Time- and Angle-resolved photoelectron spectroscopy from surfaces can be used to record the dynamics of electrons and holes in condensed matter on ultrafast time scales. However, ultrafast photoemission experiments using extreme-ultraviolet (XUV) light have previously been limited by either space-charge effects, low photon flux, or limited tuning range. In this article, we describe XUV photoelectron spectroscopy experiments with up to 5 nA of average sample current using a tunable cavity-enhanced high-harmonic source operating at 88 MHz repetition rate. The source delivers >1011 photons/s in isolated harmonics to the sample over a broad photon energy range from 18 to 37 eV with a spot size of 58 × 100 μm2. From photoelectron spectroscopy data, we place conservative upper limits on the XUV pulse duration and photon energy bandwidth of 93 fs and 65 meV, respectively. The high photocurrent, lack of strong space charge distortions of the photoelectron spectra, and excellent isolation of individual harmonic orders allow us to observe laser-induced modifications of the photoelectron spectra at the 10−4 level, enabling time-resolved XUV photoemission experiments in a qualitatively new regime.
Extensive reductive chemical doping in four conjugated polymers showed evolution of optical spectra for negative polarons and more reduced species. Delocalization lengths of the polarons, which ranged from 2 to 6 nm, were determined from measurements of bleaching of the neutrals, with the extinction coefficients measured by pulse radiolysis. A particular advantage of reductive doping is the ability to encapsulate the Na + counterions in the C 222 cryptand to control the interaction of charges with counterions. For lightly doped chains C 222 had little effect on the delocalization lengths or spectra of the two polaron transitions P 1 and P 2 , perhaps because most were completely dissociated to free ions. C 222 did strongly alter the spectra when many electrons were added to a chain. For the shortest polarons, 2 nm in poly(phenylene-vinylene) (PPV), energies of the P 1 and P 2 transitions increased with the extent of reduction. The effect on the P 1 transition was greater in the absence of C 222 indicating ion-pairing equilibria for the short PPV polarons. Highly reduced ions formed upon injection of multiple electrons included polarons compressed by factors of four or more from their normal lengths to ∼1 charge/nm: a highly reduced 60 nm long chain contained ∼60 electrons. For compressed polarons the transitions shifted with increasing reduction indicating sensitivity to counterions: ion pairing is an important determinant of the behavior upon multiple reductions.
Reduction potentials have been determined for two molecules, benzophenone (BzPh) and perylene (Per), effectively in the complete absence of electrolyte as well as in the presence of three different supporting electrolytes in the moderately polar solvent THF. A description of how this can be so, and qualifications, are described in the discussion section. The primary tool in this work, pulse radiolysis, measures electron transfer (ET) equilibria in solution to obtain differences in redox potentials. Voltammetry measures redox potentials by establishing ET equilibria at electrodes, but electrolytes are needed for current flow. Results here show that without electrolyte the redox potentials were 100-451 mV more negative than those with 100 mM electrolyte. These changes depended both on the molecule and the electrolyte. In THF the dominant contributor to stabilization of radical anions by electrolyte was ion pairing. An equation was derived to give changes in redox potentials when electrolyte is added in terms of ion pair dissociation constants and activity coefficients. Definite values were determined for energetics, ∆G d°, of ion pairing. Values of ∆Gd° for pairs with TBA + give some doubt that it is a "weaklycoordinating cation." Computations with DFT methods were moderately successful at describing the ion paring energies. I.
The nature of electron and hole polarons on poly(9,9-di-n-hexylfluorenyl-2,7-diyl) (pF) and a copolymer poly [(9,9-di-n-octylfluorenyl-2,7-diyl)-alt-(benzo[2,1,3]thiadiazol-4,8-diyl)] (F8BT) has been studied by chemical doping, pulse radiolysis, charge modulation spectroscopy, quantum chemical calculations and microwave conductivity. While pF exhibits very similar behavior in all respects for the electron and the hole, this paper explores the hypothesis that the donor acceptor (push-pull) nature of F8BT will tend to localize charges.Optical spectra and quantum chemical calculations point to an electron localized on the thiadiazole unit in polar liquids, but becoming more delocalized as the solvent polarity decreases. Indeed, in the non-polar solvent benzene, the electron mobility is only 2.7 times lower than that of the hole, which conversely is shown to be delocalized in all environments and has a similar mobility to polarons on the homopolymer polyfluorene. Advantageous modifications to the optoelectronic properties of conjugated polymers, that come about by using alternating donor acceptor repeat units, have thus been shown to not significantly hinder charge transport despite the corrugated energy landscape along the backbone.
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