A molecular triad consisting of a porphyrin (P) covalently linked to two photochromes-one from the dihydroindolizine family (DHI) and one from the dihydropyrene family (DHP)-has been synthesized and found to act as either a molecular AND logic gate or an INHIBIT gate, depending on the inputs and initial state of the photochromes. The basis of these functions is quenching of porphyrin fluorescence (output of the gates) by the photochromes. The spiro form of DHI does not quench porphyrin fluorescence, whereas its betaine isomer strongly quenches by photoinduced electron transfer. DHP also quenches porphyrin fluorescence, but its cyclophanediene isomer does not. The triad has been designed using suitable energetics and electronic interactions, so that although these quenching phenomena may be observed, independent isomerization of the attached photochromes still occurs. This makes it possible to switch porphyrin fluorescence on or off by isomerization of the photochromes using various combinations of inputs such as UV light, red light, and heat.

A molecule-based binary half-adder with optical inputs and outputs has been demonstrated. The half-adder consists of two photochromic organic molecules in solution and a third-harmonic-generating crystal. One substance acts as an AND Boolean logic gate and the other as an XOR gate. Inputs are laser pulses at 1064 or 532 nm that initiate photoisomerization reactions. Outputs are the optical absorbance of a fullerene radical anion (AND gate) and fluorescence of a porphyrin (XOR gate). The system carries out binary addition based on the laser input pulses. Half-adders in combination are capable of carrying out all mathematical operations necessary for digital computing.

Experimental galactose toxicity may serve as a model for exploring some of the basic tenets of POF.

High catalytic proficiencies observed for the native and promiscuous reaction of the Pseudomonas aeruginosa arylsulfatase (PAS; the picture shows transition states of the two substrates with corresponding binding constants K(tx)) suggest that the trade-off between high activity and tight specificity can be substantially relaxed.