Nora Y. Nelson

Synthesis, Reactivity and Structural Chemistry of 5,10,15,20-Tetraalkylporphyrins

The synthesis, reactivity and full characterization of a series of meso-tetraalkyl porphyrins and metalloporphyrins with R ≡ n-butyl (6), 2-methyl-propyl (7), isopropyl (8), l-ethyl-propyl (10) and tert-butyl (11) groups are reported. Derivatives of the last of these show considerably bathochromically shifted absorption bands and the crystal structure of Zn(II)11(pyr) exhibits a severely ruffled macrocycle conformation. Systematic crystallographic studies of the porphyrins showed that the free base porphyrins with R ≡ n-butyl (H26), 2-methyl-propyl (H27) and 1-ethyl-propyl (H210) are planar. A larger conformational variety was found for the metal complexes. While most Ni(II) derivatives and Pd(II)8 showed a ruffled macrocycle conformation with a degree of ruffling exceeding that of meso-unsubstituted porphyrins, both planar and non-planar forms were found for the related Cu(II) derivatives. The Zn(II) complexes of porphyrins with isopropyl or 1-ethyl-propyl exhibited conformations with variable degree of ...

Unusual picosecond 1(π, π∗) deactivation of ruffled nonplanar porphyrins

Abstract Time-resolved and steady-state optical data are presented for metal-free meso-tetraalkylporphyrins, H 2 T(alkyl)P. Introduction of bulky tertiary-butyl or adamantyl substituents at the meso positions induces large ruffling distortions of the macrocycle skeleton that give rise to highly novel photophysical properties of the porphyrins. These include ultrashort 1 (π, π ∗ ) lifetimes of 10–50 ps at 296 K that, intriguingly, lenghten to 10–15 ns at 78 K, the typical timescale found at both temperatures for planar derivatives such as tetra( n -pentyl) and tetraphenyl porphyrins. The ability of the nonplanar ruffled porphyrins to undergo additional structural deformations in the excited singlet state likely underlies their dramatically enhanced rates of internal conversion.

Evidence for unusually strong intramolecular hydrogen bonding in highly nonplanar porphyrins

The 1H NMR spectra of the 5,10,15,20-tetraalkylporphyrins 2–4 reveal a large downfield shift of the signal for the NH protons and a reduction in the activation energy for NH tautomerism as the alkyl substituents become larger; these unusual changes can be rationalized in terms of greatly enhanced intramolecular hydrogen bonding of the NH protons as the bulkier substituents distort the macrocycle from planarity and contract the porphyrin core.

Synthesis and unusual properties of the first 2,3,7,8,12,13,17,18-octabromo-5,10,15,20-tetraalkylporphyrin

The new perhalogenated porphyrin 2,3,7,8,12,13,17,18- octabromo-5,10,15,20-tetrakis(trifluoromethyl)porphinatonickel(II) exhibits several striking features, including an extremely ruffled macrocycle with a very short Ni–N distance, an unusually red-shifted optical spectrum, and, surprisingly, hindered rotation of the meso-trifluoromethyl substituents (ΔG‡278 = 47 kJ mol–1).

Novel products from bromination reactions of 5,10,15,20-tetraisopropylporphyrins

Bromination reactions of nickel(II) or copper(II) complexes of 5,10,15,20-tetraisopropylporphyrin afford the unusual porphodimethenes 2–5; crystallographic data for some of these compounds are presented and possible mechanisms for their formation are discussed.

Stabilization of neutral oxophlorin π-radicals by bulkymeso-alkyl groups

The 15-tert-butyl-5-oxophlorin 1 forms a stable, neutral π-radical species 2, in high yield; the 13,17-bis-unsubstituted 5-oxophlorin analogue 5, under similar conditions, affords the 15-iso-5-oxophlorin 6 and 15-hydroxy compound 7 (both characterized by X-ray crystallography), the latter demonstrating the trapping of the π-radical by dioxygen.