Origin of the Red Shifts in the Optical Absorption Bands of Nonplanar Tetraalkylporphyrins

The view that the large red shifts seen in the UV−visible absorption bands of peripherally crowded
nonplanar porphyrins are the result of nonplanar deformations of the macrocycle has recently been
challenged by the suggestion that the red shifts arise from substituent-induced changes in the macrocycle
bond lengths and bond angles, termed in-plane nuclear reorganization (IPNR). We have analyzed the
contributions to the UV−visible band shifts in a series of nickel or zinc meso-tetraalkylporphyrins to establish
the origins of the red shifts in these ruffled porphyrins. Structures were obtained using a molecular mechanics
force field optimized for porphyrins, and the nonplanar deformations were quantified by using normal-coordinate structural decomposition (NSD). Transition energies were calculated by the INDO/S semiempirical
method. These computational studies demonstrate conclusively that the large Soret band red shifts
(∼40 nm) seen for very nonplanar meso-tetra(tert-butyl)porphyrin compared to meso-tetra(methyl)porphyrin
are primarily the result of nonplanar deformations and not IPNR. Strikingly, nonplanar deformations along
the high-frequency 2B1u and 3B1u normal coordinates of the macrocycle are shown to contribute significantly
to the observed red shifts, even though these deformations are an order of magnitude smaller than the
observed ruffling (1B1u) deformation. Other structural and electronic influences on the UV−visible band
shifts are discussed and problems with the recent studies are examined (e.g., the systematic underestimation
of the 2B1u and 3B1u modes in artificially constrained porphyrin structures that leads to a mistaken attribution
of the red shift to IPNR). The effect of nonplanar deformations on the UV−visible absorption bands is then
probed experimentally with a series of novel bridled nickel chiroporphyrins. In these compounds, the
substituent effect is essentially invariant and the amount of nonplanar deformation decreases as the length
of the straps connecting adjacent meso-cyclopropyl substituents decreases (the opposite of the effect
observed for conventional strapped porphyrins). Several spectroscopic markers for nonplanarity (UV−visible bands, resonance Raman lines, and 1H NMR resonances) are found to correlate with time-averaged
deformations obtained from an NSD analysis of molecular dynamics snapshot structures. These results
suggest that UV−visible band shifts of tetrapyrroles in proteins are potentially useful indicators of changes
in nonplanarity provided other structural and electronic factors can be eliminated.