Excess Polarizability Reveals Exciton Localization/Delocalization Controlled by Linking Positions on Porphyrin Rings in Butadiyne-Bridged Porphyrin Dimers Chunxing She, † Shanmugam Easwaramoorthi, ‡ Pyosang Kim, ‡ Satoru Hiroto, § Ichiro Hisaki, § Hiroshi Shinokubo, § Atsuhiro Osuka,* ,§ Dongho Kim,* ,‡ and Joseph T. Hupp* ,† Department of Chemistry, Northwestern UniVersity, 2145 Sheridan Road, EVanston, Illinois 60208, Spectroscopy Laboratory for Functional π-Electronic Systems and Department of Chemistry, Yonsei UniVersity, Seoul 120-749, Korea, and Department of Chemistry, Graduate School of Science, Kyoto UniVersity, Kyoto 606-8502, Japan ReceiVed: September 10, 2009; ReVised Manuscript ReceiVed: February 11, 2010 We report direct measurements of the excess polarizability volumes of butadiyne-bridged zinc porphyrin dimers at singly -to- (1Zn) and doubly -to- (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 Å 3 , respectively, while those of the triplet exciton are ∼100 Å 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 -to- linked dimer lies between those of singly -to- 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  positions but large at meso positions. Two butadiyne linkages between the porphyrins at  positions (2Zn) clearly facilitate electronic communication between the two porphyrin subunits by virtue of stabilization of cumulenic charge resonance structures through enforced planarity. Introduction Conjugated porphyrin oligomers have been the subject of intense interest due to their unique optical and electronic properties for the applications in cellular imaging, 1,2 charge transport in molecular wires, 3,4 and searching for materials with possible two photon absorption (TPA) property. 5-10 These unique properties are the results of strong excitonic coupling and electronic communication among porphyrin subunits. 11-13 Two types of linkages have been employed, ethyne- and butadiyne-bridges, to connect porphyrin monomers through meso-to-meso, meso-to-, or -to- positions. On the basis of red-shifted absorption maxima and frontier molecular orbitals of zinc porphyrin dimers, it is generally known that π-conjuga- tion follows the order meso-to-meso > meso-to- > -to-. 12,13 Among such covalently linked porphyrin dimers, meso-to-meso linked dimers are well understood to be highly conjugated molecular systems with singlet excitons fully delocalized on the two porphyrin subunits as well as bridges, as indicated by optical properties, 11-13 electronic structure calculations, 11,14-16 excess polarizability measurements, 17,18 and TPA measure- ments. 5-7,9,10 While TPA cross sections of meso-to-meso linked dimers have been well-studied and also suggested to have significant contributions due to one-photon processes, 9 conju- gated -to- linked porphyrin dimers are less well-studied even though they evince enhanced TPA cross-section values. 19 Previously, we reported the synthesis of a doubly -to- 1,3- butadiyne-bridged porphyrin dimer 2Zn that shows enhanced TPA cross-section values as a result of a higher extent of π-conjugation compared to that for its singly -to- linked analogue 1Zn (Chart 1). 19 While it is well conceived that planar structures ensure π-conjugation, 19 the linking positions on porphyrin rings also play a major role in determining the extent of π-conjugation. 13,20 Although strong excitonic coupling is directly reflected in the absorption spectra, a direct measurement of excess polarizability volumes that depend on the size of excitons is crucial for understanding quantitatively the strength of excitonic coupling among multiple porphyrin subunits and the dramatic enhancement of TPA cross-section values for this type of porphyrin oligomer. 5,6 * Corresponding authors. E-mails: A.O., osuka@kuchem.kyoto-u.ac.jp; D.K., dongho@yonsei.ac.kr; J.T.H., j-hupp@northwestern.edu. † Northwestern University. ‡ Yonsei University. § Kyoto University. CHART 1: Structures of Porphyrin Dimers Linked through Different Positions J. Phys. Chem. A XXXX, xxx, 000 A 10.1021/jp908767x  XXXX American Chemical Society