Synthesis of Oligothiophene-Bridged Bisporphyrins and Study of the Linkage Dependence of the Electronic Coupling

摘要

A set of twelve porphyrin dimers has been prepared to give information on how the type of connectivity between a porphyrin core and a bridge can influence theinterporphyrin electronic interactio. The new porphyrin systems are substituted directly at the meso position with an oligothiophene chain tethered either with a single C-C sigma bond, a trans ethylenyl group, or a acetylenyl group. The compounds are easily obtained by palladium-catalyzed cross-coupling reactions (Stille, Heck, and Sonogashira) betwee 5-iodo-10,15,20-(3,5-ditert-butylphenyl)porphyrin and the appropriate oligothiophene derivative. This synthetic approach is straightforward and very effective for preparing oligothiophene-based porphyrin systems. The absorption spectra and the fluorescence properties of the dimers demonstrated the crucial importance of the characteristics of the chemical bond used to connect the bridge to the porphyri unit. The magnitude of the electronic communication can thus tbe significantly modulated by altering the type of bond connectivity used to link the chromophore to the bridge. The present work shows that an oligothiophene spacer is a viable class of linker for connecting porphyrins, and that a quaterthiophene appended with ethynyl linkages affords a high electronic interaction over a distance as large as 28 A. A detailed computational study of these dimers has clarified the conditons needed for a conjugated system to behave as a molecular wire. These conditions are full planrity of the molecule and proper energy matching between the frontier orbitals of the bridge and the prophyrin. Intermolecular energy transfer in asymmetrical dyads composed of a zic porphyrin and a freebase porphyrin has been studied by fluorescence spectroscopy. In all systems, this process is more than 98% efficient, and its rate constant decreases steadily in the order 4ZH>1ZH>3ZH approx = 2ZN. Thus, the largest rate (k_EnT=1.2X1`9~11 s~(-1)) was found in the dyad linked with bisethynyl quaterthiophene, which represents the longest bridge witin the series. These results clearly demonstrate that strong communication and also efficient photoinduced processes can bve promoted over a large distance if the electronic structure of the molecular connector is appropriately chosen.