Electron transfer reaction dynamics of p-nitroaniline in water from liquid to supercritical conditions

Photoexcitation dynamics of p-nitroaniline (pNA) have been investigated by femto-second transient absorption spectroscopy in water from liquid to supercritical conditions; along the isochoric line from the ambient condition to 664 K at 40.1 MPa and along the isothermal line from 40.1 to 36.1 MPa at 664 K. The rates of the back electron transfer reaction from the photoexcited charge transfer state to the electronic ground state was determined by the bleach recovery of the ground state absorption, and the successive vibrational relaxation in the electronic ground state was determined by the hot-band decay which was apparent at the red edge of the absorption. The variation of the back electron transfer rate was compared with the prediction based on the electron transfer theory including the Franck-Condon active vibrational modes. The results indicated that both the free energy change of the reaction and the change of the intramolecular vibrational reorganization energy cause the characteristic density (or temperature) dependence of the back electron transfer rate. The density dependence of the vibrational relaxation rate was compared with the collision frequency and the coordination number of the solvent molecule around the solute estimated by the molecular dynamics simulations. The density dependence of the coordination of a water oxygen atom to an amino hydrogen atom of pNA was found to be correlated with the density dependence of vibrational relaxation rate.