Exponential probe rotation in glass-forming liquids

Area of Science:

• Physical Chemistry
• Materials Science

Background:

• Supercooled liquids exhibit complex dynamics near the glass transition temperature (Tg).
• Understanding molecular probe behavior in these liquids is crucial for characterizing their dynamics.

Purpose of the Study:

• To directly compare the rotational dynamics of molecular probes with the dynamics of the surrounding solvent in supercooled liquids.
• To investigate the origin of exponential orientational correlation functions in probe molecules within liquids showing nonexponential structural relaxation.

Main Methods:

• Time-resolved optical depolarization to study probe molecule rotation.
• Triplet state solvation experiments to measure surrounding liquid dynamics.
• Polarization angle-dependent Stokes shift correlation functions.

Main Results:

• Probe rotation and solvent response times are locally correlated for smaller probe molecules.
• The size ratio between guest and host molecules significantly influences probe rotational behavior.
• Time averaging explains the exponential rotation of probes when their rotational time constant is much slower than solvent relaxation.

Conclusions:

• The study provides direct evidence linking probe rotation to solvent dynamics in supercooled liquids.
• Molecular size and size ratios are key factors governing probe dynamics.
• Time averaging is a significant mechanism for exponential probe rotation in complex liquid environments.