화학공학소재연구정보센터
Journal of Physical Chemistry B, Vol.117, No.1, 316-325, 2013
Photoinduced Proton-Coupled Electron Transfer of Hydrogen-Bonded p-Nitrophenylphenol-Methylamine Complex in Solution
Proton-coupled electron transfer can occur through concerted (electron-proton transfer, EPT) or sequential mechanisms, but this distinction becomes less well-defined for photoinduced reactions. These issues have been examined with transient absorption experiments on a hydrogen-bonded complex consisting of p-nitrophenylphenol and t-butylamine. These experiments revealed two spectroscopically distinct states: the higher energy excited state was interpreted to be a conventional intramolecular charge transfer (ICT) state within the p-nitrophenylphenol, whereas the lower energy state was interpreted to be an ICT-EPT state, where photoexcitation resulted in both ICT and the shifting of electronic density corresponding to effective proton transfer from the phenol to the amine. In the present work, the singlet excited states of the hydrogen bonded p-nitrophenylphenol-methylamine complex in 1,2-dichloroethane are studied with time dependent density functional theory and higher level ab initio methods. The calculations suggest that the pi pi* state, which is the S-1 state at the Franck-Condon geometry, corresponds to the state denoted ICT-EPT in the experimental analysis, whereas the n pi* state, which is the S-2 state at this geometry, likely corresponds to the state denoted ICT in the experimental analysis. According to the calculations, the pi pi* state has charge transfer character, as well as a change in electronic density on the amine, with the minimum energy structure corresponding to the proton bonded to the nitrogen acceptor, consistent with proton transfer. The n pi* state has little charge transfer character, as well as negligible change in electronic density on the amine, with the minimum energy structure corresponding to the proton bonded to the oxygen donor. The calculations also provide evidence of an avoided crossing between these two states located energetically close to the Franck-Condon point These calculations provide the foundation for future nonadiabatic molecular dynamics studies of the relaxation process.