The low-lying π σ state and its role in the intramolecular charge transferof aminobenzonitriles and aminobenzethyne

• Published in: The Journal of chemical physics 2008-04, Vol.128 (16), p.164512-164512-11
• Main Authors: Lee, Jae-Kwang, Fujiwara, Takashige, Kofron, William G., Zgierski, Marek Z., Lim, Edward C.
• Format: Article
• ISSN: 0021-9606; 1089-7690
• DOI: 10.1063/1.2901976

Electronic absorption spectra of the low-lying π π * and π σ * states of several aminobenzonitriles and 4-dimethylaminobenzethyne have been studied by time-resolved transient absorption and time-dependent density functional theory calculation. In acetonitrile, the lifetime of the π σ * -state absorption is very short (picoseconds or subpicosecond) for molecules that exhibit intramolecular charge transfer (ICT), and very long (nanoseconds) for those that do not. Where direct comparison of the temporal characteristics of the π σ * -state and the ICT-state transients could be made, the formation rate of the ICT state is identical to the decay rate of the π σ * state within the experimental uncertainty. These results are consistent with the π σ * -mediated ICT mechanism, L a ( π π * ) → π σ * → ICT , in which the decay rate of the π σ * state is determined by the rate of the solvent-controlled π σ * → ICT charge-shift reaction. The π π * → π σ * state crossing does not occur in 3-dimethylaminobenzonitrile or 2-dimethylaminobenzonitrile, as predicted by the calculation, and 4-aminobenzonitrile and 4-dimethylaminobenzethyne does not exhibit the ICT reaction, consistent with the higher energy of the ICT state relative to the π σ * state.