A molecule with two different receptors that possess four distinct, spectroscopically distinguishable states that is a molecule that indicates whether only one, the other, both, or none of the binding sites are occupied by substrates was designed. The molecule shown below shows a multifold signal expression in the presence or absence of one or two different types of metal ions i.e., Ca2+ and Ag+ (Rurack 2001).
In terms of digital action, 17 can be employed as a bimodal logic gate. A strongly decreased absorbance at 440 nm is only found in the presence of both ions (transducing a NAND event, with digital action = 1 for Ca2+/Ag+ = 0/0, 0/1, and 1/0) and a strong fluorescence signal, when excited at 440 nm, is only found for Ag+. The latter case with a digital signal = 1 only for Ca2+/Ag+ = 0/1 represents a noncommutative INHIBIT logic function.
Fig. 12.l. Photochromic switch of spiropyran
11: R1=A15C5, R2=AT415C5 18: R1=A15C5, R2=N(CH3)2 19: R1=N(CH3)2, R2=AT415C5
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