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, ^2B_1(π_1), is observed at 7.888 ± 0.002 eV, ^2B_2(π_1) of 2 is at 8.342 ± 0.002 eV, and ^2B_(1g)(π_1) of 3 is at 8.314 ± 0.002 eV. Spectra of 4−6 are reported for the first time: ^2A_2(π_1) 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, λ^(QM), 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 = λ^(QM) + λ^(SC)) 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.