Probing Confined Water with Nonphotochemical Hole Burning Spectroscopy: Aluminum Phthalocyanine Tetrasulfonate in Poly(2-hydroxyethyl methacrylate)†

Nonphotochemical hole burning is used to measure the linear electron-phonon coupling, the temperature dependence of the pure dephasing, and the zero-phonon hole growth kinetics of aluminum phthalocyanine tetrasulfonate (APT) in glassy water confined in pores (∼30 A) of films of poly(2-hydroxyethyl methacrylate) (poly-HEMA). The hole burning properties of APT in the polymer are compared with those of APT in hyperquenched glassy films of water and ethanol. Below ∼8 K in the polymer, the dephasing, which is dominated by coupling to the intrinsic two-level systems (TLS i n t ) of the glass, is found to be more similar to that of APT in unannealed hyperquenched glassy water (HGW) films than in annealed HGW films. This shows, for the first time, that confinement does not lead to a significant decrease in the TLS i n t density. At higher temperatures, dephasing due to exchange coupling with a pseudolocalized mode at 42 cm - 1 becomes dominant. This coupling is due to diagonal quadratic electron-phonon coupling that leads to a change in mode energy upon electronic excitation of APT. The 42 cm - 1 vibration is assigned to the transverse acoustic mode of confined water. In HGW the energy of this mode is 50 cm - 1 . The interaction of APT with surface-bound water and the polymer surface also leads to reduction of the energy of the linearly coupled (Franck-Condon active) phonon mode from 38 cm - 1 for HGW to 32 cm - 1 . Hole growth kinetics measurements for APT in polymer saturated with D 2 O are compared with those in polymer saturated with H 2 O. In the heavy water the hole burning is 330 times slower. The equivalent factor for heavy HGW is 800. Thus, the mechanism of hole burning involves proton tunneling associated with the extrinsic two-level systems (TLS e x t ) introduced by the dye. In contrast, dephasing data indicate that the coordinate of the TLS i n t is spatially extended and involves only small-amplitude motion of protons. Differences between the hole-burning properties of APT in poly-HEMA and in HGW and hyperquenched ethanol are discussed in terms of the interactions of APT with bound (nonfreezable) water and the hydroxyethyl groups of the polymer.