variety of optoelectronic devices. © 2002 American Institute of Physics.

Notes: The processes by which solvated electrons are generated and undergo recombination are of great interest in condensed phase physical chemistry because of their relevance to both electron transfer reactions and radiation chemistry. Although most of the work in this area has focused on aqueous systems, many outstanding questions remain, especially concerning the nature of these processes in low polarity solvents where the solvated electron has a fundamentally different structure. In this paper, we use femtosecond spectroscopic techniques to explore the dynamics of solvated electrons in tetrahydrofuran (THF) that are produced in two different ways: ejection by multiphoton ionization of the neat solvent, and detachment via the charge-transfer-to-solvent (CTTS) transition of sodide (Na−). Following multiphoton ionization of the solvent, the recombination of solvated electrons can be well described by a simple model that assumes electrons are first ejected to a given thermalization distance and then move diffusively in the presence of the Coulombic attraction with their geminate cation. The short-time transient absorption dynamics of the THF radical cation in the visible region of the spectrum do not match the kinetics of the solvated electron probed at ∼2 μm, indicating that caution is warranted when drawing conclusions about recombination based only on the dynamics of the solvent cation absorption. With ∼4 eV of excess energy, geminate recombination takes place on the hundreds of picoseconds time scale, corresponding to thermalization distances ≥40 Å. The recombination of solvated electrons ejected via CTTS detachment of Na−, on the other hand, takes place on two distinct time scales of ≤2 and ∼200 ps with kinetics that cannot be adequately fit by simple diffusive models. The fraction of electrons that undergo the fast recombination process decreases with increasing excitation energy or intensity. These facts lead us to conclude that electrons localize in the vicinity of their geminate Na atom partners, producing either directly overlapping or solvent-separated contact pairs. The distinct recombination kinetics for the two separate electron generation processes serve to emphasize the differences between them: multiphoton ionization produces a delocalized electron whose wave function samples the structure of the equilibrium fluid before undergoing localization, while CTTS is an electron transfer reaction with dynamics controlled by the motions of solvent molecules adjacent to the parent ion. All the results are compared to recent experiments on the photodetachment of electrons in aqueous systems where contact pairs are also thought to be important, allowing us to develop a qualitative picture for the mechanisms of electron generation and recombination in different solvent environments. © 2000 American Institute of Physics.

Notes: Charge-transfer-to-solvent (CTTS) transitions have been the subject of a great deal of interest recently because they represent the simplest possible charge transfer reaction: The CTTS electron transfer from an atomic ion to a cavity in the surrounding solvent involves only electronic degrees of freedom. Most of the work in this area, both experimental and theoretical, has focused on aqueous halides. Experimentally, however, halides make a challenging choice for studying the CTTS phenomenon because the relevant spectroscopic transitions are deep in the UV and because the charge-transfer dynamics can be monitored only indirectly through the appearance of the solvated electron. In this paper, we show that these difficulties can be overcome by taking advantage of the CTTS transitions in solutions of alkali metal anions, in particular, the near-IR CTTS band of sodide (Na−) in tetrahydrofuran (THF). Using femtosecond pump–probe techniques, we have been able to spectroscopically separate and identify transient absorption contributions not only from the solvated electron, but also from the bleaching dynamics of the Na− ground state and from the absorption of the neutral sodium atom. Perhaps most importantly, we also have been able to directly observe the decay of the Na−* excited CTTS state, providing the first direct measure of the electron transfer rate for any CTTS system. Taken together, the data at a variety of pump and probe wavelengths provide a direct test for several kinetic models of the CTTS process. The model which best fits the data assumes a delayed ejection of the electron from the CTTS excited state in ∼700 fs. Once ejected, a fraction of the electrons, which remain localized in the vicinity of the neutral sodium parent atom, recombine on a ∼1.5-ps time scale. The fraction of electrons that recombine depends sensitively on the choice of excitation wavelength, suggesting multiple pathways for charge transfer. The spectrum of the neutral sodium atom, which appears on the ∼700-fs charge-transfer time scale, matches well with a species of stochiometry (Na+, e−) that has been identified in the radiation chemistry literature. All the results are compared to previous studies of both CTTS dynamics and alkali metal solutions, and the implications for charge transfer are discussed. © 2000 American Institute of Physics.

Notes: We have used molecular dynamics simulation to explore aqueous solvation dynamics with a realistic quantum mechanical solute, the hydrated electron. The simulations take full account of the quantum charge distribution of the solute coupled to the dielectric and mechanical response of the solvent, providing a molecular-level description of the response of the quantum eigenstates following photoexcitation. The solvent response function is found to be characterized by a 25 fs Gaussian inertial component (40%) and a 250 fs exponential decay (60%). Despite the high sensitivity of the electronic eigenstates to solvent fluctuations and the enormous fractional Stokes' shift following photoexcitation, the solvent response is found to fall within the linear regime. The relaxation of the quantum energy gap due to solvation is shown to play a direct role in the nonradiative decay dynamics of the excited state electron, as well as in the differing relaxation physics observed between electron photoinjection and transient hole-burning (photoexcitation) experiments. A microscopic examination of the solvation response finds that low frequency translational motions of the solvent play an important role in both the inertial and diffusive portions of the relaxation. Much of the local change in solvation structure is associated with a significant change in size and shape of the electron upon excitation. These results are compared in detail both to previous studies of aqueous solvation dynamics and to ultrafast transient spectroscopic work on the hydrated electron.

Notes: Quantum nonadiabatic molecular dynamics simulations are used to directly compute the transient absorption spectroscopy following photoexcitation of equilibrium hydrated electrons. The calculated spectral transients are found to be in excellent agreement with ultrafast traces measured in recent transient spectral hole-burning experiments [Barbara and co-workers, J. Chem. Phys. 98, 5996 (1993); J. Phys. Chem. 98, 3450 (1994)], indicating that the computer model correctly captures the underlying physics. The model transients are dissected into ground state bleach, excited state absorption, and stimulated emission spectral components, each of which is examined individually and analyzed in terms of the microscopic solvent response following photoexcitation. Although there is no distinct spectral hole, bleaching dynamics are found to play an important role in the overall transient spectroscopy. The excited state absorption spectrum undergoes a complex evolution due to solvation dynamics which alters both the frequencies and the oscillator strengths of the relevant quantum transitions. Calculated excited state emission from the electron is characterized by an enormous dynamic Stokes shift as well as an overall spectral narrowing in time. In combination, these three components allow the assignment of features of the measured ultrafast spectroscopic transients in terms of specific details of the microscopic solvent response.