Non-photochemical versus photochemical hole burning in hyperquenched glassy water and cubic ice

The Shu–Small mechanism (J. Opt. Soc. Am. B 9 (1992) 724) for non-photochemical hole burning (NPHB) of electronic transitions of chromophores in amorphous hosts at low temperatures is based on a hierarchy of configurational tunneling events that begin in the outer shell and terminate in the inner shell that surrounds the chromophore. Connectivity between the intrinsic and extrinsic two level systems that stem from structural disorder and the excess free volume of glasses are key features of this mechanism. This mechanism has recently been modified to take into account multilevel extrinsic systems (J. Chem. Phys. 114 (2001) 9105). An important prediction of the Shu–Small mechanism is that NPHB should cease upon formation of a crystalline phase produced, for example, by warming of a vitrified glass. In this paper data are presented that are consistent with that prediction. The systems studied are aluminum phthalocyanine tetrasulphonate (APT) and free base phthalocyanine tetrasulphonate (PcT) in hyperquenched glassy films of water (HGW) and cubic ice (Ic). The hole-burning mechanisms for APT and PcT are non-photochemical and photochemical, respectively. Surprisingly, the hole-growth kinetics for PcT are as highly dispersive as are the kinetics for APT. This means that the kinetics for photoinduced proton tautomerization in PcT are strongly influenced by its nanoenvironment of water molecules, some of which are involved in H-bonding to the –NH groups of PcT.