Non-negligible Outer-Shell Reorganization Energy for Charge Transfer in Nonpolar Systems

Area of Science:

• Solid-state physics
• Materials science
• Physical chemistry

Background:

• Charge transport in molecular systems often involves nonpolar solids.
• Outer-shell reorganization energy (λ) is typically neglected in these systems due to low dielectric constants.
• Ignoring λ can lead to inaccurate predictions of charge transport properties.

Purpose of the Study:

• To estimate the outer-shell reorganization energy (λ) in nonpolar molecular systems.
• To investigate the influence of λ on charge transport mechanisms.
• To compare λ values obtained from experimental data and molecular dynamics simulations.

Main Methods:

• Estimation of λ from experimental ultraviolet photoelectron spectra by fitting vibronic progressions.
• Calculation of λ using molecular dynamics (MD) simulations of nonpolar molecules.
• Analysis of spectral density functions to understand the contributions of low- and high-frequency dynamics to λ.

Main Results:

• Upper bounds for λ were estimated as 505 meV (anthracene) and 549 meV (pentacene) from experimental data.
• MD simulations yielded λ values of 212 meV (anthracene) and 170 meV (pentacene).
• λ is influenced by both low-frequency (molecular motion) and high-frequency dynamics, with amorphous states showing significant high-frequency contributions.

Conclusions:

• Outer-shell reorganization energy (λ) is comparable to inner-sphere reorganization energy and significantly impacts charge transport.
• Both crystalline and amorphous nonpolar systems exhibit distinct spectral density behaviors (super-Ohmic and sub-Ohmic, respectively).
• Accurate modeling of charge transport requires the inclusion of outer-shell reorganization energy.