Porphyrin-[(E)-1,2-Diethynylethene] Scaffolding: Synthesis and Optical and Electrochemical Properties of Multinanometer-Sized Porphyrin Arrays

Two series of linearly conjugated hybrid materials, consisting of (E)-1,2-diethynylethene (DEE; hex-3-ene-1,5-diyne) and Zn^II porphyrin components, were prepared by Pd⁰-catalyzed cross-coupling reactions. In one series, one or two DEE substituents were introduced into the meso-positions of the Zn^II porphyrins, leading from 5 ⋅ Zn , to 9 and 1 (Scheme 1). The second series contains the linearly π-conjugated molecular rods 1 – 3 that span a length range from 23 Å ( 1 ) to 53 Å ( 3 ) (Fig. 1). The larger rods 2 and 3 consist of two or three porphyrin moieties, respectively, that are bridged at the meso-positions by trans-enediynediyl (hex-3-ene-1,5-diyne-1,6-diyl) linkers (Scheme 2). The UV/VIS spectra in the series 5 ⋅ Zn , 9 , and 1 (Fig. 2) showed a strong bathochromic shift of both Soret and Q bands of the Zn^II porphyrin as a result of the addition of DEE substituents. Upon changing from 1 to 2 (Fig. 3), the Q band was further bathochromically shifted, whereas the Soret band remained nearly at the same position but became broadened and displayed a shoulder on the lower-wavelength edge as a result of excitonic coupling. The close resemblance between the UV/VIS spectra of 2 and 3 suggests that saturation of the optical properties in the oligomeric series already occurs at the stage of dimeric 2 . Stationary voltammetric investigations showed that the DEE substituents act as strong electron acceptors which induce large anodic shifts in the first reduction potential upon changing from 5 ⋅ Zn to 9 (ΔE=190 mV) and to 1 (ΔE=340 mV). Increasing the number of porphyrin moieties upon changing from 1 to 2 had no effect on the first reduction potential yet the first oxidation potential was substantially lowered (ΔE=110 mV). Large differences in the potentials for one-electron oxidation of the two porphyrin moieties in 2 (ΔE=200 mV) confirmed the existence of substantial electronic communication between the two macrocycles across the trans-enediynediyl bridge.