Electronic dephasing and electron–phonon coupling of aluminum phthalocyanine tetrasulphonate in hyperquenched and annealed glassy films of ethanol and methanol over a broad temperature range

Abstract

The electronic dephasing (spectral dynamics) and electron–phonon coupling of aluminum phthalocyanine tetrasulphonate (APT) in glassy films of ethanol and methanol were investigated by nonphotochemical hole burning over a broad temperature range, ∼5–100 K. Films formed by hyperquenching (∼106 K s−1) at 4.7 K were studied as well as films that were subsequently annealed at temperatures up to ∼170 K. Results are compared against those for APT in glassy water [Kim et al., J. Phys. Chem. 99, 7300 (1995); Reinot et al., J. Chem. Phys. 104, 793 (1996)]. As in the case of water, the linear coupling is weak with a Huang–Rhys factor S∼0.4 but the mean phonon frequencies for ethanol and methanol of 26 and 17 cm−1 are considerably lower than the 38 cm−1 value for water. These modes are assigned as pseudolocalized with significant amplitude (libration) localized on APT. Below about 8 K, the electronic dephasing/spectral diffusion is dominated by coupling to the tunneling intrinsic two-level systems of the glass. At higher temperatures the electronic dephasing is dominated by the exchange coupling mechanism, which derives from diagonal quadratic electron–phonon coupling. Here, for both ethanol and water, a pseudolocalized mode(s) at ∼50 cm−1 is operative. This frequency corresponds to a peak in the spectral density of the liquids which for water is due to the transverse acoustic mode. The results show that the modes responsible for linear and quadratic coupling are distinctly different. Implications of this for optical coherence loss in liquids are considered. Novel results from annealing experiments are reported and discussed in terms of the complex phase diagrams of ethanol and methanol. Formation of the glass from the supercooled liquid just above the melting point of a crystalline phase leads to a marked reduction (∼10×) in the homogeneous width of the zero-phonon hole at 4.7 K. This is interpreted in terms of a reduction in the density of intrinsic two-level systems due to reduced structural disorder of the glass formed from the supercooled liquid. As in the case of water, the highly efficient hole burning in glassy ethanol and methanol is observed to become highly inefficient upon formation of a crystalline phase as predicted by the Shu–Small mechanism for nonphotochemical hole burning. The close connection between this mechanism and Onsager’s inverse snowball effect for solvent dynamics around an instantaneously created point charge or dipole in a liquid is emphasized.