Excess polarizability reveals exciton localization/delocalization controlled by linking positions on porphyrin rings in butadiyne-bridged porphyrin dimers.

We report direct measurements of the excess polarizability volumes of butadiyne-bridged zinc porphyrin dimers at singly beta-to-beta (1Zn) and doubly beta-to-beta (2Zn) positions using the transient dc photoconductivity (TDCP) technique. The excess polarizability volumes of the singlet exciton for 1Zn and 2Zn are 110 and 270 A(3), respectively, while those of the triplet exciton are approximately 100 A(3) for both dimers. Our measurements suggest that the singlet exciton is mainly localized on one porphyrin subunit for 1Zn, similar to the case for the porphyrin monomer. While the exciton is fully delocalized on two porphyrin subunits in the case of meso-to-meso linked dimer (3Zn), the extent of exciton localization/delocalization for doubly beta-to-beta linked dimer lies between those of singly beta-to-beta and meso-to-meso linked dimers. Electronic structure calculations show that the dramatically different extents of exciton localization/delocalization are the results of frontier orbital coefficients being small at beta positions but large at meso positions. Two butadiyne linkages between the porphyrins at beta positions (2Zn) clearly facilitate electronic communication between the two porphyrin subunits by virtue of stabilization of cumulenic charge resonance structures through enforced planarity.