Synthesis, Structure, and Spectroscopy of Phenylacetylenylene Rods Incorporatingmeso-Substituted Dipyrrin Ligands

Luminescence from open-shell, first-row transition metal dipyrrin complexes

Several first-row transition metal complexes of the 1,9-bis(2',4',6'-triphenylphenyl)-5-mesityl dipyrrinato ligand and its tetrahalogenated analogues have been synthesized and their luminescence spectra obtained. The protonated ligands, as well as the Li(i), Mn(ii), Cu(i), Cu(ii), and Zn(ii) chelates show appreciable luminescence, despite the paramagnetism of the Mn(ii) and Cu(ii) ions. Fluorescence quantum yields (ΦF) as high as 0.67 were observed for the zinc complex. Luminescence was partially quenched by the introduction of heavy halogens to the backbone of the ligand, as well as by the introduction of paramagnetic metal ions. Room-temperature, solution state phosphorescence was observed from the halogenated dipyrrinato lithium salts, as well as from the non-halogenated Mn(ii) complex.

Short-Chained Anthracene Strapped Porphyrins and their Endoperoxides

The syntheses of short-chained anthracene-strapped porphyrins and their Zn(II)complexes are reported. The key synthetic step is a [2+2] condensation between a dipyrromethane and an anthracene bisaldehyde, 2,2'-((anthracene-9,10-diylbis(methylene))bis(oxy))dibenzaldehyde. Following exposure to white light, self-sensitized singlet oxygen and the anthracene moieties underwent [4+2] cycloaddition reactions to yield the corresponding endoperoxides. 1H NMR studies demonstrate that the endoperoxide readily formed in [D]chloroform and decayed at 85 °C. X-ray crystallography and absorption spectroscopy were used to confirm macrocyclic distortion in the parent strapped porphyrins and endoperoxides. Additionally, X-ray crystallography indicated that endoperoxide formation occurred exclusively on the outside face of the anthracene moiety.

Synthesis and Characterization of a Binuclear Copper(II)-dipyriamethyrin Complex: [Cu2(dipyriamethyrin)(μ2-1,1-acetato)2]

The reaction between dipyriamethyrin and copper(II) acetate [Cu(OAc)₂] afforded what is, to our knowledge, the first transition metal-dipyriamethyrin complex. Molecular and electronic characterization of this binuclear Cu(II) complex via EPR, UV-vis, and single crystal X-ray diffraction analysis revealed marked differences between the present constructs and previously reported binuclear copper(II) hexaphyrin species. UV-vis titration analyses provided evidence for a homotropic positive allosteric effect, wherein the binuclear species is formed without significant intermediacy of a monomeric complex.

Use of the Nascent Isocyclic Ring to Anchor Assembly of the Full Skeleton of Model Chlorophylls

The chlorophyll skeleton contains a chlorin macrocycle and an annulated fifth (or isocyclic) ring bearing 13¹-oxo and 13²-carbomethoxy substituents. The isocyclic ring has traditionally been constructed by annulation of an intact tetrapyrrole macrocycle. Here, a complementary route employs reaction of a gem-dimethyl-substituted dihydrodipyrrin-carboxaldehyde (AD half) and a dipyrromethane bearing a 3-methoxy-1,3-dioxopropyl group (BC half). A McMurry-like reaction of a 2-(2-nitro-5-oxohexyl)pyrrole was employed to construct the second pyrrole ring in one of three BC halves, whereas the other two were prepared by known routes. An AD half and a BC half were joined by Knoevenagel condensation at room temperature, affording the AD,BC-substituted 2-methoxycarbonyl-2-propenone. The subsequent reaction of three AD, BC-propenones (mixture of Z,E-isomers) in CH₃CN containing InCl₃ and In(OTf)₃ at 80 °C afforded the chlorophyll skeleton as the chloroindium(III) chelate; the reaction proceeds via Nazarov cyclization (to form the isocyclic ring), S_EAr (to construct the macrocycle), and 2e^-,2H⁺ oxidation (to give the aromatic chromophore). The absorption spectra of the complexes closely resemble that of chlorophyll a. The present work exploits the nascent isocyclic ring as an anchor for directed assembly of the AD and BC halves, forming both the chlorin macrocycle and the isocyclic ring in a single-flask transformation.

Exploiting exciton coupling of ligand radical intervalence charge transfer transitions to tune NIR absorption

We detail the rational design of a series of bimetallic bis-ligand radical Ni salen complexes in which the relative orientation of the ligand radical chromophores provides a mechanism to tune the energy of intense intervalence charge transfer (IVCT) bands in the near infrared (NIR) region.

We detail the rational design of a series of bimetallic bis-ligand radical Ni salen complexes in which the relative orientation of the ligand radical chromophores provides a mechanism to tune the energy of intense intervalence charge transfer (IVCT) bands in the near infrared (NIR) region. Through a suite of experimental (electrochemistry, electron paramagnetic resonance spectroscopy, UV-vis-NIR spectroscopy) and theoretical (density functional theory) techniques, we demonstrate that bimetallic Ni salen complexes form bis-ligand radicals upon two-electron oxidation, whose NIR absorption energies depend on the geometry imposed in the bis-ligand radical complex. Relative to the oxidized monomer [1˙]⁺ (E = 4500 cm^–1, ε = 27 700 M^–1 cm^–1), oxidation of the cofacially constrained analogue 2 to [2˙˙]²⁺ results in a blue-shifted NIR band (E = 4830 cm^–1, ε = 42 900 M^–1 cm^–1), while oxidation of 5 to [5˙˙]²⁺, with parallel arrangement of chromophores, results in a red-shifted NIR band (E = 4150 cm^–1, ε = 46 600 M^–1 cm^–1); the NIR bands exhibit double the intensity in comparison to the monomer. Oxidation of the intermediate orientations results in band splitting for [3˙˙]²⁺ (E = 4890 and 4200 cm^–1; ε = 26 500 and 21 100 M^–1 cm^–1), and a red-shift for [4˙˙]²⁺ using ortho- and meta-phenylene linkers, respectively. This study demonstrates for the first time, the applicability of exciton coupling to ligand radical systems absorbing in the NIR region and shows that by simple geometry changes, it is possible to tune the energy of intense low energy absorption by nearly 400 nm.

Spacer length dependent architectural diversity in bis-dipyrrin copper(ii) complexes

A series of copper(ii) complexes (1-9 and 3') derived from bis-dipyrrin ligands (L1-L9 and L3') with diverse spacer lengths [-(CH₂)_n-] have been described. Structural diversities in these complexes have been explicitly established by spectral and structural studies on these and a closely related nickel(ii) complex (3''). All the ligands and complexes have been thoroughly characterized by spectroscopic studies (ESI-MS, IR, ¹H, ¹³C NMR, UV/vis) and structures of 2, 3', 3'', 6, 8 and 9 were determined by X-ray single crystal analyses. It has been unambiguously established that ligands with n ≤ 6 gave heteroleptic binuclear (1-5), while those with n ≥ 7 yielded homoleptic mononuclear (6-9) bis-dipyrrinato complexes. Spectral and structural studies revealed distorted square planar (1-5) and distorted tetrahedral geometries (6-9) about the copper(ii) centre in these complexes which has further been evidenced by EPR and electrochemical studies. Structural differences based on the odd and even number of methylene spacers in these complexes have been supported by DFT studies. A line between the syn- and anti-conformations of the complexes has been drawn on the basis of a limiting spacer length.

Multicomponent syntheses of functional chromophores

Multicomponent reactions are a valuable tool for the synthesis of functional π-electron systems. Two different approaches can be taken into account for accessing the target structures. In the more conventional scaffold approach an already existing chromophore is coupled with other components to give a complex functional π-system. Here, electronically monotonous components can also be introduced, which may exert synergistic electronic effects within the novel compound. The more demanding chromophore concept generates a complete π-electron system and a scaffold concurrently. The latter approach is particularly stimulating for methodologists since π-systems might be accessible from simple starting materials. This review encompasses the advances in the preparation of functional π-electron systems via multicomponent processes during the past few years, based both on the scaffold and chromophore concepts. Besides the synthetic strategies the most important properties, i.e. redox potentials, absorption and emission maxima or fluorescence quantum yields, of the synthesized molecules are highlighted.

Synthesis of dipyrrin-containing architectures

Dipyrrins are valuable precursors to dyes [dipyrrinatoboron difluoride, bis(dipyrrinato)-zinc(II) complexes] and serve as ligands in a variety of self-assembled materials. Six new dipyrrin-containing architectures have been synthesized. The architectures include bis(dipyrrinato) complexes containing copper(II) or palladium(II), a dipyrrin bearing a protected phosphonic acid unit, a porphyrin bearing two dipyrrins in a trans configuration, a linear diphenylethyne-linked dipyrromethane-dipyrrin building block, and a triad composed of two zinc porphyrins joined via an intervening bis(dipyrrinato)copper(II) complex. Two porphodimethenatozinc complexes were prepared and found to have Φ f ≤ 0.002 (in toluene at room temperature), which is substantially less than the analogous bis(dipyrrinato)zinc complexes. Taken together, the syntheses described herein should broaden access to dipyrrins for use as complexation motifs in supramolecular chemistry and as pigments in light-harvesting applications.

Nanoscale Metal Coordination Macrocycles Fabricated by Using “Dimeric” Dipyrrins

Covalently linked dipyrrin (dipyrromethene) dimers have afforded nanoscale [2+2]-type neutral coordination macrocycles with a diagonal of about 1.6 nm. Two moieties of the achiral dipyrrin-Zn(II) complex yield the chiral coordination macrocycles as minor species, as well as major meso stereoisomers by the covalent linkages. Tetrahedral Zn(II) coordination by using acyclic ligands enables the dipyrrin-metal complex units to readily rotate and pass through the cavity of the nanorings in order to reveal the transitions between the chiral and achiral isomers.

Preparation and characterization of asymmetric alpha-alkoxy dipyrrin ligands and their metal complexes

Asymmetric alpha-substituted dipyrrins have been synthesized and characterized. The compounds were formed by a metal mediated reaction involving a single alkoxy group substituted into the alpha-position of an alpha,beta-unsubstituted dipyrrin. An alpha-methoxy dipyrrin, 5-(4-cyanophenyl)-1-methoxydipyrrin (alpha-OMe-4-cydpm), was prepared from 5-(4-cyanophenyl)-4,6-dipyrromethane. Methoxy, ethoxy, and propoxy derivatives (alpha-OMe-4-mecdpm, alpha-OEt-4-mecdpm, alpha-OPr-4-mecdpm) of 5-(4-methoxycarbonylphenyl)-4,6-dipyrromethane have also been prepared. A homoleptic, bis(1-methoxy)dipyrrinato zinc(II) complex, [Zn(alpha-OMe-4-mecdpm)(2)], has been synthesized, as has a heteroleptic cobalt(III) complex with one alpha-OMe-4-cydpm ligand and two unsubstituted 5-(4-cyanophenyl)dipyrrin (4-cydpm) ligands ([Co(alpha-OMe-4-cydpm)(4-cydpm)(2)]). The rotational barrier of the meso-aryl substituent of [Zn(alpha-OMe-4-mecdpm)(2)] was found to be 17.3 kcal mol(-1) by variable-temperature NMR spectroscopy. The compounds alpha-OMe-4-cydpm and [Zn(alpha-OMe-4-mecdpm)(2)] have also been characterized by X-ray diffraction. The formation of the new dipyrrin derivatives is shown to be general and can be performed on dipyrrins with various meso-aryl substitutents, with a variety of alcohols, and can be promoted by several metal salts.

Luminescent Dipyrrinato Complexes of Trivalent Group 13 Metal Ions

Although free dipyrrins (dipyrromethenes) do not strongly luminesce, certain dipyrrinato complexes of BF2 and zinc(II) are known to be intensely luminescent species. Two new dipyrrinato fluorophores, based on complexes with gallium(III) and indium(III), are described. Using a previously described meso-mesityl-substituted dipyrrin, namely 5-mesityldipyrrin (mesdpm), the complexes [Ga(mesdpm)3] and [In(mesdpm)3] were prepared and structurally characterized. The complexes display the expected octahedral geometry about the metal ions. In some solvents, such as hexanes, the complexes emit green light upon excitation with UV light at room temperature, with quantum yields of 2.4% ([Ga(mesdpm)3]) and 7.4% ([In(mesdpm)3]) and lifetimes in the low nanosecond range. Observations are consistent with assignment to ligand-localized transitions, and this interpretation is further confirmed by density functional calculations described herein. The new complexes are important additions to the widely used family of dipyrrin-based fluorescent species and show that dipyrrinato complexes containing metals other than BF2 and zinc(II) may be useful fluorophores.