Electronic Coupling in Covalently Linked Dimers

In covalent dimers, the bridging element between the two chromophores may restrict the respective orientations of the two chromophores, and thus, influences the degree of their electronic coupling. In the absence of abridge, the degree of electronic coupling between two porphyrins linked directly at the meso position can still be controlled by the peripheral substitution. Indeed, the peripheral substitution can affect the nature of HOMO frontier orbital. 

These data can be analyzed by comparing the calculated Forster energy transfer rate to the experimental energy transfer rate A et- From this comparison, the contribution of Coulombic interactions to this process can be estimated. When discrepancies are observed, involvement of an electronic coupling contribution is claimed, and the larger the difference between A p and A et, the stronger the electronic coupling. 

Finally, direct linking of the chromophores affords species in which both chromophores are perpendicular, and thus, electronic interaction between the chromophores is minimum. In this case Coulombic interactions are responsible for a very fast ET between chromophores exhibiting independent excited states. 

The case of directly meso linked chromophores deserves more attention due to their involvement in the construction of large porphyrin arrays that will be 

The link provides a dihedral angle-dependent degree of electronic coupling similar to that observed in the directly linked meso-meso dimers described previously. Thus, communication between the chromophore does not need to be established. By using a nonneutral acceptor, the redox potential of the gold porphyrin can be tuned with the help of solvent polarity. Indeed, the cationic gold porphyrin is easier to reduce in toluene = —0.30 V vs. SCE) than in benzonitrile 

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