The spectroscopic study on solvation of charge transfer molecules in supercritical water

Femto-second transient absorption spectroscopy has been applied to investigate the back-electron transfer process after the photo-excitation of p-nitroaniline (PNA) in water from ambient to supercritical condition. After PNA was photo-excited at 400 nm, the bleach recovery signal was observed due to the back-electron transfer from the excited state to the ground state within about 1 ps. In the longer wavelength region above 400 nm, a transient absorption signal was observed due to the hot-band absorption which was produced from the back-electron transfer process to the ground state. The hot band decay rate was determined along the isochoric line from 298 K to 664 K at 40.1 MPa. The hot band decay rate increases from 298 K to 473 K, then it decreases to 664 K. The rate maximum was reproduced by the density and temperature dependence of the collision frequency, although the decrease at the higher temperature region was much more moderate for the hot band decay rate than is predicted by the collision frequency, suggesting the effects of the local density enhancement and the solute-solvent hydrogen-bonding.