Excitonic interactions in the singlet and triplet excited states of covalently linked zinc porphyrin dimers

The excess polarizability volumes, Delta V-p of the relaxed S-1 and T-1 exc ited states of several covalently bridged zinc porphyrin dimers and their c orresponding monomers have been measured using the flash photolysis time-re solved microwave conductivity technique. Delta V-p(S-1) increases from clos e to zero for a diaryl-substituted monomer up to a maximum value of 590 Ang strom(3) for a dimer coupled by a 9,10-diethynylanthracene bridge, yPyAyPy. The particularly large excess polarizability of yPyAyPy is attributed to s trong electronic coupling between the porphyrin moieties resulting from sta bilization of the cumulenic quinoidal resonance structure of the bridge. Th e strength of the electronic interaction, as indicated by the magnitude of Delta V-p(S-1), increases in the following order of bridging units: 1,4-phe nylene < single sigma-bond < 1,4-diethynylbenzene < 2,5-diethynylthiophene approximate to butadiyne < 9,10-diethynylanthracene. The results provide an example of an inverse distance effect whereby the electronic interaction b etween porphyrin moieties actually increases with increasing length of the intervening bridge. The product of the intersystem crossing efficiency and the excess polarizability volume of the triplet state, phi(isc)Delta V-p(T- 1), is more than an order of magnitude less than Delta V-p(S-1), indicating a much smaller degree of exciton delocalization in T-1 than in S-1. The mi crowave results are compared with results on the optical absorption and emi ssion spectra which provide additional information on both the electronic a nd Coulombic excitonic interactions.