This Message Will Self-Heal in 3, 2, 1...
by mitch on Feb 12 2010 (13969 Views)
Cassandra Fraser
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!
Dear Mitch,
Thank you for showcasing our "scratch the surface ink"TM on your blog. We are very honored for our work to be highlighted in this way.
We can't help but correct the record however, pertaining to this statement: "....(T)he material doesn’t seem to be completely reversible at room temperature without annealing." In fact the material IS entirely reversible at room temperature without annealing! That is what makes it quite unique compared to previous reports of this phenomenon, as far as we can tell.
As we state and you quote: "The yellow emission gradually reverted back to green again at room temperature, with much faster recovery at elevated temperature." It is spontaneously self erasing or self healing at room temperature (minutes), but the process is just faster upon heating (seconds). Then you can write again many times.
You say: "The field has, in short order, gotten tantalizingly close to a 100% reversible mechanochromic luminescent material at room temperature." In fact it is already there! It's time to celebrate!
So maybe the CIA wonks, not to mention artists, toy makers and rewritable paper enthusiasts should be interested after all!
Thanks again.
Cassandra Fraser and Guoqing Zhang
University of Virginia Chemistry Department
[...] Mitch posted about an interesting reversibly triboluminescent compound (the pictures are quite impressive) from [...]
A better application would be a pressure sensitive coating. One could potentially apply this to an aircraft model and observe accumulive air pressure points. Also when incorporated into composite materials it could detect stress before failure.