Conjugated dimers of nickel(II) octaethylporphyrin linked by extended meso,meso-alkynyl bridges. II. Redox properties and electronic spectra of electrogenerated anions and dianions

This paper describes the behaviour of two suites of meso,meso-linked diporp hyrins, namely NiOEP-C-2-X-C-2-NiOEP [where X = (C-2)(n), for n = 0, 1, or 2; trans-CH=CH; 1,4-C6H4; 1,3-C6H4; 2,5-C4H2S] and trans-NiOEP-CH=CH-Y-NiOE P [where Y = C-2; trans-C-2-CH=CH; trans-C-2-CH=CH]. Each [P-2](0) dimer ha s been converted to its electrogenerated dianion, and, as far as possible, to the intervening monoanion as well. Voltammetry of these diporphyrins, an d comparison with the corresponding monoporphyrin models possessing the pen dant bridging unit as a meso-substituent, show that binuclearity per se add s only a small anodic shift to the normal substituent effect of the electro n-withdrawing meso-alkyne moiety. The electronic absorption spectra of the singly and doubly reduced diporphyrins, [P-2](1-) and [P-2](2-), were recor ded over the range 330-3200 nm by means of in situ spectroelectrochemistry at 233 K or below. Among the diporphyrin dianions, the distinctive spectral type already established for the C-4-bridged species is retained for all t hose compounds which have a triple bond directly attached to the meso-carbo n at both ends of the bridge, except for X = 1,3-C6H4 in which conjugation is interrupted. The near-IR signature band of these dianions, with extincti on coefficients of 10(5) M-1 cm(-1) or greater, falls between 10080 and 790 0 cm(-1) (i.e. > 1000 nm). The frequency diminishes linearly with increasin g number of triple bonds. Remarkably, [P-2](2-) species where the linker X = trans-CH=CH, 1,4-C6H4 and 2,5-C4H2S have similar UV to near-IR spectra an d also conform to the inverse linear frequency-distance relationship. For c ompounds with the longest bridges, the spectrum is dominated by just two in tense bands. The dianions of the alternative diporphyrins NiOEP-CH=CH-Y-NiO EP with trans-ethene linkers attached to both the meso-carbons have a more complex Soret region, but retain the unmistakable near-IR band which shifts relatively steeply from 11250 to 9200 cm(-1) between Y = C-2-CH=CH and C-4 -CH=CH. The position of the principal [P-2](2-) absorption band in the near -IR region can thus be finely tuned by modification of the bridge length an d composition, while retaining the familiar strong absorptivity of the porp hyrin pi-system. For mixed-valence [P-2](1-) two intense near-IR bands near 3000-5000 and 10 000 cm(-1) are observed for all the symmetrical alkyne-linked diporphyrins (X varying as above, excluding 1,3-C6H4). In these odd-electron systems, th e low-frequency excitation nu(1) directly maps the intrinsic frontier orbit al separation due to conjugative interporphyrin coupling (e.g. nu(1) = 4600 and 4150 cm(-1) for the butadiyne- and octatetrayne-bridged dimers, respec tively). The optical behaviour described above, spanning three oxidation st ates, is shown to be consistent with a generic two-porphyrin eight-orbital model for the conjugated binuclear chromophore.