Multivalued Logic with a Tristable Fluorescent Switch

A fluorophore 1 −spacer−receptor−spacer−fluorophore 2 dyad was prepared and photophysically characterized. The two spectrally well-differentiated fluorophores are 1,8-naphthalimide and its 4-amino-substituted derivative, the latter showing strong ICT emission. The receptor unit consists of a tertiary amine, which leads to site-selective PET (only the 1,8-naphthalimide part gets quenched). This process can be blocked by protonation of the tertiary amine, which, on the other hand, results in enhanced singlet−singlet energy transfer between the 1,8-naphthalimide and 4-amino-1,8-naphthalimide parts (Φ EET = 0.27 and 0.59 for neutral and monoprotonated form, respectively). In acetonitrile, the monoprotonated form can be deprotonated by sufficiently basic anions like fluoride in micromolar concentration (<20 μM). The observed fluorescence quenching, due to PET-reactivation, is ca. 41%. Further addition of fluoride in concentrations >1 mM leads to deprotonation of the 4-amino group of one of the naphthalimides, which is accompanied by more fluorescence quenching (ca. 93%). The resulting double-sigmoidal titration curve enables the implementation of ternary logic. Using fluoride anions as degenerate inputs results in a ternary NOR logic gate, which is demonstrated for the first time.