Determination of energies of quasifree electron state V0 in organic solids from electron transmission spectra

Reactions of electronically excited radical cations of various simple aromatic hydrocarbons (e.g., naphthalene, biphenyl, perylene) and amines (indole and L-tryptophan) with polar solvents (alcohols, ethers, and water) were studied. The radical cations were generated by biphotonic (248 nm) photoionization of aromatic solutes in oxygen-saturated solutions and subsequently excited by 2.3 eV photons from a Nd:YAG laser. The photobleaching is initiated by a transfer of the valence electron of the solvent to the solute radical cation ("hole injection"). In the polar liquids, a proton transfer occurs concurrently with this electron transfer and may occur in several ways. Hole injection is the general mechanism for decay of photoexcited radical cations, in both polar and nonpolar media. The efficiency of this process correlates with the ionization potential (IP) of the solute and optical absorbance of the corresponding radical cation. For high-IP solutes, the quantum yield of the photobleaching is 0.3-0.5. From scavenging of the photoexcited radical cations, it was found that while some of these states have the lifetimes below 1 ps, other have lifetimes of 10-40 ps (in acetonitrile).

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“…For cyclohexane, Hiraoka gives an IP liq of 8.6 eV; 46 this estimate is close to 8.72 eV given by Böttcher and…”

Section: Introductionsupporting

confidence: 86%

Reactions of Photoexcited Aromatic Radical Cations with Polar Solvents

et al. 1998

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“…ε is the Born polarization energy for a sphere of radius a. 9,43,60 Since V 0 's are large (0.6-1 eV), 70 the potential well is deep, and boundto-bound (bb) transitions are possible. Consequently, there are two schools of thought 19.…”

Section: A Absorption Spectra and Photodetachment Cross Sectionsmentioning

confidence: 99%

Photostimulated electron detrapping and the two-state model for electron transport in nonpolar liquids

Shkrob¹,

Sauer²

2005

The Journal of Chemical Physics

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In common nonpolar liquids, such as saturated hydrocarbons, there is a dynamic equilibrium between trapped (localized) and quasifree (extended) states of the excess electron (the two-state model). Using time-resolved dc conductivity, the effect of 1064 nm laser photoexcitation of trapped electrons on the charge transport has been observed in liquid n-hexane and methylcyclohexane. The light promotes the electron from the trap into the conduction band of the liquid. From the analysis of the two-pulse, two-color photoconductivity data, the residence time of the electrons in traps has been estimated as ca. 8.3 ps for n-hexane and ca. 13 ps for methylcyclohexane (at 295 K). The rate of detrapping decreases at lower temperature with an activation energy of ca. 200 meV (280-320 K); the lifetime-mobility product for quasifree electrons scales linearly with the temperature. We suggest that the properties of trapped electrons in hydrocarbon liquids can be well accounted for using the simple spherical cavity model. The estimated localization time of the quasifree electron is 20-50 fs; both time estimates are in agreement with the "quasiballistic" model. This localization time is significantly lower than the value of 310±100 fs obtained using time-domain terahertz (THz) spectroscopy 2.for the same system [E. Knoesel et al., J. Chem. Phys. 121, 394 (2004)]. We suggest that the THz signal originates from the oscillations of electron bubbles rather than the freeelectron plasma; vibrations of these bubbles may be responsible for the deviations from the Drude behavior observed below 0.4 THz. Various implications of these results are discussed.

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“…11,12,13 The expected degree of polarization of alkane molecules appears to be insufficient to bind the excess electron.…”

Section: Introductionmentioning

confidence: 99%

“…difficulty explaining the origin of the trapping potential. 11,12,13 The expected degree of polarization of alkane molecules appears to be insufficient to bind the excess electron.…”

Section: Introductionmentioning

confidence: 99%

Toward Electron Encapsulation: Polynitrile Approach

Shkrob

Sauer

2006

J. Phys. Chem. A

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This study seeks an answer to the following question: Is it possible to design a supramolecular cage that would "solvate" the excess electron in the same fashion in which several solvent molecules do that co-operatively in polar liquids? Such an "encapsulated electron" would be instrumental for structural and dynamics studies of electron solvation and might be useful for molecular electronics. Two general strategies are outlined for "electron encapsulation," viz. electron localization using polar groups arranged on the (i) inside of the cage or (ii) outside of the cage. The second approach is more convenient from the synthetic standpoint, but it is limited to polynitriles. We demonstrate, experimentally and theoretically, that this second approach faces a formidable problem: the electron attaches to the nitrile groups forming molecular anions with bent C-C-N fragments. Since the energy cost of this bending is very high, for dinitrile anions in n-hexane, thebinding energies for the electron are very low and for mononitriles, these binding energies are lower still, and the entropy of electron 2. attachment is anomalously small. Density functional theory modeling of electron trapping by mononitriles in n-hexane suggests that the mononitrile molecules substitute for the solvent molecules at the electron cavity, "solvating" the electron by their methyl groups.We argue that such "solvated electrons" resemble multimer radical anions in which the electron density is shared (mainly) between C 2p orbitals in the solute/solvent molecules, instead of existing as cavity electrons. The way in which the excess electron density is shared by such molecules is similar to the way in which this sharing occurs in large diand poly-nitrile anions, such as 1,2,4,5,7,8,10,11-octacyano-cyclododecane -. The work thus reveals limitations of the concept of "solvated electron" for organic liquids: it might be impossible to draw a clear line between such species and a certain class of radical anions. It also demostrates the feasibility of "electron encapsulation."

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Determination of energies of quasifree electron state V0 in organic solids from electron transmission spectra

Cited by 29 publications

References 10 publications

Reactions of Photoexcited Aromatic Radical Cations with Polar Solvents

Reactions of Photoexcited Aromatic Radical Cations with Polar Solvents

Photostimulated electron detrapping and the two-state model for electron transport in nonpolar liquids

Toward Electron Encapsulation: Polynitrile Approach

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