Mimicking the Binary World: Photochromic Compounds in Molecular Logic

Licentiate thesis, 2011

The language of the binary world is used in, e.g., electronic circuits of a computer where a signal either can be on, known as 1 or off, known as 0. In the effort to make things smaller, molecules that mimic this binary world have been suggested as potential replacements for electronic circuits. These molecules are said to perform molecular logic. Photochromic compounds are molecules which by light exposure can isomerise between two different forms. These isomeric forms often have different structural and photophysical properties. Hence, light can be used to control the rates, or probabilities, of processes that depend on these properties. Often, the processes can be switched in an “on-off” fashion, why photochromic compounds are good candidates to use in molecular logic. Although molecular logic originates from the possible replacement of silicon based devices it may be used in other applications as well, e.g. biological such as cancer treatment. In this thesis two examples of photochromic compounds mimicking the binary world are presented. First, a photo-chromic dithienylethene derivative is used together with a porphyrin dimer to form a supramolecular complex where the different isomeric forms of the photochromic compound can be spectroscopically monitored without causing isomerisation. This is interpreted as a molecular memory with non-destructive readout capability. Second, a photochromic spiropyran is presented which by irradiation of UV light isomerises to an open planar form. At pH 6, but not at pH 7, the open form has been found to interact with DNA, similar to known anti-cancer drugs. The possibility to control this binding with an external source, UV light, and an internal source, acidification, can be interpreted as a molecular AND gate for the purpose of killing cancer cells.