Two-photon absorption has important advantages over conventional one-photon absorption, which has led to applications in microscopy, microfabrication, three-dimensional data storage, optical power limiting, up-converted lasing, photodynamic therapy, and for the localized release of bio-active species. These applications have generated a demand for new dyes with high two-photon absorption cross-sections. This Review introduces the theory of two-photon absorption, surveys the wide range of potential applications, and highlights emerging structure-property correlations that can serve as guidelines for the development of efficient two-photon dyes.

5,10,15,20-Tetraaryl-21-vacataporphyrin (butadieneporphyrin, an annulene-porphyrin hybrid) which contains a vacant space instead of heteroatomic bridge acts as a ligand toward palladium(II). The metal ion of square-planar coordination geometry is firmly held via three pyrrolic nitrogen atoms where the fourth coordination place is occupied by a monodentate ligand or by an annulene part of vacataporphyrin. The macrocycle reveals the unique structural flexibility triggered by coordination of palladium. The structural rearrangements engage the C(20)C(1)C(2)C(3)C(4)C(5) annulene fragment which serves as a linker between two pyrrolic rings of vacataporphyrin albeit the significant ruffling of the tripyrrolic block is also of importance. Two fundamental modes of interactions between the palladium ion and annulene moiety have been recognized. The first one resembles an eta(2)-type interaction and involves the C(2)C(3) unit of the butadiene part. Alternatively the profound conformational adjustments allowed an in-plane coordination through the deprotonated trigonally hybridized C(2) center of butadiene. The coordinated vacataporphyrin acquires Hückel or extremely rare Möbius topologies readily reflected by spectroscopic properties. The palladium vacataporphyrin complexes reveal Hückel aromaticity or Möbius antiaromaticity of [18]annulene applying the butadiene fragment of vacataporphyrin as a topology selector. The properties of specific conformers were determined using (1)H NMR and density functional theory calculations.

Complexes of zinc porphyrin oligomers with multivalent ligands can be denatured by adding a large excess of a monodentate ligand, such as quinuclidine. We have used denaturation titrations to determine the stabilities of the complexes of a cyclic zinc-porphyrin hexamer with multidentate ligands with two to six pyridyl coordination sites. The corresponding complexes of linear porphyrin oligomers were also investigated. The results reveal that the stepwise effective molarities (EMs) for the third through sixth intramolecular coordination events with the cyclic hexamer are extremely high (EM = 10(2)-10(3) M), whereas the values for the linear porphyrin oligomers are modest (EM ≈ 0.05 M). The speciation profiles for the denaturation reactions demonstrate that intermediate species are not significantly populated and that these equilibria are well described by a highly cooperative two-state model.

The aromaticity of porphyrinoids can be substantially altered by reversible modification of their original electronic structures. Well-defined modulators can be used as a means to initiate these modifications, including redox processes, acid-base chemistry, and conformational phenomena. This Focus Review emphasizes the situation for which a single macrocyclic frame alternatively adopts diatropic and paratropic features and both situations are readily and mutually exchangeable. Eventually, such a porphyrinoid transformation can be explored as a suitable element to construct switchable optoelectronic materials.

Condensation of 2,4-bis(phenylhydroxymethyl)furan with pyrrole and p-toluylaldehyde formed, instead of the expected 5,20-diphenyl-10,15-di(p-tolyl)-2-oxa-21-carbaporphyrin, a pyrrole addition product [(H,pyr)OCPH]H(2); this product can formally be considered as an effect of hydrogenation of 3-(2'-pyrrolyl)-5,20-diphenyl-10,15-di(p-tolyl)-2-oxa-21-carbaporphyrin ([(pyr)OCPH]H). The new oxacarbaporphyrinoid presents the (1)H NMR spectroscopy features of an aromatic molecule, including the upfield shift of the inner H21 atom. Insertion of NiCl(2) or PdCl(2) into [(H,pyr)OCPH]H(2) gave two structurally related organometallic complexes, [(pyr)OCP]Ni(II)] and [(pyr)OCP]Pd(II)], in which the metal ions are bound by three pyrrolic nitrogens and the trigonally hybridized C21 atom of the inverted furan. The reaction of [(H,pyr)OCPH]H(2) with silver(I) acetate yields a stable Ag(III) complex [(C(2)H(5)O,pyr)OCP]Ag(III)] substituted at the C3 position by the ethoxy and pyrrole moieties. The macrocyclic frame of [(H,pyr)OCPH]H(2) is conserved. Addition of trifluoroacetic acid to [(C(2)H(5)O,pyr)OCP]Ag(III)] yielded a new aromatic complex [(pyr)OCP]Ag(III)](+). The structures of [(pyr)OCP]Ni(II)] and [(C(2)H(5)O,pyr)OCP]Ag(III)] have been determined by X-ray crystallography. In both molecules the macrocycles are only slightly distorted from planarity and the nickel(II) and silver(III) are located in the NNNC plane. The dihedral angle between the macrocyclic and appended-pyrrole planes of [(pyr)OCP]Ni(II)] reflects the biphenyl-like arrangement with the NH group pointing out toward the adjacent phenyl ring on the C5 position. Tetrahedral geometry around the C3 atom was detected for [(C(2)H(5)O,pyr)OCP]Ag(III)]. The Ni[bond]C and Ag[bond]C bond lengths are similar to other nickel(II) or silver(III) carbaporphyrinoids where the trigonal carbon atom coordinates the metal ion. The trend detected in the (13)C chemical shifts for the appended-pyrrole resonances has been rationalized by the extent of effective conjugation between the macrocycle and the appended pyrrole moiety controlled by the hybridization of the C3 atom and the metal ion oxidation state. The dianionic or trianionic macrocyclic core of the pyrrole-appended derivatives is favored to match the oxidation state of nickel(II), palladium(II), or silver(III), respectively.