Electron-Transfer Reorganization Energies of Isolated Organic Molecules

Abstract

He I photoelectron spectra of phenanthrene (1), 1,10-phenanthroline (2), phenazine (3), dibenzo[a,c]anthracene (4), dibenzo[a,c]phenazine (5), and dipyrido[3,2-a;2‘3‘-c]phenazine (6) have been obtained. Assignment of the π ionization states was aided by electronic structure calculations: the first ionization state of 1, 2B1(π1), is observed at 7.888 ± 0.002 eV, 2B2(π1) of 2 is at 8.342 ± 0.002 eV, and 2B1g(π1) of 3 is at 8.314 ± 0.002 eV. Spectra of 4−6 are reported for the first time: 2A2(π1) of 4 is at 7.376 ± 0.002 eV, and both 5 (7.983 ± 0.002 eV) and 6 (8.289 ± 0.002 eV) exhibit quasi-degenerate first and second ionization states. Quantum-mechanical reorganization energies, λQM, were extracted from analyses of vibrational structure: values are 149 ± 5 (1), 167 ± 5 (2), 68 ± 2 (3), and 92 ± 4 (4) meV. Low-frequency modes were treated semiclassically: values of λ SC are estimated to be 21 ± 1 (1), 13 ± 1 (2), 22 ± 1 (3), 66 ± 1 (4), 27 ± 9 (5), and 16 ± 1 (6) meV. Reorganization energies (λ = λQM + λSC) of isolated molecules are 170 ± 5 (1), 180 ± 5 (2), 90 ± 2 (3), and 158 ± 4 (4) meV. Density functional calculations (B3LYP/6-311G++(d,p)) give λ values that are on average 63 meV lower than experimentally derived energies.

Assessment of density-functional models for organic molecular semiconductors: The role of Hartree–Fock exchange in charge-transfer processes

“…We particularly remark again the higher accuracy of the PBE-ak and OB95-ak models which support its future inclusion in computational studies of p-conjugated materials. Furthermore, we detect for some systems (molecules IV-VI) an asymmetry in the curvature of the potential energy curves of neutral molecules and corresponding cations, much more marked than for oligoacenes; thus, the assumption of equal potential surfaces for both neutral and charged systems [104] might be affecting the deduced experimental results and thus their values might be slightly overestimated.…”

“…In view of the promising results obtained with the newly derived models, these DFT-ak computational schemes have been applied next with the 6-31G * basis sets to a new set of molecules for which experimental results are also available [104,105]. The new set is presented in Fig.…”

An electronic time scale in chemistry

“…In charge transfer literature (e.g., ref. 43), it is the ϩ reorganization energy of the isolated molecule. In terms of ordinary spectroscopy, the nuclear rearrangement upon ionization makes necessary the distinction between a vertical IP when the nuclei are frozen and the adiabatic IP when the ion is in its equilibrium nuclear configuration.…”

Importance of Intermolecular Interactions in Assessing Hopping Mobilities in Organic Field Effect Transistors: Pentacene versus Dithiophene-tetrathiafulvalene

“…[11][12][13] The extremely low reorganization energy of pentacene, even compared to substituted pentacenes, was attributed to its higher symmetry and resultant more delocalized nature of the orbital from which an electron is removed. 5 We have performed density functional calculations to calculate λ reorg for the isolated DT-TTF molecule.…”