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Exhaustive electro-oxidation of ZnOEP in the presence of pyridine and Et₄NPF₆ affords in high yield the new tetracationic porphyrin in which the four meso protons of ZnOEP are replaced by four pyridinium groups linked via their nitrogens: [Zn-5,10,15,20-tetrakis(N-pyridiniumyl)-2,3,7,8,12,13,17,18-octaethylporphyrin]⁴⁺, 1. The compound has been characterized by elemental analysis, ¹H NMR, and crystallography. The latter confirms the preparative electrosynthetic methodology and molecular structure of 1, and shows the porphyrin skeleton to adopt a severely nonplanar saddle conformation which minimizes steric crowding between the 12 peripheral substituents. As a consequence of the nonplanarity (and the high charge) of the complex, 1 exhibits significant red-shifts of the visible and Soret bands as well as a considerable broadening and loss of intensity of the latter. 1 first undergoes four one-electron reduction steps at the four pyridinium sites before the normal one-electron reduction of the porphyrin π-system.

Molecular nonlinear optics describes the nonlinear optical properties of materials at the molecular level. At this most fundamental level, the nonlinear response can be described in terms of molecular parameters, such as number of polarizable electrons, non-centrosymmetric substitution by electron-donor or electron-accepting groups, molecular conformation, position of charge, or degree of charge transfer. We recount the shifts in molecular paradigms for second-order nonlinear optics, largely based on the instrumental possibilities offered to experimentally determine the nonlinear response. We start at the shift from the dipolar paradigm to the octopolar one, initiated by the possibilities of hyper-Rayleigh scattering, and end with the perspectives recently offered by electrochemical switching of the second-order nonlinear optical response at the molecular level.

The X-ray structure, electrochemistry, and optical spectroscopy of the porphycene analog of a benzochlorin, octaethylbenzochloracene (OEBzC) are reported. The OEBzC macrocycle is easier to oxidize and harder to reduce than octaethylporphycene (OEPc). The optical spectra of OEBzC and of the diacid salt are also reported. The optical spectrum for the one-electron oxidation of OEBzC is indicative of π-cation radical formation. In porphyrins, saturation of one pyrrole ring and addition of an exocyclic benzene ring progressively red-shifts the first absorption band. For porphycenes, saturation of one pyrrole ring results in a 35 nm blue shift in the first absorption relative to OEPc. Compared to 2,3-dihydroporphycene (DHPc), the first absorption band in OEBzC is red-shifted 24 nm. These studies further illustrate how the fused exocyclic benzene ring influences the macrocycle conformation, chemical and physical properties of porphycenes.

The metalation chemistry of the phlorin, which is a non-aromatic tetrapyrrole macrocycle containing a single sp³-hybridized meso-carbon has remained underdeveloped, as compared to that of more traditional tetrapyrroles such as porphyrins, corroles and phthalocyanines. There have been few prior efforts to prepare metallophlorins, and those that have been reported have relied on either reduction or nucleophilic attack of parent metalloporphyrins, rather than direct metalation of freebase phlorin constructs. In this work, an alternate synthetic approach for preparation of gold(III) phlorin complexes that involves the first direct metalation of two different freebase phlorin derivatives (3H(Phl${}^{\text{tBu}})$ and 3H(Phl${}^{\text{F}}))$ with AuBr₃ to produce the stable and fully isolable gold(III) phlorin complexes Au(Phl${}^{\text{tBu}})$ and Au(Phl${}^{\text{F}})$ is reported. The first structural characterization of a metallophlorin bearing geminal dimethyl substituents at the sp³-hybridized meso-carbon via X-ray crystallography is also reported. In addition to the preparation of Au(Phl${}^{\text{tBu}})$ and Au(Phl${}^{\text{F}})$, the UV-vis absorption and redox properties of these two gold(III) phlorins in comparison to those of their freebase homologues is also detailed. Notably, the metallophlorins are characterized by panchromatic absorbance profiles and intense and broad bands that span the long-visible and into the near-IR regions, as well as two fully reversible oxidation and reduction waves as probed by cyclic voltammetry. Finally, the chlorination of Au(Phl${}^{\text{tBu}})$ using PhI(Cl${}_2)$ was probed and it was found that this regioslective reaction generates monochlorinated (Au(Phl${}^{\text{tBu}}$Cl)) and dichlorinated (Au(Phl${}^{\text{tBu}}$Cl${}_2))$ products, which were both structurally characterized by X-ray crystallography.