Electron-Transfer Reorganization Energies of Isolated Organic Molecules

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, 2 B 1 (π 1 ), is observed at 7.888 ± 0.002 eV, 2 B 2 (π 1 ) of 2 is at 8.342 ± 0.002 eV, and 2 B 1g (π 1 ) of 3 is at 8.314 ± 0.002 eV. Spectra of 4 − 6 are reported for the first time: 2 A 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.