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

Abstract

This paper describes the behaviour of two suites of meso,meso-linked diporphyrins, namely NiOEP–C₂–X–C₂–NiOEP [where X=(C₂)_n, for n=0, 1, or 2; trans-CHCH; 1,4-C₆H₄; 1,3-C₆H₄; 2,5-C₄H₂S] and trans-NiOEP–CHCH–Y–NiOEP [where Y=C₂; trans-C₂–CHCH; trans-C₄–CHCH]. Each [P₂]⁰ dimer has been converted to its electrogenerated dianion, and, as far as possible, to the intervening monoanion as well. Voltammetry of these diporphyrins, and comparison with the corresponding monoporphyrin models possessing the pendant bridging unit as a meso-substituent, show that binuclearity perse adds only a small anodic shift to the normal substituent effect of the electron-withdrawing meso-alkyne moiety. The electronic absorption spectra of the singly and doubly reduced diporphyrins, [P₂]^1- and [P₂]^2-, were recorded over the range 330–3200 nm by means of insitu spectroelectrochemistry at 233 K or below. Among the diporphyrin dianions, the distinctive spectral type already established for the C₄-bridged species is retained for all those compounds which have a triple bond directly attached to the meso-carbon 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 extinction coefficients of 10⁵ M^-1 cm^-1 or greater, falls between 10080 and 7900 cm^-1 (i.e. >1000 nm). The frequency diminishes linearly with increasing number of triple bonds. Remarkably, [P₂]^2- species where the linker X=trans-CHCH, 1,4-C₆H₄ and 2,5-C₄H₂S have similar UV to near-IR spectra and also conform to the inverse linear frequency–distance relationship. For compounds with the longest bridges, the spectrum is dominated by just two intense bands. The dianions of the alternative diporphyrins NiOEP–CHCH–Y–NiOEP 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=C₂–CHCH and C₄–CHCH. The position of the principal [P₂]^2- absorption band in the near-IR region can thus be finely tuned by modification of the bridge length and composition, while retaining the familiar strong absorptivity of the porphyrin π-system.

For mixed-valence [P₂]^1-, two intense near-IR bands near 3000–5000 and 10000 cm^-1 are observed for all the symmetrical alkyne-linked diporphyrins (X varying as above, excluding 1,3-C₆H₄). In these odd-electron systems, the low-frequency excitation ν₁ directly maps the intrinsic frontier orbital separation due to conjugative interporphyrin coupling (e.g. ν₁=4600 and 4150 cm^-1 for the butadiyne- and octatetrayne-bridged dimers, respectively). The optical behaviour described above, spanning three oxidation states, is shown to be consistent with a generic two-porphyrin eight-orbital model for the conjugated binuclear chromophore.