Title: Signal Processing at the Molecular Level
Author: Francisco Maria Raymo
Last saved by: James B. Lewis
Company: University of Miami

Signal Processing and Communication at the Molecular Level
Silvia Giordani and Françisco M. Raymo 
Center for Supramolecular Science
Department of Chemistry, University of Miami

Environmental stimulations are transduced into nerve impulses in our bodies and are transmitted in this form over relatively long distances.  This sequence of events involves a concatenation of processes at the molecular level.  At each step, input signals of one form are converted into output signals of another.  This complex mechanism is related conceptually to the manipulation of binary data in microprocessors.  The data introduced into a computer are elaborated through a sequence of logic operations and converted into a specific output.  In modern microprocessors, logic circuits are built integrating electronic devices.  However, they could be fabricated combining mechanical, pneumatic or other types of devices, including molecular-sized switches. 
We have synthesized a spiropyran derivative that switches to two merocyanine forms under the influence of light and chemical stimulations.  In particular, this three-state molecular switch detects three input signals: ultraviolet light, visible light and H^+.  It responds to these stimulations generating two output signals: an absorption band at 401 nm and one at 563 nm.^1
We have then developed a simple strategy to communicate intermolecularly the two optical outputs to a compatible fluorophore.^2  The fluorescence intensity of the probe changes in the presence of the molecular switch.  Binary logic can be used to describe the behavior of the communicating ensemble of molecules.  The input signals are ultraviolet light, visible light and H^+.  The output signal is the emission band at 373 nm.  Each signal can be either on or off and can be represented by a binary digit, i.e. it can only take two values 1 or 0.  Thus, the communicating molecules transduce a string of three input data into a single output digit.  The corresponding truth table and equivalent combinational logic circuit illustrate the complexity of the operations executed by the communicating ensemble of molecules.  Seven interconnected logic gates are necessary to reproduce the operations executed by two communicating molecules.  

() Raymo, F.M.; Giordani, S. J. Am. Chem. Soc. 2001, 123, 4651–4652.
(^2) Raymo, F.M.; Giordani, S. Org.Lett. 2001, 3, 1833–1836.

Figure 1.  The switching cycle associated with the three states SP, ME and MEH and the communication between PY and ME or MEH.