Recently, Cassandra Fraser’s group reported on a very cool property, reversible mechanochromic luminescence, observed in an easy to make material.^[JACS] The molecule of interest is the difluoroboron complex of avobenzone (BF₂AVB), that UV absorbing molecule in your sunscreen minus the boron and fluorines.

In broad general language, mechanochromic luminescence describes the ability of some materials to change colors after scratching under UV light. The image below shows BF₂AVB coated on weighing paper (A), a cotton swab is used to write “Light” (B), the surface is hit with a heat-gun (C), the surface is ready to be written on again with a cotton swab (D).

The image brings up all kinds of creative ways to write secret messages, especially as the letters will fade over time even without using a heat gun. But before the CIA intelligence wonks in the audience get ahead of themselves the material doesn’t seem to be completely reversible at room temperature without annealing.

…even a small mechanical perturbation, such as a slight touch with the tip of a cotton swab, changed the green-blue BF2AVB film emission to yellow. The yellow emission gradually reverted back to green again at room temperature, with much faster recovery at elevated temperature. The written regions were no longer readable after annealing.

The field has, in short order, gotten tantalizingly close to a 100% reversible mechanochromic luminescent material at room temperature. Congrats!

Link to article: Polymorphism and Reversible Mechanochromic Luminescence for Solid-State Difluoroboron Avobenzone

Sam covered one of the first entrants to reversible mechanochromic luminescence a year ago: reversible mechanochromic luminescence is cool

Mitch

Update and Correction: Cassandra Fraser has corrected me, apparently the wording of the paper was just awkward to my ear, the material is fully reversible at room temperature!