Chirality Inversion in a Molecular Exciton

Abstract

The bichromophoric pigment bilirubin acts as a molecular exciton in its UV−visible and circular dichroism (CD) spectroscopy. The optically active analogue, (βR,β‘R)-dimethylmesobilirubin-XIIIα exhibits intense bisignate CD Cotton effects in the region of its long wavelength UV−vis absorption near 400 nm, with Δ +337, Δ −186 in the nonpolar solvent CHCl3, and nearly as intense Cotton effects in the polar, hydrogen bonding solvent CH3OH: Δ +285, Δ −177. Addition of amines leads to Cotton effect sign inversions: in isopropylamine Δ −605, Δ +375, due to an inversion of molecular chirality.

Vibrational and Electronic Circular Dichroism of Dimethyl Mesobilirubins-XIIIα

“…Also, chiral derivatives of mesobilirubin with stereogenic centers, for example, (βR,β′R)-dimethylmesobilirubin-XIIIα, have been used successfully to investigate conformational properties and conformational changes in solution. 9,10 The same exciton system detected by induced CD of bilirubin, as observed in refs 7 and 8, was also found in natural ECD of their chiral derivatives described in refs 9 and 10, and changes in the CD spectra were then used as indicators of conformational change occurring in most bilirubin systems.…”

“…Indeed, in Figure 4, one can see first that the experimental ECD spectra are well reproduced by the TDDFT-calculated spectra (and the UV spectra as well), except for the overestimate of electronic transition frequencies; second, one learns that the observed large ECD couplet is due to just two transitions with large and opposite rotational strengths (in the figure, the latter are proportional to the bars underneath the bands); the corresponding wide UV band is instead due mostly to one of the two π → π* transitions, the other bearing weaker dipole strength. Thus, the ECD spectrum, which had been previously and fruitfully interpreted in terms of an exciton couplet from dipyrrinone chromophores, 9,10 The very striking effect on the ECD spectrum due to DMSO added to the solutions of (αR,α′R)-and (βS,β′S)-dimethylmesobilirubin-XIIIα in CHCl 3 (Figure 3) clearly suggests a reorientation of the dipyrrinones themselves. Their relative orientation is governed by their ability to rotate about the central CH 2 group to which they are conjoined, with limitations imposed by intramolecular hydrogen bonds.…”

Conformational Changes and Redox Properties of Bimetallic Single Helicates of Hexapyrrole‐α,ω‐dicarbaldehydes

“…In spite of the much larger twist angle of the 2,2'-bipyrrole subunit of 1bPd (58.58)t han that of 1aPd (37.48), the Pd-Pd distance of 1bPd (3.3046 (7) )i so nly slightly longert han that of 1aPd (3.1662 (5) ). This short Pd-Pd distance of 1bPd was enabled by the twist of the dipyrrinu nits in such aw ay that the dipyrrin nitrogen atoms N (2) and N(5) wered irected towards the inside cavity.T he N(2)-C(10)-C(12)-N(3) and N(4)-C(19)-C(21)-N(5) torsion angles in the dipyrrinu nits of 1bPd (À7.2 and À2.58)a re in an opposite sense to those of 1aPd (3.6 and 7.38), which compensates the difference( % 208)i nt he twist angle of their cis-2,2'-bipyrrole subunits. The axial methyl protons (H32 and H52) of the gem-dimethylmethylene bridge of 1bPd are located in the region of ar ing current effect from the pyrrole ring with distances of 2.97 (H52-C4 (pyrrole-b) )a nd 3.18 (H32-C27 (pyrrole-b) ).…”

“…The pyrrole NH hydrogen atoms play ak ey role in buildingh elical conformationso wing to intramolecular and supramolecularh ydrogen-bonding networks. [2] Moreover,t he strongly metal-coordinating nature of oligopyrroles enables generating av ariety of mono-and multi-nuclearh elical metal complexes of single-and multi-stranded structures, M m L n (1 m, n 3). [3][4][5][6][7][8][9][10] Although an umber of those helicates of openchain tetrapyrroles including bile pigments have been known, [3][4][5][6] studies on metal complexes of long-chain oligopyrroles with more than four pyrrolic units remain rather limited, [7][8][9][10] mainly because such ligands are not easy to synthesize.…”

Chirality inversion in the bilirubin molecular exciton

“…At sufficiently high concentration the Cotton effect signs invert and the intensity grows, reaching a plateau where the magnitudes are comparable to or greater than those of 1 in pure CHCl 3 . The ammonia-promoted sign inversion of the CD couplet seen in Figure 4 appears to be a general phenomenon with amines 24 and can be seen when n-propyl amine is added to 1 in CHCl 3 , or when 1 is dissolved (with 2% v/v CHCl 3 ) in neat amine (Fig. 5).…”