James T. Engle

A new highly fluorescent and symmetric pyrrole-BF2 chromophore: BOPHY.

The new fluorescent chromophore BOPHY can be readily synthesized in two steps from commercially available reagents via the coupling of pyrrole-2-carboxaldehyde with hydrazine followed by reaction with BF3. The resultant symmetric and dimeric tetracycle is composed of two BF2 units in six-membered chelate rings appended with pyrrole units on the periphery. The quantum yields of fluorescence for the unmodified compound and the tetramethyl variant are near unity, with values of 95 and 92%, respectively, in CH2Cl2. We have probed the electronic structure of this compound via cyclic voltammetry and density functional theory analysis.

meso-Arylporpholactones and their Reduction Products

The rational syntheses of meso-tetraaryl-3-oxo-2-oxaporphyrins 5, known as porpholactones, via MnO(4)(-)-mediated oxidations of the corresponding meso-tetraaryl-2,3-dihydroxychlorins (7) is detailed. Since chlorin 7 is prepared from the parent porphyrin 1, this amounts to a 2-step replacement of a pyrrole moiety in 1 by an oxazolone moiety. The stepwise reduction of the porpholactone 5 results in the formation of chlorin analogues, meso-tetraaryl-3-hydroxy-2-oxachlorin (11) and meso-tetraaryl-2-oxachlorins (12). The reactivity of 11 with respect to nucleophilic substitution by O-, N-, and S-nucleophiles is described. The profound photophysical consequences of the formal replacement of a pyrrole with an oxazolone (porphyrin-like chromophore) or (substituted) oxazole moiety (chlorin-like chromophore with, for the parent oxazolochlorin 12, red-shifted Q(x) band with enhanced oscillator strengths) are detailed and rationalized on the basis of SAC-CI and MNDO-PSDCI molecular orbital theory calculations. The single crystal X-ray structures of the porpholactones point at a minor steric interaction between the carbonyl oxygen and the flanking phenyl group. The essentially planar structures of all chromophores in all oxidation states prove that the observed optical properties originate from the intrinsic electronic properties of the chromophores and are not subject to conformational modulation.

meso-Aryl-3-alkyl-2-oxachlorins.

The formal replacement of a pyrrole moiety of meso-tetraarylporphyrin 1 by an oxazole moiety is described. The key step is the conversion of porpholactones 4 (prepared by a known two-step oxidation procedure from 1) by addition of alkyl Grignard reagent to form meso-tetraaryl-3-alkyl-2-oxachlorins 9 (alkyloxazolochlorins; alkyl = Me, Et, iPr). Hemiacetal 9 can be converted to an acetal, reduced to an ether, or converted to bis-alkyloxazolochlorins 11. The optical properties (UV-visible and fluorescence spectroscopy) are described. The chlorin-like optical properties of the alkyloxazolochlorins are compared to regular chlorins, such as 2,3-dihydroxychlorins and nonalkylated oxazolochlorins made by reduction from porpholactone 4. The conformations of the mono- and bis-alkylated 2-oxachlorins, as determined by single crystal X-ray diffractometry, are essentially planar, thus proving that their optical properties are largely due to their intrinsic electronic properties and not affected by conformational effects. The mono- and bis-3-alkyl-2-oxachlorins are a class of readily prepared and oxidatively stable chlorins.

Triphenylene-Fused Porphyrins

Triphenylene has been successfully fused to the porphyrin periphery through a convenient oxidative ring-closure reaction. Bistriphenylene-fused porphyrins and a dibenzo[fg,op]tetracene-fused porphyrin have also been obtained using a similar approach. These π-extended porphyrins exhibited broadened and bathochromic shifted UV-vis absorptions.

Phlorins Bearing Different Substituents at the sp3-Hybridized Meso-Position

Phlorins bearing different substituents at the sp(3)-hybridized meso-position were investigated. The extent to which different substituents at this unique position can influence phlorin spectroscopic properties, structure, and stability is of interest given that such substituents are not in direct conjugation with the phlorin macrocycle. While the effect of various substituents at the sp(2)-hybridized positions has been the subject of prior investigations, the impact of different substituents at the saturated carbon atom has not been systematically examined. In this study, phlorins with different combinations of geminal methyl and phenyl substituents were prepared in yields of 24-49% via dipyrromethane + dipyrromethanedicarbinol routes, and their NMR spectra, UV-vis spectra, X-ray crystal structures, and stability toward light and air were compared. The nature of the substituents at the sp(3)-hybridized position was found to impact spectroscopic properties, structure, and stability to varying degrees. Thus, the choice of substituents at the sp(3)-hybridized meso-position provides a further option for altering phlorin properties.

The synthesis and pH-dependent behaviour of Re(CO)3 conjugates with diimine phenolic ligands

In this report we present a study of a series of Re(CO)3 pyridine-imine complexes with pendant phenol groups. We investigated the effects of the position of the phenol hydroxyl group (para, meta or ortho to the imine) on the steric and electronic characteristics of a series of Re(CO)3X(pyca-C6H4OH) compounds, where X = Cl, Br and pyca = pyridine-2-carbaldehyde imine. These compounds can be generated either via ligand synthesis followed by metal chelation (compound 4) or via a one-pot method (compounds 2, 3, 5 and 6). All six compounds show striking differences in pH-dependent UV-visible absorption based on the position of the phenol hydroxyl group.

Pentacene-Fused Diporphyrins

In this work, we report the synthesis, spectroscopic characterization, and theoretical analysis of a linearly conjugated pentacene-fused porphyrin dimer and cross-conjugated quinone-fused dinaphtho[2,3]porphyrins. These multichromophoric systems display non-typical UV-visible absorptions of either porphyrins or pentacenes/quinones. UV-visible, emission and magnetic circular dichroism (MCD) spectroscopy suggest strong electronic interactions among the multichromophores in the system. DFT calculations revealed the delocalization of the HOMOs and LUMOs spanning the entire dimer and linker assembly. The pentacene-fused porphyrin dimer is significantly more stable than both the corresponding pentacene and the heptacene derivatives. The availability of these huge π-extended and electronically highly interactive multichromophoric systems promises unprecedented electronic and photophysical properties.

Phthalocrowns: Isoindoline−Crown Ether Macrocycles

The reaction of diiminoisoindoline with amine-terminated polyethers results in the formation of phthalocrown macrocycles. For n = 1 (where n is the number of ether units), a 2 + 2 condensation takes place, but for n = 2 and 3, a 1 + 1 macrocycle formation occurs. The n = 2 phthalocrown is particularly stable due to a strong intramolecular hydrogen bond, but the n = 3 ring hydrolyzes to form a 3-imino-1-oxoisoindoline derivatized crown ether species. For the n = 1 phthalocrown, we observed dynamic behavior in the (1)H NMR spectrum, and using VTNMR were able to measure a ΔG(‡) = 44.6 kJ/mol for proton exchange.

The structures of several modified isoindolines, the building blocks of phthalocyanines

This report presents the single crystal X-ray structures of several substituted isoindolines that have been frequently used as starting materials for phthalocyanines, phthalocyanine analogs and related chelates. The structures of 1,3-diiminoisoindoline (1), 1,3-bis(hydroxyimino)isoindoline (2), 1,4-diaminophthalazine (3), 1,1,3-trichloroisoindoline (4) and 3-imino-1-oxoisoindoline (5) are reported; compounds 2 and 3 are synthesized from diiminoisoindoline (1) and 4 and 5 are produced from phthalimide. All five compounds are planar macrocycles, and localization of double bonds can be readily determined. We elucidated one of the known structures of 1 at low temperature, and observed two additional new structures of 1. For the crystal forms of 1 and compounds 2, 3, and 5, hydrogen bonding in the solid state was observed. Compounds 1, 2 and 3 form extended hydrogen bonded arrays in the solid state, whereas 5 forms discrete hydrogen bonded dimers.

Bis[bis­(3,5-dimethyl-1H-pyrazol-1-yl)­borato]cobalt(II)

The asymmetric unit of the title compound, [Co(C10H16BN4)2], comprises one unit of the complex. The geometry around the CoII ion is a distorted tetrahedron. The dihedral angles between the pyrazole rings in the two ligands are 47.19 (15) and 47.20 (16)°, while that between the coordination planes is 79.77 (7)°.