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 comp onents, were prepared by Pd degrees-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 I). The second series contains the linearly pi-conjugated mol ecular rods 1-3 that span a length range from 23 Angstrom (1) to 53 An gstrom (3) (Fig. 1).The larger rods 2 and 3 consist of two or three po rphyrin 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 porp hyrin as a result of the addition of DEE substituents. Upon changing f rom 1 to 2(Fig. 3),the Q band was further bathochromically shifted, wh ereas the Soret band remained nearly at the same position but became b roadened and displayed a shoulder on the lower-wavelength edge as a re sult of excitonic coupling. The close resemblance between the UV/VIS s pectra of 2 and 3 suggests that saturation of the optical properties i n the oligomeric series already occurs at the stage of dimeric 2. Stat ionary voltammetric investigations showed that the DEE substituents ac t as strong electron accepters which induce large anodic shifts in the first reduction potential upon changing from 5 . Zn to 9 (Delta E=190 mV) and to 1 (Delta E= 340 mV). Increasing the number of porphyrin mo ieties upon changing from 1 to 2 had no effect on the first reduction potential yet the first oxidation potential was substantially lowered (Delta E = 110 mV). Large differences in the potentials for one-electr on oxidation of the two porphyrin moieties in 2 (Delta E= 200 mV) conf irmed the existence of substantial electronic communication between th e two macrocycles across the trans-enediynediyl bridge.