Paper without a Trail: Time-Dependent Encryption using Pillar[5]arene-Based Host-Guest Invisible Ink.

A stimuli-responsive invisible ink for time-dependent encryption of information is reported. Consisting of a pillar[5]arene-based supramolecular network grafted with spiropyran moieties, these materials display time-dependent photochromic behavior with tailorable fading rates. Ultraviolet (UV) light results in isomerization of the colorless spiropyran to the corresponding colored merocyanine, while visible light or heat causes the reverse isomerization with a rate that is dependent on the density of host-guest crosslinks. The kinetics of discoloration are a function of merocyanine aggregation, which becomes more pronounced as the host-guest crosslink density is increased, leading to a reduced conversion rate and slower time-dependent fading. The degree of crosslinking, and hence the fading rate, may be modulated via the addition of unbound pillar[5]arene host or nitrile guest as competitors. Time-dependent information encryption is enabled by combining selective placement of host and guest competitors and UV patterning. UV patterning provides an initially "false" image that does not reveal the desired information, and it is only after a given time that the encrypted data appears. This work provides a unique approach to enhance the security of information storage associated with offline portable data encryption.