Reversing the arrow of time: materials that come back to life

Mark G. Kuzyk

Department of Physics and Astronomy, Washington State University, Pullman, WA 99164-2814 kuz@wsu.edu

The arrow of time is characterized by a physical process that is irreversible. For example, in a closed system, the thermodynamic arrow of time runs in the direction of entropy increase. The electrodynamic arrow of time manifests itself in the potentials, which are always retarded; i.e., a distant observer sees a delay in changes to the local potentials when distant charges and currents rearrange themselves. The psychological arrow of time allows us to remember the past and not the futures, and the expansion of the universe defines the direction of time flow.¹

Irreversible photodegradation of a molecule falls under the thermodynamic arrow of time. One mechanism of light-induced damage is photo-dissociation, where the molecule breaks up into pieces when exposed to bright light. The volume of phase space associated with the decay products is so much larger than the phase space of the original molecule that there is no chance that the pieces will ever find each other during the lifetime of the universe.

Most materials degrade under high-dose exposure to light: colored paper fades in sunlight and leaves burn at the focus of a magnifying lens. However, certain molecules that would normally photodegrade in vacuum or liquid solution² have been demonstrated to self heal when left in the dark.³ We have speculated that the host polymer matrix decreases the amount of phase space available to the molecular fragments, thus suppressing photodegradation.⁴ As a result, the irreversible degradation process is replaced by a reversible one. Interestingly, physical damage to the polymer (due to laser oblation and laser burning) is also reversible, but over a longer time scale, and only in the presence of the dopant dye chromophore.

Our laboratory has been studying photodegradation and self healing in two systems: DO11 chromophore doped in PMMA using amplified spontaneous emission as a probe; and, AF455 chromophores doped in PMMA, using two-photon fluorescence. These very different molecules and characterization techniques, which show the same kind of universal healing behavior, follow a damaged population reservoir model.⁴ We will present our newest experimental results, which seek to differentiate between several competing models of the mechanisms of photodegradation by studying the decay and recovery process in the time-temperature domain. These studies offer the tantalizing possibility of giving a glimpse into the interface between reversible and irreversible processes.

1. S. W. Hawking, Phys. Rev. D 32, 2489 (1985).

2. B. F. Howell and M. G. Kuzyk, J.Opt. Soc. Am. B 19, 1790 (2002).

3. B. F. Howell and M. G. Kuzyk, Appl. Phys. Lett. 85, 1901 (2004).

4. N. B. Embaye, S. K. Ramini, and M. G. Kuzyk, J. Chem. Phys. 129, 054504 (2008).