Diffusive torsional dynamics of malachite green molecules in solid matrices probed by fluorescence decay

The torsional dynamics of phenyl rings of malachite green molecules in the excited state is studied in polymeric and monomeric glass matrices by measuring the fluorescence decay time as a function of temperature. It is shown that the phenyl rings rotate diffusively in solid polymers (polymethyl methacrylate and polyvinyl alcohol) quite rapidly even at low temperatures. To analyze the experimental results, we used the concept of microviscosity which controls the diffusive rotational motion of phenyl rings of malachite green molecules in solid matrices. By using the reaction-rate theory, we show that a horizontal excited-state potential surface rather than a downhill potential surface for the rotation of phenyl rings can more reasonably explain the rotational motion in polymers. If we assume that the potential is horizontal, the temperature dependence of the microviscosity can be described by Andrade equation with a definite activation energy which is known to be valid for many liquids over a wide range of temperatures. This implies that the microscopic dynamics of small molecular rotations in a solid polymer resembles the behavior in many liquids. By monitoring the fluorescence decay of malachite green molecules doped in ethanol monomeric glass during its phase transition, we show that the effects of phase transition are well represented in the fluorescence decay time. We then propose to use malachite green molecules as sensitive optical microprobes of local dynamics in various solid matrices and their phase transitions, etc.