Secret
information recorded by traditional single-encrypted invisible inks
is easily cracked because the inks can switch only between “NONE”
and “TRUTH”. Developing double-encrypted systems makes
the information reversibly switchable between “FALSE”
and “TRUTH”, which is helpful to ensure the safety of
the secret information during transport. Here, we prepared heat-developed
invisible inks by hydrochromic molecules donor–acceptor Stenhouse
adducts (DASAs) and oxazolidines (OXs) and promoted the invisible
inks from single to double encryption. DASAs coordinate with water
molecules and form stable colorless cyclic DASA·xH2O molecules, which lose coordinated water molecules
after heating and switch to colored linear DASAs. In contrast, OXs
are colored with water and are colorless after heating. Single-encrypted
secrecy was realized by DASA invisible inks. The information is invisible
under the encrypted state and becomes bright purple after heating.
Vapor treating re-encrypted the information in ∼5 min. Furthermore,
the single-encryption was promoted to double-encryption by a DASA/OX
invisible inks system. Heating and vapor treating switch the information
between the “FALSE” and “TRUTH” reversibly.
The DASA/OX invisible ink system is applied for secrecy of texts,
graphic images, and quick response (QR) codes.
Understanding the relationship between
chemical structure and photoswitching
property of donor–acceptor Stenhouse adducts (DASAs) is necessary
for developments and applications of the novel photoresponsive molecule.
In the current work, we demonstrated a close relationship between
the length of carbon spacer and photoswitching property of DASAs.
A series of DASAs with barbituric acid substituted electron-withdrawing
part and N-methylaniline substituted electron-donating
part were synthesized. With shortening the carbon spacer between the
phenyl and amine groups in the electron-donating part, the efficiency
and rate of the light-induced linear-to-cyclic isomerization are improved in all the test solvents. The molecular
energy variation during the isomerization process was investigated
by density functional theory calculation to further understand the
mechanism. This work provides a reliable carbon spacer strategy to
control the photoswitching behavior of DASAs using chemical methods.
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