Excitonic Interactions in the Singlet and Triplet Excited States of Covalently Linked Zinc Porphyrin Dimers

The excess polarizability volumes, ΔVp, of the relaxed S1 and T1 excited states of several covalently bridged zinc porphyrin dimers and their corresponding monomers have been measured using the flash photolysis time-resolved microwave conductivity technique. ΔVp(S1) increases from close to zero for a diaryl-substituted monomer up to a maximum value of 590 A3 for a dimer coupled by a 9,10-diethynylanthracene bridge, yPyAyPy. The particularly large excess polarizability of yPyAyPy is attributed to strong electronic coupling between the porphyrin moieties resulting from stabilization of the cumulenic quinoidal resonance structure of the bridge. The strength of the electronic interaction, as indicated by the magnitude of ΔVp(S1), increases in the following order of bridging units:  1,4-phenylene < single σ-bond < 1,4-diethynylbenzene < 2,5-diethynylthiophene ≈ butadiyne < 9,10-diethynylanthracene. The results provide an example of an inverse distance effect whereby the electronic interaction between porphyrin...