Wlodzimierz Jarzeba

Nonexponential Solvation Dynamics of Simple Liquids and Mixtures

Abstract Novel measurements on the microscopic solvation dynamics of coumarin probes in several simple polar solvents and solvent mixtures have been made using the time dependent fluorescence Stokes shift technique. The microscopic solvent relaxation function, C(t), is observed to be poorly modeled by a single exponential decay in many cases. The average experimental solvation times, , for pure solvents and binary solvent mixtures are close to values predicted by dielectric continuum theory, but in many cases, the observed C(t) shape does not agree with that predicted by dielectric continuum theory. The results suggest that molecular motion of solvent molecules near the solute can be responsible for microscopic solvation components of C(t) that are not predicted using bulk dielectric data of the neat solvent and the dielectric continuum theory. In addition to the solvation dynamics results, some potential sources of probe molecule non-ideality are examined, and it is shown that these effects are not significant contributors to experimental error for these C(t) measurements.

A photodynamical model for the excited state electron transfer of bianthryl and related molecules

Abstract The underlying chemical dynamics of the excited state electron transfer of electronically excited 9,9′-bianthryl in polar solvents is explored via a semi-empirical comprehensive theoretical model for the reaction coordinate energy profile and the dynamics along the reaction coordinate. The predictions of the model are in excellent agreement with new femtosecond fluorescence data on bianthryl, which are presented in this paper. The model is comprised of several key elements, including: (i) an Onsager cavity/ semi-empirical treatment for the solvent coordinate; (ii) an electronically adiabatic description of the mixing between the reactant and product zero-order states; (iii) a generalized Langevin equation treatment of the reaction coordinate dynamics where the friction kernel is determined using independent experimental results on solvation dynamics of coumarin probes; and (iv) an empirical solvatochromic/vibronic description for predicting fluorescence and absorption spectra. With a limited amount of parameterization the overall model is able to account in detail for many observables for bianthryl, including the static absorption spectra, the solvent dependence of the static fluorescence spectra, and the time resolved fluorescence spectra. The model supports our previous proposal that the electron transfer kinetics of bianthryl is controlled by polar solvation dynamics.

Evidence for intermolecular hydrogen-bond rearrangement in the electron transfer dynamics of betaine-30 in n-butanol

The electron transfer and ground-state solvation dynamics of betaine-30 (B-30) in n-butanol at low temperatures are investigated by femtosecond pump-probe spectroscopy. Two separate components of the solvent response to solute excitation are observed. At higher temperatures (292-273 K), spectral evolution corresponding to diffusional solvation is evident. However, low-temperature studies (273-193 K) show the presence of a second process characterized by a ≈ 100 ps decay time and weak temperature dependence demonstrating that specific intermolecular hydrogen-bond dynamics are involved in the solvation of B-30 in protic solvents.

Ultrafast experiments on electron transfer

This paper describes new ultrafast fluorescence measurements on the excited state dynamics of polar molecules in polar solvents. Two processes are studied, excited state solvation dynamics and excited state intramolecular electron transfer.

Ultrafast electron transfer in arene - Br atom charge transfer complexes

Abstract Ultrafast pump-probe transient absorption spectroscopy was applied to study, return electron transfer in the areneBr atom charge transfer (CT) complexes. Ultrafast optical excitation of the charge transfer band, of the complex, yields an excited Franck-Condon state of the CT complex, which relaxes to form an ion pair. The rate of charge recombination between the bromide and arene cation strongly depends on arene concentration, indicating on efficient quenching of the ion pair by another donor molecule. The return electron transfer has been studied in various solvents. In general, the observed kinetics is nonexponential, with distribution of relaxation times from 1 ps to ∼1000 ps. The slowest component in the relaxation is attributed to charge recombination of the ion pair in an equilibrium configuration. The rate of charge recombination of the excited areneBr complex does not depend significantly on the donor strength.

Ultrafast experiments on intermolecular electron transfer in the benzene - bromine atom charge transfer complex

Abstract Ultrafast pump - probe measurements have been made on the benzene - bromine atom charge transfer (CT) complex in CCl4 and cyclohexane solutions. Ultrafast optical excitation of the CT band of the complex yields an ion pair, which is comprised of a benzene cation and a bromide anion. The rate of charge recombination between the bromide and the benzene cation in the ion pair has been observed to be much faster than the rate of diffusion apart. The charge recombination rate is accelerated at high benzene concentrations as a result of the formation of a benzene dimer cation - bromide ion pair which undergoes much faster charge recombination than the benzene cation - bromide ion pair.

Solvation Dynamics and Ultrafast Electron Transfer

Ultrafast fluorescence spectroscopy has been used to study two processes: (i) the transient solvation of electronically excited coumarin probes and (ii) the solvent mediated excited state intramolecular electron (charge) transfer of 9,9′—bianthryl and related compounds. The solvation measurements have been analyzed in terms of contemporary theory to gain insight on the molecular aspects of microscopic motion in polar liquids. The excited state electron transfer (et) examples are well modeled by an electronically adiabatic approach, employing an “outer sphere” generalized Langevin equation description of motion along the et reaction coordinate. The relationship between solvation and electron transfer dynamics is discussed.

Ultrafast electron transfer: The role of solvent motion

This paper shows that the electron transfer (ET) time τET of the intramolecular ET reaction of electronically excited bianthryl (BA) is not equal to the longitudinal relaxation time τ1 of the solvent in various polar aprotic solvents. It has been observed that microscopic solvation time τs is very similar to τET for a broad range of polar aprotics.