Inverse isotope effects and electron-phonon coupling in the positively charged deutero- and fluoroacenes

Electron-phonon interactions in the monocations of deutero- and fluoroacenes are studied and compared with those in the monocations of acenes and those in the monoanions of fluoroacenes. Because of the significant phase pattern difference between the highest occupied molecular orbitals (HOMO) and the lowest unoccupied molecular orbitals (LUMO), the frequency modes lower than 500 cm(-1) and the high-frequency modes around 1400 cm(-1) couple more strongly to the LUMO than to the HOMO, while the frequency modes around 500 cm(-1) and the frequency modes around 1600 cm(-1) couple more strongly to the HOMO than to the LUMO in fluoroacenes with D2h geometry. The total electron-phonon coupling constants for the monocations (l(HOMO)) are estimated and compared with those for the monoanions (l(LUMO)) in deutero- and fluoroacenes. The l(HOMO) values are estimated to be 0.418, 0.399, 0.301, 0.255, and 0.222 eV for C6F6 (1f), C10F8 (2f), C14F10 (3f), C18F12 (4f), and C22F14 (5f), respectively. The l(HOMO) values are smaller than the l(LUMO) values in small fluoroacenes. But the l(HOMO) value decreases with an increase in molecular size less rapidly than the l(LUMO) value in fluoroacenes, and the l(HOMO) value of 0.074 eV is much larger than the l(LUMO) value of 0.009 eV in polyfluoroacene. The logarithmically averaged phonon frequencies for the monocations (omega(ln,HOMO)) are estimated to be larger than those for the monoanions (omega(ln,LUMO)) in fluoroacenes. This is because the C-C stretching modes around 1600 cm(-1) couple most strongly to the HOMO, and those around 1400 cm(-1) couple the most strongly to the LUMO in fluoroacenes. The significant phase pattern difference between the HOMO and the LUMO is the main reason for the calculational results. The l(HOMO) values increase much more significantly by H-F substitution than by H-D substitution in acenes. The possible inverse isotope effects in the electron-phonon interactions as a consequence of deuteration in the monocations of nanosized molecules are suggested.