The relative steric size of methyl, ethyl, isopropyl, tert-butyl, phenyl, and benzyl groups has been determined from a sensitive tetrapyrrole model and exciton coupling circular dichroism (CD) measurements. Unlike the classical cyclohexane model, from which the relative steric demand of functional groups has been assessed quantitatively (A-values) and is based on the preference for equatorial vs axial orientations, the bilirubin model assesses substituent size from head-to-head steric compression. Thus, exciton CD amplitudes of a set of sensitive anti-chiral α(R/S)-substituted-β‘(S)-methylmesobilirubins-XIIIα (1−6) suggests an apparent relative steric size:  tert-butyl ∼ isopropyl > phenyl ∼ ethyl > benzyl > methyl. The order is somewhat different from that obtained by measuring equatorial vs axial configuration preferences on substituted cyclohexanes by NMR spectroscopy:  tert-butyl ≫ phenyl > isopropyl > ethyl ∼ benzyl ∼ methyl.

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.