Exercises in molecular gymnastics—bending, stretching and twisting porphyrins

Supramolecular electron transfer by anion binding

The pH dependencies of the UV−vis and fluorescent spectra of new water-soluble dendritic porphyrins and tetrabenzoporphyrins were studied. Because of extended π-conjugation and nonplanar distortion, the absorption and the emission bands of tetraaryltetrabenzoporphyrins (Ar4TBP) are red-shifted and do not overlap with those of regular tetraarylporphyrins (Ar4P). When encapsulated inside dendrimers with hydrophilic outer layers, Ar4Ps and Ar4TBPs become water soluble and can serve as pH indicators, with pK's adjustable by the peripheral charges on the dendrimers. Two new dendritic porphyrins, Gen 4 polyglutamic porphyrin dendrimer H2P-Glu4OH (1) with 64 peripheral carboxylates and Gen 1 poly(ester amide) Newkome-type tetrabenzoporphyrin dendrimer H2TBP-Nw1OH (2) with 36 peripheral carboxylates, were synthesized and characterized. The pK's of the encapsulated porphyrins (pKH2P-Glu4OH = 6.2 and pKH2TBP-Nw1OH = 6.3) were found to be strongly influenced by the dendrimers, revealing significant electrostatic shi...

Porphyrin and Tetrabenzoporphyrin Dendrimers: Tunable Membrane-Impermeable Fluorescent pH Nanosensors

Conformational control of cofactors in nature – the influence of protein-induced macrocycle distortion on the biological function of tetrapyrroles

The ground- and excited-state properties of a series of meso-tetraphenylporphyrin (H2TPP) diacids, [H4TPP](X)2 (X = F, Cl, Br, I), ad hoc synthesized and characterized by 1H NMR, RLS, and UV−vis spectroscopies, have been studied theoretically using density functional theory (DFT) and time-dependent density functional theory (TDDFT). Several conformations corresponding to different deformations of the porphyrin core have been explored. The nearly degenerate purely saddled (sad) and hybrid (saddled with a small superimposed ruffling:  sadruf) conformations are the preferred “gas phase” conformations. The type and degree of distortion of the macrocycle and the orientation of the phenyl rings compare well to X-ray data available for H2TPP diacids. Two electronic structure features are key to an understanding of the optical and photophysical properties. (1) Strong interaction of the π-system of the phenyls with the π-system of the porphyrin leads to an upshift of the G-a2u (G = Gouterman) orbital and, hence, t...

Effects of porphyrin core saddling, meso-phenyl twisting, and counterions on the optical properties of meso-tetraphenylporphyrin diacids: The [H4TPP](X)2 (X = F, Cl, Br, I) series as a case study

The crystal structures of three porphyrin diacid species, [H4OEP](ClO4)2, [H4TPP](ClO4)2, and [H4TMP](ClO4)2, have been determined from low-temperature X-ray diffraction data to delineate how the peripheral substituents of the porphyrin affect the overall molecular flexibility. [H4OEP](ClO4)2 (|Cb| = 0.46 A), [H4TMP](ClO4)2 (|Cb| = 0.67 A, molecule 1), and [H4TPP](ClO4)2 (|Cb| = 0.93 A) show increasingly saddled core conformations with effective D2d symmetry. The mean porphyrin−aryl group dihedral angles in [H4TPP](ClO4)2 and [H4TMP](ClO4)2 (molecule 1) are 27(2)° and 63(13)°, respectively. The steric bulk of the mesityl substituents in [H4TMP]2+ limits the range of observed porphyrin−aryl group dihedral angles to >50° and, consequently, the magnitude of the core distortion. [H4TMP]2+ is therefore less flexible than [H4TPP]2+. Molecular mechanics calculations, using a modified version of MM2(87) and a newly developed force field for porphyrin diacids, correctly predict that the flexibility of meso-tetraar...

W. Robert Scheidt

Papers

An Analysis of Porphyrin Molecular FlexibilityUse of Porphyrin Diacids