Potentiometric, Electronic Structural, and Ground- and Excited-State Optical Properties of Conjugated Bis[(Porphinato)zinc(II)] Compounds Featuring Proquinoidal Spacer Units

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Abstract
We report the synthesis, optical, electrochemical, electronic structural, and transient optical properties of conjugated (porphinato)zinc(II)-spacer-(porphinato)zinc(II) (PZn - Sp-PZn) complexes that possess intervening conjugated Sp structures having varying degrees of proquinoidal character. These supermolecular PZn - Sp-PZn compounds feature Sp moieties {(4,7-diethynylbenzo[c][1,2,5]thiadiazole (E-BTD-E), 6,13-diethynylpentacene (E-PC-E), 4,9-diethynyl-6,7-dimethyl[1,2,5]thiadiazolo[3,4-g]quinoxaline (E-TDQ-E), and 4,8-diethynylbenzo[1,2-c:4,5-c‘]bis([1,2,5]thiadiazole) (E-BBTD-E)} that regulate frontier orbital energy levels and progressively increase the extent of the quinoidal resonance contribution to the ground and electronically excited states, augmenting the magnitude of electronic communication between terminal (5,-10,20-di(aryl)porphinato)zinc(II) units, relative to that evinced for a bis[(5,5‘,-10,20-di(aryl)porphinato)zinc(II)]butadiyne benchmark (PZnE-EPZn). Electronic absorption spectra show significant red-shifts of the respective PZn - Sp-PZn x-polarized Q state (S0 → S1) transition manifold maxima (240−4810 cm-1) relative to that observed for PZnE-EPZn. Likewise, the potentiometrically determined PZn - Sp-PZn HOMO−LUMO gaps (E1/20/+ − E1/2-/0) display correspondingly diminished energy separations that range from 1.88 to 1.11 eV relative to that determined for PZnE-EPZn (2.01 eV). Electronic structure calculations provide insight into the origin of the observed PZn - Sp-PZn electronic and optical properties. Pump−probe transient spectral data for these PZn - Sp-PZn supermolecules demonstrate that the S1 → Sn transition manifolds of these species span an unusually broad spectral domain of the NIR. Notably, the absorption maxima of these S1 → Sn manifolds can be tuned over a 1000−1600 nm spectral region, giving rise to intense excited-state transitions ∼4000 cm-1 lower in energy than that observed for the analogous excited-state absorption maximum of the PZnE-EPZn benchmark; these data highlight the unusually large quinoidal resonance contribution to the low-lying electronically excited singlet states of these PZn - Sp-PZn species. The fact that the length scales of the PZn - Sp-PZn species (∼25 Å) are small with respect to those of classic conducting polymers, yet possess NIR S1 → Sn manifold absorptions lower in energy, underscore the unusual electrooptic properties of these conjugated structures.