Visible-light-absorbing chromophore

While artificial photosynthetic mimics come in many manifestations, our efforts have focused predominantly on the class of molecules represented by the structure in Fig. 10.1. These molecules consist of a visible-light-absorbing chromophore in the form of a trisbipyridineruthenium(ll) complex (C) finked by flexible polymethylene chains to one or more electron donors (D) and an electron acceptor (A). The electron acceptor is anAl,W-dialkylated-2,2 -bipyridine (a so-called diquat ) and the electron donors are Al-aUcylated phenothiazines. The diquat type acceptor was chosen because... 

One attractive area of research is the coupling of ruthenium type polyazine chromophores to reactive metals. This allows the combination of intense visible light-absorbing properties of the chromophore with the favorable reactive properties of the reactive metal center imparting unique properties to the supramolecule. Ru(II) and Os(II) polyazine light absorbers frequently possess emissive MLCT states that are discussed herein and used to explain interexcited state dynamics in the coupled reactive metal assemblies. 

First, oxidizing bleaches such as sodium hypochlorite break the molecules at the double bond. This results in either a shorter molecule that does not absorb visible light, or a molecule whose chromophore is either shorter or nonexistent. A shorter chro-mophore will absorb light of a shorter wavelength than visible light (such as ultraviolet light), and so does not appear colored. 

Second, reducing bleaches such as lemon juice (in combination with sunlight) or sulfur dioxide convert the double bonds in the chromophore into single bonds, eliminating its ability to absorb visible light. Sometimes the reaction is reversible, where oxygen in the air reacts with the molecule to repair the chromophore, and the stain returns. 

Figure 16. Flash photolysis of the viologen bilayer membrane having biphenyl chromophore. [Ci2BphC5V2+ 2Br ] = lOmM. a transient absorption spectrum 50msec after visible light irradiation, b decay of 600nm absorbance after photolysis.

Most conventional techniques for the determination of biological molecules or other species with similar properties use their ability to absorb ultraviolet or visible light, their fluorescence after excitation with light of the appropriate wavelength, or their electrochemical behaviour. It possible to enhance the detectability of some species by making them react with UV-visible absorbing or fluorescent compounds. Applied to complex matrices, these detection methods are at best only selective, because a wide variety of chromophores will give a response. 

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