Light-harvesting arrays containing one, two, or eight boron-dipyrrin (BDPY) pigments and one porphyrin (free base or Zn chelate) have been synthesized using a modular building block approach. The reaction of pyrrole and 4-(BDPY)benzaldehyde or 3,5-bis(BDPY)benzaldehyde, prepared by Pd-mediated ethynylation with the corresponding iodo-benzaldehydes, affords the desired BDPY-porphyrin array in yields of 10-58%. The arrays are soluble in organic solvents and have been characterized by static and timeresolved absorption and fluorescence spectroscopy. The blue-green BDPY absorption complements spectral coverage of the porphyrin chromophores and rivals the intensity of the porphyrin Soret band when eight BDPY accessory pigments are present. Efficient energy transfer from the BDPY pigment(s) to the porphyrin (free base or Zn-chelate) is observed in arrays containing one or two (>90%) or eight (>85%) accessory pigments per porphyrin. Biphasic excited-state decay behavior is exhibited by the BDPY pigments in isolated form and in the arrays. The time constants are ∼15 and ∼500 ps in the reference compounds (both reflecting deactivation to the ground state) and ∼2 and ∼20 ps in the arrays (both primarily reflecting energy transfer to the porphyrin). The longer-lived kinetic component comprises ∼70% of the decay in each case. Ab initio calculations suggest that the two kinetic components are associated with two energetically accessible excited-state conformers involving the boron-dipyrrin unit and the 5-aryl ring (which is integral to the linker in the arrays). The calculations and experimental results indicate that the two excited-state conformers differ from one another in structure (the planarity of the boron-dipyrrin unit and its orientation with respect to the 5-aryl ring), electronic composition (especially the electron density on the 5-aryl group of the boron-dipyrrin unit), radiative and nonradiative coupling to the ground state, and the rate of energy transfer to the porphyrin constituent in the arrays. The high energy-transfer efficiencies together with favorable light-absorption and chemical properties exhibited by the boron-dipyrrin pigments make them amenable for use in porphyrin-based arrays for molecular photonics applications.

The synthesis of multi-porphyrin light-harvesting arrays requires copper-free coupling reactions. Two studies of copper-free Pd-catalyzed coupling reactions are presented. First, we investigate the effects of different ligands in the Pd-catalyzed cross-coupling of a porphyrin bearing two aryl iodides with a porphyrin bearing an aryl ethyne. Triphenylarsine affords faster rates than triphenylphosphine or tri-2-furylphosphine when experiments are performed in conjunction with tris(dibenzylideneacetone)dipalladium(O) under anaerobic conditions. Reactions performed at 35 "C are relatively clean, fast (complete in 1-2 h), and afford the ethyne-linked porphyrin array in -70% yield with a minimal amount of higher molecular weight byproducts. This reaction is ideal for preparing multi-porphyrin arrays. Second, treatment of a free base porphyrin-ethyne with a stoichiometric amount of Pd(PPh&&, or a catalytic amount of tris(dibenzy1ideneacetone)dipalladium(0) and tri-2-furylphosphine, in toluene/triethylamine (5:l) in air at 50-60 "C affords the butadiyne-linked porphyrin dimer in -70% yield. With Pd(PPh314 in toluene under anaerobic conditions at 100 "C, the porphyrin-ethyne reacts to give the head-to-tail enyne-linked dimer (29% yield). Size exclusion chromatography is effective in analyzing reaction mixtures and purifying the porphyrin arrays. These copper-free coupling reactions afford direct access to ethyne-, butadiyne-, or enyne-linked arrays of free base and/or zinc porphyrins from readily available porphyrin building blocks.Modular synthetic approaches are of great attraction for the preparation of multicomponent systems. The syntheses of photosynthetic and related model systems require methods for joining large numbers of components into functional arrays. One modular approach employs porphyrin building blocks bearing peripheral functional groups that can be joined via specific coupling meth0ds.l In this approach the inter-porphyrin linker is formed concomitantly with the joining step, thus the nature of the linker and the coupling method are closely intertwined. Among the various linkers that have been explored, ethyne~,'-~ b~tadiynes,'*~-~' or polyynes7J1 have many attractive design features.Ethyne-linked porphyrin arrays have been constructed by the Pd-catalyzed cross-coupling reaction of a porphyrin aryl iodide and a porphyrin aryl e t h~n e . l -~,~ This reac-@

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